TRPV5: an ingeniously controlled calcium channel
2008; Elsevier BV; Volume: 74; Issue: 10 Linguagem: Inglês
10.1038/ki.2008.320
ISSN1523-1755
AutoresTheun de Groot, René J.M. Bindels, Joost G.J. Hoenderop,
Tópico(s)Ion Channels and Receptors
ResumoBody Ca2+ homeostasis is tightly controlled and slight disturbances in renal Ca2+ reabsorption can lead to excessive urine Ca2+ excretion and promote kidney stone formation. The epithelial Ca2+ channel TRPV5 constitutes the rate-limiting step of active Ca2+ reabsorption in the kidney. Elucidation of the molecular pathways controlling TRPV5 function provides important information for our understanding of renal Ca2+ handling, since active Ca2+ reabsorption fine-tunes the final amount of Ca2+ excreted into the urine. Over the last years, the molecular regulation of TRPV5 has been dismantled in detail. Various calciotropic hormones, known to alter renal Ca2+ reabsorption, affect the expression of TRPV5. Others stimulate the trafficking of TRPV5 to the plasma membrane, while a number of associated proteins and ions control channel activity at the plasma membrane. Dynamic cell surface presence of TRPV5 is largely mediated by endosomal recycling processes allowing internalized channels to reappear at the plasma membrane. We present recently identified factors shown to modulate TRPV5 activity by diverse mechanisms to ultimately control renal Ca2+ handling. The selected factors include klotho, tissue kallikrein, pH, Ca2+, Mg2+, PIP2 and WNK4. This review covers the distinctive properties and regulation of the highly Ca2+-selective TRPV5 channel and highlights the implications for our understanding of the process of Ca2+ reabsorption. Body Ca2+ homeostasis is tightly controlled and slight disturbances in renal Ca2+ reabsorption can lead to excessive urine Ca2+ excretion and promote kidney stone formation. The epithelial Ca2+ channel TRPV5 constitutes the rate-limiting step of active Ca2+ reabsorption in the kidney. Elucidation of the molecular pathways controlling TRPV5 function provides important information for our understanding of renal Ca2+ handling, since active Ca2+ reabsorption fine-tunes the final amount of Ca2+ excreted into the urine. Over the last years, the molecular regulation of TRPV5 has been dismantled in detail. Various calciotropic hormones, known to alter renal Ca2+ reabsorption, affect the expression of TRPV5. Others stimulate the trafficking of TRPV5 to the plasma membrane, while a number of associated proteins and ions control channel activity at the plasma membrane. Dynamic cell surface presence of TRPV5 is largely mediated by endosomal recycling processes allowing internalized channels to reappear at the plasma membrane. We present recently identified factors shown to modulate TRPV5 activity by diverse mechanisms to ultimately control renal Ca2+ handling. The selected factors include klotho, tissue kallikrein, pH, Ca2+, Mg2+, PIP2 and WNK4. This review covers the distinctive properties and regulation of the highly Ca2+-selective TRPV5 channel and highlights the implications for our understanding of the process of Ca2+ reabsorption. Ca2+ ions play a crucial role in various cellular processes and extracellular Ca2+ levels are kept within a narrow range through a concerted interplay of several organs, including bone, intestine, and kidney.1.Frick K.K. Bushinsky D.A. Molecular mechanisms of primary hypercalciuria.J Am Soc Nephrol. 2003; 14: 1082-1095Crossref PubMed Scopus (109) Google Scholar,2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar Disturbances in the overall Ca2+ balance could affect many vital physiological functions, including intracellular signaling, muscle contraction, neuron excitation, and bone mineralization. Ca2+ is absorbed from the diet to enter the blood compartment and excreted through the urine, which is tightly controlled by the kidney through the reabsorption of Ca2+ from the filtrate. The majority of Ca2+ reabsorption takes place in the proximal tubules and the thick ascending limb of Henle through the passive paracellular route. In this part of the nephron, Ca2+ follows local Na+ reabsorption and thus Ca2+ reabsorption is not specifically regulated here. The remaining ∼15% of Ca2+ is taken up by means of active transcellular transport.2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar The latter process is restricted to the distal convoluted tubule (DCT) and the connecting tubule (CNT).2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar Transepithelial Ca2+ reabsorption occurs in a series of consecutive steps: driven by a highly favorable electrochemical driving force Ca2+ enters the cell through the apical transient receptor potential vanilloid 5 (TRPV5) Ca2+ channel. In the cell, calbindin-D28K binds Ca2+ and thereby enlarges the capacity for Ca2+ diffusion from the apical toward the basolateral compartment. The binding kinetics of calbindin-D28K are such that it allows intracellular Ca2+ signaling to operate independently of transcellular Ca2+ movement.3.Koster H.P. Hartog A. Van Os C.H. et al.Calbindin-D28K facilitates cytosolic calcium diffusion without interfering with calcium signaling.Cell Calcium. 1995; 18: 187-196Crossref PubMed Scopus (47) Google Scholar Finally, Ca2+ is transported over the basolateral membrane by the Na+/Ca2+ exchanger (NCX1) and the plasma membrane Ca2+-ATPase (PMCA1b) to re-enter the bloodstream (Figure 1).1.Frick K.K. Bushinsky D.A. Molecular mechanisms of primary hypercalciuria.J Am Soc Nephrol. 2003; 14: 1082-1095Crossref PubMed Scopus (109) Google Scholar,2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar TRPV5 belongs to the TRP superfamily, which consists of cation-selective ion channels with similar molecular structures.4.Montell C. The venerable inveterate invertebrate TRP channels.Cell Calcium. 2003; 33: 409-417Crossref PubMed Scopus (64) Google Scholar Of all TRP channels, TRPV6 holds the highest homology with TRPV5.2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar These Ca2+-selective channels share biophysical properties highly distinct from other TRP channels but, in contrast to TRPV5, TRPV6 is more ubiquitously expressed.2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar The critical role of TRPV5 in renal Ca2+ handling was demonstrated by the generation of mice lacking this Ca2+ channel.5.Hoenderop J.G. van Leeuwen J.P. van der Eerden B.C. et al.Renal Ca2+ wasting, hyperabsorption, and reduced bone thickness in mice lacking TRPV5.J Clin Invest. 2003; 112: 1906-1914Crossref PubMed Scopus (368) Google Scholar These mice displayed several phenotypic features related to a diminished active Ca2+ reabsorption. TRPV5-knockout (TRPV5−/−) mice exhibited marked hypercalciuria, albeit their blood Ca2+ levels were kept within the normal range. Additionally, a reduced trabecular and cortical bone thickness indicated disturbed bone morphology. Finally, intestinal Ca2+ absorption was significantly increased, likely compensating for the impaired renal Ca2+ reabsorption.5.Hoenderop J.G. van Leeuwen J.P. van der Eerden B.C. et al.Renal Ca2+ wasting, hyperabsorption, and reduced bone thickness in mice lacking TRPV5.J Clin Invest. 2003; 112: 1906-1914Crossref PubMed Scopus (368) Google Scholar The gene encoding TRPV5 consists of 15 exons, which translates into a protein of 729 amino acids in human.2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar This protein contains six putative transmembrane domains and an intracellular N and C tail. Functional TRPV5 channels exist as tetramers forming together a single Ca2+-selective pore.2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar The pore is formed by the hydrophobic region between TM5 and TM6.2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar The pore-residue aspartate-542 appeared to be crucial for high-affinity Ca2+ binding and selectivity. Mutation of this aspartate into an alanine abolished Ca2+ permeation, whereas the current carried by monovalent cations remained intact.6.Nilius B. Vennekens R. Prenen J. et al.The single pore residue Asp542 determines Ca2+ permeation and Mg2+ block of the epithelial Ca2+ channel.J Biol Chem. 2001; 276: 1020-1025Crossref PubMed Scopus (154) Google Scholar Early patch clamp studies revealed a constitutively open channel, as no stimulus or ligand was required for TRPV5-mediated Ca2+ entry.6.Nilius B. Vennekens R. Prenen J. et al.The single pore residue Asp542 determines Ca2+ permeation and Mg2+ block of the epithelial Ca2+ channel.J Biol Chem. 2001; 276: 1020-1025Crossref PubMed Scopus (154) Google Scholar Furthermore, the subcellular localization of TRPV5 was studied using fluorescence microscopy and enhanced green fluorescent protein (eGFP)-tagged TRPV5. Surprisingly, distinct plasma membrane localization could not be detected, as eGFP-TRPV5 appeared to be primarily located in intracellular vesicles.7.Van de Graaf S.F. Rescher U. Hoenderop J.G. et al.TRPV5 is internalized via clathrin-dependent endocytosis to enter a Ca2+-controlled recycling pathway.J Biol Chem. 2007; 283: 4077-4086Crossref PubMed Scopus (32) Google Scholar,8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar Although not visible, cell surface expression of eGFP-TRPV5 was confirmed with biochemical and functional studies.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar A similar intracellular distribution was observed for endogenous TRPV5 in primary cultures of rabbit CNT and cortical collecting duct (CCD) cells.9.van de Graaf S.F. Chang Q. Mensenkamp A.R. et al.Direct interaction with Rab11a targets the epithelial Ca2+ channels TRPV5 and TRPV6 to the plasma membrane.Mol Cell Biol. 2006; 26: 303-312Crossref PubMed Scopus (91) Google Scholar The existence of active Ca2+ reabsorption in the CCD is specific for rabbit, whereas in other species active Ca2+ transport is predominantly located in late DCT and CNT. Altogether, TRPV5 expression at the plasma membrane is limited providing a mechanism allowing controlled Ca2+ influx. Investigations of the trafficking of TRPV5 to the plasma membrane have shed light on the cellular distribution of the channel. Total internal reflection fluorescence microscopy revealed highly mobile eGFP-TRPV5-containing vesicles in the region close to the plasma membrane.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar Presence of TRPV5 in the plasma membrane appeared to be highly dynamic.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar After an irreversible block of TRPV5 channels present at the cell surface, the TRPV5 activity restored for ∼50% within a 10 min recovery period.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar This likely resulted from rapid insertion of new, unblocked channels. In addition, Van de Graaf et al.7.Van de Graaf S.F. Rescher U. Hoenderop J.G. et al.TRPV5 is internalized via clathrin-dependent endocytosis to enter a Ca2+-controlled recycling pathway.J Biol Chem. 2007; 283: 4077-4086Crossref PubMed Scopus (32) Google Scholar recently identified several mechanisms underlying TRPV5 endocytosis. After reaching the cell surface, the TRPV5 channel was internalized through dynamin- and clathrin-dependent processes (Figure 1). First, TRPV5 appeared in numerous small vesicles, before entering larger vesicles, where it colocalized with Rab11a, a marker for recycling endosomes.7.Van de Graaf S.F. Rescher U. Hoenderop J.G. et al.TRPV5 is internalized via clathrin-dependent endocytosis to enter a Ca2+-controlled recycling pathway.J Biol Chem. 2007; 283: 4077-4086Crossref PubMed Scopus (32) Google Scholar The majority of internalized TRPV5 entered the recycling pathway, whereas only a minor part was degraded.7.Van de Graaf S.F. Rescher U. Hoenderop J.G. et al.TRPV5 is internalized via clathrin-dependent endocytosis to enter a Ca2+-controlled recycling pathway.J Biol Chem. 2007; 283: 4077-4086Crossref PubMed Scopus (32) Google Scholar TRPV5 channels present in recycling endosomes were able to reach the plasma membrane again through direct interaction with Rab11a (Figure 1). Moreover, the presence of a large and stable recycling pool of TRPV5 channels was demonstrated, as blocking protein biosynthesis did not reduce TRPV5-mediated Ca2+ influx. Currently, it is unknown whether TRPV5 is functional in recycling endosomes. Altogether, the continuous endosomal recycling of internalized TRPV5 channels enables TRPV5 reappearance at the plasma membrane providing a synthesis-independent pathway for the apical influx of Ca2+. Facing the extracellular environment, TRPV5 channels exhibit closed and open states, during which Ca2+ enters the cell. The probability of a channel being open is termed open probability (Po), whereas conductance is the amount of current during the open state. Taken together, conductance and Po determine TRPV5 single-channel activity. Here, we present an overview of factors shown to regulate TRPV5 function by changing plasma membrane abundance or single-channel activity. These factors include klotho, tissue kallikrein (TK), pH, Ca2+, PIP2, Mg2+, and WNK4. Furthermore, cAMP-elevating hormones, such as parathyroid hormone (PTH), rapidly affect Ca2+ reabsorption in the distal part of the nephron.2.Van de Graaf S.F. Bindels R.J. Hoenderop J.G. Physiology of epithelial Ca2+ and Mg2+ transport.Rev Physiol Biochem Pharmacol. 2007; 158: 77-160Crossref PubMed Scopus (59) Google Scholar,10.Shimizu T. Yoshitomi K. Nakamura M. et al.Effects of PTH, calcitonin, and cAMP on calcium transport in rabbit distal nephron segments.Am J Physiol. 1990; 259: F408-F414PubMed Google Scholar,11.Griffiths N.M. Brick-Ghannam C. Siaume-Perez S. et al.Effect of prostaglandin E2 on agonist-stimulated cAMP accumulation in the distal convoluted tubule isolated from the rabbit kidney.Pflugers Arch. 1993; 422: 577-584Crossref PubMed Scopus (11) Google Scholar The potential role of TRPV5 in this cAMP-dependent process will also be discussed. Recent studies identified the antiaging hormone klotho as an important player in Ca2+ homeostasis. First, klotho-deficient mice exhibit a reduced bone mineral density and a mild hypercalcemia.12.Kuro-o M. Klotho as a regulator of fibroblast growth factor signaling and phosphate/calcium metabolism.Curr Opin Nephrol Hypertens. 2006; 15: 437-441Crossref PubMed Scopus (186) Google Scholar Second, the active form of vitamin D, released upon Ca2+ depletion, increased renal klotho expression.13.Yoshida T. Fujimori T. Nabeshima Y. Mediation of unusually high concentrations of 1,25-dihydroxyvitamin D in homozygous klotho mutant mice by increased expression of renal 1alpha-hydroxylase gene.Endocrinology. 2002; 143: 683-689Crossref PubMed Scopus (168) Google Scholar Klotho is a single-pass transmembrane protein containing a large extracellular domain, which exhibits β-glucuronidase activity upon cleavage.14.Tohyama O. Imura A. Iwano A. et al.Klotho is a novel beta-glucuronidase capable of hydrolyzing steroid beta-glucuronides.J Biol Chem. 2004; 279: 9777-9784Crossref PubMed Scopus (177) Google Scholar In 2005, Chang et al.15.Chang Q. Hoefs S. van der Kemp A.W. et al.The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel.Science. 2005; 310: 490-493Crossref PubMed Scopus (477) Google Scholar demonstrated a stimulatory effect of klotho on TRPV5 activity, whereas overexpression of klotho markedly increased TRPV5-mediated 45Ca2+ uptake. In agreement, transepithelial Ca2+ transport in primary cultures of CNT and CCD cells was significantly elevated after treatment with supernatant from klotho-secreting cells.15.Chang Q. Hoefs S. van der Kemp A.W. et al.The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel.Science. 2005; 310: 490-493Crossref PubMed Scopus (477) Google Scholar This stimulatory effect of klotho was mimicked by extracellular application of purified β-glucuronidase, suggesting that the β-glucuronidase activity of klotho is responsible for TRPV5 activation.15.Chang Q. Hoefs S. van der Kemp A.W. et al.The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel.Science. 2005; 310: 490-493Crossref PubMed Scopus (477) Google Scholar Together with the detection of klotho in urine and serum, these data suggested that this antiaging hormone operates from the extracellular compartment through its enzymatic activity. This was further substantiated by mutation of the conserved N-glycosylation site (N358) of TRPV5, located in the extracellular loop between TM1 and TM2. The activity of N358Q-TRPV5 was not different from wild-type TRPV5; however, the stimulatory effect of klotho was abolished.15.Chang Q. Hoefs S. van der Kemp A.W. et al.The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel.Science. 2005; 310: 490-493Crossref PubMed Scopus (477) Google Scholar Cell surface protein biotinylation studies elucidated the molecular mechanism by which klotho stimulates TRPV5 activity. Klotho treatment for 16 h resulted in significant elevation of TRPV5 channels present at the plasma membrane.15.Chang Q. Hoefs S. van der Kemp A.W. et al.The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel.Science. 2005; 310: 490-493Crossref PubMed Scopus (477) Google Scholar Taken together, these data indicate that modification of TRPV5 glycosylation through locally excreted klotho induces channel accumulation in the plasma membrane of DCT and CNT. In the absence of klotho, TRPV5 may not be well expressed at the luminal membrane resulting in Ca2+ wasting as demonstrated by an increased fractional excretion of Ca2+ in klotho-deficient mice.16.Tsuruoka S. Nishiki K. Ioka T. et al.Defect in parathyroid-hormone-induced luminal calcium absorption in connecting tubules of Klotho mice.Nephrol Dial Transplant. 2006; 21: 2762-2767Crossref PubMed Scopus (32) Google Scholar A marked hypercalciuria as observed in TK-deficient mice, implicating TK as a potential regulator of TRPV5 activity.17.Picard N. Van Abel M. Campone C. et al.Tissue kallikrein-deficient mice display a defect in renal tubular calcium absorption.J Am Soc Nephrol. 2005; 16: 3602-3610Crossref PubMed Scopus (41) Google Scholar Indeed, TK was further identified as a stimulator of transcellular Ca2+ reabsorption by enhancing the plasma membrane expression of TRPV5.18.Gkika D. Topala C.N. Chang Q. et al.Tissue kallikrein stimulates Ca2+ reabsorption via PKC-dependent plasma membrane accumulation of TRPV5.EMBO J. 2006; 25: 4707-4716Crossref PubMed Scopus (61) Google Scholar TK is a serine protease that is produced as an inactive precursor in the CNT. Upon cleavage, TK is secreted into the urine and can indirectly, through the conversion of kinogens to kinins, or directly by itself, activate kinin receptors like the bradykinin (BK) 2 receptor. Activation of the BK 2 receptor is known to activate phospholipase C (PLC) by Gα signaling. This results in the formation of diacylglycerol, which in turn activates protein kinase C (PKC). To investigate whether TRPV5 activity was regulated through this pathway, first, cells were pretreated with the PLC inhibitor U73122. This abolished the previously observed stimulation of TRPV5 activity.18.Gkika D. Topala C.N. Chang Q. et al.Tissue kallikrein stimulates Ca2+ reabsorption via PKC-dependent plasma membrane accumulation of TRPV5.EMBO J. 2006; 25: 4707-4716Crossref PubMed Scopus (61) Google Scholar Second, application of a cell permeable analog of diacylglycerol mimicked TK stimulation.18.Gkika D. Topala C.N. Chang Q. et al.Tissue kallikrein stimulates Ca2+ reabsorption via PKC-dependent plasma membrane accumulation of TRPV5.EMBO J. 2006; 25: 4707-4716Crossref PubMed Scopus (61) Google Scholar Third, the role of PKC was investigated, by mutating six putative PKC phosphorylation sites in TRPV5 into alanines. Detailed functional analysis revealed two PKC phosphorylation sites to be essential for TK-mediated TRPV5 stimulation.18.Gkika D. Topala C.N. Chang Q. et al.Tissue kallikrein stimulates Ca2+ reabsorption via PKC-dependent plasma membrane accumulation of TRPV5.EMBO J. 2006; 25: 4707-4716Crossref PubMed Scopus (61) Google Scholar This stimulatory effect is most probably due to an accumulation of TRPV5 at the plasma membrane, as shown by cell surface biotinylation studies.18.Gkika D. Topala C.N. Chang Q. et al.Tissue kallikrein stimulates Ca2+ reabsorption via PKC-dependent plasma membrane accumulation of TRPV5.EMBO J. 2006; 25: 4707-4716Crossref PubMed Scopus (61) Google Scholar In addition, a delayed degradation of biotinylated TRPV5 channels was observed, suggesting that TK either delays TRPV5 retrieval from the plasma membrane or enhances the recycling capacity for TRPV5. Recently, Cha et al.19.Cha S.K. Wu T. Huang C.L. Protein kinase C inhibits caveolae-mediated endocytosis of TRPV5.Am J Physiol Renal Physiol. 2008; 294: F1212-F1221Crossref PubMed Scopus (83) Google Scholar demonstrated an essential role for caveolin-1 in the PKC-mediated TRPV5 plasma membrane accumulation. The TK effect was further studied in ex vivo conditions using primary cultures of rabbit CNT and CCD cells. Only apical application of TK elevated Ca2+ transport, as basolateral addition had no effect.18.Gkika D. Topala C.N. Chang Q. et al.Tissue kallikrein stimulates Ca2+ reabsorption via PKC-dependent plasma membrane accumulation of TRPV5.EMBO J. 2006; 25: 4707-4716Crossref PubMed Scopus (61) Google Scholar Furthermore, urinary TK excretion was increased in TRPV5−/− mice, suggesting a physiological feedback mechanism that activates TK secretion upon hypercalciuria. The difference in TK excretion between control and TRPV5−/− mice was abolished when these mice were fed a high Ca2+ diet, which implies a role for TK in the maintenance of the Ca2+ balance.18.Gkika D. Topala C.N. Chang Q. et al.Tissue kallikrein stimulates Ca2+ reabsorption via PKC-dependent plasma membrane accumulation of TRPV5.EMBO J. 2006; 25: 4707-4716Crossref PubMed Scopus (61) Google Scholar Taken together, these data indicate that TK acts in an autocrine/paracrine manner to increase active Ca2+ reabsorption through an increased TRPV5 plasma membrane expression. Changes in acid-base balance are known to affect Ca2+ homeostasis. Chronic metabolic acidosis induced Ca2+ loss from bone and elevated renal Ca2+ excretion.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar Alterations in acid–base status affected Ca2+ reabsorption in the DCT and CNT, as observed by micropuncture studies.20.Sutton R.A. Wong N.L. Dirks J.H. Effects of metabolic acidosis and alkalosis on sodium and calcium transport in the dog kidney.Kidney Int. 1979; 15: 520-533Abstract Full Text PDF PubMed Scopus (200) Google Scholar Further, transcellular Ca2+ transport at pH 5.6 in primary cultures of rabbit CNT and CCD cells was markedly lowered in comparison to transport measured at pH 7.5.21.Bindels R.J. Hartog A. Abrahamse S.L. et al.Effects of pH on apical calcium entry and active calcium transport in rabbit cortical collecting system.Am J Physiol. 1994; 266: F620-F627PubMed Google Scholar In addition to transepithelial transport, the apical 45Ca2+ influx in these renal cells exhibited a similar sensitivity to extracellular pH. Recently, several molecular mechanisms underlying the pH-sensitive regulation of the active reabsorption of Ca2+ were identified by the discovery of multiple pH-dependent pathways directly controlling TRPV5. First, TRPV5 protein expression appeared to be under direct control of the acid–base states.22.Nijenhuis T. Renkema K.Y. Hoenderop J.G. et al.Acid–base status determines the renal expression of Ca2+ and Mg2+ transport proteins.J Am Soc Nephrol. 2006; 17: 617-626Crossref PubMed Scopus (105) Google Scholar Second, Lambers et al.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar proposed a dynamic insertion of TRPV5-containing vesicles into the plasma membrane as a new mechanism by which extracellular pH determines TRPV5 activity. Using total internal reflection fluorescence microscopy, the trafficking of eGFP-TRPV5-containing vesicles toward the plasma membrane was monitored at acidic (6.0) and alkaline (8.5) pH. Upon extracellular alkalinization, a pool of TRPV5-containing vesicles was rapidly recruited to the cell surface without collapsing into the plasma membrane, as reflected by the lack of fluorescence diffusion after reaching the plasma membrane.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar These vesicles contained functional TRPV5 channels as extracellular alkalinization was accompanied by an increased TRPV5 activity, as measured electrophysiologically and by 45Ca2+ uptake assays.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar A significant reduction of the pH readily induced the internalization of TRPV5-containing vesicles from the plasma membrane, diminishing the channel activity. Plasma membrane retrieval of TRPV5 has been verified by cell surface biotinylation and total internal reflection fluorescence analysis of the lateral motility of vesicles. Extracellular acidification strongly increased the motility of single vesicles, referring to TRPV5 internalization.8.Lambers T.T. Oancea E. de Groot T. et al.Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels.Mol Cell Biol. 2007; 27: 1486-1494Crossref PubMed Scopus (55) Google Scholar In conclusion, the increased Ca2+ excretion upon urinary acidosis is probably in part due to the dynamic retrieval of TRPV5-containing vesicles from the plasma membrane. Third, both intra- and extracellular acidification reduced TRPV5 single-channel activity, resulting from a diminished Po and single-channel conductance.23.Yeh B.I. Sun T.J. Lee J.Z. et al.Mechanism and molecular determinant for regulation of rabbit transient receptor potential type 5 (TRPV5) channel by extracellular pH.J Biol Chem. 2003; 278: 51044-51052Crossref PubMed Scopus (92) Google Scholar Two amino acids in the TRPV5 protein appeared to function as pH sensors. Binding of protons to glutamate-522 or lysine-607, which localize to the extra- and intracellular environment, respectively, resulted in decreased TRPV5 activity.23.Yeh B.I. Sun T.J. Lee J.Z. et al.Mechanism and molecular determinant for regulation of rabbit transient receptor potential type 5 (TRPV5) channel by extracellular pH.J Biol Chem. 2003; 278: 51044-51052Crossref PubMed Scopus (92) Google Scholar,24.Yeh B.I. Kim Y.K. Jabbar W. et al.Conformational changes of pore helix coupled to gating of TRPV5 by protons.EMBO J. 2005; 24: 3224-3234Crossref PubMed Scopus (75) Google Scholar Yeh et al.24.Yeh B.I. Kim Y.K. Jabbar W. et al.Conformational changes of pore helix coupled to gating of TRPV5 by protons.EMBO J. 2005; 24: 3224-3234Crossref PubMed Scopus (75) Google Scholar proposed a model in which binding of protons to either sensor induces a conformational change of the TRPV5 pore helix, leading to a lowered channel activity. Mutating the extracellular pH sensor glutamate-522 into glutamine prevented the extracellular acidification-induced reduction of Po, whereas the single-channel conductance was not altered.24.Yeh B.I. Kim Y.K. Jabbar W. et al.Conformational changes of pore helix coupled to gating of TRPV5 by protons.EMBO J. 2005; 24: 3224-3234Crossref PubMed Scopus (75) Google Scholar Also intracellular acidification mainly reduced the TRPV5 activity by decreasing Po.25.Cha S.K. Jabbar W. Xie J. et al.Regulation of TRPV5 single-channel activity by intracellular pH.J Membr Biol. 2007; 220: 79-85Crossref PubMed Scopus (17) Google Scholar Here, inside-out patch clamp recordings identified, besides closed and open states, a subconductance state in which the channel was
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