Agonist of growth hormone–releasing hormone enhances retinal ganglion cell protection induced by macrophages after optic nerve injury

Optic neuropathies are leading causes of irreversible visual impairment and blindness, currently affecting more than 100 million people worldwide. Glaucoma is a group of optic neuropathies attributed to progressive degeneration of retinal ganglion cells (RGCs). We have previously demonstrated an increase in survival of RGCs by the activation of macrophages, whereas the inhibition of macrophages was involved in the alleviation on endotoxin-induced inflammation by antagonist of growth hormone–releasing hormone (GHRH). Herein, we hypothesized that GHRH receptor (GHRH-R) signaling could be involved in the survival of RGCs mediated by inflammation. We found the expression of GHRH-R in RGCs of adult rat retina. After optic nerve crush, subcutaneous application of GHRH agonist MR-409 or antagonist MIA-602 promoted the survival of RGCs. Both the GHRH agonist and antagonist increased the phosphorylation of Akt in the retina, but only agonist MR-409 promoted microglia activation in the retina. The antagonist MIA-602 reduced significantly the expression of inflammation-related genes Il1b, Il6, and Tnf. Moreover, agonist MR-409 further enhanced the promotion of RGC survival by lens injury or zymosan-induced macrophage activation, whereas antagonist MIA-602 attenuated the enhancement in RGC survival. Our findings reveal the protective effect of agonistic analogs of GHRH on RGCs in rats after optic nerve injury and its additive effect to macrophage activation, indicating a therapeutic potential of GHRH agonists for the protection of RGCs against optic neuropathies especially in glaucoma.

Optic neuropathies can occur in common and serious ocular complications, including glaucoma, traumatic injury, inflammation, ischemia, and tumors. They share a common pathology of degeneration of retinal ganglion cells (RGCs) and axonal loss. Optic neuropathies are leading causes of visual impairment and irreversible blindness, affecting more than 100 million people worldwide (1). There is no effective treatment for the complete functional recovery in optic neuropathies. Among them, glaucoma is the top cause of irreversible impaired vision affecting 5% of the world’s population across all races and regions (2). The principle of treatment for glaucoma is to minimize progression by lowering the intraocular pressure (3). There is no therapy to date that stops or reverses optic nerve (ON) damage in glaucoma so as to cure the disease (4).Mammalian retina has low intrinsic regenerative ability, and the presence of myelin-associated inhibitors, scar formation at the injury site, and lack of trophic support further contribute to the failure in regeneration after ON injury (5–7). Research aimed at enhancing the survival of RGCs and axonal regeneration has led to potential therapeutic strategies, including exogenous supplementation of neurotrophic factors (8, 9), inhibition of the apoptotic pathway (10), counteracting the inhibitory signals associated with myelin and glial scar formation (11), deletion of Pten and Socs3 genes (12), as well as stem cell therapy (13). Clinical applications of these findings are under development.Mild ocular inflammation induced by lens injury or zymosan-induced macrophage activation has been shown to promote the survival of RGCs and axonal regeneration (14, 15). Oncomodulin is the major mediator derived from activated macrophages and neutrophils in the inflammation-induced axonal regeneration (16, 17), with the elevation of cyclic adenosine monophosphate (cAMP). However, oncomodulin does not account for all promoting effects on axonal regeneration by lens injury. There are still undiscovered factors and mechanisms involved in the inflammation-induced neural recovery after ON injury.Growth hormone (GH)–releasing hormone (GHRH) controls the secretion of GH in the anterior pituitary, and GH regulates the synthesis of insulin-like growth factor-1 (IGF1) in the liver (18). GHRH signaling also exerts extrapituitary functions, including the involvement in tumor apoptosis (19). Our previous study in endotoxin-induced anterior uveitis rat model have shown that GHRH receptor (GHRH-R) is up-regulated in the ciliary body, macrophages, and leukocytes (20). The GHRH antagonist MIA-602 can reduce the infiltration of endotoxin-induced macrophages and leukocytes into aqueous humor by inhibiting inflammatory cytokine secretion, indicating the participation of GHRH signaling in the regulation of inflammatory responses (20). Notably, we observed the expression of GHRH-R in the RGC layer of the retina. While GH should be neuroprotective against excitotoxic-induced cell death in RGCs (21), the physiological role of GHRH-R signaling in RGCs remains elusive. We previously showed that GHRH agonists regulate the PI3K/Akt and MAPK/Erk pathways (22), which are important for RGC survival. Herein we hypothesized that GHRH-R signaling could be involved in the promotion of neural recovery after ON injury induced by inflammation. In this study, we investigated the effects of GHRH agonists and antagonists on RGCs after ON injury, and its involvement in the promotion of neural repair induced by inflammation after lens injury or zymosan-induced macrophage activation.