Systemic Succinate Homeostasis and Local Succinate Signaling Affect Blood Pressure and Modify Risks for Calcium Oxalate Lithogenesis
Authors: Khamaysi Ahlam , Anbtawee-Jomaa Shireen , Fremder Moran , Eini-Rider Hadar , Shimshilashvili Liana , Aharon Sara , Aizenshtein Elina , Shlomi Tomer , Noguchi Audrey , Springer Danielle , Moe Orson W. , Shcheynikov Nikolay , Muallem Shmuel , Ohana Ehud 
Published: February 2019

Significance Statement
The association between kidney stone formation and hypertension is well established, but the molecular mechanism linking the two diseases has been unclear. In this study, the authors describe a metabolic pathway that tightly modulates homeostasis of the metabolite succinate. In this pathway, succinate uptake regulates BP, possibly through regulation of the renin-angiotensin system. The same pathway also regulates urinary citrate and oxalate, thus protecting against calcium oxalate stone formation. Mice lacking the slc26a6 transporter, a major succinate transport inhibitor, exhibit reduced urinary levels of succinate and citrate, increased concentration of serum succinate, increased renin secretion, and hypertension. These findings provide a link between kidney stone formation and hypertension through impaired transport of metabolites, and suggest that succinate and citrate signaling and transport are potential therapeutic targets.

Background
In the kidney, low urinary citrate increases the risk for developing kidney stones, and elevation of luminal succinate in the juxtaglomerular apparatus increases renin secretion, causing hypertension. Although the association between stone formation and hypertension is well established, the molecular mechanism linking these pathophysiologies has been elusive.

Methods
To investigate the relationship between succinate and citrate/oxalate levels, we assessed blood and urine levels of metabolites, renal protein expression, and BP (using 24-hour telemetric monitoring) in male mice lacking slc26a6 (a transporter that inhibits the succinate transporter NaDC-1 to control citrate absorption from the urinary lumen). We also explored the mechanism underlying this metabolic association, using coimmunoprecipitation, electrophysiologic measurements, and flux assays to study protein interaction and transport activity.

Results
Compared with control mice, slc26a6−/− mice (previously shown to have low urinary citrate and to develop calcium oxalate stones) had a 40% decrease in urinary excretion of succinate, a 35% increase in serum succinate, and elevated plasma renin. Slc26a6−/− mice also showed activity-dependent hypertension that was unaffected by dietary salt intake. Structural modeling, confirmed by mutational analysis, identified slc26a6 and NaDC-1 residues that interact and mediate slc26a6’s inhibition of NaDC-1. This interaction is regulated by the scaffolding protein IRBIT, which is released by stimulation of the succinate receptor SUCNR1 and interacts with the NaDC-1/slc26a6 complex to inhibit succinate transport by NaDC-1.

Conclusions
These findings reveal a succinate/citrate homeostatic pathway regulated by IRBIT that affects BP and biochemical risk of calcium oxalate stone formation, thus providing a potential molecular link between hypertension and lithogenesis.