ResultsUsing PCR, we constructed chimeric subunits in which the α7 cytoplasmic loop immediately after TM3 and before TM4 was replaced with the homologous region of the α3 or α5 sequence (Fig. 1a). The N-terminus up to TM2 regulates intersubunit assembly into receptor complexes, and α7 subunits do not coassemble with nAChR subunits, as demonstrated for endogenous subunits in CG neurons and for chimeric α7 subunits expressed in Xenopus laevis oocytes 4,5,12,13 . In particular, we have observed that coexpression of chimeric α7 subunits containing the α3 cytoplasmic loop together with wild-type α3 and β4 subunits in Xenopus oocytes results in the formation of two distinct receptor types that have the pharmacological properties of Bgt-nAChRs and nAChRs, respectively (data not shown). Thus, a difference in the distribution of chimeric α7 as compared to wild-type α7 on the infected CG neuron surface would result from the added α3 or α5 cytoplasmic loop. It cannot be explained by assembly with an endogenous nAChR subunit that can target to the synapse. Different types of neurotransmitter receptors coexist within single neurons and must be targeted to discrete synaptic regions for proper function. In chick ciliary ganglion neurons, nicotinic acetylcholine receptors (nAChRs) containing α3 and α5 subunits are concentrated in the postsynaptic membrane, whereas α-bungarotoxin receptors composed of α7 subunits are localized perisynaptically and excluded from the synapse. Using retroviral vector-mediated gene transfer in vivo, we show that the long cytoplasmic loop of α3 targets chimeric α7 subunits to the synapse and reduces endogenous nAChR surface levels, whereas the α5 loop does neither. These results show that a particular domain of one subunit targets specific receptor subtypes to the interneuronal synapse in vivo. Moreover, our findings suggest a difference in the mechanisms that govern assembly of interneuronal synapses as compared to the neuromuscular junction in vertebrates.1998 Nature America Inc.• http://neurosci.nature.com 1998 Nature America Inc.• http://neurosci.nature.com
Normal cognitive and autonomic functions require nicotinic synaptic signaling. Despite the physiological importance of these synapses, little is known about molecular mechanisms that direct their assembly during development. We show here that the tumor-suppressor protein adenomatous polyposis coli (APC) functions in localizing ␣3-nicotinic acetylcholine receptors (nAChRs) to neuronal postsynaptic sites. Our quantitative confocal microscopy studies indicate that APC is selectively enriched at cholinergic synapses; APC surface clusters are juxtaposed to synaptic vesicle clusters and colocalize with ␣3-nAChRs but not with the neighboring synaptic glycine receptors or perisynaptic ␣7-nAChRs on chick ciliary ganglion (CG) neurons. We identify PSD (postsynaptic density)-93, -catenin, and microtubule end binding protein EB1 as APC binding partners. PSD-93 and -catenin are also enriched at ␣3-nAChR postsynaptic sites. EB1 shows close proximity to and partial overlap with ␣3-nAChR and APC surface clusters. We tested the role of APC in neuronal nicotinic synapse assembly by using retroviral-mediated in vivo overexpression of an APC dominant-negative (APC-dn) peptide to block the interaction of endogenous APC with both EB1 and PSD-93 during synapse formation in CG neurons. The overexpressed APC-dn led to dramatic decreases in ␣3-nAChR surface levels and clusters. Effects were specific to ␣3-nAChR postsynaptic sites; synaptic glycine receptor and perisynaptic ␣7-nAChR clusters were not altered. In addition, APC-dn also reduced surface membrane-associated clusters of PSD-93 and EB1. The results show that APC plays a key role in organizing excitatory cholinergic postsynaptic specializations in CG neurons. We identify APC as the first nonreceptor protein to function in localizing nAChRs to neuronal synapses in vivo.
BackgroundMalignant glioma cells are particularly motile and can travel diffusely through the brain parenchyma, apparently without following anatomical structures to guide their migration. The neural adhesion/recognition protein L1 (L1CAM; CD171) has been implicated in contributing to stimulation of motility and metastasis of several non-neural cancer types. We explored the expression and function of L1 protein as a stimulator of glioma cell motility using human high-grade glioma surgical specimens and established rat and human glioma cell lines.ResultsL1 protein expression was found in 17 out of 18 human high-grade glioma surgical specimens by western blotting. L1 mRNA was found to be present in human U-87/LacZ and rat C6 and 9L glioma cell lines. The glioma cell lines were negative for surface full length L1 by flow cytometry and high resolution immunocytochemistry of live cells. However, fixed and permeablized cells exhibited positive staining as numerous intracellular puncta. Western blots of cell line extracts revealed L1 proteolysis into a large soluble ectodomain (~180 kDa) and a smaller transmembrane proteolytic fragment (~32 kDa). Exosomal vesicles released by the glioma cell lines were purified and contained both full-length L1 and the proteolyzed transmembrane fragment. Glioma cell lines expressed L1-binding αvβ5 integrin cell surface receptors. Quantitative time-lapse analyses showed that motility was reduced significantly in glioma cell lines by 1) infection with an antisense-L1 retroviral vector and 2) L1 ectodomain-binding antibodies.ConclusionOur novel results support a model of autocrine/paracrine stimulation of cell motility in glioma cells by a cleaved L1 ectodomain and/or released exosomal vesicles containing L1. This mechanism could explain the diffuse migratory behavior of high-grade glioma cancer cells within the brain.
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