Dedifferentiation maintains melanocyte stem cells in a dynamic niche.

For unknow reasons, the melanocyte stem cell (McSC) system fails earlier than other adult stem cell populations1, which leads to hair greying in most humans and mice2,3. Current dogma states that McSCs are reserved in an undifferentiated state in the hair follicle niche, physically segregated from differentiated progeny that migrate away following cues of regenerative stimuli4-8. Here we show that most McSCs toggle between transit-amplifying and stem cell states for both self-renewal and generation of mature progeny, a mechanism fundamentally distinct from those of other self-renewing systems. Live imaging and single-cell RNA sequencing revealed that McSCs are mobile, translocating between hair follicle stem cell and transit-amplifying compartments where they reversibly enter distinct differentiation states governed by local microenvironmental cues (for example, WNT). Long-term lineage tracing demonstrated that the McSC system is maintained by reverted McSCs rather than by reserved stem cells inherently exempt from reversible changes. During ageing, there is accumulation of stranded McSCs that do not contribute to the regeneration of melanocyte progeny. These results identify a new model whereby dedifferentiation is integral to homeostatic stem cell maintenance and suggest that modulating McSC mobility may represent a new approach for the prevention of hair greying.

Bear essentials.

Strangely, American black bears come in many colours. New work by Puckett et al. shows that a missense alteration in the gene encoding tyrosinase-related protein 1 (TYRP1) likely interferes with melanin synthesis and is responsible for the cinnamon colour variant in the southwest USA. However, the adaptive significance of colour polymorphisms in this large carnivore remains opaque.

Hair-bearing human skin generated entirely from pluripotent stem cells.

The skin is a multilayered organ, equipped with appendages (that is, follicles and glands), that is critical for regulating body temperature and the retention of bodily fluids, guarding against external stresses and mediating the sensation of touch and pain1,2. Reconstructing appendage-bearing skin in cultures and in bioengineered grafts is a biomedical challenge that has yet to be met3-9. Here we report an organoid culture system that generates complex skin from human pluripotent stem cells. We use stepwise modulation of the transforming growth factor ß (TGFß) and fibroblast growth factor (FGF) signalling pathways to co-induce cranial epithelial cells and neural crest cells within a spherical cell aggregate. During an incubation period of 4-5 months, we observe the emergence of a cyst-like skin organoid composed of stratified epidermis, fat-rich dermis and pigmented hair follicles that are equipped with sebaceous glands. A network of sensory neurons and Schwann cells form nerve-like bundles that target Merkel cells in organoid hair follicles, mimicking the neural circuitry associated with human touch. Single-cell RNA sequencing and direct comparison to fetal specimens suggest that the skin organoids are equivalent to the facial skin of human fetuses in the second trimester of development. Moreover, we show that skin organoids form planar hair-bearing skin when grafted onto nude mice. Together, our results demonstrate that nearly complete skin can self-assemble in vitro and be used to reconstitute skin in vivo. We anticipate that our skin organoids will provide a foundation for future studies of human skin development, disease modelling and reconstructive surgery.

Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells.

Empirical and anecdotal evidence has associated stress with accelerated hair greying (formation of unpigmented hairs)1,2, but so far there has been little scientific validation of this link. Here we report that, in mice, acute stress leads to hair greying through the fast depletion of melanocyte stem cells. Using a combination of adrenalectomy, denervation, chemogenetics3,4, cell ablation and knockout of the adrenergic receptor specifically in melanocyte stem cells, we find that the stress-induced loss of melanocyte stem cells is independent of immune attack or adrenal stress hormones. Instead, hair greying results from activation of the sympathetic nerves that innervate the melanocyte stem-cell niche. Under conditions of stress, the activation of these sympathetic nerves leads to burst release of the neurotransmitter noradrenaline (also known as norepinephrine). This causes quiescent melanocyte stem cells to proliferate rapidly, and is followed by their differentiation, migration and permanent depletion from the niche. Transient suppression of the proliferation of melanocyte stem cells prevents stress-induced hair greying. Our study demonstrates that neuronal activity that is induced by acute stress can drive a rapid and permanent loss of somatic stem cells, and illustrates an example in which the maintenance of somatic stem cells is directly influenced by the overall physiological state of the organism.

Premature gray hair development in the interbrow region owing to the loss of maxillary first molars in young mice.

The exact sites of premature hair graying and whether tooth loss causes this condition remain unknown. In this study, we aimed to explore the effect of reduced mastication on premature hair graying. Maxillary first molars were extracted from young mice, and the mice were observed for 3 months, along with non-extraction control group mice. After 3 months, gray hair emerged in the interbrow region of mice in the tooth extraction group but not in the control group. The expression of tyrosinase-related protein-2 (TRP-2) mRNA was lower in the interbrow tissues of young mice without maxillary molars than in those with maxillary molars. Tooth loss leads to interbrow gray hair growth, possibly because of weakened trigeminal nerve input, suggesting that reduced mastication causes premature graying. Thus, prompt prosthetic treatment after molar loss is highly recommended.

mTORC1 activity negatively regulates human hair follicle growth and pigmentation.

Dysregulation of the activity of the mechanistic target of rapamycin complex 1 (mTORC1) is commonly linked to aging, cancer, and genetic disorders such as tuberous sclerosis (TS), a rare neurodevelopmental multisystemic disease characterized by benign tumors, seizures, and intellectual disability. Although patches of white hair on the scalp (poliosis) are considered as early signs of TS, the underlying molecular mechanisms and potential involvement of mTORC1 in hair depigmentation remain unclear. Here, we have used healthy, organ-cultured human scalp hair follicles (HFs) to interrogate the role of mTORC1 in a prototypic human (mini-)organ. Gray/white HFs exhibit high mTORC1 activity, while mTORC1 inhibition by rapamycin stimulated HF growth and pigmentation, even in gray/white HFs that still contained some surviving melanocytes. Mechanistically, this occurred via increased intrafollicular production of the melanotropic hormone, α-MSH. In contrast, knockdown of intrafollicular TSC2, a negative regulator of mTORC1, significantly reduced HF pigmentation. Our findings introduce mTORC1 activity as an important negative regulator of human HF growth and pigmentation and suggest that pharmacological mTORC1 inhibition could become a novel strategy in the management of hair loss and depigmentation disorders.

Genomic regions associated with coat color in Gir cattle.

Indicine cattle breeds are adapted to the tropical climate, and their coat plays an important role in this process. Coat color influences thermoregulation and the adhesion of ectoparasites and may be associated with productive and reproductive traits. Furthermore, coat color is used for breed qualification, with breeders preferring certain colors. The Gir cattle is characterized by a wide variety of coat colors. Therefore, we performed genome-wide association studies to identify candidate genes for coat color in Gir cattle. Different phenotype scenarios were considered in the analyses and regions were identified on eight chromosomes. Some regions and many candidate genes are influencing coat color in the Gir cattle, which was found to be a polygenic trait. The candidate genes identified have been associated with white spotting patterns and base coat color in cattle and other species. In addition, a possible epistatic effect on coat color determination in the Gir cattle was suggested. This is the first published study that identified genomic regions and listed candidate genes associated with coat color in Gir cattle. The findings provided a better understanding of the genetic architecture of the trait in the breed and will allow to guide future fine-mapping studies for the development of genetic markers for selection.

Eye and hair color prediction of human DNA recovered from Lucilia sericata larvae.

Forensic entomological evidence is employed to estimate minimum postmortem interval (PMImin), location, and identification of fly samples or human remains. Traditional forensic DNA analysis (i.e., STR, mitochondrial DNA) has been used for human identification from the larval gut contents. Forensic DNA phenotyping (FDP), predicting human appearance from DNA-based crime scene evidence, has become an established approach in forensic genetics in the past years. In this study, we aimed to recover human DNA from Lucilia sericata (Meigen 1826) (Diptera: Calliphoridae) gut contents and predict the eye and hair color of individuals using the HIrisPlex system. Lucilia sericata larvae and reference blood samples were collected from 30 human volunteers who were under maggot debridement therapy. The human DNA was extracted from the crop contents and quantified. HIrisPlex multiplex analysis was performed using the SNaPshot minisequencing procedure. The HIrisPlex online tool was used to assess the prediction of the eye and hair color of the larval and reference samples. We successfully genotyped 25 out of 30 larval samples, and the most SNP genotypes (87.13%) matched those of reference samples, though some alleles were dropped out, producing partial profiles. The prediction of the eye colors was accurate in 17 out of 25 larval samples, and only one sample was misclassified. Fourteen out of 25 larval samples were correctly predicted for hair color, and eight were misclassified. This study shows that SNP analysis of L. sericata gut contents can be used to predict eye and hair color of a corpse.

Canine coat color E locus updates: Identification of a new MC1R variant causing 'sable' coat color in English Cocker Spaniels and a proposed update to the E locus dominance hierarchy.

The coat color phenotype 'sable' occurs in the English Cocker Spaniel dog breed. It closely resembles other canine color patterns known as domino/grizzle/pied (eA allele) and grizzle/domino (eG allele) determined by variants in the melanocortin 1 receptor gene (MC1R; 'extension' or E locus), a key multi-allele regulator of coat color. We examined genetic variation in MC1R, and found one new non-synonymous variant, c.250G>A (p.(Asp84Asn)), consistently associated with the English Cocker Spaniel 'sable' phenotype. We propose calling this newly identified allele eH and further show that the eA , eH and eG (previously known as EG ) alleles associate with similar phenotypes in dogs impacting genotypes regulated by beta-defensin 103 gene (CBD103; K locus) and agouti signaling protein gene (ASIP; A locus) in the absence of the EM and E alleles. This suggests that all three alleles are putative reduced-function variants of the MC1R gene. We propose the revised and updated E locus dominance hierarchy to be EM > E > eA /eH /eG > e1-3 .

A new Finnish flavor of feline coat coloration, "salmiak," is associated with a 95-kb deletion downstream of the KIT gene.

Cats with a distinctive white hair pattern of unknown molecular cause have been discovered in the Finnish domestic cat population. Based on the unique appearance of these cats, we have named this phenotype salmiak ("salty licorice"). The use of a commercially available panel test to genotype four salmiak-colored cats revealed the absence of all known variants associated with white-haired phenotypic loci: full White (W), Spotting (Ws) and the Birman white Gloves associated (wg) allele of the KIT proto-oncogene (KIT) gene. Whole-genome sequencing on two salmiak-colored cats was conducted to search for candidate causal variants in the KIT gene. Despite a lack of coding variants, visual inspection of the short read alignments revealed a large ~95 kb deletion located ~65 kb downstream of the KIT gene in the salmiak cats. Additional PCR genotyping of 180 domestic cats and three salmiak-colored cats confirmed the homozygous derived variant genotype fully concordant with the salmiak phenotype. We suggest the newly identified variant be designated as wsal for "w salmiak".

Poliosis, hair pigment dilution, and premature graying of the hair: A diagnostic approach in pediatric patients and review of the literature.

Poliosis is defined as the absence of melanin in hair, and hair graying typically occurs with hair melanin reduction. Poliosis can occur at any age but presents in childhood in certain genetic and acquired conditions, with many families seeking evaluation from a pediatric dermatologist. Poliosis presents as white hair typically restricted to a certain location of the scalp. Children may also present with a reduction of expected hair pigmentation, referred to as pigment dilution, or the development of hair graying. This review aims to provide a streamlined diagnostic approach for pediatric dermatologists when presented with these hair findings. Poliosis should be recognized as a potential diagnostic feature or initial sign in many syndromes and thus can guide clinicians in diagnosing and managing conditions earlier in a patient's care. Since many of the genetic and acquired conditions that present with poliosis or hair pigment dilution have extracutaneous manifestations, early diagnosis is vital in establishing multidisciplinary care.

Human genome diversity data reveal that L564P is the predominant TPC2 variant and a prerequisite for the blond hair associated M484L gain-of-function effect.

The endo-lysosomal two-pore channel (TPC2) has been established as an intracellular cation channel of significant physiological and pathophysiological relevance in recent years. For example, TPC2-/- mice show defects in cholesterol degradation, leading to hypercholesterinemia; TPC2 absence also results in mature-onset obesity, and a role in glucagon secretion and diabetes has been proposed. Infections with bacterial toxins or viruses e.g., cholera toxin or Ebola virus result in reduced infectivity rates in the absence of TPC2 or after pharmacological blockage, and TPC2-/- cancer cells lose their ability to migrate and metastasize efficiently. Finally, melanin production is affected by changes in hTPC2 activity, resulting in pigmentation defects and hair color variation. Here, we analyzed several publicly available genome variation data sets and identified multiple variations in the TPC2 protein in distinct human populations. Surprisingly, one variation, L564P, was found to be the predominant TPC2 isoform on a global scale. By applying endo-lysosomal patch-clamp electrophysiology, we found that L564P is a prerequisite for the previously described M484L gain-of-function effect that is associated with blond hair. Additionally, other gain-of-function variants with distinct geographical and ethnic distribution were discovered and functionally characterized. A meta-analysis of genome-wide association studies was performed, finding the polymorphisms to be associated with both distinct and overlapping traits. In sum, we present the first systematic analysis of variations in TPC2. We functionally characterized the most common variations and assessed their association with various disease traits. With TPC2 emerging as a novel drug target for the treatment of various diseases, this study provides valuable insights into ethnic and geographical distribution of TPC2 polymorphisms and their effects on channel activity.

Genetic variation affects morphological retinal phenotypes extracted from UK Biobank optical coherence tomography images.

Optical Coherence Tomography (OCT) enables non-invasive imaging of the retina and is used to diagnose and manage ophthalmic diseases including glaucoma. We present the first large-scale genome-wide association study of inner retinal morphology using phenotypes derived from OCT images of 31,434 UK Biobank participants. We identify 46 loci associated with thickness of the retinal nerve fibre layer or ganglion cell inner plexiform layer. Only one of these loci has been associated with glaucoma, and despite its clear role as a biomarker for the disease, Mendelian randomisation does not support inner retinal thickness being on the same genetic causal pathway as glaucoma. We extracted overall retinal thickness at the fovea, representative of foveal hypoplasia, with which three of the 46 SNPs were associated. We additionally associate these three loci with visual acuity. In contrast to the Mendelian causes of severe foveal hypoplasia, our results suggest a spectrum of foveal hypoplasia, in part genetically determined, with consequences on visual function.

Zinc deficiency or genetic mutations?-A case report of hair heterochromia in the context of MC1R genetic mutations.

Hair heterochromia may be caused by different mechanisms. At clinical work, we found a Chinese boy whose hair colour gradually turned to red. We record the diagnosis and treatment process and follow-up situation, finally find that altered hair colour phenotype is due to MC1R genetic mutations, rather than zinc deficiency. This rarely red hair colour phenotype improve our understanding of hair heterochromia caused by genetic mutations.

Do newborn lambs with black and white hair-coats in an equatorial semi-arid environment maintain homeothermy?

We assessed newborn lambs from two hair-coat sheep breeds, the black Santa Ines (n = 29) and white Dorper (n = 26), to determine how they behaviourally and physiologically respond to the prevailing thermal conditions in an equatorial semi-arid environment. Measurements of hair-coat surface temperature, rectal temperature and the lambs' exposure to sun were recorded across the first 24 h of life every hour, after the lambs had received colostrum. Lambs and ewes were kept in a lambing pen and could freely move between a shaded area or be exposed to sun. During the study period, the air temperature ranged between 20 ºC and 34 ºC. When exposed to sun, lambs and ewes could experience as much as 1200 W m-2 of solar irradiance. Santa Ines lambs exhibited higher (highest density interval at 95%, HDI) hair-coat surface temperatures than did Dorper lambs. Overall, both Santa Ines and Dorper lambs sustained homeothermy, with a mean rectal temperature ranging from 38.7 ºC to 39.1 ºC between night and daytime phase, and a mean amplitude of ~ 0.8 ºC. Nevertheless, from 10:00 to 15:00, some lambs were found to be moderately hyperthermic. Five Santa Ines and three Dorper lambs had rectal temperatures above 40 ºC, and one Santa Ines lamb, while exposed to sun, had a rectal temperature of 41.3 ºC. Over this time period, lambs were more likely to move to shade (HDI at 95%). From 00:00 to 06:00, despite the air temperature being lower than the hair-coat surface, favouring high rates of sensible heat loss to the environment, no lamb exhibited signs of hypothermia (rectal temperature < 37.5 ºC). In conclusion, haired newborn lambs coped well with high levels of radiant heat during the daytime and lower temperatures at night. However, providing access to shade during the daytime is important to improve the welfare of newborn lambs delivered in equatorial semi-arid regions.

Thermotolerance in Angus cattle is related to hair coat characteristics but not to coat color.

This study evaluated the impact of coat color (CC) and hair coat characteristics (HC) on productive and physiological traits related to thermotolerance in Angus heifers. The goal was to determine if HC and/or CC were reliable indicators of thermotolerance on a large scale for future breeding programs. Ninety-three 15-month-old Angus heifers (52 black, 41 red) were evaluated in three periods on a beef cattle farm in Brazil. Heifers were classified by CC and HC, and body weight, body condition score (BCS), and reproductive tract score (RTS) were compared between groups. In the summer evaluation, surface temperature (infrared thermography), internal temperature (intravaginal sensors), sweating rate, and behavior were assessed in a subset of heifers. Temperature-humidity index (THI) was calculated using meteorological data. The proportion of heifers with short, fine, and smooth hair (HC1) increased (P < 0.05) over the evaluations. Heifers with thick, long, and woolly hair (HC3) had lower (P < 0.05) body weights than those with finer coats, regardless of CC. Black heifers had greater (P < 0.05) puberty rates than red heifers in the first two evaluations. At a THI of 66, black heifers with HC1 exhibited a lower (P < 0.05) internal temperature compared to black heifers with HC3. At a THI of 75, all heifers with HC1 had lower (P < 0.05) internal temperatures, regardless of CC. Red heifers and those with HC3 experienced hyperthermia for longer (P < 0.05) periods. Neither HC nor CC affected (P > 0.05) surface temperatures or sweating rates. At a THI of 72, more black heifers remained standing, suggesting behavioral adaptation. In conclusion, coat color and characteristics influence thermal stress and performance in Angus heifers, though color impact is limited. Internal temperature monitoring effectively determines thermotolerance. In tropical regions, selecting for short, fine, smooth hair may improve heat tolerance.

Melanin for Photoprotection and Hair Coloration in the Emerging Era of Nanocosmetics.

Nanotechnology is revolutionizing fields of high social and economic impact. such as human health preservation, energy conversion and storage, environmental decontamination, and art restoration. However, the possible global-scale application of nanomaterials is raising increasing concerns, mostly related to the possible toxicity of materials at the nanoscale. The possibility of using nanomaterials in cosmetics, and hence in products aimed to be applied directly to the human body, even just externally, is strongly debated. Preoccupation arises especially from the consideration that nanomaterials are mostly of synthetic origin, and hence are often seen as "artificial" and their effects as unpredictable. Melanin, in this framework, is a unique material since in nature it plays important roles that specific cosmetics are aimed to cover, such as photoprotection and hair and skin coloration. Moreover, melanin is mostly present in nature in the form of nanoparticles, as is clearly observable in the ink of some animals, like cuttlefish. Moreover, artificial melanin nanoparticles share the same high biocompatibility of the natural ones and the same unique chemical and photochemical properties. Melanin is hence a natural nanocosmetic agent, but its actual application in cosmetics is still under development, also because of regulatory issues. Here, we critically discuss the most recent examples of the application of natural and biomimetic melanin to cosmetics and highlight the requirements and future steps that would improve melanin-based cosmetics in the view of future applications in the everyday market.

Eye and hair color prediction of an early medieval adult and subadult skeleton using massive parallel sequencing technology.

Phenotypic trait prediction in ancient DNA analysis can provide information about the external appearance of individuals from past human populations. Some studies predicting eye and hair color in ancient adult skeletons have been published, but not for ancient subadult skeletons, which are more prone to decay. In this study, eye and hair color were predicted for an early medieval adult skeleton and a subadult skeleton that was anthropologically characterized as a middle-aged man and a subadult of unknown sex about 6 years old. When processing the petrous bones, precautions were taken to prevent contamination with modern DNA. The MillMix tissue homogenizer was used for grinding, 0.5 g of bone powder was decalcified, and DNA was purified in Biorobot EZ1. The PowerQuant System was used for quantification and a customized version of the HIrisPlex panel for massive parallel sequencing (MPS) analysis. Library preparation and templating were performed on the HID Ion Chef Instrument and sequencing on the Ion GeneStudio S5 System. Up to 21 ng DNA/g of powder was obtained from ancient petrous bones. Clean negative controls and no matches with elimination database profiles confirmed no contamination issue. Brown eyes and dark brown or black hair were predicted for the adult skeleton and blue eyes and brown or dark brown hair for the subadult skeleton. The MPS analysis results obtained proved that it is possible to predict hair and eye color not only for an adult from the Early Middle Ages, but also for a subadult skeleton dating to this period.

White hair in alopecia areata: Clinical forms and proposed physiopathologic mechanisms.

Alopecia areata (AA) is a common form of nonscarring hair loss. It is believed to be a consequence of an immune-mediated stimulus, probably involving autoreactive T cells against antigens present in the hair follicle. The exact antigen is still unknown; however, some authors have proposed that melanogenesis-associated molecules might trigger autoimmunity. Although transient white hair regrowth is a common and well-known situation in AA, there are other types of white hair phenomena in this context, including permanent white hair regrowth, sparing of white hair in a patchy pattern, or sparing in a diffuse pattern, giving the appearance of the so-called overnight graying phenomena or canitis subita. In this review, we aim to describe the different clinical aspects of white hair in AA, as well as the proposed pathophysiologic mechanisms involved in this phenomena.