No abstract

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.

Stem cells support tissue maintenance by balancing self-renewal and differentiation. In mice, it is believed that a homogeneous stem cell population of single spermatogonia supports spermatogenesis, and that differentiation, which is accompanied by the formation of connected cells (cysts) of increasing length, is linear and nonreversible. We evaluated this model using lineage-analysis and live-imaging and found that this putative stem cell population is not homogeneous. Instead, the stem cell pool that supports steady-state spermatogenesis is contained within a subpopulation of single spermatogonia. Also, cysts are not committed to differentiation and appear to recover stem cell potential by fragmentation. Lastly, the fate of individual spermatogonial populations was dramatically altered during regeneration following damage. Thus, there are multiple and reversible paths from stem cell to differentiation, which may also occur in other systems.Maintenance of adult tissues is supported by a small number of undifferentiated stem cells that self-renew to maintain their population and produce differentiating progeny for normal tissue function. It has generally been accepted that differentiating daughter cells progress unidirectionally towards terminal differentiation. This view has been recently challenged by data suggesting that under some circumstances differentiating cells can revert to the self-renewing stem cell pool (1-8). This apparent plasticity may add robustness to maintenance of the stem cell population during normal tissue maintenance and may play a crucial role in tissue regeneration following injury. However, the nature of the self-renewing stem cells and the plasticity of differentiating cells in the maintenance of tissue homeostasis and regeneration are mostly unknown, particularly in mammals.Germ cells share a characteristic feature across all animal species. While the most primitive cells in adult gonads are singly isolated, their differentiating progeny remain connected by intercellular bridges to form syncytial cysts of 2 n cells (9,10). Thus, the length of the cysts reflects their cell division history or lineage. This unique feature has made the germline one of the most tractable systems to study adult stem cell self-renewal and differentiation (2,3).* To whom correspondence should be addressed. shosei@nibb.ac.jp. The study of the spermatogenic stem cell compartment in mammals also relies on the heterogeneity in the cyst length (9,11,12 (Fig. S1).The prevailing rodent stem cell model (14,15) (Fig. 1A) assumes that the stem cell population resides in the A s population and that cyst length reflects the extent of differentiation in a linear manner (9,11). A corollary of this 'A s model' is that A s spermatogonia are functionally homogeneous, that all A s cells are stem cells, and that all cells are equivalent in each morphological category (9,10). This model, proposed in 1971, has provided the framework for years of germline stem cell research in mice and other animals. Despite its simplici...

The klotho gene encodes a novel type I membrane protein of beta-glycosidase family and is expressed principally in distal tubule cells of the kidney and choroid plexus in the brain. These mutants displayed abnormal calcium and phosphorus homeostasis together with increased serum 1,25-(OH)2D. In kl-/- mice at the age of 3 wk, elevated levels of serum calcium (10.9 +/- 0.31 mg/dl vs. 10.0 +/- 0.048 mg/dl in wild-type mice), phosphorus (14.7 +/- 1.1 mg/dl vs. 9.7 +/- 1.5 mg/dl in wild type) and most notably, 1,25-(OH)2D (403 +/- 99.7 mg/dl vs. 88.0 +/- 34.0 mg/dl in wild type) were observed. Reduction of serum 1,25-(OH)2D concentrations by dietary restriction resulted in alleviation of most of the phenotypes, suggesting that they are downstream events resulting from elevated 1,25-(OH)2D. We searched for the signals that lead to up-regulation of vitamin D activating enzymes. We examined the response of 1alpha-hydroxylase gene expression to calcium regulating hormones, such as PTH, calcitonin, and 1,25-(OH)2D3. These pathways were intact in klotho null mutant mice, suggesting the existence of alternate regulatory circuits. We also found that the administration of 1,25-(OH)2D3 induced the expression of klotho in the kidney. These observations suggest that klotho may participate in a negative regulatory circuit of the vitamin D endocrine system, through the regulation of 1alpha-hydroxylase gene expression.