Implication of geranylgeranyltransferase I in synapse formation

Phosphoinositide 3-kinase acts through RAC and Cdc42 during agrin-induced acetylcholine receptor clustering

The formation of the neuromuscular junction (NMJ) is regulated by the nerve-derived heparan sulfate proteoglycan agrin and the muscle-specific kinase MuSK. Agrin induces a signal transduction pathway via MuSK, which promotes the reorganization of the postsynaptic muscle membrane. Activation of MuSK leads to the phosphorylation and redistribution of acetylcholine receptors (AChRs) and other postsynaptic proteins to synaptic sites. The accumulation of high densities of AChRs at postsynaptic regions represents a hallmark of NMJ formation and is required for proper NMJ function. Here we show that phosphoinositide 3-kinase (PI3-K) represents a component of the agrin/MuSK signaling pathway. Muscle cells treated with specific PI3-K inhibitors are unable to form full-size AChR clusters in response to agrin and AChR phosphorylation is reduced. Moreover, agrin-induced activation of Rac and Cdc42 is impaired in the presence of PI3-K inhibitors. PI3-K is localized to the postsynaptic muscle membrane consistent with a role during agrin/MuSK signaling. These results put PI3-K downstream of MuSK as regulator of AChR phosphorylation and clustering. Its role during agrin-stimulated Rac and Cdc42 activation suggests a critical function during cytoskeletal reorganizations, which lead to the redistribution of actin-anchored AChRs.

Mevalonate sensitizes the nociceptive transmission in the mouse spinal cord

Isoprenylation is crucial for the biological activation of small molecular G proteins. Activation of Rho/Rho kinase (ROCK) signaling has been reported to be involved in the initiation and maintenance of hyperalgesia caused by nerve injury and inflammation. The present study was then undertaken to examine whether the protein isoprenylation could affect thermal nociceptive threshold in the mouse spinal cord. Intrathecal administration of mevalonate (0.05-5.0 micromol) dose-dependently decreased the paw-withdrawal latencies for the thermal stimulation, indicating that mevalonate induces thermal hyperalgesia. Intrathecal pretreatment with a geranylgeranyl transferase I inhibitor GGTI-2133 (0.001-1.0 nmol) or a ROCK inhibitor Y27632 (0.001-1.0 nmol) completely blocked the mevalonate-induced thermal hyperalgesia. On the other hand, mevalonate-induced thermal hyperalgesia was only slightly attenuated by a farnesyl transferase inhibitor FTI-277 (0.01-1.0 nmol). Intrathecal injection of mevalonate increased the amount of geranylgeranylated RhoA in the spinal cord, which was completely blocked by intrathecal pretreatment with GGTI-2133. Intrathecal injection of mevalonate also produced RhoA translocation from cytosol to plasma membrane. This mevalonate-induced RhoA translocation was also blocked by intrathecal pretreatment with GGTI-2133, indicating that the RhoA translocation is triggered by RhoA geranylgeranylation. Moreover, inhibition of mevalonate synthesis by HMG-CoA reductase inhibition with simvastatin attenuated the second phase, but not the first phase, of nociceptive response to formalin. Our present results suggest that mevalonate sensitizes the spinal nociceptive transmission, which is mediated by the activation of ROCK following the RhoA geranylgeranylation.

Rapsyn interaction with calpain stabilizes AChR clusters at the neuromuscular junction

Agrin induces, whereas acetylcholine (ACh) disperses, ACh receptor (AChR) clusters during neuromuscular synaptogenesis. Such counteractive interaction leads to eventual dispersal of nonsynaptic AChR-rich sites and formation of receptor clusters at the postjunctional membrane. However, the underlying mechanisms are not well understood. Here we show that calpain, a calcium-dependent protease, is activated by the cholinergic stimulation and is required for induced dispersion of AChR clusters. Interestingly, the AChR-associated protein rapsyn interacted with calpain in an agrin-dependent manner, and this interaction inhibited the protease activity of calpain. Disrupting the endogenous rapsyn/calpain interaction enhanced CCh-induced dispersion of AChR clusters. Moreover, the loss of AChR clusters in agrin mutant mice was partially rescued by the inhibition of calpain via overexpressing calpastatin, an endogenous calpain inhibitor, or injecting calpeptin, a cell-permeable calpain inhibitor. These results demonstrate that calpain participates in ACh-induced dispersion of AChR clusters, and rapsyn stabilizes AChR clusters by suppressing calpain activity.

Guanylate cyclase and cyclic GMP-dependent protein kinase regulate agrin signaling at the developing neuromuscular junction

During formation of the neuromuscular junction (NMJ), agrin secreted by motor axons signals the embryonic muscle cells to organize a postsynaptic apparatus including a dense aggregate of acetylcholine receptors (AChRs). Agrin signaling at the embryonic NMJ requires the activity of nitric oxide synthase (NOS). Common downstream effectors of NOS are guanylate cyclase (GC), which synthesizes cyclic GMP, and cyclic GMP-dependent protein kinase (PKG). Here we show that GC and PKG are important for agrin signaling at the embryonic NMJ of the frog, Xenopus laevis. Inhibitors of both GC and PKG reduced endogenous AChR aggregation in embryonic muscles by 50-85%, and blocked agrin-induced AChR aggregation in cultured embryonic muscle cells. A cyclic GMP analog, 8-bromo-cyclic GMP, increased endogenous AChR aggregation in embryonic muscles to 3- to 4-fold control levels. Overexpression of either GC or PKG in embryos increased AChR aggregate area by 60-170%, whereas expression of a dominant negative form of GC inhibited endogenous aggregation by 50%. These results indicate that agrin signaling in embryonic muscle cells requires the activity of GC and PKG as well as NOS.

MuSK signaling at the neuromuscular junction

The neuromuscular junction (NMJ) is a peripheral cholinergic synapse that conveys signals from motor neurons to muscle cells (Sanes and Lichtman, 1999; Sanes and Lichtman, 2001). The formation of the NMJ requires communication between motoneurons and muscle fibers. Three molecules are essential for NMJ formation: agrin, MuSK, and rapsyn. MuSK appears to be involved in every aspect of NMJ development and maintenance. The paper reviews agrin-MuSK cascades and its potential cross talk with Wnt signaling pathways.

Shp2 is dispensable in the formation and maintenance of the neuromuscular junction

SHP2, a protein tyrosine phosphatase with two SH2 domains, has been implicated in regulating acetylcholine receptor (AChR) gene expression and cluster formation in cultured muscle cells. To understand the role of SHP2 in neuromuscular junction (NMJ) formation in vivo, we generated mus cle-specific deficient mice by using a loxP/Cre strategy since Shp2 null mutation causes embryonic lethality. Shp2(floxed/floxed) mice were crossed with mice expressing the Cre gene under the control of the human skeletal alpha-actin (HSA) promoter. Expression of SHP2 was reduced or diminished specifically in skeletal muscles of the conditional knockout (CKO) mice. The mutant mice were viable and fertile, without apparent muscle defects. The mRNA of the AChR alpha subunit and AChR clusters in CKO mice were localized in a narrow central region surrounding the phrenic nerve primary branches, without apparent change in intensity. AChR clusters colocalized with markers of synaptic vesicles and Schwann cells, suggesting proper differentiation of presynaptic terminals and Schwann cells. In comparison with age-matched littermates, no apparent difference was observed in the size and length of AChR clusters in CKO mice. Both the frequency and amplitude of mEPPs in CKO mice were similar to those in controls, suggesting normal neurotransmission when SHP2 was deficient. These results suggest that Shp2 is not required for NMJ formation and/or maintenance.

Development of the neuromuscular junction

The differentiation of the neuromuscular junction is a multistep process requiring coordinated interactions between nerve terminals and muscle. Although innervation is not needed for muscle production, the formation of nerve-muscle contacts, intramuscular nerve branching, and neuronal survival require reciprocal signals from nerve and muscle to regulate the formation of synapses. Following the production of muscle fibers, clusters of acetylcholine receptors (AChRs) are concentrated in the central regions of the myofibers via a process termed "prepatterning". The postsynaptic protein MuSK is essential for this process activating possibly its own expression, in addition to the expression of AChR. AChR complexes (aggregated and stabilized by rapsyn) are thus prepatterned independently of neuronal signals in developing myofibers. ACh released by branching motor nerves causes AChR-induced postsynaptic potentials and positively regulates the localization and stabilization of developing synaptic contacts. These "active" contact sites may prevent AChRs clustering in non-contacted regions and counteract the establishment of additional contacts. ACh-induced signals also cause the dispersion of non-synaptic AChR clusters and possibly the removal of excess AChR. A further neuronal factor, agrin, stabilizes the accumulation of AChR at synaptic sites. Agrin released from the branching motor nerve may form a structural link specifically to the ACh-activated endplates, thereby enhancing MuSK kinase activity and AChR accumulation and preventing dispersion of postsynaptic specializations. The successful stabilization of prepatterned AChR clusters by agrin and the generation of singly innervated myofibers appear to require AChR-mediated postsynaptic potentials indicating that the differentiation of the nerve terminal proceeds only after postsynaptic specializations have formed.

Lipid rafts serve as a signaling platform for nicotinic acetylcholine receptor clustering

Agrin, a motoneuron-derived factor, and the muscle-specific receptor tyrosine kinase (MuSK) are essential for the acetylcholine receptor (AChR) clustering at the postjunctional membrane. However, the underlying signaling mechanisms remain poorly defined. We show that agrin stimulates a dynamic translocation of the AChR into lipid rafts-cholesterol and sphingolipid-rich microdomains in the plasma membrane. This follows MuSK partition into lipid rafts and requires its activation. Disruption of lipid rafts inhibits MuSK activation and downstream signaling and AChR clustering in response to agrin. Rapsyn, an intracellular protein necessary for AChR clustering, is located constitutively in lipid rafts, but its interaction with the AChR is inhibited when lipid rafts are perturbed. These results reveal that lipid rafts may regulate AChR clustering by facilitating the agrin/MuSK signaling and the interaction between the receptor and rapsyn, both necessary for AChR clustering and maintenance. These results provide insight into mechanisms of AChR cluster formation.

Isoprenoids and Alzheimer's disease: a complex relationship

Cholesterol metabolism has been linked to Alzheimer's disease (AD) neuropathology, which is characterized by amyloid plaques, neurofibrillary tangles and neuroinflammation. Indeed, the use of statins, which inhibit cholesterol and isoprenoid biosynthesis, as potential AD therapeutics is under investigation. Whether statins offer benefit for AD will be determined by the outcome of large, placebo-controlled, randomized clinical trials. However, their use as pharmacological tools has delineated novel roles for isoprenoids in AD. Protein isoprenylation regulates multiple cellular and molecular events and here we review the complex roles of isoprenoids in AD-relevant processes and carefully evaluate isoprenoid pathways as potential AD therapeutic targets.

The tetratricopeptide repeat domains of rapsyn bind directly to cytoplasmic sequences of the muscle-specific kinase

Clustering of acetylcholine receptors at the developing vertebrate neuromuscular junction is initiated by neural agrin, which stimulates the activity of the muscle-specific kinase (MuSK). Acetylcholine receptor clustering is also dependent on the postsynaptic scaffolding protein, rapsyn, which binds to acetylcholine receptors. Here, we address the possibility that MuSK and rapsyn bind directly to each other by coexpressing sequences of the cytoplasmic domain of MuSK with rapsyn in COS-7 cells and assaying for codistribution and biochemical interaction. Sequences constituting the bulk of the kinase domain can interact with rapsyn. This interaction is mediated by the tetratricopeptide repeat domains, but not the coiled coil or zinc finger domains, of rapsyn. This interaction does not require tyrosine phosphorylation of the MuSK sequences. Binding is direct, as indicated by blot overlay and surface plasmon resonance experiments. The sequence of the cytoplasmic domain of MuSK that most effectively codistributes with rapsyn confers the ability of an otherwise inactive receptor tyrosine kinase, TrkA, to associate with rapsyn. Our results support a model in which the tetratricopeptide repeat domains of rapsyn bind directly to the cytoplasmic portion of MuSK, which could thereby serve as an initial scaffold for the clustering of acetylcholine receptors.

Molecular regulation of postsynaptic differentiation at the neuromuscular junction

The neuromuscular junction (NMJ) is a synapse that develops between a motor neuron and a muscle fiber. A defining feature of NMJ development in vertebrates is the re-distribution of muscle acetylcholine (ACh) receptors (AChRs) following innervation, which generates high-density AChR clusters at the postsynaptic membrane and disperses aneural AChR clusters formed in muscle before innervation. This process in vivo requires MuSK, a muscle-specific receptor tyrosine kinase that triggers AChR re-distribution when activated; rapsyn, a muscle protein that binds and clusters AChRs; agrin, a nerve-secreted heparan-sulfate proteoglycan that activates MuSK; and ACh, a neurotransmitter that stimulates muscle and also disperses aneural AChR clusters. Moreover, in cultured muscle cells, several additional muscle- and nerve-derived molecules induce, mediate or participate in AChR clustering and dispersal. In this review we discuss how regulation of AChR re-distribution by multiple factors ensures aggregation of AChRs exclusively at NMJs.

LL5beta: a regulator of postsynaptic differentiation identified in a screen for synaptically enriched transcripts at the neuromuscular junction

In both neurons and muscle fibers, specific mRNAs are concentrated beneath and locally translated at synaptic sites. At the skeletal neuromuscular junction, all synaptic RNAs identified to date encode synaptic components. Using microarrays, we compared RNAs in synapse-rich and -free regions of muscles, thereby identifying transcripts that are enriched near synapses and that encode soluble membrane and nuclear proteins. One gene product, LL5β, binds to both phosphoinositides and a cytoskeletal protein, filamin, one form of which is concentrated at synaptic sites. LL5β is itself associated with the cytoplasmic face of the postsynaptic membrane; its highest levels border regions of highest acetylcholine receptor (AChR) density, which suggests a role in “corraling” AChRs. Consistent with this idea, perturbing LL5β expression in myotubes inhibits AChR aggregation. Thus, a strategy designed to identify novel synaptic components led to identification of a protein required for assembly of the postsynaptic apparatus.

The synaptic muscle-specific kinase (MuSK) complex: New partners, new functions

The muscle-specific kinase MuSK is part of an agrin receptor complex that stimulates tyrosine phosphorylation and drives clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane at the vertebrate neuromuscular junction (NMJ). MuSK also regulates synaptic gene transcription in subsynaptic nuclei. Over the past few years, decisive progress has been made in the identification of MuSK effectors, helping to understand its function in the formation of the NMJ. Similarly to AChR, MuSK and several of its partners are the target of mutations responsible for diseases of the NMJ, such as congenital myasthenic syndromes. This minireview will focus on the multiple MuSK effectors so far identified that place MuSK at the center of a multifunctional signaling complex involved in the organization of the NMJ and associated disorders.

14-3-3 gamma associates with muscle specific kinase and regulates synaptic gene transcription at vertebrate neuromuscular synapse

The muscle-specific receptor tyrosine kinase (MuSK) is part of a receptor complex, activated by neural agrin, that orchestrates the differentiation of the neuromuscular junction (NMJ). To gain insight into the function of the MuSK complex, we have developed a proteomic approach to identify new MuSK partners. MS analysis of MuSK crosslink products from postsynaptic membranes of the Torpedo electrocytes identified the adaptor protein 14-3-3 gamma. The 14-3-3 gamma protein was found localized at the adult rat NMJ. Cotransfection experiments in COS-7 cells showed that MuSK codistributed with the 14-3-3 gamma protein at the plasma membrane. Furthermore, 14-3-3 gamma was copurified by affinity chromatography with MuSK from transfected COS-7 cells and myotubes. The 14-3-3 gamma protein did not colocalize with agrin-elicited acetylcholine receptor (AChR) aggregates in cultured myotubes, suggesting that it is not involved in AChR clustering. Expression of 14-3-3 gamma specifically repressed the transcription of several synaptic reporter genes in cultured myotubes. This repression was potentiated by MuSK expression. Moreover, the expression of 14-3-3 gamma in muscle fibers in vivo caused both the repression of synaptic genes transcription and morphological perturbations of the NMJ. Our data extend the notion that, apart from its well documented role in AChR clustering, the MuSK complex might also be involved in the regulation of synaptic gene expression at the NMJ.

A single pulse of agrin triggers a pathway that acts to cluster acetylcholine receptors

Agrin triggers signaling mechanisms of high temporal and spatial specificity to achieve phosphorylation, clustering, and stabilization of postsynaptic acetylcholine receptors (AChRs). Agrin transiently activates the kinase MuSK; MuSK activation has largely vanished when AChR clusters appear. Thus, a tyrosine kinase cascade acts downstream from MuSK, as illustrated by the agrin-evoked long-lasting activation of Src family kinases (SFKs) and their requirement for AChR cluster stabilization. We have investigated this cascade and report that pharmacological inhibition of SFKs reduces early but not later agrin-induced phosphorylation of MuSK and AChRs, while inhibition of Abl kinases reduces late phosphorylation. Interestingly, SFK inhibition applied selectively during agrin-induced AChR cluster formation caused rapid cluster dispersal later upon agrin withdrawal. We also report that a single 5-min agrin pulse, followed by extensive washing, triggered long-lasting MuSK and AChR phosphorylation and efficient AChR clustering. Following the pulse, MuSK phosphorylation increased and, beyond a certain level, caused maximal clustering. These data reveal novel temporal aspects of tyrosine kinase action in agrin signaling. First, during AChR cluster formation, SFKs initiate early phosphorylation and an AChR stabilization program that acts much later. Second, a kinase mechanism rapidly activated by agrin acts thereafter autonomously in agrin's absence to further increase MuSK phosphorylation and cluster AChRs.

ATP potentiates agrin-induced AChR aggregation in cultured myotubes: activation of RhoA in P2Y1 nucleotide receptor signaling at vertebrate neuromuscular junctions

At vertebrate neuromuscular junctions, ATP is known to stabilize acetylcholine in the synaptic vesicles and to be co-released with it. We have shown previously that a nucleotide receptor, P2Y(1) receptor, is localized at the nmjs, and we propose that this mediates a trophic role for synaptic ATP there. In cultured myotubes, the activation of P2Y(1) receptors modulated agrin-induced acetylcholine receptor (AChR) aggregation in a potentiation manner. This potentiation effect in agrin-induced AChR aggregation was reduced by antagonizing the P2Y(1) receptors. The guanosine triphosphatase RhoA was shown to be responsible for this P2Y(1)-potentiated effect. The localization of RhoA in rat and chicken skeletal muscles was restricted at the neuromuscular junctions. Application of P2Y(1) agonists in cultured myotubes induced RhoA activation, which showed an additive effect with agrin-induced RhoA activation. Over-expression of dominant-negative mutant of RhoA in cultured myotubes diminished the agrin-induced AChR aggregation, as well as the potentiation effect of P2Y(1)-specific agonist. Application of UTP in the cultures also triggered similar responses as did 2-methylthioadenosine 5'-diphosphate, suggesting the involvement of other subtypes of P2Y receptors. These results demonstrate that RhoA could serve as a downstream mediator of signaling mediated by P2Y(1) receptor and agrin, which therefore synergizes the effects of the two neuron-derived trophic factors in modulating the formation and/or maintenance of post-synaptic apparatus at the neuromuscular junctions.