Oxygen-sensing scaffolds for 3-dimensional cell and tissue culture

Balance between the cell viability and death in 3D

Cell death is a phenomenon, frequently perceived as an absolute event for cell, tissue and the organ. However, the rising popularity and complexity of such 3D multicellular 'tissue building blocks' as heterocellular spheroids, organoids, and 'assembloids' prompts to revise the definition and quantification of cell viability and death. It raises several questions on the overall viability of all the cells within 3D volume and on choosing the appropriate, continuous, and non-destructive viability assay enabling for a single-cell analysis. In this review, we look at cell viability and cell death modalities with attention to the intrinsic features of such 3D models as spheroids, organoids, and bioprints. Furthermore, we look at emerging and promising methodologies, which can help define and understand the balance between cell viability and death in dynamic and complex 3D environments. We conclude that the recent innovations in biofabrication, biosensor probe development, and fluorescence microscopy can help answer these questions.

Review on Bioinspired Design of ECM-Mimicking Scaffolds by Computer-Aided Assembly of Cell-Free and Cell Laden Micro-Modules

Tissue engineering needs bioactive drug delivery scaffolds capable of guiding cell biosynthesis and tissue morphogenesis in three dimensions. Several strategies have been developed to design and fabricate ECM-mimicking scaffolds suitable for directing in vitro cell/scaffold interaction, and controlling tissue morphogenesis in vivo. Among these strategies, emerging computer aided design and manufacturing processes, such as modular tissue unit patterning, promise to provide unprecedented control over the generation of biologically and biomechanically competent tissue analogues. This review discusses recent studies and highlights the role of scaffold microstructural properties and their drug release capability in cell fate control and tissue morphogenesis. Furthermore, the work highlights recent advances in the bottom-up fabrication of porous scaffolds and hybrid constructs through the computer-aided assembly of cell-free and/or cell-laden micro-modules. The advantages, current limitations, and future challenges of these strategies are described and discussed.

O2-sensitive microcavity arrays: A new platform for oxygen measurements in 3D cell cultures

Oxygen concentration plays a crucial role in (3D) cell culture. However, the oxygen content in vitro is usually not comparable to the in vivo situation, which is partly due to the fact that most experiments are performed under ambient atmosphere supplemented with 5% CO₂, which can lead to hyperoxia. Cultivation under physiological conditions is necessary, but also fails to have suitable measurement methods, especially in 3D cell culture. Current oxygen measurement methods rely on global oxygen measurements (dish or well) and can only be performed in 2D cultures. In this paper, we describe a system that allows the determination of oxygen in 3D cell culture, especially in the microenvironment of single spheroids/organoids. For this purpose, microthermoforming was used to generate microcavity arrays from oxygen-sensitive polymer films. In these oxygen-sensitive microcavity arrays (sensor arrays), spheroids cannot only be generated but also cultivated further. In initial experiments we could show that the system is able to perform mitochondrial stress tests in spheroid cultures to characterize mitochondrial respiration in 3D. Thus, with the help of sensor arrays, it is possible to determine oxygen label-free and in real-time in the immediate microenvironment of spheroid cultures for the first time.

Oxygen Measurement in Microdevices

Oxygen plays a fundamental role in respiration and metabolism, and quantifying oxygen levels is essential in many environmental, industrial, and research settings. Microdevices facilitate the study of dynamic, oxygen-dependent effects in real time. This review is organized around the key needs for oxygen measurement in microdevices, including integrability into microfabricated systems; sensor dynamic range and sensitivity; spatially resolved measurements to map oxygen over two- or three-dimensional regions of interest; and compatibility with multimodal and multianalyte measurements. After a brief overview of biological readouts of oxygen, followed by oxygen sensor types that have been implemented in microscale devices and sensing mechanisms, this review presents select recent applications in organs-on-chip in vitro models and new sensor capabilities enabling oxygen microscopy, bioprocess manufacturing, and pharmaceutical industries. With the advancement of multiplexed, interconnected sensors and instruments and integration with industry workflows, intelligent microdevice-sensor systems including oxygen sensors will have further impact in environmental science, manufacturing, and medicine.

Intracellular and Intravascular Oxygen Sensing of Pancreatic Tissues Based on Phosphorescence Lifetime Imaging Microscopy Using Lipophilic and Hydrophilic Iridium(III) Complexes

Simultaneous imaging of intracellular and blood oxygen levels in tissues provides valuable information on the dynamic behavior of molecular oxygen (O₂) in normal and diseased tissues. Here, to achieve this goal, we developed green-emitting intracellular O₂ probes based on the Ir(III) complex, PPY (tris(2-phenylpyridinato)iridium(III)), and investigated the possibility of multicolor O₂ imaging by co-staining tissues with a red-emitting intravascular probe BTP-PEG₄₈. The newly synthesized complexes possess modified 2-phenylpyridinato ligand(s) with a cationic or hydrophilic substituent, such as a dimethylamino group, triphenylphosphonium cation, or hydroxy group, in order to enhance cellular uptake efficiency. The photophysical and cellular properties of these complexes were systematically investigated to evaluate their ability as O₂ probes. Among these complexes, PPYDM and PPY2OH, which have a dimethylamino group and two hydroxy groups, respectively, exhibited much higher cellular uptake efficiencies compared with PPY and showed high O₂ sensitivity in HeLa cells. Phosphorescence lifetime imaging microscopy (PLIM) measurements of HeLa cells co-stained with PPYDM and hydrophilic BTP-PEG₄₈ allowed for the evaluation of intracellular and extracellular O₂ levels in cell culture. We took PLIM images of the pancreas following intravenous administration of PPYDM and BTP-PEG₄₈ into anesthetized mice. The PLIM measurements using these probes allowed simultaneous O₂ imaging of acinar cells and capillaries in the pancreas with cellular-level resolution. From the phosphorescence lifetimes of PPYDM and BTP-PEG₄₈ and the calibration curves evaluated in rat pancreatic acinar cells and blood plasma, we found that the average oxygen partial pressures of acinar cells and capillaries were almost equal at about 30 mmHg.

3D two-photon polymerization of smart cell gelatin - collagen matrixes with incorporated ruthenium complexes for the monitoring of local oxygen tensions

The extra cellular matrix plays a major role in the biomechanical properties of tissues that impact cell behavior and fate. It is therefore crucial to mimic these complex cell-matrix interactions in 3D cell cultures. Here, two-photon polymerization is applied to produce gelatin methacryloyl (GelMA) - collagen matrixes that further enable local pO_2matrix measurement, when ruthenium complexes are used as photo-activators. The fluorescence intensity of these complexes has a direct and inverse relationship with the local pO_2matrix. The 3D structures reached their maximum size in cell culture conditions after 3H with a swelling factor of ~1.5. Their shape and the ruthenium fluorescence intensity of the alveoli walls stayed constant for at least 2 weeks in the absence of cells. They were used in time series to monitor the local pO_2matrix adjacent to cancer cells during their division, migration and the formation of a tumor tissue mass. At the presence of these cell activities that consume O₂, a significant ~3-fold increase of the ruthenium fluorescence intensity in the alveoli walls was observed. This study demonstrates that online monitoring of the local pO_2matrix is possible. The ruthenium complexes provide the bio-optical sensors that are useful for further analysis of cancer and healthy cell energy metabolism in a 3D matrix that better mimics in vivo conditions and migration paths. Unraveling the cancer cell metabolic adaptations in a changing micro-environment will help the development of new therapeutic opportunities. STATEMENT OF SIGNIFICANCE: In 3D cell cultures, monitoring pericellular pO₂ is as critical as controlling pH. This facility is currently missing. Here, we take advantage of the direct and inverse relationship between pO₂ and the fluorescence intensity of ruthenium complexes to generate stable gelatin-collagen matrixes able to continuously monitoring the pO₂ at the pericellular level. The ruthenium complexes, which are photo-activators in the two-photon polymerization of these matrixes, became covalently bind to the collagen fibers. Indeed, local O₂ consumption by cancer cells during migration, mitosis and tumor mass formation caused a 3-fold increase of the ruthenium fluorescence. In the future, incorporating ruthenium complexes with other bio-optical sensors will create new drug screening platforms that monitor cell culture parameters at the pericellular level.

A novel standalone microfluidic device for local control of oxygen tension for intestinal-bacteria interactions

The intestinal environment is unique because it supports the intestinal epithelial cells under a normal oxygen environment and the microbiota under an anoxic environment. Due to importance of understanding the interactions between the epithelium and the microbiota, there is a strong need for developing representative and simple experimental models. Current approaches do not capture the partitioned oxygen environment, require external anaerobic chambers, or are complex. Another major limitation is that with the solutions that can mimic this oxygen environment, the oxygenation level of the epithelial cells is not known, raising the question whether the cells are hypoxic or not. We report standalone microfluidic devices that form a partitioned oxygen environment without the use of an external anaerobic chamber or oxygen scavengers to coculture intestinal epithelial and bacterial cells. By changing the thickness of the device cover, the oxygen tension in the chamber was modulated. We verified the oxygen levels using several tests: microscale oxygen sensitive sensors which were integrated within the devices, immunostaining of Caco-2 cells to determine hypoxia levels, and genetically encoded bacteria to visualize the growth. Collectively, these methods monitored oxygen concentrations in the devices more comprehensively than previous reports and allowed for control of oxygen tension to match the requirements of both intestinal cells and anaerobic bacteria. Our experimental model is supported by the mathematical model that considered diffusion of oxygen into the top chamber. This allowed us to experimentally determine the oxygen consumption rate of the intestinal epithelial cells under perfusion.

Luminescence lifetime imaging of three-dimensional biological objects

A major focus of current biological studies is to fill the knowledge gaps between cell, tissue and organism scales. To this end, a wide array of contemporary optical analytical tools enable multiparameter quantitative imaging of live and fixed cells, three-dimensional (3D) systems, tissues, organs and organisms in the context of their complex spatiotemporal biological and molecular features. In particular, the modalities of luminescence lifetime imaging, comprising fluorescence lifetime imaging (FLI) and phosphorescence lifetime imaging microscopy (PLIM), in synergy with Förster resonance energy transfer (FRET) assays, provide a wealth of information. On the application side, the luminescence lifetime of endogenous molecules inside cells and tissues, overexpressed fluorescent protein fusion biosensor constructs or probes delivered externally provide molecular insights at multiple scales into protein-protein interaction networks, cellular metabolism, dynamics of molecular oxygen and hypoxia, physiologically important ions, and other physical and physiological parameters. Luminescence lifetime imaging offers a unique window into the physiological and structural environment of cells and tissues, enabling a new level of functional and molecular analysis in addition to providing 3D spatially resolved and longitudinal measurements that can range from microscopic to macroscopic scale. We provide an overview of luminescence lifetime imaging and summarize key biological applications from cells and tissues to organisms.

Electrochemical Sensing in 3D Cell Culture Models: New Tools for Developing Better Cancer Diagnostics and Treatments

Currently, conventional pre-clinical in vitro studies are primarily based on two-dimensional (2D) cell culture models, which are usually limited in mimicking the real three-dimensional (3D) physiological conditions, cell heterogeneity, cell to cell interaction, and extracellular matrix (ECM) present in living tissues. Traditionally, animal models are used to mimic the 3D environment of tissues and organs, but they suffer from high costs, are time consuming, bring up ethical concerns, and still present many differences when compared to the human body. The applications of microfluidic-based 3D cell culture models are advantageous and useful as they include 3D multicellular model systems (MCMS). These models have demonstrated potential to simulate the in vivo 3D microenvironment with relatively low cost and high throughput. The incorporation of monitoring capabilities in the MCMS has also been explored to evaluate in real time biophysical and chemical parameters of the system, for example temperature, oxygen, pH, and metabolites. Electrochemical sensing is considered as one of the most sensitive and commercially adapted technologies for bio-sensing applications. Amalgamation of electrochemical biosensing with cell culture in microfluidic devices with improved sensitivity and performance are the future of 3D systems. Particularly in cancer, such models with integrated sensing capabilities can be crucial to assess the multiple parameters involved in tumour formation, proliferation, and invasion. In this review, we are focusing on existing 3D cell culture systems with integrated electrochemical sensing for potential applications in cancer models to advance diagnosis and treatment. We discuss their design, sensing principle, and application in the biomedical area to understand the potential relevance of miniaturized electrochemical hybrid systems for the next generation of diagnostic platforms for precision medicine.

Improving the colonization and functions of Wharton's Jelly-derived mesenchymal stem cells by a synergetic combination of porous polyurethane scaffold with an albumin-derived hydrogel

The development of neo-tissues assisted by artificial scaffolds is continually progressing, but the reproduction of the extracellular environment surrounding cells is quite complex. While synthetic scaffolds can support cell growth, they lack biochemical cues that can prompt cell proliferation or differentiation. In this study, Wharton's Jelly-derived mesenchymal stem cells are seeded on a polyurethane (PU) scaffold combined with a hydrogel based on bovine serum albumin (BSA). BSA hydrogel is obtained through thermal treatment. While such treatment leads to partial unfolding of the protein, we show that the extent of denaturation is small enough to maintain its bioactivity, such as protein binding. Therefore, BSA provides a suitable playground for cells inside the scaffold, allowing higher spreading, proliferation and matrix secretions. Furthermore, the poor mechanical properties of the hydrogel are compensated for by the porous PU scaffold, whose architecture is well controlled. We show that even though PU by itself can allow cell adhesion and protein secretion, cell proliferation is 3.5 times higher in the PU + BSA scaffolds as compared to pure PU after 21 d, along with the non-collagenous protein secretions (389 versus 134 μmmg ^-1). Conversely, the secretion of sulphated glycosaminoglycans is 12.3-fold higher in the scaffold made solely of PU. Thereby, we propose a simple approach to generating a hybrid material composed of a combination of PU and BSA hydrogel as a promising scaffold for tissue regeneration.

Multiparametric Optical Bioimaging Reveals the Fate of Epoxy Crosslinked Biomeshes in the Mouse Subcutaneous Implantation Model

Biomeshes based on decellularized bovine pericardium (DBP) are widely used in reconstructive surgery due to their wide availability and the attractive biomechanical properties. However, their efficacy in clinical applications is often affected by the uncontrolled immunogenicity and proteolytic degradation. To address this issue, we present here in vivo multiparametric imaging analysis of epoxy crosslinked DBPs to reveal their fate after implantation. We first analyzed the structure of the crosslinked DBP using scanning electron microscopy and evaluated proteolytic stability and cytotoxicity. Next, using combination of fluorescence and hypoxia imaging, X-ray computed microtomography and histology techniques we studied the fate of DBPs after subcutaneous implantation in animals. Our approach revealed high resistance to biodegradation, gradual remodeling of a surrounding tissue forming the connective tissue capsule and calcification of crosslinked DBPs. These changes were concomitant to the development of hypoxia in the samples within 3 weeks after implantation and subsequent induction of angiogenesis and vascularization. Collectively, presented approach provides new insights on the transplantation of the epoxy crosslinked biomeshes, the risks associated with its applications in soft-tissue reconstruction and can be transferred to studies of other types of implants.

Three-Dimensional (3D) cell culture monitoring: Opportunities and challenges for impedance spectroscopy

Three-dimensional (3D) cell culture has developed rapidly over the past 5-10 years with the goal of better replicating human physiology and tissue complexity in the laboratory. Quantifying cellular responses is fundamental in understanding how cells and tissues respond during their growth cycle and in response to external stimuli. There is a need to develop and validate tools that can give insight into cell number, viability, and distribution in real-time, nondestructively and without the use of stains or other labelling processes. Impedance spectroscopy can address all of these challenges and is currently used both commercially and in academic laboratories to measure cellular processes in 2D cell culture systems. However, its use in 3D cultures is not straight forward due to the complexity of the electrical circuit model of 3D tissues. In addition, there are challenges in the design and integration of electrodes within 3D cell culture systems. Researchers have used a range of strategies to implement impedance spectroscopy in 3D systems. This review examines electrode design, integration, and outcomes of a range of impedance spectroscopy studies and multiparametric systems relevant to 3D cell cultures. While these systems provide whole culture data, impedance tomography approaches have shown how this technique can be used to achieve spatial resolution. This review demonstrates how impedance spectroscopy and tomography can be used to provide real-time sensing in 3D cell cultures, but challenges remain in integrating electrodes without affecting cell culture functionality. If these challenges can be addressed and more realistic electrical models for 3D tissues developed, the implementation of impedance-based systems will be able to provide real-time, quantitative tracking of 3D cell culture systems.

Visualization of Stem Cell Niche by Fluorescence Lifetime Imaging Microscopy

Fluorescence lifetime imaging microscopy (FLIM), enabling live quantitative multiparametric analyses, is an emerging bioimaging approach in tissue engineering and regenerative medicine. When combined with stem cell-derived intestinal organoid models, FLIM allows for tracing stem cells and monitoring of their proliferation, metabolic fluxes, and oxygenation. It is compatible with the use of live Matrigel-grown intestinal organoids produced from primary adult stem cells, crypts, and transgenic Lgr5-GFP mice. In this chapter we summarize available experimental protocols, imaging platforms (one- and two-photon excited FLIM, phosphorescence lifetime imaging microscopy (PLIM)) and provide the anticipated data for FLIM imaging of the live intestinal organoids, focusing on labeling of cell proliferation, its colocalization with the stem cell niche, measured local oxygenation, autofluorescence, and some other parameters. The protocol is illustrated with examples of multiparameter imaging, employing spectral and "time domain"-based separation of dyes, probes, and assays.

Electrospun Fiber Mesh for High-Resolution Measurements of Oxygen Tension in Cranial Bone Defect Repair

Tissue oxygenation is one of the key determining factors in bone repair and bone tissue engineering. Adequate tissue oxygenation is essential for survival and differentiation of the bone-forming cells and ultimately the success of bone tissue regeneration. Two-photon phosphorescence lifetime microscopy (2PLM) has been successfully applied in the past to image oxygen distributions in tissue with high spatial resolution. However, delivery of phosphorescent probes into avascular compartments, such as those formed during early bone defect healing, poses significant problems. Here, we report a multifunctional oxygen-reporting fibrous matrix fabricated through encapsulation of a hydrophilic oxygen-sensitive, two-photon excitable phosphorescent probe, PtP-C343, in the core of fibers during coaxial electrospinning. The oxygen-sensitive fibers support bone marrow stromal cell growth and differentiation and at the same time enable real-time high-resolution probing of partial pressures of oxygen via 2PLM. The hydrophilicity of the probe facilitates its gradual release into the nearby microenvironment, allowing fibers to act as a vehicle for probe delivery into the healing tissue. In conjunction with a cranial defect window chamber model, which permits simultaneous imaging of the bone and neovasculature in vivo via two-photon laser scanning microscopy, the oxygen-reporting fibers provide a useful tool for minimally invasive, high-resolution, real-time 3D mapping of tissue oxygenation during bone defect healing, facilitating studies aimed at understanding the healing process and advancing design of tissue-engineered constructs for enhanced bone repair and regeneration.

Estimation of the Mitochondrial Membrane Potential Using Fluorescence Lifetime Imaging Microscopy

Monitoring of cell metabolism represents an important application area for fluorescence lifetime imaging microscopy (FLIM). In particular, assessment of mitochondrial membrane potential (MMP) in complex three-dimensional multicellular in vitro, ex vivo, and in vivo models would enable improved segmentation and functional discrimination of cell types, directly report on the mitochondrial function and complement the quenched-phosphorescence detection of cellular O₂ and two-photon excited FLIM of endogenous NAD(P)H. Here, we report the green and orange-emitting fluorescent dyes SYTO and tetramethylrhodamine methyl ester (TMRM) as potential FLIM probes for MMP. In addition to nuclear, SYTO 16 and 24 dyes also display mitochondrial accumulation. FLIM with the culture of human colon cancer HCT116 cells allowed observation of the heterogeneity of mitochondrial polarization during the cell cycle progression. The dyes also demonstrated good performance with 3D cultures of Lgr5-GFP mouse intestinal organoids, providing efficient and quick cell staining and compatibility with two-photon excitation. Multiplexed imaging of Lgr5-GFP, proliferating cells (Hoechst 33342-aided FLIM), and TMRM-FLIM allowed us to identify the population of metabolically active cells in stem cell niche. TMRM-FLIM enabled to visualize the differences in membrane potential between Lgr5-positive and other proliferating and differentiated cell types. Altogether, SYTO 24 and TMRM dyes represent promising markers for advanced FLIM-based studies of cell bioenergetics with complex 3D and in vivo models. © 2019 International Society for Advancement of Cytometry.

Cellulose-based scaffolds for fluorescence lifetime imaging-assisted tissue engineering

Quantitative measurement of pH and metabolite gradients by microscopy is one of the challenges in the production of scaffold-grown organoids and multicellular aggregates. Herein, we used the cellulose-binding domain (CBD) of the Cellulomonas fimi CenA protein for designing biosensor scaffolds that allow measurement of pH and Ca²⁺ gradients by fluorescence intensity and lifetime imaging (FLIM) detection modes. By fusing CBD with pH-sensitive enhanced cyan fluorescent protein (CBD-ECFP), we achieved efficient labeling of cellulose-based scaffolds based on nanofibrillar, bacterial cellulose, and decellularized plant materials. CBD-ECFP bound to the cellulose matrices demonstrated pH sensitivity comparable to untagged ECFP (1.9-2.3 ns for pH 6-8), thus making it compatible with FLIM-based analysis of extracellular pH. By using 3D culture of human colon cancer cells (HCT116) and adult stem cell-derived mouse intestinal organoids, we evaluated the utility of the produced biosensor scaffold. CBD-ECFP was sensitive to increases in extracellular acidification: the results showed a decline in 0.2-0.4 pH units in response to membrane depolarization by the protonophore FCCP. With the intestinal organoid model, we demonstrated multiparametric imaging by combining extracellular acidification (FLIM) with phosphorescent probe-based monitoring of cell oxygenation. The described labeling strategy allows for the design of extracellular pH-sensitive scaffolds for multiparametric FLIM assays and their use in engineered live cancer and stem cell-derived tissues. Collectively, this research can help in achieving the controlled biofabrication of 3D tissue models with known metabolic characteristics. STATEMENT OF SIGNIFICANCE: We designed biosensors consisting of a cellulose-binding domain (CBD) and pH- and Ca²⁺-sensitive fluorescent proteins. CBD-tagged biosensors efficiently label various types of cellulose matrices including nanofibrillar cellulose and decellularized plant materials. Hybrid biosensing cellulose scaffolds designed in this study were successfully tested by multiparameter FLIM microscopy in 3D cultures of cancer cells and mouse intestinal organoids.

Imaging of oxygen and hypoxia in cell and tissue samples

Molecular oxygen (O2) is a key player in cell mitochondrial function, redox balance and oxidative stress, normal tissue function and many common disease states. Various chemical, physical and biological methods have been proposed for measurement, real-time monitoring and imaging of O2 concentration, state of decreased O2 (hypoxia) and related parameters in cells and tissue. Here, we review the established and emerging optical microscopy techniques allowing to visualize O2 levels in cells and tissue samples, mostly under in vitro and ex vivo, but also under in vivo settings. Particular examples include fluorescent hypoxia stains, fluorescent protein reporter systems, phosphorescent probes and nanosensors of different types. These techniques allow high-resolution mapping of O2 gradients in live or post-mortem tissue, in 2D or 3D, qualitatively or quantitatively. They enable control and monitoring of oxygenation conditions and their correlation with other biomarkers of cell and tissue function. Comparison of these techniques and corresponding imaging setups, their analytical capabilities and typical applications are given.

A Single-Step Self-Assembly Approach for the Fabrication of Aligned and Multilayered Three-Dimensional Tissue Constructs Using Multidomain Peptide Hydrogel

Hydrogels are homogenous materials that are limited in their ability to form oriented multilayered architecture in three-dimensional (3D) tissue constructs. Current techniques have led to advancements in this area. Such techniques often require extra devices and/or involve complex processes that are inaccessible to many laboratories. Here is described a one-step methodology that permits reliable alignment of cells into multiple layers using a self-assembling multidomain peptide (MDP) hydrogels. We characterized the structural features, viability, and molecular properties of dental pulp cells fabricated with MDP and demonstrated that manipulation of the layering of cells in the scaffolds was achieved by decreasing the weight by volume percentage (w/v%) of MDP contained within the scaffold. This approach allows cells to remodel their environment and enhanced various gene expression profiles, such as cell proliferation, angiogenesis, and extracellular matrix (ECM) remodeling-related genes. We further validated our approach for constructing various architectural configurations of tissues by fabricating cells into stratified multilayered and tubular structures. Our methodology provides a simple, rapid way to generate 3D tissue constructs with multilayered architectures. This method shows great potential to mimic in vivo microenvironments for cells and may be of benefit in modeling more complex tissues in the field of regenerative medicine.

Multi-Parametric Imaging of Hypoxia and Cell Cycle in Intestinal Organoid Culture

Dynamics of oxygenation of tissue and stem cell niches are important for understanding physiological function of the intestine in normal and diseased states. Only a few techniques allow live visualization of tissue hypoxia at cellular level and in three dimensions. We describe an optimized protocol, which uses cell-penetrating O₂-sensitive probe, Pt-Glc and phosphorescence lifetime imaging microscopy (PLIM), to analyze O₂ distribution in mouse intestinal organoids. Unlike the other indirect and end-point hypoxia stains, or point measurements with microelectrodes, this method provides high-resolution real-time visualization of O₂ in organoids. Multiplexing with conventional fluorescent live cell imaging probes such as the Hoechst 33342-based FLIM assay of cell proliferation, and immunofluorescence staining of endogenous proteins, allows analysis of key physiologic parameters under O₂ control in organoids. The protocol is useful for gastroenterology and physiology of intestinal tissue, hypoxia research, regenerative medicine, studying host-microbiota interactions and bioenergetics.

Macrophage interactions with polylactic acid and chitosan scaffolds lead to improved recruitment of human mesenchymal stem/stromal cells: a comprehensive study with different immune cells

Despite the importance of immune cell-biomaterial interactions for the regenerative outcome, few studies have investigated how distinct three-dimensional biomaterials modulate the immune cell-mediated mesenchymal stem/stromal cells (MSC) recruitment and function. Thus, this work compares the response of varied primary human immune cell populations triggered by different model scaffolds and describes its functional consequence on recruitment and motility of bone marrow MSC. It was found that polylactic acid (PLA) and chitosan scaffolds lead to an increase in the metabolic activity of macrophages but not of peripheral blood mononuclear cells (PBMC), natural killer (NK) cells or monocytes. PBMC and NK cells increase their cell number in PLA scaffolds and express a secretion profile that does not promote MSC recruitment. Importantly, chitosan increases IL-8, MIP-1, MCP-1 and RANTES secretion by macrophages while PLA stimulates IL-6, IL-8 and MCP-1 production, all chemokines that can lead to MSC recruitment. This secretion profile of macrophages in contact with biomaterials correlates with the highest MSC invasion. Furthermore, macrophages enhance stem cell motility within chitosan scaffolds by 44% but not in PLA scaffolds. Thus, macrophages are the cells that in contact with engineered biomaterials become activated to secrete bioactive molecules that stimulate MSC recruitment.

Contributions of 3D Cell Cultures for Cancer Research

Cancer cell lines have contributed immensely in understanding the complex physiology of cancers. They are excellent material for studies as they offer homogenous samples without individual variations and can be utilised with ease and flexibility. Also, the number of assays and end-points one can study is almost limitless; with the advantage of improvising, modifying or altering several variables and methods. Literally, a new dimension to cancer research has been achieved by the advent of 3Dimensional (3D) cell culture techniques. This approach increased many folds the ways in which cancer cell lines can be utilised for understanding complex cancer biology. 3D cell culture techniques are now the preferred way of using cancer cell lines to bridge the gap between the 'absolute in vitro' and 'true in vivo'. The aspects of cancer biology that 3D cell culture systems have contributed include morphology, microenvironment, gene and protein expression, invasion/migration/metastasis, angiogenesis, tumour metabolism and drug discovery, testing chemotherapeutic agents, adaptive responses and cancer stem cells. We present here, a comprehensive review on the applications of 3D cell culture systems for these aspects of cancers. J. Cell. Physiol. 232: 2679-2697, 2017. © 2016 Wiley Periodicals, Inc.

Steering surface topographies of electrospun fibers: understanding the mechanisms

A profound understanding of how to tailor surface topographies of electrospun fibers is of great importance for surface sensitive applications including optical sensing, catalysis, drug delivery and tissue engineering. Hereby, a novel approach to comprehend the driving forces for fiber surface topography formation is introduced through inclusion of the dynamic solvent-polymer interaction during fiber formation. Thus, the interplay between polymer solubility as well as computed fiber jet surface temperature changes in function of time during solvent evaporation and the resultant phase separation behavior are studied. The correlation of experimental and theoretical results shows that the temperature difference between the polymer solution jet surface temperature and the dew point of the controlled electrospinning environment are the main influencing factors with respect to water condensation and thus phase separation leading to the final fiber surface topography. As polymer matrices with enhanced surface area are particularly appealing for sensing applications, we further functionalized our nanoporous fibrous membranes with a phosphorescent oxygen-sensitive dye. The hybrid membranes possess high brightness, stability in aqueous medium, linear response to oxygen and hence represent a promising scaffold for cell growth, contactless monitoring of oxygen and live fluorescence imaging in 3-D cell models.

Use of Fluorescence Lifetime Imaging Microscopy (FLIM) as a Timer of Cell Cycle S Phase

Incorporation of thymidine analogues in replicating DNA, coupled with antibody and fluorophore staining, allows analysis of cell proliferation, but is currently limited to monolayer cultures, fixed cells and end-point assays. We describe a simple microscopy imaging method for live real-time analysis of cell proliferation, S phase progression over several division cycles, effects of anti-proliferative drugs and other applications. It is based on the prominent (~ 1.7-fold) quenching of fluorescence lifetime of a common cell-permeable nuclear stain, Hoechst 33342 upon the incorporation of 5-bromo-2'-deoxyuridine (BrdU) in genomic DNA and detection by fluorescence lifetime imaging microscopy (FLIM). We show that quantitative and accurate FLIM technique allows high-content, multi-parametric dynamic analyses, far superior to the intensity-based imaging. We demonstrate its uses with monolayer cell cultures, complex 3D tissue models of tumor cell spheroids and intestinal organoids, and in physiological study with metformin treatment.

Phosphorescence based O2 sensors - Essential tools for monitoring cell and tissue oxygenation and its impact on metabolism

Oxygenation condition at the cellular level is a critical factor in tissue physiology and common pathophysiological states including cancer, metabolic disorders, ischemia-reperfusion injury and inflammation. O₂ and ROS signalling and hypoxia research are rapidly growing areas spanning life and biomedical sciences, but still many current cell and tissue models and experimental set ups lack physiological relevance, particularly precise control of cellular O₂. Quenched-phosphorescence O₂ sensing enables implementation of such in situ control of cellular O₂ and the creation of physiological conditions in respiring samples analysed in vitro. The advantages of optical O₂ sensing are the non-invasive, contactless, real-time, quantitative monitoring of O₂ concentration, which can be performed in the gas or liquid phase, macroscopically or microscopically, by point measurement or in imaging mode, with sub-cellular spatial resolution, in a flexible manner and with various cell and tissue models. Significantly, this same technology can also be used to probe the metabolism of cells and tissue under specific oxygenation conditions and their responses to changing conditions. Here we describe the range of available O₂ sensing systems and tools, their analytical capabilities, uses in cell/tissue physiology and hypoxia research, and strategies for integration in routine experimental procedures.

The Ca2+/Mn2+-transporting SPCA2 pump is regulated by oxygen and cell density in colon cancer cells

The mammalian SPCA1 and SPCA2 ATPases localize in membranes of the secretory pathway and transport ions of Ca(2+) and Mn(2+) The role of tissue-specific SPCA2 isoform, highly expressed in lungs, mammary gland and gastrointestinal tract, is poorly understood. To elucidate the function of SPCA2, we studied human colon cancer HCT116 cells, grown under ambient and decreased O2 levels. We found that in contrast with other Ca(2+)-ATPase isoforms the expression of SPCA2 was up-regulated under hypoxia (3% O2), in both adherent (2D) and spheroid (3D) cultures. In spheroids, experiencing lowest O2 levels (30-50 μM, measured by phosphorescence lifetime imaging microscopy), we observed lower staining with reactive oxygen species (ROS)-specific fluorescent probe, which correlated with increased SPCA2. However, SPCA2 expression was up-regulated in cells exposed to reactive oxygen and nitrogen species donors, and when grown at higher density. We noticed that the culture exposed to hypoxia showed overall increase in S phase-positive cells and hypothesized that SPCA2 up-regulation under hypoxia can be linked to Mn(2+)-dependent cell cycle arrest. Consequently, we found that SPCA2-transfected cells display a higher number of cells entering S phase. Altogether, our results point at the important role of SPCA2 in regulation of cell cycle in cancer cells.

Heteronuclear Ir(III)-Ln(III) Luminescent Complexes: Small-Molecule Probes for Dual Modal Imaging and Oxygen Sensing

Luminescent, mixed metal d-f complexes have the potential to be used for dual (magnetic resonance imaging (MRI) and luminescence) in vivo imaging. Here, we present dinuclear and trinuclear d-f complexes, comprising a rigid framework linking a luminescent Ir center to one (Ir·Ln) or two (Ir·Ln2) lanthanide metal centers (where Ln = Eu(III) and Gd(III), respectively). A range of physical, spectroscopic, and imaging-based properties including relaxivity arising from the Gd(III) units and the occurrence of Ir(III) → Eu(III) photoinduced energy-transfer are presented. The rigidity imposed by the ligand facilitates high relaxivities for the Gd(III) complexes, while the luminescence from the Ir(III) and Eu(III) centers provide luminescence imaging capabilities. Dinuclear (Ir·Ln) complexes performed best in cellular studies, exhibiting good solubility in aqueous solutions, low toxicity after 4 and 18 h, respectively, and punctate lysosomal staining. We also demonstrate the first example of oxygen sensing in fixed cells using the dyad Ir·Gd, via two-photon phosphorescence lifetime imaging (PLIM).

Functionalization, preparation and use of cell-laden gelatin methacryloyl-based hydrogels as modular tissue culture platforms

Progress in advancing a system-level understanding of the complexity of human tissue development and regeneration is hampered by a lack of biological model systems that recapitulate key aspects of these processes in a physiological context. Hence, growing demand by cell biologists for organ-specific extracellular mimics has led to the development of a plethora of 3D cell culture assays based on natural and synthetic matrices. We developed a physiological microenvironment of semisynthetic origin, called gelatin methacryloyl (GelMA)-based hydrogels, which combine the biocompatibility of natural matrices with the reproducibility, stability and modularity of synthetic biomaterials. We describe here a step-by-step protocol for the preparation of the GelMA polymer, which takes 1-2 weeks to complete, and which can be used to prepare hydrogel-based 3D cell culture models for cancer and stem cell research, as well as for tissue engineering applications. We also describe quality control and validation procedures, including how to assess the degree of GelMA functionalization and mechanical properties, to ensure reproducibility in experimental and animal studies.