Phosphorylation of extracellular signal-regulated kinase and p27(KIP1) after retinal detachment

Heterogeneity in quiescent Müller glia in the uninjured zebrafish retina drive differential responses following photoreceptor ablation

Introduction

Loss of neurons in the neural retina is a leading cause of vision loss. While humans do not possess the capacity for retinal regeneration, zebrafish can achieve this through activation of resident Müller glia. Remarkably, despite the presence of Müller glia in humans and other mammalian vertebrates, these cells lack an intrinsic ability to contribute to regeneration. Upon activation, zebrafish Müller glia can adopt a stem cell-like state, undergo proliferation and generate new neurons. However, the underlying molecular mechanisms of this activation subsequent retinal regeneration remains unclear.

Methods/Results

To address this, we performed single-cell RNA sequencing (scRNA-seq) and report remarkable heterogeneity in gene expression within quiescent Müller glia across distinct dorsal, central and ventral retina pools of such cells. Next, we utilized a genetically driven, chemically inducible nitroreductase approach to study Müller glia activation following selective ablation of three distinct photoreceptor subtypes: long wavelength sensitive cones, short wavelength sensitive cones, and rods. There, our data revealed that a region-specific bias in activation of Müller glia exists in the zebrafish retina, and this is independent of the distribution of the ablated cell type across retinal regions. Notably, gene ontology analysis revealed that injury-responsive dorsal and central Müller glia express genes related to dorsal/ventral pattern formation, growth factor activity, and regulation of developmental process. Through scRNA-seq analysis, we identify a shared genetic program underlying initial Müller glia activation and cell cycle entry, followed by differences that drive the fate of regenerating neurons. We observed an initial expression of AP-1 and injury-responsive transcription factors, followed by genes involved in Notch signaling, ribosome biogenesis and gliogenesis, and finally expression of cell cycle, chromatin remodeling and microtubule-associated genes.

Discussion

Taken together, our findings document the regional specificity of gene expression within quiescent Müller glia and demonstrate unique Müller glia activation and regeneration features following neural ablation. These findings will improve our understanding of the molecular pathways relevant to neural regeneration in the retina.

Molecular pathogenesis of rhegmatogenous retinal detachment

Rhegmatogenous retinal detachment (RRD) is an ophthalmic emergency, which usually requires prompt surgery to prevent further detachment and restore sensory function. Although several individual factors have been suggested, a systems level understanding of molecular pathomechanisms underlying this severe eye disorder is lacking. To address this gap in knowledge we performed the molecular level systems pathology analysis of the vitreous from 127 patients with RRD using state-of-the art quantitative mass spectrometry to identify the individual key proteins, as well as the biochemical pathways contributing to the development of the disease. RRD patients have specific vitreous proteome profiles compared to other diseases such as macular hole, pucker, or proliferative diabetic retinopathy eyes. Our data indicate that various mechanisms, including glycolysis, photoreceptor death, and Wnt and MAPK signaling, are activated during or after the RRD to promote retinal cell survival. In addition, platelet-mediated wound healing processes, cell adhesion molecules reorganization and apoptotic processes were detected during RRD progression or proliferative vitreoretinopathy formation. These findings improve the understanding of RRD pathogenesis, identify novel targets for treatment of this ophthalmic disease, and possibly affect the prognosis of eyes treated or operated upon due to RRD.

Reprogramming Müller Glia to Regenerate Retinal Neurons

In humans, various genetic defects or age-related diseases, such as diabetic retinopathies, glaucoma, and macular degeneration, cause the death of retinal neurons and profound vision loss. One approach to treating these diseases is to utilize stem and progenitor cells to replace neurons in situ, with the expectation that new neurons will create new synaptic circuits or integrate into existing ones. Reprogramming non-neuronal cells in vivo into stem or progenitor cells is one strategy for replacing lost neurons. Zebrafish have become a valuable model for investigating cellular reprogramming and retinal regeneration. This review summarizes our current knowledge regarding spontaneous reprogramming of Müller glia in zebrafish and compares this knowledge to research efforts directed toward reprogramming Müller glia in mammals. Intensive research using these animal models has revealed shared molecular mechanisms that make Müller glia attractive targets for cellular reprogramming and highlighted the potential for curing degenerative retinal diseases from intrinsic cellular sources.

Oral Selumetinib Does Not Negatively Impact Photoreceptor Survival in Murine Experimental Retinal Detachment

Purpose

Mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling is neuroprotective in some retinal damage models but its role in neuronal survival during retinal detachment (RD) is unclear. In addition, serous RDs are a prevalent side effect of MEK inhibitors (MEKi), blocking MAPK/ERK signaling for treatment of certain cancers. We tested the hypothesis that MEKi treatment in experimental RD would increase photoreceptor death.

Methods

The MEKi selumetinib was delivered daily to C57BL/6 mice at a clinically relevant dose (10 mg/mL) starting 1 day prior to creating RD with subretinal hyaluronic acid injection. Photoreceptor TUNEL and outer nuclear layer (ONL) thickness were analyzed. Phospho-ERK1/2 (pERK) distribution, glial fibrillary acidic protein (GFAP) accumulation, and Iba-1 (microglia/macrophages) were evaluated with immunofluorescence.

Results

pERK accumulated in the Müller glia in detached retinas, but this was effectively blocked by selumetinib. Selumetinib did not induce serous RDs at day 1 and did not increase TUNEL positive photoreceptors or further decrease ONL thickness compared to controls. Retinal gliosis was not altered, but selumetinib did block the increase in intraretinal microglia/macrophage Iba-1 fluorescence intensity and acquisition of amoeboid morphology.

Conclusions

MAPK/ERK is neuroprotective in some retinal damage models; in RD, selumetinib blocked Müller pERK accumulation and changed the retinal microglia/macrophage phenotype but did not alter photoreceptor survival. This is consistent with the relatively good visual acuity seen in patients developing transient retinal detachments on MEK inhibitor therapy. Compensation by other neuroprotective pathways in the retina during retinal detachment may occur in the presence of MEK inhibition.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

Emerging role of MAP kinase pathways as therapeutic targets in COPD

Studies examining the cellular mechanisms of inflammation and protease production in the lung tissue and airways of COPD patients have shed light on the important role of kinase-based signaling cascades. These pathways can be activated by environmental stimuli such as tobacco smoke, and by endogenous signals such as cytokines, growth factors, and inflammation-derived oxidants. The three most widely characterized cascades are those directed by the classical mitogen activated protein (MAP) kinase (ERK1/2), stress activated protein kinase/c-Jun N-terminal protein kinase, and p38 enzymes. These phosphorylation cascades transmit and amplify extracellular, receptor-mediated signals through the cytoplasm of the cell to activate nuclear transcription factors which bind and induce expression of target genes. The result is tight control of diverse cellular events, and rapid responses to external stimuli. However, recent research suggests that constitutive or aberrant activation of MAP kinases contributes to several COPD-associated phenotypes, including mucus overproduction and secretion, inflammation, cytokine expression, apoptosis, T cell activation, matrix metalloproteinase production, and fibrosis. This review explores the biological functions of the MAP kinase pathways in the pathogenesis of COPD, their activation by cigarette smoke, and discusses the potential role of MAP kinase inhibitors in COPD therapy.

The activation of MEK-ERK1/2 by glutamate receptor-stimulation is involved in the regulation of RPE proliferation and morphologic transformation

Retinal pigment epithelial (RPE) cells are the main cell type involved in the pathogenesis of proliferative vitreoretinopathy (PVR). As a result from retinal detachment or surgical procedures, RPE comes in contact with glutamate from serum, glial release and the injured retina. The purpose of this study was to explore a possible role for glutamate in the development of PVR, mediated by the receptor-stimulated activation of the ERK1/2 MAPK pathway, the alteration of cell proliferation and the transdifferentiation of RPE cells, using rat RPE cells in culture as a model system. We demonstrated the expression in these cells of Group I metabotropic-and ionotropic AMPA/KA and NMDA glutamate receptors (GluRs), predominantly of the NMDA subtype, which are targeted to the membrane, and exhibit pharmacological and biochemical characteristics equivalent to those previously established in brain tissue. Proliferation was measured by MTS-reduction colorimetric assay, and actin cytoskeleton dynamics was visualized by immunoflurescence using alpha-sma specific antibodies. Activation of metabotropic, AMPA and NMDA receptors by glutamate induced the time-and dose-dependent phosphorylation of ERK1/2, assessed by Western blot analysis, in parallel to a significant increase in cell proliferation and a decrease in alpha-sma expression and its recruitment into stress fibers. These effects were all prevented by the inhibition of MEK. Hence, results suggest that glutamate could be involved in the generation of PVR, through a GluR-mediated increase in proliferation and phenotypic transformation, cause-effect related to the activation of ERK1/2.

ERK1 plays a critical protective role againstN-methyl-D-aspartate-induced retinal injury

Excitotoxicity has been implicated in several ischemic diseases of the retina, including retinal vessel occlusion and diabetic retinopathy. Glutamate signaling mediated through the N-methyl-D-aspartate (NMDA) receptor contributes to ischemic cell death. The NMDA receptor antagonists MK-801 and memantine have substantial neuroprotective effects in experimental retinal disease models, but the mechanisms by which NMDA receptor activity leads to cell death is not clear. Here we describe a previously unknown role for retinal glial cells in NMDA-induced retinal injury that involves the activation of ERK1/2. Within 1 hr after injecting NMDA intravitreally, activation of ERK1/2 and c-Fos induction were observed in retinal Müller cells. The roles of activated ERK1/2 in neuronal damage were examined using ERK1 gene deficient mice (homozygous ERK1(-/-) mice). NMDA-induced ERK1/2 activation in retina was significantly suppressed in ERK1(-/-) mice, and these mice had significantly higher numbers of TUNEL-positive retinal cells than wild-type mice 24 hr after NMDA injection. These data suggest that, during NMDA injury, Müller cells are activated and play a protective role against NMDA-induced retinal cell death. ERK1 appears to play a major role in this process. These new findings on retinal glial cell response during NMDA injury offer an important new therapeutic target for preventing many retinal disorders associated with excitotoxicity.

Expression of p27(KIP1) and cyclin D1, and cell proliferation in human pterygium

BACKGROUND

The pterygium is a growth onto the cornea of fibrovascular tissue that is continuous with the conjunctiva, whereas the mechanisms of cell proliferation in pterygium epithelium are unknown.

AIM

To analyse the histopathology and the expression of cell cycle-related molecules in pterygium tissues.

METHODS

Seven pterygia were surgically removed using the bare-sclera procedure, and three normal bulbar conjunctivas were also obtained. Formalin-fixed, paraffin-wax-embedded tissues were analysed by immunohistochemistry with anti-p27(KIP1), cyclin D1 and Ki-67 antibodies.

RESULTS

Conjunctival epithelium consisted of several layers of round cells with a few goblet cells. Nuclear immunoreactivity for p27(KIP1) was noted in many normal epithelial cells, where cyclin D1 and Ki-67-positive nuclei were intermingled. A variety of goblet cells were located in the superficial layer of the pterygium head as well as those of the body epithelia. Several pterygium epithelial cells were p27(KIP1) positive, whereas nuclear immunoreactivity for cyclin D1 and Ki-67 was detected in many epithelial cells. By contrast, immunoreactivity for p27(KIP1), cyclin D1 and Ki-67 was hardly detected in the pterygium stroma.

CONCLUSION

It is suggested that pterygium growth and development are associated with the proliferation of epithelium, which is possibly involved in the expression of cell cycle-related molecules.

Induction of intrahepatic cholangiocellular carcinoma by liver-specific disruption of Smad4 and Pten in mice

Cholangiocellular carcinoma (CC), the second most common primary liver cancer, is associated with a poor prognosis. It has been shown that CCs harbor alterations of a number of tumor-suppressor genes and oncogenes, yet key regulators for tumorigenesis remain unknown. Here we have generated a mouse model that develops CC with high penetrance using liver-specific targeted disruption of tumor suppressors SMAD4 and PTEN. In the absence of SMAD4 and PTEN, hyperplastic foci emerge exclusively from bile ducts of mutant mice at 2 months of age and continue to grow, leading to tumor formation in all animals at 4-7 months of age. We show that CC formation follows a multistep progression of histopathological changes that are associated with significant alterations, including increased levels of phosphorylated AKT, FOXO1, GSK-3beta, mTOR, and ERK and increased nuclear levels of cyclin D1. We further demonstrate that SMAD4 and PTEN regulate each other through a novel feedback mechanism to maintain an expression balance and synergistically repress CC formation. Finally, our analysis of human CC detected PTEN inactivation in a majority of p-AKT-positive CCs, while about half also lost SMAD4 expression. These findings elucidate the relationship between SMAD4 and PTEN and extend our understanding of CC formation.

Expression of glutamine synthetase and cell proliferation in human idiopathic epiretinal membrane

BACKGROUND/AIM

The mechanisms of the cellular origin and cell proliferation in the idiopathic epiretinal membrane (ERM) are unsolved. The aim of this study was to examine the expression of cell cycle related molecules and glutamine synthetase (GS), which is expressed in Müller cells and their processes, in ERM tissues.

METHODS

The ERMs were surgically removed using pars plana vitrectomy. Formalin fixed, paraffin embedded ERM tissues were analysed by immunohistochemistry with anti-cyclin D1, p27 (KIP1), proliferating cell nuclear antigen (PCNA), and GS antibodies.

RESULTS

The histopathological findings showed that all the ERMs consisted of oval or spindle mononuclear cells with thin collagen-like tissues. Immunoreactivity for GS was detected in collagen-like tissues of ERM, presenting a continuous, isodense pattern. GS immunopositive cells in all cases expressed PCNA in their nuclei. Nuclear immunoreactivity for cyclin D1 was noted in the ERM constituent cells, whereas p27 (KIP1) positive nuclei were not detected.

CONCLUSION

Cyclin D1 and PCNA were expressed in the idiopathic ERM, which was mainly derived from Müller cells and extensions of their processes.