We have identified several growth factors and cytokines that can protect neurons against dysfunction and death in experimental models of Alzheimer's disease, Parkinson's disease and stroke. These trophic factors activate signaling pathways that stimulate the expression of genes whose encoded proteins increase resistance of neurons to oxidative and metabolic stress. We are also identifying novel intracellular proteins that promote neuronal survival and plasticity, including kinases and telomerase. We have established a role for telomerase in regulating neuronal survival during brain development, and have obtained evidence that turning on telomerase in neurons in the adult brain can protect neurons in experimental models of neurodegenerative disorders. Finally, our studies of neural stem cells are revealing novel approaches for replacing lost neurons in animal models of neurodegenerative disorders. --Neuroprotective Signaling Via Integrins and Pathways Involving Akt and NF-kappaB: Integrins are integral membrane proteins that mediate adhesive interactions of cells with the extracellular matrix and with other cells. Integrin engagement results in activation of intracellular signaling cascades that effect several different cellular responses including motility, proliferation and survival. Although integrins are known to provide cell survival signaling in various types of non-neuronal cells, the possibility that integrins modulate neuron survival has not been explored. We have obtained data demonstrating a neuroprotective function of integrins in embryonic hippocampal neurons. Neurons grown on laminin, an integrin ligand, exhibit increased resistance to glutamate-induced apoptosis compared with neurons grown on polylysine. Neurons expressed integrin beta1 and treatment of cultures with an antibody against integrin beta1 abolished the protective effect of laminin. Neurons maintained on laminin exhibited a sustained activation of the Akt signaling pathway demonstrated in immunoblot analyses using an antibody that selectively recognizes phosphorylated Akt. The neuroprotective effect of integrin engagement by laminin was mimicked by an IKLLI-containing integrin-binding peptide and was abolished by treatment of neurons with the PI3 kinase inhibitor wortmanin. Levels of the anti-apoptotic protein Bcl-2 were increased in neurons grown on laminin and decreased by wortmanin, suggesting a mechanism for the neuroprotective effect of integrin-mediated signaling. The ability of integrin-mediated signaling to prevent glutamate-induced apoptosis suggests a mechanism whereby neuron-substrate interactions can promote neuron survival under conditions of glutamate receptor overactivation. Prototypical NF-kappaB consists of a transcription factor dimer of p50 and p65, and an inhibitory subunit called I-kappaB. NF-kappaB is activated in neurons in response to excitotoxic, metabolic, and oxidative stress. Cell-culture data suggest that activation of NF-kappaB can prevent neuronal apoptosis, but its role in vivo is unclear and the specific kappaB subunits involved are unknown. In Huntington's disease (HD), striatal neurons degenerate, and a similar pattern of neuronal vulnerability occurs in rats and mice following exposure to the mitochondrial toxin 3-nitropropionic acid (3NP). We report that mice lacking the p50 subunit of NF-kappaB exhibit increased damage to striatal neurons following administration of 3NP. The neuronal death occurs by apoptosis as indicated by increased caspase activation and DNA fragmentation into oligonucleosomes. NF-kappaB activity is markedly increased in striatum 24-72 h following 3NP administration in wild-type mice, but not in mice lacking p50, indicating that p50 is necessary for the vast majority of 3NP-induced NF-kappaB DNA-binding activity in striatum. Cultured striatal neurons from p50-/- mice exhibited enhanced oxidative stress, perturbed calcium regulation, and increased cell death following exposure to 3NP, suggesting a direct adverse effect of p50 deficiency in striatal neurons. --Neuroprotective Actions of BDNF. We have found that brain-derived neurotrophic factor (BDNF) is a key mediator of the neuroprotective effects of dietary restriction in animal models of Parkinson's and Huntington's diseases. BDNF protects neurons against excitotoxic and oxidative insults. In addition, we found that BDNF promotes neurogenesis in the hippocampus, and mediates the enhancement of neurogenesis by dietary restriction. BDNF activates a positive feedback loop for the regulation of neurogenesis by inducing the production of nitric oxide by newly generated neurons. The nitric oxide then acts on neural progenitor cells to induce their differentiation into neurons. These findings identify several possible ways (dietary restriction, enhancement of nitric oxide signaling) to stimulate neurogenesis, which might have beneficial effects in several neurological disorders. Interestingly, we have also obtained evidence that BDNF signaling in the brain regulates peripheral glucose metabolism by increasing insulin sensitivity. These findings suggest important links between neurotrophic factor signaling in the brain and diabetes.
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