Bicarbonate is the primary inducer of KCC3a expression in renal cortical B-type intercalated cells

A primary function of intercalated cells in the distal tubule of the kidney is to maintain pH homeostasis. For example, type B intercalated cells secrete bicarbonate largely through the action of the apical Cl-/HCO3- exchanger, pendrin, which helps correct metabolic alkalosis. Since both the K-Cl cotransporter, KCC3a and pendrin colocalize to the apical region of type B and non-A, non-B intercalated cells and since both are upregulated in models of metabolic alkalosis, such as with dietary NaHCO3 loading, we raised the possibility that apical KCC3a facilitates pendrin-mediated bicarbonate secretion, such as through apical Cl- recycling. The purpose of this study was to determine if KCC3a abundance changes through intake of bicarbonate alone or through bicarbonate plus its accompanying cation, and if it requires a direct interaction with pendrin or the renin-angiotensin-aldosterone system. We observed that KCC3a protein abundance, but not mRNA, increases in a mouse model of metabolic alkalosis, achieved with dietary NaHCO3 or KHCO3 intake. Bicarbonate ion increases KCC3a abundance, both in vivo and in vitro, independently of the accompanying cation. Moreover, bicarbonate intake upregulates KCC3a independently of aldosterone or angiotensin II. Since NaHCO3 intake increased KCC3a abundance in wild-type as well as in pendrin knockout mice, this KCC3a upregulation by bicarbonate does not depend on a direct interaction with pendrin. We conclude that increased extracellular bicarbonate, as observed in models of metabolic alkalosis, directly raises KCC3a abundance independently of angiotensin II, aldosterone, or changes in KCC3a transcription and does not involve a direct interaction with pendrin.NEW & NOTEWORTHY KCC3a expression is stimulated in alkalemia. This paper shows that bicarbonate itself is mediating this effect through a posttranscriptional mechanism. The paper also shows that this phenomenon is not mediated by aldosterone or angiotensin II.

WNK kinases sense molecular crowding and rescue cell volume via phase separation

When challenged by hypertonicity, dehydrated cells must recover their volume to survive. This process requires the phosphorylation-dependent regulation of SLC12 cation chloride transporters by WNK kinases, but how these kinases are activated by cell shrinkage remains unknown. Within seconds of cell exposure to hypertonicity, WNK1 concentrates into membraneless condensates, initiating a phosphorylation-dependent signal that drives net ion influx via the SLC12 cotransporters to restore cell volume. WNK1 condensate formation is driven by its intrinsically disordered C terminus, whose evolutionarily conserved signatures are necessary for efficient phase separation and volume recovery. This disorder-encoded phase behavior occurs within physiological constraints and is activated in vivo by molecular crowding rather than changes in cell size. This allows kinase activity despite an inhibitory ionic milieu and permits cell volume recovery through condensate-mediated signal amplification. Thus, WNK kinases are physiological crowding sensors that phase separate to coordinate a cell volume rescue response.

Sickle cell dehydration: Pathophysiology and therapeutic applications

Cell dehydration is a distinguishing characteristic of sickle cell disease and an important contributor to disease pathophysiology. Due to the unique dependence of Hb S polymerization on cellular Hb S concentration, cell dehydration promotes polymerization and sickling. In double heterozygosis for Hb S and C (SC disease) dehydration is the determining factor in disease pathophysiology. Three major ion transport pathways are involved in sickle cell dehydration: the K-Cl cotransport (KCC), the Gardos channel (KCNN4) and Psickle, the polymerization induced membrane permeability, most likely mediated by the mechano-sensitive ion channel PIEZO1. Each of these pathways exhibit unique characteristics in regulation by oxygen tension, intracellular and extracellular environment, and functional expression in reticulocytes and mature red cells. The unique dependence of K-Cl cotransport on intracellular Mg and the abnormal reduction of erythrocyte Mg content in SS and SC cells had led to clinical studies assessing the effect of oral Mg supplementation. Inhibition of Gardos channel by clotrimazole and senicapoc has led to Phase 1,2,3 trials in patients with sickle cell disease. While none of these studies has resulted in the approval of a novel therapy for SS disease, they have highlighted the key role played by these pathways in disease pathophysiology.

Kinase-KCC2 coupling: Cl- rheostasis, disease susceptibility, therapeutic target

The intracellular concentration of Cl(-) ([Cl(-)]i) in neurons is a highly regulated variable that is established and modulated by the finely tuned activity of the KCC2 cotransporter. Despite the importance of KCC2 for neurophysiology and its role in multiple neuropsychiatric diseases, our knowledge of the transporter's regulatory mechanisms is incomplete. Recent studies suggest that the phosphorylation state of KCC2 at specific residues in its cytoplasmic COOH terminus, such as Ser940 and Thr906/Thr1007, encodes discrete levels of transporter activity that elicit graded changes in neuronal Cl(-) extrusion to modulate the strength of synaptic inhibition via Cl(-)-permeable GABAA receptors. In this review, we propose that the functional and physical coupling of KCC2 to Cl(-)-sensitive kinase(s), such as the WNK1-SPAK kinase complex, constitutes a molecular "rheostat" that regulates [Cl(-)]i and thereby influences the functional plasticity of GABA. The rapid reversibility of (de)phosphorylation facilitates regulatory precision, and multisite phosphorylation allows for the control of KCC2 activity by different inputs via distinct or partially overlapping upstream signaling cascades that may become more or less important depending on the physiological context. While this adaptation mechanism is highly suited to maintaining homeostasis, its adjustable set points may render it vulnerable to perturbation and dysregulation. Finally, we suggest that pharmacological modulation of this kinase-KCC2 rheostat might be a particularly efficacious strategy to enhance Cl(-) extrusion and therapeutically restore GABA inhibition.