Ca2+ channels as targets of neurological disease:: Lambert-Eaton syndrome and other Ca2+ channelopathies

In the nervous system, voltage-gated Ca2+ channels regulate numerous processes critical to neuronal function including secretion of neurotransmitters, initiation of action potentials in dendritic regions of some neurons, growth cone elongation, and gene expression. Because of the critical role which Ca2+ channels play in signaling processes within the nervous system, disruption of their function will lead to profound disturbances in neuronal function. Voltage-gated Ca2+ channels are the targets of several relatively rare neurological or neuromuscular diseases resulting from spontaneously-occurring mutations in genes encoding for parts of the channel proteins, or from autoimmune attack on the channel protein responses. Mutations in CACNA1A, which encodes for the alpha(1A) subunit of P/Q-type Ca2+ channels, lead to symptoms seen in familial hemiplegic migraine, episodic ataxia type 2, and spinocerebellar ataxia type 6. Conversely, autoimmune attack on Ca2+ channels at motor axon terminals causes peripheral cholinergic nerve dysfunction observed in Lambert - Eaton Myasthenic Syndrome (LEMS), the best studied of the disorders targeting voltage-gated Ca2+ channels. LEMS is characterized by decreased evoked quantal release of acetylcholine (ACh) and disruption of the presynaptic active zones, the sites at which ACh is thought to be released. LEMS is generally believed to be due to circulating antibodies directed specifically at the Ca2+ channels located at or near the active zone of motor nerve terminals (P/Q-type) and hence involved in the release of ACh. However, other presynaptic proteins have also been postulated to be targets of the autoantibodies. LEMS has a high degree of coincidence (similar to60%) with small cell lung cancer; the remaining 40% of patients with LEMS have no detectable tumor. Diagnosis of LEMS relies on characteristic patterns of electromyographic changes; these changes are observable at neuromuscular junctions of muscle biopsies from patients with LEMS. In the majority of LEMS patients, those having detectable tumor, the disease is thought to occur as a result of immune response directed initially against voltage-gated Ca2+ channels found on the lung tumor cells. In these patients, effective treatment of the underlying tumor generally causes marked improvement of the symptoms of LEMS as well. Animal models of LEMS can be generated by chronic administration of plasma, serum or immunoglobulin G to mice. These models have helped dramatically in our understanding of the pathogenesis of LEMS. This "passive transfer" model mimics the electrophysiological and ultrastructural findings seen in muscle biopsies of patients with LEMS. In this model, we have shown that the reduction in amplitude of Ca2+ currents through P/Q-type channels is followed by "unmasking" of an L-type Ca2+ current not normally found at the motor nerve terminal which participates in release of ACh from terminals of mice treated with plasma from patients with LEMS. It is unclear what mechanisms underlie the development of this novel L-type Ca2+ current involved in release of ACh at motor nerve terminals during passive transfer of LEMS.