Brimonidine promotes axon growth after optic nerve injury through Erk phosphorylation

Axonal protection by combination of ripasudil and brimonidine with upregulation of p-AMPK in TNF-induced optic nerve degeneration

PURPOSE

The ROCK inhibitor ripasudil hydrochloride hydrate was shown to have axonal protective effects in TNF-induced optic nerve degeneration. The α2-adrenoreceptor agonist brimonidine was also shown to exert axonal protection. The current study aimed to elucidate whether additive axonal protection was achieved by the simultaneous injection of ripasudil and brimonidine and examine the association with AMPK activation.

METHODS

Intravitreal administration was performed in the following groups: PBS, TNF, or TNF with ripasudil, with brimonidine, or with a combination of ripasudil and brimonidine. Axon numbers were counted to evaluate the effects against axon loss. Immunoblot analysis was performed to examine phosphorylated AMPK expression in optic nerves, and immunohistochemical analysis was performed to evaluate the expression levels of p-AMPK and neurofilament in the optic nerve.

RESULTS

Both ripasudil alone or brimonidine alone resulted in significant neuroprotection against TNF-induced axon loss. The combination of ripasudil and brimonidine showed additive protective effects. Combined ripasudil and brimonidine plus TNF significantly upregulated p-AMPK levels in the optic nerve compared with the TNF groups. Immunohistochemical analysis revealed that p-AMPK is present in axons and enhanced by combination therapy.

CONCLUSION

The combination of ripasudil and brimonidine may have additive protective effects compared with single-agent treatment alone. These protective effects may be at least partially associated with AMPK activation.

Novel frontiers in neuroprotective therapies in glaucoma: Molecular and clinical aspects

In the last years, neuroprotective therapies have attracted the researcher interests as modern and challenging approach for the treatment of neurodegenerative diseases, aimed at protecting the nervous system from injuries. Glaucoma is a neurodegenerative disease characterized by progressive excavation of the optic nerve head, retinal axonal injury and corresponding vision loss that affects millions of people on a global scale. The molecular basis of the pathology is largely uncharacterized yet, and the therapeutic approaches available do not change the natural course of the disease. Therefore, in accordance with the therapeutic regimens proposed for other neurodegenerative diseases, a modern strategy to treat glaucoma includes prescription of drugs with neuroprotective activities. With respect to this, several preclinical and clinical investigations on a plethora of different drugs are currently ongoing. In this review, first, the conceptualization of the rationale for the adoption of neuroprotective strategies for retina is summarized. Second, the molecular aspects highlighting glaucoma as a neurodegenerative disease are reported. In conclusion, the molecular and pharmacological properties of most promising direct neuroprotective drugs used to delay glaucoma progression are examined, including: neurotrophic factors, NMDA receptor antagonists, the α2-adrenergic agonist, brimonidine, calcium channel blockers, antioxidant agents, nicotinamide and statins.

Regulated Cell Death of Retinal Ganglion Cells in Glaucoma: Molecular Insights and Therapeutic Potentials

Glaucoma is a group of diseases characterized by the degeneration of retinal ganglion cells (RGCs) and progressive, irreversible vision loss. High intraocular pressure (IOP) heightens the likelihood of glaucoma and correlates with RGC loss. While the current glaucoma therapy prioritizes lower the IOP; however, RGC, and visual loss may persist even when the IOP is well-controlled. As such, discovering and creating IOP-independent neuroprotective strategies for safeguard RGCs is crucial for glaucoma management. Investigating and clarifying the mechanism behind RGC death to counteract its effects is a promising direction for glaucoma control. Empirical studies of glaucoma reveal the role of multiple regulated cell death (RCD) pathways in RGC death. This review delineates the RCD of RGCs following IOP elevation and optic nerve damage and discusses the substantial benefits of mitigating RCD in RGCs in preserving visual function.

PTEN Blocking Stimulates Corticospinal and Raphespinal Axonal Regeneration and Promotes Functional Recovery After Spinal Cord Injury

The long-term disabilities associated with spinal cord injury (SCI) are primarily due to the absence of robust neuronal regeneration and functional plasticity. The inability of the axon to regenerate after SCI is contributed by several intrinsic factors that trigger a cascade of molecular growth program and modulates axonal sprouting. Phosphatase and tensin homolog (PTEN) is one of the intrinsic factors contributing to growth failure after SCI, however, the underlying mechanism is not well known. Here, we developed a novel therapeutic approach for treating SCI by suppressing the action of PTEN in a mouse model of hemisection SCI. We have used a novel peptide, PTEN antagonistic peptide (PAP) to block the critical domains of PTEN to demonstrate its ability to potentially promote axon growth. PAP treatment not only enhanced regeneration of corticospinal axons into the caudal spinal cord but also promoted the regrowth of descending serotonergic axons in SCI mice. Furthermore, expression levels of p-mTOR, p-S6, p-Akt, p-Erk, p-GSK, p-PI3K downstream of PTEN signaling pathway were increased significantly in the spinal cord of SCI mice systemically treated with PAP than control TAT peptide-treated mice. Our novel strategy of administering deliverable compounds postinjury may facilitate translational feasibility for central nervous system injury.

The Role of Adrenoceptors in the Retina

The retina is a part of the central nervous system, a thin multilayer with neuronal lamination, responsible for detecting, preprocessing, and sending visual information to the brain. Many retinal diseases are characterized by hemodynamic perturbations and neurodegeneration leading to vision loss and reduced quality of life. Since catecholamines and respective bindings sites have been characterized in the retina, we systematically reviewed the literature with regard to retinal expression, distribution and function of alpha₁ (α₁)-, alpha₂ (α₂)-, and beta (β)-adrenoceptors (ARs). Moreover, we discuss the role of the individual adrenoceptors as targets for the treatment of retinal diseases.

Fasudil attenuates glial cell-mediated neuroinflammation via ERK1/2 and AKT signaling pathways after optic nerve crush

To investigate the functional role of fasudil in optic nerve crush (ONC), and further explore its possible molecular mechanism. After ONC injury, the rats were injected intraperitoneally either with fasudil or normal saline once a day until euthanized. RGCs survival was assessed by retrograde labeling with FluoroGold. Retinal glial cells activation and population changes (GFAP, iba-1) were measured by immunofluorescence. The expressions of cleaved caspase 3 and 9, p-ERK1/2 and p-AKT were detected by western blot. The levels of the pro-inflammatory cytokines were determined using real-time polymerase chain reaction. Fasudil treatment inhibited RGCs apoptosis and reduced RGCs loss demonstrated by the decreased apoptosis-associated proteins expression and the increased fluorogold labeling of RGCs after ONC, respectively. In addition, the ONC + fasudil group compared had a significantly lower expression of GFAP and iba1 compared with the ONC group. The levels of pro-inflammatory cytokines were significantly reduced in the ONC + fasudil group than in the ONC group. Furthermore, the phosphorylation levels of ERK1/2 and AKT (p-ERK1/2 and p-AKT) were obviously elevated by the fasudil treatment. Our study demonstrated that fasudil attenuated glial cell-mediated neuroinflammation by up-regulating the ERK1/2 and AKT signaling pathways in rats ONC models. We conclude that fasudil may be a novel treatment for traumatic optic neuropathy.

Dual Specific Phosphatase 14 Deletion Rescues Retinal Ganglion Cells and Optic Nerve Axons after Experimental Anterior Ischemic Optic Neuropathy

PURPOSE

Understanding molecular changes is essential for designing effective treatments for nonarteritic anterior ischemic optic neuropathy (AION), the most common acute optic neuropathy in adults older than 50 years. We investigated changes in the mitogen-activated protein kinase (MAPK) pathway after experimental AION and focused on dual specificity phosphatase 14 (Dusp14), an atypical MAPK phosphatase that is downstream of Krüppel-like transcription factor (KLF) 9-mediated inhibition of retinal ganglion cell (RGC) survival and axonal regeneration.

MATERIALS AND METHODS

We induced severe AION in a photochemical thrombosis model in adult C57BL/6 wild-type and Dusp14 knockout mice. For comparison, some studies were performed using an optic nerve crush model. We assessed changes in MAPK pathway molecules using Western blot and immunohistochemistry, measured retinal thickness using optical coherence tomography (OCT), and quantified RGCs and axons using histologic methods.

RESULTS

Three days after severe AION, there was no change in the retinal protein levels of MAPK ERK1/2, phosphorylated-ERK1/2 (pERK1/2), downstream effector Elk-1 and phosphatase Dusp14 on Western blot. Western blot analysis of purified RGCs after a more severe model using optic nerve crush also showed no change in Dusp14 protein expression. Because of the known importance of the Dusp14 and MAPK pathway in RGCs, we examined changes after AION in Dusp14 knockout mice. Three days after AION, Dusp14 knockout mice had significantly increased pERK1/2⁺, Brn3A⁺ RGCs on immunohistochemistry. Three weeks after AION, Dusp14 knockout mice had significantly greater preservation of retinal thickness, increased number of Brn3A⁺ RGCs on whole mount preparation, and increased number of optic nerve axons compared with wild-type mice.

CONCLUSIONS

Genetic deletion of Dusp14, a MAPK phosphatase important in KFL9-mediated inhibition of RGC survival, led to increased activation of MAPK ERK1/2 and greater RGC and axonal survival after experimental AION. Inhibiting Dusp14 or activating the MAPK pathway should be examined further as a potential therapeutic approach to treatment of AION.

Abbreviations: AION: anterior ischemic optic neuropathy; Dusp14: dual specific phosphatase 14; ERK1/2: extracellular signal-regulated kinases 1/2; Elk-1: ETS Like-1 protein; GCC: ganglion cell complex; GCL: ganglion cell layer; inner nuclear layer; KO: knockout; MAPK: mitogen-activated phosphokinase; OCT: optical coherence tomography; RGC: retinal ganglion cell; RNFL: retinal nerve fiber layer.

Topical ripasudil stimulates neuroprotection and axon regeneration in adult mice following optic nerve injury

Optic nerve injury induces optic nerve degeneration and retinal ganglion cell (RGC) death that lead to visual disturbance. In this study, we examined if topical ripasudil has therapeutic potential in adult mice after optic nerve crush (ONC). Topical ripasudil suppressed ONC-induced phosphorylation of p38 mitogen-activated protein kinase and ameliorated RGC death. In addition, topical ripasudil significantly suppressed the phosphorylation of collapsin response mediator protein 2 and cofilin, and promoted optic nerve regeneration. These results suggest that topical ripasudil promotes RGC protection and optic nerve regeneration by modulating multiple signaling pathways associated with neural cell death, microtubule assembly and actin polymerization.

Brimonidine enhances inhibitory postsynaptic activity of OFF- and ON-type retinal ganglion cells in a Wistar rat chronic glaucoma model

Glaucoma is a multifactorial disease in which retinal ganglion cells (RGCs) undergo excitotoxic damage, leading to their degeneration. The α2-adrenoceptor (α2-AR) agonist brimonidine exerts a neuroprotective effect by regulating postsynaptic excitatory N-methyl-D-aspartate (NMDA) receptor activity in RGCs. However, researchers have not clearly determined whether or how brimonidine regulates inhibitory synaptic transmission in rat models of chronic glaucoma. Whole-cell voltage-clamp and current-clamp recordings were performed in ON- and OFF-type RGCs in retinal slices. Brimonidine directly and acutely enhanced γ-aminobutyric acidergic (GABAergic) transmission mediated by ionotropic GABA_A receptors in ON- and OFF-type RGCs in rat retinal slices; this effect occurred at the synaptic terminals and was independent of action potentials and multi-synaptic connections. The highly selective α2-AR antagonist yohimbine blocked the effects of brimonidine. Regarding the postsynaptic GABA receptor sensitivity, brimonidine also increased the amplitude of the GABA-induced current. Additionally, compared to RGCs from the control group, the frequencies and amplitudes of spontaneous excitatory postsynaptic currents (sEPSCs) and miniature excitatory postsynaptic currents (mEPSCs) did not change after brimonidine gravity perfusion. Brimonidine significantly decreased the spontaneous firing frequency of rat RGCs with intact synaptic inputs and decreased the resting membrane potential of RGCs, changes that were blocked by the highly selective GABA_A receptor antagonist SR95531. SR95531 alone increased spontaneous action potentials and the resting membrane potential. Based on these findings, an α2-AR agonist facilitated the frequency of the GABAergic inhibitory postsynaptic currents (IPSCs), directly increased the amplitude of the postsynaptic GABA-induced current (GABA receptor reactivity/sensitivity), suppressed the firing frequency of spontaneous action in RGCs with intact synaptic inputs and decreased the resting membrane potential of RGCs, thus deactivating RGCs from the neural network level and reducing the excitotoxic damage occurring during the pathological process of chronic glaucoma.

[Selective α2 agonists in the treatment of glaucoma: neuroprotective properties and impact on ocular blood flow]

Glaucoma is the main cause of irreversible blindness in the world. The selective α2 adrenergic receptor agonist brimonidine holds an important place among the hypotensive eye drops. The second part of this review focuses on some important effects of brimonidine that characterizes it as a medication with direct neuroprotective multifactorial action, discusses its influence on ocular blood flow and highlights its capability to maintain normal autoregulation of ocular blood flow.

Loss of Shp2 Rescues BDNF/TrkB Signaling and Contributes to Improved Retinal Ganglion Cell Neuroprotection

Glaucoma is characterized by the loss of retinal ganglion cells (RGC), and accordingly the preservation of RGCs and their axons has recently attracted significant attention to improve therapeutic outcomes in the disease. Here, we report that Src homology region 2-containing protein tyrosine phosphatase 2 (Shp2) undergoes activation in the RGCs, in animal model of glaucoma as well as in the human glaucoma tissues and that Shp2 dephosphorylates tropomyosin receptor kinase B (TrkB) receptor, leading to reduced BDNF/TrkB neuroprotective survival signaling. This was elucidated by specifically modulating Shp2 expression in the RGCs in vivo, using adeno-associated virus serotype 2 (AAV2) constructs. Shp2 upregulation promoted endoplasmic reticulum (ER) stress and apoptosis, along with functional and structural deficits in the inner retina. In contrast, loss of Shp2 decelerated the loss of RGCs, preserved their function, and suppressed ER stress and apoptosis in glaucoma. This report constitutes the first identification of Shp2-mediated TrkB regulatory mechanisms in the RGCs that can become a potential therapeutic target in both glaucoma and other neurodegenerative disorders.

Dexmedetomidine to Help Nerve Regeneration in a Rat Sciatic Nerve Injury Model

Background

Several studies have shown that dexmedetomidine (DXM), a selective α2-adrenoceptor agonist, also has neuroprotective effects. However, its effect on impaired peripheral nerve regeneration has not been studied.

Materials and Methods

Forty-five Sprague-Dawley rats were randomly assigned to three groups: group 1 (control SHAM), group 2 (sciatic nerve injury + normal saline), and group 3 (sciatic nerve injury + DXM). The rats of group 3 were subdivided into the following three groups: DXM 0.5, 6, and 20 μg·kg⁻¹ (groups 3A, 3B, and 3C, resp.). The sciatic nerve injury was assessed for nerve regeneration at 2 and 6 weeks.

Results

There were no differences between groups 2 and 3 in their sciatic functional index (SFI) values or histological findings at 2 weeks postinjury. However, SFI differences were statistically significant at 6 weeks postinjury in group 3. The gross findings with H&E staining showed that the number of axons was higher in group 3 than in group 2. There was no histological difference according to the DXM concentration.

Conclusion

The coincidental functional and histological assessment results of this study suggest that DXM for 6 weeks positively affects damaged peripheral nerves.

A Masked, Randomized, Phase 3 Comparison of Triple Fixed-Combination Bimatoprost/Brimonidine/Timolol versus Fixed-Combination Brimonidine/Timolol for Lowering Intraocular Pressure

OBJECTIVE

To evaluate the efficacy and safety of triple fixed-combination bimatoprost 0.01%/brimonidine 0.15%/timolol 0.5% (TFC) versus dual fixed-combination brimonidine 0.2%/timolol 0.5% (DFC) in primary open-angle glaucoma and ocular hypertension.

METHODS

Patients with intraocular pressure (IOP) ≥23 and ≤34 mmHg were randomized to twice-daily TFC or DFC. The primary variable is the change in worse eye mean IOP from baseline at week 12 (modified intent-to-treat (mITT) population). Secondary endpoints are mean IOP and mean change from baseline at weeks 1, 2, 4, 8, and 12 (mITT population). TFC superiority was demonstrated if the primary variable favored TFC (p ≤ 0.05). Sensitivity analyses were conducted, and safety was assessed at all visits.

RESULTS

TFC (n = 93) provided greater IOP reductions from baseline than DFC (n = 97) at week 12 (treatment difference, 0.85 mmHg; p = 0.028) and all other visits. TFC was also superior to DFC in patients with high baseline IOP (i.e., IOP ≥ 25 mmHg; p ≤ 0.011). Conjunctival hyperemia, ocular irritation, and dry eye were reported more often with TFC (p ≤ 0.016); however, discontinuations for ocular adverse events were similar between treatments.

CONCLUSIONS

TFC demonstrated IOP-lowering benefits that outweigh the risk of predominantly mild ocular side effects, which may be particularly relevant in patients who require greater IOP lowering to prevent/delay disease progression. This trial is registered with ClinicalTrials.gov registry number: NCT01241240.

Brimonidine Protects Auditory Hair Cells from in vitro-Induced Toxicity of Gentamicin

Brimonidine, an alpha-2 adrenergic receptor (α2-AR) agonist, has neuroprotective effects in the visual system and in spiral ganglion neurons. Auditory hair cells (HCs) express all 3 α2-AR subtypes, but their roles in HCs remain unknown. This study investigated the effects of brimonidine on auditory HCs that were also exposed to gentamicin, which is toxic to HCs. Organ of Corti explants were exposed to gentamicin in the presence or absence of brimonidine, and the α2-AR protein expression levels and Erk1/2 and Akt phosphorylation levels were determined. Brimonidine had a protective effect on auditory HCs against gentamicin-induced toxicity that was blocked by yohimbine. This suggested that the protective effect of brimonidine on HCs was mediated by the α2-AR. None of the treatments altered α2-AR protein expression levels, and brimonidine did not significantly change the activation levels of the Erk1/2 and Akt proteins. These observations indicated that brimonidine, acting directly via α2-AR, protects HCs from gentamicin-induced toxicity. Therefore, brimonidine shows potential for preventing or treating sensorineural hearing loss.

ATP-sensitive potassium (KATP) channel openers diazoxide and nicorandil lower intraocular pressure by activating the Erk1/2 signaling pathway

Elevated intraocular pressure is the most prevalent and only treatable risk factor for glaucoma, a degenerative disease of the optic nerve. While treatment options to slow disease progression are available, all current therapeutic and surgical treatments have unwanted side effects or limited efficacy, resulting in the need to identify new options. Previous reports from our laboratory have established a novel ocular hypotensive effect of ATP-sensitive potassium channel (KATP) openers including diazoxide (DZ) and nicorandil (NCD). In the current study, we evaluated the role of Erk1/2 signaling pathway in KATP channel opener mediated reduction of intraocular pressure (IOP). Western blot analysis of DZ and NCD treated primary normal trabecular meshwork (NTM) cells, human TM (isolated from perfusion cultures of human anterior segments) and mouse eyes showed increased phosphorylation of Erk1/2 when compared to vehicle treated controls. DZ and NCD mediated pressure reduction (p<0.02) in human anterior segments (n = 7 for DZ, n = 4 for NCD) was abrogated by U0126 (DZ + U0126: -9.7 ± 11.5%, p = 0.11; NCD + U0126: -0.1 ± 11.5%, p = 1.0). In contrast, U0126 had no effect on latanoprostfree acid-induced pressure reduction (-52.5 ± 6.8%, n = 4, p = 0.001). In mice, DZ and NCD reduced IOP (DZ, 14.9 ± 3.8%, NCD, 16.9 ± 2.5%, n = 10, p<0.001), but the pressure reduction was inhibited by U0126 (DZ + U0126, 0.7 ± 3.0%; NCD + U0126, 0.9 ± 2.2%, n = 10, p>0.1). Histologic evaluation of transmission electron micrographs from DZ + U0126 and NCD + U0126 treated eyes revealed no observable morphological changes in the ultrastructure of the conventional outflow pathway. Taken together, the results indicate that the Erk1/2 pathway is necessary for IOP reduction by KATP channel openers DZ and NCD.

Two PTP receptors mediate CSPG inhibition by convergent and divergent signaling pathways in neurons

Receptor protein tyrosine phosphatase σ (PTPσ) and its subfamily member LAR act as transmembrane receptors that mediate growth inhibition of chondroitin sulfate proteoglycans (CSPGs). Inhibition of either receptor increases axon growth into and beyond scar tissues after CNS injury. However, it is unclear why neurons express two similar CSPG receptors, nor whether they use the same or different intracellular pathways. We have now studied the signaling pathways of these two receptors using N2A cells and primary neurons derived from knockout mice. We demonstrate that both receptors share certain signaling pathways (RhoA, Akt and Erk), but also use distinct signals to mediate CSPG actions. Activation of PTPσ by CSPGs selectively inactivated CRMP2, APC, S6 kinase and CREB. By contrast LAR activation inactivated PKCζ, cofilin and LKB1. For the first time, we propose a model of the signaling pathways downstream of these two CSPG receptors. We also demonstrate that deleting both receptors exhibits additive enhancement of axon growth in adult neuronal cultures in vitro. Our findings elucidate the novel downstream pathways of CSPGs and suggest potential synergy of blocking their two PTP receptors.

Ginsenoside-Rd Promotes Neurite Outgrowth of PC12 Cells through MAPK/ERK- and PI3K/AKT-Dependent Pathways

Panax ginseng is a famous herbal medicine widely used in Asia. Ginsenosides have been identified as the principle active ingredients for Panax ginseng’s biological activity, among which ginsenoside Rd (Rd) attracts extensive attention for its obvious neuroprotective activities. Here we investigated the effect of Rd on neurite outgrowth, a crucial process associated with neuronal repair. PC12 cells, which respond to nerve growth factor (NGF) and serve as a model for neuronal cells, were treated with different concentrations of Rd, and then their neurite outgrowth was evaluated. Our results showed that 10 μM Rd significantly increased the percentages of long neurite- and branching neurite-bearing cells, compared with respective controls. The length of the longest neurites and the total length of neurites in Rd-treated PC12 cells were much longer than that of respective controls. We also showed that Rd activated ERK1/2 and AKT but not PKC signalings, and inhibition of ERK1/2 by PD98059 or/and AKT by LY294002 effectively attenuated Rd-induced neurite outgrowth. Moreover, Rd upregulated the expression of GAP-43, a neuron-specific protein involved in neurite outgrowth, while PD98059 or/and LY294002 decreased Rd-induced increased GAP-43 expression. Taken together, our results provided the first evidence that Rd may promote the neurite outgrowth of PC12 cells by upregulating GAP-43 expression via ERK- and ARK-dependent signaling pathways.

TrkB Signaling in Retinal Glia Stimulates Neuroprotection after Optic Nerve Injury

Brain-derived neurotrophic factor (BDNF) regulates neural cell survival mainly by activating TrkB receptors. Several lines of evidence support a key role for BDNF-TrkB signaling in survival of adult retinal ganglion cells in animal models of optic nerve injury (ONI), but the neuroprotective effect of exogenous BDNF is transient. Glial cells have recently attracted considerable attention as mediators of neural cell survival, and TrkB expression in retinal glia suggests its role in neuroprotection. To elucidate this point directly, we examined the effect of ONI on TrkB(flox/flox):glial fibrillary acidic protein (GFAP)-Cre+ (TrkB(GFAP)) knockout (KO) mice, in which TrkB is deleted in retinal glial cells. ONI markedly increased mRNA expression levels of basic fibroblast growth factor (bFGF) in wild-type (WT) mice but not in TrkB(GFAP) KO mice. Immunohistochemical analysis at 7 days after ONI (d7) revealed bFGF up-regulation mainly occurred in Müller glia. ONI-induced retinal ganglion cell loss in WT mice was consistently mild compared with TrkB(GFAP) KO mice at d7. On the other hand, ONI severely decreased TrkB expression in both WT and TrkB(GFAP) KO mice after d7, and the severity of retinal degeneration was comparable with TrkB(GFAP) KO mice at d14. Our data provide direct evidence that glial TrkB signaling plays an important role in the early stage of neural protection after traumatic injury.

α2-Adrenergic modulation of the glutamate receptor and transporter function in a chronic ocular hypertension model

Excitotoxicity, glutamate-induced toxic effects to retinal ganglion cells (RGCs), is one of several mechanisms of RGC loss suggested in glaucoma. In this study, we focused on the role of glutamate transporter of glial cells as well as N-methyl-d-aspartate (NMDA) receptor with regard to glutamate toxicity in glaucoma. We also investigated whether α2-adrenoceptor activation could modulate glutamate transporters and NMDA receptors in a chronic ocular hypertension model. Brimonidine 0.15% was administered topically to the eyes of experimental glaucoma and control animals twice daily. After 8 weeks of intraocular pressure (IOP) elevation, staining with terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) revealed an increase in the ganglion cell layer, and the number of TUNEL-positive cells was reduced by brimonidine treatment (P<0.05). Animals with experimentally induced glaucoma exhibited an increase in retinal stress marker glial fibrillary acidic protein (GFAP) immunoreactivity; brimonidine treatment reduced GFAP. Excitatory amino acid transporter 1(EAAT1) expression remained stable throughout the period of chronic ocular hypertension. α2-Adrenergic treatment upregulated EAAT1 protein levels (P<0.05). NMDA receptor (GluN1) expression was stimulated by chronic elevation of IOP, and GluN1-positive cells in ganglion cell layer were co-localized with TUNEL staining. Brimonidine administration suppressed GluN1 levels (P<0.05). These results indicate that brimonidine decreased RGC apoptosis, upregulating EAAT1 and downregulating NMDA receptors. We suggest that topical brimonidine treatment may decrease the glutamate excitotoxicity through modulation of glutamate transporter and NMDA receptor in glaucoma.

MicroRNA target prediction in glaucoma

Glaucoma is a progressive optic neuropathy and is one of the leading causes of blindness in the industrialized countries. The aim of this study is to investigate microRNA (miRNA) regulation in glaucoma and other neurodegenerative diseases, that share similar pathways, by means of in silico approaches such as bibliographic search and access to bioinformatic resources. First of all, data mining was carried out on Human miRNA Disease Database (HMDD) and miR2Disease databases. Then, predictions of deregulated miRNAs were carried out accessing to microrna.org database. Finally, the potential combinatorial effect of miRNAs, on regulation of biochemical pathways, was studied by an enrichment analysis performed by DIANA-miRPath v.2.0. We found, from literature search, 8 deregulated miRNAs in glaucoma and 9 and 23 in age-related macular degeneration (AMD) and Alzheimer's disease (AD), respectively. One miRNA is commonly deregulated in glaucoma and AMD (miR-23a). Two miRNAs (miR-29a, miR-29b) are common to glaucoma and AD, and four miRNAs were identified to be commonly deregulated in AMD and AD (miR-9, miR-21, miR-34a, miR-146a). The match of the miRNA common to glaucoma and the other two neurodegenerative diseases (AMD and AD) did not generate any output. Enrichment of information has been reached through miRNAs prediction: 88 predicted miRNAs are common to glaucoma and AMD, 19 are common to glaucoma and AD, and 9 are common to AMD and AD. Indeed, predicted miRNAs common to the three neurodegenerative diseases are nine (miR-107, miR-137, miR-146a, miR-181c, miR-197, miR-21, miR-22, miR-590, miR-9). DIANA-miRPath predicted that those nine miRNAs might regulate pathways involved in inflammation. The findings hereby obtained provide a valuable hint to assess deregulation of specific miRNA, as potential biomarkers and therapeutic targets, in glaucoma and other neurodegenerative diseases by means of preclinical and clinical studies.

Axonal protection by brimonidine with modulation of p62 expression in TNF-induced optic nerve degeneration

Purpose

The p62, also called sequestosome 1 (SQSTM1), plays a crucial role in tumor necrosis factor (TNF)-induced optic nerve degeneration. Brimonidine has been shown to have protective effects on retinal ganglion cell bodies, although its role in their axons remains to be examined. We determined whether brimonidine modulates axonal loss induced by TNF and affects the expression of p62 in the optic nerve.

Methods

Experiments were performed on adult male Wistar rats that received an intravitreal injection of 10 ng TNF alone or simultaneous injection of TNF and 2, 20, or 200 pmol of brimonidine tartrate. The expression of p62 in the optic nerve was examined by immunoblot analysis. The effects of brimonidine on axons were evaluated by counting axon numbers 2 weeks after intravitreal injection.

Results

Intravitreal injection of brimonidine exerted substantial axonal protection against TNF-induced optic nerve degeneration. Immunoblot analysis showed that p62 was upregulated in the optic nerve after intravitreal injection of TNF, and that this increase was completely inhibited by brimonidine. Treatment with brimonidine alone also significantly decreased p62 protein levels in the optic nerve compared with the basal level.

Conclusions

These results suggest that the modulation of p62 levels in the optic nerve by brimonidine may be involved partly in its axonal protection.

Molecular, anatomical and functional changes in the retinal ganglion cells after optic nerve crush in mice

PURPOSE

Optic nerve crush (ONC) and subsequent axonal damage can be used in rodents to study the mechanism of retinal ganglion cell (RGC) degeneration. Here, we examined electroretinograms (ERGs) in post-ONC mice to investigate changes in the positive scotopic threshold response (pSTR). We then compared these changes with molecular and morphological changes to identify early objective biomarkers of RGC dysfunction.

METHODS

Fifty 12-week-old C57BL/6 mice were included. ONC was used to induce axonal injury in the right eye of each animal, with the left eye used as a control. The expression of the RGC markers Brn3a and Brn3b was measured on days 1, 2, 3, 5 and 7 after ONC with quantitative real-time PCR. ERGs were recorded under dark adaptation with the stimulus intensity increasing from -6.2 to 0.43 log cd-s/m(2) on days 1, 2, 3, 5, 7 and 10 after ONC. The pSTR, a- and b-wave amplitudes were measured. Inner retinal thickness around the optic nerve head was measured with spectral-domain optical coherence tomography on days 0, 2, 5, 7 and 10 after ONC.

RESULTS

The expression of Brn3a and Brn3b began to significantly decrease on day 1 and day 2, respectively (P < 0.01). The amplitude of the pSTR underwent rapid, significant deterioration on day 3, after which it fell gradually (P < 0.01), while the a- and b-wave amplitudes remained unchanged throughout the experiment. Inner retinal thickness gradually decreased, with the most significant reduction on day 10 (P < 0.01).

CONCLUSIONS

Decrease in pSTR likely reflected the early loss of RGC function after ONC and that declining expression of RGC-specific genes preceded anatomical and functional changes in the RGCs.

βB2-Crystallin Promotes Axonal Regeneration in the Injured Optic Nerve in Adult Rats

The purpose of the study was to further scrutinize the potential of βB2-crystallin in supporting regeneration of injured retinal ganglion cell axons both in vitro and in vivo. Retinal explants obtained from animals after treatment either with lens injury (LI) alone or with combined LI 5 days or 3 days before or simultaneously with an optic nerve crush (ONC) were cultured for 96 h under regenerative conditions, and the regenerating axons were quantified and compared with untreated controls. These measurements were then repeated with LI replaced by intravitreal injections of γ-crystallin and β-crystallin at 5 days before ONC. Finally, βB2-crystallin-overexpressing transfected neural progenitor cells (βB2-crystallin-NPCs) in the eye were studied after crushing the optic nerve in vivo. Regeneration was monitored with the aid of immunoblotting of the retina and optic nerve both distal and proximal to the lesion site, and this was compared with controls that received injections of phosphate buffer only. LI performed 5 days or 3 days before ONC significantly promoted axonal outgrowth in vitro (p < 0.001), while LI performed alone before explantation did not. Intravitreal injections of β-crystallin and γ-crystallin mimicked the effects of LI and significantly increased axonal regeneration in culture at the same time intervals (p < 0.001). Western blot analysis revealed that crystallins were present in the proximal optic nerve stump at the lesion site in ONC, but were neither expressed in the undamaged distal optic nerve nor in uninjured tissue. βB2-crystallin-NPCs supported the regeneration of cut optic nerve axons within the distal optic nerve stump in vivo. The reported data suggest that βB2-crystallin-producing "cell factories" could be used to provide novel therapeutic drugs for central nervous system injuries.

Brimonidine prevents neurodegeneration in a mouse model of normal tension glaucoma

Glaucoma is one of the leading causes of irreversible blindness that is characterized by progressive degeneration of optic nerves and retinal ganglion cells (RGCs). In the mammalian retina, excitatory amino-acid carrier 1 (EAAC1) is expressed in neural cells, including RGCs, and the loss of EAAC1 leads to RGC degeneration without elevated intraocular pressure (IOP). Brimonidine (BMD) is an α2-adrenergic receptor agonist and it is commonly used in a form of eye drops to lower IOP in glaucoma patients. Recent studies have suggested that BMD has direct protective effects on RGCs involving IOP-independent mechanisms, but it is still controversial. In the present study, we examined the effects of BMD in EAAC1-deficient (KO) mice, an animal model of normal tension glaucoma. BMD caused a small decrease in IOP, but sequential in vivo retinal imaging and electrophysiological analysis revealed that treatment with BMD was highly effective for RGC protection in EAAC1 KO mice. BMD suppressed the phosphorylation of the N-methyl-D-aspartate receptor 2B (NR2B) subunit in RGCs in EAAC1 KO mice. Furthermore, in cultured Müller glia, BMD stimulated the production of several neurotrophic factors that enhance RGC survival. These results suggest that, in addition to lowering IOP, BMD prevents glaucomatous retinal degeneration by stimulating multiple pathways including glia–neuron interactions.

AxonQuant: A Microfluidic Chamber Culture-Coupled Algorithm That Allows High-Throughput Quantification of Axonal Damage

Published methods for imaging and quantitatively analyzing morphological changes in neuronal axons have serious limitations because of their small sample sizes, and their time-consuming and nonobjective nature. Here we present an improved microfluidic chamber design suitable for fast and high-throughput imaging of neuronal axons. We developed the AxonQuant algorithm, which is suitable for automatic processing of axonal imaging data. This microfluidic chamber-coupled algorithm allows calculation of an 'axonal continuity index' that quantitatively measures axonal health status in a manner independent of neuronal or axonal density. This method allows quantitative analysis of axonal morphology in an automatic and nonbiased manner. Our method will facilitate large-scale high-throughput screening for genes or therapeutic compounds for neurodegenerative diseases involving axonal damage. When combined with imaging technologies utilizing different gene markers, this method will provide new insights into the mechanistic basis for axon degeneration. Our microfluidic chamber culture-coupled AxonQuant algorithm will be widely useful for studying axonal biology and neurodegenerative disorders.

One Year of Glaucoma Research in Review: 2012 to 2013

PURPOSE

The objective of this study was to provide the practicing clinical ophthalmologist with an update of pertinent glaucoma literature published from 2012 to 2013.

DESIGN

Literature review.

METHODS

The authors conducted a 1-year (July 1, 2012, to September 30, 2013) English-language glaucoma literature search on PubMed using the following terms: glaucoma, automated perimetry, optic nerve imaging, optical coherence tomography, glaucoma structure and function, intraocular pressure, central corneal thickness, glaucoma medical therapy, neuroprotection, glaucoma laser treatment, secondary glaucoma, glaucoma surgery, and miscellaneous topics in glaucoma.

RESULTS

Of 2659 articles on glaucoma published during our time frame, this review selected original and review articles that reflect novel aspects and updates in the field of glaucoma, while excluding letters to the editor, unpublished works, and abstracts. Preference was given to human research.

CONCLUSIONS

This review focuses on literature that is applicable to ophthalmologists in practice and also highlights studies that may enhance the diagnosis and management of glaucoma.