Abstract: FR-PO286

TRPC1 Gene Deletion Disturbs Homeostasis of Intracellular Free Ca ([Ca 2+]i), Produces Hyperparathyroidism, Hypercalcemia, Hyperphosphatemia, and Increased Bone Mass Due to Renal Ca and P Retention

Session Information

Category: Mineral Disease

  • 1202 Mineral Disease: Vitamin D, PTH, FGF-23

Authors

  • Eby, Bonnie, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
  • Onopiuk, Marta, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
  • Humphrey, Marybeth, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
  • Tsiokas, Leonidas, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
  • Lau, Kai, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
Background

Mice deleted of the gene for transient receptor potential canonical channel 1 (TRPC1) have higher PTH (313 vs 218 pg/ml), hypercalcemia (11.3 vs.10.2 mg %) & hypocalciuria (1.2 vs 2.2 mg/d), indicating TRPC1 deficiency mimics familial hypocalciuric hypercalcemia phenotypes from CaSR inactivating mutations. Micro-CT shows increased tibia bone volume:tissue volume (11 vs 6 %). Their hind limbs were heavier (189 vs.151 mg). We now studied the potential mechanisms.

Methods

We measured Ca & P balance & clearance by standard methods, calciotropic hormones by ELISA, and [Ca 2+]i in cultured cells by fura 2.

Results

In rat parathyroid (PT) cells, [Ca 2+]i stimulation by CaSR allosteric agonists was markedly blunted by TRPC1 siRNA, showing CaSR signaling pathway depends on this store-operated Ca entry (SOCE). Without TRPC1, [Ca 2+]i is down & PTH secretion is up. In contrast, high medium [Ca] raises [Ca 2+]i & reduces PTH secretion. Transfection with plasmid overexpressing TRPC1 also inhibits PTH. The null mouse phenotypes cannot be attributed to the comparable serum calcitriol (301 vs.308 pg/ml) & calcitonin (21.9 vs.22.3 pg/ml). Renal cells transfected with TRPC1 siRNA & osteocytes from null mice also have blunted [Ca 2+]i response to CaSR agonists, indicating a generalized signaling defect in TRPC1-deficient cells. Since FGF23 synthesis &/or secretion are known to change directionally with [Ca 2+]I, we tested the thesis that P excretion is reduced in null mice, due to putatively low FGF23 from reduced [Ca 2+]i in osteocytes. At similar diet intake, 24 h urine P was similar (3.5 vs. 4.1 mg/d). Fasting serum P was elevated (7.4 vs. 6.3 mg %) due to concurrently decreased P clearance (48 vs. 67 µl/min).

Conclusion

Conclusions: 1. Our data support the model that, activated by upstream CaSR signaling, TRPC1 functions as a SOCE channel to control PT [Ca 2+]i and regulate PTH secretion. 2. Increased bone mineral accretion in TRPC1 deletion is mediated by greater renal Ca & P retention, the latter likely due to low FGF23 from reduced osteocyte [Ca 2+]i. 3. The apparent skeletal PTH resistance is explicable by the hypercalcemia, hyperphosphatemia, &/or the known impaired osteoclast proliferation, differentiation & function due to loss of TRPC1 functions.

Funding

  • NIDDK Support