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Previous electrophysiological evidence has indicated that astrocytes and oligodendrocytes express inwardly rectifying K(+) channels both in vitro and in vivo. Here, for the first time, we have undertaken light microscopic immunohistochemical studies demonstrating the location of one such channel, Kir4.1, in both cell types in regions of the rat CNS. Some astrocytes such as those in the deep cerebellar nuclei, Bergmann glia, retinal Müller cells, and a subset in hippocampus express Kir4.1 immunoreactivity, but not others including those in white matter. Oligodendrocytes also express this protein, strongly in perikarya and to a lesser extent in their processes. Expression of Kir4.1 in astrocytes and oligodendrocytes would enable these cells to clear extracellular K(+) through this channel, whereas nonexpressors might use other mechanisms., (Copyright 2000 Wiley-Liss, Inc.)

Voltage-dependent Shaker-related (Kv1) K+ channels are composed of transmembrane alpha subunits and peripheral Kv beta proteins that exist as octomers with (alpha)4(beta)4 stoichiometry. Although several alpha (designated Kv1.X) and three Kv beta subunits are known to be expressed in brain, their oligomeric combinations in neurons have yet to be deciphered. Herein, the subunits comprising a number of neuronal K+ channels from bovine brain cortex were deduced by immunoprecipitation and Western blotting, using antibodies specific for Kv1.X and Kv beta subtypes. Only a subset of the theoretically possible oligomers was detected, showing that the synthesis and/or assembly of these multisubunit K+ proteins is controlled to yield a limited variety of K+ channels. Except for a small population of Kv1.4 containing K+ channels, all the recognizable species contained Kv1.2 and beta2 subunits. Furthermore, several subpopulations were identified including a fully defined complex of Kv1.2/1.3/1.4/1.6 and Kv beta2, plus oligomers containing three or two assigned alpha subunits. Kv1.2 was also shown to occur in the absence of these other subunits as a putative homo-oligomer. Thus, for the first time, the complete subunit combination of an authentic K+ channel has been elucidated; also, the strategy employed to establish this can now be applied to closely related members of other K+ channel families.

alpha-Latrotoxin, a black widow spider neurotoxin, can bind to high affinity receptors on the presynaptic plasma membrane and stimulate massive neurotransmitter release in the absence of Ca2+. Neurexins, previously isolated as alpha-latrotoxin receptors, require Ca2+ for their interaction with the toxin and, thus, may not participate in the Ca2+-independent alpha-latrotoxin activity. We now report the isolation of a novel protein that binds alpha-latrotoxin with high affinity in the presence of various divalent cations (Ca2+, Mg2+, Ba2+, and Sr2+) as well as in EDTA. This protein, termed here latrophilin, has been purified from detergent-solubilized bovine brain membranes by affinity chromatography on immobilized alpha-latrotoxin and concentrated on a wheat germ agglutinin affinity column. The single polypeptide chain of latrophilin is N-glycosylated and has an apparent molecular weight of 120,000. Sucrose gradient centrifugations demonstrated that latrophilin and alpha-latrotoxin form a stable equimolar complex. In the presence of the toxin, anti-alpha-latrotoxin antibodies precipitated iodinated latrophilin, whose binding to immobilized toxin was characterized by a dissociation constant of 0.5-0.7 nM. This presumably membrane-bound protein is localized to and differentially distributed among neuronal tissues, with about four times more latrophilin expressed in the cerebral cortex than in the cerebellum; subcellular fractionation showed that the protein is highly enriched in synaptosomal plasma membranes. Our data suggest that latrophilin may represent the Ca2+-independent receptor and/or molecular target for alpha-latrotoxin.

alpha-Latrocrustatoxin, the crustacean-specific neurotoxin from the venom of the black widow spider Latrodectus mactans tredecimguttatus was radioactively labelled with Bolton-Hunter reagent to the specific activity of 160 Ci/mmol with retention of the biological activity. A highly specific binding of radioactive toxin on plasmatic membranes from the crayfish Astacus astacus nerve cells with Bmax = 0.04 pmol binding toxin/mg membrane protein and Kd = 0.7 x 10(-10) M was demonstrated.

A method of the isolation of a crustacea-specific neurotoxin from the venom of the Latrodectus mactans tredecimguttatus spider by means of ion exchange chromatography on Mono Q and Mono S columns and hydrophobic chromatography on Phenyl-Superose column has been developed. LD50 of the toxin has been elucidated.

The receptor protein of alpha-latrotoxin (alpha LTx, a neurotoxin with 'pure' presynaptic action isolated from black widow spider venom), was solubilized by Triton X-100 from bovine brain membranes and purified by affinity chromatography on alpha LTx-Sepharose. The purified receptor preparation contained four major polypeptides of molecular masses 200 (alpha), 160 (alpha'), 79 (beta) and 43 (gamma) kd according to SDS electrophoresis with molecular ratio alpha 1 alpha' 1 beta 2 gamma 2. The alpha- and alpha'-subunits are glycoproteins binding to wheat germ lectin and can be separated under non-denaturing conditions by anion exchange chromatography. Purified to homogeneity, both of them, though differing in the carbohydrate composition, retain the alpha LTx-binding activity and give closely related peptide maps. Anti-alpha antibodies recognize the alpha'-subunit as well. These results suggest that alpha LTx receptor is present in purified preparations in two very close forms containing the alpha- or alpha'-subunit. Beta and gamma proteins do not specifically bind alpha LTx and their physiological role is unclear. They form a complex with solubilized alpha- and alpha'-subunits independently of alpha LTx presence. The receptor proteins were purified to homogeneity by high performance gel filtration in the presence of SDS, their amino acid composition was determined.

Biospecific sorbents for isolation of the latrotoxin receptor have been obtained and studied. Binding of the receptor components solubilized from the bovine cerebral cortex membrane to the immobilized toxin critically depends on the presence of calcium ions and the solution ionic strength. The procedure for semipreparative isolation of the highly active receptor preparation with Kd = 9 x 10(-10) M and Bmax = 0.9 nmol/mg has been developed. Binding activity of the isolated receptor is inhibited by heating as well as by proteases and denaturing agents. According to electrophoretic analysis in the presence of SDS the receptor complex contains protein components of molecular mass 200, 160, 79, 43 kDa, the former two being glycoproteins.

Iodine-125 labelled alpha-latrotoxin from the venom of Central Asia black widow spider Latrodectus mactans tredecimguttatus binds specifically to the bovine brain membrane receptor producing a stable slowly dissociating complex with Kd = 1.6 x 10(-10) M and Bmax = 0.5 pmol/mg protein. Treatment of the complex with alkaline high-salt buffer induces reversible dissociation of the bound toxin. The antitoxin polyclonal antibody does not increase the dissociation rate of the bound toxin. Wheat germ lectin as well as concanavalin A inhibit the toxin binding to the membrane receptor. The receptor is solubilized with ionic and non-ionic detergents, and methods of latrotoxin binding assay are developed. The solubilized receptor is shown to retain high affinity to toxin, its binding activity being stable but critically dependent on the presence of calcium ions. Chromatographic properties of the receptor suggest its glycoprotein nature.