Nuclear atrophy of retinal ganglion cells precedes the bax-dependent stage of apoptosis

Tafluprost promotes axon regeneration after optic nerve crush via Zn2+-mTOR pathway

PURPOSE

To investigate whether Tafluprost could promote optic nerve regeneration in mice after optic nerve crush (ONC) and determine the underlying molecular mechanism.

METHODS

Tafluprost was injected into the vitreous body immediately after ONC. The level of Zn2+ in the inner plexiform layer (IPL) of the retina was stained using autometallography (AMG). The number of survival retinal ganglion cells (RGCs) was determined via dual staining with RGC markers Tuj1 and RBPMS. Individual axons that regenerated to 0.25, 0.5, 0.75 and 1 mm were manually counted in the whole-mount optic nerve labeled by cholera toxin B fragment (CTB). Immunofluorescence and Western blot were performed to detect protein expression levels. Pattern electroretinogram was used to evaluate RGCs function.

RESULTS

Tafluprost promoted RGC survival in a dose-dependent manner with an optimal concentration of 1 μM. Tafluprost significantly decreased ZnT-3 expression and Zn2+ accumulation in the IPL of retina. Tafluprost stimulated intense axonal regeneration and maintained RGCs function compared to control. Mechanistically, Tafluprost and Zn2+ elimination treatment (TPEN or ZnT-3 deletion) can activate the mTOR pathway with an improved percentage of pS6+ RGCs in the retina. However, rapamycin, a specific inhibitor of the mTOR1, inhibited the activation of the mTOR pathway and abolished the regenerative effect mediated by Tafluprost. Tafluprost also inhibited the upregulation of p62, LC3 and Beclin-1, attenuated the overactivation of microglia/macrophages and downregulated the expression of TNFα and IL-1β.

CONCLUSIONS

Our results suggest that Tafluprost promoted axon regeneration via regulation of the Zn2+-mTOR pathway, and provide novel research directions for glaucomatous optic nerve injury mechanisms.

Monocyte-derived SDF1 supports optic nerve regeneration and alters retinal ganglion cells' response to Pten deletion

Although mammalian retinal ganglion cells (RGCs) normally cannot regenerate axons nor survive after optic nerve injury, this failure is partially reversed by inducing sterile inflammation in the eye. Infiltrative myeloid cells express the axogenic protein oncomodulin (Ocm) but additional, as-yet-unidentified, factors are also required. We show here that infiltrative macrophages express stromal cell–derived factor 1 (SDF1, CXCL12), which plays a central role in this regard. Among many growth factors tested in culture, only SDF1 enhances Ocm activity, an effect mediated through intracellular cyclic AMP (cAMP) elevation and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) activation. SDF1 deficiency in myeloid cells (CXCL12flx/flxLysM-Cre−/+ mice) or deletion of the SDF1 receptor CXCR4 in RGCs (intraocular AAV2-Cre in CXCR4flx/flx mice) or SDF1 antagonist AMD3100 greatly suppresses inflammation-induced regeneration and decreases RGC survival to baseline levels. Conversely, SDF1 induces optic nerve regeneration and RGC survival, and, when combined with Ocm/cAMP, SDF1 increases axon regeneration to levels similar to those induced by intraocular inflammation. In contrast to deletion of phosphatase and tensin homolog (Pten), which promotes regeneration selectively from αRGCs, SDF1 promotes regeneration from non-αRGCs and enables the latter cells to respond robustly to Pten deletion; however, SDF1 surprisingly diminishes the response of αRGCs to Pten deletion. When combined with inflammation and Pten deletion, SDF1 enables many RGCs to regenerate axons the entire length of the optic nerve. Thus, SDF1 complements the effects of Ocm in mediating inflammation-induced regeneration and enables different RGC subtypes to respond to Pten deletion.

Evaluation of the neuroprotective effects of methylprednisolone and surgical decompression in a rodent model of traumatic optic neuropathy

PURPOSE OF THE STUDY

Traumatic optic neuropathy (TON) is a rare but serious consequence of head injuries. The optimal therapy for TON remains controversial, and standardized recommendations are lacking. The most common therapies used are steroid administration and surgical decompression procedures. The aim of the present study was to compare two common conservative and surgical therapies in a rodent model with a standardized traumatic optic nerve lesion.

MATERIALS AND METHODS

This study employed 59 male Wistar rats. After exposing the optic nerve, defined trauma was exerted on the optic nerve using a micromanipulator to trigger TON. Rats received either "megadose" methylprednisolone applied perioperatively or decompression via nerve sheath fenestration. The number of neurons was histologically evaluated in retinae explanted as whole mounts. Neuronal size was determined histomorphometrically.

RESULTS

Neuronal loss was significantly lower following perioperative "megadose" steroid therapy (p = 0.017), especially in the central retinal area (p = 0.025). Compared to the control group without therapy, on average more than 400 neurons/mm² were saved. In the central retinal area, more than 600 neurons/mm² were rescued. In contrast, neuronal loss was not significantly affected by surgical decompression; however, this procedure was associated with a reduction in neuron size (p = 0.003).

CONCLUSIONS

The present model revealed significant neuroprotective effects following administration of methylprednisolone for TON treatment. Mitigation of neuronal loss may result in functional benefits. Neuroprotective effects were not observed following surgical therapy, suggesting that this approach should be reserved for individual cases such as hematomas in the area of nerve envelopes.

Quantification and image-derived phenotyping of retinal ganglion cell nuclei in the nee mouse model of congenital glaucoma

The nee mouse model exhibits characteristic features of congenital glaucoma, a common cause of childhood blindness. The current study of nee mice had two components. First, the time course of neurodegeneration in nee retinal flat-mounts was studied over time using a retinal ganglion cell (RGC)-marker, BRN3A; a pan-nuclear marker, TO-PRO-3; and H&E staining. Based on segmentation of nuclei using ImageJ and RetFM-J, this analysis identified a rapid loss of BRN3A⁺ nuclei from 4 to 15 weeks of age, with the first statistically significant difference in average density compared to age-matched controls detected in 8-week-old cohorts (49% reduction in nee). Consistent with a model of glaucoma, no reductions in BRN3A^- nuclei were detected, but the combined analysis indicated that some RGCs lost BRN3A marker expression prior to actual cell loss. These results have a practical application in the design of experiments using nee mice to study mechanisms or potential therapies for congenital glaucoma. The second component of the study pertains to a discovery-based analysis of the large amount of image data with 748,782 segmented retinal nuclei. Using the automatedly collected region of interest feature data captured by ImageJ, we tested whether RGC density of glaucomatous mice was significantly correlated to average nuclear area, perimeter, Feret diameter, or MinFeret diameter. These results pointed to two events influencing nuclear size. For variations in RGC density above approximately 3000 nuclei/mm² apparent spreading was observed, in which BRN3A^- nuclei-regardless of genotype-became slightly larger as RGC density decreased. This same spreading occurred in BRN3A⁺ nuclei of wild-type mice. For variation in RGC density below 3000 nuclei/mm², which only occurred in glaucomatous nee mutants, BRN3A⁺ nuclei became smaller as disease was progressively severe. These observations have relevance to defining RGCs of relatively higher sensitivity to glaucomatous cell death and the nuclear dynamics occurring during their demise.

The basic science of optic nerve regeneration

Diverse insults to the optic nerve result in partial to total vision loss as the axons of retinal ganglion cells are destroyed. In glaucoma, axons are injured at the optic nerve head; in other optic neuropathies, axons can be damaged along the entire visual pathway. In all cases, as mammals cannot regenerate injured central nervous system cells, once the axons are lost, vision loss is irreversible. However, much has been learned about how retinal ganglion cells respond to axon injuries, and many of these crucial discoveries offer hope for future regenerative therapies. Here we review the current understanding regarding the temporal progression of axonal degeneration. We summarize known survival and regenerative mechanisms in mammals, including specific signaling pathways, key transcription factors, and reprogramming genes. We cover mechanisms intrinsic to retinal ganglion cells as well as their interactions with myeloid and glial cell populations in the retina and optic nerve that affect survival and regeneration. Finally, we highlight some non-mammalian species that are able to regenerate their retinal ganglion cell axons after injury, as understanding these successful regenerative responses may be essential to the rational design of future clinical interventions to regrow the optic nerve. In the end, a combination of many different molecular and cellular interventions will likely be the only way to achieve functional recovery of vision and restore quality of life to millions of patients around the world.

Non-Cell-Autonomous Regulation of Optic Nerve Regeneration by Amacrine Cells

Visual information is conveyed from the eye to the brain through the axons of retinal ganglion cells (RGCs) that course through the optic nerve and synapse onto neurons in multiple subcortical visual relay areas. RGCs cannot regenerate their axons once they are damaged, similar to most mature neurons in the central nervous system (CNS), and soon undergo cell death. These phenomena of neurodegeneration and regenerative failure are widely viewed as being determined by cell-intrinsic mechanisms within RGCs or to be influenced by the extracellular environment, including glial or inflammatory cells. However, a new concept is emerging that the death or survival of RGCs and their ability to regenerate axons are also influenced by the complex circuitry of the retina and that the activation of a multicellular signaling cascade involving changes in inhibitory interneurons - the amacrine cells (AC) - contributes to the fate of RGCs. Here, we review our current understanding of the role that interneurons play in cell survival and axon regeneration after optic nerve injury.

Dynamic changes in cell size and corresponding cell fate after optic nerve injury

Identifying disease-specific patterns of retinal cell loss in pathological conditions has been highlighted by the emergence of techniques such as Detection of Apoptotic Retinal Cells and Adaptive Optics confocal Scanning Laser Ophthalmoscopy which have enabled single-cell visualisation in vivo. Cell size has previously been used to stratify Retinal Ganglion Cell (RGC) populations in histological samples of optic neuropathies, and early work in this field suggested that larger RGCs are more susceptible to early loss than smaller RGCs. More recently, however, it has been proposed that RGC soma and axon size may be dynamic and change in response to injury. To address this unresolved controversy, we applied recent advances in maximising information extraction from RGC populations in retinal whole mounts to evaluate the changes in RGC size distribution over time, using three well-established rodent models of optic nerve injury. In contrast to previous studies based on sampling approaches, we examined the whole Brn3a-positive RGC population at multiple time points over the natural history of these models. The morphology of over 4 million RGCs was thus assessed to glean novel insights from this dataset. RGC subpopulations were found to both increase and decrease in size over time, supporting the notion that RGC cell size is dynamic in response to injury. However, this study presents compelling evidence that smaller RGCs are lost more rapidly than larger RGCs despite the dynamism. Finally, using a bootstrap approach, the data strongly suggests that disease-associated changes in RGC spatial distribution and morphology could have potential as novel diagnostic indicators.

Targeting HDAC3 in the DBA/2J spontaneous mouse model of glaucoma

High intraocular pressure (IOP) is the most common risk factor associated with glaucoma in humans. While lowering IOP is effective at reducing the rate of retinal ganglion cell (RGC) loss, to date, no treatment exists to directly preserve these cells affected by damage to the optic nerve. Recently, histone deacetylase-3 (HDAC3) has become a potential therapeutic target because it plays an important role in the early nuclear atrophic events that precede RGC death. Conditional knockout or inhibition of HDAC3 prevents histone deacetylation, heterochromatin formation, apoptosis, and eventual RGC loss following acute optic nerve injury. Using these approaches to repress HDAC3 activity, we tested whether targeting HDAC3 protects RGCs from ganglion cell-specific BRN3A expression loss, total somatic cell loss, and optic nerve degeneration in the DBA/2J mouse model of spontaneous glaucoma. Targeted ablation of Hdac3 activity did not protect RGCs from axonal degeneration or somatic cell death in the DBA/2J mouse model of glaucoma. However, inhibition of HDAC3 activity using RGFP966 conferred mild protection against somatic cell loss in the ganglion cell layer in aged DBA/2J mice. Further experimentation is necessary to determine whether other class I HDACs may serve as potential therapeutic targets in chronic models of glaucoma.

Optic nerve regeneration: A long view

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.

Neuronal Death in the Contralateral Un-Injured Retina after Unilateral Axotomy: Role of Microglial Cells

For years it has been known that unilateral optic nerve lesions induce a bilateral response that causes an inflammatory and microglial response in the contralateral un-injured retinas. Whether this contralateral response involves retinal ganglion cell (RGC) loss is still unknown. We have analyzed the population of RGCs and the expression of several genes in both retinas of pigmented mice after a unilateral axotomy performed close to the optic nerve head (0.5 mm), or the furthest away that the optic nerve can be accessed intraorbitally in mice (2 mm). In both retinas, RGC-specific genes were down-regulated, whereas caspase 3 was up-regulated. In the contralateral retinas, there was a significant loss of 15% of RGCs that did not progress further and that occurred earlier when the axotomy was performed at 2 mm, that is, closer to the contralateral retina. Finally, the systemic treatment with minocycline, a tetracycline antibiotic that selectively inhibits microglial cells, or with meloxicam, a non-steroidal anti-inflammatory drug, rescued RGCs in the contralateral but not in the injured retina. In conclusion, a unilateral optic nerve axotomy triggers a bilateral response that kills RGCs in the un-injured retina, a death that is controlled by anti-inflammatory and anti-microglial treatments. Thus, contralateral retinas should not be used as controls.

BAX-Depleted Retinal Ganglion Cells Survive and Become Quiescent Following Optic Nerve Damage

Removal of the Bax gene from mice completely protects the somas of retinal ganglion cells (RGCs) from apoptosis following optic nerve injury. This makes BAX a promising therapeutic target to prevent neurodegeneration. In this study, Bax^+/- mice were used to test the hypothesis that lowering the quantity of BAX in RGCs would delay apoptosis following optic nerve injury. RGCs were damaged by performing optic nerve crush (ONC) and then immunostaining for phospho-cJUN, and quantitative PCR were used to monitor the status of the BAX activation mechanism in the months following injury. The apoptotic susceptibility of injured cells was directly tested by virally introducing GFP-BAX into Bax^-/- RGCs after injury. The competency of quiescent RGCs to reactivate their BAX activation mechanism was tested by intravitreal injection of the JNK pathway agonist, anisomycin. Twenty-four weeks after ONC, Bax^+/- mice had significantly less cell loss in their RGC layer than Bax^+/+ mice 3 weeks after ONC. Bax^+/- and Bax^+/+ RGCs exhibited similar patterns of nuclear phospho-cJUN accumulation immediately after ONC, which persisted in Bax^+/- RGCs for up to 7 weeks before abating. The transcriptional activation of BAX-activating genes was similar in Bax^+/- and Bax^+/+ RGCs following ONC. Intriguingly, cells deactivated their BAX activation mechanism between 7 and 12 weeks after crush. Introduction of GFP-BAX into Bax^-/- cells at 4 weeks after ONC showed that these cells had a nearly normal capacity to activate this protein, but this capacity was lost 8 weeks after crush. Collectively, these data suggest that 8-12 weeks after crush, damaged cells no longer displayed increased susceptibility to BAX activation relative to their naïve counterparts. In this same timeframe, retinal glial activation and the signaling of the pro-apoptotic JNK pathway also abated. Quiescent RGCs did not show a timely reactivation of their JNK pathway following intravitreal injection with anisomycin. These findings demonstrate that lowering the quantity of BAX in RGCs is neuroprotective after acute injury. Damaged RGCs enter a quiescent state months after injury and are no longer responsive to an apoptotic stimulus. Quiescent RGCs will require rejuvenation to reacquire functionality.

Change in phospholipid species of retinal layer in traumatic optic neuropathy model

Injured optic nerves induce death in almost all retinal ganglion cells (RGC) and cause a loss of axons. To date, we have studied injured RGC axon regeneration by using a traumatic optic nerve injury (TONI) rodent model, and we revealed that axonal regeneration is induced by the graft of an autologous peripheral nerve. The efficient approach to the regeneration of axons thus needs an environmental adjustment of RGC. However, the RGC environment induced by TONI remains unknown. Here, we analyzed female and male C57BL/6 mouse retinal tissue alterations in detail after TONI and focused on the major phospholipid species that are enriched in the whole retina. Reactive astrocyte accumulation, glia scar formation, and demyelination were observed in the injured optic nerve area, while RGC cell death, astrocyte accumulation, and Glial fibrillary acidic protein (GFAP) positive Müller cell increases were detected in the retinal layer. Furthermore, phosphatidylinositol (PI) 18:0/20:4 was localized to three nuclear layer structures: the ganglion cell layer (GCL), the inner nuclear layer (INL), and the outer nuclear layer (ONL) in control retina; however, the localization of 18:0/20:4 PI in TONI was disturbed. Meanwhile, phosphatidylserine (PS) 18:0/22:6 showed that the expression was specifically in the inner plexiform layer (IPL) with similar signal intensity in both cases. Other PS species and phosphatidylethanolamine (PE) were differentially localized in the retinal layer; however, the expressions of PE including docosahexaenoic acid (DHA) were affected by TONI. These results suggest that not only GCL but also other retinal layers were influenced by TONI.

Sequential phases of RGC axonal and somatic injury in EAE mice examined using DTI and OCT

BACKGROUND

Clinical imaging modalities including optical coherence tomography (OCT) and diffusion tensor imaging (DTI) are vital in Multiple Sclerosis (MS), but their relationships during the different phases of Retinal ganglion cell (RGC) degeneration are not clear. We hypothesize that initial injury in optic nerve causes axonal degeneration leading to RGC loss in retina, which can be characterized by a combination of DTI and OCT. Our objective was to examine the correlation between noninvasive and histological data to chronicle the degeneration profile of RGCs in the retina and optic nerve in a mouse model of MS.

MATERIALS AND METHODS

Experimental Autoimmune Encephalomyelitis (EAE) was induced in 11 C57Bl/6 mice, with 8 mice reserved as controls. OCT and DTI was conducted 2-8 weeks after induction of EAE. The thickness of the retinal ganglion cell complex (GCC) was measured using OCT and compared to DTI indices measured in optic nerves. End-stage histology was used to quantify axon/myelin loss in the optic nerve and retinal thinning/RGC loss in the retina.

RESULTS

Significant changes in DTI-derived Axial Diffusivity (AD, -17.2%) and Trace Diffusivity (TR, -18.3%) began after 2 weeks of EAE. Later significant reductions in Fractional Anisotropy (FA) and AD, with increases in Radial Diffusion (RD) were apparent after 4 and 8 weeks. OCT-derived measures of GCC thickness were reduced after 4 weeks, and reached significant reduction after 8 weeks. Among EAE mice, DTI (FA, AD and RD measures) and OCT measures were all significantly correlated after 4 and 8 weeks. Among histology measures, RGC density (-23%), RGC size (-27%), and the number of SMI31+ axons (-54%) were reduced significantly. DTI measures of FA and AD along with GCC thinning were the best independent predictors of axon loss.

CONCLUSIONS

DTI and OCT measures are tightly correlated during the chronic phase of axonal degeneration (4-8 weeks) in EAE mice. After 8 weeks of EAE, both OCT and DTI measures are strong predictors of axon loss in the Optic Nerve.

iDrugs and iDevices Discovery Research: Preclinical Assays, Techniques, and Animal Model Studies for Ocular Hypotensives and Neuroprotectants

Discovery ophthalmic research is centered around delineating the molecular and cellular basis of ocular diseases and finding and exploiting molecular and genetic pathways associated with them. From such studies it is possible to determine suitable intervention points to address the disease process and hopefully to discover therapeutics to treat them. An investigational new drug (IND) filing for a new small-molecule drug, peptide, antibody, genetic treatment, or a device with global health authorities requires a number of preclinical studies to provide necessary safety and efficacy data. Specific regulatory elements needed for such IND-enabling studies are beyond the scope of this article. However, to enhance the overall data packages for such entities and permit high-quality foundation-building publications for medical affairs, additional research and development studies are always desirable. This review aims to provide examples of some target localization/verification, ocular drug discovery processes, and mechanistic and portfolio-enhancing exploratory investigations for candidate drugs and devices for the treatment of ocular hypertension and glaucomatous optic neuropathy (neurodegeneration of retinal ganglion cells and their axons). Examples of compound screening assays, use of various technologies and techniques, deployment of animal models, and data obtained from such studies are also presented.

AAV2-Mediated Transduction of the Mouse Retina After Optic Nerve Injury

Purpose

Gene therapy of retinal ganglion cells (RGCs) has promise as a powerful therapeutic for the rescue and regeneration of these cells after optic nerve damage. However, early after damage, RGCs undergo atrophic changes, including gene silencing. It is not known if these changes will deleteriously affect transduction and transgene expression, or if the therapeutic protein can influence reactivation of the endogenous genome.

Methods

Double-transgenic mice carrying a Rosa26-(LoxP)-tdTomato reporter, and a mutant allele for the proapoptotic Bax gene were reared. The Bax mutant blocks apoptosis, but RGCs still exhibit nuclear atrophy and gene silencing. At times ranging from 1 hour to 4 weeks after optic nerve crush (ONC), eyes received an intravitreal injection of AAV2 virus carrying the Cre recombinase. Successful transduction was monitored by expression of the tdTomato reporter. Immunostaining was used to localize tdTomato expression in select cell types.

Results

Successful transduction of RGCs was achieved at all time points after ONC using AAV2 expressing Cre from the phosphoglycerate kinase (Pgk) promoter, but not the CMV promoter. ONC promoted an increase in the transduction of cell types in the inner nuclear layer, including Müller cells and rod bipolar neurons. There was minimal evidence of transduction of amacrine cells and astrocytes in the inner retina or optic nerve.

Conclusions

Damaged RGCs can be transduced and at least some endogenous genes can be subsequently activated. Optic nerve damage may change retinal architecture to allow greater penetration of an AAV2 virus to transduce several additional cell types in the inner nuclear layer.

Targeting HDAC3 Activity with RGFP966 Protects Against Retinal Ganglion Cell Nuclear Atrophy and Apoptosis After Optic Nerve Injury

PURPOSE

HDAC3 regulates nuclear atrophy as an early response to axonal injury in retinal ganglion cells (RGCs) following optic nerve crush (ONC). Since conditional knockout of Hdac3 prevents nuclear atrophy post ONC, HDAC3 selective inhibition with RGFP966 through localized and systemic dosing of RGFP966 is necessary for application to acute and chronic models of optic nerve injury.

METHODS

C57BL/6 mice were injected intravitreally with 1-10 μM RGFP966 immediately following ONC, and retinas were analyzed at 5, 7, and 14 days for metrics of nuclear atrophy and cell loss. Mice were similarly assessed after intraperitoneal (IP) injections with RGFP966 doses of 2-10 mg/kg, and eyes were harvested at 5, 14, and 28 days after ONC. H&E and BrdU staining were used to analyze toxicity to off-target tissues after 14 days of daily treatment with RGFP966.

RESULTS

A single intravitreal injection of RGFP966 prevented histone deacetylation, heterochromatin formation, apoptosis, and DNA damage at 5 and 7 days post ONC. After IP injection, RGFP966 bioavailability in the retina reached peak concentration within 1 h after injection and then rapidly declined. A single IP injection of 2-10 mg/kg RGFP966, significantly prevented histone deacetylation. Repeated IP injections of 2 mg/kg RGFP966 over the course of 2 and 4 weeks post ONC prevented RGC loss. There were no significant toxic or antiproliferative effects to off-target tissues in mice treated daily for 14 days with RGFP966.

CONCLUSION

Inhibition of HDAC3 activity with systemic dosing of RGFP966 prevents apoptosis-related histone deacetylation and attenuates RGC loss after acute optic nerve injury.

Microglial dynamics after axotomy-induced retinal ganglion cell death

BACKGROUND

Microglial cells (MCs) are the sentries of the central nervous system. In health, they are known as surveying MCs because they examine the tissue to maintain the homeostasis. In disease, they activate and, among other functions, become phagocytic to clean the cellular debris. In this work, we have studied the behavior of rat retinal MCs in two models of unilateral complete intraorbital optic nerve axotomy which elicit a different time course of retinal ganglion cell (RGC) loss.

METHODS

Albino Sprague-Dawley rats were divided into these groups: (a) intact (no surgery), (b) fluorogold (FG) tracing from the superior colliculi, and (c) FG tracing + crush or transection of the left optic nerve. The retinas were dissected from 2 days to 2 months after the lesions (n = 4-12 group/lesion and time point) and then were subjected to Brn3a and Iba1 double immunodetection. In each intact retina, the total number of Brn3a+RGCs and Iba+MCs was quantified. In each traced retina (b and c groups), FG-traced RGCs and phagocytic microglial cells (PMCs, FG+Iba+) were also quantified. Topographical distribution was assessed by neighbor maps.

RESULTS

In intact retinas, surveying MCs are homogenously distributed in the ganglion cell layer and the inner plexiform layer. Independently of the axotomy model, RGC death occurs in two phases, one quick and one protracted, and there is a lineal and topographical correlation between the appearance of PMCs and the loss of traced RGCs. Furthermore, the clearance of FG+RGCs by PMCs occurs 3 days after the actual loss of Brn3a expression that marks RGC death. In addition, almost 50% of MCs from the inner plexiform layer migrate to the ganglion cell layer during the quick phase of RGC loss, returning to the inner plexiform layer during the slow degeneration phase. Finally, in contrast to what happens in mice, in rats, there is no microglial phagocytosis in the contralateral uninjured retina.

CONCLUSIONS

Axotomy-induced RGC death occurs earlier than RGC clearance and there is an inverse correlation between RGC loss and PMC appearance, both numerically and topographically, suggesting that phagocytosis occurs as a direct response to RGC death rather than to axonal damage.

BAX to basics: How the BCL2 gene family controls the death of retinal ganglion cells

Retinal ganglion cell (RGC) death is the principal consequence of injury to the optic nerve. For several decades, we have understood that the RGC death process was executed by apoptosis, suggesting that there may be ways to therapeutically intervene in this cell death program and provide a more direct treatment to the cells and tissues affected in diseases like glaucoma. A major part of this endeavor has been to elucidate the molecular biological pathways active in RGCs from the point of axonal injury to the point of irreversible cell death. A major component of this process is the complex interaction of members of the BCL2 gene family. Three distinct family members of proteins orchestrate the most critical junction in the apoptotic program of RGCs, culminating in the activation of pro-apoptotic BAX. Once active, BAX causes irreparable damage to mitochondria, while precipitating downstream events that finish off a dying ganglion cell. This review is divided into two major parts. First, we summarize the extent of knowledge of how BCL2 gene family proteins interact to facilitate the activation and function of BAX. This area of investigation has rapidly changed over the last few years and has yielded a dramatically different mechanistic understanding of how the intrinsic apoptotic program is run in mammalian cells. Second, we provided a comprehensive analysis of nearly two decades of investigation of the role of BAX in the process of RGC death, much of which has provided many important insights into the overall pathophysiology of diseases like glaucoma.

Mobile zinc increases rapidly in the retina after optic nerve injury and regulates ganglion cell survival and optic nerve regeneration

Retinal ganglion cells (RGCs), the projection neurons of the eye, cannot regenerate their axons once the optic nerve has been injured and soon begin to die. Whereas RGC death and regenerative failure are widely viewed as being cell-autonomous or influenced by various types of glia, we report here that the dysregulation of mobile zinc (Zn²⁺) in retinal interneurons is a primary factor. Within an hour after the optic nerve is injured, Zn²⁺ increases several-fold in retinal amacrine cell processes and continues to rise over the first day, then transfers slowly to RGCs via vesicular release. Zn²⁺ accumulation in amacrine cell processes involves the Zn²⁺ transporter protein ZnT-3, and deletion of slc30a3, the gene encoding ZnT-3, promotes RGC survival and axon regeneration. Intravitreal injection of Zn²⁺ chelators enables many RGCs to survive for months after nerve injury and regenerate axons, and enhances the prosurvival and regenerative effects of deleting the gene for phosphatase and tensin homolog (pten). Importantly, the therapeutic window for Zn²⁺ chelation extends for several days after nerve injury. These results show that retinal Zn²⁺ dysregulation is a major factor limiting the survival and regenerative capacity of injured RGCs, and point to Zn²⁺ chelation as a strategy to promote long-term RGC protection and enhance axon regeneration.

Quantitative measurement of retinal ganglion cell populations via histology-based random forest classification

The inner surface of the retina contains a complex mixture of neurons, glia, and vasculature, including retinal ganglion cells (RGCs), the final output neurons of the retina and primary neurons that are damaged in several blinding diseases. The goal of the current work was two-fold: to assess the feasibility of using computer-assisted detection of nuclei and random forest classification to automate the quantification of RGCs in hematoxylin/eosin (H&E)-stained retinal whole-mounts; and if possible, to use the approach to examine how nuclear size influences disease susceptibility among RGC populations. To achieve this, data from RetFM-J, a semi-automated ImageJ-based module that detects, counts, and collects quantitative data on nuclei of H&E-stained whole-mounted retinas, were used in conjunction with a manually curated set of images to train a random forest classifier. To test performance, computer-derived outputs were compared to previously published features of several well-characterized mouse models of ophthalmic disease and their controls: normal C57BL/6J mice; Jun-sufficient and Jun-deficient mice subjected to controlled optic nerve crush (CONC); and DBA/2J mice with naturally occurring glaucoma. The result of these efforts was development of RetFM-Class, a command-line-based tool that uses data output from RetFM-J to perform random forest classification of cell type. Comparative testing revealed that manual and automated classifications by RetFM-Class correlated well, with 83.2% classification accuracy for RGCs. Automated characterization of C57BL/6J retinas predicted 54,642 RGCs per normal retina, and identified a 48.3% Jun-dependent loss of cells at 35 days post CONC and a 71.2% loss of RGCs among 16-month-old DBA/2J mice with glaucoma. Output from automated analyses was used to compare nuclear area among large numbers of RGCs from DBA/2J mice (n = 127,361). In aged DBA/2J mice with glaucoma, RetFM-Class detected a decrease in median and mean nucleus size of cells classified into the RGC category, as did an independent confirmation study using manual measurements of nuclear area demarcated by BRN3A-immunoreactivity. In conclusion, we have demonstrated that histology-based random forest classification is feasible and can be utilized to study RGCs in a high-throughput fashion. Despite having some limitations, this approach demonstrated a significant association between the size of the RGC nucleus and the DBA/2J form of glaucoma.

Retinal glial responses to optic nerve crush are attenuated in Bax-deficient mice and modulated by purinergic signaling pathways

Background

Retinal ganglion cell (RGC) soma death is a consequence of optic nerve damage, including in optic neuropathies like glaucoma. The activation of the innate immune network in the retina after nerve damage has been linked to RGC pathology. Since the eye is immune privileged, innate immune functions are the responsibility of the glia, specifically the microglia, astrocytes, and Müller cells that populate the retina. Glial activation, leading to the production of inflammatory cytokines, is a hallmark feature of retinal injury resulting from optic nerve damage and purported to elicit secondary degeneration of RGC somas.

Methods

A mouse model of optic nerve crush (ONC) was used to study retinal glial activation responses. RGC apoptosis was blocked using Bax-deficient mice. Glial activation responses were monitored by quantitative PCR and immunofluorescent labeling in retinal sections of activation markers. ATP signaling pathways were interrogated using P2X receptor agonists and antagonists and Pannexin 1 (Panx1)-deficient mice with RGC-specific deletion.

Results

ONC induced activation of both macroglia and microglia in the retina, and both these responses were dramatically muted if RGC death was blocked by deletion of the Bax gene. Macroglial, but not microglial, activation was modulated by purinergic receptor activation. Release of ATP after optic nerve damage was not mediated by PANX1 channels in RGCs.

Conclusions

RGC death in response to ONC plays a principal stimulatory role in the retinal glial activation response.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0558-y) contains supplementary material, which is available to authorized users.

Role of HDACs in optic nerve damage-induced nuclear atrophy of retinal ganglion cells

Optic neuropathies are characterized by retinal ganglion cell (RGC) death, resulting in the loss of vision. In glaucoma, the most common optic neuropathy, RGC death is initiated by axonal damage, and can be modeled by inducing acute axonal trauma through procedures such as optic nerve crush (ONC) or optic nerve axotomy. One of the early events of RGC death is nuclear atrophy, and is comprised of RGC-specific gene silencing, histone deacetylation, heterochromatin formation, and nuclear shrinkage. These early events appear to be principally regulated by epigenetic mechanisms involving histone deacetylation. Class I histone deacetylases HDACs 1, 2, and 3 are known to play important roles in the process of early nuclear atrophy in RGCs, and studies using both inhibitors and genetic ablation of Hdacs also reveal a critical role in the cell death process. Select inhibitors, such as those being developed for cancer therapy, may also provide a viable secondary treatment option for optic neuropathies.

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.

Label-free nonlinear optical imaging of mouse retina

A nonlinear optical (NLO) microscopy system integrating stimulated Raman scattering (SRS), two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG) was developed to image fresh mouse retinas. The morphological and functional details of various retinal layers were revealed by the endogenous NLO signals. Particularly, high resolution label-free imaging of retinal neurons and nerve fibers in the ganglion cell and nerve fiber layers was achieved by capturing endogenous SRS and TPEF signals. In addition, the spectral and temporal analysis of TPEF images allowed visualization of different fluorescent components in the retinal pigment epithelium (RPE). Fluorophores with short TPEF lifetime, such as A2E, can be differentiated from other long-lifetime components in the RPE. The NLO imaging method would provide important information for investigation of retinal ganglion cell degeneration and holds the potential to study the biochemical processes of visual cycle in the RPE.

Tumor necrosis factor alpha has an early protective effect on retinal ganglion cells after optic nerve crush

BACKGROUND

Glaucoma is an optic neuropathy that is characterized by the loss of retinal ganglion cells (RGCs) initiated by damage to axons in the optic nerve. The degeneration and death of RGCs has been thought to occur in two waves. The first is axogenic, caused by direct insult to the axon. The second is somatic, and is thought to be caused by the production of inflammatory cytokines from the activated retinal innate immune cells. One of the cytokines consistently linked to glaucoma and RGC damage has been TNFα. Despite strong evidence implicating this protein in neurodegeneration, a direct injection of TNFα does not mimic the rapid loss of RGCs observed after acute optic nerve trauma or exposure to excitotoxins. This suggests that our understanding of TNFα signaling is incomplete.

METHODS

RGC death was induced by optic nerve crush in mice. The role of TNFα in this process was examined by quantitative PCR of Tnfα gene expression, and quantification of cell loss in Tnfα (-/-) mice or in wild-type animals receiving an intraocular injection of exongenous TNFα either before or after crush. Signaling pathways downstream of TNFα were examined by immunolabeling for JUN protein accumulation or activation of EGFP expression in NFκB reporter mice.

RESULTS

Optic nerve crush caused a modest increase in Tnfα gene expression, with kinetics similar to the activation of both macroglia and microglia. A pre-injection of TNFα attenuated ganglion cell loss after crush, while ganglion cell loss was more severe in Tnfα (-/-) mice. Conversely, over the long term, a single exposure to TNFα induced extrinsic apoptosis in RGCs. Müller cells responded to exogenous TNFα by accumulating JUN and activating NFκB.

CONCLUSION

Early after optic nerve crush, TNFα appears to have a protective role for RGCs, which may be mediated through Müller cells.

Histone deacetylase 3 (HDAC3) plays an important role in retinal ganglion cell death after acute optic nerve injury

Background

Optic nerve damage initiates a series of early atrophic events in retinal ganglion cells (RGCs) that precede the BAX-dependent committed step of the intrinsic apoptotic program. Nuclear atrophy, including global histone deacetylation, heterochromatin formation, shrinkage and collapse of nuclear structure, and the silencing of normal gene expression, comprise an important obstacle to overcome in therapeutic approaches to preserve neuronal function. Several studies have implicated histone deacetylases (HDACs) in the early stages of neuronal cell death, including RGCs. Importantly, these neurons exhibit nuclear translocation of HDAC3 shortly after optic nerve damage. Additionally, HDAC3 activity has been reported to be selectively toxic to neurons.

Results

RGC-specific conditional knockout of Hdac3 was achieved by transducing the RGCs of Hdac3^fl/fl mice with an adeno-associated virus serotype 2 carrying CRE recombinase and GFP (AAV2-Cre/GFP). Controls included similar viral transduction of Rosa26^fl/fl reporter mice. Optic nerve crush (ONC) was then performed on eyes. The ablation of Hdac3 in RGCs resulted in significant amelioration of characteristics of ONC-induced nuclear atrophy such as H4 deacetylation, heterochromatin formation, and the loss of nuclear structure. RGC death was also significantly reduced. Interestingly, loss of Hdac3 expression did not lead to protection against RGC-specific gene silencing after ONC, although this effect was achieved using the broad spectrum inhibitor, Trichostatin A.

Conclusion

Although other HDACs may be responsible for gene expression changes in RGCs, our results indicate a critical role for HDAC3 in nuclear atrophy in RGC apoptosis following axonal injury. This study provides a framework for studying the roles of other prevalent retinal HDACs in neuronal death as a result of axonal injury.

Electronic supplementary material

The online version of this article (doi:10.1186/1750-1326-9-39) contains supplementary material, which is available to authorized users.

REDD2-mediated inhibition of mTOR promotes dendrite retraction induced by axonal injury

Dendritic defects occur in neurodegenerative diseases accompanied by axonopathy, yet the mechanisms that regulate these pathologic changes are poorly understood. Using Thy1-YFPH mice subjected to optic nerve axotomy, we demonstrate early retraction of retinal ganglion cell (RGC) dendrites and selective loss of mammalian target of rapamycin (mTOR) activity, which precede soma loss. Axonal injury triggered rapid upregulation of the stress-induced protein REDD2 (regulated in development and DNA damage response 2), a potent inhibitor of mTOR. Short interfering RNA-mediated REDD2 knockdown restored mTOR activity and rescued dendritic length, area and branch complexity in a rapamycin-dependent manner. Whole-cell recordings demonstrated that REDD2 depletion leading to mTOR activation in RGCs restored their light response properties. Lastly, we show that REDD2-dependent mTOR activity extended RGC survival following axonal damage. These results indicate that injury-induced stress leads to REDD2 upregulation, mTOR inhibition and dendrite pathology causing neuronal dysfunction and subsequent cell death.

Identifying the role of microRNAs in spinal cord injury

Spinal cord injury (SCI) is medically and socioeconomically debilitating, and effective treatments are lacking. The elucidation of the pathophysiological mechanisms underlying SCI is essential for developing effective treatments for SCI. MicroRNAs (miRNAs) are small non-coding RNA molecules (18-24 nucleotides long) that regulate gene expression by interacting with specific target sequences. Recent studies suggest that miRNAs can act as post-transcriptional regulators to inhibit mRNA translation. Bioinformatic analyses indicate that the altered expression of miRNAs has an effect on critical processes of SCI physiopathology, including astrogliosis, oxidative stress, inflammation, apoptosis, and neuroplasticity. Therefore, the study of miRNAs may provide new insights into the molecular mechanisms of SCI. Current studies have also provided potential therapeutic clinical applications that involve targeting mRNAs to treat SCI. This review summarizes the biogenesis and function of miRNAs and the roles of miRNAs in SCI. We also discuss the potential therapeutic applications of miRNA-based interventions for SCI.

Hepatocyte growth factor promotes long-term survival and axonal regeneration of retinal ganglion cells after optic nerve injury: comparison with CNTF and BDNF

AIMS

Different trophic factors are known to promote retinal ganglion cell survival and regeneration, but each had their own limitations. We report that hepatocyte growth factor (HGF) confers distinct advantages in supporting ganglion cell survival and axonal regeneration, when compared to two well-established trophic factors ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF).

METHODS

Ganglion cells in adult hamster were injured by cutting the optic nerve. HGF, CNTF, or BDNF was injected at different dosages intravitreally after injury. Ganglion cell survival was quantified at 7, 14, or 28 days postinjury. Peripheral nerve (PN) grafting to the cut optic nerve of the growth factor-injected eye was performed either immediately after injury or delayed until 7 days post-injury. Expression of heat-shock protein 27 and changes in microglia numbers were quantified in different growth factor groups. The cellular distribution of c-Met in the retina was examined by anti-c-Met immunostaining.

RESULTS

Hepatocyte Growth Factor (HGF) was equally potent as BDNF in promoting short-term survival (up to 14 days post-injury) and also supported survival at 28 days post-injury when ganglion cells treated by CNTF or BDNF failed to be sustained. When grafting was performed without delay, HGF stimulated twice the number of axons to regenerate compared with control but was less potent than CNTF. However, in PN grafting delayed for 7 days after optic nerve injury, HGF maintained a better propensity of ganglion cells to regenerate than CNTF. Unlike CNTF, HGF application did not increase HSP27 expression in ganglion cells. Microglia proliferation was prolonged in HGF-treated retinas compared with CNTF or BDNF. C-Met was localized to both ganglion cells and Muller cells, suggesting HGF could be neuroprotective via interacting with both neurons and glia.

CONCLUSION

Compared with CNTF or BDNF, HGF is advantageous in sustaining long-term ganglion cell survival and their propensity to respond to favorable stimuli.