Sweet-Talking the Distal Nephron Calcium-Sensing Receptor

Like most circulating electrolytes, serum calcium concentrations must be maintained within a tight physiologic window. Key to this homeostatic process is the calcium-sensing receptor (CaSR), a plasma membrane protein with major sites of action in the parathyroid glands and kidney tubule. The CaSR is a dimeric G protein-coupled receptor with a large extracellular domain that senses extracellular Ca²⁺.^1,2 In the parathyroid gland, ionized Ca²⁺ binds to multiple sites within this domain, triggering an inhibitory intracellular signal that suppresses the release of parathyroid hormone. This process is augmented by calcimimetics, widely prescribed drugs that bind to a different site on the CaSR and enhance the Ca²⁺-dependent blockade of parathyroid hormone secretion.^2,3

In the kidney, the CaSR is highly expressed along the basolateral membrane of the thick ascending limb (TAL) of the loop of Henle, where it senses interstitial Ca²⁺ concentrations. During hypercalcemia, Ca²⁺ binding to the TAL CaSR activates signals that control the Na-K-2Cl cotransporter NKCC2, the K⁺ channel ROMK, and the tight junction protein claudin-14.^2,4 This coordinated response results in reduced paracellular Ca²⁺ reabsorption, hypercalciuria, and normalization of total body calcium. Although CaSR expression is lower in other parts of the renal tubule, prior studies have identified additional roles for the receptor in the proximal tubule (PT) and collecting duct.² In contrast to the TAL, CaSR expression in these nephron segments has been suggested to be apical (facing the urinary space), although not all studies have reproduced this.^2,5 Nevertheless, available functional evidence suggests that in certain parts of the nephron, the CaSR can sense and respond to Ca²⁺ fluctuations in the tubular fluid.²

The distal convoluted tubule (DCT) also expresses a functional CaSR, expressed on both the apical and basolateral membranes.⁵ Early studies identified a role for the DCT CaSR in transcellular calcium reabsorption and in basolateral K⁺ channel function.² In 2018, Bazúa-Valenti and colleagues found that the CaSR also regulates DCT sodium transport.⁶ Receptor activation was associated with increased activity of the thiazide-sensitive NaCl cotransporter NCC, the primary mediator of sodium reabsorption in the DCT.⁷ A mechanism was identified in which luminal calcium triggers a CaSR-dependent G protein-coupled signal that activates the WNK4-SPAK pathway, the canonical kinase cascade that switches on the cotransporter via phosphorylation.⁷

On the basis of these findings, the authors proposed that DCT Ca²⁺ sensing via the apical CaSR may protect against excessive sodium and volume losses during calciuresis.

Although an intriguing relationship between luminal calcium sensing and NCC function had been identified, questions remained. CaSR expression is lower in the DCT compared with the TAL,^5,8 so how significant is the receptor’s effect on DCT salt transport, and is it clinically relevant? To address these questions, the authors considered the possibility that the DCT CaSR may be subject to regulation by cofactors. In addition to calcium, the CaSR binds glucose, a finding that was discovered after investigators recognized that the receptor’s extracellular sensing domain shares similarities to those found in sweet taste receptors.³ In vitro, physiologic glucose concentrations trigger rapid CaSR activation that only occurs when extracellular calcium is present. Thus, rather than functioning as a bona fide CaSR agonist, glucose is a positive allosteric modulator of calcium sensing. Normally, filtered glucose is fully reabsorbed in the PT, and distal delivery is negligible. However, during hyperglycemic conditions in which the transport capacity for glucose is exceeded, or during pharmacologic sodium-glucose cotransporter 2 (SGLT2) inhibition, sugar is delivered to the DCT. The glucose isomer fructose is a constituent of modern diets and contributes significantly to obesity and the metabolic syndrome.⁹ Like glucose, filtered fructose is reabsorbed in PT,⁹ although significant fructosuria has been observed in diabetic patients with hyperglycemia¹⁰ and in patients with inherited disorders of fructose metabolism. The presence of luminal glucose or fructose in the DCT could potentially augment NCC-mediated salt reabsorption via positive allosteric regulation of the apical CaSR.

In this issue of JASN, Bahena-Lopez and coauthors¹¹ tested this hypothesis by using a multifaceted approach extending from experiments in cultured cells and rodents to a small study in human subjects. In in vitro cell culture studies controlled for osmolality, extracellular glucose and fructose activated WNK4 and SPAK in a CaSR-dependent manner. In mice, inducing glucose delivery to the DCT with the SGLT2 inhibitor dapagliflozin activated NCC, an effect that required the WNK4-SPAK pathway. Consistent with CaSR dependence, this SGLT2 inhibition effect on NCC was abrogated by pharmacologic CaSR inhibition. Similar CaSR-dependent activation of the WNK4-SPAK-NCC axis was seen when mice were subjected to diet-induced fructosuria. The effect of sugars on NCC activity was due to their presence in the DCT lumen because ex vivo rodent kidney perfusion studies confirmed an effect of filtered glucose on SPAK and NCC phosphorylation status that was CaSR dependent. Finally, normalized analysis of NCC and SPAK phosphoprotein expression in urinary extracellular vesicles was performed in healthy men subjected to various maneuvers, including a single dose of the calcimimetic cinacalcet, a single dose of dapagliflozin, or an oral fructose load. In all cases, SPAK and NCC phosphorylation status were increased. A consistent theme among all these experiments is the timescale: regardless of the approach, the positive effect of luminal sugars on CaSR signaling to NCC appeared to occur within hours, suggesting dynamic regulation.

In sum, Bahena-Lopez and colleagues present a comprehensive, well-designed study that implicates luminal glucose and fructose in the activation of NCC via the CaSR. These sugars act as allosteric CaSR activators that coax (or “sweet-talk”) the receptor into stimulating DCT salt reabsorption via the WNK4-SPAK-NCC axis when luminal calcium is present. These findings support the relevance of the CaSR in DCT salt handling, and identify a physiologic relationship between luminal glucose, calcium, and sodium chloride in the distal tubule. The study also provides clues into how long-term use of SGLT2 inhibitors might influence distal nephron adaptation and offers a mechanism that could contribute to salt-sensitive hypertension in metabolic syndromes associated with the fructose-enriched diets that are commonplace in modern society.

Disclosures

A.R. Subramanya is an editorial board member of JASN and American Journal of Physiology—Renal Physiology, and reports honoraria from Icahn School of Medicine at Mt Sinai, The Hospital for Sick Children, University of Toronto, Vanderbilt University, and the University of Tennessee Health Sciences Center.

Funding

This work was supported by the National Institutes of Health (grant R01DK119252).

Author Contributions

A.R. Subramanya was responsible for conceptualization and wrote the original draft of the manuscript.