Conditional Ablation of Nonmuscle Myosin II-B: Cre-recombinase Controlled by the Transgelin/SM22alpha Promoter Global ablation of nonmuscle myosin II-B (NM II-B;B-/B-) in mice results in lethality during embryonic development with cardiac associated defects. In an effort to produce a mouse model for human cardiac disease, we generated a NM II-B conditionally ablated mouse in which Cre-recombinase is under the control of the transgelin/SM22alphanpromoter (BSM22alpha/BSM22alpha mice). NM II-B was ablated in almost all cardiac myocytes, the majority of epicardial cells, and smooth muscle cells of the aorta as early as E11.5. Unlike B-/B- mice, BSM22alpha/BSM22alpha mice survive for up to 5 months of age. MRI analyses of the mouse hearts show severe dysfunction and dilation of the right ventricle at 4 months, while the contractile activity of the left ventricle is not impaired. EKG analyses of BSM22alpha/BSM22alpha mice at 1 month and later show cardiac arrhythmias, including defects in conduction such as atrial-ventricular dissociation and the presence of numerous premature ventricular beats indicating impairment of cardiac conduction. Sectioning of BSM22alpha/BSM22alpha hearts at 3 months demonstrates marked cardiac myocyte hypertrophy and interstitial fibrosis in the right ventricle. Cardiac myocyte hypertrophy is also observed in the left ventricle, but no obvious interstitial fibrosis is seen. In addition, BSM22alpha/BSM22alpha hearts show defects in coronary vessel development during the early embryonic stages. Large size coronary arteries are missing in the right ventricle of adult BSM22alpha/BSM22alpha hearts. Many of these mice die suddenly between 3-5 months of age due to cardiac arrhythmias. In addition both the systemic and pulmonary blood pressure of BSM22alpha/BSM22alpha mice were significantly lower compared to their control littermates, consistent with the idea that NM II-B in vascular smooth muscle is contributing to maintaining the vascular tone. Conditional Ablation of Nonmuscle Myosin II-B: Cre-recombinase Controlled by the Nestin or alpha-cardiac Myosin Heavy Chain Promotor We used a loxP/Cre recombinase strategy to specifically ablate nonmuscle myosin II-B (NM II-B) in the brains or hearts of mice. These are the two major organs affected by germ line ablation of NM II-B (B-/B- mice). Our purpose was two-fold: first to identify the cell-type(s) responsible for the defects found in the hearts and brains of B-/B- mice and second, to avoid the embryonic lethality found in B-/B- mice. Mice ablated for NM II-B in neural tissues (Bnest/Bnest mice) develop severe hydrocephalus starting at E16.5 and die between postnatal day (P)12 and 22 without showing cardiac defects. Mice deficient in NM II-B only in cardiac myocytes (BalphaMHC/BalphaMHC mice) do not show defects in the brain. However BalphaMHC/BalphaMHC mice display novel cardiac defects not seen in B-/B- mice. Between 6-10 months most BalphaMHC/BalphaMHC mice develop a dilated cardiomyopathy, which includes interstitial fibrosis and infiltration of the myocardium and pericardium with inflammatory cells. At 10 months echocardiography measures a decrease in fractional shortening of BalphaMHC/BalphaMHC hearts. In addition most of the BalphaMHC/BalphaMHC mice showed an abnormal right axis by EKG. Four of five BalphaMHC/BalphaMHC hearts develop marked widening of the intercalated discs, consistent with a role for NM II-B in myocyte cell adhesion. By avoiding the embryonic lethality found in germline-ablated mice we were able to study the role of NM II-B in adult mice and to define a role for this isoform in the intercalated disc. Generation of Nonmuscle Myosin II-B Point Mutant Mice: Nonmuscle myosin (NM) IIs play a variety of roles during mouse embryonic development and mutations in the genes encoding NM II-A and II-C are associated with human abnormalities. We generated point mutant knock-in mmice expressing motor-deficient NM II-B. Homozygous mice die at E14.5 in cardiac failure and exhibit novel abnormalities not previously observed in NM II-B null mice: a failure in midline fusion resulting in a cleft palate, ectopia cordis, and a large omphalocele, which resemble the human syndrome Pentalogy of Cantrell. Impaired apoptosis of mesenchymal cells in the fusing sternum and endocardial cushions contributes to these abnormalities and provides evidence that mutant II-B is interfering with normal NM II-A function. Surprisingly mutant NM II-B rescues the cytokinesis defect in NM II-B deficient cardiac myocytes and COS-7 cells, suggesting a structural rather than a motor function for NM II during cytokinesis. However NM II-B motor function is required for normal epicardial FGF signaling and for disassembly of myocyte cell-cell adhesions during cardiac outflow tract development. This failure in myocyte disassociation contributes to the displacement of the aorta to the right ventricle in mutant mice. Our studies reveal dual roles for NM II based on actin-crosslinking or enzymatic functions.
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