Differential Localization of Voltage-Dependent Calcium Channel α₁Subunits at the Human and Rat Neuromuscular Junction

Abstract

Neurotransmitter release is regulated by voltage-dependent calcium channels (VDCCs) at synapses throughout the nervous system. At the neuromuscular junction (NMJ) electrophysiological and pharmacological studies have identified a major role for P-and/or Q-type VDCCs in controlling acetylcholine release from the nerve terminal. Additional studies have suggested that N-type channels may be involved in neuromuscular transmission. VDCCs consist of pore-forming ␣ 1 and regulatory ␤ subunits. In this report, using fluorescence immunocytochemistry, we provide evidence that immunoreactivity to ␣ 1A , ␣ 1B , and ␣ 1E subunits is present at both rat and human adult NMJs. Using control and denervated rat preparations, we have been able to establish that the subunit thought to correspond to P/Q-type channels, ␣ 1A , is localized presynaptically in discrete puncta that may represent motor nerve terminals. We also demonstrate for the first time that ␣ 1A and ␣ 1B (which corresponds to N-type channels) may be localized in axon-associated Schwann cells and, further, that the ␣ 1B subunit may be present in perisynaptic Schwann cells. In addition, the ␣ 1E subunit (which may correspond to R/T-type channels) seems to be localized postsynaptically in the muscle fiber membrane and concentrated at the NMJ. The possibility that all three VDCCs at the NMJ are potential targets for circulating autoantibodies in amyotrophic lateral sclerosis is discussed.

Acetylcholine Release at Neuromuscular Junctions of Adult Tottering Mice Is Controlled by N-(Cav2.2) and R-Type (Cav2.3) but Not L-Type (Cav1.2) Ca2+Channels

“…The distribution and juxtaposition of the two stains is consistent with results described by Day et al (1997). However, this pattern of distribution was not observed in the tg preparations.…”

Calcium Signaling Pathways Mediating Synaptic Potentiation Triggered by Amyotrophic Lateral Sclerosis IgG in Motor Nerve Terminals

“…Strikingly, these results suggested that ALS-IgG did not interact with Ca v 2.2 channels, in agreement with previous immunoprecipitation and ELISA results (Arsac et al, 1996). Furthermore, Ca v 2.2 channels remain in denervated muscles (Day et al, 1997;Pagani et al, 2004), whereas ALS-IgG molecular target disappears completely after denervation (Fig. 4).…”

Human autoantibodies specific for the α 1A calcium channel subunit reduce both P-type and Q-type calcium currents in cerebellar neurons

“…Passive transfer of LEMS IgG to mice has been shown to cause a reduction in the quantal release of acetylcholine at the neuromuscular junction, a process that is believed to be mediated by the P͞Q-type VDCC (19,23,24). In addition, recent studies have demonstrated presynaptic expression of the ␣ 1A subunit at the human neuromuscular junction (25). Taken in conjunction with these published studies, our data provide strong evidence that antibodies directed against P͞Q-type VDCCs are responsible for the impairment of neuromuscular transmission in LEMS.…”