Paving the way for Bartter syndrome type 3 drug discovery: a hope from basic research

Understanding the genetic background and the molecular mechanisms of inherited kidney channelopathies is essential to the definition of targeted therapeutic approaches for affected patients. Bartter syndrome (BS) types 1–5 are rare tubulopathies presenting with overlapping clinical phenotypes including marked salt wasting and hypokalaemia that lead to polyuria, polydipsia, volume contraction, muscle weakness and growth retardation. BS type 3 is a clinically heterogeneous form caused by mutations in the CLCNKB gene that encodes for the ClC-Kb chloride channel involved in NaCl reabsorption in the renal tubule. Particularly, ClC-Kb is expressed in the thick ascending limb (TAL), distal convoluted tubule, and collecting duct, where it transfers chloride (Cl−) ions to the basolateral side. Impaired ClC-Kb function in TAL results in lower level of Cl− extrusion reduced NaCl reabsorption through the Na+–K+–2Cl− cotransporter and divalent cation reabsorption, accounting for the Bartter phenotype. Hypochloraemia is a characteristic hallmark for the correct diagnosis of BS type 3 among the BS variants. The stronger chloride depletion in patients with BS type 3 may be due also to defects in connecting tubule and collecting duct that can impair chloride extrusion and transepithelial chloride reabsorption through the pendrin Cl−/HCO3− exchanger. Specific therapy for BS is lacking and the only pharmacotherapy currently available is purely symptomatic and characterized by limiting side effects. Indeed, patients are treated with non-steroidal anti-inflammatory drugs such as indomethacin, as well as potassium supplements, potassium-sparing diuretics, and renin–aldosterone–angiotensin system inhibitors. In contrast to many other ion channels, electrophysiological characterization of ClC-Kb was long-hampered by technical difficulties in its expression. Indeed, only recently have the most efficient conditions for its heterologous protein expression, stability and degradation been established (Estévez et al. 2001; Scholl et al. 2006; Imbrici et al. 2014). In this issue of The Journal of Physiology, Cheng et al. (2017) significantly contribute toward advancing our understanding of the phenotype/genotype correlation and of the precise pathogenic mechanisms associated with BS type 3. As a first step, the authors collected classic BS patients carrying homozygous missense mutations with previously described functional consequences and clinical features. Then, by using a combination of electrophysiology and biochemistry, the authors examined the functional consequences of 18 CLCNKB mutations, assessing gating and protein amounts for each mutant channel expressed in cells from the human embryonic kidney (HEK) cell line. Among mutations under investigation, nine caused massive reduction of chloride current. Importantly, the functional severity of these CLCNKB mutations highly correlated with the severity of BS symptoms, such as age of diagnosis, urine calcium excretion and plasma chloride concentration, all clinical features often used to differentiate BS from Gitelman syndrome. In parallel, guided by the crystal structure of the eukaryotic CmCLC, they grouped the mutations on the basis of their position in the channel structure, gaining insight into the functional importance of structural domains. Severe mutations are located in barttin-interacting helix B, in the proposed dimer interface and in the selectivity filter, all important domains for channel activity. This paper is a relevant example of how basic and clinical research can cooperate to gain insight into the pathophysiology of a channelopathy, paving the way for a personalized therapy. The functional characterization of ClC-K mutations is indeed complementary to the genetic and clinical diagnosis of BS and is fundamental to design therapies tailored to patients’ mutations (Fig. 1). To date, only a few ClC-K ligands are known and no commercial drug targeting ClC-K channels is used in BS therapy. The identification of peculiar biophysical changes or trafficking defects of ClC-K mutations through in vitro studies can help patients’ stratification towards specific therapeutic approaches, such as gating or trafficking correctors. To this purpose, the recently solved structure of a bovine ClC-K highlighted a critical difference in the cytosolic side of the Cl− transport pathway between ClC-K channels and other ClC Cl−/H+ transporters (Park et al. 2017). This structural information could be essential for the screening and rational design of new pharmacological agents acting on ClC-K channels. In addition, novel pharmacological strategies are under investigation such as the reprofiling of commercial drugs and their repurposing in BS (Imbrici et al. 2017). Thus, we are looking forward to new discoveries in the pathophysiology and treatment of this rare renal channelopathy, and the work of Cheng et al. represents a first step that narrows the gap between basic research and the clinic, toward a precision and personalized medicine. None declared. This work was supported by Fondazione Cassa di Risparmio di Puglia (Grant FCRP 2014 to D.C.) and by Telethon-Italy (grant GGP14096 to D.C.).