Biochimica et Biophysica Acta 1833 (2013) 3046–3053 Contents lists available at ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbamcr Flow-induced activation of TRPV5 and TRPV6 channels stimulates Ca2+-activated K+ channel causing membrane hyperpolarization Seung-Kuy Cha a,b,⁎,Ji-HeeKima, Chou-Long Huang b,⁎⁎ a Department of Physiology, Institute of Lifestyle Medicine and Nuclear Receptor Research Consortium, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea b Department of Internal Medicine, UT Southwestern Medical Center, Dallas, TX 75390, USA article info abstract Article history: TRPV5 and TRPV6 channels are expressed in distal renal tubules and play important roles in the transcellular Received 20 May 2013 Ca2+ reabsorption in kidney. They are regulated by multiple intracellular factors including protein kinases A Received in revised form 9 August 2013 2+ 2+ and C, membrane phospholipid PIP2, protons, and divalent ions Ca and Mg . Here, we report that fluid Accepted 26 August 2013 flow that generates shear force within the physiological range of distal tubular fluid flow activated TRPV5 and Available online 31 August 2013 TRPV6 channels expressed in HEK cells. Flow-induced activation of channel activity was reversible and did not desensitize over 2 min. Fluid flow stimulated TRPV5 and 6-mediated Ca2+ entry and increased intracellular Keywords: 2+ fi fl fl TRPV5 Ca concentration. N-glycosylation-de cient TRPV5 channel was relatively insensitive to uid ow. In cells TRPV6 coexpressing TRPV5 (or TRPV6) and Slo1-encoded maxi-K channels, fluid flow induced membrane hyperpolari- Flow-mediated Ca2+ entry zation, which could be prevented by the maxi-K blocker iberiotoxin or TRPV5 and 6 blocker La3+.Incontrast, Ca2+-activated K+ channel fluid flow did not cause membrane hyperpolarization in cells coexpressing ROMK1 and TRPV5 or 6 channel. + Flow-mediated K secretion These results reveal a new mechanism for the regulation of TRPV5 and TRPV6 channels. Activation of TRPV5 ROMK and TRPV6 by fluid flow may play a role in the regulation of flow-stimulated K+ secretion via maxi-K channels in distal renal tubules and in the mechanism of pathogenesis of thiazide-induced hypocalciuria. © 2013 Elsevier B.V. All rights reserved. 1. Introduction acid sequence homology and present in many epithelial tissues in- cluding the kidney [10]. Compared to other TRPV subfamily members, Mechanosensation is an important mechanism for cell communica- TRPV5 and 6 are highly Ca2+-selective with the relative Ca2+ to Na+ tion with the external environment. Mechanosensitive ion channels permeability ~100 [11]. TRPV5 and 6 channels are regulated by diverse 2+ 2+ play important roles in these processes by transducing mechanical inputs mechanisms including Ca ,Mg , proton, PIP2 and protein kinases into electrical signals [1,2]. Members of the transient receptor potential [12]. Mechanosensitivity for TRPV5 and 6 and their regulation by fluid (TRP) channel superfamily have been implicated in a wide range of flow and shear stress have not been described. mechanotransduction in diverse organs and species [1]. Mammalian Normally, the kidney is the principal organ in the body for excreting TRP channels are grouped into six subfamilies based on amino acid dietary K+ load. Renal K+ excretion occurs predominantly by secretion sequence homology and biophysical and pharmacological properties: in the distal renal tubular segments including late distal convoluted TRPC (canonical or classic), TRPV (vanilloid), TRPM (melastatin), tubule (DCT2), connecting tubule (CNT), and cortical collecting duct TRPML (mucolipin), TRPA (ankyrin) and TRPP (polycystin) [3].Within (CCD) collectively referred to as the aldosterone-sensitive distal neph- the TRPV subfamily, there are six mammalian members, TRPV1–6. ron (ASDN) [13].Thefinal step of K+ secretion in ASDN involves efflux Among them, TRPV1 through 4 are known to be regulated by physical into luminal space through the apical K+ channels, which include the factors such as hypotonicity, stretch and/or temperature [4,5]. Pertinent renal outer medullary K+ channel ROMK1 and the Ca2+-activated to this study, TRPV4 is activated by shear stress generated by fluid flow maxi-K channels. ROMK is constitutively open at baseline and mainly and its function as a renal tubular fluid flow sensor has been demon- responsible for K+ secretion at low luminal flow rate. Maxi-K is quies- strated [6–9]. TRPV5 and 6 are homologous proteins with ~70% amino cent at baseline but activated by high fluid flow and mainly responsible for flow-stimulated K+ secretion [14]. 2+ fl ⁎ Correspondence to: S.K. Cha, Department of Physiology, Yonsei University Wonju Flow-stimulated Ca in ux across the apical membrane is requisite + College of Medicine, 20 Ilsan-ro, Wonju, Gangwondo, South Korea. Tel.: +82 33 741 for flow-mediated K secretion via maxi-K channel in ASDN. Several 0295; fax: +82 33 745 6461. channels including TRPV4 and polycystin-2 (TRPP2) have been pro- ⁎⁎ Correspondence to: C.L. Huang, UT Southwestern Medical Center, Department of posed as candidates for apical Ca2+ entry channels [7–9,15,16].Inaddi- Internal Medicine, 5323 Harry Hines Blvd., Dallas, TX 75390-8856, USA. Tel.: +1 214 tion, TRPV4 and TRPP2 may form a polymodal sensory channel complex 648 8627; fax: +1 214 648 2071. E-mail addresses: [email protected] (S.-K. Cha), and both or each channel subunit may be responsible for activation [email protected] (C.-L. Huang). by flow [7,16]. TRPV5 and 6 are also expressed in apical membrane of 0167-4889/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bbamcr.2013.08.017 S.-K. Cha et al. / Biochimica et Biophysica Acta 1833 (2013) 3046–3053 3047 ASDN and mediate transcellular Ca2+ reabsorption in this part of neph- 2.5. Solutions for electrophysiological recordings ron [12]. Here, we report that TRPV5 and TRPV6 are activated by fluid flow and flow-induced Ca2+ influx through the channels can stimulate For recording of TRPV5 and 6 currents, the pipette and bath solution maxi-K channel allowing K+ efflux and causing membrane hyperpolar- contained (in mM) 140 Na-Asp (sodium aspartate), 10 NaCl, 10 EDTA, ization. These results suggest that TRPV5 and TRPV6 may participate in and10HEPES(pH7.4)and140Na-Asp,10NaCl,1EDTA,and10HEPES the flow-stimulated K+ secretion in ASDN. (pH 7.4), respectively. For the recording of ROMK1 current, the pipette and bath solution contained (in mM) 140 KCl and 10 HEPES (pH 7.2) and 140 KCl, 1 MgCl , 1 CaCl , and 10 HEPES (pH 7.4), respectively. 2. Materials and methods 2 2 For current-clamp recordings, patch pipettes were filled with a solu- tion containing (in mM) 140 KCl, 5 EGTA, 10 HEPES, 0.5 CaCl ,and 2.1. DNA constructs and materials 2 5 NaCl (pH 7.2). A normal physiological salt solution (PSS) was used for bath solution that contained 135 NaCl, 5.4 KCl, 1 MgCl , 2 CaCl , GFP-tagged wild-type and N358Q mutant TRPV5, and ROMK1 have 2 2 5 HEPES, and 10 glucose (pH 7.4). been described previously [17–19]. Complementary DNA for TRPV6 is in a pEGFP-N3 vector. Slo1 channel was provided by Dr Chul-Seung 2.6. Intracellular Ca2+ ([Ca2+] ) measurement Park [20]. Fura2/AM was obtained from Molecular Probes® (Invitrogen, i Carlsbad, CA, USA). Gramicidin D and iberiotoxin were purchased from Intracellular Ca2+ concentration ([Ca2+] ) was measured using Sigma-Aldrich Co. (St. Louis, MO, USA) and Tocris Bioscience (Bristol, i Ratiomaster (Photon Technology International, Lawrencevill, NJ, USA) UK), respectively. or the Lambda DG-4 (Sutter Instruments, Novato, CA, USA) fluorescence measurement system. Cells were placed on glass coverslips and loaded 2.2. Cell culture, transfection and western blot with fura-2/AM in a normal PSS in darkness for 1 h at room tempera- ture. After dye loading, cells were washed and transferred to a perfusion HEK293 cells were cultured as described previously [18].Cells chamber on a fluorescence microscope. Fura-2 signals were obtained were transiently transfected with cDNAs as indicated in each experi- by alternating excitation at 340 or 380 nm, and detecting emission at 2+ ment. Transfection was performed using FuGENE-HD reagent (Roche, 510 nm. The fura-2 fluorescence ratio was calibrated to [Ca ]i as de- Somerville, NJ, USA) according to the manufacturer's instructions. In scribed by Grynkiewicz et al. [22]. Data were analyzed using either Felix each experiment, the total amount of DNA for transfection was balanced (Photon Technology International) or MetaFluor (Sutter Instruments) by using empty vectors. The expression of TRPV5 and TRPV6 proteins in software. transfected cells was detected by western blot using a rabbit polyclonal anti-GFP antibody as described previously [18]. 2.7. Data analysis Data are presented as mean ± SEM. Statistical comparisons between 2.3. Application of fluid flow two groups of data were made using a two-tailed unpaired Student's t-test. Multiple comparisons were determined using one-way ANOVA Pressurized flows of solutions were applied onto the single cells followed by Tukey's multiple comparison tests. Statistical significance through a microbarrel (inner diameter ~250 μm). The tip of barrel was defined as p b 0.05 for a single comparison and p b 0.01 for multiple was placed around 100 μm from the cell. Fluid flow was gravity driven comparisons. (25–85 μl/min) to generate shear stresses from 0.34 to 1.15 dyn/cm2. The shear pressure was calculated for fluid flow in cylindrical tubes 3. Results according to the equation τ =6μQ/πr3,whereτ is the shear stress or pressure (dyn/cm2), μ represents the fluid viscosity, Q is the flow 3.1. Fluid flow activates TRPV5 and increases TRPV5-mediated Ca2+ influx rate (ml/s), and r is the internal radius of the tube [21].