Integrative Physiology/Experimental Medicine MicroRNAs Are Necessary for Vascular Smooth Muscle Growth, Differentiation, and Function

Objective—Regulation of vascular smooth muscle (VSM) proliferation and contractile differentiation is an important factor in vascular development and subsequent cardiovascular diseases. Recently, microRNAs (miRNAs) have been shown to regulate fundamental cellular processes in a number of cell types, but the integrated role of miRNAs in VSM in blood vessels is unknown. Here, we investigated the role of miRNAs in VSM by deleting the rate-limiting enzyme in miRNA synthesis, Dicer. Methods and Results—Deletion of Dicer in VSM results in late embryonic lethality at embryonic day 16 to 17, associated with extensive internal hemorrhage. The loss of VSM Dicer results in dilated, thin-walled blood vessels caused by a reduction in cellular proliferation. In addition, blood vessels from VSM-deleted Dicer mice exhibited impaired contractility because of a loss of contractile protein markers. We found this effect to be associated with a loss of actin stress fibers and partly rescued by overexpression of microRNA (miR)-145 or myocardin. Conclusion—Dicer-dependent miRNAs are important for VSM development and function by regulating proliferation and contractile differentiation. Key Words: contractile proteins Ⅲ molecular biology Ⅲ vascular biology Ⅲ vascular muscle M icroRNAs (miRNAs) are short (Ϸ22 nucleotides) non-coding RNAs that are involved in the regulation of mRNA expression and protein synthesis, 1 and each miRNA potentially targets multiple transcripts. 2 Immature miRNAs are processed by the 2 RNase III endonucleases, Drosha and Dicer, and then incorporated into the RNA-induced silencing complex. Depending on the complementarity of the miRNA with the 3Ј-untranslated region of the target mRNA, the RNA-induced silencing complex will mediate either transla-tional repression/activation or degradation of the target mRNA. One way to decipher the importance of miRNAs in development and cell biology has been by Dicer gene disruption in mice. The global loss of Dicer in knockout (KO) mice results in embryonic lethality associated with a loss of pluripotent stem cells and defective blood vessel formation. 3,4 However, Dicer is not essential for developmental angiogen-esis, because endothelial-specific Dicer hypomorphic mice survive but have impaired postnatal angiogenic responses. 5 Although vascular smooth muscle (VSM)–specific KO of Dicer has not been investigated to date, important functions of miRNAs in VSM are beginning to emerge. MicroRNA (miR)-21 is induced in at least 1 VSM type by the differentiation factors transforming growth factor-␤ and bone mor-phogenic protein. 6 In contrast, platelet-derived growth factor induces miR-221 and the dedifferentiation process in VSM. 7 miR-21 and miR-221 are also upregulated in vivo after …