Regulation of keratin 9 in nonpalmoplantar keratinocytes by palmoplantar fibroblasts through epithelial-mesenchymal interactions

The use of ectopic volar fibroblasts to modify skin identity

Skin identity is controlled by intrinsic features of the epidermis and dermis and their interactions. Modifying skin identity has clinical potential, such as the conversion of residual limb and stump (nonvolar) skin of amputees to pressure-responsive palmoplantar (volar) skin to enhance prosthesis use and minimize skin breakdown. Greater keratin 9 (KRT9) expression, higher epidermal thickness, keratinocyte cytoplasmic size, collagen length, and elastin are markers of volar skin and likely contribute to volar skin resiliency. Given fibroblasts' capacity to modify keratinocyte differentiation, we hypothesized that volar fibroblasts influence these features. Bioprinted skin constructs confirmed the capacity of volar fibroblasts to induce volar keratinocyte features. A clinical trial of healthy volunteers demonstrated that injecting volar fibroblasts into nonvolar skin increased volar features that lasted up to 5 months, highlighting a potential cellular therapy.

Structural and molecular characteristics of weight-bearing volar skin can be reconstituted by micro skin tissue column grafting

For patients with lower limb amputations, prostheses are immensely helpful for mobility and the ability to perform job-related or recreational activities. However, the skin covering the amputation stump is typically transposed from adjacent areas of the leg and lacks the weight-bearing capacity that is only found in the specialized skin covering the palms and soles (a.k.a. volar skin). As a result, the skin tissue in direct contact with the prosthesis frequently breaks down, leading to the development of painful sores and other complications that limit, and often preclude, the use of prostheses. Transplanting volar skin onto amputation stumps could be a solution to these problems, but traditional skin transplantation techniques cause substantial morbidity at the donor site, such as pain and scarring, which are especially problematic for volar skin given the critical functional importance of the volar skin areas. We previously developed the technology to collect and engraft full-thickness skin tissue while avoiding long-term donor site morbidity, by harvesting the skin in the form of small (~0.5 mm diameter) cores that we termed "micro skin tissue columns" (MSTCs), so that each donor wound is small enough to heal quickly and without clinically appreciable scarring or other long-term abnormalities. The goal of this study was to establish whether a similar approach could be used to confer the structural and molecular characteristics of volar skin ectopically to other skin areas. In a human-to-mouse xenograft model, we show the long-term persistence of various human plantar MSTC-derived cell types in the murine recipient. Then in an autologous porcine model, we harvested MSTCs from the bottom of the foot and transplanted them onto excision wounds on the animals' trunks. The healing processes at both the donor and graft sites were monitored over 8 weeks, and tissue samples were taken to verify volar-specific characteristics by histology and immunohistochemistry. The volar donor sites were well-tolerated, healed rapidly, and showed no signs of scarring or any other long-term defects. The graft sites were able to maintain volar-specific histologic features and expression of characteristics protein markers, up to the 8-week duration of this study. These results suggest that MSTC grafting could be a practical approach to obtain autologous donor volar skin tissue, confer volar skin characteristics ectopically to nonvolar skin areas, improve the load-bearing capacity of amputation stump skin, and ultimately enhance mobility and quality-of-life for lower limb amputees.

Development of metabolome extraction strategy for metabolite profiling of skin tissue

INTRODUCTION

Changes in skin phenotypic characteristics are based on skin tissue. The study of the metabolic changes in skin tissue can help understand the causes of skin diseases and identify effective therapeutic interventions.

OBJECTIVES

We aimed to establish and optimize a non-targeted skin metabolome extraction system for skin tissue metabolomics with high metabolite coverage, recovery, and reproducibility using gas chromatography/mass spectrometry.

METHODS

The metabolites in skin tissues were extracted using eleven different extraction systems, which were designed using reagents with different polarities based on sequential solid-liquid extraction employing a two-step strategy and analyzed using gas chromatograph/mass spectrometry. The extraction efficiency of diverse solvents was evaluated by coefficient of variation (CV), multivariate analysis, metabolites coverage, and relative peak area analysis.

RESULTS

We identified 119 metabolites and the metabolite profiles differed significantly between the eleven extraction systems. Metabolites with high abundances in the organic extraction systems, followed by aqueous extraction, were involved in the biosynthesis of unsaturated fatty acids, while metabolites with high abundances in the aqueous extraction systems, followed by organic extraction, were involved in amino sugar and nucleotide sugar metabolism, and glycerolipid metabolism. MeOH/chloroform-H2O and MeOH/H2O-chloroform were the extraction systems that yielded the highest number of metabolites, while MeOH/acetonitrile (ACN)-H2O and ACN/H2O-IPA exhibited superior metabolite recoveries.

CONCLUSION

Our results demonstrated that our research facilitates the selection of an appropriate metabolite extraction approach based on the experimental purpose for the metabolomics study of skin tissue.

A Kaleidoscope of Keratin Gene Expression and the Mosaic of Its Regulatory Mechanisms

Keratins are a family of intermediate filament-forming proteins highly specific to epithelial cells. A combination of expressed keratin genes is a defining property of the epithelium belonging to a certain type, organ/tissue, cell differentiation potential, and at normal or pathological conditions. In a variety of processes such as differentiation and maturation, as well as during acute or chronic injury and malignant transformation, keratin expression undergoes switching: an initial keratin profile changes accordingly to changed cell functions and location within a tissue as well as other parameters of cellular phenotype and physiology. Tight control of keratin expression implies the presence of complex regulatory landscapes within the keratin gene loci. Here, we highlight patterns of keratin expression in different biological conditions and summarize disparate data on mechanisms controlling keratin expression at the level of genomic regulatory elements, transcription factors (TFs), and chromatin spatial structure.

Differential cell composition and split epidermal differentiation in human palm, sole, and hip skin

Palmoplantar skin is structurally and functionally unique, but the transcriptional programs driving this specialization are unclear. Here, we use bulk and single-cell RNA sequencing of human palm, sole, and hip skin to describe the distinguishing characteristics of palmoplantar and non-palmoplantar skin while also uncovering differences between palmar and plantar sites. Our approach reveals an altered immune environment in palmoplantar skin, with downregulation of diverse immunological processes and decreased immune cell populations. Further, we identify specific fibroblast populations that appear to orchestrate key differences in cell-cell communication in palm, sole, and hip. Dedicated keratinocyte analysis highlights major differences in basal cell fraction among the three sites and demonstrates the existence of two spinous keratinocyte populations constituting parallel, site-selective epidermal differentiation trajectories. In summary, this deep characterization of highly adapted palmoplantar skin contributes key insights into the fundamental biology of human skin and provides a valuable data resource for further investigation.

The case for considering volar skin in a "separate status" for wound healing

Foot ulcers, particularly in the diabetic setting, are a major medical and socioeconomic challenge. While the effects of diabetes and its various sequelae have been extensively studied, in the wound field it is commonly assumed that the wound healing process is essentially identical between different skin types, despite the many well-known specializations in palmoplantar skin, most of which are presumed to be evolutionary adaptations for weightbearing. This article will examine how these specializations could alter the wound healing trajectory and contribute to the pathology of foot ulcers.

The origins of skin diversity: lessons from dermal fibroblasts

Skin is largely composed of an epidermis that overlies a supporting dermis. Recent advancements in our understanding of how diverse groups of dermal fibroblasts regulate epidermal and hair follicle growth and differentiation have been fueled by tools capable of resolving molecular heterogeneity at a single-cell level. Fibroblast heterogeneity can be traced back to their developmental origin before their segregation into spatially distinct fibroblast subtypes. The mechanisms that drive this lineage diversification during development are being unraveled, with studies showing that both large- and small-scale positional signals play important roles during dermal development. Here, we first delineate what is known about the origins of the dermis and the central role of Wnt/β-catenin signaling in its specification across anatomical locations. We then discuss how one of the first morphologically recognizable fibroblast subtypes, the hair follicle dermal condensate lineage, emerges. Leveraging the natural variation of skin and its appendages between species and between different anatomical locations, these collective studies have identified shared and divergent factors that contribute to the extraordinary diversity of skin.

Biology of cancer; from cellular and molecular mechanisms to developmental processes and adaptation

Cancer research has been largely focused on the cellular and molecular levels of investigation. Recent data show that not only the cell but also the extracellular matrix plays a major role in the progression of malignancy. In this way, the cells and the extracellular matrix create a specific local microenvironment that supports malignant development. At the same time, cancer implies a systemic evolution which is closely related to developmental processes and adaptation. Consequently, there is currently a real gap between the local investigation of cancer at the microenvironmental level, and the pathophysiological approach to cancer as a systemic disease. In fact, the cells and the matrix are not only complementary structures but also interdependent components that act synergistically. Such relationships lead to cell-matrix integration, a supracellular form of biological organization that supports tissue development. The emergence of this supracellular level of organization, as a structure, leads to the emergence of the supracellular control of proliferation, as a supracellular function. In humans, proliferation is generally involved in developmental processes and adaptation. These processes suppose a specific configuration at the systemic level, which generates high-order guidance for local supracellular control of proliferation. In conclusion, the supracellular control of proliferation act as an interface between the downstream level of cell division and differentiation, and upstream level of developmental processes and adaptation. Understanding these processes and their disorders is useful not only to complete the big picture of malignancy as a systemic disease, but also to open new treatment perspectives in the form of etiopathogenic (supracellular or informational) therapies.

GC-TOF/MS-Based Metabolomics for Comparison of Volar and Non-Volar Skin Types

Skin has heterogenous identities on different body sites despite similar cellular compositions. There are two types of skin, volar (palmoplantar) and non-volar (dorsal), which are characterized by epidermal thickness, pigmentation, and presence of hair follicles. However, the mechanisms underlying the development of these different skin types remain unclear. To investigate these, we profiled the cellular metabolites of volar and non-volar skin in mice using gas chromatography-time-of-flight/mass spectrometry (GC-TOF/MS), and further assessed the metabolic differences between them. In total, 96 metabolites from both volar and non-volar skin of mice were identified using the BinBase database system. Metabolomics analysis revealed important differences associated with amino acid metabolism (phenylalanine, tyrosine, and tryptophan biosynthesis; aspartate and glutamate metabolism), sugar metabolism (pentose phosphate pathway), and nucleotide metabolism (pyrimidine metabolism) in volar skin. Fifty metabolites were identified as potential biomarkers differentiating the physiological characteristics of these skin types. Of these, nine were highly increased whereas 41 were significantly decreased in volar skin compared with those in non-volar skin. Overall, these results provide valuable information for understanding the metabolic differences between volar and non-volar skin.

Fibroblast Heterogeneity in Healthy and Wounded Skin

Fibroblasts are the main cell type in the dermis. They are responsible for the synthesis and deposition of structural proteins such as collagen and elastin, which are integrated into the extracellular matrix (ECM). Mouse and human studies using flow cytometry, cell culture, skin reconstitution, and lineage tracing experiments have shown the existence of different subpopulations of fibroblasts, including papillary fibroblasts, reticular fibroblasts, and fibroblasts comprising the dermal papilla at the base of the hair follicle. In recent years, the technological advances in single-cell sequencing have allowed researchers to study the repertoire of cells present in full-thickness skin including the dermis. Multiple groups have confirmed that distinct fibroblast populations can be identified in mouse and human dermis on the basis of differences in the transcriptional profile. Here, we discuss the current state of knowledge regarding dermal fibroblast heterogeneity in healthy mouse and human skin, highlighting the similarities and differences between mouse and human fibroblast subpopulations. We also discuss how fibroblast heterogeneity may provide insights into physiological wound healing and its dysfunction in pathological states such as hypertrophic and keloid scars.

Can plantar fibroblast implantation protect amputees from skin injury? A recipe for skin augmentation

Skin injuries remain a persistent problem for users of lower-limb prostheses despite sustained progress in prosthesis design. One factor limiting the prevention of skin injuries is that skin on the residual limb is not suited to bear the mechanical loads of ambulation. One part of the body that is suited to this task is the sole of the foot. Here, we propose a novel strategy to actively augment skin's tolerance to load, increasing its resistance to mechanically induced injuries. We hypothesise that the load tolerance of skin can be augmented by autologous transplantation of plantar fibroblasts into the residual limb dermis. We expect that introducing plantar fibroblasts will induce the overlying keratinocytes to express plantar-specific keratins leading to a tougher epidermis. Using a computational finite element model of a weight-bearing residual limb, we estimate that skin deformation (a key driver of pressure ulcer injuries) could be halved by reprogramming skin to a plantar-like phenotype. We believe this strategy could yield new progress in pressure ulcer prevention for amputees, facilitating rehabilitation and improving quality of life for patients.

Fibroblasts and macrophages: Collaborators in tissue homeostasis

Fibroblasts and macrophages are universal cell types across all mammalian tissues. These cells differ in many ways including their cellular origins; dynamics of renewal, recruitment, and motility within tissues; roles in tissue structure and secretion of signaling molecules; and contributions to the activation and progression of immune responses. However, many of the features that make these two cell types unique are not opposing, but instead complementary. This review will present cell-cell communication in this context and discuss how complementarity makes fibroblasts and macrophages highly compatible partners in the maintenance of tissue homeostasis.

Corneocytes: Relationship between Structural and Biomechanical Properties

BACKGROUND

Skin is the interface between an organism and the external environment, and hence the stratum corneum (SC) is the first to withstand mechanical insults that, in certain conditions, may lead to integrity loss and the development of pressure ulcers. The SC comprises corneocytes, which are vital elements to its barrier function. These cells are differentiated dead keratinocytes, without organelles, composed of a cornified envelope and a keratin-filled interior, and connected by corneodesmosomes (CDs).

SUMMARY

The current review focusses on the relationship between the morphological, structural, and topographical features of corneocytes and their mechanical properties, to understand how they assist the SC in maintaining skin integrity and in responding to mechanical insults. Key Messages: Corneocytes create distinct regions in the SC: the inner SC is characterized by immature cells with a fragile cornified envelope and a uniform distribution of CDs; the upper SC has resilient cornified envelopes and a honeycomb distribution of CDs, with a greater surface area and a smaller thickness than cells from the inner layer. The literature indicates that this upward maturation process is one of the most important steps in the mechanical resistance and barrier function of the SC. The morphology of these cells is dependent on the body site: the surface area in non-exposed skin is about 1,000-1,200 μm2, while for exposed skin, for example, the cheek and forehead, is about 700-800 μm2. Corneocytes are stiff cells compared to other cellular types, for example, the Young's modulus of muscle and fibroblast cells is typically a few kPa, while that of corneocytes is reported to be about hundreds of MPa. Moreover, these skin cells have 2 distinct mechanical regions: the cornified envelope (100-250 MPa) and the keratin matrix (250-500 MPa).

Understanding the impact of fibroblast heterogeneity on skin fibrosis

Tissue fibrosis is the deposition of excessive extracellular matrix and can occur as part of the body's natural wound healing process upon injury, or as a consequence of diseases such as systemic sclerosis. Skin fibrosis contributes to significant morbidity due to the prevalence of injuries resulting from trauma and burn. Fibroblasts, the principal cells of the dermis, synthesize extracellular matrix to maintain the skin during homeostasis and also play a pivotal role in all stages of wound healing. Although it was previously believed that fibroblasts are homogeneous and mostly quiescent cells, it has become increasingly recognized that numerous fibroblast subtypes with unique functions and morphologies exist. This Review provides an overview of fibroblast heterogeneity in the mammalian dermis. We explain how fibroblast identity relates to their developmental origin, anatomical site and precise location within the skin tissue architecture in both human and mouse dermis. We discuss current evidence for the varied functionality of fibroblasts within the dermis and the relationships between fibroblast subtypes, and explain the current understanding of how fibroblast subpopulations may be controlled through transcriptional regulatory networks and paracrine communications. We consider how fibroblast heterogeneity can influence wound healing and fibrosis, and how insight into fibroblast heterogeneity could lead to novel therapeutic developments and targets for skin fibrosis. Finally, we contemplate how future studies should be shaped to implement knowledge of fibroblast heterogeneity into clinical practice in order to lessen the burden of skin fibrosis.

Summary: This Review discusses the multifaceted aspects of fibroblast heterogeneity and the different roles of fibroblast subpopulations to help overcome skin scarring and fibrosis.

Altered keratinocyte differentiation is an early driver of keratin mutation-based palmoplantar keratoderma

The type I intermediate filament keratin 16 (KRT16 gene; K16 protein) is constitutively expressed in ectoderm-derived appendages and in palmar/plantar epidermis and is robustly induced when the epidermis experiences chemical, mechanical or environmental stress. Missense mutations at the KRT16 locus can cause pachyonychia congenita (PC, OMIM:167200) or focal non-epidermolytic palmoplantar keratoderma (FNEPPK, OMIM:613000), which each entail painful calluses on palmar and plantar skin. Krt16-null mice develop footpad lesions that mimic PC-associated PPK, providing an opportunity to decipher its pathophysiology, and develop therapies. We report on insight gained from a genome-wide analysis of gene expression in PPK-like lesions of Krt16-null mice. Comparison of this data set with publicly available microarray data of PPK lesions from individuals with PC revealed significant synergies in gene expression profiles. Keratin 9 (Krt9/K9), the most robustly expressed gene in differentiating volar keratinocytes, is markedly downregulated in Krt16-null paw skin, well-ahead of lesion onset, and is paralleled by pleiotropic defects in terminal differentiation. Effective prevention of PPK-like lesions in Krt16-null paw skin (via topical delivery of the Nrf2 inducer sulforaphane) involves the stimulation of Krt9 expression. These findings highlight a role for defective terminal differentiation and loss of Krt9/K9 expression as additional drivers of PC-associated PPK and highlight restoration of KRT9 expression as a worthy target for therapy. Further, we report on the novel observation that keratin 16 can localize to the nucleus of epithelial cells, implying a potential nuclear function that may be relevant to PC and FNEPPK.

Double-Stranded RNA Sensing Determines Epithelial Cell Identity

Skin epithelium is constituted by heterogeneous keratinocytes in different body areas. In this issue, Zhou et al. used transcriptome analysis to investigate the site-specific epidermal cell identity on volar skin. Keratin 9 is highly enriched in volar keratinocytes, and its expression is dependent on low expression of the double-stranded RNA receptor DDX58, suggesting that double-stranded RNA sensing could allow a potential approach to modulate skin thickness and durability.

dsRNA Sensing Induces Loss of Cell Identity

How cell and tissue identity persist despite constant cell turnover is an important biologic question with cell therapy implications. Although many mechanisms exist, we investigated the controls for site-specific gene expression in skin, given its diverse structures and functions. For example, the transcriptome of in vivo palmoplantar (i.e., volar) epidermis is globally unique, including Keratin 9 (KRT9). Although volar fibroblasts have the capacity to induce KRT9 in nonvolar keratinocytes, we show here that volar keratinocytes continue to express KRT9 in in vitro solo cultures. Despite this, KRT9 expression is lost with volar keratinocyte passaging, despite stable hypomethylation of its promoter. Coincident with KRT9 loss is a gain of the primitive keratin 7 and a signature of dsRNA sensing, including the double-stranded RNA (dsRNA) receptor DExD/H-Box Helicase 58 (DDX58/RIG-I). Exogenous dsRNA inhibits KRT9 expression in early passage volar keratinocytes or in vivo footpads of wild-type mice. Loss of DDX58 in passaged volar keratinocytes rescues KRT9 and inhibits KRT7 expression. Additionally, DDX58-null mice are resistant to the ability of dsRNA to inhibit KRT9 expression. These results show that the sensing of dsRNA is critical for loss of cell-specific gene expression; our results have important implications for how dsRNA sensing is important outside of immune pathways.

Characterization of the human ridged and non-ridged skin: a comprehensive histological, histochemical and immunohistochemical analysis

The structure of the human skin is directly dependent on its location and the mechanical forces to which it is subjected. In the present work, we have performed a comprehensive analysis of the human ridged and non-ridged skin to identify the differences and similarities between both skin types. For this purpose, human skin samples were obtained from dorsal hand skin (DHS), palmar hand skin (PHS), dorsal foot skin (DFS) and plantar foot skin (PFS) from the same cadaveric donors. Histological, histochemical and semiquantitative and quantitative immunohistochemical analyses were carried out to evaluate the epidermis, dermis and basement membrane. Results show that the epithelial layer of ridged skin had larger cell number and size than non-ridged skin for most strata. Melanocytes and Langerhans cells were more abundant in non-ridged skin, whereas Merkel cells were preferentially found in ridged skin. The expression pattern of CK5/6 was slightly differed between non-ridged and ridged skin. Involucrin expression was slightly more intense in non-ridged skin than in ridged skin. Collagen was more abundant in foot skin dermis than in hand skin, and in ridged skin as compared to non-ridged skin. Elastic fibers were more abundant in DHS. Biglycan was more abundant in foot skin than in hand skin. No differences were found for blood and lymphatic vessels. The basement membrane laminin was preferentially found in foot skin. These results revealed important differences at the epithelial, dermal and basement membrane levels that could contribute to a better knowledge of the human skin histology.

The expression pattern of keratin 24 in tissue-engineered dermo-epidermal human skin substitutes in an in vivo model

AIMS AND OBJECTIVES

The use of autologous tissue-engineered skin substitutes is a promising approach to cover large skin defects in patients. Preclinical investigation is pivotal to test and improve the quality of these bio-engineered substitutes. In the skin, the epidermis, formed mainly by keratinocytes, provides the first physical barrier protecting from the environment. Proper keratinocyte differentiation and, thus, formation of a stratified epidermis is essential for this function. Keratins, the main structural support of keratinocytes, play a vital role regarding differentiation of keratinocytes. Here, we examined the expression pattern of a recently described keratinocyte differentiation marker, namely Keratin 24, in our skin substitutes.

MATERIALS AND METHODS

Human epidermal keratinocytes, melanocytes, dermal fibroblasts, palmar fibroblasts or sweat gland cells were used to prepare skin substitutes. Fibroblast-containing collagen hydrogels were prepared, and keratinocytes or sweat gland cells and melanocytes were seeded onto the hydrogels. The generated tissue-engineered dermo-epidermal skin analogs were transplanted onto full-thickness skin wounds created on the back of immuno-incompetent rats. The skin substitutes were excised at different time points and histologically examined with regard to Keratin 24 expression.

RESULTS

We observed the expression of Keratin 24 in keratinocytes of the upper stratum spinosum of the epidermis. In particular, we observed an intensified expression of Keratin 24 13 weeks after transplantation compared to 4 weeks after transplantation. Importantly, we noticed a markedly higher presence of Keratin 24 in more spinous layers if we used palmar fibroblasts or sweat gland cells in our skin substitutes compared non-palmar fibroblasts or epidermal keratinocytes.

CONCLUSION

Our observations prove that the keratinocyte differentiation marker Keratin 24 is expressed in our dermo-epidermal skin substitutes in a normal pattern. This highlights that our bio-engineered skin analogs mature and reach homeostasis in an in vivo assay. These findings harbor favorable implications regarding future clinical application.

Fibroblast heterogeneity: implications for human disease

Fibroblasts synthesize the extracellular matrix of connective tissue and play an essential role in maintaining the structural integrity of most tissues. Researchers have long suspected that fibroblasts exhibit functional specialization according to their organ of origin, body site, and spatial location. In recent years, a number of approaches have revealed the existence of fibroblast subtypes in mice. Here, we discuss fibroblast heterogeneity with a focus on the mammalian dermis, which has proven an accessible and tractable system for the dissection of these relationships. We begin by considering differences in fibroblast identity according to anatomical site of origin. Subsequently, we discuss new results relating to the existence of multiple fibroblast subtypes within the mouse dermis. We consider the developmental origin of fibroblasts and how this influences heterogeneity and lineage restriction. We discuss the mechanisms by which fibroblast heterogeneity arises, including intrinsic specification by transcriptional regulatory networks and epigenetic factors in combination with extrinsic effects of the spatial context within tissue. Finally, we discuss how fibroblast heterogeneity may provide insights into pathological states including wound healing, fibrotic diseases, and aging. Our evolving understanding suggests that ex vivo expansion or in vivo inhibition of specific fibroblast subtypes may have important therapeutic applications.

Why location matters - site-specific factors in rheumatic diseases

Rheumatic diseases follow a characteristic anatomical pattern of joint and organ involvement. This Review explores three interconnected mechanisms that might be involved in the predilection of specific joints for developing specific forms of arthritis: site-specific local cell types that drive disease; systemic triggers that affect local cell types; and site-specific exogenous factors, such as focal mechanical stress, that activate cells locally. The embryonic development of limbs and joints is also relevant to the propensity of certain joints to develop arthritis. Additionally, location-specific homeostasis and disease occurs in skin and blood vessels, thereby extending the concept of site-specificity in human diseases beyond rheumatology. Acknowledging the importance of site-specific parameters increases the complexity of current disease paradigms and brings us closer to understanding why particular disease processes manifest at a particular location.

To Control Site-Specific Skin Gene Expression, Autocrine Mimics Paracrine Canonical Wnt Signaling and Is Activated Ectopically in Skin Disease

Despite similar components, the heterogeneity of skin characteristics across the human body is enormous. It is classically believed that site-specific fibroblasts in the dermis control postnatal skin identity by modulating the behavior of the surface-overlying keratinocytes in the epidermis. To begin testing this hypothesis, we characterized the gene expression differences between volar (ventral; palmoplantar) and nonvolar (dorsal) human skin. We show that KERATIN 9 (KRT9) is the most uniquely enriched transcript in volar skin, consistent with its etiology in genetic diseases of the palms and soles. In addition, ectopic KRT9 expression is selectively activated by volar fibroblasts. However, KRT9 expression occurs in the absence of all fibroblasts, although not to the maximal levels induced by fibroblasts. Through gain-of-function and loss-of-function experiments, we demonstrate that the mechanism is through overlapping paracrine or autocrine canonical WNT-β-catenin signaling in each respective context. Finally, as an in vivo example of ectopic expression of KRT9 independent of volar fibroblasts, we demonstrate that in the human skin disease lichen simplex chronicus, WNT5a and KRT9 are robustly activated outside of volar sites. These results highlight the complexities of site-specific gene expression and its disruption in skin disease.

Epigenetically-driven anatomical diversity of synovial fibroblasts guides joint-specific fibroblast functions

A number of human diseases, such as arthritis and atherosclerosis, include characteristic pathology in specific anatomical locations. Here we show transcriptomic differences in synovial fibroblasts from different joint locations and that HOX gene signatures reflect the joint-specific origins of mouse and human synovial fibroblasts and synovial tissues. Alongside DNA methylation and histone modifications, bromodomain and extra-terminal reader proteins regulate joint-specific HOX gene expression. Anatomical transcriptional diversity translates into joint-specific synovial fibroblast phenotypes with distinct adhesive, proliferative, chemotactic and matrix-degrading characteristics and differential responsiveness to TNF, creating a unique microenvironment in each joint. These findings indicate that local stroma might control positional disease patterns not only in arthritis but in any disease with a prominent stromal component.

Arthritis affects different joints variably despite systemic inflammatory cues. Here the authors show anatomical differences in the transcriptome, epigenome and function of synovial fibroblasts that might affect susceptibility to site-specific joint diseases.

Estrogen modulates mesenchyme-epidermis interactions in the adult nipple

Maintenance of specialized epidermis requires signals from the underlying mesenchyme; however, the specific pathways involved remain to be identified. By recombining cells from the ventral skin of the K14-PTHrP transgenic mice [which overexpress parathyroid hormone-related protein (PTHrP) in their developing epidermis and mammary glands] with those from wild type, we show that transgenic stroma is sufficient to reprogram wild-type keratinocytes into nipple-like epidermis. To identify candidate nipple-specific signaling factors, we compared gene expression signatures of sorted Pdgfrα-positive ventral K14-PTHrP and wild-type fibroblasts, identifying differentially expressed transcripts that are involved in WNT, HGF, TGFβ, IGF, BMP, FGF and estrogen signaling. Considering that some of the growth factor pathways are targets for estrogen regulation, we examined the upstream role of this hormone in maintaining the nipple. Ablation of estrogen signaling through ovariectomy produced nipples with abnormally thin epidermis, and we identified TGFβ as a negatively regulated target of estrogen signaling. Estrogen treatment represses Tgfβ1 at the transcript and protein levels in K14-PTHrP fibroblasts in vitro, while ovariectomy increases Tgfb1 levels in K14-PTHrP ventral skin. Moreover, ectopic delivery of Tgfβ1 protein into nipple connective tissue reduced epidermal proliferation. Taken together, these results show that specialized nipple epidermis is maintained by estrogen-induced repression of TGFβ signaling in the local fibroblasts.

An Automated and Minimally Invasive Tool for Generating Autologous Viable Epidermal Micrografts

OBJECTIVE:

A new epidermal harvesting tool (CelluTome; Kinetic Concepts, Inc, San Antonio, Texas) created epidermal micrografts with minimal donor site damage, increased expansion ratios, and did not require the use of an operating room. The tool, which applies both heat and suction concurrently to normal skin, was used to produce epidermal micrografts that were assessed for uniform viability, donor-site healing, and discomfort during and after the epidermal harvesting procedure.

DESIGN:

This study was a prospective, noncomparative institutional review board–approved healthy human study to assess epidermal graft viability, donor-site morbidity, and patient experience.

SETTING:

These studies were conducted at the multispecialty research facility, Clinical Trials of Texas, Inc, San Antonio.

PATIENTS:

The participants were 15 healthy human volunteers.

RESULTS:

The average viability of epidermal micrografts was 99.5%. Skin assessment determined that 76% to 100% of the area of all donor sites was the same in appearance as the surrounding skin within 14 days after epidermal harvest. A mean pain of 1.3 (on a scale of 1 to 5) was reported throughout the harvesting process.

CONCLUSIONS:

Use of this automated, minimally invasive harvesting system provided a simple, low-cost method of producing uniformly viable autologous epidermal micrografts with minimal patient discomfort and superficial donor-site wound healing within 2 weeks.

Clinical and Economic Benefits of Autologous Epidermal Grafting

Chronic wounds are an increasingly prevalent disease with a significant healthcare burden. These wounds often do not respond to standard of care therapy alone, requiring the use of adjuvant therapies. Epidermal grafting, previously utilized primarily for correction of leukoderma, is increasingly being recognized as a beneficial therapy for wounds, both acute and chronic. Epidermal grafting has been shown to be effective in the management of chronic wounds, with successful healing in refractory patients. It has not only been shown to be effective, but it is also associated with lower cost and morbidity than traditional skin grafting techniques as well as improved donor site healing. Through the use of a novel epidermal harvesting system, the CelluTome™ Epidermal Harvesting System (KCI, an Acelity company, San Antonio, TX), this treatment modality has become more standardized, reproducible, and easy to use as well as less time consuming, making its use in the clinical setting more convenient and beneficial. Epidermal grafting, therefore, represents a promising, efficacious, and cost-effective option for treatment of refractory non-healing wounds.

Functional differences between neonatal and adult fibroblasts and keratinocytes: Donor age affects epithelial-mesenchymal crosstalk in vitro

Clinical evidence suggests that healing is faster and almost scarless at an early neonatal age in comparison with that in adults. In this study, the phenotypes of neonatal and adult dermal fibroblasts and keratinocytes (nestin, smooth muscle actin, keratin types 8, 14 and 19, and fibronectin) were compared. Furthermore, functional assays (proliferation, migration, scratch wound closure) including mutual epithelial-mesenchymal interactions were also performed to complete the series of experiments. Positivity for nestin and α smooth muscle actin was higher in neonatal fibroblasts (NFs) when compared with their adult counterparts (adult fibroblasts; AFs). Although the proliferation of NFs and AFs was similar, they significantly differed in their migration potential. The keratinocyte experiments revealed small, poorly differentiated cells (positive for keratins 8, 14 and 19) in primary cultures isolated from neonatal tissues. Moreover, the neonatal keratinocytes exhibited significantly faster rates of healing the experimentally induced in vitro defects in comparison with adult cells. Notably, the epithelial/mesenchymal interaction studies showed that NFs in co-culture with adult keratinocytes significantly stimulated the adult epithelial cells to acquire the phenotype of small, non-confluent cells expressing markers of poor differentiation. These results indicate the important differences between neonatal and adult cells that may be associated with improved wound healing during the early neonatal period.

Optimization of the current self-assembled urinary bladder model: Organ-specific stroma and smooth muscle inclusion

INTRODUCTION

Due to the complications associated with the use of non-native biomaterials and the lack of local tissues, bioengineered tissues are required for surgical reconstruction of complex urinary tract diseases, including those of the urinary bladder. The self-assembly method of matrix formation using autologous stromal cells obviates the need for exogenous biomaterials. We aimed at creating novel ex-vivo multilayer urinary tissue from a single bladder biopsy.

METHODS

After isolating urothelial, bladder stromal and smooth muscle cells from bladder biopsies, we produced 2 models of urinary equivalents: (1) the original one with dermal fibroblasts and (2) the new one with bladder stromal cells. Dermal fibroblasts and bladder stromal cells were stimulated to form an extracellular matrix, followed by sequential seeding of smooth muscle cells and urothelial cells. Stratification and cellular differentiation were assessed by histology, immunostaining and electron microscopy. Barrier function was checked with the permeability test. Biomechanical properties were assessed with uniaxinal tensile strength, elastic modulus, and failure strain.

RESULTS

Both urinary equivalents could be handled easily and did not contract. Stratified epithelium, intact basement membrane, fused matrix, and prominent muscle layer were detected in both urinary equivalents. Bladder stromal cell-based constructs had terminally differentiated urothelium and more elasticity than dermal fibroblasts-based equivalents. Permeation studies showed that both equivalents were comparable to native tissues.

CONCLUSIONS

Organ-specific stromal cells produced urinary tissues with more terminally differentiated urothelium and better biomechanical characteristics than non-specific stromal cells. Smooth muscle cells could be incorporated into the self-assembled tissues effectively. This multilayer tissue can be used as a urethral graft or as a bladder model for disease modelling and pharmacotherapeutic testing.

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.

Gastrointestinal Fibroblasts Have Specialized, Diverse Transcriptional Phenotypes: A Comprehensive Gene Expression Analysis of Human Fibroblasts

BACKGROUND

Fibroblasts are the principal stromal cells that exist in whole organs and play vital roles in many biological processes. Although the functional diversity of fibroblasts has been estimated, a comprehensive analysis of fibroblasts from the whole body has not been performed and their transcriptional diversity has not been sufficiently explored. The aim of this study was to elucidate the transcriptional diversity of human fibroblasts within the whole body.

METHODS

Global gene expression analysis was performed on 63 human primary fibroblasts from 13 organs. Of these, 32 fibroblasts from gastrointestinal organs (gastrointestinal fibroblasts: GIFs) were obtained from a pair of 2 anatomical sites: the submucosal layer (submucosal fibroblasts: SMFs) and the subperitoneal layer (subperitoneal fibroblasts: SPFs). Using hierarchical clustering analysis, we elucidated identifiable subgroups of fibroblasts and analyzed the transcriptional character of each subgroup.

RESULTS

In unsupervised clustering, 2 major clusters that separate GIFs and non-GIFs were observed. Organ- and anatomical site-dependent clusters within GIFs were also observed. The signature genes that discriminated GIFs from non-GIFs, SMFs from SPFs, and the fibroblasts of one organ from another organ consisted of genes associated with transcriptional regulation, signaling ligands, and extracellular matrix remodeling.

CONCLUSIONS

GIFs are characteristic fibroblasts with specific gene expressions from transcriptional regulation, signaling ligands, and extracellular matrix remodeling related genes. In addition, the anatomical site- and organ-dependent diversity of GIFs was also discovered. These features of GIFs contribute to their specific physiological function and homeostatic maintenance, and create a functional diversity of the gastrointestinal tract.

The influence of stromal cells on the pigmentation of tissue-engineered dermo-epidermal skin grafts

It has been shown in vitro that melanocyte proliferation and function in palmoplantar skin is regulated by mesenchymal factors derived from fibroblasts. In this study, we investigated in vivo the influence of mesenchymal-epithelial interactions in human tissue-engineered skin substitutes reconstructed from palmar- and nonpalmoplantar-derived fibroblasts. Tissue-engineered dermo-epidermal analogs based on collagen type I hydrogels were populated with either human palmar or nonpalmoplantar fibroblasts and seeded with human nonpalmoplantar-derived melanocytes and keratinocytes. These skin substitutes were transplanted onto full-thickness skin wounds of immunoincompetent rats. Four weeks after transplantation the development of skin color was measured and grafts were excised and analyzed with regard to epidermal characteristics, in particular melanocyte number and function. Skin substitutes containing palmar-derived fibroblasts in comparison to nonpalmoplantar-derived fibroblasts showed (a) a significantly lighter pigmentation; (b) a reduced amount of epidermal melanin granules; and (c) a distinct melanosome expression. However, the number of melanocytes in the basal layer remained similar in both transplantation groups. These findings demonstrate that human palmar fibroblasts regulate the function of melanocytes in human pigmented dermo-epidermal skin substitutes after transplantation, whereas the number of melanocytes remains constant. This underscores the influence of site-specific stromal cells and their importance when constructing skin substitutes for clinical application.

Interrogating the integument: the role of the epidermis in hair induction

Hair follicle development is driven by interactions between the epithelium and underlying mesenchyme. These reciprocal interactions are essential for development, as a lack of response from either the mesenchyme or epithelium results in arrested growth. A large body of research has focused on the role of mesenchymal cells during hair follicle development and their inductive properties for hair neogenesis. In this commentary, the role of the epidermis during hair follicle induction will be discussed.

Alteration of skin properties with autologous dermal fibroblasts

Dermal fibroblasts are mesenchymal cells found between the skin epidermis and subcutaneous tissue. They are primarily responsible for synthesizing collagen and glycosaminoglycans; components of extracellular matrix supporting the structural integrity of the skin. Dermal fibroblasts play a pivotal role in cutaneous wound healing and skin repair. Preclinical studies suggest wider applications of dermal fibroblasts ranging from skin based indications to non-skin tissue regeneration in tendon repair. One clinical application for autologous dermal fibroblasts has been approved by the Food and Drug Administration (FDA) while others are in preclinical development or various stages of regulatory approval. In this context, we outline the role of fibroblasts in wound healing and discuss recent advances and the current development pipeline for cellular therapies using autologous dermal fibroblasts. The microanatomic and phenotypic differences of fibroblasts occupying particular locations within the skin are reviewed, emphasizing the therapeutic relevance of attributes exhibited by subpopulations of fibroblasts. Special focus is provided to fibroblast characteristics that define regional differences in skin, including the thick and hairless skin of the palms and soles as compared to hair-bearing skin. This regional specificity and functional identity of fibroblasts provides another platform for developing regional skin applications such as the induction of hair follicles in bald scalp or alteration of the phenotype of stump skin in amputees to better support their prosthetic devices.

Regionalisation of the skin

The skin displays marked anatomical variation in thickness, colour and in the appendages that it carries. These regional distinctions arise in the embryo, likely founded on a combinatorial positional code of transcription factor expression. Throughout adult life, the skin's distinct anatomy is maintained through both cell autonomous epigenetic processes and by mesenchymal-epithelial induction. Despite the readily apparent anatomical differences in skin characteristics across the body, several fundamental questions regarding how such regional differences first arise and then persist are unresolved. However, it is clear that the skin's positional code is at the molecular level far more detailed than that discernible at the phenotypic level. This provides a latent reservoir of anatomical complexity ready to surface if perturbed by mutation, hormonal changes, ageing or experiment.

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.

iPSC-derived fibroblasts demonstrate augmented production and assembly of extracellular matrix proteins

Reprogramming of somatic cells to induced pluripotent stem cells (iPSC) provides an important cell source to derive patient-specific cells for potential therapeutic applications. However, it is not yet clear whether reprogramming through pluripotency allows the production of differentiated cells with improved functional properties that may be beneficial in regenerative therapies. To address this, we compared the production and assembly of extracellular matrix (ECM) by iPSC-derived fibroblasts to that of the parental, dermal fibroblasts (BJ), from which these iPSC were initially reprogrammed, and to fibroblasts differentiated from human embryonic stem cells (hESC). iPSC- and hESC-derived fibroblasts demonstrated stable expression of surface markers characteristic of stromal fibroblasts during prolonged culture and showed an elevated growth potential when compared to the parental BJ fibroblasts. We found that in the presence of L: -ascorbic acid-2-phosphate, iPSC- and hESC-derived fibroblasts increased their expression of collagen genes, secretion of soluble collagen, and extracellular deposition of type I collagen to a significantly greater degree than that seen in the parental BJ fibroblasts. Under culture conditions that enabled the self-assembly of a 3D stromal tissue, iPSC- and hESC-derived fibroblasts generated a well organized, ECM that was enriched in type III collagen. By characterizing the functional properties of iPSC-derived fibroblasts compared to their parental fibroblasts, we demonstrate that these cells represent a promising, alternative source of fibroblasts to advance future regenerative therapies.

Mouse 3T3 fibroblasts under the influence of fibroblasts isolated from stroma of human basal cell carcinoma acquire properties of multipotent stem cells

BACKGROUND INFORMATION

Multipotent mesenchymal stem cells can participate in the formation of a microenvironment stimulating the aggressive behaviour of cancer cells. Moreover, cells exhibiting pluripotent ESC (embryonic stem cell) markers (Nanog and Oct4) have been observed in many tumours. Here, we investigate the role of cancer-associated fibroblasts in the formation of stem cell supporting properties of tumour stroma. We test the influence of fibroblasts isolated from basal cell carcinoma on mouse 3T3 fibroblasts, focusing on the expression of stem cell markers and plasticity in vitro by means of microarrays, qRT-PCR (quantitative real-time PCR) and immunohistochemistry.

RESULTS

We demonstrate the biological activity of the cancer stromal fibroblasts by influencing the 3T3 fibroblasts to express markers such as Oct4, Nanog and Sox2 and to show differentiation potential similar to mesenchymal stem cells. The role of growth factors such as IGF2 (insulin-like growth factor 2), FGF7 (fibroblast growth factor 7), LEP (leptin), NGF (nerve growth factor) and TGFβ (transforming growth factor β), produced by the stromal fibroblasts, is established to participate in their bioactivity. Uninduced 3T3 do not express the stem cell markers and show minimal differentiation potential.

CONCLUSIONS

Our observations indicate the pro-stem cell activity of cancer-associated fibroblasts and underline the role of epithelial-mesenchymal interaction in tumour biology.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Regulation of skin pigmentation and thickness by Dickkopf 1 (DKK1)

Dickkopf 1 (DKK1), an inhibitor of Wnt signaling, not only functions as a head inducer during development, but also regulates joint remodeling and bone formation, which suggests roles for DKK1 in the pathogenesis of rheumatoid arthritis and multiple myeloma. We recently demonstrated that levels of DKK1 in palmoplantar dermal fibroblasts are physiologically higher than those observed in non-palmoplantar dermal fibroblasts. Thus, the DKK1-rich mesenchyme in palmoplantar dermis affects the overlying epithelium and induces a palmoplantar phenotype in the epidermis. More specifically, DKK1 suppresses melanocyte function and growth through the regulation of microphthalmia-associated transcription factor (MITF) and beta-catenin. Furthermore, DKK1 induces the expression of keratin 9 and alpha-Kelch-like ECT2-interacting protein (alphaKLEIP) but downregulates the expression of beta-catenin, glycogen synthase kinase 3beta, protein kinase C, and proteinase-activated receptor-2 (PAR-2) in keratinocytes. Treatment of reconstructed skin with DKK1 reproduces the hypopigmentation and thickening of skin through Wnt/beta-catenin signaling. These studies elucidate why human palmoplantar skin is thicker and paler than non-palmoplantar skin through the secretion of DKK1 by fibroblasts that affect the overlying epidermis. Thus, DKK1 may be useful for reducing skin pigmentation and for thickening photo-aged skin and palmoplantar wounds caused by diabetes mellitus and rheumatic skin diseases.Journal of Investigative Dermatology Symposium Proceedings (2009) 14, 73-75; doi:10.1038/jidsymp.2009.4.

Fibroblasts-a diverse population at the center of it all

The capacity of fibroblasts to produce and organize the extracellular matrix and to communicate with other cells makes them a central component of tissue biology. Even so, fibroblasts remain a somewhat enigmatic population. Our inability to fully comprehend these cells is in large part due to the paucity of unique cellular markers and to their pervasive diversity. Much of our understanding of fibroblast diversity has evolved from studies where subpopulations of these cells have been produced without resorting to cell surface markers. In this regard, cloning and mechanical separation of tissues prior to establishing cultures has provided multiple subpopulations. Nonetheless, in isolated situations, the expression or lack of expression of Thy-1/CD90 has been used to separate fibroblast subsets. The role of fibroblasts in intercellular communication is emerging through the implementation of organotypic studies in which three-dimensional fibroblast culture are combined with other populations of cells. Such studies have revealed critical paracrine loops that are essential for organ development and for wound repair. These studies also provide a backdrop for the emerging field of tissue engineering. The participation of fibroblasts in the regulation of tissue homeostasis and their contribution to the aging process are emerging issues that require better understanding. In short, fibroblasts represent a multifaceted, complex group of cells.

Phenotypic characterization of human keratinocytes in coculture reveals differential effects of fibroblasts from benign fibrous histiocytoma (dermatofibroma) as compared to cells from its malignant form and to normal fibroblasts

BACKGROUND

Benign and malignant fibrous histiocytoma present with a considerable difference concerning cellular organization in their vicinity.

OBJECTIVE

Normally appearing epithelium covers the malignant form in contrast to hyperplastic epidermis for benign tumors. It is an open question as to whether the tumor-associated fibroblasts are capable to affect phenotypic features of normal keratinocytes, prompting this comparative analysis.

METHODS

Fibroblasts were isolated from benign and malignant fibrous histiocytomas, respectively, and also from normal dermis. The resulting cell populations were thoroughly characterized immunocytochemically using a large panel of antibodies. The three fibroblast preparations were cocultured with normal interfollicular keratinocytes. Their phenotype was characterized for distinct properties including differentiation and proliferation.

RESULTS

Fibroblasts prepared from both tumor types were phenotypically practically identical with normal dermal fibroblasts. Their activities on keratinocytes were different. Cells prepared from benign fibrous histiocytoma were capable to effect strong expression of keratin 19 and production of a galectin-1-rich extracellular matrix. Fibroblasts isolated from malignant fibrous histiocytoma led to a phenotype very similar to that when keratinocytes were cocultured with normal dermal fibroblasts.

CONCLUSION

Fibroblasts prepared from benign fibrous histiocytoma were biologically active on keratinocytes in a particular manner. Our results on fibroblast activity are suggested to be relevant for morphologic differences observed in vivo between normal epidermis and epidermis adjacent to the studied tumor types.

Physiological factors that regulate skin pigmentation

More than 150 genes have been identified that affect skin color either directly or indirectly, and we review current understanding of physiological factors that regulate skin pigmentation. We focus on melanosome biogenesis, transport and transfer, melanogenic regulators in melanocytes, and factors derived from keratinocytes, fibroblasts, endothelial cells, hormones, inflammatory cells, and nerves. Enzymatic components of melanosomes include tyrosinase, tyrosinase-related protein 1, and dopachrome tautomerase, which depend on the functions of OA1, P, MATP, ATP7A, and BLOC-1 to synthesize eumelanins and pheomelanins. The main structural component of melanosomes is Pmel17/gp100/Silv, whose sorting involves adaptor protein 1A (AP1A), AP1B, AP2, and spectrin, as well as a chaperone-like component, MART-1. During their maturation, melanosomes move from the perinuclear area toward the plasma membrane. Microtubules, dynein, kinesin, actin filaments, Rab27a, melanophilin, myosin Va, and Slp2-a are involved in melanosome transport. Foxn1 and p53 up-regulate skin pigmentation via bFGF and POMC derivatives including alpha-MSH and ACTH, respectively. Other critical factors that affect skin pigmentation include MC1R, CREB, ASP, MITF, PAX3, SOX9/10, LEF-1/TCF, PAR-2, DKK1, SCF, HGF, GM-CSF, endothelin-1, prostaglandins, leukotrienes, thromboxanes, neurotrophins, and neuropeptides. UV radiation up-regulates most factors that increase melanogenesis. Further studies will elucidate the currently unknown functions of many other pigment genes/proteins. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

A dermal HOX transcriptional program regulates site-specific epidermal fate

Reciprocal epithelial-mesenchymal interactions shape site-specific development of skin. Here we show that site-specific HOX expression in fibroblasts is cell-autonomous and epigenetically maintained. The distal-specific gene HOXA13 is continually required to maintain the distal-specific transcriptional program in adult fibroblasts, including expression of WNT5A, a morphogen required for distal development. The ability of distal fibroblasts to induce epidermal keratin 9, a distal-specific gene, is abrogated by depletion of HOXA13, but rescued by addition of WNT5A. Thus, maintenance of appropriate HOX transcriptional program in adult fibroblasts may serve as a source of positional memory to differentially pattern the epithelia during homeostasis and regeneration.

A systems biology approach to anatomic diversity of skin

Human skin exhibits exquisite site-specific morphologies and functions. How are these site-specific differences specified during development, maintained in adult homeostasis, and potentially perturbed by disease processes? Here, we review progress in understanding the anatomic patterning of fibroblasts, a major constituent cell type of the dermis and key participant in epithelial-mesenchymal interactions. The gene expression programs of human fibroblasts largely reflect the superimposition of three gene expression profiles that demarcate the fibroblast's position relative to three developmental axes. The HOX family of homeodomain transcription factors is implicated in specifying site-specific transcriptional programs. The use of gene, tiling, and tissue microarrays together gives a comprehensive view of the gene regulation involved in patterning the skin.

Bioreactor for application of subatmospheric pressure to three-dimensional cell culture

Vacuum-assisted closure (VAC) negative pressure wound therapy (NPWT) is a highly successful and widely used treatment modality for wound healing, although no apparatus exists to monitor the effects of subatmospheric pressure application in vitro. Such an apparatus is desirable to better understand the biological effects of this therapy and potentially improve upon them. This article describes the development and validation of a novel bioreactor that permits such study. Tissue analogues consisting of 3-dimensional fibroblast-containing fibrin clots were cultured in off-the-shelf disposable cell culture inserts and multi-well plates that were integrated into the bioreactor module. Negative pressure dressings, commercialized for wound therapy, were placed on top of the culture, and subatmospheric pressure was applied to the dressing. Cultures were perfused with media at controlled physiologic wound exudate flow rates. The design of this bioreactor permits observation of the culture using an inverted microscope in brightfield and fluorescence modes and sustained incubation of the system in a 5% carbon dioxide atmosphere. This closed-system mimics the wound micro-environment under VAC NPWT. Matrix compression occurs as the subatmospheric pressure draws the dressing material down. At the contact zone, surface undulations were clearly evident on the fibroblast-containing tissue analogues at 24 h and appeared to correspond to the dressing microstructure. The bioreactor design, consisting of sterilizable machined plastics and disposable labware, can be easily scaled to multiple units. Validation experiments show that cell survival in this system is comparable with that seen in cells grown in static tissue culture. After application of VAC NPWT, cell morphology changed, with cells appearing thicker and with an organized actin cytoskeleton. The development and validation of this new culture system establishes a stable platform for in vitro investigations of subatmospheric pressure application to tissues.

Dickkopf 1 (DKK1) regulates skin pigmentation and thickness by affecting Wnt/beta-catenin signaling in keratinocytes

The epidermis (containing primarily keratinocytes and melanocytes) overlies the dermis (containing primarily fibroblasts) of human skin. We previously reported that dickkopf 1 (DKK1) secreted by fibroblasts in the dermis elicits the hypopigmented phenotype of palmoplantar skin due to suppression of melanocyte function and growth via the regulation of two important signaling factors, microphthalmia-associated transcription factor (MITF) and beta-catenin. We now report that treatment of keratinocytes with DKK1 increases their proliferation and decreases their uptake of melanin and that treatment of reconstructed skin with DKK1 induces a thicker and less pigmented epidermis. DNA microarray analysis revealed many genes regulated by DKK1, and several with critical expression patterns were validated by reverse transcriptase-polymerase chain reaction and Western blotting. DKK1 induced the expression of keratin 9 and alpha-Kelch-like ECT2 interacting protein (alphaKLEIP) but down-regulated the expression of beta-catenin, glycogen synthase kinase 3beta, protein kinase C, and proteinase-activated receptor-2 (PAR-2), which is consistent with the expression patterns of those proteins in human palmoplantar skin. Treatment of reconstructed skin with DKK1 reproduced the expression patterns of those key proteins observed in palmoplantar skin. These findings further elucidate why human skin is thicker and paler on the palms and soles than on the trunk through topographical and site-specific differences in the secretion of DKK1 by dermal fibroblasts that affects the overlying epidermis.

Intractable wounds caused by arteriosclerosis obliterans with end-stage renal disease treated by aggressive debridement and epidermal grafting

BACKGROUND

Patients receiving hemodialysis (HD) need to avoid skin laceration; amputation is common because of intractability of wounds and complications with other diseases, including diabetes mellitus. We have reported the usefulness of aggressive debridement deep enough to expose bone marrow cells, occlusive dressing, and epidermal grafting for diabetic foot ulcers and wounds from rheumatic diseases.

OBJECTIVE

To test whether this experimental protocol is effective to treat intractable wounds in a patient receiving HD accompanied with arteriosclerosis obliterans (ASO).

METHODS AND RESULTS

A 78-year-old patient with chronic renal failure and arteriosclerosis obliterans suffered a wound with exposed bone on the left great toe. Aggressive combination therapy was effective despite methicillin-resistant Staphylococcus aureus infection, and the wound healed in 13 weeks.

LIMITATIONS

This study, reporting a single case, limits the interpretation of results.

CONCLUSION

Aggressive debridement exposing bone marrow cells is useful in preparing a healthy wound bed and epidermal sheet grafting may be accepted more advantageously in an ischemic environment and adopt a site-specific phenotype via mesenchymal-epithelial interactions.