The effect of the mechanodynamic lung environment on fibroblast phenotype via the Flexcell

Abstract

The lung is a highly mechanical organ as it is exposed to approximately 10⁹ strain cycles, (where strain is the length change of tissue structure per unit initial length), with an approximately 4% amplitude change during quiet tidal breathing or 10⁷ strain cycles at a 25% amplitude during heavy exercises, sighs, and deep inspirations. These mechanical indices have been reported to become aberrant in lung diseases such as acute respiratory distress syndrome (ARDS), pulmonary hypertension, bronchopulmonary dysplasia (BPD), idiopathic pulmonary fibrosis (IPF), and asthma. Through recent innovations, various in vitro systems/bioreactors used to mimic the lung's mechanical strain have been developed. Among these, the Flexcell tension system which is composed of bioreactors that utilize a variety of programs in vitro to apply static and cyclic strain on different cell-types established as 2D monolayer cultures or cell-embedded 3D hydrogel models, has enabled the assessment of the response of different cells such as fibroblasts to the lung's mechanical strain in health and disease. Fibroblasts are the main effector cells responsible for the production of extracellular matrix (ECM) proteins to repair and maintain tissue homeostasis and are implicated in the excessive deposition of matrix proteins that leads to lung fibrosis. In this review, we summarise, studies that have used the Flexcell tension bioreactor to assess effects of the mechanical lung on the structure, function, and phenotype of lung fibroblasts in homeostatic conditions and abnormal environments associated with lung injury and disease. We show that these studies have revealed that different strain conditions regulate fibroblast proliferation, ECM protein production, and inflammation in normal repair and the diseased lung.

Keywords: Cyclic and static strain; Flexcell tension bioreactor; Frequency; In vitro models; Lung fibroblasts; Mechanical lung environment; Strain amplitudes.

Fig. 1

Set up of Flexcell Tension Systems. Top—Schematic of the Flexcell® FX-6000™ System, in order of functionality 1–11. “1) Surge protected power strip, 2) power outlet, 3) Ethernet cable, 4) reinforced vacuum tubing, 5) compressed air regulator/filter, 6) vent tubing, 7) system tubing, 8) flex in tubing, 9) flex out tubing, 10) water trap, 11) baseplate”. Bottom- An enlargement of a top view of the baseplate in which the different culture plates sit for strain experiments showing white loading posts. Adapted from Flexcellint® with permission

Fig. 2

Bioflex and Tissue Train in vitro models of the Flexcell system. A Image of two wells from the 6-well Bioflex culture plate with a flexible membrane bottom and a schematic showing a 2D monolayer of cells grown in the plate across a cylindrical loading post creating equibiaxial tension. B Image of two wells out of the 6-well Tissue Train culture plate with a top view schematic of a cell-embedded gel construct connected to the anchor stems, the side view illustrates that when vacuum is applied, the rubber membrane deforms downwards to create space for the 3D linear gel to form. Adapted from Flexcellint® with permission

Fig. 3

Images of 4 different 6-well Flexcell culture plates. A Uniflex culture plate. B Bioflex culture plate. C Tissue Train Linear culture plate. D Tissue Train Circular Foam culture plate. Adapted from Flexcellint® with permision