The Effect of Cell Contractility and Packing on Extracellular Matrix and Soft Tissue Rheology

In the past decades it has become clear that the mechanical properties of tissues are important for healthy functioning. The mechanical properties of tissues and their load-bearing components found in the extracellular matrix (ECM) have been tested mechanically to provide more insight. However, there is a discrepancy between tissue and ECM mechanics. In this thesis this discrepancy is investigated with a novel multiaxial rheology method, which addresses a physiologically relevant combination of shear and axial strains. Blood clots are used to study the effect of cell traction and cell packing on ECM mechanics. The results show that ECM networks compression soften and extension stiffen in a typical asymmetric manner. The apparent Young’s moduli and shear moduli are decoupled, and are strongly influenced by a modest degree of axial strain. Cell traction induced pre-stress does not change the direction of this response but makes it more symmetrical and increases shear moduli. Close red cell packing in blood clots reverses the behavior of the clots from compression softening to stiffening, and from extension and shear strain stiffening to softening, resembling soft tissues. The same effects can be mimicked by embedding chemically inert beads into a fibrin network at densities approaching the jamming threshold for granular and colloidal materials. The overall conclusion is that cell jamming is likely to be the determining factor of soft tissue mechanics. This has implications for the understanding of tissue mechanics in physiological and pathological situations as well as the modeling of tissues

Yeast Infections after Esophagectomy:A Retrospective Analysis

Esophageal malignancy is a disease with poor prognosis. Curative therapy incorporates surgery and is burdensome with high rates of infection morbidity and mortality. The role of yeast as causative organisms of post-esophagectomy infections is poorly defined. Consequently, the benefits of specific antifungal prophylactic therapy in improving patient outcome are unclear. Therefore, this study aimed at investigating the incidence of yeast infections at the University Medical Center Groningen among 565 post-esophagectomy patients between 1991 and 2017. The results show that 7.3% of the patients developed a yeast infection after esophageal resection with significantly increased incidence among patients suffering from diabetes mellitus. For patients with yeast infections, higher Acute Physiology and Chronic Health Evaluation (APACHE) II scores, more frequent intensive care unit readmissions, prolonged hospital stays and higher mortality rates were observed. One-year survival was significantly lower for patients with a yeast infection, as well as diabetes mellitus and yeast-positive pleural effusion. We conclude that the incidence of yeast infections following esophagectomy is considerable, and that patients with diabetes mellitus are at increased risk. Furthermore, yeast infections are associated with higher complication rates and mortality. These observations encourage further prospective investigations on the possible benefits of antifungal prophylactic therapy for esophagectomy patients

Loops versus lines and the compression stiffening of cells

Both animal and plant tissue exhibit a nonlinear rheological phenomenon known as compression stiffening, or an increase in moduli with increasing uniaxial compressive strain. Does such a phenomenon exist in single cells, which are the building blocks of tissues? One expects an individual cell to compression soften since the semiflexible biopolymer-based cytoskeletal network maintains the mechanical integrity of the cell and in vitro semiflexible biopolymer networks typically compression soften. To the contrary, we find that mouse embryonic fibroblasts (mEFs) compression stiffen under uniaxial compression via atomic force microscopy (AFM) studies. To understand this finding, we uncover several potential mechanisms for compression stiffening. First, we study a single semiflexible polymer loop modeling the actomyosin cortex enclosing a viscous medium modeled as an incompressible fluid. Second, we study a two-dimensional semiflexible polymer/fiber network interspersed with area-conserving loops, which are a proxy for vesicles and fluid-based organelles. Third, we study two-dimensional fiber networks with angular-constraining crosslinks, i.e. semiflexible loops on the mesh scale. In the latter two cases, the loops act as geometric constraints on the fiber network to help stiffen it via increased angular interactions. We find that the single semiflexible polymer loop model agrees well with our AFM experiments until approximately 35% compressive strain. We also find for the fiber network with area-conserving loops model that the stress-strain curves are sensitive to the packing fraction and size distribution of the area-conserving loops, thereby creating a mechanical fingerprint across different cell types. Finally, we make comparisons between this model and experiments on fibrin networks interlaced with beads as well as discuss the tissue-scale implications of cellular compression stiffening.Comment: 19 pages, 17 figure

Emergence of tissue-like mechanics from fibrous networks confined by close-packed cells

The viscoelasticity of the crosslinked semiflexible polymer networks—such as the internal cytoskeleton and the extracellular matrix—that provide shape and mechanical resistance against deformation is assumed to dominate tissue mechanics. However, the mechanical responses of soft tissues and semiflexible polymer gels differ in many respects. Tissues stiffen in compression but not in extension1,2,3,4,5, whereas semiflexible polymer networks soften in compression and stiffen in extension6,7. In shear deformation, semiflexible polymer gels stiffen with increasing strain, but tissues do not1,2,3,4,5,6,7,8. Here we use multiple experimental systems and a theoretical model to show that a combination of nonlinear polymer network elasticity and particle (cell) inclusions is essential to mimic tissue mechanics that cannot be reproduced by either biopolymer networks or colloidal particle systems alone. Tissue rheology emerges from an interplay between strain-stiffening polymer networks and volume-conserving cells within them. Polymer networks that soften in compression but stiffen in extension can be converted to materials that stiffen in compression but not in extension by including within the network either cells or inert particles to restrict the relaxation modes of the fibrous networks that surround them. Particle inclusions also suppress stiffening in shear deformation; when the particle volume fraction is low, they have little effect on the elasticity of the polymer networks. However, as the particles become more closely packed, the material switches from compression softening to compression stiffening. The emergence of an elastic response in these composite materials has implications for how tissue stiffness is altered in disease and can lead to cellular dysfunction9,10,11. Additionally, the findings could be used in the design of biomaterials with physiologically relevant mechanical properties

Unique Role of Vimentin Networks in Compression Stiffening of Cells and Protection of Nuclei from Compressive Stress

In this work, we investigate whether stiffening in compression is a feature of single cells and whether the intracellular polymer networks that comprise the cytoskeleton (all of which stiffen with increasing shear strain) stiffen or soften when subjected to compressive strains. We find that individual cells, such as fibroblasts, stiffen at physiologically relevant compressive strains, but genetic ablation of vimentin diminishes this effect. Further, we show that unlike networks of purified F-actin or microtubules, which soften in compression, vimentin intermediate filament networks stiffen in both compression and extension, and we present a theoretical model to explain this response based on the flexibility of vimentin filaments and their surface charge, which resists volume changes of the network under compression. These results provide a new framework by which to understand the mechanical responses of cells and point to a central role of intermediate filaments in response to compression

The Duality of Pancreatic Cancer: A local and systemic disease

Pancreatic ductal adenocarcinoma (PDAC) is a devastating disease with a dismal 5-year survival of 10%. Currently, the only chance at long-term survival is surgical resection combined with chemotherapy. Approximately 30 to 40% of patients presents with locally advanced pancreatic cancer (LAPC), whereby the tumor encases major peripancreatic vasculature. Due to the vascular involvement, LAPC is deemed unresectable. However, the treatment landscape has evolved since the introduction of more potent chemotherapy regimens. In this context, high-volume centers have increasingly performed complex resections of LAPC with good short- and long-term outcomes. Part I of this thesis elaborates on different strategies to expand resectability rates, by discussing different surgical techniques and the use of improved preoperative imaging. Moreover, part I highlights that, despite advancements in surgical care, surgery carries an inherent risk of complications. Besides discussing the local aspect of PDAC, this thesis also discusses the systemic nature of PDAC and the effect on survival and the challenges associated with this (part II). Finally, it will provide recommendations regarding the use of biomarkers in the postoperative follow-up of PDAC. Part III will delve into the clinical utility of known and novel biomarkers in detecting recurrence and guiding treatment decisions

On the origin of tissue mechanics: The effect of cell contractility and packing on extracellular matrix and soft tissue rheology

In the past decades it has become clear that the mechanical properties of tissues are important for healthy functioning. The mechanical properties of tissues and their load-bearing components found in the extracellular matrix (ECM) have been tested mechanically to provide more insight. However, there is a discrepancy between tissue and ECM mechanics. In this thesis this discrepancy is investigated with a novel multiaxial rheology method, which addresses a physiologically relevant combination of shear and axial strains. Blood clots are used to study the effect of cell traction and cell packing on ECM mechanics. The results show that ECM networks compression soften and extension stiffen in a typical asymmetric manner. The apparent Young’s moduli and shear moduli are decoupled, and are strongly influenced by a modest degree of axial strain. Cell traction induced pre-stress does not change the direction of this response but makes it more symmetrical and increases shear moduli. Close red cell packing in blood clots reverses the behavior of the clots from compression softening to stiffening, and from extension and shear strain stiffening to softening, resembling soft tissues. The same effects can be mimicked by embedding chemically inert beads into a fibrin network at densities approaching the jamming threshold for granular and colloidal materials. The overall conclusion is that cell jamming is likely to be the determining factor of soft tissue mechanics. This has implications for the understanding of tissue mechanics in physiological and pathological situations as well as the modeling of tissues

On the origin of tissue mechanics: The effect of cell contractility and packing on extracellular matrix and soft tissue rheology

In the past decades it has become clear that the mechanical properties of tissues are important for healthy functioning. The mechanical properties of tissues and their load-bearing components found in the extracellular matrix (ECM) have been tested mechanically to provide more insight. However, there is a discrepancy between tissue and ECM mechanics. In this thesis this discrepancy is investigated with a novel multiaxial rheology method, which addresses a physiologically relevant combination of shear and axial strains. Blood clots are used to study the effect of cell traction and cell packing on ECM mechanics. The results show that ECM networks compression soften and extension stiffen in a typical asymmetric manner. The apparent Young’s moduli and shear moduli are decoupled, and are strongly influenced by a modest degree of axial strain. Cell traction induced pre-stress does not change the direction of this response but makes it more symmetrical and increases shear moduli. Close red cell packing in blood clots reverses the behavior of the clots from compression softening to stiffening, and from extension and shear strain stiffening to softening, resembling soft tissues. The same effects can be mimicked by embedding chemically inert beads into a fibrin network at densities approaching the jamming threshold for granular and colloidal materials. The overall conclusion is that cell jamming is likely to be the determining factor of soft tissue mechanics. This has implications for the understanding of tissue mechanics in physiological and pathological situations as well as the modeling of tissues

Ultrafast laser ablation of trapped gold nanoparticles

We investigate the interaction of femtosecond (fs) laser pulses with single gold nanoparticles, trapped in a linear Paul trap. We study the scattering response of the particles as a function of the polarization angle of a cw laser at three different wavelengths. These measurements provide a value of the visibility that we compare with Mie theory calculations in order to obtain an estimate of the particle radius. We monitor the particle size during ultrafast laser ablation, obtaining an accurate figure for the mass loss as a function of the fs-laser dose. We discuss the particle mass loss induced by a single fs-laser shot and its relation with the number of absorbed photons