Abstract In Alzheimer's disease (AD), the appearance of neurofibrillary tangles (NFTs) and massive neuronal death/loss in selected brain regions are widely accepted as the two major hallmarks. Although NFTs are coincident with cell death in several neurodegenerative diseases, known as tauopathies, and most notably in AD, the molecular mechanisms leading to this chronic loss of neuronal cells is still not understood. Ectopic expression of cell cycle markers, indicating aberrant cell cycle reentry, has been reported to be associated with neuronal death in both transgenic (Tg) mouse and human AD brains. Hence, a cell cycle hypothesis was proposed that these cell cycle events (CCEs) play a central causative role and represent the first step in the development of this devastating disease. This model complements the amyloid hypothesis.. However, direct proof of a causative role for CCEs is lacking and the upstream regulatory factors are unknown. We now identify the tumor suppressor PTEN as an upstream factor governing the CCEs in post-mitotic neurons based on our finding that nuclear PTEN regulates the neuronal cell cycle. Moreover, this cell cycle regulatory function is spatially separated from the classic role of PTEN in the PI3 kinase-Akt signaling pathway, which is mediated primarily by the cytoplasmic protein. Loss of PTEN (nuclear PTEN, in particular) was found in AD-relevant regions in both Tg mice and human diseased brains which correlated with CCEs. We speculate that this loss of nuclear PTEN is one of the early causative factors in CCEs and that lentivirus-delivered nuclear PTEN gene transfer may not only block CCEs but also halt subsequent neurodegeneration. Studies proposed herein will aim to validate the cell cycle hypothesis and examine nuclear PTEN in an AD mouse model. Four specific aims are proposed to achieve this goal: 1) Quantitative analysis of PTEN/pAkt profiles and CCEs in mild cognitive impairment (MCI) and AD brains 2) Profile the pAkt level and its kinase activity in the triple Tg mouse model of AD 3) To investigate an early possible cause for CCEs in the triple Tg model and 4) To assess the effectiveness of lentivirus-delivered nuclear PTEN in treating CCEs and neurodegeneration in the triple Tg mouse model. This work will be instrumental to the future design of new therapeutic interventions, not only aiming at symptom-relief but also at disease-modifying mechanisms. Besides therapy, this study may also help validate CCEs as an early marker for disease diagnosis Project Narrative We aim to validate two new concepts that aberrant cell cycle events are one of the important disease mechanisms in AD and that lentivirus-delivered nuclear PTEN gene transfer in CNS can prevent these events in an AD mouse model. This work should have great impact on exploring a new class of cell cycle inhibitors in future preventive and treatment strategies of AD once these concepts are validated.