Human Hair Graying Revisited: Principles, Misconceptions, and Key Research Frontiers

Graying: gerontobiology of the hair follicle pigmentary unit.

BackgroundCashmere goat is known for its precious undercoat. Being photoperiod-dictated, cashmere growth has been studied focusing mainly on hair follicle cycle phases (anagen, catagen and telogen). An accurate molecular knowledge of the goat hair follicle cycle, disentangling gene expression changes during phases and recognizing timing boundaries, could be useful to improve cashmere goat management and ultimately cashmere production.ResultsTo better describe goat’s hair follicle transcriptome we applied RNA-sequencing to isolated hair follicles from five Italian cashmere goats, during the anagen and catagen phase, identifying total of 214 differentially expressed genes (DEGs): 97 were up-regulated while 117 were down-regulated in catagen with respect to anagen. Gene Ontology and pathway analysis were performed. We detected 144 significant pathways spanning from estrogen, pluripotency of stem cells, thermogenesis and fatty acid metabolism that were strongly expressed during the hair follicle phases analysed. Finally, we validated promising DEGs by RT-qPCR in the same set of samples as well as in hair follicles and entire skin biopsies of another cashmere goats cohort accounting for early anagen, anagen, early catagen, and catagen phases.ConclusionsAs in the isolated hair follicles, some target genes were homogenously modulated during the four hair follicle phases. Ceruloplasmin (CP) and Keratin 4 (K4), confirmed their clear cut expression between growing and resting phase. In fact, K4 was almost absent in catagen phases while CP was barely expressed in anagen phases. In particular, the strong expression of K4 in early anagen makes it an eligible marker to track the beginning of a new hair cycle, and therefore defining the optimum time for cashmere harvesting.

Hair follicle growth is cyclical, and hair cycle dysfunction can lead to hair follicle-related disorders, including alopecia and hirsutism. The objective was to investigate the influence and underlying mechanism of Krüppel-like factor 4 (KLF4) overexpression on hair follicle growth and development in C57BL/6 mice. To provide a theoretical basis for the biological functions of KLF4 gene in hair follicle development and hair follicle cycle, mice were assigned to three groups: experimental, overexpressing KLF4 (Ad-KLF4); control, expressing green fluorescent protein (Ad-NC); and blank, no treatment. Fur was removed from the dorsal surface, and the mice were intradermally injected with 25 μL 1 × 1010 PFU/mL adenovirus vector (Ad-KLF4 or Ad-NC) at three points. Samples were collected for molecular biological and histological analysis. It was found that mRNA and protein levels of Wnt pathway-associated factors β-catenin, LEF1, hair follicle cell proliferation-related factor Ki67, and hair follicle inner caledrin marker AE15 were all significantly greater in the Ad-NC and blank groups than in Ad-KLF4 mice (P < 0.01). These findings were confirmed by immunohistochemical analysis. Hair growth was monitored photographically for 14days, showing an absence of growth in the injected region of the KLF4-overexpressing mice in contrast to non-overexpressing areas where hair growth was normal. HE staining showed that hair follicles in the blank and Ad-NC mice were normal, while those in the KLF4-overexpressing areas remained in telogen or early anagen with spherical dermal papillae situated at the edge of the dermis and subcutaneous tissue without an inner heel sheath. In conclusion, it was found that KLF4 downregulated key Wnt/β-catenin-associated factors during follicular regeneration in mice, reducing both follicular development and growth.

Adipocytes are widely distributed in the dermis, in a unique fat depot referred to as dermal white adipose tissue (dWAT). In rodents, dWAT is present as widespread thin layers, whereas in pigs and humans, it is present in clusters referred to as 'dermal cones' around the pilosebaceous units. This distinct layer of fat cells located above the subcutaneous white adipose tissue is important for proper hair follicle (HF) cycling in rodents. Murine HFs produce spatially restricted synchronous patches after their second postnatal cycle which correlates with the spatial heterogeneity of murine dWAT. Similarly, the cycling of HFs in humans may also be related to the spatial distribution of dWAT, making the difference between murine and human HF cycling of more quantitative than of qualitative nature. This should allow the production of small spatially correlated HF patches in human skin, and we propose that this process can be regulated by paracrine signalling involving a number of signalling modules, including the hedgehog pathway. This pathway is an established player in HF cycling, but is also involved in the regulation of adipogenesis and may therefore be a key regulator of the process across species. We also suggest that the spatial heterogeneity of dWAT is connected not only to HF cycling, but may also be related to other physiological and pathological processes in the skin.

Leptin (Lep) stimulates keratinocytes to proliferate, intervenes in the wound healing and participates to hair follicle morphogenesis and cycle. While it is secreted by skin structures including epidermis and hair follicles, intradermal adipose tissue also seems to have a role in Lep secretion and accordingly in the control of hair follicle growth in mice and humans. Lep was investigated in the skin of humans and laboratory animals but there are not data regarding bovine species. The aim of this work was to study the expression of Lep and its receptor (LepR) in the skin of bovine and, at the same time, to investigate the presence and extension of intradermal adipose tissue. A morphological evaluation of the skin was performed while the presence and localization of Lep and LepR were analyzed by RTPCR and immunohistochemistry. A high and thick dermis without adipocytes was observed. Hair follicles and sebaceous and sweat glands were located in the proximal part of the skin while a thick layer of connective tissue, lacking adipose cells, separated these structures by subcutis. RT-PCR evidenced the transcripts for both molecules. By immunohistochemistry, Lep and LepR were observed in the epidermis and hair follicles. Based on the absence of intradermal adipose tissue and the presence of both Lep and LepR in the epidermis and in the hair follicle epithelium, it can be posited that in bovine skin Lep participates to the control of epidermis growth and hair follicle cycle through a paracrine and autocrine mechanism.

Do Hair Bulb Melanocytes Undergo Apotosis During Hair Follicle Regression (Catagen)?

Sheep body size can directly reflect the growth rates and fattening rates of sheep and is also an important index for measuring the growth performance of meat sheep.Inner Mongolia Cashmere Goat is a local excellent breed of cashmere and meat dual-purpose, which is a typical heterogeneous indumentum. The hair follicles cycle through periods of vigorous growth (anagen), a regression caused by apoptosis (catagen), and relative rest (telogen). At present, it is not clear which genes affect the cycle transformation of hair follicles and unclear how proteins impact the creation and expansion of hair follicles.we using multi-omics joint analysis methodologies to investigated the possible pathways of transformation and apoptosis in goat hair follicles. The results showed that 917,1,187, and 716 proteins were specifically expressed in anagen, catagen andtelogen. The result of gene ontology (GO) annotation showed that differentially expressed proteins (DEPs) are in different growth cycle periods, and enriched GO items are mostly related to the transformation of cells and proteins. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment result indicated that the apoptosis process has a great impact on hair follicle's growth cycle. The results of the protein interaction network of differential proteins showed that the ribosomal protein family (RPL4, RPL8, RPS16, RPS18, RPS2, RPS27A, RPS3) was the core protein in the network. The results of combined transcriptome and proteomics analysis showed that there were 16,34, and 26 overlapped DEGs and DEPs in the comparison of anagen VS catagen, catagen VS telogen and anagen VS telogen, of which API5 plays an important role in regulating protein and gene expression levels. We focused on API5 and Ribosomal protein and found that API5 affected the apoptosis process of hair follicles, and ribosomal protein was highly expressed in the resting stage of hair follicles. They are both useful as molecular marker candidate genes to study hair follicle growth and apoptosis,and they both have an essential function in the cycle transition process of hair follicles. The results of this study may provide a theoretical basis for further research on the growth and development of hair follicles in Inner Mongolian Cashmere goats.

Oncogene-induced senescence acts as a barrier against tumour formation and has been implicated as the mechanism preventing the transformation of benign melanocytic lesions that frequently harbour oncogenic B-RAF or N-RAS mutations. In the present study we systematically assessed the relative importance of the tumour suppressor proteins p53, p21(Waf1), pRb and p16(INK4a) in mediating oncogene-induced senescence in human melanocytes. We now show that oncogenic N-RAS induced senescence in melanocytes is associated with DNA damage, a potent DNA damage response and the activation of both the p16(INK4a)/pRb and p53/p21(Waf1) tumour suppressor pathways. Surprisingly neither the pharmacological inhibition of the DNA damage response pathway nor silencing of p53 expression had any detectable impact on oncogene-induced senescence in human melanocytes. Our data indicate that the pRb pathway is the dominant effector of senescence in these cells, as its specific inactivation delays the onset of senescence and weakens oncogene-induced proliferative arrest. Furthermore, we show that although both p16(INK4a) and p21(Waf1) are upregulated in response to N-RAS(Q61K), the activities of these CDK inhibitors are clearly distinct and only the loss of p16(INK4a) weakens senescence. We propose that the ability of p16(INK4a) to inhibit the cyclin D-dependent kinases and DNA replication, functions not shared by p21(Waf1), contribute to its role in senescence. Thus, in melanocytes with oncogenic signalling only p16(INK4a) can fully engage the pRb pathway to alter chromatin structure and silence the genes that are required for proliferation.

DNA repair and DNA damage signaling pathways are critical for the maintenance of genomic stability. Defects of DNA repair and damage signaling contribute to tumorigenesis, but also render cancer cells vulnerable to DNA damage and reliant on remaining repair and signaling activities. Here, we review the major classes of DNA repair and damage signaling defects in cancer, the genomic instability that they give rise to, and therapeutic strategies to exploit the resulting vulnerabilities. Furthermore, we discuss the impacts of DNA repair defects on both targeted therapy and immunotherapy, and highlight emerging principles for targeting DNA repair defects in cancer therapy.

There are many studies devoted to the role of hair follicle stem cells in wound healing as well as in follicle self-restoration. At the same time, the influence of the inflammatory cells on the hair follicle cycling in both injured and intact skin is well established. Immune cells of all wound healing stages, including macrophages, γδT cells, and Tregs, may activate epidermal stem cells to provide re-epithelization and wound-induced hair follicle neogenesis. In addition to the ability of epidermal cells to maintain epidermal morphogenesis through differentiation program, they can undergo de-differentiation and acquire stem features under the influence of inflammatory milieu. Simultaneously, a stem cell compartment may undergo re-programming to adopt another fate. The proportion of skin resident immune cells and wound-attracted inflammatory cells (e.g., neutrophils and macrophages) in wound-induced hair follicle anagen and plucking-induced anagen is still under discussion to date. Experimental data suggesting the role of reactive oxygen species and prostaglandins, which are uncharacteristic of the intact skin, in the hair follicle cycling indicates the role of neutrophils in injury-induced conditions. In this review, we discuss some of the hair follicles stem cell activities, such as wound-induced hair follicle neogenesis, hair follicle cycling, and re-epithelization, through the prism of inflammation. The plasticity of epidermal stem cells under the influence of inflammatory microenvironment is considered. The relationship between inflammation, scarring, and follicle neogenesis as an indicator of complete wound healing is also highlighted. Taking into consideration the available data, we also conclude that there may exist a presumptive interlink between the stem cell activation, inflammation and the components of programmed cell death pathways.

BackgroundThe hair coat is available for the yak to live in the harsh environment of the plateau. Besides, improving the hair production of yak is necessary for its textile industry development. Hair grows from hair follicles (HFs). The HFs undergo periodic growth after birth and are regulated by the complex gene regulatory network. However, the molecular mechanism of HFs regeneration in the Tianzhu white yak remains unclear. RNA editing is a post-transcriptional mechanism that regulates gene expression and produces new transcripts. Hence, we investigated the influence of the A-to-I RNA editing events on the HFs cycle of the Tianzhu white yak.ResultsWe finally identified 54,707 adenosine-to-inosine (A-to-I) RNA editing sites (RESs) from RNA sequencing data of the HFs cycle in the Tianzhu white yak. Annotation results showed RESs caused missense amino acid changes in 7 known genes. And 202 A-to-I editing sites altered 23 target genes of 140 microRNAs. A total of 1,722 differential RESs were identified during the HFs cycle of Tianzhu white yak. GO and KEGG enrichment analysis revealed several signaling pathways and GO terms involved skin development, hair growth, and HFs cycle. Such as genes with differential RNA editing levels were significantly enriched in the peroxisome, metabolic pathways, Notch signaling pathway, and PPAR signaling pathway. Besides, the editing sites in HFs development-related genes FAS, APCDD1, WWOX, MPZL3, RUNX1, KANK2, DCN, DSC2, LEPR, HEPHL1, and PTK2B were suggested as the potential RESs involving HFs development.ConclusionThis study investigated the global A-to-I RNA editing events during the HFs cycle of yak skin tissue and expanded the knowledge of A-to-I RNA editing on the HFs cycle. Furthermore, this study revealed that RNA editing-influenced genes may regulate the HFs cycle by participating in the HFs development-related pathways. The findings might provide new insight into the regulation of RNA editing in hair growth.

Hair greying depends on the altered presence and functionality of hair follicle melanocytes. Melanocyte stem cells (MelSCs) reside in the bulge of hair follicles and give rise to migrating and differentiating progeny during the anagen phase. Ageing, genotoxic stress, redox stress and multiple behaviour-associated acute stressors have been seen to induce hair greying by depleting the MelSC pool, a phenomenon which is accompanied by ectopic pigmentation of these cells, followed by their depletion from the stem cell niche. This aberrant differentiation produces a state from which a return to stem cell-like quiescence appears to be lost. The cellular features of stress-induced hair greying have been extensively studied in murine models. Here, we describe a method to assess and quantify human hair follicle MelSC differentiation by measuring ectopically pigmented MelSCs in isolated human hair follicles exposed to specific stress signal mediators. Ionizing radiation, hydrogen peroxide and noradrenaline have been shown to cause hair greying in mice. We demonstrate here that isolated, ex vivo cultured human hair follicles exposed to these treatments display similar ectopic pigmentation within the bulge area which is accompanied by induction of differentiated melanocytic markers. This study suggests that as in murine models, stress signalling induces closely matching phenotypic changes in human hair follicles which can be monitored and studied as a surrogate model for early steps in human hair greying.

FGF5 and FGF18 are key factors in the regulation of the hair follicle cycle. FGF5 is overexpressed during the late anagen phase and serves as a crucial regulatory factor that promotes the anagen-to-catagen transition in the hair follicle cycle. FGF18, which is overexpressed during the telogen phase, mainly regulates the hair follicle cycle by maintaining the telogen phase and inhibiting the entry of hair follicles into the anagen phase. The inhibition of FGF5 may prolong the anagen phase, whereas the inhibition of FGF18 may promote the transition of the hair follicles from the telogen phase to the anagen phase. In the present study, we used siRNA to suppress FGF5 or FGF18 expression as a way to inhibit the activity of these genes. Using qPCR, we showed that FGF5-targeting siRNA modified by cholesterol was more effective than the same siRNA bound to a cell-penetrating peptide at suppressing the expression of FGF5 both in vitro and in vivo. We then investigated the effects of the cholesterol-modified siRNA targeting either FGF5 or FGF18 on the hair follicle cycle in a depilated area of the skin on the back of mice. The cholesterol-modified siRNA, delivered by intradermal injection, effectively regulated the hair follicle cycle by inhibiting the expression of FGF5 and FGF18. More specifically, intradermal injection of a cholesterol-modified FGF5-targeted siRNA effectively prolonged the anagen phase of the hair follicles, whereas intradermal injection of the cholesterol-modified FGF18-targeted siRNA led to the mobilization of telogen follicles to enter the anagen phase earlier. The inhibitory effect of the cholesterol-modified FGF18-targeted siRNA on FGF18 expression was also evaluated for a topically applied siRNA. Topical application of a cream containing the cholesterol-modified FGF18-targeted siRNA on a depilated area of the skin of the back of mice revealed comparable inhibition of FGF18 expression with that observed for the same siRNA delivered by intradermal injection. These findings suggested that alopecia could be prevented and hair regrowth could be restored either through the intradermal injection of cholesterol-modified siRNA targeting FGF5 or FGF18 or the topical application of FGF18 siRNA.

Introduction: Besides the primary role of hair follicles in producing hair shafts, these mini-organs have been shown to contribute to the healing and remodelling of the skin in the response to injury. There is an increasing number of clinical and experimental studies exploring the mechanisms by which hair follicles contribute to skin recovery. Mouse tracing experiments demonstrated that after skin injury, epithelial stem cells migrate out of the follicle to support wound re-epithelialization, while the mesenchymal cells from the hair follicle are mobilised to migrate into the wound bed and contribute to the repair of the dermis. Accordingly, there is a significant delay in wound healing in the absence of hair follicles, observed both in experimental studies in mice and in humans during clinical practice. In the healthy state, hair follicles induce changes to the surrounding skin, which occur in synchrony with the hair follicle cycle. Besides supporting wound healing and healthy skin remodelling, it remains to be investigated whether hair follicles could also remodel the fibrotic tissue formed after skin injuries. Methods: For the selection of literature covered in this chapter, we used Pubmed database with the keywords hair follicles in wound healing and skin remodelling, and hair follicles transplantation and wound healing. Publications related to the clinical use of hair follicle grafts for healing chronic cutaneous wounds are also discussed. Results: A large body of evidence supports the role of hair follicles in wound healing and skin remodelling. As demonstrated in several clinical studies, transplantation of autologous hair follicle grafts into chronic ulcers and difficult to heal wounds accelerate the healing of wounds. Conclusions: There is a great potential in exploring the role of hair follicles in wound healing and skin regeneration. Once we understand the exact mechanism by which hair follicles contribute to skin recovery, we can try to mimic this effect with therapeutic solutions. A potential approach involves deciphering the populations of hair follicle cells that contribute to the observed effect and injecting them directly into wounds or scar tissue to improve the clinical outcomes. Alternatively, elucidating the paracrine effect of transplanted follicles would open up new avenues for therapeutic discovery, to replicate the combination of required factors that facilitate healing or scar remodelling.KeywordsHair folliclesWound healingSkin remodellingHair follicle cycleAutologous hair transplantation

Objective To investigate expression characteristics in regenerating hair follicles induced by Wnt10b, and to explore mechanisms underlying Wnt10b - induced regeneration of hair follicles. Methods Both adenovirus containing the Wnt10b gene (AdWnt10b) and that containing the green fluorescent protein-encoding gene (AdGFP) were amplified in HEK-293 cells and purified by caesium chloride density gradient centrifugation. A total of 36 C57BL/6J mice were randomly and equally divided into the AdWnt10b group and AdGFP group to be intracutaneously injected with AdWnt10b and AdGFP on the back respectively. Three mice were sacrificed on day 2.5, 5, 7, 9, 14 and 28 after the injection separately, and skin samples were resected from the injected sites subsequently. Hematoxylin and eosin (HE) staining and immunohistochemical staining were performed to observe hair follicle structure, analyze expression characteristics of the Wnt signaling pathway, and to estimate proliferative activity of regenerating hair follicles induced by Wnt10b. Results As HE staining showed, new hair follicles appeared as early as day 5 after the injection, then grew normally, and entered into catagen phase on day 28 in the AdWnt10b group. Immunohistochemical staining showed that AE15 expression was observed in new hair follicles as early as day 5 after the injection in the AdWnt10b group, then increased along with the growth of hair follicles, but decreased on day 28. On day 5 after AdWnt10b injection, both β-catenin and Lef1 expressions were seen in the cell nucleus. Lef1 was expressed specifically in hair germs and hair matrix, and its expression began to decrease on day 28. In addition, Ki67 expression was observed in the epidermis and outer root sheath of hair follicles as early as day 2.5 after the injection, in the bulge region of hair follicles on day 2.5, 7, 9 and 14, and in hair matrix cells as early as day 7. Conclusion Wnt10b could induce regeneration of hair follicles with normal structure, likely by activating the canonical Wnt signaling pathway in hair follicle stem cells and their daughter cells. Key words: Hair follicle; Regeneration; Stem cells; Wnt signaling pathway; Ki-67 antigen; beta Catenin