Trafficking Defects of a Novel Autosomal Recessive Distal Renal Tubular Acidosis Mutant (S773P) of the Human Kidney Anion Exchanger (kAE1)

Abstract

Autosomal dominant and recessive distal renal tubular acidosis (dRTA) can be caused by mutations in the anion exchanger 1 (AE1 or SLC4A1) gene, which encodes the erythroid chloride/bicarbonate anion exchanger membrane glycoprotein (eAE1) and a truncated kidney isoform (kAE1). The biosynthesis and trafficking of kAE1 containing a novel recessive missense dRTA mutation (kAE1 S773P) was studied in transiently transfected HEK-293 cells, expressing the mutant alone or in combination with wild-type kAE1 or another recessive mutant, kAE1 G701D. The kAE1 S773P mutant was expressed at a three times lower level than wild-type, had a 2-fold decrease in its half-life, and was targeted for degradation by the proteasome. It could not be detected at the plasma membrane in human embryonic kidney cells and showed predominant endoplasmic reticulum immunolocalization in both human embryonic kidney and LLC-PK1 cells. The oligosaccharide on a kAE1 S773P N-glycosylation mutant (N555) was not processed to the complex form indicating impaired exit from the endoplasmic reticulum. The kAE1 S773P mutant showed decreased binding to an inhibitor affinity resin and increased sensitivity to proteases, suggesting that it was not properly folded. The other recessive dRTA mutant, kAE1 G701D, also exhibited defective trafficking to the plasma membrane. The recessive kAE1 mutants formed dimers like wild-type AE1 and could hetero-oligomerize with wild-type kAE1 or with each other. Hetero-oligomers of wild-type kAE1 with recessive kAE1 S773P or G701D, in contrast to the dominant kAE1 R589H mutant, were delivered to the plasma membrane.

Oligomeric Structure and Minimal Functional Unit of the Electrogenic Sodium Bicarbonate Cotransporter NBCe1-A

“…In contrast, mutations in AE1 cause distal renal tubular acidosis that can be inherited in a dominant or recessive manner (9 -12). In autosomal dominant AE1 mutations, dimers containing a mixed wild-type and mutant monomer are targeted to the endoplasmic reticulum-associated degradation pathway presumably because of lack of acquisition of a native conformation (12). Mutations in AE1 inherited in a recessive manner are expressed normally on the plasma membrane with the functional monomer in the mixed dimer still able to transport normally (12).…”

“…In autosomal dominant AE1 mutations, dimers containing a mixed wild-type and mutant monomer are targeted to the endoplasmic reticulum-associated degradation pathway presumably because of lack of acquisition of a native conformation (12). Mutations in AE1 inherited in a recessive manner are expressed normally on the plasma membrane with the functional monomer in the mixed dimer still able to transport normally (12). The finding that NBCe1-A is predominantly a dimer composed of functional monomers leads to the interesting prediction that in the known mixed mutant-wild-type dimers (heterozygotes; parents of patients with autosomal recessive NBCe1-A-mediated proximal renal tubular acidosis), the wild-type monomer is functional, and the mutant monomer is not capable of targeting the mixed dimer to the endoplasmic reticulum-associated degradation pathway resulting in the lack of a significant phenotype.…”

A novel SLC4A1 variant in an autosomal dominant distal renal tubular acidosis family with a severe phenotype

“…Inversely, hereditary dRTA is rare. Both autosomal dominant and autosomal recessive (with deafness or preserved hearing) inheritance patterns have been reported in primary dRTA [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Recessive dRTA (rdRTA) was mainly reported in the studies of patients from Southeast Asia; however, dominant dRTA (ddRTA) was predominantly described in Caucasian.…”

Cation transport activity of anion exchanger 1 mutations found in inherited distal renal tubular acidosis

“…Known AD dRTA-associated mutations cause disease in heterozygotes by the mutant kAE1 "capturing" the wild-type kAE1 as a heterodimer, which is retained in the ER (6) or is missorted to the apical membrane (36). Autosomal recessive (AR) dRTA-associated mutations (G701D, S773P, ⌬V850, and A858D) have been found only in Southeast Asia (5,17,36) and seem to exhibit different trafficking behavior. Polarized mammalian cells coexpressing wild-type AE1 with G701D or other AR mutants show "rescue" of the mutant and expression at the cell surface (6,7).…”