Insight into astrocyte activation after optic nerve injury

Effect of astrocyte GPER on the optic nerve inflammatory response following optic nerve injury in mice

Activated astrocytes are a primary source of inflammatory factors following traumatic optic neuropathy (TON). Accumulation of inflammatory factors in this context leads to increased axonal damage and loss of retinal ganglion cells (RGCs). Therefore, in the present study, we explored the role of the astrocyte G protein-coupled estrogen receptor (GPER) in regulating inflammatory factors following optic nerve crush (ONC), and analyzed its potential regulatory mechanisms. Overall, our results showed that GPER was abundantly expressed in the optic nerve, and co-localized with glial fibrillary acidic proteins (GFAP). Exogenous administration of G-1 led to a significant reduction in astrocyte activation and expression of inflammation-related factors (including IL-1β, TNF-α, NFκB, and p-NFκB). Additionally, it dramatically increased the survival of RGCs. In contrast, astrocytes were activated to a greater extent by exogenous G15 administration; however, RGCs survival was significantly reduced. In vitro, GPER activation significantly reduced astrocyte activation and the release of inflammation-related factors. In conclusion, activation of astrocyte GPER significantly reduced ONC inflammation levels, and should be explored as a potential target pathway for protecting the optic nerve and RGCs after TON.

Viability of mitochondria-labeled retinal ganglion cells in organotypic retinal explant cultures by two methods

Retinal explant cultures provide a valuable system to study retinal function in vitro. This study established a new retinal explant culture method to prolong the survival of retinal ganglion cells (RGCs). Explants were prepared in two different ways: with or without optic nerve. Retinas from newborn mice that had received an injection of MitoTracker Red into the contralateral superior colliculus to label axonal mitochondria were cultured as organotypic culture for 7 days in vitro. At several time points during the culture, viability of RGCs was assessed by multi-electrode array recording, and morphology by immunohistochemical methods. During the culture, the thickness of the retinal tissue in both groups gradually decreased, however, the structure of the layers of the retina could be identified. Massive apoptosis in the retinal ganglion cell layer (GCL) appeared on the first day of culture, thereafter the number of apoptotic cells decreased. Glial activation was observed throughout the culture, and there was no difference in morphology between the two groups. RGCs loss was exacerbated on 3rdday of culture, and RGCs loss in retinal explants with preserved optic nerve was significantly lower than in retinas that did not preserve the optic nerve. More and longer-lasting mitochondrial signals were observed in the injured area of the optic nerve-preserving explants. Retinal explants provide an invaluable tool for studying retinal function and developing treatments for ocular diseases. The optic nerve-preserving culture helps preserve the integrity of RGCs. The higher number of mitochondria in the nerve-preserving cultures may help maintain viability of RGCs.

Injury-induced Autophagy Delays Axonal Regeneration after Optic Nerve Damage in Adult Zebrafish

Optic neuropathies comprise a group of disorders in which the axons of retinal ganglion cells (RGCs), the retinal projection neurons conveying visual information to the brain, are damaged. This results in visual impairment or even blindness, which is irreversible as adult mammals lack the capacity to repair or replace injured or lost neurons. Despite intensive research, no efficient treatment to induce axonal regeneration in the central nervous system (CNS) is available yet. Autophagy, the cellular recycling response, was shown repeatedly to be elevated in animal models of optic nerve injury, and both beneficial and detrimental effects have been reported. In this study, we subjected spontaneously regenerating adult zebrafish to optic nerve damage (ONC) and revealed that autophagy is enhanced after optic nerve damage in zebrafish, both in RGC axons and somas, as well as in macroglial cells of the retina, the optic nerve and the visual target areas in the brain. Interestingly, the pattern of the autophagic response in the axons followed the spatiotemporal window of axonal regrowth, which suggests that autophagy is ongoing at the growth cones. Pharmacological inhibition of the recycling pathway resulted in accelerated RGC target reinnervation, possibly linked to increased mechanistic target of rapamycin (mTOR) activity, known to stimulate axonal regrowth. Taken together, these intriguing findings underline that further research is warranted to decipher if modulation of autophagy could be an effective therapeutic method to induce CNS regeneration.

Inhibition of miR-21 ameliorates excessive astrocyte activation and promotes axon regeneration following optic nerve crush

Optic nerve injury is a leading cause of irreversible visual impairment worldwide and can even cause blindness. Excessive activation of astrocytes has negative effects on the repair and recovery of retinal ganglion cells following optic nerve injury. However, the molecular and cellular mechanisms underlying astrocyte activation after optic nerve injury remain largely unknown. In the present study, we explored the effects of microRNA-21 (miR-21) on axon regeneration and flash visual evoked potential (F-VEP) and the underlying mechanisms of these effects based on astrocyte activation in the rat model of optic nerve crush (ONC). To the best of our knowledge, this article is the first to report that inhibition of miR-21 enhances axonal regeneration and promotes functional recovery in F-VEP in the rat model of ONC. Furthermore, inhibition of miR-21 attenuates excessive astrocyte activation and glial scar formation, thereby promoting axonal regeneration by regulating the epidermal growth factor receptor (EGFR) pathway. In addition, we observed that the expression of tissue inhibitor of metalloproteinase-3, a target gene of miR-21, was inhibited during this process. Taken together, these findings demonstrate that inhibition of miR-21 regulates the EGFR pathway, ameliorating excessive astrocyte activation and glial scar progression and promoting axonal regeneration and alleviating impairment in F-VEP function in a model of ONC. This study's results suggest that miR-21 may represent a therapeutic target for optic nerve injury.

Retinal Astrocytes and GABAergic Wide-Field Amacrine Cells Express PDGFRα: Connection to Retinal Ganglion Cell Neuroprotection by PDGF-AA

Purpose

Our previous experiments demonstrated that intravitreal injection of platelet-derived growth factor-AA (PDGF-AA) provides retinal ganglion cell (RGC) neuroprotection in a rodent model of glaucoma. Here we used PDGFRα-enhanced green fluorescent protein (EGFP) mice to identify retinal cells that may be essential for RGC protection by PDGF-AA.

Methods

PDGFRα-EGFP mice expressing nuclear-targeted EGFP under the control of the PDGFRα promoter were used. Localization of PDGFRα in the neural retina was investigated by confocal imaging of EGFP fluorescence and immunofluorescent labeling with a panel of antibodies recognizing different retinal cell types. Primary cultures of mouse RGCs were produced by immunopanning. Neurobiotin injection of amacrine cells in a flat-mounted retina was used for the identification of EGFP-positive amacrine cells in the inner nuclear layer.

Results

In the mouse neural retina, PDGFRα was preferentially localized in the ganglion cell and inner nuclear layers. Immunostaining of the retina demonstrated that astrocytes in the ganglion cell layer and a subpopulation of amacrine cells in the inner nuclear layer express PDGFRα, whereas RGCs (in vivo or in vitro) did not. PDGFRα-positive amacrine cells are likely to be Type 45 gamma-aminobutyric acidergic (GABAergic) wide-field amacrine cells.

Conclusions

These data indicate that the neuroprotective effect of PDGF-AA in a rodent model of glaucoma could be mediated by astrocytes and/or a subpopulation of amacrine cells. We suggest that after intravitreal injection of PDGF-AA, these cells secrete factors protecting RGCs.

Neuroinflammation as Fuel for Axonal Regeneration in the Injured Vertebrate Central Nervous System

Damage to the central nervous system (CNS) is one of the leading causes of morbidity and mortality in elderly, as repair after lesions or neurodegenerative disease usually fails because of the limited capacity of CNS regeneration. The causes underlying this limited regenerative potential are multifactorial, but one critical aspect is neuroinflammation. Although classically considered as harmful, it is now becoming increasingly clear that inflammation can also promote regeneration, if the appropriate context is provided. Here, we review the current knowledge on how acute inflammation is intertwined with axonal regeneration, an important component of CNS repair. After optic nerve or spinal cord injury, inflammatory stimulation and/or modification greatly improve the regenerative outcome in rodents. Moreover, the hypothesis of a beneficial role of inflammation is further supported by evidence from adult zebrafish, which possess the remarkable capability to repair CNS lesions and even restore functionality. Lastly, we shed light on the impact of aging processes on the regenerative capacity in the CNS of mammals and zebrafish. As aging not only affects the CNS, but also the immune system, the regeneration potential is expected to further decline in aged individuals, an element that should definitely be considered in the search for novel therapeutic strategies.

Promotion of axon regeneration and inhibition of astrocyte activation by alpha A-crystallin on crushed optic nerve

AIM

To explore the effects of αA-crystallin in astrocyte gliosis after optic nerve crush (ONC) and the mechanism of α-crystallin in neuroprotection and axon regeneration.

METHODS

ONC was established on the Sprague-Dawley rat model and αA-crystallin (10(-4) g/L, 4 µL) was intravitreously injected into the rat model. Flash-visual evoked potential (F-VEP) was examined 14d after ONC, and the glial fibrillary acidic protein (GFAP) levels in the retina and crush site were analyzed 1, 3, 5, 7 and 14d after ONC by immunohistochemistry (IHC) and Western blot respectively. The levels of beta Tubulin (TUJ1), growth-associated membrane phosphoprotein-43 (GAP-43), chondroitin sulfate proteoglycans (CSPGs) and neurocan were also determined by IHC 14d after ONC.

RESULTS

GFAP level in the retina and the optic nerve significantly increased 1d after ONC, and reached the peak level 7d post-ONC. Injection of αA-crystallin significantly decreased GFAP level in both the retina and the crush site 3d after ONC, and induced astrocytes architecture remodeling at the crush site. Quantification of retinal ganglion cell (RGC) axons indicated αA-crystallin markedly promoted axon regeneration in ONC rats and enhanced the regenerated axons penetrated into the glial scar. CSPGs and neurocan expression also decreased 14d after αA-crystallin injection. The amplitude (N1-P1) and latency (P1) of F-VEP were also restored.

CONCLUSION

Our results suggest α-crystallin promotes the axon regeneration of RGCs and suppresses the activation of astrocytes.

Neuregulin-1 protects against acute optic nerve injury in rat model

OBJECTIVES

In this study, we employed a rat model and examined the expression pattern of neuregulin-1 (NRG-1) in optic nerve and retinal ganglion cells (RGCs) in response to optic nerve injury to understand the role of NRG-1 in conferring protection against acute optic nerve injury.

METHOD

Forty-eight male rats were randomly divided into two groups, the sham-operation group (n=24) and optic nerve injury group (n=24). Flash visual evoked potentials (FVEP) and fundography images were acquired at different time points following optic nerve injury (2h, 1d, 2d, 7d, 14d and 28d). Semi-quantitative analysis of NGR-1 expression pattern was performed by immunohistochemistry (IHC) staining. In a related experiment, 100 male rats were randomly divided into NGR-1 treatment group (n=60) (treated with increasing dose of NGR-1 at 0.5μg, 1μg and 3μg), normal saline (NS) group (n=20) and negative control group (n=20). Optic nerve injury was induced in all the animals and in situ cell death was measured by detecting the apoptosis rates using TUNEL assay.

RESULTS

Fundus photography results revealed no detectable differences between the sham-operation group and optic nerve injury group at 2h, 1d, 2d and 7d. However at 2weeks, the optic discs turned pale in all animals in the optic nerve injury group. NRG-1 expression increased significantly at all time points in the optic nerve injury group (P<0.05), compared to the sham-operation group, with NRG-1 expression peaking at 14d and gradually declining by 28d. Statistically significant differences in amplitude and latency of P100 wave were also detected between the optic nerve injury and sham-operation group (P<0.05). In related experiment, compared to NS group, treatment with 1μg and 3μg of recombinant human NRG-1 resulted in statistically significant FVEP-P100 amplitude values (all P<0.05). Further, compared to the NS group, ganglion cell apoptosis was dramatically reduced in the NRG-1 group at all time points and the reduction was statistically significant in 3μg NRG-1 treatment group at 7d, 14d and 28d (all P<0.05).

CONCLUSION

Our results strongly suggest that NRG-1 is highly effective in preserving normal optic nerve function and is essential for tissue repair following optic nerve injury. Thus, NRG-1 expression confers protection against acute optic nerve injury in a dose-dependent manner.

Neuroinflammation in Glaucoma and Optic Nerve Damage

Glaucoma is a group of optic neuropathies characterized by the degeneration of retinal ganglion cell axons and somas, ultimately preventing light signals in the retina from reaching the brain. Glaucoma is a leading cause of blindness in the world, and treatment options for patients remain limited and minimally efficacious. A number of mechanisms have been linked to glaucomatous pathophysiology. A leading role is now attributed to neuroinflammatory conditions generated by the resident innate immune cells in the optic nerve and retina. Since the eye is immune privileged, the adaptation of these innate immune cells, termed glia, is crucial following trauma. In this chapter, we discuss the mechanisms associated with normal glial function in a healthy eye, and how changes in glial activation can contribute to the process of glaucomatous neurodegeneration in both the optic nerve and retina.