Development of the vertebrate neuromuscular junction

NMJ maintenance and repair in aging

As the final output of the somatic nervous system, the neuromuscular junction (NMJ) is essential for all voluntary movements. The NMJ is also necessary for connected cells to function and survive. Because of this central role, much effort has been devoted to understanding the effects of aging, diseases, and injuries on the NMJ. These efforts have revealed a close relationship between aberrant changes at NMJs and its three cellular components - the presynaptic site on motor axons, the postsynaptic region on muscle fibers and perisynaptic Schwann cells. Here, we review the morphological and molecular changes associated with aging NMJs in rodents and humans. We also provide an overview of factors with potential roles in maintaining and repairing adult and aged NMJs.

The neuromuscular junction of Xenopus tadpoles: Revisiting a classical model of early synaptogenesis and regeneration

The frog neuromuscular junction (NMJ) has been extensively used as a model system to dissect the mechanisms involved in synapse formation, maturation, maintenance, regeneration, and function. Early NMJ synaptogenesis relies on a combination of cell-autonomous and interdependent pre/postsynaptic communication processes. Due to their transparency, comparatively easy manipulation, and remarkable regenerative abilities, frog tadpoles constitute an excellent model to study NMJ formation and regeneration. Here, we aimed to contribute new aspects on the characterization of the ontogeny of NMJ formation in Xenopus embryos and to explore the morphological changes occurring at the NMJ after spinal cord injury. Following analyses of X. tropicalis tadpoles during development we found that the early pathfinding of rostral motor axons is likely helped by previously formed postsynaptic specializations, whereas NMJ formation in recently differentiated ventral muscles in caudal segments seems to rely on presynaptic inputs. After spinal cord injury of X. laevis tadpoles our results suggest that rostral motor axon projections help caudal NMJ re-innervation before spinal cord connectivity is repaired.

Sema7A/PlxnCl signaling triggers activity-dependent olfactory synapse formation

In mammals, neural circuits are formed based on a genetic program and further refined by neuronal activity during the neonatal period. We report that in the mouse olfactory system, the glomerular map is not merely refined but newly connected to second-order neurons by odorant-receptor-derived neuronal activity. Here, we analyzed a pair of molecules, Sema7A, expressed in olfactory sensory neurons (OSNs) in an activity-dependent manner, and PlxnC1, localized to dendrites of mitral/tufted (M/T) cells in the first week after birth. In Sema7A or PlxnC1 knockout (KO) mice, initiation of synapse formation and dendrite selection of M/T cells were perturbed. Reconstitution and rescue experiments demonstrated that Sema7A-PlxnC1 interaction is essential to form the post-synaptic assembly. Pharmacological blocking experiments indicated that synaptic transmission triggers primary dendrite selection by synaptic competition. We conclude that Sema7A signaling is key to inducing activity-dependent post-synapse events and dendrite selection in M/T-cells during the neonatal period.

Deletion of Nampt in Projection Neurons of Adult Mice Leads to Motor Dysfunction, Neurodegeneration, and Death

Intracellular nicotinamide phosphoribosyltransferase (iNAMPT) is the rate-limiting enzyme of the mammalian NAD⁺ biosynthesis salvage pathway. Using inducible and conditional knockout (cKO) mice, we show that Nampt gene deletion in adult projection neurons leads to a progressive loss of body weight, hypothermia, motor neuron (MN) degeneration, motor function deficits, paralysis, and death. Nampt deletion causes mitochondrial dysfunction, muscle fiber type conversion, and atrophy, as well as defective synaptic function at neuromuscular junctions (NMJs). When treated with nicotinamide mononucleotide (NMN), Nampt cKO mice exhibit reduced motor function deficits and prolonged lifespan. iNAMPT protein levels are significantly reduced in the spinal cord of amyotrophic lateral sclerosis (ALS) patients, indicating the involvement of NAMPT in ALS pathology. Our findings reveal that neuronal NAMPT plays an essential role in mitochondrial bioenergetics, motor function, and survival. Our study suggests that the NAMPT-mediated NAD⁺ biosynthesis pathway is a potential therapeutic target for degenerative MN diseases.

Roles for neuronal and glial autophagy in synaptic pruning during development

The dendritic protrusions known as spines represent the primary postsynaptic location for excitatory synapses. Dendritic spines are critical for many synaptic functions, and their formation, modification, and turnover are thought to be important for mechanisms of learning and memory. At many excitatory synapses, dendritic spines form during the early postnatal period, and while many spines are likely being formed and removed throughout life, the net number are often gradually "pruned" during adolescence to reach a stable level in the adult. In neurodevelopmental disorders, spine pruning is disrupted, emphasizing the importance of understanding its governing processes. Autophagy, a process through which cytosolic components and organelles are degraded, has recently been shown to control spine pruning in the mouse cortex, but the mechanisms through which autophagy acts remain obscure. Here, we draw on three widely studied prototypical synaptic pruning events to focus on two governing principles of spine pruning: 1) activity-dependent synaptic competition and 2) non-neuronal contributions. We briefly review what is known about autophagy in the central nervous system and its regulation by metabolic kinases. We propose a model in which autophagy in both neurons and non-neuronal cells contributes to spine pruning, and how other processes that regulate spine pruning could intersect with autophagy. We further outline future research directions to address outstanding questions on the role of autophagy in synaptic pruning.

Adenosine Receptors in Developing and Adult Mouse Neuromuscular Junctions and Functional Links With Other Metabotropic Receptor Pathways

In the last few years, we have studied the presence and involvement in synaptogenesis and mature transmitter release of the adenosine autoreceptors (AR) in the mammalian neuromuscular junction (NMJ). Here, we review and bring together the previously published data to emphasize the relevance of these receptors for developmental axonal competition, synaptic loss and mature NMJ functional modulation. However, in addition to AR, activity-dependent mediators originating from any of the three cells that make the synapse (nerve, muscle, and glial cells) cross the extracellular cleft to generate signals in target metabotropic receptors. Thus, the integrated interpretation of the complementary function of all these receptors is needed. We previously studied, in the NMJ, the links of AR with mAChR and the neurotrophin receptor TrkB in the control of synapse elimination and transmitter release. We conclude that AR cooperate with these receptors through synergistic and antagonistic effects in the developmental synapse elimination process. In the adult NMJ, this cooperation is manifested so as that the functional integrity of a given receptor group depends on the other receptors operating normally (i.e., the functional integrity of mAChR depends on AR operating normally). These observations underlie the relevance of AR in the NMJ function.

Polyethylene glycol solutions rapidly restore and maintain axonal continuity, neuromuscular structures, and behaviors lost after sciatic nerve transections in female rats

Complete severance of major peripheral mixed sensory-motor nerve proximally in a mammalian limb produces immediate loss of action potential conduction and voluntary behaviors mediated by the severed distal axonal segments. These severed distal segments undergo Wallerian degeneration within days. Denervated muscles atrophy within weeks. Slowly regenerating (∼1 mm/day) outgrowths from surviving proximal stumps that often nonspecifically reinnervate denervated targets produce poor, if any, restoration of lost voluntary behaviors. In contrast, in this study using completely transected female rat sciatic axons as a model system, we provide extensive morphometric, immunohistochemical, electrophysiological, and behavioral data to show that these adverse outcomes are avoided by microsuturing closely apposed axonal cut ends (neurorrhaphy) and applying a sequence of well-specified solutions, one of which contains polyethylene glycol (PEG). This "PEG-fusion" procedure within minutes reestablishes axoplasmic and axolemmal continuity and signaling by nonspecifically fusing (connecting) closely apposed open ends of severed motor and/or sensory axons at the lesion site. These PEG-fused axons continue to conduct action potentials and generate muscle action potentials and muscle twitches for months and do not undergo Wallerian degeneration. Continuously innervated muscle fibers undergo much less atrophy compared with denervated muscle fibers. Dramatic behavioral recovery to near-unoperated levels occurs within days to weeks, almost certainly by activating many central nervous system and peripheral nervous system synaptic and other plasticities, some perhaps to a greater extent than most neuroscientists would expect. Negative control transections in which neurorrhaphy and all solutions except the PEG-containing solution are applied produce none of these remarkably fortuitous outcomes observed for PEG-fusion.

Polyethylene glycol treated allografts not tissue matched nor immunosuppressed rapidly repair sciatic nerve gaps, maintain neuromuscular functions, and restore voluntary behaviors in female rats

Many publications report that ablations of segments of peripheral nerves produce the following unfortunate results: (1) Immediate loss of sensory signaling and motor control; (2) rapid Wallerian degeneration of severed distal axons within days; (3) muscle atrophy within weeks; (4) poor behavioral (functional) recovery after many months, if ever, by slowly-regenerating (∼1mm/d) axon outgrowths from surviving proximal nerve stumps; and (5) Nerve allografts to repair gap injuries are rejected, often even if tissue matched and immunosuppressed. In contrast, using a female rat sciatic nerve model system, we report that neurorrhaphy of allografts plus a well-specified-sequence of solutions (one containing polyethylene glycol: PEG) successfully addresses each of these problems by: (a) Reestablishing axonal continuity/signaling within minutes by nonspecific ally PEG-fusing (connecting) severed motor and sensory axons across each anastomosis; (b) preventing Wallerian degeneration by maintaining many distal segments of inappropriately-reconnected, PEG-fused axons that continuously activate nerve-muscle junctions; (c) maintaining innervation of muscle fibers that undergo much less atrophy than otherwise-denervated muscle fibers; (d) inducing remarkable behavioral recovery to near-unoperated levels within days to weeks, almost certainly by CNS and PNS plasticities well-beyond what most neuroscientists currently imagine; and (e) preventing rejection of PEG-fused donor nerve allografts with no tissue matching or immunosuppression. Similar behavioral results are produced by PEG-fused autografts. All results for Negative Control allografts agree with current neuroscience data 1-5 given above. Hence, PEG-fusion of allografts for repair of ablated peripheral nerve segments expand on previous observations in single-cut injuries, provoke reconsideration of some current neuroscience dogma, and further extend the potential of PEG-fusion in clinical practice.

CXCL12α/SDF-1 from perisynaptic Schwann cells promotes regeneration of injured motor axon terminals

The neuromuscular junction has retained through evolution the capacity to regenerate after damage, but little is known on the inter-cellular signals involved in its functional recovery from trauma, autoimmune attacks, or neurotoxins. We report here that CXCL12α, also abbreviated as stromal-derived factor-1 (SDF-1), is produced specifically by perisynaptic Schwann cells following motor axon terminal degeneration induced by α-latrotoxin. CXCL12α acts via binding to the neuronal CXCR4 receptor. A CXCL12α-neutralizing antibody or a specific CXCR4 inhibitor strongly delays recovery from motor neuron degeneration in vivo Recombinant CXCL12α in vivo accelerates neurotransmission rescue upon damage and very effectively stimulates the axon growth of spinal cord motor neurons in vitro These findings indicate that the CXCL12α-CXCR4 axis plays an important role in the regeneration of the neuromuscular junction after motor axon injury. The present results have important implications in the effort to find therapeutics and protocols to improve recovery of function after different forms of motor axon terminal damage.

Recapitulating early development of mouse musculoskeletal precursors of the paraxial mesoderm in vitro

Body skeletal muscles derive from the paraxial mesoderm, which forms in the posterior region of the embryo. Using microarrays, we characterize novel mouse presomitic mesoderm (PSM) markers and show that, unlike the abrupt transcriptome reorganization of the PSM, neural tube differentiation is accompanied by progressive transcriptome changes. The early paraxial mesoderm differentiation stages can be efficiently recapitulated in vitro using mouse and human pluripotent stem cells. While Wnt activation alone can induce posterior PSM markers, acquisition of a committed PSM fate and efficient differentiation into anterior PSM Pax3+ identity further requires BMP inhibition to prevent progenitors from drifting to a lateral plate mesoderm fate. When transplanted into injured adult muscle, these precursors generated large numbers of immature muscle fibers. Furthermore, exposing these mouse PSM-like cells to a brief FGF inhibition step followed by culture in horse serum-containing medium allows efficient recapitulation of the myogenic program to generate myotubes and associated Pax7+ cells. This protocol results in improved in vitro differentiation and maturation of mouse muscle fibers over serum-free protocols and enables the study of myogenic cell fusion and satellite cell differentiation.

Limitations and Challenges in Modeling Diseases Involving Spinal Motor Neuron Degeneration in Vitro

Pathogenic conditions involving degeneration of spinal motor neurons (MNs), such as amyotrophic lateral sclerosis, sarcopenia, and spinal cord injury, mostly occur in individuals whose spinal MNs are fully mature. There is currently no effective treatment to prevent death or promote axonal regeneration of the spinal MNs affected in these patients. To increase our understanding and find a cure for such conditions, easily controllable and monitorable cell culture models allow for a better dissection of certain molecular and cellular events that cannot be teased apart in whole organism models. To date, various types of spinal MN cultures have been described. Yet these models are all based on the use of immature neurons or neurons uncharacterized for their degree of maturity after being isolated and cultured. Additionally, studying only MNs cannot give a comprehensive and complete view of the neurodegenerative processes usually involving other cell types. To date, there is no confirmed in vitro model faithfully emulating disease or injury of the mature spinal MNs. In this review, we summarize the different limitations of currently available culture models, and discuss the challenges that have to be overcome for developing more reliable and translational platforms for the in vitro study of spinal MN degeneration.

Utrophin up-regulation by artificial transcription factors induces muscle rescue and impacts the neuromuscular junction in mdx mice

Up-regulation of the dystrophin-related gene utrophin represents a promising therapeutic strategy for the treatment of Duchenne Muscular Dystrophy (DMD). In order to re-program the utrophin expression level in muscle, we engineered artificial zinc finger transcription factors (ZF-ATFs) that target the utrophin 'A' promoter. We have previously shown that the ZF-ATF "Jazz", either by transgenic manipulation or by systemic adeno-associated viral delivery, induces significant rescue of muscle function in dystrophic "mdx" mice. We present the full characterization of an upgraded version of Jazz gene named "JZif1" designed to minimize any possible host immune response. JZif1 was engineered on the Zif268 gene-backbone using selective amino acid substitutions to address JZif1 to the utrophin 'A' promoter. Here, we show that JZif1 induces remarkable amelioration of the pathological phenotype in mdx mice. To investigate the molecular mechanisms underlying Jazz and JZif1 induced muscle functional rescue, we focused on utrophin related pathways. Coherently with utrophin subcellular localization and role in neuromuscular junction (NMJ) plasticity, we found that our ZF-ATFs positively impact the NMJ. We report on ZF-ATF effects on post-synaptic membranes in myogenic cell line, as well as in wild type and mdx mice. These results candidate our ZF-ATFs as novel therapeutic molecules for DMD treatment.

Synaptotagmin-2, and -1, linked to neurotransmission impairment and vulnerability in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is the most frequent genetic cause of infant mortality. The disease is characterized by progressive muscle weakness and paralysis of axial and proximal limb muscles. It is caused by homozygous loss or mutation of the SMN1 gene, which codes for the Survival Motor Neuron (SMN) protein. In mouse models of the disease, neurotransmitter release is greatly impaired, but the molecular mechanisms of the synaptic dysfunction and the basis of the selective muscle vulnerability are unknown. In the present study, we investigated these open questions by comparing the molecular and functional properties of nerve terminals in severely and mildly affected muscles in the SMNΔ7 mouse model. We discovered that synaptotagmin-1 (Syt1) was developmentally downregulated in nerve terminals of highly affected muscles but not in low vulnerable muscles. Additionally, the expression levels of synaptotagmin-2 (Syt2), and its interacting protein, synaptic vesicle protein 2 (SV2) B, were reduced in proportion to the degree of muscle vulnerability while other synaptic proteins, such as syntaxin-1B (Stx1B) and synaptotagmin-7 (Syt7), were not affected. Consistently with the extremely low levels of both Syt-isoforms, and SV2B, in most affected neuromuscular synapses, the functional analysis of neurotransmission revealed highly reduced evoked release, altered short-term plasticity, low release probability, and inability to modulate normally the number of functional release sites. Together, we propose that the strong reduction of Syt2 and SV2B are key factors of the functional synaptic alteration and that the physiological downregulation of Syt1 plays a determinant role in muscle vulnerability in SMA.

Agrin and LRP4 antibodies as new biomarkers of myasthenia gravis

Myasthenia gravis (MG) is a common disorder that affects the neuromuscular junction. It is caused by antibodies against acetylcholine receptor and muscle-specific tyrosine kinase; however, some MG patients do not have antibodies against either of the proteins. Recent studies have revealed antibodies against agrin and its receptor LRP4-both critical for neuromuscular junction formation and maintenance-in MG patients from various populations. Results from experimental autoimmune MG animal models indicate that anti-LRP4 antibodies are causal to MG. Clinical studies have begun to reveal the significance of the new biomarkers. With their identification, MG appears to be a complex disease entity that can be classified into different subtypes with different etiology, each with unique symptoms. Future systematic studies of large cohorts of well-diagnosed MG patients are needed to determine whether each subtype of patients would respond to different therapeutic strategies. Results should contribute to the goal of precision medicine for MG patients. Anti-agrin and anti-LRP4 antibodies are also detectable in some patients with amyotrophic lateral sclerosis or Lou Gehrig's disease; however, whether they are a cause or response to the disorder remains unclear.

Experience-Dependent Structural Plasticity in the Visual System

During development, the environment exerts a profound influence on the wiring of brain circuits. Due to the limited resolution of studies in fixed tissue, this experience-dependent structural plasticity was once thought to be restricted to a specific developmental time window. The recent introduction of two-photon microscopy for in vivo imaging has opened the door to repeated monitoring of individual neurons and the study of structural plasticity mechanisms at a very fine scale. In this review, we focus on recent work showing that synaptic structural rearrangements are a key mechanism mediating neural circuit adaptation and behavioral plasticity in the adult brain. We examine this work in the context of classic studies in the visual systems of model organisms, which have laid much of the groundwork for our understanding of activity-dependent synaptic remodeling and its role in brain plasticity.

Exciting perspectives for Translational Myology in the Abstracts of the 2018Spring PaduaMuscleDays: Giovanni Salviati Memorial - Chapter II - Abstracts of March 15, 2018

Myologists working in Padua (Italy) were able to continue a half-century tradition of studies of skeletal muscles, that started with a research on fever, specifically if and how skeletal muscle contribute to it by burning bacterial toxin. Beside main publications in high-impact-factor journals by Padua myologists, I hope to convince readers (and myself) of the relevance of the editing Basic and Applied Myology (BAM), retitled from 2010 European Journal of Translational Myology (EJTM), of the institution of the Interdepartmental Research Center of Myology of the University of Padova (CIR-Myo), and of a long series of International Conferences organized in Euganei Hills and Padova, that is, the PaduaMuscleDays. The 2018Spring PaduaMuscleDays (2018SpPMD), were held in Euganei Hills and Padua (Italy), in March 14-17, and were dedicated to Giovanni Salviati. The main event of the "Giovanni Salviati Memorial", was held in the Aula Guariento, Accademia Galileiana di Scienze, Lettere ed Arti of Padua to honor a beloved friend and excellent scientist 20 years after his premature passing. Using the words of Prof. Nicola Rizzuto, we all share his believe that Giovanni "will be remembered not only for his talent and originality as a biochemist, but also for his unassuming and humanistic personality, a rare quality in highly successful people like Giovanni. The best way to remember such a person is to gather pupils and colleagues, who shared with him the same scientific interests and ask them to discuss recent advances in their own fields, just as Giovanni have liked to do". Since Giovanni's friends sent many abstracts still influenced by their previous collaboration with him, all the Sessions of the 2018SpPMD reflect both to the research aims of Giovanni Salviati and the traditional topics of the PaduaMuscleDays, that is, basics and applications of physical, molecular and cellular strategies to maintain or recover functions of skeletal muscles. The translational researches summarized in the 2018SpPMD Abstracts are at the appropriate high level to attract approval of Ethical Committees, the interest of International Granting Agencies and approval for publication in top quality, international journals. In this chapter II are listed the abstracts of the March 15, 2018 Padua Muscle Day. All 2018SpPMD Abstracts are indexed at the end of the Chapter IV.

Imaging Analysis of the Neuromuscular Junction in Dystrophic Muscle

Duchenne muscular dystrophy (DMD), caused by the absence of the protein dystrophin, is characterized as a neuromuscular disease in which muscle weakness, increased susceptibility to muscle injury, and inadequate repair appear to underlie the pathology. Considerable attention has been dedicated to studying muscle fiber damage, but there is little information to determine if damage from contraction-induced injury also occurs at or near the nerve terminal axon. Interestingly, both human patients and the mouse model for DMD (the mdx mouse) present fragmented neuromuscular junction (NMJ) morphology. Studies of mdx mice have revealed presynaptic and postsynaptic abnormalities, nerve terminal discontinuity, as well as increased susceptibility of the NMJ to contraction-induced injury with corresponding functional changes in neuromuscular transmission and nerve-evoked electromyography. Focusing on the NMJ as a contributor to functional deficits in the muscle represents a paradigm shift from the more prevalent myocentric perspectives. Further studies are needed to determine the extent to which the nerve-muscle interaction is disrupted in DMD and the role of the NMJ in the dystrophic progression. This chapter lists the tools needed for nerve terminal and NMJ structural analysis using fluorescence imaging, and provides a step-by-step outline for how to stain, image, and analyze the NMJ in skeletal muscle, with specific attention to mdx muscle.

Caloric Restriction Mimetics Slow Aging of Neuromuscular Synapses and Muscle Fibers

Resveratrol and metformin have been shown to mimic some aspects of caloric restriction and exercise. However, it remains unknown if these molecules also slow age-related synaptic degeneration, as previously shown for caloric restriction and exercise. In this study, we examined the structural integrity of neuromuscular junctions (NMJs) in 2-year-old mice treated with resveratrol and metformin starting at 1 year of age. We found that resveratrol significantly slows aging of NMJs in the extensor digitorum longus muscle of 2-year-old mice. Resveratrol also preserved the morphology of muscle fibers in old mice. Although metformin slowed the rate of muscle fiber aging, it did not significantly affect aging of NMJs. Based on these findings, we sought to determine if resveratrol directly affects NMJs. For this, we examined postsynaptic sites, the NMJ region located on the muscle peripheral membrane, on cultured myotubes derived from C2C12 cells. We discovered that resveratrol increases the number of postsynaptic sites on myotubes exhibiting a youthful architecture, suggesting that resveratrol directly affects the NMJ. Altogether, we provide compelling evidence indicating that resveratrol slows aging of NMJs and muscle fibers.

Strict Independence of Parallel and Poly-synaptic Axon-Target Matching during Visual Reflex Circuit Assembly

The use of sensory information to drive specific behaviors relies on circuits spanning long distances that wire up through a range of axon-target recognition events. Mechanisms assembling poly-synaptic circuits and the extent to which parallel pathways can "cross-wire" to compensate for loss of one another remain unclear and are crucial to our understanding of brain development and models of regeneration. In the visual system, specific retinal ganglion cells (RGCs) project to designated midbrain targets connected to downstream circuits driving visuomotor reflexes. Here, we deleted RGCs connecting to pupillary light reflex (PLR) midbrain targets and discovered that axon-target matching is tightly regulated. RGC axons of the eye-reflex pathway avoided vacated PLR targets. Moreover, downstream PLR circuitry is maintained; hindbrain and peripheral components retained their proper connectivity and function. These findings point to a model in which poly-synaptic circuit development reflects independent, highly stringent wiring of each parallel pathway and downstream station.

Bioreactor model of neuromuscular junction with electrical stimulation for pharmacological potency testing

In vitro models of the neuromuscular junction (NMJ) are emerging as a valuable tool to study synaptogenesis, synaptic maintenance, and pathogenesis of neurodegenerative diseases. Many models have previously been developed using a variety of cell sources for skeletal muscle and motoneurons. These models can advanced by integrating beneficial features of the native developmental milieu of the NMJ. We created a functional in vitro model of NMJ by bioreactor cultivation of transdifferentiated myocytes and stem cell-derived motoneurons, in the presence of electrical stimulation. In conjunction with a coculture medium, electrical stimulation resulted in improved maturation and function of motoneurons and myocytes, as evidenced by mature cellular structures, increased expression of neuronal and muscular genes, clusterization of acetylcholine receptors (AChRs) in the vicinity of motoneurons, and the response to glutamate stimulation. To validate the model and demonstrate its utility for pharmacological testing, we documented the potency of drugs that affect key pathways during NMJ signal transduction: (i) acetylcholine (ACh) synthesis, (ii) ACh vesicular storage, (iii) ACh synaptic release, (iv) AChR activation, and (v) ACh inactivation in the synaptic cleft. The model properly responded to the drugs in a concentration-dependent manner. We thus propose that this in vitro model of NMJ could be used as a platform in pharmacological screening and controlled studies of neuromuscular diseases.

Time course of functional recovery during the first 3 mo after surgical transection and repair of nerves to the feline soleus and lateral gastrocnemius muscles

Locomotion outcomes after peripheral nerve injury and repair in cats have been described in the literature for the period immediately following the injury (muscle denervation period) and then again for an ensuing period of long-term recovery (at 3 mo and longer) resulting in muscle self-reinnervation. Little is known about the changes in muscle activity and walking mechanics during midrecovery, i.e., the early reinnervation period that takes place between 5 and 10 wk of recovery. Here, we investigated hindlimb mechanics and electromyogram (EMG) activity of ankle extensors in six cats during level and slope walking before and every 2 wk thereafter in a 14-wk period of recovery after the soleus (SO) and lateral gastrocnemius (LG) muscle nerves in one hindlimb were surgically transected and repaired. We found that the continued increase in SO and LG EMG magnitudes and corresponding changes in hindlimb mechanics coincided with the formation of neuromuscular synapses revealed in muscle biopsies. Throughout the recovery period, EMG magnitude of SO and LG during the stance phase and the duration of the stance-related activity were load dependent, similar to those in the intact synergistic medial gastrocnemius and plantaris. These results and the fact that EMG activity of ankle extensors and locomotor mechanics during level and upslope walking recovered 14 wk after nerve transection and repair suggest that loss of the stretch reflex in self-reinnervated muscles may be compensated by the recovered force-dependent feedback in self-reinnervated muscles, by increased central drive, and by increased gain in intermuscular motion-dependent pathways from intact ankle extensors. NEW & NOTEWORTHY This study provides new evidence that the timeline for functional recovery of gait after peripheral nerve injury and repair is consistent with the time required for neuromuscular junctions to form and muscles to reach preoperative tensions. Our findings suggest that a permanent loss of autogenic stretch reflex in self-reinnervated muscles may be compensated by recovered intermuscular force-dependent and oligosynaptic length-dependent feedback and central drive to regain adequate locomotor output capabilities during level and upslope walking.

Functional decline at the aging neuromuscular junction is associated with altered laminin-α4 expression

Laminin-α4 is involved in the alignment of active zones to postjunctional folds at the neuromuscular junction (NMJ). Prior study has implicated laminin-α4 in NMJ maintenance, with altered NMJ morphology observed in adult laminin-α4 deficient mice (lama4^−/−). The present study further investigated the role of laminin-α4 in NMJ maintenance by functional characterization of transmission properties, morphological investigation of synaptic proteins including synaptic laminin-α4, and neuromotor behavioral testing. Results showed maintained perturbed transmission properties at lama4^−/− NMJs from adult (3 months) through to aged (18-22 months). Hind-limb grip force demonstrated similar trends as transmission properties, with maintained weaker grip force across age groups in lama4^−/−. Interestingly, both transmission properties and hind-limb grip force in aged wild-types resembled those observed in adult lama4^−/−. Most significantly, altered expression of laminin-α4 was noted at the wild-type NMJs prior to the observed decline in transmission properties, suggesting that altered laminin-α4 expression precedes the decline of neurotransmission in aging wild-types. These findings significantly support the role of laminin-α4 in maintenance of the NMJ during aging.

The Structure of Human Neuromuscular Junctions: Some Unanswered Molecular Questions

The commands that control animal movement are transmitted from motor neurons to their target muscle cells at the neuromuscular junctions (NMJs). The NMJs contain many protein species whose role in transmission depends not only on their inherent properties, but also on how they are distributed within the complex structure of the motor nerve terminal and the postsynaptic muscle membrane. These molecules mediate evoked chemical transmitter release from the nerve and the action of that transmitter on the muscle. Human NMJs are among the smallest known and release the smallest number of transmitter "quanta". By contrast, they have the most deeply infolded postsynaptic membranes, which help to amplify transmitter action. The same structural features that distinguish human NMJs make them particularly susceptible to pathological processes. While much has been learned about the molecules which mediate transmitter release and action, little is known about the molecular processes that control the growth of the cellular and subcellular components of the NMJ so as to give rise to its mature form. A major challenge for molecular biologists is to understand the molecular basis for the development and maintenance of functionally important aspects of NMJ structure, and thereby to point to new directions for treatment of diseases in which neuromuscular transmission is impaired.

The role of spontaneous neurotransmission in synapse and circuit development

In the past, the spontaneous release of neurotransmitter from presynaptic terminals has been thought of as a side effect of evoked release, with little functional significance. As our understanding of the process of spontaneous release has increased over time, this notion has gradually changed. In this review, we focus on the importance of this form of release during neuronal development, a time of extreme levels of plasticity that includes the growth of dendrites and axons as well as the formation of new synaptic contacts. This period also encompasses high levels of neurotransmitter release from growing axons, and recent studies have found that spontaneous transmitter release plays an important role in shaping neuronal morphology as well as modulating the properties of newly forming synaptic contacts in the brain. Here, we bring together the latest findings across different species to argue that the spontaneous release of neurotransmitter is an important player in the wiring of the brain during development.

Spontaneous and Evoked Activity from Murine Ventral Horn Cultures on Microelectrode Arrays

Motor neurons are the site of action for several neurological disorders and paralytic toxins, with cell bodies located in the ventral horn (VH) of the spinal cord along with interneurons and support cells. Microelectrode arrays (MEAs) have emerged as a high content assay platform for mechanistic studies and drug discovery. Here, we explored the spontaneous and evoked electrical activity of VH cultures derived from embryonic mouse spinal cord on multi-well plates of MEAs. Primary VH cultures from embryonic day 15–16 mice were characterized by expression of choline acetyltransferase (ChAT) by immunocytochemistry. Well resolved, all-or-nothing spontaneous spikes with profiles consistent with extracellular action potentials were observed after 3 days in vitro, persisting with consistent firing rates until at least day in vitro 19. The majority of the spontaneous activity consisted of tonic firing interspersed with coordinated bursting across the network. After 5 days in vitro, spike activity was readily evoked by voltage pulses where a minimum amplitude and duration required for excitation was 300 mV and 100 μs/phase, respectively. We characterized the sensitivity of spontaneous and evoked activity to a host of pharmacological agents including AP5, CNQX, strychnine, ω-agatoxin IVA, and botulinum neurotoxin serotype A (BoNT/A). These experiments revealed sensitivity of the cultured VH to both agonist and antagonist compounds in a manner consistent with mature tissue derived from slices. In the case of BoNT/A, we also demonstrated intoxication persistence over an 18-day period, followed by partial intoxication recovery induced by N- and P/Q-type calcium channel agonist GV-58. In total, our findings suggest that VH cultures on multi-well MEA plates may represent a moderate throughput, high content assay for performing mechanistic studies and for screening potential therapeutics pertaining to paralytic toxins and neurological disorders.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.

Membrane Receptor-Induced Changes of the Protein Kinases A and C Activity May Play a Leading Role in Promoting Developmental Synapse Elimination at the Neuromuscular Junction

Synapses that are overproduced during histogenesis in the nervous system are eventually lost and connectivity is refined. Membrane receptor signaling leads to activity-dependent mutual influence and competition between axons directly or with the involvement of the postsynaptic cell and the associated glial cell/s. Presynaptic muscarinic acetylcholine (ACh) receptors (subtypes mAChR; M₁, M₂ and M₄), adenosine receptors (AR; A₁ and A_2A) and the tropomyosin-related kinase B receptor (TrkB), among others, all cooperate in synapse elimination. Between these receptors there are several synergistic, antagonic and modulatory relations that clearly affect synapse elimination. Metabotropic receptors converge in a limited repertoire of intracellular effector kinases, particularly serine protein kinases A and C (PKA and PKC), to phosphorylate protein targets and bring about structural and functional changes leading to axon loss. In most cells A₁, M₁ and TrkB operate mainly by stimulating PKC whereas A_2A, M₂ and M₄ inhibit PKA. We hypothesize that a membrane receptor-induced shifting in the protein kinases A and C activity (inhibition of PKA and/or stimulation of PKC) in some nerve endings may play an important role in promoting developmental synapse elimination at the neuromuscular junction (NMJ). This hypothesis is supported by: (i) the tonic effect (shown by using selective inhibitors) of several membrane receptors that accelerates axon loss between postnatal days P5-P9; (ii) the synergistic, antagonic and modulatory effects (shown by paired inhibition) of the receptors on axonal loss; (iii) the fact that the coupling of these receptors activates/inhibits the intracellular serine kinases; and (iv) the increase of the PKA activity, the reduction of the PKC activity or, in most cases, both situations simultaneously that presumably occurs in all the situations of singly and paired inhibition of the mAChR, AR and TrkB receptors. The use of transgenic animals and various combinations of selective and specific PKA and PKC inhibitors could help to elucidate the role of these kinases in synapse maturation.

In Vitro Innervation as an Experimental Model to Study the Expression and Functions of Acetylcholinesterase and Agrin in Human Skeletal Muscle

Acetylcholinesterase (AChE) and agrin, a heparan-sulfate proteoglycan, reside in the basal lamina of the neuromuscular junction (NMJ) and play key roles in cholinergic transmission and synaptogenesis. Unlike most NMJ components, AChE and agrin are expressed in skeletal muscle and α-motor neurons. AChE and agrin are also expressed in various other types of cells, where they have important alternative functions that are not related to their classical roles in NMJ. In this review, we first focus on co-cultures of embryonic rat spinal cord explants with human skeletal muscle cells as an experimental model to study functional innervation in vitro. We describe how this heterologous rat-human model, which enables experimentation on highly developed contracting human myotubes, offers unique opportunities for AChE and agrin research. We then highlight innovative approaches that were used to address salient questions regarding expression and alternative functions of AChE and agrin in developing human skeletal muscle. Results obtained in co-cultures are compared with those obtained in other models in the context of general advances in the field of AChE and agrin neurobiology.

Super-Resolution Microscopy Reveals a Nanoscale Organization of Acetylcholine Receptors for Trans-Synaptic Alignment at Neuromuscular Synapses

The neuromuscular junction (NMJ) is a chemical synapse formed between motoneurons and skeletal muscle fibers. The vertebrate NMJ uses acetylcholine (ACh) as the neurotransmitter and features numerous invaginations of the postsynaptic muscle membrane termed junctional folds. ACh receptors (AChRs) are believed to be concentrated on the crest of junctional folds but their spatial organization remains to be fully understood. In this study, we utilized super-resolution microscopy to examine the nanoscale organization of AChRs at NMJ. Using Structured Illumination Microscopy, we found that AChRs appear as stripes within the pretzel-shaped mouse NMJs, which however, do not correlate with the size of the crests of junctional folds. By comparing the localization of AChRs with several pre- and postsynaptic markers of distinct compartments of NMJs, we found that AChRs are not distributed evenly across the crest of junctional folds as previously thought. Instead, AChR stripes are more closely aligned with the openings of junctional folds as well as with the presynaptic active zone. Using Stochastic Optical Reconstruction Microscopy (STORM) for increased resolution, we found that each AChR stripe contains an AChR-poor slit at the center that is equivalent to the size of the opening of junctional folds. Together, these findings indicate that AChRs are largely localized to the edges of crests surrounding the opening of folds to align with the presynaptic active zones. Such a nanoscale organization of AChRs potentially enables trans-synaptic alignment for effective synaptic transmission of NMJs.

A Novel Egr-1-Agrin Pathway and Potential Implications for Regulation of Synaptic Physiology and Homeostasis at the Neuromuscular Junction

Synaptic transmission requires intricate coordination of the components involved in processing of incoming signals, formation and stabilization of synaptic machinery, neurotransmission and in all related signaling pathways. Changes to any of these components cause synaptic imbalance and disruption of neuronal circuitry. Extensive studies at the neuromuscular junction (NMJ) have greatly aided in the current understanding of synapses and served to elucidate the underlying physiology as well as associated adaptive and homeostatic processes. The heparan sulfate proteoglycan agrin is a vital component of the NMJ, mediating synaptic formation and maintenance in both brain and muscle, but very little is known about direct control of its expression. Here, we investigated the relationship between agrin and transcription factor early growth response-1 (Egr-1), as Egr-1 regulates the expression of many genes involved in synaptic homeostasis and plasticity. Using chromatin immunoprecipitation (ChIP), cell culture with cell lines derived from brain and muscle, and animal models, we show that Egr-1 binds to the AGRN gene locus and suppresses its expression. When compared with wild type (WT), mice deficient in Egr-1 (Egr-1−/−) display a marked increase in AGRN mRNA and agrin full-length and cleavage fragment protein levels, including the 22 kDa, C-terminal fragment in brain and muscle tissue homogenate. Because agrin is a crucial component of the NMJ, we explored possible physiological implications of the Egr-1-agrin relationship. In the diaphragm, Egr-1−/− mice display increased NMJ motor endplate density, individual area and area of innervation. In addition to increased density, soleus NMJs also display an increase in fragmented and faint endplates in Egr-1−/− vs. WT mice. Moreover, the soleus NMJ electrophysiology of Egr-1−/− mice revealed increased quantal content and motor testing showed decreased movement and limb muscle strength compared with WT. This study provides evidence for the potential involvement of a novel Egr-1-agrin pathway in synaptic homeostatic and compensatory mechanisms at the NMJ. Synaptic homeostasis is greatly affected by the process of aging. These and other data suggest that changes in Egr-1 expression may directly or indirectly promote age-related pathologies.

Muscle Fibers Secrete FGFBP1 to Slow Degeneration of Neuromuscular Synapses during Aging and Progression of ALS

The identity of muscle secreted factors critical for the development and maintenance of neuromuscular junctions (NMJs) remains largely unknown. Here, we show that muscle fibers secrete and concentrate the fibroblast growth factor binding protein 1 (FGFBP1) at NMJs. Although FGFBP1 expression increases during development, its expression decreases before NMJ degeneration during aging and in SOD1^G93A mice, a mouse model for amyotrophic lateral sclerosis (ALS). Based on these findings, we examined the impact of deleting FGFBP1 on NMJs. In the absence of FGFBP1, NMJs exhibit structural abnormalities in developing and middle age mice. Deletion of FGFBP1 from SOD1^G93A mice also accelerates NMJ degeneration and death. Based on these findings, we sought to identify the mechanism responsible for decreased FGFBP1 in stressed skeletal muscles. We show that FGFBP1 expression is inhibited by increased accumulation of the transforming growth factor-β1 (TGF-β1) in skeletal muscles and at their NMJs. These findings suggest that targeting the FGFBP1 and TGF-β1 signaling axis holds promise for slowing age- and disease-related degeneration of NMJs.

SIGNIFICANCE STATEMENT

The neuromuscular junction (NMJ) is critical for all voluntary movement. Its malformation during development and degeneration in adulthood impairs motor function. Therefore, it is important to identify factors that function to maintain the structural integrity of NMJs. We show that muscle fibers secrete and concentrate the fibroblast growth factor binding protein 1 (FGFBP1) at NMJs. However, FGFBP1 expression decreases in skeletal muscles during aging and before NMJ degeneration in SOD1^G93A mice, a mouse model for amyotrophic lateral sclerosis. We show that transforming growth factor-β1 is responsible for the decreased levels of FGFBP1. Importantly, we demonstrate critical roles for FGFBP1 at NMJs in developing, aging and SOD1^G93A mice.

Nerve growth factor and basic fibroblast growth factor promote reinnervation by nerve-muscle-endplate grafting

INTRODUCTION

This study was designed to test whether exogenous application of nerve growth factor (NGF) and basic fibroblast growth factor (FGF-2) to muscles reinnervated with nerve-muscle-endplate band grafting (NMEG) could promote specific outcomes.

METHODS

The right sternomastoid muscle in adult rats was experimentally denervated and immediately reinnervated by implanting an NMEG pedicle from the ipsilateral sternohyoid muscle. A fibrin sealant containing NGF and FGF-2 was focally applied to the implantation site. Maximal tetanic force, muscle weight, regenerated axons, and motor endplates were analyzed 3 months after treatment.

RESULTS

Mean tetanic force, wet muscle weight, and number of regenerated axons in the treated muscles were 91%, 92%, and 84% of the contralateral controls, respectively. The majority of endplates (86%) in the treated muscles were reinnervated by regenerated axons.

DISCUSSION

Focal administration of NGF and FGF-2 promotes efficacy of the NMEG technique. Muscle Nerve 57: 449-459, 2018.

Invadosomes are coming: new insights into function and disease relevance

Invadopodia and podosomes are discrete, actin-based molecular protrusions that form in cancer cells and normal cells, respectively, in response to diverse signaling pathways and extracellular matrix cues. Although they participate in a host of different cellular processes, they share a common functional theme of controlling pericellular proteolytic activity, which sets them apart from other structures that function in migration and adhesion, including focal adhesions, lamellipodia, and filopodia. In this review, we highlight research that explores the function of these complex structures, including roles for podosomes in embryonic and postnatal development, in angiogenesis and remodeling of the vasculature, in maturation of the postsynaptic membrane, in antigen sampling and recognition, and in cell-cell fusion mechanisms, as well as the involvement of invadopodia at multiple steps of the metastatic cascade, and how all of this may apply in the treatment of human disease states. Finally, we explore recent research that implicates a novel role for exosomes and microvesicles in invadopodia-dependent and invadopodia-independent mechanisms of invasion, respectively.

Differential regulation of polarized synaptic vesicle trafficking and synapse stability in neural circuit rewiring in Caenorhabditis elegans

Neural circuits are dynamic, with activity-dependent changes in synapse density and connectivity peaking during different phases of animal development. In C. elegans, young larvae form mature motor circuits through a dramatic switch in GABAergic neuron connectivity, by concomitant elimination of existing synapses and formation of new synapses that are maintained throughout adulthood. We have previously shown that an increase in microtubule dynamics during motor circuit rewiring facilitates new synapse formation. Here, we further investigate cellular control of circuit rewiring through the analysis of mutants obtained in a forward genetic screen. Using live imaging, we characterize novel mutations that alter cargo binding in the dynein motor complex and enhance anterograde synaptic vesicle movement during remodeling, providing in vivo evidence for the tug-of-war between kinesin and dynein in fast axonal transport. We also find that a casein kinase homolog, TTBK-3, inhibits stabilization of nascent synapses in their new locations, a previously unexplored facet of structural plasticity of synapses. Our study delineates temporally distinct signaling pathways that are required for effective neural circuit refinement.

In this study, we identify pathways that regulate the formation and maintenance of synapses, the functional connections between neurons, in the nervous system of the nematode C. elegans. Our work characterizes the interaction between molecular motors kinesin and dynein, which carry cargo and move towards opposite ends of microtubules during synapse formation. We also address the role of a protein kinase gene TTBK-3 in maintaining synapse structure once synaptic components have reached the sites of new synapses. Our findings shed mechanistic insight into the coordination of molecular motors and the cytoskeleton in neural circuit function.

Neuroinflammation and Infection: Molecular Mechanisms Associated with Dysfunction of Neurovascular Unit

Neuroinflammation is a complex inflammatory process in the central nervous system, which is sought to play an important defensive role against various pathogens, toxins or factors that induce neurodegeneration. The onset of neurodegenerative diseases and various microbial infections are counted as stimuli that can challenge the host immune system and trigger the development of neuroinflammation. The homeostatic nature of neuroinflammation is essential to maintain the neuroplasticity. Neuroinflammation is regulated by the activity of neuronal, glial, and endothelial cells within the neurovascular unit, which serves as a “platform” for the coordinated action of pro- and anti-inflammatory mechanisms. Production of inflammatory mediators (cytokines, chemokines, reactive oxygen species) by brain resident cells or cells migrating from the peripheral blood, results in the impairment of blood-brain barrier integrity, thereby further affecting the course of local inflammation. In this review, we analyzed the most recent data on the central nervous system inflammation and focused on major mechanisms of neurovascular unit dysfunction caused by neuroinflammation and infections.

Evidence for the subsynaptic zone as a preferential site for CHRN recycling at neuromuscular junctions

Vertebrate skeletal muscle contraction is mediated by nicotinic acetylcholine receptors (CHRN). Endocytosis and recycling of CHRN regulate their proper abundance at nerve-muscle synapses, i.e. neuromuscular junctions. Recent work showed that RAB5 is essential for CHRN endocytosis. Here, using in vivo-imaging of endocytosed CHRN and RAB-GFP fusion proteins, we deliver evidence for differential effects of RAB5-GFP, RAB4-GFP, and RAB11-GFP on CHRN endocytosis. Furthermore, while newly endocytosed CHRN colocalized with RAB5-GFP over large stretches of muscle fibers, RAB4-GFP and RAB11-GFP colocalized with endocytosed CHRN almost exclusively at neuromuscular junctions. In agreement with previous findings, this data suggests the existence of a specialized subsynaptic zone that is particularly relevant for CHRN recycling.

Loss of adult skeletal muscle stem cells drives age-related neuromuscular junction degeneration

Neuromuscular junction degeneration is a prominent aspect of sarcopenia, the age-associated loss of skeletal muscle integrity. Previously, we showed that muscle stem cells activate and contribute to mouse neuromuscular junction regeneration in response to denervation (Liu et al., 2015). Here, we examined gene expression profiles and neuromuscular junction integrity in aged mouse muscles, and unexpectedly found limited denervation despite a high level of degenerated neuromuscular junctions. Instead, degenerated neuromuscular junctions were associated with reduced contribution from muscle stem cells. Indeed, muscle stem cell depletion was sufficient to induce neuromuscular junction degeneration at a younger age. Conversely, prevention of muscle stem cell and derived myonuclei loss was associated with attenuation of age-related neuromuscular junction degeneration, muscle atrophy, and the promotion of aged muscle force generation. Our observations demonstrate that deficiencies in muscle stem cell fate and post-synaptic myogenesis provide a cellular basis for age-related neuromuscular junction degeneration and associated skeletal muscle decline.

DOI:http://dx.doi.org/10.7554/eLife.26464.001

Botulinum Neurotoxins: Biology, Pharmacology, and Toxicology

The study of botulinum neurotoxins (BoNT) is rapidly progressing in many aspects. Novel BoNTs are being discovered owing to next generation sequencing, but their biologic and pharmacological properties remain largely unknown. The molecular structure of the large protein complexes that the toxin forms with accessory proteins, which are included in some BoNT type A1 and B1 pharmacological preparations, have been determined. By far the largest effort has been dedicated to the testing and validation of BoNTs as therapeutic agents in an ever increasing number of applications, including pain therapy. BoNT type A1 has been also exploited in a variety of cosmetic treatments, alone or in combination with other agents, and this specific market has reached the size of the one dedicated to the treatment of medical syndromes. The pharmacological properties and mode of action of BoNTs have shed light on general principles of neuronal transport and protein-protein interactions and are stimulating basic science studies. Moreover, the wide array of BoNTs discovered and to be discovered and the production of recombinant BoNTs endowed with specific properties suggest novel uses in therapeutics with increasing disease/symptom specifity. These recent developments are reviewed here to provide an updated picture of the biologic mechanism of action of BoNTs, of their increasing use in pharmacology and in cosmetics, and of their toxicology.

Neuronal activity patterns in the developing barrel cortex

The developing barrel cortex reveals a rich repertoire of neuronal activity patterns, which have been also found in other sensory neocortical areas and in other species including the somatosensory cortex of preterm human infants. The earliest stage is characterized by asynchronous, sparse single-cell firing at low frequencies. During the second stage neurons show correlated firing, which is initially mediated by electrical synapses and subsequently transforms into network bursts depending on chemical synapses. Activity patterns during this second stage are synchronous plateau assemblies, delta waves, spindle bursts and early gamma oscillations (EGOs). In newborn rodents spindle bursts and EGOs occur spontaneously or can be elicited by sensory stimulation and synchronize the activity in a barrel-related columnar network with topographic organization at the day of birth. Interfering with this early activity causes a disturbance in the development of the cortical architecture, indicating that spindle bursts and EGOs influence the formation of cortical columns. Early neuronal activity also controls the rate of programed cell death in the developing barrel cortex, suggesting that spindle bursts and EGOs are physiological activity patterns particularly suited to suppress apoptosis. It remains to be studied in more detail how these different neocortical activity patterns control early developmental processes such as formation of synapses, microcircuits, topographic maps and large-scale networks.

Presynaptic Membrane Receptors Modulate ACh Release, Axonal Competition and Synapse Elimination during Neuromuscular Junction Development

During the histogenesis of the nervous system a lush production of neurons, which establish an excessive number of synapses, is followed by a drop in both neurons and synaptic contacts as maturation proceeds. Hebbian competition between axons with different activities leads to the loss of roughly half of the neurons initially produced so connectivity is refined and specificity gained. The skeletal muscle fibers in the newborn neuromuscular junction (NMJ) are polyinnervated but by the end of the competition, 2 weeks later, the NMJ are innervated by only one axon. This peripheral synapse has long been used as a convenient model for synapse development. In the last few years, we have studied transmitter release and the local involvement of the presynaptic muscarinic acetylcholine autoreceptors (mAChR), adenosine autoreceptors (AR) and trophic factor receptors (TFR, for neurotrophins and trophic cytokines) during the development of NMJ and in the adult. This review article brings together previously published data and proposes a molecular background for developmental axonal competition and loss. At the end of the first week postnatal, these receptors modulate transmitter release in the various nerve terminals on polyinnervated NMJ and contribute to axonal competition and synapse elimination.

Neocortical Chandelier Cells Developmentally Shape Axonal Arbors through Reorganization but Establish Subcellular Synapse Specificity without Refinement

Diverse types of cortical interneurons (INs) mediate various kinds of inhibitory control mechanisms to balance and shape network activity. Distinct IN subtypes develop uniquely organized axonal arbors that innervate different subcellular compartments of excitatory principal neurons (PNs), which critically contribute to determining their output properties. However, it remains poorly understood how they establish this peculiar axonal organization and synaptic connectivity during development. Here, taking advantage of genetic labeling of IN progenitors, we examined developmental processes of axonal arbors and synaptic connections formed by murine chandelier cells (ChCs), which innervate axon initial segments (AISs) of PNs and thus powerfully regulate their spike generation. Our quantitative analysis by light microscopy revealed that ChCs overgrow and subsequently refine axonal branches as well as varicosities. Interestingly, we found that although a significant number of axonal varicosities are formed off AISs in addition to on AISs, presynaptic markers are predominantly colocalized with those on AISs throughout development. Immunoelectron microscopic (IEM) analysis also demonstrated that only varicosities apposed to AISs contain presynaptic profiles. These results suggest that subcellular synapse specificity of ChCs is genetically predetermined while axonal geometry is shaped through remodeling. Molecular cues localized at AISs may regulate target recognition and synapse formation by ChCs.

Tumor suppressor menin is required for subunit-specific nAChR α5 transcription and nAChR-dependent presynaptic facilitation in cultured mouse hippocampal neurons

In the central nervous system (CNS), cholinergic transmission induces synaptic plasticity that is required for learning and memory. However, our understanding of the development and maintenance of cholinergic circuits is limited, as the factors regulating the expression and clustering of neuronal nicotinic acetylcholine receptors (nAChRs) remain poorly defined. Recent studies from our group have implicated calpain-dependent proteolytic fragments of menin, the product of the MEN1 tumor suppressor gene, in coordinating the transcription and synaptic clustering of nAChRs in invertebrate central neurons. Here, we sought to determine whether an analogous cholinergic mechanism underlies menin's synaptogenic function in the vertebrate CNS. Our data from mouse primary hippocampal cultures demonstrate that menin and its calpain-dependent C-terminal fragment (C-menin) regulate the subunit-specific transcription and synaptic clustering of neuronal nAChRs, respectively. MEN1 knockdown decreased nAChR α5 subunit expression, the clustering of α7 subunit-containing nAChRs at glutamatergic presynaptic terminals, and nicotine-induced presynaptic facilitation. Moreover, the number and function of glutamatergic synapses was unaffected by MEN1 knockdown, indicating that the synaptogenic actions of menin are specific to cholinergic regulation. Taken together, our results suggest that the influence of menin on synapse formation and synaptic plasticity occur via modulation of nAChR channel subunit composition and functional clustering.

USP5/Leon deubiquitinase confines postsynaptic growth by maintaining ubiquitin homeostasis through Ubiquilin

Synapse formation and growth are tightly controlled processes. How synaptic growth is terminated after reaching proper size remains unclear. Here, we show that Leon, the Drosophila USP5 deubiquitinase, controls postsynaptic growth. In leon mutants, postsynaptic specializations of neuromuscular junctions are dramatically expanded, including the subsynaptic reticulum, the postsynaptic density, and the glutamate receptor cluster. Expansion of these postsynaptic features is caused by a disruption of ubiquitin homeostasis with accumulation of free ubiquitin chains and ubiquitinated substrates in the leon mutant. Accumulation of Ubiquilin (Ubqn), the ubiquitin receptor whose human homolog ubiquilin 2 is associated with familial amyotrophic lateral sclerosis, also contributes to defects in postsynaptic growth and ubiquitin homeostasis. Importantly, accumulations of postsynaptic proteins cause different aspects of postsynaptic overgrowth in leon mutants. Thus, the deubiquitinase Leon maintains ubiquitin homeostasis and proper Ubqn levels, preventing postsynaptic proteins from accumulation to confine postsynaptic growth.

DOI:http://dx.doi.org/10.7554/eLife.26886.001

Soluble Ectodomain of Neuroligin 1 Decreases Synaptic Activity by Activating Metabotropic Glutamate Receptor 2

Synaptic cell adhesion molecules represent important targets for neuronal activity-dependent proteolysis. Postsynaptic neuroligins (NLs) form trans-synaptic complexes with presynaptic neurexins (NXs). Both NXs and NLs are cleaved from the cell surface by metalloproteases in an activity-dependent manner, releasing a soluble extracellular fragment and membrane-tethered C-terminal fragment. The cleavage of NL1 depresses synaptic transmission, but the mechanism by which this occurs is unknown. Metabotropic glutamate receptor 2 (mGluR2) are located primarily at the periphery of presynaptic terminals, where they inhibit the formation of cyclic adenosine monophosphate (cAMP) and consequently suppress the release of glutamate and decrease synaptic transmission. In the present study, we found that the soluble ectodomain of NL1 binds to and activates mGluR2 in both neurons and heterologous cells, resulting in a decrease in cAMP formation. In a slice preparation from the hippocampus of mice, NL1 inhibited the release of glutamate from mossy fibers that project to CA3 pyramidal neurons. The presynaptic effect of NL1 was abolished in the presence of a selective antagonist for mGluR2. Thus, our data suggest that the soluble extracellular domain of NL1 functionally interacts with mGluR2 and thereby decreases synaptic strength.

Schwann cells are activated by ATP released from neurons in an in vitro cellular model of Miller Fisher syndrome

The neuromuscular junction is exposed to different types of insult, including mechanical trauma, toxins and autoimmune antibodies and, accordingly, has retained through evolution a remarkable ability to regenerate. Regeneration is driven by multiple signals that are exchanged among the cellular components of the junction. These signals are largely unknown. Miller Fisher syndrome is a variant of Guillain-Barré syndrome caused by autoimmune antibodies specific for epitopes of peripheral axon terminals. Using an animal model of Miller Fisher syndrome, we recently reported that a monoclonal anti-polysialoganglioside GQ1b antibody plus complement damages nerve terminals with production of mitochondrial hydrogen peroxide, which activates Schwann cells. Several additional signaling molecules are likely to be involved in the activation of the regeneration program in these cells. Using an in vitro cellular model consisting of co-cultured primary neurons and Schwann cells, we found that ATP is released by neurons injured by the anti-GQ1b antibody plus complement. Neuron-derived ATP acts as an alarm messenger for Schwann cells, where it induces the activation of intracellular pathways, including calcium signaling, cAMP and CREB, which, in turn, produce signals that promote nerve regeneration. These results contribute to defining the cross-talk taking place at the neuromuscular junction when it is attacked by anti-gangliosides autoantibodies plus complement, which is crucial for nerve regeneration and is also likely to be important in other peripheral neuropathies.

Regulation of axon arborization pattern in the developing chick ciliary ganglion: Possible involvement of caspase 3

During a certain critical period in the development of the central and peripheral nervous systems, axonal branches and synapses are massively reorganized to form mature connections. In this process, neurons search their appropriate targets, expanding and/or retracting their axons. Recent work suggested that the caspase superfamily regulates the axon morphology. Here, we tested the hypothesis that caspase 3, which is one of the major executioners in apoptotic cell death, is involved in regulating the axon arborization. The embryonic chicken ciliary ganglion was used as a model system of synapse reorganization. A dominant negative mutant of caspase-3 precursor (C3DN) was made and overexpressed in presynaptic neurons in the midbrain to interfere with the intrinsic caspase-3 activity using an in ovo electroporation method. The axon arborization pattern was 3-dimensionally and quantitatively analyzed in the ciliary ganglion. The overexpression of C3DN significantly reduced the number of branching points, the branch order and the complexity index, whereas it significantly elongated the terminal branches at E6. It also increased the internodal distance significantly at E8. But, these effects were negligible at E10 or later. During E6-8, there appeared to be a dynamic balance in the axon arborization pattern between the "targeting" mode, which is accompanied by elongation of terminal branches and the pruning of collateral branches, and the "pathfinding" mode, which is accompanied by the retraction of terminal branches and the sprouting of new collateral branches. The local and transient activation of caspase 3 could direct the balance towards the pathfinding mode.