Early stages of HIV replication: how to hijack cellular functions for a successful infection

From the cell surface to the nucleus, the human immunodeficiency virus type 1 (HIV-1) will face multiple obstacles, crossing the plasma and nuclear membranes, but also finding its path within the cytoplasm in which elements from the cytoskeleton, organelles, and high a protein concentration, limit intracellular movements. At the same time, HIV-1 has to counteract cellular defenses--known as restriction factors--interfering with early steps of the virus cycle. Although the general outcomes of these early stages have been identified since several decades, the stepwise interactions taking place between cellular and viral components during this early journey, which will transform the incoming viral-RNA genome into a double-strand DNA competent for integration, remain largely unknown. In that sense, the uncoating process and the molecular basis of intracellular trafficking of preintegration complexes (PICs) are still poorly defined. Additionally, other key stages, which have been the focus of many reports, still require some clarifications, as is the case for the precise determinants of nuclear import of PICs. Finally, whereas the molecular mechanisms of integration, the last event of the early phase of retroviral life cycle, are now well understood, the choice of the integration site remains mysterious. Fully elucidating the early steps of HIV-1 replication is therefore crucial, not only for developing new antiretroviral drugs, but also for improving the design of lentiviral vectors for gene therapy. Since the mechanisms of HIV-1 entry and innate cell defenses were recently the topic of excellent reviews, we will focus here on uncoating and intracellular trafficking of HIV-1.