Vascular injury triggers dedifferentiation and cytoskeletal remodeling of smooth muscle cells (SMCs), culminating in vessel occlusion. Serum response factor (SRF) and its coactivator, myocardin, play a central role in the control of smooth muscle phenotypes by regulating the expression of cytoskeletal genes.
Researchers from the University of Texas Southwestern Medical Center show that SRF and myocardin regulate a cardiovascular-specific microRNA (miRNA) cluster encoding miR-143 and miR-145. To assess the functions of these miRNAs in vivo, they systematically deleted them singly and in combination in mice. Mice lacking both miR-143 and miR-145 are viable and do not display overt abnormalities in smooth muscle differentiation, although they show a significant reduction in blood pressure due to reduced vascular tone. Remarkably, however, neointima formation in response to vascular injury is profoundly impeded in mice lacking these miRNAs, due to disarray of actin stress fibers and diminished migratory activity of SMCs. These abnormalities reflect the regulation of a cadre of modulators of SRF activity and actin dynamics by miR-143 and miR-145. Thus, miR-143 and miR-145 act as integral components of the regulatory network whereby SRF controls cytoskeletal remodeling and phenotypic switching of SMCs during vascular disease.
Smooth muscle abnormalities in miR-143/145 KO mice
(A) H&E staining of the dorsal aortae of mice of the indicated genotypes. Bar, 40 μm. (B) Electron microscopy of the dorsal aortae of mice of the indicated genotypes. Note the thickened ECM in miR-145 KO and dKO. Prominent actin-based stress fibers (arrows) are apparent in wild-type (WT) and miR-143 KO SMCs, but are absent from miR-145 KO and dKO SMCs. Rough endoplasmic reticulum (RER) is evident in miR-143 KO, miR-145 KO, and dKO SMCs, indicative of a synthetic state, but is not visible in wild-type SMCs. (ecm) Extracellular matrix; (enl) endothelial layer; (sf) stress fiber; (sm) smooth muscle. (C) Immunostaining of sections of aorta of wild-type and dKO mice showing similar staining for SMA. (D) Aortic SMCs were isolated from wild-type and dKO mice, cultured in growth medium, and stained for SMA. Wild-type SMCs display highly organized actin stress fibers, whereas SMCs from dKO mice display only diffuse actin staining. (E) Expression of the smooth muscle differentiation markers SMA and SM22 in aorta of wild-type and dKO mice as detected by Western blot analysis. GAPDH was detected as a loading control. (F) Expression of smooth muscle mRNAs as detected by real-time PCR. (G) Blood pressure measurements in wild-type and miR-145 KO mice. MAP and SBP were measured as described in the Materials and Methods. (*) P < 0.05.
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Reference
Xin M, Small EM, Sutherland LB, Qi X, McAnally J, Plato CF, Richardson JA, Bassel-Duby R, Olson EN. (2009) MicroRNAs miR-143 and miR-145 modulate cytoskeletal dynamics and responsiveness of smooth muscle cells to injury. Genes Dev 23(18),2166-178. [article]