The exon junction complex (EJC) is an essential constituent and regulator of spliced messenger ribonucleoprotein particles (mRNPs) in metazoans. As a core component of the EJC, CASC3 was described to be pivotal for EJC-dependent nuclear and cytoplasmic processes. However, recent evidence suggests that CASC3 functions differently from other EJC core proteins. Here, we have established human CASC3 knockout cell lines to elucidate the cellular role of CASC3. In the knockout cells, overall EJC composition and EJC-dependent splicing are unchanged. A transcriptome-wide analysis reveals that hundreds of mRNA isoforms targeted by nonsense-mediated decay (NMD) are upregulated. Mechanistically, recruiting CASC3 to reporter mRNAs by direct tethering or via binding to the EJC stimulates mRNA decay and endonucleolytic cleavage at the termination codon. Building on existing EJC-NMD models, we propose that CASC3 equips the EJC with the persisting ability to communicate with the NMD machinery in the cytoplasm. Collectively, our results characterize CASC3 as a peripheral EJC protein that tailors the transcriptome by promoting the degradation of EJC-dependent NMD substrates.

Messenger RNA (mRNA) turnover is a crucial and highly regulated step of gene expression in mammalian cells. This includes mRNA surveillance pathways such as nonsense-mediated mRNA decay (NMD), which assesses the fidelity of transcripts and eliminates mRNAs containing a premature translation termination codon (PTC). When studying mRNA degradation pathways, reporter mRNAs are commonly expressed in cultivated cells. Traditionally, the molecular mechanism of NMD has been characterized using pairs of reporter constructs that express the same mRNA with ("PTC-containing mRNA") or without ("wild-type mRNA") a PTC. Cell lines stably expressing an NMD reporter have been reported to yield very robust and highly reproducible results, but establishing the cell lines can be very time-consuming. Therefore, transient transfection of such reporter constructs is frequently used and allows analysis of many samples within a short period of time. However, the behavior of transiently and stably transfected NMD constructs has not been systematically compared so far. Here, we report that not all commonly used human cell lines degrade NMD targets following transient transfection. Furthermore, the degradation efficiency of NMD substrates can depend on the manner of transfection within the same cell line. This has substantial implications for the interpretation of NMD assays based on transient transfections.

Nonsense-mediated mRNA decay (NMD) is a cellular process that eliminates messenger RNA (mRNA) substrates with premature translation termination codons (PTCs). In addition, NMD regulates the expression of a number of physiological mRNAs, for example transcripts containing long 3' UTRs. Current models implicate the interaction between cytoplasmic poly(A)-binding protein (PABPC1) and translation termination in NMD. Accordingly, PABPC1 present within close proximity of a termination codon antagonizes NMD. Here, we use reporter mRNAs with different NMD-inducing 3' UTRs to establish a general NMD-inhibiting property of PABPC1. NMD-inhibition is not limited to PABPC1, but can also be achieved by peptides consisting of the PABP-interacting motif 2 (PAM2) of different proteins when recruited to an NMD-inhibiting position of NMD reporter transcripts. The short PAM2 peptides efficiently suppress NMD activated by a long 3' UTR, an exon-junction complex (EJC) and individual EJC components, and stabilize a PTC-containing β-globin mRNA. In conclusion, our results establish short PABPC1-recruiting peptides as potent but position-dependent inhibitors of mammalian NMD.

Nonsense-mediated mRNA decay (NMD) eliminates different classes of mRNA substrates including transcripts with long 3' UTRs. Current models of NMD suggest that the long physical distance between the poly(A) tail and the termination codon reduces the interaction between cytoplasmic poly(A)-binding protein (PABPC1) and the eukaryotic release factor 3a (eRF3a) during translation termination. In the absence of PABPC1 binding, eRF3a recruits the NMD factor UPF1 to the terminating ribosome, triggering mRNA degradation. Here, we have used the MS2 tethering system to investigate the suppression of NMD by PABPC1. We show that tethering of PABPC1 between the termination codon and a long 3' UTR specifically inhibits NMD-mediated mRNA degradation. Contrary to the current model, tethered PABPC1 mutants unable to interact with eRF3a still efficiently suppress NMD. We find that the interaction of PABPC1 with eukaryotic initiation factor 4G (eIF4G), which mediates the circularization of mRNAs, is essential for NMD inhibition by tethered PABPC1. Furthermore, recruiting either eRF3a or eIF4G in proximity to an upstream termination codon antagonizes NMD. While tethering of an eRF3a mutant unable to interact with PABPC1 fails to suppress NMD, tethered eIF4G inhibits NMD in a PABPC1-independent manner, indicating a sequential arrangement of NMD antagonizing factors. In conclusion, our results establish a previously unrecognized link between translation termination, mRNA circularization, and NMD suppression, thereby suggesting a revised model for the activation of NMD at termination codons upstream of long 3' UTR.

Synapsins are a family of neuronal phosphoproteins associated with the cytosolic surface of synaptic vesicles. Experimental evidence suggests a role for synapsins in synaptic vesicle clustering and recycling at the presynaptic terminal, as well as in neuronal development and synaptogenesis. Synapsin knock-out (Syn1(-/-) ) mice display an epileptic phenotype and mutations in the SYN1 gene have been identified in individuals affected by epilepsy and/or autism spectrum disorder. We investigated the impact of the c.1067G>A nonsense transition, the first mutation described in a family affected by X-linked syndromic epilepsy, on the expression and functional properties of the synapsin I protein. We found that the presence of a premature termination codon in the human SYN1 transcript renders it susceptible to nonsense-mediated mRNA decay (NMD). Given that the NMD efficiency is highly variable among individuals and cell types, we investigated also the effects of expression of the mutant protein and found that it is expressed at lower levels compared to wild-type synapsin I, forms perinuclear aggregates and is unable to reach presynaptic terminals in mature hippocampal neurons grown in culture. Taken together, these data indicate that in patients carrying the W356× mutation the function of synapsin I is markedly impaired, due to both the strongly decreased translation and the altered function of the NMD-escaped protein, and support the value of Syn1(-/-) mice as an experimental model mimicking the human pathology.