Uremic toxins and osteocyte dysfunction in CKD

PROJECT SUMMARY/ABSTRACT: Chronic Kidney Disease (CKD) affects over 20 million individuals in the US and as CKD progresses, the risk of hip fractures increases by 2 to 5-fold. Renal Osteodystrophy (ROD), the bone component of CKD-Mineral Bone Disorder is complex with both high and low bone turnover and bone quality changes, and remains a treatment challenge. Over the past 25 years, the therapeutic focus has been to lower the PTH level but this has not resulted in a change in the fracture rate. This implies that there are factors in CKD that are additive to PTH in inducing bone fragility. Recent studies in patients without CKD have identified the importance of the gut derived uremic toxin indoxyl sulfate that accumulates due to reduced renal clearance. Indoxyl sulfate is a ligand for the aryl hydrocarbon receptor (AhR) important in detoxification of toxins and the promotion of multiple aging and bone phenotypes. In our rat model of CKD that parallels the human bone phenotype, our preliminary data demonstrated the oral administration of inulin, a fermentable dietary fiber, alters the gut microbiota and reduces serum levels of IS. This dietary intervention led to higher bone volume, decreased osteoclast number, and decreased cortical porosity. Using an osteocyte cell line for a model of osteoblast to osteocyte differentiation we also demonstrated that indoxyl sulfate increased AhR activity and altered genes important in osteocyte differentiation, function and mineralization. Based on these data, we propose the hypothesis that impaired bone quantity and quality in CKD is due to the interaction of the uremic toxin indoxyl sulfate on osteocyte function mediated through aryl hydrocarbon signaling. To test this hypothesis we will conduct the following aims: Specific Aim 1: To determine the interaction of indoxyl sulfate and PTH on osteocyte function and abnormal bone quantity and quality. We will manipulate the indoxyl sulfate levels through dietary inulin and PTH levels through pharmacologic therapies in vivo in our Cy/+ CKD rats. Outcomes are bone quantity (microCT), histomorphometry, osteocyte apoptosis and immunostaining, osteocyte mineralization (Raman spectroscopy, nanoindentation), and whole bone mechanical properties. We will also utilize cultured osteocytes to further understand the interaction of IS and PTH on gene expression and signaling in osteocytes. Specific Aim 2: To define the role of the aryl hydrocarbon receptor and the involved signaling pathways on osteocyte function, bone quantity, and bone quality. We will induce CKD using adenine in the diet in AhR null mice and selective AHR-/- osteocyte mice and assess bone quantity and quality. We will also utilize primary osteocytes and pharmacologic inhibitors to understand the cell signaling pathways involved. These studies will determine the role of IS and AhR signaling in the osteocyte dysfunction of CKD, offering novel targetable mechanisms to improve bone quantity and quality and prevent fractures.