The Xenopus respiratory system reveals common tetrapod mechanisms for growth, regeneration and healing

Abstract

In recent years, we have seen a significant increase in our understanding of the mechanisms of development, regeneration, and healing of the respiratory system. However, most of these studies have been limited by their focus on mammalian systems. Here, we aimed to identify the underlying molecular mechanisms that are active during lung growth and tissue repair in amphibians, specifically Xenopus tropicalis (X. tropicalis). First, we analyzed the stem cell composition and signaling pathways that are active in epithelial and mesenchymal cells during lung growth. Then, we established a protocol for lung injury to assess the types of stem cells underlying tissue repair. In mammals, Sftpc+ (AT2) cells are alveolar stem cells that can differentiate to Krt8+ cells during lung homeostasis and post-injury repair. In this study, we identified Sftpc+ cells and Krt8+ cells, along with the activity of key developmental signaling pathways, Hippo and Wnt, during lung maturation at post-metamorphosis stages. We then established a protocol for lung injury using chemically induced injury with bleomycin, which damages the lung through oxidative stress. The results show an elevation in collagen post-injury, indicating bleomycins effect in causing lung fibrosis. X. tropicalis froglets survived 42 days post-injury, with a continuous decrease in fibrosis. To explore this effect, we analyzed the distribution of lung stem cells; Sox9 protein levels and Sftp gene expression were downregulated at the alveoli 42 days post-injury. The decrease in stem cell marker expression 42 days post-injury suggests they are differentiating as part of the healing process. Nevertheless, we could still detect them after a few weeks of healing. These results suggest that X. tropicalis has a regenerative capacity for lung tissue repair and that the same signaling pathways and stem cells are active in both amphibians and mammalian lungs during lung growth, regeneration, and healing. These findings show for the first time the physiological similarities between the anuran and the mammalian lung during growth and tissue repair processes, suggesting X. tropicalis as a potential animal model to study lung regeneration.