Alleviation of fatty acid and hypoxia-reoxygenation-induced proximal tubule deenergization by ADP/ATP carrier inhibition and glutamate

Mitochondrial ROS modulate presynaptic plasticity in the drosophila neuromuscular junction

The elevated emission of reactive oxygen species (ROS) from presynaptic mitochondria is well-documented in several inflammatory and neurodegenerative diseases. However, the potential role of mitochondrial ROS in presynaptic function and plasticity remains largely understudied beyond the context of disease. Here, we investigated this potential ROS role in presynaptic function and short-term plasticity by combining optogenetics, whole cell electrophysiological recordings, and live confocal imaging using a well-established protocol for induction and measurement of synaptic potentiation in Drosophila melanogaster neuromuscular junctions (NMJ). Optogenetic induction of ROS emission from presynaptic motorneuron mitochondria expressing mitokiller red (mK) resulted in synaptic potentiation, evidenced by an increase in the frequency of spontaneous mini excitatory junction potentials. Notably, this effect was not observed in flies co-expressing catalase, a cytosolic hydrogen peroxide (H2O2) scavenging enzyme. Moreover, the increase in electrical activity did not coincide with synaptic structural changes. The absence of Wnt1/Wg release from synaptic boutons suggested involvement of alternative or non-canonical signaling pathway(s). However, in existing boutons we observed an increase in the active zone (AZ) marker Brp/Erc1, which serves as docking site for the neurotransmitter vesicle release pool. We propose the involvement of putative redox switches in AZ components as the molecular target of mitochondrial H2O2. These findings establish a novel framework for understanding the signaling role of mROS in presynaptic structural and functional plasticity, providing insights into redox-based mechanisms of neuronal communication.

Lipotoxicity and immunometabolism in ischemic acute kidney injury: current perspectives and future directions

Dysregulated lipid metabolism is implicated in the pathophysiology of a range of kidney diseases. The specific mechanisms through which lipotoxicity contributes to acute kidney injury (AKI) remain poorly understood. Herein we review the cardinal features of lipotoxic injury in ischemic kidney injury; lipid accumulation and mitochondrial lipotoxicity. We then explore a new mechanism of lipotoxicity, what we define as "immunometabolic" lipotoxicity, and discuss the potential therapeutic implications of targeting this lipotoxicity using lipid lowering medications.

Proximal Tubular Oxidative Metabolism in Acute Kidney Injury and the Transition to CKD

The proximal tubule relies on oxidative mitochondrial metabolism to meet its energy needs and has limited capacity for glycolysis, which makes it uniquely susceptible to damage during AKI, especially after ischemia and anoxia. Under these conditions, mitochondrial ATP production is initially decreased by several mechanisms, including fatty acid-induced uncoupling and inhibition of respiration related to changes in the shape and volume of mitochondria. Glycolysis is initially insufficient as a source of ATP to protect the cells and mitochondrial function, but supplementation of tricarboxylic acid cycle intermediates augments anaerobic ATP production, and improves recovery of mitochondrial oxidative metabolism. Incomplete recovery is characterized by defects of respiratory enzymes and lipid metabolism. During the transition to CKD, tubular cells atrophy but maintain high expression of glycolytic enzymes, and there is decreased fatty acid oxidation. These metabolic changes may be amenable to a number of therapeutic interventions.

Two macamide extracts relieve physical fatigue by attenuating muscle damage in mice

BACKGROUND

Macamides, the main active components contained in maca, have attracted increasing attention due to their various bioactivities. In this study, crude macamide extract (CME) and purified macamide extract (PME) were prepared by enzyme-assisted extraction and macroporous resin separation, and the anti-fatigue effects of CME and PME were evaluated in a forced swimming model.

RESULTS

The composition analysis results revealed that both CME and PME mainly contain eight kinds of macamide. Based on the results of a weight-loaded forced swimming test, compared with a control group, CME and and PME groups could prolong exhaustive swimming time, increase levels of liver glycogen (LG) and muscle glycogen (MG), accelerate fatty acid oxidation in serum to provide energy, eliminate the accumulation of blood lactic acid (BLA) and blood urea nitrogen (BUN), and decrease the serum biomarkers for muscle damage, such as lactate dehydrogenase (LDH) and creatine kinase (CK). Histological analysis also indicated that CME and PME attenuated damage to skeletal muscle and the myocardium in mice during exercise.

CONCLUSION

Two macamide extracts have a beneficial effect on relieving physical fatigue by attenuating the damage of skeletal muscle and myocardium during exercise, and a better effect was observed in the PME group. © 2018 Society of Chemical Industry.

What we need to know about lipid-associated injury in case of renal ischemia-reperfusion

Renal segmental metabolism is reflected by the complex distribution of the main energy pathways along the nephron, with fatty acid oxidation preferentially used in the cortex area. Ischemia/reperfusion injury (IRI) is due to the restriction of renal blood flow, rapidly leading to a metabolic switch toward anaerobic conditions. Subsequent unbalance between energy demand and oxygen/nutrient delivery compromises kidney cell functions, resulting in a complex inflammatory cascade including the production of reactive oxygen species (ROS). Renal IRI especially involves lipid accumulation. Lipid peroxidation is one of the major events of ROS-associated tissue injury. Here, we briefly review the current knowledge of renal cell lipid metabolism in normal and ischemic conditions. Next, we focus on renal lipid-associated injury, with emphasis on its mechanisms and consequences during the course of IRI. Finally, we discuss preclinical observations aiming at preventing and/or attenuating lipid-associated IRI.

Mitochondrial cyclophilin D ablation is associated with the activation of Akt/p70S6K pathway in the mouse kidney

The mitochondrial matrix protein cyclophilin D (CypD) is an essential component of the mitochondrial permeability transition pore (MPTP). Here we characterized the effects of CypD ablation on bioenergetics in the kidney. CypD loss triggers a metabolic shift in Ppif-/- male and female mouse kidneys towards glycolysis and Krebs cycle activity. The shift is accompanied by increased glucose consumption and a transcriptional upregulation of effectors of glucose metabolism in the kidney. These included activation of Akt, AMPK (only in males) and p70S6K kinases. Gender specific differences between the Ppif-/- male and female mouse kidneys were observed including activation of pro-surviving ERK1/2 kinase and inhibited expression of pro-apoptotic and pro-fibrotic JNK and TGFβ1 proteins in Ppif-/- females. They also showed the highest expression of phosphorylated-ERK1/2 and Akt S473 proteins of all four investigated animal groups. Furthermore, Ppif-/- females showed higher lactate concentrations and ATP/ADP-ratios in the kidney than males. These metabolic and transcriptional modifications could provide an additional level of protection to Ppif-/- females. In summary, loss of mitochondrial CypD results in a shift in bioenergetics and in activation of glucose-metabolism regulating Akt/AMPK/p70S6 kinase pathways that is expected to affect the capability of Ppif-/- mice kidneys to react to stimuli and injury.

Mitochondrial bioenergetics and oxidative status disruption in brainstem of weaned rats: Immediate response to maternal protein restriction

Mitochondrial bioenergetics dysfunction has been postulated as an important mechanism associated to a number of cardiovascular diseases in adulthood. One of the hypotheses is that this is caused by the metabolic challenge generated by the mismatch between prenatal predicted and postnatal reality. Perinatal low-protein diet produces several effects that are manifested in the adult animal, including altered sympathetic tone, increased arterial blood pressure and oxidative stress in the brainstem. The majority of the studies related to nutritional programming postulates that the increased risk levels for non-communicable diseases are associated with the incompatibility between prenatal and postnatal environment. However, little is known about the immediate effects of maternal protein restriction on the offspring's brainstem. The present study aimed to test the hypothesis that a maternal low-protein diet causes tissue damage immediately after exposure to the nutritional insult that can be assessed in the brainstem of weaned offspring. In this regard, a series of assays was conducted to measure the mitochondrial bioenergetics and oxidative stress biomarkers in the brainstem, which is the brain structure responsible for the autonomic cardiovascular control. Pregnant Wistar rats were fed ad libitum with normoprotein (NP; 17% casein) or low-protein (LP; 8% casein) diet throughout pregnancy and lactation periods. At weaning, the male offsprings were euthanized and the brainstem was quickly removed to assess the mitochondria function, reactive oxygen species (ROS) production, mitochondrial membrane electric potential (ΔΨm), oxidative biomarkers, antioxidant defense and redox status. Our data demonstrated that perinatal LP diet induces an immediate mitochondrial dysfunction. Furthermore, the protein restriction induced a marked increase in ROS production, with a decrease in antioxidant defense and redox status. Altogether, our findings suggest that LP-fed animals may be at a higher risk for oxidative metabolism impairment throughout life than NP-fed rats, due to the immediate disruption of the mitochondrial bioenergetics and oxidative status caused by the LP diet.

Neonatal SSRI exposure improves mitochondrial function and antioxidant defense in rat heart

Protein restriction during prenatal, postnatal, or in both periods has a close relationship with subsequent development of cardiovascular disease in adulthood. Elevated brain levels of serotonin and its metabolites have been found in malnourished states. The aim in the present study was to investigate whether treatment with fluoxetine (Fx), a selective serotonin reuptake inhibitor, mimics the detrimental effect of low-protein diet during the perinatal period on the male rat heart. Our hypothesis is that increased circulating serotonin as a result of pharmacologic treatment with Fx leads to cardiac dysfunction similar to that observed in protein-restricted rats. Male Wistar rat pups received daily subcutaneous injection of Fx or vehicle from postnatal day 1 to postnatal day 21. Male rats were euthanized at 60 days of age and the following parameters were evaluated in the cardiac tissue: mitochondrial respiratory capacity, respiratory control ratio, reactive oxygen species (ROS) production, mitochondrial membrane potential, and biomarkers of oxidative stress and antioxidant defense. We found that Fx treatment increased mitochondrial respiratory capacity (123%) and membrane potential (212%) and decreased ROS production (55%). In addition we observed an increase in the antioxidant capacity (elevation in catalase activity (5-fold) and glutathione peroxidase (4.6-fold)). Taken together, our results suggest that Fx treatment in the developmental period positively affects the mitochondrial bioenergetics and antioxidant defense in the cardiac tissue.

Substrate modulation of fatty acid effects on energization and respiration of kidney proximal tubules during hypoxia/reoxygenation

Kidney proximal tubules subjected to hypoxia/reoxygenation develop a nonesterified fatty acid-induced energetic deficit characterized by persistent partial mitochondrial deenergization that can be prevented and reversed by citric acid cycle substrates. To further assess the role of competition between fatty acids and substrates on inner membrane substrate carriers in the deenergization and the contribution to deenergization of fatty acid effects on respiratory function, digitonin-permeabilized rabbit and mouse tubules were studied using either addition of exogenous oleate after control normoxic incubation or increases of endogenous fatty acids produced by hypoxia/reoxygenation. The results demonstrated major effects of matrix oxaloacetate accumulation on succinate-supported energization and respiration and their modification by fatty acids. Improvements of energization in the presence of fatty acids by glutamate were shown to result predominantly from lowering matrix oxaloacetate rather than from amelioration of transmembrane cycling of fatty acids and uncoupling. Mouse tubules had 2.5 fold higher rates of succinate utilization, which resulted in stronger effects of oxaloacetate accumulation than rabbit tubules. Hypoxia/reoxygenation induced respiratory inhibition that was more severe for complex I-dependent substrates. Fatty acids themselves did not acutely contribute to this respiratory inhibition, but lowering them during 60 min. reoxygenation to allow recovery of ATP during that period alleviated it. These data clarify the basis for the nonesterified fatty acid-induced mitochondrial energetic deficit in kidney proximal tubules that impairs structural and functional recovery and provide insight into interactions that need to be considered in the design of substrate-based interventions to improve mitochondrial function.

Lipotoxic disruption of NHE1 interaction with PI(4,5)P2 expedites proximal tubule apoptosis

Chronic kidney disease progression can be predicted based on the degree of tubular atrophy, which is the result of proximal tubule apoptosis. The Na+/H+ exchanger NHE1 regulates proximal tubule cell survival through interaction with phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], but pathophysiologic triggers for NHE1 inactivation are unknown. Because glomerular injury permits proximal tubule luminal exposure and reabsorption of fatty acid/albumin complexes, we hypothesized that accumulation of amphipathic, long-chain acyl-CoA (LC-CoA) metabolites stimulates lipoapoptosis by competing with the structurally similar PI(4,5)P2 for NHE1 binding. Kidneys from mouse models of progressive, albuminuric kidney disease exhibited increased fatty acids, LC-CoAs, and caspase-2-dependent proximal tubule lipoapoptosis. LC-CoAs and the cytosolic domain of NHE1 directly interacted, with an affinity comparable to that of the PI(4,5)P2-NHE1 interaction, and competing LC-CoAs disrupted binding of the NHE1 cytosolic tail to PI(4,5)P2. Inhibition of LC-CoA catabolism reduced NHE1 activity and enhanced apoptosis, whereas inhibition of proximal tubule LC-CoA generation preserved NHE1 activity and protected against apoptosis. Our data indicate that albuminuria/lipiduria enhances lipotoxin delivery to the proximal tubule and accumulation of LC-CoAs contributes to tubular atrophy by severing the NHE1-PI(4,5)P2 interaction, thereby lowering the apoptotic threshold. Furthermore, these data suggest that NHE1 functions as a metabolic sensor for lipotoxicity.

Vasopressin and the regulation of aquaporin-2

Water excretion is regulated in large part through the regulation of osmotic water permeability of the renal collecting duct epithelium. Water permeability is controlled by vasopressin through regulation of the water channel, aquaporin-2 (AQP2). Two processes contribute: (1) regulation of AQP2 trafficking to the apical plasma membrane; and (2) regulation of the total amount of the AQP2 protein in the cells. Regulation of AQP2 abundance is defective in several water-balance disorders, including many polyuric disorders and the syndrome of inappropriate antidiuresis. Here we review vasopressin signaling in the renal collecting duct that is relevant to the two modes of water permeability regulation.

Cardiac aquaporins

Aquaporins are a group of proteins with high-selective permeability for water. A subgroup called aquaglyceroporins is also permeable to glycerol, urea and a few other solutes. Aquaporin function has mainly been studied in the brain, kidney, glands and skeletal muscle, while the information about aquaporins in the heart is still scarce. The current review explores the recent advances in this field, bringing aquaporins into focus in the context of myocardial ischemia, reperfusion, and blood osmolarity disturbances. Since the amount of data on aquaporins in the heart is still limited, examples and comparisons from better-studied areas of aquaporin biology have been used. The human heart expresses aquaporin-1, -3, -4 and -7 at the protein level. The potential roles of aquaporins in the heart are discussed, and some general phenomena that the myocardial aquaporins share with aquaporins in other organs are elaborated. Cardiac aquaporin-1 is mostly distributed in the microvasculature. Its main role is transcellular water flux across the endothelial membranes. Aquaporin-4 is expressed in myocytes, both in cardiac and in skeletal muscle. In addition to water flux, its function is connected to the calcium signaling machinery. It may play a role in ischemia-reperfusion injury. Aquaglyceroporins, especially aquaporin-7, may serve as a novel pathway for nutrient delivery into the heart. They also mediate toxicity of various poisons. Aquaporins cannot influence permeability by gating, therefore, their function is regulated by changes of expression-on the levels of transcription, translation (by microRNAs), post-translational modification, membrane trafficking, ubiquitination and subsequent degradation. Studies using mice genetically deficient for aquaporins have shown rather modest changes in the heart. However, they might still prove to be attractive targets for therapy directed to reduce myocardial edema and injury caused by ischemia and reperfusion.

Different effects of CsA and FK506 on aquaporin-2 abundance in rat primary cultured collecting duct cells

Calcineurin (Cn) inhibitors (CnI) such as cyclosporine A (CsA) and FK506 are nephrotoxic immunosuppressant drugs, which decrease tubular function. Here, we examined the direct effect of CnI on aquaporin-2 (AQP2) expression in rat primary cultured inner medullary collecting duct cells. CsA (0.5-5 μM) but not FK 506 (0.01-1 μM) decreased expression of AQP2 protein and messenger RNA (mRNA) in a concentration and time dependent manner, without affecting mRNA stability. This effect was observed despite similar inhibition of Cn activity by both CnI, thereby suggesting that the CsA-dependent decrease in AQP2 expression was Cn independent. Another inhibitor of cyclophilin A, the primary intracellular target of CsA, had no effect on AQP2 expression. In order to investigate the mechanism of decreased AQP2 transcription, we studied activation status of two suggested transcriptional regulators of AQP2, cAMP-responsive element binding protein (CREB), and tonicity enhancer binding protein (TonEBP). Localization of TonEBP, as well as TonEBP-mediated gene transcription, was not affected by CsA. Phosphorylation of CREB at an activating phosphorylation site (S133) was decreased by CsA, but not by FK506. However, both CnI did not affect cellular cAMP levels. We show that CsA decreases transcription of AQP2, a process that is in part independent of Cn or cyclophilin A and suggests dependence on decreased activity of CREB.

Cyclophilin D and the mitochondrial permeability transition in kidney proximal tubules after hypoxic and ischemic injury

Mitochondrial matrix cyclophilin D (CyPD) is known to promote development of the mitochondrial permeability transition (MPT). Kidney proximal tubule cells are especially prone to deleterious effects of mitochondrial damage because of their dependence on oxidative mitochondrial metabolism for ATP production. To clarify the role of CyPD and the MPT in proximal tubule injury during ischemia-reperfusion (I/R) and hypoxia-reoxygenation (H/R), we assessed freshly isolated tubules and in vivo injury in wild-type (WT) and Ppif(-/-) CyPD-null mice. Isolated mouse tubules developed a sustained, nonesterified fatty acid-mediated energetic deficit after H/R in vitro that could be substantially reversed by delipidated albumin and supplemental citric acid cycle substrates but was not modified by the absence of CyPD. Susceptibility of WT and Ppif(-/-) tubules to the MPT was increased by H/R but was less in normoxic and H/R Ppif(-/-) than WT tubules. Correction of the energetic deficit that developed during H/R strongly increased resistance to the MPT. Ppif(-/-) mice were resistant to I/R injury in vivo spanning a wide range of severity. The data clarify involvement of the MPT in oxygen deprivation-induced tubule cell injury by showing that the MPT does not contribute to the initial bioenergetic deficit produced by H/R but the deficit predisposes to subsequent development of the MPT, which contributes pathogenically to kidney I/R injury in vivo.

Regulation of the mitochondrial permeability transition in kidney proximal tubules and its alteration during hypoxia-reoxygenation

Development of the mitochondrial permeability transition (MPT) can importantly contribute to lethal cell injury from both necrosis and apoptosis, but its role varies considerably with both the type of cell and type of injury, and it can be strongly opposed by the normally abundant endogenous metabolites ADP and Mg(2+). To better characterize the MPT in kidney proximal tubule cells and assess its contribution to injury to them, we have refined and validated approaches to follow the process in whole kidney proximal tubules and studied its regulation in normoxic tubules and after hypoxia-reoxygenation (H/R). Physiological levels of ADP and Mg(2+) greatly decreased sensitivity to the MPT. Inhibition of cyclophilin D by cyclosporine A (CsA) effectively opposed the MPT only in the presence of ADP and/or Mg(2+). Nonesterified fatty acids (NEFA) had a large role in the decreased resistance to the MPT seen after H/R irrespective of the available substrate or the presence of ADP, Mg(2+), or CsA, but removal of NEFA was less effective at restoring normal resistance to the MPT in the presence of electron transport complex I-dependent substrates than with succinate. The data indicate that the NEFA accumulation that occurs during both hypoxia in vitro and ischemic acute kidney injury in vivo is a critical sensitizing factor for the MPT that overcomes the antagonistic effect of endogenous metabolites and cyclophilin D inhibition, particularly in the presence of complex I-dependent substrates, which predominate in vivo.

Amino acid transamination is crucial for ischaemic cardioprotection in normal and preconditioned isolated rat hearts--focus on L-glutamate

We have found that cardioprotection by l-glutamate mimics protection by classical ischaemic preconditioning (IPC). We investigated whether the effect of IPC involves amino acid transamination and whether IPC modulates myocardial glutamate metabolism. In a glucose-perfused, isolated rat heart model subjected to 40 min global no-flow ischaemia and 120 min reperfusion, the effects of IPC (2 cycles of 5 min ischaemia and 5 min reperfusion) and continuous glutamate (20 mm) administration during reperfusion on infarct size and haemodynamic recovery were studied. The effect of inhibiting amino acid transamination was evaluated by adding the amino acid transaminase inhibitor amino-oxyacetate (AOA; 0.025 mm) during reperfusion. Changes in coronary effluent, interstitial (microdialysis) and intracellular glutamate ([GLUT](i)) concentrations were measured. Ischaemic preconditioning and postischaemic glutamate administration reduced infarct size to the same extent (41 and 40%, respectively; P < 0.05 for both), without showing an additive effect. Amino-oxyacetate abolished infarct reduction by IPC and glutamate, and increased infarct size in both control and IPC hearts in a dose-dependent manner. Ischaemic preconditioning increased [GLUT](i) before ischaemia (P < 0.01) and decreased the release of glutamate during the first 10 min of reperfusion (P = 0.03). A twofold reduction in [GLUT](i) from the preischaemic state to 45 min of reperfusion (P = 0.0001) suggested increased postischaemic glutamate utilization in IPC hearts. While IPC and AOA changed haemodynamics in accordance with infarct size, glutamate decreased haemodynamic recovery despite reduced infarct size. In conclusion, ischaemic cardioprotection of the normal and IPC-protected heart depends on amino acid transamination and activity of the malate-aspartate shuttle during reperfusion. Underlying mechanisms of IPC include myocardial glutamate metabolism.

The calcineurin inhibitor FK506 (tacrolimus) is associated with transient metabolic acidosis and altered expression of renal acid-base transport proteins

Calcineurin inhibitors like FK506 (tacrolimus) are routinely used for immunosuppression following transplantation. Its use is limited by many side effects, including renal tubular acidosis (RTA), mainly of the distal type. In this study, rats were treated with FK506 and at baseline (after 9 days) systemic acid-base status was similar to that in control animals. However, FK506-treated rats given NH4Cl in the drinking water for 2 days developed a more severe metabolic acidosis than control animals. Urine pH was more alkaline, but net acid excretion was normal. After 7 days of acid load, all differences related to acid-base homeostasis were equalized in both groups. Protein abundance of type IIa Na-Pi cotransporter, type 3 Na+/H+ exchanger, and electrogenic Na+-bicarbonate cotransporter, and both a4 and B2 subunits of the vacuolar H+-ATPase were reduced under baseline conditions, while induction of metabolic acidosis enhanced protein abundance of these transporters in FK506-treated animals. In parallel, protein expression of AE1 was reduced at baseline and increased together with pendrin during NH4Cl loading in FK506 rats. Protein abundance of the Na+-bicarbonate cotransporter NBCn1 was reduced under baseline conditions but remained downregulated during metabolic acidosis. Morphological analysis revealed an increase in the relative number of non-type A intercalated cells in the connecting tubule and cortical collecting duct at the expense of principal cells. Additionally, subcellular distribution of the a4 subunit of the vacuolar H+-ATPase was affected by FK506 with less luminal localization in the connecting tubule and outer medullary collecting duct. These data suggest that FK506 impacts on several major acid-base transport proteins in the kidney, and its use is associated with transient metabolic acidosis and altered expression of key renal acid-base transport proteins.

Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models

The renal cortico-papillary osmotic gradient is generated by sodium reabsorption in the thick ascending limb. The antidiuretic hormone arginine vasopressin (AVP) increases collecting duct water permeability by enhancing aquaporin-2 (AQP2) water channel insertion in the apical membrane of principal cells, allowing water to passively flow along the osmotic gradient from the tubule lumen to the interstitium. In addition to short-term AQP2 redistribution between intracellular compartments and the cell surface, AQP2 whole cell abundance is tightly regulated. AVP is a major transcriptional activator of the AQP2 gene, and stimulation of insulin- and calcium-sensing receptors respectively potentiate and reduce its action. Extracellular tonicity is another key factor that determines the levels of AQP2 abundance. Its effect is dependent on activation of the tonicity-responsive enhancer binding protein that reinforces AVP-induced AQP2 transcriptional activation. Conversely, activation of the NF-κB transcriptional factor by proinflammatory factors reduces AQP2 gene transcription. Aldosterone additionally regulates AQP2 whole cell abundance by simultaneously reducing AQP2 gene transcription and stimulating AQP2 mRNA translation. These examples illustrate how cross talk between various stimuli regulates AQP2 abundance in collecting duct principal cells and consequently contributes to maintenance of body water homeostasis.

Controlled aquaporin-2 expression in the hypertonic environment

The corticomedullary osmolality gradient is the driving force for water reabsorption occurring in the kidney. In the collecting duct, this gradient allows luminal water to move across aquaporin (AQP) water channels, thereby increasing urine concentration. However, this same gradient exposes renal cells to great osmotic challenges. These cells must constantly adapt to fluctuations of environmental osmolality that challenge cell volume and incite functional change. This implies profound alterations of cell phenotype regarding water permeability. AQP2 is an essential component of the urine concentration mechanism whose controlled expression dictates apical water permeability of collecting duct principal cells. This review focuses on changes of AQP2 abundance and trafficking in hypertonicity-challenged cells. Intracellular mechanisms governing these events are discussed and the biological relevance of altered AQP2 expression by hypertonicity is outlined.

Evidence for involvement of nonesterified fatty acid-induced protonophoric uncoupling during mitochondrial dysfunction caused by hypoxia and reoxygenation

BACKGROUND

Proximal tubules subjected to hypoxia in vitro under conditions relevant to ischaemia in vivo develop an energetic deficit that is not corrected even after full reoxygenation. We have provided evidence that accumulation of nonesterified fatty acids (NEFA) is the primary reason for this energetic deficit. In this study, we have further investigated the mechanism for the NEFA-induced energetic deficit.

METHODS

Mitochondrial membrane potential (Deltapsi) was measured in digitonin-permeabilized, freshly isolated proximal tubules by safranin O uptake. Addition of the potassium/proton exchanger nigericin enables the determination of the mitochondrial proton motive force (Deltap) and the proton gradient (DeltapH). ATP was measured luminometrically and NEFA colorimetrically.

RESULTS

Tubule ATP content was depleted after hypoxia and recovered incompletely, even after full reoxygenation. Mitochondrial safranin O uptake was decreased in proximal tubules after hypoxia and reoxygenation (H/R). This decrease was attenuated by delipidated bovine serum albumin (dBSA) or citrate. Addition of nigericin increased safranin O uptake of mitochondria in normoxic proximal tubules, but not in proximal tubules after H/R. Addition of dBSA restored the effect of nigericin to increase mitochondrial safranin O uptake. Addition of the NEFA oleate had the same impact on mitochondrial safranin O uptake as subjecting proximal tubules to H/R.

CONCLUSION

The mechanism of the NEFA-induced energetic deficit in freshly isolated rat proximal tubules induced by H/R is characterized by impaired ATP production after full reoxygenation, impaired recovery of Deltapsi and Deltap, abrogation of DeltapH and sensitivity to citrate, consistent with involvement of the tricarboxylate carrier. The data support the concept that protonophoric uncoupling by NEFA movement on anion carriers plays a critical role in proximal tubule mitochochondrial dysfunction after H/R.

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Maleate nephrotoxicity: mechanisms of injury and correlates with ischemic/hypoxic tubular cell death

Maleate injection causes dose-dependent injury in proximal tubular cells. This study sought to better define underlying pathogenic mechanisms and to test whether maleate toxicity recapitulates critical components of the hypoxic/ischemic renal injury cascade. CD-1 mice were injected with maleate or used as a source for proximal tubule segments (PTS) for in vitro studies. Maleate induced dose-dependent PTS injury [lactate deydrogenase (LDH) release, ATP reductions, nonesterified fatty acid (NEFA) accumulation]. These changes were partially dependent on maleate metabolism (protection conferred by metabolic inhibitors: succinate, acetoacetate). Maleate toxicity reproduced critical characteristics of the hypoxia/ATP depletion-induced injury cascade: 1) glutathione (GSH) conferred protection, but due to its glycine, not cysteine (antioxidant), content; 2) ATP reductions reflected decreased production, not Na-K-ATPase-driven increased consumption; 3) cell death was completely blocked by extracellular acidosis (pH 6.6); 4) intracellular Ca(2+) chelation (BAPTA) mitigated cell death; 5) maleate and hypoxia each caused plasma membrane cholesterol shedding and in both instances, this was completely glycine suppressible; 6) maleate + hypoxia caused neither additive NEFA accumulation nor LDH release, implying shared pathogenic pathways; and 7) maleate, like ischemia, induced renal cortical cholesterol loading; increased HMG CoA reductase (HMGCR) activity (statin inhibitable), increased HMGCR mRNA levels, and increased RNA polymerase II recruitment to the HMGCR locus (chromatin immunoprecipitation, ChIP, assay) were involved. These results further define critical determinants of maleate nephrotoxicity and suggest that it can serve as a useful adjunct for studies of ischemia/ATP depletion-induced, proximal tubule-specific, cell death.