Serum sclerostin is associated with recurrent kidney stone formation independent of hypercalciuria

Background

Kidney stones are frequent in industrialized countries with a lifetime risk of 10 to 15%. A high percentage of individuals experience recurrence. Calcium-containing stones account for more than 80% of kidney stones. Diet, environmental factors, behavior, and genetic variants contribute to the development of kidney stones. Osteocytes excrete the 21 kDa glycoprotein sclerostin, which inhibits bone formation by osteoblasts. Animal data suggests that sclerostin might directly or indirectly regulate calcium excretion via the kidney. As hypercalciuria is one of the most relevant risk factors for kidney stones, sclerostin might possess pathogenic relevance in nephrolithiasis.

Methods

We performed a prospective cross-sectional observational controlled study in 150 recurrent kidney stone formers (rKSF) to analyse the association of sclerostin with known stone risk factors and important modulators of calcium-phosphate metabolism. Serum sclerostin levels were determined at the first visit. As controls, we used 388 non-stone formers from a large Swiss epidemiological cohort.

Results

Sclerostin was mildly increased in rKSF in comparison to controls. This finding was more pronounced in women compared to men. Logistic regression indicated an association of serum sclerostin with rKSF status. In hypercalciuric individuals, sclerostin levels were not different from normocalciuric patients. In Spearman correlation analysis we found a positive correlation between sclerostin, age, and BMI and a negative correlation with eGFR. There was a weak correlation with iPTH and intact FGF 23. In contrast, serum sclerostin levels were not associated with 25-OH Vitamin D3, 1,25-dihydroxy-Vitamin D3, urinary calcium and phosphate or other urinary lithogenic risk factors.

Conclusion

This is the first prospective controlled study investigating serum sclerostin in rKSF. Sclerostin levels were increased in rKSF independent of hypercalciuria and significantly associated with the status as rKSF. It appears that mechanisms other than hypercalciuria may be involved and thus further studies are required to elucidate underlying pathways.

Ovarian cancer G protein-coupled receptor 1 (OGR1) deficiency exacerbates crystal deposition and kidney injury in oxalate nephropathy in female mice

OGR1 (Gpr68) and GPR4 (Gpr4) are proton-activated G protein-coupled receptors that are stimulated upon increased extracellular acidity. These receptors have various physiological and pathophysiological roles in renal acid-base physiology, tissue inflammation, and fibrosis among others. Their function in injured renal tissue, however, remains mostly unclear. To address this, we investigated their role in crystalline nephropathy by increasing the oxalate intake of GPR4 KO and OGR1 KO mice. &#160;After 10 days of high-oxalate intake and 4 days of recovery, renal crystal content, histopathology, filtration function, and inflammation were assessed. While GPR4 deficiency did not show major alterations in disease progression, OGR1 KO mice had higher urinary calcium levels and exacerbated crystal accumulation accompanied by decreased creatinine clearance and urea excretion and a decreased presence of regulatory T (Treg) cells in kidney tissue. When lowering the severity of the kidney injury, OGR1 KO mice were more prone to develop crystalline nephropathy. In this setting, OGR1 KO mice displayed an increased activation of the immune system and a higher production of pro-inflammatory cytokines by T cells and macrophages.</p> Taken together, in the acute setting of oxalate-induced nephropathy, the lack of the proton-activated GPCR GPR4 does not influence disease. OGR1 deficiency, however, increases crystal deposition leading to impaired kidney function. Thus, OGR1 may be important to limit kidney crystal deposition, which might subsequently be relevant for the pathophysiology of oxalate kidney stones or other crystallopathies.

Kidney metabolism and acid-base control: back to the basics

Kidneys are central in the regulation of multiple physiological functions, such as removal of metabolic wastes and toxins, maintenance of electrolyte and fluid balance, and control of pH homeostasis. In addition, kidneys participate in systemic gluconeogenesis and in the production or activation of hormones. Acid-base conditions influence all these functions concomitantly. Healthy kidneys properly coordinate a series of physiological responses in the face of acute and chronic acid-base disorders. However, injured kidneys have a reduced capacity to adapt to such challenges. Chronic kidney disease patients are an example of individuals typically exposed to chronic and progressive metabolic acidosis. Their organisms undergo a series of alterations that brake large detrimental changes in the homeostasis of several parameters, but these alterations may also operate as further drivers of kidney damage. Acid-base disorders lead not only to changes in mechanisms involved in acid-base balance maintenance, but they also affect multiple other mechanisms tightly wired to it. In this review article, we explore the basic renal activities involved in the maintenance of acid-base balance and show how they are interconnected to cell energy metabolism and other important intracellular activities. These intertwined relationships have been investigated for more than a century, but a modern conceptual organization of these events is lacking. We propose that pH homeostasis indissociably interacts with central pathways that drive progression of chronic kidney disease, such as inflammation and metabolism, independent of etiology.

FC 001ACIDOSIS AND ALKALI THERAPY ARE ASSOCIATED WITH TRANSCRIPTIONAL CHANGES AND ALTERED ABUNDANCE OF GENES INVOLVED IN CELL METABOLISM AND BICARBONATE TRANSPORT IN KIDNEY TRANSPLANT RECIPIENTS

Background and Aims

Metabolic acidosis is a common event in kidney transplant recipients and has been associated to a higher risk of graft loss and mortality. In patients with CKD and acidosis, alkali therapy ameliorating acidosis appears to protect kidney function. However, it is still poorly understood how acidosis causes the detrimental effects to kidney graft function and how alkali therapy would interact with these mechanisms. Here we aim to identify transcriptomic alterations in kidney transplant recipients without metabolic acidosis in comparison to patients with metabolic acidosis with and without alkali therapy. Moreover, we examined immunolocalization of key proteins involved in acid-base base regulation in biopsies from these patients.

Method

We obtained 22 biopsies of patients 4-6 years after kidney transplantation. Among these patients, nine were not acidotic (serum [HCO3-] ≥ 22 mM), nine had acidosis ([HCO3-] &lt; 22 mM), and four had acidosis and received sodium bicarbonate (alkali therapy) fully correcting acidosis. Age, immunosuppressive drugs, time after transplantation, and eGFR were not statistically different between groups. RNA was extracted from biopsies and RNAseq was performed. Immunohistochemistry was performed for key proteins involved in the renal regulation of acid-base balance. Additionally, a control group of 6 non-transplanted healthy kidneys was included in the histology analysis.

Results

RNAseq analysis revealed 40 genes differentially expressed between acidosis and no acidosis groups. While most of the genes tended to be recovered by alkali therapy, only three fully recovered with bicarbonate supplementation (p-value &lt; 0.05 and log2(fold change) above 0.5). These genes were KCNJ15 (Kir4.2), SHMT1, and ACADSB. Renal localization of the genes was determined using single-cell RNA sequencing data (Ransick et al., Developmental Cell, 2019, doi.org/10.1016/j.devcel.2019.10.005). Most of the genes were expressed in the proximal tubule and were organized in the model shown in Figure 1A. Several of these genes participate in cell metabolism, such as beta-oxidation, and iron, folate, and methionine metabolism. Moreover, the K+-channel Kir4.2 regulates the activity of the electrogenic sodium bicarbonate cotransporter 1 (NBCe1, SLC4A4) and ammoniagenesis in renal proximal tubules. Immunofluorescence analysis showed that NBCe1 expression in proximal tubules was strongly reduced in patients who developed acidosis and was partially recovered in patients who received alkali therapy (Figure 1B). In type B intercalated cells, a similar pattern was observed for Pendrin (SLC26A4). No alteration in the expression of GDH (GLUD1), AE1 (SLC4A1), AQP2, CA2, RhCG (SLC42A3), and B1 subunit of the H+ATPase (ATP6V1B1) was observed in kidneys of treated or untreated patients with acidosis.

Conclusion

Kidney transplant recipients suffering from metabolic acidosis show distinct expression pattern of genes involved in cell metabolism and acid-base transport.

Expression of NaPi-IIb in rodent and human kidney and upregulation in a model of chronic kidney disease

Na⁺-coupled phosphate cotransporters from the SLC34 and SLC20 families of solute carriers mediate transepithelial transport of inorganic phosphate (Pi). NaPi-IIa/Slc34a1, NaPi-IIc/Slc34a3, and Pit-2/Slc20a2 are all expressed at the apical membrane of renal proximal tubules and therefore contribute to renal Pi reabsorption. Unlike NaPi-IIa and NaPi-IIc, which are rather kidney-specific, NaPi-IIb/Slc34a2 is expressed in several epithelial tissues, including the intestine, lung, testis, and mammary glands. Recently, the expression of NaPi-IIb was also reported in kidneys from rats fed on high Pi. Here, we systematically quantified the mRNA expression of SLC34 and SLC20 cotransporters in kidneys from mice, rats, and humans. In all three species, NaPi-IIa mRNA was by far the most abundant renal transcript. Low and comparable mRNA levels of the other four transporters, including NaPi-IIb, were detected in kidneys from rodents and humans. In mice, the renal expression of NaPi-IIa transcripts was restricted to the cortex, whereas NaPi-IIb mRNA was observed in medullary segments. Consistently, NaPi-IIb protein colocalized with uromodulin at the luminal membrane of thick ascending limbs of the loop of Henle segments. The abundance of NaPi-IIb transcripts in kidneys from mice was neither affected by dietary Pi, the absence of renal NaPi-IIc, nor the depletion of intestinal NaPi-IIb. In contrast, it was highly upregulated in a model of oxalate-induced kidney disease where all other SLC34 phosphate transporters were downregulated. Thus, NaPi-IIb may contribute to renal phosphate reabsorption, and its upregulation in kidney disease might promote hyperphosphatemia.

Tumor necrosis factor stimulates fibroblast growth factor 23 levels in chronic kidney disease and non-renal inflammation

Fibroblast growth factor 23 (FGF23) regulates phosphate homeostasis, and its early rise in patients with chronic kidney disease is independently associated with all-cause mortality. Since inflammation is characteristic of chronic kidney disease and associates with increased plasma FGF23 we examined whether inflammation directly stimulates FGF23. In a population-based cohort, plasma tumor necrosis factor (TNF) was the only inflammatory cytokine that independently and positively correlated with plasma FGF23. Mouse models of chronic kidney disease showed signs of renal inflammation, renal FGF23 expression and elevated systemic FGF23 levels. Renal FGF23 expression coincided with expression of the orphan nuclear receptor Nurr1 regulating FGF23 in other organs. Antibody-mediated neutralization of TNF normalized plasma FGF23 and suppressed ectopic renal Fgf23 expression. Conversely, TNF administration to control mice increased plasma FGF23 without altering plasma phosphate. Moreover, in Il10-deficient mice with inflammatory bowel disease and normal kidney function, plasma FGF23 was elevated and normalized upon TNF neutralization. Thus, the inflammatory cytokine TNF contributes to elevated systemic FGF23 levels and also triggers ectopic renal Fgf23 expression in animal models of chronic kidney disease.

Insights from Systems Biology in Physiological Studies: Learning from Context

Systems biology presents an integrated view of biological systems, focusing on the relations between elements, whether functional or evolutionary, and providing a rich framework for the comprehension of life. At the same time, many low-throughput experimental studies are performed without influence from this integrated view, whilst high-throughput experiments use low-throughput results in their validation and interpretation. We propose an inversion in this logic, and ask which benefits could be obtained from a holistic view coming from high-throughput studies-and systems biology in particular-in interpreting and designing low-throughput experiments. By exploring some key examples from the renal and adrenal physiology, we try to show that network and modularity theory, along with observed patterns of association between elements in a biological system, can have profound effects on our ability to draw meaningful conclusions from experiments.

Distinct mechanisms underlie adaptation of proximal tubule Na+/H+ exchanger isoform 3 in response to chronic metabolic and respiratory acidosis

The Na(+/)H(+) exchanger isoform 3 (NHE3) is essential for HCO(3)(-) reabsorption in renal proximal tubules. The expression and function of NHE3 must adapt to acid-base conditions. The goal of this study was to elucidate the mechanisms responsible for higher proton secretion in proximal tubules during acidosis and to evaluate whether there are differences between metabolic and respiratory acidosis with regard to NHE3 modulation and, if so, to identify the relevant parameters that may trigger these distinct adaptive responses. We achieved metabolic acidosis by lowering HCO(3)(-) concentration in the cell culture medium and respiratory acidosis by increasing CO(2) tension in the incubator chamber. We found that cell-surface NHE3 expression was increased in response to both forms of acidosis. Mild (pH 7.21 ± 0.02) and severe (6.95 ± 0.07) metabolic acidosis increased mRNA levels, at least in part due to up-regulation of transcription, whilst mild (7.11 ± 0.03) and severe (6.86 ± 0.01) respiratory acidosis did not up-regulate NHE3 expression. Analyses of the Nhe3 promoter region suggested that the regulatory elements sensitive to metabolic acidosis are located between -466 and -153 bp, where two consensus binding sites for SP1, a transcription factor up-regulated in metabolic acidosis, were localised. We conclude that metabolic acidosis induces Nhe3 promoter activation, which results in higher mRNA and total protein level. At the plasma membrane surface, NHE3 expression was increased in metabolic and respiratory acidosis alike, suggesting that low pH is responsible for NHE3 displacement to the cell surface.