Accelerated formation of multicellular 3-D structures by cell-to-cell cross-linking

Cadherin-dependent adhesion modulated 3D cell-assembly

The emergence of synthetic biology has opened new avenues in constructing cell-assembly biosystems with specific gene expression and function. The phenomena of cell spreading and detachment during tissue development and cancer metastasis are caused by surface tension, which in turn results from differences in cell-cell adhesion mediated by the dimerization of cadherin expressed on the cell surface. In this study, E- and P-cadherin plasmids were first constructed based on the differential adhesion hypothesis, then they were electroporated into K562 cells and HEK293T cells, respectively, to explore the process of cell migration and assembly regulated by cadherins. Using this approach, some special 3D cell functional components with a phase separation structure were fabricated successfully. Our work will be of potential application in the construction of self-assembling synthetic tissues and organoids.

Spatiotemporal Control Over Multicellular Migration Using Green Light Reversible Cell-Cell Interactions

The regulation of cell-cell adhesions in space and time plays a crucial role in cell biology, especially in the coordination of multicellular behavior. Therefore, tools that allow for the modulation of cell-cell interactions with high precision are of great interest to a better understanding of their roles and building tissue-like structures. Herein, the green light-responsive protein CarH is expressed at the plasma membrane of cells as an artificial cell adhesion receptor, so that upon addition of its cofactor vitamin B₁₂ specific cell-cell interactions form and lead to cell clustering in a concentration-dependent manner. Upon green light illumination, the CarH based cell-cell interactions disassemble and allow for their reversion with high spatiotemporal control. Moreover, these artificial cell-cell interactions impact cell migration, as observed in a wound-healing assay. When the cells interact with each other in the presence of vitamin B₁₂ in the dark, the cells form on a solid front and migrate collectively; however, under green light illumination, individual cells migrate randomly out of the monolayer. Overall, the possibility of precisely controlling cell-cell interactions and regulating multicellular behavior is a potential pathway to gaining more insight into cell-cell interactions in biological processes.

Cell Mechanics in Embryoid Bodies

Embryoid bodies (EBs) resemble self-organizing aggregates of pluripotent stem cells that recapitulate some aspects of early embryogenesis. Within few days, the cells undergo a transition from rather homogeneous epithelial-like pluripotent stem cell colonies into a three-dimensional organization of various cell types with multifaceted cell-cell interactions and lumen formation-a process associated with repetitive epithelial-mesenchymal transitions. In the last few years, culture methods have further evolved to better control EB size, growth, cellular composition, and organization-e.g., by the addition of morphogens or different extracellular matrix molecules. There is a growing perception that the mechanical properties, cell mechanics, and cell signaling during EB development are also influenced by physical cues to better guide lineage specification; substrate elasticity and topography are relevant, as well as shear stress and mechanical strain. Epithelial structures outside and inside EBs support the integrity of the cell aggregates and counteract mechanical stress. Furthermore, hydrogels can be used to better control the organization and lineage-specific differentiation of EBs. In this review, we summarize how EB formation is accompanied by a variety of biomechanical parameters that need to be considered for the directed and reproducible self-organization of early cell fate decisions.

Cell membrane engineering with synthetic materials: Applications in cell spheroids, cellular glues and microtissue formation

Biologically inspired materials with tunable bio- and physicochemical properties provide an essential framework to actively control and support cellular behavior. Cell membrane remodeling approaches benefit from the advances in polymer science and bioconjugation methods, which allow for the installation of un-/natural molecules and particles on the cells' surface. Synthetically remodeled cells have superior properties and are under intense investigation in various therapeutic scenarios as cell delivery systems, bio-sensing platforms, injectable biomaterials and bioinks for 3D bioprinting applications. In this review article, recent advances in the field of cell surface remodeling via bio-chemical means and the potential biomedical applications of these emerging cell hybrids are discussed. STATEMENT OF SIGNIFICANCE: Recent advances in bioconjugation methods, controlled/living polymerizations, microfabrication techniques and 3D printing technologies have enabled researchers to probe specific cellular functions and cues for therapeutic and research purposes through the formation of cell spheroids and polymer-cell chimeras. This review article highlights recent non-genetic cell membrane engineering strategies towards the fabrication of cellular ensembles and microtissues with interest in 3D in vitro modeling, cell therapeutics and tissue engineering. From a wider perspective, these approaches may provide a roadmap for future advances in cell therapies which will expedite the clinical use of cells, thereby improving the quality and accessibility of disease treatments.

Blue Light Switchable Cell-Cell Interactions Provide Reversible and Spatiotemporal Control Towards Bottom-Up Tissue Engineering

Controlling cell-cell interactions is central for understanding key cellular processes and bottom-up tissue assembly from single cells. The challenge is to control cell-cell interactions dynamically and reversibly with high spatiotemporal precision noninvasively and sustainably. In this study, cell-cell interactions are controlled with visible light using an optogenetic approach by expressing the blue light switchable proteins CRY2 or CIBN on the surfaces of cells. CRY2 and CIBN expressing cells form specific heterophilic interactions under blue light providing precise control in space and time. Further, these interactions are reversible in the dark and can be repeatedly and dynamically switched on and off. Unlike previous approaches, these genetically encoded proteins allow for long-term expression of the interaction domains and respond to nontoxic low intensity blue light. In addition, these interactions are suitable to assemble cells into 3D multicellular architectures. Overall, this approach captures the dynamic and reversible nature of cell-cell interactions and controls them noninvasively and sustainably both in space and time. This provides a new way of studying cell-cell interactions and assembling cellular building blocks into tissues with unmatched flexibility.

Engineering epithelial-stromal interactions in vitro for toxicology assessment

Crosstalk between epithelial and stromal cells drives the morphogenesis of ectodermal organs during development and promotes normal mature adult epithelial tissue homeostasis. Epithelial-stromal interactions (ESIs) have historically been examined using mammalian models and ex vivo tissue recombination. Although these approaches have elucidated signaling mechanisms underlying embryonic morphogenesis processes and adult mammalian epithelial tissue function, they are limited by the availability of tissue, low throughput, and human developmental or physiological relevance. In this review, we describe how bioengineered ESIs, using either human stem cells or co-cultures of human primary epithelial and stromal cells, have enabled the development of human in vitro epithelial tissue models that recapitulate the architecture, phenotype, and function of adult human epithelial tissues. We discuss how the strategies used to engineer mature epithelial tissue models in vitro could be extrapolated to instruct the design of organotypic culture models that can recapitulate the structure of embryonic ectodermal tissues and enable the in vitro assessment of events critical to organ/tissue morphogenesis. Given the importance of ESIs towards normal epithelial tissue development and function, such models present a unique opportunity for toxicological screening assays to incorporate ESIs to assess the impact of chemicals on mature and developing epidermal tissues.

Sequence-specific nuclease-mediated release of cells tethered by oligonucleotide phospholipids

Single-stranded oligonucleotide-conjugated lipids (ssDNA-PEG-lipids) that associate with the cell membrane confer to the cell an artificial adhesive capability via sequence-specific hybridization to complementary oligonucleotides, forming bonds of double stranded oligonucleotides (dsDNA). Such artificial tethers permit surface patterning of cells or controlled formation of cellular aggregates. However, the hybridization responsible for tethering cells to surfaces or to other cells is not trivially reversed under physiological conditions. In this study, we approach the unbinding of tethered cells by cleaving dsDNA bonds with restriction endonuclease BamHI or digesting bonds with the nonspecific nuclease Benzonase. The procedure was applied to CCRF-CEM cells bearing dsDNA suspended in isolation, cells tethered to glass substrates, and cells aggregated heterotypically with other ssDNA-bearing cells. Cells liberated from surfaces with BamHI could be flushed from flow chambers and viably recovered while the majority of cells not bearing enzyme recognition sequences were retained on the surface, and DNA-tethered cells could be nonspecifically recovered viably from surfaces after Benzonase treatment. Heterotypic aggregates of cells joined by recognition sequence DNA could be dispersed with 10 min exposure to BamHI while undispersed cells heterotypically aggregated with a control sequence remained. Likewise, 10 min exposure to Benzonase was sufficient to disperse aggregates independently of sequence. The potential to undo artificially engineered DNA-mediated adhesion offers new possibilities in the controlled arrangement of cells relative to other cells and in the study of membrane biophysics.

Macromolecular cell surface engineering for accelerated and reversible cellular aggregation

Two simple cell membrane interacting copolymers are reported that induce rapid cell aggregation and act as self-supporting “cellular glues” at minute concentrations.

Media additives to promote spheroid circularity and compactness in hanging drop platform

Three-dimensional spheroid cultures have become increasingly popular as drug screening platforms, especially with the advent of different high throughput spheroid forming technologies.

In vitro spatially organizing the differentiation in individual multicellular stem cell aggregates

With significant potential as a robust source to produce specific somatic cells for regenerative medicine, stem cells have attracted increasing attention from both academia and government. In vivo, stem cell differentiation is a process under complicated regulations to precisely build tissue with unique spatial structures. Since multicellular spheroidal aggregates of stem cells, commonly called as embryoid bodies (EBs), are considered to be capable of recapitulating the events in early stage of embryonic development, a variety of methods have been developed to form EBs in vitro for studying differentiation of embryonic stem cells. The regulation of stem cell differentiation is crucial in directing stem cells to build tissue with the correct spatial architecture for specific functions. However, stem cells within the three-dimensional multicellular aggregates undergo differentiation in a less unpredictable and spatially controlled manner in vitro than in vivo. Recently, various microengineering technologies have been developed to manipulate stem cells in vitro in a spatially controlled manner. Herein, we take the spotlight on these technologies and researches that bring us the new potential for manipulation of stem cells for specific purposes.

Fluorescent oxygen sensitive microbead incorporation for measuring oxygen tension in cell aggregates

Molecular oxygen is a main regulator of various cell functions. Imaging methods designed as screening tools for fast, in situ, 3D and non-interfering measurement of oxygen tension in the cellular microenvironment would serve great purpose in identifying and monitoring this vital and pivotal signalling molecule. We describe the use of dual luminophore oxygen sensitive microbeads to measure absolute oxygen concentrations in cellular aggregates. Stable microbead integration, a prerequisite for their practical application, was ensured by a site-specific delivery method that is based on the interactions between streptavidin and biotin. The spatial stability introduced by this method allowed for long term measurements of oxygen tension without interfering with the cell aggregation process. By making multiple calibration experiments we further demonstrated the potential of these sensors to measure local oxygen tension in optically dense cellular environments.

Aggregation promotes cell viability, proliferation, and differentiation in an in vitro model of injection cell therapy

Many cell therapy approaches aim to deliver high-density single-cell suspensions to diseased or injured sites in the body. Long term clinical success will in part be dependent on the cells that remain viable and that assume correct functionality post-administration. The research presented in this paper focuses on the potential of cell aggregate delivery to generate a more supportive environment for cells than single cell suspensions. An in vitro model of injection delivery of C2C12 myoblast cells showed a significant difference in cell function and phenotype between adhesive collagen and non-adhesive alginate, indicating that in vitro assays based on this approach can discriminate between cell-cell/cell-matrix interactions and could be valuable when assessing cell therapy systems. Contrary to single cells, aggregates maintain viability, cellular activity, and phenotype beyond that of single cells, even in non-adhesive matrices, enabling delivery of higher cell densities with enhanced proliferative and differentiation capacity.

Mimicking the inflammatory cell adhesion cascade by nucleic acid aptamer programmed cell-cell interactions

Nature has evolved effective cell adhesion mechanisms to deliver inflammatory cells to inflamed tissue; however, many culture-expanded therapeutic cells are incapable of targeting diseased tissues following systemic infusion, which represents a great challenge in cell therapy. Our aim was to develop simple approaches to program cell-cell interactions that would otherwise not exist toward cell targeting and understanding the complex biology of cell-cell interactions. We employed a chemistry approach to engineer P- or L-selectin binding nucleic acid aptamers onto mesenchymal stem cells (MSCs) to enable them to engage inflamed endothelial cells and leukocytes, respectively. We show for the first time that engineered cells with a single artificial adhesion ligand can recapitulate 3 critical cell interactions in the inflammatory cell adhesion cascade under dynamic flow conditions. Aptamer-engineered MSCs adhered on respective selectin surfaces under static conditions >10 times more efficiently than controls including scrambled-DNA modified MSCs. Significantly, engineered MSCs can be directly captured from the flow stream by selectin surfaces or selectin-expressing cells under flow conditions (≤2dyn/cm²). The simple chemistry approach and the versatility of aptamers permit the concept of engineered cell-cell interactions to be generically applicable for targeting cells to diseased tissues and elucidating the biology of cell-cell interactions.

Biomarkers for simplifying HTS 3D cell culture platforms for drug discovery: the case for cytokines

In this review, we discuss the microenvironmental cues that modulate the status of cells to yield physiologically more relevant three-dimensional (3D) cell-based high throughput drug screening (HTS) platforms for drug discovery. Evidence is provided to support the view that simplifying 3D cell culture platforms for HTS applications calls for identifying and validating ubiquitous three-dimensionality biomarkers. Published results from avascular tumorigenesis and early stages of inflammatory wound healing, where cells transition from a two-dimensional (2D) to 3D microenvironment, conclusively report regulation by cytokines, providing the physiological basis for focusing on cytokines as potential three-dimensionality biomarkers. We discuss additional support for cytokines that comes from numerous 2D and 3D comparative transcriptomic and proteomic studies, which generally report upregulation of cytokines in 3D compared with 2D culture counterparts.

Osteogenic differentiation of embryonic stem cells in 2D and 3D culture

Osteoblasts are the cells that contribute to the formation and function of bone tissue. Knowledge of their biology is important to understanding of the normal processes of bone repair, the development of diseases affecting bone tissue, and to the investigation of approaches to improve bone repair and to treat or prevent bone diseases. Osteoblasts can be readily isolated from bone tissues and grown in culture, and under relatively simple culture conditions, they will recapitulate many aspects of their normal biology. These culture conditions can be also applied to adult stem cells, such as mesenchymal/bone marrow stromal stem cells. More recently, these studies have been extended to include embryonic stem cells. This chapter provides detailed step-by-step protocols to investigate the differentiation of embryonic stem cells into osteoblasts. Several 2D and 3D culture methods are presented and enable comparisons to be made on the efficiency and mechanisms of osteogenic differentiation. Emphasis is also placed on methods to analyse and confirm osteogenic differentiation.

Engineering embryonic stem-cell aggregation allows an enhanced osteogenic differentiation in vitro

Pluripotent embryonic stem (ES) cells hold great promise for the field of tissue engineering, with numerous studies investigating differentiation into various cell types including cardiomyocytes, chondrocytes, and osteoblasts. Previous studies have detailed osteogenic differentiation via dissociated embryoid body (EB) culture in osteoinductive media comprising of ascorbic acid, beta-glycerophosphate, and dexamethasone. It is hoped that these osteogenic cultures will have clinical application in bone tissue repair and regeneration and pharmacological testing. However, differentiation remains highly inefficient and generates heterogeneous populations. We have previously reported an engineered three-dimensional culture system for controlled ES cell-ES cell interaction via the avidin-biotin binding complex. Here we investigate the effect of such engineering on ES cell differentiation. Engineered EBs exhibit enhanced osteogenic differentiation assessed by cadherin-11, Runx2, and osteopontin expression, alkaline phosphatase activity, and bone nodule formation. Results show that cultures produced from intact EBs aggregated for 3 days generated the greatest levels of osteogenic differentiation when cultured in osteoinductive media. However, when cultured in control media, only engineered samples appeared to exhibit bone nodule formation. In addition, polymerase chain reaction analysis revealed a decrease in endoderm and ectoderm expression within engineered samples. This suggests that engineered ES cell aggregation has increased mesoderm homogeneity, contributing to enhanced osteogenic differentiation.

Chemistry and material science at the cell surface

Cell surfaces are fertile ground for chemists and material scientists to manipulate or augment cell functions and phenotypes. This not only helps to answer basic biology questions but also has diagnostic and therapeutic applications. In this review, we summarize the most recent advances in the engineering of the cell surface. In particular, we focus on the potential applications of surface engineered cells for 1) targeting cells to desirable sites in cell therapy, 2) programming assembly of cells for tissue engineering, 3) bioimaging and sensing, and ultimately 4) manipulating cell biology.

Controlled embryoid body formation via surface modification and avidin-biotin cross-linking

Cell-cell interaction is an integral part of embryoid body (EB) formation controlling 3D aggregation. Manipulation of embryonic stem (ES) cell interactions could provide control over EB formation. Studies have shown a direct relationship between EB formation and ES cell differentiation. We have previously described a cell surface modification and cross-linking method for influencing cell-cell interaction and formation of multicellular constructs. Here we show further characterisation of this engineered aggregation. We demonstrate that engineering accelerates ES cell aggregation, forming larger, denser and more stable EBs than control samples, with no significant decrease in constituent ES cell viability. However, extended culture >/=5 days reveals significant core necrosis creating a layered EB structure. Accelerated aggregation through engineering circumvents this problem as EB formation time is reduced. We conclude that the proposed engineering method influences initial ES cell-ES cell interactions and EB formation. This methodology could be employed to further our understanding of intrinsic EB properties and their effect on ES cell differentiation.

Recent advances in three-dimensional multicellular spheroid culture for biomedical research

Many types of mammalian cells can aggregate and differentiate into 3-D multicellular spheroids when cultured in suspension or a nonadhesive environment. Compared to conventional monolayer cultures, multicellular spheroids resemble real tissues better in terms of structural and functional properties. Multicellular spheroids formed by transformed cells are widely used as avascular tumor models for metastasis and invasion research and for therapeutic screening. Many primary or progenitor cells on the other hand, show significantly enhanced viability and functional performance when grown as spheroids. Multicellular spheroids in this aspect are ideal building units for tissue reconstruction. Here we review the current understanding of multicellular spheroid formation mechanisms, their biomedical applications, and recent advances in spheroid culture, manipulation, and analysis techniques.

Tissue engineering: strategies, stem cells and scaffolds

Tissue engineering scaffolds are designed to influence the physical, chemical and biological environment surrounding a cell population. In this review we focus on our own work and introduce a range of strategies and materials used for tissue engineering, including the sources of cells suitable for tissue engineering: embryonic stem cells, bone marrow-derived mesenchymal stem cells and cord-derived mesenchymal stem cells. Furthermore, we emphasize the developments in custom scaffold design and manufacture, highlighting laser sintering, supercritical carbon dioxide processing, growth factor incorporation and zoning, plasma modification of scaffold surfaces, and novel multi-use temperature-sensitive injectable materials.

Transient inter-cellular polymeric linker

Three-dimensional (3D) tissue-engineered constructs with bio-mimicry cell-cell and cell-matrix interactions are useful in regenerative medicine. In cell-dense and matrix-poor tissues of the internal organs, cells support one another via cell-cell interactions, supplemented by small amount of the extra-cellular matrices (ECM) secreted by the cells. Here we connect HepG2 cells directly but transiently with inter-cellular polymeric linker to facilitate cell-cell interaction and aggregation. The linker consists of a non-toxic low molecular-weight polyethyleneimine (PEI) backbone conjugated with multiple hydrazide groups that can aggregate cells within 30 min by reacting with the aldehyde handles on the chemically modified cell-surface glycoproteins. The cells in the cellular aggregates proliferated; and maintained the cortical actin distribution of the 3D cell morphology while non-aggregated cells died over 7 days of suspension culture. The aggregates lost distinguishable cell-cell boundaries within 3 days; and the ECM fibers became visible around cells from day 3 onwards while the inter-cellular polymeric linker disappeared from the cell surfaces over time. The transient inter-cellular polymeric linker can be useful for forming 3D cellular and tissue constructs without bulk biomaterials or extensive network of engineered ECM for various applications.