In normal adult retinas, NGF receptor TrkA is expressed in retinal ganglion cells (RGC), whereas glia express p75(NTR). During retinal injury, endogenous NGF, TrkA, and p75(NTR) are up-regulated. Paradoxically, neither endogenous NGF nor exogenous administration of wild type NGF can protect degenerating RGCs, even when administered at high frequency. Here we elucidate the relative contribution of NGF and each of its receptors to RGC degeneration in vivo. During retinal degeneration due to glaucoma or optic nerve transection, treatment with a mutant NGF that only activates TrkA, or with a biological response modifier that prevents endogenous NGF and pro-NGF from binding to p75(NTR) affords significant neuroprotection. Treatment of normal eyes with an NGF mutant-selective p75(NTR) agonist causes progressive RGC death, and in injured eyes it accelerates RGC death. The mechanism of p75(NTR) action during retinal degeneration due to glaucoma is paracrine, by increasing production of neurotoxic proteins TNF-α and α(2)-macroglobulin. Antagonists of p75(NTR) inhibit TNF-α and α(2)-macroglobulin up-regulation during disease, and afford neuroprotection. These data reveal a balance of neuroprotective and neurotoxic mechanisms in normal and diseased retinas, and validate each neurotrophin receptor as a pharmacological target for neuroprotection.

In many diseases, expression and ligand-dependent activity of the p75NTR receptor can promote pericyte and vascular dysfunction, inflammation, glial activation, and neurodegeneration. Diabetic retinopathy (DR) is characterized by all of these pathological events. However, the mechanisms by which p75NTR may be implicated at each stage of DR pathology remain poorly understood. Using a streptozotocin mouse model of diabetic retinopathy, we report that p75NTR is upregulated very early in glia and in pericytes to mediate ligand-dependent induction of inflammatory cytokines, disruption of the neuro-glia-vascular unit, promotion of blood–retina barrier breakdown, edema, and neuronal death. In a mouse model of oxygen-induced retinopathy, mimicking proliferative DR, p75NTR-dependent inflammation leads to ischemia and pathological angiogenesis through Semaphorin 3A. The acute use of antagonists of p75NTR or antagonists of the ligand proNGF suppresses each distinct phase of pathology, ameliorate disease, and prevent disease progression. Thus, our study documents novel disease mechanisms and validates druggable targets for diabetic retinopathy. SIGNIFICANCE STATEMENT Diabetic retinopathy (DR) affects an estimated 250 million people and has no effective treatment. Stages of progression comprise pericyte/vascular dysfunction, inflammation, glial activation, and neurodegeneration. The pathophysiology of each stage remains unclear. We postulated that the activity of p75NTR may be implicated. We show that p75NTR in glia and in pericytes mediate ligand-dependent induction of inflammatory cytokines, disruption of the neuro-glia-vascular unit, promotion of blood–retina barrier breakdown, edema, and neuronal death. p75NTR-promoted inflammation leads to ischemia and angiogenesis through Semaphorin 3A. Antagonists of p75NTR or antagonists of proNGF suppress each distinct phase of pathology, ameliorate disease, and prevent disease progression. Our study documents novel mechanisms in a pervasive disease and validates druggable targets for treatment.

Melanomas commonly undergo a phenotype switch, from a proliferative to an invasive state. Such tumor cell plasticity contributes to immunotherapy resistance; however, the mechanisms are not completely understood and thus are therapeutically unexploited. Using melanoma mouse models, we demonstrated that blocking the MNK1/2-eIF4E axis inhibited melanoma phenotype switching and sensitized melanoma to anti–PD-1 immunotherapy. We showed that phospho-eIF4E–deficient murine melanomas expressed high levels of melanocytic antigens, with similar results verified in patient melanomas. Mechanistically, we identified phospho-eIF4E–mediated translational control of NGFR, a critical effector of phenotype switching. Genetic ablation of phospho-eIF4E reprogrammed the immunosuppressive microenvironment, exemplified by lowered production of inflammatory factors, decreased PD-L1 expression on dendritic cells and myeloid-derived suppressor cells, and increased CD8⁺ T cell infiltrates. Finally, dual blockade of the MNK1/2-eIF4E axis and the PD-1/PD-L1 immune checkpoint demonstrated efficacy in multiple melanoma models regardless of their genomic classification. An increase in the presence of intratumoral stem-like TCF1⁺PD-1⁺CD8⁺ T cells, a characteristic essential for durable antitumor immunity, was detected in mice given a MNK1/2 inhibitor and anti–PD-1 therapy. Using MNK1/2 inhibitors to repress phospho-eIF4E thus offers a strategy to inhibit melanoma plasticity and improve response to anti–PD-1 immunotherapy.

Nerve growth factor (NGF) is generated from a precursor proNGF that is proteolytically procesed. NGF preferentially binds a trophic tyrosine kinase receptor TrkA, while proNGF binds a receptor p75^NTR that can have neurotoxic activity. Previously, we and others showed that a soluble protein α₂macroglobulin (α₂M) is neurotoxic. Toxicity is due in part because α₂M binds to NGF, and inhibits trophic activity presumably by preventing NGF binding to TrkA. However, the mechanisms remained unclear. Here, we show ex vivo and in vivo three mechanisms for α₂M neurotoxicity. First, unexpectedly the [α₂M•NGF] complexes do bind TrkA receptors, but do not induce TrkA dimerization or activation, resulting in deficient trophic support. Second, α₂M makes stable complexes with proNGF, conveying resistant to proteolysis that results in more proNGF and less NGF. Third, [α₂M•proNGF] complexes bind p75^NTR, and are more potent agonists than free proNGF, inducing TNFα production. Hence, α₂M regulates proNGF/p75^NTR positively and mature NGF/TrkA negatively, causing neuronal death ex vivo. These three mechanisms are operative in vivo, and α₂M causes neurodegeneration in a p75^NTR and proNGF–dependent manner. α₂M could be exploited as a therapeutic target, or as a modifier of neurotrophin signals.