Abstract
Vertebrate neuromuscular junctions (NMJs) have been conceived as tripartite synapses composed of motor neuron, Schwann cell, and muscle fiber. Recent work has shown the presence of sympathetic neurons in the immediate vicinity of NMJs and experimental and clinical findings suggest that this plays an eminent role in adult NMJ biology. The present study examined the postnatal development and distribution of sympathetic innervation in different muscles using immunofluorescence, confocal microscopy, and Western blot. This demonstrates the proximity of sympathetic neurons in diaphragm, extensor digitorum longus, tibialis anterior, soleus, and levator auris longus muscles. In extensor digitorum longus muscle, sympathetic innervation of NMJs was quantified from perinatal to adult stage and found to increase up to two months of age. In diaphragm muscle, an extensive network of sympathetic neurons was prominent along the characteristic central synapse band. In summary, these data demonstrate that an elaborate sympathetic innervation is present in several mouse skeletal muscles and that this is often next to NMJs. Although the presence of sympathetic neurons at the perisynaptic region of NMJs increased during postnatal development, many synapses were already close to sympathetic neurons at birth. Potential implications of these findings for treatment of neuromuscular diseases are discussed.
Highlights
Congenital myasthenic syndromes (CMS) are neuromuscular transmission disorders that are due to mutations in one of several components needed for the function or maintenance of the synaptic apparatus of neuromuscular junctions (NMJs), such as choline acetyl transferase, agrin, or docking protein 7 (DOK-7) [1]
Most NMJs were in immediate proximity to tyrosine hydroxylase (TH)-positive sympathetic neurons in soleus and extensor digitorum longus muscles
In other regions, where muscle fibers were rather seen in cross-section, TH-positive signals appeared as dots. This suggests, that TH-positive axons generally align for some distance along the muscle fibers. This was confirmed by further analyses, where slices were stained with an antibody against α-actinin to highlight the muscle fiber sarcomeres (Figure 1B,C)
Summary
Congenital myasthenic syndromes (CMS) are neuromuscular transmission disorders that are due to mutations in one of several components needed for the function or maintenance of the synaptic apparatus of neuromuscular junctions (NMJs), such as choline acetyl transferase, agrin, or docking protein 7 (DOK-7) [1]. The latter studies, though, interpreted their findings such that the cholinergic motor neuron itself might have TH activity rather than suggesting the presence of another, sympathetic, neuron at the perisynaptic region Such a conclusion was reasonable in the framework of the long-held concept of a tripartite structure of NMJs and given that the observations were made on muscle cross sections, which do not allow the visualization of axons approaching the synapse. This was different in the most recent study, which used optical tissue clearing in combination with reporter mice expressing a fluorescent protein under dopamine beta-hydroxylase promoter control and which showed axons different to those of lower motor neurons approaching the NMJs [4]. This was addressed in the present work and, in addition, the list of muscles showing sympathetic innervation at NMJs was further increased
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
