Induction of acetylcholine receptor cluster formation by local application of growth factors in cultured Xenopus muscle cells

Roles of tyrosine kinases and phosphatases in the formation and dispersal of acetylcholine receptor clusters

The formation of acetylcholine receptor (AChR) clusters at the postsynaptic muscle membrane in response to motor innervation is a key event in the development of the neuromuscular junction. The synaptic AChR clustering process is initiated by motor axon-released agrin, which activates a tyrosine kinase-based signaling pathway to cause AChR aggregation. In cultured muscle cells, AChR clustering is elicited by diverse nonneural signals, and this process is also mediated by tyrosine kinases. Conversely, the formation of new AChR clusters induced by innervation or nonneural stimuli is unfailingly associated with the dispersal of pre-existing AChR clusters, and this process is mediated by tyrosine phosphatases. In this review, we address how local kinase activation leads to global phosphatase action in muscle. More specifically, we discuss the roles of Src kinase and the SH2 domain-containing tyrosine phosphatase Shp-2 in establishing a regenerative mechanism to propagate the AChR cluster dispersing signal extrasynaptically and in defining the boundary of cluster formation subsynaptically.

Combination of agrin and laminin increase acetylcholine receptor clustering and enhance functional neuromuscular junction formation In vitro

Clustering of acetylcholine receptors (AChR) at the postsynaptic membrane is a crucial step in the development of neuromuscular junctions (NMJ). During development and after denervation, aneural AChR clusters form on the sarcolemma. Recent studies suggest that these receptors are critical for guiding and initiating synaptogenesis. The aim of this study is to investigate the effect of agrin and laminin-1; agents with known AChR clustering activity; on NMJ formation and muscle maturation. Primary myoblasts were differentiated in vitro on collagen, laminin or collagen and laminin-coated surfaces in the presence or absence of agrin and laminin. The pretreated cells were then subject to innervation by PC12 cells. The number of neuromuscular junctions was assessed by immunocytochemical co-localization of AChR clusters and the presynaptic marker synaptophysin. Functional neuromuscular junctions were quantitated by analysis of the level of spontaneous as well as neuromuscular blocker responsive contractile activity and muscle maturation was assessed by the degree of myotube striation. Agrin alone did not prime muscle for innervation while a combination of agrin and laminin pretreatment increased the number of neuromuscular junctions formed and enhanced acetylcholine based neurotransmission and myotube striation. This study has direct clinical relevance for treatment of denervation injuries and creating functional neuromuscular constructs for muscle tissue repair.

Mechanisms of tumor-promoting activities of nicotine in lung cancer: synergistic effects of cell membrane and mitochondrial nicotinic acetylcholine receptors

Background

One of the major controversies of contemporary medicine is created by an increased consumption of nicotine and growing evidence of its connection to cancer, which urges elucidation of the molecular mechanisms of oncogenic effects of inhaled nicotine. Current research indicates that nicotinergic regulation of cell survival and death is more complex than originally thought, because it involves signals emanating from both cell membrane (cm)- and mitochondrial (mt)-nicotinic acetylcholine receptors (nAChRs). In this study, we elaborated on the novel concept linking cm-nAChRs to growth promotion of lung cancer cells through cooperation with the growth factor signaling, and mt-nAChRs — to inhibition of intrinsic apoptosis through prevention of opening of mitochondrial permeability transition pore (mPTP).

Methods

Experiments were performed with normal human lobar bronchial epithelial cells, the lung squamous cell carcinoma line SW900, and intact and NNK-transformed immortalized human bronchial cell line BEP2D.

Results

We demonstrated that the growth-promoting effect of nicotine mediated by activation of α7 cm-nAChR synergizes mainly with that of epidermal growth factor (EGF), α3 — vascular endothelial growth factor (VEGF), α4 — insulin-like growth factor I (IGF-I) and VEGF, whereas α9 with EGF, IGF-I and VEGF. We also established the ligand-binding abilities of mt-nAChRs and demonstrated that quantity of the mt-nAChRs coupled to inhibition of mPTP opening increases upon malignant transformation.

Conclusions

These results indicated that the biological sum of simultaneous activation of cm- and mt-nAChRs produces a combination of growth-promoting and anti-apoptotic signals that implement the tumor-promoting action of nicotine on lung cells. Therefore, nAChRs may be a promising molecular target to arrest lung cancer progression and re-open mitochondrial apoptotic pathways.

Connections of nicotine to cancer

This Opinion article discusses emerging evidence of direct contributions of nicotine to cancer onset and growth. The list of cancers reportedly connected to nicotine is expanding and presently includes small-cell and non-small-cell lung carcinomas, as well as head and neck, gastric, pancreatic, gallbladder, liver, colon, breast, cervical, urinary bladder and kidney cancers. The mutagenic and tumour-promoting activities of nicotine may result from its ability to damage the genome, disrupt cellular metabolic processes, and facilitate growth and spreading of transformed cells. The nicotinic acetylcholine receptors (nAChRs), which are activated by nicotine, can activate several signalling pathways that can have tumorigenic effects, and these receptors might be able to be targeted for cancer therapy or prevention. There is also growing evidence that the unique genetic makeup of an individual, such as polymorphisms in genes encoding nAChR subunits, might influence the susceptibility of that individual to the pathobiological effects of nicotine. The emerging knowledge about the carcinogenic mechanisms of nicotine action should be considered during the evaluation of regulations on nicotine product manufacturing, distribution and marketing.

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.

Effect of combinations of insulin, glucose and scopolamine on radial arm maze performance

Previous research has shown that glucose is an effective agent in facilitating memory performance and in attenuating scopolamine-induced amnesia. Although insulin has not been shown to facilitate unimpaired memory, a previous study has demonstrated that insulin can also attenuate scopolamine-degraded memory. The present study was designed to determine how different combinations of insulin, glucose and scopolamine affect memory. It involved nine rats whose memory was assessed through performance in a win-shift radial arm maze task under different drug treatments. A 2 x 2 x 2 (insulin x glucose x scopolamine) within-subjects design with a 5-h drug test interval was employed. Scopolamine disrupted memory performance, and both glucose and insulin counteracted this disruption. Combining the glucose and insulin treatments did not increase their ability to attenuate scopolamine deficits but slightly decreased this effect. Glucose tended to enhance memory, even in the absence of scopolamine, whereas insulin had no effect on memory in the absence of scopolamine. Blood glucose levels were measured and did not indicate changes caused by drug treatments. The memory effects may have been due to the acetylcholine-agonist actions of glucose and insulin, an interpretation consistent with previous research findings.