Post‐translational modification of POU domain transcription factor Oct‐4 by SUMO‐1

SUMO-dependent transcriptional repression by Sox2 inhibits the proliferation of neural stem cells

Sox2 is known for its roles in maintaining the stem cell state of embryonic stem cells and neural stem cells. In particular, it has been shown to slow the proliferation of these cell types. It is also known for its effects as an activating transcription factor. Despite this, analysis of published studies shows that it represses as many genes as it activates. Here, we identify a new set of target genes that Sox2 represses in neural stem cells. These genes are associated with centrosomes, centromeres and other aspects of cell cycle control. In addition, we show that SUMOylation of Sox2 is necessary for the repression of these genes and for its repressive effects on cell proliferation. Together, these data suggest that SUMO-dependent repression of this group of target genes is responsible for the role of Sox2 in regulating the proliferation of neural stem cells.

Paradoxes of Cellular SUMOylation Regulation: A Role of Biomolecular Condensates?

Protein SUMOylation is a major post-translational modification essential for maintaining cellular homeostasis. SUMOylation has long been associated with stress responses as a diverse array of cellular stress signals are known to trigger rapid alternations in global protein SUMOylation. In addition, while there are large families of ubiquitination enzymes, all small ubiquitin-like modifiers (SUMOs) are conjugated by a set of enzymatic machinery comprising one heterodimeric SUMO-activating enzyme, a single SUMO-conjugating enzyme, and a small number of SUMO protein ligases and SUMO-specific proteases. How a few SUMOylation enzymes specifically modify thousands of functional targets in response to diverse cellular stresses remains an enigma. Here we review recent progress toward understanding the mechanisms of SUMO regulation, particularly the potential roles of liquid-liquid phase separation/biomolecular condensates in regulating cellular SUMOylation during cellular stresses. In addition, we discuss the role of protein SUMOylation in pathogenesis and the development of novel therapeutics targeting SUMOylation. SIGNIFICANCE STATEMENT: Protein SUMOylation is one of the most prevalent post-translational modifications and plays a vital role in maintaining cellular homeostasis in response to stresses. Protein SUMOylation has been implicated in human pathogenesis, such as cancer, cardiovascular diseases, neurodegeneration, and infection. After more than a quarter century of extensive research, intriguing enigmas remain regarding the mechanism of cellular SUMOylation regulation and the therapeutic potential of targeting SUMOylation.

Using single cell atlas data to reconstruct regulatory networks

Inference of global gene regulatory networks from omics data is a long-term goal of systems biology. Most methods developed for inferring transcription factor (TF)-gene interactions either relied on a small dataset or used snapshot data which is not suitable for inferring a process that is inherently temporal. Here, we developed a new computational method that combines neural networks and multi-task learning to predict RNA velocity rather than gene expression values. This allows our method to overcome many of the problems faced by prior methods leading to more accurate and more comprehensive set of identified regulatory interactions. Application of our method to atlas scale single cell data from 6 HuBMAP tissues led to several validated and novel predictions and greatly improved on prior methods proposed for this task.

Protein sumoylation in normal and cancer stem cells

Stem cells with the capacity of self-renewal and differentiation play pivotal roles in normal tissues and malignant tumors. Whereas stem cells are supposed to be genetically identical to their non-stem cell counterparts, cell stemness is deliberately regulated by a dynamic network of molecular mechanisms. Reversible post-translational protein modifications (PTMs) are rapid and reversible non-genetic processes that regulate essentially all physiological and pathological process. Numerous studies have reported the involvement of post-translational protein modifications in the acquirement and maintenance of cell stemness. Recent studies underscore the importance of protein sumoylation, i.e., the covalent attachment of the small ubiquitin-like modifiers (SUMO), as a critical post-translational protein modification in the stem cell populations in development and tumorigenesis. In this review, we summarize the functions of protein sumoylation in different kinds of normal and cancer stem cells. In addition, we describe the upstream regulators and the downstream effectors of protein sumoylation associated with cell stemness. We also introduce the translational studies aiming at sumoylation to target stem cells for disease treatment. Finally, we propose future directions for sumoylation studies in stem cells.

SUMO control of nervous system development

In the last decades, the post-translational modification system by covalent attachment of the SUMO polypeptide to proteins has emerged as an essential mechanism controlling virtually all the physiological processes in the eukaryotic cell. This includes vertebrate development. In the nervous system, SUMO plays crucial roles in synapse establishment and it has also been linked to a variety of neurodegenerative diseases. However, to date, the involvement of the modification of specific targets in key aspects of nervous system development, like patterning and differentiation, has remained largely elusive. A number of recent works confirm the participation of target-specific SUMO modification in critical aspects of nervous system development. Here, we review pioneering and new findings demonstrating the essential role SUMO plays in neurogenesis and other facets of neurodevelopment, which will help to precisely understand the variety of mechanisms SUMO utilizes to control most fundamental processes in the cell.

ISG15 enhances glioma cell stemness by promoting Oct4 protein stability

The effects of ISG15 or ISGylation on tumor progression have been widely revealed; however, its roles in glioma progression are largely unknown. This study aims to explore the roles and underlying mechanisms of ISG15 in glioma progression. Here, ISG15 level was found to be upregulated in glioma tissues compared to the paired/unpaired normal tissues, and positively correlated with the level of stemness markers in glioma tissues. Loss of functional experiments indicated that ISG15 positively regulated glioma cell stemness, as evident by the increase of sphere formation ability, ALDH activity, stemness marker expression, and tumor-initiating ability. Further mechanistic studies revealed that ISG15 directly interacted with Oct4 protein, a critical stemness promoter, induced the ISGylation of Oct4 protein, and thus enhanced Oct4 protein stability. Additionally, it was found that Oct4 was ISGylated at lysine 284 (K284), which has been confirmed to be the ubiquitination site of Oct4 protein, and ISG15 knockdown did not degrade K284R mutant Oct4. Furthermore, ISG15 knockdown-induced downregulation of glioma cell stemness was rescued by Oct4 overexpression, but not by K284R mutant Oct4. Altogether, we suggest that ISG15-induced ISGylation of Oct4 protein is essential for glioma cell stemness.

Sumoylation in Physiology, Pathology and Therapy

Sumoylation is an essential post-translational modification that has evolved to regulate intricate networks within emerging complexities of eukaryotic cells. Thousands of target substrates are modified by SUMO peptides, leading to changes in protein function, stability or localization, often by modulating interactions. At the cellular level, sumoylation functions as a key regulator of transcription, nuclear integrity, proliferation, senescence, lineage commitment and stemness. A growing number of prokaryotic and viral proteins are also emerging as prime sumoylation targets, highlighting the role of this modification during infection and in immune processes. Sumoylation also oversees epigenetic processes. Accordingly, at the physiological level, it acts as a crucial regulator of development. Yet, perhaps the most prominent function of sumoylation, from mammals to plants, is its role in orchestrating organismal responses to environmental stresses ranging from hypoxia to nutrient stress. Consequently, a growing list of pathological conditions, including cancer and neurodegeneration, have now been unambiguously associated with either aberrant sumoylation of specific proteins and/or dysregulated global cellular sumoylation. Therapeutic enforcement of sumoylation can also accomplish remarkable clinical responses in various diseases, notably acute promyelocytic leukemia (APL). In this review, we will discuss how this modification is emerging as a novel drug target, highlighting from the perspective of translational medicine, its potential and limitations.

Extensive SUMO Modification of Repressive Chromatin Factors Distinguishes Pluripotent from Somatic Cells

Post-translational modification by SUMO is a key regulator of cell identity. In mouse embryonic fibroblasts (MEFs), SUMO impedes reprogramming to pluripotency, while in embryonic stem cells (ESCs), it represses the emergence of totipotent-like cells, suggesting that SUMO targets distinct substrates to preserve somatic and pluripotent states. Using MS-based proteomics, we show that the composition of endogenous SUMOylomes differs dramatically between MEFs and ESCs. In MEFs, SUMO2/3 targets proteins associated with canonical SUMO functions, such as splicing, and transcriptional regulators driving somatic enhancer selection. In contrast, in ESCs, SUMO2/3 primarily modifies highly interconnected repressive chromatin complexes, thereby preventing chromatin opening and transitioning to totipotent-like states. We also characterize several SUMO-modified pluripotency factors and show that SUMOylation of Dppa2 and Dppa4 impedes the conversion to 2-cell-embryo-like states. Altogether, we propose that rewiring the repertoire of SUMO target networks is a major driver of cell fate decision during embryonic development.

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Endogenous SUMO2/3 proteomics in ESCs and MEFs uncovers drastic SUMOylome rewiring

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In ESCs, SUMO2/3 targets densely interconnected repressive chromatin proteins

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In MEFs, SUMO2/3 targets key determinants of fibroblastic cell identity

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SUMOylation of Dppa2/4 prevents conversion of ESCs to the 2C-like state

Regulation of the protein stability and transcriptional activity of OCT4 in stem cells

OCT4 (also known as Oct3 and Oct3/4), which is encoded by Pou5f1, is expressed in early embryonic cells and plays an important role in early development, pluripotency maintenance, and self-renewal of embryonic stem cells. It also regulates the reprogramming of somatic cells into induced pluripotent stem cells. Several OCT4-binding proteins, including SOX2 and NANOG, reportedly regulate gene transcription in stem cells. An increasing number of evidence suggests that not only gene transcription but also post-translational modifications of OCT4 play a pivotal role in regulating the expression and activity of OCT4. For instance, ubiquitination and sumoylation have been reported to regulate OCT4 protein stability. In addition, the phosphorylation of Ser347 in OCT4 also stabilizes the OCT4 protein level. Recently, we identified KAP1 as an OCT4-binding protein and reported the KAP1-mediated regulation of OCT4 protein stability. KAP1 overexpression led to an increased proliferation of mouse embryonic stem cells and promoted the reprogramming of somatic cells resulting in induced pluripotent stem cells. In this review, we discuss how the protein stability and function of OCT4 are regulated by protein-protein interaction in stem cells.

Roles of OCT4 in pathways of embryonic development and cancer progression

Somatic cells may be reprogrammed to pluripotent state by ectopic expression of certain transcription factors; namely, OCT4, SOX2, KLF4 and c-MYC. However, the molecular and cellular mechanisms are not adequately understood, especially for human embryonic development. Studies during the last five years implicated importance of OCT4 in human zygotic genome activation (ZGA), patterns of OCT4 protein folding and role of specialized sequences in binding to DNA for modulation of gene expression during development. Epigenetic modulation of OCT4 gene and post translational modifications of OCT4 protein activity in the context of multiple cancers are important issues. A consensus is emerging that chromatin organization and epigenetic landscape play crucial roles for the interactions of transcription factors, including OCT4 with the promoters and/or regulatory sequences of genes associated with human embryonic development (ZGA through lineage specification) and that when the epigenome niche is deregulated OCT4 helps in cancer progression, and how OCT4 silencing in somatic cells of adult organisms may impact ageing.

Chaperones and Beyond as Key Players in Pluripotency Maintenance

Pluripotency is orchestrated by distinct players and chaperones and their partners have emerged as pivotal molecules in proteostasis control to maintain stemness. The proteostasis network consists of diverse interconnected pathways that function dynamically according to the needs of the cell to quality control and maintain protein homeostasis. The proteostasis machinery of pluripotent stem cells (PSCs) is finely adjusted in response to distinct stimuli during cell fate commitment to determine successful organism development. Growing evidence has shown different classes of chaperones regulating crucial cellular processes in PSCs. Histones chaperones promote proper nucleosome assembly and modulate the epigenetic regulation of factors involved in PSCs’ rapid turnover from pluripotency to differentiation. The life cycle of pluripotency proteins from synthesis and folding, transport and degradation is finely regulated by chaperones and co-factors either to maintain the stemness status or to cell fate commitment. Here, we summarize current knowledge of the chaperone network that govern stemness and present the versatile role of chaperones in stem cells resilience. Elucidation of the intricate regulation of pluripotency, dissecting in detail molecular determinants and drivers, is fundamental to understanding the properties of stem cells in order to provide a reliable foundation for biomedical research and regenerative medicine.

Disruption of OCT4 Ubiquitination Increases OCT4 Protein Stability and ASH2L-B-Mediated H3K4 Methylation Promoting Pluripotency Acquisition

The protein level of OCT4, a core pluripotency transcription factor, is vital for embryonic stem cell (ESC) maintenance, differentiation, and somatic cell reprogramming. However, how OCT4 protein levels are controlled during reprogramming remains largely unknown. Here, we identify ubiquitin conjugation sites of OCT4 and report that disruption of WWP2-catalyzed OCT4 ubiquitination or ablation of Wwp2 significantly promotes the efficiency of pluripotency induction from mouse embryonic fibroblasts. Mechanistically, disruption of WWP2-mediated OCT4 ubiquitination elevates OCT4 protein stability and H3K4 methylation level during the reprogramming process. Furthermore, we reveal that OCT4 directly activates expression of Ash2l-b, and that ASH2L-B is a major isoform of ASH2L highly expressed in ESCs and required for somatic cell reprogramming. Together, this study emphasizes the importance of ubiquitination manipulation of the reprogramming factor and its interplay with the epigenetic regulator for successful reprogramming, opening a new avenue to improve the efficiency of pluripotency induction.

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Five lysine residues are major ubiquitination sites of OCT4 catalyzed by WWP2

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Disruption of OCT4 ubiquitination promotes somatic cell reprogramming

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Mutation of OCT4 ubiquitination sites enhances OCT4 stability and H3K4me levels

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ASH2L-B contributes to somatic cell reprogramming as a direct target of OCT4

Diversity among POU transcription factors in chromatin recognition and cell fate reprogramming

The POU (Pit-Oct-Unc) protein family is an evolutionary ancient group of transcription factors (TFs) that bind specific DNA sequences to direct gene expression programs. The fundamental importance of POU TFs to orchestrate embryonic development and to direct cellular fate decisions is well established, but the molecular basis for this activity is insufficiently understood. POU TFs possess a bipartite 'two-in-one' DNA binding domain consisting of two independently folding structural units connected by a poorly conserved and flexible linker. Therefore, they represent a paradigmatic example to study the molecular basis for the functional versatility of TFs. Their modular architecture endows POU TFs with the capacity to accommodate alternative composite DNA sequences by adopting different quaternary structures. Moreover, associations with partner proteins crucially influence the selection of their DNA binding sites. The plentitude of DNA binding modes confers the ability to POU TFs to regulate distinct genes in the context of different cellular environments. Likewise, different binding modes of POU proteins to DNA could trigger alternative regulatory responses in the context of different genomic locations of the same cell. Prominent POU TFs such as Oct4, Brn2, Oct6 and Brn4 are not only essential regulators of development but have also been successfully employed to reprogram somatic cells to pluripotency and neural lineages. Here we review biochemical, structural, genomic and cellular reprogramming studies to examine how the ability of POU TFs to select regulatory DNA, alone or with partner factors, is tied to their capacity to epigenetically remodel chromatin and drive specific regulatory programs that give cells their identities.

Serine 347 Phosphorylation by JNKs Negatively Regulates OCT4 Protein Stability in Mouse Embryonic Stem Cells

The POU transcription factor OCT4 is critical for maintaining the undifferentiated state of embryonic stem cells (ESCs) and generating induced pluripotent stem cells (iPSCs), but its precise mechanisms of action remain poorly understood. Here, we investigated the role of OCT4 phosphorylation in the biological functions of ESCs. We observed that c-Jun N-terminal kinases (JNKs) directly interacted with and phosphorylated OCT4 at serine 347, which inhibited the transcriptional activity of OCT4. Moreover, phosphorylation of OCT4 induced binding of FBXW8, which reduced OCT4 protein stability and enhanced its proteasomal degradation. We also found that the mutant OCT4 (S347A) might delay the differentiation process of mouse ESCs and enhance the efficiency of generating iPSCs. These results demonstrated that OCT4 phosphorylation on serine 347 by JNKs plays an important role in its stability, transcriptional activities, and self-renewal of mouse ESCs.

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JNKs interact with and phosphorylate OCT4 at serine 347

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Serine 347 phosphorylation inhibits OCT4 transcriptional activity and stability

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FBXW8 can interact with the OCT4 protein phosphorylated at serine 347

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The differentiation of mouse ESCs is delayed in the presence of OCT4 (S347A)

Single-stranded DNA binding protein Ssbp3 induces differentiation of mouse embryonic stem cells into trophoblast-like cells

BACKGROUND

Intrinsic factors and extrinsic signals which control unlimited self-renewal and developmental pluripotency in embryonic stem cells (ESCs) have been extensively investigated. However, a much smaller number of factors involved in extra-embryonic trophoblast differentiation from ESCs have been studied. In this study, we investigated the role of the single-stranded DNA binding protein, Ssbp3, for the induction of trophoblast-like differentiation from mouse ESCs.

METHODS

Gain- and loss-of-function experiments were carried out through overexpression or knockdown of Ssbp3 in mouse ESCs under self-renewal culture conditions. Expression levels of pluripotency and lineage markers were detected by real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analyses. The global gene expression profile in Ssbp3-overexpressing cells was determined by affymetrix microarray. Gene ontology and pathway terms were analyzed and further validated by qRT-PCR and Western blotting. The methylation status of the Elf5 promoter in Ssbp3-overexpressing cells was detected by bisulfite sequencing. The trophoblast-like phenotype induced by Ssbp3 was also evaluated by teratoma formation and early embryo injection assays.

RESULTS

Forced expression of Ssbp3 in mouse ESCs upregulated expression levels of lineage-associated genes, with trophoblast cell markers being the highest. In contrast, depletion of Ssbp3 attenuated the expression of trophoblast lineage marker genes induced by downregulation of Oct4 or treatment with BMP4 and bFGF in ESCs. Interestingly, global gene expression profiling analysis indicated that Ssbp3 overexpression did not significantly alter the transcript levels of pluripotency-associated transcription factors. Instead, Ssbp3 promoted the expression of early trophectoderm transcription factors such as Cdx2 and activated MAPK/Erk1/2 and TGF-β pathways. Furthermore, overexpression of Ssbp3 reduced the methylation level of the Elf5 promoter and promoted the generation of teratomas with internal hemorrhage, indicative of the presence of trophoblast cells.

CONCLUSIONS

This study identifies Ssbp3, a single-stranded DNA binding protein, as a regulator for mouse ESCs to differentiate into trophoblast-like cells. This finding is helpful to understand the regulatory networks for ESC differentiation into extra-embryonic lineages.

Critical POU domain residues confer Oct4 uniqueness in somatic cell reprogramming

The POU domain transcription factor Oct4 plays critical roles in self-renewal and pluripotency of embryonic stem cells (ESCs). Together with Sox2, Klf4 and c-Myc, Oct4 can reprogram any other cell types to pluripotency, in which Oct4 is the only factor that cannot be functionally replaced by other POU family members. To investigate the determinant elements of Oct4 uniqueness, we performed Ala scan on all Ser, Thr, Tyr, Lys and Arg of murine Oct4 by testing their capability in somatic cell reprogramming. We uncovered a series of residues that are important for Oct4 functionality, in which almost all of these key residues are within the POU domains making direct interaction with DNA. The Oct4 N- and C-terminal transactivation domains (TADs) are not unique and could be replaced by the Yes-associated protein (YAP) TAD domain to support reprogramming. More importantly, we uncovered two important residues that confer Oct4 uniqueness in somatic cell reprogramming. Our systematic structure-function analyses bring novel mechanistic insight into the molecular basis of how critical residues function together to confer Oct4 uniqueness among POU family for somatic cell reprogramming.

Oct4 interaction with Hmgb2 regulates Akt signaling and pluripotency

In pluripotent stem cells, bivalent domains mark the promoters of developmentally regulated loci. Histones in these chromatin regions contain coincident epigenetic modifications of gene activation and repression. How these marks are transmitted to maintain the pluripotent state in daughter progeny remains poorly understood. Our study demonstrates that Oct4 post-translational modifications (PTMs) form a positive feedback loop, which promotes Akt activation and interaction with Hmgb2 and the SET complex. This preserves H3K27me3 modifications in daughter progeny and maintains the pluripotent gene expression signature in murine embryonic stem cells. However, if Oct4 is not phosphorylated, a negative feedback loop is formed that inactivates Akt and initiates the DNA damage response. Oct4 sumoylation then is required for G1/S progression and transmission of the repressive H3K27me3 mark. Therefore, PTMs regulate the ability of Oct4 to direct the spatio-temporal formation of activating and repressing complexes to orchestrate chromatin plasticity and pluripotency. Our work highlights a previously unappreciated role for Oct4 PTM-dependent interactions in maintaining restrained Akt signaling and promoting a primitive epigenetic state.

Cobalt and nickel stabilize stem cell transcription factor OCT4 through modulating its sumoylation and ubiquitination

Stem cell research can lead to the development of treatments for a wide range of ailments including diabetes, heart disease, aging, neurodegenerative diseases, spinal cord injury, and cancer. OCT4 is a master regulator of self-renewal of undifferentiated embryonic stem cells. OCT4 also plays a crucial role in reprogramming of somatic cells into induced pluripotent stem (iPS) cells. Given known vivo reproductive toxicity of cobalt and nickel metals, we examined the effect of these metals on expression of several stem cell factors in embryonic Tera-1 cells, as well as stem cells. Cobalt and nickel induced a concentration-dependent increase of OCT4 and HIF-1α, but not NANOG or KLF4. OCT4 induced by cobalt and nickel was due primarily to protein stabilization because MG132 stabilized OCT4 in cells treated with either metals and because neither nickel nor cobalt significantly modulated its steady-state mRNA level. OCT4 stabilization by cobalt and nickel was mediated largely through reactive oxygen species (ROS) as co-treatment with ascorbic acid abolished OCT4 increase. Moreover, nickel and cobalt treatment increased sumoylation and mono-ubiquitination of OCT4 and K123 was crucial for mediating these modifications. Combined, our observations suggest that nickel and cobalt may exert their reproductive toxicity through perturbing OCT4 activity in the stem cell compartment.

c-Yes tyrosine kinase is a potent suppressor of ES cell differentiation and antagonizes the actions of its closest phylogenetic relative, c-Src

Embryonic stem (ES) cells are derived from the inner cell mass of the blastocyst stage embryo and are characterized by self-renewal and pluripotency. Previous work has shown that Src-family tyrosine kinases display dynamic expression and activity changes during ES cell differentiation, suggesting distinct functions in the control of developmental fate. Here we used ES cells to test the hypothesis that c-Src and its closest phylogenetic relative, c-Yes, act in biological opposition despite their strong homology. Unlike c-Src, enforced expression of active c-Yes blocked ES cell differentiation to embryoid bodies by maintaining pluripotency gene expression. To explore the interplay of c-Src and c-Yes in ES cell differentiation, we engineered c-Src and c-Yes mutants that are resistant to A-419259, a potent pyrrolopyrimidine inhibitor of the Src kinase family. Previous studies have shown that A-419259 treatment blocks all Src-family kinase activity in ES cells, preventing differentiation while maintaining pluripotency. Expression of inhibitor-resistant c-Src but not c-Yes rescued the A-419259 differentiation block, resulting in a cell population with properties of both primitive ectoderm and endoderm. Remarkably, when inhibitor-resistant c-Src and c-Yes were expressed together in ES cells, c-Yes activity suppressed c-Src-mediated differentiation. These studies show that even closely related kinases such as c-Src and c-Yes have unique and opposing functions in the same cell type. Selective agonists or inhibitors of c-Src versus c-Yes activity may allow more precise pharmacological manipulation of ES cell fate and have broader applications in other biological systems that express multiple Src family members such as tumor cells.

Itch, an E3 ligase of Oct4, is required for embryonic stem cell self-renewal and pluripotency induction

Itch, a C2-WW-HECT domain ubiquitin E3 ligase, plays an important role in various biological processes. However, its role in embryonic stem cells (ESCs) remains unknown. Here, we report that Itch interacts with and targets pluripotency-associated transcription factor Oct4 for ubiquitination. Moreover, Itch enhances Oct4 transcriptional activities and controls Oct4 protein stability dependent on its catalytic activity. Importantly, silencing Itch expression compromises ESC self-renewal capacity and somatic cell reprogramming efficiency. Taken together, our study identifies Itch as a regulator of Oct4 stability and transcriptional activity, establishing a functional link between an E3 ligase and the regulation of pluripotency.

Oct transcription factors in development and stem cells: insights and mechanisms

The POU domain family of transcription factors regulates developmental processes ranging from specification of the early embryo to terminal differentiation. About half of these factors display substantial affinity for an 8 bp DNA site termed the octamer motif, and are hence known as Oct proteins. Oct4 (Pou5f1) is a well-known Oct factor, but there are other Oct proteins with varied and essential roles in development. This Primer outlines our current understanding of Oct proteins and the regulatory mechanisms that govern their role in developmental processes and concludes with the assertion that more investigation into their developmental functions is needed.

Cloning and characterization of a novel oocyte-specific gene encoding an F-Box protein in rainbow trout (Oncorhynchus mykiss)

BACKGROUND

Oocyte-specific genes play critical roles in oogenesis, folliculogenesis and early embryonic development. The objectives of this study were to characterize the expression of a novel oocyte-specific gene encoding an F-box protein during ovarian development in rainbow trout, and identify its potential interacting partners in rainbow trout oocytes.

METHODS

Through analysis of expressed sequence tags (ESTs) from a rainbow trout oocyte cDNA library, a novel transcript represented by ESTs only from the oocyte library was identified. The complete cDNA sequence for the novel gene (named fbxoo) was obtained by assembling sequences from an EST clone and a 5'RACE product. The expression and localization of fbxoo mRNA and protein in ovaries of different developmental stages were analyzed by quantitative real time PCR, immunoblotting, in situ hybridization and immunohistochemistry. Identification of Fbxoo binding proteins was performed by yeast two-hybrid screening.

RESULTS

fbxoo mRNA is specifically expressed in mature oocytes as revealed by tissue distribution analysis. The fbxoo cDNA sequence is 1,996 bp in length containing an open reading frame, which encodes a predicted protein of 514 amino acids. The novel protein sequence does not match any known protein sequences in the NCBI database. However, a search of the Pfam protein database revealed that the protein contains an F-box motif at the N-terminus, indicating that Fbxoo is a new member of the F-box protein family. The expression of fbxoo mRNA and protein is high in ovaries at early pre-vitellogenesis stage, and both fbxoo mRNA and protein are predominantly expressed in early pre-vitellogenic oocytes. Several proteins including tissue inhibitor of metalloproteinase 2 (Timp2) were identified as potential Fbxoo protein binding partners.

CONCLUSIONS

Results suggest that the novel oocyte-specific F-box protein may play an important role in early oocyte development by regulating other critical proteins involved in oogenesis in rainbow trout.

Protein markers of cancer-associated fibroblasts and tumor-initiating cells reveal subpopulations in freshly isolated ovarian cancer ascites

BACKGROUND

In ovarian cancer, massive intraperitoneal dissemination is due to exfoliated tumor cells in ascites. Tumor-initiating cells (TICs or cancer stem cells) and cells showing epithelial-mesenchymal-transition (EMT) are particularly implicated. Spontaneous spherical cell aggregates are sometimes observed, but although similar to those formed by TICs in vitro, their significance is unclear.

METHODS

Cells freshly isolated from malignant ascites were separated into sphere samples (S-type samples, n=9) and monolayer-forming single-cell suspensions (M-type, n=18). Using western blot, these were then compared for expression of protein markers of EMT, TIC, and of cancer-associated fibroblasts (CAFs).

RESULTS

S-type cells differed significantly from M-type by expressing high levels of E-cadherin and no or little vimentin, integrin-β3 or stem cell transcription factor Oct-4A. By contrast, M-type samples were enriched for CD44, Oct-4A and for CAF markers. Independently of M- and S-type, there was a strong correlation between TIC markers Nanog and EpCAM. The CAF marker α-SMA correlated with clinical stage IV. This is the first report on CAF markers in malignant ascites and on SUMOylation of Oct-4A in ovarian cancer.

CONCLUSIONS

In addition to demonstrating potentially high levels of TICs in ascites, the results suggest that the S-type population is the less tumorigenic one. Nanog(high)/EpCAM(high) samples represent a TIC subset which may be either M- or S-type, and which is separate from the CD44(high)/Oct-4A(high) subset observed only in M-type samples. This demonstrates a heterogeneity in TIC populations in vivo which has practical implications for TIC isolation based on cell sorting. The biological heterogeneity will need to be addressed in future therapeutical strategies.

SyStemCell: a database populated with multiple levels of experimental data from stem cell differentiation research

Elucidation of the mechanisms of stem cell differentiation is of great scientific interest. Increasing evidence suggests that stem cell differentiation involves changes at multiple levels of biological regulation, which together orchestrate the complex differentiation process; many related studies have been performed to investigate the various levels of regulation. The resulting valuable data, however, remain scattered. Most of the current stem cell-relevant databases focus on a single level of regulation (mRNA expression) from limited stem cell types; thus, a unifying resource would be of great value to compile the multiple levels of research data available. Here we present a database for this purpose, SyStemCell, deposited with multi-level experimental data from stem cell research. The database currently covers seven levels of stem cell differentiation-associated regulatory mechanisms, including DNA CpG 5-hydroxymethylcytosine/methylation, histone modification, transcript products, microRNA-based regulation, protein products, phosphorylation proteins and transcription factor regulation, all of which have been curated from 285 peer-reviewed publications selected from PubMed. The database contains 43,434 genes, recorded as 942,221 gene entries, for four organisms (Homo sapiens, Mus musculus, Rattus norvegicus, and Macaca mulatta) and various stem cell sources (e.g., embryonic stem cells, neural stem cells and induced pluripotent stem cells). Data in SyStemCell can be queried by Entrez gene ID, symbol, alias, or browsed by specific stem cell type at each level of genetic regulation. An online analysis tool is integrated to assist researchers to mine potential relationships among different regulations, and the potential usage of the database is demonstrated by three case studies. SyStemCell is the first database to bridge multi-level experimental information of stem cell studies, which can become an important reference resource for stem cell researchers. The database is available at http://lifecenter.sgst.cn/SyStemCell/.

SUMOylation represses Nanog expression via modulating transcription factors Oct4 and Sox2

Nanog is a pivotal transcription factor in embryonic stem (ES) cells and is essential for maintaining the pluripotency and self-renewal of ES cells. SUMOylation has been proved to regulate several stem cell markers' function, such as Oct4 and Sox2. Nanog is strictly regulated by Oct4/Sox2 heterodimer. However, the direct effects of SUMOylation on Nanog expression remain unclear. In this study, we reported that SUMOylation repressed Nanog expression. Depletion of Sumo1 or its conjugating enzyme Ubc9 increased the expression of Nanog, while high SUMOylation reduced its expression. Interestingly, we found that SUMOylation of Oct4 and Sox2 regulated Nanog in an opposing manner. SUMOylation of Oct4 enhanced Nanog expression, while SUMOylated Sox2 inhibited its expression. Moreover, SUMOylation of Oct4 by Pias2 or Sox2 by Pias3 impaired the interaction between Oct4 and Sox2. Taken together, these results indicate that SUMOylation has a negative effect on Nanog expression and provides new insights into the mechanism of SUMO modification involved in ES cells regulation.

Transient inhibition of cell proliferation does not compromise self-renewal of mouse embryonic stem cells

Embryonic stem cells (ESCs) have unlimited capacity for self-renewal and can differentiate into various cell types when induced. They also have an unusual cell cycle control mechanism driven by constitutively active cyclin dependent kinases (Cdks). In mouse ESCs (mESCs). It is proposed that the rapid cell proliferation could be a necessary part of mechanisms that maintain mESC self-renewal and pluripotency, but this hypothesis is not in line with the finding in human ESCs (hESCs) that the length of the cell cycle is similar to differentiated cells. Therefore, whether rapid cell proliferation is essential for the maintenance of mESC state remains unclear. We provide insight into this uncertainty through chemical intervention of mESC cell cycle. We report here that inhibition of Cdks with olomoucine II can dramatically slow down cell proliferation of mESCs with concurrent down-regulation of cyclin A, B and E, and the activation of the Rb pathway. However, mESCs display can recover upon the removal of olomoucine II and are able to resume normal cell proliferation without losing self-renewal and pluripotency, as demonstrated by the expression of ESC markers, colony formation, embryoid body formation, and induced differentiation. We provide a mechanistic explanation for these observations by demonstrating that Oct4 and Nanog, two major transcription factors that play critical roles in the maintenance of ESC properties, are up-regulated via de novo protein synthesis when the cells are exposed to olomoucine II. Together, our data suggest that short-term inhibition of cell proliferation does not compromise the basic properties of mESCs.

Post-translational modulation of pluripotency

The maintenance of pluripotency relies on an intricate transcriptional network hinged on a key set of transcription factors. Pluripotent stem cells have been shown to be sensitive to modulations of the cellular abundance and transcriptional activity of these key pluripotency factors. Recent evidence highlights the important role of post-translational modifications, including ubiquitination, sumoylation, phosphorylation, methylation, and acetylation, in regulating the levels and activity of pluripotency factors to achieve a balance between pluripotency and differentiation.

Effect of ectopic expression of homeoprotein EGAM1C on the cell morphology, growth, and differentiation in a mouse embryonic stem cell line, MG1.19 cells

The homeoprotein EGAM1C was identified in preimplantation mouse embryos and embryonic stem (ES) cells. To explore the impact of EGAM1C on the hallmarks of mouse ES cells, MG1.19 cells stably expressing EGAM1C at levels similar to those in blastocysts were established using an episomal expression system. In the presence of leukemia inhibitory factor (+LIF), control transfectants with an empty vector formed flattened cell colonies, whileEgam1ctransfectants formed compacted colonies with increased E-CADHERIN expression. InEgam1ctransfectants, the cellular contents of POU5F1 (OCT4), SOX2, TBX3, and NANOG increased. Cell growth was accelerated in an undifferentiated state sustained by LIF and in the course of differentiation. During clonal proliferation, EGAM1C stabilized the undifferentiated state. In adherent culture conditions, EGAM1C partly inhibited the progression of differentiation at least within a 4-day culture period in the presence of retinoic acid by preventing the downregulation of LIF signaling with a robust increase in TBX3 expression. Conversely, EGAM1C enhanced the expression of lineage marker genesFgf5(epiblast),T(mesoderm),Gata6(primitive endoderm), andCdx2(trophectoderm) in −LIF conditions. In embryoid bodies expressing EGAM1C, the expression of marker genes for extraembryonic cell lineages, includingTpbpa(spongiotrophoblast) andPlat(parietal endoderm), increased. These results demonstrated that the ectopic expression of EGAM1C is capable of affecting the stabilization of an undifferentiated state and the progression of differentiation in MG1.19 ES cells, in addition to affecting cellular morphology and growth.

Variance of gene expression identifies altered network constraints in neurological disease

Gene expression analysis has become a ubiquitous tool for studying a wide range of human diseases. In a typical analysis we compare distinct phenotypic groups and attempt to identify genes that are, on average, significantly different between them. Here we describe an innovative approach to the analysis of gene expression data, one that identifies differences in expression variance between groups as an informative metric of the group phenotype. We find that genes with different expression variance profiles are not randomly distributed across cell signaling networks. Genes with low-expression variance, or higher constraint, are significantly more connected to other network members and tend to function as core members of signal transduction pathways. Genes with higher expression variance have fewer network connections and also tend to sit on the periphery of the cell. Using neural stem cells derived from patients suffering from Schizophrenia (SZ), Parkinson's disease (PD), and a healthy control group, we find marked differences in expression variance in cell signaling pathways that shed new light on potential mechanisms associated with these diverse neurological disorders. In particular, we find that expression variance of core networks in the SZ patient group was considerably constrained, while in contrast the PD patient group demonstrated much greater variance than expected. One hypothesis is that diminished variance in SZ patients corresponds to an increased degree of constraint in these pathways and a corresponding reduction in robustness of the stem cell networks. These results underscore the role that variation plays in biological systems and suggest that analysis of expression variance is far more important in disease than previously recognized. Furthermore, modeling patterns of variability in gene expression could fundamentally alter the way in which we think about how cellular networks are affected by disease processes.

Genes are a repository of information that provides the framework for cellular processes, with the flow of information from gene (DNA) to phenotype via an intermediate molecule—the messenger RNA. We understand that sequence variations in a gene may lead to phenotypic variations, but less well understood is how variation in the information flow itself might also impact on phenotype. In this study we demonstrated that disease phenotypes were correlated with expression variance. A change in expression variance might infer that the genetic networks representing information flow were less robust—surprisingly, we found that too little and too much variance were equally detrimental in the context of neurological disease.

Regulation of mouse embryonic stem cell self-renewal by a Yes–YAP–TEAD2 signaling pathway downstream of LIF

The cytoplasmic tyrosine kinase Yes has previously been shown to have an important role in maintaining mouse and human embryonic stem (ES) self-renewal through an unknown pathway downstream of leukemia inhibitory factor (LIF) and one or more factors in serum. Here, we show that TEAD2 and its transcriptional co-activator, the Yes-associated protein YAP, co-operate in a signaling pathway downstream of Yes. We show that YAP, TEAD2 and Yes are highly expressed in self-renewing ES cells, are activated by LIF and serum, and are downregulated when cells are induced to differentiate. We also demonstrate that kinase-active Yes binds and phosphorylates YAP, and activates YAP–TEAD2-dependent transcription. We found that TEAD2 associates directly with the Oct-3/4 promoter. Moreover, activation of the Yes pathway induced activity of the Oct-3/4 and Nanog promoters, whereas suppression of this pathway inhibited promoter activity. Nanog, in turn, suppressed TEAD2-dependent promoter activity, whereas siRNA-mediated knockdown of Nanog induced it, suggesting a negative regulatory feedback loop. Episomal supertransfection of cells with inhibitory TEAD2–EnR induced endodermal differentiation, which suggests that this pathway is necessary for ES cell maintenance.

A distinct role for Pin1 in the induction and maintenance of pluripotency

The prominent characteristics of pluripotent stem cells are their unique capacity to self-renew and pluripotency. Although pluripotent stem cell proliferation is maintained by specific intracellular phosphorylation signaling events, it has not been well characterized how the resulting phosphorylated proteins are subsequently regulated. We here report that the peptidylprolyl isomerase Pin1 is indispensable for the self-renewal and maintenance of pluripotent stem cells via the regulation of phosphorylated Oct4 and other substrates. Pin1 expression was found to be up-regulated upon the induction of induced pluripotent stem (iPS) cells, and the forced expression of Pin1 with defined reprogramming factors was observed to further enhance the frequency of iPS cell generation. The inhibition of Pin1 activity significantly suppressed colony formation and induced the aberrant differentiation of human iPS cells as well as murine ES cells. We further found that Pin1 interacts with the phosphorylated Ser(12)-Pro motif of Oct4 and that this in turn facilitates the stability and transcriptional activity functions of Oct4. Our current findings thus uncover an atypical role for Pin1 as a putative regulator of the induction and maintenance of pluripotency via the control of phosphorylation signaling. These data suggest that the manipulation of Pin1 function could be a potential strategy for the stable induction and proliferation of human iPS cells.

Sumoylation by Ubc9 regulates the stem cell compartment and structure and function of the intestinal epithelium in mice

BACKGROUND & AIMS

Small ubiquitin-like modifiers (SUMOs) are attached to other proteins to regulate their function (sumoylation). We investigated the role of Ubc9, which covalently attaches SUMOs to proteins, in the gastrointestinal tract of adult mice.

METHODS

We investigated the effects of decreased sumoylation in adult mammals by generating mice with an inducible knockout (by injection of 4-hydroxytamoxifen) of the E2 enzyme Ubc9 (Ubc9fl/-/ROSA26-CreERT2 mice). We analyzed the phenotypes using a range of histologic techniques.

RESULTS

Loss of Ubc9 from adult mice primarily affected the small intestine. Ubc9fl/-/ROSA26-CreERT2 mice died within 6 days of 4-hydroxytamoxifen injection, losing 20% or less of their body weight and developing severe diarrhea on the second day after injection. Surprisingly, other epithelial tissues appeared to be unaffected at that stage. Decreased sumoylation led to the depletion of the intestinal proliferative compartment and to the rapid disappearance of stem cells. Sumoylation was required to separate the proliferative and differentiated compartments from the crypt and control differentiation and function of the secretory lineage. Sumoylation was required for nucleus positioning and polarized organization of actin in the enterocytes. Loss of sumoylation caused detachment of the enterocytes from the basal lamina, as observed in tissue fragility diseases. We identified the intermediate filament keratin 8 as a SUMO substrate in epithelial cells.

CONCLUSIONS

Sumoylation maintains intestinal stem cells and the architecture, mechanical stability, and function of the intestinal epithelium of mice.

Role of Oct4 in maintaining and regaining stem cell pluripotency

Pluripotency, a characteristic of cells in the inner cell mass of the mammalian preimplantation blastocyst as well as of embryonic stem cells, is defined as the ability of a cell to generate all of the cell types of an organism. A group of transcription factors is essential for the establishment and maintenance of the pluripotent state. Recent studies have demonstrated that differentiated somatic cells could be reverted to a pluripotent state by the overexpression of a set of transcription factors, further highlighting the significance of transcription factors in the control of pluripotency. Among these factors, a member of the POU transcription factor family, Oct4, is central to the machinery governing pluripotency. Oct4 is highly expressed in pluripotent cells and becomes silenced upon differentiation. Interestingly, the precise expression level of Oct4 determines the fate of embryonic stem cells. Therefore, to control the expression of Oct4 precisely, a variety of regulators function at multiple levels, including transcription, translation of mRNA and post-translational modification. Additionally, in cooperation with Sox2, Nanog and other members of the core transcriptional regulatory circuitry, Oct4 activates both protein-coding genes and noncoding RNAs necessary for pluripotency. Simultaneously, in association with transcriptional repressive complexes, Oct4 represses another set of targets involved in developmental processes. Importantly, Oct4 can re-establish pluripotency in somatic cells, and proper reprogramming of Oct4 expression is indispensable for deriving genuine induced pluripotent stem cell lines. In the past several years, genome-wide identification of Oct4 target genes and Oct4-centered protein interactomes has been reported, indicating that Oct4 exerts tight control over pluripotency regulator expression and protects embryonic stem cells in an undifferentiated state. Nevertheless, further investigation is required to fully elucidate the underlying molecular mechanisms through which Oct4 maintains and reinitiates pluripotency. Systemic and dynamic exploration of the protein complexes and target genes associated with Oct4 will help to elucidate the role of Oct4 more comprehensively.

Role of the BCA2 ubiquitin E3 ligase in hormone responsive breast cancer

The BCA2 protein contains a RING H2 finger and a Zn finger near the N-terminus and has E3 ligase activity. RING finger proteins play critical roles in mediating the transfer of ubiquitin and ubiquitin like modifiers to heterologous substrates as well as to the RING finger proteins themselves. Protein modification by ubiquitin and small ubiquitin-related modifier (SUMO) plays a pivotal role in protein homeostasis and is critical to regulating basic cellular processes such as proliferation, differentiation, apoptosis, intracellular signaling, and gene-transcriptional regulation. The addition of ubiquitin or SUMO can modulate the ability of proteins to interact with their partners, alter their patterns of sub-cellular localization and control their stability. It is clear that SUMO influences many different biological processes however recent data suggest that it is specifically important in the regulation of transcription. BCA2 is an E3 ligase that interacts with the SUMO conjugating enzyme Ubc9. It could therefore function as an E3 in the sumoylation of various transcription factors. We have found that the BCA2 is co-expressed with the estrogen receptor in 74% of ER-positive invasive ductal carcinomas from a 635 member breast cancer cohort (p = 0.004). At the cellular level, BCA2 co-localizes with ER and it appears that at the transcriptional level BCA2 mRNA expression is regulated by estrogen. Bioinformatic analysis of the BCA2 promoter region revealed ER and PR binding sites as well as that of other more general transcription factors. The data presented here provides an overview of the potential involvement of the BCA2 in hormone responsive breast cancer and opens up avenues that should be exploited to better understand the regulation of ER expression, growth of breast cancer cells, and the importance of BCA2.

Mir-30 reduction maintains self-renewal and inhibits apoptosis in breast tumor-initiating cells

Accumulating evidence indicates that a sub-population of cancer cells with stem-like properties, termed tumor-initiating cells (T-ICs), exist in many different kinds of malignancies, which have a pivotal role in tumorigenesis, tumor progression, metastasis and post-treatment relapse. However, how the stem-like properties of T-ICs are regulated remains obscure. Our previous study showed that reduction of let-7 microRNA (miRNA) in breast tumor-initiating cells (BT-ICs) contributes to the maintenance of their self-renewal capacity and undifferentiated status. In this study we show the effect of mir-30 reduction on the stem-like features of BT-ICs. Similar to let-7, mir-30 is reduced in BT-ICs, and the protein level of Ubc9 (ubiquitin-conjugating enzyme 9) and ITGB3 (integrin beta3), the target genes of mir-30, is markedly upregulated. Enforced constitutive expression of mir-30 in BT-ICs inhibits their self-renewal capacity by reducing Ubc9, and induces apoptosis through silencing ITGB3. On the contrary, blocking the miRNA with a specific antisense oligonucleotide (ASO) in differentiated breast cancer cells revived their self-renewal capacity. Furthermore, ectopic expression of mir-30 in BT-IC xenografts reduces tumorigenesis and lung metastasis in nonobese diabetic/severe combined immunodeficient mice, whereas blocking mir-30 expression enhances tumorigenesis and metastasis. Together, our data suggest mir-30 as one of the important miRNAs in regulating the stem-like features of T-ICs.

DNA damaging bystander signalling from stem cells, cancer cells and fibroblasts after Cr(VI) exposure and its dependence on telomerase

The bystander effect is a feature of low dose radiation exposure and is characterized by a signaling process from irradiated cells to non irradiated cells, which causes DNA and chromosome damage in these 'nearest neighbour' cells. Here we show that a low and short dose of Cr(VI) can induce stem cells, cancer cells and fibroblasts to chronically secrete bystander signals, which cause DNA damage in neighboring cells. The Cr(VI) induced bystander signaling depended on the telomerase status of either cell. Telomerase negative fibroblasts were able to receive DNA damaging signals from telomerase positive or negative fibroblasts or telomerase positive cancer cells. However telomerase positive fibroblasts were resistant to signals from Cr(VI) exposed telomerase positive fibroblasts or cancer cells. Human embryonic stem cells, with positive Oct4 staining as a marker of pluripotency, showed no significant increase of DNA damage from adjacent Cr and mitomycin C exposed fibroblasts whilst those cells that were negatively stained did. This selectivity of DNA damaging bystander signaling could be an important consideration in developing therapies against cancer and in the safety and effectiveness of tissue engineering and transplantation using stem cells.

Stk40 links the pluripotency factor Oct4 to the Erk/MAPK pathway and controls extraembryonic endoderm differentiation

Self-renewal and differentiation of embryonic stem cells (ESCs) are controlled by intracellular transcriptional factors and extracellular factor-activated signaling pathways. Transcription factor Oct4 is a key player maintaining ESCs in an undifferentiated state, whereas the Erk/MAPK pathway is known to be important for ESC differentiation. However, the manner in which intracellular pluripotency factors modulate extracellular factor-activated signaling pathways in ESCs is not well understood. Here, we report identification of a target gene of Oct4, serine/threonine kinase 40 (Stk40), which is able to activate the Erk/MAPK pathway and induce extraembryonic-endoderm (ExEn) differentiation in mouse ESCs. Interestingly, cells overexpressing Stk40 exclusively contribute to the ExEn layer of chimeric embryos when injected into host blastocysts. In contrast, deletion of Stk40 in ESCs markedly reduces ExEn differentiation in vitro. Mechanistically, Stk40 interacts with Rcn2, which also activates Erk1/2 to induce ExEn specification in mouse ESCs. Moreover, Rcn2 proteins are specifically located in the cytoplasm of the ExEn layer of early mouse embryos. Importantly, knockdown of Rcn2 blocks Stk40-activated Erk1/2 and ESC differentiation. Therefore, our study establishes a link between the pluripotency factor Oct4 and the Erk/MAPK signaling pathway, and it uncovers cooperating signals in the Erk/MAPK activation that control ExEn differentiation.

Ly-1 antibody reactive clone is an important nucleolar protein for control of self-renewal and differentiation in embryonic stem cells

Embryonic stem cells (ESCs) possess the capacity to self-renew and differentiate into all cell types of an organism. It is essential to understand how these properties are controlled for the potential usage of their derivatives in clinical settings and reprogramming of differentiated somatic cells. Although transcriptional factors, such as Oct4, Sox2, and Nanog, have been considered as a part of the core regulatory circuitry, a growing body of evidence suggests that additional factors exist and contribute to the control of ESC self-renewal and differentiation. Here, we report that Ly-1 antibody reactive clone (LYAR), a zinc finger nucleolar protein highly expressed in undifferentiated ESCs, plays a critical role in maintaining ESC identity. Its downregulation significantly reduces the rate of ESC growth and increases their apoptosis. Moreover, reduced expression of LYAR in ESCs impairs their differentiation capacity, failing to rapidly silence pluripotency markers and to activate differentiation genes upon differentiation. Mechanistically, LYAR forms a complex with another nucleolar protein, nucleolin, and prevents its self-cleavage, maintaining a normal steady-state level of nucleolin protein in undifferentiated ESCs. Interestingly, the downregulation of nucleolin is detrimental to the growth of ESCs and increases the rate of apoptosis, similarly to the knockdown of LYAR. Thus, our data emphasize the fact that other genes besides Oct4 and Nanog are uniquely required for ESC self-renewal and differentiation and demonstrate that LYAR functions to control the stability of nucleolin protein, which in turn is essential for maintaining the self-renewal of ESCs.

Stem cells, stress, metabolism and cancer: a drama in two Octs

It is a classic story of two related transcription factors. Oct4 is a potent regulator of pluripotency during early mammalian embryonic development, and is notable for its ability to convert adult somatic cells to pluripotency. The widely expressed Oct1 protein shares significant homology with Oct4, binds to the same sequences, regulates common target genes, and shares common modes of upstream regulation, including the ability to respond to cellular stress. Both proteins are also associated with malignancy, yet Oct1 cannot substitute for Oct4 in the generation of pluripotency. The molecular underpinnings of these phenomena are emerging, as are the consequences for adult stem cells and cancer, and thereby hangs a tale.

PARP1 poly(ADP-ribosyl)ates Sox2 to control Sox2 protein levels and FGF4 expression during embryonic stem cell differentiation

Transcription factors Oct4 and Sox2 are key players in maintaining the pluripotent state of embryonic stem cells (ESCs). Small changes in their levels disrupt normal expression of their target genes. However, it remains elusive how protein levels of Oct4 and Sox2 and expression of their target genes are precisely controlled in ESCs. Here we identify PARP1, a DNA-binding protein with an NAD+-dependent enzymatic activity, as a cofactor of Oct4 and Sox2 to regulate expression of their target gene FGF4. We demonstrate for the first time that PARP1 binds the FGF4 enhancer to positively regulate FGF4 expression. Our data show that PARP1 interacts with and poly(ADP-ribosyl)ates Sox2 directly, which may be a step required for dissociation and degradation of inhibitory Sox2 proteins from the FGF4 enhancer. When PARP1 activity is inhibited or absent, poly(ADP-ribosyl)ation of Sox2 decreases and association of Sox2 with FGF4 enhancers increases, accompanied by an elevated level of Sox2 proteins and reduced expression of FGF4. Significantly, specific knockdown of Sox2 expression by RNA interference can considerably abrogate the inhibitory effect of the poly(ADP-ribose) polymerase inhibitor on FGF4 expression. Interestingly, PARP1 deficiency does not affect undifferentiated ESCs but compromises cell survival and/or growth when ESCs are induced into differentiation. Addition of FGF4 can partially rescue the phenotypes caused by PARP1 deficiency during ESC differentiation. Taken together, this study uncovers new mechanisms through which Sox2 protein levels and FGF4 expression are dynamically regulated during ESC differentiation and adds a new member to the family of proteins regulating the properties of ESCs.

SUMO proteases: redox regulation and biological consequences

Small-ubiquitin modifier (SUMO) has emerged as a novel modification system that governs the activities of a wide spectrum of protein substrates. SUMO-specific proteases (SENP) are of particular interest, as they are responsible for both the maturation of SUMO precursors and for their deconjugation. The interruption of SENPs has been implicated in embryonic defects and carcinoma cells, indicating that a proper balance of SUMO conjugation and deconjugation is crucial. Recent advances in molecular and cellular biology have highlighted the distinct subcellular localization, and endopeptidase and isopeptidase activities of SENPs, suggesting that they are nonredundant. A better understanding of the molecular basis of SUMO recognition and hydrolytic cleavage has been obtained from the crystal structures of SENP-substrate complexes. While a number of proteomic studies have shown an upregulation of sumoylation, attention is now increasingly being directed towards the regulatory mechanism of sumoylation, in particular the oxidative effect. Findings on the oxidation-induced intermolecular disulfide of E1-E2 ligases and SENP1/2 have improved our understanding of the mechanism by which modification is switched up or down. More intriguingly, a growing body of evidence suggests that sumoylation cross-talks with other modifications, and that the upstream and downstream signaling pathway is co-regulated by more than one modifier.

Oct-4A isoform is expressed in human cord blood-derived CD133 stem cells and differentiated progeny

OBJECTIVES

This study aims to establish whether the pluripotent embryonic stem cell marker and nuclear transcription factor Oct-4A isoform is expressed in human umbilical cord blood CD133 stem cells (CD133 cells) and their differentiated progeny.

MATERIALS AND METHODS

CD133 cells were examined for expression of the embryonic stem cell marker Oct-4A by reverse transcription-polymerase chain reaction using primers specific for the coding region of the Oct-4A isoform. Immunocytochemistry and flow cytometry were performed using an antibody raised to a peptide from the unique amino-terminal domain of the Oct-4A isoform, that does not exist in the Oct-4B isoform. Furthermore, specificity was confirmed by pre-adsorption of the antibody with the peptide immunogen. Differentiation was determined before and after expansion in culture, by flow cytometry for haematopoietic stem cell and differentiation markers. For many studies, after 7 days of culture CD133-positive and CD133-negative cells were separated by flow cytometry for additional analyses. Multilineage haematopoietic proliferative potential was determined using colony-forming assays.

RESULTS

Freshly isolated CD133 cells expressed Oct-4A mRNA and protein. The cells proliferated rapidly in culture producing only a small proportion of CD133-positive cells and a much larger proportion of non-self-renewing CD133-negative cells. Proliferation was also associated with loss of other adult stem cell markers, gain of differentiated haematopoietic markers, and maintenance of potential to generate haematopoietic lineages. Oct-4A mRNA and protein were expressed throughout these changes.

CONCLUSIONS

Oct-4A, which is associated with self-renewal in embryonic stem cells, neither defines nor confers self-renewal to CD133 stem cells.