Surgical denervation increases protein tyrosine phosphatase activity in skeletal muscle

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.

Regulation of AChR clustering by Dishevelled interacting with MuSK and PAK1

An important aspect of synapse development is the clustering of neurotransmitter receptors in the postsynaptic membrane. Although MuSK is required for acetylcholine receptor (AChR) clustering at the neuromuscular junction (NMJ), the underlying molecular mechanisms remain unclear. We report here that in muscle cells, MuSK interacts with Dishevelled (Dvl), a signaling molecule important for planar cell polarity. Disruption of the MuSK-Dvl interaction inhibits Agrin- and neuron-induced AChR clustering. Expression of dominant-negative Dvl1 in postsynaptic muscle cells reduces the amplitude of spontaneous synaptic currents at the NMJ. Moreover, Dvl1 interacts with downstream kinase PAK1. Agrin activates PAK, and this activation requires Dvl. Inhibition of PAK1 activity attenuates AChR clustering. These results demonstrate important roles of Dvl and PAK in Agrin/MuSK-induced AChR clustering and reveal a novel function of Dvl in synapse development.

Compartmentalized NRG signaling and PDZ domain-containing proteins in synapse structure and function

The synapse-specific synthesis of the acetylcholine receptor (AChR) is mediated by multiple mechanisms including compartmentalized signaling induced by neuregulin (NRG). This paper presents evidence that NRG receptors--ErbB receptor tyrosine kinases interact with distinct PDZ domain-containing proteins that are localized at the neuromuscular junction (NMJ). ErbB4 associates with the PSD-95 (also known as SAP90)-family members including PSD-95, SAP97, and SAP102 whereas ErbB2 interacts with Erbin and PICK1. Although, ErbB kinases are concentrated at the NMJ, they are not colocalized with the AChR in cultured muscle cells even in the presence of agrin. Co-expression of PSD-95 causes ErbB4 to form clusters in COS cells. We propose that PDZ domain-containing proteins play a role in anchoring ErbB proteins at the neuromuscular junction, and/or mediating downstream signaling pathways. Such mechanisms could be important for the maintenance and function of the synapse.

Erbin is a protein concentrated at postsynaptic membranes that interacts with PSD-95

Neuregulin is a factor essential for synapse-specific transcription of acetylcholine receptor genes at the neuromuscular junction. Its receptors, ErbB receptor tyrosine kinases, are localized at the postjunctional membrane presumably to ensure localized signaling. However, the molecular mechanisms underlying synaptic localization of ErbBs are unknown. Our recent studies indicate that ErbB4 interacts with postsynaptic density (PSD)-95 (SAP90), a PDZ domain-containing protein that does not interact with ErbB2 or ErbB3. Using as bait the ErbB2 C terminus, we identified Erbin, another PDZ domain-containing protein that interacts specifically with ErbB2. Erbin is concentrated in postsynaptic membranes at the neuromuscular junction and in the central nervous system, where ErbB2 is concentrated. Expression of Erbin increases the amount of ErbB2 labeled by biotin in transfected cells, suggesting that Erbin is able to increase ErbB2 surface expression. Furthermore, we provide evidence that Erbin interacts with PSD-95 in both transfected cells and synaptosomes. Thus ErbB proteins can interact with a network of PDZ domain-containing proteins. This interaction may play an important role in regulation of neuregulin signaling and/or subcellular localization of ErbB proteins.

Regulation of neuregulin-mediated acetylcholine receptor synthesis by protein tyrosine phosphatase SHP2

Synapse-specific expression of the nicotinic acetylcholine receptor (AChR) is believed to be mediated by neuregulin, an epidermal growth factor-like trophic factor released by somatic motoneurons at the neuromuscular junction (NMJ). Neuregulin stimulates ErbB2, ErbB3, and ErbB4, members of the ErbB family of receptor tyrosine kinases. SHP2 is a cytoplasmic protein tyrosine phosphatase containing two Src homology 2 domains near its N terminus, and has been shown to be a positive mediator of mitogenic responses to various growth factors. We found that SHP2 interacted with ErbB2 and ErbB3 after neuregulin stimulation of muscle cells. Expression of SHP2 in C2C12 mouse muscle cells attenuated the neuregulin-induced expression of an AChR epsilon-promoter reporter gene, whereas a catalytically inactive SHP2 mutant or a mutant lacking the N-terminal Src homology 2 (SH2) domain enhanced reporter expression, suggesting that SHP2 negatively regulates the neuregulin signaling pathway. In fibroblast cells that express a mutant SHP2 with a targeted deletion of the N-terminal SH2 domain, neuregulin-mediated activation of the Ras/Raf/extracellular signal-regulated kinase cascade was enhanced. Furthermore, we found that SHP2 immunoreactivity colocalized with the staining of alpha-bungarotoxin, a marker of the NMJ. These results demonstrate a negative role of SHP2 in the neuregulin signal that leads to AChR gene expression at the NMJ.

Attenuation of rat diaphragm low-frequency fatigue by vanadate in vitro

Sodium vanadate inhibits protein tyrosine phosphatases, including in skeletal muscle. Vanadate increases contractile force of airway, vascular and gastrointestinal smooth muscle. The present study tested the hypothesis that vanadate augments skeletal muscle contractility. Rat diaphragm muscle strips (n=26 from 12 animals) were studied in vitro at 37 degrees C. Muscles contracted isometrically while stimulated supramaximally with one of two protocols: 30 min of continuous 0.1 Hz stimulation, or 5 min of intermittent 20 Hz stimulation (duty cycle 0.33). Vanadate (500 microM)-treated muscle strips were compared with untreated muscle. Vanadate did not affect force or isometric twitch kinetics of otherwise quiescent muscle. During prolonged 0.1 Hz stimulation, force of control muscles declined by 17 +/- 4% over 30 min, whereas muscles incubated with vanadate maintained force virtually unchanged. Force over time was significantly greater with than without vanadate (P = 0.03), with values being significantly different during the last 10 min of the 30 min stimulation period. In the absence of vanadate force declined at a rate of approximately 0.6% per min, whereas with vanadate the rate of force decline was less than 0.1% per min (P < 0.02). During intermittent 20 Hz stimulation, the degree of force decline was not affected by vanadate at any time over a course of 5 min. Isometric contractile kinetics were not altered by vanadate during either 0.1 or 20 Hz stimulation. These data suggest that vanadate ameliorates low- but not higher-frequency fatigue in diaphragm, suggesting a role for protein tyrosine phosphorylation in the regulation of muscle fatigue resistance.

Protein tyrosine phosphorylation: implications for synaptic function

The phosphorylation of proteins on tyrosine residues, initially believed to be primarily involved in cell growth and differentiation, is now recognized as having a critical role in regulating the function of mature cells. The brain exhibits one of the highest levels of tyrosine kinase activity in the adult animal and the synaptic region is particularly rich in tyrosine kinases and tyrosine phosphorylated proteins. Recent studies have described the effects of tyrosine phosphorylation on the activities of a number of proteins which are potentially involved in the regulation of synaptic function. Furthermore, it is becoming apparent that tyrosine phosphorylation is involved in the modification of synaptic activity, such as occurs during depolarization, the induction of long-term potentiation or long-term depression, and ischemia. Changes in the activities of tyrosine kinases and/or protein tyrosine phosphatases which are associated with synaptic structures may result in altered tyrosine phosphorylation of proteins located at the synapse leading to both short-term and long-lasting changes in synaptic and neuronal function.