A neuroendocrinological perspective on human hair follicle pigmentation

The role of neurohormones and neuropeptides in human hair follicle (HF) pigmentation extends far beyond the control of melanin synthesis by α‐MSH and ACTH and includes melanoblast differentiation, reactive oxygen species scavenging, maintenance of HF immune privilege, and remodeling of the HF pigmentary unit (HFPU). It is now clear that human HFs are not only a target of multiple neuromediators, but also are a major non‐classical production site for neurohormones such as CRH, proopiomelanocortin, ACTH, α‐MSH, ß‐endorphin, TRH, and melatonin. Moreover, human HFs have established a functional peripheral equivalent of the hypothalamic–pituitary–adrenal axis. By charting the author’s own meanderings through the jungle of hair pigmentation research, the current perspectives essay utilizes four clinical observations – hair repigmentation, canities, poliosis, and ‘overnight greying’– as points of entry into the enigmas and challenges of .pigmentary HF neuroendocrinology. After synthesizing key principles and defining major open questions in the field, selected research avenues are delineated that appear clinically most promising. In this context, novel neuroendocrinological strategies to retard or reverse greying and to reduce damage to the HFPU are discussed.     

Exosomes derived from uMSCs promote hair regrowth in alopecia areata through accelerating human hair follicular keratinocyte proliferation and migration

Alopecia areata (AA) is a complex genetic disease that results in hair loss due to an autoimmune-mediated attack on the hair follicle. Mesenchymal stem cells (MSCs) have great potential to induce hair regeneration due to their strong secretion ability and multidirectional differentiation. Recent studies have revealed that the therapeutic potential of MSCs comes from their secretion ability, which can produce large amounts of bioactive substances and regulate the key physiological functions of subjects. The secretion products of MSCs, such as vesicles, exosomes, and conditioned media, have significant advantages in preparing of biological products derived from stem cells. Human umbilical cord mesenchymal stem cells (uMSCs) are the best choice for exosome production. uMSCs are obtained from the human umbilical cord. The umbilical cord is easy to obtain, and the efficiency of uMSCs isolation and culture higher than that of obtaining MSCs from bone marrow or adipose tissue. In this study, we investigated the effects of exosomes released from uMSCs in AA mice. In summary, due to easy isolation and cultivation, simple preparation, and convenient storage, it is possible to obtain uMSCs, or uMSCs exosomes for research and clinical treatment.     

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

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

Melanocyte stem cells: a melanocyte reservoir in hair follicles for hair and skin pigmentation

Most mammals are coated with pigmented hair. Melanocytes in each hair follicle produce melanin pigments for the hair during each hair cycle. The key to understanding the mechanism of cyclic melanin production is the melanocyte stem cell (MelSC) population, previously known as 'amelanotic melanocytes'. The MelSCs directly adhere to hair follicle stem cells, the niche cells for MelSCs and reside in the hair follicle bulge-subbulge area, the lower permanent portion of the hair follicle, to serve as a melanocyte reservoir for skin and hair pigmentation. MelSCs form a stem cell system within individual hair follicles and provide a 'hair pigmentary unit' for each cycle of hair pigmentation. This review focuses on the identification of MelSCs and their characteristics and explains the importance of the MelSC population in the mechanisms of hair pigmentation, hair greying, and skin repigmentation.     

The biology of hair follicle

The human hair follicle is a unique appendage which results from epithelio-mesenchymal interactions initiated around the 3rd month of development. This appendage has a very complex structure, with more than 20 different cell types distributed into 6 main compartments, namely the connective tissue sheath, the dermal papilla, the outer root sheath, the inner root sheath, the shaft and the sebaceous gland. The pigmentation unit, responsible for hair color, is made of fully active melanocytes located on top of the dermal papilla. This complex appendage has a unique behavior in mammals since, after a hair production phase, it involutes in situ before entering a resting phase after which it renews in a cyclical but stochastic fashion, out of a double reservoir of pluripotent stem cells also to able regenerate epidermis. The pigmentation unit also renews in a cyclical fashion, out of a melanocyte progenitor reservoir which progressively declines with time, provoking the hair whitening process. Finally, the shape of the hair shaft is programmed from the bulb. The hair follicle thus behaves as a fully autonomous skin appendage with its own hormonal control, its own autocrine and paracrine network, its own cycle, appearing as an incredibly complex and stable structure which summarizes the main rules of tissue homeostasis.     

Activation of Notch1 in the hair follicle leads to cell-fate switch and Mohawk alopecia

The Notch signaling pathway has been shown to control cell-fate decisions during mouse development. To study the role of Notch1 in epidermal differentiation and the development of the various cell types within the mouse hair follicle, we generated transgenic mice that express a constitutive activated form of Notch1 under the control of the involucrin promoter. Transgenic animals express the transgene in the suprabasal epidermal keratinocytes and inner root sheath of the hair follicle, and develop both skin and hair abnormalities. Notch1 overexpression leads to an increase of the differentiated cell compartment in the epidermis, delays inner root sheath differentiation, and leads to hair shaft abnormalities and alopecia associated with the anagen phase of the hair cycle.     

脱发相关差异表达基因的生物信息学分析

利用生物信息学方法分析脱发相关差异表达基因,有望帮助了解脱发发生发展的分子机制。本研究从NCBI的子数据库GEO中选择基因表达谱GSE45512和GSE45513数据集,利用R语言limma工具包,筛选出两个物种斑秃样本与正常样本的共同显著差异表达基因。对这部分基因进行功能注释和蛋白互作网络分析,同时对全部差异表达基因进行基因集富集分析。结果发现,人头皮斑秃样本共筛选出225个差异表达基因;C3H/HeJ小鼠自发斑秃皮肤样本共筛选出337个差异表达基因;两个物种的共同显著差异表达基因有23个。GO功能富集分析和蛋白互作网络分析显示,这部分差异基因显著富集于免疫相关功能,并且彼此间存在蛋白互作关系。基因集富集分析显示两个物种的差异基因都能显著富集到趋化因子信号通路、细胞因子受体相互作用、金葡菌感染及抗原加工与呈递通路;而且人的下调差异基因不仅映射到了人类表型数据库的脱发表型,也映射到皮肤附属物病理相关表型。综上所述,本研究通过生物信息方法分析脱发皮肤组织与正常皮肤组织的差异表达基因,最终筛选出23个在人和小鼠中共同存在的显著差异表达基因;此外,分析发现脱发与免疫过程及皮肤附属物病变密切相关,这些结果为脱发的诊断和治疗提供了新思路。     

Human hair melanins: what we have learned and have not learned from mouse coat color pigmentation

Hair pigmentation is one of the most conspicuous phenotypes in humans. Melanocytes produce two distinct types of melanin pigment: brown to black, indolic eumelanin and yellow to reddish brown, sulfur‐containing pheomelanin. Biochemically, the precursor tyrosine and the key enzyme tyrosinase and the tyrosinase‐related proteins are involved in eumelanogenesis, while only the additional presence of cysteine is necessary for pheomelanogenesis. Other important proteins involved in melanogenesis include P protein, MATP protein, α‐MSH, agouti signaling protein (ASIP), MC1R (the receptor for MSH and ASIP), and SLC7A11, a cystine transporter. Many studies have examined the effects of loss‐of‐function mutations of those proteins on mouse coat color pigmentation. In contrast, much less is known regarding the effects of mutations of the corresponding proteins on human hair pigmentation except for MC1R polymorphisms that lead to pheomelanogenesis. This perspective will discuss what we have/have not learned from mouse coat color pigmentation, with special emphasis on the significant roles of pH and the level of cysteine in melanosomes in controlling melanogenesis. Based on these data, a hypothesis is proposed to explain the diversity of human hair pigmentation.     

Acid hydrolysis reveals a low but constant level of pheomelanin in human black to brown hair

We previously reported a constant ratio of the benzothiazole pheomelanin marker thiazole‐2,4,5‐tricarboxylic acid (TTCA) to the eumelanin marker pyrrole‐2,3,5‐tricarboxylic acid (PTCA) in eumelanic, black human hair. A constant level (20%–25%) of benzothiazole‐type pheomelanin was recently demonstrated in human skin with varying concentrations of melanin. Therefore, in this study, we aimed to investigate the origin of pheomelanin markers in black to brown human hair by developing a method to remove protein components from hair by heating with 6 M HCl at 110°C for 16 hr. For comparison, synthetic melanins were prepared by oxidizing mixtures of varying ratios of dopa and cysteine with tyrosinase. Hair melanins and synthetic melanins were subjected to acid hydrolysis followed by alkaline H₂O₂ oxidation. The results show that the hydrolysis leads to decarboxylation of the 5,6‐di‐hydroxyindole‐2‐carboxylic acid moiety in eumelanin and the benzothiazole moiety in pheomelanin and that eumelanic human hair contains 11%–17% benzothiazole‐type pheomelanin.     

皮肤毛囊发育的转录组研究进展

近年来,转录组测序技术在动物重要经济性状受复杂基因网络的调控研究领域取得了显著的成果。作为哺乳动物皮肤的衍生物,毛囊是唯一具有高度自我更新能力、独特的可再生器官,毛囊细胞经增殖分化最终形成毛发。已有的研究表明,诸多生长因子及其受体作为体内分泌协调基因的重要因素,对毛发的生长发育起着重要的调控作用。文章综述了近年来转录组测序技术在人、小鼠及羊等生物的皮肤毛囊发育和再生过程中基因调控方式的研究进展,旨在为今后人工干扰绒毛周期生长发育和分子育种提供理论依据,同时也为皮肤毛囊相关疾病的临床治疗提供新思路。     

Functional role of beta 1 integrin-mediated signalling in the human hair follicle

Integrins are transmembrane adhesion proteins that convey critical topobiological information and exert crucial signalling functions. In skin and hair follicle biology, beta1 integrins and their ligands are of particular interest. It is not yet known whether beta1 integrins play any role in the regulation of human hair growth and the expression pattern of beta1 integrin in the human pilosebaceous unit remains ill-defined. Here, we show that pilosebaceous immunoreactivity for beta1 integrin is most prominent in the outermost layer of the outer root sheath and the surrounding connective tissue sheath of human scalp hair follicles in situ and in vitro. Sites of beta1 integrin immunoreactivity co-express fibronectin and tenascin-C. Contrary to previous reports, beta1 integrin immunoreactivity in situ was not significantly upregulated in the human bulge region. Functionally, two beta1 integrin-activating antibodies (12G10, TS2/16) and ligand-mimicking RGD peptides promoted the growth of microdissected, organ-cultured human scalp hair follicles in vitro and inhibited spontaneous hair follicle regression. This supports the concept that beta1 integrin-mediated signalling is also important in human hair growth control. The physiologically relevant organ culture assay employed here is a potential research tool for exploring whether targeted stimulation of beta1 integrin-mediated signalling is a suitable candidate for human hair loss management.     

Establishment of an in vitro organoid model of dermal papilla of human hair follicle

Human hair dermal papilla (DP) cells are specialized mesenchymal cells that play a pivotal role in hair regeneration and hair cycle activation. The current study aimed to first develop three‐dimensional (3D) DP spheroids (DPS) with or without a silk–gelatin (SG) microenvironment, which showed enhanced DP‐specific gene expression, resulting in enhanced extracellular matrix (ECM) production compared with a monolayer culture. We tested the feasibility of using this DPS model for drug screening by using minoxidil, which is a standard drug for androgenic alopecia. Minoxidil‐treated DPS showed enhanced expression of growth factors and ECM proteins. Further, an attempt has been made to establish an in vitro 3D organoid model consisting of DPS encapsulated by SG hydrogel and hair follicle (HF) keratinocytes and stem cells. This HF organoid model showed the importance of structural features, cell–cell interaction, and hypoxia akin to in vivo HF. The study helped to elucidate the molecular mechanisms to stimulate cell proliferation, cell viability, and elevated expression of HF markers as well as epithelial–mesenchymal crosstalks, demonstrating high relevance to human HF biology. This simple in vitro DP organoid model system has the potential to provide significant insights into the underlying mechanisms of HF morphogenesis, distinct molecular signals relevant to different stages of the hair cycle, and hence can be used for controlled evaluation of the efficacy of new drug molecules.     

A curated gene list for expanding the horizons of pigmentation biology

Over the past century, studies of human pigmentary disorders along with mouse and zebrafish models have shed light on the many cellular functions associated with visible pigment phenotypes. This has led to numerous genes annotated with the ontology term “pigmentation” in independent human, mouse, and zebrafish databases. Comparisons among these datasets revealed that each is individually incomplete in documenting all genes involved in integument‐based pigmentation phenotypes. Additionally, each database contained inherent species‐specific biases in data annotation, and the term “pigmentation” did not solely reflect integument pigmentation phenotypes. This review presents a comprehensive, cross‐species list of 650 genes involved in pigmentation phenotypes that was compiled with extensive manual curation of genes annotated in OMIM, MGI, ZFIN, and GO. The resulting cross‐species list of genes both intrinsic and extrinsic to integument pigment cells provides a valuable tool that can be used to expand our knowledge of complex, pigmentation‐associated pathways.     

MicroRNA对皮肤毛囊发育的调控机制

MicroRNA是参与转录后水平表达调控的重要因子, 在病理上成为药物作用的潜在靶点, 在生理上成为表型调控的潜在位点。目前, 对于microRNA的功能已有一定了解, 但其在皮肤毛囊发育中的作用机制还不完全清楚。近年来, 高通量测序技术为microRNA的鉴定提供了更准确、快速的途径, 研究发现一些microRNA能够影响皮肤毛囊细胞的分化和增殖, 其相关靶基因在调控毛囊周期性生长的过程中充当重要角色。文章综述了近年来microRNA在皮肤毛囊生长发育调控机制研究领域所取得的成果, 以期为后续开展绒山羊毛囊生长相关microRNA作用机制研究提供借鉴。