Peripheral astrocyte processes: Monitoring by selective immunostaining forthe actin-binding ERM proteins

Astrocytes extend thin lamellate processes in the neuropil, in particular a round synapses, where they can modulate synaptic function or mediate glial- neuronal communication. Previous studies have shown that these lamellate pe risynaptic processes change their shape in response to neuronal activity, b ut the underlying mechanisms have remained unclear. Similarly, the molecula r composition of these fine, sheetlike astrocytic processes (often 50-100 n m wide) is not understood but has to be related to their dynamic properties . To this end, we have studied the presence of ezrin, radixin, and moesin ( ERM proteins) in the rat hippocampus and in primary cultured astrocytes, ap plying immunoperoxidase, immunofluorescence, and immunogold techniques. The se three ERM proteins are known as actin-binding proteins that link the cel l membrane to the actin cytoskeleton, particularly in microvillus-bearing e pithelial cells. In cell culture, anti-ezrin and antiradixin, but not antim oesin, antibodies were specific for astrocytes, which often displayed selec tive staining of filopodia and microvilli. Nonoverlapping visualization of astrocytic peripheral and stem processes was obtained by immunocytochemical double labeling for ezrin and GFAP, respectively. In sections of rat hippo campus, homogeneous labeling of the neuropil, but not of cell layers, resul ted from immunostaining of fine, peripheral astrocyte processes, as confirm ed ultrastructurally. Our data show that the fine peripheral processes of a strocytes, which also constitute the perisynaptic glial sheath, are special ized in that they contain characteristic actin-associated molecules, likely to contribute to their dynamic properties. Applying antiezrin and anti-rad ixin as selective markers, plasticity of these perisynaptic glial processes can be analyzed. (C) 2001 Wiley-Liss, Inc.