Close to the bone - in search of the skeletal stem cell

A Combined Treatment of BMP2 and Soluble VEGFR1 for the Enhancement of Tendon-Bone Healing by Regulating Injury-Activated Skeletal Stem Cell Lineage

BACKGROUND

Bone morphogenetic protein 2 (BMP2) is an appealing osteogenic and chondrogenic growth factor for promoting tendon-bone healing. Recently, it has been reported that soluble vascular endothelial growth factor (VEGF) receptor 1 (sVEGFR1) (a VEGF receptor antagonist) could enhance BMP2-induced bone repair and cartilage regeneration; thus, their combined application may represent a promising treatment to improve tendon-bone healing. Moreover, BMP2 could stimulate skeletal stem cell (SSC) expansion and formation, which is responsible for wounded tendon-bone interface repair. However, whether the codelivery of BMP2 and sVEGFR1 increases tendon enthesis injury-activated SSCs better than does BMP2 alone needs further research.

PURPOSE

To study the effect of BMP2 combined with sVEGFR1 on tendon-bone healing and injury-activated SSC lineage.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 128 C57BL/6 mice that underwent unilateral supraspinatus tendon detachment and repair were randomly assigned to 4 groups: (1) untreated control group; (2) hydrogel group, which received a local injection of the blank hydrogel at the injured site; (3) BMP2 group, which received an injection of hydrogel with BMP2; and (4) BMP2 with sVEGFR1 group, which received an injection of hydrogel with BMP2 and sVEGFR1. Histology, micro-computed tomography, and biomechanical tests were conducted to evaluate tendon-bone healing at 4 and 8 weeks after surgery. In addition, flow cytometry was performed to detect the proportion of SSCs and their downstream differentiated subtypes, including bone, cartilage, and stromal progenitors; osteoprogenitors; and pro-chondrogenic progenitors within supraspinatus tendon enthesis at 1 week postoperatively.

RESULTS

The repaired interface in BMP2 with sVEGFR1 group showed a significantly improved collagen fiber continuity, increased fibrocartilage, greater newly formed bone, and elevated mechanical properties compared with the other 3 groups. There were more SSCs; bone, cartilage, and stromal progenitors; osteoprogenitors; and pro-chondrogenic progenitors in the BMP2 with sVEGFR1 group than that in the other groups.

CONCLUSION

Our study suggests that the combined delivery of BMP2 and sVEGFR1 could promote tendon-bone healing and stimulate the expansion of SSCs and their downstream progeny within the injured tendon-bone interface.

CLINICAL RELEVANCE

Combining BMP2 with sVEGFR1 may be a good clinical treatment for wounded tendon enthesis healing.

Impaired function of skeletal stem cells derived from growth plates in ovariectomized mice

INTRODUCTION

Mouse skeletal stem cells (mSSCs, CD45^-Ter119^-Tie2^-CD51⁺Thy^-6C3^-CD105^-CD200⁺population) are identified in growth plates (GP) and play important roles in bone regeneration. However, the role of mSSCs in osteoporosis remains unclear.

MATERIALS AND METHODS

The GP were stained by HE staining, and the mSSC lineage was analyzed by flow cytometry at postnatal of 14 days and 30 days in wild-type mice. The mice (8 weeks) were either sham operated or ovariectomy (OVX) and then sacrificed at 2, 4 and 8 w. The GP were stained by Movat staining, and mSSC lineage was analyzed. Then, mSSCs were sorted by fluorescence-activated cell sorting (FACS); the clonal ability, chondrogenic differentiation and osteogenic differentiation were evaluated, and the changed genes were analyzed by RNA-seq.

RESULTS

The percentage of mSSCs were decreased with the narrow GP. Heights of GP were decreased significantly in 8w-ovx mice compared with 8w-sham mice. We found the percentage of mSSCs were decreased in mice at 2w after ovx, but the cell numbers were not changed. Further, the percentage and cell numbers of mSSCs were not changed at 4w and 8w after ovx. Importantly, the clonal ability, chondrogenic differentiation and osteogenic differentiation of mSSCs were impaired at 8w after ovx. We found 114 genes were down-regulated in mSSCs, including skeletal developmental genes such as Col10a1, Col2a1, Mef2c, Sparc, Matn1, Scube2 and Dlx5. On the contrary, 526 genes were up-regulated, including pro-inflammatory genes such as Csf1, Nfkbla, Nfatc2, Nfkb1 and Nfkb2.

CONCLUSION

Function of mSSCs was impaired by up-regulating pro-inflammatory genes in ovx-induced osteoporosis.

Calvarial Versus Long Bone: Implications for Tailoring Skeletal Tissue Engineering

Tissue-engineered graft substitutes have shown great potential to treat large bone defects. While we usually assume that therapeutic approaches developed for appendicular bone healing could be similarly translated for application in craniofacial reconstruction and vice versa, this is not necessarily accurate. In addition to those more well-known healing-associated factors, such as age, lifestyle (e.g., nutrition and smoking), preexisting disease (e.g., diabetes), medication, and poor blood supply, the developmental origins and surrounding tissue of the wound sites can largely affect the fracture healing outcome as well as designed treatments. Therefore, the strategies developed for long bone fracture repair might not be suitable or directly applicable to skull bone repair. In this review, we discuss aspects of development, healing process, structure, and tissue engineering considerations between calvarial and long bones to assist in designing the tailored bone repair strategies. Impact Statement We summarized, in this review, the existing body of knowledge between long bone and calvarial bone with regard to their development and healing, tissue structure, and consideration of current tissue engineering strategies. By highlighting their similarities and differences, we propose that tailored tissue engineering strategies, such as scaffold features, growth factor usage, and the source of cells for tissue or region-specific bone repair, are necessary to ensure an optimized healing outcome.