The distribution and density of ligands have a determinant role in cell adhesion on planar substrates. At the same time, planar surfaces are nonphysiological for most cells, and cell behavior on planar and topographical surfaces is significantly different, with fibrous structures being the most natural environment for cells. Despite phenomenological examinations, the role of adhesion ligand density in the fibrous scaffold for cell adhesion strength has so far not been assessed. Here, we established a method to measure the amount of cell ligands on biofunctionalized electrospun meshes and planar substrate coatings with the same chemical composition. With this as a basis for systematic comparison and pure polyester as benchmark substrates, we have cultured L929 mouse fibroblasts and measured the adhesion force to surfaces of different chemistry and topography. In every case, having fibrous structures have led to an increased adhesion force per area also at a lower ligand density, which remarks the importance of such structures in a natural extracellular environment. Conversely, cells migrate more on planar surfaces than on the tested fibrous substrates. We thus established a platform to study cell-matrix interactions on different surfaces in a precise and reproducible manner as a new tool to assess and quantify cell-matrix interactions toward 3D scaffolds

In recent decades, hybrid characterization systems have become pillars in the study of cellular biomechanics. Especially, Atomic Force Microscopy (AFM) is combined with a variety of optical microscopy techniques to discover new aspects of cell adhesion. AFM, however, is limited to the early-stage of cell adhesion, so that the forces of mature cell contacts cannot be addressed. Even though the invention of Fluidic Force Microscopy (FluidFM) overcomes these limitations by combining the precise force-control of AFM with microfluidics, the correlative investigation of detachment forces arising from spread mammalian cells has been barely achieved. Here, a novel multifunctional device integrating Fluorescence Microscopy (FL) into FluidFM technology (FL-FluidFM) is introduced, enabling real-time optical tracking of entire cell detachment processes in parallel to the undisturbed acquisition of force-distance curves. This setup, thus, allows for entailing two pieces of information at once. As proof-of-principle experiment, this method is applied to fluorescently labeled rat embryonic fibroblast (REF52) cells, demonstrating a precise matching between identified force-jumps and visualized cellular unbinding steps. This study, thus, presents a novel characterization tool for the correlated evaluation of mature cell adhesion, which has great relevance, for instance, in the development of biomaterials or the fight against diseases such as cancer.

The presence of FMS-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) is one of the most frequent mutations in acute myeloid leukemia (AML) and is associated with an unfavorable prognosis. FLT3 inhibitors, such as midostaurin, are used clinically but fail to entirely eradicate FLT3-ITD+AML. This study introduces a new perspective and highlights the impact of RAC1-dependent actin cytoskeleton remodeling on resistance to midostaurin in AML. RAC1 hyperactivation leads resistance via hyperphosphorylation of the positive regulator of actin polymerization N-WASP and antiapoptotic BCL-2. RAC1/N-WASP, through ARP2/3 complex activation, increases the number of actin filaments, cell stiffness and adhesion forces to mesenchymal stromal cells (MSCs) being identified as a biomarker of resistance. Midostaurin resistance can be overcome by a combination of midostaruin, the BCL-2 inhibitor venetoclax and the RAC1 inhibitor Eht1864 in midostaurin-resistant AML cell lines and primary samples, providing the first evidence of a potential new treatment approach to eradicate FLT3-ITD+AML. Garitano-Trojaola et al. used a combination of human acute myeloid leukemia (AML) cell lines and primary samples to show that RAC1-dependent actin cytoskeleton remodeling through BCL2 family plays a key role in resistance to the FLT3 inhibitor, Midostaurin in AML. They showed that by targeting RAC1 and BCL2, Midostaurin resistance was diminished, which potentially paves the way for an innovate treatme

The mechanisms underlying the cellular response to extracellular matrices (ECMs) that consist of multiple adhesive ligands are still poorly understood. Here, we address this topic by monitoring specific cellular responses to two different extracellular adhesion molecules - the main integrin ligand fibronectin and galectin-8, a lectin that binds beta-galactoside residues - as well as to mixtures of the two proteins. Compared with cell spreading on fibronectin, cell spreading on galectin-8-coated substrates resulted in increased projected cell area, morepronounced extension of filopodia and, yet, the inability to form focal adhesions and stress fibers. These differences can be partially reversed by experimental manipulations of small G-proteins of the Rho family and their downstream targets, such as formins, the Arp2/3 complex and Rho kinase. We also show that the physical adhesion of cells to galectin-8 was stronger than adhesion to fibronectin. Notably, galectin-8 and fibronectin differently regulate cell spreading and focal adhesion formation, yet act synergistically to upregulate the number and length of filopodia. The physiological significance of the coherent cellular response to a molecularly complex matrix is discussed.

Single cell force spectroscopy (SCFS) enables data on interaction forces to be acquired during the very early adhesion phase. However, SCFS detachment forces and energies have not been compared so far with the forces and energies after maturation of the cell-material contact on a single cell level and with comparable time resolution. We used FluidFM (R) to physically attach single cells to the cantilever by aspiration through a microfluidic channel, in order to achieve the higher forces required for detaching maturely adhering cells. Combining these two approaches allowed us to compare cell adhesion in the initial and maturation phases of adhesion for two exemplary cell-substrate combinations - L929 fibroblasts on fibronectin and MC3T3 osteoblasts on collagen type I. Uncoated glass substrates were used as a reference. For both cell lines, SCFS measurements after contact times of 5, 15 and 30 s revealed significantly higher maximum detachment forces (MDFs) and energies on glass compared to the protein-coated surfaces in the 0.5-4 nN (1-40 fJ) range. FluidFM (R) measurements after 1, 2 and 3 days of culture revealed a significant absolute increase in the MDFs and detachment energies for both cell lines on protein-coated substrates to values of about 600 nN and 10 pJ. On glass, the MDFs were similar for MC3T3 cells, while they were significantly lower for L929 cells. For both cell types, the differences in detachment energy were significant. These differences underline the importance of investigating early and mature adhesion states to obtain a holistic assessment of the cell-material interactions.

In this study, we designed a novel biomaterial ink platform based on hydrophilic poly(2-ethyl-2-oxazine) (PEtOzi) specifically for melt electrowriting (MEW). This material crosslinks spontaneously after processing via dynamic Diels-Alder click chemistry. These direct-written microperiodic structures rapidly swell in water to yield thermoreversible hydrogels. These hydrogels are robust enough for repeated aspiration and ejection through a cannula without structural damage, despite their high water content of 84%. Moreover, the scaffolds retain functional groups for modification using click chemistry and therefore can be readily functionalized as demonstrated using fluorophores and peptides to facilitate visualization and cell attachment. The PEtOzi hydrogel developed here is compatible with confocal imaging and staining protocols for cells. In summary, an advanced material platform based on PEtOzi is reported that is compatible with MEW and results in functionalizable chemically crosslinked microperiodic hydrogels.

Cell-sheet technology is a well-known method by which cells are grown on thermoswitchable substrates that become nonadhesive upon cooling, such that a complete layer of adherent cells, along with the produced extracellular matrix, detaches as a sheet. Polymers that exhibit a lower critical solution temperature (LCST) below physiological temperature in water, commonly poly(N-isopropylacrylamide) (PNIPAM), are covalently grafted or, for block copolymers, physisorbed onto substrates in a monomolecular thin film to achieve this. Consequently, such substrates, and the polymers required for film formation, can only be prepared in a chemical lab with profound macromolecular expertise. In this study, we present an easy and robust method to coat standard cell culture dishes with aqueous solutions of commercially available poly(-n-propy1-2-oxazoline) (PnPrOx), a polymer that exhibits LCST behavior. Different standard cell culture dishes were repeatedly coated with 0.1 wt % aqueous solutions of PnPrOx and dried in an oven to create a fully covered and thermoresponsive surface. Using this PnPrOx surface a variety of cell types including endothelial cells, mesenchymal stem cells, and fibroblasts, were seeded and cultured until confluency. By decreasing the temperature to 16 degrees C, viable cell sheets were detached within cell-type dependent time frames and could be harvested for biological analysis.

Intercellular adhesion plays a major role in tissue development and homeostasis. Yet, technologies to measure mature cell-cell contacts are not available. We introduce a methodology based on fluidic probe force microscopy to assess cell-cell adhesion forces after formation of mature intercellular contacts in cell monolayers. With this method we quantify that L929 fibroblasts exhibit negligible cell-cell adhesion in monolayers whereas human endothelial cells from the umbilical artery (HUAECs) exert strong intercellular adhesion forces per cell. We use a new in vitro model based on the overexpression of Muscle Segment Homeobox 1 (MSX1) to induce Endothelial-to-Mesenchymal Transition (EndMT), a process involved in cardiovascular development and disease. We reveal how intercellular adhesion forces in monolayer decrease significantly at an early stage of EndMT and we show that cells undergo stiffening and flattening at this stage. This new biomechanical insight complements and expands the established standard biomolecular analyses. Our study thus introduces a novel tool for the assessment of mature intercellular adhesion forces in a physiological setting that will be of relevance to biological processes in developmental biology, tissue regeneration and diseases like cancer and fibrosis.

Substrate stiffness, biochemical composition, and matrix topography deeply influence cell behavior, guiding motility, proliferation, and differentiation responses. The aim of this work was to determine the effect that the stiffness and protein composition of the underlying substrate has on the differentiation of induced pluripotent stem (iPS) cells and the potential synergy with specific soluble cues. With that purpose, murine iPS-derived embryoid bodies (iPS-EBs) were seeded on fibronectin- or collagen I-coated polyacrylamide (pAA) gels of tunable stiffness (0.6, 14, and 50 kPa) in the presence of basal medium; tissue culture polystyrene plates were employed as control. Specification of iPS cells toward the three germ layers was analyzed, detecting an increase of tissue-specific gene markers in the pAA matrices. Interestingly, soft matrix (0.6 kPa) coated with fibronectin favored differentiation toward cardiac and neural lineages and, in the case of neural differentiation, the effect was potentiated by the addition of specific soluble factors. The generation of mature astrocytes, neural cells, and cardiomyocytes was further proven by immunofluorescence and transmission electron microscopy. In summary, this work emphasizes the importance of using biomimetic matrices to accomplish a more specific and mature differentiation of stem cells for future therapeutic applications.

Collagen gels have been extensively used as three-dimensional (3D) cell culture systems. To enhance their mechanical properties, the manufacture of collagen-based gels with agarose has been proposed. However, little is known about the stability of these gels under cold storage conditions. The consequences of cold storage on biological tissues for clinical applications are known to be significant; yet, they have not been considered on hydrogels used for in vitro experiments. This work studies the effect of extended cold storage on the stability of collagen and collagen-agarose hydrogels using rheometry and scanning electron microscopy. In addition, cell-matrix interactions of adipose-derived stem cells (ADSC) have been studied using these gels. Results show that both the storage modulus (G') and loss modulus (G.) of pure collagen gels gradually decrease with extended cold storage along the 30 days of the study, while G' and G. increase in collagen-agarose gels under the same conditions. Moreover, significant changes in both moduli of collagen-agarose gels were only found after 30 days of cold storage, while in the case of collagen gels significant changes were already detected after 7 days. Finally, a reduction in the ability of ADSC to remodel the gel after prolonged cold storage was observed. To the best of our knowledge, this is the first work proving that cold storage of hydrogels prior to cell culture might have a significant impact on their mechanical properties and cell-matrix interactions.

Tissue Engineering is a promising emerging field that studies the intrinsic regenerative potential of the human body and uses it to restore functionality of damaged organs or tissues unable of self-healing due to illness or ageing. In order to achieve regeneration using Tissue Engineering strategies, it is first necessary to study the properties of the native tissue and determine the cause of tissue failure; second, to identify an optimum population of cells capable of restoring its functionality; and third, to design and manufacture a cellular microenvironment in which those specific cells are directed towards the desired cellular functions. The design of the artificial cellular niche has a tremendous importance, because cells will feel and respond to both its biochemical and biophysical properties very differently. In particular, the artificial niche will act as a physical scaffold for the cells, allowing their three-dimensional spatial organization; also, it will provide mechanical stability to the artificial construct; and finally, it will supply biochemical and mechanical cues to control cellular growth, migration, differentiation and synthesis of natural extracellular matrix. During the last decades, many scientists have made great contributions to the field of Tissue Engineering. Even though this research has frequently been accompanied by vast investments during extended periods of time, yet too often these efforts have not been enough to translate the advances into new clinical therapies. More and more scientists in this field are aware of the need of rational experimental designs before carrying out complex, expensive and time-consuming in vitro and in vivo trials. This review highlights the importance of computer modeling and novel biofabrication techniques as critical key players for a rational design of artificial cellular niches in Tissue Engineering.