Functional improvement of dystrophic muscle by myostatin blockade

Mice and cattle with mutations in the myostatin (GDF8) gene show a marked increase in body weight and muscle mass, indicating that this new member of the TGF-beta superfamily is a negative regulator of skeletal muscle growth. Inhibition of the myostatin gene product is predicted to increase muscle mass and improve the disease phenotype in a variety of primary and secondary myopathies. We tested the ability of inhibition of myostatin in vivo to ameliorate the dystrophic phenotype in the mdx mouse model of Duchenne muscular dystrophy (DMD). Blockade of endogenous myostatin by using intraperitoneal injections of blocking antibodies for three months resulted in an increase in body weight, muscle mass, muscle size and absolute muscle strength in mdx mouse muscle along with a significant decrease in muscle degeneration and concentrations of serum creatine kinase. The functional improvement of dystrophic muscle by myostatin blockade provides a novel, pharmacological strategy for treatment of diseases associated with muscle wasting such as DMD, and circumvents the major problems associated with conventional gene therapy in these disorders.     

Functional characterization of a potassium-selective prokaryotic glutamate receptor

Ion channels are molecular pores that facilitate the passage of ions across cell membranes and participate in a range of biological processes, from excitatory signal transmission in the mammalian nervous system to the modulation of swimming behaviour in the protozoan Paramecium. Two particularly important families of ion channels are ionotropic glutamate receptors (GluRs) and potassium channels. GluRs are permeable to Na+, K+ and Ca2+, are gated by glutamate, and have previously been found only in eukaryotes. In contrast, potassium channels are selective for K+, are gated by a range of stimuli, and are found in both prokaryotes and eukaryotes. Here we report the discovery and functional characterization of GluR0 from Synechocystis PCC 6803, which is the first GluR found in a prokaryote. GluR0 binds glutamate, forms potassium-selective channels and is related in amino-acid sequence to both eukaryotic GluRs and potassium channels. On the basis of amino-acid sequence and functional relationships between GluR0 and eukaryotic GluRs, we propose that a prokaryotic GluR was the precursor to eukaryotic GluRs. GluR0 provides evidence for the missing link between potassium channels and GluRs, and we suggest that their ion channels have a similar architecture, that GluRs are tetramers and that the gating mechanisms of GluRs and potassium channels have some essential features in common.     

Functional modulation of the nicotinic acetylcholine receptor by tyrosine phosphorylation

Tyrosine-specific protein phosphorylation has been implicated in the regulation of cell transformation and proliferation. However, recent studies have shown that the expression of protein tyrosine kinases in adult brain is very high, suggesting that tyrosine-specific protein phosphorylation may also have a role in the regulation of neuronal function. Although a number of substrate proteins are phosphorylated on tyrosine residues, the functional alteration of proteins by tyrosine phosphorylation has previously been convincingly demonstrated only for protein tyrosine kinases. The nicotinic acetylcholine receptor, a neurotransmitter-gated ion channel, is phosphorylated by a protein tyrosine kinase in post-synaptic membranes in vitro and in vivo. We demonstrate here that this tyrosine phosphorylation increases the rate of the rapid phase of desensitization of the nicotinic receptor, as measured by single channel recording of purified nicotinic acetylcholine receptor, when reconstituted in lipid vesicles. These data provide direct evidence for the regulation of ion channel properties by tyrosine phosphorylation. The results, which demonstrate a functional role of tyrosine phosphorylation in the nervous system, suggest a widespread role for tyrosine phosphorylation in neuronal signal transduction.     

Calcitonin gene-related peptide regulates muscle acetylcholine receptor synthesis

Innervation of muscle by motoneurones induces the development of a characteristic, high density cluster of acetylcholine receptors (AChRs) at the neuromuscular junction. Studies in vitro show that the accumulation of AChRs at nerve-muscle contacts results from both increased insertion of new AChRs into the muscle plasma membrane beneath nerve terminals and redistribution of preexisting AChRs; these two modes of AChR accumulation may be separately controlled since factors have been identified that influence AChR redistribution but not synthesis. Although many aspects of muscle development are regulated by nerve-dependent muscle activity, junctional AChR clusters still develop when neuromuscular transmission is blocked by either curare or alpha-bungarotoxin, suggesting that their formation is mediated by nerve-derived trophic factors other than activity. A molecule immunologically related to calcitonin gene-related peptide (CGRP-I) has been found in motoneurones in a variety of mammals including man. Here we provide indirect evidence that CGRP-I may be a motoneurone-derived trophic factor that increases AChR synthesis at vertebrate neuromuscular junctions.     

Critical role of Toll-like receptor signaling in NK cell activation

Toll-like receptors (TLRs) and NK cell receptors are the most important receptor superfamilies in innate immunity. TLRs act as the sensor of external pathogens, while NK cells detect alterations in endogenous protein expression on target cells through activating and inhibitory receptors. Accumulating data has demonstrated that TLRs and NK cell receptors can coordinate and regulate each other during immune responses, which contributes to the initiation of innate response and the priming of adaptive responses. TLRs can activate NK cell function directly or with the help of accessory cells in a cytokine or cell-to-cell contact dependent manner. More understanding of the recognition of innate receptors and interactions between them may provide important insights into the design of effective strategies to combat tumor and microbial infections. In this review, we summarize how TLRs and NK cells discriminate the self or non-self components respectively. And importantly, we pay more attention to the role of TLR sig-naling in induction of NK cell activation, responses and the crosstalk between them.     

Concentration of acetylcholine receptor mRNA in synaptic regions of adult muscle fibres

Acetylcholine receptors (AChRs) are highly concentrated in the small fraction (approximately 0.1%) of the skeletal muscle fibre surface that comprises the postsynaptic membrane of the neuromuscular junction (Fig. 1a). In adult murine muscle, for example, AChRs are packed at a density of over 15,000 per micron2 in postsynaptic membrane, whereas their density is less than 30 per micron2 in extrasynaptic membrane. Because synaptic AChRs turn over they must be replaced, and it is interesting to consider where the new AChRs that maintain synaptic aggregates are synthesized. One possibility is that AChRs are synthesized uniformly along the length of the multinucleated muscle fibre; in this case, AChRs might be redistributed to or selectively stabilized at the synapse, as probably occurs during synapse formation. Alternatively, AChRs might be preferentially synthesized near synapses, a possibility that would suggest that innervation can influence not only where AChRs are inserted or accumulate but also where they are synthesized. In support of this second possibility, we report here that AChR messenger RNA is more abundant near to than far from synapses in adult muscle fibres.