Apoptotic neuronal cell death may play a role in many acute and chronic neurologic disorders. These disorders range from acute stroke, head trauma and epilepsy to more chronic states, such as Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis, HIV-associated dementia, and glaucoma. Moreover, a contributing factor to such damage is excessive excitation of glutamate receptors, particularly (but not exclusively) the N-methyI-D-aspartate (NMDA) subtype of glutamate receptor because of its high permeability to Ca 2+ and subsequent free radical generation.
The aim of this proposed research project is to uncover the role of myocyte enhancer factor-2 (MEF2) transcription factors in this excitotoxic/apoptotic process in neurons during ischemic stroke in vivo. MEF2 transcription factors are activated by p38 mitogen-activated protein kinase during neuronal and myogenic differentiation. Recent work has shown that stimulation of this pathway is anti-apoptotic in stem cells but pro-apoptotic in mature neurons exposed to mild excitotoxic or other stresses. Here, preliminary data in vitro show that mild excitotoxic (NMDA) insults to mature cerebrocortical neurons activate caspases-3, -7, in turn cleaving MEF2A, C and D isoforms. Endogenous MEF2 cleavage fragments containing a truncated transactivation domain but preserved DNA binding domain are shown to block MEF2 transcriptional activity via dominant interference. In vitro transfection of constitutively-active/uncleavable MEF2 (MEF2-CA) rescues MEF2 transcriptional activity following NMDA insult and prevents neuronal apoptosis. Conversely, dominant-interfering MEF2 (MEF2-DN) abrogates neuroprotection by MEF2C-CA. Our underlying hypothesis is that these results obtained in vitro can now be applied in vivo using tetracycline (or doxycycline, """"""""dox"""""""")-controlled transgenic mice expressing these MEF2-CA and MEF2-DN transgenes. This grant will define a novel pathway to neuronal apoptosis in ischemia via caspase-catalyzed cleavage of MEF2.
The Specific Aims are as follows: 1. To characterize anti-apoptotic effects of MEF2-CA in stroke using dox-controlled transgenic mice. 2. To characterize the effect of caspase cleavage fragments of MEF2 as dominant interfering forms that contribute to stroke damage using dox-controlled transgenic mice that express doxycycline-controlled, MEF2 cleavage products. 3. To characterize MEF2 transcriptional activity in vivo after an hypoxic/ischemic (stroke) insult but prior to cell loss using a MEF2-indicator mouse that has been engineered to activate the LacZ gene in accord with the degree of MEF2 transcriptional activity (designated des-mef2-LacZ).
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