Melanocyte stem cells and hair graying

To explore the relationship between melanocyte stem cells in the hair follicle bulge and hair graying so as to fully understand their key role in the pathogenesis of hair graying.


| INTRODUC TI ON
Hair graying, which is induced or aggravated by a variety of internal and external factors (such as psychological and environmental factors) has a genetic basis and often causes psychological and societal issues for afflicted individuals. Some hereditary diseases (such as Hutchinson Gilford and Werner's syndrome), trace element deficiency and drugs (such as hydroxychloroquine) can also cause hair graying. 1 Hair graying often starts in bilateral side burns, followed by the forehead and top of the head, as well as at sites of occipital injuries. 2 Previous studies have mostly theorized that gray-haired disease is caused by the consumption of melanocytes in the hair bulb.
However, researches have demonstrated that both the melanocytes in the hair bulb and the melanocyte stem cells (MSCS) in the bulge of the hair follicles are involved in hair graying. 3 Therefore, this article reviewed the mechanism of MSCs involved in the pathogenesis of hair graying, which can provide new targets and ideas for increasing the understanding of the pathogenesis and therapeutic research of hair graying.

| HAIR FOLLICUL AR MSC S PHYS IOLOGY
Hair follicular MSCs were derived from neural crest-derived cells.
Melanoblasts appeared in the neural crest on Day 8.5 in mouse embryos. Melanoblasts migrate upward to the epidermal basal layer to form epidermal MSCs. In addition, melanoblasts migrate to the bulge of the hair bud and differentiate into MSCs, which are located in the bulge region and provide the cellular source for the subsequent pigmented hair shaft. [4][5][6] Under physiological conditions, quiescent MSCs in the bulge undergo cycling activation, differentiation, and migration.
In the early anagen phase, MSCs are activated, after which they complete self-renewal and proliferation and differentiate into melanoblasts that lack the capacity for melanin synthesis but still maintain certain stem cell characteristics. 4,7,8 Melanoblasts subsequently migrate along the outer root sheath to the bulb to form mature melanocytes with the ability to synthesize melanin. During the growth phase, mature melanocytes continuously provide melanin granules to keratinocytes of the hair follicle matrix. Molecular markers of melanocytes include PAX3, SOX10, DCT, and Frizzeld4, in addition to some of the same markers of MSCs, such as PAX3, SOX10, and DCT, and molecular markers of melanoblasts also include MITF, KIT, Trp1, and TYR. 9,10 It is thought that almost all melanocytes in hair follicles can undergo apoptosis during hair follicle degeneration. However, it is still unclear as to whether MITF-positive melanoblasts undergo dedifferentiation and return to the bulge during hair follicle degeneration. 10

| Environmental injuries
Environmental factors such as ionizing radiation (IR) and chemotherapy drugs (busulfide and mitomycin C) are genotoxic substances that can cause hair graying. 11,12 Some researchers believe that apoptosis and senescence are the two main methods to eliminate defective melanocytes caused by genotoxicity, 13

| Abnormal mechanism
MC Gill et al. 15 found that mice with MITF Vit experienced hair graying.
Emi K et al. believed that MITF signaling regulates MSCs by mainly participating in the renewal and differentiation of MSCs 16 ; the possible mechanism is that MITF participates in the regulation of BCL2, and its expression promotes the transcription of downstream BCL2. 17 However, Melissa L 18 also found that the high expression of MITF also results in hair graying as premature differentiation of MSCs. They used Tg(DCT-SOX10)/0; Mitfmi-vga9/+ mice to overexpress SOX10, which enhanced the upregulation of MITF. It was found that the premature and excessive differentiation of MSCs and EPMs were formed in the niche, leading to the progressive depletion of MSCs. It is believed that TGFβ signaling is also involved in the maintenance of MSCs, and its main role is to inhibit the activation and differentiation of MSCs.
Moreover, the conditional knockdown of TGFBRII caused abnormal differentiation of MSCs in the niche, and EPMs were formed at the bulge. 19 In addition, NOTCH, BMP, NFIB and other signaling pathways are also involved in the maintenance of MSCs. [20][21][22] COL17A1 is a semidesmosomal transmembrane protein expressed in the basal layer of the epidermis. 23 Some researchers 24 have found that, although COL17A1 was highly expressed in hair follicle keratinocyte stem cells, it was also of great significance for the maintenance of MSCs. In COL17A1−/− mice, the number of hair follicles decreased, and EPMs appeared in the hair follicle bulge of mice at 12 weeks, thus leading to hair graying, which indicated that COL17A1 signaling played an important role in the maintenance of MSCs.

| Excessive consumption of MSCs
Stress states, such as psychological and physical pressures, are involved in the formation of hair graying disorder. 25 Zhang et al. 26 found that the massive release of norepinephrine(NE) could lead to hair graying. They used TyrcreERT2; Adrb2 fl/fl and TyrcreERT2; GR fl/fl mouse models to confirm that NE plays an important role in stressinduced MSC depletion. They believed that the typical intensity stress could elicit hair graying after 3-5 hair cycles, and intense stress can lead to hair graying in the next hair cycle. They also indicated that the release of NE due to hyperactivation of the sympathetic system could lead to excessive proliferation and subsequent depletion of MSCs.
Moreover, Inbal et al. 27 showed that during the depletion of MSCs caused by NE, EPMs could be detected in the niche. Therefore, the excessive release of NE induces the excessive consumption of MSCs due to the production of EPMs. It is possible that persistent excessive stress pressure resulted in hair graying, and it was much more difficult to recover at a later time for the loss of MSCs.

| Hair follicle aging
Physiological hair graying may follow the "three-fifty rule"; specifically, approximately 50% of patients at the age of 50 years have at least 50% white hair. 28 The general mechanism of melanin synthesis involves a complicated chemical reaction, 29 during which a large number of ROS are produced. 30 As aging progresses, antioxidant substances, including catalase, glutathione and SOD, are downregulated. Imbalanced cell oxidation and antioxidant capacity, the accumulation of oxidative damage and downregulated BCL2 can reduce the anti-apoptosis capacity of MSCs. Moreover, stem cell pools in the niche provide an insufficient source of melanocytes in the subsequent hair cycle as MSCs undergo apoptosis, eventually leading to hair graying. 31,32 In Bcl-2 −/− mice, the morphology of hair follicles was observed to be normal at 6.5 days after birth. On Day 8.5, the MSCs in the hair follicle bulge suddenly and completely disappeared, but the mature melanocytes in the hair bulb still existed.
In the second hair cycle, DACT-LacZ + melanocytes could not be detected in the hair bulb. 16 Studies have suggested that the defective expression of PTEN (phosphate and tensin homolog deleted on chromosome 10) inhibited the formation of hair graying. 33

| Unbalanced hair follicle cycle
The melanin production ability of melanocytes decreases after 10 cycles of hair follicle reconstruction, thus resulting in hair graying. 1,35 Hair pulling can accelerate the conversion of the hair follicle cycle and the formation of hair graying. 36 Machiko et al. believed that the protooncogene RET can facilitate the conversion of the hair follicle cycle and promote MSC consumption to elicit hair graying. 37 C-Raf/B-Raf gene double knockout mice have been shown to exhibit cell cycle disorders, and MSCs were not able to enter the S phase and complete self-renewal, giving rise to the progressive loss of MSCs and the incidence of hair graying. 38 Furthermore, androgenetic alopecia has the characteristics of hair follicle miniaturization and hair cycle shortening. 39 It is possible that the incidence of hair graying may be higher than that in the normal population due to the faster follicle cycle switching such as androgenetic alopecia and trichotillomania. 40,41

| CON CLUS IONS
Previous researches on hair graying have mainly focused on the mature melanocytes of hair bulbs, but more attention has been given to MSCs in the niche. It was believed that the loss of MSCs in hair bulges is the main cause of hair graying. Therefore, the exploration of the physiological and pathological damage mechanisms of MSCs is expected to further increase the understanding of the occurrence mechanism of human white hair disorder and to provide new ideas for the development of treatments for white hair disorder.

E TH I C S S TATEM ENT
The study was approved by the ethics committee of Chongqing medical university.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available in pubmed at https://pubmed.ncbi.nlm.nih.gov/, reference number 41.