Assessing the role of the transient receptor potential A1 channel in methylmercury-induced neurotoxicity.

摘要

Methylmercury (MeHg) is an environmental contaminant which bioaccumulates in aquatic food chains and, because fish and piscivorous animal species are global commodities, the potential for human exposure to this toxicant knows no geographic boundary. Human populations exposed to MeHg, either acutely or chronically, present with severe neurologic symptoms, with the preeminent clinical sign being distal paresthesia. The definitive mechanisms by which this pathologic state arises remains elusive, though the penultimate event is widespread degeneration somatosensory neurons of the dorsal root ganglia (DRG). Moreover, following in vivo MeHg exposure, a significant reduction in the size and number of large-fiber mechanoreceptive afferents is measurable, with a relative sparing of small-fiber nociceptive afferents and motor efferents. MeHg-induced cytotoxicity has been attributed to unregulated increase in intracellular Ca2+ concentration ([Ca2+]i); this perturbation arises in kinetically distinct phases, with sources of Ca 2+ including efflux from intracellular storage organelles, and influx through Ca2+-permeable ion channels. The transient receptor potential (TRP) family of ion channels has been implicated as potential targets for MeHg due to their Ca2+ permeability, high expression in DRG neurons, and polymodal means of activation. The objective of this study was to determine whether the ankyrin 1 TRP channel isoform (TRPA1) selectively confers MeHg sensitivity on large-fiber DRG, and to characterize the contribution of TRPA1 to MeHg-induced [Ca2+]i dysregulation; TRPA1 was selected as a putative target for its role as a mechanoreceptor and potential for activation via cysteine-reactive compounds. Recombinant TRPA1, when acutely exposed to MeHg in vitro, contributed to MeHg-induced [Ca2+]i dysregulation and cell death in an extracellular Ca2+ (Ca2+e)-dependent manner. MeHg-induced [Ca2+]i elevations and neurotoxicity was also Ca 2+e-dependent in acute dissociations of primary DRG, however the definitive contribution of TRPA1 as a mediator of Ca2+ influx could not be confirmed. Rather, whole-cell current recordings of large-fiber DRG revealed Na+ as the primary charge carrier in agonist-induced activation of TRPA1, and the onset of [Ca2+]i disruption was dependent upon extracellular Na+. This work contributes to our understanding of the actions of MeHg on TRP channels and implicates a role for other cations in mediating MeHg-induced [Ca2+] i dysregulation.