Dhx34 and Nbas function in the NMD pathway and are required for embryonic development in zebrafish

The nonsense-mediated mRNA decay (NMD) pathway is a highly conserved surveillance mechanism that is present in all eukaryotes. It prevents the synthesis of truncated proteins by selectively degrading mRNAs harbouring premature termination codons (PTCs). The core NMD effectors were originally identified in genetic screens in Saccharomyces cerevisae and in the nematode Caenorhabditis elegans, and subsequently by homology searches in other metazoans. A genome-wide RNAi screen in C. elegans resulted in the identification of two novel NMD genes that are essential for proper embryonic development. Their human orthologues, DHX34 and NAG/NBAS, are required for NMD in human cells. Here, we find that the zebrafish genome encodes orthologues of DHX34 and NAG/NBAS. We show that the morpholino-induced depletion of zebrafish Dhx34 and Nbas proteins results in severe developmental defects and reduced embryonic viability. We also found that Dhx34 and Nbas are required for degradation of PTC-containing mRNAs in zebrafish embryos. The phenotypes observed in both Dhx34 and Nbas morphants are similar to defects in Upf1, Smg-5- or Smg-6- depleted embryos, suggesting that these factors affect the same pathway and confirming that zebrafish embryogenesis requires an active NMD pathway.     

m6A‐mediated alternative splicing coupled with nonsense‐mediated mRNA decay regulates SAM synthetase homeostasis

Alternative splicing of pre‐mRNAs can regulate gene expression levels by coupling with nonsense‐mediated mRNA decay (NMD). In order to elucidate a repertoire of mRNAs regulated by alternative splicing coupled with NMD (AS‐NMD) in an organism, we performed long‐read RNA sequencing of poly(A)⁺ RNAs from an NMD‐deficient mutant strain of Caenorhabditis elegans, and obtained full‐length sequences for mRNA isoforms from 259 high‐confidence AS‐NMD genes. Among them are the S‐adenosyl‐L‐methionine (SAM) synthetase (sams) genes sams‐3 and sams‐4. SAM synthetase activity autoregulates sams gene expression through AS‐NMD in a negative feedback loop. We furthermore find that METT‐10, the orthologue of human U6 snRNA methyltransferase METTL16, is required for the splicing regulation in␣vivo, and specifically methylates the invariant AG dinucleotide at the distal 3′ splice site (3′SS) in␣vitro. Direct RNA sequencing coupled with machine learning confirms m⁶A modification of endogenous sams mRNAs. Overall, these results indicate that homeostasis of SAM synthetase in C. elegans is maintained by alternative splicing regulation through m⁶A modification at the 3′SS of the sams genes.     

Identification and characterization of novel factors that act in the nonsense‐mediated mRNA decay pathway in nematodes,flies and mammals

Nonsense‐mediated mRNA decay (NMD) is a surveillance mechanism that degrades mRNAs harboring premature termination codons (PTCs). We have conducted a genome‐wide RNAi screen in Caenorhabditis elegans that resulted in the identification of five novel NMD genes that are conserved throughout evolution. Two of their human homologs, GNL2 (ngp‐1) and SEC13 (npp‐20), are also required for NMD in human cells. We also show that the C. elegans gene noah‐2, which is present in Drosophila melanogaster but absent in humans, is an NMD factor in fruit flies. Altogether, these data identify novel NMD factors that are conserved throughout evolution, highlighting the complexity of the NMD pathway and suggesting that yet uncovered novel factors may act to regulate this process.