No abstract

Neuromuscular junction (NMJ) formation requires the highly coordinated communication of several reciprocal signaling processes between motoneurons and their muscle targets. Identification of the early, spatially restricted cues in target recognition at the NMJ is still poorly documented, especially in mammals. Wnt signaling is one of the key pathways regulating synaptic connectivity. Here, we report that Wnt4 contributes to the formation of vertebrate NMJ in vivo. Results from a microarray screen and quantitative RT-PCR demonstrate that Wnt4 expression is regulated during muscle cell differentiation in vitro and muscle development in vivo, being highly expressed when the first synaptic contacts are formed and subsequently downregulated. Analysis of the mouse Wnt4−/− NMJ phenotype reveals profound innervation defects including motor axons overgrowing and bypassing AChR aggregates with 30% of AChR clusters being unapposed by nerve terminals. In addition, loss of Wnt4 function results in a 35% decrease of the number of prepatterned AChR clusters while Wnt4 overexpression in cultured myotubes increases the number of AChR clusters demonstrating that Wnt4 directly affects postsynaptic differentiation. In contrast, muscle structure and the localization of several synaptic proteins including acetylcholinesterase, MuSK and rapsyn are not perturbed in the Wnt4 mutant. Finally, we identify MuSK as a Wnt4 receptor. Wnt4 not only interacts with MuSK ectodomain but also mediates MuSK activation. Taken together our data reveal a new role for Wnt4 in mammalian NMJ formation that could be mediated by MuSK, a key receptor in synaptogenesis.

1. Male and female mice, 4 weeks old, were fed ad lib. diets containing various amounts of lard (0-300 g/kg) or various kinds of dietary fats (300 g/kg) for 13 weeks. Fat cell number and size were determined by a histological method in three different adipose sites.2. Lard at 200 g/kg diet (43% energy from lipids) was sufficient to promote fat cell hyperplasia in the parametrial fat. Hyperplasia was also observed in the subcutaneous fat in males. The relationship between fat cell hypertrophy and the level of lard in the diet was dependent on site and sex.3. Obesity was produced whatever the kind of dietary fat eaten: lard, beef tallow, sunflower oil or soya-bean oil. In the subcutaneous depot of males given lard, fat cell size and number were increased, but only cell hypertrophy was observed in those given soya-bean oil. in the female groups of mice fat cell hyperplasia or hypertrophy or both were related to the adipose site but not the kind of dietary fat.4. It is concluded that dietary fats of different origin can induce obesity in mice. The effects on adipose tissue cellularity depend on the levels and kind of fat eaten, the adipose site and sex.