*Article* **miR-21-KO Alleviates Alveolar Structural Remodeling and Inflammatory Signaling in Acute Lung Injury**

**Johanna Christine Jansing 1,2, Jan Fiedler 3,4, Andreas Pich 5, Janika Viereck 3, Thomas Thum 3,4, Christian Mühlfeld 1,2 and Christina Brandenberger 1,2,\***


Received: 13 December 2019; Accepted: 24 January 2020; Published: 27 January 2020

**Abstract:** Acute lung injury (ALI) is characterized by enhanced permeability of the air–blood barrier, pulmonary edema, and hypoxemia. MicroRNA-21 (miR-21) was shown to be involved in pulmonary remodeling and the pathology of ALI, and we hypothesized that miR-21 knock-out (KO) reduces injury and remodeling in ALI. ALI was induced in miR-21 KO and C57BL/6N (wildtype, WT) mice by an intranasal administration of 75 μg lipopolysaccharide (LPS) in saline (*n* = 10 per group). The control mice received saline alone (*n* = 7 per group). After 24 h, lung function was measured. The lungs were then excised for proteomics, cytokine, and stereological analysis to address inflammatory signaling and structural damage. LPS exposure induced ALI in both strains, however, only WT mice showed increased tissue resistance and septal thickening upon LPS treatment. Septal alterations due to LPS exposure in WT mice consisted of an increase in extracellular matrix (ECM), including collagen fibrils, elastic fibers, and amorphous ECM. Proteomics analysis revealed that the inflammatory response was dampened in miR-21 KO mice with reduced platelet and neutrophil activation compared with WT mice. The WT mice showed more functional and structural changes and inflammatory signaling in ALI than miR-21 KO mice, confirming the hypothesis that miR-21 KO reduces the development of pathological changes in ALI.

**Keywords:** acute lung injury; microRNA-21; alveolar micromechanics; structural remodeling; inflammatory signaling
