Image shows a section of adult mouse sciatic nerve seven days after crush injury. The distal segment of the nerve undergoes Wallerian degeneration. Myelin ovoid form as myelin sheaths disintegrate (labeled green with Fluoromyelin). Schwann cells (Axons) stained with anti-S100 (SCG10) are labeled red. Cell nuclei, stained with Hoechst nuclear dye, are labeled blue.
Peripheral nervous system injury – from axon degeneration to axon regeneration.
Injury to the peripheral nervous system (PNS) is common and typically elicits a substantial regenerative response from injured motor and sensory neurons. However, depending on injury severity and location, as well as age and metabolic health, regeneration is often incomplete, leading to persistent loss of voluntary movement, sensory deficits, autonomic dysfunction, and chronic pain. Nerve fibers can be damaged by compression injuries (neuropraxia) or transection injuries (neurotmesis). Regardless of injury type, PNS trauma functionally divides a nerve into three distinct compartments (Figure 1) including (1) a proximal nerve segment containing axons that remain connected to their neuronal cell bodies, (2) the injury site, where physical disruption causes immediate tissue damage including axonal transection, and (3) a distal nerve segment containing axons that have lost connection to the cell soma. While proximal axons are largely stable, axons at the injury site are physically disrupted, and axons in the distal nerve undergo SARM1-dependent Wallerian degeneration.
This compartmentalized response to nerve injury is tightly coupled to activation of the immune system, involving both the innate and adaptive branches. The nervous and immune systems are in constant dialogue, and this communication becomes particularly intense following nerve trauma or disease. PNS injury triggers a stereotypical wound-healing response characterized by the sequential recruitment of specific blood-borne immune cell populations to the injury site, while a distinct immune milieu develops in the distal nerve as axons undergo Wallerian degeneration and fiber debris is cleared. To restore function, axons proximal to the injury must regenerate through the injury site and traverse the entire length of the distal nerve to reconnect with their original targets, including neuromuscular junctions (for motor axons), skin, joints, bone, and muscle spindles (for sensory axons). Macrophages play central roles in this process and have been implicated in multiple aspects of PNS repair, including angiogenesis, clearance of apoptotic cells and myelin debris, axonal regeneration, and functional recovery. While macrophages exert many beneficial functions, their prolonged or dysregulated presence is thought to be detrimental, contributing to sustained nerve inflammation, secondary damage, and possibly post-traumatic pain.
A major challenge in defining immune cell function in the injured PNS is the highly dynamic and heterogeneous nature of the cellular landscape. To address this challenge, we performed longitudinal flow cytometry and single-cell RNA sequencing studies in mice subjected to sciatic nerve crush injury. This foundational work led to the assembly of the injured sciatic nerve atlas (iSNAT), identified multiple macrophage subpopulations, and revealed pronounced differences in immune composition at the injury site, within the distal nerve, and in axotomized dorsal root ganglia (DRGs). In an ongoing collaboration with the Department of Neurosurgery, we obtain nerve specimens from patients undergoing reconstructive surgery following brachial plexus injury. These studies enable comparative analyses of the nerve injury niche in mice and humans, allowing us to identify conserved cell types, signaling pathways, and molecular programs. Insights gained from this work are critical for evaluating experimental treatment strategies in mouse models and advancing their translational potential for human patients.
Immune-Mediated Central Nervous System Regeneration
A major goal of regenerative medicine is to develop strategies that protect injured CNS neurons from axotomy-induced death and stimulate long-distance axon regeneration to restore lost neural connections. Our laboratory studies these processes using the optic nerve as a powerful and tractable model system. Specifically, we employ retro-orbital optic nerve crush (ONC) injury to investigate mechanisms that regulate retinal ganglion cell (RGC) degeneration, survival, and axon regeneration. Although significant progress has been made in identifying molecular pathways that promote or suppress RGC survival and regeneration, no effective treatment strategies currently exist. In experimental models of ONC injury, robust axon regeneration can be induced, with most approaches focused on enhancing RGC-intrinsic (cell-autonomous) growth programs or delivering growth factors to the retina. In addition, activation of the innate immune system plays a critical role in CNS repair; however, the underlying mechanisms remain incompletely understood. Experimental induction of ocular inflammation, either by intra-ocular injection of β-glucan (the active component of zymosan) or by mechanical injury to the ocular lens, elicits a strong tissue repair program that protects axotomized RGCs and promotes long-distance axon regeneration.
Despite these striking pro-regenerative effects, the specific immune cell types involved and the molecular mechanisms by which inflammation reshapes the ocular microenvironment to support regeneration remain incompletely understood. Our work has revealed that immune activation is a double-edged sword. While ocular inflammation can promote regeneration, it can also be detrimental by increasing leakiness of the blood–retina barrier (BRB), thereby suppressing immune-mediated RGC axon regeneration. We further discovered that excessive neutrophil trafficking through retinal venules drives BRB breakdown, whereas retinal microglia play a protective role in maintaining vascular integrity. To dissect the underlying cell–cell interactions, we combine ONC injury in transgenic mice with a retinal explant system, allowing us to study adult RGC axon outgrowth both in vivo and ex vivo. In parallel, we employ single-cell and single-nucleus transcriptomics together with viral vector–mediated gene delivery to define how lens injury- or β-glucan-induced cellular programs promote RGC protection and axon regeneration. Harnessing the immune system to promote CNS repair is particularly attractive from a translational perspective, as bone marrow–derived immune cells are accessible in patients and amenable to manipulation for autologous, cell-based therapies. By defining immune mechanisms that support or limit neural repair, our work aims to uncover new opportunities for personalized strategies to restore vision after optic nerve injury.
CNS Myelination and Axon–Glia Communication
Even if long-distance axon regeneration and accurate target reinnervation can be achieved after CNS injury, these advances alone are unlikely to restore meaningful function. Regenerated axons must also be properly myelinated to support the rapid and reliable propagation of complex patterns of action potentials. Understanding how myelination is regulated is therefore essential for achieving functional CNS repair. Our laboratory studies the role of phosphoinositides, a family of signaling lipids, in developmental myelination and in axon remyelination following myelin injury. In the CNS, myelin is produced by oligodendrocytes (OLs), highly complex cells that can segmentally myelinate dozens of axons simultaneously. OLs synthesize myelin components in the cell body based on nuclear gene programs, yet how these components are trafficked over long intracellular distances and precisely delivered to growing myelin sheaths remains poorly understood.Using transgenic mouse models, we discovered that perturbation of the biosynthetic machinery for the phosphoinositide PI(3,5)P₂ either in neurons or in oligodendrocyte progenitor cells (OPCs), causes severe CNS hypomyelination and impaired action potential conduction. At the cellular level, primary OLs with reduced PI(3,5)P₂ accumulate enlarged LAMP1⁺ and Rab7⁺ vesicular structures and show a striking defect in membrane sheet expansion. Loss of PI(3,5)P₂ leads to mislocalization of myelin-associated glycoprotein (MAG), which becomes trapped in perinuclear endo-lysosomal compartments rather than being delivered to nascent myelin membranes. Live-cell imaging following genetic or pharmacological inhibition of PI(3,5)P₂ synthesis reveals profound defects in endo-lysosomal trafficking pathways in both cultured OLs and intact brain tissue. A major focus moving forward is to define the regulatory mechanisms that control PI(3,5)P₂ production in neurons, and to determine how this lipid signaling pathway promotes myelination of CNS axons.