A Novel Missense Mutation in the Sodium Bicarbonate Cotransporter (NBCe1/SLC4A4) Causes Proximal Tubular Acidosis and Glaucoma through Ion Transport Defects

Abstract

In humans and terrestrial vertebrates, the kidney controls systemic pH in part by absorbing filtered bicarbonate in the proximal tubule via an electrogenic Na ؉ /HCO 3 ؊ cotransporter (NBCe1/SLC4A4). Recently, human genetics revealed that NBCe1 is the major renal contributor to this process. Homozygous point mutations in NBCe1 cause proximal renal tubular acidosis (pRTA), glaucoma, and cataracts (Igarashi, T., Inatomi, J., Sekine, T., Cha, S. H., Kanai, Y., Kunimi, M., Tsukamoto, K., Satoh, H., Shimadzu, M., Tozawa, F., Mori, T., Shiobara, M., Seki, G., and Endou, H. (1999) Nat. Genet. 23, 264 -266). We have identified and functionally characterized a novel, homozygous, missense mutation (S427L) in NBCe1, also resulting in pRTA and similar eye defects without mental retardation. To understand the pathophysiology of the syndrome, we expressed wild-type (WT) NBCe1 and S427L-NBCe1 in Xenopus oocytes. Function was evaluated by measuring intracellular pH (HCO 3 ؊ transport) and membrane currents using microelectrodes. HCO 3 ؊ -elicited currents for S427L were ϳ10% of WT NBCe1, and CO 2 -induced acidification was ϳ4-fold faster. Na ؉ -dependent HCO 3 ؊ transport (currents and acidification) was also ϳ10% of WT. Current-voltage (I-V) analysis reveals that S427L has no reversal potential in HCO 3 ؊ , indicating that under physiological ion gradient conditions, NaHCO 3 could not move out of cells as is needed for renal HCO 3 ؊ absorption and ocular pressure homeostasis. I-V analysis without Na ؉ further shows that the S427L-mediated NaHCO 3 efflux mode is depressed or absent. These experiments reveal that voltage-and Na ؉ -dependent transport by S427L-hkNBCe1 is unfavorably altered, thereby causing both insufficient HCO 3 ؊ absorption by the kidney (proximal RTA) and inappropriate anterior chamber fluid transport (glaucoma).

Slc26a9 Is Inhibited by the R-region of the Cystic Fibrosis Transmembrane Conductance Regulator via the STAS Domain

“…Measured and calculated acid-base transport properties of clones used in this study as previously reported for other transporters (24,66). "␤" is the buffering capacity in units of mM ϫ (pH unit)…”

“…Two-electrode Voltage Clamp-For these experiments, oocytes were injected with 0.5 ng of cRNA and membrane currents were recorded with an OC-725C voltage clamp (Warner Instruments, Hamden, CT), filtered at 2-5 kHz, digitized at 10 kHz. I-V protocols consisted of 40-ms steps from V h (Ϫ60 mV) to Ϫ160 and ϩ60 mV in 20-mV steps (23,24).…”

IRBIT, an inositol 1,4,5-trisphosphate receptor-binding protein, specifically binds to and activates pancreas-type Na + /HCO 3 − cotransporter 1 (pNBC1)

“…Because NBC1 transports Ͼ1 HCO 3 Ϫ ion (2 or 3) per Na ϩ ion, the transport that is mediated by NBC1 is associated with a net movement of negative charge across the membrane. We and other researchers (4)(5)(6)(7)(8) have reported that inactivating mutations of the human NBC1 gene SLC4A4 cause proximal renal tubular acidosis associated with ocular abnormalities and stunted growth. Also, some patients with NBC1 mutations have elevated pancreatic enzyme levels and͞or mental retardation (4,5,7,8).…”

Clinical and laboratory approaches in the diagnosis of renal tubular acidosis

“…These patients require larger doses of oral bicarbonate [10] or the intravenous infusion of a 3.75 % solution of sodium HCO 3 − at rates varying from 0.3 to 0.8 ml/min to cause an increment of 2-3 mEq/l/h of plasma HCO 3 − and to minimize extracellular volume expansion, as classically reported [24]. However, even if a normal plasma HCO 3 − concentration is not achieved, the verification of massive bicarbonaturia when plasma HCO 3 − is below normal levels is likely better evidence of defective proximal reabsorption [3,31]. The measurement of urine pCO 2 when the urinary pH is higher than that of blood is a sensitive index of distal nephron H + secretion.…”