bms-690514 Most studies on d penicillamine induced MG were p

Most studies on d-penicillamine-induced MG were performed before the discovery that some MG patients have anti-MuSK antibodies. After the identification of the present case, we retrospectively tested the stored frozen sera of ten earlier identified d-penicillamine MG patients with anti-AChR bms-690514 and found that none were anti-MuSK-positive. Thus, double-positive MG seems to be rare in d-penicillamine-induced MG like in idiopathic MG. The fact that about 20% of d-penicillamine-induced MG patients do not develop detectable anti-AChR antibodies (D'Anglejan et al., 1985) leaves open the possibility of the presence of anti-MuSK antibodies in some or all of these patients. It will be interesting to screen the sera of such patients and the sera of MG-asymptomatic d-penicillamine patients for the presence of anti-MuSK antibodies.
Funding This work was supported by grants from the European Commission (FP7 Fight-MG, Contract No 242210; REGPOT NeuroSign, Contract No 264033) and the Association Francaise contre les Myopathies (AFM).
Introduction Neuromuscular synapses are formed between motor neurons and skeletal muscle fibers. AChRs are concentrated at the postsynaptic membrane, which guarantees efficient and accurate neurotransmission. AChR clustering is a dynamic process, whereby nascent clusters underneath nerve terminals are stabilized by agrin, a motor-neuron-derived glycoprotein, and the AChR-associated protein rapsyn (McMahan, 1990, Sanes and Lichtman, 1999, Sanes and Lichtman, 2001). Meanwhile, motor neurons release negative signals to disperse noninnervated clusters and refine clusters at the synapses. Genetic studies suggest that ACh may serve as a negative signal. AChR clusters are larger in mice deficient in choline acetyltransferase (ChAT), the key enzyme for ACh synthesis, and numerous AChR clusters are maintained in ChAT and agrin double mutant mice (Lin et al., 2005, Misgeld et al., 2002, Misgeld et al., 2005), whereas few AChR clusters are present in the muscle of agrin single mutant mice (Burgess et al., 1999, Gautam et al., 1996, Lin et al., 2001, Yang et al., 2001). The interplay between positive and negative signals is necessary for precise matching of nerve terminals to individual postsynaptic apparatuses (Sanes and Lichtman, 1999, Sanes and Lichtman, 2001). Recent evidence suggests that Cdk5, a cytoplasmic serine/threonine kinase, may be an effector in dispersing or refining AChR clusters. ACh activates Cdk5, and inhibition of Cdk5 by genetic ablation or pharmacological inhibition increases AChR cluster size in cultured muscle fibers and in mutant mice (Fu et al., 2005, Lin et al., 2005). The dispersing activity of ACh can be counteracted by agrin (Lin et al., 2005, Misgeld et al., 2005), presumably via the activation of MuSK, a muscle-specific tyrosine kinase receptor concentrated in the postsynaptic region of neuromuscular junctions (Glass et al., 1996). Thus, the overall inhibitory effect by ACh and Cdk5 is reversed locally by agrin signaling so that AChR clusters are stabilized at the neuromuscular junction (NMJ). Rapsyn is believed to be required for agrin-induced AChR clustering and stabilization (Apel et al., 1997, Froehner, 1991, Fuhrer et al., 1999, Gautam et al., 1996, Phillips et al., 1997). However, the effectors of rapsyn are not well defined. Here we provide evidence that rapsyn acts by inhibiting calpain to regulate AChR cluster stabilization at the NMJ. Calpains are a family of calcium-activated intracellular cysteine proteases, which are ubiquitously expressed in various mammalian cells (Goll et al., 2003). In the brain, calpains are involved in many physiological events including LTP (Carafoli and Molinari, 1998, Oliver et al., 1989, Staubli et al., 1988) or neurotoxic insults ranging from ischemia to Alzheimer's disease (Lee et al., 2000, Nixon, 2000, Patrick et al., 1999). A number of proteins have been identified as calpain substrates from various tissues (Goll et al., 2003). Calpain cleavage of p35, a regulatory partner of Cdk5, generates p25, which causes hyperactivation of Cdk5 (Ahlijanian et al., 2000, Patrick et al., 1999, Patzke et al., 2003). Transient expression of p25 in the hippocampus enhances LTP and facilitates hippocampus-dependent memory, whereas prolonged p25 expression results in loss of synapses and death of neurons (Fischer et al., 2005).