Direct cleavage of AMPA receptor subunit GluR1 and suppression of AMPA currents by caspase-3: implications for synaptic plasticity and excitotoxic neuronal death.
ABSTRACT Cysteine proteases of the caspase family play central roles in excecuting the cell death process in neurons during development of the nervous system and in neurodegenerative disorders. Recent findings suggest that caspases may also play roles in modulating neuronal plasticity in the absence of cell death. We previously reported that caspases can be activated in dendrites and synapses in response to activation of glutamate receptors. In the present study we demonstrate that the GluR1 subunit of the AMPA subtype of glutamate receptor is directly cleaved by caspase-3, and provide evidence that the cleavage of this subunit modulates neuronal excitability in ways that suggest important roles for caspases in regulating synaptic plasticity and cell survival. Whole-cell patch-clamp recordings in cultured rat hippocampal neurons showed that caspase activation in response to apoptotic stimuli selectively decreases AMPA channel activity without decreasing NMDA channel activity. Perfusion of neurons with recombinant caspase-3 resulted in a decreased AMPA current, demonstrating that caspase-3 activity is sufficient to suppress neuronal responses to glutamate. Exposure of radiolabeled GluR1 to recombinant caspase-3 resulted in cleavage of GluR1, demonstrating that this glutamate receptor protein is a direct substrate of this caspase. Our findings suggest roles for caspases in the modulation of neuronal excitability in physiological settings, and also identify a mechanism whereby caspases ensure that neurons die by apoptosis rather than excitotoxic necrosis in developmental and pathological settings.
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ABSTRACT: Recent studies suggest that increased lipid peroxidation and lipid peroxidation products, such as 4-hydroxynonenal (HNE), contribute to neuronal loss in conditions associated with oxidative stress. The focus of the present study was to determine possible neuroprotective effects of elevated cyclic nucleotide levels against lipid peroxidation and HNE-mediated neural toxicity. Application of 8-bromo derivative analogs of cAMP or cGMP resulted in attenuation of HNE-induced increases in mitochondrial calcium, reactive oxygen species, and neuron loss. Similar results were obtained when neural cells were pretreated with the phosphodiesterase inhibitors zaprinast or isobutylmethylxanthanine (IBMX). These data are consistent with a possible neuroprotective role for elevated cyclic nucleotide levels in disorders associated with increases in lipid peroxidation and HNE.Neuroreport 12/1998; 9(16):3731-4. · 1.40 Impact Factor
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ABSTRACT: Neuritic regression and cell death (neurodegeneration) are common features of both normal nervous system development and neurodegenerative disorders. Growth factors and excitatory amino acid neurotransmitters have been suggested independently to play roles in neurodegenerative processes. The present study investigated the combined effects of fibroblast growth factor (FGF) and glutamate on the development and degeneration of cultured hippocampal neurons. Consistent with previous data, we found that FGF, but not NGF, promoted neuronal survival and dendritic outgrowth. In contrast, a low level of glutamate (50 microM) caused a reduction in dendritic outgrowth, and high levels (100 microM-1 mM) reduced neuronal survival in a dose-dependent manner. When cultures were maintained in the presence of FGF, there was a striking reduction in neuronal death normally caused by 100-500 microM glutamate. FGF raised the threshold for glutamate neurotoxicity. FGF also antagonized the outgrowth-inhibiting actions of glutamate. Measurements of intracellular calcium levels with fura-2 demonstrated a direct relationship between glutamate-induced rises in intracellular calcium and neurodegeneration. FGF reduced the glutamate-induced increases in intracellular calcium levels. However, when cultures were pretreated with the RNA synthesis inhibitor actinomycin D or with the protein synthesis inhibitor cycloheximide, FGF did not prevent glutamate-induced increases in intracellular calcium or neurodegeneration. Taken together, these results suggest that (1) interactions between growth factors and neurotransmitters may be important in brain development; (2) imbalances in these systems may lead to neurodegeneration; and (3) cellular calcium-regulating systems may be a common focus of growth factor and neurotransmitter actions.Journal of Neuroscience 12/1989; 9(11):3728-40. · 6.91 Impact Factor
Article: Evidence for synaptic apoptosis.[show abstract] [hide abstract]
ABSTRACT: Increasing evidence indicates that neurons die by apoptosis, an active form of cell death involving a relatively stereotyped series of biochemical changes that culminate in nuclear fragmentation, in many different developmental and pathophysiological settings. In contrast to most other cell types, neurons have elaborate morphologies with complex neuritic arbors that often extend great distances from the cell body. Neuronal death signals are likely to be activated at remote synaptic sites and, indeed, overactivation of glutamate receptors and underactivation of trophic factor receptors are implicated in neurodegenerative disorders. We now report that biochemical changes consistent with apoptosis are engaged locally in synapses. Exposure of cortical synaptosomes to staurosporine and Fe2+ resulted in loss of membrane phospholipid asymmetry, caspase activation, and mitochondrial alterations (membrane depolarization, calcium overload, and oxyradical accumulation) characteristic of apoptosis. The caspase inhibitor zVAD-fmk prevented mitochondrial membrane depolarization in synaptosomes. Studies of the effects of cytosolic extracts from synaptosomes exposed to apoptotic insults, on isolated nuclei, showed that signals capable of inducing nuclear apoptosis are generated locally in synapses. Exposure of cultured hippocampal neurons to staurosporine and glutamate resulted in caspase activation and mitochondrial membrane depolarization in dendrites, and zVAD-fmk prevented the membrane depolarization. Glutamate-induced increases in caspase activity were first observed in dendrites and later in the cell body, and focal application of glutamate to individual dendrites resulted in local activation of caspases. Collectively, the data demonstrate that apoptotic biochemical cascades can be activated locally in synapses and dendrites and suggest a role for such local apoptotic signals in synapse loss and neuronal death in neurodegenerative disorders that involve excessive activation of glutamate receptors.Experimental Neurology 10/1998; 153(1):35-48. · 4.65 Impact Factor