Biff Forbush

A Regulatory Locus of Phosphorylation in the N Terminus of the Na-K-Cl Cotransporter, NKCC1

The secretory Na-K-Cl cotransporter NKCC1 is activated by secretagogues through a phosphorylation-dependent mechanism. We found a phosphorylation stoichiometry of 3.0 ± 0.4 phosphorylated residues/NKCC1 protein harvested from shark rectal gland tubules maximally stimulated with forskolin and calyculin A, showing that at least three sites on the cotransporter are phosphorylated upon stimulation. Three phosphoacceptor sites were identified in the N-terminal domain of the protein (at Thr184, Thr189, and Thr202) using high pressure liquid chromatography and matrix-assisted laser desorption ionization time-of-flight mass spectrometry to analyze tryptic fragments of the radiolabeled cotransporter. None of these residues occurs in the context of strong consensus sites for known Ser/Thr kinases. The threonines and the surrounding amino acids are highly conserved between NKCC1 and NKCC2, and similarities are also present in the Na-Cl cotransporter NCC (or TSC). This strongly suggests that the phosphoregulatory mechanism is conserved among isoforms. Through expression of shark NKCC1 mutants in HEK-293 cells, Thr189 was found to be necessary for activation of the protein, whereas phosphorylation at Thr184 and Thr202 was modulatory, but not required. In conjunction with the recent finding (Darmen, R. B., Flemmer, A., and Forbush, B. (2001) J. Biol. Chem. 276, 34359–34362) that protein phosphatase-1 binds to residues 107–112 in the shark NKCC1 sequence, these results demonstrate that the N terminus of NKCC1 constitutes a phosphoregulatory domain of the transporter.

Sites of Regulated Phosphorylation that Control K-Cl Cotransporter Activity

Modulation of intracellular chloride concentration ([Cl(-)](i)) plays a fundamental role in cell volume regulation and neuronal response to GABA. Cl(-) exit via K-Cl cotransporters (KCCs) is a major determinant of [Cl(-)](I); however, mechanisms governing KCC activities are poorly understood. We identified two sites in KCC3 that are rapidly dephosphorylated in hypotonic conditions in cultured cells and human red blood cells in parallel with increased transport activity. Alanine substitutions at these sites result in constitutively active cotransport. These sites are highly phosphorylated in plasma membrane KCC3 in isotonic conditions, suggesting that dephosphorylation increases KCC3s intrinsic transport activity. Reduction of WNK1 expression via RNA interference reduces phosphorylation at these sites. Homologous sites are phosphorylated in all human KCCs. KCC2 is partially phosphorylated in neonatal mouse brain and dephosphorylated in parallel with KCC2 activation. These findings provide insight into regulation of [Cl(-)](i) and have implications for control of cell volume and neuronal function.

WNK4 regulates apical and basolateral Cl– flux in extrarenal epithelia

Mutations in the serine-threonine kinase WNK4 [with no lysine (K) 4] cause pseudohypoaldosteronism type II, a Mendelian disease featuring hypertension with hyperkalemia. In the kidney, WNK4 regulates the balance between NaCl reabsorption and K+ secretion via variable inhibition of the thiazide-sensistive NaCl cotransporter and the K+ channel ROMK. We now demonstrate expression of WNK4 mRNA and protein outside the kidney. In extrarenal tissues, WNK4 is found almost exclusively in polarized epithelia, variably associating with tight junctions, lateral membranes, and cytoplasm. Epithelia expressing WNK4 include sweat ducts, colonic crypts, pancreatic ducts, bile ducts, and epididymis. WNK4 is also expressed in the specialized endothelium of the blood–brain barrier. These epithelia and endothelium all play important roles in Cl– transport. Because WNK4 is known to regulate renal Cl– handling, we tested WNK4s effect on the activity of mediators of epithelial Cl– flux whose extrarenal expression overlaps with WNK4. WNK4 proved to be a potent inhibitor of the activity of both the Na+-K+-2Cl– cotransporter (NKCC1) and the Cl–/base exchanger SLC26A6 (CFEX) (>95% inhibition of NKCC1-mediated 86Rb influx, P < 0.001; >80% inhibition of CFEX-mediated [14C] formate uptake, P < 0.001), mediators of Cl– flux across basolateral and apical membranes, respectively. In contrast, WNK4 showed no inhibition of pendrin, a related Cl–/base exchanger. These findings indicate a general role for WNK4 in the regulation of electrolyte flux in diverse epithelia. Moreover, they reveal that WNK4 regulates the activities of a diverse group of structurally unrelated ion channels, cotransporters, and exchangers.

Activation of the Na-K-Cl Cotransporter NKCC1 Detected with a Phospho-specific Antibody

The Na-K-Cl cotransporter NKCC1 is activated by phosphorylation of a regulatory domain in its N terminus. In the accompanying paper (Darman, R. B., and Forbush, B. (2002)J. Biol. Chem. 277, 37542–37550), we identify three phosphothreonines important in this process. Using a phospho-specific antibody (anti-phospho-NKCC1 antibody R5) raised against a diphosphopeptide containing Thr212 and Thr217of human NKCC, we were readily able to monitor the cotransporter activation state. In 32P phosphorylation experiments with rectal gland tubules, we show that the R5 antibody signal is proportional to the amount of 32P incorporated into NKCC1; and in experiments with NKCC1-transfected HEK-293 cells, we demonstrate that R5-detected phosphorylation directly mirrors functional activation. Immunofluorescence analysis of shark rectal gland shows activation-dependent R5 antibody staining along the basolateral membrane. In perfused rat parotid glands, isoproterenol induced staining of both acinar and ductal cells along the basolateral membrane. Isoproterenol also induced basolateral staining of the epithelial cells in rat trachea, whereas basal cells in the subepithelial tissue displayed heavy, non-polarized staining of the cell membrane. In rat colon, agonist stimulation induced staining along the basolateral membrane of crypt cells. These data provide direct evidence of NKCC1 regulation in these tissues, and they further link phosphorylation of NKCC1 with its activation in transfected cells and native tissue. The high conservation of the regulatory threonine residues among NKCC1, NKCC2, and NCC family members, together with the fact that tissues from divergent vertebrate species exhibit similar R5-binding profiles, lends further support to the role of this regulatory locus in vivo.

Inhibition of Na(+)-K(+)-Cl(-) cotransporter during focal cerebral ischemia decreases edema and neuronal damage.

Our previous study demonstrated that pharmacological inhibition of the Na(+)-K(+)-Cl(-) cotransporter isoform 1 (NKCC1) during ischemia and reperfusion attenuated neuronal damage and edema. In this study, we further investigated whether NKCC1 activity contributes to ischemic damage during either ischemia or reperfusion. Immunoblotting revealed that expression of NKCC1 protein was increased following 2-h focal ischemia in cerebral cortex. A sustained up-regulation of NKCC1 in cortex was detected at 4, 8, 12, and 24 h of reperfusion. An increase in the phosphorylated NKCC1 (NKCC1-p) was found at 4 and 8 h of reperfusion. In striatum, a significant increase in NKCC1 expression occurred between 4 and 24 h of reperfusion and no elevation of NKCC1-p signal was observed. Artificial cerebral spinal fluid (aCSF) or 100 microM bumetanide in aCSF were continuously microdialyzed into left cortices either 1 h prior to ischemia plus 2-h ischemia, or only during 24-h reperfusion. Infarction volume was significantly decreased in the pre-ischemic bumetanide-treated group (P<0.05) but not in the post-ischemic treatment group (P>0.05). In addition, pre-ischemic bumetanide treatment reduced the ipsilateral water content increase by 70% (P<0.05). Inhibition of NKCC1 did not attenuate poly (ADP-ribose) polymerase cleavage or the number of TUNEL-labeled apoptotic cells in ischemic brains. These results suggest that inhibition of NKCC1 attenuates cytotoxic edema and necrotic neuronal death during focal ischemia. Activation of NKCC1 activity plays a role in the early stage of ischemic damage.

Ion transport and ligand binding by the Na-K-Cl cotransporter, structure-function studies.

The cation-Cl cotransporters (CCCs) mediate the coupled movement of Na and/or K to that of Cl across the plasmalemma of animal cells. Eight CCCs have been identified to date: two Na-K-Cl cotransporters (NKCC), four K-Cl cotransporters (KCCs), one Na-Cl cotransporter (NCC) and one CCC interacting protein (CIP). All of the NKCCs and KCCs are inhibited by loop diuretics; mercury and other modifying agents are also known to block NKCC-mediated transport. In this work, we have utilized a mutational approach to study the interaction between different substrates and the NKCCs. We relied on the strategy of exchanging domains between functionally distinct carriers (the shark NKCCl and the human NKCCl) to identify residues or group of residues that are involved in the interaction with ions, loop diuretics and Hg. Our results show that the N- and C-termini have no role in determining the species differences in ion transport and bumetanide binding. On the other hand, the interaction between Hg and the NKCCs is found to partially involve the C-terminus through residues that contain available sulfhydryl groups. Within the transmembrane segments, variant residues in the 2nd, 4th and 7th predicted alpha-helices are shown to encode the differences in ion transport between the shark and the human cotransporters. For loop diuretic binding, several regions throughout the central domain appear to be involved. Interestingly, these regions are not the same as those involved in cation or anion transport, and in Hg binding.

The Potassium Chloride Cotransporter KCC-2 Coordinates Development of Inhibitory Neurotransmission and Synapse Structure in Caenorhabditis elegans

Chloride influx through GABA-gated chloride channels, the primary mechanism by which neural activity is inhibited in the adult mammalian brain, depends on chloride gradients established by the potassium chloride cotransporter KCC2. We used a genetic screen to identify genes important for inhibition of the hermaphrodite-specific motor neurons (HSNs) that stimulate Caenorhabditis elegans egg-laying behavior and discovered mutations in a potassium chloride cotransporter, kcc-2. Functional analysis indicates that, like mammalian KCCs, C. elegans KCC-2 transports chloride, is activated by hypotonic conditions, and is inhibited by the loop diuretic furosemide. KCC-2 appears to establish chloride gradients required for the inhibitory effects of GABA-gated and serotonin-gated chloride channels on C. elegans behavior. In the absence of KCC-2, chloride gradients appear to be altered in neurons and muscles such that normally inhibitory signals become excitatory. kcc-2 is transcriptionally upregulated in the HSN neurons during synapse development. Loss of KCC-2 produces a decrease in the synaptic vesicle population within mature HSN synapses, which apparently compensates for a lack of HSN inhibition, resulting in normal egg-laying behavior. Thus, KCC-2 coordinates the development of inhibitory neurotransmission with synapse maturation to produce mature neural circuits with appropriate activity levels.

The Residues Determining Differences in Ion Affinities among the Alternative Splice Variants F, A, and B of the Mammalian Renal Na-K-Cl Cotransporter (NKCC2)

Three alternatively spliced variants of the renal Na-K-Cl cotransporter (NKCC2) are found in distinct regions of the thick ascending limb of the mammalian kidney; these variants mediate Na+K+2Cl- transport with different ion affinities. Here, we examine the specific residues involved in the variant-specific affinity differences, utilizing a mutagenic approach to change the NKCC2B variant into the A or F variant, with functional expression in Xenopus oocytes. The splice region contains the second transmembrane domain (TM2) and the putative intracellular loop (ICL1) connecting TM2 and TM3. It is found that the B variant is functionally changed to the F variant by replacement of six residues, half of the effect brought about by three TM2 residues and half by three ICL1 residues. The involvement of the ICL1 residues strongly suggests that this region of ICL1 may actually be part of a membrane-embedded domain. Changing six residues is also sufficient to bring about the smaller functional change from the B to the A variant; three residues in TM2 appear to be primarily responsible, two of which correspond to residues involved in the B-to-F changes. A B-variant mutation reported in a mild case of Bartter disease was found to render the cotransporter inactive. These results identify the combination of amino acid variations responsible for the differences among the three splice variants of NKCC2, and they support a model in which a reentrant loop following TM2 contributes to the chloride binding and translocation domains.

Intramolecular and Intermolecular Fluorescence Resonance Energy Transfer in Fluorescent Protein-tagged Na-K-Cl Cotransporter (NKCC1) SENSITIVITY TO REGULATORY CONFORMATIONAL CHANGE AND CELL VOLUME

To examine the structure and function of the Na-K-Cl cotransporter, NKCC1, we tagged the transporter with cyan (CFP) and yellow (YFP) fluorescent proteins and measured fluorescence resonance energy transfer (FRET) in stably expressing human embryonic kidney cell lines. Fluorescent protein tags were added at the N-terminal residue between the regulatory domain and the membrane domain and within a poorly conserved region of the C terminus. Both singly and doubly tagged NKCC1s were appropriately trafficked to the cell membrane and were fully functional; regulation was normal except when YFP was inserted near the regulatory domain, in which case activation occurred only upon incubation with calyculin A. Quenching of YFP fluorescence by Cl- provided a ratiometric indicator of intracellular [Cl-]. All of the CFP/YFP NKCC pairs exhibited some level of FRET, demonstrating the presence of dimers or higher multimers in functioning NKCC1. With YFP near the regulatory domain and CFP in the C terminus, we recorded a 6% FRET change signaling the regulatory phosphorylation event. On the other hand, when the probe was placed at the extreme N terminus, such changes were not seen, presumably due to the length and predicted flexibility of the N terminus. Substantial FRET changes were observed cotemporaneous with cell volume changes, possibly reflective of an increase in molecular crowding upon cell shrinkage.

Drosophila glia use a conserved cotransporter mechanism to regulate extracellular volume.

The nervous system is protected by blood barriers that use multiple systems to control extracellular solute composition, osmotic pressure, and fluid volume. In the human nervous system, misregulation of the extracellular volume poses serious health threats. Here, we show that the glial cells that form the Drosophila blood–nerve barrier have a conserved molecular mechanism that regulates extracellular volume: the Serine/Threonine kinase Fray, which we previously showed is an ortholog of mammalian PASK/SPAK; and the Na‐K‐Cl cotransporter Ncc69, which we show is an ortholog of human NKCC1. In mammals, PASK/SPAK binds to NKCC1 and regulates its activity. In Drosophila, larvae mutant for Ncc69 develop a peripheral neuropathy, where fluid accumulates between glia and axons. The accumulation of fluid has no detectable impact on action potential conduction, suggesting that the role of Ncc69 is to maintain volume or osmotic homeostasis. Drosophila Ncc69 has kinetics similar to human NKCC1, and NKCC1 can rescue Ncc69, suggesting that they function in a conserved physiological mechanism. We show that fray and Ncc69 are coexpressed in nerve glia, interact in a yeast‐two‐hybrid assay, and have an essentially identical bulging nerve phenotype. We propose that normally functioning nerves generate extracellular solutes that are removed by Ncc69 under the control of Fray. This mechanism may perform a similar role in humans, given that NKCC1 is expressed at the blood–brain barrier.