Comparative Impact of Voltage-Gated Calcium Channels and NMDA Receptors on Mitochondria-Mediated Neuronal Injury

Abstract

Glutamate excitotoxicity, a major component of many neurodegenerative disorders, is characterized by excessive calcium influx selectively through NMDA receptors (NMDARs). However, there is a substantial uncertainty concerning why other known routes of significant calcium entry, in particular voltage-gated calcium channels (VGCCs), are not similarly toxic. Here, we report that in the majority of neurons in rat hippocampal and cortical cultures, maximal L-type VGCC activation induces much lower calcium loading than toxic NMDAR activation. Consequently, few depolarization-activated neurons exhibit calcium deregulation and cell death. Activation of alternative routes of calcium entry induced neuronal death in proportion to the degree of calcium loading. In a small subset of neurons depolarization evoked stronger calcium elevations, approaching those induced by toxic NMDA. These neurons were characterized by elevated expression of VGCCs and enhanced voltage-gated calcium currents, mitochondrial dysfunction and cell death. Preventing VGCC-dependent mitochondrial calcium loading resulted in stronger cytoplasmic calcium elevations, whereas inhibiting mitochondrial calcium clearance accelerated mitochondrial depolarization. Both observations further implicate mitochondrial dysfunction in VGCC-mediated cell death. Results indicate that neuronal vulnerability tracks the extent of calcium loading but does not appear to depend explicitly on the route of calcium entry.

Mitochondria from a mouse model of the human infantile neuroaxonal dystrophy (INAD) with genetic defects in VIA iPLA 2 have disturbed Ca 2+ regulation with reduction in Ca 2+ capacity

“…An excess of Ca 2þ over the CRC threshold leads to the opening of the permeability transition pore in mitochondria followed by release of apoptotic factors, like cytochrome C (Stanika et al, 2012;Stout et al, 1998;Azarashvili et al, 2007). We hypothesize that during pathological elevations of intracellular Ca 2þ , mitochondria from INAD animals might readily undergo Ca 2þ overload and elicit neuronal cell demise.…”

“…CRC is the maximal amount of Ca 2þ which mitochondria are able to accumulate. If mitochondria undergo a Ca 2þ overflow beyond the CRC, they open the permeability transition pore that is accompanied by complete depolarization and release of apoptotic factors, like cytochrome C (Stanika et al, 2012;Stout et al, 1998;Azarashvili et al, 2007).…”

Involvement of the GluN2A and GluN2B Subunits in Synaptic and Extrasynaptic N-methyl-d-aspartate Receptor Function and Neuronal Excitotoxicity

“…Compared with our experimental setup, these early studies did not include nifedipine (an antagonist for L-VGCCs) and CNQX (an antagonist for non-NMDA-type glutamate receptors) during NMDA stimulation. Because the activation of VGCCs (35)(36)(37)43) and non-NMDA-type glutamate receptors (38 -40) contributes significantly to excitotoxicity-induced Ca 2ϩ overload, NMDA stimulation without nifedipine and CNQX may impose a more severe pathological insult. It is possible that the protective effect from GluN2A inhibition may depend on the degree of the insults.…”

NADPH Oxidase-2: Linking Glucose, Acidosis, and Excitotoxicity in Stroke

“…Specificity for the route of Ca 2 + influx in this process has additionally been shown by studies in which, Ca 2 + elevations of comparable magnitude induced by ionomycin, as measured with both high-and lowaffinity calcium-sensitive dyes, do not induce p47 phox phosphorylation or NOX2 activation in neurons (16). However, ionomycin-induced calcium elevations, to levels much higher than occur with NMDA receptor activation, can trigger neuronal superoxide production by mechanisms independent of NOX2 (16,165).…”

“…Calcium influx has long been established as a requisite event in excitotoxicity (29), but it has been difficult to resolve whether the route of calcium influx is important in this process (23,118,165). Tymianski and colleagues showed that NMDA-induced nitric oxide production requires calcium influx specifically through NMDA receptors (157,158).…”