Effects of the HIV-1 Envelope Glycoprotein gp120 in Cerebellar Cultures. [Ca2+]i Increases in a Glial Cell Subpopulation

The various types of cells present in cultures prepared from the postnatal rat cerebellum, identified by their gross morphology and immunocytochemistry, were loaded with the specific dye fura-2 and analysed individually for Ca²⁺]_i changes induced by the HIV-1 envelope glycoprotein gp120 and a variety of other treatments. In granule neurons Ca²⁺]_i increases were induced by high KCl and glutamate (mainly through the NMDA receptor) while in type-1 astrocytes this effect was observed after serotonin, carbachol and also quisqualate. In contrast, administration of gp120 was always without effect in these cells. Type-2 astrocytes (an arborized cell type responsive to agonists targeted to the glutamatergic AMPA and cholinergic receptors) were also most often unresponsive to the viral glycoprotein. However, among the cells exhibiting the arborized phenotype, a subpopulation (-13%) responded to gp120 with conspicuous Ca²⁺]_i increases sustained by both release from intracellular stores and influx across the plasma membrane. These responses to the viral protein did not involve activation of either voltage-gated Ca²⁺ channels or glutamatergic receptors. Although not yet conclusively identified by specific cytochemical markers, the gp120-responsive cells resemble type-2 astrocytes and differ from neurons and type-1 astrocytes both in gross phenotype and in a number of receptor/channel properties: positivity to AMPA and cholinergic agonists; negativity to NMDA, serotonin and high KCl. From these results it is concluded that a subpopulation of glial cells is affected by gp120. The role of these cells in HIV brain infection and damage requires further studies to be precisely established.