Disordered calcium crystal handling in antisense CLC-5-treated collecting duct cells

Dent disease: A window into calcium and phosphate transport

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.

Hypercalciuria in patients with CLCN5 mutations

Hypercalciuria is regarded as a characteristic symptom of Dent disease, an X-linked recessive tubulopathy characterized by low molecular weight (LMW) proteinuria, nephrocalcinosis/nephrolithiasis, and progressive renal failure due to mutations in the CLCN5 gene. As the presence of hypercalciuria may affect the decision to consider a CLCN5 mutation in the differential diagnosis, the phenotypic spectrum and the relative frequency of hypercalciuria in patients with CLCN5 mutations was determined. We assessed renal calcium excretion in 34 male patients with proven CLCN5 mutations, who had been referred because of LMW proteinuria and at least one additional symptom of Dent disease. Hypercalciuria was defined as renal calcium excretion exceeding 0.1 mmol/kg per day. Data obtained were compared with all series of CLCN5-positive patients identified by a systematic literature survey. In 7 of our 19 families, at least 1 affected male had normal calcium excretion. Hypercalciuria was observed in 22 of 31 patients tested (71%) compared to 85 of 90 (94.4%) in series from Europe and North America and 74.4% from Japan. LMW proteinuria was present in all CLCN5-positive patients; 25% of the patients in European and North American series, 45% of the Japanese, and 41% in the present series had only two of the four principal symptoms of Dent disease. Therefore, a CLCN5 mutation should be considered irrespective of the presence of hypercalciuria in a patient with LMW proteinuria and one additional symptom of Dent disease.

How Bartter's and Gitelman's syndromes, and Dent's disease have provided important insights into the function of three renal chloride channels: ClC-Ka/b and ClC-5

Chloride channels are expressed in almost all cell membranes and are potentially involved in a wide variety of functions. The kidney expresses 8 of the 9 chloride channels of the ClC family that have been cloned so far to date in mammals. This review focuses on the pathophysiology of two renal disorders that have contributed recently to our understanding of the physiological role of chloride channels belonging to the ClC family. First are the related syndromes of Bartter's and Gitelman's, in which inactivating mutations of the genes encoding either of the ClC-Ks, or their regulatory beta-subunit barttin, have shown the important contribution of these chloride channels to renal tubular sodium and chloride (NaCl) transport along the loop of Henle and distal tubule. Second is the renal Fanconi syndrome known as Dent's disease, in which ClC-5 disruption has revealed the key role of this endosomal chloride channel in the megalin-mediated endocytotic pathway in the proximal tubule. The underlying pathophysiology of this inherited disorder demonstrates how ClC-5 is directly or indirectly required for the reabsorption of filtered low-molecular-weight proteins and bioactive peptides, also expression of membrane transporters, and clearance of calcium-based stone-forming crystals.

Calcium phosphate and calcium oxalate crystal handling is dependent upon CLC-5 expression in mouse collecting duct cells

Defects in an intracellular chloride channel CLC-5 cause Dent's disease, an inherited kidney stone disorder. Using a collecting duct model, mIMCD-3 cells, we show expression of dimeric mCLC-5. Transient transfection of antisense CLC-5 reduces CLC-5 protein expression. Binding of both calcium phosphate (hydroxyapatite) and calcium oxalate monohydrate (COM) crystals overlaid onto mIMCD-3 cultures was affected by altered CLC-5 expression. Calcium phosphate crystal agglomerations (>10 microm) were minimal in control (9%) and sense (13%) CLC-5-transfected cells, compared to 66% of antisense CLC-5-transfected cells (P<0.001). Small calcium phosphate crystals (<10 microm) were found associated with 45% of sense CLC-5-treated cells, of which the majority (11/14 cells) appeared to be internalised within the cell. Calcium oxalate agglomerations (>10 microm) were also largely absent for controls or sense mCLC-5 transfectants (11% and 9% of cells, respectively) with COM crystal agglomerates predominating in antisense CLC-5 transfectants (66%, P<0.0001). We conclude that collecting duct cells with reduced CLC-5 expression lead to a tendency to form calcium crystal agglomeration, which may help explain the nephrocalcinosis and nephrolithiasis seen in Dent's disease.

Inherited hypercalciuric syndromes: Dent's disease (CLC-5) and familial hypomagnesemia with hypercalciuria (paracellin-1)

Dent's disease and familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) are inherited diseases in which hypercalciuria, nephrocalcinosis, and renal failure are prominent features. Dent's disease resembles a Fanconi syndrome, with impaired reabsorption in the proximal tubule; FHHNC, with urinary loss of magnesium and calcium, is associated with impaired cation transport in the thick ascending limb of Henle's loop. Gene mapping in families and positional cloning led in both cases to identification of the responsible gene. Dent's disease is associated with mutations that disrupt function of a voltage-gated chloride channel, CLC-5, expressed in subapical endosomes of the proximal tubule and in other nephron segments. Impaired function of this channel disturbs reabsorption of filtered proteins, as well as other transport functions of the proximal tubule, and leads, apparently indirectly, to hypercalciuria and renal failure. FHHNC results from mutations in paracellin-1, a tight-junction protein that appears to be important in conducting or regulating paracellular cation transport. Impaired function of paracellin-1 leads specifically to urinary losses of magnesium and calcium, but because transcellular transport is intact these patients do not have hypokalemia or salt wasting. Identification of both genes represent triumphs of a genetic approach to solving problems of pathophysiology.

Intracellular Chloride Channels: Determinants of Function in the Endosomal Pathway

Endosomes, and related subcellular compartments, contain various Cl- channels in the ClC family. In this review, we describe the known roles of intracellular Cl- channels and also explore some of the functional implications of transmembrane Cl- flux in these organelles. Cl- influx acts to control intralumenal pH, both by shunting the effects of the proton pump on membrane potential and, possibly, through direct effects of Cl- on the proton pump. Changes in intralumenal pH likely help regulate membrane trafficking. We propose that changes in intralumenal Cl- concentration ([Cl-]) could theoretically play a direct role in regulating membrane trafficking and organellar function through effects on chloride-sensitive proteins in the vesicular membrane, which could transduce information about intralumenal [Cl-] to the outside of the vesicle and thereby recruit various signaling molecules. We present a model in which regulation of cytosolic [Cl-] and vesicular Cl- conductance could help control the amount or type of neurotransmitter stored in a particular population of synaptic vesicles.