Regulation of neuregulin expression in the injured rat brain and cultured astrocytes

In this report, we investigated whether reactive astrocytes produce neuregu lins (glial growth factor 2/heregulin/acetylcholine receptor-inducing activ ity or neu differentiation factor) and its putative receptors, ErbB2 and Er bB3 tyrosine kinases, in the injured CNS in vivo. Significant immunoreactiv ities with anti-neuregulin, anti-ErbB2, and anti-ErbB3 antibodies were dete cted on astrocytes at the injured site 4 d after injury to the adult rat ce rebral cortex. To elucidate the mechanisms for the upregulation of neuregul in expression in astrocytes, primary cultured astrocytes were treated with certain reagents, including forskolin, that are known to elevate the intrac ellular level of cAMP and induce marked morphological changes in astrocytes . Western blot analysis showed that the expression of a 52 kDa membrane-spa nning form of a neuregulin protein was enhanced in cultured astrocytes afte r administration of forskolin. The upregulation of glial fibrillary acidic protein was also observed in astrocytes treated with forskolin. In contrast , inactivation of protein kinase C because of chronic treatment with phorbo l ester 12-O-tetradecanoyl phorbol 13-acetate downregulated the expression of the 52 kDa isoform, although other splice variants with apparent molecul ar sizes of 65 and 60 kDa were upregulated. These results suggest that the enhancement of neuregulin expression at injured sites is induced, at least in part, by elevation in intracellular cAMP levels and/or a protein kinase C signaling pathway. The neuregulin expressed on reactive astrocytes may st imulate their proliferation and support the survival of neurons surrounding cortical brain wounds in vivo.