Volume regulation and intracellular calcium in the rabbit proximal convoluted tubule

Piezo1, the new actor in cell volume regulation

All animal cells control their volume through a complex set of mechanisms, both to counteract osmotic perturbations of the environment and to enable numerous vital biological processes, such as proliferation, apoptosis, and migration. The ability of cells to adjust their volume depends on the activity of ion channels and transporters which, by moving K+, Na+, and Cl- ions across the plasma membrane, generate the osmotic gradient that drives water in and out of the cell. In 2010, Patapoutian's group identified a small family of evolutionarily conserved, Ca2+-permeable mechanosensitive channels, Piezo1 and Piezo2, as essential components of the mechanically activated current that mediates mechanotransduction in vertebrates. Piezo1 is expressed in several tissues and its opening is promoted by a wide range of mechanical stimuli, including membrane stretch/deformation and osmotic stress. Piezo1-mediated Ca2+ influx is used by the cell to convert mechanical forces into cytosolic Ca2+ signals that control diverse cellular functions such as migration and cell death, both dependent on changes in cell volume and shape. The crucial role of Piezo1 in the regulation of cell volume was first demonstrated in erythrocytes, which need to reduce their volume to pass through narrow capillaries. In HEK293 cells, increased expression of Piezo1 was found to enhance the regulatory volume decrease (RVD), the process whereby the cell re-establishes its original volume after osmotic shock-induced swelling, and it does so through Ca2+-dependent modulation of the volume-regulated anion channels. More recently we reported that Piezo1 controls the RVD in glioblastoma cells via the modulation of Ca2+-activated K+ channels. To date, however, the mechanisms through which this mechanosensitive channel controls cell volume and maintains its homeostasis have been poorly investigated and are still far from being understood. The present review aims to provide a broad overview of the literature discussing the recent advances on this topic.

Role of AQP2 in activation of calcium entry by hypotonicity: implications in cell volume regulation

We previously reported in a rat cortical collecting duct cell line (RCCD1) that the presence of aquaporin 2 (AQP2) in the cell membrane is critical for the rapid activation of regulatory volume decrease mechanisms (RVD) (Ford et al. Biol Cell 97: 687–697, 2005). The aim of our present work was to investigate the signaling pathway that links AQP2 to this rapid RVD activation. Since it has been previously described that hypotonic conditions induce intracellular calcium ([Ca2+]i) increases in different cell types, we tested the hypothesis that AQP2 could have a role in activation of calcium entry by hypotonicity and its implication in cell volume regulation. Using a fluorescent probe technique, we studied [Ca2+]iand cell volume changes in response to a hypotonic shock in WT-RCCD1(not expressing aquaporins) and in AQP2-RCCD1(transfected with AQP2) cells. We found that after a hypotonic shock only AQP2-RCCD1cells exhibit a substantial increase in [Ca2+]i. This [Ca2+]iincrease is strongly dependent on extracellular Ca2+and is partially inhibited by thapsigargin (1 μM) indicating that the rise in [Ca2+]ireflects both influx from the extracellular medium and release from intracellular stores. Exposure of AQP2-RCCD1cells to 100 μM gadolinium reduced the increase in [Ca2+]isuggesting the involvement of a mechanosensitive calcium channel. Furthermore, exposure of cells to all of the above described conditions impaired rapid RVD. We conclude that the expression of AQP2 in the cell membrane is critical to produce the increase in [Ca2+]iwhich is necessary to activate RVD in RCCD1cells.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

An increase in intracellular calcium concentration that is induced by basolateral CO2 in rabbit renal proximal tubule

Working with isolated perfused S2 proximal tubules, we asked whether the basolateral CO2 sensor acts, in part, by raising intracellular Ca2+ concentration ([Ca2+]i), monitored with the dye fura 2 (or fura-PE3). In paired experiments, adding 5% CO2/22 mM HCO3- (constant pH 7.40) to the bath (basolateral) solution caused [Ca2+]i to increase from 57 +/- 3 to 97 +/- 9 nM(n = 8, P < 0.002), whereas the same maneuver in the lumen had no effect. Intracellular pH (pHi), measured with the dye BCECF, fell by 0.54 +/- 0.08 (n = 14) when we added CO2/HCO3- to the lumen. In 14 tubules in which we added CO2/HCO3- to the bath, pHi fell by 0.55 +/- 0.11 in 9 with a high initial pHi, but rose by 0.28 +/- 0.07 in the other 5 with a low initial pHi. Thus it cannot be a pHi change that triggers the [Ca2+]i increase. Introducing to the bath an out-of-equilibrium (OOE) solution containing 20% CO2/no HCO3-/pH 7.40 caused [Ca2+]i to rise by 62 +/- 17 nM (n = 10), whereas an OOE solution containing 0% CO2/22 mM HCO3-/pH 7.40 caused only a trivial increase. Removing Ca2+ from the lumen and bath, or adding 10 microM nifedipine (L- and T-type Ca2+-channel blocker) or 2 microM thapsigargin [sarco-(endo) plasmic reticulum Ca2+-ATPase inhibitor] or 4 microM rotenone (mitochondrial inhibitor) to the lumen and bath, failed to reduce the CO2-induced increase in [Ca2+]i. Adding 10 mM caffeine (ryanodine-receptor agonist) had no effect on [Ca2+]i. Thus basolateral CO2, presumably via a basolateral sensor, triggers the release of Ca2+ from a nonconventional intracellular pool.

Potassium channels in basolateral membrane vesicles from necturus enterocytes: stretch and ATP sensitivity

We have previously reported that ATP-inhibitable K(+) channels, in vesicles derived from the basolateral membrane of Necturus maculosus small intestinal cells, exhibit volume regulatory responses that resemble those found in the intact tissue after exposure to anisotonic solutions. We now report that increases in K(+) channel activity can also be elicited by exposure of these vesicles to isotonic solutions containing glucose or alanine that equilibrate across these membranes. We also demonstrate that swelling after exposure to a hypotonic solution or an isotonic solution containing alanine or glucose reduces inhibition of channel activity by ATP and that this finding cannot be simply attributed to dilution of intravesicular ATP. We conclude that ATP-sensitive, stretch-activated K(+) channels may be responsible for the well-established increase in basolateral membrane K(+) conductance of Necturus small intestinal cells after the addition of sugars or amino acids to the solution perfusing the mucosal surface, and we propose that increases in cell volume, resulting in membrane stretch, decreases the sensitivity of these channels to ATP.

The role of Ca2+ in volume regulation induced by Na+-coupled alanine uptake in single proximal tubule cells isolated from frog kidney

1. It has been suggested that epithelial cells maintain cell volume and function, in the face of changes in the rate of transepithelial transport, by activation of volume-regulatory pathways. 2. The aim of the following study was to examine directly the effect of an alteration in Na+-coupled alanine transport on cell length in single proximal tubule cells isolated from frog kidney. 3. An optical technique was used to examine the change in cell length induced by 5 mM L-alanine. 4. On addition of L-alanine to the bath there was an initial increase in cell length to a peak value. This was followed by two types of response. In eighteen out of thirty-one cells a typical volume-regulatory response was observed. The remaining cells showed no volume regulation. 5. Volume regulation was not affected by the removal of extracellular Ca2+. The mean degrees of recovery were 159 +/- 21 % (n = 18) and 144 +/- 18 % (n = 8) in the presence and absence of Ca2+, respectively. 6. Volume regulation was inhibited by depletion of intracellular Ca2+ stores, or in the presence of either Gd3+ or DIDS. The mean degrees of regulation were 55.4 +/- 9.2 % (n = 7), 68.2 +/- 18.8 % (n = 7) and 69.1 +/- 14.3 % (n = 7), respectively. 7. The alanine-induced increases in cell length were both stereospecific and Na+ dependent. 8. The evidence suggests that volume regulation induced by Na+-coupled alanine uptake may be dependent on the release of Ca2+ from intracellular stores. This is in contrast to volume regulation induced by hypotonic shock, which appears to require extracellular Ca2+. Results obtained using a hypotonic shock should, therefore, be viewed with caution.

Regulation of an inwardly rectifying ATP-sensitive K+ channel in the basolateral membrane of renal proximal tubule

Functional coupling of Na+,K+-ATPase pump activity to a basolateral membrane (BLM) K+ conductance is crucial for sustaining transport in the proximal tubule. Apical sodium entry stimulates pump activity, lowering cytosolic [ATP], which in turn disinhibits ATP-sensitive K+ (KATP) channels. Opening of these KATP channels mediates hyperpolarization of the BLM that facilitates Na+ reabsorption and K+ recycling required for continued Na+,K+-ATPase pump turnover. Despite its physiological importance, little is known about the regulation of this channel. The present study focuses on the regulation of the BLM KATP channel by second messengers and protein kinases using membrane patches from dissociated, polarized Ambystoma proximal tubule cells. The channel is regulated by protein kinases A and C, but in opposing directions. The channel is activated by forskolin in cell-attached (c/a) patches, and by PKA in inside-out (i/o) membrane patches. However, phosphorylation by PKA is not sufficient to prevent channel rundown. In contrast, the channel is inhibited by phorbol ester in c/a patches, and PKC decreases channel activity (nPo) in i/o patches. The channel is pH sensitive, and lowering cytosolic pH reduces nPo. Increasing intracellular [Ca2+] ([Ca2+]i) in c/a patches decreases nPo, and this effect is direct since [Ca2+]i inhibits nPo with a Ki of approximately 170 nM in i/o patches. Membrane stretch and hypotonic swelling do not significantly affect channel behavior, but the channel appears to be regulated by the actin cytoskeleton. Finally, the activity of this BLM KATP channel is coupled to transcellular transport. In c/a patches, maneuvers that inhibit turnover of the Na+,K+-ATPase pump reduce nPo, presumably due to a rise in intracellular [ATP], although the associated cell depolarization cannot be ruled out as the possible cause. Conversely, stimulation of transport (and thus pump turnover) leads to increases in nPo, presumably due to a fall in intracellular [ATP]. These results show that the inwardly rectifying KATP channel in the BLM of the proximal tubule is a key element in the feedback system that links cellular metabolism with transport activity. We conclude that coupling of this KATP channel to the activity of the Na+,K+-ATPase pump is a mechanism by which steady state NaCl reabsorption in the proximal tubule may be maintained.

ATP is a coupling modulator of parallel Na,K-ATPase-K-channel activity in the renal proximal tubule

A fundamental and essential property of nearly all salt-transporting epithelia is the tight parallel coupling between the magnitude of the K-conductive pathway at the basolateral membrane and the activity of the Na,K-dependent ATPase (Na,K-ATPase). In the present study, we demonstrate that the coupling response in the renal proximal tubule is governed, at least in part, through the interaction between ATP-sensitive K channels and Na,K-ATPase-mediated changes in intracellular ATP levels. First, we identified a K-selective channel at the basolateral membrane, which is inhibited by the cytosolic addition of ATP. Second, conventional microelectrode analysis in the isolated perfused proximal straight tubule revealed that these channels are the major determinant of the macroscopic K conductance so that ATP-mediated changes in the open probability of the K channel could alter the extent of K recycling. Indeed, the increase in the macroscopic K conductance upon stimulation of transcellular Na transport and pump activity was found to be paralleled by a decrease in intracellular ATP. Finally, a causal link between parallel Na,K-ATPase-K-channel activity and ATP was established by the finding that intracellular ATP loading uncoupled the response. With our recent observations that similar ATP-sensitive K channels are expressed abundantly in other epithelia, we postulate that ATP may act as a universal coupling modulator of parallel Na,K-ATPase-K-channel activity.