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Characterization of Inducible SMC-specific Drosha Knockout Mice
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Smooth muscle cells (SMCs) play an important role in the contraction of gastrointestinal (GI) smooth muscles, generating spontaneous action potentials and forming electrical and mechanical junctions coupling with interstitial cells. Recent studies have shown that microRNAs (miRNAs), a new class of small RNAs that function as guide molecules in RNA silencing, regulate and maintain the differentiation state of SMCs. A transgenic animal model to study SMC-specific knockout of Dicer, one of two RNase III proteins in miRNA biogenesis, showed that contractile motility in the mutant intestine was dramatically decreased (Park, Yan, et al., 2011). The purpose of this study was to investigate the function of Drosha, another of the two RNase III proteins in miRNA biogenesis, in intestinal SMCs using a transgenic animal model. We generated SMCspecific inducible Drosha null animals (smMHCCre-ERT2/+;Droshalox/lox) by cross-breeding a smMHC/Cre/eGFP (smMHCCre-GFP/+) male mouse and a Droshalox/lox female homozygote mouse. The inducible SMC-specific knockout (KO) mice caused severe changes in phenotype and function of the GI tract. Morphological study of the GI smooth muscle showed that the KO mice developed a thicker muscle layer of the intestinal tract from the upper duodenum to the jejunum down to the whole colon. Histological serial sections stained with hematoxylin and eosin (H&E) showed hypertrophy in both jejunum and colon SM layers. This phenotype can be at least partially explained by the loss of SMC-specific miRNAs, miR-143 and miR-145, identified as regulators of SMC growth and/or differentiation. In conclusion, the data gathered in the present research suggests that miRNAs are required for the proper maintenance of SMCs in the GI tract. Further ii research is needed to understand the complete molecular mechanisms required for development and maintenance of contractile activity in SMCs in the GI tract.