Dent disease: A window into calcium and phosphate transport

Abstract

This review examines calcium and phosphate transport in the kidney through the lens of the rare X-linked genetic disorder Dent disease. Dent disease type 1 (DD1) is caused by mutations in the CLCN5 gene encoding ClC-5, a Cl^- /H⁺ antiporter localized to early endosomes of the proximal tubule (PT). Phenotypic features commonly include low molecular weight proteinuria (LMWP), hypercalciuria, focal global sclerosis and chronic kidney disease; calcium nephrolithiasis, nephrocalcinosis and hypophosphatemic rickets are less commonly observed. Although it is not surprising that abnormal endosomal function and recycling in the PT could result in LMWP, it is less clear how ClC-5 dysfunction disturbs calcium and phosphate metabolism. It is known that the majority of calcium and phosphate transport occurs in PT cells, and PT endocytosis is essential for calcium and phosphorus reabsorption in this nephron segment. Evidence from ClC-5 KO models suggests that ClC-5 mediates parathormone endocytosis from tubular fluid. In addition, ClC-5 dysfunction alters expression of the sodium/proton exchanger NHE3 on the PT apical surface thus altering transcellular sodium movement and hence paracellular calcium reabsorption. A potential role for NHE3 dysfunction in the DD1 phenotype has never been investigated, either in DD models or in patients with DD1, even though patients with DD1 exhibit renal sodium and potassium wasting, especially when exposed to even a low dose of thiazide diuretic. Thus, insights from the rare disease DD1 may inform possible underlying mechanisms for the phenotype of hypercalciuria and idiopathic calcium stones.

Keywords: ClC-5; Dent disease; endocytosis; renal calcium transport; renal phosphate transport; sodium/proton exchanger NHE3.

Figure 1

Receptor‐mediated endocytosis of LMW proteins in the proximal tubule. LMW proteins undergo receptor‐mediated endocytosis involving megalin and cubilin. Receptors and ligands are internalized by the formation of endosomes, which are progressively acidified by V‐ATPase. Chloride influx via ClC‐5 facilitates acidification by maintaining electroneutrality of ion transport. Endosomal recycling brings receptors back to the apical surface. The absence or dysfunction of ClC‐5 can potentially disrupt endosome cycling at three points (indicated by arrows): (1) reduced rate of receptor internalization; (2) disrupt progression of endosomes to lysosomes; and (3) disrupt recycling of endosomes to the cell surface

Figure 2

Renal phosphate reabsorption in the proximal tubule. Phosphate is reabsorbed via three sodium phosphate cotransporters: NaPi2a, NaPi2c and PIT‐2. In humans, NaPi2a and NaPi2c are believed to play the most important role in phosphate reabsorption. They are positioned at the apical membrane of renal proximal tubular cells and take advantage of an inward electrochemical gradient for sodium to move phosphate from the filtrate into the cell. The amount of phosphate reabsorbed is dependent on the abundance of the sodium phosphate cotransporters and variations in their number at the brush border membrane are a primary regulatory pathway for urinary phosphate excretion

Figure 3

Renal calcium reabsorption along the nephron. The majority of filtered calcium is reabsorbed in the proximal tubule (PT) and thick ascending limb (TAL), with the final highly regulated percentage in the distal convoluted tubule (DCT) and in the connecting tubule (CNT). In the PT, calcium is mainly reabsorbed paracellularly, partially driven by activity of the sodium/proton exchanger 3 (NHE3), which allows transcellular sodium entry at the apical brush border, while the Na‐K‐ATPase pumps sodium out of the cell at the basolateral side. In TAL, calcium is reabsorbed by specialized and controlled paracellular pathways involving claudin 16, 19 and 14. The driving force for calcium is produced by the combined action of the basolateral Na‐K‐ATPase, the Na‐K‐Cl cotransporter (NKCC2) and the outward rectifying ROMK channel on the apical membrane. In DCT‐CNT, calcium enters the cell at the apical side through TRPV5 channels, binds intracellular calbindin‐D‐28k and exits the cell at the basolateral side by the Na‐Ca exchanger (NCX1) and the Ca‐ATPase PMCA429

Figure 4

PTH‐induced endocytosis of NaPi2a. In the proximal tubule, PTH binds to apical and basolateral PTH receptors. Stimulation of either receptor is known to rapidly induce phosphaturia by decreasing apical phosphate transporter activity. Activation of phospholipase C (PLC) by apical PTH receptors leads to protein kinase C (PKC)‐dependent stimulation of ERK1/2 kinases and internalization of NaPi2a. Basolateral PTH receptors are linked to adenylate cyclase (AC), protein kinase A (PKA) and ERK1/2. NaPi2a is internalized via clathrin‐coated vesicles, transported to endosomes and targeted to lysosomes for degradation

Figure 5

Hypothetical role for NHERF1 in proximal tubular cell endocytosis. NHERF1 could potentially influence proximal tubular endocytosis via the following steps: (1) megalin, via an interaction with NHERF1, associates with NHE3 within microvilli; (2) albumin binds to megalin in the microvilli of proximal tubular cells; (3) the megalin albumin complex translocates to the intravillar cleft; (4) formation of a macromolecular complex containing megalin, NHE3 and ClC‐5 occurs via NHERF2; and (5) the mature endosome is internalized which allows for ligand processing

Figure 6

Roles of megalin and cubilin in proximal tubule uptake and activation of 25(OH)D. Vitamin D‐binding protein (DBP) binds 25(OH)D in the circulation, and the complex is freely filtered across the glomerulus to be then reabsorbed in the proximal tubule by receptor‐mediated endocytosis involving megalin and cubilin. The complex is then delivered to lysosomes where DBP is degraded and the vitamin released into the cytosol. 25(OH)D is either secreted directly or trafficked to the mitochondrial membrane where it serves as a substrate for 1‐⍺‐hydroxylase and converted to 1,25(OH)₂D. 1,25(OH)₂D is then released into the interstitial fluid where it is complexed by free DBP molecules [modified from reference40]

Figure 7

Role of NHE3 in paracellular Ca²⁺ reabsorption in the proximal tubule. NHE3 is responsible for the majority of Na⁺ and water reabsorption from the proximal tubule. In exchange for a luminal H⁺, NHE3 mediates the influx of Na⁺ into the proximal tubular cell. This exchange is driven by an inward electrochemical gradient for Na⁺, driven by basolateral Na⁺/K⁺ATPase (1). This active transcellular Na⁺ flux creates the osmotic driving force for water reabsorption, which, in turn, drives paracellular Ca²⁺ reabsorption (2) either by convection/solvent drag or by creating a concentration gradient for Ca²⁺ (3) [modified from reference57]