Regulation of acetylcholine receptor clustering by ADF/cofilin-directed vesicular trafficking

Mechanism of acetylcholine receptor cluster formation induced by DC electric field

Background

The formation of acetylcholine receptor (AChR) cluster is a key event during the development of the neuromuscular junction. It is induced through the activation of muscle-specific kinase (MuSK) by the heparan-sulfate proteoglycan agrin released from the motor axon. On the other hand, DC electric field, a non-neuronal stimulus, is also highly effective in causing AChRs to cluster along the cathode-facing edge of muscle cells.

Methodology/Principal Findings

To understand its molecular mechanism, quantum dots (QDs) were used to follow the movement of AChRs as they became clustered under the influence of electric field. From analyses of trajectories of AChR movement in the membrane, it was concluded that diffuse receptors underwent Brownian motion until they were immobilized at sites of cluster formation. This supports the diffusion-mediated trapping model in explaining AChR clustering under the influence of this stimulus. Disrupting F-actin cytoskeleton assembly and interfering with rapsyn-AChR interaction suppressed this phenomenon, suggesting that these are integral components of the trapping mechanism induced by the electric field. Consistent with the idea that signaling pathways are activated by this stimulus, the localization of tyrosine-phosphorylated forms of AChR β-subunit and Src was observed at cathodal AChR clusters. Furthermore, disrupting MuSK activity through the expression of a kinase-dead form of this enzyme abolished electric field-induced AChR clustering.

Conclusions

These results suggest that DC electric field as a physical stimulus elicits molecular reactions in muscle cells in the form of cathodal MuSK activation in a ligand-free manner to trigger a signaling pathway that leads to cytoskeletal assembly and AChR clustering.

14-3-3 proteins and regulation of cytoskeleton

The proteins of the 14-3-3 family are universal adapters participating in multiple processes running in the cell. We describe the structure, isoform composition, and distribution of 14-3-3 proteins in different tissues. Different elements of 14-3-3 structure important for dimer formation and recognition of protein targets are analyzed in detail. Special attention is paid to analysis of posttranslational modifications playing important roles in regulation of 14-3-3 function. The data of the literature concerning participation of 14-3-3 in regulation of intercellular contacts and different elements of cytoskeleton formed by microfilaments are analyzed. We also describe participation of 14-3-3 in regulation of small G-proteins and protein kinases important for proper functioning of cytoskeleton. The data on the interaction of 14-3-3 with different components of microtubules are presented, and the probable role of 14-3-3 in developing of certain neurodegenerative diseases is discussed. The data of the literature concerning the role of 14-3-3 in formation and normal functioning of intermediate filaments are also reviewed. It is concluded that due to its adapter properties 14-3-3 plays an important role in cytoskeleton regulation. The cytoskeletal proteins that are abundant in the cell might compete with the other protein targets of 14-3-3 and therefore can indirectly regulate many intracellular processes that are dependent on 14-3-3.

Dopamine-depletion and increased α-synuclein load induce degeneration of cortical cholinergic fibers in mice

Cognitive dysfunction can be common among Parkinson's disease (PD) patients, and multiplication of the gene α-synuclein (αsyn) increases the risk of dementia. Here, we studied the role of dopamine-depletion and increased αsyn load and aggregation on cholinergic structures in vivo. Wild-type (WT) and mice with A30P αsyn overexpression were treated subacutely with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and the number of cholinergic cells in their nucleus basalis magnocellularis-substantia innominata (NBM-SI), their cortical fiber density and their expression of different genes 1day or 90 days after the last MPTP-injection were measured. Long-term dopamine depletion decreased the expression of choline acetyl transferase (ChAT) in the NBM-SI of WT mice, but no neuron loss was observed. In contrast, cortical cholinergic fiber density was decreased three months after MPTP-injection. Increased brain-derived neurotrophic factor expression could maintain cholinergic functions under these conditions. Expression of A30P αsyn in six-months-old transgenic mice resulted in decreased tyrosine receptor kinase B expression, and lower cortical cholinergic fiber density. Dopamine-depletion by MPTP induced cholinergic cell loss in the NBM-SI and increased cortical fiber loss. Our findings may explain why cholinergic cells are more vulnerable in PD, leading to an increased probability of dementia.

Nestin negatively regulates postsynaptic differentiation of the neuromuscular synapse

Positive and negative regulation of neurotransmitter receptor aggregation on the postsynaptic membrane is a critical event during synapse formation. Acetylcholine (ACh) and agrin are two opposing signals that regulate ACh receptor (AChR) clustering during neuromuscular junction (NMJ) development. ACh induces dispersion of AChR clusters that are not stabilized by agrin via a cyclin-dependent kinase 5 (Cdk5)-mediated mechanism, but regulation of Cdk5 activation is poorly understood. Here we show that the intermediate filament protein nestin physically interacts with Cdk5 and is required for ACh-induced association of p35, the co-activator of Cdk5, with the muscle membrane. Blockade of nestin-dependent signaling inhibits ACh-induced Cdk5 activation and the dispersion of AChR clusters in cultured myotubes. Similar to the effects of Cdk5 gene inactivation, knockdown of nestin in agrin-deficient embryos significantly restores AChR clusters. These results suggest that nestin is required for ACh-induced, Cdk5-dependent dispersion of AChR clusters during NMJ development.

GluA2 (GluR2) regulates metabotropic glutamate receptor-dependent long-term depression through N-cadherin-dependent and cofilin-mediated actin reorganization

The GluA2 (GluR2) subunit is critical for the regulation of AMPA receptor properties and synaptic plasticity, but the underlying mechanisms remain unclear. Here, we demonstrate that GluA2 regulates metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) through a previously unknown mechanism involving N-cadherin-dependent and cofilin-mediated actin reorganization. We show that GluA2 is indispensable for mGluR-LTD in the hippocampus, and surprisingly this action of GluA2 is mediated by its extracellular domain interaction with N-cadherin. Accordingly, we show that the function of N-cadherin is regulated by and required for mGluR-LTD. Furthermore, we show that the regulatory effect of GluA2/N-cadherin is mediated through activation of Rho GTPase Rac1 and its downstream actin regulator cofilin, and, importantly, the requirement for GluA2/N-cadherin can be overcome by manipulating cofilin. These results provide compelling evidence that the extracellular domain of GluA2 regulates long-lasting synaptic plasticity through a signaling mechanism that is distinct from those used by the other domains of the receptor subunit.

Compartmentalization from the outside: the extracellular matrix and functional microdomains in the brain

The extracellular matrix (ECM) of the central nervous system is well recognized as a migration and diffusion barrier that allows for the trapping and presentation of growth factors to their receptors at the cell surface. Recent data highlight the importance of ECM molecules as synaptic and perisynaptic scaffolds that direct the clustering of neurotransmitter receptors in the postsynaptic compartment and that present barriers to reduce the lateral diffusion of membrane proteins away from synapses. The ECM also contributes to the migration and differentiation of stem cells in the neurogenic niche and organizes the polarized localization of ion channels and transporters at contacts between astrocytic processes and blood vessels. Thus, the ECM contributes to functional compartmentalization in the brain.

Multiple roles of the Rho GEF ephexin1 in synapse remodeling

Synapse remodeling, which involves changes in the synaptic structure and their molecular composition, is required for the maturation and refinement of neural circuits. Although synapse remodeling is known to be tightly dependent on the assembly of local actin cytoskeleton, how actin directs the structural changes of synapse and targeting of synaptic proteins are not fully understood. Recently, we identified ephexin1, a Rho guanine nucleotide exchange factor (GEF) that regulates actin dynamics, to play an essential role in the maturation and functioning of the mammalian neuromuscular junction (NMJ). We showed that ephexin1 regulates the synaptic organization of the neurotransmitter receptor acetylcholine receptor (AChR) clusters through RhoA-dependent actin reorganization. Interestingly, ephexin1 has been implicated in the regulation of postsynaptic structure as well as the presynaptic vesicle release at various types of synapses. Our findings thus establish a novel function of ephexin1 in synapse remodeling through regulating the synaptic targeting of neurotransmitter receptors, revealing a versatile role of ephexin1 at synapses.

ADF/cofilin-mediated actin dynamics regulate AMPA receptor trafficking during synaptic plasticity

Dendritic spines undergo actin-based growth and shrinkage during synaptic plasticity, in which the actin depolymerizing factor (ADF)/cofilin family of actin-associated proteins are important. Elevated ADF/cofilin activities often lead to reduced spine size and immature spine morphology but can also enhance synaptic potentiation in some cases. Thus, ADF/cofilin may have distinct effects on postsynaptic structure and function. We found that ADF/cofilin-mediated actin dynamics regulated AMPA receptor (AMPAR) trafficking during synaptic potentiation, which was distinct from actin's structural role in spine morphology. Specifically, elevated ADF/cofilin activity markedly enhanced surface addition of AMPARs after chemically induced long-term potentiation (LTP), whereas inhibition of ADF/cofilin abolished AMPAR addition. We found that chemically induced LTP elicited a temporal sequence of ADF/cofilin dephosphorylation and phosphorylation that underlies AMPAR trafficking and spine enlargement. These findings suggest that temporally regulated ADF/cofilin activities function in postsynaptic modifications of receptor number and spine size during synaptic plasticity.

Roles of ADF/cofilin in actin polymerization and beyond

In collaboration or competition with many other actin-binding proteins, the actin-depolymerizing factor/cofilins integrate transmembrane signals to coordinate the spatial and temporal organization of actin filament assembly/disassembly (dynamics). In addition, newly discovered effects of these proteins in lipid metabolism, gene regulation, and apoptosis suggest that their roles go well beyond regulating the cytoskeleton.

To build a synapse: signaling pathways in neuromuscular junction assembly

Synapses, as fundamental units of the neural circuitry, enable complex behaviors. The neuromuscular junction (NMJ) is a synapse type that forms between motoneurons and skeletal muscle fibers and that exhibits a high degree of subcellular specialization. Aided by genetic techniques and suitable animal models, studies in the past decade have brought significant progress in identifying NMJ components and assembly mechanisms. This review highlights recent advances in the study of NMJ development, focusing on signaling pathways that are activated by diffusible cues, which shed light on synaptogenesis in the brain and contribute to a better understanding of muscular dystrophy.

Ephexin1 is required for structural maturation and neurotransmission at the neuromuscular junction

The maturation of neuromuscular junctions (NMJs) requires the topological transformation of postsynaptic acetylcholine receptor (AChR)-containing structures from a simple plaque to an elaborate structure composed of pretzel-like branches. This maturation process results in the precise apposition of the presynaptic and postsynaptic specializations. However, little is known about the molecular mechanisms underlying the plaque-to-pretzel transition of AChR clusters. In this study, we identify an essential role for the RhoGEF ephexin1 in the maturation of AChR clusters. Adult ephexin1(-/-) mice exhibit severe muscle weakness and impaired synaptic transmission at the NMJ. Intriguingly, when ephexin1 expression is deficient in vivo, the NMJ fails to mature into the pretzel-like shape, and such abnormalities can be rescued by re-expression of ephexin1. We further demonstrate that ephexin1 regulates the stability of AChR clusters in a RhoA-dependent manner. Taken together, our findings reveal an indispensible role for ephexin1 in regulating the structural maturation and neurotransmission of NMJs.

Podosomes are present in a postsynaptic apparatus and participate in its maturation

A critical step in synapse formation is the clustering of neurotransmitter receptors in the postsynaptic membrane, directly opposite the nerve terminal. At the neuromuscular junction, a widely studied model synapse, acetylcholine receptors (AChRs) initially aggregate to form an ovoid postsynaptic plaque. As the synapse matures, the plaque becomes perforated and is eventually transformed into a complex, branched structure. We found that this transformation also occurs in myotubes cultured in the absence of neurons, and used this system to seek machinery that orchestrates postsynaptic maturation. We show that perforations in the AChR aggregate bear structures resembling podosomes, dynamic actin-rich adhesive organelles involved in matrix remodeling in non-neuronal cells but not described in neural structures. The location and dynamics of synaptic podosomes are spatiotemporally correlated with changes in AChR aggregate topology, and pharmacological disruption of podosomes leads to rapid alterations in AChR organization. Our results indicate that synaptic podosomes play critical roles in maturation of the postsynaptic membrane.