Abstract
Neuroblastoma cell lines have been used extensively to screen novel compounds for neurotoxic properties and associated mechanisms. Such transformed cell lines often display morphological, developmental, and signaling characteristics that are substantially different from the parental cell type. Consequently, the response of neuroblastoma cells to toxin exposure may differ from that of neurons. An appreciation of the pharmacological and functional differences between neurons and neuron-like cell lines is therefore essential when interpreting data derived from neuroblastoma-based assays. We have compared the effects of several neurotoxins on Ca2+ homeostasis and cell viability in cerebellar granule neurons (CGN) and a neuroblastoma cell line (Neuro-2a). To explore the mechanisms underlying differential sensitivity of intact neurons and neuroblastoma cells to neurotoxins, we also compared CGN and Neuro-2a cells for expression of voltage-gated sodium channels (VGSC) and N-methyl-D-aspartate receptors (NMDAR). Cytotoxic potency in neurons was several orders of magnitude greater for Caribbean-ciguatoxin-1 (C-CTX-1) than either domoate (Dom) or brevetoxin-2 (PbTx-2). In addition, the cytotoxic potency of C-CTX-1 was two orders of magnitude greater in CGN than in Neuro-2a cells. The effect of C-CTX-1 and Dom on calcium homeostasis was compared in fluo-3 loaded neurons. Dom caused an elevation in intracellular calcium ([Ca 2+]i) at concentrations that paralleled the concentration/response relationship for cytotoxicity in CGN. Conversely, C-CTX-1 did not elevate [Ca2+]; within the dynamic concentration range for cell death. The discordance of the concentration/response relationships for C-CTX-1 induced cytotoxicity and [Ca2+]i elevation suggests that acute C-CTX-1 cytotoxicity may involve mechanisms other than Ca2+ load. C-CTX-1-induced elevation of [Ca2+]i in neurons was dependent on activation of NMDAR and the reverse mode of operation of the Na+/Ca2+ exchanger. These data demonstrate that, although C-CTX-1, domoate, and PbTx-2 share the ability to produce neurotoxicity and mobilize calcium, their respective molecular targets and mechanisms of neurotoxicity differ. Neuro-2a cells that were not pretreated with veratridine and ouabain were insensitive to C-CTX-1 and glutamatergic agonists. VGSC expression was 20-fold lower in Neuro-2a cells than in CGN, whereas NMDARs were not expressed in these neuroblastoma cells. It is therefore likely that the enhanced sensitivity of CGN, relative to Neuro-2a cells, to neurotoxins is a consequence of pronounced differences in VGSC and NMDAR expression. These results underscore the need to exercise caution in interpreting negative cytotoxicity data derived from the use of neuroblastoma cell lines.