Apoptotic Pathways Mobilized in Microglia and Neurones as a Consequence of Chromogranin A‐Induced Microglial Activation

Abstract

Senile plaques of Alzheimer's brain are characterized by activated microglia and immunoreactivity for the peptide chromogranin A. We have investigated the mechanisms by which chromogranin A activates microglia, producing modulators of neuronal survival. Primary cultures of rat brain-derived microglia display a reactive phenotype within 24 h of exposure to 10 nM chromogranin A, culminating in microglial death via apoptotic mechanisms mediated by interleukin-1␤ converting enzyme. The signalling cascade initiated by chromogranin A triggers nitric oxide production followed by enhanced microglial glutamate release, inhibition of which prevents microglial death. The plasma membrane carrier inhibitor aminoadipate and the type II/III metabotropic glutamate receptor antagonist (RS)-␣-methyl-4-sulphonophenylglycine are equally protective. A significant amount of the released glutamate occurs from bafilomycin-sensitive stores, suggesting a vesicular mode of release. Inhibition of this component of release affords significant microglial protection. Conditioned medium from activated microglia kills cerebellar granule cells by inducing caspase-3-dependent neuronal apoptosis. Brain-derived neurotrophic factor is partially neuroprotective, as are ionotropic glutamate receptor antagonists, and, when combined with boiling of conditioned medium, full protection is achieved; nitric oxide synthase inhibitors are ineffective.

Activation of Microglial Group III Metabotropic Glutamate Receptors Protects Neurons against Microglial Neurotoxicity

“…Cells were routinely used 1 d after plating. We confirmed our previous findings that, after 1 d in vitro (1 DIV), microglial cells exhibit a resting, ramified morphology with only a small number of cells staining positive for ED1 (a marker for activated microglia) and that cultures were enriched with microglia with few contaminating cells as shown by positive immunoreactivity to the microglial marker OX-42 (data not shown) (Kingham et al, 1999).…”

“…Microglial overactivation may lead to their enhanced and neurotoxic production of a number of compounds that normally are important brain cell signaling molecules. One of these, glutamate, is a major determinant of microglial-evoked neurotoxicity (Piani et al, 1991(Piani et al, , 1992Patrizio and Levi, 1994) and can cause NMDA receptor-mediated toxicity to neurons in vitro (Piani et al, 1991;Piani and Fontana 1994;Kingham et al, 1999). Glutamate release from microglia occurs via the cystine-glutamate antiporter, as well as by a bafilomycin A-sensitive mechanism (Kingham et al, 1999).…”

“…One of these, glutamate, is a major determinant of microglial-evoked neurotoxicity (Piani et al, 1991(Piani et al, , 1992Patrizio and Levi, 1994) and can cause NMDA receptor-mediated toxicity to neurons in vitro (Piani et al, 1991;Piani and Fontana 1994;Kingham et al, 1999). Glutamate release from microglia occurs via the cystine-glutamate antiporter, as well as by a bafilomycin A-sensitive mechanism (Kingham et al, 1999). Overstimulation of microglia leads to microglial apoptosis, which may have consequences for the ability of the brain to recover from insult (Kingham et al, 1999;Kingham and Pocock, 2000;Liu et al, 2001).…”

“…Glutamate release from microglia occurs via the cystine-glutamate antiporter, as well as by a bafilomycin A-sensitive mechanism (Kingham et al, 1999). Overstimulation of microglia leads to microglial apoptosis, which may have consequences for the ability of the brain to recover from insult (Kingham et al, 1999;Kingham and Pocock, 2000;Liu et al, 2001). Thus, the regulation of microglial reactivity and neurotoxicity would seem desirous for neuroprotection in neurodegenerative diseases (Nicoletti et al, 1996).…”

“…Activation of microglia with chromogranin A (CGA), a secretory peptide with enhanced expression in Alzheimer's disease (Munoz et al, 1990;Munoz, 1991;Yasuhara et al, 1994), leads to a reactive neurotoxic microglial phenotype (Ciesielski-Treska et al, 1998) and causes microglial glutamate release (Kingham et al, 1999). This glutamate release is involved in an autologous feedback loop to damage the microglia (Kingham et al, 1999) and to trigger neurotoxin release by activation of group II metabotropic glutamate (mGlu) receptors on these cells (Taylor et al, 2002). Exposure of microglia to amyloid ␤ peptide 25-35 (A␤25-35) also displays an underlying activation of group II mGlu receptors (Taylor et al, 2002).…”