Integration of technologies for hepatic tissue engineering

Thermoresponsive block copolymer brush for temperature-modulated hepatocyte separation

Hepatic tissue engineering may be an effective approach for the treatment of liver disease; however, its practical application requires hepatic cell separation technologies that do not involve cell surface modification and maintain cell activity. In this study, we developed hepatocyte cell separation materials using a thermoresponsive polymer and a polymer with high affinity to hepatocytes. A block copolymer of poly(N-p-vinylbenzyl-O-β-D-galactopyranosyl-(1→4)-D-gluconamide) (PVLA) and poly(N-isopropylacrylamide) (PNIPAAm) [PVLA-b-PNIPAAm] was prepared through two steps of atom transfer radical polymerization. On the prepared PVLA-b-PNIPAAm brush, HepG2 cells (model hepatocytes) adhered at 37 °C and detached at 20 °C, attributed to the temperature-modulated affinity between PVLA and HepG2. Cells from the immortalized human hepatic stellate cell line (TWNT-1) did not adhere to the copolymer brush, and RAW264.7 cells (mouse macrophage; model Kupffer cells) adhered to the copolymer brush, regardless of temperature. Using the difference in cell adhesion properties on the copolymer brush, temperature-modulated cell separation was successfully demonstrated. A mixture of HepG2, RAW264.7, and TWNT-1 cells was seeded on the copolymer brush at 37 °C for adherence. By reducing the temperature to 20 °C, adhered HepG2 cells were selectively recovered with a purity of approximately 85% and normal activity. In addition, induced pluripotent stem (iPS) cell-derived hepatocytes adhered on the PVLA-b-PNIPAAm brush at 37 °C and detached from the copolymer brush at 20 °C, whereas the undifferentiated iPS cells did not adhere, indicating that the prepared PVLA-b-PNIPAAm brush could be utilized to separate hepatocyte differentiated and undifferentiated cells. These results indicated that the newly developed PVLA-b-PNIPAAm brush can separate hepatic cells from contaminant cells by temperature modulation, without affecting cell activity or modifying the cell surface. Thus, the copolymer brush is expected to be a useful separation tool for cell therapy and tissue engineering using hepatocytes.

Co-Culture of Human Primary Hepatocytes and Nonparenchymal Liver Cells in the Emulate® Liver-Chip for the Study of Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a significant public health issue, but standard animal tests and clinical trials sometimes fail to predict DILI due to species differences and the relatively low number of human subjects involved in preapproval studies of a new drug, respectively. In vitro models have long been used to aid DILI prediction, with primary human hepatocytes (PHHs) being generally considered the gold standard. However, despite many efforts and decades of work, traditional culture methods have been unsuccessful in either fully preserving essential liver functions after isolation of PHHs or in emulating interactions between PHHs and hepatic nonparenchymal cells (NPCs), both of which are essential for the development of DILI under in vivo conditions. Recently, various liver-on-a-chip (Liver-Chip) systems have been developed to co-culture hepatocytes and NPCs in a three-dimensional environment on microfluidic channels, enabling better maintenance of primary liver cells and thus improved DILI prediction. The Emulate® Liver-Chip is a commercially available system that can recapitulate some in vivo DILI responses associated with certain compounds whose liver safety profile cannot be accurately evaluated using conventional approaches involving PHHs or animal models due to a lack of innate immune responses or species-dependent toxicity, respectively. Here, we describe detailed procedures for the use of Emulate® Liver-Chips for co-culturing PHHs and NPCs for the purpose of DILI evaluation. First, we describe the procedures for preparing the Liver-Chip. We then outline the steps needed for sequential seeding of PHHs and NPCs in the prepared Liver-Chips. Lastly, we provide a protocol for utilizing cells maintained in perfusion culture in the Liver-Chips to evaluate DILI, using acetaminophen as an example. In all, use of this system and the procedures described here allow better preservation of the functions of human primary liver cells, resulting in an improved in vitro model for DILI assessment. © 2022 Wiley Periodicals LLC. This article has been contributed to by US Government employees and their work is in the public domain in the USA. Basic Protocol 1: Liver-Chip preparation Basic Protocol 2: Seeding primary human hepatocytes and nonparenchymal cells on Liver-Chips Basic Protocol 3: Perfusion culture for the study of acetaminophen-induced liver injury.

The transcriptional landscape of a hepatoma cell line grown on scaffolds of extracellular matrix proteins

Background

The behavior of cells in vivo is complex and highly dynamic, as it results from an interplay between intercellular matrix proteins with surface receptors and other microenvironmental cues. Although the effects of the cellular niche have been investigated for a number of cell types using different molecular approaches, comprehensive assessments of how the global transcriptome responds to 3D scaffolds composed of various extracellular matrix (ECM) constituents at different concentrations are still lacking.

Results

In this study, we explored the effects of two diverse extracellular matrix (ECM) components, Collagen I and Matrigel, on the transcriptional profile of cells in a cell culture system. Culturing Huh-7 cells on traditional cell culture plates (Control) or on the ECM components at different concentrations to modulate microenvironment properties, we have generated transcriptomics data that may be further explored to understand the differentiation and growth potential of this cell type for the development of 3D cultures. Our analysis infers transcription factors that are most responsible for the transcriptome response to the extracellular cues.

Conclusion

Our data indicates that the Collagen I substrate induces a robust transcriptional response in the Huh-7 cells, distinct from that induced by Matrigel. Enhanced hepatocyte markers (ALB and miR-122) reveal a potentially robust remodelling towards primary hepatocytes. Our results aid in defining the appropriate culture and transcription pathways while using hepatoma cell lines. As systems mimicking the in vivo structure and function of liver cells are still being developed, our study could potentially circumvent bottlenecks of limited availability of primary hepatocytes for preclinical studies of drug targets.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07532-2.

Characterization of a long-term mouse primary liver 3D tissue model recapitulating innate-immune responses and drug-induced liver toxicity

Three-dimensional liver in vitro systems have recently attracted a lot of attention in drug development. These systems help to gain unprecedented insights into drug-induced liver injury (DILI), as they more closely reproduce liver biology, and as drug effects can be studied in isolated and controllable microenvironments. Many groups established human-based in vitro models but so far neglected the animal equivalent, although the availability of both models would be desirable. Animal in vitro models enable back- and forward translation of in vitro and in vivo findings, bridge the gap between rodent in vivo and human in vitro scenarios, and ultimately support the interpretation of data generated with preclinical species and humans. Since mice are often used in drug development and physiologically relevant in vitro systems are lacking, we established, for the first time, a mouse liver model that encompasses primary parenchymal and non-parenchymal cells with preserved viability and functionality over three weeks. Using our three-dimensional liver spheroids, we were able to predict the toxicity of known DILI compounds, demonstrated the interaction cascades between the different cell types and showed evidence of drug-induced steatosis and cholestasis. In summary, our mouse liver spheroids represent a valuable in vitro model that can be applied to study DILI findings, reported from mouse studies, and offers the potential to detect immune-mediated drug-induced liver toxicity.

Cell sheet tissue engineering for scaffold-free three-dimensional (3D) tissue reconstruction

Three-dimensional (3D) reconstruction of highly functional tissues is of great importance in advancing the clinical benefit of tissue engineering and regenerative medicine. In the last quarter century, many studies have found that by engineering a 3D microenvironment that resembles the in vivo tissue condition, cells exhibit behaviors and functions that reflect those of native tissue. Biomaterial scaffolds are a central technology for providing 3D microenvironments in vitro, and, in conjunction with diverse design and cell seeding advents, have produced highly functional and complex 3D tissues. Here, we describe a new approach to creating 3D cell-dense tissue-like constructs without a biomaterial scaffold. Cell sheet technology with cell sheet layering strategies generates highly cell dense, engineered tissue capable of direct crosstalk with the tissue-engraftment surface, in addition to paracrine-mediated signaling. In this chapter, we will introduce methods of reconstructing 3D tissue using cell sheet technology and the advantages of a scaffold-free design.

Large-scale high-density culture of hepatocytes in a liver microsystem with mimicked sinusoid blood flow

In vitro engineering of liver tissue is a rapidly developing field for various biomedical applications. However, liver tissue culture is currently performed on only a small scale with a low density of hepatocytes. In this study, a simple design was introduced in a liver microsystem to enhance the transport of nutrients (e.g., oxygen and glucose) for the three-dimensional large-scale, high-density culture of hepatocytes. In this design, convection across the cell culture zone was generated to mimic sinusoid blood flow (SBF) based on the pressure difference between two fluids flowing in a countercurrent manner on either side of the cell culture zone. First, the distributions of living and dead cells in different culture subzones under various perfusion flow rates were observed, analysed, and compared. Then, the enhanced transport of nutrients was experimentally validated in relation to the viability of cells and theoretically explained by comparing the fluid velocity and oxygen concentration distribution in the cell culture zone in counterflow and coflow modes. Finally, the functions of the SBF-mimicked liver microsystem were assessed on the basis of specific metabolites, synthesized proteins, and bilirubin detoxification of hepatocytes, with collagen and alginate as extracellular matrices. Under this design, the density of hepatocytes cultured at the 3-mm-thickness scale reached ~7 × 10⁷  cells/ml on Day 7, and the metabolism and detoxification functions of the cells worked well. In addition, a liver rope-like structure and sphere-like clusters of cells were observed. This work provides insight for the design of a bionic liver microsystem.

Bioinspired liver scaffold design criteria

Maintaining hepatic functional characteristics in-vitro is considered one of the main challenges in engineering liver tissue. As hepatocytes cultured ex-vivo are deprived of their native extracellular matrix (ECM) milieu, developing scaffolds that mimic the biomechanical and physicochemical properties of the native ECM is thought to be a promising approach for successful tissue engineering and regenerative medicine applications. On the basis that the decellularized liver matrix represents the ideal design template for engineering bioinspired hepatic scaffolds, to derive quantitative descriptors of liver ECM architecture, we characterised decellularised liver matrices in terms of their biochemical, viscoelastic and structural features along with porosity, permeability and wettability. Together, these data provide a unique set of quantitative design criteria which can be used to generate guidelines for fabricating biomaterial scaffolds for liver tissue engineering. As proof-of-concept, we investigated hepatic cell response to substrate viscoelasticity. On collagen hydrogels mimicking decellularised liver mechanics, cells showed superior morphology, higher viability and albumin secretion than on stiffer and less viscous substrates. Although scaffold properties are generally inspired by those of native tissues, our results indicate significant differences between the mechano-structural characteristics of untreated and decellularised hepatic tissue. Therefore, we suggest that design rules - such as mechanical properties and swelling behaviour - for engineering biomimetic scaffolds be re-examined through further studies on substrates matching the features of decellularized liver matrices.

Role of Bioreactor Technology in Tissue Engineering for Clinical Use and Therapeutic Target Design

Micro and small bioreactors are well described for use in bioprocess development in pre-production manufacture, using ultra-scale down and microfluidic methodology. However, the use of bioreactors to understand normal and pathophysiology by definition must be very different, and the constraints of the physiological environment influence such bioreactor design. This review considers the key elements necessary to enable bioreactors to address three main areas associated with biological systems. All entail recreation of the in vivo cell niche as faithfully as possible, so that they may be used to study molecular and cellular changes in normal physiology, with a view to creating tissue-engineered grafts for clinical use; understanding the pathophysiology of disease at the molecular level; defining possible therapeutic targets; and enabling appropriate pharmaceutical testing on a truly representative organoid, thus enabling better drug design, and simultaneously creating the potential to reduce the numbers of animals in research. The premise explored is that not only cellular signalling cues, but also mechano-transduction from mechanical cues, play an important role.

Transcriptomic, Proteomic, and Functional Long-Term Characterization of Multicellular Three-Dimensional Human Liver Microtissues

Three-Dimensional (3D) liver microtissues, specifically prepared from primary human hepatocytes (PHH) in coculture with nonparenchymal cells (NPCs), have been shown to be a valuable tool for in vitro toxicology. However, a lack of thorough characterization on a functional, transcriptomic, and proteomic level of such models during long-term cultivation is evident. By integrating multiple omics technologies, we provide in this study an in-depth long-term characterization of 3D microtissues composed of PHH from three different donors cocultured with primary NPCs. The 3D human liver microtissues (hLiMTs) exhibited stable adenosine triphosphate (ATP) content and albumin secretion over 5 weeks. Histological analysis indicated a healthy liver tissue with polarized expression of bile salt export pump (BSEP) and multidrug resistance protein 2 (MRP2) in a structure reminiscent of bile canaliculi. The 3D microtissues exhibited stable basal and inducible cytochrome P450 activities up to 5 weeks in culture. Analysis of 40,716 transcripts using RNA arrays revealed distinct similarities to native human liver gene expression. Long-term culture showed a stable phenotype up to 5 weeks, with differences in liver gene expression primarily attributed to individual donors. Proteomic profiling of 2200 unique proteins by label-free LC-MS/MS revealed a relatively stable protein expression where only 7.3% were up- or downregulated more than twofold from day 7 to 35 in culture. Taken together, these results suggest that hLiMTs represent a responsive and physiologically relevant in vitro liver model that maintains stable function over 5 weeks and is therefore well suited for repeated-dose toxicity testing.

Compartmentalized Microfluidic Platforms: The Unrivaled Breakthrough of In Vitro Tools for Neurobiological Research

Microfluidic technology has become a valuable tool to the scientific community, allowing researchers to study fine cellular mechanisms with higher variable control compared with conventional systems. It has evolved tremendously, and its applicability and flexibility made its usage grow exponentially and transversely to several research fields. This has been particularly noticeable in neuroscience research, where microfluidic platforms made it possible to address specific questions extending from axonal guidance, synapse formation, or axonal transport to the development of 3D models of the CNS to allow pharmacological testing and drug screening. Furthermore, the continuous upgrade of microfluidic platforms has allowed a deeper study of the communication occurring between different neuronal and glial cells or between neurons and other peripheral tissues, both in physiological and pathological conditions. Importantly, the evolution of microfluidic technology has always been accompanied by the development of new computational tools addressing data acquisition, analysis, and modeling.

3D Bioprinting for Tissue and Organ Fabrication

The field of regenerative medicine has progressed tremendously over the past few decades in its ability to fabricate functional tissue substitutes. Conventional approaches based on scaffolding and microengineering are limited in their capacity of producing tissue constructs with precise biomimetic properties. Three-dimensional (3D) bioprinting technology, on the other hand, promises to bridge the divergence between artificially engineered tissue constructs and native tissues. In a sense, 3D bioprinting offers unprecedented versatility to co-deliver cells and biomaterials with precise control over their compositions, spatial distributions, and architectural accuracy, therefore achieving detailed or even personalized recapitulation of the fine shape, structure, and architecture of target tissues and organs. Here we briefly describe recent progresses of 3D bioprinting technology and associated bioinks suitable for the printing process. We then focus on the applications of this technology in fabrication of biomimetic constructs of several representative tissues and organs, including blood vessel, heart, liver, and cartilage. We finally conclude with future challenges in 3D bioprinting as well as potential solutions for further development.

Isolation and co-culture of rat parenchymal and non-parenchymal liver cells to evaluate cellular interactions and response

The liver is a central organ in the human body, and first line of defense between host and external environment. Liver response to any external perturbation is a collective reaction of resident liver cells. Most of the current in vitro liver models focus on hepatocytes, the primary metabolic component, omitting interactions and cues from surrounding environment and non-parenchymal cells (NPCs). Recent studies suggest that contributions of NPCs are vital, particularly in disease conditions, and outcomes of drugs and their metabolites. Along with hepatocytes, NPCs–Kupffer (KC), sinusoidal endothelial (LSEC) and stellate cells (SC) are major cellular components of the liver. Incorporation of primary cells in in vitro liver platforms is essential to emulate the functions of the liver, and its overall response. Herein, we isolate individual NPC cell fractions from rat livers and co-culture them in a transwell format incorporating primary rat hepatocytes with LSECs, SCs, and KCs. Our results indicate that the presence and contributions of multiple cells within the co-culture capture the interactions between hepatocytes and NPC, and modulates the responses to inflammatory stimulus such as LPS. The isolation and co-culture methods could provide a stable platform for creating in vitro liver models that provide defined functionality beyond hepatocytes alone.

One step antibody-mediated isolation and patterning of multiple cell types in microfluidic devices

Cell-cell interactions play a key role in regeneration, differentiation, and basic tissue function taking place under physiological shear forces. However, current solutions to mimic such interactions by micro-patterning cells within microfluidic devices have low resolution, high fabrication complexity, and are limited to one or two cell types. Here, we present a microfluidic platform capable of laminar patterning of any biotin-labeled peptide using streptavidin-based surface chemistry. The design permits the generation of arbitrary cell patterns from heterogeneous mixtures in microfluidic devices. We demonstrate the robust co-patterning of α-CD24, α-ASGPR-1, and α-Tie2 antibodies for rapid isolation and co-patterning of mixtures of hepatocytes and endothelial cells. In addition to one-step isolation and patterning, our design permits step-wise patterning of multiple cell types and empty spaces to create complex cellular geometries in vitro. In conclusion, we developed a microfluidic device that permits the generation of perfusable tissue-like patterns in microfluidic devices by directly injecting complex cell mixtures such as differentiated stem cells or tissue digests with minimal sample preparation.

Liver sinusoid on a chip: Long-term layered co-culture of primary rat hepatocytes and endothelial cells in microfluidic platforms

We describe the generation of microfluidic platforms for the co-culture of primary hepatocytes and endothelial cells; these platforms mimic the architecture of a liver sinusoid. This paper describes a progressional study of creating such a liver sinusoid on a chip system. Primary rat hepatocytes (PRHs) were co-cultured with primary or established endothelial cells in layers in single and dual microchannel configurations with or without continuous perfusion. Cell viability and maintenance of hepatocyte functions were monitored and compared for diverse experimental conditions. When primary rat hepatocytes were co-cultured with immortalized bovine aortic endothelial cells (BAECs) in a dual microchannel with continuous perfusion, hepatocytes maintained their normal morphology and continued to produce urea for at least 30 days. In order to demonstrate the utility of our microfluidic liver sinusoid platform, we also performed an analysis of viral replication for the hepatotropic hepatitis B virus (HBV). HBV replication, as measured by the presence of cell-secreted HBV DNA, was successfully detected. We believe that our liver model closely mimics the in vivo liver sinusoid and supports long-term primary liver cell culture. This liver model could be extended to diverse liver biology studies and liver-related disease research such as drug induced liver toxicology, cancer research, and analysis of pathological effects and replication strategies of various hepatotropic infectious agents. .

Microbial-derived lithocholic acid and vitamin K2 drive the metabolic maturation of pluripotent stem cells-derived and fetal hepatocytes

The liver is the main organ responsible for the modification, clearance, and transformational toxicity of most xenobiotics owing to its abundance in cytochrome P450 (CYP450) enzymes. However, the scarcity and variability of primary hepatocytes currently limits their utility. Human pluripotent stem cells (hPSCs) represent an excellent source of differentiated hepatocytes; however, current protocols still produce fetal-like hepatocytes with limited mature function. Interestingly, fetal hepatocytes acquire mature CYP450 expression only postpartum, suggesting that nutritional cues may drive hepatic maturation. We show that vitamin K2 and lithocholic acid, a by-product of intestinal flora, activate pregnane X receptor (PXR) and subsequent CYP3A4 and CYP2C9 expression in hPSC-derived and isolated fetal hepatocytes. Differentiated cells produce albumin and apolipoprotein B100 at levels equivalent to primary human hepatocytes, while demonstrating an 8-fold induction of CYP450 activity in response to aryl hydrocarbon receptor (AhR) agonist omeprazole and a 10-fold induction in response to PXR agonist rifampicin. Flow cytometry showed that over 83% of cells were albumin and hepatocyte nuclear factor 4 alpha (HNF4α) positive, permitting high-content screening in a 96-well plate format. Analysis of 12 compounds showed an R(2) correlation of 0.94 between TC50 values obtained in stem cell-derived hepatocytes and primary cells, compared to 0.62 for HepG2 cells. Finally, stem cell-derived hepatocytes demonstrate all toxicological endpoints examined, including steatosis, apoptosis, and cholestasis, when exposed to nine known hepatotoxins.

CONCLUSION

Our work provides fresh insights into liver development, suggesting that microbial-derived cues may drive the maturation of CYP450 enzymes postpartum. Addition of these cues results in the first functional, inducible, hPSC-derived hepatocyte for predictive toxicology.

Gene transfection to spheroid culture system on micropatterned culture plate by polyplex nanomicelle: a novel platform of genetically-modified cell transplantation

Three-dimensional (3D) cellular spheroids have attracted much attention as a transplantation procedure because the increased cell-to-cell interaction in spheroids enhances cell survival and its functions after the transplantation into the body. Furthermore, the potency of spheroidal cells may be further improved by introducing transgenes to augment cellular functions as well as enhance the paracrine effects by secreting key proteins involved in the essential cellular signaling cascades. In this study, we organized a new platform for genetically-modified cell transplantation by combining a microfabricated culture system for 3D spheroid formation with a newly developed non-viral transfection system, polyplex nanomicelle. After transfection of Gaussia luciferase using the nanomicelle, the prolonged luciferase expression was obtained for more than a month with continuous albumin secretion from the hepatocyte spheroids to the level comparable with control spheroids receiving no transfection. In contrast, by the transfection using FuGENE HD, a commercially available lipid-based reagent, the luciferase expression was obtained, yet the albumin secretion was significantly decreased with disintegration of the spheroid architecture. To assess the feasibility of the hepatocyte spheroids for in vivo transplantation, the spheroids were recovered by the use of micropatterned culture plate functionalized with thermosensitive polymer and dispersed into Matrigel(TM) Matrix. The luciferase expression as well as albumin secretion was maintained for more than a month from the spheroids in the Matrix. Thus, the combination of spheroid cell culture on micropatterned plates with gene introduction using polyplex nanomicelle is a promising platform for genetically-modified cell transplantation to achieve sustained transgene expression with maintaining innate cell functions.

Long-term coculture strategies for primary hepatocytes and liver sinusoidal endothelial cells

Hepatocytes and their in vitro models are essential tools for preclinical screening studies for drugs that affect the liver. Most of the current models primarily focus on hepatocytes alone and lack the contribution of non-parenchymal cells (NPCs), which are significant through both molecular and the response of the NPCs themselves. Models that incorporate NPCs alongside hepatocytes hold the power to enable more realistic recapitulation and elucidation of cell interactions and cumulative drug response. Hepatocytes and liver sinusoidal endothelial cells (LSECs) account for ∼ 80% of the liver mass where the LSECs line the walls of blood vessels, and act as a barrier between hepatocytes and blood. Culturing LSECs with hepatocytes to generate multicellular physiologically relevant in vitro liver models has been a major hurdle since LSECs lose their phenotype rapidly after isolation. To this end, we describe the application of collagen gel (1) in a sandwich and (2) as an intervening extracellular matrix layer to coculture hepatocytes with LSECs for extended periods. These coculture configurations provide environments wherein hepatocyte and LSECs, through cell-cell contacts and/or secretion factors, lead to enhanced function and stability of the cocultures. Our results show that in these configurations, hepatocytes and LSECs maintained their phenotypes when cultured together as a mixture, and showed stable secretion and metabolic activity for up to 4 weeks. Immunostaining for sinusoidal endothelial 1 (SE-1) antibody demonstrated retention of LSEC phenotype during the culture period. In addition, LSECs cultured alone maintained high viability and SE-1 expression when cultured within a collagen sandwich configuration up to 4 weeks. Albumin production of the cocultures was 10-15 times higher when LSECs were cultured as a bottom layer (with an intervening collagen layer) and as a mixture in a sandwich configuration, and native CYP 1A1/2 activity was at least 20 times higher than monoculture controls. Together, these data suggest that collagen gel-based hepatocyte-LSEC cocultures are highly suitable models for stabilization and long-term culture of both cell types. In summary, these results indicate that collagen gel-based hepatocyte-LSEC coculture models are promising for in vitro toxicity testing, and liver model development studies.

The importance of physiological oxygen concentrations in the sandwich cultures of rat hepatocytes on gas-permeable membranes

Oxygen supply is a critical issue in the optimization of in vitro hepatocyte microenvironments. Although several strategies have been developed to balance complex oxygen requirements, these techniques are not able to accurately meet the cellular oxygen demand. Indeed, neither the actual oxygen concentration encountered by cells nor the cellular oxygen consumption rates (OCR) was assessed. The aim of this study is to define appropriate oxygen conditions at the cell level that could accurately match the OCR and allow hepatocytes to maintain liver specific functions in a normoxic environment. Matrigel overlaid rat hepatocytes were cultured on the polydimethylsiloxane (PDMS) membranes under either atmospheric oxygen concentration [20%-O2 (+)] or physiological oxygen concentrations [10%-O2 (+), 5%-O2 (+)], respectively, to investigate the effects of various oxygen concentrations on the efficient functioning of hepatocytes. In parallel, the gas-impermeable cultures (polystyrene) with PDMS membrane inserts were used as the control groups [PS-O2 (-)]. The results indicated that the hepatocytes under 10%-O2 (+) exhibited improved survival and maintenance of metabolic activities and functional polarization. The dramatic elevation of cellular OCR up to the in vivo liver rate proposed a normoxic environment for hepatocytes, especially when comparing with PS-O2 (-) cultures, in which the cells generally tolerated hypoxia. Additionally, the expression levels of 84 drug-metabolism genes were the closest to physiological levels. In conclusion, this study clearly shows the benefit of long-term culture of hepatocytes at physiological oxygen concentration, and indicates on an oxygen-permeable membrane system to provide a simple method for in vitro studies.

Bioreactor technologies to support liver function in vitro

Liver is a central nexus integrating metabolic and immunologic homeostasis in the human body, and the direct or indirect target of most molecular therapeutics. A wide spectrum of therapeutic and technological needs drives efforts to capture liver physiology and pathophysiology in vitro, ranging from prediction of metabolism and toxicity of small molecule drugs, to understanding off-target effects of proteins, nucleic acid therapies, and targeted therapeutics, to serving as disease models for drug development. Here we provide perspective on the evolving landscape of bioreactor-based models to meet old and new challenges in drug discovery and development, emphasizing design challenges in maintaining long-term liver-specific function and how emerging technologies in biomaterials and microdevices are providing new experimental models.

Biomaterials for liver tissue engineering

Liver extracellular matrix (ECM) composition, topography and biomechanical properties influence cell-matrix interactions. The ECM presents guiding cues for hepatocyte phenotype maintenance, differentiation and proliferation both in vitro and in vivo. Current understanding of such cell-guiding cues along with advancement of techniques for scaffold fabrication has led to evolution of matrices for liver tissue culture from simple porous scaffolds to more complex 3D matrices with microarchitecture similar to in vivo. Natural and synthetic polymeric biomaterials fabricated in different topographies and porous matrices have been used for hepatocyte culture. Heterotypic and homotypic cell interactions are necessary for developing an adult liver as well as an artificial liver. A high oxygen demand of hepatocytes as well as graded oxygen distribution in liver is another challenging attribute of the normal liver architecture that further adds to the complexity of engineered substrate design. A balanced interplay of cell-matrix interactions along with cell-cell interactions and adequate supply of oxygen and nutrient determines the success of an engineered substrate for liver cells. Techniques devised to incorporate these features of hepatic function and mimic liver architecture range from maintaining liver cells in mm-sized tailor-made scaffolds to a more bottoms up approach that starts from building the microscopic subunit of the whole tissue. In this review, we discuss briefly various biomaterials used for liver tissue engineering with respect to design parameters such as scaffold composition and chemistry, biomechanical properties, topography, cell-cell interactions and oxygenation.

Layered long-term co-culture of hepatocytes and endothelial cells on a transwell membrane: toward engineering the liver sinusoid

This paper presents a novel liver model that mimics the liver sinusoid where most liver activities occur. A key aspect of our current liver model is a layered co-culture of primary rat hepatocytes (PRHs) and primary rat liver sinusoidal endothelial cells (LSECs) or bovine aortic endothelial cells (BAECs) on a transwell membrane. When a layered co-culture was attempted with a thin Matrigel layer placed between hepatocytes and endothelial cells to mimic the space of Disse, the cells did not form completely separated monolayers. However, when hepatocytes and endothelial cells were cultured on the opposite sides of a transwell membrane, PRHs co-cultured with LSECs or BAECs maintained their viability and normal morphology for 39 and 57 days, respectively. We assessed the presence of hepatocyte-specific differentiation markers to verify that PRHs remained differentiated in the long-term co-culture and analyzed hepatocyte function by monitoring urea synthesis. We also noted that the expression of cytochrome P-450 remained similar in the co-cultured system from day 1 to day 48. Thus, our novel liver model system demonstrated that primary hepatocytes can be cultured for extended times and retain their hepatocyte-specific functions when layered with endothelial cells.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Fabricated micro-nano devices for in vivo and in vitro biomedical applications

In recent years, the innovative use of microelectromechanical systems (MEMSs) and nanoelectromechanical systems (NEMSs) in biomedical applications has opened wide opportunities for precise and accurate human diagnostics and therapeutics. The introduction of nanotechnology in biomedical applications has facilitated the exact control and regulation of biological environments. This ability is derived from the small size of the devices and their multifunctional capabilities to operate at specific sites for selected durations of time. Researchers have developed wide varieties of unique and multifunctional MEMS/NEMS devices with micro and nano features for biomedical applications (BioMEMS/NEMS) using the state of the art microfabrication techniques and biocompatible materials. However, the integration of devices with the biological milieu is still a fundamental issue to be addressed. Devices often fail to operate due to loss of functionality, or generate adverse toxic effects inside the body. The in vitro and in vivo performance of implantable BioMEMS such as biosensors, smart stents, drug delivery systems, and actuation systems are researched extensively to understand the interaction of the BioMEMS devices with physiological environments. BioMEMS developed for drug delivery applications include microneedles, microreservoirs, and micropumps to achieve targeted drug delivery. The biocompatibility of BioMEMS is further enhanced through the application of tissue and smart surface engineering. This involves the application of nanotechnology, which includes the modification of surfaces with polymers or the self-assembly of monolayers of molecules. Thereby, the adverse effects of biofouling can be reduced and the performance of devices can be improved in in vivo and in vitro conditions.

Geometric control of vascular networks to enhance engineered tissue integration and function

Tissue vascularization and integration with host circulation remains a key barrier to the translation of engineered tissues into clinically relevant therapies. Here, we used a microtissue molding approach to demonstrate that constructs containing highly aligned "cords" of endothelial cells triggered the formation of new capillaries along the length of the patterned cords. These vessels became perfused with host blood as early as 3 d post implantation and became progressively more mature through 28 d. Immunohistochemical analysis showed that the neovessels were composed of human and mouse endothelial cells and exhibited a mature phenotype, as indicated by the presence of alpha-smooth muscle actin-positive pericytes. Implantation of cords with a prescribed geometry demonstrated that they provided a template that defined the neovascular architecture in vivo. To explore the utility of this geometric control, we implanted primary rat and human hepatocyte constructs containing randomly organized endothelial networks vs. ordered cords. We found substantially enhanced hepatic survival and function in the constructs containing ordered cords following transplantation in mice. These findings demonstrate the importance of multicellular architecture in tissue integration and function, and our approach provides a unique strategy to engineer vascular architecture.

InVERT molding for scalable control of tissue microarchitecture

Complex tissues contain multiple cell types that are hierarchically organized within morphologically and functionally distinct compartments. Construction of engineered tissues with optimized tissue architecture has been limited by tissue fabrication techniques, which do not enable versatile microscale organization of multiple cell types in tissues of size adequate for physiological studies and tissue therapies. Here we present an 'Intaglio-Void/Embed-Relief Topographic molding' method for microscale organization of many cell types, including induced pluripotent stem cell-derived progeny, within a variety of synthetic and natural extracellular matrices and across tissues of sizes appropriate for in vitro, pre-clinical, and clinical studies. We demonstrate that compartmental placement of non-parenchymal cells relative to primary or induced pluripotent stem cell-derived hepatocytes, compartment microstructure, and cellular composition modulate hepatic functions. Configurations found to sustain physiological function in vitro also result in survival and function in mice for at least 4 weeks, demonstrating the importance of architectural optimization before implantation.

Hepatocyte-like cells differentiated from human induced pluripotent stem cells: relevance to cellular therapies

Maturation of induced pluripotent stem cells (hiPSCs) to hepatocyte-like cells (HLCs) has been proposed to address the shortage of human hepatocytes for therapeutic applications. The purpose of this study was to evaluate hiPSCs, HLCs and hepatocytes, all of human origin, in terms of performance metrics of relevance to cell therapies. hiPSCs were differentiated to HLCs in vitro using an established four-stage approach. We observed that hiPSCs had low oxygen consumption and possessed small, immature mitochondria located around the nucleus. With maturation to HLCs, mitochondria showed characteristic changes in morphology, ultrastructure, and gene expression. These changes in mitochondria included elongated morphology, swollen cristae, dense matrices, cytoplasmic migration, increased expression of mitochondrial DNA transcription and replication-related genes, and increased oxygen consumption. Following differentiation, HLCs expressed characteristic hepatocyte proteins including albumin and hepatocyte nuclear factor 4-alpha, and intrinsic functions including cytochrome P450 metabolism. But HLCs also expressed high levels of alpha fetoprotein, suggesting a persistent immature phenotype or inability to turn off early stage genes. Furthermore, the levels of albumin production, urea production, cytochrome P450 activity, and mitochondrial function of HLCs were significantly lower than primary human hepatocytes.

CONCLUSION

- hiPSCs offer an unlimited source of human HLCs. However, reduced functionality of HLCs compared to primary human hepatocytes limits their usefulness in clinical practice. Novel techniques are needed to complete differentiation of hiPSCs to mature hepatocytes.

A novel 3D liver organoid system for elucidation of hepatic glucose metabolism

Hepatic glucose metabolism is a key player in diseases such as obesity and diabetes as well as in antihyperglycemic drugs screening. Hepatocytes culture in two-dimensional configurations is limited in vitro model for hepatocytes to function properly, while truly practical platforms to perform three-dimensional (3D) culture are unavailable. In this work, we present a practical organoid culture method of hepatocytes for elucidation of glucose metabolism under nominal and stress conditions. Employing this new method of culturing cells within a hollow fiber reactor, hepatocytes were observed to self-assemble into 3D spherical organoids with preservation of tight junctions and display increased liver-specific functions. Compared to both monolayer culture and sandwich culture, the hepatocyte organoids displayed higher intracellular glycogen content, glucose consumption, and gluconeogenesis and approached the in vivo values, as also confirmed by gene expression of key enzymes. Moreover, hepatocyte organoids demonstrated more realistic sensitivity to hormonal challenges with insulin, glucagon, and dexamethasone. Finally, the exposure to high glucose demonstrated toxicities including alteration of mitochondrial membrane potential, lipid accumulation, and reactive oxygen species formation, similar to the in vivo responses, which was not captured by monolayer cultures. Collectively, hepatocyte organoids mimicked the in vivo functions better than hepatocyte monolayer and sandwich cultures, suggesting suitability for applications such as antihyperglycemic drugs screening.

Biomaterials to prevascularize engineered tissues

Tissue engineering promises to restore tissue and organ function following injury or failure by creating functional and transplantable artificial tissues. The development of artificial tissues with dimensions that exceed the diffusion limit (1-2 mm) will require nutrients and oxygen to be delivered via perfusion (or convection) rather than diffusion alone. One strategy of perfusion is to prevascularize tissues; that is, a network of blood vessels is created within the tissue construct prior to implantation, which has the potential to significantly shorten the time of functional vascular perfusion from the host. The prevascularized network of vessels requires an extracellular matrix or scaffold for 3D support, which can be either natural or synthetic. This review surveys the commonly used biomaterials for prevascularizing 3D tissue engineering constructs.

Bile canaliculi formation by aligning rat primary hepatocytes in a microfluidic device

In this study, we propose a microfluidic cell culture device mimicking the microscopic structure in liver tissue called hepatic cords. The cell culture area of the device was designed to align hepatocytes in two lines in a similar way to hepatic cords. Thanks to the structural design together with a cell seeding procedure, rat primary hepatocytes were successfully aligned in two lines and cultured under perfusion condition. It is shown that aligned hepatocytes gradually self-organize and form bile canaliculi along the hepatic cord-like structure. The present technique to culture hepatocytes with functional bile canaliculi could be used as an alternative to animal testing in the field of drug discovery and toxicological studies, and also be beneficial to tissue engineering applications.

Metabolic profiling based quantitative evaluation of hepatocellular metabolism in presence of adipocyte derived extracellular matrix

The elucidation of the effect of extracellular matrices on hepatocellular metabolism is critical to understand the mechanism of functional upregulation. We have developed a system using natural extracellular matrices [Adipogel] for enhanced albumin synthesis of rat hepatocyte cultures for a period of 10 days as compared to collagen sandwich cultures. Primary rat hepatocytes isolated from livers of female Lewis rats recover within 4 days of culture from isolation induced injury while function is stabilized at 7 days post-isolation. Thus, the culture period can be classified into three distinct stages viz. recovery stage [day 0–4], pre-stable stage [day 5–7] and the stable stage [day 8–10]. A Metabolic Flux Analysis of primary rat hepatocytes cultured in Adipogel was performed to identify the key metabolic pathways modulated as compared to collagen sandwich cultures. In the recovery stage [day 4], the collagen-soluble Adipogel cultures shows an increase in TriCarboxylic Acid [TCA] cycle fluxes; in the pre-stable stage [day 7], there is an increase in PPP and TCA cycle fluxes while in the stable stage [day 10], there is a significant increase in TCA cycle, urea cycle fluxes and amino acid uptake rates concomitant with increased albumin synthesis rate as compared to collagen sandwich cultures throughout the culture period. Metabolic analysis of the collagen-soluble Adipogel condition reveals significantly higher transamination reaction fluxes, amino acid uptake and albumin synthesis rates for the stable vs. recovery stages of culture. The identification of metabolic pathways modulated for hepatocyte cultures in presence of Adipogel will be a useful step to develop an optimization algorithm to further improve hepatocyte function for Bioartificial Liver Devices. The development of this framework for upregulating hepatocyte function in Bioartificial Liver Devices will facilitate the utilization of an integrated experimental and computational approach for broader applications of Adipogel in tissue e engineering and regenerative medicine.

Microfluidic devices for studying heterotypic cell-cell interactions and tissue specimen cultures under controlled microenvironments

Microfluidic devices allow for precise control of the cellular and noncellular microenvironment at physiologically relevant length- and time-scales. These devices have been shown to mimic the complex in vivo microenvironment better than conventional in vitro assays, and allow real-time monitoring of homotypic or heterotypic cellular interactions. Microfluidic culture platforms enable new assay designs for culturing multiple different cell populations and∕or tissue specimens under controlled user-defined conditions. Applications include fundamental studies of cell population behaviors, high-throughput drug screening, and tissue engineering. In this review, we summarize recent developments in this field along with studies of heterotypic cell-cell interactions and tissue specimen culture in microfluidic devices from our own laboratory.

Embryonic and induced pluripotent stem cells as a model for liver disease

Induced pluripotent stem (iPS) cells are human somatic cells that have been reprogrammed to a pluripotent state. Through several elegant technologies, we are now able to generate human iPS cells with disease genotypes that could serve as invaluable tools for human disease modeling. This could lead to an understanding of the root causes of a disease and to the development of effective prophylactic and therapeutic strategies for it. However, we are still far from generating fully functional liver cells from stem cells, including iPS cells, on in vitro culture systems. Tissue-engineering techniques have opened the window to inducing a functional fate for differentiated cells by providing a microenvironment that allows the maintenance of signals similar to those found in the natural microenvironment. Here we review the current technology to establish iPS cells and discuss strategies to generate human liver disease modeling using iPS cell technology in concert with bioengineering approaches.

Engineering hydrogels as extracellular matrix mimics

Extracellular matrix (ECM) is a complex cellular environment consisting of proteins, proteoglycans, and other soluble molecules. ECM provides structural support to mammalian cells and a regulatory milieu with a variety of important cell functions, including assembling cells into various tissues and organs, regulating growth and cell-cell communication. Developing a tailored in vitro cell culture environment that mimics the intricate and organized nanoscale meshwork of native ECM is desirable. Recent studies have shown the potential of hydrogels to mimic native ECM. Such an engineered native-like ECM is more likely to provide cells with rational cues for diagnostic and therapeutic studies. The research for novel biomaterials has led to an extension of the scope and techniques used to fabricate biomimetic hydrogel scaffolds for tissue engineering and regenerative medicine applications. In this article, we detail the progress of the current state-of-the-art engineering methods to create cell-encapsulating hydrogel tissue constructs as well as their applications in in vitro models in biomedicine.

Organ reengineering through development of a transplantable recellularized liver graft using decellularized liver matrix

Orthotopic liver transplantation is the only available treatment for severe liver failure, but it is currently limited by organ shortage. One technical challenge that has thus far limited the development of a tissue-engineered liver graft is oxygen and nutrient transport. Here we demonstrate a novel approach to generate transplantable liver grafts using decellularized liver matrix. The decellularization process preserves the structural and functional characteristics of the native microvascular network, allowing efficient recellularization of the liver matrix with adult hepatocytes and subsequent perfusion for in vitro culture. The recellularized graft supports liver-specific function including albumin secretion, urea synthesis and cytochrome P450 expression at comparable levels to normal liver in vitro. The recellularized liver grafts can be transplanted into rats, supporting hepatocyte survival and function with minimal ischemic damage. These results provide a proof of principle for the generation of a transplantable liver graft as a potential treatment for liver disease.

Cell delivery: from cell transplantation to organ engineering

Cell populations derived from adult tissue and stem cells possess a great expectation for the treatment of several diseases. Great efforts have been made to generate cells with therapeutic impact from stem cells. However, it is clear that the development of systems to deliver such cells to induce efficient engraftment, growth, and function is a real necessity. Biologic and artificial scaffolds have received significant attention for their potential therapeutic application when use to form tissues in vitro and facilitate engraftment in vivo. Ultimately more sophisticated methods for decellularization of organs have been successfully used in tissue engineering and regenerative medicine applications. These decellularized tissues and organs appear to provide bioactive molecules and bioinductive properties to induce homing, differentiation, and proliferation of cells. The combination of decellularized organs and stem cells may dramatically improve the survival, engraftment, and fate control of transplanted stem cells and their ultimate clinical utility, opening the doors to a new era of organ engineering.

Imaging glucose metabolism in perfluorocarbon-perfused hepatocyte bioreactors using positron emission tomography

In vitro hepatocyte bioreactor functionality depends particularly on maintaining appropriate oxygen levels and exposure to nonparenchymal cells. An attractive solution without immunological consequences to the patient is incorporating a perfluorocarbon oxygen carrier in the circulating medium and co-culturing hepatocytes with stellate cells. Since bioreactors are normally sealed sterile units, demonstrating metabolic functionality is hindered by limited access to the cells after their aggregation in the matrix. A novel possibility is to use positron emission tomography (PET) to image cellular radioactive glucose uptake under O(2)-limited conditions. In this study, primary cell isolation procedures were carried out on eight pigs. Pairs of cell-seeded and cell-free (control) bioreactors with and without perfluorocarbon were cultured under identical conditions and were oxygenated using hypoxic (5% O(2)) and ambient (20% O(2)) gas mixes. Sixteen PET scans were conducted 24 h after cell isolation, the same timescale as that involved in treating a liver failure patient with a primary-cell bioreactor. In all cases, cell-seeded bioreactors without perfluorocarbon were more radioactive, i.e., were more glycolytic, than those with perfluorocarbon. This difference was significant in the hypoxic pair of bioreactors but not in the ambient pair of bioreactors. Additionally, in the same hypoxic bioreactors, circulating extracellular steady-state glucose levels were significantly lower and lactate levels were higher than those in the ambient bioreactors. Similar findings have been made in other in vitro hepatocyte studies investigating the effects of perfluorocarbons. PET is attractive for studying in situ O(2)-dependent bioreactor metabolism because of its visual and numerically quantifiable outputs. Longer-term metabolic studies (e.g., 5-10 days) investigating the effect of perfluorocarbon on bioreactor longevity will complement these findings in the future.

Oxygen-mediated enhancement of primary hepatocyte metabolism, functional polarization, gene expression, and drug clearance

The liver is a major site for the metabolism of xenobiotic compounds due to its abundant level of phase I/II metabolic enzymes. With the cost of drug development escalating to over $400 million/drug there is an urgent need for the development of rigorous models of hepatic metabolism for preclinical screening of drug clearance and hepatotoxicity. Here, we present a microenvironment in which primary human and rat hepatocytes maintain a high level of metabolic competence without a long adaptation period. We demonstrate that co-cultures of hepatocytes and endothelial cells in serum-free media seeded under 95% oxygen maintain functional apical and basal polarity, high levels of cytochrome P450 activity, and gene expression profiles on par with freshly isolated hepatocytes. These oxygenated co-cultures demonstrate a remarkable ability to predict in vivo drug clearance rates of both rapid and slow clearing drugs with an R(2) of 0.92. Moreover, as the metabolic function of oxygenated co-cultures stabilizes overnight, preclinical testing can be carried out days or even weeks before other culture methods, significantly reducing associated labor and cost. These results are readily extendable to other culture configurations including three-dimensional culture, bioreactor studies, as well as microfabricated co-cultures.

Augmentation of EB-directed hepatocyte-specific function via collagen sandwich and SNAP

The development of implantable engineered liver tissue constructs and ex vivo hepatocyte-based therapeutic devices are limited by an inadequate hepatocyte cell source. In our previous studies, embryoid body (EB)-mediated stem cell differentiation spontaneously yielded populations of hepatocyte lineage cells expressing mature hepatocyte markers such as albumin (ALB) and cytokeratin-18 (CK18). However, these cultures neither yielded a homogenous hepatocyte lineage population nor exhibited detoxification function typical of a more mature hepatocyte lineage cell. In this study, secondary culture configurations were used to study the effects of collagen sandwich culture and oncostatin-M (OSM) or S-nitroso-N-acetylpenicillamine (SNAP) supplementation of EB-derived hepatocyte-lineage cell function. Quantitative immunofluorescence and secreted protein analyses were used to provide insights into the long-term maintenance and augmentation of existing functions. The results of these studies suggest that SNAP, independent of the collagen supplementation, maintained the highest levels of ALB expression, however, mature liver-specific CK18 was only expressed in the presence of gel sandwich culture supplemented with SNAP. In addition, albumin secretion and cytochrome P450 detoxification studies indicated that this condition was the best for the augmentation of hepatocyte-like function. Maintenance and augmentation of hepatocyte-like cells isolated from heterogeneous EB cell populations will be a critical step in generating large numbers of functional differentiated cells for therapeutic use.

High-throughput single cell arrays as a novel tool in biopreservation

Microwell array cytometry is a novel high-throughput experimental technique that makes it possible to correlate pre-stress cell phenotypes and post-stress outcomes with single cell resolution. Because the cells are seeded in a high density grid of cell-sized microwells, thousands of individual cells can be tracked and imaged through manipulations as extreme as freezing or drying. Unlike flow cytometry, measurements can be made at multiple time points for the same set of cells. Unlike conventional image cytometry, image analysis is greatly simplified by arranging the cells in a spatially defined pattern and physically separating them from one another. To demonstrate the utility of microwell array cytometry in the field of biopreservation, we have used it to investigate the role of mitochondrial membrane potential in the cryopreservation of primary hepatocytes. Even with optimized cryopreservation protocols, the stress of freezing almost always leads to dysfunction or death in part of the cell population. To a large extent, cell fate is dominated by the stochastic nature of ice crystal nucleation, membrane rupture, and other biophysical processes, but natural variation in the initial cell population almost certainly plays an important and under-studied role. Understanding why some cells in a population are more likely to survive preservation will be invaluable for the development of new approaches to improve preservation yields. For this paper, primary hepatocytes were seeded in microwell array devices, imaged using the mitochondrial dyes Rh123 or JC-1, cryopreserved for up to a week, rapidly thawed, and checked for viability after a short recovery period. Cells with a high mitochondrial membrane potential before freezing were significantly less likely to survive the freezing process, though the difference in short term viability was fairly small. The results demonstrate that intrinsic cell factors do play an important role in cryopreservation survival, even in the short term where extrinsic biophysical factors would be expected to dominate. We believe that microwell array cytometry will be an important tool for a wide range of studies in biopreservation and stress biology.

Amino acid-mediated heterotypic interaction governs performance of a hepatic tissue model

Tissue-engineered models that mimic in vivo tissue organization offer the potential of capturing complex signaling pathways in vitro. In the liver, hepatocytes and endothelial cells are closely associated but separated by the extracellular matrix of the space of Disse. This unique configuration was mimicked by embedding primary hepatocytes in collagen gel and overlaying the matrix with endothelial cells. We demonstrate that during the first few days of culture, the secretion of albumin and fibrinogen was 2-fold higher in cocultures compared to hepatocytes alone. Hepatocyte function in both cultures stabilized to a similar level during the second week, suggesting that endothelial cells can induce the early recovery of hepatocytes after isolation and seeding. Endothelial cell-conditioned medium reproduced the effect of coculture in a dose-dependent fashion, suggesting a role for endothelial cell-derived soluble factors. Endothelial cell-conditioned medium increased mRNA levels of various acute-phase proteins such as albumin, fibrinogen, transferrin, and alpha-macroglobulin in hepatocytes. Surprisingly, the effect of endothelial cell-conditioned medium was not mediated by growth factors or cytokines, or by secreted extracellular matrix, but by the release of the amino acid proline, which mediates endogenous collagen synthesis by hepatocytes. These findings suggest an important role for proline secretion by endothelial cells as a paracrine factor regulating hepatocyte function.

Transport-mediated angiogenesis in 3D epithelial coculture

Increasing interest has focused on capturing the complexity of tissues and organs in vitro as models of human pathophysiological processes. In particular, a need exists for a model that can investigate the interactions in three dimensions (3D) between epithelial tissues and a microvascular network since vascularization is vital for reconstructing functional tissues in vitro. Here, we implement a microfluidic platform to analyze angiogenesis in 3D cultures of rat primary hepatocytes and rat/human microvascular endothelial cells (rMVECs/hMVECs). Liver and vascular cells were cultured on each sidewall of a collagen gel scaffold between two microfluidic channels under static or flow conditions. Morphogenesis of 3D hepatocyte cultures was found to depend on diffusion and convection across the nascent tissue. Furthermore, rMVECs formed 3D capillary-like structures that extended across an intervening gel to the hepatocyte tissues in hepatocyte-rMVEC coculture while they formed 2D sheet-like structures in rMVEC monoculture. In addition, diffusion of fluorescent dextran across the gel scaffold was analyzed, demonstrating that secreted proteins from the hepatocytes and MVECs can be exchanged across the gel scaffold by diffusional transport. The experimental approach described here is useful more generally for investigating microvascular networks within 3D engineered tissues with multiple cell types in vitro.

Synthetic microvascular networks for quantitative analysis of particle adhesion

We have developed a methodology to study particle adhesion in the microvascular environment using microfluidic, image-derived microvascular networks on a chip accompanied by Computational Fluid Dynamics (CFD) analysis of fluid flow and particle adhesion. Microfluidic networks, obtained from digitization of in vivo microvascular topology were prototyped using soft-lithography techniques to obtain semicircular cross sectional microvascular networks in polydimethylsiloxane (PDMS). Dye perfusion studies indicated the presence of well-perfused as well as stagnant regions in a given network. Furthermore, microparticle adhesion to antibody coated networks was found to be spatially non-uniform as well. These findings were broadly corroborated in the CFD analyses. Detailed information on shear rates and particle fluxes in the entire network, obtained from the CFD models, were used to show global adhesion trends to be qualitatively consistent with current knowledge obtained using flow chambers. However, in comparison with a flow chamber, this method represents and incorporates elements of size and complex morphology of the microvasculature. Particle adhesion was found to be significantly localized near the bifurcations in comparison with the straight sections over the entire network, an effect not observable with flow chambers. In addition, the microvascular network chips are resource effective by providing data on particle adhesion over physiologically relevant shear range from even a single experiment. The microfluidic microvascular networks developed in this study can be readily used to gain fundamental insights into the processes leading to particle adhesion in the microvasculature.