Tyrosine kinase inhibitors alter composition of nicotinic receptors on neurons

Abelson family tyrosine kinases regulate the function of nicotinic acetylcholine receptors and nicotinic synapses on autonomic neurons

Abelson family kinases (AFKs; Abl1, Abl2) are non-receptor tyrosine kinases (NRTKs) implicated in cancer, but they also have important physiological roles that include regulating synaptic structure and function. Recent studies using Abl-deficient mice and the antileukemia drug STI571 [imatinib mesylate (Gleevec); Novartis], which potently and selectively blocks Abl kinase activity, implicate AFKs in regulating presynaptic neurotransmitter release in hippocampus and postsynaptic clustering of nicotinic acetylcholine receptors (nAChRs) in muscle. Here, we tested whether AFKs are relevant for regulating nAChRs and nAChR-mediated synapses on autonomic neurons. AFK immunoreactivity was detected in ciliary ganglion (CG) lysates and neurons, and STI571 application blocked endogenous Abl tyrosine kinase activity. With similar potency, STI571 specifically reduced whole-cell current responses generated by both nicotinic receptor subtypes present on CG neurons (α3*- and α7-nAChRs) and lowered the frequency and amplitude of α3*-nAChR-mediated excitatory postsynaptic currents. Quantal analysis indicated that the synaptic perturbations were postsynaptic in origin, and confocal imaging experiments revealed they were unaccompanied by changes in nAChR clustering or alignment with presynaptic terminals. The results indicate that in autonomic neurons, Abl kinase activity normally supports postsynaptic nAChR function to sustain nAChR-mediated neurotransmission. Such consequences contrast with the influence of Abl kinase activity on presynaptic function and synaptic structure in hippocampus and muscle, respectively, demonstrating a cell-specific mechanism of action. Finally, because STI571 potently inhibits Abl kinase activity, the autonomic dysfunction side effects associated with its use as a chemotherapeutic agent may result from perturbed α3*- and/or α7-nAChR function.

Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation

Alpha7 nicotinic acetylcholine receptors (nAChRs) modulate network activity in the CNS. Thus, functional regulation of alpha7 nAChRs could influence the flow of information through various brain nuclei. It is hypothesized here that these receptors are amenable to modulation by tyrosine phosphorylation. In both Xenopus oocytes and rat hippocampal interneurons, brief exposure to a broad-spectrum protein tyrosine kinase inhibitor, genistein, specifically and reversibly potentiated alpha7 nAChR-mediated responses, whereas a protein tyrosine phosphatase inhibitor, pervanadate, caused depression. Potentiation was associated with an increased expression of surface alpha7 subunits and was not accompanied by detectable changes in receptor open probability, implying that the increased function results from an increased number of alpha7 nAChRs. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated exocytosis was shown to be a plausible mechanism for the rapid delivery of additional alpha7 nAChRs to the plasma membrane. Direct phosphorylation/dephosphorylation of alpha7 subunits was unlikely because mutation of all three cytoplasmic tyrosine residues did not prevent the genistein-mediated facilitation. Overall, these data are consistent with the hypothesis that the number of functional cell surface alpha7 nAChRs is controlled indirectly via processes involving tyrosine phosphorylation.

Regulatory mechanisms that govern nicotinic synapse formation in neurons

Individual cholinoceptive neurons express high levels of different neuronal nicotinic acetylcholine receptor (nAChR) subtypes, and target them to the appropriate synaptic regions for proper function. This review focuses on the intercellular and intracellular processes that regulate nAChR expression in vertebrate peripheral nervous system (PNS) and central nervous system (CNS) neurons. Specifically, we discuss the cellular and molecular mechanisms that govern the induction and maintenance of nAChR expression-innervation, target tissue interactions, soluble factors, and activity. We define the regulatory principles of interneuronal nicotinic synapse differentiation that have emerged from these studies. We also discuss the molecular players that target nAChRs to the surface membrane and the interneuronal synapse.

A polymorphism in the mouse neuronal alpha4 nicotinic receptor subunit results in an alteration in receptor function

Nicotine-stimulated (86)Rb(+) efflux and [(3)H]cytisine binding, both of which seem to measure the nicotinic acetylcholine receptor, composed of alpha4 and beta2 subunits, were assessed in eight brain regions obtained from 14 inbred mouse strains. The potential role of a single nucleotide polymorphism (SNP) in the nicotinic receptor alpha4 subunit gene (Chrna4) on nicotinic receptor binding and function in mice was also evaluated. This SNP leads to an alanine-to-threonine variation at amino acid position 529 of the nascent alpha4 subunit polypeptide. Both nicotine-stimulated (86)Rb(+) efflux and [(3)H]cytisine binding were found to vary across brain regions and among mouse strains. Variability in nicotine-stimulated (86)Rb(+) efflux was positively correlated (r > 0.9) within each strain with the number of [(3)H]cytisine binding sites. However, the number of [(3)H]cytisine binding sites was not correlated with nicotine-stimulated (86)Rb(+) efflux across mouse strains. In contrast, the Chrna4 polymorphism was associated with receptor function across mouse strains: (86)Rb(+) efflux was greater in seven of the eight brain regions studied in those mouse strains that carry the Ala-529 variant of Chrna4. The Chrna4 SNP did not seem to influence the number of [(3)H]cytisine binding sites across mouse strains. These data indicate that inbred mouse strains exhibit differences in receptor function that cannot be attributed to variation in receptor expression but may be explained, at least in part, by the missense polymorphism in the alpha4 subunit.

Differences in the fate of neuronal acetylcholine receptor protein expressed in neurons and stably transfected cells

Ligand-gated ion channels are structurally complex transmembrane proteins that all neurons must synthesize for rapid chemical synaptic transmission. The most abundant nicotinic acetylcholine receptor serving as a ligand-gated ion channel in the nervous system is a species that contains alpha7 subunits, binds alpha-bungarotoxin, and has a high relative permeability to calcium. The ability of neurons to make such receptors was compared with that of non-neuronal cells stably transfected with an alpha7 cDNA to determine whether neuron-specific machinery is likely to aid in their assembly or stabilization. Transfected cells expressed alpha7 protein and assembled it into a species that was indistinguishable in size and pharmacology from native receptors, but much of the alpha7 protein they synthesized was rapidly degraded without becoming receptor. Neurons were not only more efficient than the best transfectants at assembling the receptors but also produced a subpopulation of receptors on the cell surface that was relatively stable and resistant to solubilization. This subpopulation, which was absent from transfected cells, may be tethered to cytoskeletal elements in the neurons. The results support the contention that neurons contain components that facilitate the production and stabilization of ligand-gated ion channels.