Reply to the ‘Comment on “Tensional homeostasis at different length scales” by J. Humphrey and C. Cyron, Soft Matter, 2022, 18, DOI: 10.1039/D1SM01151K’

In this Reply to the Comment, we discuss data from the literature which show that the idea that tensional homeostasis in focal adhesions (FAs) of living cells exists over “a central range of FAs”, which is promulgated in the Comment, is not tenable.

Comment on “Tensional homeostasis at different length scales” by D. Stamenović and M. L. Smith, Soft Matter, 2021, 17, 10274–10285, DOI: 10.1039/D0SM01911A

Assessing potential mechanical homeostasis requires appropriate solutions to the initial-boundary value problems that define the biophysical situation of interest and appropriate definitions of what is meant by homeostasis, including its range.

Distinct roles of nonmuscle myosin II isoforms for establishing tension and elasticity during cell morphodynamics

Nonmuscle myosin II (NM II) is an integral part of essential cellular processes, including adhesion and migration. Mammalian cells express up to three isoforms termed NM IIA, B, and C. We used U2OS cells to create CRISPR/Cas9-based knockouts of all three isoforms and analyzed the phenotypes on homogenously-coated surfaces, in collagen gels, and on micropatterned substrates. In contrast to homogenously-coated surfaces, a structured environment supports a cellular phenotype with invaginated actin arcs even in the absence of NM IIA-induced contractility. A quantitative shape analysis of cells on micropatterns combined with a scale-bridging mathematical model reveals that NM IIA is essential to build up cellular tension during initial stages of force generation, while NM IIB is necessary to elastically stabilize NM IIA-generated tension. A dynamic cell stretch/release experiment in a three-dimensional scaffold confirms these conclusions and in addition reveals a novel role for NM IIC, namely the ability to establish tensional homeostasis.

The stiffness-sensitive transcriptome of human tendon stromal cells

Matrix stiffness and its effects on tensional homeostasis act as major regulators of cellular states in health and disease. Stiffness-sensing studies are typically performed using cells that have acquired "mechanical memory" through prolonged propagation in rigid mechanical environments, e.g. tissue culture plastic (TCP). This may potentially mask the full extent of the stiffness-driven mechanosensing programs. To address this, we developed a biomaterial system composed of two-dimensional mechano-variant silicone substrates that is permissive to large-scale cell culture expansion processes. We broadly mapped the stiffness-mediated mechano-responses by performing RNA sequencing on human tendon-derived stromal cells. We find that matrix rigidities approximating tendon microscale stiffness range (E. ~35 kPa) distinctly favor programs related to chromatin remodeling and Hippo signaling; whereas more compliant stiffnesses (E. 2 kPa) were enriched in responses related to pluripotency, synapse assembly and angiogenesis. We also find that tendon stromal cells undergo dramatic phenotypic drift on conventional TCP, with near-complete suppression of tendon-related genes and emergence of expression signatures skewed towards fibro-inflammatory and metabolic activation. Strikingly, mechano-variant substrates abrogate fibroblasts activation, with tenogenic stiffnesses inducing a transcriptional program that strongly correlate with established tendon tissue-specific signatures. Computational inference predicted that AKT1 and ERK1/2 are major signaling hubs mediating stiffness-sensing in tendon cells. Together, our findings highlight how the underlying biophysical cues may dictate the transcriptional identity of resident cells, and how matrix mechano-reciprocity regulates diverse sets of previously underappreciated mechanosensitive processes in tendon stromal fibroblasts.

Extracellular matrix stiffness regulates degradation of the Hippo kinase MST2 via SCF βTrCP

Tumor microenvironments display disrupted mechanical properties, including altered extracellular matrix (ECM) rigidity. ECM stiffening perturbs cell tensional homeostasis resulting in activation of mechanosensing transcriptional co-activators, such as the Hippo pathway effectors YAP and TAZ. The Hippo pathway plays central roles in development and tumorigenesis, but how the proteostasis of the Hippo kinase MST2 is regulated remains unknown. We show that MST2 levels decrease upon changes in ECM rigidity via proteasome degradation. MST2 degradation is enhanced in human breast epithelial cells that are cultured in stiffer microenvironments due to integrin and integrin-linked kinase activation. MST2 knockdown resulted in increased nucleus-to-cytoplasm ratio of YAP, increased proliferation rates in a soft microenvironment and F-actin alignment in cells cultured in intermediate stiffness. We found that MST2 is ubiquitinated by the SCFβTrCP ubiquitin ligase, and site-directed mutagenesis combined with computational molecular dynamics studies revealed that βTrCP binds MST2 via a noncanonical degradation motif. Our study uncovers the underlying biochemical mechanisms controlling MST2 degradation and demonstrates how alterations in the microenvironment rigidity regulate the proteostasis of a central Hippo pathway component.

Tissue-scale tensional homeostasis in skin regulates structure and physiological function

Tensional homeostasis is crucial for organ and tissue development, including the establishment of morphological and functional properties. Skin plays essential roles in waterproofing, cushioning and protecting deeper tissues by forming internal tension-distribution patterns, which involves aligning various cells, appendages and extracellular matrices (ECMs). The balance of traction force is thought to contribute to the formation of strong and pliable physical structures that maintain their integrity and flexibility. Here, by using a human skin equivalent (HSE), the horizontal tension-force balance of the dermal layer was found to clearly improve HSE characteristics, such as the physical relationship between cells and the ECM. The tension also promoted skin homeostasis through the activation of mechano-sensitive molecules such as ROCK and MRTF-A, and these results compared favourably to what was observed in tension-released models. Tension-induced HSE will contribute to analyze skin physiological functions regulated by tensional homeostasis as an alternative animal model.

Mechanical stimulation of single cells by reversible host-guest interactions in 3D microscaffolds

Many essential cellular processes are regulated by mechanical properties of their microenvironment. Here, we introduce stimuli-responsive composite scaffolds fabricated by three-dimensional (3D) laser lithography to simultaneously stretch large numbers of single cells in tailored 3D microenvironments. The key material is a stimuli-responsive photoresist containing cross-links formed by noncovalent, directional interactions between β-cyclodextrin (host) and adamantane (guest). This allows reversible actuation under physiological conditions by application of soluble competitive guests. Cells adhering in these scaffolds build up initial traction forces of ~80 nN. After application of an equibiaxial stretch of up to 25%, cells remodel their actin cytoskeleton, double their traction forces, and equilibrate at a new dynamic set point within 30 min. When the stretch is released, traction forces gradually decrease until the initial set point is retrieved. Pharmacological inhibition or knockout of nonmuscle myosin 2A prevents these adjustments, suggesting that cellular tensional homeostasis strongly depends on functional myosin motors.