The Na+/H+ antiporter: a "melt" polymorphism allows regional mapping to the short arm of chromosome 1

Requirements for ion and solute transport, and pH regulation during enamel maturation

Transcellular bicarbonate transport is suspected to be an important pathway used by ameloblasts to regulate extracellular pH and support crystal growth during enamel maturation. Proteins that play a role in amelogenesis include members of the ABC transporters (SLC gene family and CFTR). A number of carbonic anhydrases (CAs) have also been identified. The defined functions of these genes are likely interlinked during enamel mineralization. The purpose of this study is to quantify relative mRNA levels of individual SLC, Cftr, and CAs in enamel cells obtained from secretory and maturation stages on rat incisors. We also present novel data on the enamel phenotypes for two animal models, a mutant porcine (CFTR-ΔF508) and the NBCe1-null mouse. Our data show that two SLCs (AE2 and NBCe1), Cftr, and Car2, Car3, Car6, and Car12 are all significantly up-regulated at the onset of the maturation stage of amelogenesis when compared to the secretory stage. The remaining SLCs and CA gene transcripts showed negligible expression or no significant change in expression from secretory to maturation stages. The enamel of CFTR-ΔF508 adult pigs was hypomineralized and showed abnormal crystal growth. NBCe1-null mice enamel was structurally defective and had a marked decrease in mineral content relative to wild-type. These data demonstrate the importance of many non-matrix proteins to amelogenesis and that the expression levels of multiple genes regulating extracellular pH are modulated during enamel maturation in response to an increased need for pH buffering during hydroxyapatite crystal growth.

Polymorphism of the 5' untranslated region of NHE1 gene associated with type-I diabetes

The ubiquitous form of the sodium-hydrogen exchanger, NHE1, is devoted to the regulation of intracellular pH and cell volume. In addition, NHE1 activity is stimulated by growth factors and increased NHE rates are found in both circulating and immortalized cells during diabetes or diabetic nephropathy. In this context, we searched for polymorphisms of the 5'-flanking regulatory region of NHE1 gene in subjects with type-I diabetes. We identified a C/T transition 696 bases upstream the translation initiation start site which disrupts a repeated palindromic GC sequence. The TT genotype was significantly more frequent in type-1 diabetics and may have functional importance. Genetic linkage between NHE1 and diabetes has been previously described in NOD mice strains with consequences on NHE rates. Hence, the polymorphism described hereby may act as a predisposition factor to type-I diabetes or to diabetic complications, and may be useful to investigate the genetic involvement of NHE1 in human pathophysiology.

Human Na+/H+ exchanger genes : identification of polymorphisms by radiation hybrid mapping and analysis of linkage in end-stage renal disease

The Na+/H+ exchangers (NHEs) are membrane-bound transporters that catalyze the electro-neutral movement of extracellular Na+ for intracellular H+. NHE genes play a critical role in pH homeostasis and cellular volume regulation and can be considered candidate genes for essential hypertension and renal disease. This study was performed to determine whether the NHE genes contributed to genetic susceptibility in end-stage renal disease (ESRD). To date, 5 isoforms of NHE have been cloned in mammals (NHE1 to NHE5). The complementary DNA (cDNA) sequences of NHE1 to NHE3 and NHE5 are known in humans. Because the chromosomal structure of the NHE genes is unknown, we used cDNA sequences to design polymerase chain reaction primers for use in radiation hybrid mapping. Radiation hybrid mapping of NHE genes identified nearby polymorphic markers for NHE1 to NHE3 (NHE1: D1S197, D1S2677; NHE2: D2S373, D2S1789; and NHE3: D5S678, D5S2005). We used these markers, and other previously identified polymorphic markers for NHE5, in linkage and association analyses of ESRD. The NHE1 to NHE3 and NHE5 loci did not demonstrate evidence for linkage to ESRD. However, NHE5 showed significant evidence for association (P</=1.0x10(-4)). The strongest evidence for association was observed with allele 6 of NHE5 (P</=0.001 to 0. 01). Allele 6 appeared to have a renoprotective effect, with a frequency of 0.15 in the control population and 0.06 to 0.09 in patients with ESRD. The combined approach of designing primers from cDNA and radiation hybrid mapping has proven successful in identifying polymorphisms for human genes of which only cDNA sequences were previously available. The NHE primers and associated polymorphic loci identified in this study can be used in genomic, linkage, and association analysis of NHE genes in future genetic studies of hypertension and renal failure. Given the allelic association, further evaluation of the role of NHE5 in ESRD susceptibility appears warranted.

Membrane sodium-proton exchange and primary hypertension

Recent studies have revealed that an enhancement of sodium-proton exchange is a frequently observed ion transport abnormality in essential hypertension. An altered antiport activity not only is measurable in blood cells of hypertensive subjects ex vivo but also is detectable in skeletal muscle in vivo. Several lines of argument suggest that the altered antiport activity is not an epiphenomenon of hypertension: 1) the increased activity is found only in a subgroup of patients with high blood pressure, 2) it is not tightly correlated to the severity or duration of hypertension, and 3) high sodium-proton exchange activity persists over time and is not affected by antihypertensive treatment. Available evidence suggests that enhanced sodium-proton exchange is associated with or a cause for the structural alterations found in resistance vessels of hypertensive individuals (media hypertrophy) and left ventricular hypertrophy. This review summarizes some of the physiological properties and roles of the sodium-proton exchanger and discusses its kinetic properties in essential hypertension. Furthermore, the reasons for the enhanced antiport activity and its potential implications regarding the pathogenesis of hypertension are discussed.

Progress towards completing the human linkage map

Genetic linkage maps order polymorphic loci (markers) along a chromosome and provide a measure of distance that is an inherently statistical measure of the frequency of meiotic recombination and has no simple relationship to DNA length in Mb. Few accurate and dense maps now exist. Maps with markers spaced every 2 cM providing accurate estimates of distance should be available in the next few years, however.