Acidic calponin immunoreactivity in postnatal rat brain and cultures: subcellular localization in growth cones, under the plasma membrane and along actin and glial filaments

Acidic calponin, an F-actin-binding protein, is particularly enriched in br ain, where calponin protein and mRNA are mainly expressed by neurons. The p resence of calponin immunoreactivity in cultured astroglial cells has been reported, but the presence of acidic calponin in astrocytes in vivo appears equivocal. For the present study, we raised a specific polyclonal antibody against the 16-residue synthetic peptide covering the sequence E311-Q326 ( EYPDEYPREYQYGDDQ) situated at the carboxy-terminal end of rat acidic calpon in, and we investigated the cellular and subcellular localization of the pr otein in the developing central nervous system. Our results show that acidi c calponin is particularly enriched in: (i) growth cones and submembranous fields of maturing cerebellar and cortical cells, where it codistributes wi th microfilaments and (ii) glial cells in vivo, including radial glia, glia limitans, Bergmann glia and mature astrocytes, and ex vivo, where acidic c alponin strongly colocalizes with intermediate glial fibrillary acidic prot ein (GFAP) and vimentin filaments. Finally, up to four acidic calponin subt ypes with different isoelectric point (pI) values were identified by two-di mensional gel electrophoresis of cerebellar and hippocampal extracts. The m ore acidic isoforms were developmentally regulated. As only one single mRNA for acidic calponin has been identified, these isoforms must reflect posts ynthesis changes probably related to the particular functions of acidic cal ponin in maturing cells. Although brain acidic calponin's exact role remain s uncertain, the present data suggest that it is involved in neuronal and g lial plasticity.