RBP-Jκ-dependent Notch signaling enhances retinal pigment epithelial cell proliferation in transgenic mice

Melanocyte stem cells in the skin: Origin, biological characteristics, homeostatic maintenance and therapeutic potential

Melanocyte stem cells (MSCs), melanocyte lineage-specific skin stem cells derived from the neural crest, are observed in the mammalian hair follicle, the epidermis or the sweat gland. MSCs differentiate into mature melanin-producing melanocytes, which confer skin and hair pigmentation and uphold vital skin functions. In controlling and coordinating the homeostasis, repair and regeneration of skin tissue, MSCs play a vital role. Decreased numbers or impaired functions of MSCs are closely associated with the development and therapy of many skin conditions, such as hair graying, vitiligo, wound healing and melanoma. With the advancement of stem cell technology, the relevant features of MSCs have been further elaborated. In this review, we provide an exhaustive overview of cutaneous MSCs and highlight the latest advances in MSC research. A better understanding of the biological characteristics and micro-environmental regulatory mechanisms of MSCs will help to improve clinical applications in regenerative medicine, skin pigmentation disorders and cancer therapy. KEY POINTS: This review provides a concise summary of the origin, biological characteristics, homeostatic maintenance and therapeutic potential of cutaneous MSCs. The role and potential application value of MSCs in skin pigmentation disorders are discussed. The significance of single-cell RNA sequencing, CRISPR-Cas9 technology and practical models in MSCs research is highlighted.

Zic5 stabilizes Gli3 via a non-transcriptional mechanism during retinal development

The Zic family of zinc finger transcription factors plays a critical role in multiple developmental processes. Using loss-of-function studies, we find that Zic5 is important for the differentiation of retinal pigmented epithelium (RPE) and the rod photoreceptor layer through suppressing Hedgehog (Hh) signaling. Further, Zic5 interacts with the critical Hh signaling molecule, Gli3, through the zinc finger domains of both proteins. This Zic5-Gli3 interaction disrupts Gli3/Gli3 homodimerization, resulting in Gli3 protein stabilization via a reduction in Gli3 ubiquitination. During embryonic Hh signaling, the activator form of Gli is normally converted to a repressor form through proteosome-mediated processing of Gli3, and the ratio of Gli3 repressor to full-length (activator) form of Gli3 determines the Gli3 repressor output required for normal eye development. Our results suggest Zic5 is a critical player in regulating Gli3 stability for the proper differentiation of RPE and rod photoreceptor layer during Xenopus eye development.

Notch-IGF1 signaling during liver regeneration drives biliary epithelial cell expansion and inhibits hepatocyte differentiation

In the adult liver, a population of facultative progenitor cells called biliary epithelial cells (BECs) proliferate and differentiate into cholangiocytes and hepatocytes after injury, thereby restoring liver function. In mammalian models of chronic liver injury, Notch signaling is essential for bile duct formation from these cells. However, the continual proliferation of BECs and differentiation of hepatocytes in these models have limited their use for determining whether Notch signaling is required for BECs to replenish hepatocytes after injury in the mammalian liver. Here, we used a temporally restricted model of hepatic repair in which large-scale hepatocyte injury and regeneration are initiated through the acute loss of Mdm2 in hepatocytes, resulting in the rapid, coordinated proliferation of BECs. We found that transient, early activation of Notch1- and Notch3-mediated signaling and entrance into the cell cycle preceded the phenotypic expansion of BECs into hepatocytes. Notch inhibition reduced BEC proliferation, which resulted in failure of BECs to differentiate into hepatocytes, indicating that Notch-dependent expansion of BECs is essential for hepatocyte regeneration. Notch signaling increased the abundance of the insulin-like growth factor 1 receptor (IGF1R) in BECs, and activating IGFR signaling increased BEC numbers but suppressed BEC differentiation into hepatocytes. These results suggest that different signaling mechanisms control BEC expansion and hepatocyte differentiation.

Nf2 fine-tunes proliferation and tissue alignment during closure of the optic fissure in the embryonic mouse eye

Uveal coloboma represents one of the most common congenital ocular malformations accounting for up to 10% of childhood blindness (~1 in 5000 live birth). Coloboma originates from defective fusion of the optic fissure (OF), a transient gap that forms during eye morphogenesis by asymmetric, ventral invagination. Genetic heterogeneity combined with the activity of developmentally regulated genes suggests multiple mechanisms regulating OF closure. The tumor suppressor and FERM domain protein Neurofibromin 2 (NF2) controls diverse processes in cancer, development and regeneration, via Hippo pathway and cytoskeleton regulation. In humans, NF2 mutations can cause ocular abnormalities, including coloboma, however, its actual role in OF closure is unknown. Using conditional inactivation in the embryonic mouse eye, our data indicate that loss of Nf2 function results in a novel underlying cause for coloboma. In particular, mutant eyes show substantially increased retinal pigmented epithelium (RPE) proliferation in the fissure region with concomitant acquisition of RPE cell fate. Cells lining the OF margin can maintain RPE fate ectopically and fail to transition from neuroepithelial to cuboidal shape. In the dorsal RPE of the optic cup, Nf2 inactivation leads to a robust increase in cell number, with local disorganization of the cytoskeleton components F-actin and pMLC2. We propose that RPE hyperproliferation is the primary cause for the observed defects causing insufficient alignment of the OF margins in Nf2 mutants and failure to fuse properly, resulting in persistent coloboma. Our findings indicate that limiting proliferation particularly in the RPE layer is a critical mechanism during OF closure.

Detrimental Effects of UVB on Retinal Pigment Epithelial Cells and Its Role in Age-Related Macular Degeneration

Retinal pigment epithelial (RPE) cells are an essential part of the human eye because they not only mediate and control the transfer of fluids and solutes but also protect the retina against photooxidative damage and renew photoreceptor cells through phagocytosis. However, their function necessitates cumulative exposure to the sun resulting in UV damage, which may lead to the development of age-related macular degeneration (AMD). Several studies have shown that UVB induces direct DNA damage and oxidative stress in RPE cells by increasing ROS and dysregulating endogenous antioxidants. Activation of different signaling pathways connected to inflammation, cell cycle arrest, and intrinsic apoptosis was reported as well. Besides that, essential functions like phagocytosis, osmoregulation, and water permeability of RPE cells were also affected. Although the melanin within RPE cells can act as a photoprotectant, this photoprotection decreases with age. Nevertheless, the changes in lens epithelium-derived growth factor (LEDGF) and autophagic activity or application of bioactive compounds from natural products can reverse the detrimental effect of UVB. Additionally, in vivo studies on the whole retina demonstrated that UVB irradiation induces gene and protein level dysregulation, indicating cellular stress and aberrations in the chromosome level. Morphological changes like retinal depigmentation and drusen formation were noted as well which is similar to the etiology of AMD, suggesting the connection of UVB damage with AMD. Therefore, future studies, which include mechanism studies via in vitro or in vivo and other potential bioactive compounds, should be pursued for a better understanding of the involvement of UVB in AMD.

Role of Epithelial-Mesenchymal Transition in Retinal Pigment Epithelium Dysfunction

Retinal pigment epithelial (RPE) cells maintain the health and functional integrity of both photoreceptors and the choroidal vasculature. Loss of RPE differentiation has long been known to play a critical role in numerous retinal diseases, including inherited rod-cone degenerations, inherited macular degeneration, age-related macular degeneration, and proliferative vitreoretinopathy. Recent studies in post-mortem eyes have found upregulation of critical epithelial-mesenchymal transition (EMT) drivers such as TGF-β, Wnt, and Hippo. As RPE cells become less differentiated, they begin to exhibit the defining characteristics of mesenchymal cells, namely, the capacity to migrate and proliferate. A number of preclinical studies, including animal and cell culture experiments, also have shown that RPE cells undergo EMT. Taken together, these data suggest that RPE cells retain the reprogramming capacity to move along a continuum between polarized epithelial cells and mesenchymal cells. We propose that movement along this continuum toward a mesenchymal phenotype be defined as RPE Dysfunction. Potential mechanisms include impaired tight junctions, accumulation of misfolded proteins and dysregulation of several key pathways and molecules, such as TGF-β pathway, Wnt pathway, nicotinamide, microRNA 204/211 and extracellular vesicles. This review synthesizes the evidence implicating EMT of RPE cells in post-mortem eyes, animal studies, primary RPE, iPSC-RPE and ARPE-19 cell lines.

Activation of the Notch‑Nox4‑reactive oxygen species signaling pathway induces cell death in high glucose‑treated human retinal endothelial cells

Diabetic retinopathy (DR) occurs in almost all patients with diabetes and remains as one of the major causes of vision loss worldwide. Nevertheless, the molecular mechanisms underlying the pathogenesis of DR remain elusive. The present study aimed to investigate the role and association of Notch signaling and NADPH oxidase 4 (Nox4)‑mediated oxidative stress in high glucose (HG)‑treated retinal cells. Human retinal endothelial cells were cultured for various durations in RPMI‑1640 medium containing 30 mM glucose (HG) or 30 mM mannitol (MN) as an osmotic control; apoptotic cell death and reactive oxygen species (ROS) levels were assessed, respectively. Alterations in the expression profiles of Nox and Notch proteins were evaluated using reverse transcription‑quantitative polymerase chain reaction and western blot analysis. Knockdown of Nox4 and recombination signal‑binding protein J (RBPj) was generated by transfection with specific small interfering (siRNA). Persistent activation of Notch signaling was induced via the overexpression of Notch intracellular domain (NICD). In the present study, time‑dependent increases in ROS production and cell death were detected in HG‑treated cells. Depletion of ROS by diphenyleneiodonium decreased HG‑induced cell death, and suppressed increases in caspase 3 activity and B‑cell lymphoma 2‑associated X protein levels. In HG‑treated cells, Nox4 expression was upregulated at the mRNA and protein levels, and inhibition of Nox4 by GKT137831 or knockdown of expression by siRNA Nox4 significantly reduced ROS levels and cell death. In the presence of HG, Notch1 expression levels were elevated, and increased NICD abundance was detected in whole cell lysates and nuclear fractions. Additionally, HG‑induced cell death was decreased by treatment with γ‑secretase inhibitor (GSI), but increased via the overexpression of NICD. The application of GSI or knockdown of RBPj by siRNA RBPj prevented increases in Nox4 expression within HG‑treated cells. The findings of the present study demonstrated that Nox4‑mediated ROS serves an important role in HG‑induced retinal cell damage, in which the activation of Notch signaling may be responsible for Nox4 upregulation. Therefore, inhibition of Notch signaling or Nox4 expression may be considered as potential therapeutic targets in patients with DR.

Notch3/VEGF-A axis is involved in TAT-mediated proliferation of pulmonary artery smooth muscle cells: Implications for HIV-associated PAH

The incidence of pulmonary arterial hypertension (PAH) is a significant co-morbidity observed in HIV (+) individuals. Pulmonary artery smooth muscle cells (PASMCs)—key components of the arterial vessel wall that regulate vessel diameter, demonstrate increased proliferation and hypertrophy in the lungs of HIV infected individuals, underscoring the role of these cells in the pathogenesis of HIV-associated PAH. While several pathways have been implicated in enhanced proliferation of PASMCs, detailed molecular mechanism(s) underlying HIV-associated PASMC proliferation still remain elusive. In the current study, we sought to investigate the effects HIV protein transactivator of transcription (TAT)-mediated proliferation on PASMCs. In agreement with earlier findings, our results also demonstrated TAT-mediated proliferation of human PASMCs. We identified activation of a novel Notch3 signaling pathway in TAT-mediated proliferation of PASMCs. Further validation of the Notch 3 pathway was demonstrated using both pharmacological (γ-secretase inhibitor, DAPT), as well as genetic approaches (Notch3 siRNA). Vascular endothelial growth factor A (VEGF-A) was identified as a novel downstream molecule that was induced following Notch activation. Findings from in vitro studies were further validated in archived simian immunodeficiency virus (SIV)-infected monkey lung tissues. There was increased activation of Notch3 signaling as well as enhanced expression of VEGF-A in the lungs of SIV-infected macaques compared with the lungs of SIV(−) controls. Taken together, we demonstrated that HIV-TAT increased the proliferation of PASMCs via the Notch3/VEGF-A axis. Targeting the Notch3/VEGF-A axis could thus be considered a potential therapeutic approach for the treatment of HIV-associated PAH.

Regulation of cell proliferation in the retinal pigment epithelium: Differential regulation of the death-associated protein like-1 DAPL1 by alternative MITF splice forms

Vertebrate eye development and homoeostasis critically depend on the regulation of proliferation of cells forming the retinal pigment epithelium (RPE). Previous results indicated that the death-associated protein like-1 DAPL1 cell autonomously suppresses RPE proliferation in vivo and in vitro. Here, we show in human RPE cell lines that the pigment cell transcription factor MITF regulates RPE cell proliferation by upregulating DAPL1 expression. DAPL1 regulation by MITF is, however, mediated predominantly by (-) MITF, one of two alternative splice isoforms of MITF that lacks six residues located upstream of the DNA-binding basic domain. Furthermore, we find that the regulation of DAPL1 by MITF is indirect in that (-) MITF stimulates the transcription of Musashi homolog-2 (MSI2), which negatively regulates the processing of the anti-DAPL1 microRNA miR-7. Our results provide molecular insights into the regulation of RPE cell proliferation and quiescence and may help us understand the mechanisms of normal RPE maintenance and of eye diseases associated with either RPE hyperproliferation or the lack of regenerative proliferation.

BANCR contributes to the growth and invasion of melanoma by functioning as a competing endogenous RNA to upregulate Notch2 expression by sponging miR‑204

BRAF-activated non-coding RNA (BANCR) is a long non-coding RNA (lncRNA) that contributes to the initiation and development of many solid tumors, including melanoma. However, the BANCR functions and downstream mechanisms are largely unknown. In this study, we aim to investigate how BANCR participates in the proliferation and migration of malignant melanoma and elucidate the underlying mechanism in this process. We found that the expression of the BANCR was low in melanocytic nevus and human melanocytes but high in melanoma tissues and cell lines. Knockdown of BANCR inhibited melanoma cell proliferation and invasion, and induced cell apoptosis. The decreased expression of relative marker proteins further demonstrated the inhibitory effect of BANCR siRNA in cell growth and migration. Then, we detected downregulation of microRNA-204 (miR‑204), a suppressor of melanoma growth, in melanoma tissues and cell lines. We identified that miR‑204 was a direct target of BANCR and neurogenic locus notch homolog protein 2 (Notch2) was a direct target of miR‑204. BANCR may promote melanoma cell growth through inhibition of miR‑204, leading to the activation of Notch2 pathway. By tumorigenicity assay in BALB/c nude mice, we further demonstrated that BANCR knockdown inhibited tumor growth in vivo. Our results suggest the BANCR/miR‑204/Notch2 axis mediates melanoma cell proliferation and tumor progression.

The T-Box Transcription Factor TBX2 Regulates Cell Proliferation in the Retinal Pigment Epithelium

PURPOSE

Vertebrate eye development and function critically depend on the regulation of proliferation of retinal pigment epithelium (RPE) cells. Hence, a thorough analysis of the molecular parameters controlling RPE cell proliferation is crucial for our understanding of the physiology of this cell type both in health and in disease. The T-box transcription factor TBX2 is an important cell cycle regulator in development and oncogenesis, but its specific role in RPE cell proliferation is far from clear. The purpose of the present study is to investigate whether TBX2 plays any role in regulating RPE cell proliferation.

MATERIALS AND METHODS

The expression of TBX2 in RPE cells was analyzed in wildtype mice and ARPE-19 cells by co-staining for RPE-specific markers and cell proliferation. In vitro, the role of TBX2 was studied by manipulating its levels using RNAi and analyzing the effects on DNA synthesis and cell growth and on gene expression at the RNA and protein levels.

RESULTS

Here, we find that TBX2 is expressed in RPE cells both in vivo and in vitro. Specific knockdown of TBX2 in the human RPE cell line ARPE-19 leads to an accumulation of cells at G1. This cell cycle arrest is accompanied by changes in the levels of known cell cycle regulators and, in particular, by an increase in the levels of the tumor-suppressor gene CCAAT/enhancer-binding protein delta (CEBPD). In fact, simultaneous knockdown of both TBX2 and CEBPD interferes with the reduction in cell proliferation brought about by TBX2 reduction alone.

CONCLUSIONS

Our results provide novel insights into the regulatory mechanisms of cell proliferation in the RPE and may contribute to our understanding of normal RPE maintenance and its pathology in degenerative and proliferative disorders of the eye.

The inhibition of NOTCH2 reduces UVB-induced damage in retinal pigment epithelium cells

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in the elderly. The pathogenesis of dry AMD remains indistinct and the mechanism of retinal pigment epithelium (RPE) cells death in dry AMD is controversial. The aim of the present study was to investigate the functions of Notch signaling in ultraviolet B (UVB)-induced damage of RPE cells. It was identified that, in RPE cells, UVB increased intracellular reactive oxygen species (ROS) and induced cell apoptosis. In addition, UVB activated Notch signaling in a dose dependent manner. Surprisingly, NOTCH2, but not NOTCH1, was demonstrated to be the major Notch receptor in RPE cells. Under normal conditions, the inhibition of NOTCH2 reduced cell growth and cell migration, but had no impact on intracellular ROS and cell apoptosis. However, in the presence of UVB, the inhibition of NOTCH2, but not NOTCH1, attenuated intracellular ROS and cell apoptosis. The function of Notch signaling involved in UVB damage of RPE cells may not only be significant to understanding the pathogenesis of AMD (especially dry AMD), but also useful for designing effective therapeutic agents for dry AMD.

The Retinal Pigment Epithelium Is a Notch Signaling Niche in the Mouse Retina

Notch signaling in neural progenitor cell is triggered by ligands expressed in adjacent cells. To identify the sources of active Notch ligands in the mouse retina, we negatively regulated Notch ligand activity in various neighbors of retinal progenitor cells (RPCs) by eliminating mindbomb E3 ubiquitin protein ligase 1 (Mib1). Mib1-deficient retinal cells failed to induce Notch activation in intra-lineage RPCs, which prematurely differentiated into neurons; however, Mib1 in post-mitotic retinal ganglion cells was not important. Interestingly, Mib1 in the retinal pigment epithelium (RPE) also contributed to Notch activation in adjacent RPCs by supporting the localization of active Notch ligands at RPE-RPC contacts. Combining this RPE-driven Notch signaling and intra-retinal Notch signaling, we propose a model in which one RPC daughter receives extra Notch signals from the RPE to become an RPC, whereas its sister cell receives only a subthreshold level of intra-retinal Notch signal and differentiates into a neuron.

Cell models to study regulation of cell transformation in pathologies of retinal pigment epithelium

The retinal pigment epithelium (RPE) plays a key role in the development of many eye diseases leading to visual impairment and even blindness. Cell culture models of pathological changes in the RPE make it possible to study factors responsible for these changes and signaling pathways coordinating cellular and molecular mechanisms of cell interactions under pathological conditions. Moreover, they give an opportunity to reveal target cells and develop effective specific treatment for degenerative and dystrophic diseases of the retina. In this review, data are presented on RPE cell sources for culture models, approaches to RPE cell culturing, phenotypic changes of RPE cells in vitro, the role of signal pathways, and possibilities for their regulation in pathological processes.

Blockage of Notch signaling inhibits the migration and proliferation of retinal pigment epithelial cells

The Notch signaling is an evolutionarily conserved cell-cell communication pathway that plays critical roles in the proliferation, survival, apoptosis, and fate determination of mammalian cells. Retinal pigment epithelial (RPE) cells are responsible for supporting the function of the neural retina and maintaining vision. This study investigated the function of Notch signaling in RPE cells. We found that the members of the Notch signaling pathway components were differentially expressed in RPE cells. Furthermore, blockage of Notch signaling inhibited the migration and proliferation of RPE cells and reduced the expression levels of certain Notch signaling target genes, including HES1, MYC, HEY2, and SOX9. Our data reveal a critical role of Notch signaling in RPE cells, suggesting that targeting Notch signaling may provide a novel approach for the treatment of ophthalmic diseases related to RPE cells.

Notch signaling in melanoma: interacting pathways and stromal influences that enhance Notch targeting

The Notch signaling pathway is an evolutionarily conserved, intercellular signaling cascade. Notch was first described in the early 1900s when a mutant Drosophila showed notches on the wing margins. Studies of the role of Notch signaling have ever since flourished, and the pleiotropic nature of the Notch gene is now evident. Indeed, the Notch signaling pathway plays key roles in cell fate decisions, tissue patterning, and morphogenesis during development. However, deregulation of this pathway can contribute to cell transformation and tumorigenesis. Several reports have now highlighted the role of Notch signaling in a variety of malignancies where Notch can either be an oncogene or a tumor suppressor depending on the cell context. Here, we summarize the major components of Notch signaling with an aim to emphasize the contribution of deregulated Notch signaling in melanomagenesis.

Human disease locus discovery and mapping to molecular pathways through phylogenetic profiling

By analyzing the conservation of human proteins across 87 species, we sorted proteins into clusters of coevolution. Some clusters are enriched for genes assigned to particular human diseases or molecular pathways; the other genes in the same cluster may function in related pathways and diseases.

Many genes that were thought to map to different diseases are actually coevolved together and mapped into the same phylogenetic clusters.

Many molecular pathways map to the same phylogenetic clusters as genes associated with specific human diseases.

Focusing on proteins coevolved with the microphthalmia-associated transcription factor (MITF), we identified the Notch pathway suppressor of hairless (RBP-Jk/SuH) transcription factor, and showed that RBP-Jk functions as an MITF cofactor.

Our analysis thus establishes a connectivity between different diseases and pathways, linking diseases phenotypes and functional gene groups.

Genes with common profiles of the presence and absence in disparate genomes tend to function in the same pathway. By mapping all human genes into about 1000 clusters of genes with similar patterns of conservation across eukaryotic phylogeny, we determined that sets of genes associated with particular diseases have similar phylogenetic profiles. By focusing on those human phylogenetic gene clusters that significantly overlap some of the thousands of human gene sets defined by their coexpression or annotation to pathways or other molecular attributes, we reveal the evolutionary map that connects molecular pathways and human diseases. The other genes in the phylogenetic clusters enriched for particular known disease genes or molecular pathways identify candidate genes for roles in those same disorders and pathways. Focusing on proteins coevolved with the microphthalmia-associated transcription factor (MITF), we identified the Notch pathway suppressor of hairless (RBP-Jk/SuH) transcription factor, and showed that RBP-Jk functions as an MITF cofactor.

Retinal pigment epithelium development, plasticity, and tissue homeostasis

The retinal pigment epithelium (RPE) is a simple epithelium interposed between the neural retina and the choroid. Although only 1 cell-layer in thickness, the RPE is a virtual workhorse, acting in several capacities that are essential for visual function and preserving the structural and physiological integrities of neighboring tissues. Defects in RPE function, whether through chronic dysfunction or age-related decline, are associated with retinal degenerative diseases including age-related macular degeneration. As such, investigations are focused on developing techniques to replace RPE through stem cell-based methods, motivated primarily because of the seemingly limited regeneration or self-repair properties of mature RPE. Despite this, RPE cells have an unusual capacity to transdifferentiate into various cell types, with the particular fate choices being highly context-dependent. In this review, we describe recent findings elucidating the mechanisms and steps of RPE development and propose a developmental framework for understanding the apparent contradiction in the capacity for low self-repair versus high transdifferentiation.

Vax1/2 genes counteract Mitf-induced respecification of the retinal pigment epithelium

During vertebrate eye development, the transcription factor MITF acts to promote the development of the retinal pigment epithelium (RPE). In embryos with Mitf mutations, the future RPE hyperproliferates and is respecified as retinal tissue but only in a small portion of the dorsal RPE. Using a series of genetic crosses, we show that this spatial restriction of RPE respecification is brought about by persistent expression of the anti-retinogenic ventral homeodomain gene Vax2 in the dorso-proximal and both Vax1 and Vax2 in the ventral RPE. We further show that dorso-proximal RPE respecification in Vax2/Mitf double mutants and dorso-proximal and ventral RPE respecification in Vax1/2/Mitf triple mutants result from increased FGF/MAP kinase signaling. In none of the mutants, however, does the distal RPE show signs of hyperproliferation or respecification, likely due to local JAGGED1/NOTCH signaling. Expression studies and optic vesicle culture experiments also suggest a role for NOTCH signaling within the mutant dorsal RPE domains, where ectopic JAGGED1 expression may partially counteract the effects of FGF/ERK1/2 signaling on RPE respecification. The results indicate the presence of complex interplays between distinct transcription factors and signaling molecules during eye development and show how RPE phenotypes associated with mutations in one gene are modulated by expression changes in other genes.

A role for Jag2 in promoting uveal melanoma dissemination and growth

PURPOSE

Controlling the spread of uveal melanoma is key to improving survival of patients with this common intraocular malignancy. The Notch ligand Jag2 has been shown to be upregulated in primary tumors that metastasize, and we therefore investigated its role in promoting invasion and clonogenic growth of uveal melanoma cells.

METHODS

mRNA and protein expression of Notch pathway components were measured using qPCR and Western blot in uveal melanoma cell lines. Expression of Jag2 ligand was upregulated using Jag2-GFP-MSCV constructs or downregulated by sh-Jag2 in the uveal melanoma cell lines Mel285, Mel290, 92.1, and OMM1, and the effects on growth and invasion were assessed.

RESULTS

Jag2 was introduced into Mel285 and Mel290 cells, which have low baseline levels of both this ligand and Notch activity. Overall growth of the Jag2-expressing cultures increased somewhat, and a significant 3-fold increase in clonogenic growth in soft agar was also noted. Introduction of Jag2 increased motility in both wound-healing and transwell invasion assays. We also observed a significant increase in Jag2 and Hes1 mRNA in invasive OMM1 cells that had passed through a Matrigel-coated filter in the transwell assay when compared with noninvading cells. Loss-of-function studies performed in 92.1 and OMM1 lines using Jag2 shRNAs showed that downregulation of the ligand significantly suppressed cellular growth, invasion, and migration.

CONCLUSIONS

Our data suggest that Jag2 may play an important role in promoting Notch activity, growth, and metastasis in uveal melanoma.

Evidence for baseline retinal pigment epithelium pathology in the Trp1-Cre mouse

The increasing popularity of the Cre/loxP recombination system has led to the generation of numerous transgenic mouse lines in which Cre recombinase is expressed under the control of organ- or cell-specific promoters. Alterations in retinal pigment epithelium (RPE), a multifunctional cell monolayer that separates the retinal photoreceptors from the choroid, are prevalent in the pathogenesis of a number of ocular disorders, including age-related macular degeneration. To date, six transgenic mouse lines have been developed that target Cre to the RPE under the control of various gene promoters. However, multiple lines of evidence indicate that high levels of Cre expression can be toxic to mammalian cells. In this study, we report that in the Trp1-Cre mouse, a commonly used transgenic Cre strain for RPE gene function studies, Cre recombinase expression alone leads to RPE dysfunction and concomitant disorganization of RPE layer morphology, large areas of RPE atrophy, retinal photoreceptor dysfunction, and microglial cell activation in the affected areas. The phenotype described herein is similar to previously published reports of conditional gene knockouts that used the Trp1-Cre mouse, suggesting that Cre toxicity alone could account for some of the reported phenotypes and highlighting the importance of the inclusion of Cre-expressing mice as controls in conditional gene targeting studies.

Notch signaling in ocular vasculature development and diseases

Ocular angiogenesis, characterized by the formation of new blood vessels in the avascular area in eyes, is a highly coordinated process involved in retinal vasculature formation and several ocular diseases such as age-related macular degeneration, proliferative diabetic retinopathy and retinopathy of prematurity. This process is orchestrated by complicated cellular interactions and vascular growth factors, during which endothelial cells acquire heterogeneous phenotypes and distinct cellular destinations. To date, while the vascular endothelial growth factor has been identified as the most critical angiogenic agent with a remarkable therapeutic value, the Notch signaling pathway appears to be a similarly important regulator in several angiogenic steps. Recent progress has highlighted the involvement, mechanisms and therapeutic potential of Notch signaling in retinal vasculature development and pathological angiogenesis-related eye disorders, which may cause irreversible blindness.

Notch signaling promotes growth and invasion in uveal melanoma

PURPOSE

To determine whether uveal melanoma, the most common primary intraocular malignancy in adults, requires Notch activity for growth and metastasis.

EXPERIMENTAL DESIGN

Expression of Notch pathway members was characterized in primary tumor samples and in cell lines, along with the effects of Notch inhibition or activation on tumor growth and invasion.

RESULTS

Notch receptors, ligands, and targets were expressed in all five cell lines examined and in 30 primary uveal melanoma samples. Interestingly, the three lines with high levels of baseline pathway activity (OCM1, OCM3, and OCM8) had their growth reduced by pharmacologic Notch blockade using the γ-secretase inhibitor (GSI) MRK003. In contrast, two uveal melanoma lines (Mel285 and Mel290) with very low expression of Notch targets were insensitive to the GSI. Constitutively active forms of Notch1 and Notch2 promoted growth of uveal melanoma cultures and were able to rescue the inhibitory effects of GSI. MRK003 treatment also inhibited anchorage-independent clonogenic growth and cell invasion and reduced phosphorylation levels of STAT3 and extracellular signal-regulated kinase (Erk)1/2. Suppression of canonical Notch activity using short hairpin RNA targeting Notch2 or CBF1 was also able to reduce tumor growth and invasion. Finally, intraocular xenograft growth was significantly decreased by GSI treatment.

CONCLUSION

Our findings suggest that Notch plays an important role in inducing proliferation and invasion in uveal melanoma and that inhibiting this pathway may be effective in preventing tumor growth and metastasis.

A mart-1::Cre transgenic line induces recombination in melanocytes and retinal pigment epithelium

The number of transgenic mouse lines expressing Cre in either type of pigment cells (melanocytes and retinal pigment epithelium, RPE) is limited, and the available lines do not always offer sufficient specificity. In this study, we addressed this issue and we report on the generation of a MART-1::Cre BAC transgenic mouse line, in which the expression of Cre recombinase is controlled by regulatory elements of the pigment cell-specific gene MART-1 (mlana). When MART-1::Cre BAC transgenic mice were bred with the ROSA26-R reporter line, ß-galactosidase expression was observed in RPE from E12.5 onwards, and in melanocyte precursors from E17.5, indicating that the MART-1::Cre line provides Cre recombinase activity in pigment-producing cells rather than in a particular lineage. In addition, breeding of this mouse line to mice carrying a conditional allele of RBP-Jκ corroborated the reported phenotypes in both pigment cell lineages, inducing hair greying and microphthalmia. Our results thus suggest, that the MART-1::Cre line may serve as a novel and useful tool for functional studies in melanocytes and the RPE.genesis 49:403-409, 2011.