We have characterized the expression and secretion of the acute kidney injury (AKI) biomarkers insulin-like growth factor binding protein 7 (IGFBP7) and tissue inhibitor of metalloproteinases-2 (TIMP-2) in human kidney epithelial cells in primary cell culture and tissue. We established cell culture model systems of primary kidney cells of proximal and distal tubule origin and observed that both proteins are indeed expressed and secreted in both tubule cell types in vitro. However, TIMP-2 is both expressed and secreted preferentially by cells of distal tubule origin, while IGFBP7 is equally expressed across tubule cell types yet preferentially secreted by cells of proximal tubule origin. In human kidney tissue, strong staining of IGFBP7 was seen in the luminal brush-border region of a subset of proximal tubule cells, and TIMP-2 stained intracellularly in distal tubules. Additionally, while some tubular colocalization of both biomarkers was identified with the injury markers kidney injury molecule-1 and neutrophil gelatinase-associated lipocalin, both biomarkers could also be seen alone, suggesting the possibility for differential mechanistic and/or temporal profiles of regulation of these early AKI biomarkers from known markers of injury. Last, an in vitro model of ischemia-reperfusion demonstrated enhancement of secretion of both markers early after reperfusion. This work provides a rationale for further investigation of these markers for their potential role in the pathogenesis of acute kidney injury.

WNK bodies are large punctate membraneless cytosolic signaling foci that sequester WNK serine–threonine kinases and form in renal distal tubular epithelial cells during shifts in total body potassium balance. The assembly of these structures requires KS-WNK1, a truncated isoform of the WNK1 gene that is exclusively expressed in the distal tubule.

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We characterized mouse blood pressure and ion transport in the setting of commonly used rodent diets that drive K+ intake to the extremes of deficiency and excess. Male 129S2/Sv mice were fed either K+-deficient, control, high-K+ basic, or high-KCl diets for 10 days. Mice maintained on a K+-deficient diet exhibited no change in blood pressure, whereas K+-loaded mice developed an ~10-mmHg blood pressure increase. Following challenge with NaCl, K+-deficient mice developed a salt-sensitive 8 mmHg increase in blood pressure, whereas blood pressure was unchanged in mice fed high-K+ diets. Notably, 10 days of K+ depletion induced diabetes insipidus and upregulation of phosphorylated NaCl cotransporter, proximal Na+ transporters, and pendrin, likely contributing to the K+-deficient NaCl sensitivity. While the anionic content with high-K+ diets had distinct effects on transporter expression along the nephron, both K+ basic and KCl diets had a similar increase in blood pressure. The blood pressure elevation on high-K+ diets correlated with increased Na+-K+-2Cl− cotransporter and γ-epithelial Na+ channel expression and increased urinary response to furosemide and amiloride. We conclude that the dietary K+ maneuvers used here did not recapitulate the inverse effects of K+ on blood pressure observed in human epidemiological studies. This may be due to the extreme degree of K+ stress, the low-Na+-to-K+ ratio, the duration of treatment, and the development of other coinciding events, such as diabetes insipidus. These factors must be taken into consideration when studying the physiological effects of dietary K+ loading and depletion.

The type-2 diabetes drug metformin has proven to have protective effects in several renal disease models. Here, we investigated the protective effects in a 3-day unilateral ureteral obstruction (3dUUO) mouse model. Compared with controls, ureteral obstructed animals displayed increased tubular damage and inflammation. Metformin treatment attenuated inflammation, increased the anti-oxidative response and decreased tubular damage. Hepatic metformin uptake depends on the expression of organic cation transporters (OCTs). To test whether the effects of metformin in the kidney are dependent on these transporters, we tested metformin treatment in OCT1/2−/− mice. Even though exposure of metformin in the kidney was severely decreased in OCT1/2−/− mice when evaluated with [11C]-Metformin and PET/MRI, we found that the protective effects of metformin were OCT1/2 independent when tested in this model. AMP-activated protein kinase (AMPK) has been suggested as a key mediator of the effects of metformin. When using an AMPK-β1 KO mouse model, the protective effects of metformin still occurred in the 3dUUO model. In conclusion, these results show that metformin has a beneficial effect in early stages of renal disease induced by 3dUUO. Furthermore, these effects appear to be independent of the expression of OCT1/2 and AMPK-β1, the most abundant AMPK-β isoform in the kidney.