Significance: The optic nerve conveys information from retinal ganglion cells (RGCs) to visual processing areas of the brain. Although this pathway normally cannot regenerate when injured nor in degenerative diseases such as glaucoma, this failure can be partially reversed by eliciting a controlled immune reaction in the eye. We show here that the chemokine SDF1 (stromal cell–derived factor 1) is an important contributor to this phenomenon. SDF1 is produced by infiltrative monocytes and acts through its cognate receptor to enhance RGC survival, promote optic nerve regeneration, and sensitize subtypes of RGCs that normally fail to respond to a complementary treatment to exhibit robust, long-distance regeneration. These findings establish SDF1 as an important therapeutic candidate for repairing the injured optic nerve.

The optic nerve conveys information about the outside world from the retina to multiple subcortical relay centers. Until recently, the optic nerve was widely believed to be incapable of re-growing if injured, with dire consequences for victims of traumatic, ischemic, or neurodegenerative diseases of this pathway. Over the past 10–20 years, research from our lab and others has made considerable progress in defining factors that normally suppress axon regeneration and the ability of retinal ganglion cells, the projection neurons of the retina, to survive after nerve injury. Here we describe research from our lab on the role of inflammation-derived growth factors, suppression of inter-cellular signals among diverse retinal cell types, and combinatorial therapies, along with related studies from other labs, that enable animals with optic nerve injury to regenerate damaged retinal axons back to the brain. These studies raise the possibility that vision might one day be restored to people with optic nerve damage.

Oncomodulin (Ocm) is a myeloid cell–derived growth factor that enables axon regeneration in mice and rats after optic nerve injury or peripheral nerve injury, yet the mechanisms underlying its activity are unknown. Using proximity biotinylation, coimmunoprecipitation, surface plasmon resonance, and ectopic expression, we have identified armadillo-repeat protein C10 (ArmC10) as a high-affinity receptor for Ocm. ArmC10 deletion suppressed inflammation-induced axon regeneration in the injured optic nerves of mice. ArmC10 deletion also suppressed the ability of lesioned sensory neurons to regenerate peripheral axons rapidly after a second injury and to regenerate their central axons after spinal cord injury in mice (the conditioning lesion effect). Conversely, Ocm acted through ArmC10 to accelerate optic nerve and peripheral nerve regeneration and to enable spinal cord axon regeneration in these mouse nerve injury models. We showed that ArmC10 is highly expressed in human-induced pluripotent stem cell–derived sensory neurons and that exposure to Ocm altered gene expression and enhanced neurite outgrowth. ArmC10 was also expressed in human monocytes, and Ocm increased the expression of immune modulatory genes in these cells. These findings suggest that Ocm acting through its receptor ArmC10 may be a useful therapeutic target for nerve repair and immune modulation.

Although most pathways in the mature central nervous system cannot regenerate when injured, research beginning in the late 20th century has led to discoveries that may help reverse this situation. Here, we highlight research in recent years from our laboratory identifying oncomodulin (Ocm), stromal cell-derived factor (SDF)-1, and chemokine CCL5 as growth factors expressed by cells of the innate immune system that promote axon regeneration in the injured optic nerve and elsewhere in the central and peripheral nervous systems. We also review the role of ArmC10, a newly discovered Ocm receptor, in mediating many of these effects, and the synergy between inflammation-derived growth factors and complementary strategies to promote regeneration, including deleting genes encoding cell-intrinsic suppressors of axon growth, manipulating transcription factors that suppress or promote the expression of growth-related genes, and manipulating cell-extrinsic suppressors of axon growth. In some cases, combinatorial strategies have led to unprecedented levels of nerve regeneration. The identification of some similar mechanisms in human neurons offers hope that key discoveries made in animal models may eventually lead to treatments to improve outcomes after neurological damage in patients.

Precisely localizing the spatial distribution of proteins within various brain cell types and subcellular compartments, such as the synapses, is essential for generating and testing hypotheses to elucidate their roles in brain function. While the fms-like tyrosine kinase-3 (Flt3) has been extensively studied in the context of blood cell development and leukemia pathogenesis, its role in the brain remains poorly understood. Previous efforts to address this issue were hindered by the low expression levels of Flt3 and the limited sensitivity of the standard immunolabeling method, which were insufficient to reliably detect Flt3 protein in brain tissue. In this study, we systematically characterized Flt3 protein localization during brain development using a highly sensitive immunolabeling method based on alkaline phosphatase (AP) polymer biochemistry. This approach revealed a previously unrecognized neuron-selective Flt3 expression pattern in both mouse and human cerebella, with a developmental increase in total protein levels accompanied by a shift from a cytosolic to a dendritic subcellular distribution. Combining AP-polymer-based immunohistochemistry (AP-IHC) for Flt3 with conventional immunostaining of cell type marker proteins revealed parvalbumin- and calbindin-positive Purkinje cells to be the main cell type expressing Flt3 in the cerebellum. To validate the versatility of the AP-IHC method for detecting low-abundance neuronal proteins, we demonstrated robust labeling of Kir2.1, a potassium channel protein, in brain tissue sections from mouse, pig, and human samples. We further applied the AP-IHC method to human stem cell-derived neurons, effectively visualizing the postsynaptic density scaffold protein PSD95 within synapses. To our knowledge, this is the first study to employ an AP-IHC method combined with other standard immunofluorescent staining to co-detect weakly expressed neuronal proteins and other cellular markers in brain tissue and cultured neurons. Additionally, our findings uncover a previously unrecognized neuron-specific pattern of Flt3 expression in the cerebellum, laying the foundation for future mechanistic studies on its role in normal brain development and neurological disorders.