Intrinsic BMP inhibitor Gremlin regulates lung alveolar progenitor cell proliferation and differentiation

Type 1 alveolar epithelial cells (AT1s) and type 2 alveolar epithelial cells (AT2s) regulate the structural integrity and function of alveoli. AT1s mediate gas exchange, whereas AT2s serve multiple functions, including surfactant secretion and alveolar repair through proliferation and differentiation into AT1s as progenitors. However, mechanisms regulating AT2 proliferation and differentiation remain unclear. Here we demonstrate that Gremlin, an intrinsic inhibitor of bone morphogenetic protein (BMP), induces AT2 proliferation and differentiation. Transient overexpression of Gremlin in rat lungs by adenovirus vector delivery suppressed BMP signaling, induced proliferation of AT2s and the production of Bmp2, which in turn led to the recovery of BMP signaling and induced AT2 differentiation into AT1s. Gremlin was upregulated in a bleomycin-induced lung injury model. TGF- β and IL-1 β induced Gremlin expression in fibroblasts. Taken together, our findings implicate that Gremlin expression during lung injury leads to precisely timed inhibition of BMP signaling and activates AT2s, leading to alveolar repair. ### Competing Interest Statement T. Yanagihara was funded by the Uehara Memorial Foundation Research Fellowship and Mitacs Canada, and the research institute of St Joseph's Hospital, Hamilton, ON, Canada (Post-doctoral Fellowship Award). K. Tsubouchi was funded by the Uehara Memorial Foundation Research Fellowship. K. Ask reports grants and personal fees from Boehringer Ingelheim, grants from Canadian Pulmonary Fibrosis Foundation, Synairgen, Alkermes, GlaxoSmitheKline, Pharmaxis, Unity, Avalyn, Canadian Institutes of Health Research, Ceapro, Pieris, outside the submitted work. M. Kolb reports grants from the Canadian Institute for Health Research and grants/ personal fees from Roche, Boehringer Ingelheim, Prometic, Respivert, Alkermes, and Pharmaxis and personal fees from Genoa.