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1-Methyl-4-phenylpyridinium induces autocrine excitotoxicity, protease activation, and neuronal apoptosis
1Faculty of Biology, Chair of Molecular Toxicology, University of Konstanz, D-78457 Konstanz, Germany.
Abstract:
The pathogenesis of several neurodegenerative diseases may involve indirect excitotoxic mechanisms, where glutamate receptor overstimulation is a secondary consequence of initial functional defects of neurons (e.g., impairment of mitochondrial energy generation). The neurotoxin 1-methyl-4-phenylpyridinium (MPP+) and other mitochondrial inhibitors (e.g., rotenone or 3-nitropropionic acid) elicited apoptosis in cerebellar granule cell cultures via stimulation of autocrine excitotoxicity. Cell death, increase in intracellular Ca2+ concentration, release of cytochrome c, and all biochemical and morphological signs of apoptosis were prevented by blockade of the N-methyl-D-aspartate receptor with noncompetitive, glycine-site or glutamate-site inhibitors. In addition, MPP+-induced apoptosis was reduced by high Mg2+ concentrations in the medium or by inhibiting exocytosis with clostridial neurotoxins. Two classes of cysteine proteases were involved in the execution of cell death: caspases and calpains. Inhibitors of either class of proteases prevented cell death, cleavage of intracellular proteins (i.e., fodrin), and the appearance of typical features of apoptosis such as phosphatidylserine translocation or DNA fragmentation. However, protease inhibitors did not interfere with the initial intracellular Ca2+ concentration increase. We suggest that MPP+ as well as other mitochondrial inhibitors trigger indirect excitotoxic processes, which lead to Ca2+ overload, protease activation, and subsequent neuronal apoptosis.
Insights
Mitochondrial dysfunction triggers excitotoxicity, leading to neuronal apoptosis. Blocking glutamate receptors or proteases prevents this cell death, revealing a key mechanism in neurodegeneration.
Area of Science:
- Neuroscience
- Cell Biology
- Biochemistry
Background:
- Neurodegenerative diseases may involve indirect excitotoxic mechanisms.
- Glutamate receptor overstimulation can be a secondary consequence of neuronal defects, such as impaired mitochondrial energy generation.
Purpose of the Study:
- To investigate the role of excitotoxicity and protease activation in MPP+-induced neuronal apoptosis.
- To explore potential therapeutic targets for preventing mitochondrial inhibitor-induced neurodegeneration.
Main Methods:
- Utilized cerebellar granule cell cultures treated with 1-methyl-4-phenylpyridinium (MPP+) and other mitochondrial inhibitors.
- Assessed apoptosis markers, intracellular calcium levels, and protein cleavage.
- Employed N-methyl-D-aspartate (NMDA) receptor antagonists, high Mg2+ concentrations, exocytosis inhibitors, and protease inhibitors (caspase and calpain inhibitors).
Main Results:
- MPP+ and mitochondrial inhibitors induced apoptosis via autocrine excitotoxicity, characterized by increased intracellular Ca2+ and cytochrome c release.
- NMDA receptor blockade, high Mg2+, and exocytosis inhibition prevented MPP+-induced apoptosis.
- Inhibitors of caspases and calpains blocked cell death, protein cleavage, and apoptotic features, but did not affect the initial Ca2+ increase.
Conclusions:
- Mitochondrial inhibitors like MPP+ trigger indirect excitotoxic pathways leading to Ca2+ overload.
- This Ca2+ overload activates caspases and calpains, resulting in neuronal apoptosis.
- Targeting NMDA receptors or proteases may offer neuroprotective strategies against mitochondrial dysfunction-related neurodegeneration.