Distribution of p27(KIP1), cyclin D1, and proliferating cell nuclear antigen 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.

Cellular Signaling in Müller Glia: Progenitor Cells for Regenerative and Neuroprotective Responses in Pharmacological Models of Retinal Degeneration

Retinal degenerative diseases are a leading cause of visual impairment or blindness. There are many therapies for delaying the progression of vision loss but no curative strategies currently. Stimulating intrinsic neuronal regeneration is a potential approach to therapy in retinal degenerative diseases. In contrast to stem cells, as embryonic/pluripotent stem cell-derived retinal progenitor cell or mesenchymal stem cells, Müller glia provided an endogenous cellular source for regenerative therapy in the retina. Müller glia are a major component of the retina and considerable evidence suggested these cells can be induced to produce the lost neurons in several species. Understanding the specific characteristic of Müller glia to generate lost neurons will inspire an attractive and alternative therapeutic strategy for treating visual impairment with regenerative research. This review briefly provides the different signal transduction mechanisms which are underlying Müller cell-mediated neuroprotection and neuron regeneration and discusses recent advances about regeneration from Müller glia-derived progenitors.

Proliferative reactive gliosis is compatible with glial metabolic support and neuronal function

Background

The response of mammalian glial cells to chronic degeneration and trauma is hypothesized to be incompatible with support of neuronal function in the central nervous system (CNS) and retina. To test this hypothesis, we developed an inducible model of proliferative reactive gliosis in the absence of degenerative stimuli by genetically inactivating the cyclin-dependent kinase inhibitor p27^Kip1 (p27 or Cdkn1b) in the adult mouse and determined the outcome on retinal structure and function.

Results

p27-deficient Müller glia reentered the cell cycle, underwent aberrant migration, and enhanced their expression of intermediate filament proteins, all of which are characteristics of Müller glia in a reactive state. Surprisingly, neuroglial interactions, retinal electrophysiology, and visual acuity were normal.

Conclusion

The benign outcome of proliferative reactive Müller gliosis suggests that reactive glia display context-dependent, graded and dynamic phenotypes and that reactivity in itself is not necessarily detrimental to neuronal function.

Accelerated turnover of taste bud cells in mice deficient for the cyclin-dependent kinase inhibitor p27Kip1

Background

Mammalian taste buds contain several specialized cell types that coordinately respond to tastants and communicate with sensory nerves. While it has long been appreciated that these cells undergo continual turnover, little is known concerning how adequate numbers of cells are generated and maintained. The cyclin-dependent kinase inhibitor p27^Kip1 has been shown to influence cell number in several developing tissues, by coordinating cell cycle exit during cell differentiation. Here, we investigated its involvement in the control of taste cell replacement by examining adult mice with targeted ablation of the p27^Kip1 gene.

Results

Histological and morphometric analyses of fungiform and circumvallate taste buds reveal no structural differences between wild-type and p27^Kip1-null mice. However, when examined in functional assays, mutants show substantial proliferative changes. In BrdU incorporation experiments, more S-phase-labeled precursors appear within circumvallate taste buds at 1 day post-injection, the earliest time point examined. After 1 week, twice as many labeled intragemmal cells are present, but numbers return to wild-type levels by 2 weeks. Mutant taste buds also contain more TUNEL-labeled cells and 50% more apoptotic bodies than wild-type controls. In normal mice, p27 ^Kip1 is evident in a subset of receptor and presynaptic taste cells beginning about 3 days post-injection, correlating with the onset of taste cell maturation. Loss of gene function, however, does not alter the proportions of distinct immunohistochemically-identified cell types.

Conclusions

p27^Kip1 participates in taste cell replacement by regulating the number of precursor cells available for entry into taste buds. This is consistent with a role for the protein in timing cell cycle withdrawal in progenitor cells. The equivalence of mutant and wild-type taste buds with regard to cell number, cell types and general structure contrasts with the hyperplasia and tissue disruption seen in certain developing p27^Kip1-null sensory organs, and may reflect a compensatory capability inherent in the regenerative taste system.

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.

Mitf functions as an in ovo regulator for cell differentiation and proliferation during development of the chick RPE

Mitf has been reported to play a crucial role in regulating the differentiation of pigment cells in homeothermal animals, i.e. the melanocytes and the retinal pigment epithelium (RPE). However, less is known about the functions of Mitf in the developing RPE. To elucidate such functions, we introduced wild-type and dominant-negative Mitf expression vectors into chick optic vesicles by electroporation. Over-expression of wild-type Mitf altered neural retina cells to become RPE-like and repressed the expression of neural retina markers in vivo. In contrast, dominant-negative Mitf inhibited pigmentation in the RPE. The percentage of BrdU-positive cells decreased during normal RPE development, which was followed by Mitf protein expression. The percentage of BrdU-positive cells decreased in the wild-type Mitf-transfected neural retina, but increased in the dominant-negative Mitf-transfected RPE. p27(kip1), one of the cyclin-dependent kinase inhibitors, begins to be expressed in the proximal region of the RPE at stage 16. Transfection of wild-type Mitf induced expression of p27(kip1), while transfection of dominant-negative Mitf inhibited p27(kip1) expression. We found that Mitf was associated with the endogenous p27(kip1) 5' flanking region. These results demonstrate for the first time "in vivo" that Mitf uniquely regulates both differentiation and cell proliferation in the developing RPE.

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.

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.

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

PURPOSE

The roles of the extracellular signal-regulated kinase (ERK) pathway in the expression of cyclin D1 and p27(KIP1), the phosphorylation of p27(KIP1), and proliferation activity were examined after retinal detachment.

METHODS

Normal eyes and eyes at 15 min, 2 and 4 days after retinal detachment in C57Bl6 mice were examined by immunohistochemistry using anti-phosphorylated (p) ERK1/2, anti-cyclin D1, anti-p27(KIP1), anti-p27(KIP1) phosphorylated at serine 10 (S10-phospho-p27), and anti-proliferating cell nuclear antigen (PCNA) antibodies with or without treatment with a specific ERK inhibitor, PD98059. Mouse Müller cells were isolated and examined for alteration of p27(KIP1) and cyclin D1 after exposure of basic fibroblast growth factor (bFGF) with and without treatment of PD98059 using Western blotting.

RESULTS

In the normal retina, nuclear immunoreactivity for p27(KIP1), but not S10-phospho-p27 or pERK1/2, was observed in the middle sublayer of the inner nuclear layer (INL), where Müller glial cells are situated. At 15 min after the retinal detachment, p27(KIP1), S10-phospho-p27 and pERK1/2-positive nuclei were noted in the INL, whereas immunoreactivity for pERK1/2 or S10-phospho-p27 was not observed after treatment with PD98095. Cyclin D1 was induced in the INL 2 days after the retinal detachment, and the induction was inhibited by PD98059. At 4 days after the detachment, p27(KIP1) immunoreactivity was not observed, and cyclin D1 and PCNA were expressed. The disappearance of p27(KIP1) was suppressed, whereas expression of cyclin D1 and PCNA was not observed in mice treated with PD98059. Exposure of bFGF relatively decreased the expression level of p27(KIP1) and increased the level of cyclin D1 in mouse Müller cells, compared with control level. Induction of cyclin D1 and decrease in p27(KIP1) were inhibited with treatment of PD98059.

CONCLUSION

Phosphorylation of ERK and expression of p27(KIP1) and cyclin D1 are involved in the proliferation of Müller cells after retinal detachment.

Induction of nestin, Ki-67, and cyclin D1 expression in Müller cells after laser injury in adult rat retina

PURPOSE

The purpose was to examine the expression of nestin, Ki-67, and cyclin D1 in Müller cells after laser injury in adult rat retina.

METHODS

The right eyes of adult Brown Norway rats were treated with laser photocoagulation. The eyes were removed 3, 7, and 14 days after laser treatment. The retinas were investigated immunocytochemically by confocal microscopy. Agarose-embedded sections were immunostained with antibodies to nestin, vimentin, glial fibrillary acidic protein (GFAP), glutamate-aspartate transporter (GLAST), rhodopsin, Ki-67, and cyclin D1. Cell death was examined using terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling (TUNEL) assay on agarose sections.

RESULTS

Nestin expression was induced in Müller cells following laser injury. In addition, Ki-67 and cyclin D1 expression was found in the nuclei of Müller cells after the treatment. TUNEL assay demonstrated that Müller cells were not labeled; hence these cells were not apoptotic.

CONCLUSIONS

These results suggest that dedifferentiation and proliferation of Müller cells can be induced by laser injury in adult rat retina.