Non-muscle myosin IIB (Myh10) is required for epicardial function and coronary vessel formation during mammalian development

The coronary vasculature is an essential vessel network providing the blood supply to the heart. Disruptions in coronary blood flow contribute to cardiac disease, a major cause of premature death worldwide. The generation of treatments for cardiovascular disease will be aided by a deeper understanding of the developmental processes that underpin coronary vessel formation. From an ENU mutagenesis screen, we have isolated a mouse mutant displaying embryonic hydrocephalus and cardiac defects (EHC). Positional cloning and candidate gene analysis revealed that the EHC phenotype results from a point mutation in a splice donor site of the Myh10 gene, which encodes NMHC IIB. Complementation testing confirmed that the Myh10 mutation causes the EHC phenotype. Characterisation of the EHC cardiac defects revealed abnormalities in myocardial development, consistent with observations from previously generated NMHC IIB null mouse lines. Analysis of the EHC mutant hearts also identified defects in the formation of the coronary vasculature. We attribute the coronary vessel abnormalities to defective epicardial cell function, as the EHC epicardium displays an abnormal cell morphology, reduced capacity to undergo epithelial-mesenchymal transition (EMT), and impaired migration of epicardial-derived cells (EPDCs) into the myocardium. Our studies on the EHC mutant demonstrate a requirement for NMHC IIB in epicardial function and coronary vessel formation, highlighting the importance of this protein in cardiac development and ultimately, embryonic survival.冠状动脉血管网络（coronary vasculature）是为心脏供血的关键血管网络。冠状动脉血流受阻可引发心脏疾病，而心脏疾病是全球范围内导致过早死亡的主要诱因。加深对冠状动脉血管形成背后的发育调控过程的理解，将助力心血管疾病治疗方案的研发。通过ENU诱变筛选，我们成功分离出一株表现出胚胎脑积水与心脏缺陷的小鼠突变体，该表型被命名为EHC。经位置克隆与候选基因分析，我们发现EHC表型源于Myh10基因剪接供体位点的点突变，该基因编码非肌肌球蛋白重链IIB（NMHC IIB）。互补试验验证了Myh10基因的突变正是EHC表型的致病原因。对EHC小鼠的心脏缺陷进行表型表征后，我们观察到其心肌发育存在异常，这与此前已构建的NMHC IIB基因敲除小鼠品系的观测结果一致。对EHC突变体心脏的分析还发现，其冠状动脉血管形成存在缺陷。我们将该冠状动脉血管异常归因于心外膜细胞功能受损：EHC小鼠的心外膜表现出异常细胞形态，上皮间质转化（EMT）能力下降，且心外膜来源细胞（EPDCs）向心肌层迁移的过程受到阻滞。本研究通过对EHC突变体的分析证实，NMHC IIB对于心外膜功能维持与冠状动脉血管形成不可或缺，凸显了该蛋白在心脏发育乃至胚胎存活过程中的关键作用。