O-GlcNAc cycling: emerging roles in development and epigenetics

Abstract

The nutrient-sensing hexosamine signaling pathway modulates the levels of O-linked N-acetylglucosamine (O-GlcNAc) on key targets impacting cellular signaling, protein turnover and gene expression. O-GlcNAc cycling may be deregulated in neurodegenerative disease, cancer, and diabetes. Studies in model organisms demonstrate that the O-GlcNAc transferase (OGT/Sxc) is essential for Polycomb group (PcG) repression of the homeotic genes, clusters of genes responsible for the adult body plan. Surprisingly, from flies to man, the O-GlcNAcase (OGA, MGEA5) gene is embedded within the NK cluster, the most evolutionarily ancient of three homeobox gene clusters regulated by PcG repression. PcG repression also plays a key role in maintaining stem cell identity, recruiting the DNA methyltransferase machinery for imprinting, and in X-chromosome inactivation. Intriguingly, the Ogt gene resides near the Xist locus in vertebrates and is subject to regulation by PcG-dependent X-inactivation. OGT is also an enzymatic component of the human dosage compensation complex. These 'evo-devo' relationships linking O-GlcNAc cycling to higher order chromatin structure provide insights into how nutrient availability may influence the epigenetic regulation of gene expression. O-GlcNAc cycling at promoters and PcG repression represent concrete mechanisms by which nutritional information may be transmitted across generations in the intra-uterine environment. Thus, the nutrient-sensing hexosamine signaling pathway may be a key contributor to the metabolic deregulation resulting from prenatal exposure to famine, or the 'vicious cycle' observed in children of mothers with type-2 diabetes and metabolic disease.

Figure 1

The nutrient-responsive hexosamine pathway. The concentration of UDP-GlcNAc is responsive to levels of the indicated precursors and serves as a sensor of nutrient status. Pools of UDP-GlcNAc are utilized in the nuclear/cytoplasmic compartment by O-linked GlcNAc transferase (OGT) or transported into the endoplasmic reticulum and golgi (shown in gray) for utilization in N-linked and O-linked glycoprotein biosynthesis and in the formation of glycosaminoglycans (GAGS). O-GlcNAc cycling is maintained by OGT and the O-GlcNAcase and is acutely sensitive to physiological flux of UDP-GlcNAc (green arrow).

Figure 2

O-GlcNAc cycling is essential for PcG-mediated transcriptional repression. A) The three core polycomb repressive complexes found in Drosophila are shown. The mammalian (black text) and C. elegans (orange text) homologs are also listed. The yellow stars denote catalytic activity. Ph is modified by O-GlcNAc (blue hexagon). B) Epigenetic regulation is maintained by the concerted action of PcG and Trx. In Drosophila the PcG and Trx complexes bind to PRE/TREs (yellow). The two dashed lines represent conditions of repressed (left line) and active transcription (right line). The middle area represents ‘bivalent’ or ‘poised’ genes that are characterized by the competing activity of both complexes and exhibit both active and repressive chromatin marks. O-GlcNAc cycling at PcG/Trx or RNA PolII (or both) would impart a nutrient responsive element to the epigenetic machinery.

Figure 3

The genes encoding enzymes of O-GlcNAc cycling are at discrete locations within the genomes of flies and mammals suggesting conserved regulation by higher order chromatin structure. OGT (red) is present near sites of heterochromatin-euchromatin boundaries on Chr2R in Drosophila (upper) and near the Xist locus in mammals (lower). The Oga gene (green) is present in the 93DE cluster of homeobox genes in the fly and in the orthologous NK cluster of homeobox genes in most vertebrates.

Figure 4

Whole genome analysis of O-GlcNAc cycling on promoters and altered gene expression in viable C. elegans mutants. A) Identification of 800 genes whose promoters are marked by O-GlcNAc using ChIP-on-chip tiling arrays. Strains analyzed were an oga-1 knockout (top row, red), wildtype (middle row, green) and ogt-1 knockout (bottom row, blue). The number of marks on each linkage group from the oga-1 knockout (red) and wildtype (green) is shown below. B) A representative gene from chromosome 1, daf-16, shows that O-GlcNAc is restricted to promoters. The O-GlcNAc signal is higher in fed (red line) than in starved (green line) worms, suggesting that the O-GlcNAc mark is nutrient-responsive. C.) Genes marked by O-GlcNAc (red and pink circles) are enriched in the TGFβ, insulin signaling, and MAPK pathways. A dauer worm (right panel) is shown next to a normal L4 larvae (left panel).

Figure 5

A summary of the multiple mechanisms by which O-GlcNAc cycling may impact development, stem cell fate, and metabolism. O-GlcNAc, which modifies the nuclear pore complex may function to organize the higher order structure of chromatin. X-inactivation contributes to the transcriptional regulation of OGT as described in the text. O-GlcNAc also directly modifies components of the transcription machinery and acts through polycomb repression. The resultant transcriptional effects may be manifested in changes in stem cell viability where PcG dosage is critically important (see text). In certain cell lineages such as the hematopoietic stem cell, these cell fate decisions may lead to changes in the immune system. The nutrient-responsive, fine-tuning of signaling pathways and changes in stem cell fate could contribute to the metabolic reprogramming seen in certain diet-induced maladies.