Nerve-derived sonic hedgehog defines a niche for hair follicle stem cells capable of becoming epidermal stem cells

Spatial association of SEMA3C with nerve endings/terminal Schwann cells in hair follicle isthmus region

Nerve endings and terminal Schwann cells (TSCs) specifically and densely surround hair follicle at isthmus area, forming a neuromuscular-junction-like structure called lanceolate complex. The interplay between neuronal components and epidermis in this specialized structure enables hair to properly sense complex stimuli from environments. However, how nerves precisely attach to and innervate this specific region during development remains to be elucidated. Here, we demonstrate that SEMA3C, a secreted protein member of semaphorin family responsible for axonal guidance, is localized right below sebaceous gland and in close approximation with nerve endings and TSCs processes all through the entire hair cycle. SEMA3C protein is deposited outside of epithelial cells and its expression is independent on the presence of nerve endings/TSCs. SEMA3C is also found in portions of dermal papilla at growth phase. The tight spatial association of SEMA3C with lanceolate complex suggests that it might play roles in establishment and/or maintenance of the lanceolate complex in hair follicle.

EMILIN proteins are novel extracellular constituents of the dentin-pulp complex

Odontoblasts and pulp stroma cells are embedded within supramolecular networks of extracellular matrix (ECM). Fibrillin microfibrils and associated proteins are crucial constituents of these networks, serving as contextual scaffolds to regulate tissue development and homeostasis by providing both structural and mechanical properties and sequestering growth factors of the TGF-β superfamily. EMILIN-1, -2, and -3 are microfibril-associated glycoproteins known to modulate cell behaviour, growth factor activity, and ECM assembly. So far their expression in the various cells of the dentin-pulp complex during development, in the adult stage, and during inflammation has not been investigated. Confocal immunofluorescence microscopy and western blot analysis of developing and adult mouse molars and incisors revealed an abundant presence of EMILINs in the entire dental papilla, at early developmental stages. Later in development the signal intensity for EMILIN-3 decreases, while EMILIN-1 and -2 staining appears to increase in the pre-dentin and in the ECM surrounding odontoblasts. Our data also demonstrate new specific interactions of EMILINs with fibulins in the dentin enamel junction. Interestingly, in dentin caries lesions the signal for EMILIN-3 was significantly increased in inflamed odontoblasts. Overall our findings point for the first time to a role of EMILINs in dentinogenesis, pulp biology, and inflammation.

Promotion of Axon Growth by the Secreted End of a Transcription Factor

Axon growth is regulated externally by attractive and repulsive cues generated in the environment. In addition, intrinsic pathways govern axon development, although the extent to which axons themselves can influence their own growth is unknown. We find that dorsal root ganglion (DRG) axons secrete a factor supporting axon growth and identify it as the C terminus of the ER stress-induced transcription factor CREB3L2, which is generated by site 2 protease (S2P) cleavage in sensory neurons. S2P and CREB3L2 knockdown or inhibition of axonal S2P interfere with the growth of axons, and C-terminal CREB3L2 is sufficient to rescue these effects. C-terminal CREB3L2 forms a complex with Shh and stabilizes its association with the Patched-1 receptor on developing axons. Our results reveal a neuron-intrinsic pathway downstream of S2P that promotes axon growth.

The landscape of GPCR signaling in the regulation of epidermal stem cell fate and skin homeostasis

Continuous integration of signals from the micro and macro-environment is necessary for somatic stem cells to adapt to changing conditions, maintain tissue homeostasis and activate repair mechanisms. G-protein coupled receptors (GPCRs) facilitate this integration by binding to numerous hormones, metabolites and inflammatory mediators, influencing a diverse network of pathways that regulate stem cell fate. This adaptive mechanism is particularly relevant for tissues that are exposed to environmental assault, like skin. The skin is maintained by a set of basal keratinocyte stem and progenitor cells located in the hair follicle and interfollicular epidermis, and several GPCRs and their signaling partners serve as makers and regulators of epidermal stem cell activity. GPCRs utilize heterotrimeric G protein dependent and independent pathways to translate extracellular signals into intracellular molecular cascades that dictate the activation of keratinocyte proliferative and differentiation networks, including Hedgehog GLI, Hippo YAP1 and WNT/β-catenin, ultimately regulating stem cell identity. Dysregulation of GPCR signaling underlines numerous skin inflammatory diseases and cancer, with smoothened-driven basal cell carcinoma being a main example of a GPCR associated cancer. In this review, we discuss the impact of GPCRs and their signaling partners in skin keratinocyte biology, particularly in the regulation of the epidermal stem cell compartment.

Azelaic acid stimulates catalase activation and promotes hair growth through upregulation of Gli1 and Gli2 mRNA and Shh protein

OBJECTIVE

Although azelaic acid is effective for treatment of acne and rosacea, the biological activity of azelaic acid and the effect of its combination therapy with minoxidil were not elucidated with regard to hair growth.

MATERIALS AND METHODS

In this study, mouse vibrissae follicles were dissected on day 10 after depilation. Then, the bulb and bulge cells of the hair follicle were treated with minoxidil and azelaic acid for 10 days to evaluate Sonic hedgehog (Shh) protein expression. Moreover, bulge and bulb cells of the hair follicles were cultivated and the expression of Gli1, Gli2, and Axin2 mRNA levels was evaluated using real-time polymerase chain reaction (PCR) analysis. We further investigated the protective effects of azelaic acid against ultraviolet B (UVB) irradiation in cultured bulb and bulge cells by determining catalase activity. An irradiation dose of 20 mJ/cm2 UVB for 4 sec was chosen.

RESULTS

The results showed that catalase activity significantly (p<0.05) increased in the bulge cells after exposure to 2.5 mM and 25 mM azelaic acid. Meanwhile, treatment of the bulb cells with azelaic acid (2.5 and 25 mM) did not cause significant changes in catalase activity. We also found that azelaic acid (25 mM) alone upregulated Gli1 and Gli2 expression in the bulge cells and 100 µ minoxidil caused Gli1 and Axin2 overexpression in the bulb region of the hair follicle. Moreover, minoxidil (100 µM) alone and in combination with azelaic acid (25 mM) led to Shh protein overexpression in the hair follicles in vitro and in organ culture.

CONCLUSION

Our results indicated a potential role for azelaic acid in the protection of bulge cells from UVB damage and its combination with minoxidil may activate hair growth through overexpression of Shh protein.

Space of Disse: a stem cell niche in the liver

Recent evidence indicates that the plasticity of preexisting hepatocytes and bile duct cells is responsible for the appearance of intermediate progenitor cells capable of restoring liver mass after injury without the need of a stem cell compartment. However, mesenchymal stem cells (MSCs) exist in all organs and are associated with blood vessels which represent their perivascular stem cell niche. MSCs are multipotent and can differentiate into several cell types and are known to support regenerative processes by the release of immunomodulatory and trophic factors. In the liver, the space of Disse constitutes a stem cell niche that harbors stellate cells as liver resident MSCs. This perivascular niche is created by extracellular matrix proteins, sinusoidal endothelial cells, liver parenchymal cells and sympathetic nerve endings and establishes a microenvironment that is suitable to maintain stellate cells and to control their fate. The stem cell niche integrity is important for the behavior of stellate cells in the normal, regenerative, aged and diseased liver. The niche character of the space of Disse may further explain why the liver can become an organ of extra-medullar hematopoiesis and why this organ is frequently prone to tumor metastasis.

Progress in studies of epidermal stem cells and their application in skin tissue engineering

The epidermis, which is the outermost layer of mammalian skin, provides an essential barrier that is essential for maintenance of life. The epidermis is a stratified epithelium, which is maintained by the proliferation of epidermal stem cells (EPSCs) at the basal layer of the epidermis. As a unique cell population characterized by self-renewal and differentiation capabilities, EPSCs ensure the maintenance of adult skin homeostasis and participate in repair of the epidermis after injury. Recently, the utilization of EPSCs for wound healing and tissue regeneration has been attracting increased attention from researchers. In addition, the advances in tissue engineering have increased the interest in applying EPSCs in tissue-engineered scaffolds to further reconstitute injured tissues. In this review, we introduce research developments related to EPSCs, including methods recently used in the culture and enrichment of EPSCs, as well as advanced tools to study EPSCs. The function and mechanism of the EPSC-dermal units in the development and homeostasis of the skin are also summarized. Finally, the potential applications of EPSCs in skin tissue engineering are discussed.

Cell Types Promoting Goosebumps Form a Niche to Regulate Hair Follicle Stem Cells

Piloerection (goosebumps) requires concerted actions of the hair follicle, the arrector pili muscle (APM), and the sympathetic nerve, providing a model to study interactions across epithelium, mesenchyme, and nerves. Here, we show that APMs and sympathetic nerves form a dual-component niche to modulate hair follicle stem cell (HFSC) activity. Sympathetic nerves form synapse-like structures with HFSCs and regulate HFSCs through norepinephrine, whereas APMs maintain sympathetic innervation to HFSCs. Without norepinephrine signaling, HFSCs enter deep quiescence by down-regulating the cell cycle and metabolism while up-regulating quiescence regulators Foxp1 and Fgf18. During development, HFSC progeny secretes Sonic Hedgehog (SHH) to direct the formation of this APM-sympathetic nerve niche, which in turn controls hair follicle regeneration in adults. Our results reveal a reciprocal interdependence between a regenerative tissue and its niche at different stages and demonstrate sympathetic nerves can modulate stem cells through synapse-like connections and neurotransmitters to couple tissue production with demands.

Niches for Skeletal Stem Cells of Mesenchymal Origin

With very few exceptions, all adult tissues in mammals are maintained and can be renewed by stem cells that self-renew and generate the committed progeny required. These functions are regulated by a specific and in many ways unique microenvironment in stem cell niches. In most cases disruption of an adult stem cell niche leads to depletion of stem cells, followed by impairment of the ability of the tissue in question to maintain its functions. The presence of stem cells, often referred to as mesenchymal stem cells (MSCs) or multipotent bone marrow stromal cells (BMSCs), in the adult skeleton has long been realized. In recent years there has been exceptional progress in identifying and characterizing BMSCs in terms of their capacity to generate specific types of skeletal cells in vivo. Such BMSCs are often referred to as skeletal stem cells (SSCs) or skeletal stem and progenitor cells (SSPCs), with the latter term being used throughout this review. SSPCs have been detected in the bone marrow, periosteum, and growth plate and characterized in vivo on the basis of various genetic markers (i.e., Nestin, Leptin receptor, Gremlin1, Cathepsin-K, etc.). However, the niches in which these cells reside have received less attention. Here, we summarize the current scientific literature on stem cell niches for the SSPCs identified so far and discuss potential factors and environmental cues of importance in these niches in vivo. In this context we focus on (i) articular cartilage, (ii) growth plate cartilage, (iii) periosteum, (iv) the adult endosteal compartment, and (v) the developing endosteal compartment, in that order.

Merkel Cell Polyomavirus and Merkel Cell Carcinoma

Viruses are the cause of approximately 15% of all human cancers. Both RNA and DNA human tumor viruses have been identified, with Merkel cell polyomavirus being the most recent one to be linked to cancer. This virus is associated with about 80% of Merkel cell carcinomas, a rare, but aggressive cutaneous malignancy. Despite its name, the cells of origin of this tumor may not be Merkel cells. This review provides an update on the structure and life cycle, cell tropism and epidemiology of the virus and its oncogenic properties. Putative strategies to prevent viral infection or treat virus-positive Merkel cell carcinoma patients are discussed.

A Light-Inducible Hedgehog Signaling Activator Modulates Proliferation and Differentiation of Neural Cells

The Hedgehog signaling pathway shapes our body by regulating the proliferation and differentiation of cells. The spatial and temporal distribution pattern of its ligands finely controls the activity of the Hedgehog pathway during development. To model the control of Hedgehog signaling activities in vitro, we developed a light-inducible Hedgehog signaling activator 6-nitroveratryloxy-carbonyl Smoothened agonist (NVOC-SAG). NVOC-SAG controls the proliferation of mouse cerebellar granule neuron precursor cells and ventral and neural differentiation of human iPS cells in a light dependent manner. The compound provides a new method to control Hedgehog signaling activities.

Peripheral Nerve Single-Cell Analysis Identifies Mesenchymal Ligands that Promote Axonal Growth

Peripheral nerves provide a supportive growth environment for developing and regenerating axons and are essential for maintenance and repair of many non-neural tissues. This capacity has largely been ascribed to paracrine factors secreted by nerve-resident Schwann cells. Here, we used single-cell transcriptional profiling to identify ligands made by different injured rodent nerve cell types and have combined this with cell-surface mass spectrometry to computationally model potential paracrine interactions with peripheral neurons. These analyses show that peripheral nerves make many ligands predicted to act on peripheral and CNS neurons, including known and previously uncharacterized ligands. While Schwann cells are an important ligand source within injured nerves, more than half of the predicted ligands are made by nerve-resident mesenchymal cells, including the endoneurial cells most closely associated with peripheral axons. At least three of these mesenchymal ligands, ANGPT1, CCL11, and VEGFC, promote growth when locally applied on sympathetic axons. These data therefore identify an unexpected paracrine role for nerve mesenchymal cells and suggest that multiple cell types contribute to creating a highly pro-growth environment for peripheral axons.

Innervation: the missing link for biofabricated tissues and organs

Innervation plays a pivotal role as a driver of tissue and organ development as well as a means for their functional control and modulation. Therefore, innervation should be carefully considered throughout the process of biofabrication of engineered tissues and organs. Unfortunately, innervation has generally been overlooked in most non-neural tissue engineering applications, in part due to the intrinsic complexity of building organs containing heterogeneous native cell types and structures. To achieve proper innervation of engineered tissues and organs, specific host axon populations typically need to be precisely driven to appropriate location(s) within the construct, often over long distances. As such, neural tissue engineering and/or axon guidance strategies should be a necessary adjunct to most organogenesis endeavors across multiple tissue and organ systems. To address this challenge, our team is actively building axon-based "living scaffolds" that may physically wire in during organ development in bioreactors and/or serve as a substrate to effectively drive targeted long-distance growth and integration of host axons after implantation. This article reviews the neuroanatomy and the role of innervation in the functional regulation of cardiac, skeletal, and smooth muscle tissue and highlights potential strategies to promote innervation of biofabricated engineered muscles, as well as the use of "living scaffolds" in this endeavor for both in vitro and in vivo applications. We assert that innervation should be included as a necessary component for tissue and organ biofabrication, and that strategies to orchestrate host axonal integration are advantageous to ensure proper function, tolerance, assimilation, and bio-regulation with the recipient post-implant.

Single-cell profiling reveals an endothelium-mediated immunomodulatory pathway in the eye choroid

The activity and survival of retinal photoreceptors depend on support functions performed by the retinal pigment epithelium (RPE) and on oxygen and nutrients delivered by blood vessels in the underlying choroid. By combining single-cell and bulk RNA sequencing, we categorized mouse RPE/choroid cell types and characterized the tissue-specific transcriptomic features of choroidal endothelial cells. We found that choroidal endothelium adjacent to the RPE expresses high levels of Indian Hedgehog and identified its downstream target as stromal GLI1+ mesenchymal stem cell-like cells. In vivo genetic impairment of Hedgehog signaling induced significant loss of choroidal mast cells, as well as an altered inflammatory response and exacerbated visual function defects after retinal damage. Our studies reveal the cellular and molecular landscape of adult RPE/choroid and uncover a Hedgehog-regulated choroidal immunomodulatory signaling circuit. These results open new avenues for the study and treatment of retinal vascular diseases and choroid-related inflammatory blinding disorders.

Immune modulation of hair follicle regeneration

The mammalian hair follicle undergoes repeated bouts of regeneration orchestrated by a variety of hair follicle stem cells. The last decade has witnessed the emergence of the immune niche as a key regulator of stem cell behavior and hair follicle regeneration. Hair follicles chemotactically attract macrophages and T cells so that they are in range to regulate epithelial stem cell quiescence, proliferation and differentiation during physiologic and injured states. Disruption of this dynamic relationship leads to clinically significant forms of hair loss including scarring and non-scarring alopecias. In this review, we summarize key concepts behind immune-mediated hair regeneration, highlight gaps in the literature and discuss the therapeutic potential of exploiting this relationship for treating various immune-mediated alopecias.

Sensory nerve-deficient microenvironment impairs tooth homeostasis by inducing apoptosis of dental pulp stem cells

OBJECTIVES

The aim of this study is to investigate the role of sensory nerve in tooth homeostasis and its effect on mesenchymal stromal/stem cells (MSCs) in dental pulp.

MATERIALS AND METHODS

We established the rat denervated incisor models to identify the morphological and histological changes of tooth. The groups were as follows: IANx (inferior alveolar nerve section), SCGx (superior cervical ganglion removal), IANx + SCGx and Sham group. The biological behaviour of dental pulp stromal/stem cells (DPSCs) was evaluated. Finally, we applied activin B to DPSCs from sensory nerve-deficient microenvironment to analyse the changes of proliferation and apoptosis.

RESULTS

Incisor of IANx and IANx + SCGx groups exhibited obvious disorganized tooth structure, while SCGx group only showed slight decrease of dentin thickness, implying sensory nerve, not sympathetic nerve, contributes to the tooth homeostasis. Moreover, we found sensory nerve injury led to disfunction of DPSCs via activin B/SMAD2/3 signalling in vitro. Supplementing activin B promoted proliferation and reduced apoptosis of DPSCs in sensory nerve-deficient microenvironment.

CONCLUSIONS

This research first demonstrates that sensory nerve-deficient microenvironment impairs tooth haemostasis by inducing apoptosis of DPSCs via activin B/SMAD2/3 signalling. Our study provides the evidence for the crucial role of sensory nerve in tooth homeostasis.

Coordinated hedgehog signaling induces new hair follicles in adult skin

Hair follicle (HF) development is orchestrated by coordinated signals from adjacent epithelial and mesenchymal cells. In humans this process only occurs during embryogenesis and viable strategies to induce new HFs in adult skin are lacking. Here, we reveal that activation of Hedgehog (Hh) signaling in adjacent epithelial and stromal cells induces new HFs in adult, unwounded dorsal mouse skin. Formation of de novo HFs recapitulated embryonic HF development, and mature follicles produced hair co-occurring with epithelial tumors. In contrast, Hh-pathway activation in epithelial or stromal cells alone resulted in tumor formation or stromal cell condensation respectively, without induction of new HFs. Provocatively, adjacent epithelial-stromal Hh-pathway activation induced de novo HFs also in hairless paw skin, divorced from confounding effects of pre-existing niche signals in haired skin. Altogether, cell-type-specific modulation of a single pathway is sufficient to reactivate embryonic programs in adult tissues, thereby inducing complex epithelial structures even without wounding.

We are born with all the hair follicles that we will ever have in our life. These structures are maintained by different types of cells (such as keratinocytes and fibroblasts) that work together to create hair. Follicles form in the embryo thanks to complex molecular signals, which include a molecular cascade known as the Hedgehog signaling pathway.

After birth however, these molecular signals are shut down to avoid conflicting messages – inappropriate activation of Hedgehog signaling in adult skin, for instance, leads to tumors. This means that our skin loses the ability to make new hair follicles, and if skin is severely damaged it cannot regrow hair or produce the associated sebaceous glands that keep skin moisturized.

Being able to create new hair follicles in adult skin would be both functionally and aesthetically beneficial for patients in need, for example, burn victims. Overall, it would also help to understand if and how it is possible to reactivate developmental programs after birth.

To investigate this question, Sun, Are et al. triggered Hedgehog signaling in the skin cells of genetically modified mice; this was done either in keratinocytes, in fibroblasts, or in both types of cells. The experiments showed that Hedgehog signaling could produce new hair follicles, but only when activated in keratinocytes and fibroblasts together. The process took several weeks, mirrored normal hair follicle development and resulted in new hair shafts. The follicles grew on both the back of mice, where hair normally occurs, and even in paw areas that are usually hairless.

Not unexpectedly the new hair follicles were accompanied with skin tumors. But, promisingly, treatment with Hedgehog-pathway inhibitor Vismodegib restricted tumor growth while keeping the new follicles intact. This suggests that future work on improving “when and where” Hedgehog signaling is activated may allow the formation of new follicles in adult skin with fewer adverse effects.

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.

The Neural Regulation of Cancer

The nervous system is intimately involved in physiological processes from development and growth to tissue homeostasis and repair throughout the body. It logically follows that the nervous system has the potential to play analogous roles in the context of cancer. Progress toward understanding the crucial role of the nervous system in cancer has accelerated in recent years, but much remains to be learned. Here, we highlight rapidly evolving concepts in this burgeoning research space and consider next steps toward understanding and therapeutically targeting the neural regulation of cancer.

Sonic Hedgehog Signaling and Tooth Development

Sonic hedgehog (Shh) is a secreted protein with important roles in mammalian embryogenesis. During tooth development, Shh is primarily expressed in the dental epithelium, from initiation to the root formation stages. A number of studies have analyzed the function of Shh signaling at different stages of tooth development and have revealed that Shh signaling regulates the formation of various tooth components, including enamel, dentin, cementum, and other soft tissues. In addition, dental mesenchymal cells positive for Gli1, a downstream transcription factor of Shh signaling, have been found to have stem cell properties, including multipotency and the ability to self-renew. Indeed, Gli1-positive cells in mature teeth appear to contribute to the regeneration of dental pulp and periodontal tissues. In this review, we provide an overview of recent advances related to the role of Shh signaling in tooth development, as well as the contribution of this pathway to tooth homeostasis and regeneration.

The Effects of Essential Oils and Terpenes in Relation to Their Routes of Intake and Application

Essential oils have been used in multiple ways, i.e., inhaling, topically applying on the skin, and drinking. Thus, there are three major routes of intake or application involved: the olfactory system, the skin, and the gastro-intestinal system. Understanding these routes is important for clarifying the mechanisms of action of essential oils. Here we summarize the three systems involved, and the effects of essential oils and their constituents at the cellular and systems level. Many factors affect the rate of uptake of each chemical constituent included in essential oils. It is important to determine how much of each constituent is included in an essential oil and to use single chemical compounds to precisely test their effects. Studies have shown synergistic influences of the constituents, which affect the mechanisms of action of the essential oil constituents. For the skin and digestive system, the chemical components of essential oils can directly activate gamma aminobutyric acid (GABA) receptors and transient receptor potential channels (TRP) channels, whereas in the olfactory system, chemical components activate olfactory receptors. Here, GABA receptors and TRP channels could play a role, mostly when the signals are transferred to the olfactory bulb and the brain.

The aging skin microenvironment dictates stem cell behavior

Aging manifests with architectural alteration and functional decline of multiple organs throughout an organism. In mammals, aged skin is accompanied by a marked reduction in hair cycling and appearance of bald patches, leading researchers to propose that hair follicle stem cells (HFSCs) are either lost, differentiate, or change to an epidermal fate during aging. Here, we employed single-cell RNA-sequencing to interrogate aging-related changes in the HFSCs. Surprisingly, although numbers declined, aging HFSCs were present, maintained their identity, and showed no overt signs of shifting to an epidermal fate. However, they did exhibit prevalent transcriptional changes particularly in extracellular matrix genes, and this was accompanied by profound structural perturbations in the aging SC niche. Moreover, marked age-related changes occurred in many nonepithelial cell types, including resident immune cells, sensory neurons, and arrector pili muscles. Each of these SC niche components has been shown to influence HF regeneration. When we performed skin injuries that are known to mobilize young HFSCs to exit their niche and regenerate HFs, we discovered that aged skin is defective at doing so. Interestingly, however, in transplantation assays in vivo, aged HFSCs regenerated HFs when supported with young dermis, while young HFSCs failed to regenerate HFs when combined with aged dermis. Together, our findings highlight the importance of SC:niche interactions and favor a model where youthfulness of the niche microenvironment plays a dominant role in dictating the properties of its SCs and tissue health and fitness.

EGFL6 expression in hair follicle central isthmus is dependent on the presence of terminal Schwann cells

Hair follicle central isthmus is surrounded by dense nerve endings and terminal Schwann cells (TSCs), forming a specialized sensory structure called lanceolate complexes. Extracellular matrix protein EGFL6 expressed from epidermis has been found closely associated with lanceolate complexes and important for proper alignment of nerve fibres and TSCs processes, and for proper response to light touch. However, how EGFL6 itself is specifically induced/deposited/maintained at the central isthmus remains to be elucidated. Previous reports and our results showed that nerve endings and TSCs docking at the central isthmus during hair follicle development occur before the specific depositing of EGFL6 protein. Furthermore, we found nude mice rarely maintain the lanceolate complex, and EGFL6 is lost in their aberrant hair follicle. Instead, reconstituted hair follicle in nude mice by stem cells chamber grafting assay expresses EGFL6 at the central isthmus area after hair follicle innervation. At last, long-term but not short-term cutaneous denervation leads to degeneration of TSCs and loss of EGFL6 expression. Together, our results demonstrate that EGFL6 expression in the central isthmus is dependent on the presence of TSCs, proposing that the interplay of epidermis and neuronal components is important for maintaining functional structure of lanceolate complexes.

Neuroendocrinology and neurobiology of sebaceous glands

The nervous system communicates with peripheral tissues through nerve fibres and the systemic release of hypothalamic and pituitary neurohormones. Communication between the nervous system and the largest human organ, skin, has traditionally received little attention. In particular, the neuro-regulation of sebaceous glands (SGs), a major skin appendage, is rarely considered. Yet, it is clear that the SG is under stringent pituitary control, and forms a fascinating, clinically relevant peripheral target organ in which to study the neuroendocrine and neural regulation of epithelia. Sebum, the major secretory product of the SG, is composed of a complex mixture of lipids resulting from the holocrine secretion of specialised epithelial cells (sebocytes). It is indicative of a role of the neuroendocrine system in SG function that excess circulating levels of growth hormone, thyroxine or prolactin result in increased sebum production (seborrhoea). Conversely, growth hormone deficiency, hypothyroidism, and adrenal insufficiency result in reduced sebum production and dry skin. Furthermore, the androgen sensitivity of SGs appears to be under neuroendocrine control, as hypophysectomy (removal of the pituitary) renders SGs largely insensitive to stimulation by testosterone, which is crucial for maintaining SG homeostasis. However, several neurohormones, such as adrenocorticotropic hormone and α-melanocyte-stimulating hormone, can stimulate sebum production independently of either the testes or the adrenal glands, further underscoring the importance of neuroendocrine control in SG biology. Moreover, sebocytes synthesise several neurohormones and express their receptors, suggestive of the presence of neuro-autocrine mechanisms of sebocyte modulation. Aside from the neuroendocrine system, it is conceivable that secretion of neuropeptides and neurotransmitters from cutaneous nerve endings may also act on sebocytes or their progenitors, given that the skin is richly innervated. However, to date, the neural controls of SG development and function remain poorly investigated and incompletely understood. Botulinum toxin-mediated or facial paresis-associated reduction of human sebum secretion suggests that cutaneous nerve-derived substances modulate lipid and inflammatory cytokine synthesis by sebocytes, possibly implicating the nervous system in acne pathogenesis. Additionally, evidence suggests that cutaneous denervation in mice alters the expression of key regulators of SG homeostasis. In this review, we examine the current evidence regarding neuroendocrine and neurobiological regulation of human SG function in physiology and pathology. We further call attention to this line of research as an instructive model for probing and therapeutically manipulating the mechanistic links between the nervous system and mammalian skin.

Tumour innervation and neurosignalling in prostate cancer

Prostate cancer progression has been shown to be dependent on the development of autonomic nerves into the tumour microenvironment. Sympathetic nerves activate adrenergic neurosignalling that is necessary in early stages of tumour progression and for initiating an angiogenic switch, whereas parasympathetic nerves activate cholinergic neurosignalling resulting in tumour dissemination and metastasis. The innervation of prostate cancer seems to be initiated by neurotrophic growth factors, such as the precursor to nerve growth factor secreted by tumour cells, and the contribution of brain-derived neural progenitor cells has also been reported. Current experimental, epidemiological and clinical evidence shows the stimulatory effect of tumour innervation and neurosignalling in prostate cancer. Using nerves and neurosignalling could have value in the management of prostate cancer by predicting aggressive disease, treating localized disease through denervation and relieving cancer-associated pain in bone metastases.

Sonic hedgehog signaling in epithelial tissue development

The Sonic hedgehog (SHH) signaling pathway is essential for embryonic development and tissue regeneration. The dysfunction of SHH pathway is involved in a variety of diseases, including cancer, birth defects, and other diseases. Here we reviewed recent studies on main molecules involved in the SHH signaling pathway, specifically focused on their function in epithelial tissue and appendages development, including epidermis, touch dome, hair, sebaceous gland, mammary gland, tooth, nail, gastric epithelium, and intestinal epithelium. The advance in understanding the SHH signaling pathway will give us more clues to the mechanisms of tissue repair and regeneration, as well as the development of new treatment for diseases related to dysregulation of SHH signaling pathway.

Beta-caryophyllene enhances wound healing through multiple routes

Beta-caryophyllene is an odoriferous bicyclic sesquiterpene found in various herbs and spices. Recently, it was found that beta-caryophyllene is a ligand of the cannabinoid receptor 2 (CB2). Activation of CB2 will decrease pain, a major signal for inflammatory responses. We hypothesized that beta-caryophyllene can affect wound healing by decreasing inflammation. Here we show that cutaneous wounds of mice treated with beta-caryophyllene had enhanced re-epithelialization. The treated tissue showed increased cell proliferation and cells treated with beta-caryophyllene showed enhanced cell migration, suggesting that the higher re-epithelialization is due to enhanced cell proliferation and cell migration. The treated tissues also had up-regulated gene expression for hair follicle bulge stem cells. Olfactory receptors were not involved in the enhanced wound healing. Transient Receptor Potential channel genes were up-regulated in the injured skin exposed to beta-caryophyllene. Interestingly, there were sex differences in the impact of beta- caryophyllene as only the injured skin of female mice had enhanced re-epithelialization after exposure to beta-caryophyllene. Our study suggests that chemical compounds included in essential oils have the capability to improve wound healing, an effect generated by synergetic impacts of multiple pathways.

Fracture repair requires TrkA signaling by skeletal sensory nerves

Bone is richly innervated by nerve growth factor-responsive (NGF-responsive) tropomyosin receptor kinase A-expressing (TrKa-expressing) sensory nerve fibers, which are required for osteochondral progenitor expansion during mammalian skeletal development. Aside from pain sensation, little is known regarding the role of sensory innervation in bone repair. Here, we characterized the reinnervation of tissue following experimental ulnar stress fracture and assessed the impact of loss of TrkA signaling in this process. Sequential histological data obtained in reporter mice subjected to fracture demonstrated a marked upregulation of NGF expression in periosteal stromal progenitors and fracture-associated macrophages. Sprouting and arborization of CGRP+TrkA+ sensory nerve fibers within the reactive periosteum in NGF-enriched cellular domains were evident at time points preceding periosteal vascularization, ossification, and mineralization. Temporal inhibition of TrkA catalytic activity by administration of 1NMPP1 to TrkAF592A mice significantly reduced the numbers of sensory fibers, blunted revascularization, and delayed ossification of the fracture callus. We observed similar deficiencies in nerve regrowth and fracture healing in a mouse model of peripheral neuropathy induced by paclitaxel treatment. Together, our studies demonstrate an essential role of TrkA signaling for stress fracture repair and implicate skeletal sensory nerves as an important upstream mediator of this repair process.

Hes1 regulates anagen initiation and hair follicle regeneration through modulation of hedgehog signaling

Adult hair follicles undergo repeated cycling of regression (catagen), resting (telogen), and growth (anagen), which is maintained by hair follicle stem cells (HFSCs). The mechanism underlying hair growth initiation and HFSC maintenance is not fully understood. Here, by epithelial deletion of Hes1, a major Notch downstream transcriptional repressor, we found that hair growth is retarded, but the hair cycle progresses normally. Hes1 is specifically upregulated in the lower bulge/HG during anagen initiation. Accordingly, loss of Hes1 results in delayed activation of the secondary hair germ (HG) and shortened anagen phase. This developmental delay causes reduced hair shaft length but not identity changes in follicular lineages. Remarkably, Hes1 ablation results in impaired hair regeneration upon repetitive depilation. Microarray gene profiling on HFSCs indicates that Hes1 modulates Shh responsiveness in anagen initiation. Using primary keratinocyte cultures, we demonstrated that Hes1 deletion negatively influences ciliogenesis and Smoothened ciliary accumulation upon Shh treatment. Furthermore, transient application of Smoothened agonist during repetitive depilation can rescue anagen initiation and HFSC self-renewal in Hes1-deficient hair follicles. We reveal a critical function of Hes1 in potentiating Shh signaling in anagen initiation, which allows sufficient signaling strength to expand the HG and replenish HFSCs to maintain the hair cycle homeostasis.

Polyomavirus-Positive Merkel Cell Carcinoma Derived from a Trichoblastoma Suggests an Epithelial Origin of this Merkel Cell Carcinoma

Merkel cell carcinoma (MCC), an aggressive neuroendocrine carcinoma of the skin, is to date the only human cancer known to be frequently caused by a polyomavirus. However, it is a matter of debate which cells are targeted by the Merkel cell polyomavirus (MCPyV) to give rise to the phenotypically multifaceted MCC cells. To assess the lineage of origin of MCPyV-positive MCC, genetic analysis of a very rare tumor combining benign trichoblastoma and MCPyV-positive MCC was conducted by massive parallel sequencing. Although MCPyV was found to be integrated only in the MCC part, six somatic mutations were shared by both tumor components. The mutational overlap between the trichoblastoma and MCPyV-positive MCC parts of the combined tumor implies that MCPyV integration occurred in an epithelial tumor cell before MCC development. Therefore, our report demonstrates that MCPyV-positive MCC can derive from the epithelial lineage.

Lymphatic vessels interact dynamically with the hair follicle stem cell niche during skin regeneration in vivo

Lymphatic vessels are essential for skin fluid homeostasis and immune cell trafficking. Whether the lymphatic vasculature is associated with hair follicle regeneration is, however, unknown. Here, using steady and live imaging approaches in mouse skin, we show that lymphatic vessels distribute to the anterior permanent region of individual hair follicles, starting from development through all cycle stages and interconnecting neighboring follicles at the bulge level, in a stem cell-dependent manner. Lymphatic vessels further connect hair follicles in triads and dynamically flow across the skin. At the onset of the physiological stem cell activation, or upon pharmacological or genetic induction of hair follicle growth, lymphatic vessels transiently expand their caliber suggesting an increased tissue drainage capacity. Interestingly, the physiological caliber increase is associated with a distinct gene expression correlated with lymphatic vessel reorganization. Using mouse genetics, we show that lymphatic vessel depletion blocks hair follicle growth. Our findings point toward the lymphatic vasculature being important for hair follicle development, cycling, and organization, and define lymphatic vessels as stem cell niche components, coordinating connections at tissue-level, thus provide insight into their functional contribution to skin regeneration.

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.

Mice lacking the epidermal retinol dehydrogenases SDR16C5 and SDR16C6 display accelerated hair growth and enlarged meibomian glands

Retinol dehydrogenases catalyze the rate-limiting step in the biosynthesis of retinoic acid, a bioactive lipid molecule that regulates the expression of hundreds of genes by binding to nuclear transcription factors, the retinoic acid receptors. Several enzymes exhibit retinol dehydrogenase activities in vitro; however, their physiological relevance for retinoic acid biosynthesis in vivo remains unclear. Here, we present evidence that two murine epidermal retinol dehydrogenases, short-chain dehydrogenase/reductase family 16C member 5 (SDR16C5) and SDR16C6, contribute to retinoic acid biosynthesis in living cells and are also essential for the oxidation of retinol to retinaldehyde in vivo Mice with targeted knockout of the more catalytically active SDR16C6 enzyme have no obvious phenotype, possibly due to functional redundancy, because Sdr16c5 and Sdr16c6 exhibit an overlapping expression pattern during later developmental stages and in adulthood. Mice that lack both enzymes are viable and fertile but display accelerated hair growth after shaving and also enlarged meibomian glands, consistent with a nearly 80% reduction in the retinol dehydrogenase activities of skin membrane fractions from the Sdr16c5/Sdr16c6 double-knockout mice. The up-regulation of hair-follicle stem cell genes is consistent with reduced retinoic acid signaling in the skin of the double-knockout mice. These results indicate that the retinol dehydrogenase activities of murine SDR16C5 and SDR16C6 enzymes are not critical for survival but are responsible for most of the retinol dehydrogenase activity in skin, essential for the regulation of the hair-follicle cycle, and required for the maintenance of both sebaceous and meibomian glands.

Role of Muscarinic Acetylcholine Signaling in Gastrointestinal Cancers

In the tumor microenvironment, various stromal and immune cells accumulate and interact with cancer cells to contribute to tumor progression. Among stromal players, nerves have recently been recognized as key regulators of tumor growth. More neurotransmitters, such as catecholamines and acetylcholine (ACh), are present in tumors, as the cells that secrete neurotransmitters accumulate by the release of neurotrophic factors from cancer cells. In this short review, we focus on the role of nerve signaling in gastrointestinal (GI) cancers. Given that muscarinic acetylcholine receptor signaling seems to be a dominant regulator of GI stem cells and cancers, we review the function and mechanism of the muscarinic ACh pathway as a regulator of GI cancer progression. Accumulating evidence suggests that ACh, which is secreted from nerves and tuft cells, stimulates GI epithelial stem cells and contributes to cancer progression via muscarinic receptors.

Epidermal stem cells in wound healing and their clinical applications

The skin has important barrier, sensory, and immune functions, contributing to the health and integrity of the organism. Extensive skin injuries that threaten the entire organism require immediate and effective treatment. Wound healing is a natural response, but in severe conditions, such as burns and diabetes, this process is insufficient to achieve effective treatment. Epidermal stem cells (EPSCs) are a multipotent cell type and are committed to the formation and differentiation of the functional epidermis. As the contributions of EPSCs in wound healing and tissue regeneration have been increasingly attracting the attention of researchers, a rising number of therapies based on EPSCs are currently under development. In this paper, we review the characteristics of EPSCs and the mechanisms underlying their functions during wound healing. Applications of EPSCs are also discussed to determine the potential and feasibility of using EPSCs clinically in wound healing.

CRISPR/Cas9-mediated VDR knockout plays an essential role in the growth of dermal papilla cells through enhanced relative genes

Background

Hair follicles in cashmere goats are divided into primary and secondary hair follicles (HFs). HF development, which determines the morphological structure, is regulated by a large number of vital genes; however, the key functional genes and their interaction networks are still unclear. Although the vitamin D receptor (VDR) is related to cashmere goat HF formation, its precise effects are largely unknown. In the present study, we verified the functions of key genes identified in previous studies using hair dermal papilla (DP) cells as an experimental model. Furthermore, we used CRISPR/Cas9 technology to modify the VDR in DP cells to dissect the molecular mechanism underlying HF formation in cashmere goats.

Results

The VDR expression levels in nine tissues of Shaanbei white cashmere goats differed significantly between embryonic day 60 (E60) and embryonic day 120 (E120). At E120, VDR expression was highest in the skin. At the newborn and E120 stages, the VDR protein was highly expressed in the root sheath and hair ball region of Shaanbei cashmere goats. We cloned the complete CDS of VDR in the Shaanbei white cashmere goat and constructed a VDR-deficient DP cell model by CRISPR/Cas9. Heterozygous and homozygous mutant DP cells were produced. The growth rate of mutant DP cells was significantly lower than that of wild-type DP cells (P < 0.05) and VDR mRNA levels in DP cells decreased significantly after VDR knockdown (P < 0.05). Further, the expression levels of VGF, Noggin, Lef1, and β-catenin were significantly downregulated (P < 0.05).

Conclusions

Our results indicated that VDR has a vital role in DP cells, and that its effects are mediated by Wnt and BMP4 signaling.

Schwann cell precursors contribute to skeletal formation during embryonic development in mice and zebrafish

Multipotent Schwann cell precursors (SCPs) generate numerous cell types. Here, in both mouse and zebrafish, SCPs contributed to the generation of mesenchymal, chondroprogenitor, and osteoprogenitor cells during embryonic development. These findings reveal a source of cartilage and bone cells and previously unanticipated interactions between the nervous system and skeleton during development.

Immature multipotent embryonic peripheral glial cells, the Schwann cell precursors (SCPs), differentiate into melanocytes, parasympathetic neurons, chromaffin cells, and dental mesenchymal populations. Here, genetic lineage tracing revealed that, during murine embryonic development, some SCPs detach from nerve fibers to become mesenchymal cells, which differentiate further into chondrocytes and mature osteocytes. This occurred only during embryonic development, producing numerous craniofacial and trunk skeletal elements, without contributing to development of the appendicular skeleton. Formation of chondrocytes from SCPs also occurred in zebrafish, indicating evolutionary conservation. Our findings reveal multipotency of SCPs, providing a developmental link between the nervous system and skeleton.

Hair-follicle-associated pluripotent stem cells derived from cryopreserved intact human hair follicles sustain multilineage differentiation potential

The bulge area of the hair follicle contains hair-follicle-associated pluripotent (HAP) stem cells. Here, we present effective cryopreservation procedures of the human hair follicle that preserve the differentiation potential of HAP stem cells. Whole hair follicles isolated from human scalp were cryopreserved by a slow-rate cooling medium and stored in liquid nitrogen. A careful thawing method was used to collect the upper parts of the human hair follicles which were cultured for four weeks in a Dulbecco’s Modified Eagle’s Medium with fetal bovine serum (FBS). Proliferating hair follicle cells were then shifted to DMEM/Ham’s Nutrient Mixture F-12 medium without FBS and allowed to grow for one week. These proliferating cells were able to produce HAP stem cell colonies with multilineage differentiation capacity. They produced keratinocytes, smooth muscle cells, cardiac muscle cells, neurons and glial cells. Interestingly, these cryopreserved hair follicles produced pluripotent HAP stem cell colonies similar to fresh follicles. These findings suggest that the cryopreserved whole human hair follicle preserves the ability to produce HAP stem cells, which will enable any individual to preserve a bank of these stem cells for personalized regenerative medicine.

Isolation and Characterization of Cutaneous Epithelial Stem Cells

The outer layer of mammalian skin is a multilayered epithelium that perpetually renews multiple differentiated lineages. During homeostasis, the maintenance of skin epithelial turnover is ensured by regionalized populations of stem cells that largely remain dedicated to distinct epithelial lineages including squamous, follicular, sebaceous, Merkel, and sweat glands. Cutting edge developments in this field have focused on: (1) stem cell activation cues derived from a number of extrinsic sources including neurons, dermal fibroblasts and adipocyte, and immune cells; and (2) characterization of epithelial stem cell homeostasis via hierarchical versus stochastic paradigms. The techniques outlined in this chapter are designed to facilitate such studies and describe basic procedures for cutaneous stem cell isolation and purification, which are based on leveraging their unique expression of surface proteins for simultaneous targeting and purifying of multiple subpopulations in adult skin. In addition, protocols for assessment of in vitro and ex vivo progenitor capacity as well as techniques to visualize progenitor populations in whole skin are discussed.

Functional anatomy of the hair follicle: The Secondary Hair Germ

The secondary hair germ (SHG)-a transitory structure in the lower portion of the mouse telogen hair follicle (HF)-is directly involved in anagen induction and eventual HF regrowth. Some crucial aspects of SHG functioning and ontogenetic relations with other HF parts, however, remain undefined. According to recent evidence (in contrast to previous bulge-centric views), the SHG is the primary target of anagen-inducing signalling and a source of both the outer root sheath (ORS) and ascending HF layers during the initial (morphogenetic) anagen subphase. The SHG is comprised of two functionally distinct cell populations. Its lower portion (originating from lower HF cells that survived catagen) forms all ascending HF layers, while the upper SHG (formed by bulge-derived cells) builds up the ORS. The predetermination of SHG cells to a specific morphogenetic fate contradicts their attribution to the "stem cell" category and supports SHG designation as a "germinative" or a "founder" cell population. The mechanisms of this predetermination driving transition of the SHG from "refractory" to the "competent" state during the telogen remain unknown. Functionally, the SHG serves as a barrier, protecting the quiescent bulge stem cell niche from the extensive follicular papilla/SHG signalling milieu. The formation of the SHG is a prerequisite for efficient "precommitment" of these cells and provides for easier sensing and a faster response to anagen-inducing signals. In general, the formation of the SHG is an evolutionary adaptation, which allowed the ancestors of modern Muridae to acquire a specific, highly synchronized pattern of hair cycling.

The Clinical Trials of Mesenchymal Stem Cell Therapy in Skin Diseases: An Update and Concise Review

The skin is one of the crucial body organs with anatomy and physiology linked to various disorders including congenital and acquired diseases. Nowadays, mesenchymal stem cell (MSCs)- based therapy has appeared as a promising therapeutic field, in which many see opportunities to cure the costliest and incurable diseases. However, one question to be asked is that if the use of MSCs in clinical trials studies and diseases treatment has improved. In this study, the clinical trials using MSCs in skin diseases were reviewed. A remarkable number of clinical trial studies are in progress in this field; however, only a few of them have led to tangible benefits for patients. The relevant papers and ongoing clinical trials that address MSC's therapeutic goals for various skin disorders were examined. This review can be very useful for both the dermatologists and basic skin researchers interested in contributing to stem cell-based therapeutic researches in the area of skin disorders.

CD34 defines melanocyte stem cell subpopulations with distinct regenerative properties

Melanocyte stem cells (McSCs) are the undifferentiated melanocytic cells of the mammalian hair follicle (HF) responsible for recurrent generation of a large number of differentiated melanocytes during each HF cycle. HF McSCs reside in both the CD34+ bulge/lower permanent portion (LPP) and the CD34- secondary hair germ (SHG) regions of the HF during telogen. Using Dct-H2BGFP mice, we separate bulge/LPP and SHG McSCs using FACS with GFP and anti-CD34 to show that these two subsets of McSCs are functionally distinct. Genome-wide expression profiling results support the distinct nature of these populations, with CD34- McSCs exhibiting higher expression of melanocyte differentiation genes and with CD34+ McSCs demonstrating a profile more consistent with a neural crest stem cell. In culture and in vivo, CD34- McSCs regenerate pigmentation more efficiently whereas CD34+ McSCs selectively exhibit the ability to myelinate neurons. CD34+ McSCs, and their counterparts in human skin, may be useful for myelinating neurons in vivo, leading to new therapeutic opportunities for demyelinating diseases and traumatic nerve injury.

Chemotherapy-Induced Hair Loss: The Use of Biomarkers for Predicting Alopecic Severity and Treatment Efficacy

Damage to hair follicles following exposure to toxic chemotherapeutics can cause substantial hair loss, commonly known as chemotherapy-induced alopecia (CIA). Preventive therapies remain limited; however, recent advances in the use of scalp cooling technologies have proved successful in preventing or reducing hair loss in some patients. Further improvements in scalp cooling efficacy and/or development of novel treatments to prevent chemotherapy-induced hair loss are required. To achieve this, post-chemotherapy assessment of hair follicle damage markers, with and without scalp cooling, would provide invaluable mechanistic and prognostic information. At present, the availability of such data is extremely limited. This article describes the potential utility of a combination of biomarkers in assessing drug-induced alopecia and the protective potential of existing or new treatments. A greater understanding of the precise mechanisms of anti-CIA therapies through biomarker analysis would enhance the rationale, use, and development of such treatments.

Tumor Neurobiology and the War of Nerves in Cancer

Nerves are emerging regulators of cancer progression. Cancer cells induce the outgrowth of nerves in the tumor microenvironment through the release of neurotrophic factors, and in return nerves liberate neurotransmitters that activate cancer growth and dissemination. Although sympathetic nerves drive tumor angiogenesis via the liberation of noradrenaline, sensory and parasympathetic nerves stimulate cancer stem cells. Interestingly, recent evidence indicates that parasympathetic nerves can eventually inhibit tumor progression, suggesting a yin-yang type of regulation of cancer by nerves. From a broader perspective, the question of a higher level of control of cancer development by the central nervous system should be raised. SIGNIFICANCE: Nerves are emerging regulators of cancer initiation, progression, and metastasis. Here, we review the evidence to date and explore the basic and clinical ramifications of these findings.

Treg-Cell Control of a CXCL5-IL-17 Inflammatory Axis Promotes Hair-Follicle-Stem-Cell Differentiation During Skin-Barrier Repair

Restoration of barrier-tissue integrity after injury is dependent on the function of immune cells and stem cells (SCs) residing in the tissue. In response to skin injury, hair-follicle stem cells (HFSCs), normally poised for hair generation, are recruited to the site of injury and differentiate into cells that repair damaged epithelium. We used a SC fate-mapping approach to examine the contribution of regulatory T (Treg) cells to epidermal-barrier repair after injury. Depletion of Treg cells impaired skin-barrier regeneration and was associated with a Th17 inflammatory response and failed HFSC differentiation. In this setting, damaged epithelial cells preferentially expressed the neutrophil chemoattractant CXCL5, and blockade of CXCL5 or neutrophil depletion restored barrier function and SC differentiation after epidermal injury. Thus, Treg-cell regulation of localized inflammation enables HFSC differentiation and, thereby, skin-barrier regeneration, with implications for the maintenance and repair of other barrier tissues.

Hedgehog Signaling Regulates Taste Organs and Oral Sensation: Distinctive Roles in the Epithelium, Stroma, and Innervation

The Hedgehog (Hh) pathway has regulatory roles in maintaining and restoring lingual taste organs, the papillae and taste buds, and taste sensation. Taste buds and taste nerve responses are eliminated if Hh signaling is genetically suppressed or pharmacologically inhibited, but regeneration can occur if signaling is reactivated within the lingual epithelium. Whereas Hh pathway disruption alters taste sensation, tactile and cold responses remain intact, indicating that Hh signaling is modality-specific in regulation of tongue sensation. However, although Hh regulation is essential in taste, the basic biology of pathway controls is not fully understood. With recent demonstrations that sonic hedgehog (Shh) is within both taste buds and the innervating ganglion neurons/nerve fibers, it is compelling to consider Hh signaling throughout the tongue and taste organ cell and tissue compartments. Distinctive signaling centers and niches are reviewed in taste papilla epithelium, taste buds, basal lamina, fibroblasts and lamellipodia, lingual nerves, and sensory ganglia. Several new roles for the innervation in lingual Hh signaling are proposed. Hh signaling within the lingual epithelium and an intact innervation each is necessary, but only together are sufficient to sustain and restore taste buds. Importantly, patients who use Hh pathway inhibiting drugs confront an altered chemosensory world with loss of taste buds and taste responses, intact lingual touch and cold sensation, and taste recovery after drug discontinuation.

The discovery and development of topical medicines for wound healing

INTRODUCTION

Chronic, nonhealing skin wounds claim >3% of the health-care budget in industrialized countries, and the incidence is rising. Currently, two parallel trends influence innovations within the field of wound healing: the need to reduce spread of antibiotic resistance and the emerging use of health economy and value-based models. Areas covered: This review focuses on the discovery of drug candidates and development of treatments aiming to enhance wound healing in the heterogeneous group of patients with nonhealing wounds. Expert opinion: Nonhealing wounds are multifaceted and recognized as difficult indications. The majority of products currently in use are medical device dressings, or concepts of negative pressure or hyperbaric oxygen treatment. Global best practice guidelines for the treatment of diabetic foot ulcers recommend debridement, redressing, as well as infection control, and are critical to the lack of coherent clinical evidence for many approved products in active wound care. To accelerate wound healing, there is an emerging trend toward biologics, gene therapy, and novel concepts for drug delivery in research and in the pipeline for clinical trials. Scientific delineation of the therapeutic mechanism of action is, in our opinion, vital for clinical trial success and for an increased fraction of medical products in the pharmaceutical pipeline.

Misactivation of Hedgehog signaling causes inherited and sporadic cancers

The Hedgehog pathway is critical for the development of diverse organs. Misactivation of the Hedgehog pathway can cause developmental abnormalities and cancers, including medulloblastoma, the most common pediatric brain tumor, and basal cell carcinoma, the most common cancer in the United States. Here, we review how basic, translational, and clinical studies of the Hedgehog pathway have helped reveal how cells communicate, how intercellular communication controls development, how signaling goes awry to cause cancer, and how to use targeted molecular agents to treat both inherited and sporadic cancers.

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.