Murine Fetal Serum Phosphorus is Set Independent of FGF23 and PTH, Except in the Presence of Maternal Phosphate Loading

Abstract

Fibroblast growth factor 23 (FGF23) appears to play no role until after birth, given unaltered phosphate and bone metabolism in Fgf23- and Klotho-null fetuses. However, in those studies maternal serum phosphorus was normal. We studied whether maternal phosphate loading alters fetal serum phosphorus and invokes a fetal FGF23 or parathyroid hormone (PTH) response. C57BL/6 wild-type (WT) female mice received low (0.3%), normal (0.7%), or high (1.65%) phosphate diets beginning 1 week prior to mating to WT males. Fgf23+/- female mice received the normal or high-phosphate diets 1 week before mating to Fgf23+/- males. One day before expected birth, we harvested maternal and fetal blood, intact fetuses, placentas, and fetal kidneys. Increasing phosphate intake in WT resulted in progressively higher maternal serum phosphorus and FGF23 during pregnancy, while PTH remained undetectable. Fetal serum phosphorus was independent of the maternal phosphorus and PTH remained low, but FGF23 showed a small nonsignificant increase with high maternal serum phosphorus. There were no differences in fetal ash weight and mineral content, or placental gene expression. High phosphate intake in Fgf23+/- mice also increased maternal serum phosphorus and FGF23, but there was no change in PTH. WT fetuses remained unaffected by maternal high-phosphate intake, while Fgf23-null fetuses became hyperphosphatemic but had no change in PTH, skeletal ash weight or mineral content. In conclusion, fetal phosphate metabolism is generally regulated independently of maternal serum phosphorus and fetal FGF23 or PTH. However, maternal phosphate loading reveals that fetal FGF23 can defend against the development of fetal hyperphosphatemia.

Keywords: fetus; fibroblast growth factor-23; hyperphosphatemia; parathyroid hormone; phosphate; phosphorus; pregnancy.

Figure 1.

Biochemical and hormonal responses in WT mothers exposed to low, normal, and high-phosphate diets. Increasing phosphate intake caused an increase in maternal serum phosphorus during prepregnancy and pregnancy (A), accompanied by increased FGF23 (B). Maternal phosphate intake did not alter PTH, which became suppressed to below the detection limit in all as expected during pregnancy, hence no visible error bars (C). Note that low phosphate intake caused apparent maternal hypophosphatemia during pregnancy, although the value did not reach statistical significance (A). The numbers of observations are indicated in parentheses. Abbreviations: PTH, parathyroid hormone; WT, wild-type.

Figure 2.

Fetal response to dietary phosphate loading in WT mothers. Unlike their mothers, which achieved a progressive increase in serum phosphorus in response to increased dietary intake, the fetuses maintained the same high serum phosphorus value independent of the maternal serum phosphorus (A). Fetal FGF23 and PTH remained at low values and did not change significantly in response to the maternal serum phosphorus or dietary assignment (B,C). Similarly, the incorporation of mineral into the developing fetal skeleton was unaltered by maternal phosphorus intake, as shown by the normal ash weight (D), and the normal ash content of calcium (E) and phosphate (F). Error bars in B and C are too narrow to be discerned. The numbers of observations are indicated in parentheses. Abbreviations: PTH, parathyroid hormone; WT, wild-type.

Figure 3.

Biochemical and hormonal responses in Fgf23^±/- mothers exposed to normal and high-phosphate diets. Increased phosphate intake caused a nominally higher maternal serum phosphorus during prepregnancy and pregnancy (A), which provoked a significant increase in FGF23 during pregnancy (B). Maternal phosphate intake did not alter PTH, which became suppressed as expected during pregnancy (C). The numbers of observations are indicated in parentheses. Abbreviation: PTH, parathyroid hormone.

Figure 4.

Responses of Fgf23-null fetuses to maternal dietary phosphate loading. Ffg23-null fetuses developed hyperphosphatemia in response to the increased maternal phosphate load, while the other 3 groups (Ffg23-null fetuses from the normal diet and WT fetuses from both diets) were indistinguishable by serum phosphorus (A). Fetal FGF23 was absent in Fgf23-null fetuses (hence no error bars) and did not increase in WT fetuses (B); PTH remained at low values in both (C). Error bars on the high dietary phosphate group in C are too narrow to be discerned. The amniotic fluid phosphorus concentration did not differ significantly among the 4 groups (D). The incorporation of mineral into the developing fetal skeleton was unaltered by maternal phosphate intake or genotype, as shown by the normal ash weight (E) and the normal ash content of calcium (F) and phosphate (G). The numbers of observations are indicated in parentheses. Abbreviations: PTH, parathyroid hormone; WT, wild-type.

Figure 5.

Mineral deposition in WT and Fgf23-null placentas. Phosphate deposition in placentas was visualized by von Kossa staining, in which silver ions react with phosphate to create a black precipitate. Panel A summarizes the digital analysis of all images. Phosphate deposition was significantly elevated in WT placentas from mothers consuming the high-phosphate diet compared with WT and Fgf23-null placentas from the normal phosphate diet. Fgf23-null placentas from the high-phosphate diet showed a trend for increased deposition but were not significantly different from any value. The numbers of observations are indicated in parentheses. Panel B shows representative images of placentas from the different genotypes and diets. The scale bar in the upper panel indicates 500 microns, while the bar in the lower panel indicates 200 microns. Abbreviations: PTH, parathyroid hormone; WT, wild-type.