Preclinical Validation of the Mastocitary EP2 Receptor as Anti-Asthmatic Therapeutic Diana
Author: Dr. Judith Plaza Almolda. III Prize in honor of Dr. Josep Séculi Brillas
Asthma is a chronic disease of the respiratory tract whose current therapeutic approach does not slow down its progression, and sometimes does not revert the symptomatology. The analysis of endogenous protection mechanisms can result on the identification of new effective pharmacological targets. The preclinical studies carried out so far in our laboratory (Figure 1) together with clinical and basic evidence from other groups, suggested the hypothesis that selective activation of the prostanoid EP2 receptor of PGE2 inhibits mastocyte activity, and consequently reduces the respiratory pathological process induced in mice by exposure to aeroallergens, which can suppose a synergistic anti-asthmatic therapeutic strategy to the mastocyte blockade by other mechanisms.
For its verification, a first objective was proposed: to evaluate the selective function of the prostanoid EP2 receptor in the modulation of mastocyte activity (Figure 2), and in the respiratory reactivity to aeroallergens. Inhibition of mast cells by two chemically distinct EP2 agonists in phenotypically diverse mast cell populations was observed. Inhibition that was also demonstrated in vivo in a mouse asthma model exposed to dust mite aeroallergens.
The inhibitory effect on mast cells was accompanied by the prevention of bronchial hyperreactivity (Figure 3), and pointed at a possible control of the bronchovascular inflammatory process. Protective nature of EP2 agonism was confirmed by administering antagonists in in vitro and in vivo models. These triggered an overactivity of the mast cells that aggravated the respiratory alteration. This suggests that the beneficial effect of PGE2 described in experimental clinical studies is a consequence of the mediation of the mastocyte EP2 receptor.
The second objective was the development of a transgenic mouse that overexpressed the EP2 receptor specifically in mast cells in order to have an experimental tool to determine the functional importance of the mastocyte receptor in the benefit attributable to PGE2 in asthma. We succeeded in inserting the EP2 gene and overexpressing it (Fig. 4), demonstrating in some in vitro assays that mast cells isolated from transgenic EP2 mice responded with an increase in blockade.
However, the beneficial effect attributable to the presence of a greater number of EP2 receptors could not be confirmed in vivo, probably due to the compensatory fluctuations in the expression of other related genes. The development of the transgenic colony of EP2 requires a more exhaustive characterization to investigate the importance of mastocyte EP2 in the prevention of respiratory alteration.
Finally, given the importance of the mastocyte EP2 receptor, a preclinical approach was used to determine the potential clinical interest of the activation of EP2 compared to other antiasthmatic mast cell blocking strategies. In the third objective, the effect of a mastocyte blockade was compared in two cell populations by means of two mechanisms, EP2 agonism and anti-IgE antibodies, in in vivo and in vitro allergy models. It was demonstrated in a model of cutaneous anaphylaxis in vivo and in mast cell populations in vitro (Figure 5) that EP2 agonism and neutralization of IgE exert a quantitatively comparable blockade of mast cells. For further studies of the potential synergy or therapeutic complementarity between both mechanisms in the airways of sensitized mice, an innovative model of passive respiratory anaphylaxis (PRA) was successfully developed in transgenic mice expressing the human IgE receptor (Figure 6).
Results obtained in this thesis allow to conclude that the activation of the axis “PGE2 – EP2 mastocyte – Asthma” supposes an endogenous mechanism of protection against the dysfunction of the airways induced by aeroallergens. The new in vivo experimental models developed in this project will characterize the antiasthmatic mechanism, and confirm the therapeutic target status of the EP2 receptor, or other molecules that are part of that axis.
Article published in the journal of the ACVC (Academy of Veterinary Sciences of Catalonia for its abbreviation in Spanish). Year 2017.