Aberrant PTEN signaling in chronically activated microglia drives tau pathology, neurodegeneration and cognitive decline following repetitive brain injury - Repetitive mild TBI (r-mTBI) can induce neurological impairment years after injury, increasing the risk for Alzheimer’s Disease (AD). No suitable treatments have been developed to rescue the long-term consequences of TBI. Advancing our understanding of the mechanisms driving the chronic effects of r-mTBI is critical, as this could lead to the identification of novel therapies. Microglia mediated neuroinflammation is a common feature of TBI and AD. Experimental evidence suggests that neurodegeneration and cognitive deficits are associated with the maladaptive transformation of microglia from an early neurorestorative phenotype to a dysfunctional and chronically activated proinflammatory state. Factors governing the persistence of these neuroinflammatory responses in the chronic sequelae of TBI remain elusive. We have established a WT mouse model that recapitulates many of the features of human r-mTBI and thus represents a translationally relevant preclinical platform to interrogate these factors. Having established this r-mTBI model (and given the role of tau as a pathognomonic lesion of TBI), we generated microglial transcriptomic profiles in human Tau Knock In (TauKI) mice at a range of timepoints post-injury. Firstly, when we compared microglial responses in TauKI vs WT mice, we revealed an increase in significant gene transcripts between r-mTBI and sham microglia by 3 to 9-fold at chronic timepoints. In the TauKI model, we reveal deficits in bioenergetics, cytokine signaling, lipid metabolism, and a pro- inflammatory signature of chronically activated microglia, which appear to be influenced by the activation of Phosphatase and Tensin Homolog (PTEN) signaling, and this correlated with TBI dependent Tau pathology. PTEN is a lipid phosphatase that antagonizes PI3K signaling, a critical node vital for regulating cell survival, energy bioenergetics and inflammation. PTEN is highly expressed in myeloid cells, and its dysregulation can trigger the activation of neuroinflammatory responses. PTEN activation has been reported to occur early in tauopathy models, and its inhibition, dampens microglial activation, reduces synaptic phagocytosis by microglia, and preserves neuronal integrity. We thus hypothesize that PTEN activation is a mediator of chronically activated microglia and their dysfunctional activities, and a driver of tau seeding and pathogenesis, such that genetic deletion of PTEN in microglia will mitigate the deleterious effects of chronic microglial activation on tau pathology and long-term TBI outcomes. To begin to address this, we developed a Cre-LoxP model that specifically deletes PTEN in myeloid cells between 2.5 to 3.5 mo post- injury and revealed rescue of pro-inflammatory microglial responses after r-mTBI. In this new proposal, we plan to expand on this work, using an improved tamoxifen inducible model on a human TauKI background that deletes PTEN signaling in microglia, and not other myeloid cells. We will explore the optimal time-window of intervention (early vs delayed) and generate a detailed longitudinal time-course of neurobehavior, microglial phenotype and function, tau seeding/pathogenesis and neuropathological outcomes after r-mTBI. We will also use a single cell RNAseq mining approach to compare TBI-dependent responses in the presence or absence of PTEN deletion to identify the distinctive states of microglia and reparative mechanisms that drives their favorable outcomes. Our goal is to identify molecular mechanisms driving the transformation of chronic microglia into a proinflammatory and neurotoxic phenotype that contribute to tauopathy, neurodegeneration and cognitive deficits, in order to develop new and effective delayed therapeutic interventions for TBI and AD.
