Inactivation of the microRNA-183/96/182 cluster results in syndromic retinal degeneration.

The microRNA-183/96/182 cluster is highly expressed in the retina and other sensory organs. To uncover its in vivo functions in the retina, we generated a knockout mouse model, designated "miR-183C(GT/GT)," using a gene-trap embryonic stem cell clone. We provide evidence that inactivation of the cluster results in early-onset and progressive synaptic defects of the photoreceptors, leading to abnormalities of scotopic and photopic electroretinograms with decreased b-wave amplitude as the primary defect and progressive retinal degeneration. In addition, inactivation of the miR-183/96/182 cluster resulted in global changes in retinal gene expression, with enrichment of genes important for synaptogenesis, synaptic transmission, photoreceptor morphogenesis, and phototransduction, suggesting that the miR-183/96/182 cluster plays important roles in postnatal functional differentiation and synaptic connectivity of photoreceptors.

Elevated cyclin G2 expression intersects with DNA damage checkpoint signaling and is required for a potent G2/M checkpoint arrest response to doxorubicin.

To maintain genomic integrity DNA damage response (DDR), signaling pathways have evolved that restrict cellular replication and allow time for DNA repair. CCNG2 encodes an unconventional cyclin homolog, cyclin G2 (CycG2), linked to growth inhibition. Its expression is repressed by mitogens but up-regulated during cell cycle arrest responses to anti-proliferative signals. Here we investigate the potential link between elevated CycG2 expression and DDR signaling pathways. Expanding our previous finding that CycG2 overexpression induces a p53-dependent G(1)/S phase cell cycle arrest in HCT116 cells, we now demonstrate that this arrest response also requires the DDR checkpoint protein kinase Chk2. In accord with this finding we establish that ectopic CycG2 expression increases phosphorylation of Chk2 on threonine 68. We show that DNA double strand break-inducing chemotherapeutics stimulate CycG2 expression and correlate its up-regulation with checkpoint-induced cell cycle arrest and phospho-modification of proteins in the ataxia telangiectasia mutated (ATM) and ATM and Rad3-related (ATR) signaling pathways. Using pharmacological inhibitors and ATM-deficient cell lines, we delineate the DDR kinase pathway promoting CycG2 up-regulation in response to doxorubicin. Importantly, RNAi-mediated blunting of CycG2 attenuates doxorubicin-induced cell cycle checkpoint responses in multiple cell lines. Employing stable clones, we test the effect that CycG2 depletion has on DDR proteins and signals that enforce cell cycle checkpoint arrest. Our results suggest that CycG2 contributes to DNA damage-induced G(2)/M checkpoint by enforcing checkpoint inhibition of CycB1-Cdc2 complexes.

NANOG induction of fetal liver kinase-1 (FLK1) transcription regulates endothelial cell proliferation and angiogenesis.

NANOG is a master transcription factor associated with the maintenance of stem cell pluripotency. Here, we demonstrate that transcription factor NANOG is expressed in cultured endothelial cells (ECs) and in a subset of tumor cell lines. Importantly, we provide evidence that WNT3A stimulation of ECs induces the transcription of NANOG which mediates the expression of vascular endothelial growth factor receptor-2, also known as fetal liver kinase-1 (FLK1). We defined ATTA as a minimal binding site for NANOG. Accordingly, a luciferase reporter assay showed that NANOG binds to and activates 4 ATTA binding sites identified in the FLK1 promoter after WNT3A stimulation. Consistent with this data, we found that, under basal conditions and in response to WNT3A stimulation, NANOG binding to these ATTA sequences markedly induced the expression of FLK1. Thus, our data indicate an essential role in angiogenesis for NANOG binding to these 4 ATTA sites. Surprisingly, NANOG depletion not only decreased FLK1 expression but also reduced cell proliferation and angiogenesis. These findings show the necessary and sufficient role of NANOG in inducing the transcription of FLK1 to regulate the angiogenic phenotypes of ECs.

Kruppel-like factor-4 transcriptionally regulates VE-cadherin expression and endothelial barrier function.

RATIONALE: Vascular endothelial (VE)-cadherin localized at adherens junctions (AJs) regulates endothelial barrier function. Because WNT (wingless) signaling-induced activation of the transcription factor Krüppel-like factor (KLF)4 may have an important role in mediating the expression of VE-cadherin and AJ integrity, we studied the function of KLF4 in regulating VE-cadherin expression and the control of endothelial barrier function. OBJECTIVE: The goal of this study was to determine the transcriptional role of KLF4 in regulating VE-cadherin expression and endothelial barrier function. METHODS AND RESULTS: Expression analysis, microscopy, chromatin immunoprecipitation, electrophoretic mobility shift assays, and VE-cadherin-luciferase reporter experiments demonstrated that KLF4 interacted with specific domains of VE-cadherin promoter and regulated the expression of VE-cadherin at AJs. KLF4 knockdown disrupted the endothelial barrier, indicating that KLF4 is required for normal barrier function. In vivo studies in mice showed augmented lipopolysaccharide-induced lung injury and pulmonary edema following Klf4 depletion. CONCLUSION: Our data show the key role of KLF4 in the regulation of VE-cadherin expression at the level of the AJs and in the acquisition of VE-cadherin-mediated endothelial barrier function. Thus, KLF4 maintains the integrity of AJs and prevents vascular leakage in response to inflammatory stimuli.

MicroRNA-146 inhibits thrombin-induced NF-κB activation and subsequent inflammatory responses in human retinal endothelial cells.

PURPOSE: Nuclear factor-κB (NF-κB), a key regulator of immune and inflammatory responses, plays important roles in diabetes-induced microvascular complications including diabetic retinopathy (DR). Thrombin activates NF-κB through protease-activated receptor (PAR)-1, a member of the G-protein-coupled receptor (GPCR) superfamily, and contributes to DR. The current study is to uncover the roles of microRNA (miRNA) in thrombin-induced NF-κB activation and retinal endothelial functions. METHODS: Target prediction was performed using the TargetScan algorithm. Predicted target was experimentally validated by luciferase reporter assays. Human retinal endothelial cells (HRECs) were transfected with miRNA mimics or antimiRs and treated with thrombin. Expression levels of miR-146 and related protein-coding genes were analyzed by quantitative (q)RT-PCR. Functional changes of HRECs were analyzed by leukocyte adhesion assays. RESULTS: We identified that caspase-recruitment domain (CARD)-containing protein 10 (CARD10), an essential scaffold/adaptor protein of GPCR-mediated NF-κB activation pathway, is a direct target of miR-146. Thrombin treatment resulted in NF-κB-dependent upregulation of miR-146 in HRECs; while transfection of miR-146 mimics resulted in significant downregulation of CARD10 and prevented thrombin-induced NF-κB activation, suggest that a negative feedback regulation of miR-146 on thrombin-induced NF-κB through targeting CARD10. Furthermore, overexpression of miR-146 prevented thrombin-induced increased leukocyte adhesion to HRECs. CONCLUSIONS: We uncovered a novel negative feedback regulatory mechanism on thrombin-induced GPCR-mediated NF-κB activation by miR-146. In combination with the negative feedback regulation of miR-146 on the IL-1R/toll-like receptor (TLR)-mediated NF-κB activation in RECs that we reported previously, our results underscore a pivotal, negative regulatory role of miR-146 on multiple NF-κB activation pathways and related inflammatory processes in DR.

MicroRNAs in early diabetic retinopathy in streptozotocin-induced diabetic rats.

PURPOSE: Diabetic retinopathy (DR) is one of the leading causes of blindness. However, the roles of microRNAs (miRNAs) in DR are still unknown. The aims of this study were to identify miRNAs involved in early DR and to characterize their roles in the pathogenesis of DR. METHODS: miRNA-expression profiling was performed in the retina and retinal endothelial cells (RECs) of streptozotocin (STZ)-induced diabetic rats 3 months after the onset of diabetes and miRNAs differentially expressed in diabetic rats were identified and compared with controls. Subsequently, functional annotation analysis was conducted to identify miRNA signatures of pathologic pathways of DR. In addition, in vitro functional assays were used to dissect interactions of miR-146 and NF-κB activation in a conditionally immortalized retinal capillary endothelial cell line, Tr-iBRB. RESULTS: Approximately 350 and 220 miRNAs were detected in the retinas and RECs, respectively, in both control and diabetic rats. At least 86 and 120 miRNAs were differentially expressed (P < 0.01) in the retinas and RECs of diabetic rats and controls, respectively. Upregulation of NF-κB-, VEGF-, and p53-responsive miRNAs constituted key miRNA signatures, reflecting ongoing pathologic changes of early DR. In addition, it was demonstrated that the negative feedback regulation of miR-146 on NF-κB activation may function in Tr-iBRB endothelial cells, suggesting that miR-146 is a potential therapeutic target for the treatment of DR through its inhibition on NF-κB activation in RECs. CONCLUSIONS: miRNAs are involved in the pathogenesis of DR through the modulation of multiple pathogenetic pathways and may be novel therapeutic targets for the treatment of DR.

Cyclin G2 is a centrosome-associated nucleocytoplasmic shuttling protein that influences microtubule stability and induces a p53-dependent cell cycle arrest.

Cyclin G2 is an atypical cyclin that associates with active protein phosphatase 2A. Cyclin G2 gene expression correlates with cell cycle inhibition; it is significantly upregulated in response to DNA damage and diverse growth inhibitory stimuli, but repressed by mitogenic signals. Ectopic expression of cyclin G2 promotes cell cycle arrest, cyclin dependent kinase 2 inhibition and the formation of aberrant nuclei [Bennin, D. A., Don, A. S., Brake, T., McKenzie, J. L., Rosenbaum, H., Ortiz, L., DePaoli-Roach, A. A., and Horne, M. C. (2002). Cyclin G2 associates with protein phosphatase 2A catalytic and regulatory B' subunits in active complexes and induces nuclear aberrations and a G(1)/S-phase cell cycle arrest. J Biol Chem 277, 27449-67]. Here we report that endogenous cyclin G2 copurifies with centrosomes and microtubules (MT) and that ectopic G2 expression alters microtubule stability. We find exogenous and endogenous cyclin G2 present at microtubule organizing centers (MTOCs) where it colocalizes with centrosomal markers in a variety of cell lines. We previously reported that cyclin G2 forms complexes with active protein phosphatase 2A (PP2A) and colocalizes with PP2A in a detergent-resistant compartment. We now show that cyclin G2 and PP2A colocalize at MTOCs in transfected cells and that the endogenous proteins copurify with isolated centrosomes. Displacement of the endogenous centrosomal scaffolding protein AKAP450 that anchors PP2A at the centrosome resulted in the depletion of centrosomal cyclin G2. We find that ectopic expression of cyclin G2 induces microtubule bundling and resistance to depolymerization, inhibition of polymer regrowth from MTOCs and a p53-dependent cell cycle arrest. Furthermore, we determined that a 100 amino acid carboxy-terminal region of cyclin G2 is sufficient to both direct GFP localization to centrosomes and induce cell cycle inhibition. Colocalization of endogenous cyclin G2 with only one of two GFP-centrin-tagged centrioles, the mature centriole present at microtubule foci, indicates that cyclin G2 resides primarily on the mother centriole. Copurification of cyclin G2 and PP2A subunits with microtubules and centrosomes, together with the effects of ectopic cyclin G2 on cell cycle progression, nuclear morphology and microtubule growth and stability, suggests that cyclin G2 may modulate the cell cycle and cellular division processes through modulation of PP2A and centrosomal associated activities.