Stressor-dependent Alterations in Glycoprotein 130: Implications for Glial Cell Reactivity, Cytokine Signaling and Ganglion Cell Health in Glaucoma

Consolidating data on the association of IL-6 and IL-10 polymorphisms with the development of glaucoma: a meta-analysis

BACKGROUND

The study aimed to investigate the association of IL-6 and IL-10 polymorphisms with susceptibility to glaucoma by analyzing all relevant individual studies.

MATERIALS AND METHODS

Relevant articles were gathered from PubMed, Web of Science, Embase, WanFang, and CNKI databases up to 15 October 2023. Odds ratios (ORs) were used to evaluate the association strengths, along with 95% confidence intervals (CIs).

RESULTS

Seven case-control studies involving 1408 cases and 1789 controls on the IL-6 -174 G>C polymorphism, and three studies with 675 cases and 1100 controls on the IL-6 -572 G>C were included. Moreover, three separate studies, each comprising 442 cases and 672 controls, investigated the IL-10 -592C>A, -819T>C, and -1082A>G polymorphisms. The combined data indicated a significant association between -592C>A, -819T>C, and -1082A>G at IL-10 gene and IL-6 -572 G>C with glaucoma susceptibility, with no correlation found for IL-6 -174 G>C.

CONCLUSIONS

The study found that IL-10 -592C>A, -819T>C, -1082A>G, and IL-6 -572 G>C polymorphisms were linked to glaucoma risk. However, no significant association was observed for IL-6 -174 G>C. These findings imply a possible connection between genetic variations in these genes and glaucoma risk. Further research is crucial to fully understand the underlying mechanisms and their significance in managing and preventing glaucoma.

Impairment of Membrane Repolarization Accompanies Axon Transport Deficits in Glaucoma

Glaucoma is a leading cause of blindness worldwide, resulting from degeneration of retinal ganglion cells (RGCs), which form the optic nerve. In glaucoma, axon transport deficits appear to precede structural degeneration of RGC axons. The period of time between the onset of axon transport deficits and the structural degeneration of RGC axons may represent a therapeutic window for the prevention of irreversible vision loss. However, it is unclear how deficits in axon transport relate to the electrophysiological capacity of RGCs to produce and maintain firing frequencies that encode visual stimuli. Here, we examined the electrophysiological signature of individual RGCs in glaucomatous retina with respect to axon transport facility. Utilizing the Microbead Occlusion Model of murine ocular hypertension, we performed electrophysiological recordings of RGCs with and without deficits in anterograde axon transport. We found that RGCs with deficits in axon transport have a reduced ability to maintain spiking frequency that arises from elongation of the repolarization phase of the action potential. This repolarization phenotype arises from reduced cation flux and K+ dyshomeostasis that accompanies pressure-induced decreases in Na/K-ATPase expression and activity. In vitro studies with purified RGCs indicate that elevated pressure induces early internalization of Na/K-ATPase that, when reversed, stabilizes cation flux and prevents K+ dyshomeostasis. Furthermore, pharmacological inhibition of the Na/K-ATPase is sufficient to replicate pressure-induced cation influx and repolarization phase phenotypes in healthy RGCs. These studies suggest that deficits in axon transport also likely reflect impaired electrophysiological function of RGCs. Our findings further identify a failure to maintain electrochemical gradients and cation dyshomeostasis as an early phenotype of glaucomatous pathology in RGCs that may have significant bearing on efforts to restore RGC health in diseased retina.

Ccl5 Mediates Proper Wiring of Feedforward and Lateral Inhibition Pathways in the Inner Retina

The β-chemokine Ccl5 and its receptors are constitutively expressed in neurons of the murine inner retina. Here, we examined the functional and structural significance of this constitutive Ccl5 signaling on retinal development. We compared outcomes of electrophysiology, ocular imaging and retinal morphology in wild-type mice (WT) and mice with Ccl5 deficiency (Ccl5^-/-). Assessment of retinal structure by ocular coherence tomography and histology revealed slight thinning of the inner plexiform layer (IPL) and inner nuclear layer (INL) in Ccl5^-/- mice, compared to WT (p < 0.01). Assessment of postnatal timepoints important for development of the INL (P7 and P10) revealed Ccl5-dependent alterations in the pattern and timing of apoptotic pruning. Morphological analyses of major inner retinal cell types in WT, Ccl5^-/-, gustducin^gfp and gustducin^gfp/Ccl5^-/- mice revealed Ccl5-dependent reduction in GNAT3 expression in rod bipolar cells as well as a displacement of their terminals from the IPL into the GCL. RGC dendritic organization and amacrine cell morphology in the IPL was similarly disorganized in Ccl5^-/- mice. Examination of the intrinsic electrophysiological properties of RGCs revealed higher spontaneous activity in Ccl5^-/- mice that was characterized by higher spiking frequency and a more depolarized resting potential. This hyperactive phenotype could be negated by current clamp and correlated with both membrane resistance and soma area. Overall, our findings identify Ccl5 signaling as a mediator of inner retinal circuitry during development of the murine retina. The apparent role of Ccl5 in retinal development further supports chemokines as trophic modulators of CNS development and function that extends far beyond the inflammatory contexts in which they were first characterized.

Impact of Graphene on the Efficacy of Neuron Culture Substrates

How graphene influences the behavior of living cells or tissues remains a critical issue for its application in biomedical studies, despite the general acceptance that graphene is biocompatible. While direct contact between cells and graphene is not a requirement for all biomedical applications, it is often mandatory for biosensing. Therefore, it is important to clarify whether graphene impedes the ability of cells to interact with biological elements in their environment. Here, a systematic study is reported to determine whether applying graphene on top of matrix substrates masks interactions between these substrates and retinal ganglion cells (RGCs). Six different platforms are tested for primary RGC cultures with three platforms comprised of matrix substrates compatible with these neurons, and another three having a layer of graphene placed on top of the matrix substrates. The results demonstrate that graphene does not impede interactions between RGCs and underlying substrate matrix, such that their positive or negative effects on neuron viability and vitality are retained. However, direct contact between RGCs and graphene reduces the number, but increases basal activity, of functional cation channels. The data indicate that, when proper baselines are established, graphene is a promising biosensing material for in vitro applications in neuroscience.

Interleukin-6 Deficiency Attenuates Retinal Ganglion Cell Axonopathy and Glaucoma-Related Vision Loss

The pleotropic cytokine interleukin-6 (IL-6) is implicated in retinal ganglion cell (RGC) survival and degeneration, including that associated with glaucoma. IL-6 protects RGCs from pressure-induced apoptosis in vitro. However, it is unknown how IL-6 impacts glaucomatous degeneration in vivo. To study how IL-6 influences glaucomatous RGC axonopathy, accompanying glial reactivity, and resultant deficits in visual function, we performed neural tracing, histological, and neurobehavioral assessments in wildtype (B6;129SF2/J; WT) and IL-6 knock-out mice (B6;129S2-IL6^tm1kopf/J; IL-6-/-) after 8 weeks of unilateral or bilateral microbead-induced glaucoma (microbead occlusion model). IOP increased by 20% following microbead injection in both genotypes (p < 0.05). However, deficits in wound healing at the site of corneal injection were noted. In WT mice, elevated IOP produced degenerating axon profiles and decreased axon density in the optic nerve by 15% (p < 0.01). In IL-6-/- mice, axon density in the optic nerve did not differ between microbead- and saline-injected mice (p > 0.05) and degenerating axon profiles were minimal. Preservation of RGC axons was reflected in visual function, where visual acuity decreased significantly in a time-dependent manner with microbead-induced IOP elevation in WT (p < 0.001), but not IL-6-/- mice (p > 0.05). Despite this preservation of RGC axons and visual acuity, both microbead-injected WT and IL-6-/- mice exhibited a 50% decrease in anterograde CTB transport to the superior colliculus, as compared to saline-injected controls (p < 0.01). Assessment of glial reactivity revealed no genotype- or IOP-dependent changes in retinal astrocytes. IOP elevation decreased microglia density and percent retinal area covered in WT mice (p < 0.05), while IL-6-/- mice exhibited only a decrease in density (p < 0.05). Together, our findings indicate that two defining features of RGC axonopathy—axon transport deficits and structural degeneration of axons—likely occur via independent mechanisms. Our data suggest that IL-6 is part of a mechanism that specifically leads to structural degeneration of axons. Furthermore, its absence is sufficient to prevent both structural degeneration of the optic nerve and vision loss. Overall, our work supports the proposition that functional deficits in axon transport represent a therapeutic window for RGC axonopathy and identify IL-6 signaling as a strong target for such a therapeutic.

Constitutive and Stress-induced Expression of CCL5 Machinery in Rodent Retina

Signaling by inflammatory cytokines and chemokines is associated with neurodegeneration in disease and injury. Here we examined expression of the β-chemokine CCL5 and its receptors in the mouse retina and evaluated its relevance in glaucoma, a common optic neuropathy associated with sensitivity to intraocular pressure (IOP). Using quantitative PCR, fluorescent in situ hybridization, immunohistochemistry and quantitative image analysis, we found CCL5 mRNA and protein was constitutively expressed in the inner retina and synaptic layers. CCL5 appeared to associate with Müller cells and RGCs as well as synaptic connections between horizontal cells and bipolar cells in the OPL and amacrine cells, bipolar cells and RGCs in the IPL. Although all three high-affinity receptors (CCR5, CCR3, CCR1) for CCL5 were expressed constitutively, CCR5 expression was significantly higher than CCR3, which was also markedly greater than CCR1. Localization patterns for constitutive CCR5, CCR3 and CCR1 expression differed, particularly with respect to expression in inner retinal neurons. Stress-related expression of CCL5 was primarily altered in aged DBA/2 mice with elevated IOP. In contrast, changes in expression and localization of both CCR3 and CCR5 were evident not only in aged DBA/2 mice, but also in age-matched control mice and young DBA/2 mice. These groups do not exhibit elevated IOP, but possess either the aging stress (control mice) or the genetic predisposition to glaucoma (DBA/2 mice). Together, these data indicate that CCL5 and its high-affinity receptors are constitutively expressed in murine retina and differentially induced by stressors associated with glaucomatous optic neuropathy. Localization patterns further indicate that CCL5 signaling may be relevant for modulation of synapses in both health and disease, particularly in the inner plexiform layer.

Hyper-IL-6: a potent and efficacious stimulator of RGC regeneration

Mature retinal ganglion cells (RGCs) normally fail to regenerate injured axons and die soon after optic nerve injury. Research over the last two decades has demonstrated that application of IL-6-like cytokines or activation of respective downstream signaling pathways promote neuroprotection and optic nerve regeneration. However, the overall beneficial effects of natural cytokines remain usually rather moderate, possibly due to intrinsic signaling pathway inhibitors, such as PTEN or SOCS3, or a limited expression of specific cytokine receptors in RGCs. It was recently demonstrated that directly targeting the gp130 receptor, a common signalling receptor of all IL-6-like cytokines, induces stronger RGC axon regeneration in vitro and in vivo than other known growth-promoting treatments such as inflammatory stimulation or PTEN knockout. Remarkably, continuous expression of hyper-IL-6 (hIL-6) upon intravitreal AAV injection after nerve injury enables long-distance axon regeneration, with some axons growing through the optic chiasm 6 weeks after optic nerve injury. Thus, AAV-mediated hIL-6 delivery is so far one of the strongest single, post-injury treatments for the promotion of optic nerve regeneration and may be suitable for the development of novel, clinically applicable therapeutic treatments for human patients.

Interleukin-6: A Constitutive Modulator of Glycoprotein 130, Neuroinflammatory and Cell Survival Signaling in Retina

Objective

The interleukin-6 (IL-6) family of cytokines and their signal transducer glycoprotein (gp130) are implicated in inflammatory and cell survival functions in glaucoma. There are several avenues for interdependent modulation of IL-6 family members and gp130 signaling. Here we investigated whether IL-6 modulates gp130 and related neuroinflammatory, cell survival and regulatory signaling in both healthy and glaucomatous retina.

Methods

In naïve and glaucomatous (Microbead Occlusion Model), wildtype (WT) and IL-6 knockout (IL-6−/−) mice, we examined gp130 protein expression and localization, using western blot and immunohistochemistry. Gene targets related to IL-6 and gp130 signaling and pertinent to neuroinflammation (TNFα, IL-1β), cell health (Bax, Bcl-xl) and STAT3 regulation (Socs3) were quantified using qRTPCR.

Results

In the naïve retina, IL-6−/− retina contained significantly less gp130 compared to WT retina. This IL-6-related decrease in gp130 was accompanied by a reduction in mRNA expression of TNFα, Socs3 and Bax. After 4 weeks of microbead-induced ocular hypertension, both microbead- and saline-injected (control) eyes of IL-6−/− mice exhibited higher expression of TNFα, compared to WT mice. IL-1β expression was also reduced specifically in IL-6−/− retina with microbead-induced glaucoma. While saline and microbead injection increased Bcl-xl and Socs3 mRNA in both WT and IL-6−/− mice, IL-6−/− deficiency led to smaller increases for both Bcl-xl and Socs3.

Conclusions

Our findings support a role for IL-6 in setting baseline parameters for neuroinflammatory, cell health and gp130 regulatory signaling that can impact the nature and magnitude of retinal responses to glaucoma-related stressors.

Boosting Central Nervous System Axon Regeneration by Circumventing Limitations of Natural Cytokine Signaling

Retinal ganglion cells (RGCs) do not normally regenerate injured axons, but die upon axotomy. Although IL-6-like cytokines are reportedly neuroprotective and promote optic nerve regeneration, their overall regenerative effects remain rather moderate. Here, we hypothesized that direct activation of the gp130 receptor by the designer cytokine hyper-IL-6 (hIL-6) might induce stronger RGC regeneration than natural cytokines. Indeed, hIL-6 stimulated neurite growth of adult cultured RGCs with significantly higher efficacy than CNTF or IL-6. This neurite growth promoting effect could be attributed to stronger activation of the JAK/STAT3 and PI3K/AKT/mTOR signaling pathways and was also observed in peripheral dorsal root ganglion neurons. Moreover, hIL-6 abrogated axon growth inhibition by central nervous system (CNS) myelin. Remarkably, continuous hIL-6 expression upon RGC-specific AAV transduction after optic nerve crush exerted stronger axon regeneration than other known regeneration promoting treatments such as lens injury and PTEN knockout, with some axons growing through the optic chiasm 6 weeks after optic nerve injury. Combination of hIL-6 with RGC-specific PTEN knockout further enhanced optic nerve regeneration. Therefore, direct activation of gp130 signaling might be a novel, clinically applicable approach for robust CNS repair.

Primate neural retina upregulates IL-6 and IL-10 in response to a herpes simplex vector suggesting the presence of a pro-/anti-inflammatory axis

Injection of herpes simplex virus vectors into the vitreous of primate eyes induces an acute, transient uveitis. The purpose of this study was to characterize innate immune responses of macaque neural retina tissue to the herpes simplex virus type 1-based gene delivery vector hrR3. PCR array analysis demonstrated the induction of the pro-inflammatory cytokine IL-6, as well as the anti-inflammatory cytokine IL-10, following hrR3 exposure. Secretion of IL-6 was detected by ELISA and cone photoreceptors and Muller cells were the predominant IL-6 positive cell types. RNA in situ hybridization confirmed that IL-6 was expressed in photoreceptor and Muller cells. The IL-10 positive cells in the inner nuclear layer were identified as amacrine cells by immunofluorescence staining with calretinin antibody. hrR3 challenge resulted in activation of NFκB (p65) in Muller glial cells, but not in cone photoreceptors, suggesting a novel regulatory mechanism for IL-6 expression in cone cells. hrR3 replication was not required for IL-6 induction or NFκB (p65) activation. These data suggest a pro-inflammatory (IL-6)/anti-inflammatory (IL-10) axis exists in neural retina and the severity of acute posterior uveitis may be determined by this interaction. Further studies are needed to identify the trigger for IL-6 and IL-10 induction and the mechanism of IL-6 induction in cone cells.

Pressure-Induced Alterations in PEDF and PEDF-R Expression: Implications for Neuroprotective Signaling in Glaucoma

Introduction

Alterations in neuron-glia signaling are implicated in glaucoma, a neurodegenerative disease characterized by retinal ganglion cell (RGC) death. Pigment epithelium derived factor (PEDF) is a secreted protein with potential neuroprotective qualities in retinal disease, including chronic ocular hypertension. Here we sought to determine whether moderate, short-term elevations in IOP alter PEDF signaling and whether pressure-induced PEDF signaling directly impacts RGC apoptosis.

Methods

In retina from naïve mice and mice with unilateral, microbead-induced glaucoma, we examined expression and cell type-specific localization of PEDF and its receptor (PEDF-R), using quantitative PCR and immunohistochemistry. Using primary cultures of purified RGCs and Müller cells, we examined cell type-specific expression of PEDF in response to 48 hours of elevated hydrostatic pressure, using multiplex ELISA and immunocytochemistry. We also measured pressure-induced apoptosis of RGCs in the presence or absence of atglistatin, a potent and selective inhibitor of PEDF-R, and recombinant PEDF, using TUNEL assays.

Results

PEDF and PEDF-R are constitutively expressed in naïve retina, primarily in the ganglion cell and nerve fiber layers. Elevated IOP increases PEDF and PEDF-R expression, particularly associated with RGCs and Müller cells. Elevated pressure in vitro increased PEDF secretion by 6-fold in RGCs and trended towards an increase in expression by Müller cells, as compared to ambient pressure. This was accompanied by changes in the subcellular localization of PEDF-R in both cell types. Inhibition of PEDF signaling with atglistatin increased pressure-induced apoptosis in RGCs and treatment with recombinant PEDF inhibited pressure-induced apoptosis, both in a dose-dependent manner.

Conclusion

Our findings suggest that moderate, short-term elevations in IOP promote PEDF signaling via up-regulation of both PEDF and PEDF-R. Based on in vivo and in vitro studies, this PEDF signaling likely arises from both Müller cells and RGCs, and has the potential to directly inhibit RGC apoptosis.

Early pro-inflammatory cytokine elevations in the DBA/2J mouse model of glaucoma

BACKGROUND

Neuroinflammation-astrogliosis, microglial activation, and changes in cytokine signaling-is a prominent feature of neurodegenerative disorders. Glaucoma is a group of chronic neurodegenerative conditions that make up the leading cause of irreversible blindness worldwide. Neuroinflammation has been postulated to play a significant role in the pathogenesis and progression of glaucomatous neurodegeneration. Though much is known regarding inflammation in the eye in glaucoma, little is known about cytokine activity outside of the retina where pathologies develop early.

METHODS

We traced the primary visual projection from the eye to the superior colliculus (SC) in DBA/2J and DBA/2J.Gpnmb (+) (control) mice using the anterograde tracer cholera toxin-B (CTB) to assay axonal transport deficits. Forty-eight hours later, visual structures were microdissected from fresh tissue based on transport outcome. Using magnetic bead multiplexing assays, we measured levels of 20 cytokines in the retina, proximal and distal optic nerves, CTB-positive and negative SC subdivisions, cerebellum, and serum at different ages representing different stages of pathology.

RESULTS

Pro- and anti-inflammatory cytokine levels in mice often changed in the same direction based on strain, age, and tissue. Significant elevations in retinal pro-inflammatory cytokines were observed in young DBA/2J mice compared to controls, followed by an age-dependent decrease in the DBA/2J mice. Proximal optic nerve of young DBA/2J mice showed a 50 % or greater decrease in levels of certain cytokines compared to older DBA/2J cohorts and controls, while both proximal and distal optic nerve of DBA/2Js showed elevations in IL-1β at all ages compared to controls. Pro-inflammatory cytokine IL-6 levels varied in accordance with transport outcome in the SC: IL-6 was elevated 44-80 % in glaucomatous DBA/2J collicular regions deficient in anterograde transport from retinal ganglion cells (RGCs) compared to areas with intact transport.

CONCLUSION

Dysregulation of cytokine signaling in the RGC projection of DBA/2J mice was evident early in distal retinal targets, well before intraocular pressure elevation or axonal degeneration begins.

Astrocyte Reactivity: A Biomarker for Retinal Ganglion Cell Health in Retinal Neurodegeneration

Retinal ganglion cell (RGC) loss in glaucoma is sectorial in nature and preceded by deficits in axonal transport. Neuroinflammation plays an important role in the pathophysiology of glaucoma in the retina, optic nerve and visual centers of the brain, where it similarly appears to be regulated spatially. In a murine model, we examined the spatial characteristics of astrocyte reactivity (migration/proliferation, hypertrophy and GFAP expression) in healthy retina, retina with two glaucoma-related risk factors (aging and genetic predisposition) and glaucomatous retina and established relationships between these reactivity indices and the spatial organization of astrocytes as well as RGC health. Astrocyte reactivity was quantified by morphological techniques and RGC health was determined by uptake and transport of the neural tracer cholera toxin beta subunit (CTB). We found that: (1) astrocyte reactivity occurs in microdomains throughout glaucomatous retina as well as retina with risk factors for glaucoma, (2) these astrocyte microdomains are primarily differentiated by the degree of retinal area covered by the astrocytes within them and (3) percent retinal area covered by astrocytes is highly predictive of RGC health. Our findings suggest that microdomains of astrocyte reactivity are biomarkers for functional decline of RGCs. Based on current and emerging imaging technologies, diagnostic assessment of astrocytes in the nerve fiber layer could succeed in translating axonal transport deficits to a feasible clinical application.