ORIGINAL RESEARCH published: 18 January 2016 doi: 10.3389/fendo.2015.00191 GIT2 Acts as a Systems-Level Coordinator of Neurometabolic Activity and Pathophysiological Aging Bronwen Martin1† , Wayne Chadwick2† , Jonathan Janssens3,4† , Richard T. Premont5 , Robert Schmalzigaug5 , Kevin G. Becker6 , Elin Lehrmann6 , William H. Wood6 , Yongqing Zhang6 , Sana Siddiqui2 , Sung-Soo Park2 , Wei-na Cong1 , Caitlin M. Daimon1 and Stuart Maudsley2,3,4* 1 Metabolism Unit, National Institute on Aging, National Institutes of Health, Baltimore, MD, USA, 2 Receptor Pharmacology Unit, National Institute on Aging, National Institutes of Health, Baltimore, MD, USA, 3 Translational Neurobiology Group, 4 Edited by: VIB Department of Molecular Genetics, University of Antwerp, Antwerp, Belgium, Laboratory of Neurogenetics, Institute 5 Brian Hudson, Born-Bunge, University of Antwerp, Antwerp, Belgium, Department of Medicine, Gastroenterology Division, Duke University, 6 University of Glasgow, UK Durham, NC, USA, Gene Expression and Genomics Unit, National Institutes of Health, Baltimore, MD, USA Reviewed by: Amanda E. Mackenzie, Aging represents one of the most complicated and highly integrated somatic processes. University of Glasgow, UK Keyong Du, Healthy aging is suggested to rely upon the coherent regulation of hormonal and Tufts Medical Center, USA neuronal communication between the central nervous system and peripheral tissues. *Correspondence: The hypothalamus is one of the main structures in the body responsible for sustaining Stuart Maudsley an efficient interaction between energy balance and neurological activity and therefore [email protected] likely coordinates multiple systems in the aging process. We previously identified, in †Bronwen Martin, Wayne Chadwick and Jonathan Janssens have hypothalamic and peripheral tissues, the G protein-coupled receptor kinase interacting contributed equally to this work. protein 2 (GIT2) as a stress response and aging regulator. As metabolic status profoundly affects aging trajectories, we investigated the role of GIT2 in regulating metabolic activity. Specialty section: This article was submitted to We found that genomic deletion of GIT2 alters hypothalamic transcriptomic signatures Molecular and Structural related to diabetes and metabolic pathways. Deletion of GIT2 reduced whole animal Endocrinology, respiratory exchange ratios away from those related to primary glucose usage for energy a section of the journal Frontiers in Endocrinology homeostasis. GIT2 knockout (GIT2KO) mice demonstrated lower insulin secretion levels, Received: 05 August 2015 disruption of pancreatic islet beta cell mass, elevated plasma glucose, and insulin resis- Accepted: 14 December 2015 tance. High-dimensionality transcriptomic signatures from islets isolated from GIT2KO Published: 18 January 2016 mice indicated a disruption of beta cell development. Additionally, GIT2 expression was Citation: Martin B, Chadwick W, Janssens J, prematurely elevated in pancreatic and hypothalamic tissues from diabetic-state mice Premont RT, Schmalzigaug R, (db/db), compared to age-matched wild type (WT) controls, further supporting the role Becker KG, Lehrmann E, Wood WH, of GIT2 in metabolic regulation and aging. We also found that the physical interaction of Zhang Y, Siddiqui S, Park S-S, Cong W-n, Daimon CM and pancreatic GIT2 with the insulin receptor and insulin receptor substrate 2 was diminished Maudsley S (2016) GIT2 Acts as a in db/db mice compared to WT mice. Therefore, GIT2 appears to exert a multidimen- Systems-Level Coordinator of Neurometabolic Activity and sional “keystone” role in regulating the aging process by coordinating somatic responses Pathophysiological Aging. to energy deficits. Front. Endocrinol. 6:191. doi: 10.3389/fendo.2015.00191 Keywords: GIT2, metabolism, keystone, aging, dysglycemia, trajectory Frontiers in Endocrinology | www.frontiersin.org 1 January 2016 | Volume 6 | Article 191 Martin et al. GIT2 Regulation of Neurometabolism INTRODUCTION that both neurodegenerative diseases and pathophysiological aging involve a functional interplay between a series of diverse The natural aging process is associated with an accumulation biological systems including neurological, endocrinological, of molecular perturbations affecting almost all cells, tissues and sensory, and metabolic activities (13–23). Many of these systems organs of the body. These alterations have long been known are functionally integrated together in one crucial CNS organ, to affect multiple processes related to cell survival, genomic i.e., the hypothalamus (24). The hypothalamus is responsible for instability, altered gene expression patterns, aberrant cellular the regulation of many metabolic pathways by synthesizing and replication, oxidative damage by reactive oxygen species (ROS), secreting numerous neuronal hormones that stimulate or inhibit and fluctuations in protein expression and coherent protein the secretion of trophic hormones from the anterior pituitary. post-translational modification 1( ). Life expectancy in most The hypothalamus therefore can control global metabolism, body westernized and developing countries is demonstrating an temperature, thirst, hunger, fatigue, and circadian rhythms (25). inexorable increase (2, 3), therefore either natural or pathological Not only does the hypothalamus act as a trophic master-controller aging could be considered the primary threat to global health in of the endocrine system but it also possesses neuronal projections the future. With increasing age, there is a system-wide reduction to many autonomous and higher centers of the brain (26), thereby in the ability of the body to cope with stress in-part due to a providing a crucial link between aging and age-related disorders degradation of the efficiency of energy-generating [e.g., ATP such as dementia (27). We have previously shown, through the (adenosine triphosphate)] metabolic systems (4–6). Disruption development of in cellula aging models (19), that regulation of the of the primary energy-synthesizing system, i.e., glucose catabo- G protein-coupled receptor kinase interacting protein 2 (GIT2) lism by oxidative phosphorylation, leads to both ATP depletion is highly sensitive to the chronological aging process and also to (thus affecting electrical cellular excitability, proteolytic activi- the presence of age-linked stressors such as ROS. Using a combi- ties, transmembrane transport processes, and kinase activity) natorial informatics and proteomics approach, we demonstrated as well as an increase in the deleterious effects of unregulated that GIT2 acts as a hypothalamic aging “keystone” factor (13). hyperglycemia, e.g., systemic inflammation, arterial stenosis, Subsequently, GIT2 has also been implicated in regulating DNA impaired tissue healing, neuronal damage, and renal failure. integrity via its interaction with ataxia telangiectasia-mutated Hence, one of the most characteristic factors in the aging process (ATM), a DNA damage repair (DDR) kinase (18). In response to are the generation of insulin resistance, disruptions to oxidative multiple forms of DNA damaging insults, GIT2 is rapidly phos- phosphorylation of glucose, and changes in body fat composi- phorylated by ATM and then forms a complex with multiple DDR tion (6). Therapeutic interventions, such as caloric restriction or proteins including MDC1, MRE11, and phosphorylated H2AX visceral fat depletion, designed to address features of metabolic/ (18). Genomic deletion of GIT2 in mice (GIT2KO) resulted in glycemic aging, have succeeded in improving insulinotropic a significant increase in the rate of DNA damage accumulation activity and life span in rodents (7, 8). The importance of the effi- compared to wild type (WT) age-matched control mice (18). DNA ciency in glucose metabolism in the aging process is underscored instability has been associated with an advanced rate of aging for by the demonstration that multiple glucose-regulatory factors several decades (28–30) and this has been reinforced with the in nematode worms were also major age-regulating genes, e.g., appreciation that accelerated molecular aging occurs in patients daf-2 [insulin receptor analog (9)] and age-1 [phosphoinositide possessing mutations in multiple DDR proteins including ATM 3-kinase analog – a primary downstream factor of insulin recep- (31), WRN [Werner syndrome, RecQ helicase-like (32, 33)], tor functions (10)]. Translation of these fundamental insulin/ and BLM [Bloom syndrome, RecQ helicase-like (34)]. Glucose- glucose-mediated life span alterations from nematodes to higher related diabetic pathologies often result in elevated cellular levels organisms has proven problematical: a situation likely due to the of ROS due to an attenuated glucose uptake capacity and support increased functional diversity of insulin/insulin-related ligands of oxidative phosphorylation causing cells to generate ATP via in more complex, higher organisms. Despite this however, other less-efficient synthetic mechanisms. Prematurely elevated adipose tissue-specific insulin receptor deletion is associated ROS levels, as with mutations in DDR proteins, are considered to with life span alterations in mice (11). Linked to this observa- be one of the major causes of advanced aging (35–37). Reinforcing tion, experimental paradigms in rhesus macaques that attenuate the link between dysfunctional aging, diabetic pathologies, and age-related glycemic disruption and increase insulin sensitivity DNA instability, DDR proteins