Figure 3.

Expression of NMD components during spermatogenesis. Spermatogenesis takes about 34 days in mice (top). Cell types progress from spermatogonia (that still undergo mitosis) through multiple stages of meiosis (spermatocytes), through postmeiotic differentiation stages (step 1 through step 8 round spermatids and elongating spermatids), and emerge as spermatozoa. Spermatozoa must traverse the epididymis to become competent for fertilization (not shown). In spermatocytes, cloud-like (“nuage”) structures called the intermitochondrial cement (IMC) assemble near mitochondria. These structures coalesce into perinuclear structures called chromatoid bodies (CBs) after meiosis. In the absence of an enclosing membrane, proteins such as the Tudor domain-containing TDRD6 act as organizational components of the CB; UPF1, UPF2, and UPF3B are also found there. Also during meiosis, the X and Y chromosomes are inactivated transcriptionally and sequestered into sex bodies, a process known as meiotic sex chromosome inactivation (MCSI). Thus, X-linked genes (such as Upf3b) are inactivated during meiosis while autosomal paralogs (Upf3a) are expressed. Several studies have shown that 3΄ UTR length of mRNAs becomes shorter as spermatogenesis decreases. The studies reviewed here indicate that long 3΄ UTR-mediated NMD is prominent during postmeiotic stages. We speculate that EJC-stimulated NMD is prominent during earlier stages of spermatogenesis, although the studies did not address this point directly.