Hypothyroidism and thyrotoxicosis have detrimental effects on skeletal development and adult bone strength. These effects result from thyroid hormone actions in bone, although a direct role for TSH in osteoblasts and osteoclasts was postulated following analysis of TSH receptor (TSHR) null mice. This hypothesis remains controversial as other studies failed to demonstrate osteoblast or osteoclast responses to TSH in vitro. Thyrostimulin is a heterodimeric glycoprotein hormone composed of α2 (GPA2) and β5 (GPB5) subunits that has a 10-fold higher binding-affinity for TSHR than TSH and is thought to exert paracrine effects in peripheral tissues. We hypothesized that thyrostimulin activates TSHR in bone and demonstrated expression of GPA2, GPB5 and TSHR mRNAs in primary cultured osteoblasts and osteoclasts. To investigate thyrostimulin actions in vitro, we expressed the protein in Cos7 cells. Co-expressed GPA2 and GPB5 subunits formed heterodimers and were glycosylated and secreted appropriately. Conditioned medium from Cos7 cells expressing thyrostimulin induced a robust cAMP response in CHO cells expressing the TSHR, but no response was evident in osteoblasts or osteoclasts. To investigate thyrostimulin actions in vivo, we characterized GPB5 knockout (GPB5KO) mice. In juveniles circulating T3 was normal, but T4 was reduced by 25% and TSH increased twofold. Adult GPB5KO mice were euthyroid, suggesting thyrostimulin may influence development of the hypothalamicpituitarythyroid axis. Consistent with the normal T3 level, endochondral ossification, linear growth and bone maturation were unaffected in GPB5KO mice. Bone micro-architecture, cortical thickness, mineral content, mineralization density and biomechanical properties were also normal in adult GPB5KO mice. In summary, although GPA2, GPB5 and TSHR are expressed in bone, thyrostimulin does not induce a cAMP response in primary osteoblasts or osteoclasts. Furthermore, thyrostimulin-deficient mice display a normal skeletal phenotype indicating that thyrostimulin does not have a physiological role in bone.
(GPB5KO mice were generated by Lexicon Genetics)