Activation of epidermal growth factor receptor signals induction of nitric oxide synthase-2 in human optic nerve head astrocytes in glaucomatous optic neuropathy

Molecular regulation of neuroinflammation in glaucoma: Current knowledge and the ongoing search for new treatment targets

Neuroinflammation relying on the inflammatory responses of glial cells has emerged as an impactful component of the multifactorial etiology of neurodegeneration in glaucoma. It has become increasingly evident that despite early adaptive and reparative features of glial responses, prolonged reactivity of the resident glia, along with the peripheral immune cells, create widespread toxicity to retinal ganglion cell (RGC) axons, somas, and synapses. As much as the synchronized responses of astrocytes and microglia to glaucoma-related stress or neuron injury, their bi-directional interactions are critical to build and amplify neuroinflammation and to dictate the neurodegenerative outcome. Although distinct molecular programs regulate somatic and axonal degeneration in glaucoma, inhibition of neurodegenerative inflammation can provide a broadly beneficial treatment strategy to rescue RGC integrity and function. Since inflammatory toxicity and mitochondrial dysfunction are converging etiological paths that can boost each other and feed into a vicious cycle, anti-inflammatory treatments may also offer a multi-target potential. This review presents an overview of the current knowledge on neuroinflammation in glaucoma with particular emphasis on the cell-intrinsic and cell-extrinsic factors involved in the reciprocal regulation of glial responses, the interdependence between inflammatory and mitochondrial routes of neurodegeneration, and the research aspects inspiring for prospective immunomodulatory treatments. With the advent of powerful technologies, ongoing research on molecular and functional characteristics of glial responses is expected to accumulate more comprehensive and complementary information and to rapidly move the field forward to safe and effective modulation of the glial pro-inflammatory activities, while restoring or augmenting the glial immune-regulatory and neurosupport functions.

Therapeutic Drugs and Devices for Tackling Ocular Hypertension and Glaucoma, and Need for Neuroprotection and Cytoprotective Therapies

Damage to the optic nerve and the death of associated retinal ganglion cells (RGCs) by elevated intraocular pressure (IOP), also known as glaucoma, is responsible for visual impairment and blindness in millions of people worldwide. The ocular hypertension (OHT) and the deleterious mechanical forces it exerts at the back of the eye, at the level of the optic nerve head/optic disc and lamina cribosa, is the only modifiable risk factor associated with glaucoma that can be treated. The elevated IOP occurs due to the inability of accumulated aqueous humor (AQH) to egress from the anterior chamber of the eye due to occlusion of the major outflow pathway, the trabecular meshwork (TM) and Schlemm’s canal (SC). Several different classes of pharmaceutical agents, surgical techniques and implantable devices have been developed to lower and control IOP. First-line drugs to promote AQH outflow via the uveoscleral outflow pathway include FP-receptor prostaglandin (PG) agonists (e.g., latanoprost, travoprost and tafluprost) and a novel non-PG EP2-receptor agonist (omidenepag isopropyl, Eybelis^®). TM/SC outflow enhancing drugs are also effective ocular hypotensive agents (e.g., rho kinase inhibitors like ripasudil and netarsudil; and latanoprostene bunod, a conjugate of a nitric oxide donor and latanoprost). One of the most effective anterior chamber AQH microshunt devices is the Preserflo^® microshunt which can lower IOP down to 10–13 mmHg. Other IOP-lowering drugs and devices on the horizon will be also discussed. Additionally, since elevated IOP is only one of many risk factors for development of glaucomatous optic neuropathy, a treatise of the role of inflammatory neurodegeneration of the optic nerve and retinal ganglion cells and appropriate neuroprotective strategies to mitigate this disease will also be reviewed and discussed.

A lab-on-a-chip model of glaucoma

AIMS

We developed a glaucoma-on-a-chip model to evaluate the viability of retinal ganglion cells (RGCs) against high pressure and the potential effect of neuroprotection.

METHODS

A three-layered chip consisting of interconnecting microchannels and culture wells was designed and fabricated from poly-methyl methacrylate sheets. The bottom surface of the wells was modified by air plasma and coated with different membranes to provide a suitable extracellular microenvironment. RGCs were purified from postnatal Wistar rats by magnetic assisted cell sorting up to 70% and characterized by flow cytometry and immunocytochemistry. The cultured RGCs were exposed to normal (15 mmHg) or elevated pressure (33 mmHg) for 6, 12, 24, 36, and 48 hr, with and without adding brain-derived neurotrophic factor (BDNF) or a novel BDNF mimetic (RNYK).

RESULTS

Multiple inlet ports allow culture media and gas into the wells under elevated hydrostatic pressure. PDL/laminin formed the best supporting membrane. RGC survival rates were 85%, 78%, 70%, 67%, and 61% under normal pressure versus 40%, 22%, 18%, 12%, and 10% under high pressure at 6, 12, 24, 36, and 48 hr, respectively. BDNF and RNYK separately reduced RGC death rates about twofold under both normal and elevated pressures.

CONCLUSION

This model recapitulated the effects of elevated pressure over relatively short time periods and demonstrated the neuroprotective effects of BDNF and RNYK.

Update on the genetics of primary open-angle glaucoma

Affecting nearly 80 million individuals, glaucoma is the number one cause of irreversible blindness in the world. This ocular disease describes a set of optic neuropathies of which primary open angle glaucoma (POAG) is the most common. POAG is associated with progressive visual field deterioration resulting from damage to the optic nerve and loss of retinal ganglion cells. Risk factors for POAG include elevated intraocular pressure, aging, African and Hispanic ancestry, and a positive family history of POAG. Multiple genes have been found to contribute to POAG. Much of POAG genetics and pathology has yet to be explained. Recent genome-wide association studies have identified a large number of novel loci associated with POAG and its endophenotypes. Genomic and proteomic profiling of biofluids has contributed to our knowledge of differential gene expression in POAG. Functional studies both in cell culture and animal models have confirmed the effects of variants and differential gene expression on ocular physiology while in silico analyses have increased our understanding of disease risk and progression so that we might better diagnose and treat this complex genetic illness.

Modeling the Mechanical Parameters of Glaucoma

Glaucoma is a major eye disease characterized by a progressive loss of retinal ganglion cells (RGCs). Biomechanical forces as a result of hydrostatic pressure and strain play a role in this disease. Decreasing intraocular pressure is the only available therapy so far, but is not always effective and does not prevent blindness in many cases. There is a need for drugs that protect RGCs from dying in glaucoma; to develop these, we need valid glaucoma and drug screening models. Since in vivo models are unsuitable for screening purposes, we focus on in vitro and ex vivo models in this review. Many groups have studied pressure and strain model systems to mimic glaucoma, to investigate the molecular and cellular events leading to mechanically induced RGC death. Therefore, the focus of this review is on the different mechanical model systems used to mimic the biomechanical forces in glaucoma. Most models use either cell or tissue strain, or fluid- or gas-controlled hydrostatic pressure application and apply it to the relevant cell types such as trabecular meshwork cells, optic nerve head astrocytes, and RGCs, but also to entire eyes. New model systems are warranted to study concepts and test experimental compounds for the development of new drugs to protect vision in glaucoma patients. Impact Statement The outcome of currently developed models to investigate mechanically induced retinal ganglion cell death by applying different mechanical strains varies widely. This suggests that a robust glaucoma model has not been developed yet. However, a comprehensive overview of current developments is not available. In this review, we have therefore assessed what has been done before and summarized the available knowledge in the field, which can be used to develop improved models for glaucoma research.

Rapamycin mediates mTOR signaling in reactive astrocytes and reduces retinal ganglion cell loss

Damage and loss of retinal ganglion cells (RGCs) can cause visual impairment. The underlying molecular mechanisms that mediate RGC death in ischemic retinal diseases are still unclear. In this study, we sought to understand the neuroprotective effect of rapamycin, the selective inhibitor of mTORC1, on RGC survival and the cellular mechanics that mediate this effect. Recent studies have reported that the epidermal growth factor (EGF) receptor shows an increase in expression in astrocytes after injury, and this receptor can promote their transformation into reactive astrocytes. Our results, along with previous works from others, show the colocalization of phosphor-EGF receptors with the astrocyte marker glial fibrillary acidic proteins in reactive astrocytes in the injured retina. In our in vitro studies, using primary astrocyte cultures of the optic nerve head of rats, showed that rapamycin significantly blocked EGF-induced mTOR signaling mainly through the PI3K/Akt pathway in primary astrocytes, but not through the MAPK/Erk pathway. Additionally, rapamycin dramatically inhibited the activation of mTOR signaling in our ratinal ischemia-reperfusion (I/R) injury model in vivo. Astrocyte activation was assessed by immunostaining retinal flat mounts or cross sections with antibody against GFAP, and we also used western blots to detect the expression of GFAP. Taken together, these results revealed that rapamycin decreases the activation of astrocytes after retinal ischemia-reperfusion injury. Furthermore, rapamycin can improve retinal RGC survival in rats during I/R, as detected by FluoroGold labeling. Our data reveals the neuroprotective effects of rapamycin in an experimental retina injury model, possibly through decreasing glial-dependent intracellular signaling mechanisms for suppressing apoptosis of RGCs. Our study also presents an approach to targeting reactive astrocytes for the treatment of optic neurodegenerations.

Genome-wide association study identifies seven novel susceptibility loci for primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is the leading cause of irreversible blindness worldwide for which 15 disease-associated loci had been discovered. Among them, only 5 loci have been associated with POAG in Asians. We carried out a genome-wide association study and a replication study that included a total of 7378 POAG cases and 36 385 controls from a Japanese population. After combining the genome-wide association study and the two replication sets, we identified 11 POAG-associated loci, including 4 known (CDKN2B-AS1, ABCA1, SIX6 and AFAP1) and 7 novel loci (FNDC3B, ANKRD55-MAP3K1, LMX1B, LHPP, HMGA2, MEIS2 and LOXL1) at a genome-wide significance level (P < 5.0×10-8), bringing the total number of POAG-susceptibility loci to 22. The 7 novel variants were subsequently evaluated in a multiethnic population comprising non-Japanese East Asians (1008 cases, 591 controls), Europeans (5008 cases, 35 472 controls) and Africans (2341 cases, 2037 controls). The candidate genes located within the new loci were related to ocular development (LMX1B, HMGA2 and MAP3K1) and glaucoma-related phenotypes (FNDC3B, LMX1B and LOXL1). Pathway analysis suggested epidermal growth factor receptor signaling might be involved in POAG pathogenesis. Genetic correlation analysis revealed the relationships between POAG and systemic diseases, including type 2 diabetes and cardiovascular diseases. These results improve our understanding of the genetic factors that affect the risk of developing POAG and provide new insight into the genetic architecture of POAG in Asians.

The effect of the rate of hydrostatic pressure depressurization on cells in culture

Changes in hydrostatic pressure, at levels as low as 10 mm Hg, have been reported in some studies to alter cell function in vitro; however, other studies have found no detectable changes using similar methodologies. We here investigate the hypothesis that the rate of depressurization, rather than elevated hydrostatic pressure itself, may be responsible for these reported changes. Hydrostatic pressure (100 mm Hg above atmospheric pressure) was applied to bovine aortic endothelial cells (BAECs) and PC12 neuronal cells using pressurized gas for periods ranging from 3 hours to 9 days, and then the system was either slowly (~30 minutes) or rapidly (~5 seconds) depressurized. Cell viability, apoptosis, proliferation, and F-actin distribution were then assayed. Our results did not show significant differences between rapidly and slowly depressurized cells that would explain differences previously reported in the literature. Moreover, we found no detectable effect of elevated hydrostatic pressure (with slow depressurization) on any measured variables. Our results do not confirm the findings of other groups that modest increases in hydrostatic pressure affect cell function, but we are not able to explain their findings.

Early immune responses are independent of RGC dysfunction in glaucoma with complement component C3 being protective

Various immune response pathways are altered during early, predegenerative stages of glaucoma; however, whether the early immune responses occur secondarily to or independently of neuronal dysfunction is unclear. To investigate this relationship, we used the Wlds allele, which protects from axon dysfunction. We demonstrate that DBA/2J.Wlds mice develop high intraocular pressure (IOP) but are protected from retinal ganglion cell (RGC) dysfunction and neuroglial changes that otherwise occur early in DBA/2J glaucoma. Despite this, immune pathways are still altered in DBA/2J.Wlds mice. This suggests that immune changes are not secondary to RGC dysfunction or altered neuroglial interactions, but may be directly induced by the increased strain imposed by high IOP. One early immune response following IOP elevation is up-regulation of complement C3 in astrocytes of DBA/2J and DBA/2J.Wlds mice. Unexpectedly, because the disruption of other complement components, such as C1Q, is protective in glaucoma, C3 deficiency significantly increased the number of DBA/2J eyes with nerve damage and RGC loss at an early time point after IOP elevation. Transcriptional profiling of C3-deficient cultured astrocytes implicated EGFR signaling as a hub in C3-dependent responses. Treatment with AG1478, an EGFR inhibitor, also significantly increased the number of DBA/2J eyes with glaucoma at the same early time point. These findings suggest that C3 protects from early glaucomatous damage, a process that may involve EGFR signaling and other immune responses in the optic nerve head. Therefore, therapies that target specific components of the complement cascade, rather than global inhibition, may be more applicable for treating human glaucoma.

EGFR regulates macrophage activation and function in bacterial infection

EGFR signaling regulates macrophage function, but its role in bacterial infection has not been investigated. Here, we assessed the role of macrophage EGFR signaling during infection with Helicobacter pylori, a bacterial pathogen that causes persistent inflammation and gastric cancer. EGFR was phosphorylated in murine and human macrophages during H. pylori infection. In human gastric tissues, elevated levels of phosphorylated EGFR were observed throughout the histologic cascade from gastritis to carcinoma. Deleting Egfr in myeloid cells attenuated gastritis and increased H. pylori burden in infected mice. EGFR deficiency also led to a global defect in macrophage activation that was associated with decreased cytokine, chemokine, and NO production. We observed similar alterations in macrophage activation and disease phenotype in the Citrobacter rodentium model of murine infectious colitis. Mechanistically, EGFR signaling activated NF-κB and MAPK1/3 pathways to induce cytokine production and macrophage activation. Although deletion of Egfr had no effect on DC function, EGFR-deficient macrophages displayed impaired Th1 and Th17 adaptive immune responses to H. pylori, which contributed to decreased chronic inflammation in infected mice. Together, these results indicate that EGFR signaling is central to macrophage function in response to enteric bacterial pathogens and is a potential therapeutic target for infection-induced inflammation and associated carcinogenesis.

Glaucomatous Optic Neuropathy: When Glia Misbehave

The loss of vision in the human eye disease, glaucoma, is due to degeneration of the axons of the retinal ganglion cells. In glaucoma, reactive astrocytes in the optic nerve head contain inducible nitric oxide synthase, which apparently produces excessive nitric oxide that damages the axons. The astrocytes respond to the elevated intraocular pressure that is characteristic of the disease. An important signal transduction pathway for the induction of nitric oxide synthase in response to pressure is the epidermal growth factor receptor tyrosine kinase. Pharmacological inhibition of the activity or the induction of inducible nitric oxide synthase may provide neuroprotection for the treatment of glaucoma.

Glaucoma: Current and Developing Concepts for Inflammation, Pathogenesis and Treatment

Glaucoma is a prevalent neurodegenerative disorder of the eye. However, the mechanism leading to the disease is still unclear. Increased intraocular pressure (IOP) and subsequent retinal ganglion cell (RGC) death leading to the loss of visual field characterizes the pathology of primary open angle glaucoma, which is the most common form. Possible factors leading to glaucoma include glutamate-induced neurotoxicity, nitric oxide (NO) based damage, disruption of neurotrophic factor transport and immune-induced neurodestruction. Current treatment options primarily aim at decreasing IOP by utilizing pharmacological agents, laser therapy and surgery. Developing treatments target neuroprotection with vaccines, the inhibition of NO synthesis and apoptosis. Gaining a better understanding of the pathogenesis can aid in the development of new treatment options and, perhaps, even a cure for glaucoma.

Autoimmune aspects in glaucoma

The pathogenesis of glaucoma, a common neurodegenerative disease, involves an immunologic component. Studies demonstrate changes of autoantibody concentrations against retinal and optic nerve head antigens in glaucoma patients. Furthermore we found antibody deposits in human glaucomatous retinae in a pro-inflammatory environment. Clinical studies showed up regulated, but also significantly down-regulated autoantibody levels. These antibodies belong to the natural autoimmunity. The upregulation of autoantibodies can be associated with fatal conditions, but several studies demonstrate that natural autoantibodies entail also neuroprotective characteristics and influence the protein expression of neuroretinal cells. A misbalance in the physiological equilibrium may shift from regulatory immunity into a neuroinflammatory degenerative process, what may lead to a predisposition to glaucoma. However, the protective nature of autoantibodies and the molecular mechanisms underlying the very sensitive equilibrium of natural autoimmunity between autoaggression and neuroprotection offer promising target sites for new therapeutic approaches. Finally, the changes in antibody profiles represent a new opportunity as highly sensitive and specific biomarkers for diagnostics purposes.

Discovery of novel inhibitors for the treatment of glaucoma

INTRODUCTION

Glaucoma is a neurodegenerative disease with heterogeneous causes that result in retinal ganglionic cell (RGC) death. The discovery of ocular antihypertensives has shifted glaucoma therapy, largely, from surgery to medical intervention. Indeed, several intraocular pressure (IOP)-lowering drugs, with different mechanisms of action and RGC protective property, have been developed.

AREAS COVERED

In this review, the authors discuss the main new class of kinase inhibitors used as glaucoma treatments, which lower IOP by enhancing drainage and/or lowering production of aqueous humor. The authors include novel inhibitors under preclinical evaluation and investigation for their anti-glaucoma treatment. Additionally, the authors look at treatments that are in clinics now and which may be available in the near future.

EXPERT OPINION

Treatment of glaucoma remains challenging because the exact cause is yet to be delineated. Neuroprotection to the optic nerve head is undisputable. The novel Rho-associated kinase inhibitors have the capacity to lower IOP and provide optic nerve and RGC protection. In particular, the S-isomer of roscovitine has the capacity to lower IOP and provide neuroprotection. Combinations of selected drugs, which can provide maximal and sustained IOP-lowering effects as well as neuroprotection, are paramount to the prevention of glaucoma progression. In the near future, microRNA intervention may be considered as a potential therapeutic target.

Does autoimmunity play a part in the pathogenesis of glaucoma?

Glaucoma is a chronic neurodegenerative disease and one of the leading causes of blindness. Several risk factors have been described, e.g. an elevated intraocular pressure (IOP), oxidative stress or mitochondrial dysfunction. Additionally, alterations in serum antibody profiles of glaucoma patients, upregulation (e.g. anti-HSP60, anti-MBP) and downregulation (e.g. anti-14-3-3), have been described, but it still remains elusive if the autoantibodies seen in glaucoma are an epiphenomenon or causative. However, it is known that elicited autoimmunity causes retinal ganglion cell loss resulting in glaucomatous-like damage and according to the autoaggressive nature of some autoantibodies we found antibody deposits in human glaucomatous retinae in a pro-inflammatory environment. Furthermore, glaucomatous serum has the potential to influence neuroretinal cell regulatory processes. Importantly, we demonstrate that some autoantibodies hold neuroprotective potential for neuroretinal cells. The protective nature of autoantibodies and the molecular mechanisms underlying the very sensitive equilibrium between autoaggression and protection remain subject of future examinations and offer promising target sites for new therapeutic approaches. Additionally, the changes in antibody profiles could be used as highly sensitive and specific marker for diagnostics purposes. Early diagnosis and intervention in risk patients would offer the chance of early treatment and to slow down the progression of glaucoma and delay the resulting blindness.

Receptor tyrosine kinases: molecular switches regulating CNS axon regeneration

The poor or lack of injured adult central nervous system (CNS) axon regeneration results in devastating consequences and poor functional recovery. The interplay between the intrinsic and extrinsic factors contributes to robust inhibition of axon regeneration of injured CNS neurons. The insufficient or lack of trophic support for injured neurons is considered as one of the major obstacles contributing to their failure to survive and regrow their axons after injury. In the CNS, many of the signalling pathways associated with neuronal survival and axon regeneration are regulated by several classes of receptor tyrosine kinases (RTK) that respond to a variety of ligands. This paper highlights and summarises the most relevant recent findings pertinent to different classes of the RTK family of molecules, with a particular focus on elucidating their role in CNS axon regeneration.

ErbB receptor signaling in astrocytes: a mediator of neuron-glia communication in the mature central nervous system

Astrocytes are now recognized as active players in the developing and mature central nervous system. Each astrocyte contacts vascular structures and thousands of synapses within discrete territories. These cells receive a myriad of inputs and generate appropriate responses to regulate the function of brain microdomains. Emerging evidence has implicated receptors of the ErbB tyrosine kinase family in the integration and processing of neuronal inputs by astrocytes: ErbB receptors can be activated by a wide range of neuronal stimuli; they control critical steps of glutamate-glutamine metabolism; and they regulate the biosynthesis and release of various glial-derived neurotrophic factors, gliomediators and gliotransmitters. These key properties of astrocytic ErbB signaling in neuron-glia interactions have significance for the physiology of the mature central nervous system, as exemplified by the central control of reproduction within the hypothalamus, and are also likely to contribute to pathological situations, since both dysregulation of ErbB signaling and glial dysfunction occur in many neurological disorders.

Off-target effects of epidermal growth factor receptor antagonists mediate retinal ganglion cell disinhibited axon growth

Inhibition of central nervous system axon growth is reportedly mediated in part by calcium-dependent phosphorylation of axonal epidermal growth factor receptor, with local administration of the epidermal growth factor receptor kinase inhibitors AG1478 and PD168393 to an optic nerve lesion site promoting adult retinal ganglion cell axon regeneration. Here, we show that epidermal growth factor receptor was neither constitutively expressed, nor activated in optic nerve axons in our non-regenerating and regenerating optic nerve injury models, a finding that is inconsistent with phosphorylated epidermal growth factor receptor-dependent intra-axonal signalling of central nervous system myelin-related axon growth inhibitory ligands. However, epidermal growth factor receptor was localized and activated within most glia in the retina and optic nerve post-injury, and thus an indirect glial-dependent mechanism for stimulated retinal ganglion cell axon growth by epidermal growth factor receptor inhibitors seemed plausible. Using primary retinal cultures with added central nervous system myelin extracts, we confirmed previous reports that AG1478/PD168393 blocks epidermal growth factor receptor activation and promotes disinhibited neurite outgrowth. Paradoxically, neurites did not grow in central nervous system myelin extract-containing cultures after short interfering ribonucleic acid-mediated knockdown of epidermal growth factor receptor. However, addition of AG1478 restored neurite outgrowth to short interfering ribonucleic acid-treated cultures, implying that epidermal growth factor receptor does not mediate AG1478-dependent effects. TrkA-/B-/C-Fc fusion proteins and the kinase blocker K252a abrogated the neuritogenic activity in these cultures, correlating with the presence of the neurotrophins brain derived neurotrophic factor, nerve growth factor and neurotrophin-3 in the supernatant and increased intracellular cyclic adenosine monophosphate activity. Neurotrophins released by AG1478 stimulated disinhibited retinal ganglion cell axon growth in central nervous system myelin-treated cultures by the induction of regulated intramembraneous proteolysis of p75(NTR) and Rho inactivation. Retinal astrocytes/Müller cells and retinal ganglion cells were the source of neurotrophins, with neurite outgrowth halved in the presence of glial inhibitors. We attribute AG1478-stimulated neuritogenesis to the induced release of neurotrophins together with raised cyclic adenosine monophosphate levels in treated cultures, leading to axon growth and disinhibition by neurotrophin-induced regulated intramembraneous proteolysis of p75(NTR). These off-target effects of epidermal growth factor receptor kinase inhibition suggest a novel therapeutic approach for designing treatments to promote central nervous system axon regeneration.

Friend or foe? Resolving the impact of glial responses in glaucoma

Glaucomatous vision loss results from the progressive degeneration of optic nerve axons and the death of retinal ganglion cells. This process is accompanied by dramatic alterations in the functional properties and distribution of glial cells in both the retina and the optic nerve head in a reaction commonly referred to as glial activation. The recent availability of rodent and cell culture glaucoma models has substantially contributed to our knowledge of glial activation under glaucomatous conditions. Conclusions drawn from these studies have led to the refinement of existing hypotheses and the generation of new ones. Because these hypotheses encompass both protective and injurious roles for glia, the impact of specific aspects of glial activation are current topics of intensive research, speculation, and debate in the field. With these unresolved issues in mind, this review will summarize recent progress in our understanding of the process of glial activation in the glaucomatous optic nerve head and retina.

Epidermal growth factor receptor inhibitors promote CNS axon growth through off-target effects on glia

Administration of epidermal growth factor receptor (EGFR) inhibitors (e.g. AG1478/PD168393) promotes central nervous system (CNS) axon regeneration in vivo by an unknown mechanism. Here, we show that EGFR activation is not required for AG1478-/PD168393-induced neurite outgrowth in cultures of dorsal root ganglion neurons (DRGN) with added inhibitory CNS myelin extract (CME), but is mediated by the paracrine and autocrine actions of the glia-/neuron-derived neurotrophins (NT) NGF, BDNF and NT-3 through Trk signalling in DRGN potentiated by elevated cAMP levels. The DRGN neurite growth seen in CME-inhibited cultures treated with AG1478 is eradicated by blocking Trk signalling but undiminished after siRNA knockdown of >90% EGFR. Moreover, addition of the combined triplet of NT restores neurite outgrowth in CME-inhibited cultures, when cAMP levels are raised. Accordingly, we suggest that chemical EGFR inhibitors act independently of EGFR, inducing glia and neurons to secrete NT and raising cAMP levels in DRG cultures, leading to Trk-dependent disinhibited DRGN neurite outgrowth.

Pressure activates epidermal growth factor receptor leading to the induction of iNOS via NFkappaB and STAT3 in human proximal tubule cells

Ureteral obstruction leads to increased pressure and inducible nitric oxide synthase (iNOS) expression. This study examined the involvement of epidermal growth factor (EGF) receptor (EGFR), nuclear factor-kappaB (NFkappaB), and signal transducers and activators of transcription 3 (STAT3) in iNOS induction in human proximal tubule (HKC-8) cells in response to pressure or EGF. HKC-8 cells were subjected to 60 mmHg pressure or treated with EGF for 0-36 h. iNOS was more rapidly induced in response to EGF than pressure. The addition of EGFR, NFkappaB, and STAT3 inhibitors significantly suppressed pressure- or EGF-stimulated iNOS mRNA and protein expression. Analysis of the activated states of EGFR, NFkappaB p65, and STAT3 after exposure to both stimuli demonstrated phosphorylation within 2.5 min. Anti-EGF antibody inhibited iNOS induction in pressurized HKC-8 cells, providing evidence that endogenous EGF mediates the response to pressure. In ureteral obstruction, when pressure is elevated, phosphorylated EGFR was detected in the apical surface of the renal tubules, validating the in vitro findings. These data indicate that EGFR, NFkappaB, and STAT3 are required for human iNOS gene induction in response to pressure or EGF, indicating a similar mechanism of activation.

Bioinformatic and statistical analysis of the optic nerve head in a primate model of ocular hypertension

BACKGROUND

The nonhuman primate model of glaucomatous optic neuropathy most faithfully reproduces the human disease. We used high-density oligonucleotide arrays to investigate whole genome transcriptional changes occurring at the optic nerve head during primate experimental glaucoma.

RESULTS

Laser scarification of the trabecular meshwork of cynomolgus macaques produced elevated intraocular pressure that was monitored over time and led to varying degrees of damage in different samples. The macaques were examined clinically before enucleation and the myelinated optic nerves were processed post-mortem to determine the degree of neuronal loss. Global gene expression was examined in dissected optic nerve heads with Affymetrix GeneChip microarrays. We validated a subset of differentially expressed genes using qRT-PCR, immunohistochemistry, and immuno-enriched astrocytes from healthy and glaucomatous human donors. These genes have previously defined roles in axonal outgrowth, immune response, cell motility, neuroprotection, and extracellular matrix remodeling.

CONCLUSION

Our findings show that glaucoma is associated with increased expression of genes that mediate axonal outgrowth, immune response, cell motility, neuroprotection, and ECM remodeling. These studies also reveal that, as glaucoma progresses, retinal ganglion cell axons may make a regenerative attempt to restore lost nerve cell contact.

Inducible nitric oxide synthase, Nos2, does not mediate optic neuropathy and retinopathy in the DBA/2J glaucoma model

Background

Nitric oxide synthase 2 (NOS2) contributes to neural death in some settings, but its role in glaucoma remains controversial. NOS2 is implicated in retinal ganglion cell degeneration in a rat glaucoma model in which intraocular pressure (IOP) is experimentally elevated by blood vessel cauterization, but not in a rat glaucoma model where IOP was elevated by injection of hypertonic saline. To test the importance of NOS2 for an inherited glaucoma, in this study we both genetically and pharmacologically decreased NOS2 activity in the DBA/2J mouse glaucoma model.

Methods

The expression of Nos2 in the optic nerve head was analyzed at both the RNA and protein levels at different stages of disease pathogenesis. To test the involvement of Nos2 in glaucomatous neurodegeneration, a null allele of Nos2 was backcrossed into DBA/2J mice and the incidence and severity of glaucoma was assessed in mice of each Nos2 genotype. Additionally, DBA/2J mice were treated with the NOS2 inhibitor aminoguanidine and the disease compared to untreated mice.

Results

Optic nerve head Nos2 RNA levels varied and increased during moderate but decreased at early and severe stages of disease. Despite the presence of a few NOS2 positive cells in the optic nerve head, NOS2 protein was not substantially increased during the glaucoma. Genetic deficiency of Nos2 or aminoguanidine treatment did not alter the IOP profile of DBA/2J mice. Additionally, neither Nos2 deficiency nor aminoguanidine had any detectable affect on the glaucomatous optic nerve damage.

Conclusion

Glaucomatous neurodegeneration in DBA/2J mice does not require NOS2 activity. Further experiments involving various models are needed to assess the general importance of Nos2 in glaucoma.

NCX-4016, a nitro-derivative of aspirin, inhibits EGFR and STAT3 signaling and modulates Bcl-2 proteins in cisplatin-resistant human ovarian cancer cells and xenografts

We have previously reported the inhibitory effect of NCX-4016, a nitro derivative of aspirin, on the proliferation of cisplatin-resistant human ovarian cancer cells, in vitro (Bratasz et al., Proc Natl Acad Sci USA 2006; 103:3914-9). In this report we present the results of our study on the mechanistic aspects of drug action including the molecular and signaling pathways involved in an in vitro cell line, as well as in a murine tumor xenograft. We report, for the first time, that NCX-4016 significantly inhibited the growth of cisplatin-resistant human ovarian cancer xenografts in mice. We observed that the inhibitory effect of NCX-4016 on cell proliferation was associated with G(1) phase cell cycle arrest with increased activity of p53, p21 and p27 proteins. NCX-4016 modulated the Bcl-2 family of proteins, and induced apoptosis by activating Bax and cytochrome c release in a time-dependent manner. In addition, NCX-4016 selectively down-regulated the phosphorylated forms of EGFR (Tyr845, Tyr992), pAkt (Ser473, Thr305), and STAT3 (Tyr705, Ser727), in vitro and in vivo. Taken together, the results clearly suggested that NCX-4016 causes significant induction of cell cycle arrest and apoptosis in cisplatin-resistant human ovarian cancer cells via down-regulation of EGFR/PI3K/STAT3 signaling and modulation of Bcl-2 family proteins. Thus, NCX-4016 appears to be a potential therapeutic agent for treating recurrent human ovarian carcinoma.

Pressure induces loss of gap junction communication and redistribution of connexin 43 in astrocytes

Astrocytes, the major glia in the nonmyelinated optic nerve head (ONH), connect via gap junctions built of connexin-43 (Cx43) to form a functional syncytium allowing communication and control of ionic and metabolic homeostasis of retinal ganglion cells (RGCs) axon. We examined gap junction intercellular communication (GJIC) by scrape loading assays in human ONH astrocytes exposed to hydrostatic (HP) or ambient pressure (CP) in vitro. Immunostaining, immunoprecipitation, and immunoblots were used to detect Cx43 distribution and phosphorylation in astrocytes exposed to HP with/without EGF receptor (EGFR) tyrosine kinase inhibitors AG1478 and AG82 and MAPK inhibitors U0126, PD98059, and SB203580. The data indicates that upon exposure to HP, astrocytes decrease GJIC and exhibit altered cellular localization and phosphorylation of Cx43. Inhibition of EGFR blocked the effects of HP on GJIC and HP-induced Cx43 tyrosine phosphorylation. Inhibitors of MAPK- ERK1/2 and -p38 caused partial closure of GJIC under CP and HP, which was maintained for 6 h. Inhibition of Big Mitogen-Activated Kinase 1/ERK5 (BMK1/ERK5) caused partial closure under CP and HP followed by full recovery after 6 h. Inhibition of MAPK did not affect the HP-induced increase in Cx43 serine 279/282 phosphorylation. We conclude that activation of the EGFR pathway in response to HP leads to decrease of GJIC via tyrosine phosphorylation of Cx43 in ONH astrocytes. In glaucoma under conditions of elevated intraocular pressure (IOP), astrocytes may lose GJIC altering the homeostasis of RGC axons, adopting the reactive phenotype, contributing to glaucomatous neuropathy.

Inhibiting epidermal growth factor receptor improves structural, locomotor, sensory, and bladder recovery from experimental spinal cord injury

Lack of axon regeneration in the adult CNS has been attributed partly to myelin inhibitors and the properties of astrocytes. After spinal cord injury, proliferating astrocytes not only represent a physical barrier to regenerating axons but also express and secrete molecules that inhibit nerve growth, including chondroitin sulfate proteoglycans (CSPGs). Epidermal growth factor receptor (EGFR) activation triggers astrocytes into becoming reactive astrocytes, and EGFR ligands stimulate the secretion of CSPGs as well as the formation of cribriform astrocyte arrangements that contribute to the formation of glial scars. Recently, it was shown that EGFR inhibitors promote nerve regeneration in vitro and in vivo. Blocking a novel Nogo receptor interacting mechanism and/or effects of EGFR inhibition on astrocytes may underlie these effects. Here we show that rats subjected to weight-drop spinal cord injury can be effectively treated by direct delivery of a potent EGFR inhibitor to the injured area, leading to significantly better functional and structural outcome. Motor and sensory functions are improved and bladder function is restored. The robust effects and the fact that other EGFR inhibitors are in clinical use in cancer treatments make these drugs particularly attractive candidates for clinical trials in spinal cord injury.

Activation of epidermal growth factor receptors in astrocytes: From development to neural injury

The epidermal growth factor receptor (EGFR) pathway controls the phenotypic characteristics of astrocytes. In the developing central nervous system (CNS), activation of the EGFR pathway induces astrocyte differentiation, forming the cribriform structure that surrounds axons and providing a supportive environment for neurons. In the adult CNS, the EGFR pathway is absent from astrocytes but is highly up-regulated and activated following neuronal injury. Activation of the EGFR pathway triggers quiescent astrocytes to become reactive astrocytes. Although astrocytes regulated by the EGFR pathway play constructive roles in the developing CNS, astrocytes that become reactive in response to activation of the EGFR pathway appear to be destructive to neurons in the adult CNS. The reappearance and activation of EGFRs in astrocytes under pathological conditions may activate a developmental process in an adult tissue. Regulation of EGFR function in astrocytes may be a new therapeutic strategy for the treatment of neural disorders.

Epidermal growth factor receptor activation: an upstream signal for transition of quiescent astrocytes into reactive astrocytes after neural injury

Modulating the behaviors of reactive astrocytes is a potential therapeutic strategy for neurodegenerative diseases. We found that upregulation and activation of the epidermal growth factor receptor (EGFR) occur in astrocytes after different injuries in optic nerves in vivo. Activation of EGFR regulates genes and cellular processes representing most major markers of reactive astrocytes and genes related with glaucomatous optic neuropathy and other neural disorders. These results suggest that activation of EGFR is a common, regulatory pathway that triggers quiescent astrocytes into reactive astrocytes in response to neural injuries in the optic nerve, and perhaps other parts of the CNS. Targeting EGFR activation using an EGFR tyrosine kinase inhibitor prevents the loss of retinal ganglion cells in a model of glaucomatous optic neuropathy. Because these inhibitors are currently used clinically, our results present an approach to reactive astrocytes as a potential new target for the treatment of neurodegenerations.

Nuclear interaction of EGFR and STAT3 in the activation of the iNOS/NO pathway

Epidermal growth factor receptor (EGFR) exists in the nucleus of highly proliferative cells where it functions as a transcription factor. Although EGFR has transactivational activity, it lacks a DNA binding domain and, therefore, may require a DNA binding transcription cofactor for its transcriptional function. Here, we report that EGFR physically interacts with signal transducers and activators of transcription 3 (STAT3) in the nucleus, leading to transcriptional activation of inducible nitric oxide synthase (iNOS). In breast carcinomas, nuclear EGFR positively correlates with iNOS. This study describes a mode of transcriptional control involving cooperated efforts of STAT3 and nuclear EGFR. Our work suggests that the deregulated iNOS/NO pathway may partly contribute to the malignant biology of tumor cells with high levels of nuclear EGFR and STAT3.

Activation of epidermal growth factor receptor causes astrocytes to form cribriform structures

Epidermal growth factor receptor (EGFR) is expressed in reactive astrocytes following injury in the CNS. However, the effects of activation of the EGFR pathway in astrocytes are not well established. In the present study, we demonstrate that activation of EGFR causes optic nerve astrocytes, as well as brain astrocytes, to form cribriform structures with cavernous spaces. Formation of the cribriform structures is dependent on new protein synthesis and cell proliferation. Platelet-derived growth factor and basic fibroblast growth factor were not effective. Smooth muscle cells and epithelial cells do not form cribriform structures in response to EGFR activation. The formation of the cribriform structures appears to be related to a guided migration of astrocytes and the expression of integrin beta1 and extracellular fibronectin in response to activation of EGFR. The EGFR pathway may be a specific, signal transduction pathway that regulates reactive astrocytes to form cavernous spaces in the glial scars following CNS injury and in the compressed optic nerve in glaucomatous optic nerve neuropathy.

Comparison of the signal transduction pathways for the induction of gene expression of nitric oxide synthase-2 in response to two different stimuli

Human optic nerve astrocytes induce nitric oxide synthase-2 (NOS-2) in vitro in response to cytokines (interferon-gamma/interleukin-1beta) and elevated hydrostatic pressure. Using relatively specific inhibitors, we have compared induction of NOS-2 in response to these two stimuli to determine whether the same or different signal transduction pathways participate in the responses. Using SN50 and CAGE, which inhibit the NFkappaB pathway, the induction of NOS-2 in response to both cytokines and elevated hydrostatic pressure was blocked. Using SB202190 and SB203580, which inhibit p38 mitogen-activated protein kinase, only the response to cytokines was blocked. In contrast, when inhibitors of epidermal growth factor receptor tyrosine kinase AG 82 and AG 18 were used, the induction of NOS-2 in response to pressure, but not in response to cytokines, was blocked. Signal transduction pathways presumably regulate the synthesis of NOS-2 through downstream events that induce transcription of the NOS-2 gene. Our data suggest that activation of different sites in the promoter region of the NOS-2 gene is needed for these different stimuli to induce NOS-2.