The mechanobiology of fibroblast activation in disease

Abstract

Fibroblasts play crucial roles in wound healing, cancer, and fibrosis. Many aspects of these roles are driven by the process known as fibroblast activation. The generally accepted definition of fibroblast activation is the transition from a quiescent state to a state in which fibroblasts participate in a number of active processes, including extracellular matrix (ECM) production and remodeling, elevated contractility, and enhanced migratory capacity, although there is no universal consensus on what exactly constitutes "activation." Interestingly, the time scale of activation is not consistent across tissues and disease states; some fibroblasts quickly return to quiescence after activation (e.g., in wound healing), others undergo apoptosis, while a subset become persistently activated. This activation, both acute and persistent, is inherently a mechanical process, given the increase in ECM production and remodeling and the enhanced traction force generation. Thus, there exists a dynamic reciprocity, or cell-ECM feedback, in which activated fibroblasts produce a mechanical microenvironment that in turn supports persistent activation. This has a wide variety of implications for disease, most notably fibrosis and cancer, as the fibroblasts that become persistently activated in connection with these conditions can contribute to disease state progression. Like other mechanosensitive processes, this mechanically induced persistent fibroblast activation is driven by a number of mechanotransduction signaling pathways. Thus, an opportunity exists in which the mechanosensitive underpinning of fibroblast activation can be leveraged to improve clinical outcomes. Here, we highlight these opportunities and make a call to the field to consider the mechanosensitive pathways governing fibroblast activation as an important frontier in mechanomedicine.

FIG. 1.

Phenotypic changes in fibroblast activation during fibroblast-to-myofibroblast transition (FMT). In response to biochemical signaling, fibroblasts undergo activation. While this activation occurs along a continuous spectrum of phenotypic changes, it can be broadly categorized into three main stages. In the first stage, fibroblasts are quiescent and do not actively remodel the ECM. As they activate, they grow bigger, exhibit more robust stress fibers, and develop mature focal adhesions. Fibroblasts at this stage are categorized as proto-myofibroblasts. Fully activated fibroblasts form supermature focal adhesions and express α-SMA. As the ECM is remodeled by the fibroblasts, mechanical cues including stiffness, stretch, compression, and density increase, further influencing fibroblast activation. Fibroblasts sense and respond to these changes via integrins, G protein-coupled receptors, growth factor receptors, and ion channels. One of the key mechanotransduction pathways underlying fibroblast activation is the integrin-FAK-ROCK-MRTF-YAP-TAZ signaling axis.

FIG. 2.

Similarities in the mechanical environments of cancer and fibrosis. Although the biochemical environments found in cancer and fibrosis are different, fibroblasts experience similar mechanical changes in both cancer and fibrosis, including enhanced alignment of the ECM and increased cross-linking, which can modulate the density and stiffness of the substrate. These mechanical changes result in similar cellular responses in CAFs and FAFs, including elevated $\alpha$-SMA expression, enhanced ECM production, and nuclear translocation of YAP/TAZ which has been shown to contribute to the persistent activation of fibroblasts.

FIG. 3.

Engineering techniques to study the mechanobiology of fibroblast activation. Fibroblasts experience diverse mechanical stresses in cancer and fibrosis. These mechanical stresses have been investigated through 2D and 3D systems. Engineering techniques that have been used to study fibroblast activation include the use of (a) hydrogels to vary stiffness, (b) PDMS to stretch fibroblasts, (c) weight to compress fibroblasts, (d) electrospun fibers to create matrices with varied density, and (e) micropatterned surfaces to create random and aligned topographies. These platforms have shown that dynamic changes in stiffness, stretch, compression, density, and alignment can promote fibroblast activation.

FIG. 4.

Overview of key techniques to study confined migration in vitro. (a) Linear micropatterned surfaces guide cell alignment and migration within the defined pattern, resulting in 2D confined migration. In addition, PDMS-based platforms are used to study more physiologically relevant types of confinement, including (b) pinch-point confinement using micropillars to study intravasation and extravasation, (c) vertical confinement, and (d) long microchannels to investigate sustained confinement in vitro. (e) 3D ECM fiber networks are used to study the complex interplay between confinement and matrix composition.