Effects of supported lipid monolayer fluidity on the adhesion of hematopoietic progenitor cell lines to fibronectin-derived peptide ligands for alpha5beta1 and alpha4beta1 integrins

Integrin-Targeting Peptides for the Design of Functional Cell-Responsive Biomaterials

Integrins are a family of cell surface receptors crucial to fundamental cellular functions such as adhesion, signaling, and viability, deeply involved in a variety of diseases, including the initiation and progression of cancer, of coronary, inflammatory, or autoimmune diseases. The natural ligands of integrins are glycoproteins expressed on the cell surface or proteins of the extracellular matrix. For this reason, short peptides or peptidomimetic sequences that reproduce the integrin-binding motives have attracted much attention as potential drugs. When challenged in clinical trials, these peptides/peptidomimetics let to contrasting and disappointing results. In the search for alternative utilizations, the integrin peptide ligands have been conjugated onto nanoparticles, materials, or drugs and drug carrier systems, for specific recognition or delivery of drugs to cells overexpressing the targeted integrins. Recent research in peptidic integrin ligands is exploring new opportunities, in particular for the design of nanostructured, micro-fabricated, cell-responsive, stimuli-responsive, smart materials.

The Plot Thickens: The Emerging Role of Matrix Viscosity in Cell Mechanotransduction

Cell mechanotransduction is an area of intense research focus. Until now, very limited tools have existed to study how cells respond to changes in the extracellular matrix beyond, for example, mechanical deformation studies and twisting cytometry. However, emerging are a range of elastic, viscoelastic and even purely viscous materials that deform and dissipate on cellular length and timescales. This article reviews developments in these materials, typically translating from 2D model surfaces to 3D microenvironments and explores how cells interact with them. Specifically, it focuses on emerging concepts such as the molecular clutch model, how different extracellular matrix proteins engage the clutch under viscoelastic-stress relaxation conditions, and how mechanotransduction can drive transcriptional control through regulators such as YAP/TAZ.

About Chemical Strategies to Fabricate Cell-Instructive Biointerfaces with Static and Dynamic Complexity

Properly functioning cell-instructive biointerfaces are critical for healthy integration of biomedical devices in the body and serve as decisive tools for the advancement of our understanding of fundamental cell biological phenomena. Studies are reviewed that use covalent chemistries to fabricate cell-instructive biointerfaces. These types of biointerfaces typically result in a static presentation of predefined cell-instructive cues. Chemically defined, but dynamic cell-instructive biointerfaces introduce spatiotemporal control over cell-instructive cues and present another type of biointerface, which promises a more biomimetic way to guide cell behavior. Therefore, strategies that offer control over the lateral sorting of ligands, the availability and molecular structure of bioactive ligands, and strategies that offer the ability to induce physical, chemical and mechanical changes in situ are reviewed. Specific attention is paid to state-of-the-art studies on dynamic, cell-instructive 3D materials. Future work is expected to further deepen our understanding of molecular and cellular biological processes investigating cell-type specific responses and the translational steps toward targeted in vivo applications.

Hybrid Biomimetic Interfaces Integrating Supported Lipid Bilayers with Decellularized Extracellular Matrix Components

This paper describes the functionalization of solid supported phospholipid bilayer with decellularized extracellular matrix (dECM) components, toward the development of biomimetic platforms that more closely mimic the cell surface environment. The dECM was obtained through a combination of chemical and enzymatic treatments of mouse adipose tissue that contains collagen, fibronectin, and glycosaminoglycans (GAGs). Using amine coupling chemistry, the dECM components were attached covalently to the surface of a supported lipid bilayer containing phospholipids with reactive carboxylic acid headgroups. The bilayer formation and the kinetics of subsequent dECM conjugation were monitored by quartz crystal microbalance with dissipation (QCM-D). Fluorescence recovery after photobleaching (FRAP) confirmed the fluidity of the membrane after functionalization with dECM. The resulting hybrid biomimetic interface supports the attachment and survival of the human hepatocyte Huh 7.5 and maintains the representative hepatocellular function. Importantly, the platform is suitable for monitoring the lateral organization and clustering of cell-binding ligands and growth factor receptors in the presence of the rich biochemical profile of tissue-derived ECM components.

Lower fluidity of supported lipid bilayers promotes neuronal differentiation of neural stem cells by enhancing focal adhesion formation

Extensive studies have been performed to understand how the mechanical properties of a stem cell's microenvironment influence its behaviors. Supported lipid bilayers (SLBs), a well-known biomimetic platform, have been used to mimic the dynamic characteristics of the extracellular matrix (ECM) because of their fluidity. However, the effect of the fluidity of SLBs on stem cell fate is unknown. We constructed SLBs with different fluidities to explore the influence of fluidity on the differentiation of neural stem cells (NSCs). The results showed that the behavior of NSCs was highly dependent on the fluidity of SLBs. Low fluidity resulted in enhanced focal adhesion formation, a dense network of stress fibers, stretched and elongated cellular morphology and increased neuronal differentiation, while high fluidity led to less focal adhesion formation, immature stress fibers, round cellular morphology and more astrocyte differentiation. Mechanistic studies revealed that low fluidity may have enhanced focal adhesion formation, which activated FAK-MEK/ERK signaling pathways and ultimately promoted neuronal differentiation of NSCs. This work provides a strategy for manipulating the dynamic matrix surface for the development of culture substrates and tissue-engineered scaffolds, which may aid the understanding of how the dynamic ECM influences stem cell behaviors as well as improve the efficacy of stem cell applications.

Supported lipid bilayer platforms to probe cell mechanobiology

Mammalian and bacterial cells sense and exert mechanical forces through the process of mechanotransduction, which interconverts biochemical and physical signals. This is especially important in contact-dependent signaling, where ligand-receptor binding occurs at cell-cell or cell-ECM junctions. By virtue of occurring within these specialized junctions, receptors engaged in contact-dependent signaling undergo oligomerization and coupling with the cytoskeleton as part of their signaling mechanisms. While our ability to measure and map biochemical signaling within cell junctions has advanced over the past decades, physical cues remain difficult to map in space and time. Recently, supported lipid bilayer (SLB) technologies have emerged as a flexible platform to mimic and perturb cell-cell and cell-ECM junctions, allowing one to study membrane receptor mechanotransduction. Changing the lipid composition and underlying substrate tunes bilayer fluidity, and lipid and ligand micro- and nano-patterning spatially control positioning and clustering of receptors. Patterning metal gridlines within SLBs confines lipid mobility and introduces mechanical resistance. Here we review fundamental SLB mechanics and how SLBs can be engineered as tunable cell substrates for mechanotransduction studies. Finally, we highlight the impact of this work in understanding the biophysical mechanisms of cell adhesion. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Ferritin-supported lipid bilayers for triggering the endothelial cell response

Hybrid nanoassemblies of ferritin and silica-supported lipid bilayers (ferritin-SLBs) have been prepared and tested for the adhesion, spreading and proliferation of retinal microvascular endothelial cells (ECs). Lipid membranes with varying surface charge were obtained by mixing cationic 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine (POEPC) with zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at increasing POPC/POEPC ratios. The supported bilayer formation and their subsequent interaction processes with ferritin were studied at the pH of 7.4 at different protein concentrations, by using the quartz crystal microbalance with dissipation monitoring and by atomic force microscopy. Both kinetics and viscoelastic parameters of the protein-lipid membrane interface were scrutinized, as well as surface coverage. Phase-contrast optical microscopy analyses of the ferritin-SLBs substrates after their interaction with endothelial cells evidenced the highest cell adhesion (2-4h of incubation time) and proliferation (from 24h to 5 days) for the membranes of POPC/POEPC (75:25 ratio). Moreover, ferritin increased both cell adhesion and proliferation in comparison to control glass (respectively 1.5- and 1.75-fold) as well as proliferation in comparison to bare POPC/POEPC (95:5 ratio) (2 fold). Results are very promising in the goal of modulating the endothelial cell response through the interplay of viscoelastic/charge properties of the solid-supported membranes and the SLB-conditioned ferritin activity.

Lateral Chain Length in Polyalkyl Acrylates Determines the Mobility of Fibronectin at the Cell/Material Interface

Cells, by interacting with surfaces indirectly through a layer of extracellular matrix proteins, can respond to a variety of physical properties, such as topography or stiffness. Polymer surface mobility is another physical property that is less well understood but has been indicated to hold the potential to modulate cell behavior. Polymer mobility is related to the glass-transition temperature (Tg) of the system, the point at which a polymer transitions from an amorphous solid to a more liquid-like state. This work shows that changes in polymer mobility translate to interfacial mobility of extracellular matrix proteins adsorbed on the material surface. This study has utilized a family of polyalkyl acrylates with similar chemistry but different degrees of mobility, obtained through increasing length of the side chain. These materials are used, in conjunction with fluorescent fibronectin, to determine the mobility of this interfacial layer of protein that constitutes the initial cell-material interface. Furthermore, the extent of fibronectin domain availability (III9, III10, - the integrin binding site), cell-mediated reorganization, and cell differentiation was also determined. A nonmonotonic dependence of fibronectin mobility on polymer surface mobility was observed, with a similar trend noted in cell-mediated reorganization of the protein layer by L929 fibroblasts. The availability of the integrin-binding site was higher on the more mobile surfaces, where a similar organization of the protein into networks at the material interface was observed. Finally, differentiation of C2C12 myoblasts was seen to be highly sensitive to surface mobility upon inhibition of cell contractility. Altogether, these findings show that polymer mobility is a subtle influence that translates to the cell/material interface through the protein layer to alter the biological activity of the surface.

Cell adhesion mechanisms on laterally mobile polymer films

In contrast with the majority of substrates used to study cell adhesion, the natural extracellular matrix (ECM) is dynamic and remodeled over time. Here we use amphiphilic block copolymers to create self-assembled supported films with tunable lateral mobility. These films are intended to serve as partial mimics of the ECM in order to better understand cell adhesion responses, specifically in the context of dynamic substrates. Block copolymers are end-labeled with RGD peptide ligands to allow for integrin-mediated cell adhesion, and the addition of a trace hydrophobic homopolymer is used to control the film lateral mobility. We find that NIH 3T3 fibroblasts cultured on these biomimetic films exhibit non-linear spreading behavior in response to substrate mobility. In the absence of RGD ligands, however, fibroblasts do not spread. Employing quantitative analysis of focal adhesions (FA) and integrin ligation, we discover the presence of FA-dependent and FA-independent mechanisms responsible for the biphasic cell spreading behavior. The use of designed biomimetic platforms therefore yields insight into ECM mechanosensing by revealing that cells can engage distinct mechanisms to promote adhesion onto substrates with different time-dependent properties.

Covalent immobilization of stem cell factor and stromal derived factor 1α for in vitro culture of hematopoietic progenitor cells

Hematopoietic stem cells (HSCs) are currently utilized in the treatment of blood diseases, but widespread application of HSC therapeutics has been hindered by the limited availability of HSCs. With a better understanding of the HSC microenvironment and the ability to precisely recapitulate its components, we may be able to gain control of HSC behavior. In this work we developed a novel, biomimetic PEG hydrogel material as a substrate for this purpose and tested its potential with an anchorage-independent hematopoietic cell line, 32D clone 3 cells. We immobilized a fibronectin-derived adhesive peptide sequence, RGDS; a cytokine critical in HSC self-renewal, stem cell factor (SCF); and a chemokine important in HSC homing and lodging, stromal derived factor 1α (SDF1α), onto the surfaces of poly(ethylene glycol) (PEG) hydrogels. To evaluate the system's capabilities, we observed the effects of the biomolecules on 32D cell adhesion and morphology. We demonstrated that the incorporation of RGDS onto the surfaces promotes 32D cell adhesion in a dose-dependent fashion. We also observed an additive response in adhesion on surfaces with RGDS in combination with either SCF or SDF1α. In addition, the average cell area increased and circularity decreased on gel surfaces containing immobilized SCF or SDF1α, indicating enhanced cell spreading. By recapitulating aspects of the HSC microenvironment using a PEG hydrogel scaffold, we have shown the ability to control the adhesion and spreading of the 32D cells and demonstrated the potential of the system for the culture of primary hematopoietic cell populations.

Supported Membranes Meet Flat Fluidics: Monitoring Dynamic Cell Adhesion on Pump-Free Microfluidics Chips Functionalized with Supported Membranes Displaying Mannose Domains

In this paper we demonstrate the combination of supported membranes and so-called flat microfluidics, which enables one to manipulate liquids on flat chip surfaces via “inverse piezoelectric effect”. Here, an alternating external electric field applied to the inter-digital transducers excites a surface acoustic wave on a piezoelectric substrate. Employing lithographic patterning of self-assembled monolayers of alkoxysilanes, we successfully confine a free-standing, hemi-cylindrical channel with the volume of merely 7 µL . The experimentally determined maximum flow velocity scales linearly with the acoustic power, suggesting that our current setup can drive liquids at the speed of up to 7 cm/s (corresponding to a shear rate of 280 s⁻¹) without applying high pressures using a fluidic pump. After the establishment of the functionalization of fluidic chip surfaces with supported membranes, we deposited asymmetric supported membranes displaying well-defined mannose domains and monitored the dynamic adhesion of E. Coli HB101 expressing mannose-binding receptors. Despite of the further technical optimization required for the quantitative analysis, the obtained results demonstrate that the combination of supported membranes and flat fluidics opens a large potential to investigate dynamic adhesion of cells on biofunctional membrane surfaces with the minimum amount of samples, without any fluidic pump.

Cell differentiation of pluripotent tissue sheets immobilized on supported membranes displaying cadherin-11

Investigating cohesive tissue sheets in controlled cultures still poses a challenge since the complex intercellular interactions are difficult to mimic in in vitro models. We used supported lipid membranes functionalized by the adhesive part of the extracellular domain of the cell adhesion molecule cadherin-11 for the immobilization of pluripotent tissue sheets, the animal cap isolated from Xenopus laevis blastula stage embryos. Cadherin-11 was bound via histidine tag to lipid membranes with chelator head groups. In the first step, quantitative functionalization of the membranes with cadherin-11 was confirmed by quartz crystal microbalance and high energy specular X-ray reflectivity. In the next step, animal cap tissue sheets induced to neural crest cell fate were cultured on the membranes functionalized with cadherin-11. The adhesion of cells within the cohesive tissue was significantly dependent on changes in lateral densities of cadherin-11. The formation of filopodia and lamellipodia in the cohesive tissue verified the viability and sustainability of the culture over several hours. The expression of the transcription factor slug in externally induced tissue demonstrated the applicability of lipid membranes displaying adhesive molecules for controlled differentiation of cohesive pluripotent tissue sheets.

Tether-supported biomembranes with α-helical peptide-based anchoring constructs

The strict requirement of constructing a native lipid environment to preserve the structure and functionality of membrane proteins is the starting constraint when building biomaterials and sensor systems from these biomolecules. To enhance the viability of supported biomembranes systems and build new ligand display interfaces, we apply rationally designed peptides partitioned into the lipid bilayer interface. Peptides designed to form membrane-spanning α-helical anchoring domains are synthesized using solid-phase peptide synthesis. K(3)A(4)L(2)A(7)L(2)A(3)K(2)-FITC is synthesized on the 100 mg scale for use as a biomembrane anchoring molecule, where orthogonal side-chain modifications allow us to introduce probes enabling peptide localization within supported bilayers. The peptides are found to form α-helical domains within liposomes as assessed with circular dichroism spectroscopy. These peptides are designed to be incorporated into lipid bilayers supported by microspheres and serve as biomembrane anchoring moieties to amino-terminated surfaces. Here, the silica bead surface (4.7 μm diameter) is activated with homobifunctional NHS-PEG(3000)-NHS as "polymer cushion" spacers. This tethering to a subset of the K(3)A(4)L(2)A(7)L(2)A(3)K(2)-FITC molecules present in the bilayer is achieved by the fusion of liposomes followed by coupling of the peptide amino groups to the NHS presented from the silica microsphere surfaces. The biomembrane distributions of tethered and untethered K(3)A(4)L(2)A(7)L(2)A(3)K(2)-FITC are probed with confocal microscopy and are found to give 3D reconstructions consistent with largely homogeneous supported biomembranes. The fluidity of the untethered fraction of peptides within supported membranes is quantified using the fluorescence recovery after photobleaching (FRAP) technique. The presence of the PEG(3000) polymer cushion facilitated a 28.9% increase in peptide diffusivity over untethered bilayers at the lowest peptide to lipid ratio we examined. We show that rationally designed peptide-based anchors can be used to tether lipid bilayers, creating a polymer-cushioned lipid microenvironment on surfaces with high lateral mobility and facilitating the development of a new platform for ligand displays.

Tethered fibronectin liposomes on supported lipid bilayers as a prepackaged controlled-release platform for cell-based assays

A biomimetic construct containing an extracellular matrix protein-liposome composite tethered on supported lipid bilayers (SLBs) was formed with fibronectin (FN), and polyethylene glycol (PEG) and cholesterol-containing liposomes. The construct can serve as a multifunctional platform for cell attachment and drug release. The successful fabrication of the FN-liposome-SLB model platform was analyzed in situ with a quartz crystal microbalance with dissipation. The long-term stability of the surface tethered liposomes was measured via an encapsulated fluorescent probe. Less than 20% of the fluorescent probe content was released in 8 days, which compared favorably to the release of 90% of the probe content in one day from a similar construct made without PEG and cholesterol. HeLa cells were used to study the cellular interactions with the model platform. The extracellular matrix composition, FN, was found to be essential to promote HeLa cell adhesion on the liposome-SLB surfaces. Upon cell adhesion, the liposomes were spatially reorganized and absorbed by the cells. The rate of HeLa cell apoptosis was correlated with the surface density of doxorubicin-loaded liposomes, confirming the effective drug delivery through liposomes. The multifunctional model platform could be useful as preadministered, controlled-release platforms for cell- and tissue-based assays.

Combinatorial screening of chemically defined human mesenchymal stem cell culture substrates

Self-assembled monolayers (SAMs) of alkanethiolates on gold are chemically defined substrates that can be used to evaluate the effects of an immobilized biomolecule. However, the types of biomolecules that can influence stem cell behavior are numerous and inter-related, and efficient experimental formats are a critical need. Here we employed a SAM array technology to investigate the effects of multiple, distinct peptides and peptide combinations on human mesenchymal stem cell (hMSC) behavior. Specifically, we characterized the conjugation of peptide mixtures to SAM arrays and then investigated the combined effects of a bone morphogenic protein receptor-binding peptide (BR-BP), a heparin proteoglycan-binding peptide (HPG-BP), and varied densities of the integrin-binding ligand Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) on hMSC surface coverage and alkaline phosphatase activity. Results indicate that an amine reactive fluorescent probe can be used to characterize peptide composition after immobilization in SAM array spots. Furthermore, hMSC response to BR-BP and HPG-BP is dependent on GRGDSP density and at day 7, hMSC alkaline phosphatase expression is highly dependent on GRGDSP density. Taken together, we demonstrate how a SAM array approach can be used to probe the combinatorial effects of multiple peptides and motivate further investigations into potential synergies between cell adhesion and other bioactive peptides.

A synthetic substrate to support early mesodermal differentiation of human embryonic stem cells

Our ability to guide differentiation of human pluripotent stem cells (hPSCs) toward desired lineages efficiently and reproducibly in xeno-free conditions is the key to advancing hPSC technology from the laboratory to clinical use. Here we report an engineered biomimetic substrate functionalized with both peptide ligands for α5β1 and α6β1 integrins to support efficient early mesodermal differentiation of human embryonic stem cells (hESCs) when cultured in a differentiation medium containing BMP4. In contrast, mesodermal differentiation is not induced on substrates functionalized with either ligand alone even though the culture medium is identical. Mesodermal differentiation was characterized by immunofluorescent staining, flow cytometric analysis, and RT-PCR analysis of early mesodermal markers Brachyury, Mixl1, and Wnt3. The early mesodermal progenitors derived on the substrate functionalized with both integrin ligands have the normal developmental potential to further differentiate along the hemato-endothelial and cardiac lineages. Immobilized ligands for α5β1 and α6β1 integrins both are permissive, necessary, and sufficient insoluble ligands in this engineered system to support early mesodermal differentiation of hESCs. This synthetic substrate, in conjunction with defined soluble factors, constructs a well-controlled and xeno-free early mesodermal differentiation niche that offers advantages over the previously reported niche constructed with the Matrigel-coated substrate.

Neural stem cell adhesion and proliferation on phospholipid bilayers functionalized with RGD peptides

Peptide-functionalized materials show promise in controlling stem cell behavior by mimicking cell-matrix interactions. Supported lipid bilayers are an excellent platform for displaying peptides due to their ease of fabrication and low non-specific interactions with cells. In this paper, we report on the behavior of adult hippocampal neural stem cells (NSCs) on phospholipid bilayers functionalized with different RGD-containing peptides: either GGGNGEPRGDTYRAY ('bsp-RGD(15)') or GRGDSP. Fluid supported bilayers were prepared on glass surfaces by adsorption and fusion of small lipid vesicles incorporating synthetic peptide amphiphiles. NSCs adhered to bilayers with either GRGDSP or bsp-RGD(15) peptide. After 5 days in culture, NSCs formed neurosphere-like aggregates on GRGDSP bilayers, whereas on bsp-RGD(15) bilayers a large fraction of single adhered cells were observed, comparable to monolayer growth seen on laminin controls. NSCs retained their ability to differentiate into neurons and astrocytes on both peptide surfaces. This work illustrates the utility of supported bilayers in displaying peptide ligands and demonstrates that RGD peptides may be useful in synthetic culture systems for stem cells.