Bone morphogenetic protein signaling inhibits hair follicle anagen induction by restricting epithelial stem/progenitor cell activation and expansion

Thyroid receptor β: A promising target for developing novel anti-androgenetic alopecia drugs

Androgenetic alopecia (AGA) significantly impacts the self-confidence and mental well-being of people. Recent research has revealed that thyroid receptor β (TRβ) agonists can activate hair follicles and effectively stimulate hair growth. This review aims to comprehensively elucidate the specific mechanism of action of TRβ in treating AGA from various perspectives, highlighting its potential as a drug target for combating AGA. Moreover, this review provides a thorough summary of the research advances in TRβ agonist candidates with anti-AGA efficacy and outlines the structure-activity relationships (SARs) of TRβ agonists. We hope that this review will provide practical information for the development of effective anti-alopecia drugs.

Metabolomics reveals metabolites associated with hair follicle cycle in cashmere goats

BACKGROUND

The hair follicle is a skin accessory organ that regulates hair development, and its activity varies on a regular basis. However, the significance of metabolites in the hair follicle cycle has long been unknown.

RESULTS

Targeted metabolomics was used in this investigation to reveal the expression patterns of 1903 metabolites in cashmere goat skin during anagen to telogen. A statistical analysis was used to investigate the potential associations between metabolites and the hair follicle cycle. The findings revealed clear changes in the expression patterns of metabolites at various phases and in various feeding models. The majority of metabolites (primarily amino acids, nucleotides, their metabolites, and lipids) showed downregulated expression from anagen (An) to telogen (Tn), which was associated with gene expression, protein synthesis and transport, and cell structure, which reflected, to some extent, that the cells associated with hair follicle development are active in An and apoptotic in An-Tn. It is worth mentioning that the expression of vitamin D3 and 3,3',5-triiodo-L-thyronine decreased and then increased, which may be related to the shorter and longer duration of outdoor light, which may stimulate the hair follicle to transition from An to catagen (Cn). In the comparison of different hair follicle development stages (An, Cn, and Tn) or feeding modes (grazing and barn feeding), Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that common differentially expressed metabolites (DEMs) (2'-deoxyadenosine, L-valine, 2'-deoxyuridine, riboflavin, cytidine, deoxyguanosine, L-tryptophan, and guanosine-5'-monophosphate) were enriched in ABC transporters. This finding suggested that this pathway may be involved in the hair follicle cycle. Among these DEMs, riboflavin is absorbed from food, and the expression of riboflavin and sugars (D-glucose and glycogen) in skin tissue under grazing was greater and lower than that during barn feeding, respectively, suggesting that eating patterns may also alter the hair follicle cycle.

CONCLUSIONS

The expression patterns of metabolites such as sugars, lipids, amino acids, and nucleotides in skin tissue affect hair follicle growth, in which 2'-deoxyadenosine, L-valine, 2'-deoxyuridine, riboflavin, cytidine, deoxyguanosine, L-tryptophan, and guanosine-5'-monophosphate may regulate the hair follicle cycle by participating in ABC transporters. Feeding practices may regulate hair follicle cycles by influencing the amount of hormones and vitamins expressed in the skin of cashmere goats.

CircERCC6 Positively Regulates the Induced Activation of SHF Stem Cells in Cashmere Goats via the miR-412-3p/BNC2 Axis in an m6A-Dependent Manner

The cashmere, a kind of nature protein fiber, is one of the main use of cashmere goats. The induced activation of secondary hair follicle (SHF) stem cells by the dermal papilla cell-derived signals is a key biological process for the morphogenesis and growth of cashmere fiber in cashmere goats. Previously, the circRNA-ERCC6 (circERCC6) was identified from cashmere goat SHFs; however, its biological significance is unclear in the SHF physiology process of cashmere goats. In this study, we found that circERCC6 exhibited significantly higher expression at anagen SHF bulge compared with the counterpart of telogen and harbored three m6A modified sites (named m6A-685, m6A-862, and m6A-995) through methylation immunoprecipitation using a real-time quantitative polymerase chain reaction (Me-RIP-qPCR) technique. The knockdown experiments of circERCC6 in SHF stem cells showed that circERCC6 positively regulates the induced activation of SHF stem cells in cashmere goats. Through a dual-luciferase reporter assay, we demonstrated that m6A-modified circERCC6 (m6A-circERCC6) sponged miR-412-3p to upregulate the expression of BNC2 mRNA in SHFstem cells. Through m6A-deficient mutant assay in circERCC6 knockdown SHF stem cells, we further showed that m6A modification within circERCC6 is required to mediate the miR-412-3p/BNC2 axis to finally promote the proper induced activation of SHF stem cells in cashmere goats.

Interpretation of the Yak Skin Single-Cell Transcriptome Landscape

The morphogenesis of hair follicle structure is accompanied by the differentiation of skin tissue. Mammalian coats are produced by hair follicles. The formation of hair follicles requires signal transmission between the epidermis and dermis. However, knowledge of the transcriptional regulatory mechanism is still lacking. We used single-cell RNA sequencing to obtain 26,573 single cells from the scapular skin of yaks at hair follicle telogen and anagen stages. With the help of known reference marker genes, 11 main cell types were identified. In addition, we further analyzed the DP cell and dermal fibroblast lineages, drew a single-cell map of the DP cell and dermal fibroblast lineages, and elaborated the key genes, signals, and functions involved in cell fate decision making. The results of this study provide a very valuable resource for the analysis of the heterogeneity of DP cells and dermal fibroblasts in the skin and provide a powerful theoretical reference for further exploring the diversity of hair follicle cell types and hair follicle morphogenesis.

Stem cell heterogeneity, plasticity, and regulation

As a population of homogeneous cells with both self-renewal and differentiation potential, stem cell pools are highly compartmentalized and contain distinct subsets that exhibit stable but limited heterogeneity during homeostasis. However, their striking plasticity is showcased under natural or artificial stress, such as injury, transplantation, cancer, and aging, leading to changes in their phenotype, constitution, metabolism, and function. The complex and diverse network of cell-extrinsic niches and signaling pathways, together with cell-intrinsic genetic and epigenetic regulators, tightly regulate both the heterogeneity during homeostasis and the plasticity under perturbation. Manipulating these factors offers better control of stem cell behavior and a potential revolution in the current state of regenerative medicine. However, disruptions of normal regulation by genetic mutation or excessive plasticity acquisition may contribute to the formation of tumors. By harnessing innovative techniques that enhance our understanding of stem cell heterogeneity and employing novel approaches to maximize the utilization of stem cell plasticity, stem cell therapy holds immense promise for revolutionizing the future of medicine.

Commensal microbiome promotes hair follicle regeneration by inducing keratinocyte HIF-1α signaling and glutamine metabolism

Tissue injury induces metabolic changes in stem cells, which likely modulate regeneration. Using a model of organ regeneration called wound-induced hair follicle neogenesis (WIHN), we identified skin-resident bacteria as key modulators of keratinocyte metabolism, demonstrating a positive correlation between bacterial load, glutamine metabolism, and regeneration. Specifically, through comprehensive multiomic analysis and single-cell RNA sequencing in murine skin, we show that bacterially induced hypoxia drives increased glutamine metabolism in keratinocytes with attendant enhancement of skin and hair follicle regeneration. In human skin wounds, topical broad-spectrum antibiotics inhibit glutamine production and are partially responsible for reduced healing. These findings reveal a conserved and coherent physiologic context in which bacterially induced metabolic changes improve the tolerance of stem cells to damage and enhance regenerative capacity. This unexpected proregenerative modulation of metabolism by the skin microbiome in both mice and humans suggests important methods for enhancing regeneration after injury.

Mechanistic synergy of hair growth promotion by the Avicennia marina extract and its active constituent (avicequinone C) in dermal papilla cells isolated from androgenic alopecia patients

Androgenic alopecia (AGA) is associated with an increased production of 5α-dihydrotestosterone (DHT) by steroid-5α-reductase (5α-R). Crude extracts from Avicennia marina (AM) and its active constituent, avicequinone C (AC), can inhibit 5α-R. We have, herein, explored the potential use of the AM extract and of AC as anti-AGA agents. To this end, we employed human dermal papilla cells (DPCs) isolated from AGA patients' hair that express 5α-R type-1 as well as the androgenic receptor (AR) at high levels. Our in vitro experiments revealed that the AM extract (10 μg/mL) and the AC (10 μM) exhibit multiple actions that interfere with the mechanism that causes AGA. Beside acting as 5α-R inhibitors, both preparations were able to inhibit either the DHT-AR complex formation or its translocation from the cytoplasm into the nucleus (the site of DHT's action). The treatments also increased the gene expression of growth factors in DPCs; these factors play important roles in the angiogenesis associated with hair growth. Moreover, the AM extract suppressed the apoptotic pathway, thereby postponing the initiation of the catagen phase. Taken together, our findings suggest that the AM extract and the AC could serve as natural sources for hair growth promotion and AGA treatment.

The Molecular Mechanism of Natural Products Activating Wnt/β-Catenin Signaling Pathway for Improving Hair Loss

Hair loss, or alopecia, is a dermatological disorder that causes psychological stress and poor quality of life. Drug-based therapeutics such as finasteride and minoxidil have been clinically used to treat hair loss, but they have limitations due to their several side effects in patients. To solve this problem, there has been meaningful progress in elucidating the molecular mechanisms of hair growth and finding novel targets to develop therapeutics to treat it. Among various signaling pathways, Wnt/β-catenin plays an essential role in hair follicle development, the hair cycle, and regeneration. Thus, much research has demonstrated that various natural products worldwide promote hair growth by stimulating Wnt/β-catenin signaling. This review discusses the functional role of the Wnt/β-catenin pathway and its related signaling molecules. We also review the molecular mechanism of the natural products or compounds that activate Wnt/β-catenin signaling and provide insights into developing therapeutics or cosmeceuticals that treat hair loss.

Regulation of signaling pathways in hair follicle stem cells

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

BMP-AKT-GSK3β signalling restores hair follicle stem cells decrease associated with loss of Sfrp1

Wnt signaling plays a pivotal role in regulating activation, proliferation, stem cell renewal and differentiation of hair follicle stem cells (HFSCs). Secreted frizzled related protein-1 (Sfrp1), a Wnt antagonist is up regulated in the HFSCs; however, its role in the HFSCs regulation is still obscure. Here, we show that Sfrp1 loss showed a depletion of HFSCs, enhanced HFSC proliferation and faster hair follicle cycle at PD21 to PD28, HFSC markers such as Lgr5 and Axin2 were decreased in both the Sfrp1  +/- and Sfrp1  -/- HFSCs. In addition, the second hair follicle cycle was also faster as compared to WT. Importantly, Sfrp1  -/- showed a restoration of HFSC by 2 nd telogen (PD49), while Sfrp1+/- did not show restoration with still having a decreased HFSC. Infact, restoration of HFSCs was due to a pronounced down-regulation of β-CATENIN activity mediated through a cross-talk of BMP-AKT-GSK3β signalling in Sfrp1-/- as compared to Sfrp1+/-, where down regulation was less pronounced. In cultured keratinocytes, Sfrp1 loss resulted in enhanced proliferation and clonogenicity, which were reversed by treating with either BMPR1A or GSK3β inhibitor thereby confirming BMP-AKT-GSK3β signaling involved in β-CATENIN regulation in both the Sfrp1  +/- and Sfrp1  -/- mice. Our study reveals a novel function of Sfrp1 by unravelling an in vivo molecular mechanism that regulate the HFSCs pool mediated through a hitherto unknown cross-talk of BMP-AKT-GSK3β signalling that maintain stem cell pool balance, which in turn maintain skin tissue homeostasis.

Epithelial-Mesenchymal Interaction in Hair Regeneration and Skin Wound Healing

Interactions between epithelial and mesenchymal cells influence hair follicles (HFs) during embryonic development and skin regeneration following injury. Exchanging soluble molecules, altering key pathways, and extracellular matrix signal transduction are all part of the interplay between epithelial and mesenchymal cells. In brief, the mesenchyme contains dermal papilla cells, while the hair matrix cells and outer root sheath represent the epithelial cells. This study summarizes typical epithelial–mesenchymal signaling molecules and extracellular components under the control of follicular stem cells, aiming to broaden our current understanding of epithelial–mesenchymal interaction mechanisms in HF regeneration and skin wound healing.

Vitamin A alleviates heat stress-induced damage to hair follicle development in Rex rabbits

BACKGROUND

Rex rabbits are important fur rabbits. Heat stress severely reduces the fur quality of Rex rabbits. The aim of this study was to experimentally investigate the effect of dietary vitamin A (VA) addition on hair follicle development and related signal pathways in Rex rabbits under heat stress.

RESULTS

In the experiment, 90 Rex rabbits were randomly divided into three groups: control group (20-25 °C, fed basic diet), heat stress group (30-34 °C, fed basic diet), and heat stress + VA group (20-25 °C, fed 12 000 IU/kg VA in addition to the basic diet). VA could significantly increase the hair follicle density (P < 0.01), hair length (P < 0.05), and the ratio of secondary to primary hair follicles (P < 0.05). In addition, VA could significantly inhibit the expression of BMP2, BMP4, FGF5, TGF-β1, and miR-214 in heat-stressed Rex rabbits and significantly increase the expression of noggin, IGF1, IGF1R, Wnt10b, CTNNB1, SHH, and miR-203 and the levels of Wnt10b and p-β-catenin; however, there was no significant effect of VA on the expression of EGF and miR-205.

CONCLUSION

The dietary addition of VA can increase the hair follicle density and fur quality of heat-stressed Rex rabbits. Wnt10/β-catenin, insulin-like growth factor 1 (IGF1), fibroblast growth factor 5 (FGF5), noggin-BMP, and sonic hedgehog (SHH) signaling were associated with VA regulation under heat stress. It is possible that miR-205 and miR-194 contribute to the regulation of Wnt10/β-catenin and bone morphogenetic protein (BMP) signaling. © 2021 Society of Chemical Industry.

Deficiency of Formyl Peptide Receptor 2 Retards Hair Regeneration by Modulating the Activation of Hair Follicle Stem Cells and Dermal Papilla Cells in Mice

Hair loss is one of the most common chronic diseases, with a detrimental effect on a patient’s psychosocial life. Hair loss results from damage to the hair follicle (HF) and/or hair regeneration cycle. Various damaging factors, such as hereditary, inflammation, and aging, impair hair regeneration by inhibiting the activation of hair follicle stem cells (HFSCs) and dermal papilla cells (DPCs). Formyl peptide receptor 2 (FPR2) regulates the inflammatory response and the activity of various types of stem cells, and has recently been reported to have a protective effect on hair loss. Given that stem cell activity is the driving force for hair regeneration, we hypothesized that FPR2 influences hair regeneration by mediating HFSC activity. To prove this hypothesis, we investigated the role of FPR2 in hair regeneration using Fpr2 knockout (KO) mice. Fpr2 KO mice were found to have excessive hair loss and abnormal HF structures and skin layer construction compared to wild-type (WT) mice. The levels of Sonic hedgehog (Shh) and β-catenin, which promote HF regeneration, were significantly decreased, and the expression of bone morphogenetic protein (Bmp)2/4, an inhibitor of the anagen phase, was significantly increased in Fpr2 KO mice compared to WT mice. The proliferation of HFSCs and DPCs was significantly lower in Fpr2 KO mice than in WT mice. These findings demonstrate that FPR2 impacts signaling molecules that regulate HF regeneration, and is involved in the proliferation of HFSCs and DPCs, exerting a protective effect on hair loss.

Pyridoxine regulates hair follicle development via the PI3K/Akt, Wnt and Notch signalling pathways in rex rabbits

This study was conducted to evaluate the effect of pyridoxine on the development of hair follicles in Rex rabbits and the underlying molecular mechanism. Two hundred 3-month-old Rex rabbits were randomly divided into 5 groups and fed diets supplemented with 0, 5, 10, 20, or 40 mg/kg pyridoxine. The hair follicle density on the dorsal skin and the gene and protein expression levels of components of the phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB or Akt), Wnt, Notch and bone morphogenetic protein (BMP) signalling pathways were measured. In addition, free hair follicles were isolated from Rex rabbits and cultured with pyridoxine in vitro to measure hair shaft growth. Furthermore, dermal papilla cells (DPC) were isolated from the skin of Rex rabbits and cultured with pyridoxine in vitro to measure the gene and protein expression levels of components of the PI3K/Akt, Wnt, Notch and BMP signalling pathways. The results showed that the addition of dietary pyridoxine significantly increased the total follicle density, secondary follicle density, and secondary-to-primary ratio (S/P, P < 0.05), that the growth ratio of hair stems was promoted by pyridoxine in basic culture medium, and that the growth length of tentacle hair follicles cultured in the pyridoxine group was longer than that in the control group (P < 0.05). In addition, pyridoxine changed the DPC cycle progression and promoted cell proliferation, and appropriate concentrations of pyridoxine (10 and 20 μmol/L) significantly inhibited cell apoptosis (P < 0.05). Pyridoxine significantly affected the gene expression of components of the PI3K/Akt, Wnt and Notch signalling pathways in the skin and DPC of Rex rabbits (P < 0.05), increased the levels of phosphorylated catenin beta 1 (CTNNB1) and Akt, and decreased the level of phosphorylated glycogen synthase kinase 3 beta (GSK-3β) (P < 0.05). Therefore, the molecular mechanism by which pyridoxine promotes hair follicle density in Rex rabbits probably occurs through activation of the PI3K/Akt, Wnt and Notch signalling pathways, prolonging hair follicle growth and delaying the onset of telogen.

Platelet-rich Plasma and Cell Therapy: The New Horizon in Hair Loss Treatment

Current medicinal therapies for treating hair loss have shortcomes due to variability and ineffectiveness, noncompliance, and adverse effects. The prevalence of hair loss and its associated negative psychological impact have driven research into regenerative medicine approaches, such as platelet-rich plasma (PRP) and cell-based therapies, in an attempt to find alternative, safe, effective, and reproducible treatments. Current research shows promising results from these therapies; however, more robust trials are needed to confirm the reported efficacies of PRP and cell-based therapies. Moreover, standardization of treatment preparation as well as dose and regimen are needed.

The Act of Controlling Adult Stem Cell Dynamics: Insights from Animal Models

Adult stem cells (ASCs) are the undifferentiated cells that possess self-renewal and differentiation abilities. They are present in all major organ systems of the body and are uniquely reserved there during development for tissue maintenance during homeostasis, injury, and infection. They do so by promptly modulating the dynamics of proliferation, differentiation, survival, and migration. Any imbalance in these processes may result in regeneration failure or developing cancer. Hence, the dynamics of these various behaviors of ASCs need to always be precisely controlled. Several genetic and epigenetic factors have been demonstrated to be involved in tightly regulating the proliferation, differentiation, and self-renewal of ASCs. Understanding these mechanisms is of great importance, given the role of stem cells in regenerative medicine. Investigations on various animal models have played a significant part in enriching our knowledge and giving In Vivo in-sight into such ASCs regulatory mechanisms. In this review, we have discussed the recent In Vivo studies demonstrating the role of various genetic factors in regulating dynamics of different ASCs viz. intestinal stem cells (ISCs), neural stem cells (NSCs), hematopoietic stem cells (HSCs), and epidermal stem cells (Ep-SCs).

Inhibition of pyruvate oxidation as a versatile stimulator of the hair cycle in models of alopecia

Hair follicle stem cells (HFSCs) are known to be responsible for the initiation of a new hair cycle, but typically remain quiescent for very long periods. In alopecia, or hair loss disorders, follicles can be refractory to activation for years or even permanently. Alopecia can be triggered by autoimmunity, age, chemotherapeutic treatment, stress, disrupted circadian rhythm or other environmental insults. We previously showed that hair follicle stem cells and the hair cycle can be manipulated by regulation of pyruvate entry into mitochondria for subsequent oxidation to fuel the TCA cycle in normal adult mice with typical hair cycling. Here, we present new data from our efforts to develop murine models of alopecia based on environmental triggers that have been shown to do the same in human skin. We found that inhibition of pyruvate transport into mitochondria can accelerate the hair cycle even during refractory hair cycling due to age, repeated chemotherapeutic treatment and stress. Hair cycle acceleration in these alopecia models led to the formation of histologically normal hair follicles within 30-40 days of treatment without any overt signs of toxicity or deleterious effects. Therefore, we propose inhibition of pyruvate entry into mitochondria as a versatile treatment strategy for alopecia in humans.

Stem cell-intrinsic mechanisms regulating adult hair follicle homeostasis

Adult hair follicle stem cells (HFSCs) undergo dynamic and periodic molecular changes in their cellular states throughout the hair homeostatic cycle. These states are tightly regulated by cell-intrinsic mechanisms and by extrinsic signals from the microenvironment. HFSCs are essential not only for fuelling hair growth, but also for skin wound healing. Increasing evidence suggests an important role of HFSCs in organizing multiple skin components around the hair follicle, thus functioning as an organizing centre during adult skin homeostasis. Here, we focus on recent findings on cell-intrinsic mechanisms of HFSC homeostasis, which include transcription factors, histone modifications, DNA regulatory elements, non-coding RNAs, cell metabolism, cell polarity and post-transcriptional mRNA processing. Several transcription factors are now known to participate in well-known signalling pathways that control hair follicle homeostasis, as well as in super-enhancer activities to modulate HFSC and progenitor lineage progression. Interestingly, HFSCs have been shown to secrete molecules that are important in guiding the organization of several skin components around the hair follicle, including nerves, arrector pili muscle and vasculature. Finally, we discuss recent technological advances in the field such as single-cell RNA sequencing and live imaging, which revealed HFSC and progenitor heterogeneity and brought new light to understanding crosstalking between HFSCs and the microenvironment. The field is well on its way to generate a comprehensive map of molecular interactions that should serve as a solid theoretical platform for application in hair and skin disease and ageing.

An Intrinsic Oscillation of Gene Networks Inside Hair Follicle Stem Cells: An Additional Layer That Can Modulate Hair Stem Cell Activities

This article explores and summarizes recent progress in and the characterization of main players in the regulation and cyclic regeneration of hair follicles. The review discusses current views and discoveries on the molecular mechanisms that allow hair follicle stem cells (hfSCs) to synergistically integrate homeostasis during quiescence and activation. Discussion elaborates on a model that shows how different populations of skin stem cells coalesce intrinsic and extrinsic mechanisms, resulting in the maintenance of stemness and hair regenerative potential during an organism’s lifespan. Primarily, we focus on the question of how the intrinsic oscillation of gene networks in hfSCs sense and respond to the surrounding niche environment. The review also investigates the existence of a cell-autonomous mechanism and the reciprocal interactions between molecular signaling axes in hfSCs and niche components, which demonstrates its critical driving force in either the activation of whole mini-organ regeneration or quiescent homeostasis maintenance. These exciting novel discoveries in skin stem cells and the surrounding niche components propose a model of the intrinsic stem cell oscillator which is potentially instructive for translational regenerative medicine. Further studies, deciphering of the distribution of molecular signals coupled with the nature of their oscillation within the stem cells and niche environments, may impact the speed and efficiency of various approaches that could stimulate the development of self-renewal and cell-based therapies for hair follicle stem cell regeneration.

SIRT7 activates quiescent hair follicle stem cells to ensure hair growth in mice

Hair follicle stem cells (HFSCs) are maintained in a quiescent state until activated to grow, but the mechanisms that reactivate the quiescent HFSC reservoir are unclear. Here, we find that loss of Sirt7 in mice impedes hair follicle life‐cycle transition from telogen to anagen phase, resulting in delay of hair growth. Conversely, Sirt7 overexpression during telogen phase facilitated HSFC anagen entry and accelerated hair growth. Mechanistically, Sirt7 is upregulated in HFSCs during the telogen‐to‐anagen transition, and HFSC‐specific Sirt7 knockout mice (Sirt7^f/f;K15‐Cre) exhibit a similar hair growth delay. At the molecular level, Sirt7 interacts with and deacetylates the transcriptional regulator Nfatc1 at K612, causing PA28γ‐dependent proteasomal degradation to terminate Nfatc1‐mediated telogen quiescence and boost anagen entry. Cyclosporin A, a potent calcineurin inhibitor, suppresses nuclear retention of Nfatc1, abrogates hair follicle cycle delay, and promotes hair growth in Sirt7^−/− mice. Furthermore, Sirt7 is downregulated in aged HFSCs, and exogenous Sirt7 overexpression promotes hair growth in aged animals. These data reveal that Sirt7 activates HFSCs by destabilizing Nfatc1 to ensure hair follicle cycle initiation.

Its written all over your face: The molecular and physiological consequences of aging skin

Perhaps the most recognizable consequences of tissue aging are manifested in the skin. Hair graying and loss, telltale wrinkles, and age spots are indicative of physiological aging symptoms, many of which are analogous to processes in other tissues as well with less visible outcomes. While the study of skin aging has been conducted for decades, more recent work has illuminated many of the fundamental molecular and physiological causes of aging in the skin. Recent technological advances have allowed for the detection and quantification of a variety of physiological triggers that lead to aging in the skin and molecular methods have begun to determine the etiology of these phenotypic features. This review will attempt to summarize recent work in this area and provide some speculation about the next wave of studies.

Targeting Wnt/β-Catenin Pathway for Developing Therapies for Hair Loss

Persistent hair loss is a major cause of psychological distress and compromised quality of life in millions of people worldwide. Remarkable progress has been made in understanding the molecular basis of hair loss and identifying valid intracellular targets for designing effective therapies for hair loss treatment. Whereas a variety of growth factors and signaling pathways have been implicated in hair cycling process, the activation of Wnt/β-catenin signaling plays a central role in hair follicle regeneration. Several plant-derived chemicals have been reported to promote hair growth by activating Wnt/β-catenin signaling in various in vitro and in vivo studies. This mini-review sheds light on the role of Wnt/β-catenin in promoting hair growth and the current progress in designing hair loss therapies by targeting this signaling pathway.

The Wnt/β-catenin signaling pathway is involved in regulating feather growth of embryonic chicks

Avian feathers have robust growth and regeneration capability and serve as a useful model for decoding hair morphogenesis and other developmental studies. However, the molecular signaling involved in regulating the development of feather follicles is unclear. The purpose of this study was to investigate the role of the Wnt/β-catenin pathway in regulating feather morphogenesis in embryonic chicks through in ovo injection of different doses of Dickkopf-1 (DKK1, a specific inhibitor of the target of the Wnt/β-catenin pathway). A total of 120 fertilized embryo eggs were randomly divided into 4 treatments, including a noninjection group (control group) and groups injected with 100 μL of phosphate-buffered saline (PBS)/egg (PBS control group), 100 μL of PBS/egg containing 600-ng DKK1/egg (600-ng DKK1 group), and 100-μL PBS/egg containing 1,200-ng DKK1/egg (1,200-ng DKK1 group). Feathers and skin tissues were sampled on embryonic (E) day 15 and the day of hatching to examine the feather mass, diameter and density of feather follicles, and the protein expression of the Wnt/β-catenin pathway. The results showed that, compared with CON and PBS treatment, the injection of DKK1 into the yolk sac of chick embryos had no significant effect on the hatching rate and embryo weight (P > 0.05), while it significantly decreased the relative mass of feathers in the whole body (P < 0.05). The high dose of DKK1 (1,200-ng DKK1/egg) decreased the relative mass of feathers on the back, chest, belly, neck, wings, head, and legs, which was more obvious than that in the 600-ng DKK1 group, which presented a dose-dependent effect. In addition, DKK1 injection significantly downregulated the protein expression levels of β-catenin, transcription factor 4, Cyclin D1, and c-Myc (P < 0.05). The immunofluorescence result of β-catenin was consistent with the Western blotting assay results. Altogether, these observations suggested that the Wnt/β-catenin signaling pathway is involved in regulating feather follicle development and feather growth during the embryonic development of chicks.

The functions of ocu-miR-205 in regulating hair follicle development in Rex rabbits

BACKGROUND

Hair follicles are an appendage of the vertebrate epithelium in the skin that arise from the embryonic ectoderm and regenerate cyclically during adulthood. Dermal papilla cells (DPCs) are the key dermal component of the hair follicle that directly regulate hair follicle development, growth and regeneration. According to recent studies, miRNAs play an important role in regulating hair follicle morphogenesis and the proliferation, differentiation and apoptosis of hair follicle stem cells.

RESULTS

The miRNA expression profile of the DPCs from Rex rabbits with different hair densities revealed 240 differentially expressed miRNAs (|log2(HD/LD)| > 1.00 and Q-value≤0.001). Among them, ocu-miR-205-5p was expressed at higher levels in DPCs from rabbits with low hair densities (LD) than in rabbits with high hair densities (HD), and it was expressed at high levels in the skin tissue from Rex rabbits (P < 0.05). Notably, ocu-miR-205 increased cell proliferation and the cell apoptosis rate, altered the progression of the cell cycle (P < 0.05), and modulated the expression of genes involved in the PI3K/Akt, Wnt, Notch and BMP signalling pathways in DPCs and skin tissue from Rex rabbits. It also inhibited the phosphorylation of the CTNNB1 and GSK-3β proteins, decreased the level of the noggin (NOG) protein, and increased the level of phosphorylated Akt (P < 0.05). A significant change in the primary follicle density was not observed (P > 0.05), but the secondary follicle density and total follicle density (P < 0.05) were altered upon interference with ocu-miR-205-5p expression, and the secondary/primary ratio (S/P) in the ocu-miR-205-5p interfered expression group increased 14 days after the injection (P < 0.05).

CONCLUSIONS

In the present study, ocu-miR-205 promoted the apoptosis of DPCs, altered the expression of genes and proteins involved in the PI3K/Akt, Wnt, Notch and BMP signalling pathways in DPCs and skin from Rex rabbits, promoted the transition of hair follicles from the growth phase to the regression and resting phase, and altered the hair density of Rex rabbits.

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.

Regeneration of skin appendages and nerves: current status and further challenges

Tissue-engineered skin (TES), as an analogue of native skin, is promising for wound repair and regeneration. However, a major drawback of TES products is a lack of skin appendages and nerves to enhance skin healing, structural integrity and skin vitality. Skin appendages and nerves are important constituents for fully functional skin. To date, many studies have yielded remarkable results in the field of skin appendages reconstruction and nerve regeneration. However, patients often complain about a loss of skin sensation and even cutaneous chronic pain. Restoration of pain, temperature, and touch perceptions should now be a major challenge to solve in order to improve patients’ quality of life. Current strategies to create skin appendages and sensory nerve regeneration are mainly based on different types of seeding cells, scaffold materials, bioactive factors and involved signaling pathways. This article provides a comprehensive overview of different strategies for, and advances in, skin appendages and sensory nerve regeneration, which is an important issue in the field of tissue engineering and regenerative medicine.

Overexpression of MYB in the Skin Induces Alopecia and Epidermal Hyperplasia

Skin homeostasis is controlled by a complex interplay between tightly regulated transcription factors and signaling pathways. MYB is a transcription factor expressed in hair follicle progenitor cells and found overexpressed in adnexal skin tumors. However, the biological consequences of deregulated MYB expression in the skin remain poorly understood. To address this, we generated transgenic mice that overexpress MYB in epidermal and follicular keratinocytes. These mice exhibited a normal hair coat after birth but gradually developed alopecia, accompanied by altered follicular differentiation, disrupted hair cycle, and a marked depletion of hair follicle stem cells. Additionally, transgenic mice developed massive epidermal hyperplasia and hyperkeratosis. Global expression profiling not only confirmed that the skin of these mice exhibited transcriptomic features of alopecia and epidermal differentiation, but also revealed features of psoriasis and the inflammatory response. The latter was further confirmed by the increased T-cell infiltration found in the skin of transgenic mice. Overall, these results suggest that tight regulation of MYB expression in the skin is critical to maintain skin homeostasis.

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.

Expression analysis of proteasome maturation protein (POMP) gene in Liaoning Cashmere goat

Liaoning Cashmere goat is a precious genetic resource of China. To explore the relationship between POMP and cashmere growth, we analyzed the expression of POMP. POMP encodes a hudrophilic protein which is most closely related to bos. RT-PCR showed that POMP was expressed in skin, heart, liver, spleen, lung, and kidney tissues. Real-time PCR showed that the expression of POMP was more active in the secondary hair follicles than the primary hair follicles in anagen. In situ hybridization showed that POMP was obviously expressed in the Inner Root Sheath (IRS) but no expression in Outer Root. The treatment of fibroblasts with melatonin (MT), fibroblast growth factors 5 (FGF5) and insulin-like growth factors 1 (IGF-1) showed that MT/FGF5/IGF-1 much performance for inhibiting the expression of POMP; MT + FGF5 inhibited the expression of POMP; MT + IGF-1 promoted the expression of POMP. When Noggin expression is decreased by siRNA, the expression of POMP is inhibited. To sum up, POMP strongly expressed in the root sheath of hair follicles, related to the development of the primary and secondary hair follicle; In addition, by adding MT/FGF5/IGF-1 or interfering with the Noggin expression to regulate the expression of POMP, to control the growth of Liaoning Cashmere goat cashmere.

Synthesized Ceramide Induces Growth of Dermal Papilla Cells with Potential Contribution to Hair Growth

Background

The ceramide is known to play an important role in the formation of intracellular lipids, and play a crucial role as a barrier for skin and hair cuticle. Recent study has revealed that ceramide has potential effect on hair growth in a mouse model. However, the role of ceramide in human dermal papilla cells (hDPCs) known to play an important role in hair growth is not well understood yet.

Objective

The goal of this study was to investigate the effect of synthetic ceramides (oleyl and stearyl ceramides) on hair growth using hDPCs.

Methods

hDPCs were treated with synthesized ceramides. hDPCs viability was evaluated by MTT assay. The expression of hair growth related factors were investigated by western blot, real-time polymerase chain reaction and growth factor array. The expression of β-catenin was confirmed by immunofluorescence.

Results

Treatment with ceramides increased the expression of proteins affecting cell proliferation such as Bcl-2, BAX, phosphorylated-ERK and Cyclin D1. Also, ceramides treatment were increased the expression of several growth factors, including epidermal growth factor family, and promote the expression of Wnt/β-catenin and BMP2/4 signaling.

Conclusion

Our data suggest that synthetic ceramides stimulates hair growth by induction proliferation of hDPCs via modulation of Wnt/β-catenin and BMP2/4 signaling.

Review of Human Hair Follicle Biology: Dynamics of Niches and Stem Cell Regulation for Possible Therapeutic Hair Stimulation for Plastic Surgeons

Plastic surgeons are frequently asked to manage male- and female-pattern hair loss in their practice. This article discusses the epidemiology, pathophysiology, and current management of androgenetic alopecia and emphasizes more recent knowledge of stem cell niches in hair follicles that drive hair cycling, alopecia, and its treatment. The many treatment programs available for hair loss include newer strategies that involve the usage of growth factors, platelet-rich plasma, and fat to stimulate follicle growth. Future research may clarify novel biomolecular mechanisms that target specific cells that promote hair regeneration.Level of Evidence V This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

HIF-1α-mediated BMP6 down-regulation leads to hyperproliferation and abnormal differentiation of keratinocytes in vitro

Hypoxia-inducible factor-1α (HIF-1α) has been reported to be up-regulated in psoriatic epidermis, resulting in increased proliferation and abnormal differentiation of human keratinocytes (KCs). However, the role of HIF-1α in psoriatic epidermis, which is mainly composed of KCs, is poorly understood. Here, we show that morphogenic protein 6 (BMP6) is down-regulated when HIF-1α is upregulated in patients with psoriasis skin lesions. HIF-1α overexpression in primary human KCs promoted proliferation and inhibited terminal differentiation. Furthermore, HIF1-α repressed the expression of BMP6 by binding directly to the hypoxia-response element (HRE) in the BMP6 promotor region, which shows that BMP6 is a novel target gene of HIF-1α. We also found that HIF-1α-mediated BMP6 suppression could alter the proliferation status by modulating the expression levels of cell cycle regulatory proteins and also affect the early differentiation of KCs. Therefore, we suggest that HIF-1α-dependent BMP6 suppression has a critical role in the induction of hyper-proliferation and abnormal differentiation in psoriatic KCs.

Thyroid Hormone Receptors Regulate the Expression of microRNAs with Key Roles in Skin Homeostasis

BACKGROUND

MicroRNAs (miRNAs) play a unique role in posttranscriptional regulation of gene expression and control different aspects of skin development, homeostasis, and disease. Although it is generally accepted that thyroid hormone signaling is important in skin pathophysiology, the role of their nuclear receptors (TRs) in cutaneous miRNA expression has yet to be explored.

METHODS

RNAseq was used to compare the skin miRnome of wild-type mice and genetically modified mice lacking both TRα1 and TRβ, the main thyroid hormone binding isoforms. Changes in miRNAs with a crucial role in skin physiopathology were confirmed by stem-loop quantitative polymerase chain reaction in both total skin and isolated keratinocytes, and the levels of their target mRNAs were evaluated by real-time polymerase chain reaction.

RESULTS

The skin of TRα1/TRβ knockout mice displays altered levels of >50 miRNAs. Among the downregulated species are several miRNAs, including miR-21, miR-31, miR-34, and miR-203, with crucial roles in skin homeostasis. TRα1 appears to be the main isoform responsible for their regulation. Increased levels of gene transcripts previously shown to be bona fide targets of these miRNAs are also found in the skin and keratinocytes of TR-deficient mice. This suggests that multiple miRNAs that are downregulated in the absence of TRs cooperate to regulate gene expression in the skin.

CONCLUSIONS

The miRNAs reduced in TRα1/TRβ knockout mice are known to play crucial roles in epidermal proliferation, hair cycling, wound healing, stem-cell function, and tumor development, all processes altered in the absence of TRs. These results suggest that their regulation could contribute to the skin defects found in these mice and to the skin disorders associated with altered thyroid status in humans.

Expression characteristics of BMP2, BMPR-IA and Noggin in different stages of hair follicle in yak skin

Bone morphogenetic protein 2 (BMP2), BMP receptor-IA (BMPR-IA), and the BMP2 antagonist Noggin are important proteins involved in regulating the hair follicle (HF) cycle in skin. In order to explore the expression profiles of BMP2, BMPR-IA, and Noggin in the HF cycle of yak skin, we collected adult yak skin in the telogen, proanagen, and midanagen phases of HFs and evaluated gene and protein expression by real-time quantitative polymerase chain reaction (qRT-PCR), western blotting, and immunohistochemistry. qRT-PCR and western blotting results showed that BMP2 and BMPR-IA expression levels were highest in the telogen of HFs and higher than that of Noggin in the same phase. The expression of Noggin was significantly higher in proanagen and midanagen phases of HFs than in the telogen phase, with the highest expression observed in the proanagen phase. Moreover, the expression of Noggin in the proanagen phase was significantly higher than those of BMP2 and BMPR-IA during the same phase. Immunohistochemistry results showed that BMP2, BMPR-IA, and Noggin were expressed in the skin epidermis, sweat glands, sebaceous glands, HF outer root sheath, and hair matrix. In summary, the characteristic expression profiles of BMP2, BMPR-IA, and Noggin suggested that BMP2 and BMPR-IA had inhibitory effects on the growth of HFs in yaks, whereas Noggin promoted the growth of yak HFs, mainly by affecting skin epithelial cell activity. These results provide a basis for further studies of HF development and cycle transition in yak skin.

TGF-β Family Signaling in Epithelial Differentiation and Epithelial-Mesenchymal Transition

Epithelia exist in the animal body since the onset of embryonic development; they generate tissue barriers and specify organs and glands. Through epithelial-mesenchymal transitions (EMTs), epithelia generate mesenchymal cells that form new tissues and promote healing or disease manifestation when epithelial homeostasis is challenged physiologically or pathologically. Transforming growth factor-βs (TGF-βs), activins, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs) have been implicated in the regulation of epithelial differentiation. These TGF-β family ligands are expressed and secreted at sites where the epithelium interacts with the mesenchyme and provide paracrine queues from the mesenchyme to the neighboring epithelium, helping the specification of differentiated epithelial cell types within an organ. TGF-β ligands signal via Smads and cooperating kinase pathways and control the expression or activities of key transcription factors that promote either epithelial differentiation or mesenchymal transitions. In this review, we discuss evidence that illustrates how TGF-β family ligands contribute to epithelial differentiation and induce mesenchymal transitions, by focusing on the embryonic ectoderm and tissues that form the external mammalian body lining.

Novel insights into the pathways regulating the canine hair cycle and their deregulation in alopecia X

Alopecia X is a hair cycle arrest disorder in Pomeranians. Histologically, kenogen and telogen hair follicles predominate, whereas anagen follicles are sparse. The induction of anagen relies on the activation of hair follicle stem cells and their subsequent proliferation and differentiation. Stem cell function depends on finely tuned interactions of signaling molecules and transcription factors, which are not well defined in dogs. We performed transcriptome profiling on skin biopsies to analyze altered molecular pathways in alopecia X. Biopsies from five affected and four non-affected Pomeranians were investigated. Differential gene expression revealed a downregulation of key regulator genes of the Wnt (CTNNB1, LEF1, TCF3, WNT10B) and Shh (SHH, GLI1, SMO, PTCH2) pathways. In mice it has been shown that Wnt and Shh signaling results in stem cell activation and differentiation Thus our findings are in line with the lack of anagen hair follicles in dogs with Alopecia X. We also observed a significant downregulation of the stem cell markers SOX9, LHX2, LGR5, TCF7L1 and GLI1 whereas NFATc1, a quiescence marker, was upregulated in alopecia X. Moreover, genes coding for enzymes directly involved in the sex hormone metabolism (CYP1A1, CYP1B1, HSD17B14) were differentially regulated in alopecia X. These findings are in agreement with the so far proposed but not yet proven deregulation of the sex hormone metabolism in this disease.

Delayed Hair Follicle Morphogenesis and Hair Follicle Dystrophy in a Lipoatrophy Mouse Model of Pparg Total Deletion

PPARγ regulates multiple aspects of skin physiology, including sebocyte differentiation, keratinocyte proliferation, epithelial stem cell survival, adipocyte biology, and inflammatory skin responses. However, the effects of its global deletion, namely of nonredundant key functions of PPARγ signaling in mammalian skin, are yet unknown because of embryonic lethality. Here, we describe the skin and hair phenotype of a whole-body PPARγ-null mouse (Pparg^Δ/Δ), obtained by preserving PPARγ expression in the placenta. Pparg^Δ/Δ mice exhibited total lipoatrophy and complete absence of sebaceous glands. Right after birth, hair follicle (HF) morphogenesis was transiently delayed, along with reduced expression of HF differentiation markers and of transcriptional regulators necessary for HF development. Later, adult Pparg^Δ/Δ mice developed scarring alopecia and severe perifollicular inflammation. Skin analyses in other models of lipodystrophy, AZIP^tg/+ and Adipoq-Cre^tg/+Pparg^fl/fl mice, coupled with skin graft experiments, showed that the early defects observed in hair morphogenesis were caused by the absence of adipose tissue. In contrast, the late alteration of HF cycle and appearance of inflammation were observed only in Pparg^Δ/Δ mice and likely were due to the lack sebaceous glands. Our findings underscore the increasing appreciation for the importance of adipose tissue-mediated signals in HF development and function.

Multiple-trait QTL mapping and genomic prediction for wool traits in sheep

BACKGROUND

The application of genomic selection to sheep breeding could lead to substantial increases in profitability of wool production due to the availability of accurate breeding values from single nucleotide polymorphism (SNP) data. Several key traits determine the value of wool and influence a sheep's susceptibility to fleece rot and fly strike. Our aim was to predict genomic estimated breeding values (GEBV) and to compare three methods of combining information across traits to map polymorphisms that affect these traits.

METHODS

GEBV for 5726 Merino and Merino crossbred sheep were calculated using BayesR and genomic best linear unbiased prediction (GBLUP) with real and imputed 510,174 SNPs for 22 traits (at yearling and adult ages) including wool production and quality, and breech conformation traits that are associated with susceptibility to fly strike. Accuracies of these GEBV were assessed using fivefold cross-validation. We also devised and compared three approximate multi-trait analyses to map pleiotropic quantitative trait loci (QTL): a multi-trait genome-wide association study and two multi-trait methods that use the output from BayesR analyses. One BayesR method used local GEBV for each trait, while the other used the posterior probabilities that a SNP had an effect on each trait.

RESULTS

BayesR and GBLUP resulted in similar average GEBV accuracies across traits (~0.22). BayesR accuracies were highest for wool yield and fibre diameter (>0.40) and lowest for skin quality and dag score (<0.10). Generally, accuracy was higher for traits with larger reference populations and higher heritability. In total, the three multi-trait analyses identified 206 putative QTL, of which 20 were common to the three analyses. The two BayesR multi-trait approaches mapped QTL in a more defined manner than the multi-trait GWAS. We identified genes with known effects on hair growth (i.e. FGF5, STAT3, KRT86, and ALX4) near SNPs with pleiotropic effects on wool traits.

CONCLUSIONS

The mean accuracy of genomic prediction across wool traits was around 0.22. The three multi-trait analyses identified 206 putative QTL across the ovine genome. Detailed phenotypic information helped to identify likely candidate genes.

TGF-β Family Signaling in Embryonic and Somatic Stem-Cell Renewal and Differentiation

Soon after the discovery of transforming growth factor-β (TGF-β), seminal work in vertebrate and invertebrate models revealed the TGF-β family to be central regulators of tissue morphogenesis. Members of the TGF-β family direct some of the earliest cell-fate decisions in animal development, coordinate complex organogenesis, and contribute to tissue homeostasis in the adult. Here, we focus on the role of the TGF-β family in mammalian stem-cell biology and discuss its wide and varied activities both in the regulation of pluripotency and in cell-fate commitment.

Emerging roles of transit-amplifying cells in tissue regeneration and cancer

Most regenerative tissues employ transit-amplifying cells (TACs) that are positioned in between stem cells and differentiated progeny. In a classical hierarchical model, stem cells undergo limited divisions to produce TACs, which then proliferate rapidly to expand the system and produce diverse differentiated cell types. Although TACs are indispensable for generating tissues, they have been largely viewed as a transit point between stem cells and downstream lineages. Studies in the past few years, however, have revealed some fascinating biology and unanticipated functions of TACs. In the hair follicle, recent findings have placed TACs as key players in tissue regeneration by coordinating tissue production, governing stem cell behaviors, and instructing niche remodeling. In the hematopoietic system, rather than being transient, some TACs may participate in long-term hematopoiesis under steady state. Here, we compare and summarize recent discoveries about TACs in the hair follicle and the hematopoietic system. We also discuss how TACs of these two tissues contribute to the formation of cancer. WIREs Dev Biol 2017, 6:e282. doi: 10.1002/wdev.282 For further resources related to this article, please visit the WIREs website.

Aging in hair follicle stem cells and niche microenvironment

Hair graying and hair loss are prominent and common characteristics of the elderly population. In some individuals these processes can significantly impact their quality of life, leading to depression, anxiety and other serious mental health problems. Accordingly, there has been much interest in understanding the complex physiological changes within the hair follicle in the aging individual. It is now known that hair follicles represent a prototypical stem cell niche, where both micro- and macroenvironmental influences are integrated alongside stem cell-stem cell and stem cell-stem niche interactions to determine hair growth or hair follicle senescence. Recent studies have identified imbalanced stem cell differentiation and altered stem cell activity as important factors during hair loss, indicating new avenues for the development of therapeutic agents to stimulate hair growth. Here, we pull together the latest findings on the hair follicle stem cell niche and the multifactorial interactions underlying the various forms of hair loss.

Recruiting endogenous stem cells: a novel therapeutic approach for erectile dysfunction

Transplanted stem cells (SCs), owing to their regenerative capacity, represent one of the most promising methods to restore erectile dysfunction (ED). However, insufficient source, invasive procedures, ethical and regulatory issues hamper their use in clinical applications. The endogenous SCs/progenitor cells resident in organ and tissues play critical roles for organogenesis during development and for tissue homeostasis in adulthood. Even without any therapeutic intervention, human body has a robust self-healing capability to repair the damaged tissues or organs. Therefore, SCs-for-ED therapy should not be limited to a supply-side approach. The resident endogenous SCs existing in patients could also be a potential target for ED therapy. The aim of this review was to summarize contemporary evidence regarding: (1) SC niche and SC biological features in vitro; (2) localization and mobilization of endogenous SCs; (3) existing evidence of penile endogenous SCs and their possible mode of mobilization. We performed a search on PubMed for articles related to these aspects in a wide range of basic studies. Together, numerous evidences hold the promise that endogenous SCs would be a novel therapeutic approach for the therapy of ED.

Regeneration of hair and other skin appendages: A microenvironment-centric view

Advances in skin regeneration have resulted in techniques and products that have allowed regeneration of both the dermis and epidermis. Yet complete skin regeneration requires the adnexal skin structures. Thus it is crucial to understand the regenerative potential of hair follicles where genetic, nutritional, and hormonal influences have important effects and are critical for skin regeneration. The follicular stem cell niche serves as an anatomical compartment, a structural unit, a functional integrator, and a dynamic regulator necessary to sustain internal homeostasis and respond to outside stimuli. In particular, mechanics such as pressure, compression, friction, traction, stretch, shear, and mechanical wounding can influence hair loss or growth. Relevant niche signaling pathways such as Wnt, bone morphogenetic protein, fibroblast growth factor, Shh, and Notch may yield potential targets for therapeutic interventions.

The importance of basonuclin 2 in adult mice and its relation to basonuclin 1

BNC2 is an extremely conserved zinc finger protein with important functions in the development of craniofacial bones and male germ cells. Because disruption of the Bnc2 gene in mice causes neonatal lethality, the function of the protein in adult animals has not been studied. Until now BNC2 was considered to have a wider tissue distribution than its paralog, BNC1, but the precise cell types expressing Bnc2 are largely unknown. We identify here the cell types containing BNC2 in the mouse and we show the unexpected presence of BNC1 in many BNC2-containing cells. BNC1 and BNC2 are colocalized in male and female germ cells, ovarian epithelial cells, sensory neurons, hair follicle keratinocytes and connective cells of organ capsules. In many cell lineages, the two basonuclins appear and disappear synchronously. Within the male germ cell lineage, BNC1 and BNC2 are found in prospermatogonia and undifferentiated spermatogonia, and disappear abruptly from differentiating spermatogonia. During oogenesis, the two basonuclins accumulate specifically in maturing oocytes. During the development of hair follicles, BNC1 and BNC2 concentrate in the primary hair germs. As follicle morphogenesis proceeds, cells possessing BNC1 and BNC2 invade the dermis and surround the papilla. During anagen, BNC1 and BNC2 are largely restricted to the basal layer of the outer root sheath and the matrix. During catagen, the compartment of cells possessing BNC1 and BNC2 regresses, and in telogen, the two basonuclins are confined to the secondary hair germ. During the next anagen, the BNC1/BNC2-containing cell population regenerates the hair follicle. By examining Bnc2(-/-) mice that have escaped the neonatal lethality usually associated with lack of BNC2, we demonstrate that BNC2 possesses important functions in many of the cell types where it resides. Hair follicles of postnatal Bnc2(-/-) mice do not fully develop during the first cycle and thereafter remain blocked in telogen. It is concluded that the presence of BNC2 in the secondary hair germ is required to regenerate the transient segment of the follicle. Postnatal Bnc2(-/-) mice also show severe dwarfism, defects in oogenesis and alterations of palatal rugae. Although the two basonuclins possess very similar zinc fingers and are largely coexpressed, BNC1 cannot substitute for BNC2. This is shown incontrovertibly in knockin mice expressing Bnc1 instead of Bnc2 as these mice invariably die at birth with craniofacial abnormalities undistinguishable from those of Bnc2(-/-) mice. The function of the basonuclins in the secondary hair germ is of particular interest.

Niche Regulation of Limbal Epithelial Stem Cells: Relationship between Inflammation and Regeneration

Human limbal palisades of Vogt are the ideal site for studying and practicing regenerative medicine due to their accessibility. Nonresolving inflammation in limbal stroma is common manifestation of limbal stem cell (SC) deficiency and presents as a threat to the success of transplanted limbal epithelial SCs. This pathologic process can be overcome by transplantation of cryopreserved human amniotic membrane (AM), which exerts anti-inflammatory, antiscarring and anti-angiogenic action to promote wound healing. To determine how AM might exert anti-inflammation and promote regeneration, we have purified a novel matrix, HC-HA/PTX3, responsible for the efficacy of AM efficacy. HC-HA complex is covalently formed by hyaluronan (HA) and heavy chain 1 (HC1) of inter-α-trypsin inhibitor by the catalytic action of tumor necrosis factor-stimulated gene-6 (TSG-6) and are tightly associated with pentraxin 3 (PTX3) to form HC-HA/PTX3. In vitro reconstitution of the limbal niche can be established by reunion between limbal epithelial progenitors and limbal niche cells on different substrates. In 3-dimensional Matrigel, clonal expansion indicative of SC renewal is correlated with activation of canonical Wnt signaling and suppression of canonical bone morphogenetic protein (BMP) signaling. In contrast, SC quiescence can be achieved in HC-HA/PTX3 by activation of canonical BMP signaling and non-canonical planar cell polarity (PCP) Wnt signaling, but suppression of canonical Wnt signaling. HC-HA/PTX3 is a novel matrix mitigating nonresolving inflammation and restoring SC quiescence in the niche for various applications in regenerative medicine.