SRSF1-mediated alternative splicing of Tial1/Tiar is essential for homing and self-renewal in mouse spermatogonial stem cells

Reviewer #1 (Public Review):

The authors revealed that spermatogonia-related genes (e.g., Plzf, Id4, Setdb1, Stra8, Tial1/Tiar, Bcas2, Ddx5, Srsf10, Uhrf1, and Bud31) were bound by SRSF1 in the mouse testes by Crosslinking immunoprecipitation and sequencing (CLIP-seq). Using Vasa-cre mouse line, the authors successfully evidenced that SRSF1 in the testis is essential for homing and self-renewal in mouse spermatogonial stem cells. Further evidence showed that SRSF1 directly binds and regulates the expression of Tial1/Tiar via AS to implement SSC homing and self-renewal. Immunoprecipitation mass spectrometry (IP-MS) data showed that the AS of SSC is regulated by SRSF1 coordinated with other RNA splicing-related proteins (e.g., SRSF10, SART1, RBM15, SRRM2, SF3B6, and SF3A2). The authors revealed the critical role of SRSF1-mediated AS in SSC homing and self-renewal, which may provide a framework to elucidate the molecular mechanisms of the posttranscriptional network underlying the formation of SSC pools and the establishment of niches. The experiments are well-designed and conducted, the overall conclusions are convincing. This work will be of interest to stem cell and reproductive biologists.

Reviewer #2 (Public Review):

Summary
The authors seek to characterize the role of splicing factor SRSF1 during spermatogenesis. Using a conditional deletion of Srsf1 in germ cells, they find that SRSF1 is required for male fertility. Via immunostaining and RNA-seq analysis of the Srsf1 conditional knockout (cKO) testes, combined with SRSF1 CLIP-seq and IP-MS data from the testis, they ultimately conclude that Srsf1 is required for spermatogonial stem cell (SSC) homing and self-renewal due to alternative splicing of Tial1.

Strengths
The overall methods and results are robust. The histological analysis of the Srsf1 cKO traces the origins of the fertility defect to the postnatal testis, and the authors have generated interesting datasets characterizing SRSF1's RNA targets and interacting proteins specifically in the testis.

Ultimately, the authors have shown that SRSF1's effects on alternative splicing are required to establish spermatogenesis. In the absence of Srsf1, the postnatal gonocytes/nascent spermatogonia do not properly relocate from the seminiferous tubules' lumen to basement membrane, and consequently, never initiate spermatogenesis. I believe this relocation event is what the authors are referring to as "SSC homing".

Weaknesses
I do not think there is enough evidence to support two major conclusions. First, the authors conclude that SRSF1 is required for "SSC self-renewal." Given the defect in nascent spermatogonial development, it is not evident to me that SSCs actually form in the Srsf1 cKO. Second, the authors conclude that SRSF1 controls alternative splicing of Tial1 to "implement SSC homing and self-renewal." I'm unsure as to the basis for TIAL1's role in "SSC homing and self-renewal," particularly as the only reference provided about Tial1's effects on the germ line shows that germ cells are completely lost during embryonic gestation. As a result, it's ultimately unclear to me how SRSF1 mechanistically regulates the relocation of gonocytes/nascent spermatogonia to the basement membrane.

Alternative splicing is quite pronounced in the testis relative to other tissues. The authors have presented interesting work that shows that alternative splicing is required in the testicular germ line for the establishment of spermatogenesis.

Reviewer #3 (Public Review):

In this study, Sun et al examine the role of the splicing factor SRSF1 in spermatogenesis in mice. Alternative splicing is important for spermatogenic development, but its regulation and major developmental roles during spermatogenesis are not well understood. The authors set out to better define both SRSF1 function in testes and the contribution of alternative splicing. They collect several large 'omics datasets to define SRSF1 targets in testis, including RNA interactions by CLIP-seq in whole testis, protein interactions by IP-mass spec in whole testis, and RNA sequencing to detect expression levels and splice variants. They also examine the phenotype of germline conditional knockouts (cKO) for Srsf1, using the early-acting Vasa-Cre, and find a severe depletion of germ cells starting at 7 days post partum (dpp) and culminating with a lack of germ cells (Sertoli Cell Only Syndrome) by adulthood. They detect differences in gene expression as well as differences in splicing between control and knockout, including 9 genes that are downregulated, experience alternative splicing, and whose transcripts are also bound by SRSF1, and identify the Tial1/Tiar transcript as one of these targets. They conclude that SRSF1 is required for homing and self-renewal of spermatogonial stem cells, at least in part through its regulation of Tial1/Tiar splicing.

Strengths of the paper include detailed phenotyping of the Srsf1 cKO, which convincingly supports the Sertoli Cell Only phenotype, establishes the timing of the first appearance of the spermatogonial defect, and provides new insight into the role of splicing factors and SRSF1 specifically in spermatogenesis. Another strength is the generation of CLIP-seq, IP-MS, and RNA-seq datasets which will be a useful resource for the field of germ cell development. Major weaknesses include a lack of robust support for two major claims: first, there is inadequate support for the claim of defects in either "homing" or "self-renewal" of spermatogonia in the cKO, and second, there is inadequate support for the claim that altered splicing of the Tial1 transcript mediates the effect of SRSF1 loss. A moderate weakness is the superficial discussion of the CLIP, RNA-seq, and IP-MS datasets, limiting their otherwise high utility for other researchers. Overall, the paper as it stands will have a moderate impact on the field of male reproductive biology. Specific points that should be addressed to improve support for the claims are below.

Major comments

1. In Fig 1D, it appears that SRSF1 is expressed most strongly in spermatogonia by immunofluorescence, but this is inconsistent with the sharp rise in expression detected by RT-qPCR at 20 days post partum (dpp) (Fig. 1B), which is when round spermatids are first added; this discrepancy should be explained or addressed.

2. It is important to provide a more comprehensive basic description of the CLIP-seq datasets beyond what is shown in the tracks shown in Fig. 2B. This would allow a better assessment of the data quality and would also provide information about the transcriptome-wide patterns of SRSF1 binding. No information or quality metrics are provided about the libraries, and it is not stated how replicates are handled to maximize the robustness of the analysis. The distribution of peaks across exons, introns, and other genomic elements should also be shown.

3. The claim that SRSF1 is required for "homing and self-renewal" of SSCs is made in multiple places in the manuscript. However, neither homing nor self-renewal is ever directly tested. A single image is shown in Fig. 5E of a spermatogonium at 5dpp that does not appropriately sit on the basal membrane, potentially indicating a homing defect, but this is not quantified or followed up. There is good evidence for depletion of spermatogonia starting at 7 dpp, but no further explanation of how homing and/or self-renewal fit into the phenotype.

4. In Fig. 6A (lines 258-260) very few genes downregulated in the cKO are bound by SRSF1 and undergo abnormal splicing. The small handful that falls into this overlap could simply be noise. A much larger fraction of differentially spliced genes are CLIP-seq targets (~33%), which is potentially interesting, but this set of genes is not explored.

5. The background gene set for Gene Ontology analyses is not specified. If these were done with the whole transcriptome as background, one would expect enrichment of spermatogenesis genes simply because they are expressed in testes. The more appropriate set of genes to use as background in these analyses is the total set of genes that are expressed in testis.

6. In general, the model is over-claimed: aside from interactions by IP-MS, little is demonstrated in this study about how SRSF1 affects alternative splicing in spermatogenesis, or how alternative splicing of TIAL1 specifically would result in the phenotype shown. It is not clear why Tial1/Tiar is selected as a candidate mediator of SRSF1 function from among the nine genes that are downregulated in the cKO, are bound by SRSF1, and undergo abnormal splicing. Although TIAL1 levels are reduced in cKO testes by Western blot (Fig. 7J), this could be due just be due to a depletion of germ cells from whole testis. The reported splicing difference for Tial1 seems very subtle and the ratio of isoforms does not look different in the Western blot image.