Fibrillar proteins are principal components of extracellular matrix (ECM) that confer mechanical properties to tissues. Fibrosis can result from wound repair in nearly every tissue in adults, and it associates with increased ECM density and crosslinking as well as increased tissue stiffness. Such fibrotic tissues are a major biomedical challenge, and an emerging view posits that the altered mechanical environment supports both synthetic and contractile myofibroblasts in a state of persistent activation. Here, we review the matrisome in several fibrotic diseases, as well as normal tissues, with a focus on physicochemical properties. Stiffness generally increases with the abundance of fibrillar collagens, the major constituent of ECM, with similar mathematical trends for fibrosis as well as adult tissues from soft brain to stiff bone and heart development. Changes in expression of other core matrisome and matrisome-associated proteins or proteoglycans contribute to tissue stiffening in fibrosis by organizing collagen, crosslinking ECM, and facilitating adhesion of myofibroblasts. Understanding how ECM composition and mechanics coevolve during fibrosis can lead to better models and help with antifibrotic therapies. (C) 2022 Elsevier B.V. All rights reserved.

The macrophage checkpoint interaction CD47-SIRPoc is an emerging target for cancer therapy, but clinical trials of monoclonal anti-CD47 show efficacy only in liquid tumors when combined with tumor-opsonizing IgG. Here, in challenging metastatic solid tumors, CD47 deletion shows no effect on tumor growth unless combined with otherwise ineffective tumor-opsonization, and we likewise show wild-type metastases are suppressed by SIRP alpha-blocked macrophages plus tumor-opsonization. Lung tumor nodules of syngeneic B16F10 melanoma cells with CD47 deletion show opsonization drives macrophage phagocytosis of B16F10s, consistent with growth versus phagocytosis calculus for exponential suppression of cancer. Wild-type CD47 levels on metastases in lungs of immunocompetent mice and on human metastases in livers of immunodeficient mice show that systemic injection of antibody-engineered macrophages also suppresses growth. Such in vivo functionality can be modulated by particle pre-loading of the macrophages. Thus, even though CD47-SIRP alpha disruption and tumor-opsonizing IgG are separately ineffective against established metastatic solid tumors, their combination in molecular and cellular therapies prolongs survival.

Cell migration on two-dimensional substrates is typically characterized by lamellipodia at the leading edge, mature focal adhesions and spread morphologies. These observations result from adherent cell migration studies on stiff, elastic substrates, because most cells do not migrate on soft, elastic substrates. However, many biological tissues are soft and viscoelastic, exhibiting stress relaxation over time in response to a deformation. Here, we have systematically investigated the impact of substrate stress relaxation on cell migration on soft substrates. We observed that cells migrate minimally on substrates with an elastic modulus of 2 kPa that are elastic or exhibit slow stress relaxation, but migrate robustly on 2-kPa substrates that exhibit fast stress relaxation. Strikingly, migrating cells were not spread out and did not extend lamellipodial protrusions, but were instead rounded, with filopodia protrusions extending at the leading edge, and exhibited small nascent adhesions. Computational models of cell migration based on a motor-clutch framework predict the observed impact of substrate stress relaxation on cell migration and filopodia dynamics. Our findings establish substrate stress relaxation as a key requirement for robust cell migration on soft substrates and uncover a mode of two-dimensional cell migration marked by round morphologies, filopodia protrusions and weak adhesions.It is now shown that cells migrate robustly on soft, viscoelastic substrates with fast stress relaxation using a migration mode marked by a rounded cell morphology and filopodia protrusions extending at the leading edge.

Simple Summary Hepatocellular carcinoma (HCC) is a deadly primary liver cancer that most often develops in a cirrhotic (highly scarred) liver. Cirrhosis is associated with large-scale mechanical changes in the liver, with increases in stiffness to levels that alter liver cell behavior. Importantly, recent research has yielded two key observations: that mechanical changes in the microenvironment can promote the development and progression of a variety of cancers, and that nuclear deformation can increase genetic instability and the accumulation of DNA damage in some contexts. HCC is a unique cancer given that it occurs in an environment that is already very stiff and that liver cells in cirrhosis have highly deformed nuclei. This suggests that mechanical changes in the liver could be a source of genetic instability that leads to cancer development. Hepatocellular carcinoma (HCC) is the second most deadly primary cancer in the world and is thus a major global health challenge. HCC primarily develops in patients with an underlying chronic liver disease, the vast majority with advanced cirrhosis, characterized by increased matrix deposition and liver stiffness. Liver stiffness is highly associated with cancer development and poor patient outcome and is measured clinically to assess cancer risk; cirrhotic livers greatly exceed the threshold stiffness shown to alter hepatocyte cell behavior and to increase the malignancy of cancer cells. Recent studies have shown that cirrhotic liver cells have highly irregular nuclear morphologies and that nuclear deformation mediates mechanosensitive signaling. Separate research has shown that nuclear deformation can increase genetic instability and the accumulation of DNA damage in migrating cancer cells. We hypothesize that the mechanical changes associated with chronic liver disease are drivers of oncogenesis, activating mechanosensitive signaling pathways, increasing rates of DNA damage, and ultimately inducing malignant transformation.

A genetic screen designed to capture modulators of single-cell state variability identifies a new mode of altering resistance to BRAF inhibition in melanoma that pushes cells toward a more differentiated state. Modulating cellular plasticity may provide a new avenue to overcome drug resistance.Cellular plasticity describes the ability of cells to transition from one set of phenotypes to another. In melanoma, transient fluctuations in the molecular state of tumor cells mark the formation of rare cells primed to survive BRAF inhibition and reprogram into a stably drug-resistant fate. However, the biological processes governing cellular priming remain unknown. We used CRISPR-Cas9 genetic screens to identify genes that affect cell fate decisions by altering cellular plasticity. We found that many factors can independently affect cellular priming and fate decisions. We discovered a new plasticity-based mode of increasing resistance to BRAF inhibition that pushes cells towards a more differentiated state. Manipulating cellular plasticity through inhibition of DOT1L before the addition of the BRAF inhibitor resulted in more therapy resistance than concurrent administration. Our results indicate that modulating cellular plasticity can alter cell fate decisions and may prove useful for treating drug resistance in other cancers.

This Review explores the role of viscoelasticity of tissues and extracellular matrices in cell-matrix interactions and mechanotransduction and the potential utility of viscoelastic biomaterials in regenerative medicine. Substantial research over the past two decades has established that extracellular matrix (ECM) elasticity, or stiffness, affects fundamental cellular processes, including spreading, growth, proliferation, migration, differentiation and organoid formation. Linearly elastic polyacrylamide hydrogels and polydimethylsiloxane (PDMS) elastomers coated with ECM proteins are widely used to assess the role of stiffness, and results from such experiments are often assumed to reproduce the effect of the mechanical environment experienced by cells in vivo. However, tissues and ECMs are not linearly elastic materials-they exhibit far more complex mechanical behaviours, including viscoelasticity (a time-dependent response to loading or deformation), as well as mechanical plasticity and nonlinear elasticity. Here we review the complex mechanical behaviours of tissues and ECMs, discuss the effect of ECM viscoelasticity on cells, and describe the potential use of viscoelastic biomaterials in regenerative medicine. Recent work has revealed that matrix viscoelasticity regulates these same fundamental cell processes, and can promote behaviours that are not observed with elastic hydrogels in both two- and three-dimensional culture microenvironments. These findings have provided insights into cell-matrix interactions and how these interactions differentially modulate mechano-sensitive molecular pathways in cells. Moreover, these results suggest design guidelines for the next generation of biomaterials, with the goal of matching tissue and ECM mechanics for in vitro tissue models and applications in regenerative medicine.

Non-genetic factors can cause individual cells to fluctuate substantially in gene expression levels over time. It remains unclear whether these fluctuations can persist for much longer than the time of one cell division. Current methods for measuring gene expression in single cells mostly rely on single time point measurements, making the duration of gene expression fluctuations or cellular memory difficult to measure. Here, we combined Luria and Delbrack's fluctuation analysis with population-based RNA sequencing (MemorySeq) for identifying genes transcriptome-wide whose fluctuations persist for several divisions. MemorySeq revealed multiple gene modules that expressed together in rare cells within otherwise homogeneous clonal populations. These rare cell subpopulations were associated with biologically distinct behaviors like proliferation in the face of anti-cancer therapeutics. The identification of non-genetic, multigenerational fluctuations can reveal new forms of biological memory in single cells and suggests that non-genetic heritability of cellular state may be a quantitative property.