Ca2+- or Mg2+-dependent enzymatic ATP hydrolysis associated with the microsomal fraction of frog sciatic nerves

CaV2.2 Gates Calcium-Independent but Voltage-Dependent Secretion in Mammalian Sensory Neurons

Action potential induces membrane depolarization and triggers intracellular free Ca²⁺ concentration (Ca²⁺)-dependent secretion (CDS) via Ca²⁺ influx through voltage-gated Ca²⁺ channels. We report a new type of somatic exocytosis triggered by the action potential per se-Ca²⁺-independent but voltage-dependent secretion (CiVDS)-in dorsal root ganglion neurons. Here we uncovered the molecular mechanism of CiVDS, comprising a voltage sensor, fusion machinery, and their linker. Specifically, the voltage-gated N-type Ca²⁺ channel (Ca_V2.2) is the voltage sensor triggering CiVDS, the SNARE complex functions as the vesicle fusion machinery, the "synprint" of Ca_V2.2 serves as a linker between the voltage sensor and the fusion machinery, and ATP is a cargo of CiVDS vesicles. Thus, CiVDS releases ATP from the soma while CDS releases glutamate from presynaptic terminals, establishing the Ca_V2.2-SNARE "voltage-gating fusion pore" as a novel pathway co-existing with the canonical "Ca²⁺-gating fusion pore" pathway for neurotransmitter release following action potentials in primary sensory neurons.

The effect of gossypol on fast axonal transport and microtubule assembly

Gossypol at micromolar concentrations (2 microM) was found to inhibit axonal transport and a microsomal ATPase activity in the frog sciatic nerve, although axonal microtubules and the neuronal content of AMP, ADP and ATP were not affected. At slightly higher concentrations (30-40 microM), gossypol also inhibited microtubule assembly and neuronal energy metabolism. Gossypol accumulated in the nerve and the results indicate that gossypol may act as a potent neurotoxin.

Inhibition of fast axonal transport by erythro-9-[3-(2-hydroxynonyl)]adenine

erythro-9-[3-(2-Hydroxynonyl)]adenine, an inhibitor of protein carboxylmethylation and dynein-ATPase activity, inhibited fast axonal transport in vitro in frog sciatic nerves. Its site of action might be associated with an ATPase on which transport depends, since specific carboxylmethylation inhibitors lacked effects on transport. The levels of high energy phosphates and protein synthesis were unaffected by the drug at a transport-inhibiting concentration, making disturbances due to metabolic effects less likely. An erythro-9-[3-(2-hydroxynonyl)]adenine-sensitive ATPase was looked for in various nerve fractions but has so far not been resolved.

Characterization of the Ca2+- and Mg2+-dependent ATPases in Electrophorus electroplax microsomes

Electrophorus electroplax microsomes were examined for Ca2+- and Mg2+-dependent ATPase activity. In addition to the previously reported low-affinity ATPase, a high-affinity (Ca2+,Mg2+)-ATPase was found. At low ATP and Mg2+ concentrations (200 microM or less), the high-affinity (Ca2+,Mg2+)-ATPase exhibits an activity of 18 nmol Pi mg-1 min-1 with 0.58 microM Ca2+. At higher ATP concentrations (3 mM), the low-affinity Ca2+-ATPase predominates, with an activity of 28 nmol Pi mg-1 min-1 with 1 mM Ca2+. In addition, Mg2+ can also activate the low-affinity ATPase (18 nmol Pi mg-1 min-1). The high-affinity ATPase hydrolyzes ATP at a greater rate than it does GTP, ITP, or UTP and is insensitive to ouabain, oligomycin, or dicyclohexylcarbodiimide inhibition. The high-affinity enzyme is inhibited by vanadate, trifluoperazine, and N-ethylmaleimide. Added calmodulin does not significantly stimulate enzyme activity; rinsing the microsomes with EGTA does not confer calmodulin sensitivity. Thus the high-affinity ATPase from electroplax microsomes is similar to the (Ca2+,Mg2+)-ATPase reported to be associated with Ca2+ transport, based on its affinity for calcium and its response to inhibitors. The low-affinity enzyme hydrolyzes all tested nucleoside triphosphates, as well as diphosphates, but not AMP. Vanadate and N-ethylmaleimide do not inhibit the low-affinity enzymes. The low-affinity enzyme reflects a nonspecific nucleoside triphosphatase, probably an ectoenzyme.

Mn2+-stimulated ATPase in rat brain

Divalent cation ATPases were prepared from rat brain synaptic vesicles, synaptosomal plasma membranes, and plasma membranes from the brain stem and sciatic nerve and tested for optimal stimulation by Mn2+, Mg2+, or Ca2+. ATPase in the synaptic vesicle subfraction was optimally stimulated by Mn2+. All plasma membrane preparations were optimally stimulated by Mg2+. Separate Mn2+ and Mg2+ ATPases could not be distinguished by either chemical inactivation or substrate preference criteria. Mn2+ stimulated ATPase in the micromolar range and it is suggested that Mn2+ interaction with ATPase may be of physiological and/or toxicological importance by being related to the cellular metabolism of this element.

Calcium requirement for fast axonal transport in frog motoneurons

Calcium is required to sustain fast axonal transport in sensory neurons of frog and cat. We studied the Ca2+ dependence of fast axonal transport in the motoneurons of the lower spinal cord from frog. The accumulation of acetylcholinesterase at a crush on the ventral roots was used to follow axonal transport. Two types of experiments were performed: modification of the medium bathing the ventral roots, alone, and modification of the medium bathing the spinal cord and ventral roots. Incubation (17--18 h) of the ventral roots in Ca2+-free medium markedly inhibited acetylcholinesterase transport, a finding that demonstrates a Ca2+ requirement for fast axonal transport in motoneurons; when 4 mM MgCl2 was added to the Ca2+-free medium, transport was also greatly reduced. During incubation of the ventral roots in normal medium supplemented with 0.18 mM CoCl2 transport proceeded normally; but when the Co2+ concentration was raised to 1.8 mM, transport was diminished as drastically as in the Ca2+-free medium. Incubation of the spinal cord and ventral roots in medium containing 0.18 mM CoCl2 did not reduce the accumulation of acetylcholinesterase at the crush. Similarly, accumulation of acetylcholinesterase at a crush on the dorsal root was not significantly reduced by exposure of the dorsal root ganglion and root to 0.18 mM Co2+. Exposure of sensory cell bodies to 0.18 mM Co2+ thus produces differential effects on transport of acetylcholinesterase and on transport of newly synthesized radiolabeled protein.

Ca2+- or Mg2+-stimulated ATPase activity in bullfrog spinal nerve: relation to Ca2+ requirements for fast axonal transport

Adenosine triphosphatase (ATPase) activity stimulated by Ca2+ or Mg2+ was characterized in spinal nerve and spinal sensory ganglion of bullfrog. Enzyme activity of homogenates from both sources reached a maximum at a 1-2 mM concentration of either cation, although the level of maximal activity in nerve trunks was approximately twice that in ganglia. Enzyme activation was not observed with 2 mM-Sr2+ or Ba2+. Co2+ or Mn2+, at 2 mM, depressed Ca2+ activation of the enzyme by 50-60% in nerve but had no inhibitory effect on ganglia activity. In intact spinal ganglion/spinal nerve preparations, incubated for 20 h in medium containing 0.2 mM-Co2+, no effect was detected on Ca2+/Mg2+ ATPase activity in ganglia or nerve trunks whereas fast axonal transport was inhibited by 80%. Incubation in medium containing 0.02 mM-Hg2+ depressed enzyme activity in ganglia by 64% and in nerve trunks by 44%, whereas fast transport was again inhibited by 80%. When only nerve trunks were exposed to these ions, Hg2+ but not Co2+ was observed to slow the rate of fast axonal transport. The divalent cation specificity of the Ca2+/Mg2+ ATPase activity is distinct from the ion specificities, determined in previous work, of the Ca2+ requirement during initiation of fast axonal transport in the soma, and of the Ca2+ requirement during translocation in the axon. Thus, previous observations of Ca2+-dependent events in fast axonal transport cannot be taken per se to suggest the involvement of Ca2+/Mg+ ATPase in the transport process.