Synapse formation molecules in muscle and autonomic ganglia: the dual constraint hypothesis

On the mechanism of action of muscle fibre activity in synapse competition and elimination at the mammalian neuromuscular junction

Activity-dependent competition plays a crucial role in the refinement of synaptic connections in the peripheral and central nervous system. The reduction in number of axons innervating each neuromuscular junction during development, i.e. synapse elimination, appears to be one such competitive activity-driven event. Recently, we showed that asynchronous firing of competing presynaptic terminals is a key player in synapse elimination. Although some previous studies suggested that activity of the postsynaptic cell may be an intermediary in the disposal of redundant presynaptic inputs, the mechanism involved remains unknown. In the present study, in order to assess the role of evoked muscle activity in this process, we inhibited the generation of postsynaptic action potentials in muscle fibers in vivo, through the overexpression of inwardly rectifying Kir2.1 and Kir2.2 channels, via electroporation of the soleus muscle in the mouse hindlimb. Electrophysiological and morphological data show that overexpression of potassium channels in the endplate region of neonatal muscle fibres induces membrane hyperpolarization and an increase in conductance, inhibition of the action potential mechanism and prolonged persistence of polyneuronal innervation. These changes are not seen in muscle fibres with overexpression of a non-conducting Kir2.1 mutant. Our results are compatible with the interpretation that the block of action potential generation, even in single endplates, can inhibit synapse elimination through local signalling.

[Morphological study of CNS lesions and the consequences on rat neuromuscular junction and peripheral nerve using confocal laser scanning microscopy and Koelle's technique]

STATE OF THE ART

In humans, it is currently believed that peripheral nerves remain intact after central nervous system (CNS) injuries. This should lead us to observe a lack of amyotrophy in the peripheral projection areas of CNS damage. Nevertheless, the appearance of amyotrophy, described as underuse amyotrophy, is common in victims of CNS injury. Its pathophysiology remains poorly understood and is currently being debated. Amyotrophy could result directly from the structural deterioration of a nervous fiber in the muscular area corresponding to the CNS injury caused by neuromuscular junction (NMJ) changes.

AIMS OF THIS STUDY

The aims of this study were to assess the repercussions of a CNS injury on the NMJ and peripheral nerve complex and to evaluate the involvement of peripheral nerves and NMJs in plasticity.

METHODOLOGY

Peripheral nerve and muscle biopsies were collected from a group of 35 female Wistar rats that had previously undergone a thoracic spinal cord hemisection (15 rats at the T2 level (group 1), 15 rats at the T6 level (group 2), and 5 matched rats used as controls). We studied the localization and expression of the NMJ molecular components in muscle specimens by immunohistochemistry using confocal microscopy. We also searched for signs of nerve and muscle degeneration using light and electron microscopy.

RESULTS

We observed nonpathologic NMJs coexisting with completely denervated and partially reinnervated NMJs. We also found characteristics of embryonic behavior in rat axons secondary to axonal caliber distortions. Some authors associate this decrease in axonal activity with physiological denervation.

CONCLUSION

This project was designed to improve the understanding of the mechanisms involved in the interactions between the first and second motoneurons after different types of CNS injuries, with variable functional repercussions. Our results strongly suggest that CNS injuries lead to both morphological and functional repercussions at the NMJ and the peripheral nerve.

Dual constraints on synapse formation and regression in schizophrenia: neuregulin, neuroligin, dysbindin, DISC1, MuSK and agrin

During adolescence there is a loss of approximately 30% of the synapses formed in the cortex during childhood. Comprehensive studies of the visual cortex show that this loss of synapses does not occur as a consequence of less appropriate projections being eliminated in favour of more appropriate ones. Rather it seems that synapses with low efficacy for transmission are eliminated in favour of those with higher efficacy. The loss of low-efficacy synapses is known, on theoretical grounds, to enhance the function of neural networks, but large synapse losses lead to failure of network function. In the dorsolateral prefrontal cortex (DLPC) of those suffering from schizophrenia the number of synapses is relatively very low, approximately 60% lower than that observed in normal childhood. It is not known if this is due to an additional loss over that during normal adolescence or whether it results from a failure to form a normal complement of synapses during childhood. The first study of synapse loss in the mammalian nervous system was made on the neuromuscular junction at Sydney University in 1974. Since then this junction has provided principal insights into the molecular basis of synapse formation and regression, so providing a paradigm for investigations of these phenomena in the DLPC. For example the molecules muscle-specific receptor tyrosine kinase (MuSK), agrin and neuregulin have been identified and their critical roles in the formation and maintenance of synapses elucidated. Loss of function of MuSK or agrin leads to failure of neuromuscular synapse formation as well as a loss of approximately 30% of excitatory synapses in the cortex. Similar synapse loss occurs on failure of neuregulin in vitro and of neuroligin in vivo. It is suggested that three important questions need to be answered: first, over what development period are the synapse numbers in DLPC of subjects with schizophrenia lower than normal; second, what are the relative importance of MuSK/agrin, neuregulin/ErB and neurexin/neuroligin in synapse formation and regression in the DLPC; and third, to what extent have these molecules gone awry in schizophrenia.

Neuronal cell death, nerve growth factor and neurotrophic models: 50 years on

Viktor Hamburger has just died at the age of 100. It is 50 years since he and Rita Levi-Montalcini laid the foundations for the study of naturally occurring cell death and of neurotrophic factors in the nervous system. In a period of less than 10 years, from 1949 to 1958, Hamburger and Levi-Montalcini made the following seminal discoveries: that neuron cell death occurs in dorsal root ganglia, sympathetic ganglia and the cervical column of motoneurons; that the predictions arising from this observation, namely that survival is dependent on the supply of a trophic factor, could be substantiated by studying the effects of a sarcoma on the proliferation of ganglionic processes both in vivo and in vitro; and that the proliferation of these processes could be used as an assay system to isolate the factor. This work provides a short review mostly of the early history of this subject in the context of the Hamburger/Levi-Montalcini paradigm. This acts as an introduction to a consideration of models that have been proposed to account for how the different sources of growth factors provide for the survival of neurons during development. It is suggested that what has been called the 'social-control' model provides the most parsimonious quantitative description of the contribution of trophic factors to neuronal survival, a concept for which we are in debt to Viktor Hamburger and Rita Levi-Montalcini.

The expression profiles of neurotrophins and their receptors in rat and chicken tissues during development

Neurotrophic factors are target-derived proteins that promote the survival and differentiation of the innervating neurons. Increasing evidence indicate the involvement of these factors and receptors during the formation and maturation of the neuromuscular junction. To gain further insight on the expression pattern of these factors and receptors in developing spinal cord and skeletal muscle during the critical stages of synapse formation, a systematic study was performed with chicken and rat tissues using Northern blot analysis. The expression of all the neurotrophins was detected in skeletal muscle early in development, coincidental with the appearance of their corresponding receptors in the spinal cord. Taken together, the similar regulatory patterns observed in both rat and chicken tissues suggest that the potential roles of neurotrophins at the neuromuscular synapse are conserved throughout evolution.

Evolutionary significance of myosin heavy chain heterogeneity in birds

This article reviews the complexity, expression, genetics, regulation, function, and evolution of the avian myosin heavy chain (MyHC). The majority of pertinent studies thus far published have focussed on domestic chicken and, to a much lesser extent, Japanese quail. Where possible, information available about wild species has also been incorporated into this review. While studies of additional species might modify current interpretations, existing data suggest that some fundamental properties of myosin proteins and genes in birds are unique among higher vertebrates. We compare the characteristics of myosins in birds to those of mammals, and discuss potential molecular mechanisms and evolutionary forces that may explain how avian MyHCs acquired these properties.

Developmental regulation of amyloid precursor protein at the neuromuscular junction in mouse skeletal muscle

Amyloid precursor protein (APP), associated with Alzheimer's disease plaques, is known to be present in synapses of the brain and in the adult neuromuscular junction (NMJ). In the present study we examined protein and gene expression of APP during the development of mouse skeletal muscle. Using immunocytochemical approaches, we found that APP is first detected in myotube cytoplasm at embryonic day 16 and becomes progressively concentrated at the NMJ beginning at birth until adulthood. The colocalization between APP and acetylcholine receptors at the NMJ is only partial at birth, but becomes complete upon reaching adulthood. We observed that all APP isoforms, including the Kunitz-containing (protease inhibitor or KPI) forms, are up-regulated from birth to postnatal day 5 and then decreased to reach the low levels observed in the adult. This suggests the involvement of APP during the events which lead to a mature mono-innervated synapse. A 92-kDa band, characteristic of a cleaved APP695 isoform and not due to a new muscle-specific alternative spliced form, was observed from postnatal day 15 following completion of polyneuronal synapse elimination. Taken together, these data suggest that skeletal muscle APP may well play a role in the differentiation of skeletal muscle and in the formation and maturation of NMJs.

Repression of myosin isoforms in developing and denervated skeletal muscle fibers originates near motor endplates

During development of chicken pectoralis muscle, a neonatal myosin heavy-chain isoform is supplanted progressively by an adult isoform. This expression is under neuronal control. In this study we test the hypothesis that developmental myosin transformations are initiated near the motor endplate of each muscle fiber, thereafter progressing toward the fiber ends. By using immunocytochemical methods, pectoralis muscle from chickens aged 1-115 days after hatching were labeled by antibody against neonatal isoform. Ellipse minor axis and mean optical density of labeled and/or unlabeled fiber profiles from each bird were measured by computer image analysis. Acetylcholinesterase (AChE) activity was demonstrated histochemically. Using serial cross sections, we show that smaller fiber profiles are the tapered ends of larger fiber profiles. The largest fiber profiles (central regions of the fibers) were the first to lose their neonatal myosin during development. Motor endplates were localized by AChE activity to the central regions of the fibers. The pectoralis of mature chickens was denervated for 3, 7, 15, or 21 days. After 2 weeks' denervation, neonatal myosin is first reexpressed in the fiber ends. Dev Dyn 2000;217:50-61.