Repressing PTBP1 fails to convert reactive astrocytes to dopaminergic neurons in a 6-hydroxydopamine mouse model of Parkinson’s disease

Essential revisions:

All three reviewers expressed a high level of enthusiasm for your work and made important comments/recommendations (see appended reviews) that are, for the most part, straightforward and aimed at strengthening your study. Thus, we would like to see these being addressed before your manuscript can be considered suitable for publication. Moreover, we would like you to also consider the following revisions:

(1) to tone-down your findings throughout including in the title as it was felt that you overstate the implications of this work given the fact that clearly, 6-OHDA does not capture the etiology of PD and that your data leave open the possibility that modulation of ptbp1 could facilitate dopaminergic neurogenesis if tested in other models, by other techniques, over longer time scales, or in greater Ns.

We appreciate the editor’s suggestion and have revised our title to ‘Repressing PTBP1 fails to convert reactive astrocytes to dopaminergic neurons in a mouse 6-hydroxydopamine model of Parkinson's disease’.

(2) To edit the presentation of the statistics as mentioned by Reviewer #1 throughout.

The detailed statistical information is provided in our revised manuscript.

(3) to revise your discussion. Indeed, we concur with the Reviewers that a more formal discussion would increase the potential impact of your work. In particular, it would be important that the discussion better covers the limitations of the present work and stresses the outstanding questions.

As suggested by the editor and reviewers, we have added a formal discussion in our revised manuscript that covers the limitations of the present work and the outstanding questions in this field.

Reviewer #2 (Recommendations for the authors):

Considering the following points might improve the manuscript

1. It would be useful to show which cells were targeted by the AAV construct at earlier time points, e.g. 7days following infection. There is a general agreement that hGFAP promoter may be leaky and a tighter system would use AAVs with floxed transgenes. However, even if the system lacks in specificity, the authors do show convincingly that Ptbp1 is knocked down in astrocytes. This effect, illustrated in Figure 1B should be quantified.

To identify the exact cell type targeted by our AAV construct seven days after AAV infection, we performed immunostaining and GFP⁺ cell number counting in the substantia nigra and striatum, respectively. The results showed that most of the infected cells were astrocytes (AldoC⁺), and a few other cells, such as neurons (NeuN⁺), NG2-glia (NG2⁺), and microglia (Iba-1⁺), were infected (Figure 1—figure supplement 1).

Moreover, as suggested by the reviewer, we have quantified the PTBP1 knockdown efficiency in Figure 1B, and the quantified data are included in the revised manuscript (Figure 1C).

2. In Figure 1C, the authors should provide 3D visualization of individual neurons to demonstrate co-localization of GFP with NeuN or TH more clearly.

To more clearly demonstrate the colocalization of GFP with NeuN or TH, we performed immunofluorescence experiments using anti-GFP, anti-TH, and anti-NeuN antibodies. Images were captured using a LSM780 confocal microscope (Zeiss). The enlarged 3D reconstruction of boxed individual neurons are shown in the lower panel of Figure 1D, F in the revised manuscript.

3. The authors should quantitatively compare the number of NeuN and TH positive neurons in w/o 6-OHDA controls to those following scramble or Ptbp1 knockdown treatment in figure 3.

As suggested by the reviewer, we have quantified the number of NeuN⁺ neurons and TH⁺ dopaminergic neurons in the substantia nigra before and after AAV-shPtbp1 or AAV-shscramble delivery in the 6-OHDA model and found no significant difference between the two groups, indicating that no neurons, including DA neurons, were regenerated in the substantia nigra after astroglial PTBP1 repression (Figure 3D, E).

4. Given that the authors suspect that AAV-mediated GFP expression occurs in neurons despite being driven from hGAP promoter, would it make sense to show presence of GFP mRNA in neurons, even if very little, at an time point when GFP fluorescence is still undetectable in neurons but clearly seen in astrocytes? The authors could use RNA Scope towards this. I would expect to see clear signs of low-level expression compared to massive expression in astrocytes.

We agree with the reviewer that low levels of GFP mRNA may appear in neurons at early time points, and we did observe low GFP protein levels in neurons 1 month after AAV-shPtbp1 delivery, but not AAV-shscramble (Figure 1 D, F). According to a recent study, the coding sequence of some proneural genes, such as Neurod1 could activate hGfap promoter via binding to cis-acting elements of the promoter in neurons (Wang et al., 2021). Therefore, we suppose that the shRNA sequence targeting Ptbp1 per se might function similar to Neurod1, activating the hGfap promoter to drive GFP expression in neurons at a relatively low level.

5. Given that hGFAP driven expression should eventually cease when astrocytes convert, would this have any impact on the interpretation? The authors should analyze whether there are any fate-mapped neurons that seem to lack AAV-mediated GFP expression. Or do the AAV constructs used in Figure 3 lack GFP to allow for TH IF?

We understand the reviewer’s concern. Because the hGfap promoter activity gradually ceased during AtoN conversion, we chose Rpl22^HA/HA reporter mice to permanently label astrocytes with HA-tag. If AtoN conversion occurrs, astrocyte-derived neurons or dopaminergic neurons should be marked with HA. However, we did not detect any HA⁺NeuN⁺ or HA⁺TH⁺ cells after PTBP1 repression, suggesting that no neurons or DAn were converted from astrocytes.

The two AAV constructs used in this study were attached to EGFP driven by the hGfap promoter as illustrated in Figure 1A. We apologize for the misleading comment because GFP images were not provided, which have been included in the revised manuscript (Figure 3C). The fluorescence images in the manuscript were captured on a gray scale and pseudocolored for overlay presentation.

6. The left panels in 3B must be controls, but this is not clearly indicated in the figure legend.

We appreciate the reviewer’s reminder and have revised according to reviewer’s suggestion.

7. English should be revised.

As suggested by the reviewer, we have sent our manuscript to the Editage (www.editage.cn) for English language editing.

8. Regarding the ethical endorsement of animal experiments, more clear information regarding the endorsing body should be provided. Only "university" is mentioned (Care and Use committee of university). Is there any legal number associated to this endorsement?

We supplied detailed ethical endorsement information of animal experiments including permission numbers in the Animal part of the Material and Methods section.

Reviewer #3 (Recommendations for the authors):

Chen et al., highlight a technical confound in previous publications that claim attenuation of PTBP1 results in trans-differentiation of astrocytes into dopamine neurons. While they clearly observe that previous use of recombinant AAV vectors lead to EGFP expression in dopaminergic neurons despite using an astrocyte "specific" promoter, the authors cannot conclude from the presented experiments that attenuation of PTBP1 does not achieve trans-differentiation. This is because the use of AAV could render astrocytes unable to differentiate into dopaminergic neurons in response to successful downregulation of PTBP1. Adding other, AAV independent, means of achieving PTBP1 downregulation in astrocytes in vivo would strengthen this study.

We deeply appreciate Reviewer 3’s constructive suggestions. We injected antisense oligonucleotide (ASO) against mouse Ptbp1 in the substantia nigra of lineage-tracing mice with or without 6-OHDA lesions for 2 months. While astroglial PTBP1 was efficiently repressed after ASO delivery for 2 months (Figure 4—figure supplement 1), no neurons, including DAn expressing astroglial tracing reporter, were detected, indicating that no neurons, including DAn, were converted from quiescent or reactive astrocytes after ASO-Ptbp1 delivery (Figure 4B, D). Moreover, motor deficits in 6-OHDA lesion mice did not improve 2 months after ASO-Ptbp1 delivery. These results have been added to the revised manuscript (Figure 4E).

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