Developing Therapies to Prevent Neuronal Apoptosis
Our laboratory uses basic molecular signaling pathways to prevent neuronal apoptosis and to promote neuronal survival and outgrowth during normal aging and various neurodegenerative diseases, including cerebrovascular disease (stroke) and AIDS dementia. Neuronal damage is curtailed by preventing excessive activity of the NMDA subtype of glutamate receptor and its downstream effectors (see figure). Cultures of cerebrocortical neurons as well as transgenic and knock-out animal models are used to show the involvement of calcium, free radicals, caspases, and transcription factors in NMDA receptor-mediated neuronal apoptosis. Two NMDA antagonists that we have developed are clinically tolerated because they have been designed using biophysical principles to decrease only excessive NMDA receptor activity while leaving physiological levels of activity relatively spared - these drugs are now in clinical trials. Techniques used in the laboratory include patch-clamp recording, site-directed mutagenesis of recombinant NMDA receptor subunits and GABAC subunits, multi-photon confocal imaging of mitochondrial activities, deconvolution microscopy, gene reporter assays, and various fluorogenic methods for apoptosis assessment.
Additionally, during the past few years we cloned and are currently characterizing two novel NMDA receptor subunits (one was recently published in Nature), and cloned a transcription factor, MEF2C, that controls the expression of NMDA receptor subunit genes and determines whether neurons undergo apoptosis after glutamate-related insults (recently published in PNAS and JBC). MEF2C is activated by the p38 stress kinase pathway, an active area of research in the laboratory that mediates both neuronal cell apoptosis and ischemic tolerance in the brain.
Recently, we also discovered a new action of nitric oxide-related species on cysteine residues of the NMDA receptor. This reaction, termed S-nitrosylation (transfer of the NO group to critical cysteine sulfhydryls), down-regulates NMDA receptor activity as well as caspase activity and may be useful clinically. Several other protein targets of nitrosylation are being examined in the laboratory (recently published by us in Neuron and in Nature).
We have also found a possible cause of neuronal apoptosis in AIDS brains (about one-third of AIDS patients eventually develop dementia). We discovered that the coat protein gp120 of HIV-1 produces a dramatic rise in neuronal calcium. This destructive process is primarily mediated by stimulation/activation of macrophage chemokine receptors by gp120 to release toxins that in turn trigger NMDA receptor-mediated neuronal destruction. Therefore, in some ways, this pathway resembles neuronal damage observed after stroke and other neurodegenerative diseases. (recently published in Nature, and Neuron, and JAMA). The involvement of apoptotic pathways in this type of cell death, involving reactive oxygen species, nitric oxide, mitochondrial toxins and caspases, is currently being explored.