Regulation of ROMK and channel-inducing factor (CHIF) in acute renal failure due to ischemic reperfusion injury

Interaction of ROMK2 channel with lipid kinases DGKE and AGK: Potential channel activation by localized anionic lipid synthesis

In this study, we utilized enzyme-catalyzed proximity labeling with the engineered promiscuous biotin ligase Turbo-ID to identify the proxisome of the ROMK2 channel. This channel resides in various cellular membrane compartments of the cell including the plasma membrane, endoplasmic reticulum and mitochondria. Within mitochondria, ROMK2 has been suggested as a pore-forming subunit of mitochondrial ATP-regulated potassium channel (mitoKATP). We found that ROMK2 proxisome in addition to previously known protein partners included two lipid kinases: acylglycerol kinase (AGK) and diacylglycerol kinase ε (DGKE), which are localized in mitochondria and the endoplasmic reticulum, respectively. Through co-immunoprecipitation, we confirmed that these two kinases are present in complexes with ROMK2 channels. Additionally, we found that the products of AGK and DGKE, lysophosphatidic acid (LPA) and phosphatidic acid (PA), stimulated the activity of ROMK2 channels in artificial lipid bilayers. Our molecular docking studies revealed the presence of acidic lipid binding sites in the ROMK2 channel, similar to those previously identified in Kir2 channels. Based on these findings, we propose a model wherein localized lipid synthesis, mediated by channel-bound lipid kinases, contributes to the regulation of ROMK2 activity within distinct intracellular compartments, such as mitochondria and the endoplasmic reticulum.

Urinary proteomics to support diagnosis of stroke

Accurate diagnosis in suspected ischaemic stroke can be difficult. We explored the urinary proteome in patients with stroke (n = 69), compared to controls (n = 33), and developed a biomarker model for the diagnosis of stroke. We performed capillary electrophoresis online coupled to micro-time-of-flight mass spectrometry. Potentially disease-specific peptides were identified and a classifier based on these was generated using support vector machine-based software. Candidate biomarkers were sequenced by liquid chromatography-tandem mass spectrometry. We developed two biomarker-based classifiers, employing 14 biomarkers (nominal p-value <0.004) or 35 biomarkers (nominal p-value <0.01). When tested on a blinded test set of 47 independent samples, the classification factor was significantly different between groups; for the 35 biomarker model, median value of the classifier was 0.49 (−0.30 to 1.25) in cases compared to −1.04 (IQR −1.86 to −0.09) in controls, p<0.001. The 35 biomarker classifier gave sensitivity of 56%, specificity was 93% and the AUC on ROC analysis was 0.86. This study supports the potential for urinary proteomic biomarker models to assist with the diagnosis of acute stroke in those with mild symptoms. We now plan to refine further and explore the clinical utility of such a test in large prospective clinical trials.

Inflammatory Mechanisms of Organ Crosstalk during Ischemic Acute Kidney Injury

Acute kidney injury (AKI) is a common complication during inpatient hospitalization, and clinical outcomes remain poor despite advancements in renal replacement therapy. AKI in the setting of multiple organ failure (MOF) remains a formidable challenge to clinicians and incurs an unacceptably high mortality rate. Kidney ischemia-reperfusion injury (IRI) incites a proinflammatory cascade and releases cellular and soluble mediators with systemic implications for remote organ injury. Evidence from preclinical models cites mechanisms of organ crosstalk during ischemic AKI including the expression of cellular adhesion molecules, lymphocyte trafficking, release of proinflammatory cytokines and chemokines, and modification of the host innate and adaptive immune response systems. In this paper, the influence of kidney IRI on systemic inflammation and distant organ injury will be examined. Recent experimental data and evolving concepts of organ crosstalk during ischemic AKI will also be discussed in detail.

Surgical sepsis and organ crosstalk: the role of the kidney

Acute kidney injury (AKI) is a common complication of hospitalized patients, and clinical outcomes remain poor despite advances in renal replacement therapy. The accepted pathophysiology of AKI in the setting of sepsis has evolved from one of simple decreased renal blood flow to one that involves a more complex interaction of intra-glomerular microcirculatory vasodilation combined with the local release of inflammatory mediators and apoptosis. Evidence from preclinical AKI models suggests that crosstalk occurs between kidneys and other organ systems via soluble and cellular inflammatory mediators and that this involves both the innate and adaptive immune systems. These interactions are reflected by genomic changes and abnormal rates of cellular apoptosis in distant organs including the lungs, heart, gut, liver, and central nervous system. The purpose of this article is to review the influence of AKI, particularly sepsis-associated AKI, on inter-organ crosstalk in the context of systemic inflammation and multiple organ failure (MOF).

FXYD proteins: new regulators of Na-K-ATPase

FXYD proteins belong to a family of small-membrane proteins. Recent experimental evidence suggests that at least five of the seven members of this family, FXYD1 (phospholemman), FXYD2 (gamma-subunit of Na-K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), and FXYD7, are auxiliary subunits of Na-K-ATPase and regulate Na-K-ATPase activity in a tissue- and isoform-specific way. These results highlight the complexity of the regulation of Na+ and K+ handling by Na-K-ATPase, which is necessary to ensure appropriate tissue functions such as renal Na+ reabsorption, muscle contractility, and neuronal excitability. Moreover, a mutation in FXYD2 has been linked to cases of human hypomagnesemia, indicating that perturbations in the regulation of Na-K-ATPase by FXYD proteins may be critically involved in pathophysiological states. A better understanding of this novel regulatory mechanism of Na-K-ATPase should help in learning more about its role in pathophysiological states. This review summarizes the present knowledge of the role of FXYD proteins in the modulation of Na-K-ATPase as well as of other proteins, their regulation, and their structure-function relationship.

Regulation of the renal sodium-dependent phosphate cotransporter NaPi2 (Npt2) in acute renal failure due to ischemia and reperfusion

BACKGROUND

Acute renal failure (ARF) is associated with hyperphosphatemia and decreased urinary phosphate excretion. The present study was undertaken to characterize the effects of ARF due to ischemia and reperfusion on renal phosphate transport and on gene and protein expression of type IIa NaPi cotransporter (Npt2) the physiologically most relevant renal sodium-dependent phosphate cotransporter.

METHODS

The following groups of rats with intact parathyroid glands were studied: (1) sham operated (sham); (2) after 1 h ischemia by bilateral renal artery clamping (I), and after 1 h ischemia and reperfusion of 1 h (I + R 1 h); (3) 24 h (I + R 24 h); (4) 48 h (I + R 48 h), and (5) 72 h (I + R 72 h) duration. The effect of ARF on Npt2 mRNA and protein expression was also examined after parathyroidectomy (PTX) of 2 and 4 days' duration.

RESULTS

Ischemia and reperfusion were associated with increases in plasma creatinine, hyperphosphatemia, and with decreased tubular phosphate reabsorption. Npt2 mRNA was significantly downregulated in the cortex, maximal at 24 and 48 h of reperfusion. The degree of Npt2 mRNA downregulation was not affected by PTX of 2-4 days' duration. The abundance of Npt2 protein in proximal tubular apical brush border membrane was markedly decreased after reperfusion. Npt2 protein, however, was more abundant in PTX animals than in those with intact parathyroids and a similar degree of renal insufficiency. The immunohistochemical analysis of proximal tubular apical brush border membrane showed a progressive decrease of Npt2 protein labeling after ischemia and reperfusion, with progressive regeneration after 72 h.

CONCLUSION

These results suggest that downregulation of Npt2 protein may contribute to the decreased tubular reabsorption of phosphate in acute ischemic renal failure and hyperphosphatemia.

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.

FXYD proteins: new tissue-specific regulators of the ubiquitous Na,K-ATPase

Maintenance of the Na+ and K+ gradients between the intracellular and extracellular milieus of animal cells is a prerequisite for basic cellular homeostasis and for functions of specialized tissues. The Na,K-ATPase, an oligomeric P-type adenosine triphosphatase (ATPase), is composed of a catalytic alpha subunit and a regulatory beta subunit and is the main player that fulfils these tasks. A variety of regulatory mechanisms are necessary to guarantee appropriate Na,K-ATPase expression and activity adapted to changing physiological demands. Recently, a regulatory mechanism was defined that is mediated by interaction of Na,K-ATPase with small proteins of the FXYD family, which possess a single transmembrane domain and so far have been considered as channels or regulators of ion channels. The mammalian FXYD proteins FXYD1 through FXYD7 exhibit tissue-specific distribution. Phospholemman (FXYD1) in heart and skeletal muscle, the gamma subunit of Na,K-ATPase (FXYD2) and corticosteroid hormone-induced factor (FXYD4, also known as CHIF) in the kidney, and FXYD7 in the brain associate preferentially with the widely expressed Na,K-ATPase alpha1-beta1 isozyme and modulate its transport activity in a way that conforms to tissue-specific requirements. Thus, tissue- and isozyme-specific interaction of Na,K-ATPase with FXYD proteins contributes to proper handling of Na+ and K+ by the Na,K-ATPase, and ensures correct function in such processes as renal Na+-reabsorption, muscle contraction, and neuronal excitability.