O-Glcnacylation Is Essential for Erythropoiesis

Understanding the molecular mechanisms of erythropoiesis is critical for treating anemia and other hematopoietic diseases, which affect roughly 3 million Americans and 28% of the global population. The role of post-translational modification (PTM) of proteins in regulating developmental and differentiation processes is understudied, but recently we established that O-GlcNAcylation regulates erythropoiesis. O-GlcNAc regulates numerous cellular functions, including stress response, transcription, and cell cycle progression. O-GlcNAc is a single O-linked β-N-acetyl-D-glucosamine moiety added to serine/threonine amino acids of nuclear, cytoplasmic, and mitochondrial proteins. O-GlcNAc transferase (OGT), which adds the modification, and O-GlcNAcase (OGA), which removes the modification, are responsible for the dynamic processing of the PTM. In response to environmental cues, the variable cycling of O-GlcNAc on and off proteins has potential effects on transcriptional pathways essential for differentiation. Previously, we demonstrated that O-GlcNAc plays a role in regulating human γ-globin gene transcription during development in human β-globin locus yeast artificial chromosome (β-YAC) transgenic mice and derivative immortalized bone marrow cells. O-GlcNAcylation modulates the formation of a GATA-1-FOG-1-NuRD repressor complex that binds the -566 GATA site of the ^Aγ-globin promoter when γ-globin gene expression is silent. OGT and OGA interact with GATA-1 and CHD4, a component of the NuRD complex. O-GlcNAcylation of CHD4 stimulates the formation of this repressor complex, blocking O-GlcNAcylation of CHD4 maintains ^Aγ-globin gene expression. Thus, O-GlcNAc cycling is a novel γ-globin regulatory mechanism, which might be modulated to increase fetal hemoglobin (HbF).

Since O-GlcNAcylation involves input from multiple metabolic pathways, the modification acts as a general sensor of cellular homeostasis. Thus, in response to environmental cues, the addition and removal of O-GlcNAc from proteins may be variably altered with potential effects on biochemical and transcriptional pathways essential for erythropoiesis. To better understand how O-GlcNAcylation affects erythropoiesis in vivo, we developed several new, innovative mouse models. These include erythroid-specific OGT or OGA conditional knockout mice, and transgenic mice with erythroid-specific enforced expression of human OGT or OGA. OGT is an essential gene; erythroid-specific knockout results in fetal death due to severe anemia between day E12-14. OGA is not essential for erythropoiesis; no overt phenotype is observed.

Based on previous our previous studies, we hypothesize that at the onset of erythroid lineage commitment, GATA-1 functions as an adaptor protein to deliver OGT and OGA to erythroid-specific cis-regulatory DNA elements, where they modify transcription complex or chromatin proteins responsible for directing transcriptional networks necessary for normal erythroid development and terminal differentiation. Currently, we are exploring how GATA-1-adaptor function mediates changes in the global O-GlcNAcylation pattern following the GATA-2 to GATA-1 switch that triggers erythroid differentiation. We are also examining the roles of OGT and OGA in the formation and function of the GATA-1-FOG-1-NuRD γ-globin repressor complex. Novel CRISPR/Cas9-based genome targeting tools were developed to probe these questions.

We present phenotypic and molecular data related to the hematopoietic system, including anemia, blood cell histology and morphology, standard blood indices, and β-like globin gene expression during embryonic, fetal, and adult stages of erythropoiesis in our mouse models. In addition, we will show preliminary data using the enzymatically dead dCas9 tools we have synthesized, dCas9-OGT and dCas9-OGA protein fusions that are delivered to cis-regulatory elements controlling erythroid-specific genes involved in erythropoiesis and globin gene switching. The therapeutic outcome will be the identification of erythroid-specific protein targets whose activity can be modulated by altering their O-GlcNAcylation status. We emphasize that because the O-GlcNAc cycle has pleiotropic effects within the cell, it is not a good direct target for therapeutic intervention. However, many of the target proteins are likely to be suitable for treatment venues.