Reserpine maintains photoreceptor survival in retinal ciliopathy by resolving proteostasis imbalance and ciliogenesis defects

Abstract
Editor's evaluation
eLife digest
Introduction
Results
Discussion
Materials and methods
Appendix 1
Data availability
References
Article and author information
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Abstract

Ciliopathies manifest from sensory abnormalities to syndromic disorders with multi-organ pathologies, with retinal degeneration a highly penetrant phenotype. Photoreceptor cell death is a major cause of incurable blindness in retinal ciliopathies. To identify drug candidates to maintain photoreceptor survival, we performed an unbiased, high-throughput screening of over 6000 bioactive small molecules using retinal organoids differentiated from induced pluripotent stem cells (iPSC) of rd16 mouse, which is a model of Leber congenital amaurosis (LCA) type 10 caused by mutations in the cilia-centrosomal gene CEP290. We identified five non-toxic positive hits, including the lead molecule reserpine, which maintained photoreceptor development and survival in rd16 organoids. Reserpine also improved photoreceptors in retinal organoids derived from induced pluripotent stem cells of LCA10 patients and in rd16 mouse retina in vivo. Reserpine-treated patient organoids revealed modulation of signaling pathways related to cell survival/death, metabolism, and proteostasis. Further investigation uncovered dysregulation of autophagy associated with compromised primary cilium biogenesis in patient organoids and rd16 mouse retina. Reserpine partially restored the balance between autophagy and the ubiquitin-proteasome system at least in part by increasing the cargo adaptor p62, resulting in improved primary cilium assembly. Our study identifies effective drug candidates in preclinical studies of CEP290 retinal ciliopathies through cross-species drug discovery using iPSC-derived organoids, highlights the impact of proteostasis in the pathogenesis of ciliopathies, and provides new insights for treatments of retinal neurodegeneration.

Editor's evaluation

This work provides an important pipeline for high-throughput screening platform to be used for drug discovery. The current data are complete. Validation of human patients-derived iPSC clones and functional assays in mice further strengthens the current conclusion.

https://doi.org/10.7554/eLife.83205.sa0

eLife digest

Leber congenital amaurosis (LCA) is an inherited disease that affects the eyes and causes sight loss in early childhood, which generally gets worse over time. Individuals with this condition have genetic mutations that result in the death of light-sensitive cells, known as photoreceptors, in a region called the retina at the back of the eye. Patients carrying a genetic change in the gene CEP290 account for 20-25% of all LCA.

At present, treatment options are only available for a limited number of patients with LCA. One option is to use small molecules as drugs that may target or bypass the faulty processes within the eye to help the photoreceptors survive in many different forms of LCA and other retinal diseases. However, over 90% of new drug candidates fail the first phase of clinical trials for human diseases. This in part due to the candidates having been developed using cell cultures or animal models that do not faithfully reflect how the human body works.

Recent advances in cell and developmental biology are now enabling researchers to use stem cells derived from humans to grow retina tissues in a dish in the laboratory. These tissues, known as retinal organoids, behave in a more similar way to retinas in human eyes than those of traditional animal models. However, the methods for making and maintaining human retinal organoids are time-consuming and labor-intensive, which has so far limited their use in the search for new therapies.

To address this challenge, Chen et al. developed a large-scale approach to grow retinal organoids from rd16 mutant mice stem cells (which are a good model for LCA caused by mutations to CEP290) and used the photoreceptors from these organoids to screen over 6,000 existing drugs for their ability to promote the survival of photoreceptors. The experiments found that the drug reserpine, which was previously approved to treat high blood pressure, also helped photoreceptors to survive in the diseased organoids. Reserpine also had a similar effect in retinal organoids derived from human patients with LCA and in the rd16 mice themselves.

Further experiments suggest that reserpine may help patients with LCA by partially restoring a process by which the body destroys and recycles old and damaged proteins in the cells. The next steps following on from this work will be to perform further tests to demonstrate that this use of reserpine is safe to enter clinical trials as a treatment for LCA and other similar eye diseases.

Introduction

Once considered vestigial, the primary cilium has emerged as a key microtubule-based organelle that senses the external environment and modulates diverse signaling pathways in multiple tissues. Aberrant cilium biogenesis and/or ciliary transport and functions lead to numerous diseases, collectively termed ciliopathies, which manifest from sensory abnormalities to syndromic disorders with multi-organ pathologies including aberrant kidney morphogenesis, brain malformation, and congenital retinal degeneration (Reiter and Leroux, 2017). The mammalian retina is an extension of the central nervous system that is specialized for vision (London et al., 2013). The visual information is captured by rod and cone photoreceptors, integrated and processed by interneurons, and transmitted to the cortex through the retinal ganglion cells. Inability of the retinal photoreceptors to detect and/or transmit light-triggered signals is a major cause of vision impairment in retinal and macular degenerative diseases (Veleri et al., 2015; Wright et al., 2010; Verbakel et al., 2018). Mutations in over 200 genes can lead to inherited retinal diseases (IRDs) (RetNet, https://sph.uth.edu/retnet/), with a combined prevalence of 1/3–4000 individuals (Verbakel et al., 2018; Hanany et al., 2020). Among them, up to 20% of IRD-causing genes are involved in primary cilium biogenesis or functions (Zelinger and Swaroop, 2018). Due to extensive clinical and genetic heterogeneity, high inter- and intra-family variability, and incomplete penetrance (Farrar et al., 2017), treatment options for IRD are limited (Garafalo et al., 2020). Only one gene therapy drug is currently approved by FDA for Leber congenital amaurosis (LCA, MIM204000) caused by RPE65 mutations (Pierce and Bennett, 2015). While the initial clinical outcomes were promising, the long-term data of this gene therapy drug are less encouraging (Gardiner et al., 2020). Furthermore, the development of individualized gene therapy protocols for divergent mutations in a multitude of genes for rare IRDs would be time-consuming, expensive, and labor-intensive (Tambuyzer et al., 2020). Thus, gene-agnostic paradigms are being developed for retinal and macular diseases using model organisms and/or stem cell-based approaches (Scholl et al., 2016). Small-molecule or antibody drugs represent a relatively affordable and scalable option (Tambuyzer et al., 2020). However, over 90% of drug candidates fail in Phase I clinical trials due to the lack of effective model systems which faithfully recapitulate the pathophysiology of human diseases (Horvath et al., 2016). The limited number of cells in the retina and challenges in the maintenance of primary retinal cultures have also hindered the progress of therapeutic development.

LCA is a clinically severe and genetically heterogeneous group of IRDs, leads to vision loss in early childhood (Cremers et al., 2018). LCA10 caused by mutations in the cilia-centrosomal gene CEP290 (also called NPHP6) is one of the most common types, accounting for over 20% of patients (den Hollander et al., 2008). Mutations in CEP290 exhibit pleiotropy with phenotypes ranging from LCA (affecting vision and other sensory systems) to nephronophthisis, and Joubert and Meckel syndromes involving multiple organ systems (Sayer et al., 2006; Valente et al., 2006; McEwen et al., 2007; Coppieters et al., 2010; Drivas et al., 2015). The large 290 kDa centrosome-cilia protein CEP290 is ubiquitously expressed and localized to the Y-links of the transition zone of primary cilia, where it acts as a hub for connecting major protein complexes and likely performs a gating function (Sayer et al., 2006; Craige et al., 2010; Rachel et al., 2012; Drivas et al., 2013). The rd16 mouse carries an in-frame deletion in the myosin tail of CEP290, which causes a malformed connecting cilium (equivalent to the transition zone) and rudimentary outer segment (the primary cilium of photoreceptor) structure, leading to rapid degeneration of photoreceptors (Chang et al., 2006; Rachel et al., 2015). Additional studies have also demonstrated the critical role of CEP290 in cilia biogenesis (Drivas et al., 2013; Tsang et al., 2008; Barbelanne et al., 2015; Wu et al., 2020; Prosser et al., 2022). Thus, LCA10 is considered a retinal ciliopathy caused by hypomorphic mutations (den Hollander et al., 2006; Cideciyan et al., 2007) that eliminate some functions of CEP290 (Shimada et al., 2017). Vision impairment in early childhood of LCA10 patients imposes a considerable burden on families and society (Leroy et al., 2021). Notably, these patients demonstrate sparing of the foveal cones even at late stages of life, providing an attractive target for therapy (Cideciyan et al., 2011). Gene replacement using an AAV vector is difficult because of the large coding region of CEP290. Thus, other strategies including antisense oligonucleotides are currently under investigation (Collin et al., 2012; Gerard et al., 2012; Burnight et al., 2014; Dooley et al., 2018; Dulla et al., 2018; Kim et al., 2018). However, no approved treatment is currently available for alleviating vision loss due to CEP290 defects.

Generation of three-dimensional tissue organoids from induced pluripotent stem cells (iPSCs) (Nakano et al., 2012; Sasai, 2013) has revolutionized biological and disease-modeling investigations and created opportunities for high-throughput screening (HTS) to design novel treatment paradigms (Welsbie et al., 2017). Further refinements of human retinal organoid culture protocols have permitted high efficiency and yield, developmental staging, and higher reproducibility (Zhong et al., 2014; Capowski et al., 2019; Kaya et al., 2019; Regent et al., 2022). Biogenesis of diverse cell types in retinal organoid cultures largely recapitulates structural and temporal development of in vivo human retina and can display intrinsic light responses mimicking those of primate fovea (Saha et al., 2022; Cowan et al., 2020; Sridhar et al., 2020). Patient iPSC-derived human retinal organoids demonstrate disease-associated phenotypes and are being used to evaluate various therapeutic approaches (Bell et al., 2020; Kruczek and Swaroop, 2020). However, long and tedious generation protocols spanning 150+ days, inherent variability, and lack of compatible HTS assays still pose challenges to the application of human retinal organoids for developing therapies (Struzyna and Watt, 2021).

In this study, we aimed to establish a reliable drug discovery pipeline using an organoid-based HTS platform with the goal to identify drug candidates for maintaining photoreceptor survival in retinopathies, focusing initially on LCA10. For primary HTS of over 6000 bioactive small molecules, we designed survival assays using rd16 mouse iPSC-derived retinal organoids, which showed compromised photoreceptor development and viability. An FDA-approved small molecule drug reserpine was identified as an efficacious lead compound, which also enhanced photoreceptor development/survival in LCA10 patient iPSC-derived retinal organoids as well as in the rd16 retina in vivo. Transcriptomic analyses of drug-treated patient organoids indicated modulation of cell survival pathways including p53 and proteostasis by reserpine. Further examination validated the mis-regulation of autophagy in patient organoids and rd16 retina and demonstrated partial restoration of proteostasis and improved ciliogenesis after reserpine treatment. Our study thus establishes a cross-species drug discovery pipeline using organoid-based HTS and identifies a repurposing drug candidate for maintaining photoreceptor survival in LCA10 patients. As the action mechanisms of reserpine are partially through the modulation of proteostasis, which could be a common pathological impact of ciliopathies, reserpine, and its derivatives could potentially serve as an effective treatment for patients with other retinal ciliopathies.

Results

Establishment of an organoid-based HTS platform

We designed an unbiased HTS assay to identify drug candidates that might improve photoreceptor development and/or survival. For a primary screen, we chose mouse retinal organoids because of a much shorter photoreceptor differentiation time and efficient generation of rudimentary outer segments using our modified HIPRO protocol (Chen et al., 2016a). We decided to use a karyotypically normal iPSC line derived from the Nrl-GFP containing rd16 mice (Figure 1—figure supplement 1A), which expresses green fluorescent protein (GFP) in all rods and carries an in-frame deletion in the myosin tail homology domain of CEP290 (Figure 1—figure supplement 2A). The rd16 mice are a model of photoreceptor degeneration observed in LCA10 patients (Chang et al., 2006). The iPSC lines derived from rd16 mice displayed similar morphology, proliferation rate, and stem cell properties as those derived from the wild-type (WT) (Figure 1—figure supplement 2B and C). WT and rd16 retinal organoids also showed comparable morphology at the early stages of differentiation (Figure 1A). However, at later stages, rod photoreceptors in rd16 organoids displayed aberrant morphology, with malformed or missing ciliary axoneme, connecting cilium and ciliary rootlets (Figure 1—figure supplement 2D). We then performed flow cytometry analyses to evaluate the differentiation of GFP+ rod photoreceptors in the rd16 organoid cultures, with the goal to identify quantifiable phenotypes for HTS. The rd16 organoids demonstrated as much as 50% lower organoid viability and almost 60% fewer GFP+ rod photoreceptors at day (D) 32 (Figure 1B), the end stage of differentiation in mouse retinal organoids.

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Drug discovery pipelines to identify drug candidates.

(A) Morphology of *Nrl*-GFP wild-type (WT) and *rd16* retinal organoids differentiated from mouse-induced pluripotent stem cells (iPSC) at various differentiation time points. (B) Flow cytometry analysis of GFP+ rod photoreceptors and viability at different developmental stages. Each data point summarized at least three batches of independent experiments, each of which included at least 10 organoids. One-way ANOVA was performed to compare the %GFP+ cells and viability of organoids from four different cell lines. *p<0.05; **p<0.01. (C) Fluorescent images of dissociated day (D) 28 WT and *rd16* cells stained by 4',6-diamidino-2-phenylindole (DAPI) and anti-GFP antibody. (D) Schematic outline of the drug discovery strategy.

The phenotypes in GFP+ rd16 rod photoreceptors permitted the development of an HTS screening platform to identify bioactive small-molecule candidates for augmenting rod cell differentiation and/or survival (Figure 1—figure supplement 2E). To avoid the impact of organoid variability on drug effects, we dissociated rd16 organoids into single cells at D25 and performed the treatment using two-dimensional cultures from D26 to D28. These cultures recapitulated the phenotypes detected in non-dissociated organoids, and rd16 cells displayed reduced viability and fewer GFP+ rod cells at D28 compared to the WT (Figure 1C).

Our drug discovery pipeline is illustrated in Figure 1D. In the primary screens, rd16 retinal organoid-derived cells were treated with over 6000 small molecules; of these, 114 compounds revealed higher GFP signal intensity, indicating a positive effect on rod photoreceptor survival. We then eliminated the compounds that showed toxicity or autofluorescence even in the absence of GFP expression (false positives). Fourteen small molecule compounds were then selected for further screening to identify a lead compound.

Selection of the lead compound

We then treated rd16 retinal organoids with the 14 selected small molecules in a secondary assay, which started at D22 and lasted for three days to maximize the drug effect without causing toxicity (Figure 2A). WT, untreated, and treated rd16 organoids were harvested at D29 to allow a sufficient period for the restoration of photoreceptors. The lead compound was selected based on the efficacy of the small molecule drugs to improve photoreceptor development and/or survival. Immunostaining of rhodopsin and S-opsin, markers of rod and cone photoreceptors respectively, was performed to quantify the drug effect (Figure 2B). Rhodopsin is highly expressed and polarized to the apical side of rod photoreceptors in the neural retina of WT organoids. In contrast, rd16 photoreceptors exhibited lower expression of rhodopsin, and the polarity was considerably diminished. Two of the compounds, B05 and to some extent B03, were able to enhance rhodopsin expression, with B05 improving the polarity of expression in the neural retina as well. We note that S-cone photoreceptors are difficult to be maintained in mouse organoid culture (DiStefano et al., 2018); nonetheless, compounds B01 and B05 were able to augment the expression and polarity of S-opsin. Three of the 5 compounds (B01, B03, B05) that showed no toxicity revealed a dramatic increase in rod and/or cone photoreceptors in D29 rd16 organoids (Figure 2C); of these, B05 was the most effective molecule and chosen as the lead compound. Interestingly, the other four molecules (B01-B04) were found to be derivatives of B05, further validating our results. B05 was identified as reserpine, a small-molecule drug that has been approved by the FDA for the treatment of hypertension and schizophrenia (Armitage et al., 1956; Kalliomäki and Kasanen, 1969), thereby holding a potential for drug repurposing (Figure 2D).

Figure 2

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Identification of reserpine as the lead compound.

(A) Timeline for hit validation in *rd16* retinal organoids. (B) Immunostaining of rod cell marker rhodopsin (RHO, green) and cone cell marker S-opsin (OPN1SW, red) in wild-type (WT) and *rd16* organoids treated with non-toxic positive hits (B01–B05). Drug vehicle dimethylsulfoxide (DMSO) was used as a control. Nuclei were stained by 4',6-diamidino-2-phenylindole (DAPI). Arrowheads indicate relevant staining. Images were representative of at least three independent experiments, each of which had at least three organoids. (C) Bee swarm plots show the quantification of fluorescence intensity of rhodopsin (upper) and S-opsin (lower) staining in the validation. The shape of the plot indicates the distribution of data points, which are shown by colorful circles in the center. The black diamond indicates the mean, and the error bar reveals the standard error of the mean. The pink dash line shows the mean fluorescence intensity of DMSO-treated organoids. The plot summarizes at least three independent experiments with at least three organoids in each batch. One-way ANOVA followed by the Bonferroni test was performed. *p<0.05; **p<0.01. (D) Chemical structure depiction of the selected lead compound reserpine.

Validation of reserpine on LCA10 patient organoids

To further examine the efficacy of the lead compound reserpine, we utilized iPSC-derived retinal organoids from a previously-reported LCA10 family (Shimada et al., 2017), which comprised of a heterozygous unaffected mother (referred to as control henceforth) and two affected compound heterozygous children (LCA-1 and LCA-2; Figure 3—figure supplement 1A and B). Immunoblot analysis revealed a profound reduction of full-length CEP290 in patient organoids compared to controls (Figure 3—figure supplement 1C). Defects in rod development and outer segment biogenesis were evident in patient organoids from D120 onwards, as revealed by the near loss of connecting cilium marker ARL13B and absence or mislocalization of rod-specific protein rhodopsin (Figure 3—figure supplement 1D). In concordance with the observation of retained central cones in LCA10 patients, cone photoreceptors were only slightly compromised even at a late stage of differentiation in patient organoids.

Given that photoreceptor, outer segment biogenesis begins around D120 in organoids (Kaya et al., 2019) and aberrant phenotypes in patient organoids were detectable at this stage (Figure 3—figure supplement 1D), reserpine was added to patient organoids at D107 at a concentration of 10 µM, 20 µM, or 30 µM based on the EC₅₀ (half maximal effective concentration) in the primary screens (16.7 µM) (Figure 3A). We could detect improved connecting cilium immunostaining and rod development as early as D125 with the addition of 30 µM reserpine (Figure 3—figure supplement 2A). Even using the lowest tested dose (10 µM), we observed more polarized rhodopsin and connecting cilium at the apical side of photoreceptors in treated patient organoids, demonstrating a favorable effect of the drug (Figure 3—figure supplement 2A). Although variations in drug effects on organoids derived from the two patients could be observed, reserpine treatment significantly increased the fluorescence intensity of rhodopsin in retinal organoids derived from two patients (Figure 3B, C), suggesting an improvement of rod photoreceptor development. To avoid clonal variation, we also tested reserpine on another clone of each patient. Although statistically non-significant, these retinal organoids showed an obvious positive trend of rhodopsin staining upon reserpine treatment (Figure 3—figure supplement 2B, C). Such differences could be attributed to variability in response to dose and/or treatment windows of retinal organoids derived from various iPSC lines. We then performed transmission electron microscopy to uncover additional structural details of the primary cilium in photoreceptors of patient organoids after treatment. Reserpine increased the percentage of mother centrioles harboring ciliary vesicles in LCA-1 patient organoids (Figure 3D and Figure 3—figure supplement 3A), which were reportedly missing in a substantial fraction of LCA photoreceptors (Shimada et al., 2017). LCA-2 organoids demonstrated almost 50% more elongated ciliary axonemes after reserpine treatment though ciliary vesicles were hardly noticed. Quantification of the length of the ciliary axoneme indicated a significant increase in both patient organoids after treatment (Figure 3E). Notably, well-organized disc-like structures could be distinguished in treated samples (Figure 3—figure supplement 3B), suggesting a positive impact of reserpine treatment on photoreceptor primary cilium development.

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Effect of reserpine (RSP) on *LCA10* patient retinal organoids.

(A) Timeline for RSP treatments and harvest of patient organoids. (B) Immunostaining of rhodopsin (RHO, green) and ARL13B (red). Nuclei were stained by 4',6-diamidino-2-phenylindole (DAPI). Arrowheads indicate relevant staining. Images were representative of at least three independent experiments, each of which had at least three organoids. (C) Bee swarm plots show the quantification of fluorescence intensity of rhodopsin staining. The shape of the plot indicates the distribution of data points, which are shown by colorful circles in the center. The black diamond indicates the mean, and the error bar reveals the standard error of the mean. The plot summarizes at least three independent experiments with at least one image quantified in each batch. Unpaired t-test was performed to compare untreated and treated groups. *p<0.05. (D) Transmission electron microscopy analysis of control, untreated, and treated patient organoids. Arrowheads indicate relevant staining. (E) Quantification of the number of the ciliary vesicles, elongated ciliary axoneme, and aberrant cilia (left) as well as the length of the primary cilia (right) in untreated and RSP-treated patient organoids. Aberrant cilia were defined as docked mother centrioles without ciliary vesicles or elongated ciliary axoneme. The data summarized at least four batches of independent experiments, each of which has at least two organoids and seven docked mother centrioles. The shape of the bee swamp plot indicates the distribution of data points, which are shown by colorful circles in the center. The black diamond indicates the mean, and the error bar reveals the standard error of the mean. Unpaired t-test was performed to compare untreated and treated groups. *p<0.05.

We note that reserpine reportedly interferes with the sympathetic nervous system by inhibiting the transport of neurotransmitters into presynaptic vesicles (Bernstein et al., 2014); yet, even at 30 µM, no adverse effect was detected on the development of cone photoreceptors, ribbon synapses, presynaptic vesicles, bipolar cells, and Müller glia in treated patient organoids (Figure 3—figure supplement 4).

Implication of cell survival and proteostasis pathways in reserpine-treated organoids

To elucidate the mechanism of action of reserpine and to gain insights into photoreceptor cell death in LCA10, we performed RNA-seq analyses of control and patient retinal organoids. The organoids were harvested right after the treatment with 30 µM reserpine at D150 (Figure 4—figure supplement 1A). Consistent with the moderate response of LCA-2 patient organoid to reserpine (Figure 3—figure supplement 2C), we did not identify sufficient significantly differentially expressed (DE) genes between untreated and treated groups of LCA-2 for downstream analyses (data not shown) and thereby focused only on LCA-1. Principal component analysis (PCA) revealed substantial alterations in patient organoid transcriptome compared to control and after reserpine treatment (Figure 4A). The largest principal component PC1 accounted for up to 36.2% of the total variation and is likely due to the drug treatment. PC2 explained another 21% of the total variation and seemed to be mainly contributed by the disease status. Notably, the reserpine-treated group was closer to the control compared to the untreated group in PC2. A total of 355 genes were significantly differentially expressed between untreated and treated organoids at thresholds of 5% FDR and twofold change (Figure 4—source data 1). Consistent with the PCA plot, heatmap analysis indicated that DE genes in treated patient organoids showed a relative transcriptomic shift from the untreated group toward the control ones (Figure 4B). However, compared to the controls, most of the downregulated genes (e.g. those involved in proteostasis) in untreated LCA-1 organoids had an even higher expression upon drug treatment. Similarly, the expression of genes upregulated in untreated samples was lowered even more than the controls by reserpine treatment. Curiously, treatment of LCA-1 organoids with 10 µM or 20 µM reserpine did not yield sufficient DE genes for further analyses (data not shown).

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Transcriptomic upregulation of proteasomal components induced by reserpine in patient retinal organoids.

(A) Principal component analysis (PCA) diagram of patient and control organoids shows altered retinal transcriptomes after reserpine treatment. (B) Drug-induced genes in patient organoids displayed specific trends compared to control organoids. (C) Volcano plot summarizes reserpine-induced differential gene expression changes in LCA-1 organoids. (D) ClueGO analysis of KEGG pathway enrichment showed overexpressed genes mapping to protein homeostasis, metabolism, and cellular signaling processes. The red rectangle highlights the ‘Proteasome’ pathway. (E) GSEA plot showing enrichment and significance of Proteostasis Network. (F) Histogram of the log fold change of Proteostasis Network genes upon reserpine treatment. (G) Proteasomal subunits responded strongly to reserpine treatment. (H) SQSTM1 (p62) and (I) NFE2L2 (NRF2), two key regulators of protein homeostasis, showed increased expression after reserpine treatment.

Figure 4—source data 1 Differentially expressed genes in reserpine-treated retinal organoids.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig4-data1-v2.xlsx
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We first analyzed the expression level of genes specific to different retinal cell types and those involved in phototransduction and photoreceptor outer segment structure/function. Reserpine treatment modulated the expression of marker genes for inner retina cell types, including Müller glia (Figure 4—figure supplement 1B); however, surprisingly, several rod and cone photoreceptor genes (e.g. OPN1SW and GRK7) showed lower expression (Figure 4C, Figure 4—figure supplement 1C, Figure 4—source data 1), with CEP290-associated cilia genes (Rachel et al., 2012) exhibiting varying changes in expression (Figure 4—figure supplement 1D).

As reserpine did not seem to directly regulate the expression of key components involved in disease pathology, we mapped the overexpressed genes to KEGG pathways and created an enrichment network. Using a cutoff of three genes with minimum overlap and 5% impact, the annotation network was plotted to visualize leading ontology terms (Figure 4D). Clusters of metabolism, proteostasis, and immune pathways, together with cell survival-related processes, were apparent in the network. To better understand functional connectivity among DE genes, we performed a Random-walktrap analysis on their protein-protein interactions network to identify co-functioning over- and under- expressing genes and identified three prominent modules (Figure 4—figure supplement 1E). Functional module 1 was comprised of extracellular matrix (ECM) and ECM-receptor interaction, and advanced glycation end products (AGE)-Receptor for AGE (RAGE) signaling pathway genes. Müller glia-specific genes MMPL14, TIMP1, and VIM were included in this module. Functional module 2 consisted of inflammation- and proteasome-related genes, which are consistent with the reported role of reserpine as an autophagy modulator (Lee et al., 2015). Notably, functional module 3 contained critical cell survival factors such as p53 signaling and cellular senescence-associated genes. We further investigated the trend of the p53 network to characterize its role in response to drug treatment. Expression of TP53, the gene coding p53, was found to increase and match the level of control organoids (Figure 4—figure supplement 1F). In addition, a widespread modulation of each component of the p53 signaling network was evident upon reserpine treatment (Figure 4—figure supplement 1G). Downstream targets of p53, including metabolic modulator TSC and mTOR complex genes, responded to reserpine and returned to the level of control organoids (Figure 4—figure supplement 1H). Two components of the mTORC1 signaling pathway, RHEB, and LAMTOR1, were over-expressed after the drug treatment (Figure 4—source data 1). As mTORC1 is a key regulator of cellular metabolism, we observed significantly elevated expression of SLC2A1 (GLUT) which is the primary glucose transporter in neurons (Figure 4—figure supplement 1I).

Given the reported actions of reserpine in neuronal cells (Lee et al., 2015), consistent with activation of mTORC1 activation (Kim and Guan, 2015) and ‘proteasome’ subunit-encoding genes (Figure 4D), we performed a focused gene set enrichment analysis to test the impact on the proteostasis network (PN). A PN geneset was manually curated from KEGG and Reactome using keywords reviewed from previous publications (Jayaraj et al., 2020; Klaips et al., 2018). We detected a significant positive net enrichment of PN in patient organoids after reserpine treatment (NES = 1.37, adj. p-value = 1.6e−11; Figure 4E), as measured by log₂ fold change in expression of all PN genes (Figure 4F). We also identified global overexpression of proteasomal subunits, some of which were notably induced with fold change >2 (Figure 4G). In concordance, expression of the key regulators of proteostasis and members of the p53 network, p62 (SQSTM1) and NRF2 (NFE2L2), were significantly augmented in reserpine-treated patient organoids (Figure 4H, I; Figure 4—source data 1).

Restoration of proteostasis in patient photoreceptors

To experimentally examine the role of PN in the survival of LCA10 photoreceptors, we supplemented organoid cultures with various autophagy inhibitors (MRT68921, Lys05, chloroquine, hydroxychloroquine, ROC-325) that target different steps of the autophagy pathway using the same timeline as reserpine treatment (Figure 5—figure supplement 1A, B). MRT68921 and Lys05 inhibit the initiation of autophagy (Galluzzi et al., 2017) and were highly toxic even at one-fourth of the reported EC₅₀. Hydroxychloroquine and a more specific and efficient small molecule autophagy inhibitor ROC-325 enhanced the number of RHO+ cells in patient organoids with more polarized localization of rhodopsin to the apical side (Figure 5—figure supplement 1C). Quantification of the fluorescence intensity of rhodopsin immunostaining demonstrated a consistent trend and hydroxychloroquine- and ROC-325-treated organoids had a significantly higher expression of rhodopsin (Figure 5—figure supplement 1D). Although statistically non-significant, chloroquine-treated organoids also showed a positive trend in rhodopsin expression (Figure 5—figure supplement 1D), providing further evidence in support of the autophagy pathway as a target for designing therapies.

To further determine the role of autophagy, we evaluated the level of cargo receptor sequestosome 1 (SQTM1) or p62, which recognizes cellular components and helps in the formation of autophagosomes for proteolysis (Mizushima, 2007; Figure 5A). We noted that LCA-1 patient organoids had a significantly lower level of p62 compared to the control, and reserpine treatment significantly elevated p62 levels by more than twofold (Figure 5B), consistent with previous studies (Lee et al., 2015; Bresciani et al., 2018). Significantly higher levels of LC3-II in patient organoids also indicated accumulation of autophagosomes (defects in autophagy), and reserpine treatment revealed a trend of decreasing LC3-II levels (though not statistically significant) in patient organoids (Figure 5B). The comparable ratio of LC3-II/LC3-I indicated a similar rate of autophagosome formation between control and patient organoids, suggesting that increased LC3-II levels in patient organoids could be due to overexpression of autophagy pathway components. Consistent with the reported function of reserpine as an autophagy inhibitor, the treated patient organoids had a significantly lower LC3-II/LC3-I ratio, although we did not detect a significant difference between the control and patient organoids (Figure 5B).

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Proteostasis Network in patient organoid cultures in response to reserpine treatment.

(A) Schematic diagram of autophagy.(B) Immunoblot analyses and quantification of autophagy cargo adaptor p62 and autophagosome marker LC-II in control and patient organoids treated with reserpine (RSP). (C) Schematic diagram showing the proteome balance between ubiquitin-proteasome system (UPS) and autophagy is mediated through p62 as documented in the literature. (D) Immunoblot analysis of the 20S proteasome in control, DMSO-, and RSP-treated cultures. β-Actin was used as the loading control. (E) Proteasomal chymotrypsin-like activity in organoids. (F) Immunoblot analyses and quantification of key regulators of cilium assembly/disassembly in control, untreated, and RSP-treated patient organoids. The drug vehicle dimethylsulfoxide (DMSO) was added to the cultures in the untreated group at the same volume as the drugs. β-Actin was used as the loading control. The histograms summarize data in at least three batches of experiments, each of which had at least three retinal organoids per group. Each dot in the histogram shows data in one batch of experiment and are presented as mean ± standard deviation. One-way ANOVA followed by Tukey’s test. *p<0.05; ***p<0.005.

Figure 5—source data 1 Overlay of the bright field and chemiluminescence images indicating the signal of p62. RSP stands for reserpine. The size of the protein ladders, p62, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data1-v2.zip
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Figure 5—source data 2 Overlay of the bright field and chemiluminescence images indicating the signal of LC3. RSP stands for reserpine. The size of the protein ladders, LC3, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data2-v2.zip
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Figure 5—source data 3 Overlay of the bright field and chemiluminescence images indicating the signal of β-Actin. RSP stands for reserpine. The size of the protein ladders, β-Actin, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data3-v2.zip
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Figure 5—source data 4 Overlay of the bright field and chemiluminescence images indicating the signal of 20S proteosome and β-Actin. RSP stands for reserpine. The size of the protein ladders, β-Actin, 20S proteosome, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data4-v2.zip
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Figure 5—source data 5 Overlay of the bright field and chemiluminescence images indicating the signal of IFT88. RSP stands for reserpine. The size of the protein ladders, IFT88, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data5-v2.zip
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Figure 5—source data 6 Overlay of the bright field and chemiluminescence images indicating the signal of BBS6. RSP stands for reserpine. The size of the protein ladders, BBS6, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data6-v2.zip
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Figure 5—source data 7 Overlay of the bright field and chemiluminescence images indicating the signal of CEP164. RSP stands for reserpine. The size of the protein ladders, CEP164, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data7-v2.zip
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Figure 5—source data 8 Overlay of the bright field and chemiluminescence images indicating the signal of OFD1. RSP stands for reserpine. The size of the protein ladders, OFD1, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data8-v2.zip
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Figure 5—source data 9 Overlay of the bright field and chemiluminescence images indicating the signal of HDAC6. RSP stands for reserpine. The size of the protein ladders, HDAC6, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data9-v2.zip
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Figure 5—source data 10 Overlay of the bright field and chemiluminescence images indicating the signal of β-Actin. RSP stands for reserpine. The size of the protein ladders, β-Actin, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig5-data10-v2.zip
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We then applied Bafilomycin A1 (BafA1), an inhibitor of autophagosome-lysosome fusion, to patient organoid cultures using the same timeline as reserpine treatment (Figure 5—figure supplement 2A). A short 6-hr treatment with BafA1 did not alter the autophagy pathway in the control, as shown by comparable levels of p62 and LC3-II as well as LC3-II/LC3-I ratio (Figure 5—figure supplement 2B); however, patient organoids demonstrated a significant (up to 70%) increase of LC3-II and LC3-II/LC3-I ratio, suggesting an elevated autophagic flux in patient organoids, consistent with the rescue by reserpine treatment.

The ubiquitin-proteasome system (UPS) and autophagy are the two key pathways in proteostasis and are reported to interact through p62 (Kumar et al., 2022; Liu et al., 2016; Figure 5C). We, therefore, investigated the response of UPS in patient organoids upon reserpine treatment. Untreated patient organoids revealed a high level of 20S proteasome, which could be barely detected in the control (Figure 5D). Nonetheless, both the control and untreated patient organoids showed comparable total catalytic activity and reserpine significantly elevated the proteasome activity (Figure 5E). These results suggest that an increase in 20S expression is likely a compensatory mechanism for compromised proteasome activity in patient organoids and that reserpine facilitates the clearance of accumulated cellular components and/or autophagosomes (Figure 5B).

We were intrigued by the reported links of autophagy to primary cilium biogenesis (Pampliega et al., 2013; Tang et al., 2013; Yamamoto and Mizushima, 2021) and, therefore, looked at the expression of key regulators involved in ciliary transport, cilium assembly, and disassembly (Figure 5F). We identified higher expression of OFD1 (oral-facial-digital syndrome 1), which when eliminated by autophagy is shown to promote ciliogenesis (Tang et al., 2013), in patient organoids compared to the control even though autophagy activity was higher in the latter (Figure 5F). This apparent ambiguity could be due to variations in autophagic adapter machinery for cargo identification. Nonetheless, reserpine treatment reduced OFD1 levels in patient organoids and should facilitate the initiation of cilia biogenesis. Another key regulator, histone deacetylase 6 (HDAC6), which deacetylates microtubules and destabilizes the primary cilium for disassembly (Simões-Pires et al., 2013; Pugacheva et al., 2007), was significantly elevated in patient organoids compared to the control. Dramatic reduction of HDAC6 by reserpine (Figure 5F) would also have a favorable impact on cilia biogenesis. Consistent with this hypothesis, the addition of a selective HDAC6 inhibitor Tubastatin A to patient organoids improved rod photoreceptor development as shown by a higher number of RHO + cells (Figure 5—figure supplement 2C). Tubastatin A treatment enhanced the polarity of not only rhodopsin but also S-opsin, likely due to increased stability of intracellular microtubules and improved intracellular trafficking. Quantification of the fluorescence intensity of rhodopsin indicated a significant increase of rhodopsin expression in tubastatin A-treated organoids (Figure 5—figure supplement 2D), suggesting a favorable effect on rod photoreceptor development probably through the inhibition of HDAC6.

To validate the effect of reserpine, we assessed the autophagy machineries and relevant ciliary markers in a less responsive LCA-2 clone F upon reserpine treatment. Consistent with LCA-1, reserpine treatment significantly increased the p62 level but did not alter LC3-II/LC3-I ratio in LCA-2 organoids (Figure 5—figure supplement 3A, B). Although statistically non-significant, the level of autophagosome showed a decreasing trend in organoids treated with reserpine (Figure 5—figure supplement 3A, B). As retinal organoids contain abundant retinal cell types besides photoreceptors, we performed immunostaining of p62 on control, untreated, and treated LCA1 and LCA2 organoids. As shown in Figure 5—figure supplement 3C, p62 staining could barely be observed in both patient organoids compared to the control. Reserpine treatment dramatically increased p62 staining in both photoreceptors and other retinal cell types. We also quantified the level of OFD1 and HDAC6, the two ciliary regulators that mediated the favorable effect of reserpine on outer segment biogenesis (Figure 5—figure supplement 3D). We observed a significant decrease of HDAC6 in LCA2 organoids upon reserpine treatment. Although statistically non-significant, a lower expression of OFD1 was shown in reserpine-treated LCA2 organoids. Therefore, the action mechanisms of reserpine are independent of the responsiveness of patient organoids.

Maintenance of photoreceptor survival in rd16 mice in vivo

We then performed intravitreal injection of 40 µM reserpine into rd16 mouse eyes at postnatal day (P)4, and the retinas were harvested at P21 (Figure 6A). Reserpine was able to maintain photoreceptor survival, with a significantly thicker outer nuclear layer (ONL) in both the central and peripheral retina (Figure 6B). As a small molecule drug, reserpine is highly diffusive, and we observed the effect of the drug in the contralateral eyes of the treated mice (Figure 6—figure supplement 1). Indeed, bilateral therapeutic effects have been reported following unilateral injection in clinical treatments (Michalska-Małecka et al., 2016; Rouvas et al., 2009; Calvo et al., 2016; Zlotcavitch et al., 2015; Rotsos et al., 2014). Reserpine-treated rd16 mice showed a trend of increase in scotopic a-wave and a significant increase in scotopic b-wave (Figure 6C and D), suggesting functional recovery of rod photoreceptors upon reserpine treatment. No statistically significant difference was found in photopic a-wave or b-wave (Figure 6D). Notably, no systemic toxicity was observed in the treated mice. Injection of reserpine into WT mice did not lead to structural or functional alterations (Figure 6—figure supplement 2A and B), suggesting reserpine to be a safe treatment option.

Figure 6 with 3 supplements see all

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Neuroprotective effect of reserpine (RSP) in *rd16* mouse retina.

(A) Timeline of in vivo intravitreal injection for quantification of outer nuclear layer (ONL), morphology, and immunohistochemistry studies. The eyes used for subsequent analyses were highlighted by red rectangles. (B) Immunostaining of dimethylsulfoxide (DMSO)- and reserpine (RSP)-treated *rd16* retina (left) and quantification of the GFP+ outer nuclear layer (ONL) thickness (right). The shape of the bee swamp plot indicates the distribution of data points from four different litters, in which at least one mouse was assessed. Data points are shown in colorful circles in the center. The black diamond indicates the mean, and the error bar reveals the standard error of the mean. Unpaired t-test was used to compare the mean. *p<0.05. (C) Timeline of in vivo intravitreal injection for electroretinography (ERG) studies. (D) Scotopic and photopic electroretinogram responses in DMSO- and RSP-treated *rd16* mice. The lower panel shows individual values of a- and b-wave amplitudes at 10 cd.s/m² (scotopic) or at 100 cd.s/m² (photopic). ERG was measured in both eyes of at least one mouse from five different litters in each group (12 DMSO-treated and 16 RSP-treated mice). Data were expressed as mean ± SEM, and the Mann-Whitney U test was used to compare DMSO- and RSP-treated groups. ***p<0.001; ****p<0.0001.

Figure 6—source data 1 Individual values of a- and b-wave amplitudes (µV) of scotopic and photopic electroretinogram responses in DMSO- and RSP-treated rd16 mice. RSP stands for reserpine.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig6-data1-v2.zip
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Consistent with the effects observed on human retinal organoids, reserpine treatment increased p62 levels (Figure 7A and B), dramatically reduced the 20S proteasome (Figure 7C), and significantly enhanced the proteasome activity (Figure 7D). Further analyses confirmed an improvement in the structure of photoreceptor outer segments of the treated rd16 retina. Reserpine partially restored outer segment axonemes that were largely missing in untreated mouse retina (Figure 7E). In addition, substantial ciliary rootlets were conspicuous in the inner segment (shown by GFP) and extended into the ONL. Phototransduction proteins including rhodopsin and Pde6β were transported to the outer segment (Figure 6—figure supplement 3A). In addition, the treatment with 40 µM reserpine did not alter the morphology of other retinal cell types or structures including bipolar neurons, amacrine cells, and Müller glia (Figure 6—figure supplement 3B and C). Notably, the retinal stress marker Gfap was significantly increased in untreated rd16 retina as compared to the WT and was significantly reduced upon reserpine treatment (Figure 6—figure supplement 3D and E), suggesting a favorable effect of reserpine to retinal homeostasis.

Figure 7

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*Rd16* mouse retina in response to reserpine (RSP) treatment.

(A) p62 level in dimethylsulfoxide (DMSO)- and RSP-treated photoreceptors shown by immunostaining. (B) Quantification of p62 puncta. At least three non-overlap regions were quantified in at least one mouse from at least two different litters. One-way ANOVA followed by Tukey’s test was used to compare different groups. *p<0.05. (C) Western blot analysis of 20S proteasome in wild-type (WT), DMSO-, and RSP-treated retina of *rd16* mice. γ-Tubulin was used as the loading control. (D) Proteasomal chymotrypsin-like activity in *rd16* retina. Data in the histogram were summarized from at least three mice from different litters and are presented as mean ± standard deviation. One-way ANOVA followed by Tukey’s test was used to compare different groups. *p<0.05. (E) Immunostaining of outer segment (OS) axoneme marker RP1 (magenta, left panel) and ciliary rootlet marker Rootletin (magenta, right panel). In (A) and (E), nuclei were stained by 4',6-diamidino-2-phenylindole (DAPI). Arrowheads indicate relevant staining. Images were representative of at least three mice from different litters.

Figure 7—source data 1 Overlay of the bright field and chemiluminescence images indicating the signal of 20S proteosome. WT and RSP stand for wild-type and reserpine respectively. The size of the protein ladders, 20 S proteosome, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig7-data1-v2.zip
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Figure 7—source data 2 Overlay of the bright field and chemiluminescence images indicating the signal of γ-Tubulin. WT and RSP stand for wild-type and reserpine respectively. The size of the protein ladders, γ-Tubulin, and relevant sample identity are labeled.: https://cdn.elifesciences.org/articles/83205/elife-83205-fig7-data2-v2.zip
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Discussion

Repurposing of existing drugs for new unrelated clinical modalities provides an excellent opportunity for alleviating patient sufferings in a timely and cost-effective manner (Begley et al., 2021). HTS of approved small molecule drugs can be based on simple assays that target the modulation of a disease-related phenotype or molecule. The use of patient-derived iPSCs and their derivatives, including differentiated cell types in two-dimensional or organoids in three-dimensional cultures, have substantially enhanced the prospects of successful drug discovery (Struzyna and Watt, 2021; Engle and Puppala, 2013). Exciting screening platforms are now being applied to retinal cells and organoids for drug discovery (Welsbie et al., 2017; Vergara et al., 2017). We note the success of cross-species screening, in which the drug candidates identified using one species could be effective in another species (Zhang et al., 2021a). Given the ‘orphan’ status of IRDs, extensive genetic and phenotypic heterogeneity, and predominantly early dysfunction/death of rods, we designed a simple assay using GFP-tagged rods from iPSCs of a mouse mutant that phenocopies LCA10 and established an HTS platform to identify small-molecule drugs to maintain photoreceptor survival. As a two-dimensional primary or stem cell-derived cultures demonstrate a relatively lower variation and have generated promising candidates in screenings (Welsbie et al., 2017; Kost-Alimova et al., 2020; Mills et al., 2019; Chen et al., 2016b), we dissociated mouse retinal organoids into single cells and performed HTS in two-dimensional cultures. The identified lead compound reserpine was subsequently verified to be effective on patient organoids and in mouse mutant retina in vivo, suggesting the feasibility of our approach for drug discovery.

To our knowledge, there is no other study testing reserpine in other retinal diseases. Reserpine was approved for the treatment of hypertension in 1955 and later for the treatment of schizophrenia (https://www.pubchem.gov/); however, many better-tolerated and more potent hypertensive medications have become available during the last few decades. Though potential side effects of inhibition of presynaptic vesicle formation and consequently depression have been described for reserpine treatment (Yohn et al., 2017), we did not observe adverse consequences on ribbon synapses or presynaptic vesicles in treated organoids or retinal function in WT mouse retina in vivo (Figure 3—figure supplement 4 and Figure 6—figure supplement 2B). Our results are consistent with a previous study demonstrating the little effect of reserpine on central sympathetic activity in cats (Iggo and Vogt, 1960). Additionally, the doses of reserpine for the treatment of hypertension or schizophrenia in adults range from 8 mg/kg to 80 mg/kg for a 60 kg adult (Cheung and Parmar, 2023), which is higher than the dose we used in mouse studies (9.6 mg/kg). Reserpine has also been intramuscular or intravenous injection at a dose of 1 mg/kg to assess the modulation of the outcome of intraocular pressure drug and allergic inflammation of the eye in rabbits (Okada and Shimada, 1980; Delamere and Williams, 1985). No adverse effects have been reported in both studies. Besides, local delivery by intravitreal injection or via eye drops should be sufficient for reserpine to elicit its effects in the retina due to the highly diffusive properties of small-molecule drugs. Thus, we suggest that reserpine could be a safe therapeutic approach for the treatment of LCA10 and probably other retinal ciliopathies. Future studies will focus on toxicity evaluation as well as on identifying more potent and less toxic derivatives of reserpine to initial clinical trials.

Transcriptomic analyses have permitted us to interrogate potential mechanism(s) of reserpine action in patient organoids and implicated signaling pathways involved in immune response (e.g. primary immunodeficiency, complement, and coagulation cascade), cell survival, and cell death (e.g. p53 signaling pathway, cellular senescence, apoptosis), metabolism (e.g. glutathione metabolism, purine metabolism), and proteostasis. Indeed, these pathways have highly intricate relationships. The p53 protein acts as a sensor of stress conditions and can act both as a transcriptional activator or repressor to promote cell death and cell survival decisions (Kruiswijk et al., 2015; Liu et al., 2021; Wylie et al., 2022). Gene profiles of treated patient organoids revealed a significant increase of TP53 as well as its downstream targets including key cellular metabolic regulator TSC and mTOR complexes. More importantly, the expression level of genes involved in TSC and mTOR complexes returned to the level of control organoids, suggesting a positive impact of reserpine in restoring photoreceptor metabolism. As both TSC knockout and mTOR complex 1 activation are reported to maintain photoreceptor survival (Venkatesh et al., 2015; Venkatesh et al., 2016), the trends in our experimental system suggest a cell survival effect of the p53 pathway through the modulation of cell metabolism. In addition, the expression of several Müller glia-specific genes was altered by reserpine, with GFAP levels reduced in mouse retina in vivo (Figure 6—figure supplement 3D and E). Müller glia is believed to play a major role in reactive gliosis and likely adapt their transcriptome to support photoreceptors in retinal degeneration (Tomita et al., 2021; Palko et al., 2022). Yet, we are unsure whether reserpine directly acts on Müller glia to augment photoreceptor survival, or its response is a consequence of improved microenvironment and reduced retinal stress. Notably, the primary cilium appears to have a role in Müller glia maturity and functions in primary cultures (Ferraro et al., 2015). Whether and how Müller glia are affected in LCA10 or other degenerative diseases and their role in photoreceptor survival are active areas of investigation.

CEP290 is localized at the connecting cilium of photoreceptors and serves as a protein complex hub to control the biogenesis and function of outer segments. Hypomorphic CEP290 in the mouse degenerative model rd16 leads to malformed connecting cilia, compromised development of outer segments, and mislocalization of phototransduction machineries (Chang et al., 2006; Rachel et al., 2015). Comparable phenotypes are observed in LCA10 patient-derived retinal organoids (Figure 3—figure supplement 1D; Shimada et al., 2017; Parfitt et al., 2016). Ciliary defects lead to the accumulation of outer segment proteins (e.g. opsin) in the endoplasmic reticulum (ER) and subsequently induce the unfolded protein response (Reiter and Leroux, 2017), which plays a crucial role in the regulation of autophagy, a key quality control mechanism for degradation and recycling of components in response to starvation, growth factor deprivation, ER stress, and pathogen infection (He and Klionsky, 2009). Although autophagy is a protective surveillance mechanism, chronic protein stress, such as in the case of IRD and aging retina, may dysregulate the degradation machineries and subsequently lead to cell death due to disruption of cellular proteostasis (Weinberg et al., 2022; Qiu et al., 2019). Loss of proteostasis has been shown to attribute to the pathogenesis of retinal and macular degeneration including IRD and age-related macular degeneration (AMD) (Weinberg et al., 2022; Faber and Roepman, 2019; Tolone et al., 2022). Cellular proteostasis is largely maintained by the autophagy-lysosome pathway and the ubiquitin-proteasome system (UPS). Multiple studies reveal that modulation of autophagy or UPS could be a promising therapeutic approach to maintain photoreceptor survival in IRD (Qiu et al., 2019; Sen et al., 2021; Yao et al., 2018) and AMD (Zhang et al., 2021b; Vessey et al., 2022). A favorable effect of autophagy inhibition has also been reported for retinal ganglion cell survival (Zaninello et al., 2020). Interestingly, both proteasome inhibition and activation have been shown to be able to maintain photoreceptor survival and function in a mouse degenerative model P23H carrying misfolding of rhodopsin (Qiu et al., 2019; Sen et al., 2021). This suggests a more complicated regulatory mechanism in the restoration of proteostasis. We observed increased autophagic flux in LCA10 patient organoids together with reduced p62 and accumulation of autophagosome (Figure 5B and Figure 5—figure supplement 2B). Though it seems counterintuitive to reduce autophagic flux as the accumulated protein would aggravate disease pathology, we note that reserpine treatment facilitated the clearance of autophagosome at least partially through the activation of the UPS in both retinal organoids and in vivo mouse retina (Figure 5 B-E and Figure 7 A-D). Although no significant difference in 20 S proteasome activity was found in patient-derived organoids or rd16 mouse retina as compared to the control/WT (Figures 5E and 7D), a higher expression of 20 S proteasome was observed (Figures 5D and 7C), suggesting a compromised proteosome activities in patient-derived organoids and rd16 retina. The UPS and autophagy reciprocally regulate the activity of each other through the common cargo adaptor p62 (Kumar et al., 2022; Liu et al., 2016). Consistent with a previous study (Lee et al., 2015), reserpine increased the p62 level, which should facilitate the activation of UPS. Consistently, we observed clearance of accumulated autophagosome even though an autophagy inhibitor reserpine was applied (Figure 5B). Therefore, we propose that, instead of modulating a single pathway, the proteostasis network should be considered as a whole system in developing therapies (Figure 8). Further studies are needed to dissect the proteostasis network and understand the mechanisms of recognition of targeted components for degradation.

Figure 8

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Action mechanisms of reserpine in photoreceptor survival.

*CEP29*0 mutations lead to defects in the outer segment biogenesis and consequently, ciliary vesicles carrying building blocks of the primary cilium and ciliary proteins are accumulated in patient photoreceptors, leading to the activation of autophagy to degrade unwanted materials. As an autophagy inhibitor, reserpine downregulates autophagy and increases the p62 level in photoreceptors. As p62 is a mediator and cargo adaptor of the ubiquitin-proteasome system and autophagy, upregulation of p62 not only activates the 20 S proteasome to facilitate the clearance of the accumulated autophagosome but also facilitates the degradation of histone deacetylase 6 (HDAC6), which deacetylates microtubules. Removal of HDAC6 in photoreceptors thus should improve the stability of intracellular microtubules and facilitate the transport of pre-ciliary vesicles to the mother centriole for outer segment biogenesis.

Improved outer segment biogenesis is another prominent effect of reserpine in our study, which is consistent with a previous study showing reserpine induces rod outer segment elongation (Kralj and Pipan, 1998). As a key signaling regulator, the primary cilium is essential for the activation of starvation-induced autophagy through modulation of the Hedgehog pathway and other components involved in autophagy, which can eliminate components in intraflagellar transport (Pampliega et al., 2013) as well as remove OFD1 from distal appendages to initiate cilium biogenesis (Tang et al., 2013). Thus, the outcome of the crosstalk between ciliogenesis and autophagy seems to largely depend on cell type and physiological context. Our observation of high OFD1 levels in patient organoids compared to the control, despite an elevated autophagic flux, suggests alternative mechanisms. HDAC6 has also been demonstrated to promote autophagosome maturation (Lee et al., 2010; Chang et al., 2021) as well as deacetylate microtubules to impede intracellular trafficking, which is a key driver of primary cilium disassembly. Higher expression of HDAC6 in patient organoids could explain both the failure of docking of pre-ciliary vesicles (Shimada et al., 2017) and elevated autophagic flux in patient photoreceptors. HDAC6 is shown to be a target of p62 for degradation, and an increase in p62 in photoreceptors can reduce HDAC6 leading to improved outer segment length (Toulis et al., 2020). We suggest that reserpine-induced p62 levels can facilitate the degradation of HDAC6 and augment the intracellular transport of preciliary vesicles to the mother centriole for cilia biogenesis (Figure 8).

In conclusion, we identified a repurposing small-molecule drug reserpine to maintain photoreceptor survival in retinal ciliopathies, specifically LCA10, and at least partially act by restoration of proteostasis in photoreceptors. Reserpine has been evaluated in a human context (using patient organoids) and mouse retina in vivo and thus holds promise in future clinical studies. As the loss of proteomic homeostasis is a major cause of multiple retinal degenerative diseases (Faber and Roepman, 2019), reserpine and its derivatives have clinical potential for gene-agnostic therapies. Despite the identification of pathways associated with reserpine action, it is hard to dissect the function of each pathway as they are highly intertwined with each other. We realize that transcriptomic analysis to elucidate drug effect was performed with organoids of a single patient (LCA-1). In addition, the drug candidate we identified using mouse organoids is effective on patient organoids and mouse retina in vivo; however, certain pathogenetic pathways specific to humans might be missing in the drug target search by the current HTS strategy. A major future direction will be to evaluate the effect and safety of different dosages of reserpine on photoreceptor survival and function in multiple degenerative mouse models and using patient iPSC-derived retinal organoids of other ciliopathies in IRD.

Materials and methods

Animals

B6.Cg-Cep290rd16/Boc mice (Strain #: 012283; RRID: IMSR_JAX:012283) were obtained from the Jackson Laboratory and crossed to Nrlp-eGFP mice (Akimoto et al., 2006) to generate Nrlp-EGFP; Cep290^rd16/rd16 mice (referred to as rd16). The absence of the rd8 mutation in the colony was assessed by PCR. All animal procedures were approved by the Animal Care and Use committee of the National Eye Institutes (Animal study protocol NEI-650) and adhered to ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. Mice were housed in an atmosphere controlled-environment (temperature: 22°C ± 2°C, humidity: 30–70%), under a 12 hr dark/12 hr light cycle, and supplied with food and water ad libitum. Food, water, and nesting material were changed weekly.

Reagent type (species) or resource	Designation	Source or reference	Identifiers	Additional information
Cell line (M. musculus)	Nrl-GFP WT	PMID: 27667917		Reprogrammed from mouse embryonic fibroblasts by lentivirus carrying Yamanaka factors
Cell line (M. musculus)	Nrl-GFP rd16 line 1	PMID: 30332642		Reprogrammed from mouse embryonic fibroblasts by lentivirus carrying Yamanaka factors
Cell line (M. musculus)	Nrl-GFP rd16 line 2	PMID: 30332642		Reprogrammed from mouse embryonic fibroblasts by lentivirus carrying Yamanaka factors
Cell line (Homo-sapiens)	Control	PMID: 28700940		Reprogrammed from control fibroblasts by Sendai virus carrying Yamanaka factors
Cell line (Homo-sapiens)	LCA-1	PMID: 28700940		Reprogrammed from control fibroblasts by Sendai virus carrying Yamanaka factors
Cell line (Homo-sapiens)	LCA-2	In this paper		Reprogrammed from control fibroblasts by Sendai virus carrying Yamanaka factors and maintained in Swaroop lab in National Eye Institute, National Institutes of Health, United States
Cell line (Homo-sapiens)	LCA-1-clone C	In this paper		Reprogrammed from control fibroblasts by Sendai virus carrying Yamanaka factors and maintained in Swaroop lab in National Eye Institute, National Institutes of Health, United States
Cell line (Homo-sapiens)	LCA-2-clone F	PMID: 28700940
Antibody	anti-RHO (mouse monoclonal)	Gift of Dr. Robert Molday, University of British Columbia		IF(1:500)
Antibody	anti-OPN1SW (Goat polyclonal)	Santa Cruz	Cat#: sc-14363	IF(1:500)
Antibody	anti-GT335 (Mouse monoclonal)	AdipoGen	Cat#: AG-20B- 0020	IF(1:500)
Antibody	anti-Rootletin (Chicken polyclonal)	PMID: 12427867		IF (1:200)
Antibody	anti-ARL13B (Rabbit polyclonal)	Proteintech	Cat#: 17711–1-AP	IF (1:500)
Antibody	anti-OPN1LMW (Rabbit polyclonal)	Millipore	Cat#: AB5405	IF (1:500)
Antibody	anti-CEP290 (Rabbit monoclonal)	Bethyl laboratories	Cat#: A301-659A	WB (1:500)
Antibody	anti-GAPDH (Mouse monoclonal)	Millipore	Cat#: G8795	WB (1:1000)
Antibody	anti-PDE6b (Rabbit polyclonal)	PMID: 33107904		IF (1:500)
Antibody	anti-Synaptophysin (Mouse monoclonal)	Abcam	ab8049	IF (1:200)
Antibody	anti-p62 (Rabbit polyclonal)	Abcam	ab109012	IF (1:200) WB (1:500)
Antibody	anti-LC3A/B (D3U4C) (Rabbit monoclonal)	Cell Signaling	12741	For human retinal organoids, WB (1:1000)
Antibody	anti-LC3B (Rabbit polyclonal)	Abcam	ab51520	For mouse retina, WB (1:1000)
Antibody	anti-b-Actin (Mouse monoclonal)	Millipore	A5316	WB (1:5000-1:10000)
Antibody	anti-20S (Rabbit polyclonal)	Enzo Life Sciences	BML-PW8155-0100	WB (1:500)
Antibody	anti-IFT88 (Rabbit polyclonal)	Proteintech	13967–1-AP	WB (1:500)
Antibody	anti-BBS6 (Rabbit polyclonal)	Michel Leroux	#17	WB (1:500)
Antibody	anti-CEP164 (Rabbit polyclonal)	GeneTex	GTX85298	WB (1:500)
Antibody	anti-OFD1 (Rabbit polyclonal)	Thermo	PA5-115684	WB (1:500)
Antibody	anti-HDAC6 (Rabbit polyclonal)	Abgent	AP1106A	WB (1:1000)
Antibody	anti-RP1 (Chicken polyclonal)	PMID: 11773008		IF (1:2000)
Antibody	anti-Bassoon (Rabbit monoclonal)	Cell Signaling	6897 S	IF (1:200)
Antibody	anti-CHX10 (Sheep polyclonal)	Abcam	ab16141	IF (1:200)
Antibody	anti-PKCa (Rabbit monoclonal)	Thermo Fisher	MA1-157	IF (1:1000)
Antibody	anti-CRALBP (Mouse monoclonal)	Abcam	ab15051	IF (1:200)
Antibody	anti-GFAP (Rabbit polyclonal)	Dako	Z0334	IF (1:500) WB (1:1000)
Antibody	anti-Calretinin (Rabbit monoclonal)	Millipore	MAB1568	IF (1:200)
Commercial kits	Proteasome 20 S Activity Assay Kit	Sigma-Aldrich	MAK172-1KT
Chemical compound, drug	Reserpine	Sigma Aldrich	06859
Chemical compound, drug	MRT68921	Sigma Aldrich	SML1644
Chemical compound, drug	Lys05	Sigma Aldrich	SML2097
Chemical compound, drug	Chloroquine	Sigma Aldrich	C6628

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Drug discovery pipelines to identify drug candidates.

Identification of reserpine as the lead compound.

Effect of reserpine (RSP) on LCA10 patient retinal organoids.

Transcriptomic upregulation of proteasomal components induced by reserpine in patient retinal organoids.

Figure 4—source data 1

Proteostasis Network in patient organoid cultures in response to reserpine treatment.

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Figure 5—source data 2

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Neuroprotective effect of reserpine (RSP) in rd16 mouse retina.

Figure 6—source data 1

Rd16 mouse retina in response to reserpine (RSP) treatment.

Figure 7—source data 1

Figure 7—source data 2

Action mechanisms of reserpine in photoreceptor survival.

Author details

Holly Y Chen

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Manju Swaroop

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Contributed equally with

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Samantha Papal

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Contributed equally with

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Anupam K Mondal

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Contributed equally with

Competing interests

Hyun Beom Song

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Competing interests

Laura Campello

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Gregory J Tawa

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Florian Regent

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Hiroko Shimada

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Kunio Nagashima

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Natalia de Val

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Samuel G Jacobson (deceased)

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Wei Zheng

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Anand Swaroop

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For correspondence

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