Abstract
SummaryGlobal transcriptome reprogramming during spermatogenesis ensures timely expression of factors in each phase of male germ cell differentiation. Spermatocytes and spermatids require particularly extensive reprogramming of gene expression to switch from mitosis to meiosis and to support gamete morphogenesis. Here, we uncovered an extensive alternative splicing program during this transmeiotic differentiation. Notably, intron retention was largely the most enriched pattern, with spermatocytes showing generally higher levels of retention compared with spermatids. Retained introns are characterized by weak splice sites and are enriched in genes with strong relevance for gamete function. Meiotic intron-retaining transcripts (IRTs) were exclusively localized in the nucleus. However, differently from other developmentally regulated IRTs, they are stable RNAs, showing longer half-life than properly spliced transcripts. Strikingly, fate-mapping experiments revealed that IRTs are recruited onto polyribosomes days after synthesis. These studies reveal an unexpected function for regulated intron retention in modulation of the timely expression of select transcripts during spermatogenesis.
Highlights
Spermatogenesis is a remarkable cell differentiation process that yields the male gamete and remains active throughout the adult life of mammals (Griswold, 2016)
A Regulated Alternative Splicing Program Characterizes the Meiotic Transition of Male Germ Cells To achieve a comprehensive characterization of the splicing changes occurring during transmeiotic differentiation of male germ cells, we performed high-throughput RNA sequencing (RNA-seq) analyses of poly(A)+ RNA isolated from highly purified populations of meiotic spermatocytes (n = 2) and post-meiotic spermatids (n = 3) (Figure 1A)
By using the FAST-DB splicing patterns annotation tool, we identified 4,090 alternative splicing (AS) events in 1,753 genes that are modulated across meiosis in male germ cells, with the majority (65.1%) being upregulated in spermatocytes (Figure 1C and Table S1)
Summary
Spermatogenesis is a remarkable cell differentiation process that yields the male gamete and remains active throughout the adult life of mammals (Griswold, 2016). Each stage of spermatogenesis requires a specific repertoire of factors to accomplish the profound genomic and morphological modifications that characterize male germ cell differentiation. Proper expression of these factors is dynamically controlled at transcriptional and post-transcriptional levels (Paronetto and Sette, 2010; Chalmel and Rolland, 2015). Since transcription is not continuously active during male germ cell differentiation (Monesi, 1964), translational regulation of stored mRNAs ensures timely expression of proteins in the transcriptionally silent stages of spermatogenesis (Paronetto and Sette, 2010)
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