The binding of two classes of neurotoxins to axolemma of mammalian brain

Analysis of the effect of the scorpion toxin AaH-II on action potential generation in the axon initial segment

The toxin AaH-II, from the scorpion Androctonus australis Hector venom, is a 64 amino acid peptide that targets voltage-gated Na+ channels (VGNCs) and slows their inactivation. While at macroscopic cellular level AaH-II prolongs the action potential (AP), a functional analysis of the effect of the toxin in the axon initial segment (AIS), where VGNCs are highly expressed, was never performed so far. Here, we report an original analysis of the effect of AaH-II on the AP generation in the AIS of neocortical layer-5 pyramidal neurons from mouse brain slices. After determining that AaH-II does not discriminate between Nav1.2 and Nav1.6, i.e. between the two VGNC isoforms expressed in this neuron, we established that 7 nM was the smallest toxin concentration producing a minimal detectable deformation of the somatic AP after local delivery of the toxin. Using membrane potential imaging, we found that, at this minimal concentration, AaH-II substantially widened the AP in the AIS. Using ultrafast Na+ imaging, we found that local application of 7 nM AaH-II caused a large increase in the slower component of the Na+ influx in the AIS. Finally, using ultrafast Ca2+ imaging, we observed that 7 nM AaH-II produces a spurious slow Ca2+ influx via Ca2+-permeable VGNCs. Molecules targeting VGNCs, including peptides, are proposed as potential therapeutic tools. Thus, the present analysis in the AIS can be considered a general proof-of-principle on how high-resolution imaging techniques can disclose drug effects that cannot be observed when tested at the macroscopic level.

Gangliosides modulate Schwann cell proliferation and morphology

We examined the effect of gangliosides on Schwann cell cultures isolated from neonatal rat sciatic nerves. Addition of gangliosides (GM1, GM3, and ganglioside mixture) at concentrations between 0.25 and 2 mg/ml significantly diminished both the baseline rate of proliferation of the Schwann cells and their response to two types of mitogens, the axolemmal fragments and derivatives of adenosine 3'-5'-monophosphate (cAMP). Gangliosides, the sialic acid residue of which had been removed, were highly toxic to the Schwann cells, which went to indicate that sialic acid is necessary to produce the inhibitory effects. Gangliosides also produced prominent changes in the morphological appearance of the Schwann cells. Most of the Schwann cells treated with gangliosides had an elongated shape with long processes and an alignment of end-to-end or side-by-side cell adhesion. These effects of gangliosides apparently were not mediated by cAMP, since intracellular cyclic adenosine monophosphate (cAMP) of Schwann cells at a basal- and forskolin-stimulated level was not altered by the exogenous gangliosides. These findings indicate that the direct effect of gangliosides on Schwann cells should also be considered as a background mechanism of ganglioside-induced facilitation of neuronal regeneration.

A novel method for the detection of receptors and membrane proteins by scintillation proximity radioassay

A rapid and convenient binding assay for receptors and membrane proteins has been developed. It is based on the binding of 125I-labeled ligands to membrane proteins adsorbed to polyvinyltoluene plastic scintillation microspheres. Membranes or isolated membrane proteins adsorb to the beads upon mixing, and addition of 125I-labeled ligand induces photon emission which is proportional to the amount of added receptor or membrane protein. The interaction of acetylcholine receptor with 125I-labeled alpha-bungarotoxin and antigens with 125I-labeled antibodies or protein A were used as models to test the system. As little as 1 ng of acetylcholine receptor is detected by the assay and a linear relationship with receptor concentration is observed up to 50 ng of receptor per 250 microliter reaction medium. The effects of detergents, salts, soluble proteins, and neutral membranes were studied. Inclusion of bovine serum albumin up to 1 mg/ml, sodium chloride up to 0.5 M, and membranes up to 10 micrograms/ml cause little or no effect on the assay. Detergents at 10-fold below their critical micelle concentrations had little or no effect on the assay. The pharmacological effects of agonists such as acetylcholine were conveniently studied by following the displacement of the 125I-labeled ligand. Similarly, the amount of toxin in crude snake venom can be assayed by measuring competition with the labeled toxin. Only a few seconds are required to perform each binding assay.

Experimental lead neuropathy: inorganic lead inhibits proliferation but not differentiation of Schwann cells

Schwann cells were prepared from the sciatic nerves of newborn rats and cultured in a monolayer. Addition of lead acetate at concentrations between 0.4 and 10.0 micrograms/ml, levels comparable to those occurring in neural tissues and physiological fluids of lead-intoxicated rats, diminished both the baseline rate of proliferation of the Schwann cells and their response to the mitogens, axolemmal fragments, glial growth factor, and the adenosine 3':5'-cyclic monophosphate (cAMP) analogues 8-bromo-cAMP and dibutyryl-cAMP. This demonstrates a direct toxic effect of inorganic lead on Schwann cells. Lead acetate in this concentration range did not, however, inhibit the cAMP analogue-induced appearance of the "myelin marker" lipid galactocerebroside on the surfaces of the cultured Schwann cells.

Characterization of an antiserum against an axolemma-enriched fraction

An antiserum was raised to rat central nervous system (CNS) axolemma-enriched fractions (AEF), which showed no cross-reactivity with myelin proteins or liver microsomes yet gave an endpoint titer of 1:51 200 to CNS AEF by the enzyme-linked immunosorbent assay (ELISA). Immunochemical staining of electroblotted proteins from rat CNS and peripheral nervous system (PNS) AEFs separated by gel electrophoresis identified a major reactive band at 38.5 kD. CNS AEF also showed major immunoreactivity at 91 kD (+/- 3 kD) and a broad band from 110 kD to 130 kD. By immunoperoxidase staining the antiserum specifically recognized the axolemma of peripheral nerve and synaptic terminals in the CNS. The significance of the specificity is discussed with respect to anti-synaptosome antisera.

Astroglial proliferation and phenotype are modulated by neuronal plasma membrane

The rate of proliferation of rat astroglia cultured in a serum-free medium, estimated by tritiated thymidine radioautography, was diminished by more than 50% by addition of rat central nervous system axolemmal fragments to the culture medium. Addition of the axolemmal fragments also induced a phenotypic alteration of the cultured astroglia, from cells of irregular shape containing a fine meshwork of intracytoplasmic glial fibrils to star-shaped cells with thicker, cable-like glial fibrils.

Presence of phospholipid-N-methyltransferases and base-exchange enzymes in rat central nervous system axolemma-enriched fractions

Rat CNS myelinated axons were fractionated by sucrose density gradient centrifugation with a zonal rotor. Fraction VI, obtained at 28-30% sucrose, appeared, on the basis of the presence of related marker enzymes, to be enriched in axolemma. Phospholipid-N-methyltransferases (PMTs) and base-exchange enzymes were associated with fraction VI. PMT activity was significantly stimulated by the addition of either phosphatidylmonomethylethanolamine or phosphatidyldimethylethanolamine but the PMT activity of the homogenate or the myelinated axons was unresponsive. Recoveries of the ethanolamine, serine, and choline base-exchange activities were 14.4%, 13.8%, and 3.4%, respectively, of that present in the myelinated axons. The myelin-rich fraction obtained simultaneously seems contaminated with other membrane fractions.

Subcellular sites of enzymes catalyzing the phosphorylation-dephosphorylation of dolichol in the central nervous system

The subcellular locations of several enzymes involved in dolichyl monophosphate (Dol-P) metabolism in brain have been investigated. Dolichol kinase is highly enriched in a heavy microsomal fraction from calf brain, while 71% of the Dol-P phosphatase activity was recovered with the light microsomes. Lower amounts of the phosphatase activity were also found in the heavy microsomal, mitochondrial-lysosomal, and synaptic plasma membrane fractions. Since the light microsomal fraction also contained substantial acetylcholinesterase activity, an axon plasma membrane marker, an axolemma-enriched fraction, was prepared from rat brain by a second procedure. A comparison with microsomal and mitochondrial-lysosomal fractions revealed that the axolemma-enriched fraction contained the highest specific activity of Dol-P phosphatase, indicating that the enzyme was present in the axon plasma membrane. The tunicamycin-sensitive UDP-N-acetylglucosamine:Dol-P N- acetylglucosaminylphosphotransferase , glucosyl- phosphoryldolichol (Glc-P-Dol) synthase, Glc-P-Dol:oligosaccharide glucosyltransferase, and the oligosaccharyltransferase were all found predominantly in the heavy microsomes. These results indicate that the enzymes responsible for the initiation and termination of biosynthesis, as well as the transfer of dolichol-linked oligosaccharides, reside in the rough endoplasmic reticulum (ER) of central nervous tissue. Evidence that at least some Dol-P molecules formed by dolichol kinase are accessible to multiple glycosyltransferases in the rough ER of brain is also presented.

Specific and potent mitogenic effect of axolemmal fraction on Schwann cells from rat sciatic nerves in serum-containing and defined media

Using cultures of Schwann cells from neonatal rat sciatic nerves, we examined the mitogenic activity of an axolemmal fraction from adult rat CNS. Axolemmal fraction proved a potent mitogen, stimulating [3H]thymidine incorporation into Schwann cell DNA 13.5-fold over control values when axolemmal fraction equivalent to 16 micrograms of protein per culture microwell or more was added. Half maximal stimulation was obtained with addition of axolemmal fraction equivalent to 4 micrograms of protein. The concentration-dependence and magnitude of the mitogenic response of the cultured cells were nearly identical whether they were maintained in vitro for 1 day or for 2 weeks prior to addition of the axolemmal fraction. A study of the time-course of the effect of axolemmal fraction on Schwann cell mitosis showed that maximal [3H]thymidine incorporation took place during the fifth day after addition of axolemmal fraction. Axolemmal fraction also produced stimulation of [3H]thymidine incorporation into Schwann cells, seeded and cultured in a serum-free defined medium. Though the concentration-dependence of the mitogenic effect in the absence of serum was similar to that in a serum-containing medium, maximal stimulation in the defined medium was only 2.8-fold. The mitogenic activity of axolemmal fraction was rapidly and almost totally inactivated by sonication or homogenization, and was partially lost after exposure to heat. The mitogenic activities of plasma membrane fragments from rat skeletal muscle or rat erythrocytes, and of mitochondrial fragments (the major contaminant of the axolemmal fraction) were one-tenth that of axolemmal fraction or less. In contrast to glial growth factor prepared from bovine pituitaries (GGF-BP), which stimulates proliferation of both fibroblasts and Schwann cells, axolemmal fraction induced proliferation of Schwann cells but not of endoneurial fibroblasts; cultures treated with axolemmal fraction demonstrated a 3-fold increase in Schwann cell population in 10 days without detectable increase in number of fibroblasts. Also in contrast to GGF-BP, the mitogenic effect of which is considerably enhanced by simultaneous addition of cholera toxin to the medium, cholera toxin had no effect on the Schwann cell proliferative response to axolemmal fraction.

Identification of an axolemma-enriched fraction from peripheral nerve

A method has been devised for the fractionation of whole peripheral nerve. The procedure utilizes differential centrifugation and separation on a linear sucrose gradient (10-40%, wt/wt). A membrane fraction localized between 26% and 29% sucrose was not only enriched for the plasma membrane markers, 5'-nucleotidase and acetylcholinesterase (AChE), but also possessed the highest binding of [3H]saxitoxin, a specific marker for sodium channels. Neurons in the lumbar dorsal roots and ventral horns of rats were injected with [3H]fucose to label glycoproteins associated with the axolemma from sciatic nerve. Fractionation of the labeled nerves demonstrated a coincidence in the distribution of [3H]fucose-labeled material and AChE activity in the sucrose density gradient. The increase in the specific activity of marker enzymes for plasma membrane, sodium channels, and labeled membrane, previously demonstrated to be of axolemmal origin, identified the 26-29% region of the sucrose gradient as enriched for axolemma derived from peripheral nerve.

The presence of phospholipase D in rat central nervous system axolemma

An axolemma-enriched fraction prepared from a purified myelinated axon fraction isolated from rat CNS was found to contain phospholipase D at a specific activity similar to that of a microsomal fraction isolated from whole brain. There was a concomitant threefold enrichment in the specific activity of phospholipase D and acetylcholinesterase in the axolemma-enriched fraction compared with the specific activities of these enzymes in the starting white matter whole homogenate. This axonal phospholipase D may be involved in remodeling of phospholipid, which in turn may affect axonal functions such as ion translocation.