Functional expression of human NKCC1 from a synthetic cassette-based cDNA: introduction of extracellular epitope tags and removal of cysteines

The biogenesis of potassium transporters: implications of disease-associated mutations

The concentration of intracellular and extracellular potassium is tightly regulated due to the action of various ion transporters, channels, and pumps, which reside primarily in the kidney. Yet, potassium transporters and cotransporters play vital roles in all organs and cell types. Perhaps not surprisingly, defects in the biogenesis, function, and/or regulation of these proteins are linked to range of catastrophic human diseases, but to date, few drugs have been approved to treat these maladies. In this review, we discuss the structure, function, and activity of a group of potassium-chloride cotransporters, the KCCs, as well as the related sodium-potassium-chloride cotransporters, the NKCCs. Diseases associated with each of the four KCCs and two NKCCs are also discussed. Particular emphasis is placed on how these complex membrane proteins fold and mature in the endoplasmic reticulum, how non-native forms of the cotransporters are destroyed in the cell, and which cellular factors oversee their maturation and transport to the cell surface. When known, we also outline how the levels and activities of each cotransporter are regulated. Open questions in the field and avenues for future investigations are further outlined.

Pharmacology of Compounds Targeting Cation-Chloride Cotransporter Physiology

Transporters of the solute carrier family 12 (SLC12) carry inorganic cations such as Na+ and/or K+ alongside Cl across the plasma membrane of cells. These tightly coupled, electroneutral, transporters are expressed in almost all tissues/organs in the body where they fulfil many critical functions. The family includes two key transporters participating in salt reabsorption in the kidney: the Na-K-2Cl cotransporter-2 (NKCC2), expressed in the loop of Henle, and the Na-Cl cotransporter (NCC), expressed in the distal convoluted tubule. NCC and NKCC2 are the targets of thiazides and "loop" diuretics, respectively, drugs that are widely used in clinical medicine to treat hypertension and edema. Bumetanide, in addition to its effect as a loop diuretic, has recently received increasing attention as a possible therapeutic agent for neurodevelopmental disorders. This chapter also describes how over the past two decades, the pharmacology of Na+ independent transporters has expanded significantly to provide novel tools for research. This work has indeed led to the identification of compounds that are 100-fold to 1000-fold more potent than furosemide, the first described inhibitor of K-Cl cotransport, and identified compounds that possibly directly stimulate the function of the K-Cl cotransporter. Finally, the recent cryo-electron microscopy revolution has begun providing answers as to where and how pharmacological agents bind to and affect the function of the transporters.

NKCC1 and KCC2 Chloride Transporters Have Different Membrane Dynamics on the Surface of Hippocampal Neurons

Na-K-2Cl cotransporter 1 (NKCC1) regulates chloride influx in neurons and thereby GABAA receptor activity in normal and pathological conditions. Here, we characterized in hippocampal neurons the membrane expression, distribution and dynamics of exogenous NKCC1a and NKCC1b isoforms and compared them to those of the chloride extruder K-Cl cotransporter 2 (KCC2). We found that NKCC1a and NKCC1b behave quite similarly. NKCC1a/1b but not KCC2 are present along the axon initial segment where they are confined. Moreover, NKCC1a/1b are detected in the somato-dendritic compartment at a lower level than KCC2, where they form fewer, smaller and less compact clusters at perisynaptic and extrasynaptic sites. Interestingly, ~60% of dendritic clusters of NKCC1a/1b are colocalized with KCC2. They are larger and brighter than those devoid of KCC2, suggesting a particular NKCC1a/1b-KCC2 relationship. In agreement with the reduced dendritic clustering of NKCC1a/1b compared with that of KCC2, NKCC1a/1b are more mobile on the dendrite than KCC2, suggesting weaker cytoskeletal interaction. NKCC1a/b are confined to endocytic zones, where they spend more time than KCC2. However, they spend less time in these compartments than at the synapses, suggesting that they can rapidly leave endocytic zones to increase the membrane pool, which can happen in pathological conditions. Thus, NKCC1a/b have different membrane dynamics and clustering from KCC2, which helps to explain their low level in the neuronal membrane, while allowing a rapid increase in the membrane pool under pathological conditions.

Choroid plexus-targeted NKCC1 overexpression to treat post-hemorrhagic hydrocephalus

Post-hemorrhagic hydrocephalus (PHH) refers to a life-threatening accumulation of cerebrospinal fluid (CSF) that occurs following intraventricular hemorrhage (IVH). An incomplete understanding of this variably progressive condition has hampered the development of new therapies beyond serial neurosurgical interventions. Here, we show a key role for the bidirectional Na-K-Cl cotransporter, NKCC1, in the choroid plexus (ChP) to mitigate PHH. Mimicking IVH with intraventricular blood led to increased CSF [K+] and triggered cytosolic calcium activity in ChP epithelial cells, which was followed by NKCC1 activation. ChP-targeted adeno-associated viral (AAV)-NKCC1 prevented blood-induced ventriculomegaly and led to persistently increased CSF clearance capacity. These data demonstrate that intraventricular blood triggered a trans-choroidal, NKCC1-dependent CSF clearance mechanism. Inactive, phosphodeficient AAV-NKCC1-NT51 failed to mitigate ventriculomegaly. Excessive CSF [K+] fluctuations correlated with permanent shunting outcome in humans following hemorrhagic stroke, suggesting targeted gene therapy as a potential treatment to mitigate intracranial fluid accumulation following hemorrhage.

Lateral Diffusion of NKCC1 Contributes to Chloride Homeostasis in Neurons and Is Rapidly Regulated by the WNK Signaling Pathway

An upregulation of the Na⁺-K⁺-2Cl^- cotransporter NKCC1, the main chloride importer in mature neurons, can lead to depolarizing/excitatory responses mediated by GABA type A receptors (GABA_ARs) and, thus, to hyperactivity. Understanding the regulatory mechanisms of NKCC1 would help prevent intra-neuronal chloride accumulation that occurs in pathologies with defective inhibition. The cell mechanisms regulating NKCC1 are poorly understood. Here, we report in mature hippocampal neurons that GABAergic activity controls the membrane diffusion and clustering of NKCC1 via the chloride-sensitive WNK lysine deficient protein kinase 1 (WNK1) and the downstream Ste20 Pro-line Asparagine Rich Kinase (SPAK) kinase that directly phosphorylates NKCC1 on key threonine residues. At rest, this signaling pathway has little effect on intracellular Cl^- concentration, but it participates in the elevation of intraneuronal Cl^- concentration in hyperactivity conditions associated with an up-regulation of NKCC1. The fact that the main chloride exporter, the K⁺-Cl^- cotransporter KCC2, is also regulated in mature neurons by the WNK1 pathway indicates that this pathway will be a target of choice in the pathology.

Structural basis for inhibition of the Cation-chloride cotransporter NKCC1 by the diuretic drug bumetanide

Cation-chloride cotransporters (CCCs) NKCC1 and NKCC2 catalyze electroneutral symport of 1 Na⁺, 1 K⁺, and 2 Cl⁻ across cell membranes. NKCC1 mediates trans-epithelial Cl⁻ secretion and regulates excitability of some neurons and NKCC2 is critical to renal salt reabsorption. Both transporters are inhibited by the so-called loop diuretics including bumetanide, and these drugs are a mainstay for treating edema and hypertension. Here, our single-particle electron cryo-microscopy structures supported by functional studies reveal an outward-facing conformation of NKCC1, showing bumetanide wedged into a pocket in the extracellular ion translocation pathway. Based on these and the previously published inward-facing structures, we define the translocation pathway and the conformational changes necessary for ion translocation. We also identify an NKCC1 dimer with separated transmembrane domains and extensive transmembrane and C-terminal domain interactions. We further define an N-terminal phosphoregulatory domain that interacts with the C-terminal domain, suggesting a mechanism whereby (de)phosphorylation regulates NKCC1 by tuning the strength of this domain association.

Loop diuretics including bumetanide inhibit Na⁺-K⁺-Cl⁻-cotransporters (NKCCs) and are used for the treatment of edema and hypertension. Here, Zhao et. al. report structures of NKCC1 with bumetanide bound, revealing its mechanism of action that would facilitate design of novel diuretics.

Novel γ-sarcoglycan interactors in murine muscle membranes

BACKGROUND

The sarcoglycan complex (SC) is part of a network that links the striated muscle cytoskeleton to the basal lamina across the sarcolemma. The SC coordinates changes in phosphorylation and Ca⁺⁺-flux during mechanical deformation, and these processes are disrupted with loss-of-function mutations in gamma-sarcoglycan (Sgcg) that cause Limb girdle muscular dystrophy 2C/R5.

METHODS

To gain insight into how the SC mediates mechano-signaling in muscle, we utilized LC-MS/MS proteomics of SC-associated proteins in immunoprecipitates from enriched sarcolemmal fractions. Criteria for inclusion were co-immunoprecipitation with anti-Sgcg from C57BL/6 control muscle and under-representation in parallel experiments with Sgcg-null muscle and with non-specific IgG. Validation of interaction was performed in co-expression experiments in human RH30 rhabdomyosarcoma cells.

RESULTS

We identified 19 candidates as direct or indirect interactors for Sgcg, including the other 3 SC proteins. Novel potential interactors included protein-phosphatase-1-catalytic-subunit-beta (Ppp1cb, PP1b) and Na⁺-K⁺-Cl^--co-transporter NKCC1 (SLC12A2). NKCC1 co-localized with Sgcg after co-expression in human RH30 rhabdomyosarcoma cells, and its cytosolic domains depleted Sgcg from cell lysates upon immunoprecipitation and co-localized with Sgcg after detergent permeabilization. NKCC1 localized in proximity to the dystrophin complex at costameres in vivo. Bumetanide inhibition of NKCC1 cotransporter activity in isolated muscles reduced SC-dependent, strain-induced increases in phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). In silico analysis suggests that candidate SC interactors may cross-talk with survival signaling pathways, including p53, estrogen receptor, and TRIM25.

CONCLUSIONS

Results support that NKCC1 is a new SC-associated signaling protein. Moreover, the identities of other candidate SC interactors suggest ways by which the SC and NKCC1, along with other Sgcg interactors such as the membrane-cytoskeleton linker archvillin, may regulate kinase- and Ca⁺⁺-mediated survival signaling in skeletal muscle.

High-Resolution Views and Transport Mechanisms of the NKCC1 and KCC Transporters

Cation-chloride cotransporters (CCCs) are responsible for the coupled co-transport of Cl- with K+ and/or Na+ in an electroneutral manner. They play important roles in myriad fundamental physiological processes--from cell volume regulation to transepithelial solute transport and intracellular ion homeostasis--and are targeted by medicines commonly prescribed to treat hypertension and edema. After several decades of studies into the functions and pharmacology of these transporters, there have been several breakthroughs in the structural determination of CCC transporters. The insights provided by these new structures for the Na+/K+/Cl- cotransporter NKCC1 and the K+/Cl- cotransporters KCC1, KCC2, KCC3 and KCC4 have deepened our understanding of their molecular basis and transport function. This focused review discusses recent advances in the structural and mechanistic understanding of CCC transporters, including architecture, dimerization, functional roles of regulatory domains, ion binding sites, and coupled ion transport.

WNK1 is an assembly factor for the human ER membrane protein complex

The assembly of nascent proteins into multi-subunit complexes is a tightly regulated process that must occur at high fidelity to maintain cellular homeostasis. The ER membrane protein complex (EMC) is an essential insertase that requires seven membrane-spanning and two soluble cytosolic subunits to function. Here, we show that the kinase with no lysine 1 (WNK1), known for its role in hypertension and neuropathy, functions as an assembly factor for the human EMC. WNK1 uses a conserved amphipathic helix to stabilize the soluble subunit, EMC2, by binding to the EMC2-8 interface. Shielding this hydrophobic surface prevents promiscuous interactions of unassembled EMC2 and directly competes for binding of E3 ubiquitin ligases, permitting assembly. Depletion of WNK1 thus destabilizes both the EMC and its membrane protein clients. This work describes an unexpected role for WNK1 in protein biogenesis and defines the general requirements of an assembly factor that will apply across the proteome.

The structural basis of function and regulation of neuronal cotransporters NKCC1 and KCC2

NKCC and KCC transporters mediate coupled transport of Na++K++Cl- and K++Cl- across the plasma membrane, thus regulating cell Cl- concentration and cell volume and playing critical roles in transepithelial salt and water transport and in neuronal excitability. The function of these transporters has been intensively studied, but a mechanistic understanding has awaited structural studies of the transporters. Here, we present the cryo-electron microscopy (cryo-EM) structures of the two neuronal cation-chloride cotransporters human NKCC1 (SLC12A2) and mouse KCC2 (SLC12A5), along with computational analysis and functional characterization. These structures highlight essential residues in ion transport and allow us to propose mechanisms by which phosphorylation regulates transport activity.

Choroid plexus NKCC1 mediates cerebrospinal fluid clearance during mouse early postnatal development

Cerebrospinal fluid (CSF) provides vital support for the brain. Abnormal CSF accumulation, such as hydrocephalus, can negatively affect perinatal neurodevelopment. The mechanisms regulating CSF clearance during the postnatal critical period are unclear. Here, we show that CSF K+, accompanied by water, is cleared through the choroid plexus (ChP) during mouse early postnatal development. We report that, at this developmental stage, the ChP showed increased ATP production and increased expression of ATP-dependent K+ transporters, particularly the Na+, K+, Cl-, and water cotransporter NKCC1. Overexpression of NKCC1 in the ChP resulted in increased CSF K+ clearance, increased cerebral compliance, and reduced circulating CSF in the brain without changes in intracranial pressure in mice. Moreover, ChP-specific NKCC1 overexpression in an obstructive hydrocephalus mouse model resulted in reduced ventriculomegaly. Collectively, our results implicate NKCC1 in regulating CSF K+ clearance through the ChP in the critical period during postnatal neurodevelopment in mice.

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na⁺–K⁺–Cl⁻ cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na⁺, K⁺, and Cl⁻ ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na⁺, Cl⁻, Ca²⁺, Mg²⁺, and HCO₃⁻ loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na⁺–K⁺–Cl⁻ cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Variants encoding a restricted carboxy-terminal domain of SLC12A2 cause hereditary hearing loss in humans

Hereditary hearing loss is challenging to diagnose because of the heterogeneity of the causative genes. Further, some genes involved in hereditary hearing loss have yet to be identified. Using whole-exome analysis of three families with congenital, severe-to-profound hearing loss, we identified a missense variant of SLC12A2 in five affected members of one family showing a dominant inheritance mode, along with de novo splice-site and missense variants of SLC12A2 in two sporadic cases, as promising candidates associated with hearing loss. Furthermore, we detected another de novo missense variant of SLC12A2 in a sporadic case. SLC12A2 encodes Na⁺, K⁺, 2Cl⁻ cotransporter (NKCC) 1 and plays critical roles in the homeostasis of K⁺-enriched endolymph. Slc12a2-deficient mice have congenital, profound deafness; however, no human variant of SLC12A2 has been reported as associated with hearing loss. All identified SLC12A2 variants mapped to exon 21 or its 3’-splice site. In vitro analysis indicated that the splice-site variant generates an exon 21-skipped SLC12A2 mRNA transcript expressed at much lower levels than the exon 21-included transcript in the cochlea, suggesting a tissue-specific role for the exon 21-encoded region in the carboy-terminal domain. In vitro functional analysis demonstrated that Cl⁻ influx was significantly decreased in all SLC12A2 variants studied. Immunohistochemistry revealed that SLC12A2 is located on the plasma membrane of several types of cells in the cochlea, including the strial marginal cells, which are critical for endolymph homeostasis. Overall, this study suggests that variants affecting exon 21 of the SLC12A2 transcript are responsible for hereditary hearing loss in humans.

Sounds are perceived by auditory sensory cells, owing to tissues surrounding them, including the cochlear lateral wall. Part of the cochlear lateral wall, the stria vascularis, is critical for production and maintenance of inner-ear fluid with a high potassium concentration, and for generating the positive voltage in the inner ear, important for sound perception, by stimulating secretion of potassium from marginal cells. The gene SLC12A2 encodes a protein involved in sodium, potassium, and chloride transport essential for proper function of specific cells in the stria vascularis; however, human variants of SLC12A2 have not previously been associated with hearing loss. By comprehensive genetic analysis of protein-coding sequences, we identified four candidate changes in SLC12A2 in four families with congenital, severe-to-profound hearing loss. Intriguingly, all four genetic variants were either within or at the 3’-splice site of the exon 21 which encodes a part of the carboxy terminal intracellular domain of SLC12A2. Experiments in cultured cells showed that skipping or mutation of exon 21 significantly decreased chloride influx mediated by the SLC12A2 protein. Overall, our results strongly indicate that mutations influencing exon 21 of SLC12A2 represent a novel mechanism underlying deafness in humans.

Structure of the human cation-chloride cotransporter NKCC1 determined by single-particle electron cryo-microscopy

The secondary active cation-chloride cotransporters (CCCs) utilize the existing Na+ and/or K+ gradients to move Cl- into or out of cells. NKCC1 is an intensively studied member of the CCC family and plays fundamental roles in regulating trans-epithelial ion movement, cell volume, chloride homeostasis and neuronal excitability. Here, we report a cryo-EM structure of human NKCC1 captured in a partially loaded, inward-open state. NKCC1 assembles into a dimer, with the first ten transmembrane (TM) helices harboring the transport core and TM11-TM12 helices lining the dimer interface. TM1 and TM6 helices break α-helical geometry halfway across the lipid bilayer where ion binding sites are organized around these discontinuous regions. NKCC1 may harbor multiple extracellular entryways and intracellular exits, raising the possibility that K+, Na+, and Cl- ions may traverse along their own routes for translocation. NKCC1 structure provides a blueprint for further probing structure-function relationships of NKCC1 and other CCCs.

KCC2 membrane diffusion tunes neuronal chloride homeostasis

Neuronal Cl^- homeostasis is regulated by the activity of two cation chloride co-transporters (CCCs), the K⁺-Cl^- cotransporter KCC2 and the Na⁺-K⁺-Cl^- cotransporter NKCC1, which are primarily extruding and importing chloride in neurons, respectively. Several neurological and psychiatric disorders including epilepsy, neuropathic pain, schizophrenia and autism are associated with altered neuronal chloride (Cl^-) homeostasis. A current view is that the accumulation of intracellular Cl^- in neurons as a result of KCC2 down-regulation and/or NKCC1 up-regulation may weaken inhibitory GABA signaling and thereby promote the development of pathological activities. CCC activity is determined mainly by their level of expression in the plasma membrane. Furthermore, CCCs undergo "diffusion-trapping" in the membrane, a mechanism that is rapidly adjusted by activity-dependent post-translational modifications i.e. phosphorylation/dephosphorylation of key serine and threonine residues. This represents probably the most rapid cellular mechanism for adapting CCC function to changes in neuronal activity. Therefore, interfering with these mechanisms may help restoring Cl^- homeostasis and inhibition under pathological conditions. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Molecular and evolutionary insights into the structural organization of cation chloride cotransporters

Cation chloride cotransporters (CCC) play an essential role for neuronal chloride homeostasis. K(+)-Cl(-) cotransporter (KCC2), is the principal Cl(-)-extruder, whereas Na(+)-K(+)-Cl(-) cotransporter (NKCC1), is the major Cl(-)-uptake mechanism in many neurons. As a consequence, the action of the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine strongly depend on the activity of these two transporters. Knowledge of the mechanisms involved in ion transport and regulation is thus of great importance to better understand normal and disturbed brain function. Although no overall 3-dimensional crystal structures are yet available, recent molecular and phylogenetic studies and modeling have provided new and exciting insights into structure-function relationships of CCC. Here, we will summarize our current knowledge of the gross structural organization of the proteins, their functional domains, ion binding and translocation sites, and the established role of individual amino acids (aa). A major focus will be laid on the delineation of shared and distinct organizational principles between KCC2 and NKCC1. Exploiting the richness of recently generated genome data across the tree of life, we will also explore the molecular evolution of these features.

Molecular motions involved in Na-K-Cl cotransporter-mediated ion transport and transporter activation revealed by internal cross-linking between transmembrane domains 10 and 11/12

We examined the relationship between transmembrane domain (TM) 10 and TM11/12 in NKCC1, testing homology models based on the structure of AdiC in the same transporter superfamily. We hypothesized that introduced cysteine pairs would be close enough for disulfide formation and would alter transport function: indeed, evidence for cross-link formation with low micromolar concentrations of copper phenanthroline or iodine was found in 3 of 8 initially tested pairs and in 1 of 26 additionally tested pairs. Inhibition of transport was observed with copper phenanthroline and iodine treatment of P676C/A734C and I677C/A734C, consistent with the proximity of these residues and with movement of TM10 during the occlusion step of ion transport. We also found Cu(2+) inhibition of the single-cysteine mutant A675C, suggesting that this residue and Met(382) of TM3 are involved in a Cu(2+)-binding site. Surprisingly, cross-linking of P676C/I730C was found to prevent rapid deactivation of the transporter while not affecting the dephosphorylation rate, thus uncoupling the phosphorylation and activation steps. Consistent with this, (a) cross-linking of P676C/I730C was dependent on activation state, and (b) mutants lacking the phosphoregulatory domain could still be activated by cross-linking. These results suggest a model of NKCC activation that involves movement of TM12 relative to TM10, which is likely tied to movement of the large C terminus, a process somehow triggered by phosphorylation of the regulatory domain in the N terminus.