The Role of Nonmuscle Myosin II-B in Brain Development

我们产生了在非肌肉肌球蛋白重链II-B(NMHC II-B)的运动区存在单一氨基酸突变的小鼠。纯合子突变小鼠步态异常，难以保持平衡。与它们的运动缺陷相一致的是，突变小鼠表现出了小脑叶状结构的异常模式。对纯合子突变小鼠大脑的分析显示，小脑颗粒细胞、面神经元和小脑前壁前壁外前迁移流中的神经元迁移存在显著缺陷。NMHC II-B在这些神经元中的高水平表达表明，这种分子在它们在发育中的大脑中的迁移过程中发挥着重要作用。与静止的脑桥神经元相比，迁移过程中肌球蛋白调节轻链的磷酸化增加支持了肌球蛋白在调节其迁移过程中的积极作用。这些研究表明，NMHC II-B对于小鼠大脑发育过程中不同组神经元的正常迁移特别重要。我们目前正在鉴定在移除Neo-R盒后表现出与野生型相同水平表达的突变小鼠。这些小鼠之所以令人感兴趣，是因为与我们在NMHC II-B零突变小鼠中的发现相反，大约一半的杂合突变小鼠出现脐疝和横隔疝，并在出生后不久死亡。其余的杂合子小鼠存活到成年并具有生育能力，尽管仍观察到脐带闭合和横隔膜发育的细微缺陷。纯合子小鼠大约在胚胎16.5天死亡，表现出涉及腹侧体壁、横隔膜、心脏和大脑的异常。所有这些小鼠都显示出严重的中线融合缺陷，并伴有包含肝脏和肠道大部分的大腹股沟，胸骨分裂和心脏外化。突变小鼠的横隔膜非常薄。与野生型相比，变异心脏的主动脉和肺动脉都发自右室，室间隔缺陷，心脏垫发育异常，室壁和房壁的致密区都要薄得多。与在突变等位基因中保留新盒的亚型突变小鼠类似，这些小鼠也出现了脑积水和某些神经元组的异常迁移。然而，与亚型突变小鼠不同的是，在非亚型R709C突变小鼠中，胚胎心脏的心肌细胞胞质分裂没有明显的缺陷。

We generated mice harboring a single amino acid mutation in the motor domain of nonmuscle myosin heavy chain II-B (NMHC II-B). Homozygous mutant mice had an abnormal gait and difficulties in maintaining balance. Consistent with their motor defects, the mutant mice displayed an abnormal pattern of cerebellar foliation. Analysis of the brains of homozygous mutant mice showed significant defects in neuronal migration involving granule cells in the cerebellum, the facial neurons and the anterior extramural precerebellar migratory stream. A high level of NMHC II-B expression in these neurons suggests an important role for this molecule during their migration in the developing brain. Increased phosphorylation of the myosin regulatory light chain in migrating, compared to stationary pontine neurons supports an active role for myosin in regulating their migration. These studies demonstrate that NMHC II-B is particularly important for normal migration of distinct groups of neurons during mouse brain development. We are presently characterizing mutant mice which show the same level of expression as wild-types following removal of the Neo-R cassette. These mice are of interest since, in contrast to our findings in mice with a null mutation for NMHC II-B, about one-half of the heterozygous mutant mice developed umbilical and diaphragmatic hernias, and died shortly after the birth. The remainder of the heterozygous mice survived to adulthood and were fertile, although subtle defects in closure of the umbilical area and diaphragm development were still observed. Homozygous mice died at approximately embryonic day 16.5, displaying abnormalities involving the ventral body wall, diaphragm, heart and brain. All these mice show severe defects in midline fusion with a large hernia containing most of the liver and intestine, a split sternum and externalized heart. The diaphragm of the mutant mice was very thin. The mutant heart had both the aorta and pulmonary artery emanating from the right ventricle, a ventricular septal defect, abnormal development of the cardiac cushions, and a much thinner compact zone of both the ventricle and atrial walls compared to the wild-type littermate. Similar to the hypomorphic mutant mice, which retain the Neo-cassette in the mutant alleles, hydrocephalus and abnormal migration of certain groups of neurons were also seen in these mice. Unlike the hypomorphic mutant mouse however, no obvious defects in cytokinesis of the cardiomyocytes in the embryonic heart were seen in the nonhypomorphic R709C mutant mice.