Bone is a dynamic tissue which continuously undergoes remodeling primarily through osteoblast-mediated bone formation and osteoclast-mediated bone resorption [1]. The close interplay between osteoblast and osteoclast activity ultimately determines the turnover, remodeling, and overall health of bone tissue. Disruption of this homeostasis due to alterations in molecular signaling, physiological aging, or injury can lead to various bone pathologies and compromised healing. Purinergic signaling has been identified as a key molecular pathway involved in bone remodeling and homeostasis [2]. Purinergic receptors can be categorized into either P2- or P1-type based on the activation ligand. P2 receptors are activated by nucleotides whereas P1 receptors are activated by the nucleoside adenosine. While P2 and P1 purinergic receptors are expressed by osteoblasts and osteoclasts, this review will focus on the role played by purinergic P1 receptors and adenosine signaling in bone tissue health and regeneration. For a more comprehensive review on P2 purinergic signaling and its effect on bone homeostasis, refer to other reviews [3].

Adenosine is a nucleoside that is produced mostly through enzymatic degradation of adenine nucleotides (ATP, ADP, and AMP). Typically, healthy physiological concentrations of adenosine are maintained between 20 and 300 nM [4]. Extracellular adenosine levels can be drastically increased in response to pathological conditions such as injury, inflammation, or factors that contribute to cellular stresses; where cells can produce over 10 times the basal level of adenosine, resulting in micromolar concentrations [5]. This effect is a result of a stress-induced increase in phosphatase activity along with activation of ecto-5′-nucleotidase (CD73) and ectonucleoside triphosphate diphosphohydrolase-1 (CD39) [6]. Specifically, upon injury or mechanical stress, intracellular ATP is released to the extracellular space through pannexin/connexin channels [7]. The extracellular ATP is hydrolyzed to ADP and AMP by CD39, and subsequently hydrolyzed to the adenosine metabolite via CD73. In addition, tissue nonspecific alkaline phosphatase (TNAP) has also been shown to contribute to the extracellular adenosine via degradation of ATP to adenosine [8]. Once in the extracellular space, adenosine functions through G-protein-coupled receptors, specifically binding to four distinct adenosine receptors – ADORA1, ADORA2A, ADORA2B, and ADORA3. Different concentrations of adenosine are needed to activate these receptors. At typical physiological adenosine concentration of 0.3–3 nM and 1–20 nM, ADORA1 and ADORA2A can be activated, respectively [9]. Contrarily, ADORA2B and ADORA3 require much higher levels of adenosine, with activation occurring at concentrations of 10 μM and 1 μM, respectively. Although a majority of adenosine is produced from nucleotide metabolism, alternative pathways also contribute to adenosine production. For instance, adenosine can be intracellularly generated from hydrolysis of S-adenosyl-homocysteine (SAH) in the methionine cycle, and subsequently transported to the extracellular space through equilibrative nucleoside transporters (ENT1/2) [4]. Interestingly, molecules other than adenosine, such as Netrin, can also enhance extracellular adenosine signaling. For example, Netrin-1 can activate ADORA2B and has been shown to promote revascularization during limb ischemia [10].

This mini review will discuss how alterations in adenosine signaling contribute to changes in bone health and repair outcomes. Although the significant role played by adenosine in bone homeostasis and its potential therapeutic impact has been widely recognized, severe side effects caused by systemic administration have limited its clinical use. This review will also discuss the technological advancements in the field to circumvent the challenges associated with adenosine delivery.