Implications for Extracellular Matrix Interactions With Human Lung Basal Stem Cells in Lung Development, Disease, and Airway Modeling

Abstract

The extracellular matrix (ECM) is not simply a quiescent scaffold. This three-dimensional network of extracellular macromolecules provides structural, mechanical, and biochemical support for the cells of the lung. Throughout life, the ECM forms a critical component of the pulmonary stem cell niche. Basal cells (BCs), the primary stem cells of the airways capable of differentiating to all luminal cell types, reside in close proximity to the basolateral ECM. Studying BC-ECM interactions is important for the development of therapies for chronic lung diseases in which ECM alterations are accompanied by an apparent loss of the lung's regenerative capacity. The complexity and importance of the native ECM in the regulation of BCs is highlighted as we have yet to create an in vitro culture model that is capable of supporting the long-term expansion of multipotent BCs. The interactions between the pulmonary ECM and BCs are, therefore, a vital component for understanding the mechanisms regulating BC stemness during health and disease. If we are able to replicate these interactions in airway models, we could significantly improve our ability to maintain basal cell stemness ex vivo for use in in vitro models and with prospects for cellular therapies. Furthermore, successful, and sustained airway regeneration in an aged or diseased lung by small molecules, novel compounds or via cellular therapy will rely upon both manipulation of the airway stem cells and their immediate niche within the lung. This review will focus on the current understanding of how the pulmonary ECM regulates the basal stem cell function, how this relationship changes in chronic disease, and how replicating native conditions poses challenges for ex vivo cell culture.

Keywords: airway modeling; basal stem cells; cellular niche; chronic lung disease; extracellular matrix; regeneration.

FIGURE 1

Strategies for utilizing airway basal stem cells in regenerative medicine approaches. (A) Methods for ex vivo expansion and differentiation of airway basal cells. Basal cells (BCs) can be isolated from the human lung epithelium and expanded in 3-D culture, co-culture with 3T3 fibroblasts, or potentially on specifically designed hydrogels or extracellular matrix (ECM)-derived from decellularized lungs. Expanded basal cells are capable of complete differentiation into a pseudostratified airway epithelium at the air-liquid interface, in 3-D spheroid cultures or in decellularized lung scaffolds. (B) Decellularization and recellularization of human lungs for autologous transplantation. Decellularization of human donor lungs, followed by recellularization with patient-derived cells (generated via methods described in panels c (primary airway cell expansion) and d (iPSC-derived basal cell generation. The concept is designed to achieve functional lung tissues or indeed, entire lungs, for autologous transplantation. (C) Ex vivo expansion of human airway basal cells. BCs can be isolated from human lung biopsy or bronchoscopy and expanded on defined ECM to maintain stemness and phenotype. The goal is to engraft these stem cells into the lung to restore airway function. Generation of autologous iPSC-derived basal cells. Using accessible cells such as peripheral blood mononucleocytes (PBMCs) or fibroblasts from skin biopsies, iPSC can be generated via reprogramming. Protocols now exist for their precise differentiation to lung basal cells (Hawkins et al., 2020) generating a potentially unlimited source of autologous cells for engraftment.