Ultrastructural and immunohistochemical localization of plasma membrane Ca2+-ATPase 4 in Ca2+-transporting epithelia

Calcium signalling and transport in the kidney

The kidney plays a pivotal role in regulating calcium levels within the body. Approximately 98% of the filtered calcium is reabsorbed in the nephron, and this process is tightly controlled to maintain calcium homeostasis, which is required to facilitate optimal bone mineralization, preserve serum calcium levels within a narrow range, and support intracellular signalling mechanisms. The maintenance of these functions is attributed to a delicate balance achieved by various calcium channels, transporters, and calcium-binding proteins in renal cells. Perturbation of this balance due to deficiency or dysfunction of calcium channels and calcium-binding proteins can lead to severe complications. For example, polycystic kidney disease is linked to aberrant calcium transport and signalling. Furthermore, dysregulation of calcium levels can promote the formation of kidney stones. This Review provides an updated description of the key aspects of calcium handling in the kidney, focusing on the function of various calcium channels and the physiological stimuli that control these channels or are communicated through them. A discussion of the role of calcium as an intracellular second messenger and the pathophysiology of renal calcium dysregulation, as well as a summary of gaps in knowledge and future prospects, are also included.

Maternal Epidermal Growth Factor Promotes Neonatal Claudin-2 Dependent Increases in Small Intestinal Calcium Permeability

A higher concentration of calcium in breast milk than blood favors paracellular calcium absorption enabling growth during postnatal development. We aimed to determine whether suckling animals have greater intestinal calcium permeability to maximize absorption and to identify the underlying molecular mechanism. We examined intestinal claudin expression at different ages in mice and in human intestinal epithelial (Caco-2) cells in response to hormones or human milk. We also measured intestinal calcium permeability in wildtype, Cldn2 and Cldn12 KO mice and Caco-2 cells in response to hormones or human milk. Bone mineralization in mice was assessed by μCT. Calcium permeability across the jejunum and ileum of mice were 2-fold greater at 2 wk than 2 mo postnatal age. At 2 wk, Cldn2 and Cldn12 expression were greater, but only Cldn2 KO mice had decreased calcium permeability compared to wildtype. This translated to decreased bone volume, cross-sectional thickness, and tissue mineral density of femurs. Weaning from breast milk led to a 50% decrease in Cldn2 expression in the jejunum and ileum. Epidermal growth factor (EGF) in breast milk specifically increased only CLDN2 expression and calcium permeability in Caco-2 cells. These data support intestinal permeability to calcium, conferred by claudin-2, being greater in suckling mice and being driven by EGF in breast milk. Loss of the CLDN2 pathway leads to suboptimal bone mineralization at 2 wk of life. Overall, EGF-mediated control of intestinal claudin-2 expression contributes to maximal intestinal calcium absorption in suckling animals.

The importance of kidney calcium handling in the homeostasis of extracellular fluid calcium

Extracellular fluid calcium concentration must be maintained within a narrow range in order to sustain many biological functions, encompassing muscle contraction, blood coagulation, and bone and tooth mineralization. Blood calcium value is critically dependent on the ability of the renal tubule to reabsorb the adequate amount of filtered calcium. Tubular calcium reabsorption is carried out by various and complex mechanisms in 3 distinct segments: the proximal tubule, the cortical thick ascending limb of the loop of Henle, and the late distal convoluted/connecting tubule. In addition, calcium reabsorption is tightly controlled by many endocrine, paracrine, and autocrine factors, as well as by non-hormonal factors, in order to adapt the tubular handling of calcium to the metabolic requirements. The present review summarizes the current knowledge of the mechanisms and factors involved in calcium handling by the kidney and, ultimately, in extracellular calcium homeostasis. The review also highlights some of our gaps in understanding that need to be addressed in the future.

The contribution of regulated colonic calcium absorption to the maintenance of calcium homeostasis

Calcium absorption and secretion can occur along the length of the small and large intestine. To date, the focus of research into intestinal calcium absorption has been the small intestine, the site contributing the majority of intestinal calcium absorption. However, evidence that the colon contributes as much as 10% of enteral calcium transport has been available for decades. Transcellular calcium absorption and bidirectional paracellular calcium flux contributing to either net absorption or secretion have been observed in the colon, depending on the physiological state. Moreover, the calcium transport pathways contributing to colonic absorption or secretion are regulated by a variety of hormones, including calcitriol, plasma calcium and dietary factors, including prebiotics. Herein we review historical and recent research highlighting the role of colonic calcium transport in overall maintenance of calcium balance, and suggest these data are consistent with the colon being a site of significant regulated transepithelial calcium transport.

Differential parathyroid and kidney Ca2+-sensing receptor activation in autosomal dominant hypocalcemia 1

Background

Parathyroid Ca²⁺-sensing receptor (CaSR) activation inhibits parathyroid hormone (PTH) release, while activation of renal CaSRs attenuates Ca²⁺ transport and increases expression of the pore-blocking claudin-14. Patients with autosomal dominant hypocalcemia 1 (ADH1), due to activating CASR mutations, exhibit hypocalcemia but not always hypercalciuria (elevated Ca²⁺ in urine). The latter promotes nephrocalcinosis and renal insufficiency. Although CaSRs throughout the body including the kidney harbor activating CASR mutations, it is not understood why only some ADH1 patients display hypercalciuria.

Methods

Activation of the CaSR was studied in mouse models and a ADH1 patient. In vitro CaSR activation was studied in HEK293 cells.

Findings

Cldn14 showed blood Ca²⁺ concentration-dependent regulation, which was absent in mice with kidney-specific Casr deletion, indicating Cldn14 is a suitable marker for chronic CaSR activation in the kidney. Mice with a gain-of-function mutation in the Casr (Nuf) were hypocalcemic with low plasma PTH levels. However, renal CaSRs were not activated at baseline but only after normalizing blood Ca²⁺ levels. Similarly, significant hypercalciuria was not observed in a ADH1 patient until blood Ca²⁺ was normalized. In vitro experiments indicate that increased CaSR expression in the parathyroid relative to the kidney could contribute to tissue-specific CaSR activation thresholds.

Interpretation

Our findings suggest that parathyroid CaSR overactivity can reduce plasma Ca²⁺ to levels insufficient to activate renal CaSRs, even when an activating mutation is present. These findings identify a conceptually new mechanism of CaSR-dependent Ca²⁺ balance regulation that aid in explaining the spectrum of hypercalciuria in ADH1 patients.

Funding

Erasmus+ 2018/E+/4458087, the Canadian Institutes for Health research, the Novo Nordisk Foundation, the Beckett Foundation, the Carlsberg Foundation and Independent Research Fund Denmark.

Gentamicin Inhibits Ca2+ Channel TRPV5 and Induces Calciuresis Independent of the Calcium-Sensing Receptor-Claudin-14 Pathway

BACKGROUND

Treatment with the aminoglycoside antibiotic gentamicin can be associated with severe adverse effects, including renal Ca²⁺ wasting. The underlying mechanism is unknown but it has been proposed to involve activation of the Ca²⁺-sensing receptor (CaSR) in the thick ascending limb, which would increase expression of claudin-14 (CLDN14) and limit Ca²⁺ reabsorption. However, no direct evidence for this hypothesis has been presented.

METHODS

We studied the effect of gentamicin in vivo using mouse models with impaired Ca²⁺ reabsorption in the proximal tubule and the thick ascending limb. We used a Cldn14 promoter luciferase reporter assay to study CaSR activation and investigated the effect of gentamicin on activity of the distal nephron Ca²⁺ channel transient receptor potential vanilloid 5 (TRPV5), as determined by patch clamp in HEK293 cells.

RESULTS

Gentamicin increased urinary Ca²⁺ excretion in wild-type mice after acute and chronic administration. This calciuretic effect was unaltered in mice with genetic CaSR overactivation and was present in furosemide-treated animals, whereas the calciuretic effect in Cldn14^-/- mice and mice with impaired proximal tubular Ca²⁺ reabsorption (claudin-2 [CLDN2]-deficient Cldn2^-/- mice) was equivalent to that of wild-type mice. In vitro, gentamicin failed to activate the CaSR. In contrast, patch clamp analysis revealed that gentamicin strongly inhibited rabbit and human TRPV5 activity and chronic gentamicin administration downregulated distal nephron Ca²⁺ transporters.

CONCLUSIONS

Gentamicin does not cause hypercalciuria via activation of the CaSR-CLDN14 pathway or by interfering with proximal tubular CLDN2-dependent Ca²⁺ reabsorption. Instead, gentamicin blocks distal Ca²⁺ reabsorption by direct inhibition of the Ca²⁺ channel TRPV5. These findings offer new insights into Ca²⁺ wasting in patients treated with gentamicin.

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

Claudin-2 and claudin-12 form independent, complementary pores required to maintain calcium homeostasis

Calcium (Ca²⁺) homeostasis is maintained through coordination between intestinal absorption, renal reabsorption, and bone remodeling. Intestinal and renal (re)absorption occurs via transcellular and paracellular pathways. The latter contributes the bulk of (re)absorption under conditions of adequate intake. Epithelial paracellular permeability is conferred by tight-junction proteins called claudins. However, the molecular identity of the paracellular Ca²⁺ pore remains to be delineated. Claudins (Cldn)-2 and -12 confer Ca²⁺ permeability, but deletion of either claudin does not result in a negative Ca²⁺ balance or increased calciotropic hormone levels, suggesting the existence of additional transport pathways or parallel roles for the two claudins. To test this, we generated a Cldn2/12 double knockout mouse (DKO). These animals have reduced intestinal Ca²⁺ absorption. Colonic Ca²⁺ permeability is also reduced in DKO mice and significantly lower than single-null animals, while small intestine Ca²⁺ permeability is unaltered. The DKO mice display significantly greater urinary Ca²⁺ wasting than Cldn2 null animals. These perturbations lead to hypocalcemia and reduced bone mineral density, which was not observed in single-KO animals. Both claudins were localized to colonic epithelial crypts and renal proximal tubule cells, but they do not physically interact in vitro. Overexpression of either claudin increased Ca²⁺ permeability in cell models with endogenous expression of the other claudin. We find claudin-2 and claudin-12 form partially redundant, independent Ca²⁺ permeable pores in renal and colonic epithelia that enable paracellular Ca²⁺ (re)absorption in these segments, with either one sufficient to maintain Ca²⁺ balance.

Mechanisms Underlying Calcium Nephrolithiasis

Nephrolithiasis is a worldwide problem with increasing prevalence, enormous costs, and significant morbidity. Calcium-containing kidney stones are by far the most common kidney stones encountered in clinical practice. Consequently, hypercalciuria is the greatest risk factor for kidney stone formation. Hypercalciuria can result from enhanced intestinal absorption, increased bone resorption, or altered renal tubular transport. Kidney stone formation is complex and driven by high concentrations of calcium-oxalate or calcium-phosphate in the urine. After discussing the mechanism mediating renal calcium salt precipitation, we review recent discoveries in renal tubular calcium transport from the proximal tubule, thick ascending limb, and distal convolution. Furthermore, we address how calcium is absorbed from the intestine and mobilized from bone. The effect of acidosis on bone calcium resorption and urinary calcium excretion is also considered. Although recent discoveries provide insight into these processes, much remains to be understood in order to provide improved therapies for hypercalciuria and prevent kidney stone formation. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Activation of the calcium sensing receptor increases claudin-14 expression via a PLC -p38-Sp1 pathway

Activation of the basolateral calcium sensing receptor (CaSR) in the renal tubular thick ascending limb (TAL) increases claudin‐14 expression, which reduces paracellular calcium (Ca²⁺) permeability, thus increasing urinary Ca²⁺ excretion. However, the upstream signaling pathway contributing to altered CLDN14 gene expression is unknown. To delineate this pathway, we identified and then cloned the CaSR responsive region including the promoter of mouse Cldn14 into a luciferase reporter vector. This 1500 bp sequence upstream of the 5′ UTR of Cldn14 variant 1, conferred increased reporter activity in the presence of high extracellular Ca²⁺ (5 mM) relative to a lower (0.5 mM) concentration. Assessment of Cldn14 reporter activity in response to increased extracellular Ca²⁺ in the presence or absence of specific inhibitors confirmed signaling through PLC and p38, but not JNK. Overexpression of SP1 attenuated Cldn14 reporter activity in response to CasR signaling. SP1 is expressed in the TAL and phosphorylation was attenuated by CaSR signaling. Finally, activating mutations in the CaSR increased Cldn14 reporter activity while a dominant negative mutation in the CaSR inhibited it. Together, these studies suggest that basolateral activation of the CASR leads to increased Cldn14 expression via a PLC‐ stimulated p38 pathway that prevents Sp1 mediated repression.

A TRPV6 expression atlas for the mouse

The transient receptor potential vanilloid 6 (TRPV6) channel is highly Ca²⁺-selective and has been implicated in mediating transcellular Ca²⁺ transport and thus maintaining the Ca²⁺ balance in the body. To characterize its physiological function(s), a detailed expression profile of the TRPV6 channel throughout the body is essential. Capitalizing on a recently established murine Trpv6-reporter strain, we identified primary TRPV6 channel-expressing cells in an organism-wide manner. In a complementary experimental approach, we characterized TRPV6 expression in different tissues of wild-type mice by TRPV6 immunoprecipitation (IP) followed by mass spectrometry analysis and correlated these data with the reporter gene expression. Taken together, we present a TRPV6 expression atlas throughout the entire body of juvenile and adult mice, providing a novel resource to investigate the role of TRPV6 channels in vivo.

Differential localization patterns of claudin 10, 16, and 19 in human, mouse, and rat renal tubular epithelia

Functional properties of the paracellular pathway depend critically on the set of claudins (CLDN) expressed at the tight junction. Two syndromes are causally linked to loss-of-function mutations of claudins: hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia (HELIX) syndrome caused by genetic variations in the CLDN10 gene and familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by genetic variations in the CLDN16 or CLDN19 genes. All three genes are expressed in the kidney, particularly in the thick ascending limb (TAL). However, localization of these claudins in humans and rodents remains to be delineated in detail. We studied the segmental and subcellular expression of CLDN10, CLDN16, and CLDN19 in both paraffin-embedded and frozen kidney sections from the adult human, mouse, and rat using immunohistochemistry and immunofluorescence, respectively. Here, CLDN10 was present in a subset of medullary and cortical TAL cells, localizing to basolateral domains and tight junctions in human and rodent kidneys. Weak expression was detected at the tight junction of proximal tubular cells. CLDN16 was primarily expressed in a subset of TAL cells in the cortex and outer stripe of outer medulla, restricted to basolateral domains and tight junctional structures in both human and rodent kidneys. CLDN19 predominantly colocalized with CLDN16 in tight junctions and basolateral domains of the TAL but was also found in basolateral and junctional domains in more distal sites. CLDN10 expression at tight junctions almost never overlapped with that of CLND16 and CLDN19, consistent with distinct junctional pathways with different permeation profiles in both human and rodent kidneys.NEW & NOTEWORTHY This study used immunohistochemistry and immunofluorescence to investigate the distribution of claudin 10, 16, and 19 in the human, mouse, and rat kidney. The findings showed distinct junctional pathways in both human and rodent kidneys, supporting the existence of different permeation profiles in all species investigated.

Moderation of doxorubicin-induced nephrotoxicity in Wistar rats by aqueous leaf-extracts of Chromolaena odorata and Tridax procumbens

BACKGROUND

The major draw-back of doxorubicin's use in chemotherapy is its toxicity on various organs including the kidneys. This study investigated the potential protective role of aqueous leaf-extracts of Chromolaena odorata and Tridax procumbens against nephrotoxicity induced by doxorubicin.

METHODS

To this end, their impact on plasma biomarkers of kidney function, as well as renal lipid profile, biomarkers of oxidative stress, electrolyte profile and activities of renal ATPases was monitored in doxorubicin treated rats. Metformin (250 mg/kg body weight, orally) and the extracts (50, 75 and 100 mg/kg, orally) were daily administered for 14 days; while nephrotoxicity was induced with doxorubicin (15 mg/kg, intra-peritioneally), once on the 12th day of study.

RESULTS

The plasma concentrations of creatinine, and urea; as well as the renal malondialdehyde, cholesterol, calcium and sodium concentrations in the Test control, were significantly (P < .05) higher than those of all the other groups. However, the renal concentrations of ascorbic acid, chloride, magnesium and potassium, and the renal activities of catalase, glutathione peroxidase superoxide dismutase, Ca2+-ATPase, Mg2+-ATPase and Na+,K+-ATPase in the Test control were significantly (P < .05) lower than those of all the other groups.

CONCLUSIONS

Pre-treatment with the extracts and metformin boosted endogenous antioxidants, and prevented doxorubicin-induced renal damage, as indicated by the attenuation of doxorubicin-induced renal oxidative stress, as well as the attenuation of doxorubicin-induced adverse alterations in renal cholesterol, ATPases and electrolyte balance, and plasma biomarkers of kidney function, and keeping them at near-normal values.

Localization and regulation of claudin-14 in experimental models of hypercalcemia

Variations in the claudin-14 (CLDN14) gene have been linked to increased risk of hypercalciuria and kidney stone formation. However, the exact cellular localization of CLDN14 and its regulation remain to be fully delineated. To this end, we generated a novel antibody that allowed the detection of CLDN14 in paraffin-embedded renal sections. This showed CLDN14 to be detectable in the kidney only after induction of hypercalcemia in rodent models. Protein expression in the kidney is localized exclusively to the thick ascending limbs (TALs), mainly restricted to the cortical and upper medullary portion of the kidney. However, not all cells in the TALs expressed the tight junction protein. In fact, CLDN14 was primarily expressed in cells also expressing CLDN16 but devoid of CLDN10. CLDN14 appeared in very superficial apical cell domains and near cell junctions in a belt-like formation along the apical cell periphery. In transgenic mice, Cldn14 promotor-driven LacZ activity did not show complete colocalization with CLDN14 protein nor was it increased by hypercalcemia, suggesting that LacZ activity cannot be used as a marker for CLDN14 localization and regulation in this model. In conclusion, CLDN14 showed a restricted localization pattern in the apical domain of select cells of the TAL.

TRPV6 and Cav1.3 Mediate Distal Small Intestine Calcium Absorption Before Weaning

BACKGROUND & AIMS

Intestinal Ca²⁺ absorption early in life is vital to achieving optimal bone mineralization. The molecular details of intestinal Ca²⁺ absorption have been defined in adults after peak bone mass is obtained, but they are largely unexplored during development. We sought to delineate the molecular details of transcellular Ca²⁺ absorption during this critical period.

METHODS

Expression of small intestinal and renal calcium transport genes was assessed by using quantitative polymerase chain reaction. Net calcium flux across small intestinal segments was measured in Ussing chambers, including after pharmacologic inhibition or genetic manipulation of TRPV6 or Ca_v1.3 calcium channels. Femurs were analyzed by using micro-computed tomography and histology.

RESULTS

Net TRPV6-mediated Ca²⁺ flux across the duodenum was absent in pre-weaned (P14) mice but present after weaning. In contrast, we found significant transcellular Ca²⁺ absorption in the jejunum at 2 weeks but not 2 months of age. Net jejunal Ca²⁺ absorption observed at P14 was not present in either Trpv6 mutant (D541A) mice or Ca_v1.3 knockout mice. We observed significant nifedipine-sensitive transcellular absorption across the ileum at P14 but not 2 months. Ca_v1.3 knockout pups exhibited delayed bone mineral accrual, compensatory nifedipine-insensitive Ca²⁺ absorption in the ileum, and increased expression of renal Ca²⁺ reabsorption mediators at P14. Moreover, weaning pups at 2 weeks reduced jejunal and ileal Ca_v1.3 expression.

CONCLUSIONS

We have detailed novel pathways contributing to transcellular Ca²⁺ transport across the distal small intestine of mice during development, highlighting the complexity of the multiple mechanisms involved in achieving a positive Ca²⁺ balance early in life.

The kidney anion exchanger 1 affects tight junction properties via claudin-4

In the renal collecting duct, intercalated cells regulate acid-base balance by effluxing protons through the v-H⁺-ATPase, and bicarbonate via apical pendrin or the basolateral kidney anion exchanger 1 (kAE1). Additionally, collecting duct cells play an essential role in transepithelial absorption of sodium and chloride. Expression of kAE1 in polarized MDCK I cells was previously shown to decrease trans-epithelial electrical resistance (TEER), suggesting a novel role for kAE1 in paracellular permeability. In our study, we not only confirmed that inducible expression of kAE1 in mIMCD3 cells decreased TEER but we also observed (i) increased epithelial absolute permeability to both sodium and chloride, and (ii) that this effect was dependent on kAE1 activity. Further, kAE1 regulated tight junction properties through the tight junction protein claudin-4, a protein with which it physically interacts and colocalizes. These findings unveil a novel interaction between the junctional protein claudin-4 and the kidney anion exchanger, which may be relevant to ion and/or pH homeostasis.

Transcriptional regulation of Hepatic Stellate Cell activation in NASH

Non-alcoholic steatohepatitis (NASH) signified by hepatic steatosis, inflammation, hepatocellular injury, and fibrosis is a growing cause of chronic liver disease, cirrhosis, and hepatocellular carcinoma. Hepatic fibrosis resulting from accumulation of extracellular matrix proteins secreted by hepatic myofibroblasts plays an important role in disease progression. Activated hepatic stellate cells (HSCs) have been identified as the primary source of myofibroblasts in animal models of hepatotoxic liver injury; however, so far HSC activation and plasticity have not been thoroughly investigated in the context of NASH-related fibrogenesis. Here we have determined the time-resolved changes in the HSC transcriptome during development of Western diet- and fructose-induced NASH in mice, a NASH model recapitulating human disease. Intriguingly, HSC transcriptional dynamics are highly similar across disease models pointing to HSC activation as a point of convergence in the development of fibrotic liver disease. Bioinformatic interrogation of the promoter sequences of activated genes combined with loss-of-function experiments indicates that the transcriptional regulators ETS1 and RUNX1 act as drivers of NASH-associated HSC plasticity. Taken together, our results implicate HSC activation and transcriptional plasticity as key aspects of NASH pathophysiology.

H+-ATPase B1 subunit localizes to thick ascending limb and distal convoluted tubule of rodent and human kidney

The vacuolar-type H⁺-ATPase B1 subunit is heavily expressed in the intercalated cells of the collecting system, where it contributes to H⁺ transport, but has also been described in other segments of the renal tubule. This study aimed to determine the localization of the B1 subunit of the vacuolar-type H⁺-ATPase in the early distal nephron, encompassing thick ascending limbs (TAL) and distal convoluted tubules (DCT), in human kidney and determine whether the localization differs between rodents and humans. Antibodies directed against the H⁺-ATPase B1 subunit were used to determine its localization in paraffin-embedded formalin-fixed mouse, rat, and human kidneys by light microscopy and in sections of Lowicryl-embedded rat kidneys by electron microscopy. Abundant H⁺-ATPase B1 subunit immunoreactivity was observed in the human kidney. As expected, intercalated cells showed the strongest signal, but significant signal was also observed in apical membrane domains of the distal nephron, including TAL, macula densa, and DCT. In mouse and rat, H⁺-ATPase B1 subunit expression could also be detected in apical membrane domains of these segments. In rat, electron microscopy revealed that the H⁺-ATPase B1 subunit was located in the apical membrane. Furthermore, the H⁺-ATPase B1 subunit colocalized with other H⁺-ATPase subunits in the TAL and DCT. In conclusion, the B1 subunit is expressed in the early distal nephron. The physiological importance of H⁺-ATPase expression in these segments remains to be delineated in detail. The phenotype of disease-causing mutations in the B1 subunit may also relate to its presence in the TAL and DCT.

Endothelial mineralocorticoid receptor ablation does not alter blood pressure, kidney function or renal vessel contractility

Aldosterone blockade confers substantial cardiovascular and renal protection. The effects of aldosterone on mineralocorticoid receptors (MR) expressed in endothelial cells (EC) within the renal vasculature have not been delineated. We hypothesized that lack of MR in EC may be protective in renal vasculature and examined this by ablating the Nr3c2 gene in endothelial cells (EC-MR) in mice. Blood pressure, heart rate and PAH clearance were measured using indwelling catheters in conscious mice. The role of the MR in EC on contraction and relaxation was investigated in the renal artery and in perfused afferent arterioles. Urinary sodium excretion was determined by use of metabolic cages. EC-MR transgenics had markedly decreased MR expression in isolated aortic endothelial cells as compared to littermates (WT). Blood pressure and effective renal plasma flow at baseline and following AngII infusion was similar between groups. No differences in contraction and relaxation were observed between WT and EC-MR KO in isolated renal arteries during baseline or following 2 or 4 weeks of AngII infusion. The constriction or dilatations of afferent arterioles between genotypes were not different. No changes were found between the groups with respect to urinary excretion of sodium after 4 weeks of AngII infusion, or in urinary albumin excretion and kidney morphology. In conclusion, deletion of the EC-MR does not confer protection towards the development of hypertension, endothelial dysfunction of renal arteries or renal function following prolonged AngII-infusion.

Deficiency of Carbonic Anhydrase II Results in a Urinary Concentrating Defect

Carbonic anhydrase II (CAII) is expressed along the nephron where it interacts with a number of transport proteins augmenting their activity. Aquaporin-1 (AQP1) interacts with CAII to increase water flux through the water channel. Both CAII and aquaporin-1 are expressed in the thin descending limb (TDL); however, the physiological role of a CAII-AQP1 interaction in this nephron segment is not known. To determine if CAII was required for urinary concentration, we studied water handling in CAII-deficient mice. CAII-deficient mice demonstrate polyuria and polydipsia as well as an alkaline urine and bicarbonaturia, consistent with a type III renal tubular acidosis. Natriuresis and hypercalciuria cause polyuria, however, CAII-deficient mice did not have increased urinary sodium nor calcium excretion. Further examination revealed dilute urine in the CAII-deficient mice. Urinary concentration remained reduced in CAII-deficient mice relative to wild-type animals even after water deprivation. The renal expression and localization by light microscopy of NKCC2 and aquaporin-2 was not altered. However, CAII-deficient mice had increased renal AQP1 expression. CAII associates with and increases water flux through aquaporin-1. Water flux through aquaporin-1 in the TDL of the loop of Henle is essential to the concentration of urine, as this is required to generate a concentrated medullary interstitium. We therefore measured cortical and medullary interstitial concentration in wild-type and CAII-deficient mice. Mice lacking CAII had equivalent cortical interstitial osmolarity to wild-type mice: however, they had reduced medullary interstitial osmolarity. We propose therefore that reduced water flux through aquaporin-1 in the TDL in the absence of CAII prevents the generation of a maximally concentrated medullary interstitium. This, in turn, limits urinary concentration in CAII deficient mice.

The serum protein renalase reduces injury in experimental pancreatitis

Acute pancreatitis is a disease associated with inflammation and tissue damage. One protein that protects against acute injury, including ischemic injury to both the kidney and heart, is renalase, which is secreted into the blood by the kidney and other tissues. However, whether renalase reduces acute injury associated with pancreatitis is unknown. Here, we used both in vitro and in vivo murine models of acute pancreatitis to study renalase's effects on this condition. In isolated pancreatic lobules, pretreatment with recombinant human renalase (rRNLS) blocked zymogen activation caused by cerulein, carbachol, and a bile acid. Renalase also blocked cerulein-induced cell injury and histological changes. In the in vivo cerulein model of pancreatitis, genetic deletion of renalase resulted in more severe disease, and administering rRNLS to cerulein-exposed WT mice after pancreatitis onset was protective. Because pathological increases in acinar cell cytosolic calcium levels are central to the initiation of acute pancreatitis, we also investigated whether rRNLS could function through its binding protein, plasma membrane calcium ATPase 4b (PMCA4b), which excretes calcium from cells. We found that PMCA4b is expressed in both murine and human acinar cells and that a PMCA4b-selective inhibitor worsens pancreatitis-induced injury and blocks the protective effects of rRNLS. These findings suggest that renalase is a protective plasma protein that reduces acinar cell injury through a plasma membrane calcium ATPase. Because exogenous rRNLS reduces the severity of acute pancreatitis, it has potential as a therapeutic agent.

Expression of transcellular and paracellular calcium and magnesium transport proteins in renal and intestinal epithelia during lactation

Significant alterations in maternal calcium (Ca2+) and magnesium (Mg2+) balance occur during lactation. Ca2+ is the primary divalent cation mobilized into breast milk by demineralization of the skeleton and alterations in intestinal and renal Ca2+ transport. Mg2+ is also concentrated in breast milk, but the underlying mechanisms are not well understood. To determine the molecular alterations in Ca2+ and Mg2+ transport in the intestine and kidney during lactation, three groups of female mice consisting of either nonpregnant controls, lactating mice, or mice undergoing involution were examined. The fractional excretion of Ca2+, but not Mg2+, rose significantly during lactation. Renal 1-α hydroxylase and 24-OHase mRNA levels increased markedly, as did plasma 1,25 dihydroxyvitamin D levels. This was accompanied by significant increases in intestinal expression of Trpv6 and S100g in lactating mice. However, no alterations in the expression of cation-permeable claudin-2, claudin-12, or claudins-15 were found in the intestine. In the kidney, increased expression of Trpv5 and Calb1 was observed during lactation, while no changes in claudins involved in Ca2+ and Mg2+ transport (claudin-2, claudin-14, claudin-16, or claudin-19) were found. Consistent with the mRNA expression, expression of both calbindin-D28K and transient receptor potential vanilloid 5 (TRPV5) proteins increased. Colonic Trpm6 expression increased during lactation, while renal Trpm6 remained unaltered. In conclusion, proteins involved in transcellular Ca2+ and Mg2+ transport pathways increase during lactation, while expression of paracellular transport proteins remained unchanged. Increased fractional Ca2+ excretion can be explained by vitamin D-dependent intestinal hyperabsorption and bone demineralization, despite enhanced transcellular Ca2+ uptake by the kidney.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

Effect of diuretics on renal tubular transport of calcium and magnesium

Calcium (Ca2+) and Magnesium (Mg2+) reabsorption along the renal tubule is dependent on distinct trans- and paracellular pathways. Our understanding of the molecular machinery involved is increasing. Ca2+ and Mg2+ reclamation in kidney is dependent on a diverse array of proteins, which are important for both forming divalent cation-permeable pores and channels, but also for generating the necessary driving forces for Ca2+ and Mg2+ transport. Alterations in these molecular constituents can have profound effects on tubular Ca2+ and Mg2+ handling. Diuretics are used to treat a large range of clinical conditions, but most commonly for the management of blood pressure and fluid balance. The pharmacological targets of diuretics generally directly facilitate sodium (Na+) transport, but also indirectly affect renal Ca2+ and Mg2+ handling, i.e., by establishing a prerequisite electrochemical gradient. It is therefore not surprising that substantial alterations in divalent cation handling can be observed following diuretic treatment. The effects of diuretics on renal Ca2+ and Mg2+ handling are reviewed in the context of the present understanding of basal molecular mechanisms of Ca2+ and Mg2+ transport. Acetazolamide, osmotic diuretics, Na+/H+ exchanger (NHE3) inhibitors, and antidiabetic Na+/glucose cotransporter type 2 (SGLT) blocking compounds, target the proximal tubule, where paracellular Ca2+ transport predominates. Loop diuretics and renal outer medullary K+ (ROMK) inhibitors block thick ascending limb transport, a segment with significant paracellular Ca2+ and Mg2+ transport. Thiazides target the distal convoluted tubule; however, their effect on divalent cation transport is not limited to that segment. Finally, potassium-sparing diuretics, which inhibit electrogenic Na+ transport at distal sites, can also affect divalent cation transport.

Intestinal absorption and renal reabsorption of calcium throughout postnatal development

Calcium is vital for many physiological functions including bone mineralization. Postnatal deposition of calcium into bone is greatest in infancy and continues through childhood and adolescence until peek mineral density is reached in early adulthood. Thereafter, bone mineral density remains static until it eventually declines in later life. A positive calcium balance, i.e. more calcium absorbed than excreted, is crucial to bone deposition during growth and thus to peek bone mineral density. Dietary calcium is absorbed from the intestine into the blood. It is then filtered by the renal glomerulus and either reabsorbed by the tubule or excreted in the urine. Calcium can be (re)absorbed across intestinal and renal epithelia via both transcellular and paracellular pathways. Current evidence suggests that significant intestinal and renal calcium transport changes occur throughout development. However, the molecular details of these alterations are incompletely delineated. Here we first briefly review the current model of calcium transport in the intestine and renal tubule in the adult. Then, we describe what is known with regard to calcium handling through postnatal development, and how alterations may aid in mediating a positive calcium balance. The role of transcellular and paracellular calcium transport pathways and the contribution of specific intestinal and tubular segments vary with age. However, the current literature highlights knowledge gaps in how specifically intestinal and renal calcium (re)absorption occurs early in postnatal development. Future research should clarify the specific changes in calcium transport throughout early postnatal development including mediators of these alterations enabling appropriate bone mineralization. Impact statement This mini review outlines the current state of knowledge pertaining to the molecules and mechanisms maintaining a positive calcium balance throughout postnatal development. This process is essential to achieving optimal bone mineral density in early adulthood, thereby lowering the lifetime risk of osteoporosis.

Correlative study on the JAK-STAT/PSMβ3 signal transduction pathway in asthenozoospermia

The aim of the present study was to investigate the possible mechanism of Janus kinase (JAK)-signal transduction and activator of transcription (STAT)/PSMβ3 signaling in the occurrence of asthenozoospermia. We examined seminal fluid samples from 30 cases of asthenozoospermia and 30 healthy controls. Sperm was collected using the Percoll density gradient centrifugation method. The expression of JAK, STAT and PSMβ3 mRNA was assessed by reverse-transcription quantitative PCR and the protein levels of p-JAK, p-STAT and PSMβ3 were measured by western blot analysis. The PSMβ3 mRNA and protein expression levels were also measured after application of a JAK inhibitor, AG-490, to the control group, with a FITC-labeled monoclonal rabbit anti-human PSMβ3 primary antibody. The cells were observed under a laser confocal microscope. The mRNA levels of JAK, STAT and PSMβ3 in asthenozoospermia were decreased significantly (P<0.05). The protein levels of p-JAK, p-STAT and PSMβ3 in asthenozoospermia were also reduced and the differences were statistically significant (P<0.05). The PSMβ3 mRNA and protein expression levels were decreased in the control group after treatment with the JAK inhibitor, and levels were approximately equal to those of the asthenozoospermia group. PSMβ3 was mainly expressed in round-headed sperm, and less in asthenozoospermia. In conclusion, the JAK-STAT/PSMβ3 signaling transduction pathway may be involved in the pathogenic mechanism of asthenozoospermia.

Acidosis and Urinary Calcium Excretion: Insights from Genetic Disorders

Metabolic acidosis is associated with increased urinary calcium excretion and related sequelae, including nephrocalcinosis and nephrolithiasis. The increased urinary calcium excretion induced by metabolic acidosis predominantly results from increased mobilization of calcium out of bone and inhibition of calcium transport processes within the renal tubule. The mechanisms whereby acid alters the integrity and stability of bone have been examined extensively in the published literature. Here, after briefly reviewing this literature, we consider the effects of acid on calcium transport in the renal tubule and then discuss why not all gene defects that cause renal tubular acidosis are associated with hypercalciuria and nephrocalcinosis.

Calcium Extrusion Pump PMCA4: A New Player in Renal Calcium Handling?

Calcium (Ca²⁺) is vital for multiple processes in the body, and maintenance of the electrolyte concentration is required for everyday physiological function. In the kidney, and more specifically, in the late distal convoluted tubule and connecting tubule, the fine-tuning of Ca²⁺ reabsorption from the pro-urine takes place. Here, Ca²⁺ enters the epithelial cell via the transient receptor potential vanilloid receptor type 5 (TRPV5) channel, diffuses to the basolateral side bound to calbindin-D_28k and is extruded to the blood compartment via the Na⁺/Ca²⁺ exchanger 1 (NCX1) and the plasma membrane Ca²⁺ ATPase (PMCA). Traditionally, PMCA1 was considered to be the primary Ca²⁺ pump in this process. However, in recent studies TRPV5-expressing tubules were shown to highly express PMCA4. Therefore, PMCA4 may have a predominant role in renal Ca²⁺ handling. This study aimed to elucidate the role of PMCA4 in Ca²⁺ homeostasis by characterizing the Ca²⁺ balance, and renal and duodenal Ca²⁺-related gene expression in PMCA4 knockout mice. The daily water intake of PMCA4 knockout mice was significantly lower compared to wild type littermates. There was no significant difference in serum Ca²⁺ level or urinary Ca²⁺ excretion between groups. In addition, renal and duodenal mRNA expression levels of Ca²⁺-related genes, including TRPV5, TRPV6, calbindin-D_28k, calbindin-D_9k, NCX1 and PMCA1 were similar in wild type and knockout mice. Serum FGF23 levels were significantly increased in PMCA4 knockout mice. In conclusion, PMCA4 has no discernible role in normal renal Ca²⁺ handling as no urinary Ca²⁺ wasting was observed. Further investigation of the exact role of PMCA4 in the distal convoluted tubule and connecting tubule is required.

Multifaceted plasma membrane Ca(2+) pumps: From structure to intracellular Ca(2+) handling and cancer

Plasma membrane Ca(2+) ATPases (PMCAs) are intimately involved in the control of intracellular Ca(2+) concentration. They reduce Ca(2+) in the cytosol not only by direct ejection, but also by controlling the formation of inositol-1,4,5-trisphosphate and decreasing Ca(2+) release from the endoplasmic reticulum Ca(2+) pool. In mammals four genes (PMCA1-4) are expressed, and alternative RNA splicing generates more than twenty variants. The variants differ in their regulatory characteristics. They localize into highly specialized membrane compartments and respond to the incoming Ca(2+) with distinct temporal resolution. The expression pattern of variants depends on cell type; a change in this pattern can result in perturbed Ca(2+) homeostasis and thus altered cell function. Indeed, PMCAs undergo remarkable changes in their expression pattern during tumorigenesis that might significantly contribute to the unbalanced Ca(2+) homeostasis of cancer cells. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.