HIV-1 gp120 Glycoprotein Induces [Ca2+]i Responses not only in Type-2 but also Type-1 Astrocytes and Oligodendrocytes of the Rat Cerebellum

Cultures of cerebellar cortex cells were exposed to the HIV‐1 envelope glycoprotein, gp120, and investigated for cytosolic Ca2+ ion concentration ([Ca2+]i) changes by the fura‐2 ratio videoimaging technique while bathed in complete, Na+‐free or Mg2+‐free Krebs‐Ringer media. At the end of the [Ca2+]i experiments the cells were fixed and immunoidentified through the revelation of markers specific for neurons (microtubule associated protein‐2), type‐2 (A2B5) or all (glial fibrillary acidic protein) astrocytes, oligodendrocytes (galactocerebroside) or microglia (F4/80 antibody). In complete medium, rapid biphasic (spike‐plateau) responses induced by gp120 (0.1–1 nM) were observed in a subpopulation of type‐2 astrocytes. In addition, slow but progressive responses were observed in other type‐2 cells and oligodendrocytes, whereas type‐1 astrocytes showed small responses, if any, and granule neurons did not respond at all. Use of Na+‐free medium (a condition that blocked another gp120‐induced response, cytosolic alkalinization) resulted in an increase in [Ca2+]i response that was appreciable not only in type‐2 but also in most type‐1 astrocytes, possibly because of the inhibition of the Na+/Ca2+ exchanger and the ensuing decrease in Ca2+ extrusion. Granule neurons, including those in direct contact with responsive astrocytes, remained unresponsive, even when the experiments were carried out in Mg2+‐free medium supplemented with glycine, a condition that favours activation of the glutamatergic N‐methyl‐D‐aspartate (NMDA) receptor. The results obtained demonstrate that sensitivity to gp120 is a property of not only a few type‐2 astrocytes but of the majority of cerebellar glial cells, which, however, do not respond to the protein with glutamate release, as indicated by the negative results obtained with NMDA‐receptor‐expressing granule neurons. Single glial cell [Ca2+]i increase, the faster and most sensitive effect of gp120 revealed in the brain so far, could be ultimately employed to reveal CD4‐independent transmembrane signalling machanisms of the viral protein that, at the moment, remain almost entirely unknown.