Isolation of cystatin C via functional cloning of astrocyte differentiation factors

We screened for factors upregulating glial fibrillary acidic protein (GFAP) promoter activity by functional cloning with an immature astrocyte cell line (HB108-10) harboring a GFAP-lacZ construct. One cDNA clone that repeatedly upregulated lacZ expression encoded cystatin C (CysC), a cysteine protease inhibitor. TGF-beta induced CysC and GFAP expression in AP-16 cells, an astrocyte progenitor-like cell line expressing GLAST (a glutamate transporter subtype specifically expressed in immature astrocytes). CysC gene expression started earlier than that of GFAP in the mouse forebrain. It started in the ventricular zone at a similar period as (or slightly after) GLAST expression, but before GFAP expression. Although previous data showed that CysC is involved in the maintenance of adult neural stem cells, our data indicate that it is involved in astrocyte differentiation during mouse brain development.