David Loane - US grants

Project Summary Background: Elderly individuals are particularly vulnerable to traumatic brain injury (TBI), and numerous studies report clinically worse outcomes in elderly TBI patients. As the aged population continues to grow towards an estimated 71.5 million in 2030 in the US, the burden of TBI in the elderly is expected to dramatically increase, and will have a major impact on the national health care system. Despite these increasing challenges, research on the underlying mechanisms responsible for worse outcomes in elderly TBI patients is limited. Microglial activation is a key secondary injury mechanism that contributes to chronic neurodegeneration and loss of neurological function after TBI. Microglia have multiple activation phenotypes, including a classically activated/M1 state that may to lead to exacerbation of injury and progressive tissue destruction, and an alternatively activated/M2 state that serves to dampen the inflammatory response and promote tissue repair. Whether microglia differentiate into an M1 state that contributes to secondary injury or into an M2 state that promotes repair depends on the signals within the lesion microenvironment (pro- vs. anti- inflammatory), as well as systemic factors; both of which may be influenced by age. Description: The goal of the proposed research is to investigate the underlying molecular mechanisms that contribute to increased microglial activation and associated neurotoxicity in the aged brain following TBI. We hypothesis that increased NADPH oxidase activity during aging tips the M1/M2 microglia balance from favoring an anti-inflammatory M2 state in young, to a pro-inflammatory M1 state in elderly. This shift towards an M1 state will result in increased neurodegeneration and worse neurological outcomes in the elderly. This hypothesis is based on our recently published and preliminary data showing that aged mice have excessive M1 microglial activation in response to TBI compared to young mice, and studies in a knockout mouse which revealed that the polarization of microglia toward an M1 activation state following TBI depends on the activity of a critical molecular switch, NADPH oxidase. Here, we propose to investigate a novel molecular intervention (NADPH oxidase inhibition) to concurrently down-regulate the M1 state and up-regulate the M2 state, and we will determine its potential to promote functional recovery and repair in the young and aged TBI brain. Specific aims include: 1) Determine if aging alters NADPH oxidase activity and M1/M2-polarization of microglia after TBI, 2) Determine whether NADPH oxidase is a key molecular switch for M1 polarization of microglia after TBI, and 3) Assess whether inhibition of NADPH oxidase will attenuate poorer outcomes in aged TBI mice. Impact: Understanding the molecular mechanisms that polarize microglia towards an M2 state will be crucial to unlock the endogenous potential of microglia to promote repair during the chronic phase of recovery after TBI. Given the impact aging has on neuroinflammation after TBI, targeting NADPH oxidase may be particularly beneficial for the estimated 142,000 elderly individuals that attend the ER each year because of TBI.

Project Summary: Traumatic brain injury (TBI) triggers delayed molecular secondary injury cascades, including chronic neuroinflammation, that contribute to progressive tissue loss and neurological deficits, including dementia. We have shown that microglia are chronically activated for months-to-years following experimental TBI in mice, contributing to progressive neurodegeneration associated with cognitive decline. Microglia also undergo changes in their activation profile that may contribute to cognitive decline during neurodegenerative diseases, including Alzheimer?s disease (AD) and dementias of non-AD type. An important component of these pathological states is the maladaptive transformation of microglia from a neurorestorative/neuroprotective phenotype to a persistent, dysfunctional neurotoxic activation state. Our new studies show that microglia isolated from chronically injured brain display deficits in phagocytosis in parallel with elevations of pro-inflammatory cytokines and senescence markers, indicative of a chronic dysfunctional/neurotoxic activation state. Furthermore, we identify specific histone acetylation (H3K9ac) and methylation (H3K27me3) changes in neurotoxic microglia, which implicate intrinsic epigenetic mechanisms as drivers of this chronic phenotype. Importantly, new pilot data show that global removal of microglia from the chronically injured brain by short-term administration of a CSF1R inhibitor (PLX5622) starting at 1-month post-injury results in the repopulation of the injured brain with microglia with an anti-inflammatory phenotype. This process of resetting microglial activation after TBI dampens the chronic neuroinflammatory environment and improves long-term motor and cognitive function recovery. Thus, our data indicates that erasing posttraumatic immunological memory, by removing microglia epigenetically programmed toward a neurotoxic activation state, promotes neuroprotective microglial activation responses and improves long-term neurological recovery. Therefore, we hypothesize that moderate- severe TBI induces specific epigenetic mechanisms in microglia that promote a chronic neurotoxic activation state, causing progressive neurodegeneration and cognitive deficits. Moreover, we predict that strategies that eliminate this microglial phenotype and/or targeted inhibition of pro-inflammatory epigenetic mechanisms, even at highly delayed time points after TBI, can substantially improve long-term cognitive recovery. Here, we will use neurobehavioral, immunological, and molecular approaches to test our novel hypotheses as outlined in following specific aims: 1) To elucidate TBI-induced intrinsic epigenetic changes that lead to chronic microglial dysfunction, with a shift toward a pro-inflammatory, neurotoxic phenotype. 2) To demonstrate that microglia that repopulate the injured brain following delayed administration of CSF1R inhibitor are reprogramed toward a neurorestorative and neuroprotective phenotype that improves cognitive function. 3) To determine whether delayed interventions that target specific epigenetic mechanisms promote the neurorestorative/neuroprotective microglial phenotype and improve long-term functional recovery after TBI.