Matrix elasticity, cytoskeletal forces and physics of the nucleus: how deeply do cells 'feel' outside and in?

Synthetic hydrogels formed by thiol-ene crosslinking of vinyl sulfone-functional poly(methyl vinyl ether-alt-maleic acid) with α,ω-dithio-polyethyleneglycol

Polymer hydrogels formed by rapid thiol-ene coupling of macromolecular gel formers can offer access to versatile new matrices. This paper describes the efficient synthesis of cysteamine vinyl sulfone (CVS) trifluoroacetate, and its incorporation into poly(methyl vinyl ether-alt-maleic anhydride) (PMMAn) to form a series of CVS-functionalized poly(methyl vinyl ether-alt-maleic acid) polymers (PMM-CVS_x) containing 10 to 30 mol% pendant vinyl sulfone groups. Aqueous mixtures of these PMM-CVS and a dithiol crosslinker, α,ω-dithio-polyethyleneglycol (HS-PEG-SH, M_n = 1 kDa), gelled through crosslinking by Michael addition within seconds to minutes, depending on pH, degree of functionalization, and polymer loading. Gelation efficiency, Young's modulus, equilibrium swelling and hydrolytic stability are described, and step-wise hydrogel post-functionalization with a small molecule thiol, cysteamine, was demonstrated. Cytocompatibility of these crosslinked hydrogels towards entrapped 3T3 fibroblasts was confirmed using a live/dead fluorescence assay.

Self-assembled phosphate-polyamine networks as biocompatible supramolecular platforms to modulate cell adhesion

The modulation of cell adhesion via biologically inspired materials plays a key role in the development of realistic platforms to envisage not only mechanistic descriptions of many physiological and pathological processes but also new biointerfacial designs compatible with the requirements of biomedical devices. In this work, we show that the cell adhesion and proliferation of three different cell lines can be easily manipulated by using a novel biologically inspired supramolecular coating generated via dip coating of the working substrates in an aqueous solution of polyallylamine in the presence of phosphate anions-a simple one-step modification procedure. Our results reveal that selective cell adhesion can be controlled by varying the deposition time of the coating. Cell proliferation experiments showed a cell type-dependent quasi-exponential growth demonstrating the nontoxic properties of the supramolecular platform. After reaching a certain surface coverage, the supramolecular films based on phosphate-polyamine networks displayed antiadhesive activity towards cells, irrespective of the cell type. However and most interestingly, these antiadherent substrates developed strong adhesive properties after thermal annealing at 37 °C for 3 days. These results were interpreted based on the changes in the coating hydrophilicity, topography and stiffness, with the latter being assessed by atomic force microscopy imaging and indentation experiments. The reported approach is simple, robust and flexible, and would offer opportunities for the development of tunable, biocompatible interfacial architectures to control cell attachment for various biomedical applications.

Cell density overrides the effect of substrate stiffness on human mesenchymal stem cells' morphology and proliferation

The effect of substrate stiffness on the cellular morphology, proliferation, and differentiation of human mesenchymal stem cells (hMSCs) has been extensively researched and well established. However, the majority of these studies are done with a low seeding density where cell to cell interactions do not play a significant role. While these conditions permit an analysis of cell-substratum interactions at the single cell level, such a model system fails to capture a critical aspect of the cellular micro-environment in vivo, i.e. the cell-cell interaction via matrix deformation (i.e., strain). To address this question, we seeded hMSCs on soft poly-acrylamide (PAA) gels, at a seeding density that permits cells to be mechanically interacting via the underlying substrate. We found that as the intercellular distance decreases with the increasing seeding density, cellular sensitivity towards the substrate rigidity becomes significantly diminished. With the increasing seeding density, the cell spread area increased on a soft substrate (500 Pa) but reduced on an even slightly stiffer substrate (2 kPa) as well as on glass making them indistinguishable at a high seeding density. Not only in terms of cell spread area but also at a high seeding density, cells formed mature focal adhesions and prominent stress fibres on a soft substrate similar to that of the cells being cultured on a stiff substrate. The decreased intercellular distance also influenced the proliferation rate of the cells: higher seeding density on the soft substrate showed cell cycle progression similar to that of the cells on glass substrates. In summary, this paper demonstrates how the effect of substrate rigidity on the cell morphology and fate is a function of inter-cellular distance when seeded on a soft substrate. Our AFM data suggest that such changes happen due to local strain stiffening of the soft PAA gel, an effect that has been rarely reported in the literature so far.

Cancer invasion into musculature: Mechanics, molecules and implications

Tumor invasion along structural interphases of surrounding tumor-free tissue represents a key process during tumor progression. Much attention has been devoted to mechanisms of tumor cell migration within extracellular matrix (ECM)-rich connective tissue, however a comprehensive understanding of tumor invasion into tissue of higher structural complexity, such as muscle tissue, is lacking. Muscle invasion in cancer patients is often associated with destructive growth and worsened prognosis. Here, we review biochemical, geometrical and mechanical cues of smooth and skeletal muscle tissues and their relevance for guided invasion of cancer cells. As integrating concept, muscle-organizing ECM-rich surfaces of the epi-, peri- and endomysium provide cleft-like confined spaces along interfaces between dynamic muscle cells, which provide molecular and physical cues that guide migrating cancer cells, forming a possible contribution to cancer progression.

An optimized culture system for notochordal cell expansion with retention of phenotype

BACKGROUND

Notochordal (NC) cells display therapeutic potential in treating degeneration of the intervertebral disc. However, research on their phenotype and function is limited by low-cell yields and a lack of appropriate methodology for cell expansion. Utilizing porcine cells, this study aimed to develop an optimized culture system which allows expansion of NC cell populations with retention of phenotype.

METHODS

Post-natal porcine and foetal human nucleus pulposus tissue was compared histologically and expression of known NC cell marker genes by porcine NC cells was analyzed. Porcine NC cells were isolated from six-week post-natal discs and cultured in vitro under varied conditions: (1) DMEM vs αMEM; (2) laminin-521, fibronectin, gelatin and uncoated tissue culture-treated polystyrene (TCP); (3) 2% O2 vs normoxia; (4) αMEM (300 mOsm/L) vs αMEM (400 mOsm/L); (5) surface stiffness of 0.5 and 4 kPa and standard TCP. Adherence, proliferation, morphology and expression of NC cell markers were assessed over a 14-day culture period.

RESULTS

Native porcine nucleus pulposus tissue demonstrated similar morphology to human foetal tissue and porcine NC cells expressed known notochordal markers (CD24, KRT8, KRT18, KRT19, and T). Use of αMEM media and laminin-521-coated surfaces showed the greatest cell adherence, proliferation and retention of NC cell morphology and phenotype. Proliferation of NC cell populations was further enhanced in hypoxia (2%) and phenotypic retention was improved on 0.5 kPa culture surfaces.

DISCUSSION

Our model has demonstrated an optimized system in which NC cell populations may be expanded while retaining a notochordal phenotype. Application of this optimized culture system will enable NC cell expansion for detailed phenotypic and functional study, a major advantage over current culture methods described in the literature. Furthermore, the similarities identified between porcine and human NC cells suggest this system will be applicable in human NC cell culture for investigation of their therapeutic potential.

From individual to collective 3D cancer dissemination: roles of collagen concentration and TGF-β

Cancer cells have the ability to migrate from the primary (original) site to other places in the body. The extracellular matrix affects cancer cell migratory capacity and has been correlated with tissue-specific spreading patterns. However, how the matrix orchestrates these behaviors remains unclear. Here, we investigated how both higher collagen concentrations and TGF-β regulate the formation of H1299 cell (a non-small cell lung cancer cell line) spheroids within 3D collagen-based matrices and promote cancer cell invasive capacity. We show that at low collagen concentrations, tumor cells move individually and have moderate invasive capacity, whereas when the collagen concentration is increased, the formation of cell clusters is promoted. In addition, when the concentration of TGF-β in the microenvironment is lower, most of the clusters are aggregates of cancer cells with a spheroid-like morphology and poor migratory capacity. In contrast, higher concentrations of TGF-β induced the formation of clusters with a notably higher invasive capacity, resulting in clear strand-like collective cell migration. Our results show that the concentration of the extracellular matrix is a key regulator of the formation of tumor clusters that affects their development and growth. In addition, chemical factors create a microenvironment that promotes the transformation of idle tumor clusters into very active, invasive tumor structures. These results collectively demonstrate the relevant regulatory role of the mechano-chemical microenvironment in leading the preferential metastasis of tumor cells to specific tissues with high collagen concentrations and TFG-β activity.

Thiol-Ene Alginate Hydrogels as Versatile Bioinks for Bioprinting

Bioprinting is a powerful technique that allows precise and controlled 3D deposition of biomaterials in a predesigned, customizable, and reproducible manner. Cell-laden hydrogel ("bioink") bioprinting is especially advantageous for tissue engineering applications as multiple cells and biomaterial compositions can be selectively dispensed to create spatially well-defined architectures. Despite this promise, few hydrogel systems are easily available and suitable as bioinks, with even fewer systems allowing for molecular design of mechanical and biological properties. In this study, we report the development of a norbornene functionalized alginate system as a cell-laden bioink for extrusion-based bioprinting, with a rapid UV-induced thiol-ene cross-linking mechanism that avoids acrylate kinetic chain formation. The mechanical and swelling properties of the hydrogels are tunable by varying the concentration, length, and structure of dithiol PEG cross-linkers and can be further modified by postprinting secondary cross-linking with divalent ions such as calcium. The low concentrations of alginate needed (<2 wt %), coupled with their rapid in situ gelation, allow both the maintenance of high cell viability and the ability to fabricate large multilayer or multibioink constructs with identical bioprinting conditions. The modularity of this bioink platform design enables not only the rational design of materials properties but also the gel's biofunctionality (as shown via RGD attachment) for the expected tissue-engineering application. This modularity enables the creation of multizonal and multicellular constructs utilizing a chemically similar bioink platform. Such tailorable bioink platforms will enable increased complexity in 3D bioprinted constructs.

Molecular Therapies for Muscular Dystrophies

PURPOSE OF REVIEW

To construct a framework to understand the different molecular interventions for muscular dystrophy.

RECENT FINDINGS

The recent approval of antisense oligonucleotides treatment for Duchenne muscular dystrophy and spinal muscular atrophy and current clinical trials using recombinant adeno-associated virus for the treatment of those diseases suggests that we are at a tipping point where we are able to treat and potentially cure muscular dystrophies. Understanding the basic molecular pathogenesis of muscular dystrophies and the molecular biology of the treatment allows for critical evaluation of the proposed therapies.

Perspective: Biophysical regulation of cancerous and normal blood cell lineages in hematopoietic malignancies

It is increasingly appreciated that physical forces play important roles in cancer biology, in terms of progression, invasiveness, and drug resistance. Clinical progress in treating hematological malignancy and in developing cancer immunotherapy highlights the role of the hematopoietic system as a key model in devising new therapeutic strategies against cancer. Understanding mechanobiology of the hematopoietic system in the context of cancer will thus yield valuable fundamental insights that can information about novel cancer therapeutics. In this perspective, biophysical insights related to blood cancer are defined and detailed. The interactions with immune cells relevant to immunotherapy against cancer are considered and expounded, followed by speculation of potential regulatory roles of mesenchymal stromal cells (MSCs) in this complex network. Finally, a perspective is presented as to how insights from these complex interactions between matrices, blood cancer cells, immune cells, and MSCs can be leveraged to influence and engineer the treatment of blood cancers in the clinic.

Design and Applications of Cell-Selective Surfaces and Interfaces

Tissue regeneration involves versatile types of cells. The accumulation and disorganized behaviors of undesired cells impair the natural healing process, leading to uncontrolled immune response, restenosis, and/or fibrosis. Cell-selective surfaces and interfaces can have specific and positive effects on desired types of cells, allowing tissue regeneration with restored structures and functions. This review outlines the importance of surfaces and interfaces of biomaterials with cell-selective properties. The chemical and biological cues including peptides, antibodies, and other molecules, physical cues such as topography and elasticity, and physiological cues referring mainly to interactions between cells-cells and cell-chemokines or cytokines are effective modulators for achieving cell selectivity upon being applied into the design of biomaterials. Cell-selective biomaterials have also shown practical significance in tissue regeneration, in particular for endothelialization, nerve regeneration, capture of stem cells, and regeneration of tissues of multiple structures and functions.

Rheological characterization of dynamic remodeling of the pericellular region by human mesenchymal stem cell-secreted enzymes in well-defined synthetic hydrogel scaffolds

Human mesenchymal stem cells (hMSCs) dynamically remodel their microenvironment during basic processes, such as migration and differentiation. Migration requires extracellular matrix invasion, necessitating dynamic cell-material interactions. Understanding these interactions is critical to advancing materials designs that harness and manipulate these processes for applications including wound healing and tissue regeneration. In this work, we encapsulate hMSCs in a cell-degradable poly(ethylene glycol)-peptide hydrogel to determine how cell-secreted enzymes, specifically matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), create unique pericellular microenvironments. Using multiple particle tracking microrheology (MPT), we characterize spatio-temporal rheological properties in the pericellular region during cell-mediated remodeling. In MPT, the thermal motion of probes embedded in the network is measured. A newly designed sample chamber that limits probe drift during degradation and minimizes high value antibody volumes required for cell treatments enables MPT characterization. Previous MPT measurements around hMSCs show that directly around the cell the scaffold remains intact with the cross-link density decreasing as distance from the cell increases. This degradation profile suggests that hMSCs are simultaneously secreting TIMPs, which are inactivating MMPs through MMP-TIMP complexes. By neutralizing TIMPs using antibodies, we characterize the changes in matrix degradation. TIMP inhibited hMSCs create a reaction-diffusion type degradation profile where MMPs are actively degrading the matrix immediately after secretion. In this profile, the cross-link density increases with increasing distance from the cell. This change in material properties also increases the speed of migration. This simple treatment could increase delivery of hMSCs to injuries to aid wound healing and tissue regeneration.

Actin and myosin II modulate differentiation of pluripotent stem cells

Use of stem cell-based therapies in tissue engineering and regenerative medicine is hindered by efficient means of directed differentiation. For pluripotent stem cells, an initial critical differentiation event is specification to one of three germ lineages: endoderm, mesoderm, and ectoderm. Differentiation is known to be regulated by numerous extracellular and intracellular factors, but the role of the cytoskeleton during specification, or early differentiation, is still unknown. In these studies, we used agonists and antagonists to modulate actin polymerization and the actin-myosin molecular motor during spontaneous differentiation of embryonic stem cells in embryoid bodies. We found that inhibiting either actin polymerization or actin-myosin interactions led to a decrease in differentiation to the mesodermal lineage and an increase in differentiation to the endodermal lineage. Thus, targeting processes that regulate cytoskeletal tension may be effective in enhancing or inhibiting differentiation towards cells of the endodermal or mesodermal lineages, which include hepatocytes, islets, cardiomyocytes, endothelial cells, and osteocytes. Therefore, these fundamental findings demonstrate that modulation of the cytoskeleton may be useful in production for a range of cell-based therapies, including for liver, pancreatic, cardiac, vascular, and orthopedic applications.

Functional brain-specific microvessels from iPSC-derived human brain microvascular endothelial cells: the role of matrix composition on monolayer formation

BACKGROUND

Transwell-based models of the blood-brain barrier (BBB) incorporating monolayers of human brain microvascular endothelial cells (dhBMECs) derived from induced pluripotent stem cells show many of the key features of the BBB, including expression of transporters and efflux pumps, expression of tight junction proteins, and physiological values of transendothelial electrical resistance. The fabrication of 3D BBB models using dhBMECs has so far been unsuccessful due to the poor adhesion and survival of these cells on matrix materials commonly used in tissue engineering.

METHODS

To address this issue, we systematically screened a wide range of matrix materials (collagen I, hyaluronic acid, and fibrin), compositions (laminin/entactin), protein coatings (fibronectin, laminin, collagen IV, perlecan, and agrin), and soluble factors (ROCK inhibitor and cyclic adenosine monophosphate) in 2D culture to assess cell adhesion, spreading, and barrier function.

RESULTS

Cell coverage increased with stiffness of collagen I gels coated with collagen IV and fibronectin. On 7 mg mL^-1 collagen I gels coated with basement membrane proteins (fibronectin, collagen IV, and laminin), cell coverage was high but did not reliably reach confluence. The transendothelial electrical resistance (TEER) on collagen I gels coated with basement membrane proteins was lower than on coated transwell membranes. Agrin, a heparin sulfate proteoglycan found in basement membranes of the brain, promoted monolayer formation but resulted in a significant decrease in transendothelial electrical resistance (TEER). However, the addition of ROCK inhibitor, cAMP, or cross-linking the gels to increase stiffness, resulted in a significant improvement of TEER values and enabled the formation of confluent monolayers.

CONCLUSIONS

Having identified matrix compositions that promote monolayer formation and barrier function, we successfully fabricated dhBMEC microvessels in cross-linked collagen I gels coated with fibronectin and collagen IV, and treated with ROCK inhibitor and cAMP. We measured apparent permeability values for Lucifer yellow, comparable to values obtained in the transwell assay. During these experiments we observed no focal leaks, suggesting the formation of tight junctions that effectively block paracellular transport.

Unifying in vitro and in vivo IVT mRNA expression discrepancies in skeletal muscle via mechanotransduction

The translational efficiency of an in vitro transcribed (IVT) mRNA was measured upon delivery to primary skeletal muscle cells and to a mouse model system, towards the development of a predictive in vitro assay for the screening and validation of intramuscular mRNA-based vaccines. When IVT mRNA was delivered either naked or complexed with novel aminoglycoside-based delivery vehicles, significant differences in protein expression in vitro and in vivo were observed. We hypothesized that this previously anticipated discrepancy was due to differences in the mechanism of IVT mRNA endosomal entry and release following delivery. To address this, IVT mRNA was fluorescently labeled prior to delivery, to visualize its distribution. Colocalization with endosomal markers indicated that different entry pathways were utilized in vivo and in vitro, depending on the delivery vehicle, resulting in variations in protein expression levels. Since extracellular matrix stiffness (ECM) influences mRNA entry, trafficking and release, the effect of mechanotransduction on mRNA expression was investigated in vitro upon delivery of IVT mRNA alone, and complexed with delivery vehicles to skeletal muscle cells grown on ∼10 kPa hydrogels. This in vitro hydrogel model more accurately recapitulated the results obtained in vivo upon IM injection, indicating that this approach may assist in the characterization of mRNA based vaccines.

Cellular and Nuclear Forces: An Overview

Biological cells sample their surrounding microenvironments using nanoscale force sensors on the cell surfaces. These surface-based force and stress sensors generate physical and chemical responses inside the cell. The inherently well-connected cytoskeleton and its physical contacts with the force elements on the nuclear membrane lead these physicochemical responses to cascade all the way inside the cell nucleus, physically altering the nuclear state. These physical alterations of the cell nucleus, through yet-unknown complex steps elicit physical and functional response from the chromatin in the form of altered gene expression profiles. This mechanism of force/stress sensing by the cell and then its nuclear response has been shown to play a vital role in maintaining robust cellular homeostasis, controlling gene expression profiles during developmental phases as well as cell differentiation. Over the last few years, there has been appreciable progress toward identification of the molecular players responsible for force sensing. However, the actual sensing mechanism of cell surface bound force sensors and more importantly cascading of the signals, both physical (via cytosolic force sensing elements such as microtubule and actin framework) and chemical (cascade of biochemical signaling from cell surface to nuclear surface and further to the chromatin), inside the cell is poorly understood. In this chapter, we present a review of the currently known molecular players in cellular as well as nuclear force sensing repertoire and their possible mechanistic aspects. We also introduce various biophysical concepts that are used to describe the force/stress sensing and response of a cell. We hope this will help asking clearer questions and designing pointed experiments for better understanding of the force-dependent design principles of the cell surface, nuclear surface, and gene expression.

Advances in Protein-Based Materials: From Origin to Novel Biomaterials

Biomaterials play a very important role in biomedicine and tissue engineering where they directly affect the cellular activities and their microenvironment . Myriad of techniques have been employed to fabricate a vast number natural, artificial and recombinant polymer s in order to harness these biomaterials in tissue regene ration , drug delivery and various other applications. Despite of tremendous efforts made in this field during last few decades, advanced and new generation biomaterials are still lacking. Protein based biomaterials have emerged as an attractive alternatives due to their intrinsic properties like cell to cell interaction , structural support and cellular communications. Several protein based biomaterials like, collagen , keratin , elastin , silk protein and more recently recombinant protein s are being utilized in a number of biomedical and biotechnological processes. These protein-based biomaterials have enormous capabilities, which can completely revolutionize the biomaterial world. In this review, we address an up-to date review on the novel, protein-based biomaterials used for biomedical field including tissue engineering, medical science, regenerative medicine as well as drug delivery. Further, we have also emphasized the novel fabrication techniques associated with protein-based materials and implication of these biomaterials in the domain of biomedical engineering .

Cellular mechanosensing of the biophysical microenvironment: A review of mathematical models of biophysical regulation of cell responses

Cells in vivo reside within complex microenvironments composed of both biochemical and biophysical cues. The dynamic feedback between cells and their microenvironments hinges upon biophysical cues that regulate critical cellular behaviors. Understanding this regulation from sensing to reaction to feedback is therefore critical, and a large effort is afoot to identify and mathematically model the fundamental mechanobiological mechanisms underlying this regulation. This review provides a critical perspective on recent progress in mathematical models for the responses of cells to the biophysical cues in their microenvironments, including dynamic strain, osmotic shock, fluid shear stress, mechanical force, matrix rigidity, porosity, and matrix shape. The review highlights key successes and failings of existing models, and discusses future opportunities and challenges in the field.

Cell Mechanics of Craniosynostosis

Craniosynostosis is the premature fusion of the calvarial sutures that is associated with a number of physical and intellectual disabilities spanning from pediatric to adult years. Over the past two decades, techniques in molecular genetics and more recently, advances in high-throughput DNA sequencing have been used to examine the underlying pathogenesis of this disease. To date, mutations in 57 genes have been identified as causing craniosynostosis and the number of newly discovered genes is growing rapidly as a result of the advances in genomic technologies. While contributions from both genetic and environmental factors in this disease are increasingly apparent, there remains a gap in knowledge that bridges the clinical characteristics and genetic markers of craniosynostosis with their signaling pathways and mechanotransduction processes. By linking genotype to phenotype, outlining the role of cell mechanics may further uncover the specific mechanotransduction pathways underlying craniosynostosis. Here, we present a brief overview of the recent findings in craniofacial genetics and cell mechanics, discussing how this information together with animal models is advancing our understanding of craniofacial development.

Coordinated increase of nuclear tension and lamin-A with matrix stiffness outcompetes lamin-B receptor that favors soft tissue phenotypes

Matrix stiffness that is sensed by a cell or measured by a purely physical probe reflects the intrinsic elasticity of the matrix and also how thick or thin the matrix is. Here, mesenchymal stem cells (MSCs) and their nuclei spread in response to thickness-corrected matrix microelasticity, with increases in nuclear tension and nuclear stiffness resulting from increases in myosin-II and lamin-A,C. Linearity between the widely varying projected area of a cell and its nucleus across many matrices, timescales, and myosin-II activity levels indicates a constant ratio of nucleus-to-cell volume, despite MSCs' lineage plasticity. Nuclear envelope fluctuations are suppressed on the stiffest matrices, and fluctuation spectra reveal a high nuclear tension that matches trends from traction force microscopy and from increased lamin-A,C. Transcriptomes of many diverse tissues and MSCs further show that lamin-A,C's increase with tissue or matrix stiffness anti-correlates with lamin-B receptor (LBR), which contributes to lipid/sterol biosynthesis. Adipogenesis (a soft lineage) indeed increases LBR:lamin-A,C protein stoichiometry in MSCs versus osteogenesis (stiff). The two factors compete for lamin-B in response to matrix elasticity, knockdown, myosin-II inhibition, and even constricted migration that disrupts and segregates lamins in situ. Matrix stiffness-driven contractility thus tenses the nucleus to favor lamin-A,C accumulation and suppress soft tissue phenotypes.

Investigation of surface topography and stiffness on adhesion and neurites extension of PC12 cells on crosslinked silica aerogel substrates

Fundamental understanding and characterization of neural response to substrate topography is essential in the development of next generation biomaterials for nerve repair. Aerogels are a new class of materials with great potential as a biomaterial. In this work, we examine the extension of neurites by PC12 cells plated on matrigel-coated and collagen-coated mesoporous aerogel surfaces. We have successfully established the methodology for adhesion and growth of PC12 cells on polyurea crosslinked silica aerogels. Additionally, we have quantified neurite behaviors and compared their response on aerogel substrates with their behavior on tissue culture (TC) plastic, and polydimethylsiloxane (PDMS). We found that, on average, PC12 cells extend longer neurites on crosslinked silica aerogels than on tissue culture plastic, and, that the average number of neurites per cluster is lower on aerogels than on tissue culture plastic. Aerogels are an attractive candidate for future development of smart neural implants and the work presented here creates a platform for future work with this class of materials as a substrate for bioelectronic interfacing.

Supramolecular Peptide Nanofiber Morphology Affects Mechanotransduction of Stem Cells

Chirality and morphology are essential factors for protein function and interactions with other biomacromolecules. Extracellular matrix (ECM) proteins are also similar to other proteins in this sense; however, the complexity of the natural ECM makes it difficult to study these factors at the cellular level. The synthetic peptide nanomaterials harbor great promise in mimicking specific ECM molecules as model systems. In this work, we demonstrate that mechanosensory responses of stem cells are directly regulated by the chirality and morphology of ECM-mimetic peptide nanofibers with strictly controlled characteristics. Structural signals presented on l-amino acid containing cylindrical nanofibers (l-VV) favored the formation of integrin β1-based focal adhesion complexes, which increased the osteogenic potential of stem cells through the activation of nuclear YAP. On the other hand, twisted ribbon-like nanofibers (l-FF and d-FF) guided the cells into round shapes and decreased the formation of focal adhesion complexes, which resulted in the confinement of YAP proteins in the cytosol and a corresponding decrease in osteogenic potential. Interestingly, the d-form of twisted-ribbon like nanofibers (d-FF) increased the chondrogenic potential of stem cells more than their l-form (l-FF). Our results provide new insights into the importance and relevance of morphology and chirality of nanomaterials in their interactions with cells and reveal that precise control over the chemical and physical properties of nanostructures can affect stem cell fate even without the incorporation of specific epitopes.

The Role of Titanium Surface Nanostructuring on Preosteoblast Morphology, Adhesion, and Migration

Surface structuring of titanium-based implants is known to modulate the behavior of adherent cells, but the influence of different nanotopographies is poorly understood. The aim is to investigate preosteoblast proliferation, adhesion, morphology, and migration on surfaces with similar surface chemistry but distinct nanotopographical features. Sonochemical treatment and anodic oxidation are employed to fabricate disordered, mesoporous titania (TMS) and ordered titania nanotubular (TNT) topographies on titanium, respectively. Morphological evaluation reveals that cells are polygonal and well-spread on TMS, but display an elongated, fibroblast-like morphology on TNT surfaces, while they are much flatter on glass. Both nanostructured surfaces impair cell adhesion, but TMS is more favorable for cell growth due to its support of cell attachment and spreading in contrast to TNT. A quantitative wound healing assay in combination with live-cell imaging reveals that cell migration on TMS surfaces has a more collective character than on other surfaces, probably due to a closer proximity between neighboring migrating cells on TMS. The results indicate distinctly different cell adhesion and migration on ordered and disordered titania nanotopographies, providing important information that can be used in optimizing titanium-based scaffold design to foster bone tissue growth and repair while allowing for the encapsulation of drugs into porous titania layer.

Twisted nanoribbons from a RGD-bearing cholic acid derivative

In light of the biomedical interest for self-assembling amphiphiles bearing the tripeptide Arg-Gly-Gly (RGD), a cholic acid derivative was synthesized by introducing an aromatic moiety on the steroidal skeleton and the RGD sequence on the carboxylic function of its chain 17-24, thus forming a peptide amphiphile with the unconventional rigid amphiphilic structure of bile salts. In aqueous solution, the compound self-assembled into long twisted ribbons characterized by a very low degree of polydispersity in terms of width (≈25nm), thickness (≈4.5nm) and pitch (≈145nm). It was proposed that in the ribbon the molecules are arranged in a bilayer structure with the aromatic moieties in the interior, strongly involved in the intermolecular interaction, whereas the RGD residues are located at the bilayer-water interface. The nanostructure is significantly different from those generally provided by RGD-containing amphiphiles with the conventional peptide-tail structure, for which fibers with a circular cross-section were observed, and successfully tested as scaffolds for tissue regeneration. From previous work on the use of this kind of nanostructures, it is known that features like morphology, rigidity, epitope spacing and periodicity are important factors that dramatically affect cell adhesion and signaling. Within this context, the reported results demonstrate that bile salt-based peptide surfactants are promising building blocks in the preparation of non-trivial RGD-decorated nanoaggregates with well-defined morphologies and epitope distributions.

Vascular Mechanobiology: Towards Control of In Situ Regeneration

The paradigm of regenerative medicine has recently shifted from in vitro to in situ tissue engineering: implanting a cell-free, biodegradable, off-the-shelf available scaffold and inducing the development of functional tissue by utilizing the regenerative potential of the body itself. This approach offers a prospect of not only alleviating the clinical demand for autologous vessels but also circumventing the current challenges with synthetic grafts. In order to move towards a hypothesis-driven engineering approach, we review three crucial aspects that need to be taken into account when regenerating vessels: (1) the structure-function relation for attaining mechanical homeostasis of vascular tissues, (2) the environmental cues governing cell function, and (3) the available experimental platforms to test instructive scaffolds for in situ tissue engineering. The understanding of cellular responses to environmental cues leads to the development of computational models to predict tissue formation and maturation, which are validated using experimental platforms recapitulating the (patho)physiological micro-environment. With the current advances, a progressive shift is anticipated towards a rational and effective approach of building instructive scaffolds for in situ vascular tissue regeneration.

An Integrated Stochastic Model of Matrix-Stiffness-Dependent Filopodial Dynamics

The ways that living cells regulate their behavior in response to their local mechanical environment underlie growth, development, and healing and are important to critical pathologies such as metastasis and fibrosis. Although extensive experimental evidence supports the hypothesis that this regulation is governed by the dependence of filopodial dynamics upon extracellular matrix stiffness, the pathways for this dependence are unclear. We therefore developed a model to relate filopodial focal adhesion dynamics to integrin-mediated Rho signaling kinetics. Results showed that focal adhesion maturation, i.e., focal adhesion links reinforcement and integrin clustering, dominates over filopodial dynamics. Downregulated focal adhesion maturation leads to the biphasic relationship between extracellular matrix stiffness and retrograde flow that has been observed in embryonic chick forebrain neurons, whereas upregulated maturation leads to the monotonically decreasing relationship that has been observed in mouse embryonic fibroblasts. When integrin-mediated Rho activation and stress-dependent focal adhesion maturation are combined, the model shows how filopodial dynamics endows cells with exquisite mechanosensing. Taken together, the results support the hypothesis that mechanical and structural factors combine with signaling kinetics to enable cells to probe their environments via filopodial dynamics.

Nuclear mechanotransduction: sensing the force from within

The cell nucleus is a hallmark of eukaryotic evolution, where gene expression is regulated and the genome is replicated and repaired. Yet, in addition to complex molecular processes, the nucleus has also evolved to serve physical tasks that utilize its optical and mechanical properties. Nuclear mechanotransduction of externally applied forces and extracellular stiffness is facilitated by the physical connectivity of the extracellular environment, the cytoskeleton and the nucleoskeletal matrix of lamins and chromatin. Nuclear mechanosensor elements convert applied tension into biochemical cues that activate downstream signal transduction pathways. Mechanoregulatory networks stabilize a contractile cell state with feedback to matrix, cell adhesions and cytoskeletal elements. Recent advances have thus provided mechanistic insights into how forces are sensed from within, that is, in the nucleus where cell-fate decision-making is performed.

Stretchable impedance sensor for mammalian cell proliferation measurements

This paper presents the fabrication and testing of a novel stretchable electric cell-substrate impedance sensing (ECIS) lab on a chip device. This is the first time that ECIS electrodes were fabricated on a stretchable polydimethylsiloxane (PDMS) substrate and ECIS measurements were performed on mammalian cells exposed to cyclic strain. The stretchable ECIS biosensors simulate in vitro the dynamic environment of organisms, such as pulsation, bending and stretching, which enables investigations on cell behavior that undergoes mechanical stimuli in biological tissue. Usually cell-based assays used in cell mechanobiology rely on endpoint cell tests, which provide a limited view on dynamic cellular mechanisms. The ECIS technique is a label-free, real-time and noninvasive method to monitor the cellular response to mechanical stimuli. Bovine aortic endothelial cells (BAECs) have been used in this research because the BAECs are exposed in vivo to cyclic physiologic elongation produced by blood circulation in the arteries. These innovative stretchable ECIS biosensors were used to analyze the proliferation of BAECs under different cyclic mechanical stimulations. The results of fluorescence based cell proliferation assays confirmed that the stretchable ECIS sensors were able to analyze in real-time the BAEC proliferation. The novel stretchable ECIS sensor has the ability to analyse cell proliferation, determine the cell number and density, and apply mechanical stimulation at the same time.

Improvement of paracellular transport in the Caco-2 drug screening model using protein-engineered substrates

The Caco-2 assay has achieved wide popularity among pharmaceutical companies in the past two decades as an in vitro method for estimation of in vivo oral bioavailability of pharmaceutical compounds during preclinical characterization. Despite its popularity, this assay suffers from a severe underprediction of the transport of drugs which are absorbed paracellularly, that is, which pass through the cell-cell tight junctions of the absorptive cells of the small intestine. Here, we propose that simply replacing the collagen I matrix employed in the standard Caco-2 assay with an engineered matrix, we can control cell morphology and hence regulate the cell-cell junctions that dictate paracellular transport. Specifically, we use a biomimetic engineered extracellular matrix (eECM) that contains modular protein domains derived from two ECM proteins found in the small intestine, fibronectin and elastin. This eECM allows us to independently tune the density of cell-adhesive RGD ligands presented to Caco-2 cells as well as the mechanical stiffness of the eECM. We observe that lower amounts of RGD ligand presentation as well as decreased matrix stiffness results in Caco-2 morphologies that more closely resemble primary small intestinal epithelial cells than Caco-2 cells cultured on collagen. Additionally, these matrices result in Caco-2 monolayers with decreased recruitment of actin to the apical junctional complex and increased expression of claudin-2, a tight junction protein associated with higher paracellular permeability that is highly expressed throughout the small intestine. Consistent with these morphological differences, drugs known to be paracellularly transported in vivo exhibited significantly improved transport rates in this modified Caco-2 model. As expected, permeability of transcellularly transported drugs remained unaffected. Thus, we have demonstrated a method of improving the physiological accuracy of the Caco-2 assay that could be readily adopted by pharmaceutical companies without major changes to their current testing protocols.

T-plastin is essential for basement membrane assembly and epidermal morphogenesis

The establishment of epithelial architecture is a complex process involving cross-talk between cells and the basement membrane. Basement membrane assembly requires integrin activity but the role of the associated actomyosin cytoskeleton is poorly understood. Here, we identify the actin-bundling protein T-plastin (Pls3) as a regulator of basement membrane assembly and epidermal morphogenesis. In utero depletion of Pls3 transcripts in mouse embryos caused basement membrane and polarity defects in the epidermis but had little effect on cell adhesion and differentiation. Loss-of-function experiments demonstrated that the apicobasal polarity defects were secondary to the disruption of the basement membrane. However, the basement membrane itself was profoundly sensitive to subtle perturbations in the actin cytoskeleton. We further show that Pls3 localized to the cell cortex, where it was essential for the localization and activation of myosin II. Inhibition of myosin II motor activity disrupted basement membrane organization. Our results provide insights into the regulation of cortical actomyosin and its importance for basement membrane assembly and skin morphogenesis.

Poly (glycerol sebacate) elastomer supports bone regeneration by its mechanical properties being closer to osteoid tissue rather than to mature bone

Mechanical load influences bone structure and mass. Arguing the importance of load-transduction, we investigated the mechanisms inducing bone formation using an elastomeric substrate. We characterized Poly (glycerol sebacate) (PGS) in vitro for its mechanical properties, compatibility with osteoprogenitor cells regarding adhesion, proliferation, differentiation under compression versus static cultures and in vivo for the regeneration of a rabbit ulna critical size defect. The load-transducing properties of PGS were compared in vitro to a stiffer poly lactic-co-glycolic-acid (PLA/PGA) scaffold of similar porosity and interconnectivity. Under cyclic compression for 7days, we report focal adhesion kinase overexpression on the less stiff PGS and upregulation of the transcription factor Runx2 and late osteogenic markers osteocalcin and bone sialoprotein (1.7, 4.0 and 10.0 folds increase respectively). Upon implanting PGS in the rabbit ulna defect, histology and micro-computed tomography analysis showed complete gap bridging with new bone by the PGS elastomer by 8weeks while minimal bone formation was seen in empty controls. Immunohistochemical analysis demonstrated the new bone to be primarily regenerated by recruited osteoprogenitors cells expressing periostin protein during early phase of maturation similar to physiological endochondral bone development. This study confirms PGS to be osteoconductive contributing to bone regeneration by recruiting host progenitor/stem cell populations and as a load-transducing substrate, transmits mechanical signals to the populated cells promoting differentiation and matrix maturation toward proper bone remodeling. We hence conclude that the material properties of PGS being closer to osteoid tissue rather than to mineralized bone, allows bone maturation on a substrate mechanically closer to where osteoprogenitor/stem cells differentiate to develop mature load-bearing bone.

SIGNIFICANCE OF SIGNIFICANCE

The development of effective therapies for bone and craniofacial regeneration is a foremost clinical priority in the mineralized tissue engineering field. Currently at risk are patients seeking treatment for craniofacial diseases, traumas and disorders including birth defects such as cleft lip and palate, (1 in 525 to 714 live births), craniosynostosis (300-500 per 1,000,000 live births), injuries to the head and face (20 million ER visits per year), and devastating head and neck cancers (8000 deaths and over 30,000 new cases per year). In addition, approximately 6.2 million fractures occur annually in the United States, of which 5-10% fail to heal properly, due to delayed or non-union [1], and nearly half of adults aged 45-65 have moderate to advanced periodontitis with associated alveolar bone loss, which, if not reversed, will lead to the loss of approximately 6.5 teeth/individual [2]. The strategies currently available for bone loss treatment largely suffer from limitations in efficacy or feasibility, necessitating further development and material innovation. Contemporary materials systems themselves are indeed limited in their ability to facilitate mechanical stimuli and provide an appropriate microenvironment for the cells they are designed to support. We propose a strategy which aims to leverage biocompatibility, biodegradability and material elasticity in the creation of a cellular niche. Within this niche, cells are mechanically stimulated to produce their own extracellular matrix. The hypothesis that mechanical stimuli will enhance bone regeneration is supported by a wealth of literature showing the effect of mechanical stimuli on bone cell differentiation and matrix formation. Using mechanical stimuli, to our knowledge, has not been explored in vivo in bone tissue engineering applications. We thus propose to use an elastomeric platform, based on poly(glycerol sebacate (PGS), to mimic the natural biochemical environment of bone while enabling the transmission of mechanical forces. In this study we report the material's load-transducing ability as well as falling mechanically closer to bone marrow and osteoid tissue rather than to mature bone, allowed osteogenesis and bone maturation. Defying the notion of selecting bone regeneration scaffolds based on their relative mechanical comparability to mature bone, we consider our results in part novel for the new application of this elastomer and in another fostering for reassessment of the current selection criteria for bone scaffolds.

Nucleosome-nucleosome interactions via histone tails and linker DNA regulate nuclear rigidity

A force-calibrated microneedle setup and controlled biochemical perturbation reveal that chromatin acts as a spring-like mechanical module that controls the rigidity of cell nuclei. The underlying molecular mechanism involves linker DNA and internucleosomal interaction via histone tails.

Cells, as well as the nuclei inside them, experience significant mechanical stress in diverse biological processes, including contraction, migration, and adhesion. The structural stability of nuclei must therefore be maintained in order to protect genome integrity. Despite extensive knowledge on nuclear architecture and components, however, the underlying physical and molecular mechanisms remain largely unknown. We address this by subjecting isolated human cell nuclei to microneedle-based quantitative micromanipulation with a series of biochemical perturbations of the chromatin. We find that the mechanical rigidity of nuclei depends on the continuity of the nucleosomal fiber and interactions between nucleosomes. Disrupting these chromatin features by varying cation concentration, acetylating histone tails, or digesting linker DNA results in loss of nuclear rigidity. In contrast, the levels of key chromatin assembly factors, including cohesin, condensin II, and CTCF, and a major nuclear envelope protein, lamin, are unaffected. Together with in situ evidence using living cells and a simple mechanical model, our findings reveal a chromatin-based regulation of the nuclear mechanical response and provide insight into the significance of local and global chromatin structures, such as those associated with interdigitated or melted nucleosomal fibers.

The rotation of mouse myoblast nuclei is dependent on substrate elasticity

The complex interplay of biochemical signaling and mechanical traction forces regulate the position of cellular nuclei. Although the phenomenon of nuclear rotation has been observed for many years, the influence of substrate elasticity was unknown. We discovered another layer of complexity to this phenomenon: nuclear rotation is dependent on substrate elasticity. Nuclear rotation is drastically reduced on physiologically relevant stiffnesses. Here, we studied nuclear rotation in mouse C2C12 myoblasts cultured on soft substrates designed to mimic resting tissue (∼26 kPa) and on hard glass substrates. We examined the roles of the actin and microtubule cytoskeleton on the presence and dynamics of nuclear rotation in these two different microenvironments. We demonstrated the clear dependence of nuclear rotation dynamics on matrix stiffness. These results will have important implications for the design of future studies of nuclear rotation and our understanding of the phenomenon as a whole. Unnaturally, hard substrates do not only fail to mimic the in vivo microenvironment, but can also induce cellular processes that would not normally occur in the natural cellular environment.

Influence of Additive Manufactured Scaffold Architecture on the Distribution of Surface Strains and Fluid Flow Shear Stresses and Expected Osteochondral Cell Differentiation

Scaffolds for regenerative medicine applications should instruct cells with the appropriate signals, including biophysical stimuli such as stress and strain, to form the desired tissue. Apart from that, scaffolds, especially for load-bearing applications, should be capable of providing mechanical stability. Since both scaffold strength and stress–strain distributions throughout the scaffold depend on the scaffold’s internal architecture, it is important to understand how changes in architecture influence these parameters. In this study, four scaffold designs with different architectures were produced using additive manufacturing. The designs varied in fiber orientation, while fiber diameter, spacing, and layer height remained constant. Based on micro-CT (μCT) scans, finite element models (FEMs) were derived for finite element analysis (FEA) and computational fluid dynamics (CFD). FEA of scaffold compression was validated using μCT scan data of compressed scaffolds. Results of the FEA and CFD showed a significant impact of scaffold architecture on fluid shear stress and mechanical strain distribution. The average fluid shear stress ranged from 3.6 mPa for a 0/90 architecture to 6.8 mPa for a 0/90 offset architecture, and the surface shear strain from 0.0096 for a 0/90 offset architecture to 0.0214 for a 0/90 architecture. This subsequently resulted in variations of the predicted cell differentiation stimulus values on the scaffold surface. Fluid shear stress was mainly influenced by pore shape and size, while mechanical strain distribution depended mainly on the presence or absence of supportive columns in the scaffold architecture. Together, these results corroborate that scaffold architecture can be exploited to design scaffolds with regions that guide specific tissue development under compression and perfusion. In conjunction with optimization of stimulation regimes during bioreactor cultures, scaffold architecture optimization can be used to improve scaffold design for tissue engineering purposes.

Cell infiltrative hydrogel fibrous scaffolds for accelerated wound healing

Development of natural protein-based fibrous scaffolds with tunable physical properties and biocompatibility is highly desirable to construct three-dimensional (3D), fully cellularized scaffolds for wound healing. Herein, we demonstrated a simple and effective technique to construct electrospun 3D fibrous scaffolds for accelerated wound healing using a photocrosslinkable hydrogel based on gelatin methacryloyl (GelMA). We found that the physical properties of the photocrosslinkable hydrogel including water retention, stiffness, strength, elasticity and degradation can be tailored by changing the light exposure time. We further observed that the optimized hydrogel fibrous scaffolds which were soft and elastic could support cell adhesion, proliferation and migration into the whole scaffolds, facilitating regeneration and formation of cutaneous tissues within two weeks. Such tunable characteristics of the fibrous GelMA scaffolds distinguished them from other reported substrates developed for reconstruction of wound defects including glutaraldehyde-crosslinked gelatin or poly (lactic-co-glycolic acid) (PLGA), whose physical and chemical properties were difficult to modify to allow cell infiltration into the 3D scaffolds for tissue regeneration. We anticipate that the ability to become fully cellularized will make the engineered GelMA fibrous scaffolds suitable for widespread applications as skin substitutes or wound dressings.

STATEMENT OF SIGNIFICANCE

In present study, we generate three-dimensional photocrosslinkable gelatin (GelMA)-based fibrous scaffolds with tunable physical and biological properties by using a combined photocrosslinking/electrospinning approach. The developed GelMA fibrous scaffolds can not only support cell viability and cell adhesion, but also facilitate cell migration and proliferation, accelerating regeneration and formation of cutaneous tissues. In addition, the physical properties of the engineered fibrous GelMA hydrogel including water retention capability, mechanical properties and biodegradability can be tuned to accommodate different patients' needs, making it a promising candidate for skin tissue engineering.

Mineralization potential of cellulose-nanofibrils reinforced gelatine scaffolds for promoted calcium deposition by mesenchymal stem cells

Cellulose-nanofibrils (CNFs) enriched gelatine (GEL) scaffolds were fabricated in-situ by the combined freeze-thawing process and carbodiimide crosslinking chemistry. The original- and variously surface anionised CNFs (carboxylated/CNF-COOH/, and phosphonated with 3-AminoPropylphosphoric Acid/CNF-COOH-ApA/) were used in order to tune the scaffolds' biomimetic structure towards a more intensive mineralization process. The pore size reduction (from 208±35μm to 91±35μm) after 50% v/v of CNFs addition to GEL was identified, while separated pore-walls' alignment vs. shorter, dense and elongated pores are observed when using 80% v/v of original-CNFs vs. anionised-CNFs, all of them possessed osteoid-like compressive strength (0.025-0.40MPa) and elasticity (0.04-0.15MPa). While randomly distributed Ca²⁺-deficient hydroxyapatite/HAp/(Ca/P≈1.4) aggregates were identified in the case of original-CNF prevalent scaffolds after four weeks of incubation in SBF, the more uniform and intensified deposition with HAp-like (Ca/P≈1.69) structures were established using CNF-COOH-Apa. The growth of Mesenchymal Stem Cells (MSCs) was observed on all CNF-containing scaffolds, resulting in more extensive Ca²⁺ deposition compared to the positive control or pure GEL scaffold. Among them, the scaffold prepared with the 50% v/v CNF-COOH-ApA showed significantly increased mineralization kinetic as well as the capacity for bone-like patterning in bone tissue regeneration.

Biofluids, cell mechanics and epigenetics: Flow-induced epigenetic mechanisms of endothelial gene expression

Epigenetics is the regulation of gene expression (transcription) in response to changes in the cell environment through genomic modifications that largely involve the non-coding fraction of the human genome and that cannot be attributed to modification of the primary DNA sequence. Epigenetics is dominant in establishing cell fate and positioning during programmed embryonic development. However the same pathways are used by mature postnatal and adult mammalian cells during normal physiology and are implicated in disease mechanisms. Recent research demonstrates that blood flow and pressure are cell environments that can influence transcription via epigenetic pathways. The principal epigenetic pathways are chemical modification of cytosine residues of DNA (DNA methylation) and of the amino tails of histone proteins associated with DNA in nucleosomes. They also encompass the post-transcriptional degradation of mRNA transcripts by non-coding RNAs (ncRNA). In vascular endothelium, epigenetic pathways respond to temporal and spatial variations of flow and pressure, particularly hemodynamic disturbed blood flow, with important consequences for gene expression. The biofluid environment is linked by mechanotransduction and solute transport to cardiovascular cell phenotypes via signaling pathways and epigenetic regulation for which there is an adequate interdisciplinary infrastructure with robust tools and methods available. Epigenetic mechanisms may be less familiar than acute genomic signaling to Investigators at the interface of biofluids, biomechanics and cardiovascular biology. Here we introduce a biofluids / cellular biomechanics readership to the principal epigenetic pathways and provide a contextual overview of endothelial epigenetic plasticity in the regulation of flow-responsive transcription.

Medical applications of the intrinsic mechanical properties of single cells

The mechanical properties of single cells have been recently identified as the basis of an emerging approach in medical applications because they are closely related to the biological processes of cells and, ultimately, human health conditions. In this article, we provide a brief review of the intrinsic mechanical properties of single cells related to cancer and aging. The mechanical properties can be used as biomarkers for early cancer diagnosis because cancer cells have a lower Young's modulus, indicating higher elasticity or softness than their counterpart normal cells. The metastatic potential of cancer cells is inversely correlated with their elastic properties. Aging induces stiffness through an increased amount of cytoskeletal fiber. Changes in the mechanical properties also show potential for drug screening. Although there are several challenges to be met before clinical applications can be made, such mechanical properties of single cells may provide new approaches to human diseases.

MEKK1-dependent phosphorylation of calponin-3 tunes cell contractility

MEKK1 (also known as MAP3K1), which plays a major role in MAPK signaling, has been implicated in mechanical processes in cells, such as migration. Here, we identify the actin-binding protein calponin-3 as a new MEKK1 substrate in the signaling that regulates actomyosin-based cellular contractility. MEKK1 colocalizes with calponin-3 at the actin cytoskeleton and phosphorylates it, leading to an increase in the cell-generated traction stress. MEKK1-mediated calponin-3 phosphorylation is attenuated by the inhibition of myosin II activity, the disruption of actin cytoskeletal integrity and adhesion to soft extracellular substrates, whereas it is enhanced upon cell stretching. Our results reveal the importance of the MEKK1-calponin-3 signaling pathway to cell contractility.

Gelatin-Hyaluronic Acid Hydrogels with Tuned Stiffness to Counterbalance Cellular Forces and Promote Cell Differentiation

Cells interact mechanically with their environment, exerting mechanical forces that probe the extracellular matrix (ECM). The mechanical properties of the ECM determine cell behavior and control cell differentiation both in 2D and 3D environments. Gelatin (Gel) is a soft hydrogel into which cells can be embedded. This study shows significant 3D Gel shrinking due to the high traction cellular forces exerted by the cells on the matrix, which prevents cell differentiation. To modulate this process, Gel with hyaluronic acid (HA) has been combined in an injectable crosslinked hydrogel with controlled Gel-HA ratio. HA increases matrix stiffness. The addition of small amounts of HA leads to a significant reduction in hydrogel shrinking after cell encapsulation (C2C12 myoblasts). We show that hydrogel stiffness counterbalanced traction forces of cells and this was decisive in promoting cell differentiation and myotube formation of C2C12 encapsulated in the hybrid hydrogels.

Parallelized cytoindentation using convex micropatterned surfaces

Here we present a high-throughput, parallelized cytoindentor for local compression of live cells. The cytoindentor uses convex lens-induced confinement (CLiC) to indent micrometer-sized areas in single cells and/or populations of cells with submicron precision. This is accomplished using micropatterned poly(dimethylsiloxane) (PDMS) films that are adhered to a convex lens to create arrays of extrusions referred to here as "posts." These posts caused local deformation of subcellular regions without any evidence of cell lysis upon CLiC indentation. Our micropost arrays were also functionalized with glycoproteins, such as fibronectin, to both pull and compress cells under customized confinement geometries. Measurements of Chinese hamster ovary (CHO-K1) cell migration trajectories and oxidative stress showed that the CLiC device did not damage or significantly stress the cells. Our novel tool opens a new area of investigation for visualizing mechanobiology and mechanochemistry within living cells, and the high-throughput nature of the technique will streamline investigations as current tools for mechanically probing material properties and molecular dynamics within cells, such as traditional cytoindentors and atomic force microscopy (AFM), are typically restricted to single-cell manipulation.

3D Culture of Chondrocytes in Gelatin Hydrogels with Different Stiffness

Gelatin hydrogels can mimic the microenvironments of natural tissues and encapsulate cells homogeneously, which makes them attractive for cartilage tissue engineering. Both the mechanical and biochemical properties of hydrogels can affect the phenotype of chondrocytes. However, the influence of each property on chondrocyte phenotype is unclear due to the difficulty in separating the roles of these properties. In this study, we aimed to study the influence of hydrogel stiffness on chondrocyte phenotype while excluding the role of biochemical factors, such as adhesion site density in the hydrogels. By altering the degree of methacryloyl functionalization, gelatin hydrogels with different stiffnesses of 3.8, 17.1, and 29.9 kPa Young's modulus were prepared from the same concentration of gelatin methacryloyl (GelMA) macromers. Bovine articular chondrocytes were encapsulated in the hydrogels and cultured for 14 days. The influence of hydrogel stiffness on the cell behaviors including cell viability, cell morphology, and maintenance of chondrogenic phenotype was evaluated. GelMA hydrogels with high stiffness (29.9 kPa) showed the best results on maintaining chondrogenic phenotype. These results will be useful for the design and preparation of scaffolds for cartilage tissue engineering.

Enzymatic Crosslinking of Polymer Conjugates is Superior over Ionic or UV Crosslinking for the On-Chip Production of Cell-Laden Microgels

Cell-laden micrometer-sized hydrogels (microgels) hold great promise for improving high throughput ex-vivo drug screening and engineering biomimetic tissues. Microfluidics is a powerful tool to produce microgels. However, only a limited amount of biomaterials have been reported to be compatible with on-chip microgel formation. Moreover, these biomaterials are often associated with mechanical instability, cytotoxicity, and cellular senescence. To resolve this challenge, dextran-tyramine has been explored as a novel biomaterial for on-chip microgel formation. In particular, dextran-tyramine is compared with two commonly used biomaterials, namely, polyethylene-glycol diacrylate (PEGDA) and alginate, which crosslink through enzymatic reaction, UV polymerization, and ionic interaction, respectively. Human mesenchymal stem cells (hMSCs) encapsulated in dextran-tyramine microgels demonstrate significantly higher (95%) survival as compared to alginate (81%) and PEGDA (69%). Long-term cell cultures demonstrate that hMSCs in PEGDA microgels become senescent after 7 d. Alginate microgels dissolve within 7 d due to Ca²⁺ loss. In contrast, dextran-tyramine based microgels remain stable, sustain hMSCs metabolic activity, and permit for single-cell level analysis for at least 28 d of culture. In conclusion, enzymatically crosslinking dextran-tyramine conjugates represent a novel biomaterial class for the on-chip production of cell-laden microgels, which possesses unique advantages as compared to the commonly used UV and ionic crosslinking biomaterials.

Numerical modeling of cell differentiation and proliferation in force-induced substrates via encapsulated magnetic nanoparticles

BACKGROUND AND OBJECTIVE

Cell migration, differentiation, proliferation and apoptosis are the main processes in tissue regeneration. Mesenchymal Stem Cells have the potential to differentiate into many cell phenotypes such as tissue- or organ-specific cells to perform special functions. Experimental observations illustrate that differentiation and proliferation of these cells can be regulated according to internal forces induced within their Extracellular Matrix. The process of how exactly they interpret and transduce these signals is not well understood.

METHODS

A previously developed three-dimensional (3D) computational model is here extended and employed to study how force-free substrates and force-induced substrate control cell differentiation and/or proliferation during the mechanosensing process. Consistent with experimental observations, it is assumed that cell internal deformation (a mechanical signal) in correlation with the cell maturation state directly triggers cell differentiation and/or proliferation. The Extracellular Matrix is modeled as Neo-Hookean hyperelastic material assuming that cells are cultured within 3D nonlinear hydrogels.

RESULTS

In agreement with well-known experimental observations, the findings here indicate that within neurogenic (0.1-1kPa), chondrogenic (20-25kPa) and osteogenic (30-45kPa) substrates, Mesenchymal Stem Cells differentiation and proliferation can be precipitated by inducing the substrate with an internal force. Therefore, cells require a longer time to grow and maturate within force-free substrates than within force-induced substrates. In the instance of Mesenchymal Stem Cells differentiation into a compatible phenotype, the magnitude of the net traction force increases within chondrogenic and osteogenic substrates while it reduces within neurogenic substrates. This is consistent with experimental studies and numerical works recently published by the same authors. However, in all cases the magnitude of the net traction force considerably increases at the instant of cell proliferation because of cell-cell interaction.

CONCLUSIONS

The present model provides new perspectives to delineate the role of force-induced substrates in remotely controlling the cell fate during cell-matrix interaction, which open the door for new tissue regeneration methodologies.

Organic Bioelectronics: Bridging the Signaling Gap between Biology and Technology

The electronics surrounding us in our daily lives rely almost exclusively on electrons as the dominant charge carrier. In stark contrast, biological systems rarely use electrons but rather use ions and molecules of varying size. Due to the unique combination of both electronic and ionic/molecular conductivity in conducting and semiconducting organic polymers and small molecules, these materials have emerged in recent decades as excellent tools for translating signals between these two realms and, therefore, providing a means to effectively interface biology with conventional electronics-thus, the field of organic bioelectronics. Today, organic bioelectronics defines a generic platform with unprecedented biological recording and regulation tools and is maturing toward applications ranging from life sciences to the clinic. In this Review, we introduce the field, from its early breakthroughs to its current results and future challenges.