The NMDA subtype of the glutamate receptor (NMDARs) plays essential and diverse roles in normal CNS function. However over-activation of these receptors leads to excessive Ca2+ entry, mitochondrial calcium (Ca2+) overload and dysfunction, and is a crucial early event in excitotoxic injury. This recommends NMDARs as targets for anti-excitotoxic therapies, but the generally disappointing patient outcomes for such approaches strongly suggest that factors beyond the over-activation of NMDARs are at play. This information prompts searches for other important Ca2+-dependent injury pathways. Recent progress and the Specific Aims of ongoing work are summarized next.
Aim #1 : To define the role of voltage-gated calcium channels in calcium-dependent neurodegeneration. Although NMDARs clearly play the dominant role in toxic Ca2+ loading, evidence from our lab and others indicates that alternative routes of Ca2+ entry, for example, through voltage-gated calcium channels (VGCCs), can contribute significantly to toxicity in developmentally mature neurons. Thus, we find that while in general in hippocampal and cortical cultured neurons VGCC activation does not promote significant cell death, VGCC activation does evoke much stronger calcium elevations in a small but important subset of neurons. These neurons are characterized by elevated expression of VGCCs, which leads to cell death by mechanisms that are reminiscent of the classical excitotoxicity pathway, namely, excessive Ca2+ loading and mitochondrial dysfunction that precedes neuronal degeneration. The results demonstrate one ancillary pathway of glutamate toxicity, one whose significance is likely to increase during brain aging or in age-related dementia.
Aim #2 : To determine the role of zinc in glutamate excitotoxicity and ischemic injury. Elevation of intracellular zinc (Zn2+) following transient ischemia contributes to neuronal injury, but the mechanism(s) of Zn2+ toxicity remain unclear. In analogy to Ca2+, Zn2+ has been proposed to induce toxicity via mitochondrial dysfunction and/or ROS generation. Recent experiments in cultured hippocampal neurons reveal that Zn2+ can enter neurons through VGCCs and Ca2+-permeable AMPA receptors, but accumulated Zn2+ is only toxic when the extracellular medium contains unusually high (200 uM) exogenous Zn2+ concentrations, as might occur after ischemia. Under conditions that favor Zn2+ uptake, both Ca2+ and Zn2+ accumulate and co-precipitate within mitochondria, but only Ca2+ is capable of inducing mitochondrial swelling, depolarization, and free radical generation, which are the main hallmarks of mitochondrial damage. These data support the working hypothesis that classical mechanisms of excitotoxic mitochondrial dysfunction are necessarily Ca2+ dependent, so that mechanism(s) of Zn2+ toxicity, although still unknown, must be different from that of Ca2+.
