Paracellin-1 is critical for magnesium and calcium reabsorption in the human thick ascending limb of Henle

Background. A new protein, named paracellin 1 (PCLN-1), expressed in human thick ascending limb (TAL) tight junctions, possibly plays a critical role in the control of magnesium and calcium reabsorption, since mutations of PC LN-1 are present in the hypomagnesemia hypercalciuria syndrome (HHS). Howev er, no functional experiments have demonstrated that TAL magnesium and calc ium reabsorption were actually impaired in patients with HHS. Methods. Genetic studies were performed in the kindred of two unrelated pat ients with HHS. Renal magnesium and calcium reabsorption in TAL were analyz ed in one homozygous affected patient of each family, one patient with extr arenal hypomagnesemia (ERH), and two control subjects (CSs). Results. We found two yet undescribed mutations of PCLN-1 (Gly 162 Val, Ala 139 Val). In patients with HHS, renal magnesium and calcium reabsorptions were impaired as expected; NaCl renal conservation during NaCl deprivation and NaCl tubular reabsorption in diluting segment were intact. Furosemide i nfusion in CS markedly increased NaCl, Mg, and Ca urinary excretion rates. In HHS patients, furosemide similarly increased NaCl excretion, but failed to increase Mg and Ca excretion. Acute MgCl2 infusion in CS and ERH patient provoked a dramatic increase in urinary calcium excretion without change i n NaCl excretion. When combined with MgCl2 infusion, furosemide infusion re mained able to induce normal natriuretic response, but was unable to increa se urinary magnesium and calcium excretion further. In HHS patients, calciu ric response to MgCl2 infusion was blunted. Conclusion. This study is the first to our knowledge to demonstrate that ho mozygous mutations of PCLN-1 result in a selective defect in paracellular M g and Ca reabsorption in the TAL, with intact NaCl reabsorption ability at this site. In addition, the study supports a selective physiological effect of basolateral Mg2+ and Ca2+ concentration on TAL divalent cation paracell ular permeability, that is, PCLN-1 activity.