Pharmacological Manipulation of Early Zebrafish Skeletal Development Shows an Important Role for Smad9 in Control of Skeletal Progenitor Populations.

Georgina L K Mcdonald,Chrissy L Hammond,Dylan J M Bergen,Mengdi Wang

doi:10.3390/biom11020277

Abstract

Osteoporosis and other conditions associated with low bone density or quality are highly prevalent, are increasing as the population ages and with increased glucocorticoid use to treat conditions with elevated inflammation. There is an unmet need for therapeutics which can target skeletal precursors to induce osteoblast differentiation and osteogenesis. Genes associated with high bone mass represent interesting targets for manipulation, as they could offer ways to increase bone density. A damaging mutation in SMAD9 has recently been associated with high bone mass. Here we show that Smad9 labels groups of osteochondral precursor cells, which are not labelled by the other Regulatory Smads: Smad1 or Smad5. We show that Smad9+ cells are proliferative, and that the Smad9+ pocket expands following osteoblast ablation which induced osteoblast regeneration. We further show that treatment with retinoic acid, prednisolone, and dorsomorphin all alter Smad9 expression, consistent with the effects of these drugs on the skeletal system. Taken together these results demonstrate that Smad9+ cells represent an undifferentiated osteochondral precursor population, which can be manipulated by commonly used skeletal drugs. We conclude that Smad9 represents a target for future osteoanabolic therapies.

Highlights

Bone is a mineralised tissue, in which dynamic remodelling is tightly regulated by the activity of osteoclasts and osteoblasts and osteocytes
We show that Smad9 is expressed in a population of osteochondral precursors, which are capable of proliferation and contribute to regeneration of osteoblasts following their ablation
Between human and zebrafish) and showed striking sequence similarity in the MH1 domain, MH2 domain, and the C-terminal SSVS BMPR phosphorylation motif, whereas the linker domain exhibited more sequence variability between human, mouse and zebrafish (Figure S2). This gave us further confidence that zebrafish Smad9 protein function is likely to be highly similar to land vertebrates

Summary

Introduction

Bone is a mineralised tissue, in which dynamic remodelling is tightly regulated by the activity of osteoclasts (which resorb matrix) and osteoblasts and osteocytes (which deposit bone matrix). This tight coupling of bone resorption and deposition is essential to maintain a healthy skeleton capable of functioning efficiently. When this balance is perturbed it can lead to various bone remodelling related diseases such as osteopetrosis (excessive bone strength) or osteoporosis (OP; reduced bone strength) [1].

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