Coordinated Control of Metabolism by Enzymes and Transporters

About This Grant

Karthikeyani Chellappa PhD, Brown University Coordinated Control of Metabolism by Transporters and Enzymes The transport of molecules across the plasma membrane is a highly regulated process that impacts metabolism at subcellular to organismal scales. The physical association of enzymes in a pathway or the association of transporters and enzymes can increase the efficiency of metabolic pathways. Here, we address the broader implications of coordination between transporters and enzymes by investigating the transport mechanism of a precursor for nicotinamide adenine dinucleotide (NAD). NAD, a critical redox cofactor in over 400 biochemical reactions, also acts as a substrate for signaling enzymes like sirtuins, ADP-ribosyltransferases, and cyclic ADP-ribose synthases. The decrease in NAD levels is associated with a broad range of diseases, including obesity, diabetes, aging, and cardiovascular and neurodegenerative diseases. While boosting NAD levels by precursor supplementation has shown benefits in rodent disease models, its effectiveness in human clinical trials is limited. Understanding the biology of NAD metabolism has implications for basic science and translational research. Nicotinamide (NAM) is a primary precursor for NAD biosynthesis in most tissues in mammals. NAD-consuming enzymes cleave NAD molecules to release NAM, which is constantly exchanged between tissues and circulation. Our previous research has shown that NAM transport is a critical site of regulation in NAD homeostasis with potential implications in aging. We have further established that the transport of NAM is crucial for NAD synthesis in the gut microbiome and precursor supplementation. Our current work has discovered that NAM is transported into mammalian cells via a biphasic transport system. We also found that nicotinamide phosphoribosyl transferase (NAMPT), a rate-limiting enzyme that uses NAM as a substrate, regulates NAM transport. Hence, we hypothesize that a coordination of NAM transport and metabolism is essential to maintain cellular function. Therefore, it is important to understand how NAM, an essential intermediate in NAD metabolism, is transported across the plasma membrane in mammalian cells. Our aim is to address fundamental questions such as how NAM is transported in mammals, how NAMPT controls NAM transport, and what is the significance of NAM transport control by NAMPT. By unraveling the mechanisms of NAM transport, we can develop novel genetic and pharmacological tools to accelerate the next phase of discoveries in NAD biology, microbiome research, and nutrient supplementation. An over-arching long-term goal of this research program is to fill the gap in understanding the implications of coordination between transporter and enzymes to cellular metabolism and function at the genome level using a systems biology approach.