The effect of agrin and laminin on acetylcholine receptor dynamics in vitro

Engineering skeletal muscle: Building complexity to achieve functionality

Volumetric muscle loss (VML) VML is defined as the loss of a critical mass of skeletal muscle that overwhelms the muscle's natural healing mechanisms, leaving patients with permanent functional deficits and deformity. The treatment of these defects is complex, as skeletal muscle is a composite structure that relies closely on the action of supporting tissues such as tendons, vasculature, nerves, and bone. The gold standard of treatment for VML injuries, an autologous muscle flap transfer, suffers from many shortcomings but nevertheless remains the best clinically available avenue to restore function. This review will consider the use of composite tissue engineered constructs, with multiple components that act together to replicate the function of an intact muscle, as an alternative to autologous muscle flaps. We will discuss recent advances in the field of tissue engineering that enable skeletal muscle constructs to more closely reproduce the functionality of an autologous muscle flap by incorporating vasculature, promoting innervation, and reconstructing the muscle-tendon boundary. Additionally, our understanding of the cellular composition of skeletal muscle has evolved to recognize the importance of a diverse variety of cell types in muscle regeneration, including fibro/adipogenic progenitors and immune cells like macrophages and regulatory T cells. We will address recent advances in our understanding of how these cell types interact with, and can be incorporated into, implanted tissue engineered constructs.

HDAC6 regulates microtubule stability and clustering of AChRs at neuromuscular junctions

Microtubules (MTs) are known to be post-translationally modified at the neuromuscular junction (NMJ), hence increasing their stability. To date however, the function(s) of the dynamic MT network and its relative stability in the formation and maintenance of NMJs remain poorly described. Stabilization of the MT is dependent in part on its acetylation status, and HDAC6 is capable of reversing this post-translational modification. Here, we report that HDAC6 preferentially accumulates at NMJs and that it contributes to the organization and the stability of NMJs. Indeed, pharmacological inhibition of HDAC6 protects against MT disorganization and reduces the size of acetylcholine receptor (AChR) clusters. Moreover, the endogenous HDAC6 inhibitor paxillin interacts with HDAC6 in skeletal muscle cells, colocalizes with AChR aggregates, and regulates the formation of AChR. Our findings indicate that the focal insertion of AChRs into the postsynaptic membrane is regulated by stable MTs and highlight how an MT/HDAC6/paxillin axis participates in the regulation of AChR insertion and removal to control the structure of NMJs.

The Metabolic Stability of the Nicotinic Acetylcholine Receptor at the Neuromuscular Junction

The clustering and maintenance of nicotinic acetylcholine receptors (AChRs) at high density in the postsynaptic membrane is a hallmark of the mammalian neuromuscular junction (NMJ). The regulation of receptor density/turnover rate at synapses is one of the main thrusts of neurobiology because it plays an important role in synaptic development and synaptic plasticity. The state-of-the-art imaging revealed that AChRs are highly dynamic despite the overall structural stability of the NMJ over the lifetime of the animal. This review highlights the work on the metabolic stability of AChRs at developing and mature NMJs and discusses the role of synaptic activity and the regulatory signaling pathways involved in the dynamics of AChRs.

Engineering 3D skeletal muscle primed for neuromuscular regeneration following volumetric muscle loss

Volumetric muscle loss (VML) overwhelms the native regenerative capabilities of skeletal muscle and has few effective treatments to regain lost muscle mass and function. Tissue engineered muscle constructs designed to promote neuromuscular regeneration are a promising therapeutic avenue. To date, there has been no engineered muscle construct for VML treatment that has incorporated a pharmacologic agent to promote neuromuscular regeneration. Here, we have modified electrospun fibrin microfiber bundles, which have demonstrated muscle regenerative potential, with the heparan sulfate proteoglycan, agrin, to stimulate innervation post-VML. Myoblasts cultured on microfiber bundles with either soluble or chemically tethered agrin demonstrated statistically significant increased clustering of acetylcholine receptors (AChRs) with soluble agrin displaying AChR clusters throughout the myofiber bundles, and tethered agrin displaying AChR clusters only at 10 μm from the substrate surface. Following implantation into murine VML defects for 4 weeks, constructs pre-treated with soluble or tethered agrin resulted in statistically significant increased neuromuscular junctions, regenerating myofibers, vascular infiltration, neural infiltration, and nuclear yes-associated protein (YAP) expression within the defect site compared to the control without agrin. The agrin-tethered microfiber bundles provided sustained agrin signaling within the regenerating site during the 4-week post-implantation periods and further augmented the density of regenerating myofibers in regenerated tissue with statistical significance compared to constructs with soluble agrin. These data demonstrate the neuromuscular regenerative potential of engineered muscle constructs pre-treated to induce AChR clustering with locally delivered agrin at the site of VML regeneration.

Roles of tyrosine kinases and phosphatases in the formation and dispersal of acetylcholine receptor clusters

The formation of acetylcholine receptor (AChR) clusters at the postsynaptic muscle membrane in response to motor innervation is a key event in the development of the neuromuscular junction. The synaptic AChR clustering process is initiated by motor axon-released agrin, which activates a tyrosine kinase-based signaling pathway to cause AChR aggregation. In cultured muscle cells, AChR clustering is elicited by diverse nonneural signals, and this process is also mediated by tyrosine kinases. Conversely, the formation of new AChR clusters induced by innervation or nonneural stimuli is unfailingly associated with the dispersal of pre-existing AChR clusters, and this process is mediated by tyrosine phosphatases. In this review, we address how local kinase activation leads to global phosphatase action in muscle. More specifically, we discuss the roles of Src kinase and the SH2 domain-containing tyrosine phosphatase Shp-2 in establishing a regenerative mechanism to propagate the AChR cluster dispersing signal extrasynaptically and in defining the boundary of cluster formation subsynaptically.

Vascularized and Innervated Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a devastating loss of muscle tissue that overwhelms the native regenerative properties of skeletal muscle and results in lifelong functional deficits. There are currently no treatments for VML that fully recover the lost muscle tissue and function. Tissue engineering presents a promising solution for VML treatment and significant research has been performed using tissue engineered muscle constructs in preclinical models of VML with a broad range of defect locations and sizes, tissue engineered construct characteristics, and outcome measures. Due to the complex vascular and neural anatomy within skeletal muscle, regeneration of functional vasculature and nerves is vital for muscle recovery following VML injuries. This review aims to summarize the current state of the field of skeletal muscle tissue engineering using 3D constructs for VML treatment with a focus on studies that have promoted vascular and neural regeneration within the muscle tissue post-VML.

Laminin is instructive and calmodulin dependent kinase II is non-permissive for the formation of complex aggregates of acetylcholine receptors on myotubes in culture

Previous work has shown that myotubes cultured on laminin-coated substrates form complex aggregates of synaptic proteins that are similar in shape and composition to neuromuscular junctions (NMJs). Here we show that laminin instructs the location of complex aggregates which form only on the lower surface when laminin is coated onto culture dishes but over the entire cell when laminin is added in solution. Silencing of myotubes by agents that block electrical activity (tetrodotoxin, verapamil) or by inhibitors of calmodulin dependent kinase (CaMKII) render the myotube permissive for the formation of complex aggregates. Treatment with laminin alone will facilitate the formation of complex aggregates hours later when myotubes are made permissive by inhibiting CaMKII. The AChR agonist carbachol disperses pre formed aggregates suggesting that non-permissiveness may involve active dispersal of AChRs. The permissive period requires ongoing protein synthesis. The latter may reflect a requirement for rapsyn, which turns over rapidly, and is necessary for aggregation. Consistent with this geldanamycin, an agent that increases rapsyn turnover disrupts complex aggregates. Agrin is well known to induce small clusters of AChRs but does not induce complex aggregates even though aggregate formation requires MuSK, a receptor tyrosine kinase activated by agrin. Dystroglycan (DG) is the major laminin receptor mediating complex aggregate formation with some contribution from β1 integrins. In addition, there is a pool of CaMKII associated with DG. We discuss how these permissive and instructive mechanisms bear on NMJ formation in vivo.

Dynein disruption perturbs post-synaptic components and contributes to impaired MuSK clustering at the NMJ: implication in ALS

The neuromuscular junction (NMJ) allows the transformation of a neuronal message into a mechanical force by muscle contraction and is the target of several neuromuscular disorders. While the neuronal side is under extensive research, the muscle appeared recently to have a growing role in the formation and integrity of the neuromuscular junction. We used an in vitro model of mature myofibers to study the role of dynein on major postsynaptic proteins. We found that dynein affects the expression and the clustering of acetylcholine receptors (AChRs), muscle specific tyrosine kinase (MuSK) and Rapsyn. We also show that myofibers with dynein impairment or from an amyotrophic lateral sclerosis (ALS) model (SOD1^G93A) show similar defects in myofiber formation and agrin-induced AChR clustering suggesting a role for dynein impairment in ALS progression. Finally, we found that dynein can affect MuSK traffic through the endosomal pathway. Collectively, our studies show that defects in dynein can lead to impairment of muscle NMJ components’ expression and clustering. We propose that NMJ defects could happen via defective MuSK traffic and that this could be one of the pathological features involved in neurodegeneration such as ALS.

The formation of complex acetylcholine receptor clusters requires MuSK kinase activity and structural information from the MuSK extracellular domain

Efficient synaptic transmission at the neuromuscular junction (NMJ) requires the topological maturation of the postsynaptic apparatus from an oval acetylcholine receptor (AChR)-rich plaque into a complex pretzel-shaped array of branches. However, compared to NMJ formation very little is known about the mechanisms that regulate NMJ maturation. Recently the process of in vivo transformation from plaque into pretzel has been reproduced in vitro by culturing myotubes aneurally on laminin-coated substrate. It was proposed that the formation of complex AChR clusters is regulated by a MuSK-dependent muscle intrinsic program. To elucidate the structure–function role of MuSK in the aneural maturation of AChR pretzels, we used muscle cell lines expressing MuSK mutant and chimeric proteins. Here we report, that besides its role during agrin-induced AChR clustering, MuSK kinase activity is also necessary for substrate-dependent cluster formation. Constitutive-active MuSK induces larger AChR clusters, a faster cluster maturation on laminin and increases the anchorage of AChRs to the cytoskeleton compared to MuSK wild-type. In addition, we find that the juxtamembrane region of MuSK, which has previously been shown to regulate agrin-induced AChR clustering, is unable to induce complex AChR clusters on laminin substrate. Most interestingly, MuSK kinase activity is not sufficient for laminin-dependent AChR cluster formation since the MuSK ectodomain is also required suggesting a so far undiscovered instructive role for the extracellular domain of MuSK.

Neuregulin/ErbB regulate neuromuscular junction development by phosphorylation of α-dystrobrevin

Neuregulin/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Neuregulin (NRG)/ErbB signaling is involved in numerous developmental processes in the nervous system, including synapse formation and function in the central nervous system. Although intensively investigated, its role at the neuromuscular synapse has remained elusive. Here, we demonstrate that loss of neuromuscular NRG/ErbB signaling destabilized anchoring of acetylcholine receptors (AChRs) in the postsynaptic muscle membrane and that this effect was caused by dephosphorylation of α-dystrobrevin1, a component of the postsynaptic scaffold. Specifically, in mice in which NRG signaling to muscle was genetically or pharmacologically abolished, postsynaptic AChRs moved rapidly from the synaptic to the perisynaptic membrane, and the subsynaptic scaffold that anchors the AChRs was impaired. These defects combined compromised synaptic transmission. We further show that blockade of NRG/ErbB signaling abolished tyrosine phosphorylation of α-dystrobrevin1, which reduced the stability of receptors in agrin-induced AChR clusters in cultured myotubes. Our data indicate that NRG/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Formation of complex AChR aggregates in vitro requires alpha-dystrobrevin

Efficient function at the neuromuscular junction requires high-density aggregates of acetylcholine receptors (AChRs) to be precisely aligned with the motor nerve terminal. A collaborative effort between the motor neuron and muscle intrinsic factors drives the formation and maintenance of these AChR aggregates. alpha-Dystrobrevin (alpha DB), a cytoplasmic protein found at the postsynaptic membrane, has been implicated in the regulation of AChR aggregate density and patterning. To investigate the contribution of alpha DB to the muscle intrinsic program regulating AChR aggregate development, we analyzed the formation of complex, pretzel-like AChR aggregates on primary muscle cell cultures derived from alpha DB knockout (alpha DB-KO) mice in the absence of nerve or agrin. In myotubes lacking alpha DB, complex AChR aggregates failed to form, whereas aggregates formed readily in wildtype myotubes. Five major isoforms of alpha DB are expressed in skeletal muscle: alpha DB1, alpha DB1(-), alpha DB2, alpha DB2(-), and alpha DB3. Expression of alpha DB1 or alpha DB1(-) in alpha DB-KO myotubes restored formation of complex AChR aggregates similar to those in wildtype myotubes. In contrast, individual expression of alpha DB2, alpha DB2(-), alpha DB3, or an alpha DB1 phosphorylation mutant resulted in the formation of few, if any, complex AChR aggregates. Collectively, these data suggest that alpha DB is a significant component of the muscle intrinsic program that mediates the formation of complex AChR aggregates and that alpha DB's tyrosine phosphorylation sites are of particular functional importance to this program. Although the muscle intrinsic program appears to influence synaptogenesis, the formation of complex mature AChR aggregates in alpha DB-KO mice (with the motor neuron present) suggests the motor neuron, not the muscle intrinsic program, is the major stimulus driving the maturation of AChRs from plaque to pretzel in vivo.

Receptor-associated proteins and synaptic plasticity

Changes in synaptic strength are important for synaptic development and synaptic plasticity. Most directly responsible for these synaptic changes are alterations in synaptic receptor number and density. Although alterations in receptor density mediated by the insertion, lateral mobility, removal, and recycling of receptors have been extensively studied, the dynamics and regulators of intracellular scaffolding proteins have only recently begun to be illuminated. In particular, a closer look at the receptor-associated proteins, which bind to receptors and are necessary for their synaptic localization and clustering, has revealed broader functions than previously thought and some rather unexpected thematic similarities. More than just "placeholders" or members of a passive protein "scaffold," receptor-associated proteins in every synapse studied have been shown to provide a number of signaling roles. In addition, the most recent state-of-the-art imaging has revealed that receptor-associated proteins are highly dynamic and are involved in regulating synaptic receptor density. Together, these results challenge the view that receptor-associated proteins are members of a static and stable scaffold and argue that their dynamic mobility may be essential for regulating activity-dependent changes in synaptic strength.

Rapid and modifiable neurotransmitter receptor dynamics at a neuronal synapse in vivo

Synaptic plasticity underlies the adaptability of the mammalian brain, but has been difficult to study in living animals. Here we imaged the synapses between pre- and postganglionic neurons in the mouse submandibular ganglion in vivo, focusing on the mechanisms that maintain and regulate neurotransmitter receptor density at postsynaptic sites. Normally, synaptic receptor densities were maintained by rapid exchange of receptors with nonsynaptic regions (over minutes) and by continual turnover of cell surface receptors (over hours). However, after ganglion cell axons were crushed, synaptic receptors showed greater lateral mobility and there was a precipitous decline in insertion. These changes led to near-complete loss of synaptic receptors and synaptic depression. Disappearance of postsynaptic spines and presynaptic terminals followed this acute synaptic depression. Therefore, neurotransmitter receptor dynamism associated with rapid changes in synaptic efficacy precedes long-lasting structural changes in synaptic connectivity.

Localized acetylcholine receptor clustering dynamics in response to microfluidic focal stimulation with agrin

Agrin is a proteoglycan secreted by the motor neuron's growing axon terminal upon contact with the muscle during embryonic development. It was long thought that agrin's role was to trigger the clustering of acetylcholine receptors (AChRs) to nascent synapse sites. However, agrin-predating, protosynaptic AChR clusters are present well before innervation in the embryo and in myotube cultures, yet no role has been conclusively ascribed to agrin. We used a microfluidic device to focally deliver agrin to protosynaptic AChR clusters in micropatterned myotube cultures. The distribution of AChRs labeled with fluorescent bungarotoxin was imaged at various time points over >24 h. We find that a 4-h focal application of agrin (100 nM) preferentially reduces AChR loss at agrin-exposed clusters by 17% relative to the agrin-deprived clusters on the same myotube. In addition, the focal application increases the addition of AChRs preferentially at the clusters by 10% relative to the agrin-exposed, noncluster areas. Taken together, these findings suggest that a focal agrin stimulus can play a key stabilizing role in the aggregation of AChRs at the early stages of synapse formation. This methodology is generally applicable to various developmental processes and cell types, including neurons and stem cells.

Running to stand still: ionotropic receptor dynamics at central and peripheral synapses

For synapses to form and function, neurotransmitter receptors must be recruited to a location on the postsynaptic cell in direct apposition to presynaptic neurotransmitter release. However, once receptors are inserted into the postsynaptic membrane, they are not fixed in place but are continually exchanged between synaptic and extrasynaptic regions, and they cycle between the surface and intracellular compartments. This article highlights and compares the current knowledge about the dynamics of acetylcholine receptors at the vertebrate peripheral neuromuscular junction and AMPA, N-methyl-D-aspartate, and gamma-aminobutyric acid receptors in central synapses.

The dynamics of the rapsyn scaffolding protein at individual acetylcholine receptor clusters

Rapsyn, a cytoplasmic receptor-associated protein, is required for the clustering of acetylcholine receptors (AChRs). Although AChR dynamics have been extensively studied, little is known about the dynamics of rapsyn. Here, we used a rapsyn-green fluorescent protein (GFP) fusion protein and quantitative fluorescent imaging to study the dynamics of rapsyn in transfected C2C12 myotubes. First, we found that rapsyn-GFP expression at clusters did not alter AChR aggregation, function, or turnover. Quantification of rapsyn immunofluorescence indicated that the expression of rapsyn-GFP proteins at clusters does not increase the overall rapsyn density compared with untransfected myotube clusters. Using time lapse imaging and fluorescence recovery after photobleaching, we demonstrated that the recovery of rapsyn-GFP fluorescence at clusters was very fast, with a halftime of about approximately 1.5 h (approximately 3 times faster than AChRs). Inhibition of protein kinase C significantly altered receptor insertion, but it had no effect on rapsyn insertion. When cells were treated with the broad spectrum kinase inhibitor staurosporine, receptor insertion was decreased even further. However, inhibition of protein kinase A had no effect on insertion of either rapsyn or receptors. Finally, when cells were treated with neural agrin, rapsyn and AChRs were both directed away from preexisting clusters and accumulated together in new small clusters. These results demonstrate the remarkable dynamism of rapsyn, which may underlie the stability and maintenance of the postsynaptic scaffold and suggest that the insertion of different postsynaptic proteins may be operating independently.

Phosphorylation of the nicotinic acetylcholine receptor in myotube-cholinergic neuron cocultures

Acetylcholine receptor (AChR) stability in the postsynaptic membrane is affected by serine kinases. AChR are phosphorylated by protein kinase C (PKC) and PKA, and we have shown that activation of PKA and PKC have opposite effects on AChR stability and that this may play some role in the selective, activity-dependent synapse loss that occurs during development of the neuromuscular junction. Myotube cultures with and without added spinal motor neurons were probed with immunoaffinity-purified antibodies prepared against phosphorylated peptides with amino acid sequences from different AChR subunits. Different treatments activating PKC (phorbol 12-myristate 13-acetate; PMA) or PKA (dibutyryl cyclic adenosine monophosphate; cAMP) or blocking electrical activity (tetrodotoxin; TTX) of the cocultures were chosen because of their known effects, direct or indirect, on receptor stability. We asked whether the phospho-specific antibody staining in conjunction with alpha-bungarotoxin (BTX) identification of AChR aggregates could provide a direct demonstration of changes in receptor phosphorylation produced by the treatments. We found that PMA treatment did increase phosphorylation of the delta subunit and cAMP increased phosphorylation of the epsilon subunit relative to total BTX labeling in muscle-nerve cocultures, but not in muscle-only cultures. Blockade of electrical activity with TTX increased the incidence of aggregates that showed no phospho-epsilon staining. Myotube cultures grown in the absence of neurons did not show the responses of myotubes in cocultures. The results show that manipulations that alter receptor stability also produce changes in receptor phosphorylation. We suggest that phosphorylation may be a mechanism mediating the changes in receptor stability.

Variability, compensation and homeostasis in neuron and network function

Neurons in most animals live a very long time relative to the half-lives of all of the proteins that govern excitability and synaptic transmission. Consequently, homeostatic mechanisms are necessary to ensure stable neuronal and network function over an animal's lifetime. To understand how these homeostatic mechanisms might function, it is crucial to understand how tightly regulated synaptic and intrinsic properties must be for adequate network performance, and the extent to which compensatory mechanisms allow for multiple solutions to the production of similar behaviour. Here, we use examples from theoretical and experimental studies of invertebrates and vertebrates to explore several issues relevant to understanding the precision of tuning of synaptic and intrinsic currents for the operation of functional neuronal circuits.