Adenosine-dependent airway inflammation and hyperresponsiveness in partially adenosine deaminase-deficient mice.

Adenosine is a signaling nucleoside that is elevated in the lungs of asthmatics. We have engineered a mouse model that has elevated levels of adenosine as a result of the partial expression of the enzyme that metabolizes adenosine, adenosine deaminase (ADA). Mice with lowered levels of ADA enzymatic activity were generated by the ectopic expression of an ADA minigene in the gastrointestinal tract of otherwise ADA-deficient mice. These mice developed progressive lung inflammation and damage and died at 4-5 mo of age from respiratory distress. Associated with this phenotype was a progressive increase in lung adenosine levels. Examination of airway physiology at 6 wk of age revealed alterations in airway hyperresponsiveness. This was reversed following the lowering of adenosine levels using ADA enzyme therapy and also through the use of the adenosine receptor antagonist theophylline, implicating both the nucleoside and its receptors in airway physiological alterations. All four adenosine receptors were expressed in the lungs of both control and partially ADA-deficient mice. However, transcript levels for the A(1), A(2B), and A(3) adenosine receptors were significantly elevated in partially ADA-deficient lungs. There was a significant increase in alveolar macrophages, and monocyte chemoattractant protein-3 was found to be elevated in the bronchial epithelium of these mice, which may have important implications in the regulation of pulmonary inflammation and airway hyperresponsiveness. Collectively, these findings suggest that elevations in adenosine can directly impact lung inflammation and physiology.

Adenosine-mediated mast cell degranulation in adenosine deaminase-deficient mice.

Adenosine is a signaling nucleoside that has been suggested to play a role in asthma in part through its ability to influence mediator release from mast cells. Adenosine levels are elevated in the lungs of asthmatics, further implicating this molecule in the regulation of lung inflammation and suggesting that animal models exhibiting endogenous increases in adenosine will be useful for the analysis of adenosine function. Adenosine deaminase (ADA) is a purine catabolic enzyme responsible for regulating the levels of adenosine in tissues and cells. ADA-deficient mice develop lung inflammation and damage reminiscent of that seen in asthma in association with elevated adenosine levels. In the current study, we investigated the status of mast cells in ADA-deficient lungs. ADA-deficient mice exhibited extensive lung mast cell degranulation concurrent with elevated adenosine levels. ADA enzyme therapy prevented the accumulation of lung adenosine as well as mast cell degranulation, suggesting that this process was dependent on elevated lung adenosine levels. Consistent with this, treatment of ADA-deficient mice with broad spectrum adenosine receptor antagonists attenuated degranulation by 30 to 40%, supporting the involvement of adenosine receptor signaling. Moreover, these studies demonstrate the ability of endogenously generated adenosine to influence lung mast cell degranulation in a receptor-mediated manner and establish ADA-deficient mice as a model system to investigate the specific adenosine receptor responses involved in the degranulation of lung mast cells.

Transitory expression of the A2b adenosine receptor during implantation chamber development.

Adenosine is a short-range signal molecule that surges in the mouse uterus immediately after blastocyst implantation (Blackburn et al. [1992] Dev. Dyn. 194:155-168). The present study has investigated patterns of uterine adenosine receptor expression during early post-implantation development. Strong expression of the A2b adenosine receptor was observed. Utilizing northern blot analysis, in situ hybridization, and immunostaining, the source of expression was mapped to the primary and secondary decidua of the antimesometrial region, between days 4-8 of gestation. Distribution of the A2b receptor protein followed that of the corresponding transcript by about one gestational day and reflected the dynamics of antimesometrial tissue organization during implantation chamber development. Uterine adenosine surges to levels sufficient for A2b receptor engagement during a defined period (i.e., days 4-6) after blastocyst implantation. Decidual A2b receptor expression thus defines a transitory window of murine gestation that corresponds to a period of human gestation encompassing most spontaneous pregnancy losses. Because adenosine receptors are sensitive to metabolically stable adenosine analogues, their differential expression during implantation chamber development may hold therapeutic potential in the prevention of early pregnancy loss. Dev Dyn 1999;216:127-136.