UPF2-Dependent Nonsense-Mediated mRNA Decay Pathway Is Essential for Spermatogenesis by Selectively Eliminating Longer 3'UTR Transcripts

Abstract

During transcription, most eukaryotic genes generate multiple alternative cleavage and polyadenylation (APA) sites, leading to the production of transcript isoforms with variable lengths in the 3’ untranslated region (3’UTR). In contrast to somatic cells, male germ cells, especially pachytene spermatocytes and round spermatids, express a distinct reservoir of mRNAs with shorter 3’UTRs that are essential for spermatogenesis and male fertility. However, the mechanisms underlying the enrichment of shorter 3’UTR transcripts in the developing male germ cells remain unknown. Here, we report that UPF2-mediated nonsense-mediated mRNA decay (NMD) plays an essential role in male germ cells by eliminating ubiquitous genes-derived, longer 3’UTR transcripts, and that this role is independent of its canonical role in degrading “premature termination codon” (PTC)-containing transcripts in somatic cell lineages. This report provides physiological evidence supporting a noncanonical role of the NMD pathway in achieving global 3’UTR shortening in the male germ cells during spermatogenesis.

Stress and the nonsense-mediated RNA decay pathway

“…In most known cases, NMD is downregulated as development proceeds [37, 43–45], but notable exceptions are B-cell development and mesoderm differentiation, where NMD is upregulated [28, 38]. Male germ cell development is also likely to be accompanied by shifts in NMD magnitude, but it has not been resolved whether NMD is up- or down-regulated [26, 46, 47]. …”

Alternative polyadenylation of mRNA precursors

“…Second, mRNA decay mechanisms are highly potent and responsible for the global elimination of RNAs during the transition from spermatocytes to spermatids, which contain ∼12 pg and ∼2.5 pg total RNA per cell, respectively 168 . Thus, long isoforms that contain destabilizing elements are rapidly degraded, probably by multiple decay mechanisms, including the degradation of mRNAs containing transposable elements, mediated by PIWI-interacting RNAs 129,169–171 , and the nonsense-mediated decay pathway, which degrades transcripts with abnormally long 3′ UTRs (among others) 172,173 . Importantly, transcripts with short 3′ UTRs, which lack these destabilizing elements or features and thus escape mRNA degradation, are thought to be stored for translation at a later developmental stage, when transcription in globally inhibited.…”

RNA decay, evolution, and the testis

“…Time-course analysis of the first wave of spermatogenesis confirmed that Upf1 and Upf2 are upregulated at the spermatocyte stage, with sustained or elevated expression in round spermatids. 4,14 However, there is also evidence that NMD is instead downregulated in spermatocytes (Fig. 3B, Model II).…”

“…Evidence that NMD is upregulated as germ cell development proceeds comes from Bao et al, who found that many NMD genes, including Upf1 and Upf2, are upregulated in enriched spermatocytes and round spermatids compare with enriched immature germ cells (spermatogonia) 4 (Fig. 3A, Model I).…”

“…To address whether NMD functions in spermatogenesis, Bao et al conditionally knocked out the essential NMD factor gene, Upf2, in male germ cells. 4 They first used Vasa-Cre mice to conditionally knock out Upf2 in fetal germ cells (specifically at the prospermatogonia stage). These Upf2-cKO mice had small testes with few germ cells and were infertile.…”