Polycomb-group complex 1 acts as an E3 ubiquitin ligase for Geminin to sustain hematopoietic stem cell activity

How CBX proteins regulate normal and leukemic blood cells

Hematopoietic stem cell (HSC) fate decisions are dictated by epigenetic landscapes. The Polycomb Repressive Complex 1 (PRC1) represses genes that induce differentiation, thereby maintaining HSC self-renewal. Depending on which chromobox (CBX) protein (CBX2, CBX4, CBX6, CBX7, or CBX8) is part of the PRC1 complex, HSC fate decisions differ. Here, we review how this occurs. We describe how CBX proteins dictate age-related changes in HSCs and stimulate oncogenic HSC fate decisions, either as canonical PRC1 members or by alternative interactions, including non-epigenetic regulation. CBX2, CBX7, and CBX8 enhance leukemia progression. To target, reprogram, and kill leukemic cells, we suggest and describe multiple therapeutic strategies to interfere with the epigenetic functions of oncogenic CBX proteins. Future studies should clarify to what extent the non-epigenetic function of cytoplasmic CBX proteins is important for normal, aged, and leukemic blood cells.

RNF2 regulates Wnt/ß-catenin signaling via TCF7L1 destabilization

The Wnt signaling pathway is a crucial regulator of various biological processes, such as development and cancer. The downstream transcription factors in this pathway play a vital role in determining the threshold for signaling induction and the length of the response, which vary depending on the biological context. Among the four transcription factors involved in canonical Wnt/ß-catenin signaling, TCF7L1 is known to possess an inhibitory function; however, the underlying regulatory mechanism remains unclear. In this study, we identified the E3 ligase, RNF2, as a novel positive regulator of the Wnt pathway. Here, we demonstrate that RNF2 promotes the degradation of TCF7L1 through its ubiquitination upon activation of Wnt signaling. Loss-of-function studies have shown that RNF2 consistently destabilizes nuclear TCF7L1 and is required for proper Wnt target gene transcription in response to Wnt activation. Furthermore, our results revealed that RNF2 controls the threshold, persistence, and termination of Wnt signaling by regulating TCF7L1. Overall, our study sheds light on the previously unknown degradation mechanism of TCF7L1 by a specific E3 ligase, RNF2, and provides new insights into the variability in cellular responses to Wnt activation.

HPIP protooncogene differentially regulates metabolic adaptation and cell fate in breast cancer cells under glucose stress via AMPK and RNF2 dependent pathways

While cancer cells rewire metabolic pathways to sustain growth and survival under metabolic stress in solid tumors, the molecular mechanisms underlying these processes remain largely unknown. In this study, cancer cells switched from survival to death during the early to late phases of metabolic stress by employing a novel signaling switch from AMP activated protein kinase (AMPK)-Forkhead box O3 (FOXO3a)-hematopoietic PBX1-interacting protein (HPIP) to the ring finger protein 2 (RNF2)-HPIP-ubiquitin (Ub) pathway. Acute metabolic stress induced proto-oncogene HPIP expression in an AMPK-FOXO3a-dependent manner in breast cancer (BC) cells. HPIP depletion reduced cell survival and tumor formation in mouse xenografts, which was accompanied by diminished intracellular ATP levels and increased apoptosis in BC cells in response to metabolic (glucose) stress. Glutamine flux (¹³C-labeled) analysis further suggested that HPIP rewired glutamine metabolism by controlling the expression of the solute carrier family 1 member 5 (SLC1A5) and glutaminase (GLS) genes by acting as a coactivator of MYC to ensure cell survival upon glucose deprivation. However, in response to chronic glucose stress, HPIP was ubiquitinated by the E3-Ub ligase, RNF2, and was concomitantly degraded by the proteasome-mediated pathway, ensuring apoptosis. In support of these data, clinical analyses further indicated that elevated levels of HPIP correlated with AMPK activation in BC. Taken together, these data suggest that HPIP is a signal coordinator during metabolic stress and thus serves as a potential therapeutic target in BC.

Molecular Genetics of Microcephaly Primary Hereditary: An Overview

MicroCephaly Primary Hereditary (MCPH) is a rare congenital neurodevelopmental disorder characterized by a significant reduction of the occipitofrontal head circumference and mild to moderate mental disability. Patients have small brains, though with overall normal architecture; therefore, studying MCPH can reveal not only the pathological mechanisms leading to this condition, but also the mechanisms operating during normal development. MCPH is genetically heterogeneous, with 27 genes listed so far in the Online Mendelian Inheritance in Man (OMIM) database. In this review, we discuss the role of MCPH proteins and delineate the molecular mechanisms and common pathways in which they participate.

Association between RNF2+P-AKT expression in pretreatment biopsy specimens, and poor survival following radiotherapy in patients with esophageal squamous cell carcinoma

The protein expression levels of Ring finger protein 2 (RNF2) and phosphor-protein kinase B (P-AKT) were determined in esophageal squamous cell carcinoma (ESCC) tissues, and the association between patient clinicopathological characteristics and survival time following definitive intensity-modulated radiotherapy was assessed. Cancerous biopsy tissues were collected from patients with ESCC at The Fourth Affiliated Hospital of Hebei Medical University between January 2010 and December 2013. Of these 99 cases, 83 were used to analyze the protein expression level of RNF2 (89.2% positive), 85 for P-AKT (65.9% positive) and 80 for RNF2+P-AKT protein expression levels (62.5% both positive). The expression levels of RNF2 protein in ESCC were associated with tumor volume (P=0.024), whilst those of P-AKT and RNF2+PAKT were associated with sex (P=0.041 and P=0.003, respectively). There were no significant differences in overall survival (OS) or progression-free survival (PFS) rate between the RNF2^- and the RNF2^+-+++ groups (P=0.134 and P=0.366, respectively), or between the P-AKT^- group and P-AKT^+-+++ group (P=0.468; P=0.580, respectively). The 1-, 3- and 5-year OS rates were 68.0, 28.0, and 20.0%, and 86.7, 53.3, and 31.1%, in the RNF2/P-AKT⁺ group and Other group, respectively (χ²=4.205; P=0.040). Multivariate analysis revealed that age, T stage and RNF2+P-AKT expression were independent prognostic factors for ESCC (P=0.010, P=0.008 and P=0.010, respectively). The expression of RNF2+P-AKT combined was an independent prognostic factor affecting survival rate, and therefore presents a potential prognostic indicator for patients with ESCC, treated with definitive radiotherapy.

Regulation of geminin by neuropeptide Y in vascular smooth muscle cell proliferation : A current review

Geminin, a key regulator of DNA replication licensing in the cell cycle, plays an essential role in determining the fate of cells via suppression of cell proliferation and cellular differentiation. Neuropeptide Y (NPY) intensifies the proliferation of vascular smooth muscle cells (VSMCs) directly by binding with Y1 receptors. In vitro experiments have shown that stimulation of NPY on VSMCs via regulation of geminin is a double-edged sword. Given that the proliferation and the phenotypic transformation of VSMCs increase the risk for progression of atherosclerosis, we focus on the role of geminin interference in determining the fate of VSMCs. Furthermore, we discuss the therapeutic potential of peripheral neurotransmitter interference, thus pointing toward future research directions in the treatment of atherosclerosis.

Cell Signaling in Neuronal Stem Cells

The defining characteristic of neural stem cells (NSCs) is their ability to multiply through symmetric divisions and proliferation, and differentiation by asymmetric divisions, thus giving rise to different types of cells of the central nervous system (CNS). A strict temporal space control of the NSC differentiation is necessary, because its alterations are associated with neurological dysfunctions and, in some cases, death. This work reviews the current state of the molecular mechanisms that regulate the transcription in NSCs, organized according to whether the origin of the stimulus that triggers the molecular cascade in the CNS is internal (intrinsic factors) or whether it is the result of the microenvironment that surrounds the CNS (extrinsic factors).

Redox-dependent BMI1 activity drives in vivo adult cardiac progenitor cell differentiation

Accumulation of reactive oxygen species (ROS) is associated with several cardiovascular pathologies and with cell cycle exit by neonanatal cardiomyocytes, a key limiting factor in the regenerative capacity of the adult mammalian heart. The polycomb complex component BMI1 is linked to adult progenitors and is an important partner in DNA repair and redox regulation. We found that high BMI1 expression is associated with an adult Sca1⁺ cardiac progenitor sub-population with low ROS levels. In homeostasis, BMI1 repressed cell fate genes, including a cardiogenic differentiation program. Oxidative damage nonetheless modified BMI1 activity in vivo by derepressing canonical target genes in favor of their antioxidant and anticlastogenic functions. This redox-mediated mechanism is not restricted to damage situations, however, and we report ROS-associated differentiation of cardiac progenitors in steady state. These findings demonstrate how redox status influences the cardiac progenitor response, and identify redox-mediated BMI1 regulation with implications in maintenance of cellular identity in vivo.

The E3 Ligase RING1 Targets p53 for Degradation and Promotes Cancer Cell Proliferation and Survival

As a component of the transcriptional repression complex 1 (PRC1), the ring finger protein RING1 participates in the epigenetic regulation in cancer. However, the contributions of RING1 to cancer etiology or development are unknown. In this study, we report that RING1 is a critical negative regulator of p53 homeostasis in human hepatocellular and colorectal carcinomas. RING1 acts as an E3 ubiquitin (Ub) ligase to directly interact with and ubiquitinate p53, resulting in its proteasome-dependent degradation. The RING domain of RING1 was required for its E3 Ub ligase activity. RING1 depletion inhibited the proliferation and survival of the p53 wild-type cancer cells by inducing cell-cycle arrest, apoptosis, and senescence, with only modest effects on p53-deficient cells. Its growth inhibitory effect was partially rescued by p53 silencing, suggesting an important role for the RING1-p53 complex in human cancer. In clinical specimens of hepatocellular carcinoma, RING1 upregulation was evident in association with poor clinical outcomes. Collectively, our results elucidate a novel PRC1-independent function of RING1 and provide a mechanistic rationale for its candidacy as a new prognostic marker and/or therapeutic target in human cancer.Significance: These results elucidate a novel PRC1-independent function of RING1 and provide a mechanistic rationale for its candidacy as a new prognostic marker and/or therapeutic target in human cancer. Cancer Res; 78(2); 359-71. ©2017 AACR.

Targeting MUC1-C suppresses polycomb repressive complex 1 in multiple myeloma

The polycomb repressive complex 1 (PRC1) includes the BMI1, RING1 and RING2 proteins. BMI1 is required for survival of multiple myeloma (MM) cells. The MUC1-C oncoprotein is aberrantly expressed by MM cells, activates MYC and is also necessary for MM cell survival. The present studies show that targeting MUC1-C with (i) stable and inducible silencing and CRISPR/Cas9 editing and (ii) the pharmacologic inhibitor GO-203, which blocks MUC1-C function, downregulates BMI1, RING1 and RING2 expression. The results demonstrate that MUC1-C drives BMI1 transcription by a MYC-dependent mechanism. MUC1-C thus promotes MYC occupancy on the BMI1 promoter and thereby activates BMI1 expression. We also show that the MUC1-C→MYC pathway induces RING2 expression. Moreover, in contrast to BMI1 and RING2, we found that MUC1-C drives RING1 by an NF-κB p65-dependent mechanism. Targeting MUC1-C and thereby the suppression of these key PRC1 proteins was associated with downregulation of the PRC1 E3 ligase activity as evidenced by decreases in ubiquitylation of histone H2A. Targeting MUC1-C also resulted in activation of the PRC1-repressed tumor suppressor genes, PTEN, CDNK2A and BIM. These findings identify a heretofore unrecognized role for MUC1-C in the epigenetic regulation of MM cells.

MUC1-C activates BMI1 in human cancer cells

BMI1 is a component of the PRC1 complex that is overexpressed in breast and other cancers, and promotes self-renewal of cancer stem-like cells. The oncogenic mucin 1 (MUC1) C-terminal (MUC1-C) subunit is similarly overexpressed in human carcinoma cells and has been linked to their self-renewal. There is no known relationship between MUC1-C and BMI1 in cancer. The present studies demonstrate that MUC1-C drives BMI1 transcription by a MYC-dependent mechanism in breast and other cancer cells. In addition, we show that MUC1-C blocks miR-200c-mediated downregulation of BMI1 expression. The functional significance of this MUC1-C→BMI1 pathway is supported by the demonstration that targeting MUC1-C suppresses BMI1-induced ubiquitylation of H2A and thereby derepresses homeobox HOXC5 and HOXC13 gene expression. Notably, our results further show that MUC1-C binds directly to BMI1 and promotes occupancy of BMI1 on the CDKN2A promoter. In concert with BMI1-induced repression of the p16^INK4a tumor suppressor, we found that targeting MUC1-C is associated with induction of p16^INK4a expression. In support of these results, analysis of three gene expresssion datasets demonstrated highly significant correlations between MUC1-C and BMI1 in breast cancers. These findings uncover a previously unrecognized role for MUC1-C in driving BMI1 expression and in directly interacting with this stem cell factor, linking MUC1-C with function of the PRC1 in epigenetic gene silencing.

Role of Geminin in cell fate determination of hematopoietic stem cells (HSCs)

Geminin exerts two distinct molecular roles. Geminin negatively regulates DNA replication licensing through the direct interaction with Cdt1 to prevent re-replication in proliferating cells. Geminin also regulates chromatin remodeling through the direct interaction with Brahma/Brg1 to maintain undifferentiated states of stem cells. We previously uncovered that Polycomb-group complex 1 and Hoxb4/Hoxa9, well-known intrinsic factors that are essential for maintaining the hematopoietic stem cell (HSC) activity, alternatively act as ubiquitin-proteasome systems for Geminin protein to reduce the protein expression level, and sustain the HSC activity. Thus, Geminin is presumed to play an important role in determining cell fate, i.e., turning on and off cellular quiescence and proliferation/differentiation, in HSCs. We recently generated recombinant cell-penetrating Geminin (CP-Geminin), enabling rapid incorporation and withdraw of Geminin protein in cells. CP-Geminin may be useful in regulating the cell cycle and chromatin configuration. In this article, we summarize current information on the molecular functions of Geminin and the regulatory system for Geminin protein expression, and argue for the molecular role of Geminin in cell fate determination of HSCs, and future perspective of a new technology for manipulating the activities of HSCs and cancer stem cells (CSCs).

Manipulation of Cell Cycle and Chromatin Configuration by Means of Cell-Penetrating Geminin

Geminin regulates chromatin remodeling and DNA replication licensing which play an important role in regulating cellular proliferation and differentiation. Transcription of the Geminin gene is regulated via an E2F-responsive region, while the protein is being closely regulated by the ubiquitin-proteasome system. Our objective was to directly transduce Geminin protein into cells. Recombinant cell-penetrating Geminin (CP-Geminin) was generated by fusing Geminin with a membrane translocating motif from FGF4 and was efficiently incorporated into NIH 3T3 cells and mouse embryonic fibroblasts. The withdrawal study indicated that incorporated CP-Geminin was quickly reduced after removal from medium. We confirmed CP-Geminin was imported into the nucleus after incorporation and also that the incorporated CP-Geminin directly interacted with Cdt1 or Brahma/Brg1 as the same manner as Geminin. We further demonstrated that incorporated CP-Geminin suppressed S-phase progression of the cell cycle and reduced nuclease accessibility in the chromatin, probably through suppression of chromatin remodeling, indicating that CP-Geminin constitutes a novel tool for controlling chromatin configuration and the cell cycle. Since Geminin has been shown to be involved in regulation of stem cells and cancer cells, CP-Geminin is expected to be useful for elucidating the role of Geminin in stem cells and cancer cells, and for manipulating their activity.

Inactivation of Geminin in neural crest cells affects the generation and maintenance of enteric progenitor cells, leading to enteric aganglionosis

Neural crest cells comprise a multipotent, migratory cell population that generates a diverse array of cell and tissue types, during vertebrate development. Enteric Nervous System controls the function of the gastrointestinal tract and is mainly derived from the vagal and sacral neural crest cells. Deregulation on self-renewal and differentiation of the enteric neural crest cells is evident in enteric nervous system disorders, such as Hirschsprung disease, characterized by the absence of ganglia in a variable length of the distal bowel. Here we show that Geminin is essential for Enteric Nervous System generation as mice that lacked Geminin expression specifically in neural crest cells revealed decreased generation of vagal neural crest cells, and enteric neural crest cells (ENCCs). Geminin-deficient ENCCs showed increased apoptosis and decreased cell proliferation during the early stages of gut colonization. Furthermore, decreased number of committed ENCCs in vivo and the decreased self-renewal capacity of enteric progenitor cells in vitro, resulted in almost total aganglionosis resembling a severe case of Hirschsprung disease. Our results suggest that Geminin is an important regulator of self-renewal and survival of enteric nervous system progenitor cells.

A Novel Aspect of Tumorigenesis-BMI1 Functions in Regulating DNA Damage Response

BMI1 plays critical roles in maintaining the self-renewal of hematopoietic, neural, intestinal stem cells, and cancer stem cells (CSCs) for a variety of cancer types. BMI1 promotes cell proliferative life span and epithelial to mesenchymal transition (EMT). Upregulation of BMI1 occurs in multiple cancer types and is associated with poor prognosis. Mechanistically, BMI1 is a subunit of the Polycomb repressive complex 1 (PRC1), and binds the catalytic RING2/RING1b subunit to form a functional E3 ubiquitin ligase. Through mono-ubiquitination of histone H2A at lysine 119 (H2A-K119Ub), BMI1 represses multiple gene loci; among these, the INK4A/ARF locus has been most thoroughly investigated. The locus encodes the p16INK4A and p14/p19ARF tumor suppressors that function in the pRb and p53 pathways, respectively. Its repression contributes to BMI1-derived tumorigenesis. BMI1 also possesses other oncogenic functions, specifically its regulative role in DNA damage response (DDR). In this process, BMI1 ubiquitinates histone H2A and γH2AX, thereby facilitating the repair of double-stranded DNA breaks (DSBs) through stimulating homologous recombination and non-homologous end joining. Additionally, BMI1 compromises DSB-induced checkpoint activation independent of its-associated E3 ubiquitin ligase activity. We review the emerging role of BMI1 in DDR regulation and discuss its impact on BMI1-derived tumorigenesis.

Polycomb Group (PcG) Proteins and Human Cancers: Multifaceted Functions and Therapeutic Implications

Polycomb group (PcG) proteins are transcriptional repressors that regulate several crucial developmental and physiological processes in the cell. More recently, they have been found to play important roles in human carcinogenesis and cancer development and progression. The deregulation and dysfunction of PcG proteins often lead to blocking or inappropriate activation of developmental pathways, enhancing cellular proliferation, inhibiting apoptosis, and increasing the cancer stem cell population. Genetic and molecular investigations of PcG proteins have long been focused on their PcG functions. However, PcG proteins have recently been shown to exert non-classical-Pc-functions, contributing to the regulation of diverse cellular functions. We and others have demonstrated that PcG proteins regulate the expression and function of several oncogenes and tumor suppressor genes in a PcG-independent manner, and PcG proteins are associated with the survival of patients with cancer. In this review, we summarize the recent advances in the research on PcG proteins, including both the Pc-repressive and non-classical-Pc-functions. We specifically focus on the mechanisms by which PcG proteins play roles in cancer initiation, development, and progression. Finally, we discuss the potential value of PcG proteins as molecular biomarkers for the diagnosis and prognosis of cancer, and as molecular targets for cancer therapy.

MEL-18 loss mediates estrogen receptor-α downregulation and hormone independence

The polycomb protein MEL-18 has been proposed as a tumor suppressor in breast cancer; however, its functional relevance to the hormonal regulation of breast cancer remains unknown. Here, we demonstrated that MEL-18 loss contributes to the hormone-independent phenotype of breast cancer by modulating hormone receptor expression. In multiple breast cancer cohorts, MEL-18 was markedly downregulated in triple-negative breast cancer (TNBC). MEL-18 expression positively correlated with the expression of luminal markers, including estrogen receptor-α (ER-α, encoded by ESR1). MEL-18 loss was also associated with poor response to antihormonal therapy in ER-α-positive breast cancer. Furthermore, whereas MEL-18 loss in luminal breast cancer cells resulted in the downregulation of expression and activity of ER-α and the progesterone receptor (PR), MEL-18 overexpression restored ER-α expression in TNBC. Consistently, in vivo xenograft experiments demonstrated that MEL-18 loss induces estrogen-independent growth and tamoxifen resistance in luminal breast cancer, and that MEL-18 overexpression confers tamoxifen sensitivity in TNBC. MEL-18 suppressed SUMOylation of the ESR1 transactivators p53 and SP1, thereby driving ESR1 transcription. MEL-18 facilitated the deSUMOylation process by inhibiting BMI-1/RING1B-mediated ubiquitin-proteasomal degradation of SUMO1/sentrin-specific protease 1 (SENP1). These findings demonstrate that MEL-18 is a SUMO-dependent regulator of hormone receptors and suggest MEL-18 expression as a marker for determining the antihormonal therapy response in patients with breast cancer.

Geminin deletion increases the number of fetal hematopoietic stem cells by affecting the expression of key transcription factors

Balancing stem cell self-renewal and initiation of lineage specification programs is essential for the development and homeostasis of the hematopoietic system. We have specifically ablated geminin in the developing murine hematopoietic system and observed profound defects in the generation of mature blood cells, leading to embryonic lethality. Hematopoietic stem cells (HSCs) accumulated in the fetal liver following geminin ablation, while committed progenitors were reduced. Genome-wide transcriptome analysis identified key HSC transcription factors as being upregulated upon geminin deletion, revealing a gene network linked with geminin that controls fetal hematopoiesis. In order to obtain mechanistic insight into the ability of geminin to regulate transcription, we examined Hoxa9 as an example of a key gene in definitive hematopoiesis. We demonstrate that in human K562 cells geminin is associated with HOXA9 regulatory elements and its absence increases HOXA9 transcription similarly to that observed in vivo. Moreover, silencing geminin reduced recruitment of the PRC2 component SUZ12 to the HOXA9 locus and resulted in an increase in RNA polymerase II recruitment and H3K4 trimethylation (H3K4me3), whereas the repressive marks H3K9me3 and H3K27me3 were reduced. The chromatin landscape was also modified at the regulatory regions of HOXA10 and GATA1. K562 cells showed a reduced ability to differentiate to erythrocytes and megakaryocytes upon geminin silencing. Our data suggest that geminin is indispensable for fetal hematopoiesis and regulates the generation of a physiological pool of stem and progenitor cells in the fetal hematopoietic system.

Regulation of the Tumor-Suppressor Function of the Class III Phosphatidylinositol 3-Kinase Complex by Ubiquitin and SUMO

The occurrence of cancer is often associated with a dysfunction in one of the three central membrane-involution processes—autophagy, endocytosis or cytokinesis. Interestingly, all three pathways are controlled by the same central signaling module: the class III phosphatidylinositol 3-kinase (PI3K-III) complex and its catalytic product, the phosphorylated lipid phosphatidylinositol 3-phosphate (PtdIns3P). The activity of the catalytic subunit of the PI3K-III complex, the lipid-kinase VPS34, requires the presence of the membrane-targeting factor VPS15 as well as the adaptor protein Beclin 1. Furthermore, a growing list of regulatory proteins associates with VPS34 via Beclin 1. These accessory factors define distinct subunit compositions and thereby guide the PI3K-III complex to its different cellular and physiological roles. Here we discuss the regulation of the PI3K-III complex components by ubiquitination and SUMOylation. Especially Beclin 1 has emerged as a highly regulated protein, which can be modified with Lys11-, Lys48- or Lys63-linked polyubiquitin chains catalyzed by distinct E3 ligases from the RING-, HECT-, RBR- or Cullin-type. We also point out other cross-links of these ligases with autophagy in order to discuss how these data might be merged into a general concept.

Geminin loss causes neural tube defects through disrupted progenitor specification and neuronal differentiation

Geminin is a nucleoprotein that can directly bind chromatin regulatory complexes to modulate gene expression during development. Geminin knockout mouse embryos are preimplantation lethal by the 32-cell stage, precluding in vivo study of Geminin's role in neural development. Therefore, here we used a conditional Geminin allele in combination with several Cre-driver lines to define an essential role for Geminin during mammalian neural tube (NT) formation and patterning. Geminin was required in the NT within a critical developmental time window (embryonic day 8.5-10.5), when NT patterning and closure occurs. Geminin excision at these stages resulted in strongly diminished expression of genes that mark and promote dorsal NT identities and decreased differentiation of ventral motor neurons, resulting in completely penetrant NT defects, while excision after embryonic day 10.5 did not result in NT defects. When Geminin was deleted specifically in the spinal NT, both NT defects and axial skeleton defects were observed, but neither defect occurred when Geminin was excised in paraxial mesenchyme, indicating a tissue autonomous requirement for Geminin in developing neuroectoderm. Despite a potential role for Geminin in cell cycle control, we found no evidence of proliferation defects or altered apoptosis. Comparisons of gene expression in the NT of Geminin mutant versus wild-type siblings at embryonic day 10.5 revealed decreased expression of key regulators of neurogenesis, including neurogenic bHLH transcription factors and dorsal interneuron progenitor markers. Together, these data demonstrate a requirement for Geminin for NT patterning and neuronal differentiation during mammalian neurulation in vivo.

RNF2 is recruited by WASH to ubiquitinate AMBRA1 leading to downregulation of autophagy

WASH (Wiskott-Aldrich syndrome protein (WASP) and SCAR homolog) was identified to function in endosomal sorting via Arp2/3 activation. We previously demonstrated that WASH is a new interactor of BECN1 and present in the BECN1-PIK3C3 complex with AMBRA1. The AMBRA1-DDB1-CUL4A complex is an E3 ligase for K63-linked ubiquitination of BECN1, which is required for starvation-induced autophagy. WASH suppresses autophagy by inhibition of BECN1 ubiquitination. However, how AMBRA1 is regulated during autophagy remains elusive. Here, we found that RNF2 associates with AMBRA1 to act as an E3 ligase to ubiquitinate AMBRA1 via K48 linkage. RNF2 mediates ubiquitination of AMBRA1 at lysine 45. Notably, RNF2 deficiency enhances autophagy induction. Upon autophagy induction, RNF2 potentiates AMBRA1 degradation with the help of WASH. WASH deficiency impairs the association of RNF2 with AMBRA1 to impede AMBRA1 degradation. Our findings reveal another novel layer of regulation of autophagy through WASH recruitment of RNF2 for AMBRA1 degradation leading to downregulation of autophagy.

Transcription of the Geminin gene is regulated by a negative-feedback loop

Geminin transcription, regulated by E2Fs, is negatively regulated by Geminin through the inhibition of chromatin remodeling. Geminin transcription is thus regulated by a negative-feedback loop through the chromatin configuration. Homeostatically regulated Geminin may help couple regulation of DNA replication and transcription.

Geminin performs a central function in regulating cellular proliferation and differentiation in development and also in stem cells. Of interest, down-regulation of Geminin induces gene transcription regulated by E2F, indicating that Geminin is involved in regulation of E2F-mediated transcriptional activity. Because transcription of the Geminin gene is reportedly regulated via an E2F-responsive region (E2F-R) located in the first intron, we first used a reporter vector to examine the effect of Geminin on E2F-mediated transcriptional regulation. We found that Geminin transfection suppressed E2F1- and E2F2-mediated transcriptional activation and also mildly suppressed such activity in synergy with E2F5, 6, and 7, suggesting that Geminin constitutes a negative-feedback loop for the Geminin promoter. Of interest, Geminin also suppressed nuclease accessibility, acetylation of histone H3, and trimethylation of histone H3 at lysine 4, which were induced by E2F1 overexpression, and enhanced tri­methylation of histone H3 at lysine 27 and monoubiquitination of histone H2A at lysine 119 in E2F-R. However, Geminin5EQ, which does not interact with Brahma or Brg1, did not suppress accessibility to nuclease digestion or transcription but had an overall dominant-negative effect. These findings suggest that E2F-mediated activation of Geminin transcription is negatively regulated by Geminin through the inhibition of chromatin remodeling.

Aurora-A controls pre-replicative complex assembly and DNA replication by stabilizing geminin in mitosis

Geminin, an essential factor for DNA replication, directly binds to the licensing factor Cdt1 and inhibits pre-replicative complex formation to prevent re-replication. In G1, geminin levels are controlled by the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase complex, which targets geminin for proteasomal degradation to allow pre-replicative complex formation. Conversely, from S to G2, geminin is stabilized due to APC/C ubiquitin ligase complex inhibition, ensuring the inhibition of pre-replicative complex formation. However, mitotic regulation of geminin has hitherto not been described. Here we show that Aurora-A phosphorylates geminin on Thr25 during M phase, and this event induces geminin stabilization by preventing its APC/C ubiquitin ligase complex-mediated degradation during mitosis. In turn, stabilized geminin inhibits SCF^Skp2-mediated degradation of Cdt1 to ensure pre-replicative complex formation in the ensuing S phase. The Aurora-A–geminin–Cdt1 axis therefore represents a critical regulator of proper DNA replication.

Geminin blocks the inappropriate assembly of pre-replication complexes on DNA, and this activity is inhibited in G1 by its proteasomal degradation. Tsunematsu et al. demonstrate that geminin is stabilized during mitosis due to its phosphorylation by the mitotic kinase Aurora-A.

Aberrant epigenetic regulators control expansion of human CD34+ hematopoietic stem/progenitor cells

Transcription is a tightly regulated process ensuring the proper expression of numerous genes regulating all aspects of cellular behavior. Transcription factors regulate multiple genes including other transcription factors that together control a highly complex gene network. The transcriptional machinery can be “hijacked” by oncogenic transcription factors, thereby leading to malignant cell transformation. Oncogenic transcription factors manipulate a variety of epigenetic control mechanisms to fulfill gene regulatory and cell transforming functions. These factors assemble epigenetic regulators at target gene promoter sequences, thereby disturbing physiological gene expression patterns. Retroviral vector technology and the availability of “healthy” human hematopoietic CD34+ progenitor cells enable the generation of pre-leukemic cell models for the analysis of aberrant human hematopoietic progenitor cell expansion mediated by leukemogenic transcription factors. This review summarizes recent findings regarding the mechanism by which leukemogenic gene products control human hematopoietic CD34+ progenitor cell expansion by disrupting the normal epigenetic program.

Molecular profiling of human mammary gland links breast cancer risk to a p27(+) cell population with progenitor characteristics

Early full-term pregnancy is one of the most effective natural protections against breast cancer. To investigate this effect, we have characterized the global gene expression and epigenetic profiles of multiple cell types from normal breast tissue of nulliparous and parous women and carriers of BRCA1 or BRCA2 mutations. We found significant differences in CD44(+) progenitor cells, where the levels of many stem cell-related genes and pathways, including the cell-cycle regulator p27, are lower in parous women without BRCA1/BRCA2 mutations. We also noted a significant reduction in the frequency of CD44(+)p27(+) cells in parous women and showed, using explant cultures, that parity-related signaling pathways play a role in regulating the number of p27(+) cells and their proliferation. Our results suggest that pathways controlling p27(+) mammary epithelial cells and the numbers of these cells relate to breast cancer risk and can be explored for cancer risk assessment and prevention.

Multilocus loss of DNA methylation in individuals with mutations in the histone H3 lysine 4 demethylase KDM5C

BACKGROUND

A number of neurodevelopmental syndromes are caused by mutations in genes encoding proteins that normally function in epigenetic regulation. Identification of epigenetic alterations occurring in these disorders could shed light on molecular pathways relevant to neurodevelopment.

RESULTS

Using a genome-wide approach, we identified genes with significant loss of DNA methylation in blood of males with intellectual disability and mutations in the X-linked KDM5C gene, encoding a histone H3 lysine 4 demethylase, in comparison to age/sex matched controls. Loss of DNA methylation in such individuals is consistent with known interactions between DNA methylation and H3 lysine 4 methylation. Further, loss of DNA methylation at the promoters of the three top candidate genes FBXL5, SCMH1, CACYBP was not observed in more than 900 population controls. We also found that DNA methylation at these three genes in blood correlated with dosage of KDM5C and its Y-linked homologue KDM5D. In addition, parallel sex-specific DNA methylation profiles in brain samples from control males and females were observed at FBXL5 and CACYBP.

CONCLUSIONS

We have, for the first time, identified epigenetic alterations in patient samples carrying a mutation in a gene involved in the regulation of histone modifications. These data support the concept that DNA methylation and H3 lysine 4 methylation are functionally interdependent. The data provide new insights into the molecular pathogenesis of intellectual disability. Further, our data suggest that some DNA methylation marks identified in blood can serve as biomarkers of epigenetic status in the brain.

RNF2/Ring1b negatively regulates p53 expression in selective cancer cell types to promote tumor development

Large numbers of studies have focused on the posttranslational regulation of p53 activity. One of the best-known negative regulators for p53 is MDM2, an E3 ubiquitin ligase that promotes p53 degradation through proteasome degradation pathways. Additional E3 ligases have also been reported to negatively regulate p53. However, whether these E3 ligases have distinct/overlapping roles in the regulation of p53 is largely unknown. In this study, we identify RNF2 (ring finger protein 2) as an E3 ligase that targets p53 for degradation. The E3 ligase activity of RNF2 requires Bmi1 protein, a component of the polycomb group (PcG) complex. The up-regulation of p53 does not affect RNF2 expression. Unlike Mdm2, RNF2 only degrades p53 in selective cell lines, such as those from germ-cell tumors. The knockdown of RNF2 induces apoptosis, which can be rescued through the reduction of p53 expression. Moreover, the down-regulation of RNF2 expression in germ-cell tumors significantly reduces tumor cell growth, while the simultaneous down-regulation of both genes restores tumor cell growth in vitro and in tumor xenograft models. Furthermore, a reverse correlation between RNF2 and p53 expression was detected in human ovarian cancer tissues. Together, these results indicate that RNF2 is an E3 ligase for p53 degradation in selective cells, implicating RNF2 as a therapeutic target to restore tumor suppression through p53 in certain tumor cells.

Hoxa9 transduction induces hematopoietic stem and progenitor cell activity through direct down-regulation of geminin protein

Hoxb4, a 3′-located Hox gene, enhances hematopoietic stem cell (HSC) activity, while a subset of 5′-located Hox genes is involved in hematopoiesis and leukemogenesis, and some of them are common translocation partners for Nucleoporin 98 (Nup98) in patients with leukemia. Although these Hox gene derivatives are believed to act as transcription regulators, the molecular involvement of the Hox gene derivatives in hematopoiesis and leukemogenesis remains largely elusive. Since we previously showed that Hoxb4 forms a complex with a Roc1-Ddb1-Cul4a ubiquitin ligase core component and functions as an E3 ubiquitin ligase activator for Geminin, we here examined the E3 ubiquitin ligase activities of the 5′-located Hox genes, Hoxa9 and Hoxc13, and Nup98-Hoxa9. Hoxa9 formed a similar complex with the Roc1-Ddb1-Cul4a component to induce ubiquitination of Geminin, but the others did not. Retroviral transduction-mediated overexpression or siRNA-mediated knock-down of Hoxa9 respectively down-regulated or up-regulated Geminin in hematopoietic cells. And Hoxa9 transduction-induced repopulating and clonogenic activities were suppressed by Geminin supertransduction. These findings suggest that Hoxa9 and Hoxb4 differ from Hoxc13 and Nup98-Hoxa9 in their molecular role in hematopoiesis, and that Hoxa9 induces the activity of HSCs and hematopoietic progenitors at least in part through direct down-regulation of Geminin.

Emerging role of the ubiquitin proteasome system in the control of shoot apical meristem function(f)

The shoot apical meristem (SAM) is a population of undifferentiated cells at the tip of the shoot axis that establishes early during plant embryogenesis and gives rise to all shoot organs throughout the plant's life. A plethora of different families of transcription factors (TFs) play a key role in establishing the equilibrium between cell differentiation and stem cell maintenance in the SAM. Fine tuning of these regulatory proteins is crucial for a proper and fast SAM response to environmental and hormonal cues, and for development progression. One effective way to rapidly inactivate TFs involves regulated proteolysis by the ubiquitin/26S proteasome system (UPS). However, a possible role of UPS-dependent protein degradation in the regulation of key SAM TFs has not been thoroughly investigated. Here, we summarize recent evidence supporting a role for the UPS in SAM maintenance and function. We integrate this survey with an in silico analysis of publicly-available microarray databases which identified ubiquitin ligases that are expressed in specific areas within the SAM, suggesting that they may regulate or act downstream of meristem-specific factors.

Scmh1 has E3 ubiquitin ligase activity for geminin and histone H2A and regulates geminin stability directly or indirectly via transcriptional repression of Hoxa9 and Hoxb4

Polycomb-group (PcG) complex 1 acts as an E3 ubiquitin ligase both for histone H2A to silence transcription and for geminin to regulate its stability. Scmh1 is a substoichiometric component of PcG complex 1 that provides the complex with an interaction domain for geminin. Scmh1 is unstable and regulated through the ubiquitin-proteasome system, but its molecular roles are unknown, so we generated Scmh1-deficient mice to elucidate its function. Loss of Scmh1 caused derepression of Hoxb4 and Hoxa9, direct targets of PcG complex 1-mediated transcriptional silencing in hematopoietic cells. Double knockdown of Hoxb4 and Hoxa9 or transduction of a dominant-negative Hoxb4N→A mutant caused geminin accumulation. Age-related transcriptional downregulation of derepressed Hoxa9 also leads to geminin accumulation. Transduction of Scmh1 lacking a geminin-binding domain restored derepressed expression of Hoxb4 and Hoxa9 but did not downregulate geminin like full-length Scmh1. Each of Hoxb4 and Hoxa9 can form a complex with Roc1-Ddb1-Cul4a to act as an E3 ubiquitin ligase for geminin. We suggest that geminin dysregulation may be restored by derepressed Hoxb4 and Hoxa9 in Scmh1-deficient mice. These findings suggest that PcG and a subset of Hox genes compose a homeostatic regulatory system for determining expression level of geminin.

Differential chromatin structure encompassing replication origins in transformed and normal cells

This study examines the chromatin structure encompassing replication origins in transformed and normal cells. Analysis of the global levels of histone H3 acetylated at K9&14 (open chromatin) and histone H3 trimethylated at K9 (closed chromatin) revealed a higher ratio of open to closed chromatin in the transformed cells. Also, the trithorax and polycomb group proteins, Brg-1 and Bmi-1, respectively, were overexpressed and more abundantly bound to chromatin in the transformed cells. Quantitative comparative analyses of episomal and in situ chromosomal replication origin activity as well as chromatin immunoprecipitation (ChIP) assays, using specific antibodies targeting members of the pre-replication complex (pre-RC) as well as open/closed chromatin markers encompassing both episomal and chromosomal origins, revealed that episomal origins had similar levels of in vivo activity, nascent DNA abundance, pre-RC protein association, and elevated open chromatin structure at the origin in both cell types. In contrast, the chromosomal origins corresponding to 20mer1, 20mer2, and c-myc displayed a 2- to 3-fold higher activity and pre-RC protein abundance as well as higher ratios of open to closed chromatin and of Brg-1 to Bmi-1 in the transformed cells, whereas the origin associated with the housekeeping lamin B2 gene exhibited similar levels of activity, pre-RC protein abundance, and higher ratios of open to closed chromatin and of Brg-1 to Bmi-1 in both cell types. Nucleosomal positioning analysis, using an MNase-Southern blot assay, showed that all the origin regions examined were situated within regions of inconsistently positioned nucleosomes, with the nucleosomes being spaced farther apart from each other prior to the onset of S phase in both cell types. Overall, the results indicate that cellular transformation is associated with differential epigenetic regulation, whereby chromatin structure is more open, rendering replication origins more accessible to initiator proteins, thus allowing increased origin activity.

A long ncRNA links copy number variation to a polycomb/trithorax epigenetic switch in FSHD muscular dystrophy

Repetitive sequences account for more than 50% of the human genome. Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal-dominant disease associated with reduction in the copy number of the D4Z4 repeat mapping to 4q35. By an unknown mechanism, D4Z4 deletion causes an epigenetic switch leading to de-repression of 4q35 genes. Here we show that the Polycomb group of epigenetic repressors targets D4Z4 in healthy subjects and that D4Z4 deletion is associated with reduced Polycomb silencing in FSHD patients. We identify DBE-T, a chromatin-associated noncoding RNA produced selectively in FSHD patients that coordinates de-repression of 4q35 genes. DBE-T recruits the Trithorax group protein Ash1L to the FSHD locus, driving histone H3 lysine 36 dimethylation, chromatin remodeling, and 4q35 gene transcription. This study provides insights into the biological function of repetitive sequences in regulating gene expression and shows how mutations of such elements can influence the progression of a human genetic disease.

Cdt1 and geminin in DNA replication initiation

One of the mechanisms controlling the initiation of DNA replication is the dynamic interaction between Cdt1, which promotes assembly of the pre-replication license complex, and Geminin, which inhibits it. Specifically, Cdt1 cooperates with the cell cycle protein Cdc6 to promote loading of the minichromosome maintenance helicases (MCM) onto the chromatin-bound origin recognition complex (ORC), by directly interacting with the MCM complex, and by modulating histone acetylation and inducing chromatin unfolding. Geminin, on the other hand, prevents the loading of the MCM onto the ORC both by directly binding to Cdt1, and by modulating Cdt1 stability and activity. Protein levels of Geminin and Cdt1 are tightly regulated through the cell cycle, and the Cdt1-Geminin complex likely acts as a molecular switch that can enable or disable the firing of each origin of replication. In this review we summarize structural studies of Cdt1 and Geminin and subsequent insights into how this molecular switch may function to ensure DNA is faithfully replicated only once during S phase of each cell cycle.

Geminin regulates cortical progenitor proliferation and differentiation

During cortical development, coordination of proliferation and differentiation ensures the timely generation of different neural progenitor lineages that will give rise to mature neurons and glia. Geminin is an inhibitor of DNA replication and it has been proposed to regulate cell proliferation and fate determination during neurogenesis via interactions with transcription factors and chromatin remodeling complexes. To investigate the in vivo role of Geminin in the maintenance and differentiation of cortical neural progenitors, we have generated mice that lack Geminin expression in the developing cortex. Our results show that loss of Geminin leads to the expansion of neural progenitor cells located at the ventricular and subventricular zones of the developing cortex. Early cortical progenitors lacking Geminin exhibit a longer S-phase and a reduced ability to generate early born neurons, consistent with a preference on self-renewing divisions. Overexpression of Geminin in progenitor cells of the cortex reduces the number of neural progenitor cells, promotes cell cycle exit and subsequent neuronal differentiation. Our study suggests that Geminin has an important role during cortical development in regulating progenitor number and ultimately neuron generation.

Polycomb group proteins: multi-faceted regulators of somatic stem cells and cancer

Polycomb Group (PcG) proteins are transcriptional repressors that epigenetically modify chromatin and participate in the establishment and maintenance of cell fates. These proteins play important roles in both stem cell self-renewal and in cancer development. Our understanding of their mechanism of action has greatly advanced over the past 10 years, but many unanswered questions remain. In this review, we present the currently available experimental data that connect PcG protein function with some of the key processes which govern somatic stem cell activity. We also highlight recent studies suggesting that a delicate balance in PcG gene dosage is crucial for proper stem cell homeostasis and prevention of cancer stem cell development.

Hoxb4 transduction down-regulates Geminin protein, providing hematopoietic stem and progenitor cells with proliferation potential

Retrovirus-mediated transduction of Hoxb4 enhances hematopoietic stem cell (HSC) activity and enforced expression of Hoxb4 induces in vitro development of HSCs from differentiating mouse embryonic stem cells, but the underlying molecular mechanism remains unclear. We previously showed that the HSC activity was abrogated by accumulated Geminin, an inhibitor for the DNA replication licensing factor Cdt1 in mice deficient in Rae28 (also known as Phc1), which encodes a member of Polycomb-group complex 1. In this study we found that Hoxb4 transduction reduced accumulated Geminin in Rae28-deficient mice, despite increasing the mRNA, and restored the impaired HSC activity. Supertransduction of Geminin suppressed the HSC activity induced by Hoxb4 transduction, whereas knockdown of Geminin promoted the clonogenic and replating activities, indicating the importance of Geminin regulation in the molecular mechanism underlying Hoxb4 transduction-mediated enhancement of the HSC activity. This facilitated our investigation of how transduced Hoxb4 reduced Geminin. We showed in vitro and in vivo that Hoxb4 and the Roc1 (also known as Rbx1)-Ddb1-Cul4a ubiquitin ligase core component formed a complex designated as RDCOXB4, which acted as an E3 ubiquitin ligase for Geminin and down-regulated Geminin through the ubiquitin-proteasome system. Down-regulated Geminin and the resultant E2F activation may provide cells with proliferation potential by increasing a DNA prereplicative complex loaded onto chromatin. Here we suggest that transduced Hoxb4 down-regulates Geminin protein probably by constituting the E3 ubiquitin ligase for Geminin to provide hematopoietic stem and progenitor cells with proliferation potential.

Keeping cell identity in Arabidopsis requires PRC1 RING-finger homologs that catalyze H2A monoubiquitination

Polycomb group (PcG) proteins form conserved regulatory complexes that modify chromatin to repress the genes that are not required in a specific differentiation status [1]. In animals, the two best-characterized PcG complexes are PRC2 and PRC1, which respectively possess histone 3 lysine 27 (H3K27) trimethyltransferase [2-4] and histone 2A lysine 119 (H2AK119) E3 ubiquitin ligase activities [5-7]. In Arabidopsis, PRC2 activity is also required for the gene silencing mechanism [8]; however, the existence of PRC1 has been questioned, because plant genomes do not encode clear PRC1 components and H2A monoubiquitination has not been detected [6, 9]. Conversely, recent reports have unveiled the presence of homologs to PRC1 components that together with plant-specific proteins could be part of the long-sought PRC1-like complexes [10, 11]. Here we show that the PRC1 RING-finger homologs AtBMI1A and AtBMI1B are implicated in the repression of embryonic and stem cell regulators. Plants impaired in AtBMI1A and AtBMI1B show derepression of embryonic traits in somatic cells, displaying a phenotype similar to plants mutant in PRC2 components [12-14]. Our data demonstrate that the AtBMI1A/B proteins mediate H2A monoubiquitination in Arabidopsis and that this mark, together with PRC2-mediated H3K27 trimethylation, plays a key role in maintaining cell identity.

Life without geminin

The interplay of proliferation and differentiation is essential for normal development and organogenesis. Geminin is a cell cycle regulator which controls licensing of origins for DNA replication, safeguarding genomic stability. Geminin has also been shown to regulate cellular decisions of self-renewal versus commitment of neuronal progenitor cells. We discuss here our recent analysis of mice with conditional inactivation of the Geminin gene in the immune system. Our data indicate that Geminin is not indispensable for every cell division: in the absence of Geminin, development of progenitor T cells appears largely unaffected. In contrast, rapid cell divisions, taking place in vitro upon TCR receptor activation or in vivo during homeostatic proliferation, are defective.

Id1 enhances RING1b E3 ubiquitin ligase activity through the Mel-18/Bmi-1 polycomb group complex

The helix-loop-helix inhibitor of differentiation and DNA binding (Id1) is well known as an oncogene in various tumors. Although it has been reported that Id1 promotes several oncogenic processes, it is still unclear whether Id1 functions through epigenetic transcriptional regulation. In this study, we examined the effect of Id1 on polycomb group (PcG) proteins, which are crucial epigenetic gene silencers, and found that Id1 regulated the expression of Mel-18 and Bmi-1, both of which belong to polycomb repressive complex 1. We also confirmed that Id1 induced Mel-18 downregulation, which was mediated by the Akt pathway, and consequently upregulated the transcription of its target gene, c-Myc. Using a promoter-reporter, we demonstrated that Id1 regulated Bmi-1 transcription through c-Myc binding to its E-box in the promoter. Finally, we examined the activity of E3 ligase RING1b, whose catalytic activity is increased by binding with the RING finger protein Bmi-1, and found that Id1 overexpression enhanced RING1b E3 ligase activity leading to accumulation of H2A ubiquitination and ubiquitin/proteasome-mediated degradation of geminin. Taken together, our study provided a novel link between Id1 and PcG proteins and suggested that Id1 may contribute to tumor development through PcG-mediated epigenetic regulation.

Polycomb group proteins as epigenetic mediators of neuroprotection in ischemic tolerance

Exposing the brain to sublethal ischemia affects the response to a subsequent, otherwise injurious ischemia, resulting in transcriptional suppression and neuroprotection, a response called ischemic tolerance. Here, we show that the proteomic signature of the ischemic-tolerant brain is characterized by increased abundance of transcriptional repressors, particularly polycomb group (PcG) proteins. Knocking down PcG proteins precluded the induction of ischemic tolerance, whereas in an in vitro model, overexpressing the PcG proteins SCMH1 or BMI1 induced tolerance to ischemia without preconditioning. We found that PcG proteins are associated with the promoter regions of genes encoding two potassium channel proteins that show decreased abundance in ischemic-tolerant brains. Furthermore, PcG proteins decreased potassium currents in cultured neuronal cells, and knocking down potassium channels elicited tolerance without preconditioning. These findings reveal a previously unknown mechanism of neuroprotection that involves gene repressors of the PcG family.

Differential geminin requirement for proliferation of thymocytes and mature T cells

Stem/progenitor cells coordinate proliferation and differentiation, giving rise to appropriate cell numbers of functionally specialized cells during organogenesis. In different experimental systems, Geminin was shown to maintain progenitor cells and participate in fate determination decisions and organogenesis. Although the exact mechanisms are unclear, Geminin has been postulated to influence proliferation versus differentiation decisions. To gain insight into the in vivo role of Geminin in progenitor cell division and differentiation, we have generated mice that specifically lack Geminin in cells of lymphoid lineage through Cre-mediated recombination. T cells lacking Geminin expression upregulate early activation markers efficiently upon TCR stimulation in vitro and are able to enter the S phase of cell cycle, but show a marked defect in completing the cycle, leading to a large proportion of T cells accumulating in S/G2/M phases. Accordingly, T cells deficient in Geminin show a reduced ability to repopulate lymphopenic hosts in vivo. Contrary to expectations, Geminin deficiency does not alter development and differentiation of T cells in vivo. Our data suggest that Geminin is required for the proliferation events taking place either in vitro upon TCR receptor activation or during homeostatic expansion, but appears to be redundant for the proliferation and differentiation of the majority of progenitor T cell populations.

Talking to chromatin: post-translational modulation of polycomb group function

Polycomb Group proteins are important epigenetic regulators of gene expression. Epigenetic control by polycomb Group proteins involves intrinsic as well as associated enzymatic activities. Polycomb target genes change with cellular context, lineage commitment and differentiation status, revealing dynamic regulation of polycomb function. It is currently unclear how this dynamic modulation is controlled and how signaling affects polycomb-mediated epigenetic processes at the molecular level. Experimental evidence on regulation of polycomb function by post-translational mechanisms is steadily emerging: Polycomb Group proteins are targeted for ubiquitylation, sumoylation and phosphorylation. In addition, specific Polycomb Group proteins modify other (chromatin) associated proteins via similar post-translational modifications. Such modifications affect protein function by affecting protein stability, protein-protein interactions and enzymatic activities. Here, we review current insights in covalent modification of Polycomb Group proteins in the context of protein function and present a tentative view of integrated signaling to chromatin in the context of phosphorylation. Clearly, the available literature reveals just the tip of the iceberg, and exact molecular mechanisms in, and the biological relevance of post-translational regulation of polycomb function await further elucidation. Our understanding of causes and consequences of post-translational modification of polycomb proteins will gain significantly from in vivo validation experiments. Impaired polycomb function has important repercussions for stem cell function, development and disease. Ultimately, increased understanding of signaling to chromatin and the mechanisms involved in epigenetic remodeling will contribute to the development of therapeutic interventions in cell fate decisions in development and disease.

MicroRNA-98 and let-7 confer cholangiocyte expression of cytokine-inducible Src homology 2-containing protein in response to microbial challenge

Posttranscriptional gene regulation by microRNAs (miRNAs) has been implicated in the fine-tuning of TLR-mediated inflammatory response. The cytokine-inducible Src homology 2-containing protein (CIS), one member of the suppressors of cytokine signaling family of proteins, is an important negative regulator for inflammatory cytokine signaling. Using in vitro models using normal human biliary epithelial cells (cholangiocytes), we demonstrated that LPS stimulation or infection with the parasitic protozoan Cryptosporidium parvum induced expression of CIS protein without a change in CIS mRNA levels by activating the TLR signaling pathway. Of those miRNAs expressed in cholangiocytes, we found that targeting of the 3'-untranslated region of CIS by microRNA-98 (miR-98) or let-7 resulted in translational repression, but not CIS mRNA degradation. LPS stimulation or C. parvum infection decreased cholangiocyte expression of miR-98 and let-7. Down-regulation of miR-98 and let-7 relieved miRNA-mediated translational suppression of CIS and contributed to LPS- and C. parvum-stimulated CIS protein expression. Moreover, gain-of-function (by overexpression of CIS) and loss-of-function (by siRNA interference) studies revealed that CIS could enhance IkappaBalpha degradation and regulate NF-kappaB activation in cholangiocytes in response to LPS stimulation or C. parvum infection. Our data suggest that miR-98 and let-7 confer cholangiocyte expression of CIS in response to microbial challenge, a process that may be relevant to the regulation of TLR-mediated epithelial innate immune response.