CaSR is the principal sensor coupling extracellular calcium to parathyroid hormone (PTH) secretion; however, CaSR activity is modified by heteromerization with the type B γ-aminobutyric acid receptor 1 (GABAB1R). The CaSR/GABAB1R heteromer controls tonic PTH secretion, a secretory mode functionally distinct from phasic PTH secretion mediated by homodimeric CaSR in response to rising extracellular calcium. In hyperplastic parathyroid glands from patients with primary and secondary hyperparathyroidism (HPT), CaSR/GABAB1R heteromerization increases in association with reduced CaSR and increased GABAB1R expression. In mouse models of primary HPT, parathyroid-specific ablation of GABAB1R suppresses tonic PTH hypersecretion and produces hypocalcemia.
The CaSR/GABAB1R heteromer responds to endogenous ligands amyloid-beta peptide (Aβ42), a proteolytic product of the amyloid precursor protein (APP), and GABA secreted by parathyroid cells. APP and Aβ42 expression are increased in both primary and secondary HPT, particularly under vitamin D-deficient conditions. Consistent with this relationship, parathyroid-specific ablation of Vdr promotes tonic PTH hypersecretion and increases circulating PTH. These effects are attenuated by concurrent deletion of parathyroid App or Mapt, which encodes Tau, a downstream effector of the Aβ42 signaling cascade. In a mouse model of secondary HPT induced by adenine-associated chronic kidney disease, genetic ablation of parathyroid App or treatment with the Aβ42-neutralizing antibody aducanumab suppresses tonic PTH secretion, lowers circulating PTH, mitigates renal injury, and reduces HPT-associated vertebral bone loss. Likewise, in mice with aging-associated HPT, co-treatment with aducanumab and the calcimimetic cinacalcet, which enhances homodimeric CaSR-mediated calcium sensing, synergistically suppresses tonic and phasic PTH secretion and lowers circulating PTH with significantly less hypocalcemia than cinacalcet monotherapy.
Together, these studies support a new paradigm in which vitamin D deficiency activates an Aβ42/CaSR/GABAB1R/Tau signaling cascade to sustain pathological tonic PTH secretion. Targeting Aβ42 or CaSR/GABAB1R heteromerization may therefore complement conventional calcimimetic therapy by selectively suppressing the tonic component of PTH hypersecretion.