To investigate the physiological function of nonmuscle myosin heavy chain II-B (II-B), we generated a point mutant II-B mouse where amino acid arginine-709 was replaced by cysteine (R709C). Due to the presence of the Neo cassette in the targeted allele intron, both heterozygous (B-R/CN) and homozygous mutant (B-CN/CN) mice showed decreased expression of mutant II-B. The B-R/CN mice were generally healthy and fertile. However, most of the B-CN/CN mice died within the first two days after birth, and the remainder survived for up to 15 days suffering from cardiac and brain abnormalities similar to the B-/B- mice which die prior to birth. The B-CN/CN mice developed an early postnatal ataxia coupled with abnormal development of the cerebellum, which was remarkably reduced in size and had abnormal foliation. Detailed histological examination and neuronal migration assays revealed significant neuronal migration defects in the B-CN/CN mouse. These included an abnormal protrusion of the facial nuclei into the 4th ventricle, premature arrest of the pontine migrational stream, and slowed migration of cerebellar granule cells. This study provides evidence that NMHC II-B is required for migration of certain groups of neuronal cells. Removal of the Neo cassette from B-R/CN mice using Cre-recombinase rescued the decrease in II-B expression (i.e., increased the mutant protein) and generated B-R/C mice, which revealed a novel phenotype. Most of the B-R/C mice (90%) died shortly after birth showing absence of the small intestines from the duodenum through the sigmoid colon and collapsed lung alveoli. The unexpected phenotype was due to the herniation of the liver and intestines during embryonic development and the ingestion of these organs by the mother. Present studies are directed at understanding the role of NMHC II-B in generating this abnormal herniation.
