The proposed research is directed at the study of growth-associated protein GAP-43, an essential presynaptic protein with critical functions in neural development (axonal pathfinding), and plasticity (long-term potentiation, learning and memory). Because GAP-43 is central to plasticity mechanisms, brain site-selective manipulations targeting GAP-43 in a temporally and spatially controlled fashion can offer unique insights into memory acquisition, storage and recall. GAP-43 is critical for brain development as its knockout is perinatal lethal probably because the null mutation disrupts synaptic targeting (see Benowitz and Routtenberg, 1997, for review). GAP-43 bidirectionally regulates adult memory: reduction by half impairs (Rekart et aI., 2005), while its overexpression can enhance (Routtenberg et aI., 2000), information storage processes. The proposed research tests the over-arching Aim that in wild type animals the level of endogenous phosphorylatable GAP- 43 plays a pivotal role in regulating brain information storage. Evidence in support of this hypothesis has been gained in transgenic animals, yet no information is currently available as to the site specificity of the phenotype. The specific questions we will address are: What level of region-specific knockdown in GAP-43, using RNAi methods, in adult wild type rodents will impair acquisition or retention of 3 different learning tasks? The siRNA will be delivered using lipofectamine or lentiviral transfer, with bilateral intracranial injections targeting hippocampus, amygdala, anterior cingulate cortex or medial prefrontal cortex. We will thus determine the kinetics of this silencing using RNAi methods to deliver the knockdown targeting nodal points in the 'memory circuit'at different time points in the learning and retention process to pinpoint the mnemonic function served by GAP-43 in each brain location.
In transgenic mice that are termed G-Perm, because they have a permanently mutated growth protein linked to memory, there is a dramatically long-lasting contextual fear that does not extinguish. This parallels closely inextinguishable fear as well as a rodent model for PTSD. Such a model may be useful in screening novel therapeutics for this stress disorder. When this same growth protein in its native form is overexpressed at high levels animals show profoundly impaired memory, paralleling our observations made in the past project period in Alzheimer's post-mortem tissue in which abnormally elevated levels of GAP-43 were observed in brain areas involved in information storage. A novel `burden of plasticity'view of memory loss in AD may involve the excessive process outgrowth leading to the scrambling of synaptic connections.
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