Spatial and temporal changes in extracellular elastin and laminin distribution during lung alveolar development

Yongfeng Luo,Nan Li,David Warburton,Gloria S. Pryhuber,Rex Moats,Wei Shi,Hui Chen,Robert P. Mecham,G. Esteban Fernandez,Daria Krenitsky

doi:10.1038/s41598-018-26673-1

Abstract

Lung alveolarization requires precise coordination of cell growth with extracellular matrix (ECM) synthesis and deposition. The role of extracellular matrices in alveogenesis is not fully understood, because prior knowledge is largely extrapolated from two-dimensional structural analysis. Herein, we studied temporospatial changes of two important ECM proteins, laminin and elastin that are tightly associated with alveolar capillary growth and lung elastic recoil respectively, during both mouse and human lung alveolarization. By combining protein immunofluorescence staining with two- and three-dimensional imaging, we found that the laminin network was simplified along with the thinning of septal walls during alveogenesis, and more tightly associated with alveolar endothelial cells in matured lung. In contrast, elastin fibers were initially localized to the saccular openings of nascent alveoli, forming a ring-like structure. Then, throughout alveolar growth, the number of such alveolar mouth ring-like structures increased, while the relative ring size decreased. These rings were interconnected via additional elastin fibers. The apparent patches and dots of elastin at the tips of alveolar septae found in two-dimensional images were cross sections of elastin ring fibers in the three-dimension. Thus, the previous concept that deposition of elastin at alveolar tips drives septal inward growth may potentially be conceptually challenged by our data.

Highlights

Pulmonary alveoli are organized clusters of sac-like structures distal to the broncho-alveolar duct junction, arising from both the sides and the tips of terminal ducts, which provide the large gas exchange surface that fulfills respiratory function
extracellular matrix (ECM) is classified into two distinct types based on its location and composition: the interstitial connective tissue matrix and the basement membrane
The basement membrane is a thin layer of ECM between the epithelia/endothelia and surrounding stromal cells, and is composed of two molecular networks: laminin and type IV collagen[24]

Summary

Introduction

Pulmonary alveoli are organized clusters of sac-like structures distal to the broncho-alveolar duct junction, arising from both the sides and the tips of terminal ducts, which provide the large gas exchange surface that fulfills respiratory function. Approximately 300 proteins constitute the core matrisome, which are usually covalently attached with glycosaminoglycan chains (proteoglycans) or glycans (glycoproteins)[9] During lung development, these ECM components constantly interact with a variety of lung cells in a coordinated and dynamic manner. The deposition and dynamic remodeling of elastin fibers at the tip of a secondary septal crest, based on a 2-D image, is assumed to be a driving force for the subdivision of pre-alveolar spaces, leading to the growth of alveolar septa This hypothesis is supported by the findings that inhibition of elastin protein expression or perturbation of elastin crosslinking causes attenuation or absence of secondary septal crests and definitive alveoli[16,17,18]

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Conclusion