TNFa/Glutamate Induced Cell Death in Alzheimers Disease

ABSTRACT:

Although abundant reactive microglia are found associated with b-amyloid (Ab) plaques in Alzheimer’s disease (AD) brains their contribution to cell loss remains speculative. A variety of studies have documented the ability of Ab fibrils to directly stimulate microglia in vitro to assume a neurotoxic phenotype characterized by secretion of a plethora of proinflammatory molecules. Collectively, these data suggest that activated microglia play a direct role in contributing to neuron death in AD rather than simply a role in clearance following plaque deposition. In spite of continuing characterization of Ab-stimulated microglial secretory products, the identity of the specific neurotoxic agents both in vitro and in vivo remains unclear. We have utilized primary mouse microglial and neuronal cultures to mechanistically link Ab fibril stimulation of microglia to specific neurotoxic product secretion. It appears that neuron death induced by microglial conditioned media in our cultures is a consequence of increased microglial secretion of TNFa and glutamate acting upon the neuronal NMDA and TNFa receptors. Neuron death occurs in an oxidative damage-dependent fashion requiring activity of inducible nitric oxide synthase and transient activation of the extracellular signal regulated kinases (ERKs). Toxicity results from coincident stimulation of the TNFa and NMDA receptors since stimulations of either alone, are insufficient to initiate cell death.

We have identified a specific mechanism by which two secretory products from Ab-activated microglia can directly lead to oxidative damage-dependent death. We predict that neurons that co-express specific TNFa and NMDA receptors in human adult brain are susceptible to the particular inflammation-associated death we describe. More importantly, the specific cellular localization of TNFa receptor subtypes with certain NMDA receptor subunits may identify vulnerable neuronal populations in the AD brain. In correlative support of this hypothesis, it is known that levels of TNFa and neuronal TNFRI expression in the AD brain are elevated and NMDA receptor expressing neurons represent a population vulnerable for loss during disease. Additionally, neurons in the AD brain display increased immunoreactivity for both iNOS and tyrosine nitrated proteins similar to our in vitro observations. The microglial-mediated death mechanism we observed provides not only molecular markers for identifying neurons vulnerable to loss in AD but also specific molecular targets amenable to targeting for neuroprotective therapeutic design. For example, the success of the noncompetitive NMDA receptor antagonist, memantine for treatment of AD patients supports the idea that NMDA receptor activity is required for disease progression. Finally, it is conceivable that the inflammation-associated death we have characterized may be mechanistically important for neuron loss in a variety of inflammation-associated neurodegenerative conditions besides AD.

Colin K. Combs, Ph.D.