Adenosine levels in the postimplantation mouse uterus: Quantitation by HPLC-fluorometric detection and spatiotemporal regulation by 5′-nucleotidase and adenosine deaminase

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Abstract
Extracellular adenosine has the potential to influence many aspects of target cell metabolism. The present study has determined the endogenous levels of adenosine in the pregnant mouse uterus and developing embryodecidual unit with respect to the expression of two key enzymes of adenosine metabolism, 5′-nucleotidase (5′-NT; EC 3.1.3.5) and adenosine deaminase (ADA; EC 3.5.4.4). To measure adenosine levels, nucleoside extracts were etheno-derivatized and quantitated by high-performance liquid chromatography-fluorescence detection (0.03 pmol/mg protein sensitivity). Adenosine levels were determined to be 0.18 nmol/mg protein in the nonpregnant uterus; however, two statistically significant changes were identified in the pregnant uterus: (1) a periimplantation surge between day 3 (0.24 nmol/mg protein) and day 5 (0.59 nmol/mg protein) of gestation (plug day 0; implantation day 4); and (2) an early postimplantation decline between day 6 (0.54 nmol/mg protein) and day 7 (0.10 nmol/mg protein). The periimplantation adenosine surge coincided with uterine expression of 5′-NT, an enzyme which catalyzes the irreversible dephos-phorylation of 5′-AMP to adenosine. 5′-NT expression was shown by Northern blot analysis to peak in the embryo-decidual unit on day 5 of gestation and then to decline through day 9; transcripts remained elevated in the placenta between day 9 and day 13 (the latest day examined in this study). By use of specific enzyme histochemistry, most 5′-NT activity was localized to the primary decidual zone on day 5. This expression subsequently declined during regression of the primary decidua; however, 5′-NT appeared on giant trophoblast (days 7–13) and the metrial gland (days 11–13). Other purine catabolic enzymes degrading AMP (adenylate deaminase) or generating adenosine (S-adenosylhomocysteine hydrolase) were not detected in the embryo-decidual unit suggesting that the net flux of utero-placental AMP catabolism proceeds with adenosine as an intermediate, this being the major pathway of adenosine formation. The sharp drop in adenosine levels between day 6 and day 7 coincided with a rise in the activity and mRNA expression of ADA, an enzyme which catalyzes the irreversible deamination of adenosine to inosine. ADA was previously localized to the secondary decidual zone (days 6–11), secondary giant cells (days 7–13), and spongiotrophoblasts (days 8–13) in the mouse (Knudsen et al., 1991). Results of developmental Northern blot analysis demonstrated a direct correlation of relative 5′-NT/ADA mRNA band intensity to adenosine content between day 4 and day 9 of gestation, suggesting that the local availability of adenosine in the antimesometrium is dependent upon the distribution of these enzymatic activities. Purine nucleoside phosphorylase and xanthine oxidase, which are two catabolic enzymes acting subsequent to 5′-NT and ADA in the sequential degradation of AMP to xanthine, remained low and constant in the tissues examined suggesting that the catabolic pathway is geared toward regulation of adenosine levels. These results suggest the establishment of an adenosine gradient across the developing antimesometrium. It is proposed that the source of adenosine is AMP released during uterine cell death, and that adenosine, in turn, serves as a regulatory signal to coordinate early postimplantation morphogenetic events with the progression of cell death at the uterine-embryo interface.

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