Date Published: August 29, 2019
Publisher: Public Library of Science
Author(s): Sara Rodríguez-Mora, Flore De Wit, Javier García-Perez, Mercedes Bermejo, María Rosa López-Huertas, Elena Mateos, Pilar Martí, Susana Rocha, Lorena Vigón, Frauke Christ, Zeger Debyser, Juan Jesús Vílchez, Mayte Coiras, José Alcamí, Michael Emerman.
The causative mutation responsible for limb girdle muscular dystrophy 1F (LGMD1F) is one heterozygous single nucleotide deletion in the stop codon of the nuclear import factor Transportin 3 gene (TNPO3). This mutation causes a carboxy-terminal extension of 15 amino acids, producing a protein of unknown function (TNPO3_mut) that is co-expressed with wild-type TNPO3 (TNPO3_wt). TNPO3 has been involved in the nuclear transport of serine/arginine-rich proteins such as splicing factors and also in HIV-1 infection through interaction with the viral integrase and capsid. We analyzed the effect of TNPO3_mut on HIV-1 infection using PBMCs from patients with LGMD1F infected ex vivo. HIV-1 infection was drastically impaired in these cells and viral integration was reduced 16-fold. No significant effects on viral reverse transcription and episomal 2-LTR circles were observed suggesting that the integration of HIV-1 genome was restricted. This is the second genetic defect described after CCR5Δ32 that shows strong resistance against HIV-1 infection.
Productive HIV-1 infection requires the interaction with cellular co-factors at virtually all the steps of the viral replication cycle . Viral entry depends on fusion of viral and cellular membranes through successive interactions with CD4 receptor combined with CXC chemokine receptor type 4 (CXCR4) or CC chemokine receptor type 5 (CCR5) . Once the core is released into the cytosol, the reverse transcriptase converts the viral RNA genome into a double-stranded copy DNA (cDNA) and the capsid (CA) uncoating process is initiated. HIV-1 cDNA gains access to the nucleus through the cellular nuclear transport machinery located at the nuclear pore, in the form of a pre-integration complex (PIC). These PICs consist of viral cDNA and other HIV-1 components like integrase (IN), matrix, nucleocapsid, CA and viral protein R (Vpr), as well as various host proteins, such as the high mobility group protein B1 (HMGB1), barrier to autointegration factor 1 (BAF1), lamina-associated polypeptide 2α (LAP2α) and lens-epithelium derived growth factor (LEDGF/p75) [3–7]. Several cellular import factors, including importin-7, importin-α3 and Transportin 3 (TNPO3, also called TRN-SR2) have also been involved in HIV-1 nuclear import . Apart from its implication in nuclear import of the viral PIC, it has been confirmed that N-terminal end of TNPO3 protein act as a direct binding partner of HIV-1 IN . Interaction with the viral CA has also been documented [10,11] and nearly 30 CA-mutants able to modify HIV-1 dependence on TNPO3 have been described .
HIV-1 infection remains incurable despite efficient antiretroviral treatments that tackle HIV-1 enzymes and proteins. One potential strategy to develop new therapeutic targets can be based on the study of the interaction between viral proteins and their cellular cofactors. In this regard, TNPO3 and other importins have been previously described as essential cellular proteins for HIV-1 infection [23–25,37]. However the exact mechanism of action of TNPO3 still remains a matter of controversy . Some studies suggest that TNPO3 participates in the nuclear import of PICs [9,25,39,40] whereas other authors propose that TNPO3 promotes HIV-1 infection though the interaction with HIV-1 CA [41–43] or indirectly through the interaction of CPSF6 with HIV-1 CA . Besides an indirect role for TNPO3 in viral integration through its interaction with CA and/or the IN and their respective cellular partners such as CPSF6 or LEDGF/p75  has been proposed. These cellular host factors may affect the nuclear landscape of HIV-1 infection  by targeting the viral genome to silent or actively transcribed chromatin  opening a new perspective in the mechanisms of HIV-1 latency and reactivation.