Retinoic acid and mouse skin morphogenesis. I. Expression pattern of retinoic acid receptor genes during hair vibrissa follicle, plantar, and nasal gland development

CRYAA and GJA8 promote visual development after whisker tactile deprivation

Deprivation of one sense can be followed by enhanced development of other senses via cross-modal plasticity mechanisms. To study the effect of whisker tactile deprivation on vision during the early stages of development, we clipped the bilateral whiskers of young mice and found that their vision was impaired but later recovered to normal levels. Our results demonstrate that inhibition of the PI3K/AKT/ERK signaling pathway caused short-term visual impairment during early development, while high expression levels of Crystallin Alpha A (CRYAA) and Gap Junction Protein Alpha 8 (GJA8) in the retina led to the recovery of developmental visual acuity. Interestingly, analysis of single-cell sequencing results from human embryonic retinas at 9-19 gestational weeks (GW) revealed that CRYAA and GJA8 display stage-specific peak expression during human embryonic retinal development, suggesting potential functions in visual development. Our data show that high expression levels of CRYAA and GJA8 in the retina after whisker deprivation rescue impaired visual development, which may provide a foundation for further research on the mechanisms of cross-modal plasticity and in particular, offer new insights into the mechanisms underlying tactile-visual cross-modal development.

Noncoding dsRNA induces retinoic acid synthesis to stimulate hair follicle regeneration via TLR3

How developmental programs reactivate in regeneration is a fundamental question in biology. We addressed this question through the study of Wound Induced Hair follicle Neogenesis (WIHN), an adult organogenesis model where stem cells regenerate de novo hair follicles following deep wounding. The exact mechanism is uncertain. Here we show that self-noncoding dsRNA activates the anti-viral receptor toll like receptor 3 (TLR3) to induce intrinsic retinoic acid (RA) synthesis in a pattern that predicts new hair follicle formation after wounding in mice. Additionally, in humans, rejuvenation lasers induce gene expression signatures for dsRNA and RA, with measurable increases in intrinsic RA synthesis. These results demonstrate a potent stimulus for RA synthesis by non-coding dsRNA, relevant to their broad functions in development and immunity.

During wound induced hair follicle neogenesis (WIHN), stem cells regenerate hair follicles but how this arises is unclear. Here, the authors show that self-noncoding dsRNA activates the antiviral receptor TLR3 to induce intrinsic retinoic acid, which stimulates WIHN in mice, and in isolated human keratinocyte cells.

New facets of keratin K77: interspecies variations of expression and different intracellular location in embryonic and adult skin of humans and mice

The differential expression of keratins is central to the formation of various epithelia and their appendages. Structurally, the type II keratin K77 is closely related to K1, the prototypical type II keratin of the suprabasal epidermis. Here, we perform a developmental study on K77 expression in human and murine skin. In both species, K77 is expressed in the suprabasal fetal epidermis. While K77 appears after K1 in the human epidermis, the opposite is true for the murine tissue. This species-specific pattern of expression is also found in conventional and organotypic cultures of human and murine keratinocytes. Ultrastructure investigation shows that, in contrast to K77 intermediate filaments of mice, those of the human ortholog are not attached to desmosomes. After birth, K77 disappears without deleterious consequences from human epidermis while it is maintained in the adult mouse epidermis, where its presence has so far gone unnoticed. After targeted Krt1 gene deletion in mice, K77 is normally expressed but fails to functionally replace K1. Besides the epidermis, both human and mouse K77 are present in luminal duct cells of eccrine sweat glands. The demonstration of a K77 ortholog in platypus but not in non-mammalian vertebrates identifies K77 as an evolutionarily ancient component of the mammalian integument that has evolved different patterns of intracellular distribution and adult tissue expression in primates.

Endogenous retinoids in the hair follicle and sebaceous gland

Vitamin A and its derivatives (retinoids) are critically important in the development and maintenance of multiple epithelial tissues, including skin, hair, and sebaceous glands, as shown by the detrimental effects of either vitamin A deficiency or toxicity. Thus, precise levels of retinoic acid (RA, active metabolite) are needed. These precise levels of RA are achieved by regulating several steps in the conversion of dietary vitamin A (retinol) to RA and RA catabolism. This review discusses the localization of RA synthesis to specific sites within the hair follicle and sebaceous gland, including their stem cells, during both homeostasis and disease states. It also discusses what is known about the specific roles of RA within the hair follicle and sebaceous gland. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Dynamic expression of the retinoic acid-synthesizing enzyme retinol dehydrogenase 10 (rdh10) in the developing mouse brain and sensory organs

Organs develop through many tissue interactions during embryogenesis, involving numerous signaling cascades and gene products. One of these signaling molecules is retinoic acid (RA), an active vitamin A derivative, which in mammalian embryos is synthesized from maternal retinol by two oxidative reactions involving alcohol/retinol dehydrogenases (ADH/RDHs) and retinaldehyde dehydrogenases (RALDHs), respectively. The activity of RALDHs is known to be crucial for RA synthesis; however, recently a retinol dehydrogenase (RDH10) has been shown to represent a new limiting factor in this synthesis. We investigated the spatiotemporal distribution of Rdh10 gene transcripts by in situ hybridization and quantitative polymerase chain reaction (PCR) during development of the brain and sensory organs. Although Rdh10 relative mRNA levels decline throughout brain development, we show a strong and lasting expression in the meninges and choroid plexuses. Rdh10 expression is also specifically seen in the striatum, a known site of retinoid signaling. In the eye, regional expression is observed both in the prospective pigmented epithelium and neural retina. In the inner ear Rdh10 expression is specific to the endolymphatic system and later the stria vascularis, both organs being involved in endolymph homeostasis. Furthermore, in the peripheral olfactory system and the vibrissae follicles, expression is present from early stages in regions where sensory receptors appear and mesenchymal/epithelial interactions take place. The distribution of Rdh10 transcripts during brain and sensory organ development is consistent with a role of this enzyme in generating region-specific pools of retinaldehyde that will be used by the various RALDHs to refine the patterns of RA synthesis.

Mouse epidermal development: effects of retinoic acid exposure in utero

Epidermal morphogenesis was studied in vivo following prenatal exposure to retinoic acid (RA). In pregnant mice, a single oral dose of RA on day 11.5 of gestation failed to induce histological changes in fetal epidermal development except in epidermal thickness. Epidermal thickness increased from 16.5 days post-coitum (dpc) onwards, and temporal and spatial epidermal modifications in keratins K5 and K14 related to proliferative activity of keratinocytes were observed. An RA effect on cell proliferation was supported by a statistically significant increase in the number of epidermal S-phase cells, containing BrdU-incorporated DNA in RA-exposed mice compared with nonexposed animals. The prolonged in utero action of RA on epidermal proliferative activity in fetuses and newborns suggests a long-term RA effect that may play a role on the development and evolution of diseases in adult skin.

Towards dissecting the pathogenesis of retinoid-induced hair loss: all-trans retinoic acid induces premature hair follicle regression (catagen) by upregulation of transforming growth factor-beta2 in the dermal papilla

Diffuse hair loss ranks among the most frequent and psychologically most distressing adverse effects of systemic therapy with retinoids, which severely limits their therapeutic use even where clinically desired. Since the underlying mechanisms of retinoid-induced effluvium are as yet unknown, we have investigated the influence of the prototypic retinoid all-trans retinoic acid (ATRA, tretinoin) on the growth of human scalp hair follicles (HF) in culture. HF in the anagen VI stage of the hair cycle were cultured in the presence of 10(-8) or 10(-10) M ATRA. Compared with controls, hair shaft elongation declined significantly already after 2 d in the ATRA-treated group, and approximately 80% of the ATRA-treated HF had prematurely entered catagen-like stage at day 6, compared with 30% in the control group. This corresponded to an upregulation of apoptotic and a downregulation of Ki67-positive cells in ATRA-treated HF. Since transforming growth factor (TGF)-beta has been implicated as a key inducer of catagen, we next studied whether ATRA treatment had any effect on follicular expression. TGF-beta2 immunoreactivity was detected in the outer root sheath of anagen VI scalp HF. In catagen follicles, TGF-beta2 was also expressed in the regressing epithelial strand. After 4 d of ATRA treatment, TGF-beta2 was significantly upregulated in anagen HF in the dermal papilla (DP) and the dermal sheath, 7, and TGF-beta neutralizing antibody partially abrogated at RA induced hair growth inhibition. Real-time PCR confirmed a significant upregulation of TGF-beta2 transcripts in ATRA-treated hair bulbs. This study is the first to provide direct evidence that ATRA can indeed induce a catagen-like stage in human HF and suggests that this occurs, at least in part, via upregulation of TGF-beta2 in the DP. Therefore, topical TGF-beta2/TGF-beta receptor II antagonists deserve to be explored for the prevention and management of retinoid-induced hair loss.

The RAIG Family Member, GPRC5D, Is Associated with Hard-Keratinized Structures

Retinoic acid-inducible gene-1 was originally identified as an orphan G-protein coupled receptor induced by retinoic acid. Three highly homologous oGPCR (GPRC5B, GPRC5C, and GPRC5D) have since been classified into the RAIG1 family. We describe here, the unique tissue distribution of GPRC5D and its mechanism of expression. Hybridization in situ has shown that GPRC5D is expressed in differentiating cells that produce hard keratin, including cortical cells of the hair shaft, the keratogenous zone of the nail, and in a central region of the filiform papillae of the tongue. The GPRC5D transcript is expressed in hair follicles during mid- and late anagen, and catagen but not at telogen and early anagen phases. The differentiation-inducer, all-trans retinoic acid, induces GPRC5D expression in cultured hair bulb cells. Because the tissue distribution of GPRC5D indicates a relationship with hard keratins that constitute the major structural proteins of hard epithelial tissues, we investigated the effect of GPRC5D on acid hard keratins. Analyses of cultured cells showed that transient overexpression resulted in suppression of Ha3 and stimulation of Ha4 hair keratin gene expression. The expression was maintained in the hair follicles of whn-deficient (nude) mice, suggesting that this gene is regulated by a signal pathway different from that of hair keratin synthesis. Collectively, these data provide a framework for understanding the molecular mechanisms of GPRC5D function in hard keratinization.

Effects of in utero retinoic acid exposure on mouse pelage hair follicle development

We investigated in vivo the histological and immunohistochemical responses of mouse hair pelage follicle morphogenesis to prenatal exposure to a potentially nonteratogenic dose of all-trans-retinoic acid (RA), as a basis studying the preventive effect of RA on adult mouse skin carcinogenesis. In pregnant mice, a single oral dose of RA at 30 mg kg-1 body weight given on day 11.5 of gestation caused no RA-induced changes in the morphology or temporal expression patterns of keratins during pelage hair follicle morphogenesis. The only differential effect of RA was a statistically significant increase in the number of BrdU-positive nuclei in hair bulbs from RA exposed fetuses compared with nonexposed mice. The absence of adverse RA effects suggests that this experimental design may represent a valuable protocol for use in studies on the in vivo effects of this retinoid on different skin diseases.

Notch signalling is linked to epidermal cell differentiation level in basal cell carcinoma, psoriasis and wound healing

BACKGROUND

Epidermal homeostasis involves the monitoring of continuous proliferative and differentiative processes as keratinocytes migrate from the basal layer to the skin surface. Recently, differentiation of epidermal stem cells was shown to be promoted by the Notch pathway. This pathway is characterised by cell-cell interactions between transmembrane proteins and was first implicated in lateral inhibition, patterning and cell binary choices during embryogenesis.

METHODS

By in situ hybridisation, we investigated the in vivo expression of related genes, namely; Notch 1-3, Delta 1, Jagged 1, Lunatic Fringe, Radical Fringe and Manic Fringe during keratinocyte proliferation and differentiation in humans in basal cell carcinoma, psoriasis and in wound healing experiments, compared with normal adult skin.

RESULTS

We show that the highest level of transcription of these genes is in the basal cell layer of non-lesional skin. Conversely, when keratinocytes were hyperproliferating, as in basal cell carcinoma, psoriasis, and during the first step of re-epithelialisation, expression was weak or non-existent. Furthermore, normal levels of transcripts were rescued in psoriatic plaques when treated by phototherapy, as well as in newly regenerated stratified epidermis following wound healing.

CONCLUSION

The Notch signalling involved in the differentiation programme of normal adult human epidermis is altered under experimental conditions and pathologies which modify this programme.

Stage- and tissue-specific expression of a beta-1,4-galactosyltransferase in the embryonic epidermis

Changes in oligosaccharide structures of glycoconjugates have been observed, and are postulated to have key roles in embryonic development and differentiation. N-Acetylglucosamine (GlcNAc) beta-1,4-galactosyltransferase (beta4GalT) AKI showed different expression patterns in time and space, and different enzymatic activity from the other known family members. The epidermis of mouse embryo included a high level of AKI activities, which transferred galactose (Gal) to endogenous glycoprotein (molecular weight 130 kDa) (GP130). The maximum activity was for 13.5-d postcoitum embryos. Specific antibody against AKI inhibited 81% of GlcNAc betaGalT activities, which indicates that AKI represents the major part of the embryonic epidermis enzymes. AKI shows 2.2 times higher galactosyltransferase activity toward Gal-acceptor glucose with alpha-lactalbumin (alpha-LA) than toward GlcNAc without alpha-LA. AKI is also expressed in mouse melanoma and leukemia cell lines and in human basal cell carcinoma specimens. The GP130 Gal acceptor once galactosylated by AKI may be directly involved in epidermal differentiation and oncogenesis.

Controls of hair follicle cycling

Nearly 50 years ago, Chase published a review of hair cycling in which he detailed hair growth in the mouse and integrated hair biology with the biology of his day. In this review we have used Chase as our model and tried to put the adult hair follicle growth cycle in perspective. We have tried to sketch the adult hair follicle cycle, as we know it today and what needs to be known. Above all, we hope that this work will serve as an introduction to basic biologists who are looking for a defined biological system that illustrates many of the challenges of modern biology: cell differentiation, epithelial-mesenchymal interactions, stem cell biology, pattern formation, apoptosis, cell and organ growth cycles, and pigmentation. The most important theme in studying the cycling hair follicle is that the follicle is a regenerating system. By traversing the phases of the cycle (growth, regression, resting, shedding, then growth again), the follicle demonstrates the unusual ability to completely regenerate itself. The basis for this regeneration rests in the unique follicular epithelial and mesenchymal components and their interactions. Recently, some of the molecular signals making up these interactions have been defined. They involve gene families also found in other regenerating systems such as fibroblast growth factor, transforming growth factor-beta, Wnt pathway, Sonic hedgehog, neurotrophins, and homeobox. For the immediate future, our challenge is to define the molecular basis for hair follicle growth control, to regenerate a mature hair follicle in vitro from defined populations, and to offer real solutions to our patients' problems.

Localisation of members of the notch system and the differentiation of vibrissa hair follicles: receptors, ligands, and fringe modulators

Hair vibrissa follicle morphogenesis involves several cell segregation phases, in the dermis as well as in the epidermis. The expression of Notch-related genes, which are well established mediators of multiple cell segregation events in Drosophila development, was studied by in situ hybridisation during embryonic mouse vibrissa follicle morphogenesis and the first adult hair cycle. The results show that two receptors, Notch1 and -2, three ligands, Delta1, Serrate1, and -2, and the three Fringe regulators, Lunatic, Manic, and Radical, are expressed in different locations and morphogenetic stages. First, the appearance of hair vibrissa primordia involves the expression of complementary patterns of Notch2, Delta1, and Lunatic Fringe in the dermis and of Notch1, Serrate2, and Lunatic Fringe in the epidermis. Second, this expression pattern is no longer found after stage 3 in the dermis. Meanwhile, in the epidermis, the expression of Notch1, Serrate2, and Lunatic Fringe before the formation of the placode may be involved in determining two populations of epidermal cells in the developing follicle. Third, complementary expression patterns for Notch1, Manic, and Lunatic Fringe, as well as Serrate1 and -2 as previously shown (Powell et al., 1998), are progressively established from stage 4 of embryonic development both in the outer root sheath and in the hair matrix. These patterns are consistent with the one found in the adult anagen phase. During the hair vibrissa cycle, Notch1 and Manic Fringe display temporal and spatial changes of expression, suggesting that they may intervene as modulators of trichocyte activities.

Differential expression pattern of the three Fringe genes is associated with epidermal differentiation

Epidermal differentiation, as keratinocytes go through different layers to the skin surface, may imply a differential activation of Notch transmembrane proteins. In mouse, as recently shown in Drosophila, Notch activation by its ligands may be modulated by Fringe secreted proteins. Therefore, we cloned the mouse homolog of Radical-fng, synthesized riboprobes for Lunatic-fng, Manic-fng, and Radical-fng, and examined their expression during epidermal differentiation. Expression of all three genes is differentially activated during embryonic epidermal stratification. Manic-fng and Lunatic-fng are expressed in the basal layer, whereas Lunatic-fng is expressed in the granular layer and Radical-fng is restricted to the most differentiated nucleated layer. This expression decreases by a few days postnatally and can be reactivated by retinoic acid treatment, which triggers a new distribution of Fringe transcripts and a thickening of the granular layer. Therefore, Manic, Lunatic, and Radical Fringe by modulating the Notch pathway may play a key role in defining the different steps of keratinocyte differentiation.

Both retinoic acid receptors alpha (RARalpha) and gamma (RARgamma) are able to initiate mouse upper-lip skin glandular metaplasia

Embryonic mouse upper-lip skin explants treated with 16.7 microM all-trans retinoic acid (tRA) give rise to a glandular metaplasia of hair vibrissa follicles; however, at this concentration, tRA can activate not only the three retinoic acid receptors (RARalpha, beta, and gamma), but also the retinoid X receptors (RXRalpha, beta, and gamma) as a consequence of its isomerization to 9-cis retinoic acid. We therefore studied the respective roles of the RXR and RAR by treating RARalpha(-/-), beta(-/-), and gamma(-/-) skin explants with tRA and wild-type explants with synthetic retinoids specific for RXR or for each of the RAR. The null mutation of the RARalpha, RARbeta, and RARgamma genes did not prevent tRA-induced hair glandular metaplasia, but RARgamma inactivation dramatically reduced its ratio. As demonstrated by treating explants with a RAR- or a RXR-specific panagonist (CD367 and Ro25-7386, respectively), RAR are primarily responsible for this metaplasia. The use of two retinoids (Ro40-6055, 8 x 10(-3) microM, or CD437, 7.7 x 10(-2) microM) that are believed to act, respectively, as a RARalpha- or a RARgamma-specific agonist showed that both these receptors can initiate a metaplasia. In contrast, BMS453, a RARbeta-specific agonist, was unable to give rise to any metaplasia. Nevertheless, the highest degrees and ratios of metaplasia were only obtained after treatment with the CD367 RAR panagonist, or with either Ro40-6055 or CD437 at a concentration sufficient to allow the activation of the three RAR, suggesting that RARbeta activation is required for a metaplasia of all vibrissae.

The role of keratin proteins and their genes in the growth, structure and properties of hair

The importance of wool in the textile industry has inspired extensive research into its structure since the 1960s. Over the past several years, however, the hair follicle has increased in significance as a system for studying developmental events and the process of terminal differentiation. The present chapter seeks to integrate the expanding literature and present a broad picture of what we know of the structure and formation of hair at the cellular and molecular level. We describe in detail the hair keratin proteins and their genes, their structure, function and regulation in the hair follicle, and also the major proteins and genes of the inner and outer root sheaths. We discuss hair follicle development with an emphasis on the factors involved and describe some hair genetic diseases and transgenic and gene knockout models because, in some cases, they stimulate natural mutations that are advancing our understanding of cellular interactions in the formation of hair.

Hair follicle growth controls

Research in hair biology has embarked in the pursuit for molecules that control hair growth. Many molecules already have been associated with the controls of hair patterning, hair maturation, and hair cycling and differentiation. Knowing how these molecules work gives us the tools for understanding and treating patients with hair disorders.

Characterization of cDNAs encoding the chick retinoic acid receptor gamma 2 and preferential distribution of retinoic acid receptor gamma transcripts during chick skin development

Retinoic acid receptors alpha, beta and gamma (RAR alpha, beta and gamma) are ligand-inductible transcriptional activators which belong to the steroid/thyroid hormone receptor superfamily. At least two major isoforms (1 and 2) of each RAR arise by differential use of two promoters and alternative splicing. In mouse, the three RAR genes are expressed in stage- and tissue-specific patterns during embryonic development. In order to understand the role of the different RARs in chick, RAR gamma 2 cDNAs were isolated from an 8.5-day (stage 35 of Hamburger and Hamilton) chick embryo skin library. The deduced chick RAR gamma 2 amino acid sequence displays uncommon features such as 21 specific amino acid replacements, 12 of them being clustered in the amino-terminal region (domains A2 and B), and a truncated acidic carboxy-terminal region (F domain). However, the pattern of RAR gamma expression in chick embryo resembles that reported in mouse, particularly in skin where RAR gamma expression occurs in both the dermal and epidermal layers at the beginning of feather formation, and is subsequently restricted to the differentiating epidermal cells. Northern blot analysis suggests that different RAR gamma isoforms could be successively required during chick development.