Voltage-gated calcium channels are abnormal in cultured spinal motoneurons in the G93A-SOD1 transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of motoneurons. Hyperexcitability and excitotoxicity have been implicated in the early pathogenesis of ALS. Studies addressing excitotoxic motoneuron death and intracellular Ca²⁺ overload have mostly focused on Ca²⁺ influx through AMPA glutamate receptors. However, intrinsic excitability of motoneurons through voltage-gated ion channels may also have a role in the neurodegeneration. In this study we examined the function and localization of voltage-gated Ca²⁺ channels in cultured spinal cord motoneurons from mice expressing a mutant form of human superoxide dismutase-1 with a Gly93 → Ala substitution (G93A-SOD1). Using whole-cell patch-clamp recordings, we showed that high voltage activated (HVA) Ca²⁺ currents are increased in G93A-SOD1 motoneurons, but low voltage activated Ca²⁺ currents are not affected. G93A-SOD1 motoneurons also have altered persistent Ca²⁺ current mediated by L-type Ca²⁺ channels. Quantitative single-cell RT-PCR revealed higher levels of Ca1a, Ca1b, Ca1c, and Ca1e subunit mRNA expression in G93A-SOD1 motoneurons, indicating that the increase of HVA Ca²⁺ currents may result from upregulation of Ca²⁺ channel mRNA expression in motoneurons. The localizations of the Ca1B N-type and Ca1D L-type Ca²⁺ channels in motoneurons were examined by immunocytochemistry and confocal microscopy. G93A-SOD1 motoneurons had increased Ca1B channels on the plasma membrane of soma and dendrites. Ca1D channels are similar on the plasma membrane of soma and lower on the plasma membrane of dendrites of G93A-SOD1 motoneurons. Our study demonstrates that voltage-gated Ca²⁺ channels have aberrant functions and localizations in ALS mouse motoneurons. The increased HVA Ca²⁺ currents and PC_Ca current could contribute to early pathogenesis of ALS.