Alveolar repair following LPS-induced injury requires cell-ECM interactions

Abstract

During alveolar repair, alveolar type 2 (AT2) epithelial cell progenitors rapidly proliferate and differentiate into flat AT1 epithelial cells. Failure of normal alveolar repair mechanisms can lead to loss of alveolar structure (emphysema) or development of fibrosis, depending on the type and severity of injury. To test if β 1 -containing integrins are required during repair following acute injury, we administered E . coli lipopolysaccharide (LPS) by intratracheal injection to mice with a postdevelopmental deletion of β 1 integrin in AT2 cells. While control mice recovered from LPS injury without structural abnormalities, β 1 -deficient mice had more severe inflammation and developed emphysema. In addition, recovering alveoli were repopulated with an abundance of rounded epithelial cells coexpressing AT2 epithelial, AT1 epithelial, and mixed intermediate cell state markers, with few mature type 1 cells. AT2 cells deficient in β 1 showed persistently increased proliferation after injury, which was blocked by inhibiting NF-κB activation in these cells. Lineage tracing experiments revealed that β 1 -deficient AT2 cells failed to differentiate into mature AT1 epithelial cells. Together, these findings demonstrate that functional alveolar repair after injury with terminal alveolar epithelial differentiation requires β 1 -containing integrins.

Human alveolar hydrogels promote morphological and transcriptional differentiation in iPSC-derived alveolar type 2 epithelial cells

“…While previous studies have not directly characterized the impact of ECM proteins on iAT2s, studies employing iAT2-fibroblast co-cultures have demonstrated using single-cell RNA sequencing that fibroblasts actively express ECM genes concurrent with the emergence of transitional cell state and AT1-like subpopulations of iAT2s, suggesting a potential role of ECM in iAT2 differentiation 25 , 26 . Further, a recent study has demonstrated the role of ECM-binding through β1-integrin as an important process in AT2 morphological and transcriptional differentiation 9 . Within this study, a mouse model of β1-integrin null AT2s found that, following injury to the alveolar epithelium, AT2 differentiation into elongated squamous AT1s was significantly inhibited 9 .…”

“…Further, a recent study has demonstrated the role of ECM-binding through β1-integrin as an important process in AT2 morphological and transcriptional differentiation 9 . Within this study, a mouse model of β1-integrin null AT2s found that, following injury to the alveolar epithelium, AT2 differentiation into elongated squamous AT1s was significantly inhibited 9 . This resulted in the accumulation of enlarged, round AT2s with transitional cell state genetic markers.…”

“…Under normal lung conditions, AT2s are primarily quiescent cells that secrete surfactant through lamellar bodies in order to reduce alveolar surface tension during respiration, while additionally maintaining low levels of cellular differentiation to alveolar type 1 epithelial cells (AT1s) to preserve the alveolar epithelium 3 , 4 . However, following injury to the alveolar epithelium, AT2s are capable of responding to extracellular biochemical and mechanical signals by increasing cellular proliferation and differentiation into AT1s through a recently described transitional cell state intermediate 5 – 9 . The necessity of alveolar regeneration by AT2s is highlighted in several detrimental lungs conditions, notably pulmonary fibrosis, wherein dysfunctional AT2s stalled in a transitional cell state promote disease progression 8 , 10 – 12 .…”

Alveolar Organoids in Lung Disease Modeling

“…AT2 cell differentiation to AT1 cells is regulated by the Notch, Hedgehog, and Wnt signaling pathways. In addition, the AT2 cell progenitor gains an AT2-to-AT1 transitional cell state expressing KRT8, HBEGF, or AREG genes [52]. Any deficiencies in surfactant protein impair normal lung functions [53].…”