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）的运动域携带单个氨基酸突变的小鼠。纯合子突变小鼠步态异常，难以保持平衡。与它们的运动缺陷一致，突变小鼠表现出小脑叶状叶的异常模式。纯合子突变小鼠的大脑分析显示，涉及小脑、面神经细胞和前外小脑前脑迁移流的颗粒细胞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该分子在发育中的大脑迁移过程中起着重要作用。与静止的脑桥神经元相比，迁移过程中肌凝蛋白调节轻链磷酸化的增加支持了肌凝蛋白在调节其迁移中的积极作用。这些研究表明，在小鼠大脑发育过程中，NMHC II-B对不同神经元群的正常迁移尤为重要。我们目前正在描述在移除Neo-R磁带后表现出与野生型相同表达水平的突变小鼠。这些小鼠引起了人们的兴趣，因为与我们在NMHC II-B零突变小鼠中发现的结果相反，大约一半的杂合突变小鼠出现了脐疝和膈疝，并在出生后不久死亡。其余的杂合小鼠存活到成年并具有生育能力，尽管仍然观察到脐带闭合和横膈膜发育的细微缺陷。纯合子小鼠大约在胚胎第16.5天死亡，表现出涉及腹壁、隔膜、心脏和大脑的异常。所有这些小鼠都表现出严重的中线融合缺陷，伴有包含大部分肝脏和肠道的大疝，胸骨分裂和心脏外化。突变小鼠的横膈膜非常薄。突变心脏的主动脉和肺动脉都从右心室发出，室间隔缺损，心脏垫发育异常，心室和心房壁的致密区比野生型的幼崽要薄得多。与在突变等位基因中保留Neo-cassette的半胚突变小鼠类似，在这些小鼠中也观察到脑积水和某些神经元组的异常迁移。然而，与半形突变小鼠不同的是，非半形R709C突变小鼠胚胎心脏心肌细胞的胞质分裂未见明显缺陷。

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.