Selection of variant neuroblastoma clones with missing or altered sodium channels

Pharmacological characterization of sodium channels in the primary culture of individual Drosophila embryos: neurons of a mutant deficient in a putative sodium channel gene

Sodium channels in Drosophila embryonic neurons were characterized pharmacologically in the primary culture of individual gastrulae. In normal cultures, presence of sodium channels was demonstrated by neuronal degeneration in the presence of veratridine and ouabain, which was inhibited by tetrodotoxin. Embryonic neurons of Df(2R)M-c33a homozygotes that lack a putative sodium channel gene at 60E region showed normal neurotoxin sensitivity. Therefore, sodium channel genes other than 60E must be functional at this developmental stage. We also examined para(ts1) and nap(ts) mutants and found that they were also sensitive to the neurotoxins. To determine the genotypes of single embryo cultures by histochemical staining, we utilized a special chromosome bearing a hsp70-lacZ fusion gene. The lacZ expression in the culture was studied in detail.

Altered sensitivity to sodium channel-specific neurotoxins in cultured neurons from temperature-sensitive paralytic mutants of Drosophila

In vitro culture of central nervous system neurons from Drosophila larvae enables direct examination of effects of neurological mutations at a single-cell level not readily amenable to in vivo experimentation. Using this system, we examined the cytotoxic effect of veratridine, which selectively causes persistent activation of sodium channels, on the mutants parats1 and napts known to have a temperature-dependent block in propagation of nerve action potentials. Even at a permissive temperature (22 degrees C) for the mutant flies, the veratridine-induced neuronal lethality was significantly lower in both parats1 and napts cultures than in normal cultures. At a temperature (35 degrees C) causing paralysis of mutant flies, napts neurons showed the same high degree of resistance to veratridine; while parats1 neurons showed an increased resistance to a level similar to that of napts neurons. A similar reduction in the veratridine-induced neuronal death was also observed in normal cultures that were pretreated with the sodium channel blocker tetrodotoxin. These results support the idea that both parats and napts affect sodium channel functions at the level of isolated single neurons. It was also found that parats1 and napts mutations, like the sodium channel blocker tetrodotoxin, do not affect the morphological differentiation and survival of central nervous system neurons in culture. These findings indicate that functional sodium channels are not required for neurite outgrowth and survival of neurons at this developmental stage.

Reduced [3H]-tetrodotoxin binding in the napts paralytic mutant of Drosophila

The temperature-sensitive paralytic mutant of Drosophila, napts, has been shown to have defects in axonal physiology which suggest that it codes for a component of the voltage-sensitive sodium channel. Ligand binding studies using tritiated tetrodotoxin, a sodium channel antagonist, show a decrease in the apparent number of sodium channels in napts, providing additional support for this hypothesis.

Genetics of the nervous system in Drosophila

The genetic analysis ofDrosophilaneurobiology has expanded into a considerable field. A host of genetic variants is now available for analyzing the development, structure, physiology, and chemistry of the fly's excitable cells and tissues and how they control the animal's behavior.

Covalent labeling of protein components of the sodium channel with a photoactivable derivative of scorpion toxin

Azidonitrobenzoyl mono[125I]iodo scorpion toxin can be covalently attached to its receptor site in electrically excitable neuroblastoma cells or synaptosomes by photolysis. A polypeptide of Mr approximately 250,000 is specifically labeled in neuroblastoma cells. Labeling is blocked by unlabeled scorpion toxin and by depolarization. This polypeptide is not labeled in a variant neuroblastoma clone lacking voltage-sensitive sodium channels. Polypeptides of Mr approximately 250,000 and Mr approximately 32,000 are specifically labeled in synaptosomes. This labeling is also blocked by unlabeled toxin and by depolarization. These results identify specific polypeptides of Mr approximately 250,000 and Mr approximately 32,000 that are components of the sodium channel.

Interaction of polypeptide neurotoxins with a receptor site associated with voltage-sensitive sodium channels

Anthopleurin A, a polypeptide toxin from the Pacific sea anemone Anthopleura xanthogrammica, enhances persistent activation of voltage-sensitive sodium channels by the alkaloid toxins veratridine and batrachotoxin with K0.5 = 20 nM. This effect is inhibited by depolarization. There is a close correlation between enhancement of sodium channel activation and block of [125I]scorpion toxin binding by unlabeled scorpion toxin, sea anemone toxin II from Anemonia sulcata, and Anthopleurin A, indicating that these three polypeptide toxins interact with a common receptor site in modifying sodium channel function. Photo-activable derivatives of scorpion toxin label a single Mr approximately 250,000 polypeptide chain at the polypeptide toxin receptor site. Labeling is blocked by unlabeled scorpion toxin or depolarization and is not observed in variant neuroblastoma clones, which lack sodium channels. These results identify a protein component of the polypeptide toxin receptor site of voltage-sensitive sodium channels.