Humans possess three major isoforms of the apolipoprotein E (ApoE) gene coded by three alleles??2, ?3, and ?4?that confer differential susceptibility to late-onset Alzheimer's disease (AD), with ApoE4 currently recognized as the most potent genetic risk factor for AD and ApoE2 as a neuroprotective variant. While there exists an abundance of research demonstrating the neurotoxic impact caused by ApoE4, far less is known about the mechanisms by which ApoE2 delegates neuroprotection. To address this research gap, our laboratory has recently initiated a series of new studies attempted to identify the molecular bases that distinguish an ApoE2 brain from ApoE3 and ApoE4 brains and that could contribute to the neuroprotective nature of ApoE2. Results from our studies in human ApoE2, ApoE3, and ApoE4 gene targeted-replacement mice demonstrate that the three ApoE brains differ primarily in two areas: cellular bioenergetics and synaptic transmission. Of particular significance, we show for the first time that (1) the three ApoE brains differentially express a key component of the catalytic domain of the V-type H+-ATPase (V-ATPase), and (2) ApoE appears to interact with energy metabolism to modulate the activity of synaptic V-ATPase, the synaptic vesicular membrane-bound ATP-dependent proton pump that mediates the concentration of neurotransmitters into synaptic vesicles and thus is vital for synaptic transmission. These preliminary findings lead us to hypothesize that synaptic V-ATPase could serve as a key mechanism linking brain energy metabolism and synaptic transmission, two major areas that separate the three ApoE brains. Specifically, we hypothesize that (1) bioenergetic changes alter the function of synaptic V-ATPase, and (2) human ApoE isoforms differentially modulate synaptic V-ATPase by either directly affecting the expression/structure of V-ATPase or by indirectly modifying the activity of V-ATPase through altered energy metabolism, leading to differential outcomes in synaptic transmission and cognitive function. This application is proposed to investigate these hypotheses in two specific aims. Studies proposed in Specific Aim 1 are designed to examine how neuronal V-ATPase V1/V0 assembly and enzymatic activity alter in response to bioenergetic changes, and how these events are modulated by ApoE status. Studies proposed in Specific Aim 2 are designed to examine how synaptic V-ATPase multi-subunit expression profile and its function to facilitate proton-pumping and neurotransmitter uptake into synaptic vesicles alter in response to bioenergetic changes, and how these events are modulated by ApoE status. Results generated from this research will provide insights into a possible mechanism involving synaptic vesicular V-ATPase that could contribute to the differential impact of human ApoE genotypic status in cognitive aging and the risk for AD. Moreover, the results may shed light on a possible bioenergetic strategy that can be used to delay the cognitive aging process associated with ApoE4.