Interfacial Tension Effect on Cell Partition in Aqueous Two-Phase Systems

Exploring the potential of all-aqueous immiscible systems for preparing complex biomaterials and cellular constructs

All-aqueous immiscible systems derived from liquid-liquid phase separation of incompatible hydrophilic agents such as polymers and salts have found increasing interest in the biomedical and tissue engineering fields in the last few years. The unique characteristics of aqueous interfaces, namely their low interfacial tension and elevated permeability, as well as the non-toxic environment and high water content of the immiscible phases, confer to these systems optimal qualities for the development of biomaterials such as hydrogels and soft membranes, as well as for the preparation of in vitro tissues derived from cellular assembly. Here, we overview the main properties of these systems and present a critical review of recent strategies that have been used for the development of biomaterials with increased levels of complexity using all-aqueous immiscible phases and interfaces, and their potential as cell-confining environments for micropatterning approaches and the bioengineering of cell-rich structures. Importantly, due to the relatively recent emergence of these areas, several key design considerations are presented, in order to guide researchers in the field. Finally, the main present challenges, future directions, and adaptability to develop advanced materials with increased biomimicry and new potential applications are briefly evaluated.

Inhibiting BRAF/EGFR/MEK suppresses cancer stemness and drug resistance of primary colorectal cancer cells

Drug resistance is a major barrier against successful treatments of cancer patients. Gain of stemness under drug pressure is a major mechanism that renders treatments ineffective. Identifying approaches to target cancer stem cells (CSCs) is expected to improve treatment outcomes for patients. To elucidate the role of cancer stemness in resistance of colorectal cancer cells to targeted therapies, we developed spheroid cultures of patient-derived BRAFmut and KRASmut tumor cells and studied resistance mechanisms to inhibition of MAPK pathway through phenotypic and gene and protein expression analysis. We found that treatments enriched the expression of CSC markers CD166, ALDH1A3, CD133, and LGR5 and activated PI3K/Akt pathway in cancer cells. We examined various combination treatments to block these activities and found that a triple combination against BRAF, EGFR, and MEK significantly reduced stemness and activities of oncogenic signaling pathways. This study demonstrates the feasibility of blocking stemness-mediated drug resistance and tumorigenic activities in colorectal cancer.

Multimodal peptide ligand extracts parvovirus from interface in affinity aqueous two-phase system

Aqueous two-phase systems (ATPS) have found various applications in bioseparations and microencapsulation. The primary goal of this technique is to partition target biomolecules in a preferred phase, rich in one of the phase-forming components. However, there is a lack of understanding of biomolecule behavior at the interface between the two phases. Biomolecule partitioning behavior is studied using tie-lines (TL), where each TL is a group of systems at thermodynamic equilibrium. Across a TL, a system can either have a bulk PEG-rich phase with citrate-rich droplets, or the opposite can occur. We found that porcine parvovirus (PPV) was recovered at a higher amount when PEG was the bulk phase and citrate was in droplets and that the salt and PEG concentrations are high. To improve the recovery, A PEG 10 kDa-peptide conjugate was formed using the multimodal WRW ligand. When WRW was present, less PPV was caught at the interface of the two-phase system, and more was recovered in the PEG-rich phase. While WRW did not significantly increase the PPV recovery in the high TL system, which was found earlier to be optimal for PPV recovery, the peptide did greatly enhance recovery at a lower TL. This lower TL has a lower viscosity and overall system PEG and citrate concentration. The results provide both a method to increase virus recovery in a lower viscosity system, as well as provide interesting thoughts into the interfacial phenomenon and how to recover virus in a phase and not at the interface.

Aqueous Two-Phase Enabled Low Viscosity 3D (LoV3D) Bioprinting of Living Matter

Embedded 3D bioprinting has great value for the freeform fabrication of living matter. However, embedded 3D bioprinting is currently limited to highly viscous liquid baths or liquid-like solid baths. In contrast, prior to crosslinking, most hydrogels are formulated as low-viscosity solutions and are therefore not directly compatible with bioprinting due to low shape fidelity and poor print stability. The authors here present a method to enable low-viscosity ink 3D (LoV3D) bioprinting, based on aqueous two-phase stabilization of the ink-bath interface. LoV3D allows for the printing of living constructs at high extrusion speeds (up to 1.8 m s^-1 ) with high viability due to its exceedingly low-viscosity. Moreover, LoV3D liquid/liquid interfaces offer unique advantages for fusing printed structures, creating intricate vasculature, and modifying surfaces at higher efficiencies than traditional systems. Furthermore, the low interfacial tension of LoV3D bioprinting offers unprecedented nozzle-independent control over filament diameter via large-dimension strand-thinning, which allows for the printing of an exceptionally wide range of diameters down to the width of a single cell. Overall, LoV3D bioprinting is a unique all-aqueous approach with broad material compatibility without the need for rheological ink adaption, which opens new avenues of application in cell patterning, drug screening, engineered meat, and organ fabrication.

Therapeutic Targeting of Cancer Stem Cells Prevents Resistance of Colorectal Cancer Cells to MEK Inhibition

Drug resistance is a leading cause for the failure of cancer treatments. Plasticity of cancer cells to acquire stem cell-like properties enables them to escape drug toxicity through different adaptive mechanisms. Eliminating cancer stem cells (CSCs) can potentially improve treatment outcomes for patients. To determine the role of CSCs in resistance of colorectal cancer cells to targeted therapies and identify treatment strategies, we treated spheroids of BRAFmut and KRASmut colorectal cancer cells with inhibitors of the mitogen-activated protein kinase pathway and studied resistance mechanisms through gene and protein expression analyses. We found that treatments activated several oncogenic pathways and expression of CSC markers CD166 and ALDH1A3. We identified a specific combination treatment using trametinib and mithramycin A to simultaneously inhibit the CSC phenotype and activities of several pathways in cancer cells. This study demonstrates the feasibility of therapeutic targeting of CSCs as a strategy to block tumorigenic activities of cancer cells.

Tunable particle/cell separation across aqueous two-phase system interface by electric pulse in microfluidics

HYPOTHESIS

Separations of particles and cells are indispensable in many microfluidic systems and have numerous applications in chemistry and biomedicine. The interface of aqueous two-phase system (ATPS) can act as a liquid filter. Under electric field stimuli, the selective transfer of targets across the liquid-liquid interface are expected for particles and cells separation.

EXPERIMENTS

The separations of particles and cells based on ATPS electrophoresis in a microfluidic chip were investigated. A systematical study of the mechanism of ATPS electrophoresis was performed first by employing polystyrene (PS) particles. Subsequently, the separations of particles and microalgae cells were demonstrated.

FINDINGS

The electrophoretic transfer of particles across the interface of ATPS is determined by multi-parameters, including the strength of electric pulse, particle size, zeta potential, and hydrophobicity of the particle. The continuous separations of particles/cells can be achieved through the controllable transfer of target particles/cells across the interface under electric pulses in a microfluidic chip. By simply turning the magnitude of the applied electric pulse, the technique is suitable for different purposes, for example, the separations of particles and cells, purification of cells, and viability identification of cells. This tunable separation approach opens opportunities in multidimensional particle and cell sorting for the fields of seed selection of microorganisms, environmental assessment, and biomedical research.

Characterization of an engineered mucus microenvironment for in vitro modeling of host-microbe interactions

The human mucus layer plays a vital role in maintaining health by providing a physical barrier to pathogens. This biological hydrogel also provides the microenvironment for commensal bacteria. Common models used to study host–microbe interactions include gnotobiotic animals or mammalian–microbial co-culture platforms. Many of the current in vitro models lack a sufficient mucus layer to host these interactions. In this study, we engineered a mucus-like hydrogel Consisting of a mixed alginate-mucin (ALG-MUC) hydrogel network by using low concentration calcium chloride (CaCl₂) as crosslinker. We demonstrated that the incorporation of ALG-MUC hydrogels into an aqueous two-phase system (ATPS) co-culture platform can support the growth of a mammalian monolayer and pathogenic bacteria. The ALG-MUC hydrogels displayed selective diffusivity against macromolecules and stability with ATPS microbial patterning. Additionally, we showed that the presence of mucin within hydrogels contributed to an increase in antimicrobial resistance in ATPS patterned microbial colonies. By using common laboratory chemicals to generate a mammalian–microbial co-culture system containing a representative mucus microenvironment, this model can be readily adopted by typical life science laboratories to study host–microbe interaction and drug discovery.

Vascularized Co-Culture Clusteroids of Primary Endothelial and Hep-G2 Cells Based on Aqueous Two-Phase Pickering Emulsions

Three-dimensional cell culture has been extensively involved in biomedical applications due to its high availability and relatively mature biochemical properties. However, single 3D cell culture models based on hydrogel or various scaffolds do not meet the more in-depth requirements of in vitro models. The necrotic core formation inhibits the utilization of the 3D cell culture ex vivo as oxygen permeation is impaired in the absence of blood vessels. We report a simple method to facilitate the formation of angiogenic HUVEC (human umbilical vein endothelial cells) and Hep-G2 (hepatocyte carcinoma model) co-culture 3D clusteroids in a water-in-water (w/w) Pickering emulsions template which can overcome this limitation. This method enabled us to manipulate the cells proportion in order to achieve the optimal condition for stimulating the production of various angiogenic protein markers in the co-cultured clusteroids. The HUVEC cells respond to the presence of Hep-G2 cells and their byproducts by forming endothelial cell sprouts in Matrigel without the exogenous addition of vascular endothelial growth factor (VEGF) or other angiogenesis inducers. This culture method can be easily replicated to produce other types of cell co-culture spheroids. The w/w Pickering emulsion template can facilitate the fabrication of 3D co-culture models to a great extent and be further utilized in drug testing and tissue engineering applications.

Chemical communication at the synthetic cell/living cell interface

Although the complexity of synthetic cells has continued to increase in recent years, chemical communication between protocell models and living organisms remains a key challenge in bottom-up synthetic biology and bioengineering. In this Review, we discuss how communication channels and modes of signal processing can be established between living cells and cytomimetic agents such as giant unilamellar lipid vesicles, proteinosomes, polysaccharidosomes, polymer-based giant vesicles and membrane-less coacervate micro-droplets. We describe three potential modes of chemical communication in consortia of synthetic and living cells based on mechanisms of distributed communication and signal processing, physical embodiment and nested communication, and network-based contact-dependent communication. We survey the potential for applying synthetic cell/living cell communication systems in biomedicine, including the in situ production of therapeutics and development of new bioreactors. Finally, we present a short summary of our findings.

Characterization of Patterned Microbial Growth Dynamics in Aqueous Two-Phase Polymer Scaffolds

Microbial growth confinement using liquid scaffolds based on an aqueous two-phase system (ATPS) is a promising technique to overcome the challenges in microbial-mammalian co-culture in vitro. To better understand the potential use of the ATPS in studying these complex interactions, the goal of this research was to characterize the effects of bacteria loading and biofilm maturation on the stability of a polyethylene glycol (PEG) and dextran (DEX) ATPS. Two ATPS formulations, consisting of 5% PEG/5% DEX and 10% PEG/10% DEX (w/v), were prepared. To test the containment limits of each ATPS formulation, Escherichia coli MG1655 overnight cultures were resuspended in DEX at optical densities (ODs) of 1, 0.3, 0.1, 0.03, and 0.01. Established E. coli colonies initially seeded at lower densities were contained within the DEX phase to a greater extent than E. coli colonies initially seeded at higher densities. Furthermore, the 10% PEG/10% DEX formulation demonstrated longer containment time of E. coli compared to the 5% PEG/5% DEX formulation. E. coli growth dynamics within the ATPS were found to be affected by the initial bacterial density, where colonies of lower initial seeding densities demonstrate more dynamic growth trends compared to colonies of higher initial seeding densities. However, the addition of DEX to the existing ATPS during the growth phase of the bacterial colony does not appear to disrupt the growth inertia of E. coli. We also observed that microbial growth can disrupt ATPS stability below the physical carrying capacity of the DEX droplets. In both E. coli and Streptococcus mutans UA159 colonies, the ATPS interfacial tensions are reduced, as suggested by the loss of fluorescein isothiocyanate (FITC)-DEX confinement and contact angel measurements, while the microbial colony remained well defined. In general, we observed that the stability of the ATPS microbial colony is proportional to polymer concentrations and inversely proportional to seeding density and culture time. These parameters can be combined as part of a toolset to control microbial growth in a heterotypic co-culture platform and should be considered in future work involving mammalian-microbial cell interactions.

Production of Multiple Cell-Laden Microtissue Spheroids with a Biomimetic Hepatic-Lobule-Like Structure

The construction of an in vitro 3D cellular model to mimic the human liver is highly desired for drug discovery and clinical applications, such as patient-specific treatment and cell-based therapy in regenerative medicine. However, current bioprinting strategies are limited in their ability to generate multiple cell-laden microtissues with biomimetic structures. This study presents a method for producing hepatic-lobule-like microtissue spheroids using a bioprinting system incorporating a precursor cartridge and microfluidic emulsification system. The multiple cell-laden microtissue spheroids can be successfully generated at a speed of approximately 45 spheroids min-1 and with a uniform diameter. Hepatic and endothelial cells are patterned in a microtissue spheroid with the biomimetic structure of a liver lobule. The spheroids allow long-term culture with high cell viability, and the structural integrity is maintained longer than that of non-structured spheroids. Furthermore, structured spheroids show high MRP2, albumin, and CD31 expression levels. In addition, the in vivo study reveals that structured microtissue spheroids are stably engrafted. These results demonstrate that the method provides a valuable 3D structured microtissue spheroid model with lobule-like constructs and liver functions.

Modeling adaptive drug resistance of colorectal cancer and therapeutic interventions with tumor spheroids

Drug resistance is a major barrier against successful treatments of cancer patients. Various intrinsic mechanisms and adaptive responses of tumor cells to cancer drugs often lead to failure of treatments and tumor relapse. Understanding mechanisms of cancer drug resistance is critical to develop effective treatments with sustained anti-tumor effects. Three-dimensional cultures of cancer cells known as spheroids present a biologically relevant model of avascular tumors and have been increasingly incorporated in tumor biology and cancer drug discovery studies. In this review, we discuss several recent studies from our group that utilized colorectal tumor spheroids to investigate responses of cancer cells to cytotoxic and molecularly targeted drugs and uncover mechanisms of drug resistance. We highlight our findings from both short-term, one-time treatments and long-term, cyclic treatments of tumor spheroids and discuss mechanisms of adaptation of cancer cells to the treatments. Guided by mechanisms of resistance, we demonstrate the feasibility of designing specific drug combinations to effectively block growth and resistance of cancer cells in spheroid cultures. Finally, we conclude with our perspectives on the utility of three-dimensional tumor models and their shortcomings and advantages for phenotypic and mechanistic studies of cancer drug resistance.

Enhancing the Sensitivity of DNA and Aptamer Probes in the Dextran/PEG Aqueous Two-Phase System

Increasing the local concentration of DNA-based probes is a convenient way to improve the sensitivity of biosensors. Instead of using organic solvents or ionic liquids that phase-separate with water based on hydrophobic interactions, we herein studied a classic aqueous two-phase system (ATPS) comprising polyethylene glycol (PEG) and dextran. Polymers of higher molecular weights and higher concentrations favored phase separation. DNA oligonucleotides are selectively enriched in the dextran-rich phase unless the pH was increased to 12. A higher volume ratio of PEG-to-dextran and a higher concentration of PEG also enrich more DNA probes in the dextran-rich phase. The partition efficiency of the T₁₅ DNA was enriched around seven times in the dextran phase when the volume ratio of dextran and PEG reached 1:10. The detection of limit improved by 3.6-fold in a molecular beacon-based DNA detection system with the ATPS. The ATPS also increased the sensitivity for the detection of Hg²⁺ and adenosine triphosphate, although these target molecules alone distributed equally in the two phases. This work demonstrates a simple method using water soluble polymers to improve biosensors.

Electrically controllable cargo delivery with dextran-rich droplets

The controllable delivery of cargo is of great importance in many areas, ranging from medicine and materials science to food and cosmetic industries. To fulfil the requirements in different areas, the development of new methods for cargo delivery in a controllable manner is always essential. A novel technique of cargo delivery controlled by electric pulse was developed in this paper. In an aqueous two-phase system, the dextran-rich droplets were fabricated as droplet carriers in a continuous polyethylene glycol-rich phase. The loading and releasing of model cargos (polystyrene particles) across the surface of the droplet carriers under electric pulses were demonstrated in microfluidic chips. By controlling the amplitude of the applied electric pulses, the cargos with designed sizes were sorted and loaded into the droplet carriers; hence, the targeted delivery of cargos by size can be achieved. The exchange of cargos between droplet carriers under reversed electric pulses was also investigated, and the results indicated the flexibility of this method in cargo delivery. Moreover, possible application of this method to biological cargos was demonstrated by controlling the loading and releasing of yeast cells under electric pulses. With the advantages of easy operation and fast response, this approach provides a novel route for controllable cargo delivery with droplets.

Photosynthetic hydrogen production by droplet-based microbial micro-reactors under aerobic conditions

The spontaneous self-assembly of multicellular ensembles into living materials with synergistic structure and function remains a considerable challenge in biotechnology and synthetic biology. Here, we exploit the aqueous two-phase separation of dextran-in-PEG emulsion micro-droplets for the capture, spatial organization and immobilization of algal cells or algal/bacterial cell communities to produce discrete multicellular spheroids capable of both aerobic (oxygen producing) and hypoxic (hydrogen producing) photosynthesis in daylight under air. We show that localized oxygen depletion results in hydrogen production from the core of the algal microscale reactor, and demonstrate that enhanced levels of hydrogen evolution can be achieved synergistically by spontaneously enclosing the photosynthetic cells within a shell of bacterial cells undergoing aerobic respiration. Our results highlight a promising droplet-based environmentally benign approach to dispersible photosynthetic microbial micro-reactors comprising segregated cellular micro-niches with dual functionality, and provide a step towards photobiological hydrogen production under aerobic conditions.

The development of techniques capable of orchestrating the assembly of living cells into multicellular ensembles with synergistic and function is challenge. Here, the authors construct algal or algal/bacterial cells-based core shell-like structure based on aqueous two-phase system for synergic photosynthetic H₂ production.

Advanced biomedical applications based on emerging 3D cell culturing platforms

It is of great value to develop reliable in vitro models for cell biology and toxicology. However, ethical issues and the decreasing number of donors restrict the further use of traditional animal models in various fields, including the emerging fields of tissue engineering and regenerative medicine. The huge gap created by the restrictions in animal models has pushed the development of the increasingly recognized three-dimensional (3D) cell culture, which enables cells to closely simulate authentic cellular behaviour such as close cell-to-cell interactions and can achieve higher functionality. Furthermore, 3D cell culturing is superior to the traditional 2D cell culture, which has obvious limitations and cannot closely mimic the structure and architecture of tissues. In this study, we review several methods used to form 3D multicellular spheroids. The extracellular microenvironment of 3D spheroids plays a role in many aspects of biological sciences, including cell signalling, cell growth, cancer cell generation, and anti-cancer drugs. More recently, they have been explored as basic construction units for tissue and organ engineering. We review this field with a focus on the previous research in different areas using spheroid models, emphasizing aqueous two-phase system (ATPS)-based techniques. Multi-cellular spheroids have great potential in the study of biological systems and can closely mimic the in vivo environment. New technologies to form and analyse spheroids such as the aqueous two-phase system and magnetic levitation are rapidly overcoming the technical limitations of spheroids and expanding their applications in tissue engineering and regenerative medicine.

Reconstituting bacterial cell division assemblies in crowded, phase-separated media

Here we have summarized several strategies to reconstruct complexes containing the FtsZ protein, a central element of the cell division machinery in most bacteria, and to test their functional organization in minimal membrane systems and cell-like containers, as vesicles and droplets produced by microfluidics. These synthetic systems have been devised to mimic elements of the intracellular complexity, as excluded volume effects due to natural crowding, and macromolecular condensation resulting from biologically regulated liquid-liquid phase separation, in media of known and controllable composition. This integrative approach has allowed to demonstrate that macromolecular phase separation and crowding may also help to dynamically organize FtsZ in the intracellular space thus modulating its functional reactivity in cell division.

Quantitative Size-Based Analysis of Tumor Spheroids and Responses to Therapeutics

Drug resistance remains a major clinical problem despite advances in targeted therapies. In recent years, methods to culture cancer cells in three-dimensional (3D) environments to better mimic native tumors have gained increasing popularity. Nevertheless, unlike traditional two-dimensional (2D) cell cultures, analysis of 3D cultures is not straightforward. Most biochemical assays developed for 2D cultures have to be optimized for use with 3D cultures. We addressed this important problem by presenting a simple method of quantitative size-based analysis of growth and drug responses of 3D cultures of cancer cells as tumor spheroids. We used an aqueous two-phase system to form consistently sized tumor spheroids of colorectal cancer cells. Using spheroid images, we computed the size of spheroids over time and demonstrated that growth of spheroids from this analysis strongly correlates with that using a PrestoBlue biochemical assay optimized for 3D cultures. Next, we cyclically treated the tumor spheroids with a MEK inhibitor, trametinib, for 6-day periods with a recovery phase in between. This inhibitor was selected because of mutation of colon cancer cells in the MEK/ERK pathway. We used size measurements to evaluate the efficacy of trametinib and predict development of resistance of colon cancer cells during the cyclical treatment and recovery regimen. This size-based analysis closely matched the biochemical analysis of drug responses of spheroids. We performed molecular analysis and showed that resistance to trametinib emerged due to feedback activation of the PI3K/AKT signaling pathway. Therefore, we combined trametinib with a PI3K/AKT inhibitor, dactolisib, and demonstrated that size-based analysis of spheroids reliably allowed quantifying the effect of the combination treatment to prevent drug resistance. This study established that size measurements of spheroids can be used as a straightforward method for quantitative studies of drug responses of tumor spheroids and identifying drug combinations that block resistance.

Three-dimensional models of breast cancer-fibroblasts interactions

IMPACT STATEMENT

Tumor stroma plays an important role in progression of cancers to a fatal metastatic disease. Modern treatment strategies are considering targeting tumor stroma to improve outcomes for cancer patients. A current challenge to develop stroma-targeting therapeutics is the lack of preclinical physiologic tumor models. Animal models widely used in cancer research lack human stroma and are not amenable to screening of chemical compounds for cancer drug discovery. In this review, we outline in vitro three-dimensional tumor models that we have developed to study the interactions among cancer cells and stromal cells. We describe development of the tumor models in a modular fashion, from a spheroid model to a sophisticated organotypic model, and discuss the importance of using correct physiologic models to recapitulate tumor-stromal signaling. These biomimetic tumor models will facilitate understanding of tumor-stromal signaling biology and provide a scalable approach for testing and discovery of cancer drugs.

Cell-Inspired All-Aqueous Microfluidics: From Intracellular Liquid-Liquid Phase Separation toward Advanced Biomaterials

Living cells have evolved over billions of years to develop structural and functional complexity with numerous intracellular compartments that are formed due to liquid-liquid phase separation (LLPS). Discovery of the amazing and vital roles of cells in life has sparked tremendous efforts to investigate and replicate the intracellular LLPS. Among them, all-aqueous emulsions are a minimalistic liquid model that recapitulates the structural and functional features of membraneless organelles and protocells. Here, an emerging all-aqueous microfluidic technology derived from micrometer-scaled manipulation of LLPS is presented; the technology enables the state-of-art design of advanced biomaterials with exquisite structural proficiency and diversified biological functions. Moreover, a variety of emerging biomedical applications, including encapsulation and delivery of bioactive gradients, fabrication of artificial membraneless organelles, as well as printing and assembly of predesigned cell patterns and living tissues, are inspired by their cellular counterparts. Finally, the challenges and perspectives for further advancing the cell-inspired all-aqueous microfluidics toward a more powerful and versatile platform are discussed, particularly regarding new opportunities in multidisciplinary fundamental research and biomedical applications.

Nonspecific characteristics of macromolecules create specific effects in living cells

Recently, the important role of microphase separation in living cells has been attracting considerable interest in relation to cell organization and function. For example, many studies have focused on liquid-liquid phase separation (LLPS) as a very plausible mechanism for the presence of membraneless organelles. To confirm the role of phase separation in living cells, experimental studies on models and/or reconstructed systems are needed. In this short review, we discuss current paradigms of LLPS and provide some example "review data" to demonstrate particular points relating to the specific localization of biological macromolecules like DNAs and actin proteins with spontaneous domain formation in microdroplets emerging in an aqueous two-phase system (ATPS) (we use polyethylene glycol (PEG)/dextran (DEX)-a binary polymer solution). We also suggest that phase separation and transition may play basic roles in regulation of the biochemical reactivity of individual long genomic DNAs.

Phytochemicals inhibit migration of triple negative breast cancer cells by targeting kinase signaling

BACKGROUND

Cell migration and invasion are essential processes for metastatic dissemination of cancer cells. Significant progress has been made in developing new therapies against oncogenic signaling to eliminate cancer cells and shrink tumors. However, inherent heterogeneity and treatment-induced adaptation to drugs commonly enable subsets of cancer cells to survive therapy. In addition to local recurrence, these cells escape a primary tumor and migrate through the stroma to access the circulation and metastasize to different organs, leading to an incurable disease. As such, therapeutics that block migration and invasion of cancer cells may inhibit or reduce metastasis and significantly improve cancer therapy. This is particularly more important for cancers, such as triple negative breast cancer, that currently lack targeted drugs.

METHODS

We used cell migration, 3D invasion, zebrafish metastasis model, and phosphorylation analysis of 43 protein kinases in nine triple negative breast cancer (TNBC) cell lines to study effects of fisetin and quercetin on inhibition of TNBC cell migration, invasion, and metastasis.

RESULTS

Fisetin and quercetin were highly effective against migration of all nine TNBC cell lines with up to 76 and 74% inhibitory effects, respectively. In addition, treatments significantly reduced 3D invasion of highly motile TNBC cells from spheroids into a collagen matrix and their metastasis in vivo. Fisetin and quercetin commonly targeted different components and substrates of the oncogenic PI3K/AKT pathway and significantly reduced their activities. Additionally, both compounds disrupted activities of several protein kinases in MAPK and STAT pathways. We used molecular inhibitors specific to these signaling proteins to establish the migration-inhibitory role of the two phytochemicals against TNBC cells.

CONCLUSIONS

We established that fisetin and quercetin potently inhibit migration of metastatic TNBC cells by interfering with activities of oncogenic protein kinases in multiple pathways.

Emerging aqueous two-phase systems: from fundamentals of interfaces to biomedical applications

This review summarizes recent advances of aqueous two-phase systems (ATPSs), particularly their interfaces, with a focus on biomedical applications.

Aqueous Two-Phase System Formation in Small Droplets by Shirasu Porous Glass Membrane Emulsification Followed by Water Extraction

By utilizing water transport phenomena between two different water-in-oil (W/O) emulsion droplets through continuous oil phase, we developed a novel method of aqueous two-phase system (ATPS) formation in small droplets prepared by Shirasu porous glass (SPG) membrane emulsification technique. When we mixed W/O emulsion droplets containing poly(ethylene glycol) (PEG) and dextran (DEX) at concentrations below the threshold of the phase separation, with droplets containing other solutes at high concentrations, water extraction from the droplets containing PEG and DEX to those containing the other solutes occurred, owing to the osmotic pressure difference. This effect increased the concentrations of PEG and DEX in the droplets above the phase separation threshold. We demonstrated the feasibility of the preparation method by varying the pore sizes of the SPG membranes, the solutes, and their concentrations. Only when the concentration of the solute was high enough to extract sufficient amounts of water did the homogeneous disperse phase consisting of PEG and DEX in droplets turn into a PEG-rich phase and DEX-rich phase, showing ATPS. This result was irrespective of the solute itself and pore size of the SPG membrane. In particular, we successfully demonstrated monodisperse ATPS droplets with diameters of approximately 10 μm under a certain condition.

Synergistic Inhibition of Kinase Pathways Overcomes Resistance of Colorectal Cancer Spheroids to Cyclic Targeted Therapies

Cancer cells often adapt to single-agent treatments with chemotherapeutics. Activation of alternative survival pathways is a major mechanism of drug resistance. A potential approach to block this feedback signaling is using combination treatments of a pair of drugs, although toxicity has been a limiting factor. Preclinical tumor models to identify mechanisms of drug resistance and determine low but effective combination doses are critical to effectively suppress tumor growth with reduced toxicity to patients. Using our aqueous two-phase system microtechnology, we developed colorectal tumor spheroids in high-throughput and evaluated resistance of cancer cells to three mitogen-activated protein kinase inhibitors (MAPKi) in long-term cyclic treatments. Our quantitative analysis showed that the efficacy of MAPKi significantly reduced over time, leading to an increase in proliferation of HCT116 colorectal cancer cells and growth of spheroids. We established that resistance was due to feedback activation of PI3K/AKT/mTOR pathway. Using high-throughput, dose-dependent combinations of each MAPKi and a PI3K/mTOR inhibitor, we identified low-dose, synergistic combinations that blocked resistance to MAPKi and effectively suppressed the growth of colorectal tumor spheroids in long-term treatments. Our approach to study drug resistance offers the potential to determine high priority treatments to test in animal models.

Fabrication of Human Keratinocyte Cell Clusters for Skin Graft Applications by Templating Water-in-Water Pickering Emulsions

Most current methods for the preparation of tissue spheroids require complex materials, involve tedious physical steps and are generally not scalable. We report a novel alternative, which is both inexpensive and up-scalable, to produce large quantities of viable human keratinocyte cell clusters (clusteroids). The method is based on a two-phase aqueous system of incompatible polymers forming a stable water-in-water (w/w) emulsion, which enabled us to rapidly fabricate cell clusteroids from HaCaT cells. We used w/w Pickering emulsion from aqueous solutions of the polymers dextran (DEX) and polyethylene oxide (PEO) and a particle stabilizer based on whey protein (WP). The HaCaT cells clearly preferred to distribute into the DEX-rich phase and this property was utilized to encapsulate them in the water-in-water (DEX-in-PEO) emulsion drops then osmotically shrank to compress them into clusters. Prepared formulations of HaCaT keratinocyte clusteroids in alginate hydrogel were grown where the cells percolated to mimic 3D tissue. The HaCaT cell clusteroids grew faster in the alginate film compared to the individual cells formulated in the same matrix. This methodology could potentially be utilised in biomedical applications.

All-Aqueous Thin-Film-Flow-Induced Cell-Based Monolayers

Cells in vitro usually require a solid scaffold to attach and form two-dimensional monolayer structures. To obtain a substrate-free cell monolayer, long culture time and specific detaching procedures are required. In this study, a thin-film-flow-induced strategy is reported to overcome the challenges of assembling in vitro scaffold-free monolayered cell aggregates. The assembly is driven by a dewetting-like thin-film withdrawal along all-aqueous interfaces characterized by a low interfacial tension. The withdrawal process drives the cells adsorbed on the liquid film to aggregate and assemble into an organized and compact monolayer. This strategy is not limited to biological cells but also colloidal particles, as demonstrated by the assembly of hybrid cell-particle monolayers. The versatility offered by this approach suggests new opportunities in understanding early tissue formation and functionalizing cell monolayer aggregates by colloidal particles with customized functions.

Precision cell delivery in biphasic polymer systems enhances growth of keratinocytes in culture and promotes their attachment on acellular dermal matrices

Current approaches for precision deposition of cells are not optimized for moist environments or for substrates with complex surface topographic features, for example, the surface of dermal matrices and other biomaterials. To overcome these challenges, an approach is presented that utilizes cell confinement in phase-separating polymer solutions of polyethylene glycol and dextran to precisely deliver keratinocytes in well-defined colonies. Using this approach, keratinocyte colonies are produced with superior viability, proliferative capacity, and barrier formation compared with the same number of cells dispersedly seeded across substrate surfaces. It is further demonstrated that keratinocytes delivered in colonies to the surface of acellular dermal matrices form an intact epidermal basal layer more rapidly and more completely than cells delivered by conventional dispersed seeding. These findings demonstrate that delivery of keratinocytes in phase-separating polymer solutions holds potential for enhancing growth of keratinocytes in culture and production of functional skin equivalents.

Aqueous/Aqueous Micro Phase Separation: Construction of an Artificial Model of Cellular Assembly

To artificially construct a three-dimensional cell assembly, we investigated the availability of long-duration microdroplets that emerged near a critical point in an aqueous two-phase system (ATPS) with the hydrophilic binary polymers, polyethylene glycol (PEG), and dextran (DEX), as host containers. We found that erythrocytes (horse red blood cells; RBCs) and NAMRU mouse mammary gland epithelial cells (NMuMG cells) were completely and spontaneously entrapped inside DEX-rich microdroplets. RBCs and NMuMG cells were located in the interior and at the periphery of the droplets at PEG/DEX = 5%:5%. In contrast, the cells exhibited opposite localizations at PEG/DEX = 10%:5%, where, interestingly, NMuMG cells apparently assembled to achieve cell adhesion. We simply interpreted such specific localizations by considering the alternative responses of these cells to the properties of the PEG/DEX interfaces with different gradients in polymer concentrations.

Extraction and purification of isochlorogenic acid C from Chrysanthemum morifolium using ionic liquid-based ultrasound-assisted extraction and aqueous two-phase system

Ionic liquid-based ultrasonic-assisted extraction (IL-UAE) was developed to extract and separate the isochlorogenic acid C (ICGA) from a cultivar of Chrysanthemum morifolium (Chrysanthemummorifolium Ra Tnat.). The influencing parameters, including IL concentration, liquid-to-solid ratio, and ultrasonic time, were optimized using response surface methodology. Of the ILs studied, 1-butyl-3-methylimidazolium bromide [(Bmim)Br] exhibited the best extraction ability. The optimized conditions included liquid-to-solid ratio of 23.44:1, ultrasonic time of 48.99 min, and IL concentration of 0.65 mol/L. Under the optimal conditions, the extraction yield of ICGA could reach to 4.20 mg/g. An aqueous two-phase system was applied for purification and separation of ICGA. The maximum extraction efficiency of 98.18% was obtained under the conditions of (NH 4)2 SO 4 of 4.5 g, pH of 3.0, and a temperature of 20°C at aqueous solution. Furthermore, the thermodynamic parameters showed that the purification of ICGA from salt-rich phase to IL-rich phase was a spontaneous and exothermic process. The results indicated that the proposed system is simple, rapid, and effective to serve as a viable and sustainable platform for the extraction and purification of ICGA from Chrysanthemum morifolium flowers.

Collagen Partition in Polymeric Aqueous Two-Phase Systems for Tissue Engineering

Aqueous two-phase systems (ATPS) of polyethylene glycol (PEG) and dextran (DEX) are commonly used to partition proteins. Protein partition in ATPS is a complex phenomenon and depends on factors including molecular weight of polymers, and electrochemical and ionic properties of the phases. We studied the effect of polymer molecular weight on the partition of a natural protein, collagen, in several ATPS formulations made with non-ionic polymers polyethylene glycol (PEG) and dextran (DEX). We found that partitioning of collagen to an aqueous phase significantly increases when the molecular weight of the corresponding phase polymer decreases. Additionally, a large difference between the molecular weight of the phase-forming polymers was necessary to cause a significant uneven collagen distribution between the aqueous phases. We then employed one of the systems to create a three-dimensional breast cancer microtissue by entrapping a spheroid of breast cancer cells within the partitioned collagen. This convenient technique to generate 3D microtissues offers a convenient and promising approach for tissue engineering applications.

Stem cell colony interspacing effect on differentiation to neural cells

Efforts to enhance the efficiency of neural differentiation of stem cells are primarily focused on exogenous modulation of physical niche parameters such as surface topography and extracellular matrix proteins, or addition of certain growth factors or small molecules to culture media. We report a novel neurogenic niche to enhance the neural differentiation of embryonic stem cells (ESCs) without any external intervention by micropatterning ESCs into spatially organized colonies of controlled size and interspacing. Using an aqueous two-phase system cell microprinting technology, we generated pairs of uniformly sized isolated ESC colonies at defined interspacing distances over a layer of differentiation-inducing stromal cells. Our comprehensive analysis of temporal expression of neural genes and proteins of cells in colony pairs showed that interspacing two colonies at approximately 0.66 times the colony diameter (0.66D) significantly enhanced neural differentiation of ESCs. Cells in these colonies displayed higher expression of neural genes and proteins and formed thick neurite bundles between the two colonies. A computational model of spatial distribution of soluble factors of cells in interspaced colony pairs showed that the enhanced neural differentiation is due to the presence of stable concentration gradients of soluble signalling factors between the two colonies. Our results indicate that culturing ESCs in colony pairs with defined interspacing is a promising approach to efficiently derive neural cells. Additionally, this approach provides a platform for quantitative studies of molecular mechanisms that regulate neurogenesis of stem cells.

Emerging Biotechnology Applications of Aqueous Two-Phase Systems

Liquid-liquid phase separation between aqueous solutions containing two incompatible polymers, a polymer and a salt, or a polymer and a surfactant, has been exploited for a wide variety of biotechnology applications throughout the years. While many applications for aqueous two-phase systems fall within the realm of separation science, the ability to partition many different materials within these systems, coupled with recent advances in materials science and liquid handling, has allowed bioengineers to imagine new applications. This progress report provides an overview of the history and key properties of aqueous two-phase systems to lend context to how these materials have progressed to modern applications such as cellular micropatterning and bioprinting, high-throughput 3D tissue assembly, microscale biomolecular assay development, facilitation of cell separation and microcapsule production using microfluidic devices, and synthetic biology. Future directions and present limitations and design considerations of this adaptable and promising toolkit for biomolecule and cellular manipulation are further evaluated.

Microprinted Stem Cell Niches Reveal Compounding Effect of Colony Size on Stromal Cells-Mediated Neural Differentiation

Microenvironmental factors have a major impact on differentiation of embryonic stem cells (ESCs). Here, a novel phenomenon that size of ESC colonies has a significant regulatory role on stromal cells induced differentiation of ESCs to neural cells is reported. Using a robotic cell microprinting technology, defined densities of ESCs are confined within aqueous nanodrops over a layer of supporting stromal cells immersed in a second, immiscible aqueous phase to generate ESC colonies of defined sizes. Temporal protein and gene expression studies demonstrate that larger ESC colonies generate disproportionally more neural cells and longer neurite processes. Unlike previous studies that attribute neural differentiation of ESCs solely to interactions with stromal cells, it is found that increased intercellular signaling of ESCs significantly enhances neural differentiation. This study offers an approach to generate neural cells with improved efficiency for potential use in translational research.

Biomaterials-Based Approaches to Tumor Spheroid and Organoid Modeling

Evolving understanding of structural and biological complexity of tumors has stimulated development of physiologically relevant tumor models for cancer research and drug discovery. A major motivation for developing new tumor models is to recreate the 3D environment of tumors and context-mediated functional regulation of cancer cells. Such models overcome many limitations of standard monolayer cancer cell cultures. Under defined culture conditions, cancer cells self-assemble into 3D constructs known as spheroids. Additionally, cancer cells may recapitulate steps in embryonic development to self-organize into 3D cultures known as organoids. Importantly, spheroids and organoids reproduce morphology and biologic properties of tumors, providing valuable new tools for research, drug discovery, and precision medicine in cancer. This Progress Report discusses uses of both natural and synthetic biomaterials to culture cancer cells as spheroids or organoids, specifically highlighting studies that demonstrate how these models recapitulate key properties of native tumors. The report concludes with the perspectives on the utility of these models and areas of need for future developments to more closely mimic pathologic events in tumors.

Enrichment and Recovery of Mammalian Cells from Contaminated Cultures Using Aqueous Two-Phase Systems

This Article describes a density-based method for removing contaminants, including microorganisms and nonviable cells, from mammalian cell cultures using an aqueous two-phase system (ATPS). The properties of a 7% w/w polyethylene glycol (PEG)-11% w/w Ficoll ATPS can be tuned to prepare a biocompatible system that removes contaminants with little to no adverse effects on the viability or growth of the cultured cells after treatment. This system can be used to enrich cell culture populations for viable cells and to reduce the number of microorganism contaminants in a culture, which increases the chances of subsequent antibiotic treatments being successful. We test the effectiveness of our method in model contaminated cultures of both adherent (HeLa) and suspension (HL-60 II) mammalian cells contaminated with bacteria (E. coli) and yeast (S. cerevisiae). An average of 70.2 ± 4.6% of HeLa cells added to the system are subsequently recovered, and 55.9 ± 2.1% of HL-60 II cells are recovered. After sedimenting to the interface of the ATPS, these cells have an average viability of 98.0 ± 0.2% and 95.3 ± 2.2%, respectively. By removing unwanted cells, desired cell populations can be recovered, and cultures that would otherwise need to be discarded can continue to be used.

Three-dimensional tumor model mimics stromal - breast cancer cells signaling

Tumor stroma is a major contributor to the biological aggressiveness of cancer cells. Cancer cells induce activation of normal fibroblasts to carcinoma-associated fibroblasts (CAFs), which promote survival, proliferation, metastasis, and drug resistance of cancer cells. A better understanding of these interactions could lead to new, targeted therapies for cancers with limited treatment options, such as triple negative breast cancer (TNBC). To overcome limitations of standard monolayer cell cultures and xenograft models that lack tumor complexity and/or human stroma, we have developed a high throughput tumor spheroid technology utilizing a polymeric aqueous two-phase system to conveniently model interactions of CAFs and TNBC cells and quantify effects on signaling and drug resistance of cancer cells. We focused on signaling by chemokine CXCL12, a hallmark molecule secreted by CAFs, and receptor CXCR4, a driver of tumor progression and metastasis in TNBC. Using three-dimensional stromal-TNBC cells cultures, we demonstrate that CXCL12 – CXCR4 signaling significantly increases growth of TNBC cells and drug resistance through activation of mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways. Despite resistance to standard chemotherapy, upregulation of MAPK and PI3K signaling sensitizes TNBC cells in co-culture spheroids to specific inhibitors of these kinase pathways. Furthermore, disrupting CXCL12 – CXCR4 signaling diminishes drug resistance of TNBC cells in co-culture spheroid models. This work illustrates the capability to identify mechanisms of drug resistance and overcome them using our engineered model of tumor-stromal interactions.

All-Aqueous Assemblies via Interfacial Complexation: Toward Artificial Cell and Microniche Development

In nature, the environment surrounding biomolecules and living cells can dictate their structure, function, and properties. Confinement is a key means to define and regulate such environments. For example, the confinement of appropriate constituents in compartments facilitates the assembly, dynamics, and function of biochemical machineries as well as subcellular organelles. Membraneless organelles, in particular, are thought to form via thermodynamic cues defined within the interior space of cells. On larger length scales, the confinement of living cells dictates cellular function for both mammalian and bacterial cells. One promising class of artificial structures that can recapitulate these multiscale confinement effects is based on aqueous two-phase systems (ATPSs). This feature article highlights recent developments in the production and stabilization of ATPS-droplet-based systems, with a focus on interfacial complexation. These systems enable structure formation, modulation, and triggered (dis)assembly, thereby allowing structures to be tailored to fit the desired function and designed for particular confinement studies. Open issues for both synthetic cells and niche studies are identified.

Microprinted tumor spheroids enable anti-cancer drug screening

Spheroids present a biologically relevant model of avascular tumors and a unique tool for discovery of anti-cancer drugs. Despite being used in research laboratories for several decades, spheroids are not routinely used for drug discovery primarily due to the difficulty of mass-producing uniformly-sized spheroids and intense labor involved in handling, drug treatment, and analyzing them. We overcome this barrier using a novel technology to robotically microprint spheroids in standard 384-well plates. An aqueous drop containing cancer cells is dispensed into a bath of a second, immiscible aqueous phase. The drop maintains cells in close proximity to aggregate into a single spheroid. Using U-87 MG brain cancer cells, we show that this approach produces spheroids of well-defined size with ~10% deviation from their mean diameter. We demonstrate the feasibility of robotic, high throughput compound screening against tumor spheroids using a collection of 25 standard chemotherapeutics and molecular inhibitors against U-87 MG spheroids. Each drug is used in a wide range of concentrations. Viability of cancer cells in drug-treated spheroids is measured using a PrestoBlue assay. Morphological changes are used as a secondary measure for analysis of drug effect. We identify several compounds that effectively inhibit growth of spheroids. To generate a scoring system for effectiveness of drugs, we use half-maximum inhibitory concentration (IC50), maximum inhibition (Emax), and area under the dose-response curve (AUC) to present a multi-parametric approach that takes into account both potency and efficacy of drugs. Our robotic technology offers a low cost and convenient platform for screening large collections of chemical compounds against realistic tumor models prior to expensive and tedious in vivo tests, dramatically improving testing throughput and efficiency, and reducing costs.

Self-regulatory factors of embryonic stem cells in co-culture with stromal cells enhance neural differentiation

Embryonic stem cells (ESCs), due to their intrinsic capability to generate somatic cells of all three germ layers, are potential sources of neural cells for cell replacement therapies. However, the empirical differentiation protocols and the lack of mechanistic understanding of the neural differentiation of ESCs have limited the utility of ESCs as a developmental model or as a cell source for neural cell populations for replacement therapies. Co-culturing ESCs with stromal cells is one of the extensively used methods to induce neural differentiation. Despite several studies to identify neural inducing factors in stromal cell induced neural differentiation, the self-regulatory effects of ESCs in the neural differentiation process remain unexplored. For the first time, we elucidate the self-regulatory role of mESCs in their neural cell differentiation by supplementing conditioned media from differentiating mESCs to mESC-PA6 co-cultures and quantitatively evaluating the change in neural differentiation. Moreover, we use statistical tools to analyze the expression of various growth and trophic factors and distinguish the factors produced primarily by PA6 cells versus mESCs in co-culture. We observe that addition of the medium containing mESC-secreted factors to a single mESC colony co-cultured with PA6 cells significantly enhances the neural differentiation of mESCs compares to the medium extracted from the stromal cells only. Hierarchical clustering of gene expression data from PA6 and co-cultured mESCs segregates two groups of factors that are produced by the stromal cells and differentiating mESCs. Identifying the major soluble factors that drive and regulate the neural differentiation process in the mESC-PA6 co-culture niche will help understand molecular mechanisms of neural development. Moreover, it can be a major step toward developing novel protocols to differentiate stem cells with mESC derived factor supplementation without using feeder cells and with greater efficiency compared to existing approaches.

Single and Combination Drug Screening with Aqueous Biphasic Tumor Spheroids

Spheroids of cancer cells represent a physiologic model of solid tumors for cancer drug screening. Despite this known benefit, difficulties with generating large quantities of uniformly sized spheroids in standard plates, individually addressing spheroids with drug compounds, and quantitatively analyzing responses of cancer cells have hindered the use of spheroids in high-throughput screening applications. Recently, we addressed this challenge by using an aqueous two-phase system technology to generate a spheroid within an aqueous drop immersed in a second, immiscible aqueous phase. Integrating this approach with robotics resulted in convenient formation, maintenance, and drug treatment of spheroids. Here, we demonstrate the feasibility of high-throughput compound screening against colon cancer spheroids using 25 anticancer compounds. Using a strictly standardized mean difference and based on a preliminary testing with each compound, we select effective compounds for further dose-response testing. Finally, we use molecular inhibitors to target upregulated protein kinases and use them for drug combination studies against spheroids. We quantitatively analyze the combination treatment results using statistical metrics to identify synergy between pairs of inhibitors in compromising viability of colon cancer cells. This study demonstrates the utility of our spheroid culture technology for identification of effective drug compounds, dose-response analysis, and combination drug treatments.

Colony size effect on neural differentiation of embryonic stem cells microprinted on stromal cells

Controlling cellular microenvironment to induce neural differentiation of embryonic stem cells (ESCs) remains a major challenge. We address this need by introducing a micro-engineered co-culture system that resembles embryonic development in terms of direct intercellular interactions and induces neural differentiation of ESCs. A polymeric aqueous two-phase system (ATPS)-mediated robotic microprinting technology allows precise localization of mouse ESCs (mESCs) over a layer of supporting stromal cells. mESCs proliferate over a 2-week culture period into a single colony of defined size. Physical and chemical cues from the stromal cells guide mESCs to differentiate toward specific neural lineages. We generated mESC colonies of three different sizes from 100, 250 and 500 single cells and showed that size of mESC colonies is an important factor determining the yield of neural cells. Expression of early neural cell markers nestin denoting neural stem cells, NCAM specifying neural progenitors, and β-III tubulin (TuJ) indicating post mitotic neurons escalated from day 4. Differentiation into specific neural cells astrocytes marked by GFAP, oligodendrocytes indicated by CNPase, and TH-positive dopaminergic neurons was observed during the second week of culture. Unexpectedly, analysis of protein expression revealed a disproportionate increase in neural differentiation of mESCs by increase in the colony size. For the first time, our study establishes colony size as an important regulator of fate of ESCs in this heterocellular niche. This approach of deriving neural cells may make a major impact on stem cell research for treating neurodegenerative diseases.