Background:  HIV-1 accessory protein Vpr has been associated with the induction of programmed cell death (apoptosis) and cell cycle arrest in vitro. However, the relevance of the effect of Vpr observed in vitro on HIV-1 neuropathogenesis, remains unknown in vivo. Due to the narrow host range of HIV-1 infection, no animal model is currently available. This has prompted us to consider a small animal model to evaluate the effects of Vpr on CNS in vivo through surrogate viruses expressing HIV-1Vpr.

Methods:  To investigate the apoptosis-inducing capabilities of HIV-1Vpr in vivo, a single round of replication competent viral vectors expressing Vpr was generated. Viral particles pseudotyped with VSV.G or N2c envelopes (neurotropic rabies envelope) were generated from spleen necrosis virus (SNV) and HIV-1-based vectors to transduce central nervous system (CNS) cells. The apoptotic analyses of transduced cells were performed by TUNNEL assay. Concentrated viral particles were also injected into the neonatal mice brain through the frontal cortex and ventricles. The mice were sacrificed 8 weeks after the injection. The paraffin-fixed brain sections of 0.5 µm were subjected to immunostaining to localize the Vpr expression using antibodies against Vpr.

Results:  This is the first in vivo study, demonstrating that Vpr generated from HIV-1-based vectors had profound apoptotic effects on CNS cells, particularly neurons, as compared with SNV-based vector system, both ex vivo and in vivo. Another interesting finding was that Vpr induced apoptosis in transduced cells, as well as in nearby cells through a bystander effect. This bystander effect was also observed in vivo, in cortical, hippocampal neurons and choroids plexus, when Vpr expressed by HIV-1-based vectors was delivered through the ventricles. These results suggest that Vpr may be responsible for the induction of apoptosis in CNS-based cells at a very early age, however, the devastating outcome in the form of neuronal damage is associated with the presence of HIV-1-based vectors, suggesting that other lentiviral elements in addition to Vpr, are also involved in neuropathogenesis.

Conclusions:  This is the first in vivo study and the results suggest that Vpr induces apoptosis in CNS cells in vitro and in vivo. This inexpensive animal model may be of value to understanding the role of HIV proteins in neuropathogenesis as well as for designing targeted neuroprotective strategies.

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