Molecular Pathways for Thyroid Hormone Regulation of Secondary Center Ossification at the Epiphysis

Presentation Number: OR05-4
Date of Presentation: April 3rd, 2017

Patrick Aghajanian, Weirong Xing, Shaohong Cheng and Subburaman Mohan*
VA Loma Linda Healthcare System, Loma Linda, CA

Abstract

We have recently demonstrated that the rapid increase in thyroid hormone (TH) levels that occurs during the prepubertal growth period in mice is obligatory for initiation and progression of secondary ossification centers (SOC) in the epiphysis. Immunohistochemical, gene expression and cell lineage tracking studies revealed that at postnatal day 5 (P5), much of the epiphysis is occupied by immature chondrocytes that express high levels of Sox9 and Col2, markers of immature chondrocytes. At P7, when serum levels of TH start to increase, immature chondrocytes transition into prehypertrophic chondrocytes as revealed by expression of Ihh and TRβ1. At P8, cells at the center of the epiphysis became hypertrophic as revealed by expression of Col10, MMP13 and subsequently bone formation markers (BSP, DMP1). At this time, no vascular invasion was apparent where bone was being formed. At P9 and P10, chondrocyte hypertrophy and bone formation continued to occur in a circular fashion towards the periphery of the epiphysis. Lineage tracing using Rosa-td tomatoCol2CreERT2 mice treated with tamoxifen indicated that the same Col2 expressing chondrocytes expressed prehypertrophic, hypertrophic, and subsequently bone formation markers in a sequential manner, thus providing evidence for chondrocyte to osteoblast transdifferentiation. Chondrocyte hypertrophy and osteoblast formation failed to occur in TH deficient mice. In terms of mechanisms for TH regulation of secondary center ossification, we determined that SHH was mainly expressed in proliferating immature chondrocytes while IHH expression was seen in the maturing hypertrophic chondrocytes. In vitro studies revealed that TH acting via TRα1 promoted expression of SHH while TRβ1 activation by TH promoted expression of IHH but inhibited SHH expression. SHH promoted proliferation and increased expression of markers of immature chondrocytes but inhibited chondrocyte hypertrophy while IHH promoted chondrocyte hypertrophy by increasing osterix expression. Based on our data, we propose a model in which the increase in TH levels during the prepubertal growth period positively regulates TRβ1 expression to fine tune levels of SHH and IHH and, thereby control the transit of proliferating immature chondrocytes into mature hypertrophic chondrocytes to become osteoblasts which promote bone formation at the epiphysis during secondary center ossification.

 

Nothing to Disclose: PA, WX, SC, SM