John A. Hanover, Ph.D., G. Gilbert Ashwell Distinguished Scientist

Current Research

Our laboratory focuses on (1) the molecular features of a novel, glycan-dependent, signal transduction cascade and (2) the mechanism of nuclear transport. The nuclear transport of transcription factors, nuclear kinases, steroid hormone receptors, and replication factors often serves a critical regulatory function. We are examining the mechanisms of nuclear import, export, and subnuclear targeting. We identified a novel nuclear transport pathway involving calmodulin. This pathway has been shown to play a role in mammalian sex determination and stem cell differentiation. We are identifying additional components of this pathway using yeast genetics and chemical biology approaches.

The nuclear pore complex (NPC) mediates the transport of mRNA and proteins across the nuclear envelope. Many components of the nuclear pore are modified by a novel modification: O-linked N-acetylglucosamine (O-linked GlcNAc). The modification also occurs on transcription factors and certain oncogenes and tumor suppressors. Current evidence suggests that the O-linked GlcNAc transferase (OGT) mediates a novel glycan-dependent signal transduction pathway. We have molecularly cloned and characterized the human OGT responsible for glycosylating nuclear pore proteins. This enzyme is expressed as differentially targeted isoforms in man and is localized to both the nucleus and the mitochondria. When expressed in E. coli, the human OGT is catalytically active. We recently solved the X-ray structure of the substrate recognition domain of OGT and we are beginning to understand how it recognizes its many intracellular targets. Although the enzyme is found in a number of target tissues, it is most highly expressed in human pancreatic beta cells, consistent with a role in glucose-sensing. Based on its substrate specificity and molecular features, we have proposed that OGT is the terminal step in a glucose-responsive pathway that becomes dysregulated in diabetes mellitus (NIDDM).

Applying our Research

Our studies may provide insight into how nutrition may impact the health of both mother and offspring.

Need for Further Study

The enzyme catalyzing O-GlcNAc removal, O-GlcNAcase, has also been identified, expressed, and shown to exist as differentially targeted isoforms in man. We are also using the genetically amenable C. elegans model to examine the physiological impact of the enzymes of O-GlcNAc cycling. Using reverse genetics, knockout, and other transgenic models, we are currently exploring the role of these essential genes in signal transduction and pathogenesis of diabetes mellitus. Our studies further suggest that O-GlcNAc cycling plays a key role in the regulation of chromatin structure and may be a key player in epigenetic reprogramming in response to nutrition and other environmental influences.

Research in Plain Language

The storage and control center of a cell’s genetic information is its nucleus, where instructions for building and maintaining an organism are organized, sent, and received. Between 2,000 to 4,000 small channels or pores in the nuclear membrane provide the main gateway between the outside (the cytoplasm) and inside of the nucleus, allowing passage of proteins and messenger RNA (mRNA). Relative to other protein structures in the cell, the nuclear pore complex (NPC) is immense, making study of its function and activity challenging.

Using chemical biology, genetics in yeast and C. elegans roundworms, and other models, the Hanover lab studies enzymes and molecular pathways that control how important proteins find their way to the nucleus, how they interact with the NPC, and how material is imported and exported. We identified a new nuclear transport pathway that plays a role in determining the sex of mammals and in how stem cells become more specialized.

The lab also researches a specific enzyme called O-linked GlcNAc transferase (OGT) that modifies the NPC. Although this enzyme is found in a number of tissues in the body, it is most highly expressed in human pancreas cells that produce insulin. We think that activity of this OGT protein is the final step in a pathway that becomes impaired in diabetes mellitus (NIDDM).