Molecular Basis of Decreased Kir4.1 Function in SeSAME/EAST Syndrome

Abstract

SeSAME/EAST syndrome is a channelopathy consisting of a hypokalemic, hypomagnesemic, metabolic alkalosis associated with seizures, sensorineural deafness, ataxia, and developmental abnormalities. This disease links to autosomal recessive mutations in KCNJ10, which encodes the Kir4.1 potassium channel, but the functional consequences of these mutations are not well understood. In Xenopus oocytes, all of the disease-associated mutant channels (R65P, R65P/R199X, G77R, C140R, T164I, and A167V/R297C) had decreased K ϩ current (0 to 23% of wild-type levels). Immunofluorescence demonstrated decreased surface expression of G77R, C140R, and A167V expressed in HEK293 cells. When we coexpressed mutant and wild-type subunits to mimic the heterozygous state, R199X, C140R, and G77R currents decreased to 55, 40, and 20% of wild-type levels, respectively, suggesting that carriers of these mutations may present with an abnormal phenotype. Because Kir4.1 subunits can form heteromeric channels with Kir5.1, we coexpressed the aforementioned mutants with Kir5.1 and found that currents were reduced at least as much as observed when we expressed mutants alone. Reduction of pH i from approximately 7.4 to 6.8 significantly decreased currents of all mutants except R199X but did not affect wild-type channels. In conclusion, perturbed pH gating may underlie the loss of channel function for the disease-associated mutant Kir4.1 channels and may have important physiologic consequences.

Renal phenotype in mice lacking the Kir5.1 ( Kcnj16 ) K + channel subunit contrasts with that observed in SeSAME/EAST syndrome

“…In the DCT, the basolateral K + channel has been shown to be a heteromeric Kir4.1/Kir5.1 channel (6). Therefore, the loss-of-function mutations in Kir4.1 associated with SeSAME/EAST syndrome are predicted to reduce the functional activity of this basolateral K + channel (13,14,27). However, the functional properties of heteromeric Kir4.1/Kir5.1 channels (10,18,21,28,29) predict that genetic inactivation of Kir5.1 subunit will have a different effect on this conductance because the partnering Kir4.1 subunits still remain, and can form functional homomeric channels.…”

“…This syndrome is caused by mutations in the human KCNJ10 gene encoding the Kir4.1 subunit and results in a loss of function in both homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels (13,14,27). The clinical features of SeSAME/EAST syndrome are reminiscent of Gitelman syndrome, which is caused by mutations in the NCC in the DCT (32).…”

“…Recent studies have reported that rare homozygous missense mutations of the human KCNJ10 gene, which encodes the Kir4.1 subunit, underlie SeSAME/EAST syndrome, a rare disorder in which patients experience neurological and renal symptoms (11,12). In the kidney, it is thought that these loss-of-function mutations in Kir4.1 impair the function of heteromeric Kir4.1/Kir5.1 channels (13,14), thereby dramatically impairing salt reuptake from the DCT, and increasing downstream K + and H + secretion. This results in a Gitelman-like syndrome, characterized by salt wasting, hypocalciuria, hypomagnesaemia, and hypokalemic metabolic alkalosis (11,12,15).…”

KCNJ10 Mutations Display Differential Sensitivity to Heteromerisation with KCNJ16

“…Our data provide new insights into mutant KCNJ10 assembly and function, as one mutation, p.A167V, had been previously described as non-pathogenic on its own due to its mild effect on KCNJ10-mediated currents [4]. …”

Caveolin-1 Deficiency Inhibits the Basolateral K+ Channels in the Distal Convoluted Tubule and Impairs Renal K+ and Mg2+ Transport

“…8,9 This 40 pS K + channel is a heterotetramer of Kir4.1 and Kir5.1 because the coexpression of Kir4.1/5.1 produced an inwardly rectifying K + channel with the same biophysical properties as the 40 pS K + channel expressed in the basolateral membrane of the native DCT. [10][11][12] The role of Kir4.1 in forming the basolateral 40 pS K + channel of the DCT was convincingly demonstrated by the observation that the disruption of Kcnj10 (Kir.4.1) completely eliminated the 40 pS K + channel. 13 Moreover, the basolateral K + conductance was largely abolished in the early DCT (DCT1) of Kcnj10 2/2 mice, suggesting that Kcnj10 (Kir.4.1) is a major contributor to the basolateral K + conductance in the DCT1.…”