Collagen XIII Is Required for Neuromuscular Synapse Regeneration and Functional Recovery after Peripheral Nerve Injury.

Collagen XIII occurs as both a transmembrane-bound and a shed extracellular protein and is able to regulate the formation and function of neuromuscular synapses. Its absence results in myasthenia: presynaptic and postsynaptic defects at the neuromuscular junction (NMJ), leading to destabilization of the motor nerves, muscle regeneration and atrophy. Mutations in COL13A1 have recently been found to cause congenital myasthenic syndrome, characterized by fatigue and chronic muscle weakness, which may be lethal. We show here that muscle defects in collagen XIII-deficient mice stabilize in adulthood, so that the disease is not progressive until very late. Sciatic nerve crush was performed to examine how the lack of collagen XIII or forced expression of its transmembrane form affects the neuromuscular synapse regeneration and functional recovery following injury. We show that collagen XIII-deficient male mice are unable to achieve complete NMJ regeneration and functional recovery. This is mainly attributable to presynaptic defects that already existed in the absence of collagen XIII before injury. Shedding of the ectodomain is not required, as the transmembrane form of collagen XIII alone fully rescues the phenotype. Thus, collagen XIII could serve as a therapeutic agent in cases of injury-induced PNS regeneration and functional recovery. We conclude that intrinsic alterations at the NMJ in Col13a1-/- mice contribute to impaired and incomplete NMJ regeneration and functional recovery after peripheral nerve injury. However, such alterations do not progress once they have stabilized in early adulthood, emphasizing the role of collagen XIII in NMJ maturation.SIGNIFICANCE STATEMENT Collagen XIII is required for gaining and maintaining the normal size, complexity, and functional capacity of neuromuscular synapses. Loss-of-function mutations in COL13A1 cause congenital myasthenic syndrome 19, characterized by postnatally progressive muscle fatigue, which compromises patients' functional capacity. We show here in collagen XIII-deficient mice that the disease stabilizes in adulthood once the NMJs have matured. This study also describes a relevant contribution of the altered NMJ morphology and function to neuromuscular synapses, and PNS regeneration and functional recovery in collagen XIII-deficient mice after peripheral nerve injury. Correlating the animal model data on collagen XIII-associated congenital myasthenic syndrome, it can be speculated that neuromuscular connections in congenital myasthenic syndrome patients are not able to fully regenerate and restore normal functionality if exposed to peripheral nerve injury.

Collagen XIII secures pre- and postsynaptic integrity of the neuromuscular synapse.

Both transmembrane and extracellular cues, one of which is collagen XIII, regulate the formation and function of the neuromuscular synapse, and their absence results in myasthenia. We show that the phenotypical changes in collagen XIII knock-out mice are milder than symptoms in human patients, but the Col13a1-/- mice recapitulate major muscle findings of congenital myasthenic syndrome type 19 and serve as a disease model. In the lack of collagen XIII neuromuscular synapses do not reach full size, alignment, complexity and function resulting in reduced muscle strength. Collagen XIII is particularly important for the preterminal integrity, and when absent, destabilization of the motor nerves results in muscle regeneration and in atrophy especially in the case of slow muscle fibers. Collagen XIII was found to affect synaptic integrity through binding the ColQ tail of acetylcholine esterase. Although collagen XIII is a muscle-bound transmembrane molecule, it also undergoes ectodomain shedding to become a synaptic basal lamina component. We investigated the two forms' roles by novel Col13a1tm/tm mice in which ectodomain shedding is impaired. While postsynaptic maturation, terminal branching and neurotransmission was exaggerated in the Col13a1tm/tm mice, the transmembrane form's presence sufficed to prevent defects in transsynaptic adhesion, Schwann cell invagination/retraction, vesicle accumulation and acetylcholine receptor clustering and acetylcholinesterase dispersion seen in the Col13a1-/- mice, pointing to the transmembrane form as the major conductor of collagen XIII effects. Altogether, collagen XIII secures postsynaptic, synaptic and presynaptic integrity, and it is required for gaining and maintaining normal size, complexity and functional capacity of the neuromuscular synapse.

Correct expression and localization of collagen XIII are crucial for the normal formation and function of the neuromuscular system.

Transmembrane collagen XIII has been linked to maturation of the musculoskeletal system. Its absence in mice (Col13a1-/- ) results in impaired neuromuscular junction (NMJ) differentiation and function, while transgenic overexpression (Col13a1oe ) leads to abnormally high bone mass. Similarly, loss-of-function mutations in COL13A1 in humans produce muscle weakness, decreased motor synapse function and mild dysmorphic skeletal features. Here, analysis of the exogenous overexpression of collagen XIII in various muscles revealed highly increased transcript and protein levels, especially in the diaphragm. Unexpectedly, the main location of exogenous collagen XIII in the muscle was extrasynaptic, in fibroblast-like cells, while some motor synapses were devoid of collagen XIII, possibly due to a dominant negative effect. Concomitantly, phenotypical changes in the NMJs of the Col13a1oe mice partly resembled those previously observed in Col13a1-/- mice. Namely, the overall increase in collagen XIII expression in the muscle produced both pre- and postsynaptic abnormalities at the NMJ, especially in the diaphragm. We discovered delayed and compromised acetylcholine receptor (AChR) clustering, axonal neurofilament aggregation, patchy acetylcholine vesicle (AChV) accumulation, disrupted adhesion of the nerve and muscle, Schwann cell invagination and altered evoked synaptic function. Furthermore, the patterns of the nerve trunks and AChR clusters in the diaphragm were broader in the adult muscles, and already prenatally in the Col13a1oe mice, suggesting collagen XIII involvement in the development of the neuromuscular system. Overall, these results confirm the role of collagen XIII at the neuromuscular synapses and highlight the importance of its correct expression and localization for motor synapse formation and function.

Collagen XIII and Other ECM Components in the Assembly and Disease of the Neuromuscular Junction.

Alongside playing structural roles, the extracellular matrix (ECM) acts as an interaction platform for cellular homeostasis, organ development, and maintenance. The necessity of the ECM is highlighted by the diverse, sometimes very serious diseases that stem from defects in its components. The neuromuscular junction (NMJ) is a large peripheral motor synapse differing from its central counterparts through the ECM included at the synaptic cleft. Such synaptic basal lamina (BL) is specialized to support NMJ establishment, differentiation, maturation, stabilization, and function and diverges in molecular composition from the extrasynaptic ECM. Mutations, toxins, and autoantibodies may compromise NMJ integrity and function, thereby leading to congenital myasthenic syndromes (CMSs), poisoning, and autoimmune diseases, respectively, and all these conditions may involve synaptic ECM molecules. With neurotransmission degraded or blocked, muscle function is impaired or even prevented. At worst, this can be fatal. The article reviews the synaptic BL composition required for assembly and function of the NMJ molecular machinery through the lens of studies primarily with mouse models but also with human patients. In-depth focus is given to collagen XIII, a postsynaptic-membrane-spanning but also shed ECM protein that in recent years has been revealed to be a significant component for the NMJ. Its deficiency in humans causes CMS, and autoantibodies against it have been recognized in autoimmune myasthenia gravis. Mouse models have exposed numerous details that appear to recapitulate human NMJ phenotypes relatively faithfully and thereby can be readily used to generate information necessary for understanding and ultimately treating human diseases. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc.

Collagen XIII-derived ectodomain regulates bone angiogenesis and intracortical remodeling.

Osteoporosis is the most common degenerative bone disease that occurs when the balance of bone production and resorption is perturbed. Loss of bone mass or alteration in its quality leads to significant weakening of the bones and subsequently to higher fracture risk. Collagen XIII (ColXIII) is a conserved transmembrane protein expressed in many mesenchymal tissues. Here we show that ColXIII is a regulator of bone remodeling niche. In this study, we found that ColXIII expression is significantly upregulated in osteoporotic patients. In view of that, we studied bone homeostasis in ColXIII-overexpressing mice (Col13a1oe) up to 72 weeks of age and observed a cortical bone overgrowth followed by a drastic bone loss, together with increased bone vascularization. Moreover, our results demonstrate that the ColXIII-derived ectodomain enhances angiogenesis through ß1-integrins and the JNK pathway. Consequently, these data suggest that ColXIII has a role in age-dependent cortical bone deterioration with possible implications for osteoporosis and fracture risk.