PTBP2-dependent alternative splicing regulates protein transport and mitochondria morphology in post-meiotic germ cells.

The RNA binding protein PTBP2 (polypyrimidine tract binding protein 2) is a key regulator of tissue-specific alternative RNA splicing. In the testis, PTBP2 is expressed in meiotic and post-meiotic germ cells (spermatocytes and spermatids, respectively). In these cells, PTBP2 is required for proper alternative mRNA splicing for over 200 genes, disproportionately affecting genes encoding proteins involved in protein trafficking via transport vesicles. In this study, we used electron microscopy to test the hypothesis that protein trafficking is impaired in the absence of PTBP2, and to further investigate why spermatogenesis abruptly halts in PTBP2-deficient spermatids. Ultrastructural analysis shows that protein trafficking in spermatids is aberrant in the absence of PTBP2. Unexpectedly, we also found that mitochondria morphology and number are significantly altered in PTBP2-deficient spermatids, consistent with increased mitochondria fission. Furthermore, we show that genes with key roles in mitochondria dynamics and function are post-transcriptionally regulated by PTBP2 and in different stages of spermatogenesis. Collectively, the data provide ultrastructural evidence that alternative splicing regulation by PTBP2 during spermatogenesis is critical for proper regulation of protein trafficking and mitochondria morphology.