The role of macrophage metabolism and age in recovery from spinal cord injury

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PROJECT SUMMARY/ABSTRACT The average age at the time of spinal cord injury (SCI) has increased to 50.5 years old in the US. Observations from several independent laboratories demonstrate that inflammation, specifically sustained pro-inflammatory macrophage activation, contributes to age-related SCI deficits. In addition, it was recently discovered that the efficacy of immunomodulatory SCI therapies is age-dependent. There is an urgent need to understand the age-dependent mechanisms of sustained pro-inflammatory macrophage activation in SCI. The purpose of this proposal is to investigate the role of macrophage bioenergetics in age-dependent inflammation and SCI pathophysiology. The central hypothesis is that age-dependent impairments in macrophage metabolism drive pro-inflammatory macrophage activation and contribute to secondary injury after SCI. The premise is that the pyruvate dehydrogenase (PDH) pathway is a key regulator of pro- or anti-inflammatory macrophage activation as a connecting link between glycolysis (pro-inflammatory) or oxidative phosphorylation (OXPHOS, i.e. TCA or Krebs cycle activity; anti-inflammatory). PDH kinase (PDK) regulates PDH thereby serving as the gatekeeper for OXPHOS. The hypothesis to be tested is that PDK inhibition will drive metabolic processes (i.e. increased OXPHOS) required for reparative macrophage activation and improved SCI recovery by PDK. Accordingly, three independent Aims are designed to selectively target macrophage metabolism mechanistically, using chimerization with PDK knockout mice, and therapeutically, using a newly generated liposomal drug formulation of dichloroacetate (DCA), a PDK inhibitor. 4 and 14-month-old mice will undergo T9 contusion SCI to model the current SCI demographic. Aim 1 will identify PDH as a bottleneck for SCI macrophage metabolism using a newly optimized purification approach that allows for isolation of macrophages after SCI and assessment of extracellular acidification (ECAR, i.e. glycolysis) and oxygen consumption (OCR, i.e. OXPHOS) rates using the Seahorse bioanalyzer. Aim 2 will identify metabolic targets for macrophage dysfunction after SCI using state-of-the-art techniques including single-cell RNA-sequencing and in vivo Stable Isotope-Resolved Metabolomics to determine the metabolic profiles of resident microglia and peripherally derived macrophages. Aim 3 will identify PDK inhibition as a therapeutic target to treat SCI through evaluation of anatomic and functional recovery. Completion of the proposed work will identify ways to harness the reparative functions of CNS macrophages and improve clinical practice by refining translational treatment strategies including age as a potential influence in SCI treatment and recovery. Understanding the extent to which metabolic activity regulates macrophage function will provide insight into age-dependent CNS inflammation, thereby advancing the fields of neurotrauma, neuroscience, and aging. Macrophage metabolism is a contributing factor in a host of inflammatory conditions including cardiovascular disease, diabetes, cancer, etc., and the completion of the proposed aims will advance the study of human health on multiple fronts.