Development and degeneration of dorsal root ganglia in the absence of the HMG-domain transcription factor Sox10.

The HMG-domain transcription factor Sox10 is essential for the development of various neural crest derived lineages including glia and neurons of the peripheral nervous system (PNS). Within the PNS the most striking defect is the complete absence of glial differentiation whereas neurogenesis seemed initially normal. A degeneration of motoneurons and sensory neurons occurred later in development. The mechanism that leads to the dramatic effects on the neural crest derived cell lineages in the dorsal root ganglia (DRG), however, has not been examined up to now. Here, we provide a detailed analysis of proliferation and apoptosis in the DRG during the time of their generation and lineage segregation (between E 9.5 and E 11.5). We show that both increased apoptosis as well as decreased proliferation of neural crest cells contribute to the observed hypomorphism.

Peripheral nervous system defects in erbB2 mutants following genetic rescue of heart development.

The ErbB2 tyrosine kinase functions as coreceptor for the neuregulin receptors ErbB3 and ErbB4 and can participate in signaling of EGF receptor (ErbB1), interleukin receptor gp130, and G-protein coupled receptors. ErbB2(-/-) mice die at midgestation because of heart malformation. Here, we report a genetic rescue of their heart development by myocardial expression of erbB2 cDNA that allows survival of the mutants to birth. In rescued erbB2 mutants, Schwann cells are lacking. Motoneurons form and can project to muscle, but nerves are poorly fasciculated and disorganized. Neuromuscular junctions form, as reflected in clustering of AChR and postsynaptic expression of the genes encoding the alpha-AChR, AChE, epsilon-AChR, and the RI subunit of the cAMP protein kinase. However, a severe loss of motoneurons on cervical and lumbar, but not on thoracic levels occurs. Our results define the roles of Schwann cells during motoneuron and synapse development, and reveal different survival requirements for distinct motoneuron populations.

Infertile spermatozoa of c-ros tyrosine kinase receptor knockout mice show flagellar angulation and maturational defects in cell volume regulatory mechanisms.

Homozygous c-ros knockout male mice that lack prepubertal differentiation of the epididymal initial segment are healthy but sterile, despite normal sperm production and mating. Detailed computerized analysis of the motility of spermatozoa maturing in the epididymis revealed only minor defects. However, the majority of motile mature sperm released from the cauda epididymidis showed various extents of flagellar angulation that could not be corrected by raising extracellular osmolality. Measurement of the osmolality of cauda epididymal fluid showed no difference from the wild type. Studies in wild-type mice indicated a maturational change in the ability of motile sperm to maintain straight flagella during incubation, but angulation was induced in cauda sperm by the volume-sensitive ion channel blockers quinine, 5-nitro-2-(3-phenylpropylamino)-benzoic acid and BaCl(2), or by exposure to hypotonic media. Flagellar angulation, induced in the wild type or intrinsic to the knockout, was relieved upon demembranation by Triton X-100, confirming that it was a cell swelling phenomenon. A lack of response of immature wild-type sperm and mature knockout sperm to the channel blockers suggests that there is normally a development of the volume regulatory mechanisms upon maturation that is defective in sperm from the knockout animal. The resultant flagellar angulation may account for the reduction in sperm numbers in the oviduct of mated females and the failure to fertilize in vivo.

The role of SF/HGF and c-Met in the development of skeletal muscle.

Hypaxial skeletal muscles develop from migratory and non-migratory precursor cells that are generated by the lateral lip of the dermomyotome. Previous work shows that the formation of migratory precursors requires the c-Met and SF/HGF genes. We show here that in mice lacking c-Met or SF/HGF, the initial development of the dermomyotome proceeds appropriately and growth and survival of cells in the dermomyotome are not affected. Migratory precursors are also correctly specified, as monitored by the expression of Lbx1. However, these cells remain aggregated and fail to take up long range migration. We conclude that parallel but independent cues converge on the migratory hypaxial precursors in the dermomyotomal lip after they are laid down: a signal given by SF/HGF that controls the emigration of the precursors, and an as yet unidentified signal that controls Lbx1. SF/HGF and c-Met act in a paracrine manner to control emigration, and migratory cells only dissociate from somites located close to SF/HGF-expressing cells. During long range migration, prolonged receptor-ligand-interaction appears to be required, as SF/HGF is expressed both along the routes and at the target sites of migratory myogenic progenitors. Mice that lack c-Met die during the second part of gestation due to a placental defect. Rescue of the placental defect by aggregation of tetraploid (wild type) and diploid (c-Met-/-) morulae allows development of c-Met mutant animals to term. They lack muscle groups that derive from migratory precursor cells, but display otherwise normal skeletal musculature.

Receptor tyrosine kinase c-ros knockout mice as a model for the study of epididymal regulation of sperm function.

Despite recent advances in understanding sperm function and characterization of epididymal secretion products, little is known about the mechanisms regulating the fertilizing capacity of spermatozoa during maturation. The recently produced receptor tyrosine kinase c-ros knockout mouse has provided the first transgenic model for such study. The only abnormalities in these transgenic mice are shown in homozygous mutant males whose epididymis fails to develop the initial segment. Normal matings by these mice do not result in oocyte fertilization, but in vitro fertilization is successful. Detailed analysis of the development of sperm motility per se did not reveal any gross abnormalities that could explain infertility in vivo. Studies on c-ros, which is expressed temporarily in a few embryonic organs but solely and at high levels in the epididymis in adults, are few and nothing is known about its putative ligand or substrates. Review of the literature on other family members of receptor tyrosine kinases throws hardly any light on its role in epididymal function affecting sperm maturation. The preliminary observations that the majority of motile spermatozoa exhibit angulation in the tail and further findings suggest a defect in the volume regulation mechanism which would normally develop during sperm maturation. This finding has provided a starting point for further research to establish the link between abnormal epididymides and sterility.

Severe neuropathies in mice with targeted mutations in the ErbB3 receptor.

Neuregulins and their specific receptors, members of the ErbB family of tyrosine kinases, have been implicated in the control of growth and development of Schwann cells, specialized cells that wrap around nerve axons to provide electrical insulation. Here we use gene targeting to generate mice that lack ErbB3, a high-affinity neuregulin receptor. Homozygous erbB3 mutant embryos lack Schwann-cell precursors and Schwann cells that accompany peripheral axons of sensory and motor neurons. The initial development of motor neurons and sensory neurons of dorsal root ganglia occurs as it should, but at later stages most motor neurons (79%) and sensory neurons in dorsal root ganglia (82%) undergo cell death in erbB3 mutant embryos. Degeneration of the peripheral nervous system in erbB3 mutant pups is thus much more severe than the cell death in mice that lack neurotrophins or neurotrophin receptors. We also show that ErbB3 functions in a cell-autonomous way during the development of Schwann cells, but not in the survival of sensory or motor neurons. Our results indicate that sensory and motor neurons require factors for their survival that are provided by developing Schwann cells.

The c-ros tyrosine kinase receptor controls regionalization and differentiation of epithelial cells in the epididymis.

The c-ros gene was originally identified in mutant form as an oncogene. The proto-oncogene encodes a tyrosine kinase receptor that is expressed in a small number of epithelial cell types, including those of the epididymis. Targeted mutations of c-ros in the mouse reveal an essential role of the gene in male fertility. Male c-ros -/- animals do not reproduce, whereas the fertility of female animals is not affected. We demonstrate that c-ros is not required in a cell autonomous manner for male germ cell development or function. The gene, therefore, does not affect sperm generation or function in a direct manner. The primary defect in the mutant animals was located in the epididymis, showing that c-ros controls appropriate development of the epithelia, particularly regionalization and terminal differentiation. The epididymal defect does not interfere with production or storage of sperm but, rather, with sperm maturation and the ability of sperm to fertilize in vivo. Interestingly, sperm isolated from c-ros -/- animals can fertilize in vitro. Our results highlight the essential role of the epididymis in male fertility and demonstrate a highly specific function of the c-ros receptor tyrosine kinase during development of distinct epithelial cells.