Contractile proteins in pericytes at the blood-brain and blood-retinal barriers

Evidence from a variety of sources suggests that pericytes have contractile properties and may therefore function in the regulation of capillary blood flow. However, it has been suggested that contractility is not a ubiquitou s function of pericytes, and that pericytes surrounding true capillaries ap parently lack the machinery for contraction. The present study used a varie ty of techniques to investigate the expression of contractile proteins in t he pericytes of the CNS. The results of immunocytochemistry on cryosections of brain and retina, retinal whole-mounts and immunoblotting of isolated b rain capillaries indicate strong expression of the smooth muscle isoform of actin (alpha -SM actin) in a significant number of mid-capillary pericytes . Immunogold labelling at the ultrastructural level showed that alpha -SM a ctin expression in capillaries was exclusive to pericytes, and endothelial cells were negative. Compared to alpha -SM actin, non-muscle myosin was pre sent in lower concentrations. By contrast, smooth muscle myosin isoforms, w ere absent. Pericytes were strongly positive for the intermediate filament protein vimentin, but lacked desmin which was consistently found in vascula r smooth muscle cells. These results add support for a contractile role in pericytes of the CNS microvasculature, similar to that of vascular smooth m uscle cells.