Stimulation of the calcium-sensing receptor stabilizes the podocyte cytoskeleton, improves cell survival, and reduces toxin-induced glomerulosclerosis

Multiscale effects of the calcimimetic drug, etelcalcetide on bone health of rats with secondary hyperparathyroidism induced by chronic kidney disease

Chronic kidney disease-induced secondary hyperparathyroidism (CKD-SHPT) heightens fracture risk through impaired mineral homeostasis and elevated levels of uremic toxins (UTs), which in turn enhance bone remodeling. Etelcalcetide (Etel), a calcium-sensing receptor (CaSR) agonist, suppresses parathyroid hormone (PTH) in hyperparathyroidism to reduce excessive bone resorption, leading to increased bone mass. However, Etel's effect on bone quality, chemical composition, and strength is not well understood. To address these gaps, we established a CKD-SHPT rat model and administered Etel at a human-equivalent dose concurrently with disease induction. The effects on bone and mineral homeostasis were compared with a CKD-SHPT (vehicle-treated group) and a control group (rats without SHPT). Compared with vehicle-treated CKD-SHPT rats, Etel treatment improved renal function, reduced circulating UT levels, improved mineral homeostasis parameters, decreased PTH levels, and prevented mineralization defects. The upregulation of mineralization-promoting genes by Etel in CKD-SHPT rats might explain its ability to prevent mineralization defects. Etel preserved both trabecular and cortical bones with attendant suppression of osteoclast function, besides increasing mineralization. Etel maintained the number of viable osteocytes to the control level, which could also contribute to its beneficial effects on bone. CKD-SHPT rats displayed increased carbonate substitution of matrix and mineral, decreased crystallinity, mineral-to-matrix ratio, and collagen maturity, and these changes were mitigated by Etel. Further, Etel treatment prevented CKD-SHPT-induced deterioration in bone strength and mechanical behavior. Based on these findings, we conclude that in CKD-SHPT rats, Etel has multiscale beneficial effects on bone that involve remodeling suppression, mineralization gene upregulation, and preservation of osteocytes.

Sweroside attenuates podocyte injury and proteinuria in part by activating Akt/BAD signaling in mice

In this study, we investigated the effects of sweroside on podocyte injury in diabetic nephropathy (DN) mice and elucidated its molecular mechanisms. We conducted in vivo experiments using a C57BL/6 mice model of DN to explore the effects of sweroside on proteinuria and podocyte injury in DN mice. In in vitro experiments, conditionally immortalized mouse podocytes were treated with high glucose and sweroside, and the protective effects of sweroside on podocyte injury were analyzed. In vitro, Akt/BAD pathways were detected using gene siRNA silencing assays and found to be involved in the protective roles of sweroside in high glucose-mediated podocyte injury. In vivo, sweroside significantly decreased albuminuria in DN mice (p < 0.01). periodic acid-Schiff staining showed that sweroside alleviated the glomerular volume and mesangium expansion in DN mice. Consistently, western blot and reverse transcription-polymerase chain reaction analyses showed that the profibrotic molecule expression in the glomeruli declined in sweroside-treated DN mice. Immunofluorescent results showed that sweroside preserved nephrin and podocin expression, and transmission electron microscopy showed that sweroside attenuated podocyte injury. In DN mice, sweroside decreased podocyte apoptosis, and increased nephrin, podocin expression and decreased desmin and HIF1α expression. These results confirmed that sweroside ameliorated albuminuria, glomerulomegaly, and glomerulosclerosis in these mice. Experiments in vitro revealed that sweroside improved HG-induced podocyte injury and apoptosis. Sweroside stimulated activation of the Akt/BAD pathway and upregulated Bcl-2-associated death promoter (BAD) and p-Akt. Overall, sweroside protected podocytes from injury and prevented the progression of DN, providing a novel strategy for the treatment of DN.

P90 ribosomal S6 kinases: A bona fide target for novel targeted anticancer therapies?

The 90 kDa ribosomal S6 kinase (RSK) family of proteins is a group of highly conserved Ser/Thr kinases. They are downstream effectors of the Ras/ERK/MAPK signaling cascade. ERK1/2 activation directly results in the phosphorylation of RSKs, which further, through interaction with a variety of different downstream substrates, activate various signaling events. In this context, they have been shown to mediate diverse cellular processes like cell survival, growth, proliferation, EMT, invasion, and metastasis. Interestingly, increased expression of RSKs has also been demonstrated in various cancers, such as breast, prostate, and lung cancer. This review aims to present the most recent advances in the field of RSK signaling that have occurred, such as biological insights, function, and mechanisms associated with carcinogenesis. We additionally present and discuss the recent advances but also the limitations in the development of pharmacological inhibitors of RSKs, in the context of the use of these kinases as putative, more efficient targets for novel anticancer therapeutic approaches.

Anti-inflammatory role of extracellular l-arginine through calcium sensing receptor in human renal proximal tubular epithelial (HK-2) cells

Renal tubular epithelial cells are capable of synthesizing interleukins (IL) in response to a variety of proinflammatory cytokines. Moreover, elevated urinary levels of IL have been shown in patients with various forms of nephritic diseases. However, the underlying intracellular signaling mechanism is unclear. Here we show the immunological signaling role of l-Arginine (l-Arg) through Ca2+-sensing receptor (CaSR) in human kidney 2 (HK-2) renal proximal tubular epithelial cells, using Ca2+ imaging and patch clamp techniques and its mechanistic link to the downstream cellular function. Both pharmacological and siRNA inhibitors support the activation CaSR by extracellular l-Arg to induced Ca2+ entry via a Transient receptor potential canonical (TRPC) channel in HK-2 cells mainly through the receptor operated Ca2+ entry (ROCE). Activation of CaSR by l-Arg led to the rise in p-p38/p38 expression suggesting [Ca2+]i as a regulator for p38-signaling pathways. Notably, l-Arg activated CaSR-induced Ca2+ signaling reduced the expressions of key fibrotic, inflammatory, and apoptotic genes, suggesting its nephroprotective role via Ca2+ signaling through CaSR in HK-2 cells. Since we found that the IL-6 expressions were inversely proportional to the increasing concentrations of l-Arg in HK-2 cells, we measured the release of IL-6, which steadily decreased as the concentrations of l-Arg were elevated. Taken together, extracellular l-Arg is a negative regulator for IL-6-induced inflammatory process, through the activation of CaSR and TRPC channel by ROCE pathway and can have a potential to alleviate inflammatory renal diseases.

Should we consider calcimimetics as a therapeutic option for nephrotic syndrome?

Calcimimetics allosterically increase the calcium ion sensitivity of the calcium-sensing receptor (CaSR). Using a CaSR knockdown in podocytes and a podocyte-specific CaSR knockout in mice, Mühlig et al. uncovered a stabilizing role for actin cytoskeleton and cell adhesion. Short-term alleviation of albuminuria and proteinuria was observed in 4 children treated with cinacalcet. Here we discuss the potential mechanisms whereby CaSR displays a favorable effect in podocytes and the context in which calcimimetics may alleviate nephrotic syndrome.

Crosstalk between Renal and Vascular Calcium Signaling: The Link between Nephrolithiasis and Vascular Calcification

Calcium (Ca²⁺) is an important mediator of multicellular homeostasis and is involved in several diseases. The interplay among the kidney, bone, intestine, and parathyroid gland in Ca²⁺ homeostasis is strictly modulated by numerous hormones and signaling pathways. The calcium-sensing receptor (CaSR) is a G protein–coupled receptor, that is expressed in calcitropic tissues such as the parathyroid gland and the kidney, plays a pivotal role in Ca²⁺ regulation. CaSR is important for renal Ca²⁺, as a mutation in this receptor leads to hypercalciuria and calcium nephrolithiasis. In addition, CaSR is also widely expressed in the vascular system, including vascular endothelial cells (VECs) and vascular smooth muscle cells (VSMCs) and participates in the process of vascular calcification. Aberrant Ca²⁺ sensing by the kidney and VSMCs, owing to altered CaSR expression or function, is associated with the formation of nephrolithiasis and vascular calcification. Based on emerging epidemiological evidence, patients with nephrolithiasis have a higher risk of vascular calcification, but the exact mechanism linking the two conditions is unclear. However, a dysregulation in Ca²⁺ homeostasis and dysfunction in CaSR might be the connection between the two. This review summarizes renal calcium handling and calcium signaling in the vascular system, with a special focus on the link between nephrolithiasis and vascular calcification.

Astragaloside IV inhibits palmitic acid-induced apoptosis through regulation of calcium homeostasis in mice podocytes

Loss of podocytes is a hallmark of diabetic nephropathy, and a growing body of evidence indicates that podocytes are susceptible to palmitic acid (PA). We have previously shown that AS-IV inhibited PA-induced podocyte apoptosis by activating sarcoendoplasmic reticulum Ca²⁺ ATPase (SERCA), which indicate calcium regulation may involve in the process. Immunofluorescence staining, Western blot and flow cytometry were used to measure the protective efficacy of AS-IV to ameliorate PA-induced ER stress and podocyte apoptosis. Meanwhile, AS-IV inhibited cytochrome c release, decreased mitochondrial membrane potential, accompany with the depletion of endoplasmic reticulum Ca²⁺ and elevation of cytosolic and mitochondrial Ca²⁺. Sequestration of cytosolic calcium with BAPTA-AM limited the response of podocyte apoptosis, while during the process the effect of AS-IV was also restrained. In contrast, elevation of cytosolic calcium with calcium ionophore ionomycin was depressed by AS-IV addition. Furthermore, inhibiting TRPC6 expression with SKF96365 or TRPC6 siRNA counteracted the beneficial effect of AS-IV. Our study provides further evidence to conclude the inhibitory effect of AS-IV to podocyte apoptosis is Ca²⁺-dependent. And the efficacy correlates with inhibiting TRPC6-mediated Ca²⁺ influx, and then cellular Ca²⁺ disturbance was coordinated.

MYDGF attenuates podocyte injury and proteinuria by activating Akt/BAD signal pathway in mice with diabetic kidney disease

AIMS/HYPOTHESIS

Myeloid-derived growth factor (MYDGF), mainly secreted by bone marrow-derived cells, has been known to promote glucagon-like peptide-1 production and improve glucose/lipid metabolism in mouse models of diabetes, but little is known about the functions of MYDGF in diabetic kidney disease (DKD). Here, we investigated whether MYDGF can prevent the progression of DKD.

METHODS

In vivo experiments, both loss- and gain-of-function strategies were used to evaluate the effect of MYDGF on albuminuria and pathological glomerular lesions. We used streptozotocin-treated Mydgf knockout and wild-type mice on high fat diets to induce a model of DKD. Then, albuminuria, glomerular lesions and podocyte injury were evaluated in Mydgf knockout and wild-type DKD mice treated with adeno-associated virus-mediated Mydgf gene transfer. In vitro and ex vivo experiments, the expression of slit diaphragm protein nephrin and podocyte apoptosis were evaluated in conditionally immortalised mouse podocytes and isolated glomeruli from non-diabetic wild-type mice treated with recombinant MYDGF.

RESULTS

MYDGF deficiency caused more severe podocyte injury in DKD mice, including the disruption of slit diaphragm proteins (nephrin and podocin) and an increase in desmin expression and podocyte apoptosis, and subsequently caused more severe glomerular injury and increased albuminuria by 39.6% compared with those of wild-type DKD mice (p < 0.01). Inversely, MYDGF replenishment attenuated podocyte and glomerular injury in both wild-type and Mydgf knockout DKD mice and then decreased albuminuria by 36.7% in wild-type DKD mice (p < 0.01) and 34.9% in Mydgf knockout DKD mice (p < 0.01). Moreover, recombinant MYDGF preserved nephrin expression and inhibited podocyte apoptosis in vitro and ex vivo. Mechanistically, the renoprotection of MYDGF was attributed to the activation of the Akt/Bcl-2-associated death promoter (BAD) pathway.

CONCLUSIONS/INTERPRETATION

The study demonstrates that MYDGF protects podocytes from injury and prevents the progression of DKD, providing a novel strategy for the treatment of DKD. Graphical abstract.

Treatment of nephrotic syndrome: going beyond immunosuppressive therapy

It is indisputable that immunosuppressive therapy and pathological diagnosis of renal biopsy have greatly improved the prognosis of childhood nephrotic syndrome. Unfortunately, there is no "one-size-fits-all" approach for precise patient stratification and treatment when facing the huge challenges posed by steroid-resistant nephrotic syndrome (SRNS). But genomic medicine has brought a glimmer of light, and the cognition of SRNS has entered a new stage. Based on this, identification of single genetic variants of SRNS has recognized the key role of podocyte injury in its pathogenesis. Targeted treatment of podocyte injury is paramount, and immunosuppressant with podocyte-targeted therapy seems to be more suitable as the first choice for SRNS, that is, we need to pay attention to their additional non-immunosuppressive effects. In the same way, other effect factors of nephrotic syndrome and the related causes of immunosuppressive therapy resistance require us to select reasonable and targeted non-immunosuppressive therapies, instead of only blindly using steroids and immunosuppressants, which may be ineffective and bring significant side effects. This article provides a summary of the clinical value of identification of genetic variants in podocytes and non-immunosuppressive therapy for nephrotic syndrome in children.

Associations between echocardiographic findings and prospective changes in residual renal function in patients new to peritoneal dialysis

Although echocardiograms are often performed when peritoneal dialysis is started, associations between commonly reported findings and prospective changes in renal function remain understudied. Ninety-nine of 101 patients in the Trio Trial had transthoracic echocardiograms within 6 months of dialysis initiation, and measurements of residual renal function every six weeks for up to two years. Generalized mixed modelling linear regression in STATA was used to examine associations between left atrial size, left ventricular hypertrophy, left ventricular ejection fraction, right ventricular systolic pressure, and left valvular calcification with subsequent slopes in renal function. After echocardiography (performed a median of 16 days following peritoneal dialysis initiation) right ventricular systolic pressure was associated with faster, while declining left ventricular ejection fraction and valvular calcification were associated with slower declines in residual renal function. Future studies could be conducted to confirm these findings, and identify pathophysiological mechanisms.

IL-17 and CD40 ligand synergistically stimulate the chronicity of diabetic nephropathy

Background

Early stages of diabetic nephropathy (DN) are characterized by an influx of inflammatory cells. Interactions between infiltrating T cells and podocytes may play an important role in the ongoing inflammatory response and remodelling. The aim of this study was to explore the role of IL-17 and CD40 ligand (CD40L) in DN.

Methods

The study design involved a case series. Kidney biopsy samples of 69 patients with type 2 diabetes were assessed for the presence of CD4+ IL-17+ T cells. The number of CD4+ IL-17+ T cells were counted and correlated with clinical and laboratory findings. Additionally, advanced glycation end-products (AGEs) were added to cultured podocytes to imitate diabetic conditions and thus to elucidate the role of CD4+ IL-17+ T cells in renal sclerosis.

Results

CD80 expression was detected in early phases of DN but was absent during diffused glomerurosclerosis in DN kidney specimens. In DN samples, CD40 expression was not only observed in most of the infiltrating cells, but also increased in podocytes and tubular epithelial cells. CD40L is locally expressed on infiltrating cells. CD4+ IL-17+ T cells were found in DN, and the number of CD4+ IL-17+ T cells was positively correlated with the deterioration in glomerular filtration rate (GFR). IL-17A was the key cytokine produced by CD4+ IL-17+ T cells. IL-17A levels were elevated in DN renal tissue and were correlated with declining GFR. IL-17 and CD40L synergistically enhanced IL-6, monocyte chemoattractant protein-1 (MCP-1), regulated on activation, normal T cell expressed and secreted (RANTES), transforming growth factor beta 1 (TGF-β1) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) production in vitro. AGEs induced podocyte activation with increasing expression of IL-17A, CD40 and TGF-β1 in vitro. Blockade with an anti-IL-17 monoclonal antibody reduced the expression of CD40 and TGF-β1, but increased the viability of cultured podocytes.

Conclusions

IL-17 and CD40L synergistically mediate the inflammatory response and remodelling associated with tissue injury and glomerular sclerosis in DN.

Calcium-sensing receptor activation attenuates collagen expression in renal proximal tubular epithelial cells

316: F1006-F1015, 2019. First published March 6, 2019; doi: 10.1152/ajprenal.00413.2018 .-Experimental studies have shown that pharmacological activation of calcium-sensing receptor (CaSR) attenuates renal fibrosis in some animal models beyond modification of bone and mineral homeostasis; however, its underlying mechanisms remain largely unknown. Since excessive collagen deposition is the key feature of fibrosis, the present study aimed to examine whether CaSR was involved in the regulation of collagen expression in rats with adenine diet-induced renal fibrosis and in profibrotic transforming growth factor (TGF)-β₁-treated renal proximal tubular epithelial cells (PTECs). The results showed that the CaSR agonist cinacalcet significantly attenuated renal collagen accumulation and tubular injury in adenine diet-fed rats. Additionally, the in vitro experiment showed that profibrotic TGF-β₁ significantly increased the expression of collagen and decreased CaSR expression at the mRNA and protein levels in a concentration- and time-dependent manner. Furthermore, the CaSR CRISPR activation plasmid and cinacalcet partially abrogated the upregulation of collagen induced by TGF-β₁ treatment. Blockade of CaSR by the CRISPR/Cas9 KO plasmid or the pharmacological antagonist Calhex231 further enhanced TGF-β₁-induced collagen expression. Mechanistic experiments found that Smad2 phosphorylation and Snail expression were markedly increased in PTECs treated with TGF-β₁, whereas the CaSR CRISPR activation plasmid and cinacalcet substantially suppressed this induction. In summary, this study provides evidence for a direct renal tubular epithelial protective effect of CaSR activation in renal fibrosis, possibly through suppression of collagen expression in PTECs.

Use of calcimimetics in children with normal kidney function

The calcium-sensing receptor (CaSR) plays an important role in the homeostasis of serum ionized calcium by regulating parathyroid hormone (PTH) secretion and tubular calcium handling. Calcimimetics, which act by allosteric modulation of the CaSR, mimic hypercalcemia resulting in suppression of PTH release and increase in calciuria. Mostly used in children to treat secondary hyperparathyroidism associated with advanced renal failure, we have shown that calcimimetics can also be successfully used in children with bone and mineral disorders in which elevated PTH plays a detrimental role in skeletal pathophysiology in the face of normal kidney function. The current review briefly discusses the role of the CaSR and calcimimetics in calcium homeostasis, and then addresses the potential applications of calcimimetics in children with normal kidney function with disorders in which suppression of PTH is beneficial.

Role of p90RSK in Kidney and Other Diseases

The 90 kDa ribosomal s6 kinases (RSKs) are a group of serine/threonine kinases consisting of 4 RSK isoforms (RSK1-4), of which RSK1 is also designated as p90RSK. p90RSK plays an important role in the Ras-mitogen-activated protein kinase (MAPK) signalling cascade and is the direct downstream effector of Ras-extracellular signal-regulated kinase (ERK1/2) signalling. ERK1/2 activation directly phosphorylates and activates p90RSK, which, in turn, activates various signalling events through selection of different phosphorylation substrates. Upregulation of p90RSK has been reported in numerous human diseases. p90RSK plays an important role in the regulation of diverse cellular processes. Thus, aberrant activation of p90RSK plays a critical role in the pathogenesis of organ dysfunction and damage. In this review, we focus on the current understanding of p90RSK functions and roles in the development and progression of kidney diseases. Roles of p90RSK, as well as other RSKs, in cardiovascular disorders and cancers are also discussed.

Group I metabotropic glutamate receptor activation induces TRPC6-dependent calcium influx and RhoA activation in cultured human kidney podocytes

Researches have shown that mice lacking the metabotropic glutamate receptor 1 (mGluR) showed albuminuria, remodeling of F-actin, with loss of stress fibers. Selective group I mGluRs agonist (S)-3,5-dihydroxyphenylglycine (DHPG) attenuated albuminuria in several rodent models of nephrotic syndrome. However, the molecular mechanism is obscure. Using a human podocyte cell line, we here investigated the molecular mechanisms of group I mGluRs-induced calcium influx and the formation of stress fibers. Our data showed that group I mGluRs activation by DHPG induced a significant calcium influx, and promoted cytoskeletal stress fiber formation and focal adhesions in podocytes. Pre-incubating podocytes with non-selective inhibitor of transient receptor potential channels (TRPC), or the knockdown of TRPC6 attenuated the calcium influx and the stress fiber formation induced by DHPG. Further, DHPG resulted in an increase of active RhoA expression. However, the knockdown of RhoA by siRNA abolished the DHPG-induced increase in stress fibers. Additionally, nonselective inhibitors of TRPC or TRPC6 knockdown clearly inhibited RhoA activation induced by DHPG, as assessed by Glutathione-S-transferase pull-down assay followed by Western blotting. Taken together, our findings suggest TRPC6 regulates actin stress fiber formation and focal adhesions via the RhoA pathway in response to group I mGluRs activation. Our data can potentially explain the mechanism of protective action of group I mGluRs in glomerular podocyte injury.

Induced Pluripotent Stem Cell-Derived Podocyte-Like Cells as Models for Assessing Mechanisms Underlying Heritable Disease Phenotype: Initial Studies Using Two Alport Syndrome Patient Lines Indicate Impaired Potassium Channel Activity

Renal podocyte survival depends upon the dynamic regulation of a complex cell architecture that links the glomerular basement membrane to integrins, ion channels, and receptors. Alport syndrome is a heritable chronic kidney disease where mutations in α3, α4, or α5 collagen genes promote podocyte death. In rodent models of renal failure, activation of the calcium-sensing receptor (CaSR) can protect podocytes from stress-related death. In this study, we assessed CaSR function in podocyte-like cells derived from induced-pluripotent stem cells from two patients with Alport Syndrome (AS1 & AS2) and a renal disease free individual [normal human mesangial cell (NHMC)], as well as a human immortalized podocyte-like (HIP) cell line. Extracellular calcium elicited concentration-dependent elevations of intracellular calcium in all podocyte-like cells. NHMC and HIP, but not AS1 or AS2 podocyte-like cells, also showed acute reductions in intracellular calcium prior to elevation. In NHMC podocyte-like cells this acute reduction was blocked by the large-conductance potassium channel (KCNMA1) inhibitors iberiotoxin (10 nM) and tetraethylammonium (5 mM), as well as the focal adhesion kinase inhibitor PF562271 (N-methyl-N-(3-((2-(2-oxo-2,3-dihydro-1H-indol-5-ylamino)-5-trifluoromethyl-pyrimidin-4-ylamino)-methyl)-pyridin-2-yl)-methanesulfonamide, 10 nM). Quantitative polymerase chain reaction (qPCR) and immunolabeling showed the presence of KCNMA1 transcript and protein in all podocyte-like cells tested. Cultivation of AS1 podocytes on decellularized plates of NHMC podocyte-like cells partially restored acute reductions in intracellular calcium in response to extracellular calcium. We conclude that the AS patient-derived podocyte-like cells used in this study showed dysfunctional integrin signaling and potassium channel function, which may contribute to podocyte death seen in Alport syndrome.

Cinacalcet-mediated activation of the CaMKKβ-LKB1-AMPK pathway attenuates diabetic nephropathy in db/db mice by modulation of apoptosis and autophagy

Apoptosis and autophagy are harmoniously regulated biological processes for maintaining tissue homeostasis. AMP-activated protein kinase (AMPK) functions as a metabolic sensor to coordinate cellular survival and function in various organs, including the kidney. We investigated the renoprotective effects of cinacalcet in high-glucose treated human glomerular endothelial cells (HGECs), murine podocytes and C57BLKS/J-db/db mice. In cultured HGECs and podocytes, cinacalcet decreased oxidative stress and apoptosis and increased autophagy that were attributed to the increment of intracellular Ca²⁺ concentration and the phosphorylation of Ca²⁺/calmodulin-dependent protein kinase kinaseβ (CaMKKβ)-Liver kinase B1 (LKB1)-AMPK and their downstream signals including the phosphorylation of endothelial nitric oxide synthase (eNOS) and increases in superoxide dismutases and B cell leukemia/lymphoma 2/BCL-2-associated X protein expression. Interestingly, intracellular chelator BAPTA-AM reversed cinacalcet-induced CaMKKβ elevation and LKB1 phosphorylation. Cinacalcet reduced albuminuria without influencing either blood glucose or Ca²⁺ concentration and ameliorated diabetes-induced renal damage, which were related to the increased expression of calcium-sensing receptor and the phosphorylation of CaMKKβ-LKB1. Subsequent activation of AMPK was followed by the activation of peroxisome proliferator-activated receptor γ coactivator-1α and phospho-Ser¹¹⁷⁷eNOS-nitric oxide, resulting in a decrease in apoptosis and oxidative stress as well as an increase in autophagy.

Our results suggest that cinacalcet increases intracellular Ca²⁺ followed by an activation of CaMKKβ-LKB1-AMPK signaling in GECs and podocytes in the kidney, which provides a novel therapeutic means for type 2 diabetic nephropathy by modulation of apoptosis and autophagy.

Just Look! Intravital Microscopy as the Best Means to Study Kidney Cell Death Dynamics

Kidney cell death plays a key role in the progression of life-threatening renal diseases, such as acute kidney injury and chronic kidney disease. Injured and dying epithelial and endothelial cells take part in complex communication with the innate immune system, which drives the progression of cell death and the decrease in renal function. To improve our understanding of kidney cell death dynamics and its impact on renal disease, a study approach is needed that facilitates the visualization of renal function and morphology in real time. Intravital multiphoton microscopy of the kidney has been used for more than a decade and made substantial contributions to our understanding of kidney physiology and pathophysiology. It is a unique tool that relates renal structure and function in a time- and spatial-dependent manner. Basic renal function, such as microvascular blood flow regulation and glomerular filtration, can be determined in real time and homeostatic alterations, which are linked inevitably to cell death and can be depicted down to the subcellular level. This review provides an overview of the available techniques to study kidney dysfunction and inflammation in terms of cell death in vivo, and addresses how this novel approach can be used to improve our understanding of cell death dynamics in renal disease.

Treatment of Genetic Forms of Nephrotic Syndrome

Idiopathic steroid-resistant nephrotic syndrome (SRNS) is most frequently characterized by focal segmental glomerulosclerosis (FSGS) but also other histological lesions, such as diffuse mesangial sclerosis. In the past two decades, a multitude of genetic causes of SRNS have been discovered raising the question of effective treatment in this cohort. Although no controlled studies are available, this review will discuss treatment options including pharmacologic interventions aiming at the attenuation of proteinuria in genetic causes of SRNS, such as inhibitors of the renin-angiotensin-aldosterone system and indomethacin. Also, the potential impact of other interventions to improve podocyte stability will be addressed. In this respect, the treatment with cyclosporine A (CsA) is of interest, since a podocyte stabilizing effect has been demonstrated in various experimental models. Although clinical response to CsA in children with genetic forms of SRNS is inferior to sporadic SRNS, some recent studies show that partial and even complete response can be achieved even in individual patients inherited forms of nephrotic syndrome. Ideally, improved pharmacologic and molecular approaches to induce partial or even complete remission will be available in the future, thus slowing or even preventing the progression toward end-stage renal disease.

Etelcalcetide, A Novel Calcimimetic, Prevents Vascular Calcification in A Rat Model of Renal Insufficiency with Secondary Hyperparathyroidism

Etelcalcetide, a novel peptide agonist of the calcium-sensing receptor, prevents vascular calcification in a rat model of renal insufficiency with secondary hyperparathyroidism. Vascular calcification occurs frequently in patients with chronic kidney disease (CKD) and is a consequence of impaired mineral homeostasis and secondary hyperparathyroidism (SHPT). Etelcalcetide substantially lowers parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF23) levels in SHPT patients on hemodialysis. This study compared the effects of etelcalcetide and paricalcitol on vascular calcification in rats with adenine-induced CKD and SHPT. Uremia and SHPT were induced in male Wistar rats fed a diet supplemented with 0.75% adenine for 4 weeks. Rats were injected with vehicle, etelcalcetide, or paricalcitol for 4 weeks from the beginning of adenine diet. Rats fed an adenine-free diet were included as nonuremic controls. Similar reductions in plasma PTH and parathyroid chief cell proliferation were observed in both etelcalcetide- and paricalcitol-treated rats. Serum calcium and phosphorus were significantly lower in etelcalcetide-treated uremic rats and was unchanged in paricalcitol-treated rats. Both serum FGF23 and aortic calcium content were significantly lower in etelcalcetide-treated uremic rats compared with either vehicle- or paricalcitol-treated uremic rats. The degree of aortic calcium content for etelcalcetide-treated rats was similar to that in nonuremic controls and corroborated findings of lack of histologic aortic mineralization in those groups. In conclusion, etelcalcetide and paricalcitol similarly attenuated progression of SHPT in an adenine rat model of CKD. However, etelcalcetide differentially prevented vascular calcification, at least in part, due to reductions in serum FGF23, calcium, and phosphorus levels.

Drug discovery in focal and segmental glomerulosclerosis

Despite the high medical burden experienced by patients with focal segmental glomerulosclerosis, the etiology of the condition remains largely unknown. Focal segmental glomerulosclerosis is highly heterogeneous in clinical and morphologic manifestations. While this presents challenges for the development of new treatments, research investments over the last 2 decades have yielded a surfeit of potential avenues for therapeutic intervention. The development of many of those ideas and concepts into new therapies, however, has been very disappointing. Here, we describe some of the factors that have potentially contributed to the poor translational performance from this research investment, including the confidence we ascribe to a target, the conduct of experimental studies, and the availability of selective reagents to test hypotheses. We will discuss the significance of genetic and systems traits as well as other methods for reducing bias. We will analyze the limitations of a successful drug development. We will use specific examples hoping that these will guide a consensus for investment and drive greater translational quality. We hope that this substrate will serve to exemplify the tremendous opportunity for intervention as well as facilitate greater collaborative effort between industry, academia, and private foundations in promoting appropriate validation of these targets. Only then will we have achieved our goal for curative therapies for this devastating disease.

Calpain-Mediated Cleavage of Calcineurin in Puromycin Aminonucleoside-Induced Podocyte Injury

The calcineurin inhibitors cyclosporine A (CsA) and tacrolimus are widely used in the treatment of proteinuria diseases. As the direct target of these drugs, calcineurin has previously been demonstrated to play a role in proteinuria diseases. However, aside from its immune-related effects, the local status of calcineurin in renal inherent cells has not been fully explored in the settings of proteinuria disease and podocyte injury. In this study, calcineurin activity and protein expression in the well-known puromycin aminonucleoside (PAN)-induced podocyte injury model were examined. Interestingly, we found that calcineurin activity was abnormally increased in PAN-treated podocytes, whereas the expression of the full-length 60-kDa calcineurin protein was decreased. This result suggests that there may be another activated form of calcineurin that is independent of the full-length phosphatase. To investigate whether calpain is involved in regulating calcineurin, we exposed PAN-treated podocytes to both pharmacological inhibitors of calpain and specific siRNAs against calpain. Calpain blockade reduced the enhanced calcineurin activity and restored the down-regulated expression of 60-kDa calcineurin. In addition, purified calpain protein was incubated with podocyte extracts, and a 45-kDa fragment of calcineurin was identified; this finding was confirmed in PAN-induced podocyte injury and calpain inhibition experiments. We conclude that calcineurin activity is abnormally increased during PAN-induced podocyte injury, whereas the expression of the full-length 60-kDa calcineurin protein is down-regulated due to over-activated calpain that cleaves calcineurin to form a 45-kDa fragment.

Comparative expression of the extracellular calcium-sensing receptor in the mouse, rat, and human kidney

The calcium-sensing receptor (CaSR) was cloned over 20 years ago and functionally demonstrated to regulate circulating levels of parathyroid hormone by maintaining physiological serum ionized calcium concentration ([Ca(2+)]). The receptor is highly expressed in the kidney; however, intrarenal and intraspecies distribution remains controversial. Recently, additional functions of the CaSR receptor in the kidney have emerged, including parathyroid hormone-independent effects. It is therefore critical to establish unequivocally the localization of the CaSR in the kidney to relate this to its proposed physiological roles. In this study, we determined CaSR expression in mouse, rat, and human kidneys using in situ hybridization, immunohistochemistry (using 8 different commercially available and custom-made antibodies), and proximity ligation assays. Negative results in mice with kidney-specific CaSR ablation confirmed the specificity of the immunohistochemistry signal. Both in situ hybridization and immunohistochemistry showed CaSR expression in the thick ascending limb, distal tubule, and collecting duct of all species, with the thick ascending limb showing the highest levels. Within the collecting ducts, there was significant heterogeneity of expression between cell types. In the proximal tubule, lower levels of immunoreactivity were detected by immunohistochemistry and proximity ligation assays. Proximity ligation assays were the only technique to demonstrate expression within glomeruli. This study demonstrated CaSR expression throughout the kidney with minimal discrepancy between species but with significant variation in the levels of expression between cell and tubule types. These findings clarify the intrarenal distribution of the CaSR and enable elucidation of the full physiological roles of the receptor within this organ.

Calcium-sensing receptor activation in chronic kidney disease: effects beyond parathyroid hormone control

Secondary hyperparathyroidism (SHPT) is an important complication of advanced chronic kidney disease (CKD). Cinacalcet, an allosteric modulator of the calcium-sensing receptor (CaSR) expressed in parathyroid glands, is the only calcimimetic approved to treat SHPT in patients on dialysis. By enhancing CaSR sensitivity for plasma extracellular calcium (Ca(2+)0), cinacalcet reduces serum parathyroid hormone, Ca(2+)0, and serum inorganic phosphorous concentrations, allowing better control of SHPT and CKD-mineral and bone disorders. Of interest, the CaSR also is expressed in a variety of tissues where its activation regulates diverse cellular processes, including secretion, apoptosis, and proliferation. Thus, the existence of potential off-target effects of cinacalcet cannot be neglected. This review summarizes our current knowledge concerning the potential role(s) of the CaSR expressed in various tissues in CKD-related disorders, independently of parathyroid hormone control.

The podocyte cytoskeleton in health and in disease

The podocyte is a key cell in the selective filtering action of the glomerular capillary wall. Podocyte injury is of pathogenetic and prognostic significance in human glomerular disease; podocyte repair and regeneration are important therapeutic targets. In particular, podocyte function is dependent on the cells' actin cytoskeleton: this maintains their complex structure. Alterations in the actin cytoskeleton arise from a variety of genetic and acquired causes. Therapeutic agents that are beneficial in proteinuric disease may act at least partly by restoring the cell shape via effects on the actin cytoskeleton. Recent studies of podocytes in vivo and in vitro are described, highlighting clinically relevant observations and those that help us understand the ways in which we may harness nature's own mechanisms to repair and/or renew these specialized glomerular cells, with a particular focus on their actin cytoskeleton. Drugs that have beneficial effects on podocytes can improve our ability to treat important renal diseases including diabetic nephropathy. Currently available agents can be applied in this way and the rapid progress in the study of podocytes is highlighting new therapeutic targets that can bring even more specificity.

Antialbuminuric actions of calcilytics in the remnant kidney

Hyperphosphatemia accelerates the progression of chronic kidney diseases. In the present study, the effects of ronacaleret, a calcilytic agent, on renal injury were assessed in the following four groups of rats: 5/6-nephrectomized Wistar rats as a control (C group), rats treated with ronacaleret (3 mg·kg(-1)·day(-1); R group), rats treated with calcitriol (30 ng·kg(-1)·day(-1); V group), and rats treated with both ronacaleret and calcitriol (R + V group). Three months later, rats were euthanized under anesthesia, and the remnant kidneys were harvested for analysis. Albuminuria was lower in the R and V groups than in the C group (P < 0.05). Creatinine clearance was elevated in the R and V groups compared with the C group (P < 0.05). Serum Ca(2+) and renal ANG II were higher in the R + V group than in the C group (P < 0.05 for each), and serum phosphate was reduced in the R group compared with the C group (P < 0.05). Fibroblast growth factor-23 was lower in the R group and higher in the V and R + V groups than in the C group. However, parathyroid hormone did not differ significantly among the four groups. Renal klotho expression was elevated in the R and V groups compared with the C group (P < 0.05). The present data indicate that ronacaleret preserves klotho expression and renal function with reductions in serum phosphate and albuminuria in 5/6-nephrectomized rats. Our findings demonstrate that vitamin D prevents declines in klotho expression and renal function, suppressing albuminuria.

New functional aspects of the extracellular calcium-sensing receptor

PURPOSE OF REVIEW

Variations in extracellular calcium level have a large impact on kidney function. Most of the effects seen are attributed to the calcium-sensing receptor (CaSR), a widely expressed G-protein-coupled cell surface protein with an important function in bone mineral homeostasis. The purpose of this review is to recapitulate the novel functional aspects of CaSR.

RECENT FINDINGS

Results from mouse models demonstrate important functions for CaSR in various tissues. In the kidney, the main role of CaSR is the regulation of calcium reabsorption in the thick ascending limb, independently of its role on parathyroid hormone secretion. CaSR modulates claudin 14, the gatekeeper of paracellular ion transport in the thick ascending limb that is associated with urinary calcium excretion. One intracellular signaling pathway by which CaSR alters tight junction permeability is the calcineurin-NFAT1c-microRNA-claudin14 axis.

SUMMARY

The main function of CaSR in the kidney is the regulation of calcium excretion in the thick ascending limb, independently of parathyroid hormone. CaSR modulates paracellular cation transport by altering expression of the tight junction protein claudin 14. Still more work is needed to fully understand all functions of CaSR in the kidney. Alternative pathways of calcium 'sensing' in the kidney need to be investigated.

Calcium sensing receptor modulates extracellular calcium entry and proliferation via TRPC3/6 channels in cultured human mesangial cells

Calcium-sensing receptor (CaSR) has been demonstrated to be present in several tissues and cells unrelated to systemic calcium homeostasis, where it regulates a series of diverse cellular functions. A previous study indicated that CaSR is expressed in mouse glomerular mesangial cells (MCs), and stimulation of CaSR induces cell proliferation. However, the signaling cascades initiated by CaSR activation in MCs are currently unknown. In this study, our data demonstrate that CaSR mRNA and protein are expressed in a human mesangial cell line. Activating CaSR with high extracellular Ca²⁺ concentration ([Ca²⁺]_o) or spermine induces a phospholipase C (PLC)-dependent increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i). Interestingly, the CaSR activation-induced increase in [Ca²⁺]_i results not only from intracellular Ca²⁺ release from internal stores but also from canonical transient receptor potential (TRPC)-dependent Ca²⁺ influx. This increase in Ca²⁺ was attenuated by treatment with a nonselective TRPC channel blocker but not by treatment with a voltage-gated calcium blocker or Na⁺/Ca²⁺ exchanger inhibitor. Furthermore, stimulation of CaSR by high [Ca²⁺]_o enhanced the expression of TRPC3 and TRPC6 but not TRPC1 and TRPC4, and siRNA targeting TRPC3 and TRPC6 attenuated the CaSR activation-induced [Ca²⁺]_i increase. Further experiments indicate that 1-oleoyl-2-acetyl-sn-glycerol (OAG), a known activator of receptor-operated calcium channels, significantly enhances the CaSR activation-induced [Ca²⁺]_i increase. Moreover, under conditions in which intracellular stores were already depleted with thapsigargin (TG), CaSR agonists also induced an increase in [Ca²⁺]_i, suggesting that calcium influx stimulated by CaSR agonists does not require the release of calcium stores. Finally, our data indicate that pharmacological inhibition and knock down of TRPC3 and TRPC6 attenuates the CaSR activation-induced cell proliferation in human MCs. With these data, we conclude that CaSR activation mediates Ca²⁺ influx and cell proliferation via TRPC3 and TRPC6 in human MCs.

Control of renal calcium, phosphate, electrolyte, and water excretion by the calcium-sensing receptor

Through regulation of excretion, the kidney shares responsibility for the metabolic balance of calcium (Ca(2+)) with several other tissues including the GI tract and bone. The balances of Ca(2+) and phosphate (PO4), magnesium (Mg(2+)), sodium (Na(+)), potassium (K(+)), chloride (Cl(-)), and water (H2O) are linked via regulatory systems with overlapping effects and are also controlled by systems specific to each of them. Cloning of the calcium-sensing receptor (CaSR) along with the recognition that mutations in the CaSR gene are responsible for two familial syndromes characterized by abnormalities in the regulation of PTH secretion and Ca(2+) metabolism (Familial Hypocalciuric Hypercalcemia, FHH, and Autosomal Dominant Hypocalcemia, ADH) made it clear that extracellular Ca(2+) (Ca(2+)o) participates in its own regulation via a specific, receptor-mediated mechanism. Demonstration that the CaSR is expressed in the kidney as well as the parathyroid glands combined with more complete characterizations of FHH and ADH established that the effects of elevated Ca(2+) on the kidney (wasting of Na(+), K(+), Cl(-), Ca(2+), Mg(2+) and H2O) are attributable to activation of the CaSR. The advent of positive and negative allosteric modulators of the CaSR along with mouse models with global or tissue-selective deletion of the CaSR in the kidney have allowed a better understanding of the functions of the CaSR in various nephron segments. The biology of the CaSR is more complicated than originally thought and difficult to define precisely owing to the limitations of reagents such as anti-CaSR antibodies and the difficulties inherent in separating direct effects of Ca(2+) on the kidney mediated by the CaSR from associated CaSR-induced changes in PTH. Nevertheless, renal CaSRs have nephron-specific effects that contribute to regulating Ca(2+) in the circulation and urine in a manner that assures a narrow range of Ca(2+)o in the blood and avoids excessively high concentrations of Ca(2+) in the urine.

The calcimimetic R-568 prevents podocyte loss in uninephrectomized ApoE-/- mice

Calcimimetics are indicated for secondary hyperparathyroidism in chronic kidney disease, and some data have suggested their protective role for progression of renal damage. We aimed to evaluate whether a calcimimetic can slow the progression of kidney damage in uninephrectomized apolipoprotein E (ApoE)-deficient (ApoE-/-) mice. To this end, we compared its effect with that of calcitriol. Male ApoE-/- mice (12 wk old) were randomized to undergo sham operation (sham) or unilateral nephrectomy (UNX) and subsequently received the calcimimetic R-568 (4 μg·kg⁻¹·day⁻¹), calcitriol (0.03 μg·kg⁻¹·day⁻¹), or vehicle intraperitoneally. Glomerular number and volume, damage indexes (glomerular, vascular, and interstitial), and glomerular (podocytes, mesangial, and endothelial) cell number and volume were assessed in perfused kidneys after a 12-wk treatment period. Lower numbers of podocytes per glomerulus were observed in the UNX + vehicle group compared with the sham group, and this was prevented in the UNX + R-568 group but not in the UNX + calcitriol group. In parallel, albuminuria was higher in the untreated UNX group compared with the sham group, and the increase was prevented in the UNX + R-568 group. Interstitial fibrosis was more prevalent in the vehicle-treated UNX group compared with the sham group, and this was prevented in the UNX group treated with R-568 and less effectively with calcitriol treatment. In all UNX groups, the weight of the residual kidney was significantly higher compared with all sham groups. No differences were observed in serum ionized calcium and systolic blood pressure between the groups. The calcimimetic R-568 prevented interstial fibrosis and podocyte loss after uninephrectomy in ApoE-/- mice. Minor renal dysfunction, lack of secondary hyperparathyroidism, and hypertension in this model support the hypothesis of direct effects of this compound on glomerular cells.

Calcium-sensing receptor-dependent activation of CREB phosphorylation in HEK293 cells and human parathyroid cells

In addition to its acute effects on hormone secretion, epithelial transport, and shape change, the calcium-sensing receptor (CaSR) modulates the expression of genes that control cell survival, proliferation, and differentiation as well as the synthesis of peptide hormones and enzymes. In the present study, we investigated the impacts of a CaSR agonist and several CaSR modulators on phosphorylation of transcription factor CREB residue Ser(133) in CaSR-expressing HEK293 (HEK-CaSR) cells and human adenomatous parathyroid cells. Elevated Ca(2+)o concentration had no effect on CREB phosphorylation (p-CREB) in control HEK293 cells but stimulated p-CREB in both HEK-CaSR cells and human parathyroid cells. In addition, p-CREB was stimulated by the positive modulator cinacalcet and inhibited by the negative modulator NPS 2143 in both CaSR-expressing cell types. Two positive modulators that bind in the receptor's Venus Fly Trap domain, l-phenylalanine and S-methylglutathione, had no effect on p-CREB in HEK-CaSR cells, demonstrating the existence of pronounced signaling bias. Analysis of the signaling pathways using specific inhibitors demonstrated that phosphoinositide-specific phospholipase C and conventional protein kinase C isoforms make major contributions to Ca(2+)o-induced p-CREB in both cell-types, suggesting key roles for Gq/11. In addition, in parathyroid cells but not HEK-CaSR cells, activation of p-CREB was dependent on Gi/o, demonstrating the existence of cell type-specific signaling.

Educational paper: Progression in chronic kidney disease and prevention strategies

Chronic kidney disease (CKD) in children is a rare but devastating condition. Once a critical amount of nephron mass has been lost, progression of CKD is irreversible and results in end-stage renal disease (ESRD) and need of renal replacement therapy. The time course of childhood CKD is highly variable. While in children suffering from congenital anomalies of the kidneys and the urinary tract, progression of CKD in general is slow, in children with acquired glomerulopathies, disease progression can be accelerated resulting in ESRD within months. However, irrespective of the underlying kidney disease, hypertension and proteinuria are independent risk factors for progression. Thus, in order to prevent progression, the primary objective of treatment should always aim for efficient control of blood pressure and reduction of urinary protein excretion. Blockade of the renin-angiotensin-aldosterone system preserves kidney function not only by lowering blood pressure, but also by reducing proteinuria and exerting additional anti-proteinuric, anti-fibrotic, and anti-inflammatory effects. Besides, intensified blood pressure control, aiming for a target blood pressure below the 50th percentile, may exert additive renoprotective effects. Additionally, other modifiable risk factors, such as anemia, metabolic acidosis, dyslipidemia, and altered bone-mineral homeostasis may also contribute to CKD progression. In conclusion, beyond strict blood pressure control and reduction of urinary protein excretion, identification and treatment of both, renal disease-related and conventional risk factors are mandatory in children with CKD in order to prevent deterioration of kidney function.

Deficiency of the calcium-sensing receptor in the kidney causes parathyroid hormone-independent hypocalciuria

Rare loss-of-function mutations in the calcium-sensing receptor (Casr) gene lead to decreased urinary calcium excretion in the context of parathyroid hormone (PTH)-dependent hypercalcemia, but the role of Casr in the kidney is unknown. Using animals expressing Cre recombinase driven by the Six2 promoter, we generated mice that appeared grossly normal but had undetectable levels of Casr mRNA and protein in the kidney. Baseline serum calcium, phosphorus, magnesium, and PTH levels were similar to control mice. When challenged with dietary calcium supplementation, however, these mice had significantly lower urinary calcium excretion than controls (urinary calcium to creatinine, 0.31±0.03 versus 0.63±0.14; P=0.001). Western blot analysis on whole-kidney lysates suggested an approximately four-fold increase in activated Na(+)-K(+)-2Cl(-) cotransporter (NKCC2). In addition, experimental animals exhibited significant downregulation of Claudin14, a negative regulator of paracellular cation permeability in the thick ascending limb, and small but significant upregulation of Claudin16, a positive regulator of paracellular cation permeability. Taken together, these data suggest that renal Casr regulates calcium reabsorption in the thick ascending limb, independent of any change in PTH, by increasing the lumen-positive driving force for paracellular Ca(2+) transport.

Calcium sensing in podocytes

Besides its primary function in maintaining systemic calcium homeostasis, the calcium-sensing receptor (CaSR) is expressed by many cell types, with different, sometimes opposite, regulatory functions. Novel work from Oh and collaborators shows that activation of CaSR in podocytes has prosurvival effects and protects the cell from puromycin aminonucleoside damage. Given that the cellular consequences of CaSR activation are largely context-dependent, further studies will be required to elucidate its precise role in podocyte physiology and pathophysiology.

Cell biology and pathology of podocytes

As an integral member of the filtration barrier in the kidney glomerulus, the podocyte is in a unique geographical position: It is exposed to chemical signals from the urinary space (Bowman's capsule), it receives and transmits chemical and mechanical signals to/from the glomerular basement membrane upon which it elaborates, and it receives chemical and mechanical signals from the vascular space with which it also communicates. As with every cell, the ability of the podocyte to receive signals from the surrounding environment and to translate them to the intracellular milieu is dependent largely on molecules residing on the cell membrane. These molecules are the first-line soldiers in the ongoing battle to sense the environment, to respond to friendly signals, and to defend against injurious foes. In this review, we take a membrane biologist's view of the podocyte, examining the many membrane receptors, channels, and other signaling molecules that have been implicated in podocyte biology. Although we attempt to be comprehensive, our goal is not to capture every membrane-mediated pathway but rather to emphasize that this approach may be fruitful in understanding the podocyte and its unique properties.

Mineral and bone disorders in children with chronic kidney disease

As children with chronic kidney disease (CKD) have a long lifespan, optimal control of bone and mineral homeostasis is essential not only for the prevention of debilitating skeletal complications and for achieving adequate growth but also for preserving long-term cardiovascular health. As the growing skeleton is highly dynamic and at particular risk of deterioration, close control of bone and mineral homeostasis is required in children with CKD. However, assessment of bone disease is hampered by the limited validity of biochemical parameters-major controversy exists on key issues such as parathyroid hormone target ranges and the lack of useful imaging techniques. The role of newly discovered factors in bone and mineral homeostasis, such as fibroblast growth factor 23, is not yet established. Even though scientific evidence is limited in children with CKD, ergocalciferol or cholecalciferol supplementation and the use of calcium-free phosphate binders is recommended. The new drug cinacalcet is highly promising; however, pediatric experience is still limited to observational data and the effect of cinacalcet on longitudinal growth and pubertal development is unknown. Randomized, controlled trials are underway, including studies of cinacalcet pharmacokinetics and pharmacodynamics in infants.