Partial purification and characterization of neutrophic substance affecting tetrodotoxin sensitivity of organ-cultured mouse muscle

[My research life: from synaptic transmission to behavior]

I have studied signal transmission at synapses and the effects of drugs on it at the molecular and cellular levels. Specific areas of research interest are outlined here. 1) Electrophysiological experiments in cats and rabbits suggested that a new type of analgesic, the phenothiazine derivative levomepromazine, exerts analgesic effects by depressing emotional responses accompanying the sensation of pain. 2) It was hypothesized that motoneurons had long-term effects on muscle cell membrane properties, in addition to controlling moment-to-moment activities. The substance to recover the post-denervation changes in muscle properties in culture was partially purified from mouse nerve extract, which suggested that trophic influences were exerted by substances released from motoneurons. 3) Muscles innervated by adrenergic fibers had sites responsive to acetylcholine as well as to adrenaline in early life in chicks, but only the adrenaline-responsive sites remained during development. Acetylcholine receptor clusters on Xenopus muscles were concentrated at the cholinergic neuromuscular junctions by the movement of receptors from outside the junctions during development. The passive diffusion-trap mechanism explained the accumulation of synaptic receptors at synapses. 4) We found two endocytic pathways and pools of synaptic vesicles contributing to low- and high-frequency synaptic transmission at Drosophila nerve terminals. We then identified two Ca2+ channels designated for the low- and high-frequency endocytosis of synaptic vesicles, straightjacket Ca2+ channels in the active zone and La3+-sensitive Ca2+ channels in the inactive zone at the terminals, respectively. Recently, Drosophila melanogaster has been used as a model for studying the social brain, and the heat avoidance response of the flies was found to be socially enhanced. Future studies are expected to reveal mechanisms underlying social brain functions at the gene level.

Regulation by an extract of embryonic chick brain of the densities of voltage-dependent Na+ and Ca2+ channels in embryonic chick skeletal muscle cells during their development in culture

We studied the chronic effects of a brain extract (BE) prepared from chick embryonic brains on voltage-dependent Na+ channels (VDNCs) and Ca2+ channels (VDCCs) during the development of chick skeletal muscle cells in culture. The maximum rates of rise of Na+ and Ca2+ action potentials were measured electrophysiologically in an attempt to determine the effects of BE on the densities of these channels. The basic culture medium was supplemented with chick transferrin instead of whole-embryo extract and skeletal muscle cells were grown in the absence or in the presence of crude BE or fractionated BE. Long-term inclusion of BE to the culture medium increased the densities of both VDNCs and L-type VDCCs. By contrast, BE apparently decreased the density of T-type VDCCs. Our results indicate that BE contains some protein(s) that has a negative effect on the density of T-type VDCCs of skeletal muscle cells at a less differentiated stage and that this effect of BE is closely associated with subsequent regulation of the densities of VDNCs and L-type VDCCs. Possible roles of the influx of Ca2+ ions through T-type and L-type VDCCs in the control of the densities of VDNCs and L-type VDCCs are discussed.

Brain extract induces the tetrodotoxin-sensitive action potentials in a rat skeletal muscle cell line (L6)

Tetrodotoxin (TTX) usually blocks the action potential in adult vertebrate innervated skeletal muscle, i.e. the action potential is operated by a TTX-sensitive sodium channel. The L6 cultured myotube, however, has TTX-resistant action potential that is usually found in skeletal muscle following denervation or during fetal and neonatal stages in rat. The TTX-resistivity of the L6 myotubes might be due to a lack of a neurotrophic substance. The present study revealed that brain extract had a trophic substance that is capable of inducing the TTX-sensitive sodium channels in uninnervated cultured L6 myotubes. Seventeen to 18 days old cultures were used and the maximum rate of rise of action potential was measured as an index of the sodium channel density. In control cultures, the maximum rate of rise was about 120 V/s and did not change by TTX application. However, in cultures with the brain extract the value was 142 V/s and reduced to 86 V/s by TTX. The reduced maximum rate of rise by TTX in the latter cultures means that exposure of L6 myotubes to brain extract resulted in an increase in TTX-sensitive sodium channels and a decrease in TTX-resistant sodium channels. Therefore, exposure to a factor in brain extract can elicit development of TTX-sensitive sodium channels in isolated L6 muscle cells that have been grown as a cell line in the absence of neurons for over 10 years.

Inhibition by alpha-amanitin of development of tetrodotoxin-sensitive spike induced by brain extract in cultured chick skeletal muscle cells

The enhancing effect of brain extract on the development of tetrodotoxin-sensitive sodium channels in cultured chick skeletel muscle cells is blocked by treatment of culture with alpha-amanitin, and RNA polymerase inhibitor. The result is interpreted as indicating that the trophic substance in the brain extract exerts its effect on the development of sodium channels through regulation of gene transcription.

Neurotrophic substance develops tetrodotoxin-sensitive action potential and increases curare-sensitivity of acetylcholine response in cultured rat myotubes

We examined the trophic effects of a partially purified trophic substance from mouse spinal cord extract on the tetrodotoxin (TTX)-sensitivity of action potentials and on acetylcholine-sensitivity of rat skeletal myotubes in 7- and 8-day-old cultures. Many myotubes grown in control medium generate action potentials in the presence of TTX (10(-6) M). The addition of fraction E (Fr.E) from a Biogel P2 column, which exhibited trophic activity on adult denervated muscle in organ culture, decreased TTX-resistivity of action potentials of myotubes in cell culture. The trophic substance was also effective when further purified by paper chromatography and electrophoresis. The response to iontophoretically applied acetylcholine of Fr.E-treated myotubes was much more reduced by D-tubocurarine (10(-7) g/ml) than those of control cultured myotubes. No difference in morphological differentiation, protein synthesis, creatine phosphokinase activity or specific binding of [125I]alpha-bungarotoxin was observed between control and Fr.E-treated cultures. These results suggest that the trophic substance in Fr.E may be involved in the normal development of TTX-sensitive sodium channels and of acetylcholine receptor properties.

Characterization and partial purification of a neuronal factor which increases acetylcholine receptor aggregation on cultured muscle cells

Medium conditioned by NG108-15 neuroblastoma x glioma hybrid cells contains a factor which increases the number of acetylcholine receptor (AChR) aggregates on cultured myotubes. Protease digestion indicates that the AChR aggregation factor is a protein, and the molecular weight is from 150,000 to 250,000 daltons as estimated by ultrafiltration and gel filtration. Preparative isoelectrofocusing indicates that the aggregation factor has a pI of about 4.7. The factor is found in the soluble cytoplasmic fraction but not in the plasma membrane fraction of NG108-15 cells. Aggregation activity is not detected in the cytoplasm of liver cells or in the cytoplasm of C6BU-1 glioma cells. A possible developmental role for the aggregation factor is suggested by its presence in embryonic rat brain but not in adult rat brain. AChR aggregation factors found in the cytoplasm or conditioned medium of NG108-15 cells or in the cytoplasmic fraction of fetal brain have similar molecular weights and isoelectric points.

Dependence of in vitro myogenesis on a trophic protein present in chicken embryo extract

A trophic protein (sciatin) purified from sciatic nerves has been shown to enhance the morphological development and to promote the maintenance of skeletal muscle cells in vitro [Markelonis, G. J. & Oh, T. H. (1979) Proc. Natl. Acad. Sci. USA 76, 2470-2474]. We have elicited a specific antiserum against purified sciatin in rabbits. By using this antiserum, we have examined whether sciatin is also required for the initial differentiation of avian myogenic cells in vitro. Sciatin was found to be a component of chicken embryo extract, a constituent of culture medium required for myogenesis in vitro, by an immunodiffusion assay and by NaDodSO4 gel electrophoresis of immunoprecipitates. The removal of sciatin from chicken embryo extracts by immunoprecipitation with antiserum against sciatin completely inhibited myogenesis. When myogenic cells were grown in culture medium from which sciatin had been removed, the cells failed to differentiate beyond the myoblast stage. However, when sciatin (25 microgram/ml) was added to the sciatin-absorbed culture medium, normal myogenesis ensued. Furthermore, myogenic cells underwent normal myogenesis in the absence of embryo extract if sciatin (25 microgram/ml) was added to the culture medium. These results demonstrate that sciatin is the component of chicken embryo extract required for myogenesis and that the protein influences the initial differentiation of myogenic cells in vitro.