Post-transcriptional regulation of HIV-1 gene expression by the host factor MATR3

The post-transcriptional regulation of HIV-1 replication is finely controlled by both viral and host factors. Among the former, Rev is involved in the nuclear ex-port of viral intron-containing mRNAs, a group of transcripts which encodes for viral enzymes and structural proteins, but also constitute the viral genome that will be encapsidated into nascent virions. To avoid the nuclear retention of these intron-containing transcripts, Rev en-gages an alternative export route which ultimately involves the CRM-1 export factor; during this process Rev requires the concerted action of several host fac-tors but the details of its interactions are still not fully understood. To dissect this pathway a novel proteomic approach for the immunoprecipitation of the viral RNA was developed in our laboratory : we thus identified the nuclear matrix protein MATR3 as a Rev co-factor which was recruited after mRNA biogenesis during this process (Kula et al., 2011, 2013). We could assess that MATR3 acts in the post-transcriptional steps of viral replication and we could demonstrate its role as a Rev-cofactor during the nuclear export of viral mRNAs. To establish the functional role of MATR3 during acute viral infection we modu-late its levels in Jurkat cells by both knockdown or overexpression. We found that, while MATR3 depletion resulted in the drastic reduction of viral replication, its overexpression leads to enhanced viral particle production. We applied the same approach to primary PBLs and obtained a similar result concluding that MATR3 is a positive regulator of viral replication. To investigate a possible role for MATR3 in the establishment of viral latency we depleted MATR3 from J-Lat cells, a well-established model of latency. We ob-served that MATR3 depletion did not impede transcriptional reactivation of the integrated provirus upon TNFα stimulation, but strongly impaired intracellular viral protein production and full viral rescue. This observation demonstrated that MATR3 depletion affects the post-transcriptional steps of latency reversal suggesting that this factor could play a crucial role during the maintenance of latency. Since most of the attempts done with Latency Reversal Agents (LRAs), an heterogeneous group of drugs proposed to restore viral transcription, failed to induce full reactivation of the latent provirus (Darcis et al., 2015; Spina et al., 2013) we reasoned that there could be a post-transcriptional block to full viral reactivation in latently infected cells and we suppose that MATR3 could represent a limiting factor to this process. We confirmed that MATR3 was almost undetectable in resting PBLs but could be promptly upregulated upon cellular activation. MATR3 was not induced by treatment with LRAs, such as SAHA or Romidepsin within PBL from healthy donors and HIV-infected patients. We propose that the restoration of proper MATR3 levels within latently infected cells could enhance latency reversal in LRAs-treated cells.