Intracellular Human Immunodeficiency Virus Tat Expression in Astrocytes Promotes Astrocyte Survival but Induces Potent Neurotoxicity at Distant Sites via Axonal Transport

Abstract

The human immunodeficiency virus (HIV)-Tat protein has been implicated in the neuropathogenesis of HIV infection. However, its role in modulating astroglialneuronal relationships is poorly understood. Astrocyte infection with HIV has been associated with rapid progression of dementia. We thus initially transfected astrocytes with HIV proviral DNA and confirmed Tat production in these cells. Subsequently, using stably Tat-producing asytocyte cell lines, we observed that Tat promoted astrocyte survival by causing a prominent antioxidant effect and resistance to cell injury in these cells. Tat was released extracellularly where it could be taken up by other cells. Tat remained functionally active following uptake and caused long terminal repeat (LTR) transactivation in lymphocytic and astrocytic cell lines. Tat released from astrocytes caused mitochondrial dysfunction, trimming of neurites, and cell death in neurons. Tat neurotoxicity was attenuated by antiTat antibodies, kynurenate or heparan sulfate. The neurotoxic effects of Tat were caused at concentrations lower than that needed to cause LTR transactivation. When Tat-expressing cells were injected into the rat dentate gyrus, Tat was taken up by granule cells and transported along neuronal pathways to the CA3 region where it caused glial cell activation and neurotoxicity. The arginine-rich domain of Tat was essential for both the LTR transactivation and the neurotoxic properties of Tat. Thus HIV-Tat is a potent neurotoxin that may act at distant sites while at the same time it assures its production by preventing cell death in astrocytes where it is produced.

The taming of the cell penetrating domain of the HIV Tat: Myths and realities

“…Intriguingly, in our investigations on stable SVGA-Tat cells, co-culture with SVGA-LTRCAT or SVGA-LTRGFP did not result in LTR activation [135]. This may be due either to low production of Tat in stable cells or astrocyte-to-astrocyte transcellular Tat blockage.…”

“…When transiently Tat-transfected SVGA cells were co-cultured with D3R5-LTRGFP cells [150], however, we observed sporadic LTRmediated GFP expression. Upon treatment with Tat antibody, partial inhibition of LTR activity was observed [135]. These results may be explained, at least in part, by the following: Tat expressed in high concentrations may be secreted from these cells and taken up by the bystander reporter cells but not transcellularly [144] (see supplemental data in ref.…”

“…Many studies on Tat as well as PTD fusion proteins, including our own, have demonstrated that these proteins function only when lysosomotropic agents are used [63,135], although this may not be the case with every protein. Endocytosed PTD-fusion proteins are retained in endosomes.…”

“…Nonetheless, it has not been established definitively that PTD has transcellular property. Owing to the strong NLS property of PTD [135,146] (Figs. 2, 3, 4 and 5), it should not be used for transcellular delivery nor for cytoplasmic targeting of fusion proteins unless prior testing in expression experiments reveals that fusion protein has strong cytoplasmic localization domain(s) that can overcome the PTD 's NLS strength (Table 3).…”

“…It has been demonstrated that Tat binds strongly in the nuclei (Fig. 2,3 and 4) and hence its exit from the nucleus would be difficult unless the cell ruptures [135,[142][143][144][145][146][147][148].…”