Roles of TGFbeta signaling in epidermal/appendage development

Materials-based hair follicle engineering: Basic components and recent advances

The hair follicle (HF) is a significant skin appendage whose primary function is to produce the hair shaft. HFs are a non-renewable resource; skin damage or follicle closure may lead to permanent hair loss. Advances in biomaterials and biomedical engineering enable the feasibility of manipulating the HF-associated cell function for follicle reconstruction via rational design. The regeneration of bioengineered HF addresses the issue of limited resources and contributes to advancements in research and applications in hair loss treatment, HF development, and drug screening. Based on these requirements, this review summarizes the basic and recent advances in hair follicle regulation, including four components: acquisition of stem cells, signaling pathways, materials, and engineering methods. Recent studies have focused on efficiently combining these components and reproducing functionality, which would boost fabrication in HF rebuilding ex vivo, thereby eliminating the obstacles of transplantation into animals to promote mature development.

Keratin Microspheres as Promising Tool for Targeting Follicular Growth

Skin is the body barrier that constrains the infiltration of particles and exogenous aggression, in which the hair follicle plays an important role. Recent studies have shown that small particles can penetrate the skin barrier and reach the hair follicle, making them a potential avenue for delivering hair growth-related substances. Interestingly, keratin-based microspheres are widely used as drug delivery carriers in various fields. In this current study, we pursue the effect of newly synthesized 3D spherical keratin particles on inducing hair growth in C57BL/6 male mice and in human hair follicle dermal papilla cells. The microspheres were created from partially sulfonated, water-soluble keratin. The keratin microspheres swelled in water to form spherical gels, which were used for further experiments. Following topical application for a period of 20 days, we observed a regrowth of hair in the previously depleted area on the dorsal part of the mice in the keratin microsphere group. This observation was accompanied by the regulation of hair-growth-related pathways as well as changes in markers associated with epidermal cells, keratin, and collagen. Interestingly, microsphere keratin treatment enhanced the cell proliferation and the expression of hair growth markers in dermal papilla cells. Based on our data, we propose that 3D spherical keratin has the potential to specifically target hair follicle growth and can be employed as a carrier for promoting hair growth-related agents.

Single-Cell Transcriptome Sequence Profiling on the Morphogenesis of Secondary Hair Follicles in Ordos Fine-Wool Sheep

The Ordos fine-wool sheep is a high-quality breed in China that produces superior natural textiles and raw materials such as wool and lamb meat. However, compared to the Australian Merino sheep, there is still a gap in terms of the wool fiber fineness and wool yield. The hair follicle is the main organ that controls the type of wool fiber, and the morphological changes in the secondary hair follicle are crucial in determining wool quality. However, the process and molecular mechanisms of hair follicle morphogenesis in Ordos fine-wool sheep are not yet clear. Therefore, analyzing the molecular mechanisms underlying the process of follicle formation is of great significance for improving the fiber diameter and wool production of Ordos fine-wool sheep. The differential expressed genes, APOD, POSTN, KRT5, and KRT15, which related to primary hair follicles and secondary hair follicles, were extracted from the dermal papillae. Based on pseudo-time analysis, the differentiation trajectories of dermal lineage cells and epidermal lineage cells in the Ordos fine-wool sheep were successfully constructed, providing a theoretical basis for breeding research in Ordos fine-wool sheep.

Identification of key pathways and genes that regulate cashmere development in cashmere goats mediated by exogenous melatonin

The growth of secondary hair follicles in cashmere goats follows a seasonal cycle. Melatonin can regulate the cycle of cashmere growth. In this study, melatonin was implanted into live cashmere goats. After skin samples were collected, transcriptome sequencing and histological section observation were performed, and weighted gene co-expression network analysis (WGCNA) was used to identify key genes and establish an interaction network. A total of 14 co-expression modules were defined by WGCNA, and combined with previous analysis results, it was found that the blue module was related to the cycle of cashmere growth after melatonin implantation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that the first initiation of exogenous melatonin-mediated cashmere development was related mainly to the signaling pathway regulating stem cell pluripotency and to the Hippo, TGF-beta and MAPK signaling pathways. Via combined differential gene expression analyses, 6 hub genes were identified: PDGFRA, WNT5A, PPP2R1A, BMPR2, BMPR1A, and SMAD1. This study provides a foundation for further research on the mechanism by which melatonin regulates cashmere growth.

Regulation of signaling pathways in hair follicle stem cells

Hair follicle stem cells (HFSCs) reside in the bulge region of the outer root sheath of the hair follicle. They are considered slow-cycling cells that are endowed with multilineage differentiation potential and superior proliferative capacity. The normal morphology and periodic growth of HFSCs play a significant role in normal skin functions, wound repair and skin regeneration. The HFSCs involved in these pathophysiological processes are regulated by a series of cell signal transduction pathways, such as lymphoid enhancer factor/T-cell factor, Wnt/β-catenin, transforming growth factor-β/bone morphogenetic protein, Notch and Hedgehog. The mechanisms of the interactions among these signaling pathways and their regulatory effects on HFSCs have been previously studied, but many mechanisms are still unclear. This article reviews the regulation of hair follicles, HFSCs and related signaling pathways, with the aims of summarizing previous research results, revealing the regulatory mechanisms of HFSC proliferation and differentiation and providing important references and new ideas for treating clinical diseases.

Characterization and functional analysis of SMAD2 regulation in hair follicle cycle in Angora rabbits

Hair follicle (HF) development is characterized by periodic growth cycles regulated by numerous factors. We previously showed that SMAD2 might be involved in the HF growth cycle in Angora rabbits. However, its extra role in the HF growth and development remains obscure. In this study, we cloned the complete coding sequence (CDS) of the Angora rabbit SMAD2 gene. Within SMAD2 CDS, we identified the open reading frame (ORF) had a length of 1314 bp and encoding 437 amino acids. Bioinformatics analyses revealed that the SMAD2 protein is unstable and hydrophilic, and predominatelylocalizesin the cell nucleus. We identified that SMAD2 expression was elevated in the telogen phase of the during HF cycle. The knockdown and overexpression of SMAD2 could regulate HF growth and development related genes, such as WNT2, FGF2, and LEF1.Furthermore, SMAD2 may upregulate TGF-β signaling pathway-related genes, including TFDP1, E2F4, and RBL1. In conclusion, our results indicate that SMAD2 plays a vital role in HF development by regulating the TGF-β signaling pathway.

Identification of key genes and pathways associated with duck (Anas platyrhynchos) embryonic skin development using weighted gene co-expression network analysis

Skin and feather follicle morphogenesis are important processes for duck development; however, the mechanisms underlying morphogenesis at the embryonic stage remain unclear. To improve the understanding of these processes, we used transcriptome and weighted gene co-expression network analyses to identify the critical genes and pathways involved in duck skin development. Five modules were found to be the most related to five key stages in skin development that span from embryonic day 8 (E8) to postnatal day 7 (D7). Using STEM software, 6519 genes from five modules were clustered into 10 profiles to reveal key genes. Above all, we obtained several key module genes including WNT3A, NOTCH1, SHH, BMP2, NOG, SMAD3, and TGFβ2. Furthermore, we revealed that several pathways play critical roles throughout the skin development process, including the Wnt pathway and cytoskeletal rearrangement-related pathways, whereas others are involved in specific stages of skin development, such as the Notch, Hedgehog, and TGF-beta signaling pathways. Overall, this study identified the pathways and genes that play critical roles in skin development, which may provide a basis for high-quality down-type meat duck breeding.

A novel miRNA-4484 is up-regulated on microarray and associated with increased MMP-21 expression in serum of systemic sclerosis patients

Systemic sclerosis (SSc) is a complex, heterogeneous connective tissue disease, characterized by fibrosis and ECM deposition in skin and internal organs, autoimmunity, and changes in the microvasculature. Profiling of circulating miRNAs in serum has been found to be changed in pathological states, creating new possibilities for molecular diagnostics as blood-based biomarkers. This study was designed to identify miRNAs that are differentially expressed in SSc and might be potentially contributing to the disease etiopathogenesis or be used for diagnostic purposes. Thus, we compared the expression pattern of multiple miRNAs in serum of 10 SSc patients to 6 healthy controls using microarray analysis, and RT-qPCR to confirm the obtained results. In addition, bioinformatics analysis was performed to explore miRNAs target genes and the signaling pathways that may be potentially involved in SSc pathogenesis. Our study shows a different expression of 15 miRNAs in SSc patients. We identified that miR-4484, located on chromosome 10q26.2, was an 18-fold up-regulated in SSc patients compared to a control group. Bioinformatics analysis of the miR-4484 target genes and the signaling pathways showed that it might be potentially involved in the TGF-β signaling pathway, ECM-receptor interaction, and metalloproteinases expression. Based on the chromosomal location, the most interesting target gene of miR-4484 may be MMP-21. We found that the expression of MMP-21 significantly increased in SSc patients compared to healthy subjects (P < 0.05). Our results suggest that miR-4484, and MMP-21 might be novel serum biomarkers that may correspond to pathological fibrosis in SSc, but it needs to be validated in further studies.

Single-Cell ID-seq Reveals Dynamic BMP Pathway Activation Upstream of the MAF/MAFB-Program in Epidermal Differentiation

Epidermal homeostasis requires balanced and coordinated adult stem cell renewal and differentiation. These processes are controlled by both extracellular signaling and by cell intrinsic transcription regulatory networks, yet how these control mechanisms are integrated to achieve this is unclear. Here, we developed single-cell Immuno-Detection by sequencing (scID-seq) and simultaneously measured 69 proteins (including 34 phosphorylated epitopes) at single-cell resolution to study the activation state of signaling pathways during human epidermal differentiation. Computational pseudo-timing inference revealed dynamic activation of the JAK-STAT, WNT, and BMP pathways along the epidermal differentiation trajectory. We found that during differentiation, cells start producing BMP2-ligands and activate the canonical intracellular effectors SMAD1/5/9. Mechanistically, the BMP pathway is responsible for activating the MAF/MAFB/ZNF750 transcription factor network to drive late-stage epidermal differentiation. Our work indicates that incorporating signaling pathway activation into this transcription regulatory network enables coordination of transcription programs during epidermal differentiation.

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scID-seq allows quantification of 70 (phospho-)proteins at single-cell level

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Pseudo-time inference reveals signaling dynamics during epidermal differentiation

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BMP signaling drives a late differentiation transcription program

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BMP signaling activates the MAF/MAFB/ZNF750 transcription factor network

An RNAi Screen Reveals an Essential Role for HIPK4 in Human Skin Epithelial Differentiation from iPSCs

Molecular mechanisms responsible for the development of human skin epithelial cells are incompletely understood. As a consequence, the efficiency to establish a pure skin epithelial cell population from human induced pluripotent stem cells (hiPSCs) remains poor. Using an approach including RNAi and high-throughput imaging of early epithelial cells, we identified candidate kinases involved in their differentiation from hiPSCs. Among these, we found HIPK4 to be an important inhibitor of this process. Indeed, its silencing increased the amount of generated skin epithelial precursors at an early time point, increased the amount of generated keratinocytes at a later time point, and improved growth and differentiation of organotypic cultures, allowing for the formation of a denser basal layer and stratification with the expression of several keratins. Our data bring substantial input regarding regulation of human skin epithelial differentiation and for improving differentiation protocols from pluripotent stem cells.

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High-throughput RNAi screen setup during human skin epithelial differentiation

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Identification of HIPK4 as a crucial blocker of human skin epithelial differentiation

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Improvement of human organotypic epithelial cultures after HIPK4 silencing

Topical 5-azacytidine accelerates skin wound healing in rats

The development of new methods to improve skin wound healing may affect the outcomes of a number of medical conditions. Here, we evaluate the molecular and clinical effects of topical 5-azacytidine on wound healing in rats. 5-Azacytidine decreases the expression of follistatin-1, which negatively regulates activins. Activins, in turn, promote cell growth in different tissues, including the skin. Eight-week-old male Wistar rats were submitted to 8.0-mm punch-wounding in the dorsal region. After 3 days, rats were randomly assigned to receive either a control treatment or the topical application of a solution containing 5-azacytidine (10 mM) once per day. Photo documentation and sample collection were performed on days 5, 9, and 15. Overall, 5-azacytidine promoted a significant acceleration of complete wound healing (99.7% ± 0.7.0 vs. 71.2% ± 2.8 on day 15; n = 10; p < 0.01), accompanied by up to threefold reduction in follistatin expression. Histological examination of the skin revealed efficient reepithelization and cell proliferation, as evaluated by the BrdU incorporation method. 5-Azacytidine treatment also resulted in increased gene expression of transforming growth factor-beta and the keratinocyte markers involucrin and cytokeratin, as well as decreased expression of cytokines such as tumor necrosis factor-alpha and interleukin-10. Lastly, when recombinant follistatin was applied to the skin in parallel with topical 5-azacytidine, most of the beneficial effects of the drug were lost. Thus, 5-azacytidine acts, at least in part through the follistatin/activin pathway, to improve skin wound healing in rodents.

Signaling involved in hair follicle morphogenesis and development

Hair follicle morphogenesis depends on Wnt, Shh, Notch, BMP and other signaling pathways interplay between epithelial and mesenchymal cells. The Wnt pathway plays an essential role during hair follicle induction, Shh is involved in morphogenesis and late stage differentiation, Notch signaling determines stem cell fate while BMP is involved in cellular differentiation. The Wnt pathway is considered to be the master regulator during hair follicle morphogenesis. Wnt signaling proceeds through EDA/EDAR/NF-κB signaling. NF-κB regulates the Wnt pathway and acts as a signal mediator by upregulating the expression of Shh ligand. Signal crosstalk between epithelial and mesenchymal cells takes place mainly through primary cilia. Primary cilia formation is initiated with epithelial laminin-511 interaction with dermal β-1 integrin, which also upregulates expression of downstream effectors of Shh pathway in dermal lineage. PDGF signal transduction essential for crosstalk is mediated through epithelial PDGF-A and PDGFRα expressed on the primary cilia. Dermal Shh and PDGF signaling up-regulates dermal noggin expression; noggin is a potent inhibitor of BMP signaling which helps in counteracting BMP mediated β-catenin inhibition. This interplay of signaling between the epithelial and dermal lineage helps in epithelial Shh signal amplification. The dermal Wnt pathway helps in upregulation of epithelial Notch expression. Dysregulation of these pathways leads to certain abnormalities and in some cases even tumor outgrowth.

De Novo Transcriptome Assembly and Differential Gene Expression Profiling of Three Capra hircus Skin Types during Anagen of the Hair Growth Cycle

Despite that goat is one of the best nonmodel systems for villus growth studies and hair biology, limited gene resources associated with skin or hair follicles are available. In the present study, using Illumina/Solexa sequencing technology, we de novo assembled 130 million mRNA-Seq reads into a total of 49,115 contigs. Searching public databases revealed that about 45% of the total contigs can be annotated as known proteins, indicating that some of the assembled contigs may have previously uncharacterized functions. Functional classification by KOG and GO showed that activities associated with metabolism are predominant in goat skin during anagen phase. Many signaling pathways was also created based on the mapping of assembled contigs to the KEGG pathway database, some of which have been previously demonstrated to have diverse roles in hair follicle and hair shaft formation. Furthermore, gene expression profiling of three skin types identified ~6,300 transcript-derived contigs that are differentially expressed. These genes mainly enriched in the functional cluster associated with cell cycle and cell division. The large contig catalogue as well as the genes which were differentially expressed in different skin types provide valuable candidates for further characterization of gene functions.

Differential response of epithelial stem cell populations in hair follicles to TGF-β signaling

Epidermal stem cells residing in different locations in the skin continuously self-renew and differentiate into distinct cell lineages to maintain skin homeostasis during postnatal life. Murine epidermal stem cells located at the bulge region are responsible for replenishing the hair lineage, while the stem cells at the isthmus regenerate interfollicular epidermis and sebaceous glands. In vitro cell culture and in vivo animal studies have implicated TGF-β signaling in the maintenance of epidermal and hair cycle homeostasis. Here, we employed a triple transgenic animal model that utilizes a combined Cre/loxP and rtTA/TRE system to allow inducible and reversible inhibition of TGF-β signaling in hair follicle lineages and suprabasal layer of the epidermis. Using this animal model, we have analyzed the role of TGF-β signaling in distinct phases of the hair cycle. Transient abrogation of TGF-β signaling does not prevent catagen progression; however, it induces aberrant proliferation and differentiation of isthmus stem cells to epidermis and sebocyte lineages and a blockade in anagen re-entry as well as results in an incomplete hair shaft development. Moreover, ablation of TGF-β signaling during anagen leads to increased apoptosis in the secondary hair germ and bulb matrix cells. Blocking of TGF-β signaling in bulge stem cell cultures abolishes their colony-forming ability, suggesting that TGF-β signaling is required for the maintenance of bulge stem cells. At the molecular level, inhibition of TGF-β signaling results in a decrease in both Lrig1-expressing isthmus stem cells and Lrg5-expressing bulge stem cells, which may account for the phenotypes seen in our animal model. These data strongly suggest that TGF-β signaling plays an important role in regulating the proliferation, differentiation, and apoptosis of distinct epithelial stem cell populations in hair follicles.

Diverse epigenetic strategies interact to control epidermal differentiation

It is becoming clear that interconnected functional gene networks, rather than individual genes, govern stem cell self-renewal and differentiation. To identify epigenetic factors that impact on human epidermal stem cells we performed siRNA-based genetic screens for 332 chromatin modifiers. We developed a Bayesian mixture model to predict putative functional interactions between epigenetic modifiers that regulate differentiation. We discovered a network of genetic interactions involving EZH2, UHRF1 (both known to regulate epidermal self-renewal), ING5 (a MORF complex component), BPTF and SMARCA5 (NURF complex components). Genome-wide localization and global mRNA expression analysis revealed that these factors impact two distinct but functionally related gene sets, including integrin extracellular matrix receptors that mediate anchorage of epidermal stem cells to their niche. Using a competitive epidermal reconstitution assay we confirmed that ING5, BPTF, SMARCA5, EZH2 and UHRF1 control differentiation under physiological conditions. Thus, regulation of distinct gene expression programs through the interplay between diverse epigenetic strategies protects epidermal stem cells from differentiation.

A pair of transmembrane receptors essential for the retention and pigmentation of hair

Hair follicles are simple, accessible models for many developmental processes. Here, using mutant mice, we show that Bmpr2, a known receptor for bone morphogenetic proteins (Bmps), and Acvr2a, a known receptor for Bmps and activins, are individually redundant but together essential for multiple follicular traits. When Bmpr2/Acvr2a function is reduced in cutaneous epithelium, hair follicles undergo rapid cycles of hair generation and loss. Alopecia results from a failure to terminate hair development properly, as hair clubs never form, and follicular retraction is slowed. Hair regeneration is rapid due to premature activation of new hair-production programs. Hair shafts differentiate aberrantly due to impaired arrest of medullary-cell proliferation. When Bmpr2/Acvr2a function is reduced in melanocytes, gray hair develops, as melanosomes differentiate but fail to grow, resulting in organelle miniaturization. We conclude that Bmpr2 and Acvr2a normally play cell-type-specific, necessary roles in organelle biogenesis and the shutdown of developmental programs and cell division.

Integrative multicellular biological modeling: a case study of 3D epidermal development using GPU algorithms

BACKGROUND

Simulation of sophisticated biological models requires considerable computational power. These models typically integrate together numerous biological phenomena such as spatially-explicit heterogeneous cells, cell-cell interactions, cell-environment interactions and intracellular gene networks. The recent advent of programming for graphical processing units (GPU) opens up the possibility of developing more integrative, detailed and predictive biological models while at the same time decreasing the computational cost to simulate those models.

RESULTS

We construct a 3D model of epidermal development and provide a set of GPU algorithms that executes significantly faster than sequential central processing unit (CPU) code. We provide a parallel implementation of the subcellular element method for individual cells residing in a lattice-free spatial environment. Each cell in our epidermal model includes an internal gene network, which integrates cellular interaction of Notch signaling together with environmental interaction of basement membrane adhesion, to specify cellular state and behaviors such as growth and division. We take a pedagogical approach to describing how modeling methods are efficiently implemented on the GPU including memory layout of data structures and functional decomposition. We discuss various programmatic issues and provide a set of design guidelines for GPU programming that are instructive to avoid common pitfalls as well as to extract performance from the GPU architecture.

CONCLUSIONS

We demonstrate that GPU algorithms represent a significant technological advance for the simulation of complex biological models. We further demonstrate with our epidermal model that the integration of multiple complex modeling methods for heterogeneous multicellular biological processes is both feasible and computationally tractable using this new technology. We hope that the provided algorithms and source code will be a starting point for modelers to develop their own GPU implementations, and encourage others to implement their modeling methods on the GPU and to make that code available to the wider community.

Recent insights into the role of dipeptidyl aminopeptidase IV (DPIV) and aminopeptidase N (APN) families in immune functions

BACKGROUND

In the past, different research groups could show that treatment of immune cells with inhibitors of post-proline splitting dipeptidyl aminopeptidases leads to functional changes in the immune system consistent with immunosuppression. This is due to the inhibition of proliferation of lymphocytes and the production of inflammatory cytokines of the TH1, TH2, and TH17, cells as well as the induction of immunosuppressive cytokines, such as transforming growth factor-beta1 (TGF-beta1) and interleukin (IL)-1RA. Until recently, most of the effects of these inhibitors on immune functions were attributed to the inhibition of dipeptidyl aminopeptidase IV (DPIV/CD26). With the identification of new peptidases of the DPIV family (DASH) with the same or similar substrate specificity [fibroblast activation protein (FAP), DP8/9], the question arose whether and to what extent the inhibition of intracellularly localized enzymes, DP8 and DP9, contribute to the observed immunosuppression. In addition, members of the aminopeptidase N (APN) family are also involved in the regulation of immune functions. Hence, the concept of a combined targeting of both families of peptidases for treatment of inflammatory diseases is a promising strategy.

RESULTS/CONCLUSIONS

Summarizing data obtained from the usage of different non-selective and selective inhibitors of DPIV, DP8/9, FAP, and DPII, this review provides evidence that in addition to DPIV, DP8/9 also regulate the immune response via modulation of cell cycle progression and cytokine production. The strongest and most consistent effects in vitro were, however, observed with non-selective inhibitors for the suppression of DNA synthesis and cytokine production. Similar effects were provoked by APN inhibitors, which were also found to suppress DNA synthesis and the production of inflammatory cytokines in vitro. However, different mechanisms and signaling pathways appear to mediate the cellular effects resulting from the inhibition of either APN or DPIV family members. In particular, members of the APN family uniquely influence the function of CD4+CD25+ regulatory T-cells. Consequently, the concomitant inhibition of both APN and DPIV enzyme families by means of two separate inhibitors or by binary inhibitors with specificity for both enzyme families (PETIR, peptidase targeted immunoregulation) synergistically affects immune cells on the level of cell cycle regulation, suppression of TH1, TH2, and TH17 cytokines as well as the activation of regulatory T-cells. Besides leukocytes, dermal cells as sebocytes, keratinocytes, and fibroblasts are also targeted by these inhibitors. This strongly suggests a broad potential of the multiple anti-inflammatory effects of PETIR in treatment of chronic inflammatory diseases, such as autoimmune diseases, allergies, and transplant rejections, as well as of inflammatory skin diseases, such as acne, psoriasis, rosacea or atopic dermatitis. The first active dual inhibitor, IP10.C8, has been developed by IMTM for the treatment of inflammatory skin diseases and has just entered the first phase II study.

Smad4-dependent desmoglein-4 expression contributes to hair follicle integrity

We have previously shown that keratinocyte-specific deletion of Smad4, a TGFbeta/Activin/BMP signaling mediator, results in a progressive alopecia. To further assess the molecular mechanisms of Smad4 loss-mediated alopecia, we examined expression levels of key molecules associated with hair follicle differentiation in Smad4-deleted skin. Among them, Desmoglein 4 (Dsg4) was down-regulated in Smad4-deleted skin prior to the onset of hair follicle abnormalities with gradual depletion coinciding with hair follicle degeneration. Chromatin immunoprecipitation (ChIP) assay showed that Smad4, together with the BMP mediators Smad1 and Smad5, but not the TGFbeta/Activin mediators Smad2 or Smad3, bound to the Smad Binding Element (SBE) of the Dsg4 promoter. A Dsg4 reporter assay revealed that Smad4 was required for the maximal transactivation of Dsg4 in cooperation with Smad1 and Smad5. Mutating the SBE of the Dsg4 promoter abrogated Smad4 transactivation of Dsg4. Furthermore, BMP ligands, but not ligands of TGFbeta and Activin, induced endogenous Dsg4 expression. Our data demonstrate that in the presence of Smad4, BMP signaling participated in transcriptional regulation of Dsg4. Thus, Smad4 loss-associated Dsg4 depletion contributed, at least in part, to hair follicles degeneration in Smad4 deficient skin.

IKKalpha, a critical regulator of epidermal differentiation and a suppressor of skin cancer

IκB kinase α (IKKα), one of the two catalytic subunits of the IKK complex involved in nuclear factor κB (NF-κB) activation, also functions as a molecular switch that controls epidermal differentiation. This unexpected function requires IKKα nuclear translocation but does not depend on its kinase activity, and is independent of NF-κB signalling. Ikkα^–/– mice present with a hyperproliferative and undifferentiated epidermis characterized by complete absence of a granular layer and stratum corneum. Ikkα-deficient keratinocytes do not express terminal differentiation markers and continue to proliferate even when subjected to differentiation-inducing stimuli. This antiproliferative function of IKKα is also important for the suppression of squamous cell carcinogenesis. The exact mechanisms by which nuclear IKKα controls keratinocyte proliferation and differentiation remained mysterious for some time. Recent studies, however, have revealed that IKKα is a major cofactor in a TGFβ–Smad2/3 signalling pathway that is Smad4 independent. This pathway controls cell cycle withdrawal during keratinocyte terminal differentiation. Although these are not the only functions of nuclear IKKα, this multifunctional protein is a key regulator of keratinocyte and epidermal differentiation and a critical suppressor of skin cancer.

A potential role for bone morphogenetic protein signalling in glial cell fate determination following adult central nervous system injury in vivo

Bone morphogenetic proteins (BMPs) and their endogenous inhibitors, including noggin, chordin and follistatin, have roles in pattern formation and fate specification of neuronal and glial cells during nervous system development. We have examined their influence on glial reactions in the injured central nervous system (CNS). We show that penetrating injuries to the brain and spinal cord resulted in the upregulation of BMP-2/4, BMP-7, and noggin, with the latter being expressed almost exclusively by reactive astrocytes at the injury site, and we show that astrocytes in vitro produce noggin. As BMPs have been shown to drive cultured NG2-positive oligodendrocyte precursors (OPCs) towards a multipotential phenotype (type II astrocytes), we investigated the effects of inhibiting noggin with a function-blocking antibody (noggin-FbAb). In vitro, BMP-driven conversion of OPCs to type 2 astrocytes was inhibited by noggin, an effect that was reversed by noggin-FbAb. Noggin-FbAb also increased the number of type 2 astrocytes generated from cultured OPCs exposed to an astrocyte feeder layer, consistent with astrocytes producing both BMPs and noggin. In knife cut injuries in vivo, noggin-FbAb treatment resulted in an increase in the number of NG2-positive cells and small GFAP-positive cells in the injury site, and the appearance of glial cells with the morphological and antigenic characteristics of type 2 astrocytes (as generated in vitro), with coexpression of both GFAP and NG2. This potential conversion of inhibitory OPCs to type 2 astrocyte-like cells in vivo suggests that endogenous BMPs, unmasked by noggin antagonism, might be exploited to manipulate cell fate following CNS trauma.

Major advances associated with hormone and growth factor regulation of mammary growth and lactation in dairy cows

In recent years, the number of researchers interested in mammary development and mammary function in dairy animals has declined. More importantly this cadre of workers has come to rely more than ever on scientists focused on and funded by breast cancer interests to provide fundamental mechanistic and basic cellular insights. Philosophically and practically this is a risky path to better understand, manipulate, and control a national resource as important as the dairy cow. The efficiency, resourcefulness, and dedication of dairy scientists have mirrored the actions of many dairy producers but there are limits. Many of the applications of research, use of bovine somatotropin, management of transition cows, estrus synchronization techniques, and so on, are based on decades-old scientific principles. Specific to dairy, do rodents or breast cancer cell lines adequately represent the dairy cow? Will these results inspire the next series of lactation-related dairy improvements? These are key unanswered questions. Study of the classic mammogenic and lactogenic hormones has served dairy scientists well. But there is an exciting, and bewildering universe of growth factors, transcription factors, receptors, intracellular signaling intermediates, and extracellular molecules that must ultimately interact to determine the size of the mature udder and the functional capacity of mammary gland in the lactating cow. We can only hope that enough scientific, fiscal, and resource scraps fall from the biomedical research banquet table to allow dairy-focused mammary gland research to continue.

Overlapping and distinct transcriptional regulator properties of the GLI1 and GLI2 oncogenes

The GLI transcription factors mediate the hedgehog signal in development and carcinogenesis. Basal cell carcinoma can be caused by overexpression of either GLI1 or GLI2. Though GLI1 and GLI2 have identical or very similar DNA binding specificities, some of their activities are overlapping, some are clearly distinct. We analyzed target gene specificities of GLI1 and constitutively active GLI2 (GLI2DeltaN) by global expression profiling in an inducible, well-characterized HaCaT keratinocyte expression system. Four hundred fifty-six genes up- or downregulated at least twofold were identified. GLI target gene profiles correlated well with the biological activities of these transcription factors in hair follicles and basal cell carcinoma. Upregulation of largely overlapping sets of target genes was effected by both factors, repression occurred predominantly in response to GLI2. Also, significant quantitative differences in response to GLI1 and GLI2DeltaN were found for a small number of activated genes. Since we have not detected a putative processed GLI2 repressor, these results point to specific but indirect target gene repression by GLI2DeltaN via preferential activation of one or more negative regulators.

Proliferation and cornification during development of the mammalian epidermis

The skin is the largest organ of the body and consists of the underlying dermis and outer epidermis. Proper embryonic development and continual renewal of the adult epidermis are essential to provide an impenetrable barrier against fluid loss and serve as our most important defense against insults from the external environment. During mammalian embryogenesis the epidermis develops from the surface ectoderm, which initially consists of a multipotent single-layer epithelium. Once these epithelial cells receive the appropriate developmental cues, they become committed to stratify, initiate a massive expansion program, and finally embark on a journey of terminal differentiation to produce the morphologically distinct layers of the epidermis. The culmination of this journey is the formation of an impervious cornified envelope via a highly specialized form of programmed cell death, termed "cornification" (reviewed in Candi et al.), which is distinct in many ways from the classic apoptotic pathways. The epidermal developmental program that is first seen in the fetus is recapitulated for the entire life of the organism. The basal layer of adult skin harbors stem cells, which can divide to produce daughter stem cells and transit amplifying (TA) cells that go on to differentiate and cornify (reviewed in Fuchs and Raghavan). In this review we summarize current knowledge about the molecular regulation of proliferation and cornification in the developing mammalian epidermis. We focus on events in the interfollicular epidermis, with special emphasis on transcriptional regulation by p63, Notch, NF-kappaB/IKK, Hox, AP-1, AP-2, and C/EBP factors. We end with a discussion about perturbations in epidermal proliferation and cornification as they pertain to human skin pathologies.

Hair follicle defects and squamous cell carcinoma formation in Smad4 conditional knockout mouse skin

Smad4 is the common mediator for TGFbeta signals, which play important functions in many biological processes. To study the role of Smad4 in skin development and epidermal tumorigenesis, we disrupted this gene in skin using the Cre-loxP approach. We showed that absence of Smad4 blocked hair follicle differentiation and cycling, leading to a progressive hair loss of mutant (MT) mice. MT hair follicles exhibited diminished expression of Lef1, and increased proliferative cells in the outer root sheath. Additionally, the skin of MT mice exhibited increased proliferation of basal keratinocytes and epidermal hyperplasia. Furthermore, we provide evidence that the absence of Smad4 resulted in a block of both TGFbeta and bone morphogenetic protein (BMP) signaling pathways, including p21, a well-known cyclin-dependent kinase inhibitor. Consequently, all MT mice developed spontaneous malignant skin tumors from 3 months to 13 months of age. The majority of tumors are malignant squamous cell carcinomas. A most notable finding is that tumorigenesis is accompanied by inactivation of phosphatase and tensin homolog deleted on chromosome 10 (Pten), activation of AKT, fast proliferation and nuclear accumulation of cyclin D1. These observations revealed the essential functions of Smad4-mediated signals in repressing skin tumor formation through the TGFbeta/BMP pathway, which interacts with the Pten signaling pathway.

Role of TGFβ in skin inflammation and carcinogenesis

The functions of transforming growth factor beta-1(TGFbeta1) are cell-context specific. We have found that TGFbeta1 expression in human skin squamous cell carcinoma (SCC) samples has two distinct distribution patterns: (1) either predominantly in suprabasal layers or (2) throughout tumor epithelia including basal proliferative cells. To understand whether the spatial TGFbeta1 expression patterns affect its functions, we have generated several keratinocyte-specific transgenic mouse models in which TGFbeta1 overexpression can be induced either predominantly in the suprabasal epidermis or in the basal layer of the epidermis and hair follicles. Suprabasal TGFbeta1 overexpression inhibits keratinocyte proliferation, suppresses skin carcinogenesis at early stages, but promotes tumor invasion at later stages. In contrast, TGFbeta1 overexpression in the basal layer of the epidermis and hair follicles causes a severe inflammatory skin disorder and epidermal hyperproliferation. Given the importance of inflammation in cancer development, our data suggest that TGFbeta1-induced skin inflammation may override its tumor suppressive effect at early stages during skin carcinogenesis. This hypothesis is further suggested by our recent study that Smad3 knockout mice are resistant to skin chemical carcinogenesis at least in part via abrogation of endogenous TGFbeta1-induced inflammation. This review intends to summarize current insights into the role of TGFbeta1 in skin inflammation and carcinogenesis.

Current view of the role of transforming growth factor beta 1 in skin carcinogenesis

Previously, we have shown that transforming growth factor beta 1 (TGFbeta1) overexpression in suprabasal epidermis suppresses skin carcinogenesis at early stages, but promotes tumor invasion at later stages. To elucidate the role of TGFbeta1 overexpression in naturally occurring human squamous cell carcinomas (SCC), we screened TGFbeta1 expression patterns in human skin SCC samples and found that TGFbeta1 was overexpressed with two distinct patterns: either predominantly in suprabasal layers or throughout tumor epithelia including basal proliferative cells. To determine the effect of TGFbeta1 overexpression in basal keratinocytes, we generated transgenic mice expressing wild-type TGFbeta1 in basal keratinocytes and hair follicles using the K5 promoter (K5.TGFbeta1(wt)). Surprisingly, these mice developed a severe inflammatory skin disorder. Inflammation was also observed in head and neck tissue when TGFbeta1 transgene expression was inducibly expressed in head and neck epithelia in our gene-switch-TGFbeta1 transgenic mice. Given the importance of inflammation in cancer development, our data suggest that TGFbeta1-induced inflammation may override its tumor-suppressive effect even at early stages of skin carcinogenesis. This notion is further suggested by our recent study that Smad3 knockout mice were resistant to skin chemical carcinogenesis at least in part via abrogation of endogenous TGFbeta1-induced inflammation.

RNF11 is a multifunctional modulator of growth factor receptor signalling and transcriptional regulation

Our laboratory has found that the 154aa RING finger protein 11 (RNF11), has modular domains and motifs including a RING-H2 finger domain, a PY motif, an ubiquitin interacting motif (UIM), a 14-3-3 binding sequence and an AKT phosphorylation site. RNF11 represents a unique protein with no other known immediate family members yet described. Comparative genetic analysis has shown that RNF11 is highly conserved throughout evolution. This may indicate a conserved and non-redundant role for the RNF11 protein. Molecular binding assays using RNF11 have shown that RNF11 has important roles in growth factor signalling, ubiquitination and transcriptional regulation. RNF11 has been shown to interact with HECT-type E3 ubiquitin ligases Nedd4, AIP4, Smurf1 and Smurf2, as well as with Cullin1, the core protein in the multi-subunit SCF E3 ubiquitin ligase complex. Work done in our laboratory has shown that RNF11 is capable of antagonizing Smurf2-mediated inhibition of TGFbeta signalling. Furthermore, RNF11 is capable of degrading AMSH, a positive regulator of both TGFbeta and EGFR signalling pathways. Recently, we have found that RNF11 can directly enhance TGFbeta signalling through a direct association with Smad4, the common signal transducer and transcription factor in the TGFbeta, BMP, and Activin pathways. Through its association with Smad4 and other transcription factors, RNF11 may have a role in direct transcriptional regulation. Our laboratory and others have found nearly 80 protein interactions for RNF11, placing RNF11 at the cross-roads of cell signalling and transcriptional regulation. RNF11 is highly expressed in breast tumours. Deregulation of RNF11 function may prove to be harmful to patient therapeutic outcomes. RNF11 may therefore provide a novel target for cancer therapeutics. The purpose of this review is to discuss the role of RNF11 in cell signalling and transcription factor modulation with special attention given to the ubiquitin-proteasomal pathway, TGFbeta pathway and EGFR pathway.

BMP signaling in the control of skin development and hair follicle growth

Bone morphogenetic proteins (BMPs), their antagonists, and BMP receptors are involved in controlling a large number of biological functions including cell proliferation, differentiation, cell fate decision, and apoptosis in many different types of cells and tissues during embryonic development and postnatal life. BMPs exert their biological effects via using BMP-Smad and BMP-MAPK intracellular pathways. The magnitude and specificity of BMP signaling are regulated by a large number of modulators operating on several levels (extracellular, cytoplasmic, nuclear). In developing and postnatal skin, BMPs, their receptors, and BMP antagonists show stringent spatio-temporal expressions patterns to achieve proper regulation of cell proliferation and differentiation in the epidermis and in the hair follicle. Genetic studies assert an essential role for BMP signaling in the control of cell differentiation and apoptosis in developing epidermis, as well as in the regulation of key steps of hair follicle development (initiation, cell fate decision, cell lineage differentiation). In postnatal hair follicles, BMP signaling plays an important role in controlling the initiation of the growth phase and is also involved in the regulation of apoptosis-driven hair follicle involution. However, additional efforts are required to fully understand the mechanisms and targets involved in the realization of BMP effects on distinct cell population in the skin and hair follicle. Progress in this area of research will hopefully lead to the development of new therapeutic approaches for using BMPs and BMP antagonists in the treatment of skin and hair growth disorders.

Essential roles of BMPR-IA signaling in differentiation and growth of hair follicles and in skin tumorigenesis

Hair differentiation and growth are controlled by complex reciprocal signaling between epithelial and mesenchymal cells. To better understand the requirement and molecular mechanism of BMP signaling in hair follicle development, we performed genetic analyses of bone morphogenetic protein receptor 1A (BMPR-IA) function during hair follicle development by using a conditional knockout approach. The conditional mutation of Bmpr1a in ventral limb ectoderm and its derivatives (epidermis and hair follicles) resulted in a lack of hair outgrowth from the affected skin regions. Mutant hair follicles exhibited abnormal morphology and lacked hair formation and pigment deposition during anagen. The timing of the hair cycle and the proliferation of hair matrix cells were also affected in the mutant follicles. We demonstrate that signaling via epithelial BMPR-IA is required for differentiation of both hair shaft and inner root sheath from hair matrix precursor cells in anagen hair follicles but is dispensable for embryonic hair follicle induction. Surprisingly, aberrant de novo hair follicle morphogenesis together with hair matrix cell hyperplasia was observed in the absence of BMPR-IA signaling within the affected skin of adult mutants. They developed hair follicle tumors from 3 months of age, indicating that inactivation of epidermal BMPR-IA signaling can lead to hair tumor formation. Taken together, our data provide genetic evidence that BMPR-IA signaling plays critical and multiple roles in controlling cell fate decisions or maintenance, proliferation, and differentiation during hair morphogenesis and growth, and implicate Bmpr1a as a tumor suppressor in skin tumorigenesis.

Matrix metalloproteinase-21 is expressed epithelially during development and in cancer and is up-regulated by transforming growth factor-beta1 in keratinocytes

Human matrix metalloproteinase-21 (MMP-21), the newest member of the MMP gene family, has been suggested to play an important role in embryogenesis and tumor progression and to be a target of the Wnt, Pax, and Notch signaling pathways. Here we report detection of MMP-21 by RT-PCR in mouse embryos aged 10.5, 12.5, 13.5, and 16.5 days, as well as in various adult murine organs. In both humans and mice, MMP-21 protein was detected in the epithelial cells of developing kidney, intestine, neuroectoderm, and skin but not in normal adult skin using immunohistochemistry with two unrelated antibodies. However, it was present in invasive cancer cells of aggressive subtypes of basal and squamous cell carcinomas, although it was not expressed in skin disorders characterized by mere keratinocyte hyperproliferation. Of several cytokines tested, transforming growth factor-beta1 induced MMP-21 in vitro in HaCaTs and keratinocytes as judged by real-time quantitative TaqMan PCR. Although suprabasal differentiating keratinocytes expressed MMP-21 in developing skin in vivo, MMP-21-positive keratinocytes were detected by immunohistochemistry in both low and high calcium cultures. MMP-21 expression was not up-regulated by ras transformation in HaCaT cell lines (HaCaT, A5, II-4, and RT3); in skin and colon cancers, its expression did not associate with apoptosis, beta-catenin transactivation, or epithelial MMPs-9 and -10. However, MMP-21 protein was found in the same regions as MMP-7 but not in the same cells. Our results suggest that during development, MMP-21 expression is temporally and spatially tightly controlled. Unlike many classical MMPs, it is present in various normal adult tissues. Among epithelial MMPs, MMP-21 has a unique expression pattern in cancer.

Photopheresis: clinical applications and mechanism of action

Photopheresis is a leukapheresis-based therapy that utilizes 8-methoxypsoralen and ultraviolet A irradiation. Photopheresis is currently available at approximately 150 medical centers worldwide. Recent evidence suggests that this therapy used as a single agent may significantly prolong life, as well as induce a 50%-75% response rate among individuals with advanced cutaneous T cell lymphoma (CTCL). Furthermore, a 20%-25% complete response rate with photopheresis alone, or in combination with other biologic response modifiers, has been obtained at our institution among patients with Sezary syndrome. These complete responses have been characterized by the complete disappearance of morphologically atypical cells from the skin and blood. The use of sensitive molecular techniques has also confirmed the sustained disappearance of the malignant T cell clone from the blood of patients with complete responses. In addition to the treatment of CTCL, numerous reports indicate that photopheresis is a potent agent in the therapy of acute allograft rejection among cardiac, lung, and renal transplant recipients. Chronic graft versus host disease also appears to be quite responsive to photopheresis therapy. Likewise, there may also be a potential role for photopheresis in the therapy of certain autoimmune diseases that are poorly responsive to conventional therapy. The immunologic basis for the responses of patients with these conditions is likely due to the induction of anticlonotypic immunity directed against pathogenic clones of T lymphocytes. Treatment-induced apoptotic death of pathogenic T cells and activation of antigen presenting cells are postulated to have important effects in this therapeutic process.