Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease, resulting in loss of motor neurons, denervation of target muscles, muscle atrophy and paralysis. Understanding ALS pathogenesis may require a fuller understanding of the bidirectional signaling between motor neurons and skeletal muscle fibers at neuromuscular synapses. Here we show that a key regulator of this signaling is miR-206, a skeletal muscle-specific microRNA that is dramatically induced in a mouse model of ALS. Mice genetically deficient in miR-206 form normal neuromuscular synapses during development, but deficiency of miR-206 in the ALS mouse model accelerates disease progression. MiR-206 is required for efficient regeneration of neuromuscular synapses after acute nerve injury, which likely accounts for its salutary effects in ALS. MiR-206 mediates these effects, at least in part, through histone deacetylase 4 and fibroblast growth factor signaling pathways. Thus, miR-206 slows ALS progression by sensing motor neuron injury and promoting compensatory regeneration of neuromuscular synapses.Amyotrophic lateral sclerosis (ALS) is the most common adult motor neuron disease (1). Symptoms of the disease include atrophy and paralysis of lower limb and respiratory muscles due to degeneration of motor neurons. There is currently no effective treatment. Thus, identification of the signaling pathways and cellular mediators of ALS remains a major challenge in the search for novel therapeutics (2).In light of recent studies implicating microRNAs (miRNAs) in stress responses in muscle (3), we investigated whether disease progression in a mouse model of ALS was accompanied
The cellular basis of age-related behavioral decline remains obscure but alterations in synapses are likely candidates. Accordingly, the beneficial effects on neural function of caloric restriction and exercise, which are among the most effective anti-aging treatments known, might also be mediated by synapses. As a starting point in testing these ideas, we studied the skeletal neuromuscular junction (NMJ), a large, accessible peripheral synapse. Comparison of NMJs in young adult and aged mice revealed a variety of age-related structural alterations, including axonal swellings, sprouting, synaptic detachment, partial or complete withdrawal of axons from some postsynaptic sites, and fragmentation of the postsynaptic specialization. Alterations were significant by 18 mo of age and severe by 24 mo. A life-long calorie-restricted diet significantly decreased the incidence of pre-and postsynaptic abnormalities in 24-mo-old mice and attenuated age-related loss of motor neurons and turnover of muscle fibers. One month of exercise (wheel running) in 22-mo-old mice also reduced age-related synaptic changes but had no effect on motor neuron number or muscle fiber turnover. Time-lapse imaging in vivo revealed that exercise partially reversed synaptic alterations that had already occurred. These results demonstrate a critical effect of aging on synaptic structure and provide evidence that interventions capable of extending health span and lifespan can partially reverse these age-related synaptic changes.aging | neuromuscular junction | muscle | motor neuron | sarcopenia A ging is accompanied by numerous functional alterations of both the central and peripheral nervous systems (1). Until recently, it was thought that many of these age-associated changes were secondary to neuronal degeneration. Recent studies show, however, that little neuronal death occurs in most areas of the aging nervous system (2). Although many possible explanations exist (3-6), a particularly attractive hypothesis is that some agerelated alterations in mental function result from synaptic alterations. Supporting this idea, alterations in synapse number, spine densities, and synaptic plasticity have been documented in the brains of aging humans and experimental animals (1,7,8).If synaptic changes underlie age-related defects in neural function, one might look to synapses as targets for treatments that minimize the decline. Two lifestyle regimens that have been consistently demonstrated to extend lifespan and mitigate agerelated changes in neural function are caloric restriction and exercise (9). In that the cellular bases of age-related changes in mental activity are obscure, it is not surprising that the means by which exercise and caloric restriction attenuate these changes are also unknown. For both regimens, however, synaptic alterations have figured prominently among proposed mechanisms (9, 10).An obstacle to progress in this area is the complexity and diversity of synaptic neuropil in the brain, which impedes detailed analysis of aging central syna...
A fundamental question in developmental biology is how a limited number of growth factors and their cognate receptors coordinate the formation of tissues and organs endowed with enormous morphological complexity. We report that the related neurotrophins NGF and NT-3, acting through a common receptor, TrkA, are required for sequential stages of sympathetic axon growth and, thus, innervation of target fields. Yet, while NGF supports TrkA internalization and retrograde signaling from distal axons to cell bodies to promote neuronal survival, NT-3 cannot. Interestingly, final target-derived NGF promotes expression of the p75 neurotrophin receptor, in turn causing a reduction in the sensitivity of axons to intermediate target-derived NT-3. We propose that a hierarchical neurotrophin signaling cascade coordinates sequential stages of sympathetic axon growth, innervation of targets, and survival in a manner dependent on the differential control of TrkA internalization, trafficking, and retrograde axonal signaling.
Normal aging and neurodegenerative diseases both lead to structural and functional alterations in synapses. Comparison of synapses that are generally similar but respond differently to insults could provide the basis for discovering mechanisms that underlie susceptibility or resistance to damage. Here, we analyzed skeletal neuromuscular junctions (NMJs) in 16 mouse muscles to seek such differences. We find that muscles respond in one of three ways to aging. In some, including most limb and trunk muscles, age-related alterations to NMJs are progressive and extensive during the second postnatal year. NMJs in other muscles, such as extraocular muscles, are strikingly resistant to change. A third set of muscles, including several muscles of facial expression and the external anal sphinter, succumb to aging but not until the third postnatal year. We asked whether susceptible and resistant muscles differed in rostrocaudal or proximodistal position, source of innervation, motor unit size, or fiber type composition. Of these factors, muscle innervation by brainstem motor neurons correlated best with resistance to age-related decline. Finally, we compared synaptic alterations in normally aging muscles to those in a mouse model of amyotrophic lateral sclerosis (ALS). Patterns of resistance and susceptibility were strikingly correlated in the two conditions. Moreover, damage to NMJs in aged muscles correlated with altered expression and distribution of CRMP4a and TDP-43, which are both altered in motor neurons affected by ALS. Together, these results reveal novel structural, regional and molecular parallels between aging and ALS.
Abbreviations used in this paper: AChR, acetylcholine receptor; Bcam, basal cell adhesion molecule/Lutheran blood group antigen; BTX, ␣ -bungarotoxin; LG, laminin globular; MuSK, muscle-specifi c kinase; NMJ, neuromuscular junction.The online version of this article contains supplemental material.
Acentral tenet of nerve growth factor (NGF) action that is poorly understood is its ability to mediate cytoplasmic signaling, through its receptor TrkA, that is initiated at the nerve terminal and conveyed to the soma. We identified an NGF-induced protein that we termed Pincher (pinocytic chaperone) that mediates endocytosis and trafficking of NGF and its receptor TrkA. In PC12 cells, overexpression of Pincher dramatically stimulated NGF-induced endocytosis of TrkA, unexpectedly at sites of clathrin-independent macropinocytosis within cell surface ruffles. Subsequently, a system of Pincher-containing tubules mediated the delivery of NGF/TrkA-containing vesicles to cytoplasmic accumulations. These vesicles selectively and persistently mediated TrkA-erk5 mitogen-activated protein kinase signaling. A dominant inhibitory mutant form of Pincher inhibited the NGF-induced endocytosis of TrkA, and selectively blocked TrkA-mediated cytoplasmic signaling of erk5, but not erk1/2, kinases. Our results indicate that Pincher mediates pinocytic endocytosis of functionally specialized NGF/TrkA endosomes with persistent signaling potential.
Retrograde signaling by neurotrophins is crucial for regulating neuronal phenotype and survival. The mechanism responsible for retrograde signaling has been elusive, because the molecular entities that propagate Trk receptor tyrosine kinase signals from the nerve terminal to the soma have not been defined. Here, we show that the membrane trafficking protein Pincher defines the primary pathway responsible for neurotrophin retrograde signaling in neurons. By both immunofluorescence confocal and immunoelectron microscopy, we find that Pincher mediates the formation of newly identified clathrin-independent macroendosomes for Trk receptors in soma, axons, and dendrites. Trk macroendosomes are derived from plasma membrane ruffles and subsequently processed to multivesicular bodies. Pincher similarly mediates macroendocytosis for NGF (TrkA) and BDNF (TrkB) in both peripheral (sympathetic) and central (hippocampal) neurons. A unique feature of Pincher-Trk endosomes is refractoriness to lysosomal degradation, which ensures persistent signaling through a critical effector of retrograde survival signaling, Erk5 (extracellular signal-regulated kinase 5). Using sympathetic neurons grown in chamber cultures, we find that block of Pincher function, which prevents Trk macroendosome formation, eliminates retrogradely signaled neuronal survival. Pincher is the first distinguishing molecular component of a novel mechanistic pathway for endosomal signaling in neurons.
As synapses form and mature the synaptic partners produce organizing molecules that regulate each other’s differentiation and ensure precise apposition of pre- and post-synaptic specializations. At the skeletal neuromuscular junction (NMJ), these molecules include agrin, a nerve-derived organizer of postsynaptic differentiation, and synaptic laminins, muscle-derived organizers of presynaptic differentiation. Both become concentrated in the synaptic cleft as the NMJ develops and are retained in adulthood. Here, we used mutant mice to ask whether these organizers are also required for synaptic maintenance. Deletion of agrin from a subset of adult motor neurons resulted in the loss of acetylcholine receptors and other components of the postsynaptic apparatus and synaptic cleft. Nerve terminals also atrophied and eventually withdrew from muscle fibers. On the other hand, mice lacking the presynaptic organizer laminin-α4 retained most of the synaptic cleft components but exhibited synaptic alterations reminiscent of those observed in aged animals. Although we detected no marked decrease in laminin or agrin levels at aged NMJs, we observed alterations in the distribution and organization of these synaptic cleft components suggesting that such changes could contribute to age-related synaptic disassembly. Together, these results demonstrate that pre- and post-synaptic organizers actively function to maintain the structure and function of adult NMJs.
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