Abstract
Loss of function mutations in kidney Kir1.1 (renal outer medullary potassium channel, KCNJ1) inwardly rectifying potassium channels can be found in patients suffering from hyperprostaglandin E syndrome (HPS), the antenatal form of Bartter syndrome. A novel mutation found in a sporadic case substitutes an asparagine by a positively charged lysine residue at amino acid position 124 in the extracellular M1-H5 linker region. When heterologously expressed in Xenopus oocytes and mammalian cells, current amplitudes from mutant Kir1.1a[N124K] channels were reduced by a factor of approximately 12 as compared with wild type. A lysine at the equivalent position is present in only one of the known Kir subunits, the newly identified Kir1.3, which is also poorly expressed in the recombinant system. When the lysine residue in guinea pig Kir1.3 (gpKir1.3) isolated from a genomic library was changed to an asparagine (reverse HPS mutation), mutant channels yielded macroscopic currents with amplitudes increased 6-fold. From single channel analysis it became apparent that the decrease in mutant Kir1.1 channels and the increase in mutant gpKir1.3 macroscopic currents were mainly due to the number of expressed functional channels. Coexpression experiments revealed a dominant-negative effect of Kir1.1a[N124K] and gpKir1.3 on macroscopic current amplitudes when coexpressed with wild type Kir1.1a and gpKir[K110N], respectively. Thus we postulate that in Kir1.3 channels the extracellular positively charged lysine is of crucial functional importance. The HPS phenotype in man can be explained by the lower expression of functional channels by the Kir1. 1a[N124K] mutant.
Highlights
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AS049076
The hyperprostaglandin E syndrome, a renal disorder resulting from impairment of tubular reabsorption, can be caused by either mutations in the furosemide-sensitive Na-K-2Cl cotransporter (NKCC2) or by mutations in Kir1.1 (20 –23)
After heterologous expression and functional characterization of Kir1.1a and Kir1.3 channels, we demonstrate that these lysine residues are responsible for impaired channel function in mutant Kir1.1a and for low macroscopic currents in Kir1.3 channels
Summary
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AS049076. When mutant Kir1.1a[N124K] channels were expressed in oocytes, the kinetics of the macroscopic current were indistinguishable from WT Kir1.1a, but amplitudes were dramatically reduced to ϳ8% (3.9 Ϯ 1.2 A; n ϭ 5) under the same recording conditions (Fig. 1A). Cloning and Sequence Analysis of a Guinea Pig Kir1.3 Subunit—It is conceivable that in analogy to other Kir channels, kidney Kir1.1 subunits coassemble with other subfamily members expressed in the same cells.
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