Extracellular Calcium Deprivation in Astrocytes: Regulation of mRNA Expression and Apoptosis

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Abstract
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 Ca2+ influx but also with the extracellular Ca2+ deprivation.

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