Interactions between KSHV ORF57 and the novel human TREX proteins, CHTOP and CIP29

mRNA nuclear retention reduces AMPAR expression and promotes autistic behavior in UBE3A-overexpressing mice

UBE3A is a common genetic factor in ASD etiology, and transgenic mice overexpressing UBE3A exhibit typical autistic-like behaviors. Because AMPA receptors (AMPARs) mediate most of the excitatory synaptic transmission in the brain, and synaptic dysregulation is considered one of the primary cellular mechanisms in ASD pathology, we investigate here the involvement of AMPARs in UBE3A-dependent ASD. We show that expression of the AMPAR GluA1 subunit is decreased in UBE3A-overexpressing mice, and that AMPAR-mediated neuronal activity is reduced. GluA1 mRNA is trapped in the nucleus of UBE3A-overexpressing neurons, suppressing GluA1 protein synthesis. Also, SARNP, an mRNA nuclear export protein, is downregulated in UBE3A-overexpressing neurons, causing GluA1 mRNA nuclear retention. Restoring SARNP levels not only rescues GluA1 mRNA localization and protein expression, but also normalizes neuronal activity and autistic behaviors in mice overexpressing UBE3A. These findings indicate that SARNP plays a crucial role in the cellular and behavioral phenotypes of UBE3A-induced ASD by regulating nuclear mRNA trafficking and protein translation of a key AMPAR subunit.

Nuclear export of plant pararetrovirus mRNAs involves the TREX complex, two viral proteins and the highly structured 5' leader region

In eukaryotes, the major nuclear export pathway for mature mRNAs uses the dimeric receptor TAP/p15, which is recruited to mRNAs via the multisubunit TREX complex, comprising the THO core and different export adaptors. Viruses that replicate in the nucleus adopt different strategies to hijack cellular export factors and achieve cytoplasmic translation of their mRNAs. No export receptors are known in plants, but Arabidopsis TREX resembles the mammalian complex, with a conserved hexameric THO core associated with ALY and UIEF proteins, as well as UAP56 and MOS11. The latter protein is an orthologue of mammalian CIP29. The nuclear export mechanism for viral mRNAs has not been described in plants. To understand this process, we investigated the export of mRNAs of the pararetrovirus CaMV in Arabidopsis and demonstrated that it is inhibited in plants deficient in ALY, MOS11 and/or TEX1. Deficiency for these factors renders plants partially resistant to CaMV infection. Two CaMV proteins, the coat protein P4 and reverse transcriptase P5, are important for nuclear export. P4 and P5 interact and co-localise in the nucleus with the cellular export factor MOS11. The highly structured 5′ leader region of 35S RNAs was identified as an export enhancing element that interacts with ALY1, ALY3 and MOS11 in vitro.

Viral regulation of mRNA export with potentials for targeted therapy

Eukaryotic gene expression begins with transcription in the nucleus to synthesize mRNA (messenger RNA), which is subsequently exported to the cytoplasm for translation to protein. Like transcription and translation, mRNA export is an important regulatory step of eukaryotic gene expression. Various factors are involved in regulating mRNA export, and thus gene expression. Intriguingly, some of these factors interact with viral proteins, and such interactions interfere with mRNA export of the host cell, favoring viral RNA export. Hence, viruses hijack host mRNA export machinery for export of their own RNAs from nucleus to cytoplasm for translation to proteins for viral life cycle, suppressing host mRNA export (and thus host gene expression and immune/antiviral response). Therefore, the molecules that can impair the interactions of these mRNA export factors with viral proteins could emerge as antiviral therapeutic agents to suppress viral RNA transport and enhance host mRNA export, thereby promoting host gene expression and immune response. Thus, there has been a number of studies to understand how virus hijacks mRNA export machinery in suppressing host gene expression and promoting its own RNA export to the cytoplasm for translation to proteins required for viral replication/assembly/life cycle towards developing targeted antiviral therapies, as concisely described here.

Overlapping motifs on the herpes viral proteins ICP27 and ORF57 mediate interactions with the mRNA export adaptors ALYREF and UIF

The TREX complex mediates the passage of bulk cellular mRNA export to the nuclear export factor TAP/NXF1 via the export adaptors ALYREF or UIF, which appear to act in a redundant manner. TREX complex recruitment to nascent RNA is coupled with 5′ capping, splicing and polyadenylation. Therefore to facilitate expression from their intronless genes, herpes viruses have evolved a mechanism to circumvent these cellular controls. Central to this process is a protein from the conserved ICP27 family, which binds viral transcripts and cellular TREX complex components including ALYREF. Here we have identified a novel interaction between HSV-1 ICP27 and an N-terminal domain of UIF in vivo, and used NMR spectroscopy to locate the UIF binding site within an intrinsically disordered region of ICP27. We also characterized the interaction sites of the ICP27 homolog ORF57 from KSHV with UIF and ALYREF using NMR, revealing previously unidentified binding motifs. In both ORF57 and ICP27 the interaction sites for ALYREF and UIF partially overlap, suggestive of mutually exclusive binding. The data provide a map of the binding sites responsible for promoting herpes virus mRNA export, enabling future studies to accurately probe these interactions and reveal the functional consequences for UIF and ALYREF redundancy.

Targeting the ATP-dependent formation of herpesvirus ribonucleoprotein particle assembly as an antiviral approach

Human herpesviruses are responsible for a range of debilitating acute and recurrent diseases, including a number of malignancies. Current treatments are limited to targeting the herpesvirus DNA polymerases, but with emerging viral resistance and little efficacy against the oncogenic herpesviruses, there is an urgent need for new antiviral strategies. Here, we describe a mechanism to inhibit the replication of the oncogenic herpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV), by targeting the ATP-dependent formation of viral ribonucleoprotein particles (vRNPs). We demonstrate that small-molecule inhibitors which selectively inhibit the ATPase activity of the cellular human transcription/export complex (hTREX) protein UAP56 result in effective inhibition of vRNP formation, viral lytic replication and infectious virion production. Strikingly, as all human herpesviruses use conserved mRNA processing pathways involving hTREX components, we demonstrate the feasibility of this approach for pan-herpesvirus inhibition.

Chtop (Chromatin target of Prmt1) auto-regulates its expression level via intron retention and nonsense-mediated decay of its own mRNA

Chtop (chromatin target of Prmt1) regulates various aspects of gene expression including transcription and mRNA export. Despite these important functions, the regulatory mechanism underlying Chtop expression remains undetermined. Using Chtop-expressing human cell lines, we demonstrate that Chtop expression is controlled via an autoregulatory negative feedback loop whereby Chtop binds its own mRNA to retain intron 2 during splicing; a premature termination codon present at the 5′ end of intron 2 leads to nonsense-mediated decay of the mRNA. We also show that Chtop interacts with exon 2 of Chtop mRNA via its arginine-glycine-rich (RG) domain, and with intron 2 via its N-terminal (N1) domain; both are required for retention of intron 2. In addition, we show that hnRNP H accelerates intron 2 splicing of Chtop mRNA in a manner dependent on Chtop expression level, suggesting that Chtop and hnRNP H regulate intron 2 retention of Chtop mRNA antagonistically. Thus, the present study provides a novel molecular mechanism by which mRNA and protein levels are constitutively regulated by intron retention.