Macrophages induce AKT/β-catenin-dependent Lgr5+ stem cell activation and hair follicle regeneration through TNF

[Wound-induced hair follicle neogenesis: a new perspective on hair follicles regeneration in adult mammals]

Objective

To explore the research progress of the cell sources and related signaling pathways of the wound-induced hair follicle neogenesis (WIHN) in recent years.

Methods

The literature related to WIHN in recent years was reviewed, and the cell sources and molecular mechanism were summarized and discussed.

Results

Current research shows that WIHN is a rare regeneration phenomenon in the skin of adult mammals, with multiple cell origins, both hair follicle stem cells and epithelial stem cells around the wound. Its molecular mechanism is complicated, which is regulated by many signaling pathways. Besides, the process is closely related to the immune response, the immunocytes and their related cytokines provide suitable conditions for this process.

Conclusion

There are still many unsolved problems on the cellular origins and molecular mechanisms of the WIHN. Further study on the mechanisms will enhance the understanding of adult mammals' hair follicle regeneration and may provide new strategy for functional healing of the human skin.

Functional complexity of hair follicle stem cell niche and therapeutic targeting of niche dysfunction for hair regeneration

Stem cell activity is subject to non-cell-autonomous regulation from the local microenvironment, or niche. In adaption to varying physiological conditions and the ever-changing external environment, the stem cell niche has evolved with multifunctionality that enables stem cells to detect these changes and to communicate with remote cells/tissues to tailor their activity for organismal needs. The cyclic growth of hair follicles is powered by hair follicle stem cells (HFSCs). Using HFSCs as a model, we categorize niche cells into 3 functional modules, including signaling, sensing and message-relaying. Signaling modules, such as dermal papilla cells, immune cells and adipocytes, regulate HFSC activity through short-range cell-cell contact or paracrine effects. Macrophages capacitate the HFSC niche to sense tissue injury and mechanical cues and adipocytes seem to modulate HFSC activity in response to systemic nutritional states. Sympathetic nerves implement the message-relaying function by transmitting external light signals through an ipRGC-SCN-sympathetic circuit to facilitate hair regeneration. Hair growth can be disrupted by niche pathology, e.g. dysfunction of dermal papilla cells in androgenetic alopecia and influx of auto-reacting T cells in alopecia areata and lichen planopilaris. Understanding the functions and pathological changes of the HFSC niche can provide new insight for the treatment of hair loss.

Phagocytosis of Wnt inhibitor SFRP4 by late wound macrophages drives chronic Wnt activity for fibrotic skin healing

Human and murine skin wounding commonly results in fibrotic scarring, but the murine wounding model wound-induced hair neogenesis (WIHN) can frequently result in a regenerative repair response. Here, we show in single-cell RNA sequencing comparisons of semi-regenerative and fibrotic WIHN wounds, increased expression of phagocytic/lysosomal genes in macrophages associated with predominance of fibrotic myofibroblasts in fibrotic wounds. Investigation revealed that macrophages in the late wound drive fibrosis by phagocytizing dermal Wnt inhibitor SFRP4 to establish persistent Wnt activity. In accordance, phagocytosis abrogation resulted in transient Wnt activity and a more regenerative healing. Phagocytosis of SFRP4 was integrin-mediated and dependent on the interaction of SFRP4 with the EDA splice variant of fibronectin. In the human skin condition hidradenitis suppurativa, phagocytosis of SFRP4 by macrophages correlated with fibrotic wound repair. These results reveal that macrophages can modulate a key signaling pathway via phagocytosis to alter the skin wound healing fate.

Lgr4 Deletion Delays the Hair Cycle and Inhibits the Activation of Hair Follicle Stem Cells

It is known that LGR4 plays an important role in hair follicle (HF) development, but the impact of LGR4 on the hair cycle is still unclear. In this study, we have found that K14-Cre-mediated skin epithelia-specific deletion of Lgr4 results in delayed anagen entry during the physiological hair cycle and compromised HF regeneration upon transplantation. We show that, although Lgr4 deletion does not appear to affect the number of quiescent HF stem cells, it leads to reduced numbers of LGR5⁺ and actively proliferating stem cells in the HFs. Moreover, LGR4-deficient HFs show molecular changes consistent with decreased mTOR and Wnt signaling but upregulated BMP signaling. Importantly, the reactivation of the protein kinase B pathway by injecting the protein kinase B activator SC79 in Lgr4^-/- mice can effectively reverse the hair cycle delay. Together, these data suggest that LGR4 promotes the normal hair cycle by activating HF stem cells and by influencing the activities of multiple signaling pathways that are known to regulate HF stem cells. Our study also implicates LGR4 as a potential target for treating hair disorder in the future.

Immune cell regulation of the hair cycle

The ability to manipulate the mammalian hair cycle will lead to novel therapies and strategies to combat all forms of alopecia. Thus, in addition to the epithelial-mesenchymal interactions in the hair follicle, niche and microenvironmental signals that accompany the phases of growth, regression and rest need to be scrutinized. Immune cells are well described in skin homeostasis and wound healing and have recently been shown to play an important role in the mammalian hair cycle. In this review, we will summarize our current knowledge of the role of immune cells in hair cycle control and discuss their relevance to human hair cycling disorders. Increased attention to this aspect of the hair cycle will provide new avenues to manipulate hair regeneration in humans and provide better insight into developing better ex vivo models of hair growth.

Scars or Regeneration?-Dermal Fibroblasts as Drivers of Diverse Skin Wound Responses

Scarring and regeneration are two physiologically opposite endpoints to skin injuries, with mammals, including humans, typically healing wounds with fibrotic scars. We aim to provide an updated review on fibroblast heterogeneity as determinants of the scarring-regeneration continuum. We discuss fibroblast-centric mechanisms that dictate scarring-regeneration continua with a focus on intercellular and cell-matrix adhesion. Improved understanding of fibroblast lineage-specific mechanisms and how they determine scar severity will ultimately allow for the development of antiscarring therapies and the promotion of tissue regeneration.

Platelet sonicates activate hair follicle stem cells and mediate enhanced hair follicle regeneration

An increasing number of studies show that platelet-rich plasma (PRP) is effective for androgenic alopecia (AGA). However, the underlying cellular and molecular mechanisms along with its effect on hair follicle stem cells are poorly understood. In this study, we designed to induce platelets in PRP to release factors by calcium chloride (PC) or by sonication where platelet lysates (PS) or the supernatants of platelet lysate (PSS) were used to evaluate their effect on the hair follicle activation and regeneration. We found that PSS and PS exhibited a superior effect in activating telogen hair follicles than PC. In addition, PSS injection into the skin activated quiescent hair follicles and induced K15⁺ hair follicle stem cell proliferation in K14-H2B-GFP mice. Moreover, PSS promoted skin-derived precursor (SKP) survival in vitro and enhanced hair follicle formation in vivo. In consistence, protein array analysis of different PRP preparations revealed that PSS contained higher levels of 16 growth factors (out of 41 factors analysed) than PC, many of them have been known to promote hair follicle regeneration. Thus, our data indicate that sonicated PRP promotes hair follicle stem cell activation and de novo hair follicle regeneration.

Macrophages Regulate Schwann Cell Maturation after Nerve Injury

Pro-regenerative macrophages are well known for their role in promoting tissue repair; however, their specific roles in promoting regeneration of the injured nerve are not well defined. Specifically, how macrophages interact with Schwann cells following injury during remyelination has been largely unexplored. We demonstrate that after injury, including in humans, macrophages function to clear debris and persist within the nerve microenvironment. Macrophage ablation immediately preceding remyelination results in an increase in immature Schwann cell density, a reduction in remyelination, and long-term deficits in conduction velocity. Targeted RNA-seq of macrophages from injured nerve identified Gas6 as one of several candidate factors involved in regulating Schwann cell dynamics. Functional studies show that the absence of Gas6 within monocyte lineage cells impairs Schwann cell remyelination within the injured nerve. These results demonstrate a role for macrophages in regulating Schwann cell function during nerve regeneration and highlight a molecular mechanism by which this occurs.

Prostaglandin E2 Receptor 2 Modulates Macrophage Activity for Cardiac Repair

Background

Prostaglandin E₂ has long been known to be an immune modulator. It is released after tissue injury and plays a role in modulating macrophage activities, which are essential for tissue regeneration. However, the involvement of prostaglandin E₂ receptor 2 (EP2)–dependent regulation of macrophages in postischemic heart is unclear. This study aims to evaluate the role of EP2 in damaged heart.

Methods and Results

The effect of EP2 in postischemic heart was evaluated using EP2‐deficient transgenic mice. We demonstrated that cardiac function was worse after myocardial injury on loss of EP2. Furthermore, EP2 deficiency also altered proinflammatory response and resulted in a defect in macrophage recruitment to the injured myocardium. Transcriptome analysis revealed that the expression of erythroid differentiation regulator 1 (Erdr1) was significantly induced in EP2‐deficient macrophages. Knocking down Erdr1 expression restored migration ability of EP2‐deficient cells both in vitro and in vivo. By using a genetic fate‐mapping approach, we showed that abolishment of EP2 expression effectively attenuated cell replenishment.

Conclusions

The EP2‐dependent signaling pathway plays a critical role in regulating macrophage recruitment to the injured myocardium, thereby exerting a function in modulating the inflammatory microenvironment for cardiac repair.

Ginkgolide B and bilobalide promote the growth and increase β-catenin expression in hair follicle dermal papilla cells of American minks

The regeneration of proliferation potential of dermal papilla (DP) cells contributes to the treatment of hair loss disorders. Ginkgolide B (GKB) and bilobalide (BB) are two functional components isolated from Ginkgo biloba that can promote hair growth. In the current study, the effect of GKB or BB on DP cell viability and the related signaling was assessed. Hair follicles were isolated from minks, and the growth of hair follicles was measured under the administration of GKB or BB. DP cells isolated from minks were also subjected to GKB or BB. The administration of GKB or BB induced the growth of hair follicles. The viability of DP cells was also increased by GKB or BB as illustrated by methyl thiazolyl tetrazolium and flow cytometry detection. Moreover, the secretion of VEGF was enhanced by GKB or BB. At molecular level, the activities of Akt, ERK1/2, and β-catenin were induced by GKB, whereas BB only increased the activities of Akt and β-catenin. In conclusion, although the two components influenced the β-catenin signaling activity in distinct mechanisms, they both increased the viability of DP cells and promoted the cycle of hair follicles.

Boehmite enhances hair follicle growth via stimulation of dermal papilla cells by upregulating β-catenin signalling

Hair growth, a complex process, has long been the subject of intense research. Recent developments in material technology have revealed boehmite as a new therapeutic modality for use in wound healing and scar reduction, indicating its beneficial effects. Nonetheless, the biological bases of the beneficial effects of boehmite remain unknown. We investigated the hair growth properties of boehmite in vitro and in vivo and observed dose-dependent proliferation of human dermal papilla cells (hDPCs) in vitro and hair regrowth in a mouse model. To investigate the effects of boehmite on the promotion of cell transition to the anagen phase, we evaluated hDPC viability, alkaline phosphatase (ALP) activity, protein expression and vascular endothelial growth factor (VEGF) secretion in vitro and assessed the anagen-promoting effects of boehmite via gross observation and histological analysis in a mouse model. Boehmite increased hDPC viability, ALP activity, AKT/GSK3ß/ß-catenin pathway activity, anagen-related gene expression and VEGF secretion; moreover, it accelerated hair regrowth in a catagen-anagen transition model via upregulation of β-catenin signalling and follicular cell proliferation. Collectively, our results indicate that boehmite accelerates hair growth, partly via its effects on critical events in the active phase of the hair follicle cycle, including the promotion of the proliferation of hDPCs and their immediate progeny to the follicle base.

Pten loss in Lgr5+ hair follicle stem cells promotes SCC development

Accumulating data support that tissue stem cells give rise to cancer cells. Hair follicle stem cells (HFSCs) undergo cyclic quiescence and activation and may sever as the origin of cutaneous squamous cell carcinoma (SCC). Pten is a tumor suppressor gene that is frequently mutated in hereditary cancer syndromes such as Cowden disease, which is featured with papillomatosis in cutaneous tissues and hyperkeratosis in the acral region of the skin. Additionally, mice with keratinocyte-specific Pten deficiency (k5-Pten^-/- mice) show epidermal hyperplasia and spontaneous tumor formation. However, the impact of Pten mutation in HFSCs, such as in Lgr5⁺ HFSCs, on SCC formation is unclear.

Methods: We established experiments with wildtype and Lgr5-CreER; Pten^flox/flox mice, and used DMBA/TPA two-stage skin carcinogenesis model to explore the effect of Pten loss in Lgr5⁺ HFSCs of 3 weeks old mice in skin carcinogenesis. In vitro experiments (cell culture and protein expression analysis) are employed to investigate molecular mechanisms involved.

Results: Pten loss in Lgr5⁺ HFSCs promoted SCC formation, which was attenuated in TNF^-/- mice. Notably, β-catenin loss in Lgr5⁺ HFSCs decreased the formation of SCC. In addition, Pten loss in cultured epidermal stem cells upregulated the levels of both phospho-Akt and β-catenin.

Conclusion: Pten loss in Lgr5⁺ cells induced Akt/β-catenin signaling, and SCCs can subsequently be raised as progeny from these primed Lgr5⁺ stem cells.

Through the lens of hair follicle neogenesis, a new focus on mechanisms of skin regeneration after wounding

Wound-induced hair follicle neogenesis (WIHN) is a phenomenon that occurs in adult mammalian skin, where fully functional hair follicles are regenerated in the center of large full-thickness excisional wounds. Although originally discovered over 50 years ago in mice and rabbits, within the last decade it has received renewed interest, as the molecular mechanism has begun to be defined. This de novo regeneration of hair follicles largely recapitulates embryonic hair development, requiring canonical Wnt signaling in the epidermis, however, important differences between the two are beginning to come to light. TLR3 mediated double stranded RNA sensing is critical for the regeneration, activating retinoic acid signaling following wounding. Inflammatory cells, including Fgf9-producing γ-δ T cells and macrophages, are also emerging as important mediators of WIHN. Additionally, while dispensable in embryonic hair follicle development, Shh signaling plays a major role in WIHN and may be able to redirect cells fated to scarring wounds into a regenerative phenotype. The cellular basis of WIHN is also becoming clearer, with increasing evidence suggesting an incredible level of cellular plasticity. Multiple stem cell populations, along with lineage switching of differentiated cells all contribute towards the regeneration present in WIHN. Further study of WIHN will uncover key steps in mammalian development and regeneration, potentially leading to new clinical treatments for hair-related disorders or fibrotic scarring.

Stem cells in tissues, organoids, and cancers

Stem cells give rise to all cells and build the tissue structures in our body, and heterogeneity and plasticity are the hallmarks of stem cells. Epigenetic modification, which is associated with niche signals, determines stem cell differentiation and somatic cell reprogramming. Stem cells play a critical role in the development of tumors and are capable of generating 3D organoids. Understanding the properties of stem cells will improve our capacity to maintain tissue homeostasis. Dissecting epigenetic regulation could be helpful for achieving efficient cell reprograming and for developing new drugs for cancer treatment. Stem cell-derived organoids open up new avenues for modeling human diseases and for regenerative medicine. Nevertheless, in addition to the achievements in stem cell research, many challenges still need to be overcome for stem cells to have versatile application in clinics.

Androgenetic alopecia: combing the hair follicle signaling pathways for new therapeutic targets and more effective treatment options

Introduction: In the past 30 years, only two drugs have received FDA approval for the treatment of androgenetic alopecia reflecting a lack of success in unraveling novel targets for pharmacological intervention. However, as our knowledge of hair biology improves, new signaling pathways and organogenesis processes are being uncovered which have the potential to yield more effective therapeutic modalities. Areas covered: This review focuses on potential targets for drug development to treat hair loss. The physiological processes underlying the promise of regenerative medicine to recreate new functional hair follicles in bald scalp are also examined. Expert opinion: The discovery of promising new targets may soon enable treatment options that modulate the hair cycle to preserve or extend the growth phase of the hair follicle. These new targets could also be leveraged to stimulate progenitor cells and morphogenic pathways to reactivate miniaturized follicles in bald scalp or to harness the potential of wound healing and embryogenic development as an emerging paradigm to generate new hair follicles in barren skin.

Choreographing Immunity in the Skin Epithelial Barrier

The skin interfaces with the external environment and is home to a myriad of immune cells that patrol the barrier to ward off harmful agents and aid in tissue repair. The formation of the cutaneous immune arsenal begins before birth and evolves throughout our lifetime, incorporating exogenous cues from microbes and inflammatory encounters, to achieve optimal fitness and function. Here, we discuss the context-specific signals that drive productive immune responses in the skin epithelium, highlighting key modulators of these reactions, including hair follicles, neurons, and commensal microbes. We thus also discuss the causal and mechanistic underpinning of inflammatory skin diseases that have been revealed in recent years. Finally, we discuss the non-canonical functions of cutaneous immune cells including their burgeoning role in epithelial regeneration and repair. The rapidly growing field of cutaneous immunity is revealing immune mechanisms and functions that can be harnessed to boost skin health and treat disease.

The origins and non-canonical functions of macrophages in development and regeneration

The discovery of new non-canonical (i.e. non-innate immune) functions of macrophages has been a recurring theme over the past 20 years. Indeed, it has emerged that macrophages can influence the development, homeostasis, maintenance and regeneration of many tissues and organs, including skeletal muscle, cardiac muscle, the brain and the liver, in part by acting directly on tissue-resident stem cells. In addition, macrophages play crucial roles in diseases such as obesity-associated diabetes or cancers. Increased knowledge of their regulatory roles within each tissue will therefore help us to better understand the full extent of their functions and could highlight new mechanisms modulating disease pathogenesis. In this Review, we discuss recent studies that have elucidated the developmental origins of various macrophage populations and summarize our knowledge of the non-canonical functions of macrophages in development, regeneration and tissue repair.

Mechanical stretch induces hair regeneration through the alternative activation of macrophages

Tissues and cells in organism are continuously exposed to complex mechanical cues from the environment. Mechanical stimulations affect cell proliferation, differentiation, and migration, as well as determining tissue homeostasis and repair. By using a specially designed skin-stretching device, we discover that hair stem cells proliferate in response to stretch and hair regeneration occurs only when applying proper strain for an appropriate duration. A counterbalance between WNT and BMP-2 and the subsequent two-step mechanism are identified through molecular and genetic analyses. Macrophages are first recruited by chemokines produced by stretch and polarized to M2 phenotype. Growth factors such as HGF and IGF-1, released by M2 macrophages, then activate stem cells and facilitate hair regeneration. A hierarchical control system is revealed, from mechanical and chemical signals to cell behaviors and tissue responses, elucidating avenues of regenerative medicine and disease control by demonstrating the potential to manipulate cellular processes through simple mechanical stimulation.

IL-1 and TNFα Contribute to the Inflammatory Niche to Enhance Alveolar Regeneration

Inflammatory responses are known to facilitate tissue recovery following injury. However, the precise mechanisms that enhance lung alveolar regeneration remain unclear. Here, using an organoid-based screening assay, we find that interleukin-1 (IL-1) and tumor necrosis factor α (TNFα) enhance the proliferation of AEC2s while maintaining their differentiation capacity. Furthermore, we find that expression of IL-1β and TNFα are induced in the AEC2 niche following influenza-induced injury in vivo, and lineage tracing analysis revealed that surviving AEC2s around the damaged area contribute to alveolar regeneration. Through genetic and pharmacological modulation of multiple components of the IL-1-nuclear factor κB (NF-κB) signaling axis, we show that cell-intrinsic as well as stromal mediated IL-1 signaling are essential for AEC2 mediated lung regeneration. Taken together, we propose that the IL-1/TNFα-NF-κB signaling axis functions as a component of an inflammation-associated niche to regulate proliferation of surviving AEC2s and promote lung regeneration.

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IL-1/TNFα enhance the growth of lung alveolar stem cells (AEC2s) in organoid culture

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AEC2s treated with IL-1 or TNFα maintain differentiation ability

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AEC2s proliferate and contribute to lung repair after influenza virus infection

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NF-κB pathway is activated in AEC2s treated with IL-1 or TNFα

Single-cell analysis reveals fibroblast heterogeneity and myeloid-derived adipocyte progenitors in murine skin wounds

During wound healing in adult mouse skin, hair follicles and then adipocytes regenerate. Adipocytes regenerate from myofibroblasts, a specialized contractile wound fibroblast. Here we study wound fibroblast diversity using single-cell RNA-sequencing. On analysis, wound fibroblasts group into twelve clusters. Pseudotime and RNA velocity analyses reveal that some clusters likely represent consecutive differentiation states toward a contractile phenotype, while others appear to represent distinct fibroblast lineages. One subset of fibroblasts expresses hematopoietic markers, suggesting their myeloid origin. We validate this finding using single-cell western blot and single-cell RNA-sequencing on genetically labeled myofibroblasts. Using bone marrow transplantation and Cre recombinase-based lineage tracing experiments, we rule out cell fusion events and confirm that hematopoietic lineage cells give rise to a subset of myofibroblasts and rare regenerated adipocytes. In conclusion, our study reveals that wounding induces a high degree of heterogeneity among fibroblasts and recruits highly plastic myeloid cells that contribute to adipocyte regeneration.

The diversity of fibroblasts contributing to wound healing is unclear. Here, the authors use single-cell RNA-sequencing to identify heterogeneity among murine fibroblasts in the wound and find that recruited myeloid cells contribute to adipocyte regeneration during healing.

Macrophages are necessary for skin regeneration during tissue expansion

Background

Tissue expansion is a procedure that promotes skin regeneration by mechanical stretch. During the stress and relaxation cycle, the skin undergoes a repeated microtrauma which triggers an immune response leading to the recruitment of macrophages to repair the damaged tissue. Macrophages have been found to be necessary for tissue repair and wound healing, but their effects on skin regeneration during mechanical stretch remain unclear.

Methods

The dynamic changes of macrophages in the rat skin tissues undergoing expansion were quantitatively determined by immunohistochemistry staining. The area of the expanded skin, skin thickness, dermal collagen density, cell proliferation and tissue vascularization were examined to determine the effects of macrophages on the expanding skin. The phenotypes of macrophages and the growth factors related to skin regeneration were also examined to evaluate the underlying mechanisms for the involvement of macrophages in skin regeneration. As a comparison, the tissue samples of expanding skin in which the macrophages were depleted by topically utilizing clodronate liposomes were also evaluated.

Results

The number of skin macrophages in skin maintained in the high level during the skin expansion compared to non-expanded skin. We found that a switch from an M1- to M2-dominant response during tissue expansion. After the macrophages were depleted, the skin regeneration was inhibited, as evidenced by a smaller expansion area, thinner skin layers and decreased cell proliferation rate, collagen synthesis and, skin vascularization. The secretion of epidermal growth factor (EGF), fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF) were decreased when macrophages were depleted.

Conclusions

Our findings suggest that macrophages are necessary for skin regeneration during tissue expansion. Modulating inflammation may provide a key therapeutic strategy to promote skin growth under mechanical strain.

Electronic supplementary material

The online version of this article (10.1186/s12967-019-1780-z) contains supplementary material, which is available to authorized users.

Insights into the regeneration of skin from Acomys, the spiny mouse

Members of the Acomys genus, known as spiny mice, are unique among mammals in being perfectly capable of regenerating large areas of skin that have been removed. During this regenerative process hairs, sebaceous glands, erector pili muscles, adipocytes and the panniculus carnosus all regenerate and the dermis does not scar. We review here the processes that the epidermis and the individual components of the dermis undergo during spiny mouse regeneration as well as the molecules that have been identified as potentially important in regeneration. We then relate this to what has been proposed as playing a role in studies from the laboratory mouse, Mus musculus. Differences in the immune systems of spiny mice and laboratory mice are also highlighted as this is suggested to play a part not only in the perfect wound healing that embryos display but also in regeneration in lower vertebrates.

Regulation of melanocyte stem cells in the pigmentation of skin and its appendages: Biological patterning and therapeutic potentials

Skin evolves essential appendages and indispensable types of cells that synergistically insulate the body from environmental insults. Residing in the specific regions in the skin such as epidermis, dermis and hair follicle, melanocytes perform an array of vital functions including defending the ultraviolet radiation and diversifying animal appearance. As one of the adult stem cells, melanocyte stem cells in the hair follicle bulge niche can proliferate, differentiate and keep quiescence to control and coordinate tissue homeostasis, repair and regeneration. In synchrony with hair follicle stem cells, melanocyte stem cells in the hair follicles undergo cyclic activation, degeneration and resting phases, to pigment the hairs and to preserve the stem cells. Disorder of melanocytes results in severe skin problems such as canities, vitiligo and even melanoma. Here, we compare and summarize recent discoveries about melanocyte in the skin, particularly in the hair follicle. A better understanding of the physiological and pathological regulation of melanocyte and melanocyte stem cell behaviours will help to guide the clinical applications in regenerative medicine.

Novel Biodegradable Polymeric Microparticles Facilitate Scarless Wound Healing by Promoting Re-epithelialization and Inhibiting Fibrosis

Despite decades of research, the goal of achieving scarless wound healing remains elusive. One of the approaches, treatment with polymeric microcarriers, was shown to promote tissue regeneration in various in vitro models of wound healing. The in vivo effects of such an approach are attributed to transferred cells with polymeric microparticles functioning merely as inert scaffolds. We aimed to establish a bioactive biopolymer carrier that would promote would healing and inhibit scar formation in the murine model of deep skin wounds. Here we characterize two candidate types of microparticles based on fibroin/gelatin or spidroin and show that both types increase re-epithelialization rate and inhibit scar formation during skin wound healing. Interestingly, the effects of these microparticles on inflammatory gene expression and cytokine production by macrophages, fibroblasts, and keratinocytes are distinct. Both types of microparticles, as well as their soluble derivatives, fibroin and spidroin, significantly reduced the expression of profibrotic factors Fgf2 and Ctgf in mouse embryonic fibroblasts. However, only fibroin/gelatin microparticles induced transient inflammatory gene expression and cytokine production leading to an influx of inflammatory Ly6C+ myeloid cells to the injection site. The ability of microparticle carriers of equal proregenerative potential to induce inflammatory response may allow their subsequent adaptation to treatment of wounds with different bioburden and fibrotic content.

Hedgehog stimulates hair follicle neogenesis by creating inductive dermis during murine skin wound healing

Mammalian wounds typically heal by fibrotic repair without hair follicle (HF) regeneration. Fibrosis and regeneration are currently considered the opposite end of wound healing. This study sought to determine if scar could be remodeled to promote healing with HF regeneration. Here, we identify that activation of the Sonic hedgehog (Shh) pathway reinstalls a regenerative dermal niche, called dermal papilla, which is required and sufficient for HF neogenesis (HFN). Epidermal Shh overexpression or constitutive Smoothened dermal activation results in extensive HFN in wounds that otherwise end in scarring. While long-term Wnt activation is associated with fibrosis, Shh signal activation in Wnt active cells promotes the dermal papilla fate in scarring wounds. These studies demonstrate that mechanisms of scarring and regeneration are not distant from one another and that wound repair can be redirected to promote regeneration following injury by modifying a key dermal signal.

On wounding, scar formation in mammals arises causing no hair follicle regeneration, but it is unclear if scarring precludes regeneration. Here, the authors show that if Sonic hedgehog signaling is activated in the wound, an inductive dermal niche forms, enabling regeneration and hair follicle formation.

Parallels between vertebrate cardiac and cutaneous wound healing and regeneration

The cellular events that contribute to tissue healing of non-sterile wounds to the skin and ischaemic injury to internal organs such as the heart share remarkable similarities despite the differences between these injury types and organs. In adult vertebrates, both injuries are characterised by a complex series of overlapping events involving multiple different cell types and cellular interactions. In adult mammals both tissue-healing processes ultimately lead to the permanent formation of a fibrotic, collagenous scar, which can have varying effects on tissue function depending on the site and magnitude of damage. Extensive scarring in the heart as a result of a severe myocardial infarction contributes to ventricular dysfunction and the progression of heart failure. Some vertebrates such as adult zebrafish, however, retain a more embryonic capacity for scar-free tissue regeneration in many tissues including the skin and heart. In this review, the similarities and differences between these different types of wound healing are discussed, with special attention on recent advances in regenerative, non-scarring vertebrate models such as the zebrafish.

Embryonic-like regenerative phenomenon: wound-induced hair follicle neogenesis

Wound-induced hair follicle neogenesis (WIHN) is a regenerative phenomenon that occurs widely in the skin of adult mammalians. A fully functional follicle can regenerate in the center of a full-thickness wound with a large enough size. The cellular origin of this process is similar to embryonic process. Many growth and development-related pathways are involved in WIHN. Studying WIHN can deeply explore the mechanism of biological growth, development and regeneration, and can identify new treatments for hair-related disorders. Our review aims to enlighten future study by summarizing the clinical manifestation of WIHN, as well as the cellular and molecular mechanism of WIHN in recent studies.

PPARβ/δ: Linking Metabolism to Regeneration

In contrast to the general belief that regeneration is a rare event, mainly occurring in simple organisms, the ability of regeneration is widely distributed in the animal kingdom. Yet, the efficiency and extent of regeneration varies greatly. Humans can recover from blood loss as well as damage to tissues like bone and liver. Yet damage to the heart and brain cannot be reversed, resulting in scaring. Thus, there is a great interest in understanding the molecular mechanisms of naturally occurring regeneration and to apply this knowledge to repair human organs. During regeneration, injury-activated immune cells induce wound healing, extracellular matrix remodeling, migration, dedifferentiation and/or proliferation with subsequent differentiation of somatic or stem cells. An anti-inflammatory response stops the regenerative process, which ends with tissue remodeling to achieve the original functional state. Notably, many of these processes are associated with enhanced glycolysis. Therefore, peroxisome proliferator-activated receptor (PPAR) β/δ—which is known to be involved for example in lipid catabolism, glucose homeostasis, inflammation, survival, proliferation, differentiation, as well as mammalian regeneration of the skin, bone and liver—appears to be a promising target to promote mammalian regeneration. This review summarizes our current knowledge of PPARβ/δ in processes associated with wound healing and regeneration.

Macrophage-derived GPNMB accelerates skin healing

Healing is a vital response important for the re-establishment of the skin integrity following injury. Delayed or aberrant dermal wound healing leads to morbidity in patients. The development of therapies to improve dermal healing would be useful. Currently, the design of efficient treatments is stalled by the lack of detailed knowledge about the cellular and molecular mechanisms involved in wound healing. Recently, using state-of-the-art technologies, it was revealed that macrophages signal via GPNMB to mesenchymal stem cells, accelerating skin healing. Strikingly, transplantation of macrophages expressing GPNMB improves skin healing in GPNMB-mutant mice. Additionally, topical treatment with recombinant GPNMB restored mesenchymal stem cells recruitment and accelerated wound closure in the diabetic skin. From a drug development perspective, this GPNMB is a new candidate for skin healing.

Macrophages Promote Wound-Induced Hair Follicle Regeneration in a CX3CR1- and TGF-β1-Dependent Manner

Hair follicle stem cells are regulated by intrafollicular and extrafollicular niche signals. Appropriate hair follicle regeneration relies on the coordinated release and integration of these signals. How immune cells, particularly cutaneous macrophages, influence the hair follicle stem cell niche and regeneration is not well understood. We took advantage of wound-induced hair growth (WIHG) to explore the relationship between wound macrophages and hair follicle regeneration. First, we showed that WIHG is dependent on CD11b⁺F4/80⁺ macrophages at 7-11 days after injury. Next, using CX₃CR1^gfp/+:CCR2^rfp/+ mice to capture the dynamic spectrum of macrophage phenotypes during wound healing, we showed that wound macrophages transition from a CX₃CR1^lo/med to a CX₃CR1^hi phenotype at the onset of WIHG. Finally, WIHG is abolished in mice deficient for CX₃CR1, delayed with pharmacological inhibition of transforming growth factor-β receptor type 1, and rescued with exogenous transforming growth factor-β1. Overall, we propose a model in which transforming growth factor-β1 and CX₃CR1 are critical for recruiting and maintaining the CCR2⁺CX₃CR1^hiLy6C^loTNFα⁺ macrophages critical for stimulating WIHG.

Wound Regeneration Deficit in Rats Correlates with Low Morphogenetic Potential and Distinct Transcriptome Profile of Epidermis

Large excisional wounds in mice prominently regenerate new hair follicles (HFs) and fat, yet humans are deficient for this regenerative behavior. Currently, wound-induced regeneration remains a clinically desirable, but only partially understood phenomenon. We show that large excisional wounds in rats across seven strains fail to regenerate new HFs. We compared wound transcriptomes between mice and rats at the time of scab detachment, which coincides with the onset of HF regeneration in mice. In both species, wound dermis and epidermis share core dermal and epidermal transcriptional programs, respectively, yet prominent interspecies differences exist. Compared with mice, rat epidermis expresses distinct transcriptional and epigenetic factors, markers of epidermal repair, hyperplasia, and inflammation, and lower levels of WNT signaling effectors and regulators. When recombined on the surface of excisional wounds with vibrissa dermal papillae, partial-thickness skin grafts containing distal pelage HF segments, but not interfollicular epidermis, readily regenerated new vibrissa-like HFs. Together, our findings establish rats as a nonregenerating rodent model for excisional wound healing and suggest that low epidermal competence and associated transcriptional profile may contribute to its regenerative deficiency. Future comparison between rat and mouse may lend further insight into the mechanism of wounding-induced regeneration and causes for its deficit.

Isolation and Cultivation of Epidermal (Stem) Cells

Recent studies have shown that epidermal stem cells derived from the epidermis of are able to form hair follicles in the presence of hair follicle-inducing cells. Here we describe the method that we have used to isolate and cultivate epidermal stem cells.

Expression of LGR5 in oral squamous cell carcinoma and its correlation to vasculogenic mimicry

BACKGROUND

LGR5, also named as GPR49, is considered as a biomarker of cancer stem cells which have been responsible for the initiation, progression, metastasis, and recurrence of cancers. Vasculogenic mimicry (VM) which defines the formation of fluid-conducting tubes by highly progressive and genetically dysregulated cancer cells has been considered as useful biomarker for metastasis and prognosis in various cancers. In this study, we analyzed associations between LGR5 and VM in oral squamous cell carcinoma (OSCC), and their association with clinicopathological characters in OSCC.

METHODS

Positive rates of LGR5 and VM in 190 OSCC tissue samples and correspondence normal tissues were detected by immunohistochemical and histochemical staining. Patients' clinical data were also collected.

RESULTS

Positive rates of LGR5 and VM were significantly higher in OSCC tissues than those in normal tissues. Positive rates of LGR5 and VM were positively related to tumor size, grades, lymph node metastasis, and TNM stages, and inversely with patients overall survival time. And there was a positive association between the expression of LGR5 and positive rate of VM. In multivariate analysis, high expression of LGR5 and positive VM and lymph node metastasis, as well as TNM stages were to be considered as independent prognosis factors for overall survival time in patients with OSCC.

CONCLUSIONS

The expression of LGR5 and VM represent potential biomarkers for metastasis and prognosis, as well as therapeutic targets for OSCC.

Extrinsic control of Wnt signaling in the intestine

The canonical Wnt/β-catenin signaling pathway is a central regulator of development and tissue homeostasis. In the intestine, Wnt signaling is primarily known as the principal organizer of epithelial stem cell identity and proliferation. Within the last decade, numerous scientific breakthroughs have shed light on epithelial self-organization in the gut, and organoids are now routinely used to study stem cell biology and intestinal pathophysiology. The contribution of non-epithelial cells to Wnt signaling in the gut has received less attention. However, there is mounting evidence that stromal cells are a rich source of Wnt pathway activators and inhibitors, which can dynamically shape Wnt signaling to control epithelial proliferation and restitution. Elucidating the extent and mechanisms of paracrine Wnt signaling in the intestine has the potential to broaden our understanding of epithelial homeostasis, and may be of particular relevance for disorders such as inflammatory bowel diseases and colitis-associated cancers.