Intermediate filament protein synemin is transiently expressed in a subsetof astrocytes during development

Synemin, a developmentally regulated protein first described in muscle cell s, has recently been recognized as an intermediate filament (IF) protein. B ecause IF proteins are invaluable markers of cell origin within the nervous system, we were interested in determining the expression pattern of synemi n in the brain. Our results show that, during development of the rat cortex , synemin is expressed only in a subpopulation of astrocytic cells expressi ng GFAP as well as vimentin and nestin. Uhlike GFAP, however, synemin is no t expressed in mature astrocytes and, unlike vimentin and nestin, synemin i s not present in astrocytic precursors before GFAP expression. Taken togeth er with morphological evidence, the time course of synemin expression, as d etermined by Western blotting, suggests that synemin is expressed in radial glial cells undergoing morphological transformation into astrocytes. Studi es of synemin expression in vitro demonstrate that, early in primary cultur e, the majority of polygonal astrocytes are derived from synemin(+) radial glial cells. With time in culture, however, polygonal astrocytes either sto p expressing synemin or are overgrown by cells not expressing synemin. The unique pattern of synemin expression, both in vivo and in vitro, suggests t hat the use of synemin as a marker will add a new dimension to studies of a strocytic differentiation. GLIA 30:143-153, 2000. (C) 2000 Wiley-Liss, Inc.