Graduation Year

2009

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Molecular Pharmacology and Physiology

Major Professor

Javier Cuevas, Ph.D.

Keywords

Intracellular calcium, Whole-cell currents, AKAP150, Calcineurin, G protein, Neurotransmission, Glutamate, VGCC, NMDA receptor

Abstract

Acid-sensing ion channels (ASIC) are a class of ligand gated plasma membrane ion channels that are activated by low extracellular pH. During ischemia, ASIC1a are activated and contribute to the demise of neurons. Pharmacological block of ASIC1a provides neuroprotection at delayed time points. However, no endogenous receptors have been implicated in the modulation of ASIC1a activity. The hypothesis presented is that sigma receptor activation inhibits ASIC1a function and ASIC1a-induced [Ca²?]i elevations during acidosis and ischemia, which may be a mechanism by which sigma ligands provide neuroprotection following stroke. This hypothesis is based on the following observations: First, sigma receptors regulate multiple ion channels that become activated during ischemia. Second, ASIC1a remain functionally active hours beyond the ischemic insult and sigma receptors have been shown to be neuroprotective at delayed time points following stroke.

Ratiometric Ca²? fluorometry and whole-cell patch clamp experiments showed that sigma-1 receptor activation depresses elevations in [Ca²?]i and membrane currents mediated by ASIC1a channels in cortical neurons. Furthermore, most of the elevations in [Ca²?]i triggered by acidosis are the result of Ca²? channels opening downstream of ASIC1a activation. Stimulation of sigma-1 receptors effectively suppressed these secondary Ca²? fluxes both by inhibiting ASIC1a and the other channels directly. The signaling cascade linking sigma-1 receptors and ASIC1a was determined to involve a pertussis toxin-sensitive G protein and A-Kinase Anchoring Protein 150/calcineurin complex, which resulted in a decrease of acid-induced [Ca²?]i elevations and ASIC1a-mediated currents.

Furthermore, immunohistochemical studies confirmed that sigma-1 receptors, ASIC1a and AKAP150 colocalize in the plasma membrane of cortical neuron cell bodies and in the dendritic processes of these cells. Additionally, concurrent exposure to acidosis and ischemia resulted in synergistic potentiation of [Ca²?]i dysregulation. Although ASIC1a activation does not induce long-lived priming of synaptic vesicles for release, channel activation does have a temporal effect on ischemia-mediated [Ca²?]i increases after ischemia onset. Moreover, presynaptic ASIC1a channels promote synaptic transmission during ischemia by overcoming block of neurotransmission and thus enhance postsynaptic [Ca²?]i elevations. Sigma-1 receptor activation decreased ischemia-mediated Ca²? dysregulation at pH values of 7.4 - 6.0 and prevented the synergistic interaction between ischemia and acidosis.

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