Structural changes in the extracellular loop 2 of the murine KCC2 potassium chloride cotransporter modulate ion transport

Highly conserved ion binding sites are not all functionally relevant in mouse KCC4

Introduction

The potassium chloride cotransporter 4 (KCC4) is expressed in various tissues and plays an important role in distal renal acidification and hearing development. Although KCCs transport K+ and Cl- in a 1:1 stoichiometry, two Cl- coordination sites were indicated via cryo-electron microscopy (CryoEM).

Methods

In a comprehensive analysis, we analyzed here the consequences of point mutation of residues coordinating potassium, and chloride in the first (Cl1) and second (Cl2) coordinating site in KCC4 using Tl+ based flux measurements.

Results

Surprisingly, not all highly conserved coordination sites in KCC4 are essential. Three out of five residues (N131, Y216, and T432) are functionally relevant for potassium coordination. For chloride coordination in Cl1, all three residues (G134, V135, and I136) are important, whereas three out of four residues (G433, M435, and Y589) are relevant for chloride binding in Cl2. As all ion coordination sites are important in KCC2, this indicates that there is a certain flexibility in the stringency of ion coordination in KCC4. One possible reason for the different relevance of ion coordination sites could be the large extracellular loop (LEL). The LEL is structured differently within KCCs and is directly linked to the transmembrane domain (TM) 6, where most of the coordination sites reside. Substitution of ion coordination sites in the KCC22-4-2 chimera, in which the LEL from mouse KCC4 is exchanged with the LEL of rat KCC2, have the same effect as substitutions in rat KCC2. An exception is the substitution of the potassium coordination site I111 in TM1, which shows enhanced activity in the KCC22-4-2 chimera compared to the impaired activity in rat KCC2 and not affected activity in mouse KCC4.

Conclusion

Thus, the different relevance of the ion coordination sites between KCC2 and KCC4 cannot be attributed solely to the different structured LEL; other structural elements must also be involved here.

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.

Serine 937 phosphorylation enhances KCC2 activity and strengthens synaptic inhibition

The potassium chloride cotransporter KCC2 is crucial for Cl- extrusion from mature neurons and thus key to hyperpolarizing inhibition. Auditory brainstem circuits contain well-understood inhibitory projections and provide a potent model to study the regulation of synaptic inhibition. Two peculiarities of the auditory brainstem are (i) posttranslational activation of KCC2 during development and (ii) extremely negative reversal potentials in specific circuits. To investigate the role of the potent phospho-site serine 937 therein, we generated a KCC2 Thr934Ala/Ser937Asp double mutation, in which Ser937 is replaced by aspartate mimicking the phosphorylated state, and the neighbouring Thr934 arrested in the dephosphorylated state. This double mutant showed a twofold increased transport activity in HEK293 cells, raising the hypothesis that auditory brainstem neurons show lower [Cl-]i. and increased glycinergic inhibition. This was tested in a mouse model carrying the same KCC2 Thr934Ala/Ser937Asp mutation by the use of the CRISPR/Cas9 technology. Homozygous KCC2 Thr934Ala/Ser937Asp mice showed an earlier developmental onset of hyperpolarisation in the auditory brainstem. Mature neurons displayed stronger glycinergic inhibition due to hyperpolarized ECl-. These data demonstrate that phospho-regulation of KCC2 Ser937 is a potent way to interfere with the excitation-inhibition balance in neural circuits.

K+/Cl- cotransporter 2 (KCC2) and Na+/HCO3- cotransporter 1 (NBCe1) interaction modulates profile of KCC2 phosphorylation

K+/Cl- cotransporter 2 (KCC2) is a major Cl- extruder in mature neurons and is responsible for the establishment of low intracellular [Cl-], necessary for fast hyperpolarizing GABAA-receptor mediated synaptic inhibition. Electrogenic sodium bicarbonate cotransporter 1 (NBCe1) is a pH regulatory protein expressed in neurons and glial cells. An interactome study identified NBCe1 as a possible interaction partner of KCC2. In this study, we investigated the putative effect of KCC2/NBCe1 interaction in baseline and the stimulus-induced phosphorylation pattern and function of KCC2. Primary mouse hippocampal neuronal cultures from wildtype (WT) and Nbce1-deficient mice, as well as HEK-293 cells stably transfected with KCC2WT, were used. The results show that KCC2 and NBCe1 are interaction partners in the mouse brain. In HEKKCC2 cells, pharmacological inhibition of NBCs with S0859 prevented staurosporine- and 4-aminopyridine (4AP)-induced KCC2 activation. In mature cultures of hippocampal neurons, however, S0859 completely inhibited postsynaptic GABAAR and, thus, could not be used as a tool to investigate the role of NBCs in GABA-dependent neuronal networks. In Nbce1-deficient immature hippocampal neurons, baseline phosphorylation of KCC2 at S940 was downregulated, compared to WT, and exposure to staurosporine failed to reduce pKCC2 S940 and T1007. In Nbce1-deficient mature neurons, baseline levels of pKCC2 S940 and T1007 were upregulated compared to WT, whereas after 4AP treatment, pKCC2 S940 was downregulated, and pKCC2 T1007 was further upregulated. Functional experiments showed that the levels of GABAAR reversal potential, baseline intracellular [Cl-], Cl- extrusion, and baseline intracellular pH were similar between WT and Nbce1-deficient neurons. Altogether, our data provide a primary description of the properties of KCC2/NBCe1 protein-protein interaction and implicate modulation of stimulus-mediated phosphorylation of KCC2 by NBCe1/KCC2 interaction-a mechanism with putative pathophysiological relevance.

Both chloride-binding sites are required for KCC2-mediated transport

The K+-Cl- cotransporter 2 (KCC2) plays an important role in inhibitory neurotransmission, and its impairment is associated with neurological and psychiatric disorders, including epilepsy, schizophrenia, and autism. Although KCCs transport K+ and Cl- in a 1:1 stoichiometry, two Cl- coordination sites were indicated via cryo-EM. In a comprehensive analysis, we analyzed the consequences of point mutations of residues coordinating Cl- in Cl1 and Cl2. Individual mutations of residues in Cl1 and Cl2 reduce or abolish KCC2WT function, indicating a crucial role of both Cl- coordination sites for KCC2 function. Structural changes in the extracellular loop 2 by inserting a 3xHA tag switches the K+ coordination site to another position. To investigate, whether the extension of the extracellular loop 2 with the 3xHA tag also affects the coordination of the two Cl- coordination sites, we carried out the analogous experiments for both Cl- coordinating sites in the KCC2HA construct. These analyses showed that most of the individual mutation of residues in Cl1 and Cl2 in the KCC2HA construct reduces or abolishes KCC2 function, indicating that the coordination of Cl- remains at the same position. However, the coupling of K+ and Cl- in Cl1 is still apparent in the KCC2HA construct, indicating a mutual dependence of both ions. In addition, the coordination residue Tyr569 in Cl2 shifted in KCC2HA. Thus, conformational changes in the extracellular domain affect K+ and Cl--binding sites. However, the effect on the Cl--binding sites is subtler.

SLC12A cryo-EM: analysis of relevant ion binding sites, structural domains, and amino acids

The solute carrier family 12A (SLC12A) superfamily of membrane transporters modulates the movement of cations coupled with chloride across the membrane. In doing so, these cotransporters are involved in numerous aspects of human physiology: cell volume regulation, ion homeostasis, blood pressure regulation, and neurological action potential via intracellular chloride concentration modulation. Their physiological characterization has been largely studied; however, understanding the mechanics of their function and the relevance of structural domains or specific amino acids has been a pending task. In recent years, single-particle cryogenic electron microscopy (cryo-EM) has been successfully applied to members of the SLC12A family including all K+:Cl- cotransporters (KCCs), Na+:K+:2Cl- cotransporter NKCC1, and recently Na+:Cl- cotransporter (NCC); revealing structural elements that play key roles in their function. The present review analyzes the data provided by these cryo-EM reports focusing on structural domains and specific amino acids involved in ion binding, domain interactions, and other important SCL12A structural elements. A comparison of cryo-EM data from NKCC1 and KCCs is presented in the light of the two recent NCC cryo-EM studies, to propose insight into structural elements that might also be found in NCC and are necessary for its proper function. In the final sections, the importance of key coordination residues for substrate specificity and their implication on various pathophysiological conditions and genetic disorders is reviewed, as this could provide the basis to correlate structural elements with the development of novel and selective treatments, as well as mechanistic insight into the function and regulation of cation-coupled chloride cotransporters (CCCs).

The European and Japanese eel NaCl cotransporter β exhibit chloride currents and are resistant to thiazide type diuretics

The thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) is the major pathway for salt reabsorption in the mammalian distal convoluted tubule, and the inhibition of its function with thiazides is widely used for the treatment of arterial hypertension. In mammals and teleosts, NCC is present as one ortholog that is mainly expressed in the kidney. One exception, however, is the eel, which has two genes encoding NCC. The eNCCa is located in the kidney and eNCCb, which is present in the apical membrane of the rectum. Interestingly, the European eNCCb functions as a NaCl cotransporter that is nevertheless resistant to thiazides and is not activated by low-chloride hypotonic stress. However, in the Japanese eel rectal sac, a thiazide-sensitive NaCl transport mechanism has been described. The protein sequences between eNCCβ and jNCCβ are 98% identical. Here, by site-directed mutagenesis, we transformed eNCCβ into jNCCβ. Our data showed that jNCCβ, similar to eNCCβ, is resistant to thiazides. In addition, both NCCβ proteins have high transport capacity with respect to their renal NCC orthologs, and in contrast to known NCCs, exhibit electrogenic properties that are reduced when residue I172 is substituted by A, G or M. This is considered a key residue for the chloride ion-binding sites of NKCC and KCC. We conclude that NCCb proteins are not sensitive to thiazides and have electrogenic properties dependent on Cl^-, and site I172 is important for the function of NCCβ.