Background:  One of the pathological hallmarks in HIV-associated dementia is neuron death.  Nevertheless, neurons are not directly infected by HIV-1.  Thus, indirect and soluble factors produced by HIV-infected microglia/macrophages and astrocytes have been proposed for HIV-induced neuropathologies.  Among them is HIV-1 Tat protein.  Tat protein can be secreted from HIV-infected and Tat-expressing cells, and is able to be taken up into neighboring cells.  Tat is neurotoxic.  Our recent studies have shown that targeted Tat expression in the brain results in neuropathologies reminiscent of those noted in the brain of HIV-1 infected individuals.  In this study, we attempted to determine the potential effects of Tat protein on the dynamic interaction and communication between astrocytes and neurons.

Methods:  Inducible Tat-expressing astroglial cell line U373.MG, mouse embryonic primary astrocytes isolated from the brain of 16 days old Tat transgenic mice, and mouse embryonic primary neurons isolated from the brain of 16-day-old wild-type mice were used.  Direct effects of Tat expression on astrocyte functions were assessed by [3H] thymidine incorporation, expression of GFAP, and glutamate uptake assay, whereas effects of Tat expression in astrocytes on neurons were investigated by expressing Tat in astrocytes, collecting the culture supernatants, applying them onto the neurons, and determining neuron survival.

Results:  The results showed that Tat expression resulted in growth inhibition, but no apparent cell death of astrocytes.  Tat expression also up-regulated GFAP expression, and the up-regulation was due to direct interaction of Tat protein with the GFAP promoter.  Furthermore, the results showed that Tat expression led to a significant decrease in the glutamate uptake by astrocytes.  Importantly, the culture supernatants from Tat-expressing astrocytes caused rapid and massive neuron death, which did not appear to be associated with Tat secretion in these supernatants.

Conclusions:  Taken together, these results demonstrate that Tat expression adversely alters astrocyte function via multiple mechanisms involving growth inhibition, up-regulation of GFAP expression, and inhibition of glutamate uptake.  Induction of neuron death by the culture supernatants of Tat-expressing astrocytes suggests that astrocytes may potentiate Tat neurotoxicity.

Keywords: Tat protein; Astrocytosis; Neuron death