This hypothesis proposes that increased extracellular glutamate in Amyotrophic Lateral Sclerosis (ALS) and cerebral ischemia, currently viewed as a trigger for excitotoxicity, is actually beneficial as it stimulates the utilization of glutamate as metabolic fuel.Renewed appreciation of glutamate oxidation by ischemic neurons has raised questions regarding the role of extracellular glutamate in ischemia. Is it detrimental, as suggested by excitotoxicity in early in vitro studies, or beneficial, as suggested by its oxidation in later in vivo studies? The answer may depend on the activity of N-methyl-Daspartate (NMDA) glutamate receptors. Early in vitro procedures co-activated NMDA receptors (NMDARs) containing 2A (GluN2A) and 2B (GluN2B) subunits, an event now believed to trigger excitotoxicity; however, during in vivo ischemia D-serine and zinc molecules are released and these ensure only GluN2B receptors are stimulated. This not only prevents excitotoxicity but also initiates signaling cascades that allow ischemic neurons to import and oxidize glutamate.
This etiologic model proposes that Alzheimers Disease (AD) arises when an unusually rapid increase in ventricle volume triggers axon stretch that culminates in the physical separation of trans-synaptic proteins. As a result, these proteins, such as neurexin, neuroligin, N-Cadherin and Amyloid Precursor Protein (APP), experience a change in the configuration of their cytoplasmic tail, so that instead of transmitting signals to create and maintain synaptic structure they activate enzymes, and generate molecules, that stimulate neurite growth; for example, the transformation of the N-Cadherin tail dissolves its complex with presenilin and β-catenin triggering activation of glycogen synthase kinase 3 beta (GSK3β) and cytoskeletal disruption. This disruption leads to an increase in pro nerve growth factor (proNGF), a molecule that stimulates neurite growth via the p75 neurotrophin receptor (p75). GSK3β contributes to this growth by increasing microtubule instability through the phosphorylation of tau. Separation of trans-synaptic APPs leads to their cis dimerization and this stimulates production of amyloid beta (Aβ), an autocrine growth factor that interacts with both the p75 and alpha 7 nicotinic acetylcholine receptors. Cis dimerization of APPs may also allow the autophosphorylation of Y682 and T668 in the APP cytoplasmic tail, triggering activation of c-Jun N terminal kinase, Abelson kinase and cyclin dependent kinase 5, all of which play a role in neurite growth. ProNGF, Aβ and the kinase cascades work together to transform synapses into growth cones and stimulate sprouting of neuropil threads in an attempt to reconnect axons and dendrites. Neurofibrillary tangles, located in neural cell soma, consist of neurofilaments and microtubules needed to fuel this renewal of neurite growth. The model suggests that the best way to treat AD is to prevent synaptic separation by identifying individuals experiencing unusually high rates of ventricle growth and reducing this to more normal levels by shunting or some other technique.
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