DIFFERENTIAL EXPRESSION OF ALPHA-ACTIN MESSENGER-RNA AND IMMUNOREACTIVE PROTEIN IN BRAIN MICROVASCULAR PERICYTES AND SMOOTH-MUSCLE CELLS

Hypertension has been linked to opening of the blood-brain barrier and may be related to the expression of the smooth muscle alpha-actin gen e in contractile cells at the brain microvasculature. However, the cel lular origin (i.e., endothelial cells, pericytes, smooth muscle cells) of the alpha-actin mRNA in the brain microvasculature is not clearly identified. Therefore, we investigated the abundance of actin mRNA by Northern blot analysis in isolated brain microvessels and in brain mic rovascular endothelia or pericytes in tissue culture. All samples show ed the characteristic 2.1 kb transcript corresponding to cytoplasmic b eta and gamma isoform mRNA. The 1.7 kb transcript corresponding to smo oth muscle alpha-actin was detected in freshly isolated bovine brain m icrovessels, in primary cultures of brain microvascular pericytes, or endothelial cells; the latter cultures contain both endothelial cells and pericytes. The alpha-actin mRNA was absent in a cloned bovine brai n endothelial cell line. The relative abundance of the alpha/(beta + g amma) actin transcript ratio was: cultured pericytes > freshly isolate d microvessels > endothelial primary. The cellular distribution of the smooth muscle alpha-actin immunoreactive protein was studied by immun ocytochemistry in cytospun/methanol-fixed isolated bovine brain microv essels with a monoclonal antibody directed to the amino-terminal decap eptide of the smooth muscle alpha-actin isoform. This antibody reacted strongly with precapillary arterioles of isolated microvessels, where as no immunostaining was observed in either capillary endothelial cell s or in pericytes. In conclusion, the alpha-actin mRNA is expressed in brain microvascular pericytes in tissue culture, but the immunoreacti ve alpha-actin protein is not expressed in brain microvascular pericyt es in vivo. These data suggest that either 1) alpha-actin gene express ion is induced in capillary pericytes in tissue culture or 2) alpha-ac tin mRNA in brain capillary pericytes in vivo is subject to translatio nal repression resulting in no detectable alpha-actin protein under no rmal conditions. (C) 1994 Wiley-Liss, Inc.