Soft tissue accumulations of phosphate are not always associated with serum phosphate or with calcifications in mouse models of hyperphosphatemia.

High serum levels of phosphate (hyperphosphatemia) can target vascular smooth muscle cells (VSMC) and induce calcium-phosphate precipitations and vascular calcification, thereby contributing to high cardiovascular mortality rates in chronic kidney disease (CKD). Calcifications within soft tissues beyond the vasculature are not well described, and the involvement of cell types other than VSMCs is not clear. Here, we studied extravascular calcifications in various soft tissues from mouse models with hyperphosphatemia. We found that klotho-deficient (kl/kl) mice without CKD not only developed calcifications in the aorta, but also in the kidney and stomach, which was accompanied by significant elevations in tissue content of phosphate. Administration of a magnesium-rich diet, which blocks the formation of calcium-phosphate crystals, prevented calcifications in these tissues. Liver, heart, skeletal muscle, spleen, brain and skin showed no signs of calcifications. Similarly, CKD mice with global deletion of Col4a3 (Col4a3^-/-) showed significant increases in phosphate content and calcifications in aorta, kidney and stomach, but only when administered a high phosphate diet, which was not accompanied by further elevations in serum phosphate levels. In Col4a3^-/- mice on normal chow, we could only detect an increase in liver phosphate content that occurred in the absence of hepatic calcifications. Our findings indicate that soft tissue calcifications are not always associated with increases in serum phosphate concentrations. Furthermore, in some tissues, the accumulation of phosphate occurs independently of serum phosphate elevations and is not necessarily accompanied by calcifications. Overall, our findings indicate that soft tissues differ in their response to hyperphosphatemia. KEY POINTS: It is known that high serum phosphate levels (hyperphosphatemia) cause vascular calcifications, which are accompanied by increases in tissue phosphate content. Here, we show in mouse models of hyperphosphatemia that calcifications also occur in soft tissue areas outside of the vasculature (extravascular calcification). We also found that in some soft tissues the accumulation of phosphate is not accompanied by calcifications and occurs independently of serum phosphate elevations. Our findings indicate that soft tissues differ in their response to hyperphosphatemia. Because tissue elevations of phosphate drive the calcification process, CKD studies to determine causalities between hyperphosphatemia, tissue injuries and mortality should not be restricted to measuring phosphate levels in the circulation. Our finding that the administration of a magnesium-rich diet protects hyperphosphatemic mice from phosphate accumulations and calcifications in various soft tissues supports ongoing clinical trials that aim to determine the beneficial effects of magnesium elevations in patients with CKD.