Joseph A. BAUR, PhD
Institute for Diabetes, Obesity, and Metabolism, and Department of Physiology, Perelman School of Medicine, University of Pennsylvania

Modulating NAD+ availability in a rodent model of citrin deficiency

Introduction

The combination of citrin (aspartate-glutamate carrier 2, AGC2) deficiency and reductive stress blocks production of cytosolic aspartate (Asp). Citrin deficiency directly prevents export of Asp from the mitochondria to the cytosol. Under basal conditions, patients with citrin deficiency likely compensate via aspartate transaminase, which generates aspartate from oxaloacetate. However, this mechanism is sensitive to nicotinamide adenine dinucleotide (NAD+/NADH) redox status, as high NADH triggers conversion of oxaloacetate to malate, making it unavailable for conversion to Asp. Citrin is required for the malate-aspartate shuttle that maintains cytosolic redox status. Thus, it would be predicted that in the context of citrin deficiency, metabolites that generate cytosolic NADH, such as sugars and alcohol, would disrupt NAD+/NADH redox balance and cut off the supply of Asp, leading to hyperammonemia. This is borne out in animal models [1, 2] and by the pronounced tendency of citrin deficient patients to avoid these foods [3].

Nicotinamide riboside (NR) is a direct precursor to NAD+ that is orally available and can bypass the rate-limiting and most energetically costly step in NAD+ synthesis [4]. It is currently sold as a nutraceutical formulation and is being tested in human clinical trials based on promising results in mice for heart failure, diabetes, and cognitive disorders [5]. KL1333 is a synthetic substrate for NAD(P)H quinone dehydrogenase 1 (NQO1) [6, 7]. KL1333 and metabolites such as citrate or ethyl pyruvate can regenerate NAD+ from NADH, alleviating reductive stress in the cytosol. Thus, each of these interventions has the potential to restore metabolic homeostasis in the absence of citrin.

The goals of the proposal are:

  1. To develop standardized tests for prevention of acute hyperammonemia, treatment of acute hyperammonemia, and sustainable chronic treatments that confer metabolic resilience.
  2. Test nicotinamide riboside (NR) supplementation, an NAD(P)H quinone dehydrogenase (NQO1) substrate, citrate, and ethyl pyruvate in the above tests.
  3. To optimize promising treatments and/or test additional candidate therapies.

In addition to providing direct information on the candidate approaches that we have chosen, this work will result in the generation of a pipeline for testing and refining these and other interventions to guide the development of therapeutics for intervention in human patients.

Please find the list of references here.

(Updated October 2022)