Alzheimer's disease (AD) remains one of the most pressing public health issues facing our nation, and is associated with enormous societal and personal costs. At present, it is estimated that as many as 4 million Americans suffer from the clinical manifestations of AD, with an annual cost approaching $100 billion dollars. The incidence of AD increases with age, approximately doubling every five years after the age of 60. Thus,as life expectancies continue to increase, the personal and societal impact of this disease is likely to intensify in the future with the aging of our population. AD results in a slow yet progressive decline in cognitive ability and is the most prevelant type of dimentia. The disease pathology can be characterized histopathologically by the presence of neuritic plaques,a largely extracellular lesion consisting of beta-amyloid (A-beta), and intracellular neurofibrillary tangles (NFTs). At present, there are no effective disease-modifying therapies for AD. One of the factors contributing to this therapeutic limitation has been our incomplete understanding of the pathogenesis of this disease. Several genes have been associated with the development of the disease. One of these, apolipoprotein E (apoE), a 299 amino acid protein that was initially identified in the context of cholesterol metabolism, has been shown to be a susceptibility gene for AD. There are three common human isoforms of apoE, designated E2, E3, and E4 which differ by single amino acid interchanges. Although these amino acid substitutions have minimal effect on the physical structure of apoE, the apoE4 isoform is associated with late onset familial and sporadic AD. Similarly, mutations of the human amyloid precursor protein(APP) gene have been associated with the development of familial AD. APP expression is observed in many tissues and in the brain occurs mainly in neurons. The proteolytic processing of APP produces the major protein constituent found in senile plaques, and results in the production of the amyloid-beta proteins, A-beta(1-40) and A-beta(1-42). Amyloid formation by A-beta(1-42)is believed to play a major role in promoting AD. Neuroinflammatory mechanisms have come under increasing scrutiny in the pathogenesis of AD, and this may be one mechanism by which genetic factors, such as apoE isoform, influence disease susceptibility. Activation of microglial cells and the release of proinflammatory cytokines can promote central nervous system (CNS) inflammation and maturation of senile plaques. We have previously demonstrated that apoE can act in an isoform-specific fashion as an immunomodulatory protein capable of suppressing both systemic and brain inflammation. Furthermore, small peptide therapeutics derived from the receptor binding region of apoE can reduce glial activation and CNS inflammation in vitro and in vivo,improve functional and histological injury after brain trauma, and are as effective as the native apoE protein in this capacity. Insight into these genetic associations has led to the development of transgenic mouse models, in which the relevant human gene is inserted into the mouse genome. One limitation to the use of transgenic models of AD is the complex interactions that may exist between genetic factors such as APOE genotype or mutations in the APP gene. Another shortcoming associated with current animal models of AD is that there is often an inconsistent, age-related development of AD pathology in these mice. This variability in the expression of AD pathology makes it particularly difficult to gauge the effect of a specific therapeutic intervention given at a defined time point. To address this dilemma, we have characterized a survival model of murine closed head injury which produces reproducible histological changes and behavioral deficits characteristic of AD. Moreover,when this closed head injury model is tested using lines of transgenic animals bearing the human apoE and mutated APP genes, a reproducible increase in APP secretion and amyloid deposition is observed in young APP/APOE3 and APP/APOE4 animals that would not otherwise have exhibited AD pathology. Thus, the use of APP/APOE double transgenics in a murine model of closed head injury is an ideal model for testing the efficacy of novel therapeutic strategies, such as the apoE-mimetic peptide, in blocking the development of AD pathology and improving histological and behavioral outcomes. SCIENTIFIC ABSTRACT Mutations of the amyloid precursor protein (APP) and the APOE4 polymorphism are implicated in pathogenesis of AD. Neuroinflammatory mechanisms have come under increasing scrutiny for a role in AD pathology, and this may be one mechanism by which apoE isoform influences disease susceptibility. We have recently demonstrated that small peptide therapeutics derived from the receptor binding region of apoE can function like the intact apoE holoprotein to reduce glial activation and CNS inflammation in vitro and in vivo, and improve functional and histological injury after brain trauma. The development of transgenic models of AD has allowed for the testing of novel disease-modifying therapeutic interventions. An important limitation to the use of these transgenic models is that the age-dependent development of AD neuropathology is highly variable with respect to age-of-onset and quantitative progression of pathological and functional changes. This variability complicates the assignment of changes that result from a novel therapeutic treatment as opposed to those typically normal variations in pathology and behavior observed in these transgenic models. Wide windows of variability also encourage investigators to employ months of prolonged treatments in an effort to ensure a greater likelihood that the therapeutic window has been covered. Since the testing of novel treatments is so important to developing an effective anti-AD therapy, we have recently created a murine survival closed head injury paradigm which causes reproducible deposition of amyloid pathology and functional deficits in younger APP transgenic mice over a defined time period. Our results are consistent with work done in other laboratories (Hartman et al., 2002) permitting a more efficient paradigm to assay for treatments affecting histology and function over a short and defined time period. A great advantage of our controlled application of closed head injury is that the rapid acquisition of AD-like pathology and behavioral changes can be induced in a more reproducible fashion and at a younger age than previously described models. We have recently demonstrated that a small therapeutic peptide derived from the receptor binding region of apoE downregulates glial activation and reduces the neuroinflammatory response in vitro and in vivo (Laskowitz et al., 2001, Lynch et al., 2003). We will assess whether administration of this novel therapeutic peptide reduces glial activation, amyloid deposition and improves behavioral outcomes in APP/APOE mice. These results may also translate into a novel therapeutic strategy to slow disease progression in patients with AD. In our experiments, we will use double transgenic mice that overexpress a mutated human APP protein together with either the human apoE3 or E4 protein isoforms. This will allow us to examine the interaction of APOE genotype and the APP mutation, and to determine whether there is a pharmacogenomic interaction between our therapeutic intervention and the humanized apoE background. Thus, the use of APP/APOE double transgenics in a murine model of closed head injury is an ideal model for testing the efficacy of novel therapeutic strategies, such as the apoE-mimetic peptide in blocking the development of AD pathology and improving behavioral outcomes.