DESCRIPTION (provided by applicant): Alzheimer's disease (AD) is a neurodegenerative process characterized by the deposition of beta- amyloid proteins (Aβ) in the brain and cerebrovasculature. While AD has always been a concern for our aging Veterans, the propensity for head injuries sustained in combat has placed our Veterans at even greater risk for developing AD than the general population. Mounting evidence now suggests the excessive accumulation of Aβ in AD is the result of impaired Aβ clearance from the brain. Lipoprotein receptors within the blood-brain barrier contribute to the brain-to-blood elimination of Aβ. However, these receptors are vulnerable to proteolysis at the cell surface (i.e., ectodomain shedding), which diminishes their ability to transport ligands, such as Aβ. One of the ligands closely associated with the lipoprotein receptors is apolipoprotein E (apoE), which exists as three isoforms in humans (apoE2, apoE3, and apoE4). Importantly, possession of the apoE4 allele represents the strongest genetic risk factor for late-onset AD. Recent reports, including our own, indicate Aβ clearance from the brain is differentially regulate by the type of apoE isoform expressed. Our preliminary studies demonstrate that apoE4 is less adept than the other isoforms in protecting lipoprotein receptors from the shedding process, which results in reduced Aβ clearance from the brain. At this stage, however, the mechanism by which apoE influences lipoprotein receptor proteolysis has yet to be elucidated. MMP9 is an endopeptidase that can bind and proteolyze (i.e., shedding) lipoprotein receptors. In AD, MMP9 levels are significantly elevated in the brain and periphery compared to control subjects. To this stage there has been little investigation into the relationship between apoE and MMP9, but our preliminary data suggest a key role for apoE in mediating the effect of MMP9 on lipoprotein receptor shedding. In our preliminary studies, cerebral vessels isolated from apoE2 transgenic mice showed reduced lipoprotein receptor shedding compared to apoE4 cerebrovessels following exposure to activated MMP9. In addition, we found that administration of an MMP9 inhibitor to apoE4 transgenic mice reduced lipoprotein receptor shedding in the brain and improved Aβ clearance across the BBB to levels near that observed in untreated apoE3 animals. These studies suggest an isoform-specific relationship between apoE and MMP9. The hypothesis of this proposal is that apoE4 is less efficient than other apoE isoforms in regulating MMP9 function, which leads to increased lipoprotein receptor shedding and reduced Aβ elimination from the brain. The goal of this proposal is to elucidate the mechanism by which apoE regulates MMP9 function and influences lipoprotein receptor shedding and Aβ removal from the brain. The current proposal will examine several mechanisms to determine the relationship between apoE and MMP9. Aim1 will evaluate the impact of apoE gen...