Extracellular calcium deprivation in astrocytes: Regulation of mRNA expression and apoptosis

Cell viability and gene expression were studied in primary astroglial cells cultured in a nominally calcium-free medium. Ca²⁺ deprivation reduced progressively the astrocytes' viability, starting from 12 h; the restoration of a normal Ca²⁺ concentration (1.8 mM) in the medium after 12-h deprivation reversed the degenerative effect within 24 h. Biochemical and morphological examinations indicated that cell death induced by Ca²⁺ deprivation was mediated by apoptosis. This was associated with the expression of c-fos, c-jun, and c-myc, which, with different time courses, were induced in astrocytes after Ca²⁺ deprivation. Furthermore, shifting to a Ca²⁺-free medium modified the expression of Ich-1s transcript and rapidly increased intracellular cyclic AMP, which has been implicated in the transcriptional activation of immediate-early genes. The absence of Ca²⁺ in the medium reduced the expression of constitutive proteins such as α-actin, clusterin, glial fibrillary acidic protein, amyloid precursor protein, and glucose-6-phosphate dehydrogenase. The expression of these mRNAs was reduced >50% after 8 h of Ca²⁺ deprivation, when the effect on cell viability was negligible. When Ca²⁺ deprivation was prolonged for 24 h the expression of mRNA dropped completely, and restoration of the Ca²⁺ ions in the medium for 48 h did not reverse this effect. In contrast with general assumption, the apoptotic machinery in astrocytes is activated similarly not only by increased Ca influx but also with the Ca²⁺ extracellular Ca deprivation.