The Late Osteoblast/Preosteocyte Cell Line MLO-A5 Displays Mesenchymal Lineage PlasticityIn VitroandIn Vivo

Dongqing Yang,Sandra Isenmann,Howard A Morris,Gerald J Atkins,Stan Gronthos

doi:10.1155/2019/9838167

Abstract

The process of osteoblast switching to alternative mesenchymal phenotypes is incompletely understood. In this study, we tested the ability of the osteoblast/preosteocyte osteogenic cell line, MLO-A5, to also differentiate into either adipocytes or chondrocytes. MLO-A5 cells expressed a subset of skeletal stem cell markers, including Sca-1, CD44, CD73, CD146, and CD166. Confluent cultures of cells underwent differentiation within 3 days upon the addition of osteogenic medium. The same cultures were capable of undergoing adipogenic and chondrogenic differentiation under lineage-appropriate culture conditions, evidenced by lineage-specific gene expression analysis by real-time reverse-transcription-PCR, and by Oil Red O and alcian blue (pH 2.5) staining, respectively. Subcutaneous implantation of MLO-A5 cells in a gel foam into NOD SCID mice resulted in a woven bone-like structure containing embedded osteocytes and regions of cartilage-like tissue, which stained positive with both alcian blue (pH 2.5) and safranin O. Together, our findings show that MLO-A5 cells, despite being a strongly osteogenic cell line, exhibit characteristics of skeletal stem cells and display mesenchymal lineage plasticity in vitro and in vivo. These unique characteristics suggest that this cell line is a useful model with which to study aging and disease-related changes to the mesenchymal lineage composition of bone.

Highlights

Osteoblasts derive from mesenchymal skeletal stem cells (SSCs) and actively lay down the organic phase of bone matrix and regulate its mineralisation
Surface markers STRO-1, CD90, CD105, and CD106 were barely detectable in MLO-A5 cells, the SSC markers Sca-1, CD44, CD73, CD146, and CD166 were expressed at high levels (Figure 1(a))
Our findings demonstrate that MLO-A5 cells are able to undergo a limited degree of adipogenic and chondrogenic differentiation when cultured under the appropriate culture conditions in vitro

Summary

Introduction

Osteoblasts derive from mesenchymal skeletal stem cells (SSCs) and actively lay down the organic phase of bone matrix and regulate its mineralisation. The differentiating osteoblast is commonly thought to have three possible fates, including apoptosis or differentiation into either bone lining cells or mature osteocytes. Osteoblasts, rather than being terminally differentiated, have been shown to retain mesenchymal lineage plasticity [1] and have been shown capable of differentiating into adipocytes and chondrocytes [2,3,4,5,6]. The phenomenon of lineage plasticity among osteoblast lineage cells is of increasing interest with the emerging knowledge surrounding the important regulatory role of the osteocyte in bone homeostasis. The observation that the tightly regulated process of bone formation (osteogenesis) decreases with age concomitant with an increase in bone marrow fat accumulation warrants the characterisation of cell line models permissive for the study of both cell fates

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Conclusion