Excess of O-linked N-acetylglucosamine modifies human pluripotent stem cell differentiation

O-GlcNAcase regulates pluripotency states of human embryonic stem cells

Understanding the regulation of human embryonic stem cells (hESCs) pluripotency is critical to advance the field of developmental biology and regenerative medicine. Despite the recent progress, molecular events regulating hESC pluripotency, especially the transition between naive and primed states, still remain unclear. Here we show that naive hESCs display lower levels of O-linked N-acetylglucosamine (O-GlcNAcylation) than primed hESCs. O-GlcNAcase (OGA), the key enzyme catalyzing the removal of O-GlcNAc from proteins, is highly expressed in naive hESCs and is important for naive pluripotency. Depletion of OGA accelerates naive-to-primed pluripotency transition. OGA is transcriptionally regulated by EP300 and acts as a transcription regulator of genes important for maintaining naive pluripotency. Moreover, we profile protein O-GlcNAcylation of the two pluripotency states by quantitative proteomics. Together, this study identifies OGA as an important factor of naive pluripotency in hESCs and suggests that O-GlcNAcylation has a broad effect on hESCs homeostasis.

The role of O-GlcNAcylation in development

O-GlcNAcylation is a dynamic post-translational modification performed by two opposing enzymes: O-GlcNAc transferase and O-GlcNAcase. O-GlcNAcylation is generally believed to act as a metabolic integrator in numerous signalling pathways. The stoichiometry of this modification is tightly controlled throughout all stages of development, with both hypo/hyper O-GlcNAcylation resulting in broad defects. In this Primer, we discuss the role of O-GlcNAcylation in developmental processes from stem cell maintenance and differentiation to cell and tissue morphogenesis.

The O-GlcNAc cycling in neurodevelopment and associated diseases

Proper neuronal development is essential to growth and adult brain function. Alterations at any step of this highly organized sequence of events, due to genetic mutations or environmental factors, triggers brain malformations, which are leading causes of diseases including epilepsy, intellectual disabilities, and many others. The role of glycosylation in neuronal development has been emphasized for many years, notably in studying human congenital disorders of glycosylation (CDGs). These diseases highlight that genetic defects in glycosylation pathways are almost always associated with severe neurological abnormalities, suggesting that glycosylation plays an essential role in early brain development. Congenital disorders of O-GlcNAcylation are no exception, and all mutations of the O-GlcNAc transferase (OGT) are associated with X-linked intellectual disabilities (XLID). In addition, mouse models and in vitro mechanistic studies have reinforced the essential role of O-GlcNAcylation in neuronal development and signaling. In this review, we give an overview of the role of O-GlcNAcylation in this critical physiological process and emphasize the consequences of its dysregulation.

Lipid desaturation regulates the balance between self-renewal and differentiation in mouse blastocyst-derived stem cells

Stem cells are defined by their ability to self-renew and differentiate, both shown in multiple studies to be regulated by metabolic processes. To decipher metabolic signatures of self-renewal in blastocyst-derived stem cells, we compared early differentiating embryonic stem cells (ESCs) and their extra-embryonic counterparts, trophoblast (T)SCs to their self-renewing counterparts. A metabolomics analysis pointed to the desaturation of fatty acyl chains as a metabolic signature of differentiating blastocyst-derived SCs via the upregulation of delta-6 desaturase (D6D; FADS2) and delta-5 desaturase (D5D; FADS1), key enzymes in the biosynthesis of polyunsaturated fatty acids (PUFAs). The inhibition of D6D or D5D by specific inhibitors or SiRNA retained stemness in ESCs and TSCs, and attenuated endoplasmic reticulum (ER) stress-related apoptosis. D6D inhibition in ESCs upregulated stearoyl-CoA desaturase-1 (Scd1), essential to maintain ER homeostasis. In TSCs, however, D6D inhibition downregulated Scd1. TSCs show higher Scd1 mRNA expression and high levels of monounsaturated fatty acyl chain products in comparison to ESCs. The addition of oleic acid, the product of Scd1 (essential for ESCs), to culture medium, was detrimental to TSCs. Interestingly, TSCs express a high molecular mass variant of Scd1 protein, hardly expressed by ESCs. Taken together, our data suggest that lipid desaturation is a metabolic regulator of the balance between differentiation and self-renewal of ESCs and TSCs. They point to lipid polydesaturation as a driver of differentiation in both cell types. Monounsaturated fatty acids (MUFAs), essential for ESCs are detrimental to TSCs.

O-GlcNAcylation and Regulation of Galectin-3 in Extraembryonic Endoderm Differentiation

The regulation of proteins through the addition and removal of O-linked β-N-acetylglucosamine (O-GlcNAc) plays a role in many signaling events, specifically in stem cell pluripotency and the regulation of differentiation. However, these post-translational modifications have not been explored in extraembryonic endoderm (XEN) differentiation. Of the plethora of proteins regulated through O-GlcNAc, we explored galectin-3 as a candidate protein known to have various intracellular and extracellular functions. Based on other studies, we predicted a reduction in global O-GlcNAcylation levels and a distinct galectin expression profile in XEN cells relative to embryonic stem (ES) cells. By conducting dot blot analysis, XEN cells had decreased levels of global O-GlcNAc than ES cells, which reflected a disbalance in the expression of genes encoding O-GlcNAc cycle enzymes. Immunoassays (Western blot and ELISA) revealed that although XEN cells (low O-GlcNAc) had lower concentrations of both intracellular and extracellular galectin-3 than ES cells (high O-GlcNAc), the relative secretion of galectin-3 was significantly increased by XEN cells. Inducing ES cells toward XEN in the presence of an O-GlcNAcase inhibitor was not sufficient to inhibit XEN differentiation. However, global O-GlcNAcylation was found to decrease in differentiated cells and the extracellular localization of galectin-3 accompanies these changes. Inhibiting global O-GlcNAcylation status does not, however, impact pluripotency and the ability of ES cells to differentiate to the XEN lineage.

Immunoprecipitation and Western blot-based detection of protein O-GlcNAcylation in cells

Detection of protein O-GlcNAcylation could be challenging. By using the host-cell factor 1 (HCF-1), a known O-GlcNAcylated protein, we immunoprecipitated HCF-1 from transfected HEK293T cells or endogenous HCF-1 from HeLa cells to detect its O-GlcNAc levels by Western blotting. We also take advantage of RNAi or chemical inhibitors to modulate OGT and OGA activities before HCF-1 immunoprecipitation. For complete details on the use and execution of this protocol, please refer to Daou et al. (2011).

The Mechanism of Stem Cell Aging

Stem cells have self-renewal ability and multi-directional differentiation potential. They have tissue repair capabilities and are essential for maintaining the tissue homeostasis. The depletion of stem cells is closely related to the occurrence of body aging and aging-related diseases. Therefore, revealing the molecular mechanisms of stem cell aging will set new directions for the therapeutic application of stem cells, the study of aging mechanisms, and the prevention and treatment of aging-related diseases. This review comprehensively describes the molecular mechanisms related to stem cell aging and provides the basis for further investigations aimed at developing new anti-stem cell aging strategies and promoting the clinical application of stem cells.

Nutrients in the fate of pluripotent stem cells

Pluripotent stem cells model certain features of early mammalian development ex vivo. Medium-supplied nutrients can influence self-renewal, lineage specification, and earliest differentiation of pluripotent stem cells. However, which specific nutrients support these distinct outcomes, and their mechanisms of action, remain under active investigation. Here, we evaluate the available data on nutrients and their metabolic conversion that influence pluripotent stem cell fates. We also discuss key questions open for investigation in this rapidly expanding area of increasing fundamental and practical importance.

A narrative review of the roles of muscle segment homeobox transcription factor family in cancer

Deregulation of many homeobox genes has been observed in various cancers and has caused functional implications in the tumor progression. In this review, we will focus on the roles of the human muscle segment homeobox (MSX) transcription factor family in the process of tumorigenesis. The MSX transcription factors, through complex downstream regulation mechanisms, are promoters or inhibitors of diverse cancers by participating in cell proliferation, cell invasion, cell metastasis, cell apoptosis, cell differentiation, drug resistance of tumors, maintenance of tumor stemness, and tumor angiogenesis. Moreover, their upstream regulatory mechanisms in cancers may include: gene mutation and chromosome aberration; DNA methylation and chromatin modification; regulation by non-coding RNAs; regulation by other transcription factors and post-translational modification. These mechanisms may provide a better understanding of why MSX transcription factors are abnormally expressed in tumors. Notably, intermolecular interactions and post-translational modification can regulate the transcriptional activity of MSX transcription factors. It is also crucial to know what affects the transcriptional activity of MSX transcription factors in tumors for possible interventions in them in the future. This systematic summary of the regulatory patterns of the MSX transcription factor family may help to further understand the mechanisms involved in transcriptional regulation and also provide new therapeutic approaches for tumor progression.

Stem cell fate determination through protein O-GlcNAcylation

Embryonic and adult stem cells possess the capability of self-renewal and lineage-specific differentiation. The intricate balance between self-renewal and differentiation is governed by developmental signals and cell-type-specific gene regulatory mechanisms. A perturbed intra/extracellular environment during lineage specification could affect stem cell fate decisions resulting in pathology. Growing evidence demonstrates that metabolic pathways govern epigenetic regulation of gene expression during stem cell fate commitment through the utilization of metabolic intermediates or end products of metabolic pathways as substrates for enzymatic histone/DNA modifications. UDP-GlcNAc is one such metabolite that acts as a substrate for enzymatic mono-glycosylation of various nuclear, cytosolic, and mitochondrial proteins on serine/threonine amino acid residues, a process termed protein O-GlcNAcylation. The levels of GlcNAc inside the cells depend on the nutrient availability, especially glucose. Thus, this metabolic sensor could modulate gene expression through O-GlcNAc modification of histones or other proteins in response to metabolic fluctuations. Herein, we review evidence demonstrating how stem cells couple metabolic inputs to gene regulatory pathways through O-GlcNAc-mediated epigenetic/transcriptional regulatory mechanisms to govern self-renewal and lineage-specific differentiation programs. This review will serve as a primer for researchers seeking to better understand how O-GlcNAc influences stemness and may catalyze the discovery of new stem-cell-based therapeutic approaches.

l-Serine links metabolism with neurotransmission

Brain energy metabolism is often considered as a succession of biochemical steps that metabolize the fuel (glucose and oxygen) for the unique purpose of providing sufficient ATP to maintain the huge information processing power of the brain. However, a significant fraction (10-15 %) of glucose is shunted away from the ATP-producing pathway (oxidative phosphorylation) and may be used to support other functions. Recent studies have pointed to the marked compartmentation of energy metabolic pathways between neurons and glial cells. Here, we focused our attention on the biosynthesis of l-serine, a non-essential amino acid that is formed exclusively in glial cells (mostly astrocytes) by re-routing the metabolic fate of the glycolytic intermediate, 3-phosphoglycerate (3PG). This metabolic pathway is called the phosphorylated pathway and transforms 3PG into l-serine via three enzymatic reactions. We first compiled the available data on the mechanisms that regulate the flux through this metabolic pathway. We then reviewed the current evidence that is beginning to unravel the roles of l-serine both in the healthy and diseased brain, leading to the notion that this specific metabolic pathway connects glial metabolism with synaptic activity and plasticity. We finally suggest that restoring astrocyte-mediated l-serine homeostasis may provide new therapeutic strategies for brain disorders.

Lectin Profile Variation in Mesenchymal Stem Cells Derived from Different Sources

Human mesenchymal stem cells (MSCs), a promising source of stem cells for regenerative medicine, have different morphological and functional characteristics. Carbohydrate moieties on the cell surface play an important role, including cell-cell interaction and cell recognition. The objective of this study was to determine possible differences in glycoconjugate distribution patterns of MSCs derived from various sources. MSCs were isolated from adipose tissue, bone marrow, Wharton's jelly, and cord blood. Then, they were stained with FITC-conjugated wheat germ agglutinin (WGA), peanut agglutinin (PNA), concanavalin A (ConA), Ulex europaeus (UEA), Dolichos biflorus (DBA), and Atto-488 conjugated Phytolacca americana (PWM) lectins. The intensity of the reactions was scored using ImageJ software. Flow cytometry was performed to detect the expression of the endothelial marker CD144. The obtained data were analyzed by ANOVA and LSD. Cord blood-derived MSCs showed the most significant staining intensities with all lectins. All MSCs were also moderately stained with PNA. Bone marrow-derived MSCs failed to react with UEA, DBA, and ConA. Wharton's jelly-derived MSCs could also not be stained with ConA. Cord blood-derived MSCs contained 2 subpopulations: osteoclast- and fibroblast-like cells. Both lectin staining intensity and distribution pattern were different in these 2 cell types; therefore, the central part of osteoclast-like cells stained more intensive with PNA and PWM, while that part in fibroblast-like cells stained more intensive with ConA. None of them expressed CD144. The glycoconjugate content of MSCs derived from various sources is different.

O-GlcNAcylation regulates the methionine cycle to promote pluripotency of stem cells

Methionine metabolism is critical for the maintenance of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) pluripotency. However, little is known about the regulation of the methionine cycle to sustain ESC pluripotency. Here, we show that adenosylhomocysteinase (AHCY), an important enzyme in the methionine cycle, is critical for the maintenance and differentiation of mouse embryonic stem cells (mESCs). We show that mESCs exhibit high levels of methionine metabolism, whereas decreasing methionine metabolism via depletion of AHCY promotes mESCs to differentiate into the three germ layers. AHCY is posttranslationally modified with an O-linked β-N-acetylglucosamine sugar (O-GlcNAcylation), which is rapidly removed upon differentiation. O-GlcNAcylation of threonine 136 on AHCY increases its activity and is important for the maintenance of trimethylation of histone H3 lysine 4 (H3K4me3) to sustain mESC pluripotency. Blocking glycosylation of AHCY decreases the ratio of S-adenosylmethionine versus S-adenosylhomocysteine (SAM/SAH), reduces the level of H3K4me3, and poises mESC for differentiation. In addition, blocking glycosylation of AHCY reduces somatic cell reprogramming. Thus, our findings reveal a critical role of AHCY and a mechanistic understanding of O-glycosylation in regulating ESC pluripotency and differentiation.

Next-generation unnatural monosaccharides reveal that ESRRB O-GlcNAcylation regulates pluripotency of mouse embryonic stem cells

Unnatural monosaccharides such as azidosugars that can be metabolically incorporated into cellular glycans are currently used as a major tool for glycan imaging and glycoproteomic profiling. As a common practice to enhance membrane permeability and cellular uptake, the unnatural sugars are per-O-acetylated, which, however, can induce a long-overlooked side reaction, non-enzymatic S-glycosylation. Herein, we develop 1,3-di-esterified N-azidoacetylgalactosamine (GalNAz) as next-generation chemical reporters for metabolic glycan labeling. Both 1,3-di-O-acetylated GalNAz (1,3-Ac₂GalNAz) and 1,3-di-O-propionylated GalNAz (1,3-Pr₂GalNAz) exhibit high efficiency for labeling protein O-GlcNAcylation with no artificial S-glycosylation. Applying 1,3-Pr₂GalNAz in mouse embryonic stem cells (mESCs), we identify ESRRB, a critical transcription factor for pluripotency, as an O-GlcNAcylated protein. We show that ESRRB O-GlcNAcylation is important for mESC self-renewal and pluripotency. Mechanistically, ESRRB is O-GlcNAcylated by O-GlcNAc transferase at serine 25, which stabilizes ESRRB, promotes its transcription activity and facilitates its interactions with two master pluripotency regulators, OCT4 and NANOG.

Per-O-acetylated unnatural monosaccharides are popular tools for glycan labeling in live cells but can undergo unwanted side reactions with cysteines. Here, the authors develop unnatural sugars in a partially esterified form that are inert towards cysteines, and use them to probe O-GlcNAcylation in mESCs.

The Nutrient-Sensing Hexosamine Biosynthetic Pathway as the Hub of Cancer Metabolic Rewiring

Alterations in glucose and glutamine utilizing pathways and in fatty acid metabolism are currently considered the most significant and prevalent metabolic changes observed in almost all types of tumors. Glucose, glutamine and fatty acids are the substrates for the hexosamine biosynthetic pathway (HBP). This metabolic pathway generates the “sensing molecule” UDP-N-Acetylglucosamine (UDP-GlcNAc). UDP-GlcNAc is the substrate for the enzymes involved in protein N- and O-glycosylation, two important post-translational modifications (PTMs) identified in several proteins localized in the extracellular space, on the cell membrane and in the cytoplasm, nucleus and mitochondria. Since protein glycosylation controls several key aspects of cell physiology, aberrant protein glycosylation has been associated with different human diseases, including cancer. Here we review recent evidence indicating the tight association between the HBP flux and cell metabolism, with particular emphasis on the post-transcriptional and transcriptional mechanisms regulated by the HBP that may cause the metabolic rewiring observed in cancer. We describe the implications of both protein O- and N-glycosylation in cancer cell metabolism and bioenergetics; focusing our attention on the effect of these PTMs on nutrient transport and on the transcriptional regulation and function of cancer-specific metabolic pathways.

O-GlcNAc cycling in the developing, adult and geriatric brain

Hundreds of proteins in the nervous system are modified by the monosaccharide O-GlcNAc. A single protein is often O-GlcNAcylated on several amino acids and the modification of a single site can play a crucial role for the function of the protein. Despite its complexity, only two enzymes add and remove O-GlcNAc from proteins, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Global and local regulation of these enzymes make it possible for O-GlcNAc to coordinate multiple cellular functions at the same time as regulating specific pathways independently from each other. If O-GlcNAcylation is disrupted, metabolic disorder or intellectual disability may ensue, depending on what neurons are affected. O-GlcNAc's promise as a clinical target for developing drugs against neurodegenerative diseases has been recognized for many years. Recent literature puts O-GlcNAc in the forefront among mechanisms that can help us better understand how neuronal circuits integrate diverse incoming stimuli such as fluctuations in nutrient supply, metabolic hormones, neuronal activity and cellular stress. Here the functions of O-GlcNAc in the nervous system are reviewed.

Higher O-GlcNAc Levels Are Associated with Defects in Progenitor Proliferation and Premature Neuronal Differentiation during in-Vitro Human Embryonic Cortical Neurogenesis

The nutrient responsive O-GlcNAcylation is a dynamic post-translational protein modification found on several nucleocytoplasmic proteins. Previous studies have suggested that hyperglycemia induces the levels of total O-GlcNAcylation inside the cells. Hyperglycemia mediated increase in protein O-GlcNAcylation has been shown to be responsible for various pathologies including insulin resistance and Alzheimer's disease. Since maternal hyperglycemia during pregnancy is associated with adverse neurodevelopmental outcomes in the offspring, it is intriguing to identify the effect of increased protein O-GlcNAcylation on embryonic neurogenesis. Herein using human embryonic stem cells (hESCs) as model, we show that increased levels of total O-GlcNAc is associated with decreased neural progenitor proliferation and premature differentiation of cortical neurons, reduced AKT phosphorylation, increased apoptosis and defects in the expression of various regulators of embryonic corticogenesis. As defects in proliferation and differentiation during neurodevelopment are common features of various neurodevelopmental disorders, increased O-GlcNAcylation could be one mechanism responsible for defective neurodevelopmental outcomes in metabolically compromised pregnancies such as diabetes.

Chemical Modulation of Protein O-GlcNAcylation via OGT Inhibition Promotes Human Neural Cell Differentiation

The enzymes that determine protein O-GlcNAcylation, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), act on key transcriptional and epigenetic regulators, and both are abundantly expressed in the brain. However, little is known about how alterations in O-GlcNAc cycling affect human embryonic stem cell (hESC) neural differentiation. Here, we studied the effects of perturbing O-GlcNAcylation during neural induction of hESCs using the metabolic inhibitor of OGT, peracetylated 5-thio-N-acetylglucosamine (Ac₄-5SGlcNAc). Treatment of hESCs with Ac₄-5SGlcNAc during induction limited protein O-GlcNAcylation and also caused a dramatic decrease in global levels of UDP-GlcNAc. Concomitantly, a subpopulation of neural progenitor cells (NPCs) acquired an immature neuronal morphology and expressed early neuronal markers such as β-III tubulin (TUJ1) and microtubule associated protein 2 (MAP2), phenotypes that took longer to manifest in the absence of OGT inhibition. These data suggest that chemical inhibition of OGT and perturbation of protein O-GlcNAcylation accelerate the differentiation of hESCs along the neuronal lineage, thus providing further insight into the dynamic molecular mechanisms involved in neuronal development.

Nutrient-driven O-linked N-acetylglucosamine (O-GlcNAc) cycling impacts neurodevelopmental timing and metabolism

Nutrient-driven O-GlcNAcylation is strikingly abundant in the brain and has been linked to development and neurodegenerative disease. We selectively targeted the O-GlcNAcase (Oga) gene in the mouse brain to define the role of O-GlcNAc cycling in the central nervous system. Brain knockout animals exhibited dramatically increased brain O-GlcNAc levels and pleiotropic phenotypes, including early-onset obesity, growth defects, and metabolic dysregulation. Anatomical defects in the Oga knockout included delayed brain differentiation and neurogenesis as well as abnormal proliferation accompanying a developmental delay. The molecular basis for these defects included transcriptional changes accompanying differentiating embryonic stem cells. In Oga KO mouse ES cells, we observed pronounced changes in expression of pluripotency markers, including Sox2, Nanog, and Otx2. These findings link the O-GlcNAc modification to mammalian neurogenesis and highlight the role of this nutrient-sensing pathway in developmental plasticity and metabolic homeostasis.

O-GlcNAc is required for the survival of primed pluripotent stem cells and their reversion to the naïve state

"Naïve" mouse embryonic stem cells (ESCs) are derived from pre-implantation embryos and possess pluripotency, the ability to differentiate into any cell type of the body. "Primed" mouse epiblast stem cells (EpiSCs) are also pluripotent but are derived from post-implantation embryos. ESC-derived EpiSCs (ESD-EpiSCs) are "primed" pluripotent stem cells and can revert to naïve reverted ESCs (rESCs). O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is a posttranslational modification in the cytoplasm and nucleus. O-GlcNAc is transferred to serine and threonine residues of proteins by O-GlcNAc transferase (Ogt) and removed from them by O-GlcNAcase (Oga). In naïve ESCs, O-GlcNAc contributes to maintain the undifferentiated state. In the transition from naïve state to primed state, Ogt maintains cell survival, whereas Oga has no function. However, the function of O-GlcNAc in primed ESD-EpiSCs and during the reversion from the primed state to naïve rESCs remains unclear. Here, we show that Ogt is required for the survival of primed ESD-EpiSCs. The expression of cytosolic Oga was significantly increased during induction from naïve ESCs to primed ESD-EpiSCs. Furthermore, both Ogt and Oga were required for the reversion from primed ESD-EpiSCs to naïve rESCs. These findings indicate that O-GlcNAcylation plays an important role in the survival of primed ESD-EpiSCs and in their reversion to naïve rESCs.

O-GlcNAcylation Negatively Regulates Cardiomyogenic Fate in Adult Mouse Cardiac Mesenchymal Stromal Cells

In both preclinical and clinical studies, cell transplantation of several cell types is used to promote repair of damaged organs and tissues. Nevertheless, despite the widespread use of such strategies, there remains little understanding of how the efficacy of cell therapy is regulated. We showed previously that augmentation of a unique, metabolically derived stress signal (i.e., O-GlcNAc) improves survival of cardiac mesenchymal stromal cells; however, it is not known whether enhancing O-GlcNAcylation affects lineage commitment or other aspects of cell competency. In this study, we assessed the role of O-GlcNAc in differentiation of cardiac mesenchymal stromal cells. Exposure of these cells to routine differentiation protocols in culture increased markers of the cardiomyogenic lineage such as Nkx2.5 and connexin 40, and augmented the abundance of transcripts associated with endothelial and fibroblast cell fates. Differentiation significantly decreased the abundance of O-GlcNAcylated proteins. To determine if O-GlcNAc is involved in stromal cell differentiation, O-GlcNAcylation was increased pharmacologically during the differentiation protocol. Although elevated O-GlcNAc levels did not significantly affect fibroblast and endothelial marker expression, acquisition of cardiomyocyte markers was limited. In addition, increasing O-GlcNAcylation further elevated smooth muscle actin expression. In addition to lineage commitment, we also evaluated proliferation and migration, and found that increasing O-GlcNAcylation did not significantly affect either; however, we found that O-GlcNAc transferase--the protein responsible for adding O-GlcNAc to proteins--is at least partially required for maintaining cellular proliferative and migratory capacities. We conclude that O-GlcNAcylation contributes significantly to cardiac mesenchymal stromal cell lineage and function. O-GlcNAcylation and pathological conditions that may affect O-GlcNAc levels (such as diabetes) should be considered carefully in the context of cardiac cell therapy.

Cell Surface Glycan Changes in the Spontaneous Epithelial-Mesenchymal Transition of Equine Amniotic Multipotent Progenitor Cells

Amniotic epithelial cells (AECs) spontaneously transform into amniotic mesenchymal cells (AMCs) in vitro during cell culture. Glycocalyx was analyzed to identify the glycan pattern in AECs, AMCs and epithelial-mesenchymal transdifferentiated cells (EMTCs). Pure cell cultures were derived using cloned AEC and AMC cell lines obtained by the dilution technique from amniotic membranes. Mesenchymal cells generated by differentiation of clonal epithelial cells were considered transdifferentiated. Immunocytoscreen, in vitro multipotent differentiation and molecular characterization of EMTCs were performed. In combination with saponification and sialidase digestion, a panel of 12 lectins was used to analyze the glycan pattern of AEC, AMC and EMTC glycocalyx. Cytokeratin cell markers were lost in EMTCs and typical mesenchymal markers, such as vimentin, appeared. These cells retained their differentiation potential. Lectin histochemistry revealed a cell-specific glycan profile. Galactose (Gal)β1,4GlcNAc, Neu5Acα2,6Gal/GalNAc and N-acetyl neuraminic (sialic) acid (NeuNAc)α2,3Galβ1,3(±NeuNAcα2,6)GalNAc were highly expressed on the surface of all the amniotic cell cultures. AECs expressed asialoglycans with terminal GalNAc and GlcNAc. More highly mannosylated N-linked glycans and NeuNAcα2,3Galβ1,3GalNAc in O-linked glycans were expressed by EMTCs, but these cells had fewer glycans ending with fucose (Fuc), Gal, GlcNAc and GalNAc than AECs. GlcNAc- and GalNAc-terminating glycans were similarly expressed on the glycocalyx of the mesenchymal cell populations (EMTCs and AMCs). These results demonstrate for the first time that the spontaneous epithelial-mesenchymal transition (EMT) of equine amnion cells is characterized by cell surface glycan remodeling and that glycosylation changes result in a cell type-specific glycan profile. The glycopattern of equine amnion spontaneous EMTCs differs from EMT of tumoral cells.

Global increase in O-linked N-acetylglucosamine modification promotes osteoblast differentiation

The balance between bone formation and bone resorption is maintained by osteoblasts and osteoclasts, and an imbalance in this bone metabolism leads to osteoporosis. Here, we found that osteoblast differentiation in MC3T3-E1 cells is promoted by the inactivation of O-linked β-N-acetylglucosaminidase (O-GlcNAcase) and suppressed by the inactivation of O-GlcNAc transferase, as indicated by extracellular matrix calcification. The expression of osteogenic genes such as alp, ocn, and bsp during osteoblast differentiation was positively regulated in a O-GlcNAc glycosylation-dependent manner. Because it was confirmed that Ets1 and Runx2 are the two key transcription factors responsible for the expression of these osteogenic genes, their transcriptional activity might therefore be regulated by O-GlcNAc glycosylation. However, osteoclast differentiation of RAW264 cells, as indicated by the expression and activity of tartrate-resistant acid phosphatase, was unaffected by the inactivation of either O-GlcNAcase or O-GlcNAc transferase. Our findings suggest that an approach to manipulate O-GlcNAc glycosylation could be useful for developing the therapeutics for osteoporosis.

You are what you eat: O-linked N-acetylglucosamine in disease, development and epigenetics

PURPOSE OF REVIEW

The O-linked N-acetylglucosamine (O-GlcNAc) modification is both responsive to nutrient availability and capable of altering intracellular cellular signalling. We summarize data defining a role for O-GlcNAcylation in metabolic homeostasis and epigenetic regulation of development in the intrauterine environment.

RECENT FINDINGS

O-GlcNAc transferase (OGT) catalyzes nutrient-driven O-GlcNAc addition and is subject to random X-inactivation. OGT plays key roles in growth factor signalling, stem cell biology, epigenetics and possibly imprinting. The O-GlcNAcase, which removes O-GlcNAc, is subject to tight regulation by higher order chromatin structure. O-GlcNAc cycling plays an important role in the intrauterine environment wherein OGT expression is an important biomarker of placental stress.

SUMMARY

Regulation of O-GlcNAc cycling by X-inactivation, epigenetic regulation and nutrient-driven processes makes it an ideal candidate for a nutrient-dependent epigenetic regulator of human disease. In addition, O-GlcNAc cycling influences chromatin modifiers critical to the regulation and timing of normal development including the polycomb repression complex and the ten-eleven translocation proteins mediating DNA methyl cytosine demethylation. The pathway also impacts the hypothalamic-pituitary-adrenal axis critical to intrauterine programming influencing disease susceptibility in later life.

O-GlcNAc modification of the runt-related transcription factor 2 (Runx2) links osteogenesis and nutrient metabolism in bone marrow mesenchymal stem cells

Runx2 is the master switch controlling osteoblast differentiation and formation of the mineralized skeleton. The post-translational modification of Runx2 by phosphorylation, ubiquitinylation, and acetylation modulates its activity, stability, and interactions with transcriptional co-regulators and chromatin remodeling proteins downstream of osteogenic signals. Characterization of Runx2 by electron transfer dissociation tandem mass spectrometry revealed sites of O-linked N-acetylglucosamine (O-GlcNAc) modification, a nutrient-responsive post-translational modification that modulates the action of numerous transcriptional effectors. O-GlcNAc modification occurs in close proximity to phosphorylated residues and novel sites of arginine methylation within regions known to regulate Runx2 transactivation. An interaction between Runx2 and the O-GlcNAcylated, O-GlcNAc transferase enzyme was also detected. Pharmacological inhibition of O-GlcNAcase (OGA), the enzyme responsible for the removal of O-GlcNAc from Ser/Thr residues, enhanced basal (39.9%) and BMP2/7-induced (43.3%) Runx2 transcriptional activity in MC3T3-E1 pre-osteoblasts. In bone marrow-derived mesenchymal stem cells differentiated for 6 days in osteogenic media, inhibition of OGA resulted in elevated expression (24.3%) and activity (65.8%) of alkaline phosphatase (ALP) an early marker of bone formation and a transcriptional target of Runx2. Osteogenic differentiation of bone marrow-derived mesenchymal stem cells in the presence of BMP2/7 for 8 days culminated in decreased OGA activity (39.0%) and an increase in the abundance of O-GlcNAcylated Runx2, as compared with unstimulated cells. Furthermore, BMP2/7-induced ALP activity was enhanced by 35.6% in bone marrow-derived mesenchymal stem cells differentiated in the presence of the OGA inhibitor, demonstrating that direct or BMP2/7-induced inhibition of OGA is associated with increased ALP activity. Altogether, these findings link O-GlcNAc cycling to the Runx2-dependent regulation of the early ALP marker under osteoblast differentiation conditions.

Functional O-GlcNAc modifications: implications in molecular regulation and pathophysiology

O-linked β-N-acetylglucosamine (O-GlcNAc) is a regulatory post-translational modification of intracellular proteins. The dynamic and inducible cycling of the modification is governed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) in response to UDP-GlcNAc levels in the hexosamine biosynthetic pathway (HBP). Due to its reliance on glucose flux and substrate availability, a major focus in the field has been on how O-GlcNAc contributes to metabolic disease. For years this post-translational modification has been known to modify thousands of proteins implicated in various disorders, but direct functional connections have until recently remained elusive. New research is beginning to reveal the specific mechanisms through which O-GlcNAc influences cell dynamics and disease pathology including clear examples of O-GlcNAc modification at a specific site on a given protein altering its biological functions. The following review intends to focus primarily on studies in the last half decade linking O-GlcNAc modification of proteins with chromatin-directed gene regulation, developmental processes, and several metabolically related disorders including Alzheimer's, heart disease and cancer. These studies illustrate the emerging importance of this post-translational modification in biological processes and multiple pathophysiologies.