Higher BMP Expression in Tendon Stem/Progenitor Cells Contributes to the Increased Heterotopic Ossification in Achilles Tendon With Aging.

Guangchun Dai,Junyan Liu,Yingjuan Li,Yunfeng Rui,Cheng Zhang,Panpan Lu,Minhao Chen

doi:10.3389/fcell.2020.570605

Abstract

Although the mineralization in tendon tissue has been reported in a series of aging and disease models, the underlying mechanisms remain unknown. This study aimed to describe the appearance of heterotopic ossification in rat Achilles tendon and further verify whether this tissue metaplasia is related to the enhanced osteogenic differentiation of tendon stem/progenitor cells (TSPCs) owing to the higher expression of bone morphogenetic proteins (BMP-2/4/7) with aging. The male SD rats, aged 4, 8, and 20 months (M), were used. The analyses of ossification and BMP expression in tendon were tested by radiological view (X-ray and CT), histological staining [hematoxylin and eosin (HE), Alcian blue, and Alizarin red], immunohistochemistry, and Western blot. The osteogenic differentiation potential and BMP expression of TSPCs were examined by Alizarin red S staining and real-time PCR. TSPCs were treated with BMP-2 or noggin, and the osteogenic differentiation potential was also examined. X-ray and CT showed the appearance of heterotopic ossification in tendon, and the volume and density of ossification was increased with aging. Histological staining showed the appearance of calcified region surrounded by chondrocyte-like cells and the increased osteogenesis-related gene and BMP expression in ossified tendon with aging. Moreover, the osteogenic differentiation potential and BMP expression in TSPCs isolated from ossified tendon were increased with aging. Additionally, BMP-2 increased the calcium nodule formation and osteogenesis-related gene expression in TSPCs. The addition of noggin inhibited BMP-induced enhancement of osteogenic differentiation. Thus, these findings suggested that the enhanced osteogenic differentiation of TSPCs contributes to the increased heterotopic ossification in aged tendon, which might be induced by the higher expression of BMPs with aging.

Highlights

In our ever-aging population, a deep understanding of our musculoskeletal unit is of utmost importance
X-ray showed the formation of heterotopic ossification, which was presented by visible radiopaque area in tendon, and the visible radiopaque area was increased in size with aging (Figures 1A–C)
Alcian blue (Figures 2L–N) and Alizarin red staining (Figures 2O–Q) showed the appearance of spontaneous ossification region, the accumulation of cartilaginous matrix by Alcian blue staining and calcium nodules formation by Alizarin red staining, and the ossification region stained by Alcian blue and Alizarin red was increased in size with aging

Summary

Introduction

In our ever-aging population, a deep understanding of our musculoskeletal unit is of utmost importance. Tenocytes were considered the only cell present in tendon and played a critical role in tendon metabolism, repair, and regeneration This hypothesis did not change until the isolation and identification of TSPCs in tendons, including mouse (Bi et al, 2007), human (Bi et al, 2007), rat (Rui et al, 2010), rabbit (Zhang and Wang, 2010), and fetal bovine (Yang et al, 2016). Rui et al (2011a) proposed that erroneous differentiation potential of TSPCs contributes to pathological alterations in calcified tendinopathy, which was consistent with the view of another study (Zhang X. et al, 2016) These findings supported the important role of altered TSPCs fate in pathological changes of chronic tendinopathy. There is no definite conclusion of these variations, the potential roles of altered TSPC differentiation capacity for age-related pathological changes in tendon were speculated (Dai et al, 2019; Li et al, 2019)

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