Background: HIV-1 infection causes neurodegeneration through toxic effects mediated by HIV-1 proteins such as Nef. While most studies have focused on neuron and astrocyte survival, the effects of HIV infection on neural stem cell survival have not been investigated to date. Our objective was to examine the effects of HIV-1 Nef on neural stem cells (NSC).

Methods: HIV-1 Nef sequences amplified from blood and brain of HIV/AIDS patients with and without HIV-1 associated dementia (blood n=10, brain n=5 each group), were subjected to phylogenetic analyses. The Nef proteins of prototypic brain-derived  (YU-2) and blood-derived (NL4-3) HIV-1 strains were expressed in a Sindbis virus (SIN) expression vector (SINrep5-NefYU2 and SINrep5-NefNL4-3). We infected murine NSC with the SIN vectors and subsequently analysed cell viability and NSC markers. The in vivo effects of the HIV-1 gene expression on NSCs were assessed in the brains of SCID/NOD mice following striatal stereotaxic implantation of SIN vectors expressing HIV proteins.

Results: Phylogenetic analysis of brain-derived Nef sequences revealed distinguishing features between clinical groups that were indicative of positive selection, depending on the individual protein domain. Infection of NSCs with the Nef-expressing SIN vectors resulted in a significant decrease in cell viability (p<0.001). The Nef protein of the brain- derived YU2 strain was more cytotoxic to NSC than the blood derived NL4-3 strain or the vector control (p<0.001). In addition, analysis of neural stem cell markers revealed marked differences in NSC differentiation after Nef expression compared to controls. Implantation of the HIV-expressing SIN vectors into SCID/NOD mice, induced inflammation and neuronal loss with associated neurobehavioral abnormalities compared to controls. Moreover, reduced immunoreactivity of NSC markers including PSA-NCAM and nestin was apparent in anterior subventricular zone of animals implanted with the HIV expressing vectors.

Conclusions: These observations suggest that HIV-1 Nef diminishes neural stem cell viability in vitro in sequence-dependent manner and that HIV gene expression reduces the in vivo expression of NCS markers. Viral gene-dependent NSC viability and function warrants further consideration in the pathogenesis of HIV-induced neurodegeneration.

Keywords: HIV-associated dementia; Nef; Neural stem cells