TLR-mediated Small Fiber Neuropathy

Project Summary Peripheral small fiber neuropathy is a significant health problem; symptoms include abnormal sensation, hypersensitivity, or a loss of sensation. Spontaneous pain, burning, tingling, and itch are additional complications, as is loss of sensation associated with injury or infection. Intraepidermal nerve fiber density (IENFD) assessment allows for quantifying epidermal axon density from skin biopsies. Despite its prevalence, mechanisms that lead to IENF loss and small fiber neuropathy are poorly understood. We propose a novel framework to explain widespread IENFD loss in chronic disease. We will explore whether epidermal axons comprise a surveillance network that responds to “threats” on body surfaces, leading to IENFD loss. This framework involves 1) pattern recognition receptors, 2) metabolic changes, and 3) SARM1-mediated axon degeneration to alter axons in the skin. Threats include pathogen-associated molecular patterns (PAMPS) and damage-associated molecular patterns (DAMPS) that lead to axon loss. We hypothesize that DAMPs lead to epidermal axon degeneration via toll-like receptors (TLRs) that involve changes in glycolysis, oxidative phosphorylation, cytokine signaling, and SARM1 activation. Aim 1 will address whether activation of pattern recognition receptors on sensory neurons stimulates epidermal axon loss. Experiments will identify DAMPs that stimulate axon degeneration in vivo and identify changes in cytokine responses. Experiments will identify signaling steps and use antagonists to TLR receptors to block TLR-induced degeneration. Experiments will test whether non-neuronal cells are required to induce TLR-induced degeneration using genetic mice that eliminate macrophages. Aim 2 will determine whether epidermal axon degeneration requires elevated glycolysis. This aim will test if TLR activation on DRG neurons in vitro leads to increased glycolysis enzymes and HIF-1a. Aim 3 will test whether TLR-induced axon degeneration requires SARM1. Experiments will use SARM1 genetic null mutant mice to determine if SARM1 is required for TLR-induced axon degeneration using various DAMPs. We will assess changes in NAD+ levels in settings of TLR-induced stimulation to see if NAD+ could explain SARM1 involvement. These experiments will provide new information about a novel surveillance function of sensory axons to respond to perceived threats. This aim will block TLR activation using antagonists and prevent elevations in glycolysis using 2-deoxyglucose. Finally, these findings will inform us about potential therapeutic targets and provide new information about axon degeneration and sensory dysfunction in patients with small fiber IENFD loss. Work in this proposal is directly translatable to clinical studies, and many of the proposed experiments are organized to aid in designing a future clinical study, as these surveillance pathways are druggable.