Use of hydrogel scaffolds to develop an in vitro 3D culture model of human intestinal epithelium

Multiscale Materials Engineering via Self-Assembly of Pentapeptide Derivatives from SARS CoV E Protein

Short peptide-based supramolecular hydrogels hold enormous potential for a wide range of applications. However, the gelation of these systems is very challenging to control. Minor changes in the peptide sequence can significantly influence the self-assembly mechanism and thereby the gelation propensity. The involvement of SARS CoV E protein in the assembly and release of the virus suggests that it may have inherent self-assembling properties that can contribute to the development of hydrogels. Here, three pentapeptide sequences derived from C-terminal of SARS CoV E protein are explored with same amino acid residues but different sequence distributions and discovered a drastic difference in the gelation propensity. By combining spectroscopic and microscopic techniques, the relationship between peptide sequence arrangement and molecular assembly structure are demonstrated, and how these influence the mechanical properties of the hydrogel. The present study expands the variety of secondary structures for generating supramolecular hydrogels by introducing the 310-helix as the primary building block for gelation, facilitated by a water-mediated structural transition into β-sheet conformation. Moreover, these Fmoc-modified pentapeptide hydrogels/supramolecular assemblies with tunable morphology and mechanical properties are suitable for tissue engineering, injectable delivery, and 3D bio-printing applications.

The in vitro intestinal cell model: different co-cultured cells create different applications

As a vitro absorption model, the Caco-2 cells originate from a human colon adenocarcinomas and can differentiate into a cell layer with enterocyte-like features. The Caco-2 cell model is popularly applied to explore drug transport mechanisms, to evaluate the permeability of drug and to predict the absorption of drugs or bioactive substances in the gut. However, there are limitations to the application of Caco-2 cell model due to lack of a mucus layer, the long culture period and the inability to accurately simulate the intestinal environment. The most frequent way to expand the Caco-2 cell model and address its limitations is by co-culturing it with other cells or substances. This article reviews the culture methods and applications of 3D and 2D co-culture cell models established around Caco-2 cells. It also concludes with a summary of model strengths and weaknesses.

Tissue regeneration properties of hydrogels derived from biological macromolecules: A review

The successful tissue engineering depends on the development of biologically active scaffolds that possess optimal characteristics to effectively support cellular functions, maintain structural integrity and aid in tissue regeneration. Hydrogels have emerged as promising candidates in tissue regeneration due to their resemblance to the natural extracellular matrix and their ability to support cell survival and proliferation. The integration of hydrogel scaffold into the polymer has a variable impact on the pseudo extracellular environment, fostering cell growth/repair. The modification in size, shape, surface morphology and porosity of hydrogel scaffolds has consequently paved the way for addressing diverse challenges in the tissue engineering process such as tissue architecture, vascularization and simultaneous seeding of multiple cells. The present review provides a comprehensive update on hydrogel production using natural and synthetic biomaterials and their underlying mechanisms. Furthermore, it delves into the application of hydrogel scaffolds in tissue engineering for cardiac tissues, cartilage tissue, adipose tissue, nerve tissue and bone tissue. Besides, the present article also highlights various clinical studies, patents, and the limitations associated with hydrogel-based scaffolds in recent times.

Development and Characterization of an In Vitro Intestinal Model Including Extracellular Matrix and Macrovascular Endothelium

In vitro intestinal models are used to study biological processes, drug and food absorption, or cytotoxicity, minimizing the use of animals in the laboratory. They usually consist of enterocytes and mucus-producing cells cultured for 3 weeks, e.g., on Transwells, to obtain a fully differentiated cell layer simulating the human epithelium. Other important components are the extracellular matrix (ECM) and strong vascularization. The former serves as structural support for cells and promotes cellular processes such as differentiation, migration, and growth. The latter includes endothelial cells, which coordinate vascularization and immune cell migration and facilitate the transport of ingested substances or drugs to the liver. In most cases, animal-derived hydrogels such as Matrigel or collagen are used as ECM in in vitro intestinal models, and endothelial cells are only partially considered, if at all. However, it is well-known that animal-derived products can lead to altered cell behavior and incorrect results. To circumvent these limitations, synthetic and modifiable hydrogels (Peptigel and Vitrogel) were studied here to mimic xenofree ECM, and the data were compared with Matrigel. Careful rheological characterization was performed, and the effect on cell proliferation was investigated. The results showed that Vitrogel exhibited shear-thinning behavior with an internal structure recovery of 78.9 ± 11.2%, providing the best properties among the gels investigated. Therefore, a coculture of Caco-2 and HT29-MTX cells (ratio 7:3) was grown on Vitrogel, while simultaneously endothelial cells were cultured on the basolateral side by inverse cultivation. The model was characterized in terms of cell proliferation, differentiation, and drug permeability. It was found that the cells cultured on Vitrogel induced a 1.7-fold increase in cell proliferation and facilitated the formation of microvilli and tight junctions after 2 weeks of cultivation. At the same time, the coculture showed full differentiation indicated by high alkaline phosphatase release of Caco-2 cells (95.0 ± 15.9%) and a mucus layer produced by HT29-MTX cells. Drug tests led to ex vivo comparable permeability coefficients (Papp) (i.e., Papp; antipyrine = (33.64 ± 5.13) × 10-6 cm/s, Papp; atenolol = (0.59 ± 0.16) × 10-6 cm/s). These results indicate that the newly developed intestinal model can be used for rapid and efficient assessment of drug permeability, excluding unexpected results due to animal-derived materials.

The shape of our gut: Dissecting its impact on drug absorption in a 3D bioprinted intestinal model

The small intestine is a complex organ with a characteristic architecture and a major site for drug and nutrient absorption. The three-dimensional (3D) topography organized in finger-like protrusions called villi increases surface area remarkably, granting a more efficient absorption process. The intestinal mucosa, where this process occurs, is a multilayered and multicell-type tissue barrier. In vitro intestinal models are routinely used to study different physiological and pathological processes in the gut, including compound absorption. Still, standard models are typically two-dimensional (2D) and represent only the epithelial barrier, lacking the cues offered by the 3D architecture and the stromal components present in vivo, often leading to inaccurate results. In this work, we studied the impact of the 3D architecture of the gut on drug transport using a bioprinted 3D model of the intestinal mucosa containing both the epithelial and the stromal compartments. Human intestinal fibroblasts were embedded in a previously optimized hydrogel bioink, and enterocytes and goblet cells were seeded on top to mimic the intestinal mucosa. The embedded fibroblasts thrived inside the hydrogel, remodeling the surrounding extracellular matrix. The epithelial cells fully covered the hydrogel scaffolds and formed a uniform cell layer with barrier properties close to in vivo. In particular, the villus-like model revealed overall increased permeability compared to a flat counterpart composed by the same hydrogel and cells. In addition, the efflux activity of the P-glycoprotein (P-gp) transporter was significantly reduced in the villus-like scaffold compared to a flat model, and the genetic expression of other drugs transporters was, in general, more relevant in the villus-like model. Globally, this study corroborates that the presence of the 3D architecture promotes a more physiological differentiation of the epithelial barrier, providing more accurate data on drug absorbance measurements.

Comparison of the Potential Relative Bioaccessibility of Zinc Supplements-In Vitro Studies

The aim of this study was to determine the potential relative bioaccessibility of zinc (Zn) from selected dietary supplements during in vitro digestion. The bioaccessibility of Zn was evaluated in dietary supplements differing in the pharmaceutical form, content, dose, and chemical form of the element. The content of Zn was determined by flame atomic absorption spectrometry. The applied method was validated, and results were characterised by good linearity (R² = 0.998), recovery (109%), and accuracy (0.02%). As a result of the tests conducted, it was found that the bioaccessibility of Zn from dietary supplements varied and ranged from 1.1% to 9.4%. The highest bioaccessibility was found for zinc diglycinate and the lowest for zinc sulphate. In 9 out of 10 tested dietary supplements, the determined Zn content was higher than the one declared by the producer (up to 161%). The estimated tolerable upper intake level (UL) was exceeded by five of the analysed dietary supplements (123-146%). The analysed dietary supplements were assessed in terms of compliance with the information contained on the product packaging, based on current Polish and European legal regulations. The qualitative assessment was performed according to the United States Pharmacopoeia (USP) guidelines.

Nanotechnology for research and treatment of the intestine

The establishment of intestinal in vitro models is crucial for elucidating intestinal cell-microbe intrinsic connections and interaction mechanisms to advance normalized intestinal diagnosis and precision therapy. This review discusses the application of nanomaterials in mucosal therapy and mechanism research in combination with the study of nanoscaffold in vitro models of the gut. By reviewing the original properties of nanomaterials synthesized by different physicochemical principles and modifying the original properties, the contribution of nanomaterials to solving the problems of short survival period, low cell differentiation rate, and poor reduction ability in traditional intestinal models is explored. According to nanomaterials’ different diagnostic mediators and therapeutic targets, the current diagnostic principles in inflammatory bowel disease, intestinal cancer, and other diseases are summarized inductively. In addition, the mechanism of action of nanomedicines in repairing mucosa, inhibiting inflammation, and alleviating the disease process is also discussed. Through such systematic elaboration, it offers a basis for nanomaterials to help advance in vitro research on the intestine and provide precision treatments in the clinic.

Bioinspired in vitro intestinal mucus model for 3D-dynamic culture of bacteria

The intestinal mucus is a biological barrier that supports the intestinal microbiota growth and filters molecules. To perform these functions, mucus possesses optimized microstructure and viscoelastic properties and it is steadily replenished thus flowing along the gut. The available in vitro intestinal mucus models are useful tools in investigating the microbiota-human cells interaction, and are used as matrices for bacterial culture or as static component of microfluidic devices like gut-on-chips. The aim of this work is to engineer an in vitro mucus models (I-Bac3Gel) addressing in a single system physiological viscoelastic properties (i.e., 2-200 Pa), 3D structure and suitability for dynamic bacterial culture. Homogeneously crosslinked alginate hydrogels are optimized in composition to obtain target viscoelastic and microstructural properties. Then, rheological tests are exploited to assess a priori the hydrogels capability to withstand the flow dynamic condition. We experimentally assess the suitability of I-Bac3Gels in the evolving field of microfluidics by applying a dynamic flow to a bacterial-loaded mucus model and by monitoring E. coli growth and survival. The engineered models represent a step forward in the modelling of the mucus, since they can answer to different urgent needs such as a 3D structure, bioinspired properties and compatibility with dynamic system.

Poly(N-isopropylacrylamide)-Based Hydrogels for Biomedical Applications: A Review of the State-of-the-Art

A prominent research topic in contemporary advanced functional materials science is the production of smart materials based on polymers that may independently adjust their physical and/or chemical characteristics when subjected to external stimuli. Smart hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) demonstrate distinct thermoresponsive features close to a lower critical solution temperature (LCST) that enhance their capability in various biomedical applications such as drug delivery, tissue engineering, and wound dressings. Nevertheless, they have intrinsic shortcomings such as poor mechanical properties, limited loading capacity of actives, and poor biodegradability. Formulation of PNIPAM with diverse functional constituents to develop hydrogel composites is an efficient scheme to overcome these defects, which can significantly help for practicable application. This review reports on the latest developments in functional PNIPAM-based smart hydrogels for various biomedical applications. The first section describes the properties of PNIPAM-based hydrogels, followed by potential applications in diverse fields. Ultimately, this review summarizes the challenges and opportunities in this emerging area of research and development concerning this fascinating polymer-based system deep-rooted in chemistry and material science.

Well-free agglomeration and on-demand three-dimensional cell cluster formation using guided surface acoustic waves through a couplant layer

Three-dimensional cell agglomerates are broadly useful in tissue engineering and drug testing. We report a well-free method to form large (1.4-mm) multicellular clusters using 100-MHz surface acoustic waves (SAW) without direct contact with the media or cells. A fluid couplant is used to transform the SAW into acoustic streaming in the cell-laden media held in a petri dish. The couplant transmits longitudinal sound waves, forming a Lamb wave in the petri dish that, in turn, produces longitudinal sound in the media. Due to recirculation, human embryonic kidney (HEK293) cells in the dish are carried to the center of the coupling location, forming a cluster in less than 10 min. A few minutes later, these clusters may then be translated and merged to form large agglomerations, and even repeatedly folded to produce a roughly spherical shape of over 1.4 mm in diameter for incubation-without damaging the existing intercellular bonds. Calcium ion signaling through these clusters and confocal images of multiprotein junctional complexes suggest a continuous tissue construct: intercellular communication. They may be formed at will, and the method is feasibly useful for formation of numerous agglomerates in a single petri dish.

Mucus-producing 3D cell culture models

In vitro cell-based models have been used for a long time since they are normally easily obtained and have an advantageous cost-benefit. Besides, they can serve a variety of ends, from studying drug absorption and metabolism to disease modeling. However, some in vitro models are too simplistic, not accurately representing the living tissues. It has been shown, mainly in the last years, that fully mimicking a tissue composition and architecture can be paramount for cellular behavior and, consequently, for the outcomes of the studies using such models. Because of this, 3D in vitro cell models have been gaining much attention, since they are able to better replicate the in vivo environment. In this review we focus on 3D models that contain mucus-producing cells, as mucus can play a pivotal role in drug absorption. Being frequently overlooked, this viscous fluid can have an impact on drug delivery. Thus, the aim of this review is to understand to which extent can mucus affect mucosal drug delivery and to provide a state-of-the-art report on the existing 3D cell-based mucus models.

An overview of in vitro, ex vivo and in vivo models for studying the transport of drugs across intestinal barriers

Oral administration is the most commonly used route for drug delivery owing to its cost-effectiveness, ease of administration, and high patient compliance. However, the absorption of orally delivered compounds is a complex process that greatly depends on the interplay between the characteristics of the drug/formulation and the gastrointestinal tract. In this contribution, we review the different preclinical models (in vitro, ex vivo and in vivo) from their development to application for studying the transport of drugs across intestinal barriers. This review also discusses the advantages and disadvantages of each model. Furthermore, the authors have reviewed the selection and validation of these models and how the limitations of the models can be addressed in future investigations. The correlation and predictability of the intestinal transport data from the preclinical models and human data are also explored. With the increasing popularity and prevalence of orally delivered drugs/formulations, sophisticated preclinical models with higher predictive capacity for absorption of oral formulations used in clinical studies will be needed.

In vitro models replicating the human intestinal epithelium for absorption and metabolism studies: A systematic review

Absorption, distribution, metabolism and excretion (ADME) studies represent a fundamental step in the early stages of drug discovery. In particular, the absorption of orally administered drugs, which occurs at the intestinal level, has gained attention since poor oral bioavailability often led to failures for new drug approval. In this context, several in vitro preclinical models have been recently developed and optimized to better resemble human physiology in the lab and serve as an animal alternative to accomplish the 3Rs principles. However, numerous models are ineffective in recapitulating the key features of the human small intestine epithelium and lack of prediction potential for drug absorption and metabolism during the preclinical stage. In this review, we provide an overview of in vitro models aimed at mimicking the intestinal barrier for pharmaceutical screening. After briefly describing how the human small intestine works, we present i) conventional 2D synthetic and cell-based systems, ii) 3D models replicating the main features of the intestinal architecture, iii) micro-physiological systems (MPSs) reproducing the dynamic stimuli to which cells are exposed in the native microenvironment. In this review, we will highlight the benefits and drawbacks of the leading intestinal models used for drug absorption and metabolism studies.

Advances in reconstructing intestinal functionalities in vitro: From two/three dimensional-cell culture platforms to human intestine-on-a-chip

Standard two/three dimensional (2D/3D)-cell culture platforms have facilitated the understanding of the communications between various cell types and their microenvironments. However, they are still limited in recapitulating the complex functionalities in vivo, such as tissue formation, tissue-tissue interface, and mechanical/biochemical microenvironments of tissues and organs. Intestine-on-a-chip platforms offer a new way to mimic intestinal behaviors and functionalities by constructing in vitro intestinal models in microfluidic devices. This review summarizes the advances and limitations of the state-of-the-art 2D/3D-cell culture platforms, animal models, intestine chips, and the combined multi-organ chips related with intestines. Their applications to studying intestinal functions, drug testing, and disease modeling are introduced. Different intestinal cell sources are compared in terms of gene expression abilities and the recapitulated intestinal morphologies. Among these cells, cells isolated form human intestinal tissues and derived from pluripotent stem cells appear to be more suitable for in vitro reconstruction of intestinal organs. Key challenges of current intestine-on-a-chip platforms and future directions are also discussed.

Preparation of Biological Scaffolds and Primary Intestinal Epithelial Cells to Efficiently 3D Model the Fish Intestinal Mucosa

Tissue engineering is an elegant tool to create organs in vitro, that can help obviate the lack of organ donors in transplantation medicine and provide the opportunity of studying complex biological systems in vitro, thereby reducing the need for animal experiments. Artificial intestine models are at the core of Fish-AI, an EU FET-Open research project dedicated to the development of a 3D in vitro platform that is intended to enable the aquaculture feed industry to predict the nutritional and health value of alternative feed sources accurately and efficiently.At present, it is impossible to infer the health and nutrition value through the chemical characterization of any given feed. Therefore, each new feed must be tested through in vivo growth trials. The procedure is lengthy, expensive and requires the use of many animals. Furthermore, although this process allows for a precise evaluation of the final effect of each feed, it does not improve our basic knowledge of the cellular and molecular mechanisms determining such end-results. In turn, this lack of mechanistic knowledge severely limits the capacity to understand and predict the biological value of a single raw material and of their different combinations.The protocol described herein allows to develop the two main components essential to produce a functional platform for the efficient and reliable screening of feeds that the feed industry is currently developing for improving their health and nutritional value. It is here applied to the Rainbow Trout, but it can be fruitfully used to many other fish species.

Advances in Engineering Human Tissue Models

Research in cell biology greatly relies on cell-based in vitro assays and models that facilitate the investigation and understanding of specific biological events and processes under different conditions. The quality of such experimental models and particularly the level at which they represent cell behavior in the native tissue, is of critical importance for our understanding of cell interactions within tissues and organs. Conventionally, in vitro models are based on experimental manipulation of mammalian cells, grown as monolayers on flat, two-dimensional (2D) substrates. Despite the amazing progress and discoveries achieved with flat biology models, our ability to translate biological insights has been limited, since the 2D environment does not reflect the physiological behavior of cells in real tissues. Advances in 3D cell biology and engineering have led to the development of a new generation of cell culture formats that can better recapitulate the in vivo microenvironment, allowing us to examine cells and their interactions in a more biomimetic context. Modern biomedical research has at its disposal novel technological approaches that promote development of more sophisticated and robust tissue engineering in vitro models, including scaffold- or hydrogel-based formats, organotypic cultures, and organs-on-chips. Even though such systems are necessarily simplified to capture a particular range of physiology, their ability to model specific processes of human biology is greatly valued for their potential to close the gap between conventional animal studies and human (patho-) physiology. Here, we review recent advances in 3D biomimetic cultures, focusing on the technological bricks available to develop more physiologically relevant in vitro models of human tissues. By highlighting applications and examples of several physiological and disease models, we identify the limitations and challenges which the field needs to address in order to more effectively incorporate synthetic biomimetic culture platforms into biomedical research.

Development of an Improved 3D in vitro Intestinal Model to Perform Permeability Studies of Paracellular Compounds

The small intestine is the primary site of drug absorption following oral administration, making paramount the proper monitoring of the absorption process. In vitro tools to predict intestinal absorption are particularly important in preclinical drug development since they are less laborious and cost-intensive and raise less ethical considerations compared to in vivo studies. The Caco-2 model is considered the gold standard of in vitro intestinal models regarding the prediction of absorption of orally delivered compounds. However, this model presents several drawbacks, such as the expression of tighter tight junctions, not being suitable to perform permeability of paracellular compounds. Besides, cells are representative of only one intestinal cell type, without considering the role of non-absorptive cells on the absorption pathway of drugs. In the present study, we developed a new three-dimensional (3D) intestinal model that aims to bridge the gap between in vitro tools and animal studies. Our 3D model comprises a collagen layer with human intestinal fibroblasts (HIFs) embedded, mimicking the intestinal lamina propria and providing 3D support for the epithelium, composed of Caco-2 cells and mucus-producing HT29-MTX cells, creating a model that can better resemble, both in terms of composition and regarding the outcomes of drug permeability when testing paracellular compounds, the human small intestine. The optimization of the collagen layer with HIFs was performed, testing different collagen concentrations and HIF seeding densities in order to avoid collagen contraction before day 14, maintaining HIF metabolically active inside the collagen disks during time in culture. HIF morphology and extracellular matrix (ECM) deposition were assessed, confirming that fibroblasts presented a normal and healthy elongated shape and secreted fibronectin and laminin, remodeling the collagen matrix. Regarding the epithelial layer, transepithelial electrical resistance (TEER) values decreased when cells were in the 3D configuration, comparing with the 2D analogs (Caco-2 and coculture of Caco-2+HT29-MTX models), becoming more similar with in vivo values. The permeability assay with fluorescein isothiocyanate (FITC)–Dextran 4 kDa showed that absorption in the 3D models is significantly higher than that in the 2D models, confirming the importance of using a more biorelevant model when testing the paracellular permeability of compounds.

The Intestinal Barrier and Current Techniques for the Assessment of Gut Permeability

The intestinal barrier is essential in human health and constitutes the interface between the outside and the internal milieu of the body. A functional intestinal barrier allows absorption of nutrients and fluids but simultaneously prevents harmful substances like toxins and bacteria from crossing the intestinal epithelium and reaching the body. An altered intestinal permeability, a sign of a perturbed barrier function, has during the last decade been associated with several chronic conditions, including diseases originating in the gastrointestinal tract but also diseases such as Alzheimer and Parkinson disease. This has led to an intensified interest from researchers with diverse backgrounds to perform functional studies of the intestinal barrier in different conditions. Intestinal permeability is defined as the passage of a solute through a simple membrane and can be measured by recording the passage of permeability markers over the epithelium via the paracellular or the transcellular route. The methodological tools to investigate the gut barrier function are rapidly expanding and new methodological approaches are being developed. Here we outline and discuss, in vivo, in vitro and ex vivo techniques and how these methods can be utilized for thorough investigation of the intestinal barrier.

Tools for probing host-bacteria interactions in the gut microenvironment: From molecular to cellular levels

Healthy function of the gut microenvironment is dependent on complex interactions between the bacteria of the microbiome, epithelial and immune (host) cells, and the surrounding tissue. Misregulation of these interactions is implicated in disease. A range of tools have been developed to study these interactions, from mechanistic studies to therapeutic evaluation. In this Digest, we highlight select tools at the cellular and molecular level for probing specific cell-microenvironment interactions. Approaches are overviewed for controlling and probing cell-cell interactions, from transwell and microfluidic devices to engineered bacterial peptidoglycan fragments, and cell-matrix interactions, from three-dimensional scaffolds to chemical handles for in situ modifications.

Use of l-pNIPAM hydrogel as a 3D-scaffold for intestinal crypts and stem cell tissue engineering

Intestinal stem cells hold great potential in tissue regeneration of the intestine, however, there are key limitations in their culture in vitro. We previously reported a novel synthetic non-biodegradable hydrogel as a 3D culture model for intestinal epithelium using Caco2 and HT29-MTX cells. Here, we investigated the potential of this system as a 3D scaffold for crypts and single intestinal stem cells to support long-term culture and differentiation. Intestinal crypts were extracted from murine small intestines and Lgr5+ stem cells isolated by magnetic activated cell sorting. Crypts and stem cells were suspended within Matrigel or l-pNIPAM for 14 days or suspended within Matrigel for 7 days then released, dissociated, and suspended within, or on l-pNIPAM hydrogel for 28 days. Cellular behaviour and phenotype were determined by histology and immunohistochemistry for stem cell and differentiation markers: Lgr5, E-cadherin MUC2 chromograninA and lysozymes. Isolated crypts and Lgr5+ intestinal stem cells formed enteroids with a central lumen surrounded by multiple crypt-like buds when cultured in Matrigel. In contrast, when crypts and stem cells were directly suspended within, or layered on l-pNIPAM hydrogel under dynamic culture conditions they formed spherical balls of cells, with no central lumen. When enteroids were initially formed in Matrigel from crypts or single Lgr5+ intestinal stem cells and dissociated into small fragments or single cells and transferred to l-pNIPAM hydrogel they formed new larger enteroids with numerous crypt-like buds. These crypt-like buds showed the presence of mucin-producing cells, which resembled goblet cells, scattered throughout their structures. Immunohistochemistry staining also showed the expression of Lgr5 and differentiation markers of all the main intestinal cell types including: enterocytes, goblet cells, enteroendocrine and Paneth cells. This demonstrated that l-pNIPAM hydrogel supported long-term culture of crypts and Lgr5+ stem cells and promoted intestinal cell differentiation.

A Guide to Human Zinc Absorption: General Overview and Recent Advances of In Vitro Intestinal Models

Zinc absorption in the small intestine is one of the main mechanisms regulating the systemic homeostasis of this essential trace element. This review summarizes the key aspects of human zinc homeostasis and distribution. In particular, current knowledge on human intestinal zinc absorption and the influence of diet-derived factors on bioaccessibility and bioavailability as well as intrinsic luminal and basolateral factors with an impact on zinc uptake are discussed. Their investigation is increasingly performed using in vitro cellular intestinal models, which are continually being refined and keep gaining importance for studying zinc uptake and transport via the human intestinal epithelium. The vast majority of these models is based on the human intestinal cell line Caco-2 in combination with other relevant components of the intestinal epithelium, such as mucin-secreting goblet cells and in vitro digestion models, and applying improved compositions of apical and basolateral media to mimic the in vivo situation as closely as possible. Particular emphasis is placed on summarizing previous applications as well as key results of these models, comparing their results to data obtained in humans, and discussing their advantages and limitations.

Repair and regeneration of small intestine: A review of current engineering approaches

The small intestine (SI) is difficult to regenerate or reconstruct due to its complex structure and functions. Recent developments in stem cell research, advanced engineering technologies, and regenerative medicine strategies bring new hope of solving clinical problems of the SI. This review will first summarize the structure, function, development, cell types, and matrix components of the SI. Then, the major cell sources for SI regeneration are introduced, and state-of-the-art biofabrication technologies for generating engineered SI tissues or models are overviewed. Furthermore, in vitro models and in vivo transplantation, based on intestinal organoids and tissue engineering, are highlighted. Finally, current challenges and future perspectives are discussed to help direct future applications for SI repair and regeneration.

Recent Strategies in Fabrication of Gradient Hydrogels for Tissue Engineering Applications

Hydrogels are widely used as scaffold in tissue engineering field because of their ability to mimic the cellular microenvironment. However, mimicking a completely natural cellular environment is complicated due to the differences in various physical and chemical properties of cellular environments. Recently, gradient hydrogels provide excellent heterogeneous environment to mimic the different cellular microenvironments. To create hydrogels with an anisotropic distribution, gradient hydrogels have been widely developed by adopting several gradient generation techniques. Herein, the various gradient hydrogel fabrication techniques, including dual syringe pump systems, microfluidic device, photolithography, diffusion, and bio-printing are summarized. As the effects of gradient 3D hydrogels with stems have been reviewed elsewhere, this review focuses principally on gradient hydrogel fabrication for multi-model tissue regeneration. This review provides new insights into the key points for fabrication of gradient hydrogels for multi-model tissue regeneration.

A human-on-a-chip approach to tackling rare diseases

Drug development for rare diseases, classified as diseases with a prevalence of < 200 000 patients, is limited by the high cost of research and low target population. Owing to a lack of representative disease models, research has been challenging for orphan drugs. Human-on-a-chip (HoaC) technology, which models human tissues in interconnected in vitro microfluidic devices, has the potential to lower the cost of preclinical studies and increase the rate of drug approval by introducing human phenotypic models early in the drug discovery process. Advances in HoaC technology can drive a new approach to rare disease research and orphan drug development.

Decellularized Human Gut as a Natural 3D Platform for Research in Intestinal Fibrosis

BACKGROUND

The current methodologies for the identification of therapeutic targets for inflammatory bowel disease (IBD) are limited to conventional 2-dimensional (2D) cell cultures and animal models. The use of 3D decellularized human intestinal scaffolds obtained from surgically resected intestine and engineered with human intestinal cells may provide a major advancement in the development of innovative intestinal disease models. The aim of the present study was to design and validate a decellularization protocol for the production of acellular 3D extracellular matrix (ECM) scaffolds from the human duodenum.

METHODS

Scaffolds were characterized by verifying the preservation of the ECM protein composition and 3D architecture of the native intestine and were employed for tissue engineering with primary human intestinal myofibroblasts for up to 14 days.

RESULTS

Engrafted cells showed the ability to grow and remodel the surrounding ECM. mRNA expression of key genes involved in ECM turnover was significantly different when comparing primary human intestinal myofibroblasts cultured in 3D scaffolds with those cultured in standard 2D cultures on plastic dishes. Moreover, incubation with key profibrogenic growth factors such as TGFβ1 and PDGF-BB resulted in markedly different effects in standard 2D vs 3D cultures, further emphasizing the importance of using 3D cell cultures.

CONCLUSIONS

These results confirm the feasibility of 3D culture of human intestinal myofibroblasts in intestinal ECM scaffolds as an innovative platform for disease modeling, biomarker discovery, and drug testing in intestinal fibrosis.

Advances and Current Challenges in Intestinal in vitro Model Engineering: A Digest

The physiological environment of the intestine is characterized by its variegated composition, numerous functions and unique dynamic conditions, making it challenging to recreate the organ in vitro. This review outlines the requirements for engineering physiologically relevant intestinal in vitro models, mainly focusing on the importance of the mechano-structural cues that are often neglected in classic cell culture systems. More precisely: the topography, motility and flow present in the intestinal epithelium. After defining quantitative descriptors for these features, we describe the current state of the art, citing relevant approaches used to address one (or more) of the elements in question, pursuing a progressive conceptual construction of an "ideal" biomimetic intestinal model. The review concludes with a critical assessment of the currently available methods to summarize the important features of the intestinal tissue in the light of their different applications.

Mini-gut: a promising model for drug development

Until recently, major advances in drug development have been hampered by a lack of proper cell and tissue models; but the introduction of organoid technology has revolutionized this field. At the level of the gastrointestinal tract, the so-called mini-gut comprises all major cell types of native intestine and recapitulates the composition and function of native intestinal epithelium. The mini-gut can be classified as an intestinal organoid (IO), derived from pluripotent stem cells, or as an enteroid, consisting only of epithelial cells and generated from adult stem cells. Both classifications have been used as models to develop drugs against cystic fibrosis, cancer and infectious disease, as well as for drug screening, personalized medicine and the development of new medical tools. In this review, we highlight and discuss the importance of mini-guts for drug development and point out their limitations and future prospects.

High-throughput acoustofluidic fabrication of tumor spheroids

Three-dimensional (3D) culture of multicellular spheroids, offering a desirable biomimetic microenvironment, is appropriate for recapitulating tissue cellular adhesive complexity and revealing a more realistic drug response. However, current 3D culture methods are suffering from low-throughput, poor controllability, intensive-labor, and variation in spheroid size, thus not ready for many high-throughput screening applications including drug discovery and toxicity testing. Herein, we developed a high-throughput multicellular spheroid fabrication method using acoustofluidics. By acoustically-assembling cancer cells with low-cost and disposable devices, our method can produce more than 12 000 multicellular aggregates within several minutes and allow us to transfer these aggregates into ultra-low attachment dishes for long-term culture. This method can generate more than 6000 tumor spheroids per operation, and reduce tumor spheroid formation time to one day. Our platform has advantages in forming spheroids with high throughput, short time, and long-term effectiveness, and is easy-to-operation. This acoustofluidic spheroid assembly method provides a simple and efficient way to produce large numbers of uniform-sized spheroids for biomedical applications in translational medicine, pharmaceutical industry and basic life science research.

Oral delivery of non-viral nucleic acid-based therapeutics - do we have the guts for this?

Gene therapy with RNA and pDNA-based drugs is limited by poor enzymatic stability and poor cellular permeation. The delivery of nucleic acids, in particular by the oral route, remains a major hurdle. This review will focus on the barriers to the oral delivery of nucleic acids and the strategies, in particular formulation strategies, which have been developed to overcome these barriers. Due to their very low oral bioavailability, the most obvious and most investigated biomedical applications for their oral delivery are related to the local treatment of inflammatory bowel diseases and colorectal cancers. Preclinical data but not yet clinical studies support the potential use of the oral route for the local delivery of formulated nucleic acid-based drugs.

Long-term in vitro 3D hydrogel co-culture model of inflammatory bowel disease

The in vitro study of the pathogenesis of inflammatory bowel disease (IBD) requires a cell model which closely reflects the characteristics of the in vivo intestinal epithelium. This study aimed to investigate the application of L-pNIPAM hydrogel as a scaffold to develop a long-term 3D co-culture model of Caco-2 and HT29-MTX cells under conditions analogous to inflammation, to determine its potential use in studying IBD. Monocultures and co-cultures were layered on L-pNIPAM hydrogel scaffolds and maintained under dynamic culture conditions for up to 12 weeks. Treatments with IL-1β, TNFα, and hypoxia for 1 week were used to create an inflammatory environment. Following prolonged culture, the metabolic activity of Caco-2 monoculture and 90% Caco-2/10% HT29-MTX co-cultures on L-pNIPAM hydrogels were increased, and finger-like structures, similar in appearance to villi were observed. Following treatment with IL-1β, TNFα and hypoxia, ALP and ZO-1 were decreased, MUC2 increased, and MUC5AC remained unchanged. ADAMTS1 was increased in response to hypoxia. Caspase 3 expression was increased in response to TNFα and hypoxic conditions. In conclusion, L-pNIPAM hydrogel supported long-term co-culture within a 3D model. Furthermore, stimulation with factors seen during inflammation recapitulated features seen during IBD.

Gut-microbiota-on-a-chip: an enabling field for physiological research

Overwhelming scientific evidence today confirms that the gut microbiota is a central player in human health. Knowledge about interactions between human gut microbiota and human health has evolved rapidly in the last decade, based on experimental work involving analysis of human fecal samples or animal models (mainly rodents). A more detailed and cost-effective description of this interplay is now being enabled by the use of in vitro systems (i.e., gut-microbiota-on-chip systems) that recapitulate key aspects of the interaction between microbiota and human cells. Here, we review recent examples of the design and use of pioneering on-chip platforms for the study of the cross-talk between representative members of human microbiota and human microtissues. In these systems, the combined use of state-of-the-art microfluidics, biomaterials, cell culture techniques, classical microbiology, and a touch of genetic expression profiling have converged for the development of gut-on-chip platforms capable of recreating key features of the interplay between human microbiota and host human tissues. We foresee that the integration of novel microfabrication techniques and stem cell technologies will further accelerate the development of more complex and physiologically relevant microbiota-on-chip platforms. In turn, this will foster the faster acquisition of knowledge regarding human microbiota and will enable important advances in the understanding of how to control or prevent disease.