The mechanical memory of lung myofibroblasts

Abstract

Fibroblasts differentiate into the highly synthetic and contractile myofibroblast phenotype when exposed to substrates with an elastic modulus corresponding to pathologically stiff fibrotic tissue. Cellular responses to changes in substrate stiffness are typically analyzed after hours or days, which does not enable the monitoring of myofibroblast persistence, a hallmark of fibrosis. To determine long-lasting effects on the fibrotic behavior of lung fibroblasts, we followed a novel approach of explanting and repeatedly passaging fibroblasts on silicone substrates with stiffness representing various states of lung health. Fibrotic activity was determined by assaying for myofibroblast proliferation, cell contractility, expression of α-smooth muscle actin, extracellular matrix and active TGFβ1. As predicted, myofibroblast activity was low on healthy soft substrates and increased with increasing substrate stiffness. However, explanting and mechanically priming lung fibroblasts for 3 weeks on pathologically stiff substrates resulted in sustained myofibroblast activity even after the cells were returned to healthy soft cultures for 2 weeks. Such primed cells retained higher fibrotic activity than cells that had been exclusively cultured on soft substrates, and were not statistically different from cells continuously passaged on stiff surfaces. Inversely, priming lung fibroblasts for 3 weeks on soft substrates partially protected from myofibroblast activation after the shift to stiff substrates. Hence, mechano-sensed information relating to physical conditions of the local cellular environment could permanently induce fibrotic behavior of lung fibroblasts. This priming effect has important implications for the progression and persistence of aggressive fibrotic diseases such as idiopathic pulmonary fibrosis.

Matrices of Physiologic Stiffness Potently Inactivate Idiopathic Pulmonary Fibrosis Fibroblasts

“…Despite this potential epigenetic regulation of differences between IPF and normal lung-tissue derived PGE 2 production, we note that variations in matrix stiffness retained the capacity to alter PGE 2 concentrations in both groups, and did so with approximately equal potency (although from different baselines), consistent with previous evidence that matrix stiffness regulates COX-2 expression levels (17). Although recent work by Balestrini and colleagues demonstrated that contact with stiff mechanical environments can lead to long-term mechanical memory in fibroblasts (32), out study indicates that in addition to such persistent effects, IPF-derived cells remain acutely responsive to physiologic reductions in matrix stiffness.…”

The Rho Kinases: Critical Mediators of Multiple Profibrotic Processes and Rational Targets for New Therapies for Pulmonary Fibrosis

“…Rather than simply being a consequence of tissue fibrosis, this increased tissue stiffness in turn amplifies myofibroblast differentiation and matrix production Wipff et al, 2007;Liu et al, 2010;Balestrini et al, 2012;Huang et al, 2012), creating a feed-forward loop that could by itself drive fibrosis progression (Liu et al, 2010). Myofibroblast differentiation driven by matrix stiffness is also ROCK-dependent .…”

Hallmarks of the ageing lung

“…Lung-specific loss of ECM components, such as α 3 -laminin, worsens bleomycin-induced pulmonary fibrosis, as recently shown [273]. The role of cell-matrix interactions represents an area of active investigation on the ability of lung matrix to prime cell behaviour, such as myofibroblast activation [274], but it would also impact on exogenous progenitor cell recruitment [275][276][277]. Along these lines, increased numbers of fibrocytes in the bone marrow, as well as in the lung, have been reported in older mice upon lung injury.…”