Myogenin is a member of the basic helix-loop-helix (bHLH) gene family and converts multipotential mesodermal cells to myoblasts. The four members of the myoD family show unique spatio-temporal expression patterns and therefore may have different functions during myogenesis. Here we inactivate the myogenin gene in order to understand its role in myogenesis. Homozygous mutations are lethal perinatally owing to the resulting major defects in skeletal muscle. The extent of disorganization of muscle tissue differs in three regions. In the latero-ventral body wall, most cells, including myogenic cells, disappear and there is rapid accretion of fluid. In the limbs, cells of the myogenic lineage exist, but they are severely disrupted, and some of them are mono-nucleate with properties of myoblasts. In contrast, there are many axial, intercostal and back muscle fibres to be seen, although fibres are mainly disorganized and Z-lines are not present in most myofibrils. These findings are evidence that myogenin is crucial for muscle development in utero and demonstrate that other members of the myogenic gene family cannot compensate for the defect.

No abstract

Using the gene targeting technique, we have generated a new mouse model of congenital muscular dystrophy (CMD), a null mutant for the laminin oc2 chain. These homozygous mice, designated dy 3K ldy 3K , are characterized by growth retardation and severe muscular dystrophic symptoms and die by 5 weeks of age. Light microscopy revealed that muscle fiber degeneration in these mice begins no later than postnatal day 9. In degenerating muscles, considerable amounts of TUNEL positive nuclei were detected as well as DNA laddering, suggesting increased apoptotic cell death was involved in the process of muscle fiber degeneration.

Human chromosomes or chromosome fragments derived from normal fibroblasts were introduced into mouse embryonic stem (ES) cells via microcell-mediated chromosome transfer (MMCT) and viable chimaeric mice were produced from them. Transferred chromosomes were stably retained, and human genes, including immunoglobulin (Ig) kappa, heavy, lambda genes, were expressed in proper tissue-specific manner in adult chimaeric tissues. In the case of a human chromosome (hChr.) 2-derived fragment, it was found to be transmitted to the offspring through the germline. Our study demonstrates that MMCT allows for introduction of very large amounts of foreign genetic material into mice. This novel procedure will facilitate the functional analyses of human genomes in vivo.

Embryonic stem (ES) cell self-renewal and pluripotency are maintained by several signaling cascades and by expression of intrinsic factors, such as Oct3/4 and Nanog. The signaling cascades are activated by extrinsic factors, such as leukemia inhibitory factor, bone morphogenic protein, and Wnt. However, the mechanism that regulates extrinsic signaling in ES cells is unknown. Heparan sulfate (HS) chains are ubiquitously present as the cell surface proteoglycans and are known to play crucial roles in regulating several signaling pathways. Here we investigated whether HS chains on ES cells are involved in regulating signaling pathways that are important for the maintenance of ES cells. RNA interference-mediated knockdown of HS chain elongation inhibited mouse ES cell self-renewal and induced spontaneous differentiation of the cells into extraembryonic endoderm. Furthermore, autocrine/paracrine Wnt/␤-catenin signaling through HS chains was found to be required for the regulation of Nanog expression. We propose that HS chains are important for the extrinsic signaling required for mouse ES cell self-renewal and pluripotency.