The role of miR-135b in normal and aberrant late lung development
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Bronchopulmonary dysplasia (BPD) is a chronic lung disease that occurs in premature infants. BPD is a consequence of oxygen supplementation and/or mechanical ventilation and is characterized by fewer, larger alveoli and thus associated with decreased surface area for gas exchange and increased alveolar wall thickness. The pathogenesis of BPD is not fully understood but several reports indicate microRNAs as potential key players during normal and aberrant lung development. In the present study, the expression of microRNA (miR)-135b-5p was significantly increased in lungs of BPD patients and in an experimental mouse model of BPD. MiR-135b-5p was found expressed in alveolar epithelial type II (AEII) cells and targeted Smad5, a regulatory protein of transforming growth factor-β (TGF-β)/bone morphogenetic protein (BMP) signaling. The AEII cells exposed to hyperoxia in vivo and in vitro presented higher miR-135b-5p expression and consequently reduced levels of Smad5. The same pattern was observed when human alveolar epithelial cells were exposed to hyperoxic conditions. Overexpression of miR-135b-5p in A549 cells revealed Smad5 as a target, and that increased expression of miR-135b-5p reduced cell proliferation. The inhibition of miR-135b-5p in vivo using a locked-nucleic acid (LNA)-stabilized antimiR directed against miR-135b-5p revealed a significant improvement in lung architecture after hyperoxic exposure. The same grade of improvement in lung structure was observed when miR-135b was genetically ablated using a Cre-ERT2 driver line. Moreover, to assess if Smad5 played a crucial role in lung development, Smad5 was genetically ablated and revealed a worsening of lung structure. The use of a target site blocker, a compound which should block the binding of miR-135b-5p to Smad5, dramatically reduced Smad5 expression and completely disrupted the lung architecture. The present study revealed the importance of miR-135b-5p and the ability to target and to regulate Smad5 during normal and aberrant lung development. The inhibition of miR-135b-5p is suggested as a potential therapeutic drug to treat premature infants with BPD.