Receptor-interacting protein kinase 3 (RIPK3)-mediated necroptosis is thought to be the pathophysiologically predominant pathway that leads to regulated necrosis of parenchymal cells in ischemia-reperfusion injury (IRI), and loss of either Fas-associated protein with death domain (FADD) or caspase-8 is known to sensitize tissues to undergo spontaneous necroptosis. Here, we demonstrate that renal tubules do not undergo sensitization to necroptosis upon genetic ablation of either FADD or caspase-8 and that the RIPK1 inhibitor necrostatin-1 (Nec-1) does not protect freshly isolated tubules from hypoxic injury. In contrast, irondependent ferroptosis directly causes synchronized necrosis of renal tubules, as demonstrated by intravital microscopy in models of IRI and oxalate crystal-induced acute kidney injury. To suppress ferroptosis in vivo, we generated a novel third-generation ferrostatin (termed 16-86), which we demonstrate to be more stable, to metabolism and plasma, and more potent, compared with the firstin-class compound ferrostatin-1 (Fer-1). Even in conditions with extraordinarily severe IRI, 16-86 exerts strong protection to an extent which has not previously allowed survival in any murine setting. In addition, 16-86 further potentiates the strong protective effect on IRI mediated by combination therapy with necrostatins and compounds that inhibit mitochondrial permeability transition. Renal tubules thus represent a tissue that is not sensitized to necroptosis by loss of FADD or caspase-8. Finally, ferroptosis mediates postischemic and toxic renal necrosis, which may be therapeutically targeted by ferrostatins and by combination therapy.

Loss of kidney function in renal ischemia/reperfusion injury is due to programmed cell death, but the contribution of necroptosis, a newly discovered form of programmed necrosis, has not been evaluated. Here, we identified the presence of death receptor-mediated but caspase-independent cell death in murine tubular cells and characterized it as necroptosis by the addition of necrostatin-1, a highly specific receptor-interacting protein kinase 1 inhibitor. The detection of receptor-interacting protein kinase 1 and 3 in whole-kidney lysates and freshly isolated murine proximal tubules led us to investigate the contribution of necroptosis in a mouse model of renal ischemia/reperfusion injury. Treatment with necrostatin-1 reduced organ damage and renal failure, even when administered after reperfusion, resulting in a significant survival benefit in a model of lethal renal ischemia/reperfusion injury. Unexpectedly, specific blockade of apoptosis by zVAD, a pan-caspase inhibitor, did not prevent the organ damage or the increase in urea and creatinine in vivo in renal ischemia/reperfusion injury. Thus, necroptosis is present and has functional relevance in the pathophysiological course of ischemic kidney injury and shows the predominance of necroptosis over apoptosis in this setting. Necrostatin-1 may have therapeutic potential to prevent and treat renal ischemia/reperfusion injury.

The paracellular claudin channel of the thick ascending limb (TAL) of Henle is critical for Ca++ reabsorption in the kidney. Genome‐wide association studies (GWASs) have identified claudin‐14 associated with hypercalciuric nephrolithiasis. Here, we show that claudin‐14 promoter activity and transcript are exclusively localized in the TAL. Under normal dietary condition, claudin‐14 proteins are suppressed by two microRNA molecules (miR‐9 and miR‐374). Both microRNAs directly target the 3′‐UTR of claudin‐14 mRNA; induce its mRNA decay and translational repression in a synergistic manner. Through physical interaction, claudin‐14 blocks the paracellular cation channel made of claudin‐16 and ‐19, critical for Ca++ reabsorption in the TAL. The transcript and protein levels of claudin‐14 are upregulated by high Ca++ diet, while downregulated by low Ca++ diet. Claudin‐14 knockout animals develop hypermagnesaemia, hypomagnesiuria, and hypocalciuria under high Ca++ dietary condition. MiR‐9 and miR‐374 transcript levels are regulated by extracellular Ca++ in a reciprocal manner as claudin‐14. The Ca++ sensing receptor (CaSR) acts upstream of the microRNA‐claudin‐14 axis. Together, these data have established a key regulatory role for claudin‐14 in renal Ca++ homeostasis.

Calcifying echinoid larvae respond to changes in seawater carbonate chemistry with reduced growth and developmental delay. To date, no information exists on how ocean acidification acts on pH homeostasis in echinoderm larvae. Understanding acid-base regulatory capacities is important because intracellular formation and maintenance of the calcium carbonate skeleton is dependent on pH homeostasis. Using H + -selective microelectrodes and the pHsensitive fluorescent dye BCECF, we conducted in vivo measurements of extracellular and intracellular pH (pH e and pH i ) in echinoderm larvae. We exposed pluteus larvae to a range of seawater CO 2 conditions and demonstrated that the extracellular compartment surrounding the calcifying primary mesenchyme cells (PMCs) conforms to the surrounding seawater with respect to pH during exposure to elevated seawater pCO 2 . Using FITC dextran conjugates, we demonstrate that sea urchin larvae have a leaky integument. PMCs and spicules are therefore directly exposed to strong changes in pH e whenever seawater pH changes. However, measurements of pH i demonstrated that PMCs are able to fully compensate an induced intracellular acidosis. This was highly dependent on Na + and HCO 3 − , suggesting a bicarbonate buffer mechanism involving secondary active Na + -dependent membrane transport proteins. We suggest that, under ocean acidification, maintained pH i enables calcification to proceed despite decreased pH e . However, this probably causes enhanced costs. Increased costs for calcification or cellular homeostasis can be one of the main factors leading to modifications in energy partitioning, which then impacts growth and, ultimately, results in increased mortality of echinoid larvae during the pelagic life stage. pH microelectrode | Strongylocentrotus droebachiensis | acid-base regulation | Na + -HCO 3 − transport | epithelial transport S ea urchin larvae have been shown to react with particular sensitivity to CO 2 -induced reductions in seawater pH (1-4). When larvae are chronically exposed to elevated seawater pCO 2 of >0.1 kPa, e.g., as is predicted to occur during the next century in response to anthropogenic CO 2 emissions or through upwelling of low-pH deep water, this sensitivity is reflected in reduced growth and developmental rates (5, 6). Echinoderm larvae are considered to be especially vulnerable to seawater pH reduction and to the associated changes in calcium carbonate saturation state of seawater (Ω Cal ) because their internal skeleton is composed of high magnesium calcite, a highly soluble form of CaCO 3 (7, 8). However, long-term reductions in growth and development might just as well be evoked by other physiological mechanisms that are also sensitive to hypercapnia and the related acid-base disturbances. Recent studies conducted on several marine taxa including mollusks (9) and echinoderms (10) demonstrated increased metabolic rates in response to elevated seawater pCO 2 . It was concluded that reductions in somatic growth and rate of development were caused by a sh...

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The thick ascending limb (TAL) of Henle's loop drives paracellular Na + , Ca 2+ , and Mg 2+ reabsorption via the tight junction (TJ). The TJ is composed of claudins that consist of four transmembrane segments, two extracellular segments (ECS1 and -2), and one intracellular loop. Claudins interact within the same (cis) and opposing (trans) plasma membranes. The claudins Cldn10b, -16, and -19 facilitate cation reabsorption in the TAL, and their absence leads to a severe disturbance of renal ion homeostasis. We combined electrophysiological measurements on microperfused mouse TAL segments with subsequent analysis of claudin expression by immunostaining and confocal microscopy. Claudin interaction properties were examined using heterologous expression in the TJ-free cell line HEK 293, live-cell imaging, and Förster/FRET. To reveal determinants of interaction properties, a set of TAL claudin protein chimeras was created and analyzed. Our main findings are that (i) TAL TJs show a mosaic expression pattern of either cldn10b or cldn3/cldn16/cldn19 in a complex; (ii) TJs dominated by cldn10b prefer Na + over Mg 2+ , whereas TJs dominated by cldn16 favor Mg 2+ over Na + ; (iii) cldn10b does not interact with other TAL claudins, whereas cldn3 and cldn16 can interact with cldn19 to form joint strands; and (iv) further claudin segments in addition to ECS2 are crucial for trans interaction. We suggest the existence of at least two spatially distinct types of paracellular channels in TAL: a cldn10b-based channel for monovalent cations such as Na + and a spatially distinct site for reabsorption of divalent cations such as Ca 2+ and Mg 2+ .T he kidney regulates the salt and water balance of the body by filtration and subsequent reabsorption or secretion of ions and water. Thereby it controls blood pressure and maintains acid-base homeostasis. The thick ascending limb (TAL) of Henle's loop drives reabsorption of Na + , Cl − , Ca 2+ , and Mg 2+ from the tubular fluid into the blood. Na + and Cl − are reabsorbed via the transcellular pathway, involving the renal-specific isoform of the Na + /K + /2Cl − cotransporter (NKCC2) in the apical epithelial cell membrane and Na + /K + -ATPase and chloride channel ClC-Kb in the basolateral membrane. K + is circulated via NKCC2 and the renal outer medullary K + channel ROMK1, across the apical cell membrane. These transport processes generate a lumen-positive transepithelial potential that drives additional paracellular reabsorption of Na + as well as the reabsorption of divalent cations, mainly Ca 2+ and Mg 2+ . Paracellular transport is regulated by the tight junction (TJ) in a size-, charge-, and water-selective manner. The main functional constituent of the TJ is the family of claudins with 27 members in mammals. Claudins consist of a four-transmembrane helix bundle, two extracellular segments that expand into the paracellular cleft, and intracellular N and C termini. Claudins interact in cis (within the same plasma membrane) and in trans (with claudins in the plasma membrane of neighbori...

PHYSIOLOGYCorrection for "Deletion of claudin-10 (Cldn10) in the thick ascending limb impairs paracellular sodium permeability and leads to hypermagnesemia and nephrocalcinosis," by Tilman Breiderhoff, Nina Himmerkus, Marchel Stuiver, Kerim Mutig,

In skeletal muscle the oligomeric ␣ 1S , ␣ 2 /␦-1 or ␣ 2 /␦-2, ␤1, and ␥1 L-type Ca 2؉ channel or dihydropyridine receptor functions as a voltage sensor for excitation contraction coupling and is responsible for the L-type Ca 2؉ current. The ␥1 subunit, which is tightly associated with this Ca 2؉ channel, is a membrane-spanning protein exclusively expressed in skeletal muscle. Previously, heterologous expression studies revealed that ␥1 might modulate Ca 2؉ currents expressed by the pore subunit found in heart, ␣ 1C , shifting steady state inactivation, and increasing current amplitude. To determine the role of ␥1 assembled with the skeletal subunit composition in vivo, we used gene targeting to establish a mouse model, in which ␥1 expression is eliminated. Comparing litter-matched mice with control mice, we found that, in contrast to heterologous expression studies, the loss of ␥1 significantly increased the amplitude of peak dihydropyridine-sensitive I Ca in isolated myotubes. Whereas the activation kinetics of the current remained unchanged, inactivation of the current was slowed in ␥1-deficient myotubes and, correspondingly, steady state inactivation of I Ca was shifted to more positive membrane potentials. These results indicate that ␥1 decreases the amount of Ca 2؉ entry during stimulation of skeletal muscle.