In vitro expansion of immature melanoblasts and their ability to repopulate melanocyte stem cells in the hair follicle

Effect of An Atmospheric Plasma Jet on the Differentiation of Melanoblast Progenitor

OBJECTIVE

Melanoblasts are the cell source of regeneration for pigment restoration. The ability to differentiate into mature melanocytes is the essential feature of melanoblasts in depigmentation diseases. Cold atmospheric plasma is an ionized gas with near-room temperature and highly reactive species that has been shown to induce stem cell differentiation. The aim of the study was to explore the effect of cold atmospheric plasma on the differentiation of melanoblast progenitor cells.

METHODS

In this study, melanoblasts were exposed to the plasma jet and the cell morphology was observed. The cell cycle and cell proliferation were detected. Furthermore, the cell immunofluorescence and the detection of melanin particle and nitric oxide were carried out to investigate the differentiation of melanoblast progenitor cells.

RESULTS

Cells that were treated with the plasma had longer and more synaptic structures, and the G1 phase of cell cycle was prolonged in the treated group. More melanin synthesis-related proteins and melanin particles were produced after plasma treatment. Nitric oxide was one of the active components generated by the plasma jet, and the nitric oxide content in the cell culture medium of the treated group increased.

CONCLUSION

These results indicate that an increase in nitric oxide production caused by a plasma jet can promote cell differentiation. The application of plasma provides an innovative strategy for the treatment of depigmentation diseases.

Melanocyte Chitosan/Gelatin Composite Fabrication with Human Outer Root Sheath-Derived Cells to Produce Pigment

The hair follicle serves as a melanocyte reservoir for both hair and skin pigmentation. Melanocyte stem cells (MelSCs) and melanocyte progenitors reside in the bulge/sub-bulge region of the lower permanent portion of the hair follicle and play a vital role for repigmentation in vitiligo. It would be beneficial to isolate MelSCs in order to further study their function in pigmentary disorders; however, due to the lack of specific molecular surface markers, this has not yet been successfully accomplished in human hair follicles (HuHF). One potential method for MelSCs isolation is the “side population” technique, which is frequently used to isolate hematopoietic and tumor stem cells. In the present study, we decided to isolate HuHF MelSCs using “side population” to investigate their melanotic function. By analyzing mRNA expression of TYR, SOX10, and MITF, melanosome structure, and immunofluorescence with melanocyte-specific markers, we revealed that the SP-fraction contained MelSCs with an admixture of differentiated melanocytes. Furthermore, our in vivo studies indicated that differentiated SP-fraction cells, when fabricated into a cell-chitosan/gelatin composite, could transiently repopulate immunologically compromised mice skin to regain pigmentation. In summary, the SP technique is capable of isolating HuHF MelSCs that can potentially be used to repopulate skin for pigmentation.

Regeneration of Mouse Skin Melanocyte Stem Cells In Vivo and In Vitro

Coordinated regeneration of melanocyte stem cells (McSCs) and hair follicle stem cells (HSCs) contributes to generation of pigmented hairs. Synchronous regeneration of McSCs with activation of HSCs occurs not only during initiation of a new hair cycle in vivo but also during reconstitution of hair follicles in vitro. The duration of the quiescent state of these stem cells becomes longer and longer in lifespan of mammals, leading to a decreased regenerative ability to form hair follicles. Here, we describe methods to activate McSCs during hair follicle regeneration in vivo, and isolate melanocytes from neonatal mouse skin to generate an immortalized cell line of melanocyte progenitors in vitro, aiming to use them for studying melanogenesis and future clinical application.

Isolating primary melanocyte-like cells from the mouse heart

We identified a novel population of melanocyte-like cells (also known as cardiac melanocytes) in the hearts of mice and humans that contribute to atrial arrhythmia triggers in mice. To investigate the electrical and biological properties of cardiac melanocytes we developed a procedure to isolate them from mouse hearts that we derived from those designed to isolate neonatal murine cardiomyocytes. In order to obtain healthier cardiac melanocytes suitable for more extensive patch clamp or biochemical studies, we developed a refined procedure for isolating and plating cardiac melanocytes based on those originally designed to isolate cutaneous melanocytes. The refined procedure is demonstrated in this review and produces larger numbers of healthy melanocyte-like cells that can be plated as a pure population or with cardiomyocytes.

Ribosomal stress, p53 activation and the tanning response

Melanocytes (MC) sit along the epidermal basal layer, largely quiescent except for constitutive melanin production. They are usually only activated after sun exposure. The recent paper by McGowan et al. (1) describes a novel mechanism by which melanocytes are induced to proliferate upon p53 activation in adjacent keratinocytes (KC). In this study, small subunit ribosomal protein mutations cause a dramatic activation of p53 that we propose mimics important aspects of the skin sunburn response after ultraviolet radiation (UVR) exposure. McGowan et al. show that the phenotype of their hyperpigmented mouse mutants results from p53-dependent upregulation of KITLG, a cytokine that binds to the KIT receptor on melanocytes and influences melanin synthesis, melanocyte proliferation, and dictates MC localization at the dermo-epidermal junction. These findings extend our knowledge about skin stress responses, in particular, how p53 activity in keratinocytes is central to the regulation of melanocyte behaviour.

Detailed histological structure of human hair follicle bulge region at different ages: a visible niche for nesting adult stem cells

In the bulge region of the hair follicle, a densely and concentrically packed cell mass is encircled by the arrector pili muscle (APM), which offers a specilized microenvironment (niche) for housing heterogeneous adult stem cells. However, the detailed histological architecture and the cellular composition of the bulge region warrants intensive study and may have implications for the regulation of hair follicle growth regulation. This study was designed to define the gene-expression profiles of putative stem cells and lineage-specific precursors in the mid-portions of plucked hair follicles prepared according to the presence of detectable autofluorescence. The structure was also characterized by using a consecutive sectioning technique. The bulge region of the hair follicle with autofluorescence was precisely excised by employing a micro-dissection procedure. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) was performed to identify the gene expression profiles specific for epithelial, melanocyte and stromal stem cells in the bulge region of the hair follicle visualized by autofluorescence. The morphology and its age-dependent changes of bulge region of the hair follicles with autofluorescence segment were also examined in 9 scalp skin specimens collected from patients aged 30 weeks to 75 years, by serial sectioning and immuno-staining. Gene expression profile analysis revealed that there were cells with mRNA transcripts of Dct(Hi)Tyrase(Lo)-Tyrp1(Lo)MC1R(Lo)MITF(Lo)/K15(Hi)/NPNT(Hi) in the bulge region of the hair follicle with autofluorescence segments, which differed from the patterns in hair bulbs. Small cell-protrusions that sprouted from the outer root sheath (ORS) were clearly observed at the APM inserting level in serial sections of hair follicles by immunohistological staining, which were characteristically replete with K15+/K19+expressing cells. Likewise, the muscle bundles of APM positive for smooth muscle actin intimately encircled these cell-protrusions, and the occurrence frequency of the cell-protrusions was increased in fetal scalp skin compared with adult scalp skin. This study provided the evidence that the cell-protrusions occurring at the ORS relative to the APM insertion are more likely to be characteristic of the visible niches that are filled with abundant stem cells. The occurrence frequency of these cell-protrusions was significantly increased in fetal scalp skin samples (128%) as compared with the scalp skins of younger (49.4%) and older (25.4%) adults (P<0.01), but difference in the frequency between the two adult groups were not significant. These results indicated that these cell-protrusions function as a niche house for the myriad stem cells and/or precursors to meet the needs of the development of hair follicles in an embryo. The micro-dissection used in this study was simple and reliable in excising the bulge region of the hair follicle with autofluorescence segments dependent on their autofluorescence is of value for the study of stem cell culture.

Toward the isolation and culture of melanocyte stem cells

A new level of understanding of pigment cell biology and pathology will require the ability to culture and manipulate melanocyte stem cells (MCSCs) in vitro. In this issue, Nishikawa-Torikai et al. report progress toward this end. MCSCs isolated from mouse hair follicles can be expanded in vitro in a feeder-layer culture system. Application to human systems can be expected.

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.

Functionally distinct melanocyte populations revealed by reconstitution of hair follicles in mice

Hair follicle reconstitution analysis was used to test the contribution of melanocytes or their precursors to regenerated hair follicles. In this study, we first confirmed the process of chimeric hair follicle regeneration by both hair keratinocytes and follicular melanocytes. Then, as first suggested from the differential growth requirements of epidermal skin melanocytes and non-cutaneous or dermal melanocytes, we confirmed the inability of the latter to be involved as follicular melanocytes to regenerate hair follicles during the hair reconstitution assay. This clear functional discrimination between non-cutaneous or dermal melanocytes and epidermal melanocytes suggests the presence of two different melanocyte cell lineages, a finding that might be important in the pathogenesis of melanocyte-related diseases and melanomas.

Adult stem cells exhibit global suppression of RNA polymerase II serine-2 phosphorylation

Adult stem cells, which are characterized by their capacity for self-renewal and differentiation, participate in tissue homeostasis and response to injury. They are thought to enter a state of relative quiescence, known as reversible cell cycle arrest, but the underlying molecular mechanisms remain poorly characterized. Previous data from our laboratory has shown that housekeeping gene expression is downregulated in melanocyte stem cells (MelSCs), suggesting a global suppression of mRNA transcription. We now show, using antibodies against specific phosphorylated forms of RNA polymerase II (RNApII), that adult MelSCs do not undergo productive mRNA transcription elongation, while RNApII is activated and initialized, ready to synthesize mRNA upon stimulation, and that the RNApII kinase CDK9 is absent in adult MelSCs. Interestingly, other adult stem cells also, including keratinocyte, muscle, spermatogonia, and hematopoietic stem cells, showed a similar absence of RNApII phosphorylation. Although it is difficult to show the functional significance of this observation in vivo, CDK9 inhibition resulted in enhanced survival of cells that are deprived from survival factors. We conclude that the absence of productive mRNA transcription is an early, specific, and conserved characteristic of adult stem cells. Downregulation of mRNA transcription may lead to decreased rates of metabolism, and protection from cellular and genetic damage. Screening heterogeneous tissues, including tumors, for transcriptionally quiescent cells may result in the identification of cells with stem cell-like phenotypes.

Two distinct types of mouse melanocyte: differential signaling requirement for the maintenance of non-cutaneous and dermal versus epidermal melanocytes

Unlike the thoroughly investigated melanocyte population in the hair follicle of the epidermis, the growth and differentiation requirements of the melanocytes in the eye, harderian gland and inner ear - the so-called non-cutaneous melanocytes - remain unclear. In this study, we investigated the in vitro and in vivo effects of the factors that regulate melanocyte development on the stem cells or the precursors of these non-cutaneous melanocytes. In general, a reduction in KIT receptor tyrosine kinase signaling leads to disordered melanocyte development. However, melanocytes in the eye, ear and harderian gland were revealed to be less sensitive to KIT signaling than cutaneous melanocytes. Instead, melanocytes in the eye and harderian gland were stimulated more effectively by endothelin 3 (ET3) or hepatocyte growth factor (HGF) signals than by KIT signaling, and the precursors of these melanocytes expressed the lowest amount of KIT. The growth and differentiation of these non-cutaneous melanocytes were specifically inhibited by antagonists for ET3 and HGF. In transgenic mice induced to express ET3 or HGF in their skin and epithelial tissues from human cytokeratin 14 promoters, the survival and differentiation of non-cutaneous and dermal melanocytes, but not epidermal melanocytes, were enhanced, apparently irrespective of KIT signaling. These results provide a molecular basis for the clear discrimination between non-cutaneous or dermal melanocytes and epidermal melanocytes, a difference that might be important in the pathogenesis of melanocyte-related diseases and melanomas.

Cutaneous melanoma: how does ultraviolet light contribute to melanocyte transformation?

Ascribing a causal role to ultraviolet radiation in melanoma induction is problematic, as the relationship between total lifetime sun exposure and melanoma risk is not as strong as for some other skin cancers. Epidemiological studies show that heightened melanoma risk is most associated with intermittent sunburns. Despite this, lesions can develop on anatomical locations receiving intermittent (e.g., the trunk) or chronic exposures (e.g., the head and neck). Individuals developing melanoma on truncal sites tend to have more nevi, suggesting that in addition to the differences in forms of sun exposure, there may also be innate variation that makes one more susceptible to one or other mechanism of melanoma development. Such differences may depend upon different responses at the time of exposure (e.g., pigmentation characteristics, DNA repair capability and melanocyte proliferative response), and/or the role of the skin microenvironment in limiting proliferation of a ‘primed’ or mutated melanocyte during the latent period leading up to the appearance of a melanocytic lesion.

Human hair pigmentation--biological aspects

Skin and hair colour contribute significantly to our overall visual appearance and to social/sexual communication. Despite their shared origins in the embryologic neural crest, the hair follicle and epidermal pigmentary units occupy distinct, although open, cutaneous compartments. They can be distinguished principally on the basis of the former's stringent coupling to the hair growth cycle compared with the latter's continuous melanogenesis. The biosynthesis of melanin and its subsequent transfer from melanocyte to hair bulb keratinocytes depend on the availability of melanin precursors and on a raft of signal transduction pathways that are both highly complex and commonly redundant. These signalling pathways can be both dependent and independent of receptors, act through auto-, para- or intracrine mechanisms and can be modified by hormonal signals. Despite many shared features, follicular melanocytes appear to be more sensitive than epidermal melanocytes to ageing influences. This can be seen most dramatically in hair greying/canities and this is likely to reflect significant differences in the epidermal and follicular microenvironments. The hair follicle pigmentary unit may also serve as an important environmental sensor, whereby hair pigment contributes to the rapid excretion of heavy metals, chemicals and toxins from the body by their selective binding to melanin; rendering the hair fibre a useful barometer of exposures. The recent availability of advanced cell culture methodologies for isolated hair follicle melanocytes and for intact anagen hair follicle organ culture should provide the research tools necessary to elucidate the regulatory mechanisms of hair follicle pigmentation. In the longer term, it may be feasible to develop hair colour modifiers of a biological nature to accompany those based on chemicals.

Murine neonatal melanocytes exhibit a heightened proliferative response to ultraviolet radiation and migrate to the epidermal basal layer

Melanocytes respond to UVR not only by producing melanin, but also by proliferating. This is essentially a protective response. We have studied the melanocyte proliferative response after a single UVR exposure to neonatal mice. At 3 days post-UVR in wild-type neonates we observed a marked melanocyte activation not seen in adults. Melanocytes migrated to the epidermal basal layer, their numbers peaking at 3-5 days after UVR then diminishing. They appeared to emanate from the hair follicle, migrating to the epidermis via the outer root sheath. In melanoma-prone mice with melanocyte-specific overexpression of Hras(G12V), basal layer melanocytes were increased in size and dendricity compared to UVR-treated wild-type mice. Melanocytes in mice carrying a pRb pathway cell-cycle defect (oncogenic Cdk4(R24C)) did not show an enhanced response to UVR such as those carrying Hras(G12V). The exquisite sensitivity to UVR-induced proliferation and migration that characterizes neonatal mouse melanocytes may partly explain the utility of this form of exposure for inducing melanoma in mice that carry oncogenic mutations.

Cancer stem cells and human malignant melanoma

Cancer stem cells (CSC) have been identified in hematological malignancies and several solid cancers. Similar to physiological stem cells, CSC are capable of self-renewal and differentiation and have the potential for indefinite proliferation, a function through which they may cause tumor growth. Although conventional anti-cancer treatments might eradicate most malignant cells in a tumor, they are potentially ineffective against chemoresistant CSC, which may ultimately be responsible for recurrence and progression. Human malignant melanoma is a highly aggressive and drug-resistant cancer. Detection of tumor heterogeneity, undifferentiated molecular signatures, and increased tumorigenicity of melanoma subsets with embryonic-like differentiation plasticity strongly suggest the presence and involvement of malignant melanoma stem cells (MMSC) in the initiation and propagation of this malignancy. Here, we review these findings in the context of functional properties ascribed to melanocyte stem cells and CSC in other cancers. We discuss the association of deregulated signaling pathways, genomic instability, and vasculogenic mimicry phenomena observed in melanoma subpopulations in light of the CSC concept. We propose that a subset of MMSC may be responsible for melanoma therapy-resistance, tumor invasiveness, and neoplastic progression and that targeted abrogation of a MMSC compartment could therefore ultimately lead to stable remissions and perhaps cures of metastatic melanoma.

[Melanocyte stem cells in adults]

Melanocyte stem cells have been recently localized in mice, in the outer root sheath of the lower permanent portion of the hair follicle. Specific depletion of melanocyte stem cell population is responsible for natural hair greying in aging mice and humans. Melanocyte stem cells also seem to drive the growth of malignant melanomas. A few mutations, either spontaneous or genetically engineered, accelerate the natural process of hair greying with age. These mutations allowed the identification of genes and signalling pathways controlling emergence, maintenance and/or differentiation of melanocyte stem cells. This review summarizes recent studies on the melanocyte stem cells and defines a few major unanswered questions in the field.