Feedback facilitation by adenosine A2A receptors of ATP release from mouse hippocampal nerve terminals

The adenosine modulation system is mostly composed by inhibitory A₁ receptors (A₁R) and the less abundant facilitatory A_2A receptors (A_2AR), the latter selectively engaged at high frequency stimulation associated with synaptic plasticity processes in the hippocampus. A_2AR are activated by adenosine originated from extracellular ATP through ecto-5'-nucleotidase or CD73-mediated catabolism. Using hippocampal synaptosomes, we now investigated how adenosine receptors modulate the synaptic release of ATP. The A_2AR agonist CGS21680 (10-100 nM) enhanced the K⁺-evoked release of ATP, whereas both SCH58261 and the CD73 inhibitor α,β-methylene ADP (100 μM) decreased ATP release; all these effects were abolished in forebrain A_2AR knockout mice. The A₁R agonist CPA (10-100 nM) inhibited ATP release, whereas the A₁R antagonist DPCPX (100 nM) was devoid of effects. The presence of SCH58261 potentiated CPA-mediated ATP release and uncovered a facilitatory effect of DPCPX. Overall, these findings indicate that ATP release is predominantly controlled by A_2AR, which are involved in an apparent feedback loop of A_2AR-mediated increased ATP release together with dampening of A₁R-mediated inhibition. This study is a tribute to María Teresa Miras-Portugal.

CD73 activity of mesenchymal stromal cell-derived extracellular vesicle preparations is detergent-resistant and does not correlate with immunomodulatory capabilities

BACKGROUND AIMS

Extracellular vesicles (EVs) derived from human mesenchymal stromal cells (MSCs) show immunomodulatory activity in different assays both in vitro and in vivo. In previous work, the authors compared the immunomodulatory potential of independent MSC-EV preparations in a multi-donor mixed lymphocyte reaction (mdMLR) assay and an optimized steroid-refractory acute graft-versus-host disease (aGVHD) mouse model. The authors observed that only a proportion of the MSC-EV preparations showed immunomodulatory capabilities and demonstrated that only MSC-EV preparations with mdMLR immunomodulating activities were able to suppress aGVHD symptoms in vivo and vice versa. Since the mdMLR assay is complex and depends on primary human cells of different donors, the authors sought to establish an assay that is much easier to standardize and fulfills the requirements for becoming qualified as a potency assay.

METHODS

The bona fide MSC antigen CD73 possesses ecto-5'-nucleotidase activity that cleaves pro-inflammatory extracellular adenosine monophosphate into anti-inflammatory adenosine and free phosphate. To test whether the ecto-5'-nucleotidase activity of the MSC-EV preparations reflected their immunomodulatory potential, the authors adopted an enzymatic assay that monitors the ecto-5'-nucleotidase activity of CD73 in a quantitative manner and compared the activity of well-characterized MSC-EV preparations containing or lacking mdMLR immunomodulatory activity.

RESULTS

The authors showed that the ecto-5'-nucleotidase activity of the MSC-EV preparations did not correlate with their ability to modulate T-cell responses in the mdMLR assay and thus with their potency in improving disease symptomatology in the optimized mouse aGVHD model. Furthermore, the ecto-5'-nucleotidase activity was resistant to EV-destroying detergent treatment.

CONCLUSIONS

Ecto-5'-nucleotidase activity neither reflects the potency of the authors' MSC-EV preparations nor provides any information about the integrity of the respective EVs. Thus, ecto-5'-nucleotidase enzyme activity is not indicative for the immunomodulatory potency of the authors' MSC-EV products. The development of appropriate potency assays for MSC-EV products remains challenging.

Nucleotide Analog ARL67156 as a Lead Structure for the Development of CD39 and Dual CD39/CD73 Ectonucleotidase Inhibitors

Nucleoside triphosphate diphosphohydrolase1 (NTPDase1, CD39) inhibitors have potential as novel drugs for the (immuno)therapy of cancer. They increase the extracellular concentration of immunostimulatory ATP and reduce the formation of AMP, which can be further hydrolyzed by ecto-5'-nucleotidase (CD73) to immunosuppressive, cancer-promoting adenosine. In the present study, we synthesized analogs and derivatives of the standard CD39 inhibitor ARL67156, a nucleotide analog which displays a competitive mechanism of inhibition. Structure-activity relationships were analyzed at the human enzyme with respect to substituents in the N ⁶- and C8-position of the adenine core, and modifications of the triphosph(on)ate chain. Capillary electrophoresis coupled to laser-induced fluorescence detection employing a fluorescent-labeled ATP derivative was employed to determine the compounds' potency. Selected inhibitors were additionally evaluated in an orthogonal, malachite green assay versus the natural substrate ATP. The most potent CD39 inhibitors of the present series were ARL67156 and its derivatives 31 and 33 with K_i values of around 1 µM. Selectivity studies showed that all three nucleotide analogs additionally blocked CD73 acting as dual-target inhibitors. Docking studies provided plausible binding modes to both targets. The present study provides a full characterization of the frequently applied CD39 inhibitor ARL67156, presents structure-activity relationships, and provides a basis for future optimization towards selective CD39 and dual CD39/CD73 inhibitors.

The Ecto-5'-Nucleotidase/CD73 Inhibitor, α,β-Methylene Adenosine 5'-Diphosphate, Exacerbates Carrageenan-Induced Pleurisy in Rat

The ecto-5’-nucleotidase (ecto-5’NT/CD73) represents a crucial enzyme for endogenous adenosine generation. Several findings have shown that CD73 plays an important role in regulating vascular permeability and immune cell function. Adenosine 5’-(α,β-methylene)diphosphate (APCP) is a CD73 inhibitor, widely used as pharmacological tool to investigate the role of CD73/adenosine pathway in several in vitro and in vivo models, although it has been also shown to inhibit other ectoenzymes involved in adenosinergic pathway. Here, we evaluated the effect of APCP in the development of inflammation in carrageenan-induced pleurisy model. We found that treatment with APCP (400 µg/rat) significantly increased cell accumulation, exudate formation, and pro-inflammatory cytokine content into the pleural cavity in the acute phase (4 h) of inflammation, with no differences in the sub-acute phase (72 h) except for the regulation of monocyte chemotactic protein-1 levels. In addition, cells collected by pleural lavage fluids of APCP-treated rats, 4 h following carrageenan injection, showed increased ability to migrate in vitro, both in presence and in absence of N-formyl-L-methionyl-L-leucyl-L-phenylalanine as chemotactic stimulus, compared to cells obtained by control rats. Our results demonstrate that APCP exacerbates the early phase of carrageenan-induced pleurisy by controlling pleural effusion and polymorphonuclear migration in vivo and ex vivo. This effect is likely dependent upon CD73 inhibition, although an inhibitory effect of other ectoenzymes cannot be ruled out.