Sodium-phosphate cotransporters, nephrolithiasis and bone demineralization

We discuss how recent findings obtained in disorders of phosphate metabolism in humans and in animal models have provided insights into the pathogenesis of renal stone formation and bone demineralization.

Mice that are null for the sodium-phosphate cotransporter (NPT)2a gene (NPT2a(-/-) mice) exhibit hypophosphataemia, increased urinary phosphate excretion, hypercalciuria and nephrolithiasis, but no bone demineralization. Mice null for the sodium-hydrogen exchanger regulatory factor (NHERF)1 (NHERF1(-/-) mice) also exhibit hypophosphataemia and increased renal phosphate excretion with decreased renal NPT2a expression, but they present with a severe sex-dependent bone demineralization. Heterozygous loss-of-function mutations in the NPT2a gene in humans induce hypophosphataemia, increased urinary phosphate excretion, hypercalciuria, nephrolithiasis in males (to date) and bone demineralization of variable severity in both sexes. Patients and experimental animals with increased circulating levels of fibroblast growth factor 23 present with hypophosphataemia, increased urinary phosphate excretion, inappropriate calcitriol synthesis and rickets/osteomalacia, but no nephrolithiasis except when treated. Low-phosphate diet in spontaneously hypercalciuric rats and disruption of the 1-alpha-hydroxylase gene in NPT2a mice prevent renal stone formation.

Increased urinary phosphate excretion is a risk factor for renal calcium stone formation when it is associated with hypercalciuria. As yet undefined interplay between NPT2a, NHERF1 and possibly other cotransporters or associated proteins in bone cells may account for the diversity of bone phenotypes observed in disorders of phosphate metabolism with impaired renal phosphate reabsorption. The pathogenesis of both renal stone and bone demineralization appear to be affected by species, sex and mutation type, among other factors.