Lors d’une blessure, les cellules migrent et prolifèrent collectivement pour rétablir la continuité de l’épithélium. En migrant, les cellules exercent des forces entre elles ainsi que sur le substrat et de nombreuses études suggèrent un couplage mécanique entre la migration et la prolifération. Récemment découvert, le cofacteur de transcription Yap (Yes-associated protein) est régulé par des signaux mécaniques. L’activation de YAP se traduit par sa rétention nucléaire et augmente la prolifération cellulaire. D’un point de vue mécanique, l’engagement des intégrines dans les adhésions focales, l’aire d’étalement des cellules et la contractilité de l’actomyosine activent YAP. Au contraire, l’engagement des cadhérines dans les jonctions intercellulaires inhibent Yap. A ce jour, les contributions respectives des contacts cellulaires et de l’actomyosine pour la régulation de Yap de de la prolifération restent inexplorées.Au cours de cette thèse, nous nous sommes intéressés au rôle des adhésions au substrat, des jonctions cellule-cellule, du cytosquelette d’actomyosine et de la tension mécanique inter- et intracellulaire sur l’activation de YAP et sur la prolifération cellulaire pendant la cicatrisation épithéliale.D’abord, nous avons étudié le rôle de l’étalement cellulaire et des forces transmises par les contacts cellule-substrat sur la régulation de la localisation de Yap. En confinement sur des motifs adhésifs micro-fabriqués, les kératinocytes humains (HaCaT) adoptent un mouvement collectif oscillatoire. En combinant la vidéomicroscopie, la microscopie à force de traction (TFM) et l’analyse quantitative d’images, nous avons d’abord montré que la migration des cellules est alternativement divergente et convergente ce qui régule l’étalement des cellules. Nous avons ensuite montré que l’étalement d’une cellule est corrélé aux forces de traction sur le substrat et à la relocalisation nucléaire de Yap. Bien qu’ils soient encore préliminaires, ces résultats suggèrent que Yap est régulé par les forces transmises au contacts cellule-substrat pendant la migration épithéliale.Ensuite, nous nous sommes intéressés à la régulation de Yap et de la prolifération cellulaire pendant la migration épitheliale en absence de contacts cellule-substrat. Pour cela, nous avons forcé la migration de monocouches de cellules HaCaT sur des bandes adhérentes séparées de bandes cytorépulsives. Lorsqu’elles migrent sur les bandes adhérentes, les cellules HaCaT étendent une couche de cellules suspendues au-dessus des bandes cytorépulsives. Les cellules suspendues sont cohésives entre elles mais n’interagissent pas avec le substrat. Dans le feuillet de cellules suspendues, les fibres de stress d’actine se réorganisent à l’échelle du tissu grâce au renforcement des contacts cellule-cellule et la contractilité est augmentée. Ce modèle est le premier qui permet de découpler la contractilité de l’actomyosine et les adhésions cellule-substrat pendant la migration épithéliale. Malgré l’augmentation des contraintes d’étirement, l’absence de contacts cellule-substrat empêche la localisation nucléaire de YAP et inhibe la prolifération des cellules suspendues. En conclusion, l’engagement de contacts cellule-substrat sont nécessaires à la localisation nucléaire de Yap et à l’augmentation de la prolifération pendant la cicatrisation épithéliale in vitro.Ces travaux démontrent que les forces de traction sur le substrat sont associées à la localisation nucléaire de Yap et à l’augmentation de la prolifération pendant la migration épithéliale in vitro / After a wound, cells both migrate and proliferate collectively to restore epithelial continuity and to heal the wound. While migrating, cells exert forces on the substrate and pull on each other. Several previous studies suggest a mechanical coupling between collective cell migration and proliferation. Recently discovered, the transcription co-factor Yap (Yes-associated protein) is regulated by mechanical signal. Yap activation induces its nuclear retention and cell cycle progression. Integrin engagement on cell-substrate contacts, cell spreading and actin contractility are related to Yap activation. In turn, cadherin engagement and forces in cell-cell contacts induces Yap nuclear exclusion and reduce cell proliferation. Integrins and cadherins anchor actomyosin cytoskeleton and to date, and the respective contributions of cell-substrate adhesions, cell-cell junctions and actin cytoskeleton on regulation Yap and cell proliferation remain unexplored.In this thesis, we interested in the role of substrate adhesions, cell-cell junctions, actomyosin cytoskeleton and cell mechanical loading on Yap activation and cell proliferation during epithelial wound healing.First, we aim to understand the role of cell spreading and mechanical loading of cell-substrate contacts on the regulation of Yap localisation. Confined on microfabricated adhesive patterns, human keratinocytes HaCaT adopt an oscillatory collective motion. Combining videomicroscopy, traction force microscopy (TFM) and quantitative image analysis, we show that collective cell movements are alternatively divergent and convergent which regulate local cell spreading. Then, we show that cell spreading correlate with traction forces on the substrate and nuclear localisation of Yap. While it remains preliminary, our data show that forces at cell-substrate contacts and cell spreading induce nuclear localisation of Yap during collective cell movements.In the second part of the thesis, we interested on Yap localisation and proliferation during epithelial migration in absence of cell-substrate contacts. To do so, we forced migration of monolayer of HaCaT keratinocytes on micropattern comprising alternatively adherent and cytorepulsive stripes. While migrating on adherent line, cells extend a multicellular layer over the non-adherent areas. Suspended cells are cohesive with each other but do not engage cell-substrate adhesion. In the suspended cell layer, actin stress fibres reorganise at the tissue level thanks to reinforcement of cell-cell contacts and contractility is increased. This model is the first one that allow to decouple actomyosin contractility and cell-substrate contact during epithelial migration. Despite increased stretching stress, absence of cell-substrate contacts induces Yap cytoplasmic localisation and inhibits cell proliferation. To conclude, cell-substrate contact engagement is necessary to induce Yap nuclear localisation and increase cell proliferation during epithelial wound healing in vitro.This work demonstrates that traction forces through cell-substrate contacts are associated to nuclear localisation of Yap and to increased cell proliferation during epithelial wound healing in vitro

Indiana University-Purdue University Indianapolis (IUPUI) / The role of mechanical forces in the fate and function of adherent cells has been revealed to be a pivotal factor in understanding cell biology. Cells require certain physical cues to be present in their microenvironment or the cell will begin apoptosis. Mechanical signals from the environment are interpreted at the cellular level and biochemical responses are made due to the information from outside the cell, this process is known as mechanotransduction. Misinterpretation of physical cues has been indicated in many disease states, including heart disease and asthma. When a cell is bound to the ECM, proteins such as integrins are engaged at static and stable adhesion sites. These tight and static anchoring points found at the ECM exist in stark contrast to the dynamic conditions seen at intercellular junctions. Intercellular junctions, such as gap and adherens junctions, are formed between cells to act as a mechanism to relay information and exchange material. Due to the important role intercellular junctions play in processes of wound healing, epithelial-mesenchymal transition and cancer metastasis developing more sophisticated levels of understanding of these mechanisms would provide valuable insight. Complex biological processes, including immune cell signaling and cellular ECM adhesions, have been effectively replicated in model systems. These model systems have included the use of solid supported lipid bilayers and polymeric hydrogels that display cell adhesion molecules. Studies of cellular mechanotransduction at ECM adhesion sites has also been completed with covalently functionalized polymeric substrates of adjustable elasticity. However, developing model systems that allow the accurate reproduction of properties seen at intercellular junctions, while also allowing the investigation of cellular mechanosensitivity has proven to be a difficult task. Previous work has shown that polymer-tethered lipid bilayers (PTLBs) are a viable material to allow the replication of the dynamics and adhesion seen at intercellular junctions. Although efforts have been made to produce PTLBs with different mechanical properties, there is currently not a material with sufficient tunable elastic properties for the study of cellular mechanotransduction. To establish a system that allows the study of stiffness effects across a biologically relevant range (~0.50 – 40 kPa) while maintaining the dynamic properties seen at cell-to-cell junctions, polymer gel-tethered bilayers (PGTBs) were developed. A fabrication strategy was established to allow the incorporation of a hydrogel support with easily tunable stiffness and a tethered lipid bilayer coating, which produced a powerful platform to study the effects of stiffness at intercellular junctions. Careful attention was given to maintain the beneficial properties of membrane diffusion, and it was shown that on different linking architectures lipid bilayers could be established and diffusion was preserved. Microscopy-based FCS and FRAP methodology were utilized to measure lipid diffusion in these systems, while confocal microscopy was used to analyze cell spreading and adhesion. Three distinct architectures to link the lipid membrane to the underlying polyacrylamide hydrogel were pursued in this work, a non-covalent biotin-streptavidin system, a covalently linked design with fibronectin, and a direct covalent linkage utilizing crosslinker chemistry. In this work, it was shown that cells were able to spread and adhere on these substrates, with cell adhesion zones visualized under plated cells that demonstrate the capability of the cell to rearrange the presented linkers, while maintaining a stable material. Also confirmed is the tunability of the polymer hydrogel across a wide range of stiffness, this was shown by quantitative changes in cell spreading area in response to polymer properties.

In this dissertation, I utilize digital holographic microscopy (DHM) to study the motility of biological cells. As an important feature of DHM, quantitative phase microscopy by digital holography (DH-QPM) is applied to study the cell-substrate interactions and migratory behavior of adhesive cells. The traction force exerted by biological cells is visualized as distortions in flexible substrata. Motile fibroblasts produce wrinkles when attached to a silicone rubber film. For the non-wrinkling elastic substrate polyacrylamide (PAA), surface deformation due to fibroblast adhesion and motility is visualized as tangential and vertical displacement. This surface deformation and the associated cellular traction forces are measured from phase profiles based on the degree of distortion. Intracellular fluctuations in amoeba cells are also analyzed statistically by DH-QPM. With the capacity of yielding quantitative measures directly, DH-QPM provides efficient and versatile means for quantitative analysis of cellular or intracellular motility. Three-dimensional profiling and tracking by DHM enable label-free and quantitative analysis of the characteristics and dynamic processes of objects, since DHM can record real-time data for micro-scale objects and produce a single hologram containing all the information about their three-dimensional structure. Here, I utilize DHM to visualize suspended microspheres and microfibers in three dimensions, and record the four-dimensional trajectories of free-swimming cells in the absence of mechanical focus adjustment. The displacement of microfibers due to interactions with cells in three spatial dimensions is measured as a function of time at sub-second and micrometer levels in a direct and straightforward manner. It has thus been shown that DHM is a highly efficient and versatile means for quantitative tracking and analysis of cell motility.

No description available.

Cell-substrate interactions influence various cellular processes such as morphology, motility, proliferation and differentiation. Actin dynamics within cells have been shown to be influenced by substrate stiffness, as NIH 3T3 fibroblasts grown on stiffer substrates tend to exhibit more prominent actin stress fiber formation. Circular dorsal ruffles (CDRs) are transient actin-rich ring-like structures within cells, induced by various growth factors, such as the platelet-derived growth factor (PDGF). CDRs grow and shrink in size after cells are stimulated with PDGF, eventually disappearing ten of minutes after stimulation. As substrate stiffness affect actin structures and cell motility, and CDRs are actin structures which have been previously linked to cell motility and macropinocytosis, the role of substrate stiffness on the properties of CDRs in NIH 3T3 fibroblasts and how they proceed to affect cell behavior is investigated. Cells were seeded on Poly-dimethylsiloxane (PDMS) substrates of various stiffnesses and stimulated with PDGF to induce CDR formation. It was found that an increase in substrate stiffness increases the lifetime of CDRs, but did not affect their size. A mathematical model of the signaling pathways involved in CDR formation is developed to provide insight into this lifetime and size dependence, and is linked to substrate stiffness via Rac-Rho antagonism. CDR formation did not affect the motility of cells seeded on 10 kPa stiff substrates, but is shown to increase localized lamellipodia formation in the cell via the diffusion of actin from the CDRs to the lamellipodia. To further probe the influence of cell-substrate interactions on cell behavior and actin dynamics, a two dimensional system which introduces a dynamically changing, reversible and localized substrate stiffness environment is constructed. Cells are seeded on top of thin PDMS nano-membranes, and are capable of feeling through the thin layer, experiencing the stiffness of the polyacrylamide substrates below the nano-membrane. The membranes are carefully re-transplanted on top of other polyacrylamide substrates with differing stiffnesses. This reversible dynamic stiffness system is a novel approach which would help in the investigation of the influence of reversible dynamic stiffness environments on cell morphology, motility, proliferation and differentiation in various cells types.

Engineering of biofunctional scaffolds to precisely regulate cell behavior and tissue growth is of significance in regenerative medicine. Protein-based biomaterials are attractive candidates for functionalization of scaffold surfaces since the ability to precisely control protein sequence and structure allows for fine-tuning of cell-substrate interactions that regulate cell behavior. In this thesis, a series of de novo proteins for bio-functionalization of interfaces was designed, synthesized, and studied. These proteins are based on a diblock motif consisting of a surface-active, amphiphilic block β-sheet domain linked to a disordered, water-soluble block with a terminal functional domain. Several types of functional domains were investigated, including sequences that act as ligands for cell surface receptors and sequences that act as templates for the growth of inorganic particles. Under moderate temperature and pH conditions, the amphiphilic β-sheet block was shown to have a strong affinity to a variety of scaffold materials and to form stable protein coatings on hydrophobic materials by self-assembly. Moreover, the surface adsorption of the proteins was shown to have minimal impact on the presentation of the functional end domains in the soluble block. For the case of diblocks with the RGDS integrin binding sequence, the capability for mediating cell attachment and spreading was demonstrated via control over ligand density on hydrophobic polymer surfaces. The case of diblock proteins with templating domains for inorganic materials was investigated for two systems. First, hydroxyapatite-binding domains were ligated to the end terminus of the water-soluble block to develop proteins for possible bone regeneration applications. It was demonstrated that the hydroxyapatite-binding domain had strong affinity to hydroxyapatite nanoparticles and was able to induce calcium phosphate mineralization on the surfaces coated with diblock proteins from dilute solutions with Ca2+.and PO43-. Next, a silver-binding domain was ligated to the end terminus to create a diblock protein for potential antimicrobial surface applications. The silver-binding domain was shown to accumulate and reduce silver ions, resulting in the formation of silver nanoparticles on the surfaces functionalized by the protein.

<p>The role of mechanical forces in the fate and function of adherent cells has been revealed to be a pivotal factor in understanding cell biology. Cells require certain physical cues to be present in their microenvironment or the cell will begin apoptosis. Mechanical signals from the environment are interpreted at the cellular level and biochemical responses are made due to the information from outside the cell, this process is known as mechanotransduction. Misinterpretation of physical cues has been indicated in many disease states, including heart disease and asthma. When a cell is bound to the ECM, proteins such as integrins are engaged at static and stable adhesion sites. These tight and static anchoring points found at the ECM exist in stark contrast to the dynamic conditions seen at intercellular junctions. Intercellular junctions, such as gap and adherens junctions, are formed between cells to act as a mechanism to relay information and exchange material. Due to the important role intercellular junctions play in processes of wound healing, epithelial-mesenchymal transition and cancer metastasis developing more sophisticated levels of understanding of these mechanisms would provide valuable insight.</p> <p>Complex biological processes, including immune cell signaling and cellular ECM adhesions, have been effectively replicated in model systems. These model systems have included the use of solid supported lipid bilayers and polymeric hydrogels that display cell adhesion molecules. Studies of cellular mechanotransduction at ECM adhesion sites has also been completed with covalently functionalized polymeric substrates of adjustable elasticity. However, developing model systems that allow the accurate reproduction of properties seen at intercellular junctions, while also allowing the investigation of cellular mechanosensitivity has proven to be a difficult task. Previous work has shown that polymer-tethered lipid bilayers (PTLBs) are a viable material to allow the replication of the dynamics and adhesion seen at intercellular junctions. Although efforts have been made to produce PTLBs with different mechanical properties, there is currently not a material with sufficient tunable elastic properties for the study of cellular mechanotransduction.</p> <p>To establish a system that allows the study of stiffness effects across a biologically relevant range (~0.50 – 40 kPa) while maintaining the dynamic properties seen at cell-to-cell junctions, polymer gel-tethered bilayers (PGTBs) were developed. A fabrication strategy was established to allow the incorporation of a hydrogel support with easily tunable stiffness and a tethered lipid bilayer coating, which produced a powerful platform to study the effects of stiffness at intercellular junctions. Careful attention was given to maintain the beneficial properties of membrane diffusion, and it was shown that on different linking architectures lipid bilayers could be established and diffusion was preserved. Microscopy-based FCS and FRAP methodology were utilized to measure lipid diffusion in these systems, while confocal microscopy was used to analyze cell spreading and adhesion. Three distinct architectures to link the lipid membrane to the underlying polyacrylamide hydrogel were pursued in this work, a non-covalent biotin-streptavidin system, a covalently linked design with fibronectin, and a direct covalent linkage utilizing crosslinker chemistry. In this work, it was shown that cells were able to spread and adhere on these substrates, with cell adhesion zones visualized under plated cells that demonstrate the capability of the cell to rearrange the presented linkers, while maintaining a stable material. Also confirmed is the tunability of the polymer hydrogel across a wide range of stiffness, this was shown by quantitative changes in cell spreading area in response to polymer properties.</p>

A fundamental understanding of bio-interfaces will facilitate improvement in the design and application of biomaterials that can beneficially interact with biological objects such as nucleic acids, molecules, bacteria, and mammalian cells. Currently, there exist analytical instruments to investigate material properties and report information on electrical, chemical, physical, and mechanical natures of biomaterials and biological samples. The overall goal of this research was to utilize advanced spectroscopy techniques coupled with data mining to elucidate specific characteristic properties for biological objects and how these properties imply interaction with environmental biomaterials. My studies of interfacial electron transfer (ET) of DNA-modified gold electrodes aided in understanding that DNA surface density is related to the step-wise order of which a self-assembled monolayer is created on a gold substrate. Further surface modification plays a role in surface conductivity, and I found that electro-oxidation of the DNA involved the oxidation of guanine and adenine nucleotides. Scanning tunneling microscopy (STM) was used to create topography and current images of the SAM surfaces. I also used Raman microspectroscopy (RM) to obtain spectra and spectral maps of DNA-modified gold surfaces. For studies of bacteria, atomic force microscopy (AFM) and scanning electron microscopy (SEM) images showed similar morphological features of Gram-positive and Gram-negative bacteria. Direct classical least squares (DCLS) analysis aided to distinguish co-cultured strains. Fourier transform infrared (FTIR) spectroscopy proved insightful for characteristic bands for Gram-positive bacteria and a combined AFM/RM image revealed a relationship between culture height/density and peak Raman intensity. In our mammalian cell studies we focused on human lung adenocarcinoma epithelial cells (A549), metastatic human breast carcinoma cells MDA-MB-435 (435), and non-metastatic MDA-MB-435/BRMS1 (435/BRMS1). RM revealed similarities between metastatic 435 and non-metastatic 435/BRMS1 cells compared to epithelial A549 cells. AFM showed increases in biomechanical properties for 435/BRMS1 in the areas of cell adhesion, cell spring constant, and Young’s modulus. Fluorescent staining illustrates F-actin rearrangement for 435 and 435/BRMS1. Electric cell-substrate impedance sensing (ECIS) revealed that 435 cells adhere tightly to substrata and migrate rapidly compared with 435/BRMS1. For ECIS, ≤10-fold diesel exhaust particles (DEP) concentration exposure caused clastogenic DNA degradation whereas ≥25-fold DEP exposure caused cytotoxic results. Resveratrol (RES) at 10 μM showed minimal to mild protection against DEP before and after exposure and aided in improving injury recovery.

No description available.

The objective of this research is to develop fundamental understanding of cell-substrate (CS) and cell-cell (CC) interactions in the culture space for time evolving cell cultures. Space resolved CC and CS interactions are important indicators of cell-density distribution, localized cellular behavior, and multiple cell-layers which are differentiators of normal and abnormal cell behavior. In this research, CS and CC interactions and the variations therein due to a) Cell growth, 2) cell-drug interaction, and 3) effect of Cytotoxin were studied using multielectrode, multi-frequency Electrochemical Impedance Spectroscopy (EIS). Contemporary impedance based methods sense either CC or CS interaction as a space averaged macroscopic quantity. A major contribution of this research is that, both CC and CS interactions are recorded and analyzed with spatio-temporal resolution. This research led to the development of an automated cell culture monitoring system, namely, CellMap. A planar eight electrode sensor was fabricated on a glass substrate and interfaced with a switching circuit. The switching circuit sequentially selects consecutive electrodes upon input of a 5V trigger pulse which is generated by the frequency response analyzer at the end of each frequency scan, thereby facilitating automated switching and recording of multielectrode dataset. Calibration standards and protocols were developed to null the channel parasitics of individual channels. A set of eight impedance measurements for eight electrodes constitutes a "frame". Frames are recorded at regular time intervals over the desired course of time. Impedance mapping of adhesion, spreading, motility and detachment of OvCa429 ovarian cancer cells was performed over a period of 70 hours. The cell-layer resistance, which indicates cell-cell contact, increased as a function of time until confluence, and decreased thereafter due to cell death and detachment. This was also confirmed by optical microscopy observations. Similarly, the cell layer Constant Phase Element (CPE) parameters, which were found to correlate well with cell density distribution, also increased as a function of time until confluence and decreased thereafter. Additionally, the cell-growth mapping revealed that the CellMap system is able to resolve non-uniform cell distributions in the culture space, which may be useful in differentiating between normal and pathological cells.