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The liver has always been considered the primary site of endogenous oxalate synthesis, and the only site where this metabolism has been demonstrated to occur. Endogenous oxalate synthesis is heightened in those afflicted with the Primary Hyperoxaluria (PH) disorders. The excessive oxalate generated in these diseases results in robust urinary oxalate excretion promoting calcium oxalate kidney stone formation, nephrocalcinosis, and renal damage which often leads to kidney failure. The terminal step in oxalate synthesis is the oxidation of glyoxylate catalyzed by hepatic lactate dehydrogenase (LDHA). Glyoxylate can be diverted from this metabolism via alanine glyoxylate aminotransferase (AGT), the enzyme absent/mis-targeted or defective in PH1. Knock down of hepatic LDHA via siRNA has been reported to promote a substantial reduction in urinary oxalate excretion in those afflicted with PH1, but not in PH2 patients, a group who have defective glyoxylate reductase (GR). This parallels experiments we have conducted in murine knockout (KO) models of PH1 and PH2, which have demonstrated a differential response, a significant reduction in urinary oxalate excretion in the PH1 mice, but limited decrease in the PH2 animals. In addition, there was no reduction in urinary oxalate excretion with hepatic LDHA knockdown in wild type mice. The aforementioned findings challenge the dogma that the liver is the only sight of endogenous oxalate synthesis, thus providing a suitable and potentially high yield target for the research proposed in this submission. Continuous intravenous infusions of the stable carbon-13 isotope of glyoxylate (13C2-glyoxylate) in wild type, GR KO, and AGT KO mice, together with hepatic LDHA siRNA knockdown, will be undertaken to examine the contribution of non-hepatic tissues to oxalate synthesis. Successful completion of the proposed studies will generate a better understanding of the relative importance of oxalate synthesis outside the liver under normal conditions and in diseases states (PH1, PH2). This may eventuate in identification of novel approaches for treating individuals with PH2, PH1, and perhaps those with idiopathic calcium oxalate kidney stone disease where oxalate excretion plays an important role in calculus formation.
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