Template:TimeCourse

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Overview

The formation of bone is a multi-stage process. It begins with mesenchymal cells committed to becoming cartilage. These cells condense into nodules and differentiate into chondrocytes which proliferate rapidly to form the template for the developing bone. They secrete a cartilege-specific extracellular matrix. blood vessels sinvade the cartilage structure. Mesenchymal precursors surrounding the cartilage cells then differentiate into osteoblasts which begin to form an extracelular matrix specific for bone. At the same time the chondrocytes within the osteoblast shell begin to die from apoptosis leaving a space that becomes the bone marrow. osteoblasts themselves become embedded into the bone matrix and differentiate further into osteocytes. Bone is constantly being remodelled by the action of osteoclasts, cells of the monocyte lineage that degrade the bone and osteoblasts on the surface which replace it with new bone. Thus the formation and maintenance of bone is a dynamic process. During the formation of bone, osteoblasts lay down a matrix of hydroxyapatite, a mineral containing calciumand phosphorous which makes up to 50% of the weight of bone. this process of mineralisation can be simulated in vitro in cell cultures treated with chemicals to induce calcification. Many cell lines, primarily derived from osteosarcomas, can be induced to mineralise in this way, as can primary vascular smooth muscle cells. It is important to understand the process of mineralisation because formation of the bony skeleton is critical to the proper functioning of the vertebrate organism and because ectopic mineralisation can occur in genetic and environmental disease. For example, calcification is a key finding in forms of arterial disease including atherosclerosis.

Sample description

In this study we used the human osteosarcoma cell line Saos-2. This line was first established from an osteosarcoma isolated from an 11 year old female patient in 1973 [1][2]. The line has been commonly used to study extracellular matrix and mechanical mechanisms in both expression and mineralisation studies [3] although the utility of Saos-2 as a human model system for normal osteoblast formation and function has been debated. Varying cellular morphology and proliferation has been observed [4], but Saos-2 demonstrates physiological levels of multiple osteoblastic markers including, osteocalcin (OC), bone sialoprotein (BSP) and decorin (DCN) [3]. In addition, active levels of alkaline phosphatase (ALPL) and the capacity to mineralize render Saos-2 a useful model system in the study of osteoblast function and ECM formation [5]. This experiment was designed to determine promoter specificity and gene expression patterns of key regulators early and late in the mineralization process using this human model system. To control for effects of serum cell proliferation and hyperconfluence we also submitted two control samples: mock-treated Saos-2 cells that had medium changes at the same time points but were not treated with BGP/ascorbic acid and MG63 osteosarcoma cells that were treated but failed ot mineralise. MG63 cells lack high levels of alkaline phosphatase and are not able to mineralise under these conditions, but they may respond to the signal by upregulating some required genes. [3][6]. The experiment was designed across 18 time points: 0, 15, 30, 45, 60, 80,100,120,150 and 180 minutes, 4, 8, and 24 hour as well as 4, 7, 14, 21, and 28 days following calcification induction. Cells were plated into 6 well plates (NUNC) or flasks (T25 or T75) at approximately 100,000 cells / well and mineralization was induced two days later with 50µg/ml ascorbic acid (Sigma) and 2.5mM ß-glycerophosphate (BGP) (Sigma) in medium with 10% serum. Medium was replaced with fresh medium including 10% serum and BGP/ascorbic acid every two to three days. All time point samples were processed for RNA at the same time relative to medium-change. RNA from the 18 time point samples were isolated through the following methods. The cells were lifted using 1x Trypsin-EDTA solution (Sigma) and RNA was isolated using the RNA Bee protocol (Ambisco). The sample RNA was quantified using a Nanodrop spectrophotometer (Nanodrop, USA). Three biological replicates were performed.

Quality control
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Profiled time course samples

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