Activin A induces terminal differentiation of cultured human keratinocytes

Therapeutic modulation of KIT ligand in melanocytic disorders with implications for mast cell diseases

KIT ligand and its associated receptor KIT serve as a master regulatory system for both melanocytes and mast cells controlling survival, migration, proliferation and activation. Blockade of this pathway results in cell depletion, while overactivation leads to mastocytosis or melanoma. Expression defects are associated with pigmentary and mast cell disorders. KIT ligand regulation is complex but efficient targeting of this system would be of significant benefit to those suffering from melanocytic or mast cell disorders. Herein, we review the known associations of this pathway with cutaneous diseases and the regulators of this system both in skin and in the more well-studied germ cell system. Exogenous agents modulating this pathway will also be presented. Ultimately, we will review potential therapeutic opportunities to help our patients with melanocytic and mast cell disease processes potentially including vitiligo, hair greying, melasma, urticaria, mastocytosis and melanoma.

Development of novel microenvironments for promoting enhanced wound healing

PURPOSE OF REVIEW

Nonhealing wounds are a significant issue facing the healthcare industry. Materials that modulate the wound microenvironment have the potential to improve healing outcomes.

RECENT FINDINGS

A variety of acellular and cellular scaffolds have been developed for regulating the wound microenvironment, including materials for controlled release of antimicrobials and growth factors, materials with inherent immunomodulative properties, and novel colloidal-based scaffolds. Scaffold construction methods include electrospinning, 3D printing, decellularization of extracellular matrix, or a combination of techniques. Material application methods include layering or injecting at the wound site.

SUMMARY

Though these techniques show promise for repairing wounds, all material strategies thus far struggle to induce regeneration of features such as sweat glands and hair follicles. Nonetheless, innovative technologies currently in the research phase may facilitate future attainment of these features. Novel methods and materials are constantly arising for the development of microenvironments for enhanced wound healing.

Activin-mediated alterations of the fibroblast transcriptome and matrisome control the biomechanical properties of skin wounds

Matrix deposition is essential for wound repair, but when excessive, leads to hypertrophic scars and fibrosis. The factors that control matrix deposition in skin wounds have only partially been identified and the consequences of matrix alterations for the mechanical properties of wounds are largely unknown. Here, we report how a single diffusible factor, activin A, affects the healing process across scales. Bioinformatics analysis of wound fibroblast transcriptome data combined with biochemical and histopathological analyses of wounds and functional in vitro studies identify that activin promotes pro-fibrotic gene expression signatures and processes, including glycoprotein and proteoglycan biosynthesis, collagen deposition, and altered collagen cross-linking. As a consequence, activin strongly reduces the wound and scar deformability, as identified by a non-invasive in vivo method for biomechanical analysis. These results provide mechanistic insight into the roles of activin in wound repair and fibrosis and identify the functional consequences of alterations in the wound matrisome at the biomechanical level.

The relationship between histopathology, gene expression, and biochemical and mechanical properties of wounds is largely unknown. Here, the authors show that activin A alters wound healing at multiple levels by promoting pro-fibrotic gene expression and matrix deposition, thereby affecting biomechanical properties of skin wounds.

Effects of Silver Nanoparticles on Burn Wound Healing in a Mouse Model

Healing of injuries caused by exposure to heat has been discussed in many studies, although a few drugs have been shown to produce satisfactory results. In this study, 100 healthy mice randomly allocated into four categories (each = 25 mice) were analyzed. A deep second-degree burn on the back of each mouse was created. The burns were dressed daily with either AgNPs or silver sulfadiazine over 28 days of treatment. Safety evaluation of the AgNP treatment was performed by measuring the deposition rate of silver in the liver, brain, and kidney of treated mice. In the murine burn model, the speed of wound healing and the antibacterial effect of AgNPs were better than those in the silver sulfadiazine group. Burn wounds treated with SSD appeared to display a greater degree of inflammation as notable by the three clinical signs of the inflammatory process such as redness and swelling which appeared to be less after wounds treated with AgNPs. Also, AgNP treatment modified leukocytic infiltration and reduced collagen degeneration in treated mice and enhanced healing processes that were confirmed by morphological and histological investigations. Beside the potential significant effects of AgNPs on reduction of some microorganism counts that routinely isolated from burn wounds included aerobic organisms as Staphylococcus aureus and Escherichia coli when compared to both SSD and control groups. The deposition kinetics of AgNPs revealed lower distribution in the liver, brain, and kidney than that in silver sulfadiazine-treated mice with respect to both SSD and control groups.

Combination of Root Surface Modification with BMP-2 and Collagen Hydrogel Scaffold Implantation for Periodontal Healing in Beagle Dogs

Objective : Biomodification of the root surface plays a major role in periodontal wound healing. Root surface modification with bone morphogenetic protein (BMP) stimulates bone and cementum-like tissue formation; however, severe ankylosis is simultaneously observed. Bio-safe collagen hydrogel scaffolds may therefore be useful for supplying periodontal ligament cells and preventing ankylosis. We examined the effects of BMP modification in conjunction with collagen hydrogel scaffold implantation on periodontal wound healing in dogs. Material and Methods: The collagen hydrogel scaffold was composed of type I collagen sponge and collagen hydrogel. One-wall infrabony defects (5 mm in depth, 3 mm in width) were surgically created in six beagle dogs. In the BMP/Col group, BMP-2 was applied to the root surface (loading dose; 1 µg/µl), and the defects were filled with collagen hydrogel scaffold. In the BMP or Col group, BMP-2 coating or scaffold implantation was performed. Histometric parameters were evaluated at 4 weeks after surgery. Results: Single use of BMP stimulated formation of alveolar bone and ankylosis. In contrast, the BMP/Col group frequently enhanced reconstruction of periodontal attachment including cementum-like tissue, periodontal ligament and alveolar bone. The amount of new periodontal ligament in the BMP/Col group was significantly greater when compared to all other groups. In addition, ankylosis was rarely observed in the BMP/Col group. Conclusion: The combination method using root surface modification with BMP and collagen hydrogel scaffold implantation facilitated the reestablishment of periodontal attachment. BMP-related ankylosis was suppressed by implantation of collagen hydrogel.

Stimulatory effect of fibroblast-derived prostaglandin E₂ on keratinocyte stratification in the skin equivalent

Epidermal-dermal interaction plays important roles in physiological events such as wound healing. In this study, we examined a double paracrine mechanism between keratinocytes and fibroblasts through interleukin-1 (IL-1) and an IL-1-induced inflammatory mediator prostaglandin E₂ (PGE₂) using the skin equivalent. The epidermal layer of the skin equivalent expressed high levels of IL-1α mRNA (IL1A mRNA) and relatively low levels of IL-1β mRNA (IL1B mRNA). IL1A mRNA was not detected in fibroblasts. Fibroblasts also expressed low but not negligible levels of IL1B mRNA only in the presence of keratinocytes. Expression of prostaglandin-endoperoxide synthase 2 mRNA (PTGS2 mRNA) and production of PGE₂ in three-dimensionally cultured fibroblasts were noticeably stimulated by co-culture with keratinocytes, whereas PTGS2 mRNA expression in the epidermal layer was very low. In addition, hydroxyprostaglandin dehydrogenase 15-(NAD) mRNA was highly expressed in keratinocytes but not in fibroblasts, and exogenous IL-1β stimulated PTGS2 mRNA expression in the dermal equivalent. The thickness of the epidermal layer and the number of MKI67-positive keratinocytes in the skin equivalent were decreased by treatment with indomethacin, and the decrease recovered when exogenous PGE₂ was added. These results indicate that keratinocytes stimulate their own proliferation through a double paracrine mechanism mediated by IL-1 and PGE₂.

Regulation of FSHβ induction in LβT2 cells by BMP2 and an Activin A/BMP2 chimera, AB215

Activins and bone morphogenetic proteins (BMPs) share activin type 2 signaling receptors but utilize different type 1 receptors and Smads. We designed AB215, a potent BMP2-like Activin A/BMP2 chimera incorporating the high-affinity type 2 receptor-binding epitope of Activin A. In this study, we compare the signaling properties of AB215 and BMP2 in HEK293T cells and gonadotroph LβT2 cells in which Activin A and BMP2 synergistically induce FSHβ. In HEK293T cells, AB215 is more potent than BMP2 and competitively blocks Activin A signaling, while BMP2 has a partial blocking activity. Activin A signaling is insensitive to BMP pathway antagonism in HEK293T cells but is strongly inhibited by constitutively active (CA) BMP type 1 receptors. By contrast, the potencies of AB215 and BMP2 are indistinguishable in LβT2 cells and although AB215 blocks Activin A signaling, BMP2 has no inhibitory effect. Unlike HEK293T, Activin A signaling is strongly inhibited by BMP pathway antagonism in LβT2 cells but is largely unaffected by CA BMP type 1 receptors. BMP2 increases phospho-Smad3 levels in LβT2 cells, in both the absence and the presence of Activin A treatment, and augments Activin A-induced FSHβ. AB215 has the opposite effect and sharply decreases basal phospho-Smad3 levels and blocks Smad2 phosphorylation and FSHβ induction resulting from Activin A treatment. These findings together demonstrate that while AB215 activates the BMP pathway, it has opposing effects to those of BMP2 on FSHβ induction in LβT2 cells apparently due to its ability to block Activin A signaling.

The bright and the dark sides of activin in wound healing and cancer

Activin was initially described as a protein that stimulates release of follicle stimulating hormone from the pituitary, and it is well known for its important roles in different reproductive functions. In recent years, this multifunctional factor has attracted the attention of researchers in other fields, as new functions of activin in angiogenesis, inflammation, immunity, fibrosis and cancer have been discovered. Studies from our laboratory have identified activin as a crucial regulator of wound healing and skin carcinogenesis. On the one hand, it strongly accelerates the healing process of skin wounds but, on the other hand, it enhances scar formation and the susceptibility to skin tumorigenesis. Finally, results from several laboratories have revealed that activin enhances tumour formation and/or progression in some other organs, in particular through its effect on the tumour microenvironment, and that it also promotes cancer-induced bone disruption and muscle wasting. These findings provide the basis for the use of activin or its downstream targets for the improvement of impaired wound healing, and of activin antagonists for the prevention and treatment of fibrosis and of malignant tumours that overexpress activin. Here, we summarize the previously described roles of activin in wound healing and scar formation and discuss functional studies that revealed different functions of activin in the pathogenesis of cancer. The relevance of these findings for clinical applications will be highlighted.

Keratinocyte-derived follistatin regulates epidermal homeostasis and wound repair

Activin is a growth and differentiation factor that controls development and repair of several tissues and organs. Transgenic mice overexpressing activin in the skin were characterized by strongly enhanced wound healing, but also by excessive scarring. In this study, we explored the consequences of targeted activation of activin in the epidermis and hair follicles by generation of mice lacking the activin antagonist follistatin in keratinocytes. We observed enhanced keratinocyte proliferation in the tail epidermis of these animals. After skin injury, an earlier onset of keratinocyte hyperproliferation at the wound edge was observed in the mutant mice, resulting in an enlarged hyperproliferative epithelium. However, granulation tissue formation and scarring were not affected. These results demonstrate that selective activation of activin in the epidermis enhances reepithelialization without affecting the quality of the healed wound.

GLI2-specific transcriptional activation of the bone morphogenetic protein/activin antagonist follistatin in human epidermal cells

Hedgehog (HH) signaling in the epidermis is primarily mediated by the zinc finger transcription factors GLI1 and GLI2. Exquisite regulation of HH/GLI signaling is crucial for proper specification of the epidermal lineage and development of its derivatives, whereas dysregulation of HH/GLI signaling disrupts tissue homeostasis and causes basal cell carcinoma (BCC). Similarly, bone morphogenetic proteins (BMPs) and activins have been described as key signaling factors in the complex regulation of epidermal fate decisions, although their precise interplay with HH/GLI is largely elusive. Here we show that, in human epidermal cells, expression of the activin/BMP antagonist follistatin (FST) is predominantly up-regulated by the HH effector GLI2. Consistently, we found strong FST expression in the outer root sheath of human hair follicles and BCC. Detailed promoter analysis showed that two sequences with homology to the GLI consensus binding site are required for GLI2-mediated activation. Interestingly, activation of the FST promoter is highly GLI2-specific, because neither GLI1 nor GLI3 can significantly increase FST transcription. GLI2 specificity requires the presence of a 518-bp fragment in the proximal FST promoter region. On the protein level, sequences C-terminal to the zinc finger are responsible for GLI2-specific activation of FST transcription, pointing to the existence of GLI-interacting cofactors that modulate GLI target specificity. Our results reveal a key role of GLI2 in activation of the activin/BMP antagonist FST in response to HH signaling and provide new evidence for a regulatory interaction between HH and activin/BMP signaling in hair follicle development and BCC.

Keratinocyte-fibroblast interactions in wound healing

Cutaneous tissue repair aims at restoring the barrier function of the skin. To achieve this, defects need to be replaced by granulation tissue to form new connective tissue, and epithelial wound closure is required to restore the physical barrier. Different wound-healing phases are recognized, starting with an inflammation-dominated early phase giving way to granulation tissue build-up and scar remodeling after epithelial wound closure has been achieved. In the granulation tissue, mesenchymal cells are maximally activated, cells proliferate, and synthesize huge amounts of extracellular matrix. Epithelial cells also proliferate and migrate over the provisional matrix of the underlying granulation tissue, eventually closing the defect. This review focuses on the role of keratinocyte-fibroblast interactions in the wound-healing process. There is ample evidence that keratinocytes stimulate fibroblasts to synthesize growth factors, which in turn will stimulate keratinocyte proliferation in a double paracrine manner. Moreover, fibroblasts can acquire a myofibroblast phenotype under the control of keratinocytes. This depends on a finely tuned balance between a proinflammatory or a transforming growth factor (TGF)-beta-dominated environment. As the phenotype of fibroblasts from different tissues or body sites becomes better defined, we may understand their individual contribution in wound healing in more detail and possibly explain different clinical outcomes.

The role of the activin system in keloid pathogenesis

Keloid scars represent a pathological response to cutaneous injury under the regulation of many growth factors. Activin-A, a dimeric protein and a member of the transforming growth factor-β superfamily, has been shown to regulate various aspects of cell growth and differentiation in the repair of the skin mesenchyme and the epidermis. Thus our aim was to study the role of activin and its antagonist, follistatin, in keloid pathogenesis. Increased mRNA expression for activin was observed in keloid scar tissue by performing RNase protection assay. Immunohistochemistry showed increased localization of both activin-A and follistatin in the basal layer of epidermis of keloid tissue compared with normal tissue. ELISA demonstrated a 29-fold increase in concentration of activin-A and an ∼5-fold increase in follistatin in conditioned media in keloid fibroblasts compared with normal fibroblasts. Although keloid keratinocytes produced 25% more follistatin than normal keratinocytes, the amounts of activin-A, in contrast, was ∼77% lower. Proliferation of fibroblasts was stimulated when treated with exogenous activin-A (46% increase in keloids fibroblasts) or following co-culture with hβAHaCaT cells (66% increase). Activin-A upregulated key extracellular matrix components, namely collagen, fibronectin, and α-smooth muscle actin, in normal and keloid fibroblasts. Co-treatment of follistatin with activin-A blocked the stimulatory effects of activin on extracellular matrix components. These findings emphasize the importance of the activin system in keloid biology and pathogenesis and suggest a possible therapeutic potential of follistatin in the prevention and treatment of keloids.

Regulation of epidermal homeostasis and repair by phosphoinositide 3-kinase

The epidermis undergoes continuous self-renewal to maintain its protective function. Whereas growth factors are known to modulate overall skin homeostasis, the intracellular signaling pathways, which control the delicate balance between proliferation and differentiation in keratinocytes, are largely unknown. Here we show transient upregulation of the phosphoinositide 3-kinase (PI3K) catalytic subunits p110α and p110β in differentiating keratinocytes in vitro, expression of these subunits in the epidermis of normal and wounded skin, and enhanced Akt phosphorylation in the hyperproliferative wound epidermis. Stimulation of PI3K activity in cultured keratinocytes by stable expression of an inducible, constitutively active PI3K mutant promoted cell proliferation and inhibited terminal differentiation in keratinocyte monocultures and induced the formation of a hyperplastic, disorganized and poorly differentiated epithelium in organotypic skin cultures. Activation of PI3K signaling also caused reorganization of the actin cytoskeleton and induced keratinocyte migration in vitro and in skin organ cultures. The identification of 122 genes, which are differentially expressed after induction of PI3K signaling provides insight into the molecular mechanisms underlying the observed effects of active PI3K on keratinocytes and indicates that hyperproliferation may be achieved at the expense of genome integrity. These results identify PI3K as an important intracellular regulator of epidermal homeostasis and repair.

Id proteins: novel targets of activin action, which regulate epidermal homeostasis

Activin is a member of the transforming growth factor beta (TGF-beta) family, which plays a crucial role in skin morphogenesis and wound healing. To gain insight into the underlying mechanisms of action, we searched for activin-regulated genes in cultured keratinocytes. One of the identified target genes encodes Id1, a negative regulator of helix-loop-helix transcription factors. We show that Id1, Id2, and Id3 are strongly downregulated by activin in keratinocytes in vitro and in vivo. To determine the role of Id1 in keratinocyte biology, we generated stable HaCaT keratinocyte cell lines overexpressing this protein. Our results revealed that enhanced levels of Id1 do not affect proliferation of keratinocytes in monoculture under exponential culture conditions or in response to activin or TGF-beta1. However, in three-dimensional organotypic cultures, Id1-overexpressing HaCaT cells formed a hyperthickened and disorganized epithelium that was characterized by enhanced keratinocyte proliferation, abnormal differentiation, and an increased rate of apoptosis. These results identify an important function of Id1 in the regulation of epidermal homeostasis.

Activin controls skin morphogenesis and wound repair predominantly via stromal cells and in a concentration-dependent manner via keratinocytes

The transforming growth factor-beta family member activin is a potent regulator of skin morphogenesis and repair. Transgenic mice overexpressing activin in keratinocytes display epidermal hyper-thickening and dermal fibrosis in normal skin and enhanced granulation tissue formation after wounding. Mice overexpressing the secreted activin antagonist follistatin, however, have the opposite wound-healing phenotype. To determine whether activin affects skin morphogenesis and repair via activation of keratinocytes and/or stromal cells, we generated transgenic mice expressing a dominant-negative activin receptor IB mutant (dnActRIB) in keratinocytes. The architecture of adult skin was unaltered in these mice, but delays were observed in postnatal pelage hair follicle morphogenesis and in the first catagen-telogen transformation of hair follicles. Although dnActRIB-transgenic mice showed slightly delayed wound re-epithelialization after skin injury, the strong inhibition of granulation tissue formation seen in follistatin-transgenic mice was not observed. Therefore, although endogenous activin appeared to affect skin morphogenesis and repair predominantly via stromal cells, overexpressed activin strongly affected the epidermis. The epidermal phenotype of activin-overexpressing mice was partially rescued by breeding these animals with dnActRIB-transgenic mice. These results demonstrate that activin affects both stromal cells and keratinocytes in normal and wounded skin and that the effect on keratinocytes is dose-dependent in vivo.

Activin: an important regulator of wound repair, fibrosis, and neuroprotection

We recently identified the gene encoding the activin betaA chain as a novel injury-regulated gene. We showed that activin over-expression in the skin of transgenic mice enhances the speed of wound healing but also the scarring response. By contrast, inhibition of activin action by over-expression of the activin antagonist follistatin caused a severe delay in wound repair, but the quality of the healed wound was improved. In a search for activin-regulated genes in keratinocytes we identified the Mad1 transcription factor as a direct target of activin in these cells. Since Mad1 inhibits proliferation and induces differentiation of various cell types, our results suggest that activin regulates these processes in keratinocytes via induction of mad1. In addition to its role in the skin, we recently identified activin as a novel neuroprotective factor in vivo. Together with results from other laboratories, these findings suggest that activin is an important player in inflammation, repair and cytoprotection in various organs.

Regulation of wound healing by growth factors and cytokines

Cutaneous wound healing is a complex process involving blood clotting, inflammation, new tissue formation, and finally tissue remodeling. It is well described at the histological level, but the genes that regulate skin repair have only partially been identified. Many experimental and clinical studies have demonstrated varied, but in most cases beneficial, effects of exogenous growth factors on the healing process. However, the roles played by endogenous growth factors have remained largely unclear. Initial approaches at addressing this question focused on the expression analysis of various growth factors, cytokines, and their receptors in different wound models, with first functional data being obtained by applying neutralizing antibodies to wounds. During the past few years, the availability of genetically modified mice has allowed elucidation of the function of various genes in the healing process, and these studies have shed light onto the role of growth factors, cytokines, and their downstream effectors in wound repair. This review summarizes the results of expression studies that have been performed in rodents, pigs, and humans to localize growth factors and their receptors in skin wounds. Most importantly, we also report on genetic studies addressing the functions of endogenous growth factors in the wound repair process.

The roles of activin in cytoprotection and tissue repair

Activin is a member of the transforming growth factor beta family of growth and differentiation factors. Initially discovered as a protein that stimulates release of follicle-stimulating hormone, it is now well accepted as an important regulator of cell growth and differentiation. Most interestingly, a series of previous studies have revealed novel roles of activin in inflammation and repair. Our own results have provided evidence for an important function of activin in cutaneous wound repair as well as in neuroprotection, and these data will be summarized and discussed in this chapter.

Dynamics and localization of activin A expression in rat gastric ulcers

BACKGROUND

Activin A, the homodimer of the activin/inhibin betaA subunit, has been shown to participate in cutaneous wound healing. In this study we intended to determine its part in gastric ulceration.

METHODS

Activin A expression was studied by immunohistochemistry and in situ hybridization in acetic-acid-induced chronic gastric ulcers in rat. The dynamics of this process were also assessed by quantitative real time RT-PCR and RNase protection assays (RPA). The effects of different doses of this cytokine on epithelial and mesenchymal cell proliferation were quantitated in vitro.

RESULTS

Low amounts of activin A and its mRNA were expressed by epithelia, endothelia and fibroblasts in intact gastric tissue. Granulation tissue of gastric ulcers and gastric glands adjacent to the ulcer rim expressed markedly increased amounts of activin protein as well as activin/inhibin betaA mRNA. RPA and RT-PCR studies revealed a more than 3-fold increase in the relative abundance of this mRNA. Activin A did not affect the proliferation rate of fibroblasts and epithelial cells in vitro.

CONCLUSIONS

Activin A participates in gastric ulcer healing in a similar fashion as in cutaneous wounding. Its expression on protein and mRNA level is markedly increased in ulcer base and rim.

Bone morphogenetic proteins and their antagonists in skin and hair follicle biology

Bone morphogenetic proteins (BMP) are members of the transforming growth factor-beta superfamily regulating a large variety of biologic responses in many different cells and tissues during embryonic development and postnatal life. BMP exert their biologic effects via binding to two types of serine/threonine kinase BMP receptors, activation of which leads to phosphorylation and translocation into the nucleus of intracellular signaling molecules, including Smad1, Smad5, and Smad8 ("canonical" BMP signaling pathway). BMP effects are also mediated by activation of the mitogen-activated protein (MAP) kinase pathway ("noncanonical" BMP Signaling pathway). BMP activity is regulated by diffusible BMP antagonists that prevent BMP interactions with BMP receptors thus modulating BMP effects in tissues. During skin development, BMPs its receptors and antagonists show stringent spatiotemporal expressions patterns to achieve proper regulation of cell proliferation and differentiation in the epidermis and in the hair follicle. In normal postnatal skin, BMP are involved in the control of epidermal homeostasis, hair follicle growth, and melanogenesis. Furthermore, BMP are implicated in a variety of pathobiologic processes in skin, including wound healing, psoriasis, and carcinogenesis. Therefore, BMPs represent new important players in the molecular network regulating homeostasis in normal and diseased skin. Pharmacologic modulation of BMP signaling may be used as a new approach for managing skin and hair disorders.

The Mad1 transcription factor is a novel target of activin and TGF-beta action in keratinocytes: possible role of Mad1 in wound repair and psoriasis

Activin A, a member of the transforming growth factor beta (TGF-beta) superfamily, affects keratinocyte proliferation and differentiation in vitro and in vivo. However, little is known about the mechanisms of activin action in keratinocytes, and its target genes have not been identified. In this study, we demonstrate that activin A and TGF-beta1 directly induce the expression and activity of Mad1, an antagonist of the c-Myc transcription factor, in the human HaCaT keratinocyte cell line. Expression and activity of Mad1 was strongly induced by both factors in keratinocytes, although the intensity of induction was different for activin A and TGF-beta1. To determine a possible role of activin and TGF-beta in the regulation of mad1 expression in vivo, we analysed its expression during cutaneous wound repair when high levels of these factors are present. Expression of mad1 mRNA and protein, but not of other mad genes, increased significantly after skin injury, particularly in polymorphonuclear leukocytes and in suprabasal keratinocytes of the hyperproliferative epithelium. Elevated levels of mad1 mRNA were also detected in the hyperthickened epidermis of psoriatic patients. Since Mad1 regulates proliferation and/or differentiation of various cell types, our results suggest that this transcription factor mediates at least in the part the anti-mitotic and/or differentiation-inducing activities of TGF-beta and activin in keratinocytes.

Impaired wound healing in transgenic mice overexpressing the activin antagonist follistatin in the epidermis

Recently, we demonstrated a strong upregulation of activin expression after skin injury. Furthermore, overexpression of this transforming growth factor beta family member in the skin of transgenic mice caused dermal fibrosis, epidermal hyperthickening and enhanced wound repair. However, the role of endogenous activin in wound healing has not been determined. To address this question we overexpressed the soluble activin antagonist follistatin in the epidermis of transgenic mice. These animals were born with open eyes, and the adult mice had larger ears, longer tails and reduced body weight compared with non-transgenic littermates. Their skin was characterized by a mild dermal and epidermal atrophy. After injury, a severe delay in wound healing was observed. In particular, granulation tissue formation was significantly reduced, leading to a major reduction in wound breaking strength. The wounds, however, finally healed, and the resulting scar area was smaller than in control animals. These results implicate an important function of endogenous activin in the control of wound repair and scar formation.

The roles of activins in repair processes of the skin and the brain

A recent study from our laboratory demonstrated a strong upregulation of activin expression during cutaneous wound healing. To further analyze the role of activin A in skin morphogenesis and wound repair, we generated transgenic mice that overexpress activin A under the control of the keratin 14 promoter. The latter targets expression of transgenes to the basal, proliferating layer of the epidermis. Hetero- as well as homozygous transgenic animals were viable and fertile. However, they were smaller than non-transgenic littermates and they had smaller ears and shorter tails. Histological analysis of their skin revealed dermal hyperthickening, mainly due to the replacement of fatty tissue by connective tissue, and an increase in suprabasal, partially differentiated epidermal layers. After cutaneous injury, a strong enhancement of granulation tissue formation was observed. Furthermore, the extent of re-epithelialization was increased in some of the wounds. These data demonstrate that activin A is a potent stimulator of the wound healing process. Using an in vivo model of local brain injury, we found that activin A also plays a significant role in the early cellular response to neuronal damage. Expression of activin mRNA and protein is markedly upregulated within a few hours of injury. If applied exogenously, recombinant activin A is capable of rescuing neurons from acute cell death. Studying the interaction between bFGF, a well-established neuroprotective agent, which is currently being tested in stroke patients, and activin A, we arrived at the unexpected conclusion that it is the strong induction of activin A by bFGF which endows the latter with its beneficial actions in patients. These findings suggest that the development of substances directly targeting activin expression or receptor binding should offer new possibilities in the acute treatment of stroke and brain trauma.

Expression and function of keratinocyte growth factor and activin in skin morphogenesis and cutaneous wound repair

Reepithelialization and granulation tissue formation during cutaneous wound repair are mediated by a wide variety of growth and differentiation factors. Recent studies from our laboratory provided evidence for an important role of keratinocyte growth factor (KGF) in the repair of the injured epithelium and for a novel function of the transforming growth factor-beta superfamily member activin in granulation tissue formation. KGF is weakly expressed in human skin, but is strongly upregulated in dermal fibroblasts after skin injury. Its binding to a transmembrane receptor on keratinocytes induces proliferation and migration of these cells. Furthermore, KGF has been shown to protect epithelial cells from the toxic effects of reactive oxygen species. We have identified a series of KGF-regulated genes that are likely to play a role in these processes. In addition to KGF, activin seems to be a novel player in wound healing. Activin expression is hardly detectable in nonwounded skin, but this factor is highly expressed in redifferentiating keratinocytes of the hyperproliferative wound epithelium as well as in cells of the granulation tissue. To gain insight into the role of activin in wound repair, we generated transgenic mice that overexpress activin in basal keratinocytes of the epidermis. These mice were characterized by a hyperthickened epidermis and by dermal fibrosis. Most importantly, overexpression of activin strongly enhanced the process of granulation tissue formation, demonstrating a novel and important role of activin in cutaneous wound repair.

Facilitated wound healing by activation of the Transglutaminase 1 gene

Transglutaminase 1 (TGase 1) is a Ca(2+)-dependent enzyme which catalyzes epsilon-(gamma-glutamyl)lysine cross-linking of substrate proteins such as involucrin and loricrin to generate the cornified envelope at the cell periphery of the stratum corneum. We have shown that disruption of the TGase 1 gene in mice results in neonatal lethality, absence of the cornified envelope, and impaired skin barrier function. Based on the importance of TGase 1 in epidermal morphogenesis, we have now assessed its role in wound healing. In neonatal mouse skin, TGase 1 mRNA as well as keratin 6alpha was induced in the epidermis at the wound edges as early as 2 hours after injury and that expression continued in the migrating epidermis until completion of re-epithelialization. The TGase 1 enzyme co-localized on the plasma membrane of migrating keratinocytes with involucrin, but not with loricrin, which suggests the premature assembly of the cornified envelope. Similar injuries to TGase 1 knockout mouse skins grafted on athymic nude mice showed substantial delays in wound healing concomitant with sustained K6alpha mRNA induction. From these results, we suggest that activation of the TGase 1gene is essential for facilitated repair of skin injury.