Activation of the Calcium-Sensing Receptor Corrects the Impaired Mitochondrial Energy Status Observed in Renal Polycystin-1 Knockdown Cells Modeling Autosomal Dominant Polycystic Kidney Disease

Calcium-Sensing Receptor (CaSR)-Mediated Intracellular Communication in Cardiovascular Diseases

The calcium-sensing receptor (CaSR), a G-protein-coupled receptor (GPCR), is a cell-surface-located receptor that can induce highly diffusible messengers (IP3, Ca²⁺, cAMP) in the cytoplasm to activate various cellular responses. Recently, it has also been suggested that the CaSR mediates the intracellular communications between the endoplasmic reticulum (ER), mitochondria, nucleus, protease/proteasome, and autophagy-lysosome, which are involved in related cardiovascular diseases. The complex intracellular signaling of this receptor challenges it as a valuable therapeutic target. It is, therefore, necessary to understand the mechanisms behind the signaling characteristics of this receptor in intracellular communication. This review provides an overview of the recent research progress on the various regulatory mechanisms of the CaSR in related cardiovascular diseases and the heart-kidney interaction; the associated common causes are also discussed.

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Disruption of polycystin-1 cleavage leads to cardiac metabolic rewiring in mice

Cardiovascular manifestations account for marked morbi-mortality in autosomal dominant polycystic kidney disease (ADPKD). Pkd1- and Pkd2-deficient mice develop cardiac dysfunction, however the underlying mechanisms remain largely unclear. It is unknown whether impairment of polycystin-1 cleavage at the G-protein-coupled receptor proteolysis site, a significant ADPKD mutational mechanism, is involved in this process. We analyzed the impact of polycystin-1 cleavage on heart metabolism using Pkd1^V/V mice, a model unable to cleave this protein and with early cardiac dysfunction. Pkd1^V/V hearts showed lower levels of glucose and amino acids and higher lipid levels than wild-types, as well as downregulation of p-AMPK, p-ACCβ, CPT1B-Cpt1b, Ppara, Nppa and Acta1. These findings suggested decreased fatty acid β-oxidation, which was confirmed by lower oxygen consumption by Pkd1^V/V isolated mitochondria using palmitoyl-CoA. Pkd1^V/V hearts also presented increased oxygen consumption in response to glucose, suggesting that alternative substrates may be used to generate energy. Pkd1^V/V hearts displayed a higher density of decreased-size mitochondria, a finding associated with lower MFN1, Parkin and BNIP3 expression. These derangements were correlated with increased apoptosis and inflammation but not hypertrophy. Notably, Pkd1^V/V neonate cardiomyocytes also displayed shifts in oxygen consumption and p-AMPK downregulation, suggesting that, at least partially, the metabolic alterations are not induced by kidney dysfunction. Our findings reveal that disruption of polycystin-1 cleavage leads to cardiac metabolic rewiring in mice, expanding the understanding of heart dysfunction associated with Pkd1 deficiency and likely with human ADPKD.

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.

Pre-clinical evaluation of dual targeting of the GPCRs CaSR and V2R as therapeutic strategy for autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD), caused by mutations of PKD1 or PKD2 genes, is characterized by development and growth of cysts causing progressive kidney enlargement. Reduced resting cytosolic calcium and increased cAMP levels associated with the tonic action of vasopressin are two central biochemical defects in ADPKD. Here we show that co‐targeting two GPCRs, the vasopressin V2 receptor (V2R) and the calcium sensing receptor, using the novel V2R antagonist lixivaptan in combination with the calcimimetic R‐568, reduced cyst progression in two animal models of human PKD. Lixivaptan is expected to have a safer liver profile compared to tolvaptan, the only drug approved to delay PKD progression, based on computational model results and initial clinical evidence. PCK rat and Pkd1^RC/RC mouse littermates were fed without or with lixivaptan (0.5%) and R‐568 (0.025% for rats and 0.04% for mice), alone or in combination, for 7 (rats) or 13 (mice) weeks. In PCK rats, the combined treatment strongly decreased kidney weight, cyst and fibrosis volumes by 20%, 49%, and 73%, respectively, compared to untreated animals. In Pkd1^RC/RC mice, the same parameters were reduced by 20%, 56%, and 69%, respectively. In both cases the combined treatment appeared nominally more effective than the individual drugs used alone. These data point to an intriguing new application for two existing drugs in PKD treatment. The potential for synergy between these two compounds suggested in these animal studies, if confirmed in appropriate clinical investigations, would represent a welcome advancement in the treatment of ADPKD.

The vasopressin-aquaporin-2 pathway syndromes

Vasopressin is the key hormone involved in water conservation and regulation of water balance, essential for life. In the renal collecting duct, vasopressin binds to the V2 receptor, increasing water permeability through activation of aquaporin-2 redistribution to the luminal membrane. This mechanism promotes rapid water reabsorption, important for immediate survival; however, only recently it has become clear that long-term adverse effects are associated with alterations of the vasopressin-aquaporin-2 pathway, leading to several syndromes associated with water balance disorders. The kidney resistance to the vasopressin action may cause severe dehydration for patients and, conversely, nonosmotic release of vasopressin is associated with water retention and increasing the circulatory blood volume. This chapter discusses the relevance of the altered vasopressin-aquaporin-2 pathway in some diseases associated with water balance disorders, including congenital nephrogenic diabetes insipidus, syndrome of inappropriate secretion of antidiuretic hormone, nephrogenic syndrome of inappropriate antidiuresis, and autosomal dominant polycystic kidney disease. The emerging picture suggests that targeting the vasopressin-AQP2 axis can provide therapeutic benefits in those patients.

Impaired endoplasmic reticulum-mitochondrial signaling in ataxia-telangiectasia

There is evidence that ATM mutated in ataxia-telangiectasia (A-T) plays a key role in protecting against mitochondrial dysfunction, the mechanism for which remains unresolved. We demonstrate here that ATM-deficient cells are exquisitely sensitive to nutrient deprivation, which can be explained by defective cross talk between the endoplasmic reticulum (ER) and the mitochondrion. Tethering between these two organelles in response to stress was reduced in cells lacking ATM, and consistent with this, Ca²⁺ release and transfer between ER and mitochondria was reduced dramatically when compared with control cells. The impact of this on mitochondrial function was evident from an increase in oxygen consumption rates and a defect in mitophagy in ATM-deficient cells. Our findings reveal that ER-mitochondrial connectivity through IP3R1-GRP75-VDAC1, to maintain Ca²⁺ homeostasis, as well as an abnormality in mitochondrial fusion defective in response to nutrient stress, can account for at least part of the mitochondrial dysfunction observed in A-T cells.

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Hypersensitivity to glucose deprivation in ATM-deficient cells

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Defective ER-mitochondrion cross talk after nutrient stress in these cells

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Markedly reduced Ca²⁺ transfer between these two organelles in ATM-deficient cells

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Mitochondrial dysfunction in response to nutrient stress in the absence of ATM

Increased mTOR and suppressed autophagic flux in the heart of a hypomorphic Pkd1 mouse model of autosomal dominant polycystic kidney disease

Cardiac hypertrophy is common in autosomal dominant polycystic kidney disease (ADPKD) patients. We found increased heart weight in Pkd1^RC/RC and Pkd2^WS25/+ mouse models of ADPKD. As there is a link between increased heart weight and mammalian target of rapamycin (mTOR), the aim of the study was to determine mTOR complex 1 and 2 signaling proteins in the heart in the Pkd1^RC/RC mouse model of PKD. In 70 day old Pkd1^RC/RC hearts, on immunoblot analysis, there was a large increase in p-AMPK^Thr172, a known autophagy inducer, and an increase in p-Akt^Ser473 and p-Akt^Thr308, but no increase in other mTORC1/2 proteins (p-S6^Ser240/244, p-mTOR^Ser2448). In 150 day old Pkd1^RC/RC hearts, there was an increase in mTORC1 (p-S6^Ser240/244) and mTOR-related proteins (p-Akt^Thr308, p-GSK3β^Ser9, p-AMPK^Thr172). As the mTOR pathway is the master regulator of autophagy, autophagy proteins were measured. There was an increase in p-Beclin-1 (BECN1), an autophagy regulator and activating molecule in Beclin-1-regulated autophagy (AMBRA1), a regulator of Beclin that play a role in autophagosome formation, an early stage of autophagy. There was a defect in the later stage of autophagy, the fusion of the autophagosome with the lysosome, known as autophagic flux, as evidenced by the lack of an increase in LC3-II, a marker of autophagosomes, with the lysosomal inhibitor bafilomycin, in both 70 day old and 150 day old hearts. To determine the role of autophagy in causing increased heart weight, Pkd1^RC/RC were treated with 2-deoxyglucose (2-DG) or Tat-Beclin1 peptide, agents known to induce autophagy. 2-DG treatment from 150 to 350 days of age, a time period when increased heart weight developed, did not reduce the increased heart weight. Unexpectedly, Tat-Beclin 1 peptide treatment from 70 to 120 days of age resulted in increased heart weight. In summary, there is suppressed autophagic flux in the heart at an early age in Pkd1^RC/RC mice. Increased mTOR signaling in older mice is associated suppressed autophagic flux. There was a large increase in p-AMPK^Thr172, a known autophagy inducer, in both young and old mice. 2-DG treatment did not impact increased heart weight and Tat-Beclin1 peptide increased heart weight.

Postnatal growth retardation is associated with intestinal mucosa mitochondrial dysfunction and aberrant energy status in piglets

Individuals with postnatal growth retardation (PGR) are prone to developing chronic disease. Abnormal development in small intestine is casually implicated in impaired growth performance. However, the exact mechanism is still unknown. In this present study, PGR piglets (aged 42 days) were employed as a good model to analyse changes in nutrient absorption and energy metabolism in the intestinal mucosa. The results showed lower serum concentrations of free amino acids, and lipid metabolites in PGR piglets, which were in accordance with the down‐regulated mRNA expressions involved in fatty acid and amino acid transporters in the jejunal and ileal mucosa. The decreased activities of digestive enzymes and the marked swelling in mitochondria were also observed in the PGR piglets. In addition, it was found that lower ATP production, higher AMP/ATP ratio, deteriorated mitochondrial complex III and ATP synthase, and decreased manganese superoxide dismutase activity in the intestinal mucosa of PGR piglets. Furthermore, altered gene expression involved in energy metabolism, accompanied by decreases in the protein abundance of SIRT1, PGC‐1α and PPARγ, as well as phosphorylations of AMPKα, mTOR, P70S6K and 4E‐BP1 were observed in intestinal mucosa of PGR piglets. In conclusion, decreased capability of nutrient absorption, mitochondrial dysfunction, and aberrant energy status in the jejunal and ileal mucosa may contribute to PGR piglets.

The Impact of Acetyl-CoA and Aspartate Shortages on the N-Acetylaspartate Level in Different Models of Cholinergic Neurons

N-acetylaspartate is produced by neuronal aspartate N-acetyltransferase (NAT8L) from acetyl-CoA and aspartate. In cholinergic neurons, acetyl-CoA is also utilized in the mitochondrial tricarboxylic acid cycle and in acetylcholine production pathways. While aspartate has to be shared with the malate–aspartate shuttle, another mitochondrial machinery together with the tricarboxylic acid cycle supports the electron transport chain turnover. The main goal of this study was to establish the impact of toxic conditions on N-acetylaspartate production. SN56 cholinergic cells were exposed to either Zn²⁺ overload or Ca²⁺ homeostasis dysregulation and male adult Wistar rats’ brains were studied after 2 weeks of challenge with streptozotocin-induced hyperglycemia or daily theophylline treatment. Our results allow us to hypothesize that the cholinergic neurons from brain septum prioritized the acetylcholine over N-acetylaspartate production. This report provides the first direct evidence for Zn²⁺-dependent suppression of N-acetylaspartate synthesis leading to mitochondrial acetyl-CoA and aspartate shortages. Furthermore, Zn²⁺ is a direct concentration-dependent inhibitor of NAT8L activity, while Zn²⁺-triggered oxidative stress is unlikely to be significant in such suppression.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

Renal Ca2+ and Water Handling in Response to Calcium Sensing Receptor Signaling: Physiopathological Aspects and Role of CaSR-Regulated microRNAs

Calcium (Ca²⁺) is a universal and vital intracellular messenger involved in a diverse range of cellular and biological processes. Changes in the concentration of extracellular Ca²⁺ can disrupt the normal cellular activities and the physiological function of these systems. The calcium sensing receptor (CaSR) is a unique G protein-coupled receptor (GPCR) activated by extracellular Ca²⁺ and by other physiological cations, aminoacids, and polyamines. CaSR is the main controller of the extracellular Ca²⁺ homeostatic system by regulating parathyroid hormone (PTH) secretion and, in turn, Ca²⁺ absorption and resorption. Recent advances highlight novel signaling pathways activated by CaSR signaling involving the regulation of microRNAs (miRNAs). miRNAs are naturally-occurring small non-coding RNAs that regulate post-transcriptional gene expression and are involved in several diseases. We previously described that high luminal Ca²⁺ in the renal collecting duct attenuates short-term vasopressin-induced aquaporin-2 (AQP2) trafficking through CaSR activation. Moreover, we demonstrated that CaSR signaling reduces AQP2 abundance via AQP2-targeting miRNA-137. This review summarizes the recent data related to CaSR-regulated miRNAs signaling pathways in the kidney.

Mitochondrial TRPC3 promotes cell proliferation by regulating the mitochondrial calcium and metabolism in renal polycystin-2 knockdown cells

PURPOSE

Previous studies indicate that autosomal dominant polycystic kidney disease (ADPKD) cells exhibited dysregulated calcium homeostasis and enhanced cell proliferation. TRPC3 has been shown to function in the modulation of calcium and sodium entry, but whether TRPC3 plays a role in cellular abnormalities of ADPKD cells has not been defined.

METHODS

Human conditionally immortalized proximal tubular epithelial cells and mouse IMCD3 cells were used with polycystin-2 (PC2, TRPP2) knockdown. Cell proliferation assay was used to detect the cell proliferations upon different treatments. QRT-PCR and western blotting were used to measure the expression profiles of TRPP2 and other proteins. High-resolution respirometry, enzymic activities and ROS levels were detected to reflect the mitochondrial functions. Calcium and sodium uptakes were measured using Fura2-AM and SBFI dyes.

RESULTS

We showed that PC2 knockdown promoted cell proliferation, ROS productions and ERK phosphorylation, compared with negative control. Meanwhile, we demonstrated that receptor-operated calcium entry (ROCE) exhibited less reductions compared with store-operated calcium entry (SOCE) upon PC2 knockdown. Inhibition of ROCE and SOCE by specific inhibitors partially reversed the enhanced cell proliferation, ROS productions and ERK phosphorylation induced by PC2 knockdown. Moreover, TRPC3 upregulation was observed upon PC2 knockdown, which acted as both SOC and ROC, promoting cation entry, cell proliferation and ERK phosphorylation. Furthermore, we showed that mitochondrial located TRPC3 was upregulated and modulating mitochondrial calcium uptake, thus promoting the ROS productions in the presence of PC2 knockdown.

CONCLUSIONS

We demonstrated that TRPC3 upregulation upon PC2 knockdown aggravated the mitochondrial abnormalities and cell proliferation by modulating mitochondrial calcium uptake. Targeting TRPC3 might be a promising target for ADPKD treatment.

Vasopressin-aquaporin-2 pathway: recent advances in understanding water balance disorders

The alteration of water balance and related disorders has emerged as being strictly linked to the state of activation of the vasopressin–aquaporin-2 (vasopressin–AQP2) pathway. The lack of responsiveness of the kidney to the vasopressin action impairs its ability to concentrate the urine, resulting in polyuria, polydipsia, and risk of severe dehydration for patients. Conversely, non-osmotic release of vasopressin is associated with an increase in water permeability in the renal collecting duct, producing water retention and increasing the circulatory blood volume. This review highlights some of the new insights and recent advances in therapeutic intervention targeting the dysfunctions in the vasopressin–AQP2 pathway causing diseases characterized by water balance disorders such as congenital nephrogenic diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion, nephrogenic syndrome of inappropriate antidiuresis, and autosomal dominant polycystic kidney disease. The recent clinical data suggest that targeting the vasopressin–AQP2 axis can provide therapeutic benefits in patients with water balance disorders.