The importance of some specific miRNAs for SMC phenotypic modulation has been described previously and recently three separate reports were published presenting vascular phenotype of global miR-143/145 KO mice. miR-143/145 are the first miRNAs suggested to be relatively specific for SMCs and play an important role for the regulation of SMC fate and maintenance of the contractile phenotype. In accordance, we found that over-expression of miR-145 rescued the loss of contractile differentiation in isolated Dicer KO SMCs. Interestingly, although not lethal, the phenotype of miR-143/145 KO mouse closely resembles that of the inducible SM-Dicer KO mouse in several aspects. Firstly, Nutlin-3 systolic blood pressure of miR-145 and miR143/145 KO mice is reduced by approximately 15�C 20 mmHg while systolic blood pressure in SM-Dicer KO mice is reduced by 27.7 mmHg. In both mouse models this is associated with a decreased heart weight, likely secondary to a decreased after-load, while heart rate is unchanged. Secondly, miR-143/145 KO mice exhibit reduced contractile responses to KCl and phenylephrine while these responses are nearly abolished in SM-Dicer KO mice 10 weeks post tamoxifen. MK-0683 Thirdly, both miR-143/145 KO mice and SM-Dicer KO mice have a decreased medial thickness and a reduced SMC contractile differentiation. These similarities indicate that although miR-143/145 are not the essential miRNAs for SMC development they are important determinants of SMC differentiation and function in vivo. Alternatively, the milder phenotype of the constitutive miR-143/145 KO mice could partly be due to compensatory mechanisms. As mentioned previously, we suggest that the decrease in blood pressure in SM-Dicer KO mice is likely due to a loss in SMC contractile differentiation. Herein, we found that deletion of Dicer in smooth muscle also resulted in reduced levels of myocardin mRNA. It has been suggested previously that myocardin expression is regulated by miR-145, either via direct binding to the 3��UTR of myocardin and translational activation or via down-regulation of KLF5, a repressor of myocardin expression. The loss of SMC contractile differentiation in SM-Dicer KO mice may thus be initially caused by a reduced miR-145 and myocardin expression. In addition, we previously reported that deletion of Dicer resulted in a dramatic loss of actin stress fibers, which was rescued by over-expression of miR-145. A similar loss of actin stress fibers was also observed in miR-145 KO SMC. We also found that the potentiating effect of miR-145 on SMC contractile differentiation was abolished in Dicer KO SMCs pretreated with an inhibitor of actin polymerization. Actin polymerization is known to be an important regulator of SMC contractile differentiation and we have previously reported that actin dynamics is involved in stretch-induced contractile differentiation of vascular smooth muscle.