Claire Legay

Genesis of muscle fiber-type diversity during mouse embryogenesis relies on Six1 and Six4 gene expression

Adult skeletal muscles in vertebrates are composed of different types of myofibers endowed with distinct metabolic and contraction speed properties. Genesis of this fiber-type heterogeneity during development remains poorly known, at least in mammals. Six1 and Six4 homeoproteins of the Six/sine oculis family are expressed throughout muscle development in mice, and Six1 protein is enriched in the nuclei of adult fast-twitch myofibers. Furthermore, Six1/Six4 proteins are known to control the early activation of fast-type muscle genes in myocytes present in the mouse somitic myotome. Using double Six1:Six4 mutants (SixdKO) to dissect in vivo the genesis of muscle fiber-type heterogeneity, we analyzed here the phenotype of the dorsal/epaxial muscles remaining in SixdKO. We show by electron microscopy analysis that the absence of these homeoproteins precludes normal sarcomeric organization of the myofiber leading to a dystrophic aspect, and by immunohistochemistry experiments a deficiency in synaptogenesis. Affymetrix transcriptome analysis of the muscles remaining in E18.5 SixdKO identifies a major role for these homeoproteins in the control of genes that are specifically activated in the adult fast/glycolytic myofibers, particularly those controlling Ca(2+) homeostasis. Absence of Six1 and Six4 leads to the development of dorsal myofibers lacking expression of fast-type muscle genes, and mainly expressing a slow-type muscle program. The absence of restriction of the slow-type program during the fetal period in SixdKO back muscles is associated with a decreased HDAC4 protein level, and subcellular relocalization of the transcription repressor Sox6. Six genes thus behave as essential global regulators of muscle gene expression, as well as a central switch to drive the skeletal muscle fast phenotype during fetal development.

Wnt4 Participates in the Formation of Vertebrate Neuromuscular Junction

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.

ColQ Controls Postsynaptic Differentiation at the Neuromuscular Junction

CollagenQ (ColQ) plays an important structural role at vertebrate neuromuscular junctions (NMJs) by anchoring and accumulating acetylcholinesterase (AChE) in the extracellular matrix (ECM). Moreover, ColQ interacts with perlecan/dystroglycan and the muscle-specific receptor tyrosine kinase (MuSK), key molecules in the NMJ formation. MuSK promotes acetylcholine receptor (AChR) clustering in a process mediated by rapsyn, a cytoplasmic protein that stimulates AChR packing in clusters and regulates synaptic gene transcription. Here, we investigated a regulatory role for ColQ by comparing the clustering and expression of synaptic proteins in wild type and ColQ-deficient muscle cells in culture and at NMJ. We show first that AChR clusters are smaller and more densely packed in the absence of ColQ both in vitro and in vivo. Second, we find that like AChRs and rapsyn, MuSK mRNA levels are increased in cultured cells but not in muscles lacking ColQ. However, membrane-bound MuSK is decreased both in vitro and in vivo suggesting that ColQ controls MuSK sorting or stabilization in the muscle membrane. In line with this, our data show that activation of the MuSK signaling pathway is altered in the absence of ColQ leading to (1) perturbation of AChR clustering and/or β-AChR subunit phosphorylation and (2) modifications of AChR mRNA level due to the lack of ColQ-MuSK interaction. Together, our results demonstrate that ColQ, in addition to its structural role, has important regulatory functions at the synapse by controlling AChR clustering and synaptic gene expression through its interaction with MuSK.

MuSK frizzled-like domain is critical for mammalian neuromuscular junction formation and maintenance.

The muscle-specific kinase MuSK is one of the key molecules orchestrating neuromuscular junction (NMJ) formation. MuSK interacts with the Wnt morphogens, through its Frizzled-like domain (cysteine-rich domain [CRD]). Dysfunction of MuSK CRD in patients has been recently associated with the onset of myasthenia, common neuromuscular disorders mainly characterized by fatigable muscle weakness. However, the physiological role of Wnt-MuSK interaction in NMJ formation and function remains to be elucidated. Here, we demonstrate that the CRD deletion of MuSK in mice caused profound defects of both muscle prepatterning, the first step of NMJ formation, and synapse differentiation associated with a drastic deficit in AChR clusters and excessive growth of motor axons that bypass AChR clusters. Moreover, adult MuSKΔCRD mice developed signs of congenital myasthenia, including severe NMJs dismantlement, muscle weakness, and fatigability. We also report, for the first time, the beneficial effects of lithium chloride, a reversible inhibitor of the glycogen synthase kinase-3, that rescued NMJ defects in MuSKΔCRD mice and therefore constitutes a novel therapeutic reagent for the treatment of neuromuscular disorders linked to Wnt-MuSK signaling pathway deficiency. Together, our data reveal that MuSK CRD is critical for NMJ formation and plays an unsuspected role in NMJ maintenance in adulthood.

5-HT metabolism in the intestinal wall of the rat—I. The mucosa

Serotonin (5-HT) metabolism was studied on isolated mucosa of the rat caecum in order to determine its characteristics in enterochromaffin cells. High levels of 5-HT were found in these cells and weak catabolic processes were demonstrated. Enterochromaffin cells of the mucosa are able to synthesize ((3)H)5-HT from ((3)H)tryptophan in vitro indicating that these cells might contain tryptophan hydroxylase. In addition, a high affinity uptake of exogenous ((3)H)5-HT is demonstrated. These results show that enterochromaffin cells of the mucosa present similar biochemical properties compared to those described for serotoninergic neurons of the central nervous system.

Wnt proteins contribute to neuromuscular junction formation through distinct signaling pathways

Understanding the developmental steps that shape formation of the neuromuscular junction (NMJ) connecting motoneurons to skeletal muscle fibers is crucial. Wnt morphogens are key players in the formation of this specialized peripheral synapse, but their individual and collaborative functions and downstream pathways remain poorly understood at the NMJ. Here, we demonstrate through Wnt4 and Wnt11 gain-of-function studies in cell culture or in mice that Wnts enhance acetylcholine receptor (AChR) clustering and motor axon outgrowth. By contrast, loss of Wnt11 or Wnt-dependent signaling in vivo decreases AChR clustering and motor nerve terminal branching. Both Wnt4 and Wnt11 stimulate AChR mRNA levels and AChR clustering downstream of activation of the β-catenin pathway. Strikingly, Wnt4 and Wnt11 co-immunoprecipitate with Vangl2, a core component of the planar cell polarity (PCP) pathway, which accumulates at embryonic NMJs. Moreover, mice bearing a Vangl2 loss-of-function mutation (loop-tail) exhibit fewer AChR clusters and overgrowth of motor axons bypassing AChR clusters. Together, our results provide genetic and biochemical evidence that Wnt4 and Wnt11 cooperatively contribute to mammalian NMJ formation through activation of both the canonical and Vangl2-dependent core PCP pathways. Summary: Wnt4 and Wnt11 cooperatively contribute to NMJ formation in mice through activation of both the canonical and Vangl2-dependent core planar cell polarity pathways.

5-HT metabolism in the intestinal wall of the rat—II. The nerves plexuses—interactions between 5-HT containing cells

The presence of serotonin (5-HT) in dissected intestinal muscular wall of the caecum was demonstrated by the determination of endogenous level of the amine by both spectrofluorimetric and radioenzymatic assays. Biosynthesis of [(3)H]5-HT from [(3)H]tryptophan in in vitro conditions revealed the presence of tryptophan hydroxylase in these muscular layers and therefore strongly suggest the presence of serotoninergic neurons. Following dissection of the mucosa from the muscular layers containing the nerve plexuses, endogenous 5-HT and 5-hydroxyindol acetic acid levels as well as amounts of [(3)H]5-HT synthesized from [(3)H]tryptophan were always higher than those found in intact fragments of the caecum. These results are discussed in terms of metabolic processes involved in the regulation between the two 5-HT containing compartments.

Amines and choline acetyltransferase in rat intestine

A biochemical study of the endogenous levels of serotonin (5-HT), noradrenaline (NA) and the activity of choline acetyltransferase (CAT) was carried out in the intestinal tract of the rat. High levels of 5-HT and NA were detected in the caecum and the colon. These anatomical regions also presented the highest activity of CAT. Similar activities of CAT were detected, after dissection, in the mucosa and the muscular layers containing the enteric plexuses. During the day-night cycle, 5-HT and NA amounts showed significant variations as a function of time. Treatment with pargyline (75 mg kg(?1)), a monoamine oxidase inhibitor, resulted in an increase in 5-HT content with parallel modifications in CAT activity. In spite of an important decrease in 5-HT endogenous level in the caecum of rats pretreated with parachlorophenylalanine (300 mg kg(?1)), no significant change in CAT activity was detected whatever was the duration of the treatment. ?-Methylparatyrosine (100 mg kg(?1)), known to block the synthesis of NA, did not affect the CAT activity in the caecum.

Cholinesterases regulation in the absence of ColQ.

Normal physiological activity of the neuromuscular junction (NMJ) requires that key molecules are clustered at the synapse. One of these molecules is acetylcholinesterase (AChE) that regulates acetylcholine levels. This enzyme exists under different isoforms but the predominant form at the NMJ is a collagen-tailed enzyme. The collagen associated to AChE (ColQ) fulfills two functions. It anchors and accumulates AChE in the extracellular matrix. Mutations in ColQ lead to faint or no activity of AChE in the synaptic cleft. As a consequence, normal NMJ functioning is impaired and myasthenic syndromes are observed in patients bearing these mutations. Here, we investigated the effects of ColQ deficiency on cholinesterases mRNA levels and cluster formation. We show that overexpression of AChE but not ColQ in muscle cells is sufficient to drive the formation of AChE clusters. The absence of ColQ in muscle cells in vitro and in vivo leads to an increase in AChE(R) and AChE(T) mRNAs, corresponding to two isoforms of AChE. However, AChE activity is decreased in the medium of ColQ-deficient cells suggesting that AChE secretion is impaired. Butyrylcholinesterase (BChE) mRNAs are also upregulated in vivo. Since AChE and BChE can associate with PRiMA, a membrane anchor, we explored the pattern of expression of PRiMA in vitro and in vivo. The level of PRiMA transcripts is downregulated in the absence of ColQ. Therefore, AChE, BChE and PRiMA mRNA level modifications found in the absence of ColQ cannot compensate for the physiological defects observed at the ColQ-deficient NMJs.

Moving forward with the neuromuscular junction

The neuromuscular junction (NMJ) is indispensable for survival. This synapse between motoneurons and skeletal muscle fibers allows posture, movement and respiration. Therefore, its dysfunction creates pathologies than can be lethal. The molecular mechanisms of NMJ development and maintenance are the subject of intensive studies. This mini‐review focuses on some of the most recent discoveries. An unexpected role for a protein, rapsyn, which has been known for 40 years to aggregate acetylcholine receptors has emerged. A new cell partner at NMJ has been unmasked and is challenging our understanding of the functioning of this synapse. Toxins are now used as new tools to study degeneration/regeneration. The possibility of creating human NMJ in vitro is within reach with major consequences for drug screening. Wnts are secreted neurogenic factors that have been involved in vitro in acetylcholine receptor clustering, but their precise role in vivo remains to be clarified. All these data are raising new and exciting perspectives in the field and are discussed in this Review.