Red Blood Cell (RBC) membrane proteomics — Part II: Comparative proteomics and RBC patho-physiology

Repository of Enriched Structures of Proteins Involved in the Red Blood Cell Environment (RESPIRE)

The Red Blood Cell (RBC) is a metabolically-driven cell vital for processes such a gas transport and homeostasis. RBC possesses at its surface exposing antigens proteins that are critical in blood transfusion. Due to their importance, numerous studies address the cell function as a whole but more and more details of RBC structure and protein content are now studied using massive state-of-the art characterisation techniques. Yet, the resulting information is frequently scattered in many scientific articles, in many databases and specialized web servers. To provide a more compendious view of erythrocytes and of their protein content, we developed a dedicated database called RESPIRE that aims at gathering a comprehensive and coherent ensemble of information and data about proteins in RBC. This cell-driven database lists proteins found in erythrocytes. For a given protein entry, initial data are processed from external portals and enriched by using state-of-the-art bioinformatics methods. As structural information is extremely useful to understand protein function and predict the impact of mutations, a strong effort has been put on the prediction of protein structures with a special treatment for membrane proteins. Browsing the database is available through text search for reference gene names or protein identifiers, through pre-defined queries or via hyperlinks. The RESPIRE database provides valuable information and unique annotations that should be useful to a wide audience of biologists, clinicians and structural biologists.

Database URL:http://www.dsimb.inserm.fr/respire

Proteomic analysis of red blood cells and the potential for the clinic: what have we learned so far?

INTRODUCTION

Red blood cells (RBC) are the most abundant host cells in the human body. Mature erythrocytes are devoid of nuclei and organelles and have always been regarded as circulating 'bags of hemoglobin'. The advent of proteomics has challenged this assumption, revealing unanticipated complexity and novel roles for RBCs not just in gas transport, but also in systemic metabolic homeostasis in health and disease. Areas covered: In this review we will summarize the main advancements in the field of discovery mode and redox/quantitative proteomics with respect to RBC biology. We thus focus on translational/clinical applications, such as transfusion medicine, hematology (e.g. hemoglobinopathies) and personalized medicine. Synergy of omics technologies - especially proteomics and metabolomics - are highlighted as a hallmark of clinical metabolomics applications for the foreseeable future. Expert commentary: The introduction of advanced proteomics technologies, especially quantitative and redox proteomics, and the integration of proteomics data with omics information gathered through orthogonal technologies (especially metabolomics) promise to revolutionize many biomedical areas, from hematology and transfusion medicine to personalized medicine and clinical biochemistry.

Multi-omics Evidence for Inheritance of Energy Pathways in Red Blood Cells

Each year over 90 million units of blood are transfused worldwide. Our dependence on this blood supply mandates optimized blood management and storage. During storage, red blood cells undergo degenerative processes resulting in altered metabolic characteristics which may make blood less viable for transfusion. However, not all stored blood spoils at the same rate, a difference that has been attributed to variable rates of energy usage and metabolism in red blood cells. Specific metabolite abundances are heritable traits; however, the link between heritability of energy metabolism and red blood cell storage profiles is unclear. Herein we performed a comprehensive metabolomics and proteomics study of red blood cells from 18 mono- and di-zygotic twin pairs to measure heritability and identify correlations with ATP and other molecular indices of energy metabolism. Without using affinity-based hemoglobin depletion, our work afforded the deepest multi-omic characterization of red blood cell membranes to date (1280 membrane proteins and 330 metabolites), with 119 membrane protein and 148 metabolite concentrations found to be over 30% heritable. We demonstrate a high degree of heritability in the concentration of energy metabolism metabolites, especially glycolytic metabolites. In addition to being heritable, proteins and metabolites involved in glycolysis and redox metabolism are highly correlated, suggesting that crucial energy metabolism pathways are inherited en bloc at distinct levels. We conclude that individuals can inherit a phenotype composed of higher or lower concentrations of these proteins together. This can result in vastly different red blood cells storage profiles which may need to be considered to develop precise and individualized storage options. Beyond guiding proper blood storage, this intimate link in heritability between energy and redox metabolism pathways may someday prove useful in determining the predisposition of an individual toward metabolic diseases.

A comparative protein profile of mammalian erythrocyte membranes identified by mass spectrometry

A comparative analysis of erythrocyte membrane proteins of economically important animals, goat (Capra aegagrus hircus), buffalo (Bubalus bubalis), pig (Sus scrofa), cow (Bos tauras), and human (Homo sapiens) was performed. Solubilized erythrocyte membrane proteins were separated by sodium dodecyl sulfate-polyacryamide gel electrophoresis (SDS-PAGE), visualized by staining the gels with Commassie Brilliant Blue (CBB), and identified by matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/MS). Emerging results show that all major erythrocyte membrane proteins present in human are also seen in all the animals except for band 4.5 which could not be identified. Band 3 is seen as more intense and compact, band 4.1 appears as a doublet in all the animal erythrocyte membranes, band 4.2 exhibits a slightly higher molecular weight (Mr) in buffalo, and cow and band 4.9 has a higher Mr in all the animals relative to the human protein. In addition, there are two new bands in the goat membrane, band G1, identified as HSP 90α, and band G2 identified as HSP 70. A new band C2 identified as HSP 70 is also seen in cow membranes. Peroxiredoxin II is of lower intensity and/or higher Mr in the animals. The difference in size of the proteins possibly indicates the variations in the composition of the amino acids. The difference in intensity of the proteins among these mammalians highlights the presence of less or more number of copies of that protein per cell. This data complement the earlier observations of differences in the sialoglycoprotein profile and effect of proteases and neuraminidase on agglutination among the mammalian erythrocytes. This study provides a platform to understand the molecular architecture of the individual erythrocytes, and in turn the dependent disorders, their phylogenetic relationship and also generates a database of erythrocyte membrane proteins of mammals. The animals selected for this study are of economic importance as they provide milk for the dairy industry and raw material for leather industry and are routinely sacrificed to obtain non vegetarian food worldwide.

Red blood cell vesiculation in hereditary hemolytic anemia

Hereditary hemolytic anemia encompasses a heterogeneous group of anemias characterized by decreased red blood cell survival because of inherited membrane, enzyme, or hemoglobin disorders. Affected red blood cells are more fragile, less deformable, and more susceptible to shear stress and oxidative damage, and show increased vesiculation. Red blood cells, as essentially all cells, constitutively release phospholipid extracellular vesicles in vivo and in vitro in a process known as vesiculation. These extracellular vesicles comprise a heterogeneous group of vesicles of different sizes and intracellular origins. They are described in literature as exosomes if they originate from multi-vesicular bodies, or as microvesicles when formed by a one-step budding process directly from the plasma membrane. Extracellular vesicles contain a multitude of bioactive molecules that are implicated in intercellular communication and in different biological and pathophysiological processes. Mature red blood cells release in principle only microvesicles. In hereditary hemolytic anemias, the underlying molecular defect affects and determines red blood cell vesiculation, resulting in shedding microvesicles of different compositions and concentrations. Despite extensive research into red blood cell biochemistry and physiology, little is known about red cell deformability and vesiculation in hereditary hemolytic anemias, and the associated pathophysiological role is incompletely assessed. In this review, we discuss recent progress in understanding extracellular vesicles biology, with focus on red blood cell vesiculation. Also, we review recent scientific findings on the molecular defects of hereditary hemolytic anemias, and their correlation with red blood cell deformability and vesiculation. Integrating bio-analytical findings on abnormalities of red blood cells and their microvesicles will be critical for a better understanding of the pathophysiology of hereditary hemolytic anemias.

Hyperspectral enhanced dark field microscopy for imaging blood cells

In this work, a novel methodology based on hyperspectral imagery with enhanced Darkfield microscopy for probing and characterizing changes in blood cell components was tested. Two main categories of blood cells were analyzed, red and white blood cells. Unique spectral signatures of ordinary and most common deformed morphologies of red blood cells were identified. Moreover, examination of white blood cells allowed to characterize and differentiate active from inactive cells. The findings indicate the ability of this technique to detect changes in light scattering property of blood cells due to their morphological properties Since pathological states can alterate the discocyte shape, this preliminary, but promising application of the hyperspectral analysis to blood cells can be useful to evaluate significant correlations of blood cell spectral features in healthy and pathological conditions. The combination of the quali- and quantitative spectral signatures of hyperspectral imaging microscopy with the information of the subject health conditions may provide a new tool for clinical applications.

Isolation technique and proteomic analysis of the erythrocyte ghosts of red-eared turtle (Trachemys scripta)

To proceed proteomic analysis of erythrocyte of the red-eared turtle Trachemys scripta, a method for obtaining turtle erythrocyte ghosts (TEG) was first developed. After hypotonic lysis using a special buffer, forcing the erythrocyte through the syringe with an "N"-shaped needle, applying low speed homogenizing and differential centrifugation, highly purified TEG fractions were obtained. The isolated TEG proteins were treated with in-gel digestion separated by SDS-PAGE or in-solution digestion followed by HPLC predissociation, and then identified by nano-ESI-LC MS/MS techniques. A total of 169 TEG proteins was identified, validated, and categorized. Among these proteins, tubulins, and cell-surface-located F-type ATP synthase revealed important information into the super tolerance of Trachemys scripta in anoxia and low temperature exposure. Altogether, this study not only provided a method to isolate high quality TEG and a dataset of comprehensive characterization of TEG proteins, but also provides a tool for proteomic research of all nucleated red blood cells, and thus opened a new research field for exploring the mechanisms of super tolerance of turtles in harsh environment.

The proteomics and interactomics of human erythrocytes

In this minireview, we focus on advances in our knowledge of the human erythrocyte proteome and interactome that have occurred since our seminal review on the topic published in 2007. As will be explained, the number of unique proteins has grown from 751 in 2007 to 2289 as of today. We describe how proteomics and interactomics tools have been used to probe critical protein changes in disorders impacting the blood. The primary example used is the work done on sickle cell disease where biomarkers of severity have been identified, protein changes in the erythrocyte membranes identified, pharmacoproteomic impact of hydroxyurea studied and interactomics used to identify erythrocyte protein changes that are predicted to have the greatest impact on protein interaction networks.

Membrane proteins of human fetal primitive nucleated red blood cells

In humans, primitive fetal nucleated red blood cells (FNRBCs) are thought to be as vital for embryonic life as their counterpart, adult red blood cells (adult RBCs) are in later-gestation fetuses and adults. Unlike adult RBCs, the identity and functions of FNRBC proteins are poorly understood owing to a scarcity of FNRBCs for proteomic investigations. The study aimed to investigate membrane proteins of this unique cell type. We present here, the first report on the membrane proteome of human primitive FNRBCs investigated by two-dimensional liquid chromatography coupled with mass-spectrometry (2D-LCMS/MS) and bioinformatics analysis. A total of 273 proteins were identified, of which 133 (48.7%) were membrane proteins. We compared our data with membrane proteins of adult RBCs to identify common, and unique, surface membrane proteins. Twelve plasma membrane proteins with transmembrane domains and eight proteins with transmembrane domains but without known sub-cellular location were identified as unique-to-FNRBCs. Except for the transferrin receptor, all other 19 unique-to-FNRBC membrane proteins have never been described in RBCs. Reverse-transcriptase PCR (RT-PCR) and immunocytochemistry validated the 2D-LCMS/MS data. Our findings provide potential surface antigens for separation of primitive FNRBCs from maternal blood for noninvasive prenatal diagnosis, and to understand the biology of these rare cells.

Profiling the erythrocyte membrane proteome isolated from patients diagnosed with chronic obstructive pulmonary disease

Structural and metabolic alterations in erythrocytes play an important role in the pathophysiology of Chronic Obstructive Pulmonary Disease (COPD). Whether these dysfunctions are related to the modulation of erythrocyte membrane proteins in patients diagnosed with COPD remains to be determined. Herein, a comparative proteomic profiling of the erythrocyte membrane fraction isolated from peripheral blood of smokers diagnosed with COPD and smokers with no COPD was performed using differential (16)O/(18)O stable isotope labeling. A total of 219 proteins were quantified as being significantly differentially expressed within the erythrocyte membrane proteomes of smokers with COPD and healthy smokers. Functional pathway analysis showed that the most enriched biofunctions were related to cell-to-cell signaling and interaction, hematological system development, immune response, oxidative stress and cytoskeleton. Chorein (VPS13A), a cytoskeleton related protein whose defects had been associated with the presence of cell membrane deformation of circulating erythrocytes was found to be down-regulated in the membrane fraction of erythrocytes obtained from COPD patients. Methemoglobin reductase (CYB5R3) was also found to be underexpressed in these cells, suggesting that COPD patients may be at higher risk for developing methemoglobinemia. This article is part of a Special Issue entitled: Integrated omics.

Palmitoylation of MPP1 (membrane-palmitoylated protein 1)/p55 is crucial for lateral membrane organization in erythroid cells

S-Acylation of proteins is a ubiquitous post-translational modification and a common signal for membrane association. The major palmitoylated protein in erythrocytes is MPP1, a member of the MAGUK family and an important component of the ternary complex that attaches the spectrin-based skeleton to the plasma membrane. Here we show that DHHC17 is the only acyltransferase present in red blood cells (RBC). Moreover, we give evidence that protein palmitoylation is essential for membrane organization and is crucial for proper RBC morphology, and that the effect is specific for MPP1. Our observations are based on the clinical cases of two related patients whose RBC had no palmitoylation activity, caused by a lack of DHHC17 in the membrane, which resulted in a strong decrease of the amount of detergent-resistant membrane (DRM) material. We confirmed that this loss of detergent-resistant membrane was due to the lack of palmitoylation by treatment of healthy RBC with 2-bromopalmitic acid (2-BrP, common palmitoylation inhibitor). Concomitantly, fluorescence lifetime imaging microscopy (FLIM) analyses of an order-sensing dye revealed a reduction of membrane order after chemical inhibition of palmitoylation in erythrocytes. These data point to a pathophysiological relationship between the loss of MPP1-directed palmitoylation activity and perturbed lateral membrane organization.

Computational identification of phospho-tyrosine sub-networks related to acanthocyte generation in neuroacanthocytosis

Acanthocytes, abnormal thorny red blood cells (RBC), are one of the biological hallmarks of neuroacanthocytosis syndromes (NA), a group of rare hereditary neurodegenerative disorders. Since RBCs are easily accessible, the study of acanthocytes in NA may provide insights into potential mechanisms of neurodegeneration. Previous studies have shown that changes in RBC membrane protein phosphorylation state affect RBC membrane mechanical stability and morphology. Here, we coupled tyrosine-phosphoproteomic analysis to topological network analysis. We aimed to predict signaling sub-networks possibly involved in the generation of acanthocytes in patients affected by the two core NA disorders, namely McLeod syndrome (MLS, XK-related, Xk protein) and chorea-acanthocytosis (ChAc, VPS13A-related, chorein protein). The experimentally determined phosphoproteomic data-sets allowed us to relate the subsequent network analysis to the pathogenetic background. To reduce the network complexity, we combined several algorithms of topological network analysis including cluster determination by shortest path analysis, protein categorization based on centrality indexes, along with annotation-based node filtering. We first identified XK- and VPS13A-related protein-protein interaction networks by identifying all the interactomic shortest paths linking Xk and chorein to the corresponding set of proteins whose tyrosine phosphorylation was altered in patients. These networks include the most likely paths of functional influence of Xk and chorein on phosphorylated proteins. We further refined the analysis by extracting restricted sets of highly interacting signaling proteins representing a common molecular background bridging the generation of acanthocytes in MLS and ChAc. The final analysis pointed to a novel, very restricted, signaling module of 14 highly interconnected kinases, whose alteration is possibly involved in generation of acanthocytes in MLS and ChAc.

Proteomic analysis of membrane proteins from Streptococcus pneumoniae with multiple separation methods plus high accuracy mass spectrometry

Abstract Streptococcus pneumoniae is a Gram-positive human pathogen that causes a variety of serious mucosal and invasive diseases in human. Bacterial membrane proteins play crucial roles in host-pathogen interactions and bacterial pathogenesis, and thus are potential drug targets or vaccine candidates. In this study, membranes from Streptococcus pneumoniae D39 were enriched by mechanical grinding and ultracentrifugation, and then the membrane proteins were extracted with trifluroethanol and chloroform. Around 60% of the extracted proteins were identified to be membrane proteins with 2-DE coupled with MALDI-MS/MS and 2D-LC-ESI-MS/MS. These identified membrane proteins can be functionally categorized into various groups involved in nutriment transport, signal transduction, protein folding or secretion, oxidation, carbohydrate metabolism, and other physiological processes. A protein interaction network was constructed for understanding the regulation relationship of the membrane proteins. This study represents the first global characterization of membrane proteome from Gram-positive streptococcus species of bacteria, providing valuable clues for further investigation aiming at identifying drug/vaccine targets for the bacterial infection.

Depletion of hemoglobin and carbonic anhydrase from erythrocyte cytosolic samples by preparative clear native electrophoresis

Proteomic analysis of red cells is compromised by the presence of high-abundance proteins (hemoglobin and carbonic anhydrase-1), which completely obscure low-abundance species. The depletion method presented here involves performing native gel electrophoresis in a polyacrylamide gel tube using a modified electroelution cell. The electrophoretic run is interrupted intermittently to allow the recovery of at least three different liquid fractions, which can be analyzed by both native PAGE and 2D isoelectric focusing SDS-PAGE, or by shotgun mass spectrometry analysis after trypsin in-solution protein digestion. This low-cost, reproducible technique can be used to process large amounts of sample, and it increases the likelihood of detecting low-abundance proteins, thereby resulting in greater proteome coverage. The separation procedure takes approximately 6-7 h.

In-depth analysis of cysteine oxidation by the RBC proteome: advantage of peroxiredoxin II knockout mice

Peroxiredoxin II (Prdx II, a typical 2-Cys Prdx) has been originally isolated from erythrocytes, and its structure and peroxidase activity have been adequately studied. Mice lacking Prdx II proteins had heinz bodies in their peripheral blood, and morphologically abnormal cells were detected in the dense red blood cell (RBC) fractions, which contained markedly higher levels of reactive oxygen species (ROS). In this study, a labeling experiment with the thiol-modifying reagent biotinylated iodoacetamide (BIAM) in Prdx II-/- mice revealed that a variety of RBC proteins were highly oxidized. To identify oxidation-sensitive proteins in Prdx II-/- mice, we performed RBC comparative proteome analysis in membrane and cytosolic fractions by nano-UPLC-MSE shotgun proteomics. We found oxidation-sensitive 54 proteins from 61 peptides containing cysteine oxidation, and analyzed comparative expression pattern in healthy RBCs of Prdx II+/+ mice, healthy RBCs of Prdx II-/- mice, and abnormal RBCs of Prdx II-/- mice. These proteins belonged to cellular functions related with RBC lifespan maintain, such as cytoskeleton, stress-induced proteins, metabolic enzymes, signal transduction, and transporters. Furthermore, protein networks among identified oxidation-sensitive proteins were analyzed to associate with various diseases. Consequently, we expected that RBC proteome might provide clues to understand redox-imbalanced diseases.

Erythrocyte membrane changes of chorea-acanthocytosis are the result of altered Lyn kinase activity

Acanthocytic RBCs are a peculiar diagnostic feature of chorea-acanthocytosis (ChAc), a rare autosomal recessive neurodegenerative disorder. Although recent years have witnessed some progress in the molecular characterization of ChAc, the mechanism(s) responsible for generation of acanthocytes in ChAc is largely unknown. As the membrane protein composition of ChAc RBCs is similar to that of normal RBCs, we evaluated the tyrosine (Tyr)-phosphorylation profile of RBCs using comparative proteomics. Increased Tyr phosphorylation state of several membrane proteins, including band 3, β-spectrin, and adducin, was noted in ChAc RBCs. In particular, band 3 was highly phosphorylated on the Tyr-904 residue, a functional target of Lyn, but not on Tyr-8, a functional target of Syk. In ChAc RBCs, band 3 Tyr phosphorylation by Lyn was independent of the canonical Syk-mediated pathway. The ChAc-associated alterations in RBC membrane protein organization appear to be the result of increased Tyr phosphorylation leading to altered linkage of band 3 to the junctional complexes involved in anchoring the membrane to the cytoskeleton as supported by coimmunoprecipitation of β-adducin with band 3 only in ChAc RBC-membrane treated with the Lyn-inhibitor PP2. We propose this altered association between membrane skeleton and membrane proteins as novel mechanism in the generation of acanthocytes in ChAc.

Elevated levels of redox regulators, membrane-bound globin chains, and cytoskeletal protein fragments in hereditary spherocytosis erythrocyte proteome

OBJECTIVE

Hereditary spherocytosis (HS), a common inherited hemolytic anemia characterized by decreased deformability, reduced surface to volume ratio, and increased osmotic fragility of the spheroidal erythrocytes, is associated with several mutations of α- and β-spectrin, ankyrin, band 3, band 4.2. HS manifests itself with high degrees of clinical heterogeneity and the molecular events leading to premature hemolysis of the spherocytes are unclear. We have employed proteomic techniques to identify differentially regulated proteins in the membrane and hemoglobin-depleted cytosol of HS erythrocytes.

METHODS

We have employed 2-D gel electrophoresis and tandem matrix assisted laser desorption ionization-time of flight/time of flight mass spectrometry to investigate the differential proteome profiling of membrane and hemoglobin-depleted cytosol of erythrocytes isolated from the peripheral blood samples of HS patients and normal volunteers.

RESULTS

Our study showed that redox regulators are up-regulated; while a co-chaperone and a nucleotide kinase are down-regulated in HS erythrocyte cytosol. We observed elevated levels of membrane-associated globin chains and low-molecular weight fragments of several major cytoskeletal proteins.

CONCLUSION

The observed changes in the erythrocyte proteomes indicate altered redox regulation, nucleotide metabolism, protein aggregation and/or degradation, cytoskeletal disorganization, and severe oxidative stress in HS. Taken together, this study could enlighten upon disease progression and pathophysiology of HS.

Red blood cell proteomics

Since its discovery in the 17th century, the red blood cell, recognized in time as the critical cell component for survival, has been the focus of much attention. Its unique role in gas exchange (oxygen/CO(2) transport) and its distinct characteristics (absence of nucleus; biconcave cell shape) together with an - in essence - unlimited supply lead to extensive targeted biochemical, molecular and structural studies. A quick PubMed query with the word "erythrocyte" results in 198 013 scientific articles of which 162 are red blood cell proteomics studies, indicating that this new technique has been only recently applied to the red blood cell and related fields. Standard and comparative proteomics have been widely used to study different blood components. A growing body of proteomics literature has since developed, which deals with the characterization of red blood cells in health and disease. The possibility offered by proteomics to obtain a global snapshot of the whole red blood cell protein make-up, has provided unique insights to many fields including transfusion medicine, anaemia studies, intra-red blood cell parasite biology and translational research. While the contribution of proteomics is beyond doubt, a full red blood cell understanding will ultimately require, in addition to proteomics, lipidomics, glycomics, interactomics and study of post-translational modifications. In this review we will briefly discuss the methodology and limitations of proteomics, the contribution it made to the understanding of the erythrocyte and the advances in red blood cell-related fields brought about by comparative proteomics.

Stimulation of human and mouse erythrocyte Na(+)-K(+)-2Cl(-) cotransport by osmotic shrinkage does not involve AMP-activated protein kinase, but is associated with STE20/SPS1-related proline/alanine-rich kinase activation

This study was undertaken to investigate whether the mechanism of increased Na(+)-K(+)-2Cl(-) (NKCC1) cotransporter activity by osmotic shrinkage involved AMP-activated protein kinase (AMPK) activation. AMPK was found to phosphorylate a recombinant GST-dogfish (1-260) NKCC1 fragment at Ser38 and Ser214, corresponding to Ser77 and Ser242 in human NKCC1, respectively. Incubation of human erythrocytes with 20 microM A769662 AMPK activator increased Ser242 NKCC1 phosphorylation but did not stimulate (86)Rb(+) uptake. Under hypertonic conditions in human red blood cells (RBCs) incubated with 0.3 M sucrose, NKCC1 activity increased as measured by bumetanide-sensitive (86)Rb(+) uptake and AMPK was activated. However, there was no effect of AMPKalpha1 deletion in mouse RBCs on the increased rate of (86)Rb(+) uptake induced by hyperosmolarity. AMPK activation by osmotic shrinkage of mouse RBCs was abrogated by 10 microM STO-609 CaMKKbeta inhibitor, but incubation with STO-609 did not affect the increase in (86)Rb(+) uptake induced by hyperosmolarity. Osmotic shrinkage of human and mouse RBCs led to activation loop phosphorylation of the STE20/SPS1-related proline/alanine-rich kinase (SPAK) at Thr233, which was accompanied by phosphorylation of NKCC1 at Thr203/207/212, one of which (Thr207) is responsible for cotransporter activation. Therefore, phosphorylation-induced activation of NKCC1 by osmotic shrinkage does not involve AMPK and is likely to be due to SPAK activation.