Different HIV-1 gp120 envelope protein mutants differentially affect calcium responses in cultured cortical astrocytes

The HIV-1 envelope glycoprotein gp120 is involved in HIV-1 cell entry. It is released from infected cells within the brain and plays a crucial role in the pathogenesis of HIV- encephalopathy. Depending on its sequence, gp120 binds to alpha (CXCR4)- or beta (CCR5)- chemokine receptors, which are present on neurons, astrocytes and microglial cells.

Activation of chemokine-receptors resulted in calcium increase due to calcium release from internal stores. In cultured cortical astrocytes, incubation with gp120 led to a significant reduction of calcium transients elicited by physiological stimuli (Köller et al., J. NeuroVirol 2002;8: 411–419). In the present study, we investigated whether this effect depended on the structure and chemokine receptor usage of gp120.

Therefore we incubated cultured cortical rat astrocytes with gp120 from X4, R5 and dual tropic isolates and glycosylation mutants thereof and measured the intracellular calcium responses upon glutamate and ATP application. For this, HeLa-T4+ cells were transiently transfected with the respective gp120 expression vectors and supernatants were used for the experiments.

Incubation with supernatant containing the X4 gp120 induced a marked reduction of glutamate and ATP elicited calcium responses.

In contrast, incubation with R5 gp120 resulted in an increase of calcium responses. Gp120 from dual tropic isolates only led to slight increase of calcium responses upon glutamate application. In dual tropic gp120 mutants the effect depended on the presence of glycosylation at a single position: loss of glycosylation resulted in a physiologically ineffective gp120 protein.

Our data show that astroglial calcium responses are affected by HIV-1 envelope protein gp120 depending on their chemokine receptor usage and glycosylation pattern. Since calcium responses and calcium waves in astrocytic syncytia are supposed to contribute to synaptic efficacy the disturbance of intracellular calcium regulation in astrocytes may contribute to the pathogenesis of HIV-1 associated encephalopathy.

(Supported in part by a grant of the Hedwig and Waldemar Hort Stipendienstiftung to R.S.)