Our translational research program focuses on immunometabolism using inborn errors of metabolism (IEM) as a model system to examine the interaction between metabolism and immune activation. Using IEM as a model system is a unique approach to studying immunometabolism. Patients with IEM experience life-threatening episodes of acute metabolic instability and organ dysfunction due to infection. Our goal is to understand how immune activation during infection contributes to this pathophysiology. This research also has implications for organ dysfunction during sepsis, a common problem in critical care. Since infection is important in patients with IEM, we also wish to understand the role of metabolism in lymphocyte function. Many enzymes involved in metabolism are expressed in lymphocytes, and have associated IEM. These studies will not only provide insight into immune function in IEM patients, but also help define the role of various metabolic enzymes in lymphocyte function. Specifically, our aims are: 1) To understand the effects of immune system activation on organ metabolism. 2) To understand the role of intermediary metabolism in immune cell function. Research: 1) The effects of immune system activation on organ metabolism. Acute metabolic decompensation in IEM is characterized by rapid onset deterioration in metabolic status leading to life threatening biochemical perturbations (e.g. hyperammonemia). Infection is the major cause of acute metabolic decompensation in patients with IEM. For example, in patients with urea cycle disorders (UCD), a disorder of hepatic amino acid metabolism, infection causes acute life-threatening hyperammonemia and is associated with increased morbidity (McGuire et al, J Peds, 2013). We postulated that activation of the immune system during infection accounted for this increased morbidity. To address the effects of immune activation on organ metabolism, we established a model system of acute metabolic decompensation due to influenza infection using a mouse model of UCD (McGuire et al, Dis Model Mech, 2014). Via this model, we described some of the mechanisms involved in precipitating hyperammonemia, including inhibition of mitochondrial urea cycle enzymes and depletion of urea cycle intermediates in the liver. These findings suggest that supplementation with urea cycle intermediates during infection may be beneficial for UCD patients. Perturbations in amino acid metabolism in UCD due to infection may also extend beyond the liver. Cerebral amino acid depletion can also be seen during systemic immune activation and may account for the neuropathology seen in UCD during infection (Tarasenko et al, in review). From these animal studies of altered amino acid metabolism, we have developed mathematical simulations of whole body nitrogen metabolism in humans during infection. The model allows us to test therapeutic interventions for restoring nitrogen (and amino acid) balance during infection, a major problem in many IEM (manuscript in preparation). In our infection model above, we see hepatic metabolic perturbations in wild-type (WT) mice. These data led us to suggest that acute metabolic decompensation in IEM may represent a failure to adapt to normal physiologic mechanisms during infection. To answer this question, we used a metabolomic approach to identify hepatic metabolic pathways that may be impacted by immune activation in WT mice (manuscript in preparation). Using the same influenza infection model above, long chain fatty acids were identified as the most significantly changed metabolites. Defects at numerous steps in long chain fatty acid oxidation including the carnitine cycle, acyl-CoA dehydrogenases, and the electron transport flavoprotein were found. These data predict that patients with fatty acid oxidation disorders receive an additional metabolic insult during infection. Since the effects described aboove involve mitochondrial metabolism, we asked about the effects of immune activation on mitochondrial energy metabolism. Using a model system of viremia, (Poly I:C), we have demonstrated perturbations in hepatic pyruvate and respiratory chain metabolism in WT mice (manuscript in preparation). Importantly, these perturbations can be improved by depleting tissue macrophages. Moving forward, our immunometabolism studies are concentrating on hepatic mitochondrial metabolism and its adaptation to and recovery from inflammatory insults. Many IEM have mitochondrial dysfunction at baseline. In addition, we chose this strategy due to its broad application to other disease states such as multi-organ system dysfunction syndrome (MODS) due to sepsis. MODS involves mitochondrial dysfunction. We anticipate that our model system will serve as a platform for the evaluation of various interventions aimed at alleviating mitochondrial metabolic dysfunction. The role of intermediary metabolism in immune cell function. Since infection has serious consequences for patients with IEM, and to describe the interactions between intermediary metabolism and immune function, we developed a clinical protocol in the NIH Clinical Center in 2013, The NIH MINI Study: Metabolism Infection and Immunity in IEM (NCT01780168). This protocol is the first organized effort to examine immune function in IEM and is being performed with the Center for Human Immunology at NIH. To date, we have recruited over 45 patients with IEM and are identifying immune perturbations in organic acidemias, fatty acid oxidation defects, and mitochondrial diseases. Moving forward, we are focusing on IEM where cell intrinsic defects are expected, i.e. mitochondrial enzyme deficiencies. We initiated this line of investigation after identifying patients with disorders of mitochondrial metabolism and immune dysfunction. In collaboration with Drs. Susan Pacheco and Mary Kay Koenig at the University of Texas, Houston, we are evaluating immune function in a large population of patients with mitochondrial disease at the NIH Clinical Center. Immune cells undergo great changes in metabolism for proliferation and differentiation. This area of immunometabolism has recently seen a resurgence, however, using IEM as a translational model for understanding immune cell metabolism is unique. Initial studies in the laboratory helped define the role of arginine metabolism in T-cells. Using a mouse model of argininosuccinate synthetase (ASS1) deficiency, we were able to demonstrate the role of this enzyme, which is involved in arginine synthesis, in T-cell differentiation and function (Tarasenko et al., in review). In keeping with our clinical observations and focus, we have developed animal models to address the role of mitochondrial energy metabolism in immune cell function. To induce complex IV deficiency (COX10flox mice), T-lymphocytes and B-lymphocytes are being targeted using CD4-Cre and CD19-Cre respectively. The B-cell specific COX10 deficient mice have recently been produced and display abnormal immune responses, similar to our findings in patients. We will continue to characterize these mice and seek to develop interventions aimed at improving B-cell function.
Showing the most recent 10 out of 17 publications
