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

Synemin, a developmentally regulated protein first described in muscle cells, has recently been recognized as an intermediate filament (IF) protein. Because IF proteins are invaluable markers of cell origin within the nervous system, we were interested in determining the expression pattern of synemin in the brain. Our results show that, during development of the rat cortex, synemin is expressed only in a subpopulation of astrocytic cells expressing GFAP as well as vimentin and nestin. Unlike GFAP, however, synemin is not expressed in mature astrocytes and, unlike vimentin and nestin, synemin is not present in astrocytic precursors before GFAP expression. Taken together with morphological evidence, the time course of synemin expression, as determined by Western blotting, suggests that synemin is expressed in radial glial cells undergoing morphological transformation into astrocytes. Studies of synemin expression in vitro demonstrate that, early in primary culture, the majority of polygonal astrocytes are derived from synemin(+) radial glial cells. With time in culture, however, polygonal astrocytes either stop expressing synemin or are overgrown by cells not expressing synemin. The unique pattern of synemin expression, both in vivo and in vitro, suggests that the use of synemin as a marker will add a new dimension to studies of astrocytic differentiation.