Immune Metabolism of Peripheral Nervous Tissue Degeneration and Regeneration

University Of Michigan At Ann Arbor, Ann Arbor MI

Investigators

Abstract

Abstract: Trauma to the adult mammalian peripheral nervous system (PNS) typically results in motor and sensory deficits. While PNS injury induces a strong regenerative response in axotomized neurons, functional recovery in humans is often incomplete and frequently accompanied by the development of chronic pain. PNS injury also triggers nerve inflammation, with immune cells playing vital roles in tissue healing and functional restoration. To better understand the immune response to sciatic nerve injury in mice, we performed a longitudinal analysis using a combination of flow cytometry and single-cell RNA sequencing. Our analysis revealed distinct and location-specific immune milieus at the site of injury and in the distal nerve. In addition to tissue-resident macrophages, blood-borne monocyte-derived macrophages (MDMs) massively accumulate at the injury site and in the distal nerve, where axons undergo Wallerian degeneration. Previous studies have shown that immune cells regulate processes such as nerve debridement, angiogenesis, axon regeneration, and pain. However, the specific roles, ontogeny, and functions of macrophage subpopulations remain poorly understood. To differentiate between blood-borne MDMs and tissue-resident macrophages, we propose experiments with transgenic mice (qMCP-/-) deficient for five monocyte chemotactic proteins. Preliminary studies with qMCP-/- mice demonstrated absence of MDMs in the injured PNS. Pathway analysis of single cell transcriptomes from injured sciatic nerves of wildtype mice revealed that MDMs undergo profound changes in their energy metabolism and inflammatory state. The metabolic state of macrophages is of particular interest because it determines their function: high glycolytic activity is a hallmark of pro-inflammatory cells, while elevated mitochondrial oxidative phosphorylation (OXPHOS) characterizes anti-inflammatory cells. Thus, genetic manipulations that disrupt specific metabolic pathways for ATP production offer an opportunity to investigate the roles of specific macrophage activation states in PNS injury. Our preliminary data show that selective disruption of glycolytic flux in MDMs impairs motor and sensory axon regeneration and function. Our central hypothesis is that MDMs are essential for PNS repair, and that metabolic reprogramming from high glycolytic flux to OXPHOS is critical for successful nerve fiber regeneration and the mitigation of neuropathic pain. To test this hypothesis, we propose two Specific Aims. Aim 1: Conduct a detailed analysis of the repair response in qMCP-/- mice, subjected to different types of PNS injuries. Aim 2: Use conditional gene ablation of key regulators of glycolysis and nerve inflammation, to assess their impact on location-specific immune milieus in the injured PNS, regenerative outcomes, and chronic pain. The proposed studies represent the first systematic analysis of Mac subpopulations in the injured mammalian PNS, their ontogeny, and metabolic plasticity. Insights gained will elucidate how impaired immune cell function impacts tissue repair, functional recovery, and chronic pain.