Increasing evidence suggests that neurons may die by a form of programmed cell death called apoptosis in several age-related disorders. A major effort in the LNS is aimed at identifying the factors that may trigger neuronal apoptosis in neurodegenerative disorders, as well as the specific molecular cascades responsible for executing the cell death program. --Novel Apoptotic Mechanisms in Amyotrophic Lateral Sclerosis (ALS): Prostate apoptosis response-4 (Par-4), a protein containing a leucine zipper domain within a death domain, is up-regulated in prostate cancer cells and hippocampal neurons induced to undergo apoptosis. We have found higher Par-4 levels in lumbar spinal cord samples from patients with ALS, and have shown that Par-4 increases the vulnerability of motor neurons to oxidative and excitotoxic insults. In other studies we have identified abnormalities in sphingolipid and cholesterol metabolism in the spinal cords of ALS patients and in a transgenic mouse model (Cu/Zn-SOD mutant mice), which manifest increased levels of sphingomyelin, ceramides and cholesterol esters; in the ALS mice these abnormalities precede the clinical phenotype. In ALS patients and mice increased oxidative stress occurs in association with the lipid alterations, and exposure of cultured motor neurons to oxidative stress increases the accumulation of sphingomyelin, ceramides and cholesterol esters. Pharmacological inhibition of sphingolipid synthesis prevents accumulation of ceramides, sphingomyelin and cholesterol esters and protects motor neurons against death induced by oxidative and excitotoxic insults. These findings suggest a pivotal role for altered sphingolipid metabolism in the pathogenesis of ALS. --The Transcription Factor NF-kappaB and Neuronal Apoptosis: We have discovered that inhibition of endoplasmic reticulum (ER) d-myo-inositol 1,4,5-trisphosphate (IP(3))-gated calcium release caused decreased basal NF-kappaB DNA-binding activity in cultured rat cortical neurons. Activation of NF-kappaB in response to tumor necrosis factor-alpha and glutamate was completely abolished when IP(3) receptors were blocked, and NF-kappaB activation in response to depletion of ER calcium by thapsigargin treatment was also decreased by IP(3) receptor blockade. We further investigated the relationship between IP(3) receptor activation and NF-kappaB activity using a cell-free system. Microsomes enriched in the ER were isolated from adult rat cerebral cortex, resuspended, and treated with agents that induce or inhibit ER calcium release. They were then recentrifuged, and the supernatant was added to cytoplasmic extract isolated from the same source tissue. We found that microsomes released an NF-kappaB-stimulating signal in response to activation of IP(3) receptors or inhibition of the ER Ca(2+)-ATPase, but not in response to ryanodine. Studies of intact cells and cell-free preparations indicated that the signal released from the ER was not calcium and was heat- and trypsin-sensitive. Our data suggest that activation of IP(3) receptors is required for a major component of both constitutive and inducible NF-kappaB binding activity in neurons and that decreasing ER intraluminal calcium levels triggers release of a diffusible NF-kappaB-activating signal from the ER. --Mechanisms of Neuronal Apoptosis in Stroke: Stroke is a major cause of long-term disability, the severity of which is directly related to the numbers of neurons that succumb to the ischemic insult. We found that transient focal cerebral ischemia induces large increases in acidic sphingomyelinase (ASMase) activity, ceramide levels, and production of inflammatory cytokines in wild-type mice, but not in mice lacking ASMase. The extent of brain tissue damage is decreased and behavioral outcome improved in mice lacking ASMase and in mice treated with a drug that blocks ceramide production. We have found that mice given diazoxide, an activator of mitochondrial ATP-sensitive potassium channels, exhibited a large (60% to 70%) decrease in cortical infarct size after permanent occlusion of the middle cerebral artery. Diazoxide decreases neuronal apoptosis and increases astrocyte survival and activation in the penumbral region of the ischemic cortex. The neuroprotective effect of diazoxide is abolished by 5-hydroxydecanoate, a selective antagonist of mitochondrial ATP-sensitive potassium channels. Studies of cultured hippocampal neurons reveal that diazoxide depolarizes mitochondria, prevents cytochrome c release, and protects cells against death induced by staurosporine and chemical hypoxia. Diazoxide increased the levels of Bcl2 and inhibited the association of Bax with mitochondria in neurons exposed to an apoptotic insult, suggesting that activation of mitochondrial ATP-sensitive potassium channels may stabilize mitochondrial function by differentially modulating proapoptotic and antiapoptotic proteins. Collectively, the data suggest that mitochondrial ATP-sensitive potassium channels play a key role in modulating neuronal survival under ischemic conditions, and identify agents that activate mitochondrial ATP-sensitive potassium channels as potential therapeutics for stroke and related neurodegenerative conditions. --DNA Damage Responses and Neuronal Apoptosis: DNA damage has been documented in neurodegenerative disorders including AD, PD and stroke. We have found that DNA damage occurs in cultured neurons subjected to conditions relevant to AD and PD including exposure to amyloid beta-peptide and mitochondrial toxins. The DNA damage is reduced when cultured neurons or adult mice are treated with folic acid, and the DNA damage is increased by homocysteine. Additional data suggest that homocysteine impairs the ability of the neurons to repair damaged DNA. DNA-dependent protein kinase (DNA-PK) is involved in V(D)J recombination and DNA double strand break repair, and may play a role in cell death induced by DNA damage. We found that cultured hippocampal neurons from severe combined immunodeficient (scid) mice which lack DNA-PK activity are hypersensitive to apoptosis induced by exposure to topoisomerase inhibitors, amyloid beta peptide (A beta) and glutamate. A similar increased vulnerability of hippocampal CA1 and CA3 neurons was observed in adult scid mice after kainate-induced seizures. Our results suggest that DNA-PK activity is important for neuron survival under conditions that may occur in neurological disorders. --Apoptosis in Neurogenesis: Neural progenitor cells (NPC) can proliferate, differentiate into neurons or glial cells, or undergo a form of programmed cell death called apoptosis. Although death of NPC occurs during development of the nervous system and in the adult, the underlying mechanisms are unknown. We discovered that nitric oxide can induce death of C17.2 NPC by a mechanism requiring activation of p38 MAP kinase, poly(ADP-ribose) polymerase and caspase-3. In related studies we have found that brain-derived neurotrophic factor prevents apoptosis of neural stem cells.. In another study we have found that Numb regulates the sensitivity of cells to neurotrophin-induced differentiation and cell survival dependency in an isoform-specific manner. Numb isoforms containing a short PTB enhance the differentiation response to NGF, and enhance apoptosis in response to NGF withdrawal by a mechanism dependent upon release of calcium from endoplasmic reticulum stores. These findings suggest that isoform-specific modulation of neurotrophin responses by Numb may play important roles in the development and plasticity of the nervous system.
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