ITPR3 and aberrant mitochondrial calcium signaling in alcoholic hepatitis

Yale University, New Haven CT

Investigators

Abstract

Excessive alcohol intake is the third leading cause of preventable death in the US and is the leading risk factor for death globally among people aged 15–49 years. Moreover, cirrhosis-related mortality among people aged 25–34 years has been increasing by >10% per year in recent years, and this is driven entirely by alcohol- associated liver disease (AALD). Alcohol-associated hepatitis (AAH) is a severe and potentially life-threatening complication of AALD, with a short-term mortality ranging from 20% to 50%. Mitochondrial Ca2+ signals regulate pathological events in hepatocytes that relate to alcohol metabolism, such as steatosis and cell damage, and this project will examine the pathophysiological consequences of altered mitochondrial Ca2+ signaling for the development of AAH. Our relevant previous observations are: (1) Ca2+ signals in hepatocytes are principally regulated by the inositol trisphosphate receptor (ITPR), which is an intracellular Ca2+ release channel in the endoplasmic reticulum, (2) the ability of Ca2+ signals to regulate cell function is determined by which of the three ITPR isoforms are expressed, and the subcellular regions in which they are localized, (3) hepatocytes normally express two of the three ITPR isoforms, ITPR1 and ITPR2, but not ITPR3, (4) Although ITPR2 is the most heavily expressed isoform in hepatocytes, it is excluded from the ER-mitochondrial junction, (5) In contrast, ITPR1 is concentrated in the ER-mitochondrial junction, where it regulates mitochondrial Ca2+, metabolism and apoptosis, (6) although ITPR3 is not in hepatocytes under normal conditions, it becomes expressed in AAH, and (7) like ITPR1, ITPR3 is concentrated in the ER-mitochondrial junction, where it also is positioned to modulate mitochondrial Ca2+ signals and cell metabolism. The hypothesis of this project is that hepatocellular expression and subcellular localization of ITPR3 in AAH is responsible for pathological changes in mitochondrial Ca2+ signals that result in steatosis, metabolic stress, and hepatocellular injury. We will test this hypothesis through three specific aims: (1) at a molecular level, we will compare Ca2+ release from ITPR1 and ITPR3 at the single channel level, determine why each isoform localizes to the ER-mitochondria interface in hepatocytes, and identify the proteins that modulate their Ca2+ channel activity; (2) at a cellular level, we will compare the effects of ITPR1 and ITPR3 on mitochondrial Ca2+ signals, mitochondrial physiology, hepatocyte metabolism, lipid droplet accumulation, and cell injury; and (3) at a whole-organ level, we will examine the roles of ITPR1 and ITPR3 in the development of AAH in animal models and in patients with AAH. Because mitochondrial Ca2+ overload appears to be a prerequisite for the most critical pathological events in AAH, this project has the potential to identify novel, highly specific, and effective treatment strategies, a goal which will have a very broad impact on this important health problem.