Organotypic cocultures as skin equivalents: A complex and sophisticated in vitro system

Beyond skin deep: Revealing the essence of iPS cell-generated skin organoids in regeneration

Various methods have been used for in vivo and in vitro skin regeneration, including stem cell therapy, tissue engineering, 3D printing, and platelet-rich plasma (PRP) injection therapy. However, these approaches are rooted in the existing knowledge of skin structures, which overlook the normal physiological processes of skin development and fall short of replicating the skin's regenerative processes outside the body. This comprehensive review primarily focuses on skin organoids derived from human pluripotent stem cells, which have the capacity to regenerate human skin tissue by restoring the embryonic skin structure, thus offering a novel avenue for producing in vitro skin substitutes. Furthermore, they contribute to the repair of damaged skin lesions in patients with systemic sclerosis or severe burns. Particular emphasis will be placed on the origins, generations, and applications of skin organoids, especially in dermatology, and the challenges that must be addressed before clinical implementation.

Dipeptidyl Peptidase-4-Mediated Fibronectin Processing Evokes a Profibrotic Extracellular Matrix

Fibronectin serves as a platform to guide and facilitate deposition of collagen and fibrillin microfibrils. During development of fibrotic diseases, altered fibronectin deposition in the extracellular matrix (ECM) is generally an early event. After this, dysregulated organization of fibrillins and fibrillar collagens occurs. Because fibronectin is an essential orchestrator of healthy ECM, perturbation of its ECM-organizational capacity may be involved in development of fibrosis. To investigate this, we employed recessive dystrophic epidermolysis bullosa as a disease model with progressive, severe dermal fibrosis. Fibroblasts from donors with recessive dystrophic epidermolysis bullosa in 2-dimensional and 3-dimensional cultures displayed dysregulated fibronectin deposition. Our analyses revealed that increase of profibrotic dipeptidyl peptidase-4-positive fibroblasts coincides with altered fibronectin deposition. Dipeptidyl peptidase-4 inhibitors normalized deposition of fibronectin and subsequently of fibrillin microfibrils and collagen I. Intriguingly, proteomics and inhibitor and mutagenesis studies disclosed that dipeptidyl peptidase-4 modulates ECM deposition through the proteolysis of the fibronectin N-terminus. Our study provides mechanistic insights into the observed profibrotic activities of dipeptidyl peptidase-4 and extends the understanding of fibronectin-guided ECM assembly in health and disease.

3D modeling of normal skin and cutaneous squamous cell carcinoma. A comparative study in 2D cultures, spheroids, and 3D bioprinted systems

The current cancer research and drug testing are primarily based on 2D cell cultures and animal models. However, these methods have limitations and yield distinct drug response patterns. This study addressed this gap by developing an innovativein vitrohuman three-dimensional (3D) normal skin model and a multicellular model of human cutaneous squamous cell carcinoma (cSCC) using 3D bioprinting technology. Comparative analyzes were performed between bioprinted 3D-cSCC model, consisting of HaCaT keratinocytes, primary normal human dermal fibroblasts and A431 cancer cells (tricellular), bioprinted 3D-A431 model composed of A431 cancer cells only (monocellular), A431 cancer cell spheroids, and conventional 2D models. The models were structurally characterized by light microscopy, immunofluorescence (LIVE/DEAD assay, confocal microscopy) and immunohistochemistry (hematoxylin/eosin, p63, vimentin, Ki67, epidermal growth factor receptor stainings). The spatial arrangement of the 3D models was analyzed using the ARIVIS scientific image analysis platform. All models were also functionally assessed by cetuximab (CTX) response testing with the MTS assay. 3D-cSCC models were maintained for up to 16 weeks. Morphological and histological examinations confirmed the presence of skin-like layers in the bioprinted 3D models of normal skin, and the intricate and diverse features of the bioprinted skin cancer model, replicating the criticalin vivocharacteristics. In both mono- and tricellular bioprinted tumor constructs, there was a gradual formation and continuous growth of spheroid-like clusters of cancer cells, significantly influencing the morphology of the models. Cancer cells in the 3D bioprinted constructs showed reduced sensitivity to CTX compared to spheroids and 2D cultures. This study underscores the potential of 3D multicellular models in elucidating drug responses and gaining a better understanding the intricate interplay of cellular components within the tumor microenvironment. Developing the multicellular 3D tumor model paves the way for new research critical to advancing fundamental cancer research and future clinical applications, particularly drug response testing.

Comparative Study of Two-Dimensional (2D) vs. Three-Dimensional (3D) Organotypic Kertatinocyte-Fibroblast Skin Models for Staphylococcus aureus (MRSA) Infection

The invasion of skin tissue by Staphylococcus aureus is mediated by mechanisms that involve sequential breaching of the different stratified layers of the epidermis. Induction of cell death in keratinocytes is a measure of virulence and plays a crucial role in the infection progression. We established a 3D-organotypic keratinocyte-fibroblast co-culture model to evaluate whether a 3D-skin model is more effective in elucidating the differences in the induction of cell death by Methicillin-resistant Staphylococcus aureus (MRSA) than in comparison to 2D-HaCaT monolayers. We investigated the difference in adhesion, internalization, and the apoptotic index in HaCaT monolayers and our 3D-skin model using six strains of MRSA representing different clonal types, namely, ST8, ST30, ST59, ST22, ST45 and ST239. All the six strains exhibited internalization in HaCaT cells. Due to cell detachment, the invasion study was limited up to two and a half hours. TUNEL assay showed no significant difference in the cell death induced by the six MRSA strains in the HaCaT cells. Our 3D-skin model provided a better insight into the interactions between the MRSA strains and the human skin during the infection establishment as we could study the infection of MRSA in our skin model up to 48 h. Immunohistochemical staining together with TUNEL assay in the 3D-skin model showed co-localization of the bacteria with the apoptotic cells demonstrating the induction of apoptosis by the bacteria and revealed the variation in bacterial transmigration among the MRSA strains. The strain representing ST59 showed maximum internalization in HaCaT cells and the maximum cell death as measured by Apoptotic index in the 3D-skin model. Our results show that 3D-skin model might be more likely to imitate the physiological response of skin to MRSA infection than 2D-HaCaT monolayer keratinocyte cultures and will enhance our understanding of the difference in pathogenesis among different MRSA strains.

Human pluripotent stem cells: An alternative for 3D in vitro modelling of skin disease

To effectively study the skin and its pathology, various platforms have been used to date, with in vitro 3D skin models being considered the future gold standard. These models have generally been engineered from primary cell lines. However, their short life span leading to the use of various donors, imposes issues with genetic variation. Human pluripotent stem cell (hPSC)-technology holds great prospects as an alternative to the use of primary cell lines to study the pathophysiology of human skin diseases. This is due to their potential to generate an unlimited number of genetically identical skin models that closely mimic the complexity of in vivo human skin. During the past decade, researchers have therefore started to use human embryonic and induced pluripotent stem cells (hESC/iPSC) to derive skin resident-like cells and components. These have subsequently been used to engineer hPSC-derived 3D skin models. In this review, we focus on the advantages, recent developments, and future perspectives in using hPSCs as an alternative cell source for modelling human skin diseases in vitro.

Skin Disease Models In Vitro and Inflammatory Mechanisms: Predictability for Drug Development

Investigative skin biology, analysis of human skin diseases, and numerous clinical and pharmaceutical applications rely on skin models characterized by reproducibility and predictability. Traditionally, such models include animal models, mainly rodents, and cellular models. While animal models are highly useful in many studies, they are being replaced by human cellular models in more and more approaches amid recent technological development due to ethical considerations. The culture of keratinocytes and fibroblasts has been used in cell biology for many years. However, only the development of co-culture and three-dimensional epidermis and full-skin models have fundamentally contributed to our understanding of cell-cell interaction and cell signalling in the skin, keratinocyte adhesion and differentiation, and mechanisms of skin barrier function. The modelling of skin diseases has highlighted properties of the skin important for its integrity and cutaneous development. Examples of monogenic as well as complex diseases including atopic dermatitis and psoriasis have demonstrated the role of skin models to identify pathomechanisms and drug targets. Recent investigations have indicated that 3D skin models are well suitable for drug testing and preclinical studies of topical therapies. The analysis of skin diseases has recognized the importance of inflammatory mechanisms and immune responses and thus other cell types such as dendritic cells and T cells in the skin. Current developments include the production of more complete skin models comprising a range of different cell types. Organ models and even multi-organ systems are being developed for the analysis of higher levels of cellular interaction and drug responses and are among the most recent innovations in skin modelling. They promise improved robustness and flexibility and aim at a body-on-a-chip solution for comprehensive pharmaceutical in vitro studies.

Efficient CRISPR-Cas9-Mediated Gene Ablation in Human Keratinocytes to Recapitulate Genodermatoses: Modeling of Netherton Syndrome

Current efforts to find specific genodermatoses treatments and define precise pathogenesis mechanisms require appropriate surrogate models with human cells. Although transgenic and gene knockout mouse models for several of these disorders exist, they often fail to faithfully replicate the clinical and histopathological features of the human skin condition. We have established a highly efficient method for precise deletion of critical gene sequences in primary human keratinocytes, based on CRISPR-Cas9-mediated gene editing. Using this methodology, in the present study we generated a model of Netherton syndrome by disruption of SPINK5. Gene-edited cells showed absence of LEKTI expression and were able to recapitulate a hyperkeratotic phenotype with most of the molecular hallmarks of Netherton syndrome, after grafting to immunodeficient mice and in organotypic cultures. To validate the model as a platform for therapeutic intervention, we tested an ex vivo gene therapy approach using a lentiviral vector expressing SPINK5. Re-expression of SPINK5 in an immortalized clone of SPINK5-knockout keratinocytes was capable of reverting from Netherton syndrome to a normal skin phenotype in vivo and in vitro. Our results demonstrate the feasibility of modeling genodermatoses, such as Netherton syndrome, by efficiently disrupting the causative gene to better understand its pathogenesis and to develop novel therapeutic approaches.

A two-layer skin construct consisting of a collagen hydrogel reinforced by a fibrin-coated polylactide nanofibrous membrane

Background: Repairs to deep skin wounds continue to be a difficult issue in clinical practice. A promising approach is to fabricate full-thickness skin substitutes with functions closely similar to those of the natural tissue. For many years, a three-dimensional (3D) collagen hydrogel has been considered to provide a physiological 3D environment for co-cultivation of skin fibroblasts and keratinocytes. This collagen hydrogel is frequently used for fabricating tissue-engineered skin analogues with fibroblasts embedded inside the hydrogel and keratinocytes cultivated on its surface. Despite its unique biological properties, the collagen hydrogel has insufficient stiffness, with a tendency to collapse under the traction forces generated by the embedded cells. Methods: The aim of our study was to develop a two-layer skin construct consisting of a collagen hydrogel reinforced by a nanofibrous poly-L-lactide (PLLA) membrane pre-seeded with fibroblasts. The attractiveness of the membrane for dermal fibroblasts was enhanced by coating it with a thin nanofibrous fibrin mesh. Results: The fibrin mesh promoted the adhesion, proliferation and migration of the fibroblasts upwards into the collagen hydrogel. Moreover, the fibroblasts spontaneously migrating into the collagen hydrogel showed a lower tendency to contract and shrink the hydrogel by their traction forces. The surface of the collagen was seeded with human dermal keratinocytes. The keratinocytes were able to form a basal layer of highly mitotically-active cells, and a suprabasal layer. Conclusion: The two-layer skin construct based on collagen hydrogel with spontaneously immigrated fibroblasts and reinforced by a fibrin-coated nanofibrous membrane seems to be promising for the construction of full-thickness skin substitute.

Bioengineering the microanatomy of human skin

Recreating the structure of human tissues in the laboratory is valuable for fundamental research, testing interventions, and reducing the use of animals. Critical to the use of such technology is the ability to produce tissue models that accurately reproduce the microanatomy of the native tissue. Current artificial cell-based skin systems lack thorough characterisation, are not representative of human skin, and can show variation. In this study, we have developed a novel full thickness model of human skin comprised of epidermal and dermal compartments. Using an inert porous scaffold, we created a dermal construct using human fibroblasts that secrete their own extracellular matrix proteins, which avoids the use of animal-derived materials. The dermal construct acts as a foundation upon which epidermal keratinocytes were seeded and differentiated into a stratified keratinised epithelium. In-depth morphological analyses of the model demonstrated very close similarities with native human skin. Extensive immunostaining and electron microscopy analysis revealed ultrastructural details such as keratohyalin granules and lamellar bodies within the stratum granulosum, specialised junctional complexes, and the presence of a basal lamina. These features reflect the functional characteristics and barrier properties of the skin equivalent. Robustness and reproducibility of in vitro models are important attributes in experimental practice, and we demonstrate the consistency of the skin construct between different users. In summary, a new model of full thickness human skin has been developed that possesses microanatomical features reminiscent of native tissue. This skin model platform will be of significant interest to scientists researching the structure and function of human skin.

Isolation and Culture of Epidermolysis Bullosa Cells and Organotypic Skin Models

Isolation and culture of keratinocytes from patients with different types of epidermolysis bullosa are sometimes challenging, because of the inherent adhesion defects of these cells. We routinely employ a well-established protocol for in vitro culture of these cells from small skin samples remaining after diagnostic procedures are performed. Keratinocytes and fibroblast can be used for downstream expression and functional studies or for construction of in vitro organotypic cocultures. These cells maintain main common characteristics of spreading, adhesion, migration, and survival, which depend on the underlying molecular defect.

Characterization of the Growth of Chlamydia trachomatis in In Vitro-Generated Stratified Epithelium

Chlamydia infection targets the mucosal epithelium, where squamous and columnar epithelia can be found. Research on Chlamydia-epithelia interaction has predominantly focused on columnar epithelia, with very little known on how Chlamydia interacts with the squamous epithelium. The stratification and differentiation processes found in the squamous epithelium might influence chlamydial growth and infection dissemination. For this reason, three-dimensional (3D) organotypic stratified squamous epithelial cultures were adapted to mimic the stratified squamous epithelium and chlamydial infection was characterized. Chlamydia trachomatis infection in monolayers and 3D cultures were monitored by immunofluorescence and transmission electron microscopy to evaluate inclusion growth and chlamydial interconversion between elementary and reticulate body. We observed that the stratified epithelium varied in susceptibility to C. trachomatis serovars L2 and D infection. The undifferentiated basal cells were susceptible to infection by both serovars, while the terminally differentiated upper layers were resistant. The differentiating suprabasal cells exhibited different susceptibilities to serovars L2 and D, with the latter unable to establish a successful infection in this layer. Mature elementary body-containing inclusions were much more prevalent in these permissive basal layers, while the uppermost differentiated layers consistently harbored very few reticulate bodies with no elementary bodies, indicative of severely limited bacterial replication and development. For serovar D, the differentiation state of the host cell was a determining factor, as calcium-induced differentiation of cells in a monolayer negatively affected growth of this serovar, in contrast to serovar L2. The apparent completion of the developmental cycle in the basal layers of the 3D cultures correlated with the greater degree of dissemination within and the level of disruption of the stratified epithelium. Our studies indicate that the squamous epithelium is a suboptimal environment for growth, and thus potentially contributing to the protection of the lower genital tract from infection. The relatively more fastidious serovar D exhibited more limited growth than the faster-growing and more invasive L2 strain. However, if given access to the more hospitable basal cell layer, both strains were able to produce mature inclusions, replicate, and complete their developmental cycle.

3D Organotypic Culture Model to Study Components of ERK Signaling

Organotypic models are 3D in vitro representations of an in vivo environment. Their complexity can range from an epidermal replica to the establishment of a cancer microenvironment. These models have been used for many years, in an attempt to mimic the structure and function of cells and tissues found inside the body. Methods for developing 3D organotypic models differ according to the tissue of interest and the experimental design. For example, cultures may be grown submerged in culture medium and or at an air-liquid interface. Our group is focusing on an air-liquid interface 3D organotypic model. These cultures are grown on a nylon membrane-covered metal grid with the cells embedded in a Collagen-Matrigel gel. This allows cells to grow in an air-liquid interface to enable diffusion and nourishment from the medium below. Subsequently, the organotypic cultures can be used for immunohistochemical staining of various components of ERK signaling, which is a key player in mediating communication between cells and their microenvironment.

Construction and characterization of a multilayered gingival keratinocyte culture model: the TURK-U model

In construction of epithelial cells as multilayers, the cells are grown submerged to confluence on fibroblast-embedded collagen gels and, then, lifted to air to promote their stratification. We recently demonstrated that gingival epithelial cells form uniform monolayers on semi-permeable nitrocellulose membranes, supported with a semi-solid growth medium, which allows the cells to grow at an air-liquid-solid interface from the beginning of the culturing protocol. In this study, the aim was to further develop our previous model to form a multilayered gingival epithelial culture model. Two different epithelial cell lines (HaCaT from skin and HMK from gingiva) were used in all experiments. Both cell lines were grown first as monolayers for 3 days. After that, keratinocytes were trypsinized, counted and seeded on a sterile semi-permeable nitrocellulose membrane placed on the top of a semi-solid growth medium, forming an air-liquid-solid interface for the cells to grow. At days 1, 4, and 7, epithelial cells were fixed, embedded in paraffin, and sectioned for routine Hematoxylin-Eosin staining and immunohistochemistry for cytokeratin (Ck). At day 1, HMK cells grew as monolayers, while HaCaT cells stratified forming an epithelium with two to three layers. At day 4, a stratified epithelium in the HMK model had four to five layers and its proliferation continued up to day 7. HaCaT cells formed a dense and weakly proliferating epithelium with three to four layers of stratification at day 4 but the proliferation disappeared at day 7. At all days, both models were strongly positive for Ck5, Ck7, and Ck 19, and weakly positive for Ck10. Gingival epithelial cells stratify successfully on semi-permeable nitrocellulose membranes, supported with a semi-solid growth medium. This technique allows researchers to construct uniform gingival epithelial cell multilayers at an air-liquid-solid interface, without using collagen gels, resulting in a more reproducible method.

Growth of Self-Assembled Bio-Organic Nanomatrices for Skin Tissue Engineering — An in vitro Study

In this paper, we have developed self-assembled nanoscale assemblies that were prepared by conjugating furan-2-carboxylic acid-3-aminopropyl amide with the short peptide sequence Gly-His (abbreviated Gly-His-FCAP). To mimic the extracellular matrix of mammalian fibroblasts and keratinocytes, the assemblies were then conjugated with Type I collagen. We then integrated the collagen bound Gly-His-FCAP assemblies with a short peptide sequence derived from salamander skin into the nanoscale assemblies for the first time to impart regenerative and wound healing properties to the composites. The antioxidant, antimicrobial and biodegradable properties were examined and results indicate that the nanocomposites displayed antioxidant properties as displayed by 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The biodegradability was found to be gradual. The nanocomposites were also found to inhibit the growth of the fungus Rhizopus sporangia over an 18[Formula: see text]h growth period. As proof of concept, to demonstrate the development of three-dimensional (3D) engineered skin in vitro, 3D printed PLA scaffolds of 2.5[Formula: see text]mm thickness were submerged in media containing nanocomposites and co-cultures of dermal fibroblasts with epidermal keratinocytes mimicking three dimensional skin substitute was examined. Our results indicated that the nanocomposites adhered to and supported cell proliferation and mimicked the components of skin and may have potential applications in skin tissue regeneration.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

Development of a Full-Thickness Human Skin Equivalent In Vitro Model Derived from TERT-Immortalized Keratinocytes and Fibroblasts

Currently, human skin equivalents (HSEs) used for in vitro assays (e.g., for wound healing) make use of primary human skin cells. Limitations of primary keratinocytes and fibroblasts include availability of donor skin and donor variation. The use of physiologically relevant cell lines could solve these limitations. The aim was to develop a fully differentiated HSE constructed entirely from human skin cell lines, which could be applied for in vitro wound-healing assays. Skin equivalents were constructed from human TERT-immortalized keratinocytes and fibroblasts (TERT-HSE) and compared with native skin and primary HSEs. HSEs were characterized by hematoxylin–eosin and immunohistochemical stainings with markers for epidermal proliferation and differentiation, basement membrane (BM), fibroblasts, and the extracellular matrix (ECM). Ultrastructure was determined with electron microscopy. To test the functionality of the TERT-HSE, burn and cold injuries were applied, followed by immunohistochemical stainings, measurement of reepithelialization, and determination of secreted wound-healing mediators. The TERT-HSE was composed of a fully differentiated epidermis and a fibroblast-populated dermis comparable to native skin and primary HSE. The epidermis consisted of proliferating keratinocytes within the basal layer, followed by multiple spinous layers, a granular layer, and cornified layers. Within the TERT-HSE, the membrane junctions such as corneosomes, desmosomes, and hemidesmosomes were well developed as shown by ultrastructure pictures. Furthermore, the BM consisted of a lamina lucida and lamina densa comparable to native skin. The dermal matrix of the TERT-HSE was more similar to native skin than the primary construct, since collagen III, an ECM marker, was present in TERT-HSEs and absent in primary HSEs. After wounding, the TERT-HSE was able to reepithelialize and secrete inflammatory wound-healing mediators. In conclusion, the novel TERT-HSE, constructed entirely from human cell lines, provides an excellent opportunity to study in vitro skin biology and can also be used for drug targeting and testing new therapeutics, and ultimately, for incorporating into skin-on-a chip in the future.

Wnt-3a-activated human fibroblasts promote human keratinocyte proliferation and matrix destruction

Aberrant Wnt regulation, detectable by nuclear translocation of beta-catenin, is a hallmark of many cancers including skin squamous cell carcinomas (SCCs). By analyzing primary human skin SCCs, we demonstrate that nuclear beta-catenin is not restricted to SCC cells but also detected in stromal fibroblasts, suggesting an important role for aberrant Wnt regulation also in the tumor microenvironment. When human keratinocytes and fibroblasts were treated with Wnt-3a, fibroblasts proved to be more responsive. Accordingly, Wnt-3a did not alter HaCaT cell functions in a cell-autonomous manner. However, when organotypic cultures (OTCs) were treated with Wnt-3a, HaCaT keratinocytes responded with increased proliferation. As nuclear beta-catenin was induced only in the fibroblasts, this argued for a Wnt-dependent, paracrine keratinocyte stimulation. Global gene expression analysis of Wnt-3a-stimulated fibroblasts identified genes encoding interleukin-8 (IL-8) and C-C motif chemokine 2 (CCL-2) as well as matrix metalloproteinase-1 (MMP-1) as Wnt-3a targets. In agreement, we show that IL-8 and CCL-2 were secreted in high amounts by Wnt-3a-stimulated fibroblasts also in OTCs. The functional role of IL-8 and CCL-2 as keratinocyte growth regulators was confirmed by directly stimulating HaCaT cell proliferation in conventional cultures. Most important, neutralizing antibodies against IL-8 and CCL-2 abolished the Wnt-dependent HaCaT cell hyperproliferation in OTCs. Additionally, MMP-1 was expressed in high amounts in Wnt-3a-stimulated OTCs and degraded the stromal matrix. Thus, our data show that Wnt-3a stimulates fibroblasts to secrete both keratinocyte proliferation-inducing cytokines and stroma-degrading metalloproteinases, thereby providing evidence for a novel Wnt deregulation in the tumor-stroma directly contributing to skin cancer progression.

Three-Dimensional Human Skin Models to Understand Staphylococcus aureus Skin Colonization and Infection

Staphylococcus aureus is both a major bacterial pathogen as well as a common member of the human skin microbiota. Due to its widespread prevalence as an asymptomatic skin colonizer and its importance as a source of skin and soft tissue infections, an improved understanding of how S. aureus attaches to, grows within, and breaches the stratified layers of the epidermis is of critical importance. Three-dimensional organotypic human skin culture models are informative and tractable experimental systems for future investigations of the interactions between S. aureus and the multi-faceted skin tissue. We propose that S. aureus virulence factors, primarily appreciated for their role in pathogenesis of invasive infections, play alternative roles in promoting asymptomatic bacterial growth within the skin. Experimental manipulations of these cultures will provide insight into the many poorly understood molecular interactions occurring at the interface between S. aureus and stratified human skin tissue.

Shaping oral cell plasticity to osteogenic differentiation by human mesenchymal stem cell coculture

In the context of cell-based oral hard tissue regeneration, especially assumed plasticity of oral host tissue cells in response to human mesenchymal stem cells (hMSCs), is poorly understood. To investigate this area, we assess osteogenic features in various oral cell types during hMSC coculture, including human alveolar osteoblasts (hOAs), periodontal ligament cells (hPDLs) and gingival fibroblasts (hGFs). Interactive hMSC coculture globally enhanced the transcription of osteogenic genes, in all oral cell types under study, as revealed by qRT-PCR and did not affect oral cell proliferation compared with controls in a transwell coculture system as evaluated by 5-bromo-2'-deoxyuridine proliferation assay. 3D gel-derived hMSC cocultures exhibited an abundance of bone-related key molecules in oral cells, which followed the ranking hOAs > hGFs > hPDLs. Compared to matched controls, this hierarchy also applied for the presence of higher amounts of extracellular matrix deposits and mineralization nodules in interactive hMSC coculture. Our results show for the first time that in the context of prospective periodontal tissue regeneration strategies, hMSCs influence oral cells by gradually shaping their plasticity, particularly features associated with an osteogenic phenotype. These novel findings contribute another piece to the conceptual hMSC action puzzle and valuably support the notion that hMSCs trigger osteogenesis in the oral cell context.

Building a microphysiological skin model from induced pluripotent stem cells

The discovery of induced pluripotent stem cells (iPSCs) in 2006 was a major breakthrough for regenerative medicine. The establishment of patient-specific iPSCs has created the opportunity to model diseases in culture systems, with the potential to rapidly advance the drug discovery field. Current methods of drug discovery are inefficient, with a high proportion of drug candidates failing during clinical trials due to low efficacy and/or high toxicity. Many drugs fail toxicity testing during clinical trials, since the cells on which they have been tested do not adequately model three-dimensional tissues or their interaction with other organs in the body. There is a need to develop microphysiological systems that reliably represent both an intact tissue and also the interaction of a particular tissue with other systems throughout the body. As the port of entry for many drugs is via topical delivery, the skin is the first line of exposure, and also one of the first organs to demonstrate a reaction after systemic drug delivery. In this review, we discuss our strategy to develop a microphysiological system using iPSCs that recapitulates human skin for analyzing the interactions of drugs with the skin.

Osteogenesis of heterotopically transplanted mesenchymal stromal cells in rat models of chronic kidney disease

The current study is based on the hypothesis of mesenchymal stromal cells (MSCs) contributing to soft-tissue calcification and ectopic osteogenesis in chronic kidney disease (CKD). Rat MSCs were transplanted intraperitoneally in an established three-dimensional collagen-based model in healthy control animals and two rat models of CKD and vascular calcification: (1) 5/6 nephrectomy + high phosphorus diet; and (2) adenine nephropathy. As internal controls, collagen gels without MSCs were transplanted in the same animals. After 4 and 8 weeks, MSCs were still detectable and proliferating in the collagen gels (fluorescence-activated cell sorting [FACS] analysis and confocal microscopy after fluorescence labeling of the cells). Aortas and MSC-containing collagen gels in CKD animals showed distinct similarities in calcification (micro-computed tomography [µCT], energy-dispersive X-ray [EDX] analysis, calcium content), induction of osteogenic markers, (ie, bone morphogenic protein 2 [BMP-2], Runt related transcription factor 2 [Runx2], alkaline phosphatase [ALP]), upregulation of the osteocytic marker sclerostin and extracellular matrix remodeling with increased expression of osteopontin, collagen I/III/IV, fibronectin, and laminin. Calcification, osteogenesis, and matrix remodeling were never observed in healthy control animals and non-MSC-containing collagen gels in all groups. Paul Karl Horan 26 (PKH-26)-labeled, 3G5-positive MSCs expressed Runx2 and sclerostin in CKD animals whereas PKH-26-negative migrated cells did not express osteogenic markers. In conclusion, heterotopically implanted MSCs undergo osteogenic differentiation in rat models of CKD-induced vascular calcification, supporting our hypothesis of MSCs as possible players in heterotopic calcification processes of CKD patients.

Natural corneal cell-based microenvironment as prerequisite for balanced 3D corneal epithelial morphogenesis: a promising animal experiment-abandoning tool in ophthalmology

To achieve durable recognition as a promising animal experiment-abandoning tool in ophthalmology, in vitro engineered tissue equivalents of the human cornea should exhibit proper morphogenesis. Regarding this issue, we were seeking for the natural cell microenvironment fulfilling the minimum requirements to allow human corneal keratinocytes to develop a balanced epithelial morphology with regular spatial appearance of tissue homeostatic biomarkers. Hence, we established cocultures of 3D cell-based collagen scaffolds comprising immortalized corneal keratinocytes combined with a gradual cornea-derived in vivo-like cell microenvironment, together with immortalized stromal fibroblasts alone (nonholistic) or fibroblasts and immortalized endothelial cells (holistic). With matched non-holistic microenvironments revealing mostly flattened cells and putative apical cell ablation foci at day 6, and 9 in HE stains, holistic counterparts yielded proper epithelial stratification with cell flattening restricted to apical layers. Concordantly, RT(2)-PCR showed a tremendous increase in gene expression for progressive and terminal biomarkers of corneal keratinocyte differentiation, cytokeratin (CK) 12, and filaggrin (FIL), in response to nonholistic environments, while involucrin (INV) was moderately but significantly upregulated. Although visible, this increase was moderate in corneal keratinocytes with a holistic environment. On the protein level, indirect immunofluorescence revealed that only epithelia of holistic environments showed diminishment in CK19, counteracted by CK12 rising over time. This time-dependent progression in differentiation coincided with declined proliferation and tissue-regular focus of differentiation biomarkers inv and fil to suprabasal and apical cell layers. Our novel findings suggest the interplay of native tissue forming cell entities, important for balanced corneal epithelial morphogenesis. In addition, they provide evidence for a holistic cell microenvironment as a prerequisite for development of an in vitro engineered corneal epithelial tissue equivalent, exhibiting a regular appearance of tissue homeostatic biomarkers. Such equivalents will be promising tools in ophthalmology, for example, for mechanistic studies in basic research and/or testing of generics or preclinical validation of innovative cornea-tailored biomaterials, desired for regenerative strategies.

Fibrous star poly(ε-caprolactone) melt-electrospun scaffolds for wound healing applications

Polymeric fibrous scaffolds based on the biocompatible and biodegradable three-arm-branched star poly(ε-caprolactone) (Mw = 189,000 g/mol) were prepared by a melt electrospinning technique. The possibility of processing polymers without the use of organic solvents is one of the main advantages over solution electrospinning. Scaffolds were biologically tested for their ability of supporting skin tissue regeneration. For this purpose, mouse embryo fibroblast (BALB/3T3 clone A31) and human keratinocyte (HaCaT) cell lines were selected as models, and seeded onto the polymeric supports both as single and co-culture. Cell viability, proliferation, and collagen production were assessed by WST-1 assay and Direct Red 80 dye, respectively. Cell morphology and colonization of the supports were evaluated by scanning electron microscopy and confocal laser scanning microscopy. Results highlighted that the star poly(ε-caprolactone) scaffolds were able to promote collagen production by fibroblasts. In co-culture studies, scaffolds supported adhesion, proliferation, and spatial organization of both cell lines. By virtue of the observed results, the developed polymeric scaffolds appeared suitable as biodegradable and biocompatible three-dimensional supports for skin tissue regeneration in wound healing dressing.

Matriptase regulates proliferation and early, but not terminal, differentiation of human keratinocytes

Genetic defects in matriptase are linked to two congenital ichthyoses: autosomal recessive ichthyosis with hypotrichosis (ARIH, OMIM 610765) and ichthyosis, follicular atrophoderma, hypotrichosis, and hypohidrosis (IFAH, OMIM 602400). Mouse models with matriptase deficiency indicate an involvement of matriptase in suprabasal keratinocytes in the maintenance of the epidermal barrier. In contrast to what has been reported for mouse skin, we show that in human skin matriptase is primarily expressed in the basal and spinous keratinocytes, but not in the more differentiated keratinocytes of the granular layer. In addition, matriptase zymogen activation was predominantly detected in the basal cells. Furthermore, by using skin organotypic cultures as a model system to monitor the course of human epidermal differentiation, we found elevated matriptase zymogen activation during early stages of epidermal differentiation, coupled with a loss of matriptase expression in the late stages of this process. We also show here that matriptase deficiency in HaCaT cells modestly reduces cell proliferation and temporally affects calcium-induced expression of differentiation markers. These collective data suggest that, unlike mouse matriptase, human matriptase may be involved in the regulation of keratinocyte growth and early differentiation, rather than terminal differentiation, providing mechanistic insights into the pathology of the two congenital ichthyoses: ARIH and IFAH.

Connective tissue fibroblast properties are position-dependent during mouse digit tip regeneration

A key factor that contributes to the regenerative ability of regeneration-competent animals such as the salamander is their use of innate positional cues that guide the regeneration process. The limbs of mammals has severe regenerative limitations, however the distal most portion of the terminal phalange is regeneration competent. This regenerative ability of the adult mouse digit is level dependent: amputation through the distal half of the terminal phalanx (P3) leads to successful regeneration, whereas amputation through a more proximal location, e.g. the subterminal phalangeal element (P2), fails to regenerate. Do the connective tissue cells of the mammalian digit play a role similar to that of the salamander limb in controlling the regenerative response? To begin to address this question, we isolated and cultured cells of the connective tissue surrounding the phalangeal bones of regeneration competent (P3) and incompetent (P2) levels. Despite their close proximity and localization, these cells show very distinctive profiles when characterized in vitro and in vivo. In vitro studies comparing their proliferation and position-specific interactions reveal that cells isolated from the P3 and P2 are both capable of organizing and differentiating epithelial progenitors, but with different outcomes. The difference in interactions are further characterized with three-dimension cultures, in which P3 regenerative cells are shown to lack a contractile response that is seen in other fibroblast cultures, including the P2 cultures. In in vivo engraftment studies, the difference between these two cell lines is made more apparent. While both P2 and P3 cells participated in the regeneration of the terminal phalanx, their survival and proliferative indices were distinct, thus suggesting a key difference in their ability to interact within a regeneration permissive environment. These studies are the first to demonstrate distinct positional characteristics of connective tissue cells that are associated with their regenerative capabilities.

Integrating macrophages into organotypic co-cultures: a 3D in vitro model to study tumor-associated macrophages

Tumor progression is controlled by signals from cellular and extra-cellular microenvironment including stromal cells and the extracellular matrix. Consequently, three-dimensional in vitro tumor models are essential to study the interaction of tumor cells with their microenvironment appropriately in a biologically relevant manner. We have previously used organotypic co-cultures to analyze the malignant growth of human squamous cell carcinoma (SCC) cell lines on a stromal equivalent in vitro. In this model, SCC cell lines are grown on a collagen-I gel containing fibroblasts. Since macrophages play a critical role in the progression of many tumor types, we now have expanded this model by integrating macrophages into the collagen gel of these organotypic tumor co-cultures. This model was established as a murine and a human system of skin SCCs. The effect of macrophages on tumor progression depends on their polarization. We demonstrate that macrophage polarization in organotypic co-cultures can be modulated towards and M1 or an M2 phenotype by adding recombinant IFN-γ and LPS or IL-4 respectively to the growth medium. IL-4 stimulation of macrophage-containing cultures resulted in enhanced tumor cell invasion evidenced by degradation of the basement membrane, enhanced collagenolytic activity and increased MMP-2 and MMP-9. Interestingly, extended co-culture with tumor cells for three weeks resulted in spontaneous M2 polarization of macrophages without IL-4 treatment. Thus, we demonstrate that macrophages can be successfully integrated into organotypic co-cultures of murine or human skin SCCs and that this model can be exploited to analyze macrophage activation towards a tumor supporting phenotype.

Remodeling of the dermal-epidermal junction in bilayered skin constructs after silencing the expression of the p.R2622Q and p.G2623C collagen VII mutants

The integrity of skin depends on a complex system of extracellular matrix molecules that form a biological scaffold. One of its elements is the dermal basement membrane that provides a link between the epidermis and the dermis. Mutations in collagen VII, a key component of the dermal membrane zone, are associated with dystrophic epidermolysis bullosa. Although it has been proposed that silencing the mutated COL7A1 allele is a promising approach to restore the dermal basement membrane zone formed in the presence of collagen VII mutants, limitations exist to testing this proposal. Here, we employed a model that utilized skin-like constructs in which engineered collagen VII mutant chains harboring the R2622Q or G2623C substitution were expressed conditionally, but the wild-type chains were expressed unconditionally. We demonstrated that switching off the production of the mutant collagen VII chains in skin constructs restores the organization of collagen VII and laminin 332 deposits in the dermal-epidermal junction to the level of control. We also demonstrated that remodeling of collagen IV deposits was not fully effective after silencing the expression of collagen VII mutants. Thus, our study suggests that while silencing mutant alleles of COL7A1 may repair critical elements of the affected dermal basement membrane, it may not be sufficient to fully remodel its entire architecture initially formed in the presence of the mutant collagen VII chains.

Epithelial morphogenesis of germline-derived pluripotent stem cells on organotypic skin equivalents in vitro

For tissue engineering, cultivation of pluripotent stem cells on three-dimensional scaffolds allows the generation of organ-like structures. Previously, we have established an organotypic culture system of skin to induce epidermal differentiation in adult stem cells. Multipotent stem cells are not able to differentiate across germinal boundaries. In contrast, pluripotent stem cells readily differentiate into tissues of all three germ layers. Germline-derived pluripotent stem cells (gPS cells) can be generated by induction of pluripotency in mouse unipotent germline stem cells without the introduction of exogenous transcription factors. In the current study, we analyzed the influence of organotypic culture conditions of skin on the epithelial differentiation of gPS cells in comparison to the well-established HM1 ES cell line. Quantitative RT-PCR data of the pluripotency gene Oct4 showed that gPS cells are characterized by an accelerated Oct4-downregulation compared to HM1 ES cells. When subjected to the organotypic culture conditions of skin, gPS cells formed tubulocystic structures lined by stratified (CK5/6(+), CK14(+), CK8/18(-)) epithelia. HM1 ES cells formed only small tubulocystic structures lined by simple, CK8/18(+) epithelia. BMP-4, an epidermal morphogen, significantly enhanced the expression of epithelial markers in HM1 ES cells, but did not significantly affect the formation of complex (squamous) epithelia in gPS cells. In HM1 ES cells the differentiation into squamous epithelium was only inducible in the presence of mature dermal fibroblasts. Both pluripotent stem cell types spontaneously differentiated into mesodermal, endodermal and into neuroectodermal cells at low frequency, underlining their pluripotent differentiation capacity. Concluding, the organotypic culture conditions of skin induce a multilayered, stratified epithelium in gPS cells, in HM1 ES cells only in the presence of dermal fibroblasts. Thus, our data show that differentiation protocols strongly depend on the stem cell type and have to be modified for each specific stem cell type.

Development of a 3D pigmented skin model to evaluate RNAi-induced depigmentation

Because current skin whitening agents often have insufficient efficacy and side effects, we aim to develop effective and safe therapeutics using RNA interference (RNAi). We established a pigmented human-reconstructed skin model as a first step in the development of novel siRNA-based depigmenting agents. Histological characterization revealed that our model had a similar morphology as normal human skin, expressed keratinocyte differentiation as well as basement membrane markers, and showed a high degree of pigmentation. The utility of the model to study RNAi-induced depigmentation was validated by incorporation of melanocytes transfected with siRNA against tyrosinase, a key enzyme in skin pigmentation. This resulted in a strong reduction in pigmentation and inhibition of melanin transfer proving that siRNA-mediated gene silencing in melanocytes worked successfully in our model. Therefore, this self-made 3D skin model will be a useful and easy tool to validate the whitening potential of candidate genes with a presumed function in melanin synthesis or transfer.

Exposure to uremic serum induces a procalcific phenotype in human mesenchymal stem cells

OBJECTIVE

Medial artery calcification in patients with chronic kidney disease proceeds through intramembranous ossification resulting from osteoblast-induced calcification of the collagen extracellular matrix. The current study is based on the hypothesis that mesenchymal stem cells (MSC) constitute critical cells for procalcific extracellular matrix remodeling in patients with chronic kidney disease.

METHODS AND RESULTS

Human MSC were cultured in media supplemented with pooled sera from either healthy or uremic patients (20%). Exposure to uremic serum enhanced the proliferation of MSC (cell counting, BrdU incorporation) whereas apoptosis and necrosis were not affected (annexin V and 7-amino-actinomycin staining). Uremic serum-exposed MSC recapitulated osteogenesis by matrix calcification and expression of bone-related genes (bone morphogenetic protein [BMP]-2 receptor, alkaline phosphatase, osteopontin, and Runx2) in 35 days. The uremic serum-induced osteogenesis was completely blocked by a BMP-2/4 neutralizing antibody or the natural antagonist NOGGIN. Calcification and matrix remodeling were further analyzed in a collagen-embedded osteogenesis model recapitulating the vascular collagen I/III environment. The uremic serum-induced calcification was shown to occur along collagen fibers as shown by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and von Kossa staining and was accompanied by extensive matrix remodeling.

CONCLUSIONS

Uremic serum induced in a BMP-2/4-dependent manner an osteoblast-like phenotype in MSC accompanied by matrix remodeling and calcification.

Three-dimensional skin models as tools for transdermal drug delivery: challenges and limitations

INTRODUCTION

Transdermal drug delivery has several known advantages over the oral route and hypodermic injections. The number of drugs that can be taken up transdermally is, however, limited owing to the innate barrier function of the skin. New transdermal drug candidates need to be tested extensively before being used on humans. In this regard, in vitro permeation methods are highly important to predict in vivo permeation of drugs.

AREAS COVERED

This review illustrates how different types of reconstructed skin models are being used as alternatives to human and pig skin for in vitro permeation testing of drugs. Insights into how various factors (including the physicochemical nature of molecules and formulations) or skin properties might affect the permeability of drugs in reconstructed skin models are provided. Also, opportunities and pitfalls of reconstructed skin models are highlighted.

EXPERT OPINION

Many studies have revealed that the permeability of reconstructed skin models is much higher compared with human excised skin. This is in accordance with the incomplete barrier found in these models. Nevertheless, the reconstructed skin models available today are useful tools for estimating the rank order of percutaneous absorption of a series of compounds with different physicochemical properties. A major challenge in the further development of reconstructed skin models for drug delivery studies is to obtain a barrier function similar to in vivo skin. Whether this goal will be achieved in the near future is uncertain and will be, in the authors' opinion, a very difficult task.

Artificial skin in perspective: concepts and applications

Skin, the largest organ of the human body, is organized into an elaborate layered structure consisting mainly of the outermost epidermis and the underlying dermis. A subcutaneous adipose-storing hypodermis layer and various appendages such as hair follicles, sweat glands, sebaceous glands, nerves, lymphatics, and blood vessels are also present in the skin. These multiple components of the skin ensure survival by carrying out critical functions such as protection, thermoregulation, excretion, absorption, metabolic functions, sensation, evaporation management, and aesthetics. The study of how these biological functions are performed is critical to our understanding of basic skin biology such as regulation of pigmentation and wound repair. Impairment of any of these functions may lead to pathogenic alterations, including skin cancers. Therefore, the development of genetically controlled and well characterized skin models can have important implications, not only for scientists and physicians, but also for manufacturers, consumers, governing regulatory boards and animal welfare organizations. As cells making up human skin tissue grow within an organized three-dimensional (3D) matrix surrounded by neighboring cells, standard monolayer (2D) cell cultures do not recapitulate the physiological architecture of the skin. Several types of human skin recombinants, also called artificial skin, that provide this critical 3D structure have now been reconstructed in vitro. This review contemplates the use of these organotypic skin models in different applications, including substitutes to animal testing.

Regulation of the matriptase-prostasin cell surface proteolytic cascade by hepatocyte growth factor activator inhibitor-1 during epidermal differentiation

Matriptase, a membrane-tethered serine protease, plays essential roles in epidermal differentiation and barrier function, largely mediated via its activation of prostasin, a glycosylphosphatidylinositol-anchored serine protease. Matriptase activity is tightly regulated by its inhibitor hepatocyte growth factor activator inhibitor-1 (HAI-1) such that free active matriptase is only briefly available to act on its substrates. In the current study we provide evidence for how matriptase activates prostasin under this tight control by HAI-1. When primary human keratinocytes are induced to differentiate in a skin organotypic culture model, both matriptase and prostasin are constitutively activated and then inhibited by HAI-1. These processes also occur in HaCaT human keratinocytes when matriptase activation is induced by exposure of the cells to a pH 6.0 buffer. Using this acid-inducible activation system we demonstrate that prostatin activation is suppressed by matriptase knockdown and by blocking matriptase activation with sodium chloride, suggesting that prostatin activation is dependent on matriptase in this system. Kinetics studies further reveal that the timing of autoactivation of matriptase, prostasin activation, and inhibition of both enzymes by HAI-1 binding are closely correlated. These data suggest that, during epidermal differentiation, the matriptase-prostasin proteolytic cascade is tightly regulated by two mechanisms: 1) prostasin activation temporally coupled to matriptase autoactivation and 2) HAI-1 rapidly inhibiting not only active matriptase but also active prostasin, resulting in an extremely brief window of opportunity for both active matriptase and active prostasin to act on their substrates.

The role of biomaterials in the direction of mesenchymal stem cell properties and extracellular matrix remodelling in dermal tissue engineering

Recently, a new generation of dermal equivalents (DE) was presented which are solely generated on a human fibroblast-derived dermal matrix. Since human mesenchymal stem cells from bone marrow (BM-MSC) and Wharton's Jelly of the umbilical cord (UC-MSC) are characterised by a distinct biosynthetic and paracrine activity, they are an appealing alternative approach for generating cell-based DE. This study compares the epithelial-mesenchymal interaction and extracellular matrix (ECM) remodelling of cell-based and collagen-based DE using fibroblasts, BM-MSC or UC-MSC, respectively, in co-culture with the keratinocyte cell line HaCaT. While fibroblast-based DE exhibit normal matrix synthesis, proliferation and differentiation of keratinocytes, mesenchymal stem cell-based DE resulted in excessive production of inhomogenous matrix aggregates, loss of polarisation of the epidermal cell layer and an inconstant paracrine activity. In contrast, collagen-embedded MSC revealed a homogenous growth pattern as well as a constant expression of growth factors and ECM proteins without a negative influence on the epidermal layer as shown by histology, electron microscopy, immunohistochemistry and realtime-RT-PCR. These results indicate the necessity of an instructive biomaterial-based scaffold to direct stem cell differentiation, proliferation, paracrine activity as well as regulation of ECM deposition.

Fluorescence spectroscopy of an in vitro model of human cervical neoplasia identifies graded spectral shape changes with neoplastic phenotype and a differential effect of acetic acid

BACKGROUND

The clinical utility of spectroscopic methods for the diagnosis of cervical cancer is limited by significant inter-patient variation in the spectroscopic properties of the cervix. Improved understanding of the contributions of the components of cervical tissue to the observed spectra would therefore be helpful in the development of spectroscopic approaches to the study of cervical disease in vivo.

METHODS

In this study, we used organotypic epithelial raft culture as an in vitro model system to analyse the fluorescence properties of the surface squamous epithelium specifically. The spectrum of cervical dysplasia was modelled by producing rafts lined by primary human keratinocytes (PHKs) and the HaCaT, SiHa and CaSki human keratinocyte cell lines and fluorescence emission spectra were recorded at a wide range of excitation wavelengths.

RESULTS

Statistically significant differences in spectral shape were identified between the different rafts at excitation wavelengths between 250nm and 310nm. A graded, differential effect of acetic acid on fluorescence intensity was also observed, consistent with the visible effects of acetic acid on clinical examination at colposcopy.

CONCLUSION

These data suggest that the development of neoplastic changes in the squamous epithelium of the cervix are associated with alterations in its fluorescence properties and that the application of acetic acid has a demonstrable effect on these properties. Identification of these alterations may aid the discrimination of cervical lesions in vivo.

Organotypic modelling as a means of investigating epithelial-stromal interactions during tumourigenesis

The advent of co-culture approaches has allowed researchers to more accurately model the behaviour of epithelial cells in cell culture studies. The initial work on epidermal modelling allowed the development of reconstituted epidermis, growing keratinocytes on top of fibroblasts seeded in a collagen gel at an air-liquid interface to generate terminally differentiated 'skin equivalents'. In addition to developing ex vivo skin sheets for the treatment of burns victims, such cultures have also been used as a means of investigating both the development and repair of the epidermis, in more relevant conditions than simple two-dimensional culture, but without the use of animals. More recently, by varying the cell types used and adjusting the composition of the matrix components, this physiological system can be adapted to allow the study of interactions between tumour cells and their surrounding stroma, particularly with regards to how such interactions regulate invasion. Here we provide a summary of the major themes involved in tumour progression and consider the evolution of the approaches used to study cancer cell behaviour. Finally, we review how organotypic models have facilitated the study of several key pathways in cancer development and invasion, and speculate on the exciting future roles for these models in cancer research.

Phosphorylation of claudin-4 is required for tight junction formation in a human keratinocyte cell line

Extensive studies have identified a large number of the molecular components of cellular tight junctions (TJ), including the claudins, occludin and ZO-1/2, and also many of the physical interactions between these molecules. However, the regulatory mechanisms of TJ formation are as yet poorly understood. In HaCaT, a human epidermal keratinocyte cell line, TJ were newly formed when cells were cultured in the presence of SP600125, a JNK inhibitor. Moreover, claudin-4 was newly phosphorylated during this process. We found that claudin-4 contains a sequence which is phosphorylated by atypical PKC (aPKC). Kinase assay demonstrated that the 195th serine (serine195) of mouse claudin-4 was phosphorylated by aPKC in vitro. The 194th serine (serine194) of human claudin-4 corresponding to serine195 of mouse claudin-4 was phosphorylated in HaCaT cells when TJ were formed, and the phosphorylated claudin-4 co-localized with ZO-1 at TJ. aPKC activity was required for both the claudin-4 phosphorylation and TJ formation in HaCaT. Furthermore, overexpression of mutant claudin-4 protein S195A, which was not phosphorylated by aPKC, perturbed the TJ formation mediated by SP600125. These findings suggest that aPKC regulates TJ formation through the phosphorylation of claudin-4.

Use of organotypic coculture to study keloid biology

BACKGROUND

Keloids are pathologic scars afflicting a large segment of our population and for which there is no definitive therapy. The lack of an animal model for keloid formation has hampered study. We developed an in vitro organotypic skin model to simulate normal keloid biology, which may allow us to study keloid formation without an animal model.

METHODS

Normal (NFs) and keloid (KFs) human fibroblasts were cultured in a collagen matrix to create a 3-dimensional dermal structure. Normal human keratinocytes (NKs) were cultured as a second layer on top and exposed to an air-fluid interface to allow differentiation into a mature keratinocyte layer. The organotypic skin was maintained for 28 days in Dulbecco's modified eagle medium with 10% fetal calf serum. Samples were collected, processed, sectioned, stained with hematoxylin and eosin, and then measured for qualitative analysis. alpha-smooth-muscle actin was also evaluated by immunoblotting.

RESULTS

KF/NK organotypic skin showed increased collagen deposition, based on significantly denser collagen staining, with increased dermal thickness compared with NF/NK organotypic skin. We saw increased contracture in the KF/NK construct, and this correlated with increased organization of alpha-smooth-muscle actin fibers in the dermal layer of KF/NK organotypic skin compared with NF/NK skin.

CONCLUSIONS

We have shown that coculture of KFs with keloid keratinocytes leads to an increased collagen production and dermal contracture compared with NFs and NKs, consistent with known keloid behavior. Given the lack of an animal model, we believe that organotypic skin culture can serve as a surrogate to study keloid formation.

Three-dimensional epidermis-like growth of human mesenchymal stem cells on dermal equivalents: contribution to tissue organization by adaptation of myofibroblastic phenotype and function

Human mesenchymal stem cells (hMSC) are able to differentiate into mature cells of various mesenchymal tissues. Recent studies have reported that hMSC may even give rise to cells of ectodermal origin. This indication of plasticity makes hMSC a promising donor source for cell-based therapies. This study explores the differentiation potential of hMSC in a tissue-specific microenvironment simulated in vitro. HMSC were cultured air-exposed on dermal equivalents (DEs) consisting of collagen types I and III with dermal fibroblasts and subjected to conditions similar to those used for tissue engineering of skin with keratinocytes. Culture conditions were additionally modified by pre-treating the cells with 5-azacytidine or supplementing the medium with all trans retinoic acid (RA). HMSC were capable of adaptation to epidermis-specific conditions without losing their mesenchymal multipotency. However, despite the viability and evident three-dimensional epidermis-like growth pattern, hMSC showed a persistent expression of mesenchymal but not of epithelial markers, thus indicating a lack of epidermal (trans) differentiation. Further, electron microscopy and immunohistochemical analyses demonstrated that hMSC cultured under epidermis-specific conditions adopted a myofibroblastic phenotype and function, promoted in particular by air exposure. In conclusion, multipotent hMSC failed to differentiate into E-cadherin- or cytokeratin-expressing cells under optimized organotypic culture conditions for keratinocytes but differentiated into myofibroblast-like cells contracting the extracellular matrix, a phenomenon that was enhanced by RA and 5-azacytidine. These results indicate that hMSC might contribute to wound-healing processes by extracellular matrix reorganization and wound contraction but not by differentiation into keratinocytes.

The clinical application of autologous bioengineered skin based on a hyaluronic acid scaffold

The aim of this work was to generate an in vitro skin substitute harbouring autologous fibroblasts, keratinocytes and melanocytes, to establish a new one-step clinical method in problems associated with skin disorders. Here we present a case of a nine-year-old girl with a congenital giant nevus treated by surgical approach, with primary co-cultures of keratinocytes, melanocytes and fibroblasts obtained from autologous skin biopsy. Generally these lesions need to be removed to avoid the risk of transformation into malignant melanoma. With this purpose we analyzed the melanocytes contained in the new skin substitute for the presence of genetic alterations correlated to increased risk for melanoma. The organotypical cultures were designed including an engineered scaffold of a non-woven mesh of hyaluronic acid (HYAFF11). This biomaterial has been previously demonstrated to be the most suitable to maintain polarity and to support the in vitro constructs. Six dermal-epidermal skin substitutes were transplanted and 14 days after surgery the re-epithelialized area was about 90%. Our results suggest that this new dermal-epidermal construct not only reduces hospitalization time and ameliorates scar retraction, but might also represent a solution for the high risk of developing a tumour derived from the original nevus.

Quantitative gene expression assessment identifies appropriate cell line models for individual cervical cancer pathways

BACKGROUND

Cell lines have been used to study cancer for decades, but truly quantitative assessment of their performance as models is often lacking. We used gene expression profiling to quantitatively assess the gene expression of nine cell line models of cervical cancer.

RESULTS

We find a wide variation in the extent to which different cell culture models mimic late-stage invasive cervical cancer biopsies. The lowest agreement was from monolayer HeLa cells, a common cervical cancer model; the highest agreement was from primary epithelial cells, C4-I, and C4-II cell lines. In addition, HeLa and SiHa cell lines cultured in an organotypic environment increased their correlation to cervical cancer significantly. We also find wide variation in agreement when we considered how well individual biological pathways model cervical cancer. Cell lines with an anti-correlation to cervical cancer were also identified and should be avoided.

CONCLUSION

Using gene expression profiling and quantitative analysis, we have characterized nine cell lines with respect to how well they serve as models of cervical cancer. Applying this method to individual pathways, we identified the appropriateness of particular cell lines for studying specific pathways in cervical cancer. This study will allow researchers to choose a cell line with the highest correlation to cervical cancer at a pathway level. This method is applicable to other cancers and could be used to identify the appropriate cell line and growth condition to employ when studying other cancers.

Functional significance of non-neuronal acetylcholine in skin epithelia

In recent years, the physiological role of non-neuronal acetylcholine (ACh) and its receptors (AChR) in epidermal physiology has been under intense investigation. However, little is known about the role of the non-neuronal cholinergic system in inflammatory skin diseases. We chose the clinically nicotine-dependent skin disease hidradenitis suppurativa (HS) as model to study the influence of long term nicotine ingestion on epidermal morphology and AChR expression. HS is a chronic inflammatory, disabling disease of unknown pathogenesis emerging from the pilosebaceous unit of the intertriginous areas. In order to correlate our findings to specific nicotine effects, we used the organotypical coculture system (OTC) and raised artificial epidermis in the presence of nicotine. After 12 days in culture control OTC showed a mature epithelium, while nicotine treated OTCs were significantly thicker. Using immunofluorescence analysis, nicotine treated OTCs produced significantly stronger immunoreactivity (IR) for the alpha3, M(3) and M(5) AChR antisera than control. In contrast, the alpha7 nAChR antiserum showed a slightly reduced IR in the granular layer and the alpha9 nAChR IR retracted to the lower suprabasal layers. In HS epidermis we found the strongest IR for all AChR around the follicular infundibulum while in the sinus epithelia it was only weak. In contrast to the nicotine treated OTC, the alpha7 nAChR IR in the hyperplastic HS epidermis was clearly extended to all living layers. Altogether we provide first hints for a causative role of the non-neuronal cholinergic system in the pathogenesis of HS by promoting infundibular epithelial hyperplasia and thus follicular plugging.

Fluorescence spectroscopy of anin vitro model of human cervical precancer identifies neoplastic phenotype

The clinical diagnosis of cervical neoplasia by spectroscopic methods is potentially a reliable, fast and cost-effective alternative to the conventional smear test. However, it is currently limited by significant inter-patient variation in the spectroscopic properties of the cervix. Characterisation of suitable in vitro models of the spectroscopic changes that take place during neoplastic progression may prove to be a significant step towards the successful development of reliable in vivo systems. In this study, we used organotypic epithelial raft culture as an in vitro model of cervical tissue to analyse changes in the fluorescence properties of surface squamous epithelium that are associated with the development of neoplastic disease. Collagen plugs lined by primary human keratinocytes (PHKs) were used to model the normal cervical epithelium, and plugs lined by cells of the SiHa line were used as a model of neoplastic cervical tissue. Fluorescence emission spectra of these rafts were recorded at excitation wavelengths in the 250-330 nm range, complementing previous work published at longer wavelengths. Normalised, truncated emission spectra were analysed using multivariate principal component analysis. We successfully distinguished between in vitro models of normal and neoplastic cervical tissue and demonstrated a differential effect of acetic acid, which enhances the discrimination of normal from neoplastic tissue. Identification of these differences between in vitro organotypic epithelial rafts may ultimately aid the discrimination of cervical lesions in vivo.