Tat protein induced neurocognitive dysfunction.

Abstract

Human immunodeficiency virus type 1 (HIV-1) is a major health threat in South Africa. Studies have shown that HIV affects the central nervous system (CNS), and can initiate a progressive neurodegenerative process culminating in HIV associated dementia. Tat protein and gp120 are two viral proteins that have been linked to the neurotoxic effects of HIV. The present study used an animal model to study the effects of tat protein on the brain. The study also investigated the ability of picolinic acid to block the damaging effects of Tat protein. Sprague-Dawley rats were divided into four groups. The control group received a bilateral injection of saline (100μl), and the experimental group received a bilateral injection of tat protein (5μg/100μl) into the hippocampus. The other two groups of rats were treated similarly, but also received picolinic acid at a concentration of 3.36mg/3ml, injected intraperitoneally (i.p) before the intrahippocampal injections. Two tests were performed to assess the cognitive behaviour of all the animals, namely the light/dark box and Morris water maze. After the behavioural tests, the brains of animals were collected for evaluating the activity of the enzyme caspase 3 by various means. Some brains were used to determine caspase 3 mRNA transcriptions by polymerase chain reaction; others were used to measure the expression of caspase 3 protein by Western blotting techniques, while the rest of the brains were used for flow cytometry analysis where a caspase 3 specific staining kit was used. Our behavioural results indicated that tat protein caused impairment in learning and memory in the experimental group, when compared to controls. The group that received both tat protein and picolinic acid showed a significant improvement in learning and memory in Morris water maze test, in comparison to the tat protein treated group. Our gene expression data showed a significant up regulation of caspase 3 gene in the tat protein treated group, compared to controls. This result was supported by the Western blot data that showed significantly increased caspase 3 protein expression in the tat protein treated group. While these increases in caspase 3 expression strongly point to an apoptotic mode of cell death in the hippocampus of animals that were treated with tat protein, our flow cytometry results were less convincing with marginal levels of caspase 3 staining being observed. Both increases in gene and protein expression were inhibited in animals that were pretreated with picolinic acid. Our data led us to conclude that tat protein can cause cognitive abnormalities through toxic sequelae that may include apoptosis. It is therefore likely that this viral protein may be one of the etiological factors of HIV associated neurocognitive impairment. Our results further suggest that picolinic acid may be considered as an adjunct therapy for HAND.