There has been increasing interest recently in the role purinergic signalling in the physiology and pathophysiology of musculoskeletal tissues, especially bone. Whereas the role of ATP in bone metabolism [1,2] has been revealed to some extent and quite a few papers have been published in this respect, functions of its metabolite adenosine are not understood. Bone remodelling is a continuous, life-long process that maintains skeletal integrity. It is orchestrated by the three bone cell types (osteoblasts, osteoclasts and osteocytes), and impairments eventually result in diseases such as osteoporosis. Although there is much insight into the systems that maintain healthy bone, there is still a need to develop new therapies. The hypothesis that the adenosine signalling pathways might provide new targets for bone disease has gained further momentum recently with the publication of the following two papers that are commented on by Bronwen A. J. Evans of Cardiff University. Interestingly enough, the first article deals with findings obtained using CD73 null mice, that have also been the subject of another recently published Highlight [3] that focused on the role of local and systemic adenosine in modulation of antitumour responses in vivo. Here, CD73 null mice have been exploited to unveil a potential role of adenosine in osteoblast differentiation. In the second commented article, by using adenosine A2B receptor knockout mice, this receptor is identified as a main target for adenosine in promoting the differentiation of mesenchymal stem cell to osteoblasts.
Article summaryM. Takedachi and colleagues hypothesized that CD73 may be involved in regulating function of the bone forming cell (osteoblast) through modulating nucleotide metabolism and generating extracellular adenosine that can activate adenosine receptors (ARs). To address this hypothesis, they asked whether CD73 functionally regulates bone metabolism in vivo by characterizing the bone phenotype of cd73−/− mice. In addition, they investigated the involvement of CD73 and AR signalling in osteoblast differentiation in vitro.Micro-computed tomography (μCT) showed that 13 week old male mice had osteopenia, and that this was due to reduced trabecular bone volume, decreased trabecular number and thickness, and increased trabecular separation in cd73−/− mice when compared to wild-type controls. Subsequent work measuring biochemical markers of bone formation showed that osteocalcin (bone formation marker) was decreased in cd73−/− mice whereas bone resorption markers (TRAP5b and fragments of type I collagen) were comparable to wild-type animals. The association of the osteoblast to the impared bone phenotype seen in cd73−/− mice was further substantiated using quantitative RT-PCR. These