Cholinesterases regulation in the absence of ColQ

Congenital myasthenic syndromes with acetylcholinesterase deficiency, the pathophysiological mechanisms

The neuromuscular junction (NMJ) is a cholinergic synapse in vertebrates. This synapse connects motoneurons to muscles and is responsible for muscle contraction, a physiological process that is essential for survival. A key factor for the normal functioning of this synapse is the regulation of acetylcholine (ACh) levels in the synaptic cleft. This is ensured by acetylcholinesterase (AChE), which degrades ACh. A number of mutations in synaptic genes expressed in motoneurons or muscle cells have been identified and are causative for a class of neuromuscular diseases called congenital myasthenic syndromes (CMSs). One of these CMSs is due to deficiency in AChE, which is absent or diffuse in the synaptic cleft. Here, I focus on the origins of the syndrome. The role of ColQ, a collagen that anchors AChE in the synaptic cleft, is discussed in this context. Studies performed on patient biopsies, transgenic mice, and muscle cultures have provided a more comprehensive view of the connectome at the NMJ that should be useful for understanding the differences in the symptoms observed in specific CMSs due to mutated proteins in the synaptic cleft.

Clinical and research strategies for limb-girdle congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a group of rare disorders that cause fatigable muscle weakness due to defective signal transmission at the neuromuscular junction, a specialized synapse between peripheral motor neurons and their target muscle fibers. There are now over 30 causative genes that have been reported for CMS. Of these, there are 10 that are associated with a limb-girdle pattern of muscle weakness and are thus classed as LG-CMS. Next-generation sequencing and advanced methods of data sharing are likely to uncover further genes that are associated with similar clinical phenotypes, contributing to better diagnosis and effective treatment of LG-CMS patients. This review highlights clinical and pathological hallmarks of LG-CMS in relation to the underlying genetic defects and pathways. Tailored animal and cell models are essential to elucidate the exact function and pathomechanisms at the neuromuscular synapse that underlie LG-CMS. The integration of genomics and proteomics data derived from these models and patients reveals new and often unexpected insights that are relevant beyond the rare genetic disorder of LG-CMS and may extend to the functioning of mammalian synapses in health and disease more generally.

Neuromuscular junction immaturity and muscle atrophy are hallmarks of the ColQ-deficient mouse, a model of congenital myasthenic syndrome with acetylcholinesterase deficiency

The collagen ColQ anchors acetylcholinesterase (AChE) in the synaptic cleft of the neuromuscular junction (NMJ). It also binds MuSK and perlecan/dystroglycan, 2 signaling platforms of the postsynaptic domain. Mutations in ColQ cause a congenital myasthenic syndrome (CMS) with AChE deficiency. Because the absence of AChE does not fully explain the complexity of the syndrome and there is no curative treatment for the disease, we explored additional potential targets of ColQ by conducting a large genetic screening of ColQ-deficient mice, a model for CMS with AChE deficiency, and analyzed their NMJ and muscle phenotypes. We demonstrated that ColQ controls the development and the maturation of the postsynaptic domain by regulating synaptic gene expression. Notably, ColQ deficiency leads to an up-regulation of the 5 subunits of the nicotinic acetylcholine receptor (AChR), leading to mixed mature and immature AChRs at the NMJ of adult mice. ColQ also regulates the expression of extracellular matrix (ECM) components. However, whereas the ECM mRNAs were down-regulated in vitro, compensation seemed to occur in vivo to maintain normal levels of these mRNAs. Finally, ColQ deficiency leads to a general atrophic phenotype and hypoplasia that affect fast muscles. This study points to new specific hallmarks for this CMS.-Sigoillot, S. M., Bourgeois, F., Karmouch, J., Molgó, J., Dobbertin, A., Chevalier, C., Houlgatte, R., Léger, J., Legay, C. Neuromuscular junction immaturity and muscle atrophy are hallmarks of the ColQ-deficient mouse, a model of congenital myasthenic syndrome with acetylcholinesterase deficiency.

Developmental consequences of the ColQ/MuSK interactions

CollagenQ (ColQ) is a specific collagen that anchors acetylcholinesterase (AChE) in the synaptic basal lamina of the neuromuscular junction (NMJ). Over 30 mutations in the COLQ gene have been identified that are responsible for a congenital myasthenic syndrome with AChE deficiency, highlighting the importance of this collagen in the physiology of the NMJ. The anchoring of AChE at the synapse requires the interaction of ColQ with MuSK (Muscle-Specific Kinase), a tyrosine kinase expressed on the muscle membrane that is necessary for the formation and the maintenance of the NMJ. MuSK forms with its co-receptor LRP4, a member of the Low-density Related Protein family, a receptor complex for agrin and Wnts, representing the core system from which the postsynaptic domain is built, the growth cone attracted and the presynaptic element instructed for some aspects of its differentiation. Therefore, the discovery that ColQ binds to MuSK prompted us to study a possible regulatory function of ColQ during NMJ development. In this review, after a brief survey on ColQ, we summarize our recent data demonstrating that ColQ, in addition to its anchoring role, exerts signaling functions and controls some aspects of postsynaptic differentiation such as the clustering of acetylcholine receptors. Our results also strengthen the hypothesis that the defects observed in synaptic congenital myasthenic syndromes might be linked, at least in part, to alterations of ColQ signaling functions and not only to AChE deficiency. Finally, we discuss future research directions to understand how ColQ may modulate the action of the other ligands of the MuSK/LRP4 complex and cooperate with them to coordinate the different steps of NMJ formation and maintenance.

Extracellular matrix: functions in the nervous system

An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.