Abstract Despite the on-going debate as to whether there are changes in gut barrier function during healthy aging, clinical studies consistently support an increased risk of infection, sepsis, and multiple organ dysfunction in elderly patients following trauma and major surgical intervention. Among many contributing factors, impaired barrier integrity of an aging gut is the essential factor to this increased risk. However, our current understanding of the mechanisms underlying the increased vulnerability to gut barrier dysfunction during aging remains poor and, as a result, effective therapies to preserve intestinal barrier integrity in old patients are limited. Maintaining the gut epithelial barrier function is a complex process that requires epithelial cells to rapidly alter gene expression patterns to regulate their interactions and survival, adaption to stress, and to maintain homeostasis. Posttranscriptional events, particularly altered mRNA turnover and translation, are major mechanisms by which mammalian cells control gene expression in response to various stresses. Regulation of mRNA stability and translation is predominantly governed by RNA-binding proteins (RBPs) and noncoding RNAs (ncRNAs) including circular RNAs (circRNAs). Recently, we have demonstrated that the RBP HuR regulates the epithelium host defense in the intestine via Paneth cells and that the circRNA circHIPK3 enhances injury-induced regeneration of the intestinal epithelium by reducing microRNA-29b function. However, the exact roles of HuR and circHIPK3 in regulating the intestinal barrier function in the aging epithelium remain unknown and how these new findings can be exploited to benefit old patients is still unclear. Our preliminary results show that the level of circHIPK3 decreased remarkedly in the small intestinal mucosa of old mice and aged individuals, whereas aged mice with HuR-deficiency exhibited an improved gut barrier function relative to age-matched control mice. Our preliminary studies further revealed that HuR was potentially involved in promoting excessive mitochondrial clearance in the old intestinal epithelium and that circHIPK3 acted as an enhancer of mitochondrial biogenesis. Based on our exciting preliminary results and support from scientific literatures, we hypothesize circHIPK3 and HuR play distinct roles in modulating mitochondrial function in aging intestinal epithelium, ultimately impacting gut barrier function and vulnerability in the elderly undergoing pathological stress. Two specific aims are proposed to test this hypothesis: 1) to test the hypothesis that circHIPK3 and HuR play key roles in the altered gut barrier function observed in aging mice and humans under pathological stress compared to normal homeostasis; and 2) to test the hypothesis that circHIPK3 and HuR regulate the epithelial barrier of the aged gut by altering IEC mitochondrial metabolism. Completion of these specific aims will make a significant conceptual advance by linki...