Localization of Shh expression by Wnt and Eda affects axial polarity and shape of hairs

Cyclical dermal micro-niche switching governs the morphological infradian rhythm of mouse zigzag hair

Biological rhythms are involved in almost all types of biological processes, not only physiological processes but also morphogenesis. Currently, how periodic morphological patterns of tissues/organs in multicellular organisms form is not fully understood. Here, using mouse zigzag hair, which has 3 bends, we found that a change in the combination of hair progenitors and their micro-niche and subsequent bend formation occur every three days. Chimeric loss-of-function and gain-of-function of Ptn and Aff3, which are upregulated immediately before bend formation, resulted in defects in the downward movement of the micro-niche and the rhythm of bend formation in an in vivo hair reconstitution assay. Our study demonstrates the periodic change in the combination between progenitors and micro-niche, which is vital for the unique infradian rhythm.

Expression variations in ectodysplasin-A gene (eda) may contribute to morphological divergence of scales in haplochromine cichlids

BACKGROUND

Elasmoid scales are one of the most common dermal appendages and can be found in almost all species of bony fish differing greatly in their shape. Whilst the genetic underpinnings behind elasmoid scale development have been investigated, not much is known about the mechanisms involved in moulding of scales. To investigate the links between gene expression differences and morphological divergence, we inferred shape variation of scales from two different areas of the body (anterior and posterior) stemming from ten haplochromine cichlid species from different origins (Lake Tanganyika, Lake Malawi, Lake Victoria and riverine). Additionally, we investigated transcriptional differences of a set of genes known to be involved in scale development and morphogenesis in fish.

RESULTS

We found that scales from the anterior and posterior part of the body strongly differ in their overall shape, and a separate look on scales from each body part revealed similar trajectories of shape differences considering the lake origin of single investigated species. Above all, nine as well as 11 out of 16 target genes showed expression differences between the lakes for the anterior and posterior dataset, respectively. Whereas in posterior scales four genes (dlx5, eda, rankl and shh) revealed significant correlations between expression and morphological differentiation, in anterior scales only one gene (eda) showed such a correlation. Furthermore, eda displayed the most significant expression difference between species of Lake Tanganyika and species of the other two younger lakes. Finally, we found genetic differences in downstream regions of eda gene (e.g., in the eda-tnfsf13b inter-genic region) that are associated with observed expression differences. This is reminiscent of a genetic difference in the eda-tnfsf13b inter-genic region which leads to gain or loss of armour plates in stickleback.

CONCLUSION

These findings provide evidence for cross-species transcriptional differences of an important morphogenetic factor, eda, which is involved in formation of ectodermal appendages. These expression differences appeared to be associated with morphological differences observed in the scales of haplochromine cichlids indicating potential role of eda mediated signal in divergent scale morphogenesis in fish.

NF-κB determines Paneth versus goblet cell fate decision in the small intestine

Although the role of the transcription factor NF-κB in intestinal inflammation and tumor formation has been investigated extensively, a physiological function of NF-κB in sustaining intestinal epithelial homeostasis beyond inflammation has not been demonstrated. Using NF-κB reporter mice, we detected strong NF-κB activity in Paneth cells, in ‘+4/+5’ secretory progenitors and in scattered Lgr5⁺ crypt base columnar stem cells of small intestinal (SI) crypts. To examine NF–κB functions in SI epithelial self-renewal, mice or SI crypt organoids (‘mini-guts’) with ubiquitously suppressed NF-κB activity were used. We show that NF-κB activity is dispensable for maintaining SI epithelial proliferation, but is essential for ex vivo organoid growth. Furthermore, we demonstrate a dramatic reduction of Paneth cells in the absence of NF-κB activity, concomitant with a significant increase in goblet cells and immature intermediate cells. This indicates that NF-κB is required for proper Paneth versus goblet cell differentiation and for SI epithelial homeostasis, which occurs via regulation of Wnt signaling and Sox9 expression downstream of NF-κB. The current study thus presents evidence for an important role for NF-κB in intestinal epithelial self-renewal.

Summary: The transcription factor NF-κB, together with downstream Wnt and Sox9, is required for Paneth and goblet cell fate decisions and for maintenance of the small intestinal stem cell niche.

Boehmite enhances hair follicle growth via stimulation of dermal papilla cells by upregulating β-catenin signalling

Hair growth, a complex process, has long been the subject of intense research. Recent developments in material technology have revealed boehmite as a new therapeutic modality for use in wound healing and scar reduction, indicating its beneficial effects. Nonetheless, the biological bases of the beneficial effects of boehmite remain unknown. We investigated the hair growth properties of boehmite in vitro and in vivo and observed dose-dependent proliferation of human dermal papilla cells (hDPCs) in vitro and hair regrowth in a mouse model. To investigate the effects of boehmite on the promotion of cell transition to the anagen phase, we evaluated hDPC viability, alkaline phosphatase (ALP) activity, protein expression and vascular endothelial growth factor (VEGF) secretion in vitro and assessed the anagen-promoting effects of boehmite via gross observation and histological analysis in a mouse model. Boehmite increased hDPC viability, ALP activity, AKT/GSK3ß/ß-catenin pathway activity, anagen-related gene expression and VEGF secretion; moreover, it accelerated hair regrowth in a catagen-anagen transition model via upregulation of β-catenin signalling and follicular cell proliferation. Collectively, our results indicate that boehmite accelerates hair growth, partly via its effects on critical events in the active phase of the hair follicle cycle, including the promotion of the proliferation of hDPCs and their immediate progeny to the follicle base.

A systematic simulation-based meta-analytical framework for prediction of physiological biomarkers in alopecia

Background

Alopecia or hair loss is a complex polygenetic and psychologically devastating disease affecting millions of men and women globally. Since the gene annotation and environmental knowledge is limited for alopecia, a systematic analysis for the identification of candidate biomarkers is required that could provide potential therapeutic targets for hair loss therapy.

Results

We designed an interactive framework to perform a meta-analytical study based on differential expression analysis, systems biology, and functional proteomic investigations. We analyzed eight publicly available microarray datasets and found 12 potential candidate biomarkers including three extracellular proteins from the list of differentially expressed genes with a p-value < 0.05. After expression profiling and functional analysis, we studied protein–protein interactions and observed functional associations of source proteins including WIF1, SPON1, LYZ, GPRC5B, PTPRE, ZFP36L2, HBB, PHF15, LMCD1, KRT35 and VAV3 with target proteins including APCDD1, WNT1, WNT3A, SHH, ESRI, TGFB1, and APP. Pathway analysis of these molecules revealed their role in major physiological reactions including protein metabolism, signal transduction, WNT, BMP, EDA, NOTCH and SHH pathways. These pathways regulate hair growth, hair follicle differentiation, pigmentation, and morphogenesis. We studied the regulatory role of β-catenin, Nf-kappa B, cytokines and retinoic acid in the development of hair growth. Therefore, the differential expression of these significant proteins would affect the normal level and could cause aberrations in hair growth.

Conclusion

Our integrative approach helps to prioritize the biomarkers that ultimately lessen the economic burden of experimental studies. It will also be valuable to discover mutants in genomic data in order to increase the identification of new biomarkers for similar problems.

Electronic supplementary material

The online version of this article (10.1186/s40709-019-0094-x) contains supplementary material, which is available to authorized users.

Skin transcriptome reveals the intrinsic molecular mechanisms underlying hair follicle cycling in Cashmere goats under natural and shortened photoperiod conditions

The growth of cashmere exhibits a seasonal pattern arising from photoperiod change. However, the underlying molecular mechanism remains unclear. We profiled the skin transcriptome of six goats at seven time points during hair follicle cycling via RNA-seq. The six goats comprised three goats exposed to a natural photoperiod and three exposed to a shortened photoperiod. During hair cycle transition, 1713 genes showed differential expression, and 332 genes showed a pattern of periodic expression. Moreover, a short photoperiod induced the hair follicle to enter anagen early, and 246 genes overlapped with the periodic genes. Among these key genes, cold-shock domain containing C2 (CSDC2) was highly expressed in the epidermis and dermis of Cashmere goat skin, although its function in hair-follicle development remains unknown. CSDC2 silencing in mouse fibroblasts resulted in the decreased mRNA expression of two key hair-follicle factors, leading to reduced cell numbers and a lower cell density. Cashmere growth or molting might be controlled by a set of periodic regulatory genes. The appropriate management of short light exposure can induce hair follicles to enter full anagen early through the activation of these regulators. The CSDC2 gene is a potentially important transcription factor in the hair growth cycle.

NF-κB Participates in Mouse Hair Cycle Control and Plays Distinct Roles in the Various Pelage Hair Follicle Types

The transcription factor NF-κB controls key features of hair follicle (HF) development, but the role of NF-κB in adult HF cycle regulation remains obscure. Using NF-κB reporter mouse models, strong NF-κB activity was detected in the secondary hair germ of late telogen and early anagen HFs, suggesting a potential role for NF-κB in HF stem/progenitor cell activation during anagen induction. At mid-anagen, NF-κB activity was observed in the inner root sheath and unilaterally clustered in the HF matrix, which indicates that NF-κB activity is also involved in hair fiber morphogenesis during HF cycling. A mouse model with inducible NF-κB suppression in the epithelium revealed pelage hair-type-dependent functions of NF-κB in cycling HFs. NF-κB participates in telogen-anagen transition in awl and zigzag HFs, and is required for zigzag hair bending and guard HF cycling. Interestingly, zigzag hair shaft bending depends on noncanonical NF-κB signaling, which previously has only been associated with lymphoid cell biology. Furthermore, loss of guard HF cycling suggests that in this particular hair type, NF-κB is indispensable for stem cell activation, maintenance, and/or growth.

Gata6 promotes hair follicle progenitor cell renewal by genome maintenance during proliferation

Cell proliferation is essential to rapid tissue growth and repair, but can result in replication-associated genome damage. Here, we implicate the transcription factor Gata6 in adult mouse hair follicle regeneration where it controls the renewal of rapidly proliferating epithelial (matrix) progenitors and hence the extent of production of terminally differentiated lineages. We find that Gata6 protects against DNA damage associated with proliferation, thus preventing cell cycle arrest and apoptosis. Furthermore, we show that in vivo Gata6 stimulates EDA-receptor signaling adaptor Edaradd level and NF-κB pathway activation, known to be important for DNA damage repair and stress response in general and for hair follicle growth in particular. In cultured keratinocytes, Edaradd rescues DNA damage, cell survival, and proliferation of Gata6 knockout cells and restores MCM10 expression. Our data add to recent evidence in embryonic stem and neural progenitor cells, suggesting a model whereby developmentally regulated transcription factors protect from DNA damage associated with proliferation at key stages of rapid tissue growth. Our data may add to understanding why Gata6 is a frequent target of amplification in cancers.

Two waves of anisotropic growth generate enlarged follicles in the spiny mouse

BACKGROUND

Mammals exhibit a remarkable variety of phenotypes and comparative studies using novel model species are needed to uncover the evolutionary developmental mechanisms generating this diversity. Here, we undertake a developmental biology and numerical modeling approach to investigate the development of skin appendages in the spiny mouse, Acomys dimidiatus.

RESULTS

We demonstrate that Acomys spines, possibly involved in display and protection, are enlarged awl hairs with a concave morphology. The Acomys spines originate from enlarged placodes that are characterized by a rapid downwards growth which results in voluminous follicles. The dermal condensation (dermal papilla) at the core of the follicle is very large and exhibits a curved geometry. Given its off-centered position, the dermal papilla generates two waves of anisotropic proliferation, first of the posterior matrix, then of the anterior inner root sheath (IRS). Higher in the follicle, the posterior and anterior cortex cross-section areas substantially decrease due to cortex cell elongation and accumulation of keratin intermediate filaments. Milder keratinization in the medulla gives rise to a foamy material that eventually collapses under the combined compression of the anterior IRS and elongation of the cortex cells. Simulations, using linear elasticity theory and the finite-element method, indicate that these processes are sufficient to replicate the time evolution of the Acomys spine layers and the final shape of the emerging spine shaft.

CONCLUSIONS

Our analyses reveal how hair follicle morphogenesis has been altered during the evolution of the Acomys lineage, resulting in a shift from ancestral awl follicles to enlarged asymmetrical spines. This study contributes to a better understanding of the evolutionary developmental mechanisms that generated the great diversity of skin appendage phenotypes observed in mammals.

Hair curvature: a natural dialectic and review

Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of just three types: straight, bent/curly, or twisted. Hair forms can thus be regarded as resulting from genetic pathways that induce, reverse or modulate these basic curvature modes. However, physical interconversions between twists and curls demonstrate that strict one-to-one correspondences between them and their genetic causes do not exist. Current hair-curvature theories do not distinguish between bending and twisting mechanisms. We here introduce a multiple papillary centres (MPC) model which is particularly suitable to explain twisting. The model combines previously known features of hair cross-sectional morphology with partially/completely separated dermal papillae within single follicles, and requires such papillae to induce differential growth rates of hair cortical material in their immediate neighbourhoods. The MPC model can further help to explain other, poorly understood, aspects of hair growth and morphology. Separate bending and twisting mechanisms would be preferentially affected at the major or minor ellipsoidal sides of fibres, respectively, and together they exhaust the possibilities for influencing hair-form phenotypes. As such they suggest dialectic for hair-curvature development. We define a natural-dialectic (ND) which could take advantage of speculative aspects of dialectic, but would verify its input data and results by experimental methods. We use this as a top-down approach to first define routes by which hair bending or twisting may be brought about and then review evidence in support of such routes. In particular we consider the wingless (Wnt) and mammalian target of rapamycin (mTOR) pathways as paradigm pathways for molecular hair bending and twisting mechanisms, respectively. In addition to the Wnt canonical pathway, the Wnt/Ca(2+) and planar cell polarity (PCP) pathways, and others, can explain many alternatives and specific variations of hair bending phenotypes. Mechanisms for hair papilla budding or its division by bisection or fission can explain MPC formation. Epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial (MET) transitions, acting in collaboration with epithelial-mesenchymal communications are also considered as mechanisms affecting hair growth and its bending and twisting. These may be treated as sub-mechanisms of an overall development from neural-crest stem cell (NCSC) lineages to differentiated hair follicle (HF) cell types, thus providing a unified framework for hair growth and development.

Generation and characterization of function-blocking anti-ectodysplasin A (EDA) monoclonal antibodies that induce ectodermal dysplasia

Development of ectodermal appendages, such as hair, teeth, sweat glands, sebaceous glands, and mammary glands, requires the action of the TNF family ligand ectodysplasin A (EDA). Mutations of the X-linked EDA gene cause reduction or absence of many ectodermal appendages and have been identified as a cause of ectodermal dysplasia in humans, mice, dogs, and cattle. We have generated blocking antibodies, raised in Eda-deficient mice, against the conserved, receptor-binding domain of EDA. These antibodies recognize epitopes overlapping the receptor-binding site and prevent EDA from binding and activating EDAR at close to stoichiometric ratios in in vitro binding and activity assays. The antibodies block EDA1 and EDA2 of both mammalian and avian origin and, in vivo, suppress the ability of recombinant Fc-EDA1 to rescue ectodermal dysplasia in Eda-deficient Tabby mice. Moreover, administration of EDA blocking antibodies to pregnant wild type mice induced in developing wild type fetuses a marked and permanent ectodermal dysplasia. These function-blocking anti-EDA antibodies with wide cross-species reactivity will enable study of the developmental and postdevelopmental roles of EDA in a variety of organisms and open the route to therapeutic intervention in conditions in which EDA may be implicated.

Ectodysplasin A (EDA) - EDA receptor signalling and its pharmacological modulation

The TNF family ligand ectodysplasin A (EDA) regulates the induction, morphogenesis and/or maintenance of skin-derived structures such as teeth, hair, sweat glands and several other glands. Deficiencies in the EDA - EDA receptor (EDAR) signalling pathway cause hypohidrotic ectodermal dysplasia (HED). This syndrome is characterized by the absence or malformation of several skin-derived appendages resulting in hypotrychosis, hypodontia, heat-intolerance, dry skin and dry eyes, susceptibility to airways infections and crusting of various secretions. The EDA-EDAR system is an important effector of canonical Wnt signalling in developing skin appendages. It functions by stimulating NF-κB-mediated transcription of effectors or inhibitors of the Wnt, Sonic hedgehog (SHH), fibroblast growth factor (FGF) and transforming growth factor beta (TGFβ) pathways that regulate interactions within or between epithelial and mesenchymal cells and tissues. In animal models of Eda-deficiency, soluble EDAR agonists can precisely correct clinically relevant symptoms with low side effects even at high agonist doses, indicating that efficient negative feedback signals occur in treated tissues. Hijacking of the placental antibody transport system can help deliver active molecules to developing foetuses in a timely manner. EDAR agonists may serve to treat certain forms of ectodermal dysplasia.

Wnt signaling in skin organogenesis

While serving as the interface between an organism and its environment, the skin also can elaborate a wide range of skin appendages to service specific purposes in a region-specific fashion. As in other organs, Wnt signaling plays a key role in regulating the proliferation, differentiation and motility of skin cells during their morphogenesis. Here I will review some of the recent work that has been done on skin organogenesis. I will cover dermis formation, the development of skin appendages, cycling of appendages in the adult, stem cell regulation, patterning, orientation, regional specificity and modulation by sex hormone nuclear receptors. I will also cover their roles in wound healing, hair regeneration and skin related diseases. It appears that Wnt signaling plays essential but distinct roles in different hierarchical levels of morphogenesis and organogenesis. Many of these areas have not yet been fully explored but are certainly promising areas of future research.

Cutaneous retinoic acid levels determine hair follicle development and downgrowth

Retinoic acid (RA) is essential during embryogenesis and for tissue homeostasis, whereas excess RA is well known as a teratogen. In humans, excess RA is associated with hair loss. In the present study, we demonstrate that specific levels of RA, regulated by Cyp26b1, one of the RA-degrading enzymes, are required for hair follicle (hf) morphogenesis. Mice with embryonic ablation of Cyp26b1 (Cyp26b1(-/-)) have excessive endogenous RA, resulting in arrest of hf growth at the hair germ stage. The altered hf development is rescued by grafting the mutant skin on immunodeficient mice. Our results show that normalization of RA levels is associated with reinitiation of hf development. Conditional deficiency of Cyp26b1 in the dermis (En1Cre;Cyp26b1f/-) results in decreased hair follicle density and specific effect on hair type, indicating that RA levels also influence regulators of hair bending. Our results support the model of RA-dependent dermal signals regulating hf downgrowth and bending. To elucidate target gene pathways of RA, we performed microarray and RNA-Seq profiling of genes differentially expressed in Cyp26b1(-/-) skin and En1Cre;Cyp26b1f/- tissues. We show specific effects on the Wnt-catenin pathway and on members of the Runx, Fox, and Sox transcription factor families, indicating that RA modulates pathways and factors implicated in hf downgrowth and bending. Our results establish that proper RA distribution is essential for morphogenesis, development, and differentiation of hfs.

A function for Rac1 in the terminal differentiation and pigmentation of hair

The small GTPase Rac1 is ubiquitously expressed in proliferating and differentiating layers of the epidermis and hair follicles. Previously, Rac1 was shown to regulate stem cell behaviour in these compartments. We have asked whether Rac1 has, in addition, a specific, stem-cell-independent function in the regulation of terminal hair follicle differentiation. To address this, we have expressed a constitutively active mutant of Rac1, L61Rac1, only in the basal epidermal layer and outer root sheath of mice possessing an epidermis-specific deletion of endogenous Rac1, which experience severe hair loss. The resulting 'rescue' mice exhibited a hair coat throughout their lives. Therefore, expression of Rac1 activity in the keratin-14-positive compartment of the skin is sufficient for the formation of hair follicles and hair in normal quantities. The quality of hair formed in rescue mice was, however, not normal. Rescue mice showed a grey, dull hair coat, whereas that of wild-type and L61Rac1-transgenic mice was black and shiny. Hair analysis in rescue mice revealed altered structures of the hair shaft and the cuticle and disturbed organization of medulla cells and pigment distribution. Disorganization of medulla cells correlates with the absence of cortical, keratin-filled spikes that normally protrude from the cortex into the medulla. The desmosomal cadherin Dsc2, which normally decorates these protrusions, was found to be reduced or absent in the hair of rescue mice. Our study demonstrates regulatory functions for Rac1 in the formation of hair structure and pigmentation and thereby identifies, for the first time, a role for Rac1 in terminal differentiation.

Dkk4 and Eda regulate distinctive developmental mechanisms for subtypes of mouse hair

The mouse hair coat comprises protective “primary” and thermo-regulatory “secondary” hairs. Primary hair formation is ectodysplasin (Eda) dependent, but it has been puzzling that Tabby (Eda^-/y) mice still make secondary hair. We report that Dickkopf 4 (Dkk4), a Wnt antagonist, affects an auxiliary pathway for Eda-independent development of secondary hair. A Dkk4 transgene in wild-type mice had no effect on primary hair, but secondary hairs were severely malformed. Dkk4 action on secondary hair was further demonstrated when the transgene was introduced into Tabby mice: the usual secondary follicle induction was completely blocked. The Dkk4-regulated secondary hair pathway, like the Eda-dependent primary hair pathway, is further mediated by selective activation of Shh. The results thus reveal two complex molecular pathways that distinctly regulate subtype-based morphogenesis of hair follicles, and provide a resolution for the longstanding puzzle of hair formation in Tabby mice lacking Eda.

Molecular aspects of hypohidrotic ectodermal dysplasia

Hypohidrotic (anhidrotic) ectodermal dysplasia (HED) is a congenital syndrome characterized by sparse hair, oligodontia, and reduced sweating. It is caused by mutations in any of the three Eda pathway genes: ectodysplasin (Eda), Edar, and Edaradd which encode a ligand, a receptor, and an intracellular signal mediator of a single linear pathway, respectively. In rare cases, HED is associated with immune deficiency caused by mutations in further downstream components of the Eda pathway that are necessary for the activation of the transcription factor NF-kappaB. Here I present a brief research update on the molecular aspects of this evolutionarily conserved pathway. The developmental role of Eda will be discussed in light of loss- and gain-of-function mouse models with emphasis on the past few years.

Epidermal patterning and induction of different hair types during mouse embryonic development

An intriguing question in developmental biology is how epidermal pattern formation processes are established and what are the molecular mechanisms involved in these events. The establishment of the pattern is concomitant with the formation of ectodermal appendages, which involves complex interactions between the epithelium and the underlying mesenchyme. Among ectodermal appendages, hair follicles are the "mini organs" that produce hair shafts. Several developmental and structural features are common to all hair follicles and to the hair shaft they produce. However, many different hair types are produced in a single organism. Also, different characteristics can be observed depending on the part of the body where the hair follicle is formed. Here, we review the mechanisms involved in the patterning of different hair types during mouse embryonic development as well as the influence of the body axes on hair patterning.

Biological activity of ectodysplasin A is conditioned by its collagen and heparan sulfate proteoglycan-binding domains

Mutations in the TNF family ligand EDA1 cause X-linked hypohidrotic ectodermal dysplasia (XLHED), a condition characterized by defective development of skin appendages. The EDA1 protein displays a proteolytic processing site responsible for its conversion to a soluble form, a collagen domain, and a trimeric TNF homology domain (THD) that binds the receptor EDAR. In-frame deletions in the collagen domain reduced the thermal stability of EDA1. Removal of the collagen domain decreased its activity about 100-fold, as measured with natural and engineered EDA1-responsive cell lines. The collagen domain could be functionally replaced by multimerization domains or by cross-linking antibodies, suggesting that it functions as an oligomerization unit. Surprisingly, mature soluble EDA1 containing the collagen domain was poorly active when administered in newborn, EDA-deficient (Tabby) mice. This was due to a short stretch of basic amino acids located at the N terminus of the collagen domain that confers EDA1 with proteoglycan binding ability. In contrast to wild-type EDA1, EDA1 with mutations in this basic sequence was a potent inducer of tail hair development in vivo. Thus, the collagen domain activates EDA1 by multimerization, whereas the proteoglycan-binding domain may restrict the distribution of endogeneous EDA1 in vivo.

The hair follicle as a dynamic miniorgan

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. The hair shaft consists of terminally differentiated keratinocytes that are produced by the hair follicle. Hair follicle development takes place during fetal skin development and relies on tightly regulated ectodermal-mesodermal interactions. After birth, mature and actively growing hair follicles eventually become anchored in the subcutis, and periodically regenerate by spontaneously undergoing repetitive cycles of growth (anagen), apoptosis-driven regression (catagen), and relative quiescence (telogen). Our molecular understanding of hair follicle biology relies heavily on mouse mutants with abnormalities in hair structure, growth, and/or pigmentation. These mice have allowed novel insights into important general molecular and cellular processes beyond skin and hair biology, ranging from organ induction, morphogenesis and regeneration, to pigment and stem cell biology, cell proliferation, migration and apoptosis. In this review, we present basic concepts of hair follicle biology and summarize important recent advances in the field.

Ectodysplasin signaling in cutaneous appendage development: dose, duration, and diversity

The development of several types of skin appendages is guided by prenatal ectodysplasin signaling. In this issue, Cui et al. report on the dose and duration of ectodysplasin signaling required for the maintenance and morphogenesis of different types of appendages. They report that achievement of an intimate arrangement between epithelial and mesenchymal cell populations correlates with the acquisition of autonomy from ectodysplasin stimulation.

Enhanced ectodysplasin-A receptor (EDAR) signaling alters multiple fiber characteristics to produce the East Asian hair form

Hair morphology differs dramatically between human populations: people of East Asian ancestry typically have a coarse hair texture, with individual fibers being straight, of large diameter, and cylindrical when compared to hair of European or African origin. Ectodysplasin-A receptor (EDAR) is a cell surface receptor of the tumor necrosis factor receptor (TNFR) family involved in the development of hair follicles, teeth, and sweat glands. Analyses of genome-wide polymorphism data from multiple human populations suggest that EDAR experienced strong positive selection in East Asians. It is likely that a nonsynonymous SNP in EDAR, rs3827760, was the direct target of selection as the derived p.Val370Ala variant is seen at high frequencies in populations of East Asian and Native American origin but is essentially absent from European and African populations. Here we demonstrate that the derived EDAR370A common in East Asia has a more potent signaling output than the ancestral EDAR370 V in vitro. We show that elevation of Edar activity in transgenic mice converts their hair phenotype to the typical East Asian morphology. The coat texture becomes coarse, with straightening and thickening of individual hairs and conversion of fiber cross-sectional profile to a circular form. These thick hair fibers are produced by enlarged hair follicles, which in turn develop from enlarged embryonic organ primordia. This work shows that the multiple differences in hair form between East Asian and other human populations can be explained by the simplest of genetic alterations.

Impaired hair placode formation with reduced expression of hair follicle-related genes in mice lacking Lgr4

We observed that Lgr4K5 KO mice had sparse head hair and focal alopecia behind their ears, as observed in tabby (Eda) and downless (Edar) mice, which are models of the human genetic disorder hypohidrotic (anhidrotic) ectodermal dysplasia (HED). Lgr4-deficient mice showed partial impairment in hair follicle development with reduced expression of Edar, Lef1, and Shh, which were essential for hair follicle morphogenesis, in the epidermis. Immunohistochemical analysis of Lgr4-/- mice epidermis using shh antibody showed reduced numbers of hair placodes, and we also detected higher phosphorylation of Smad1/5/8, which is required to suppress normal hair follicle induction. We suspected that Lgr4 might be a novel gene class regulating the development of hair follicles.

Analysis of the temporal requirement for eda in hair and sweat gland development

EDA signaling is important in skin appendage initiation. Its possible involvement in appendage subtype determination and postinduction stage appendage development, however, has not been studied systematically. To address these issues we manipulated Eda-A1 transgene expression in a tetracycline-regulated conditional mouse model, where the transgene is the only source of active ectodysplasin (Eda). We find that Eda-A1 restores sweat glands and all hair subtypes in Tabby, but each requires its action at an idiosyncratic time of development: by E17 for guard, by E19 for awl, and starting at E18 for zigzag/auchen hair. Guard and awl hairs were indistinguishable from their wild-type counterparts; but restored zigzag and auchen hairs, although recognizable, were somewhat smaller and lacked characteristic bends. Notably, secondary hair follicle formation of awl, auchen, and zigzag hairs required higher Eda-A1 expression level than did guard hair or sweat glands. Furthermore, Eda-A1 expression is required until the early dermal papilla stage for guard hair germs to make follicles, but is dispensable for their maturation. Similarly, sweat gland pegs require Eda-A1 at an early stage to form mature glands. Thus we infer that EDA signaling is needed for the determination and development of various skin appendages at spatiotemporally restricted intervals.

A periodic pattern generator for dental diversity

Background

Periodic patterning of iterative structures is a fundamental process during embryonic organization and development. Studies have shown how gene networks are employed to pattern butterfly eyespots, fly bristles and vertebrate epithelial appendages such as teeth, feathers, hair and mammary glands. Despite knowledge of how these features are organized, little is known about how diversity in periodic patterning is generated in nature. We address this problem through the molecular analysis of oral jaw dental diversity in Lake Malawi cichlids, where closely related species exhibit from 1 to 20 rows of teeth, with total teeth counts ranging from around 10 to 700.

Results

We investigate the expression of conserved gene networks (involving bmp2, bmp4, eda, edar, fgf8, pax9, pitx2, runx2, shh and wnt7b) known to pattern iterative structures and teeth in other vertebrates. We show that spatiotemporal variation in expression pattern reflects adult morphological diversity among three closely related Malawi cichlid species. Combinatorial epithelial expression of pitx2 and shh appears to govern the competence both of initial tooth sites and future tooth rows. Epithelial wnt7b and mesenchymal eda are expressed in the inter-germ and inter-row regions, and likely regulate the spacing of these shh-positive units. Finally, we used chemical knockdown to demonstrate the fundamental role of hedgehog signalling and initial placode formation in the organization of the periodically patterned cichlid dental programme.

Conclusion

Coordinated patterns of gene expression differ among Malawi species and prefigure the future-ordered distribution of functional teeth of specific size and spacing. This variation in gene expression among species occurs early in the developmental programme for dental patterning. These data show how a complex multi-rowed vertebrate dentition is organized and how developmental tinkering of conserved gene networks during iterative pattern formation can impact upon the evolution of trophic novelty.

Characterization of the structural and molecular defects in fibres and follicles of the Merino felting lustre mutant

The felting lustre (FL) mutation found in Merino sheep results in a fleece that has a lustrous appearance and readily felts. This phenotype was described 50 years ago to result from the mutation of a single gene, but the molecular and cellular changes in the wool are not well understood. In this study, follicle and fibre material of FL mutant (n = 3) and normal control (n = 5) Merino ewes was compared using histological analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), real-time polymerase chain reaction (qPCR) and electron microscopy [scanning electron microscopy (SEM) and transmission electron microscopy (TEM)]. Histological examination suggested that follicle structure in FL mutants is essentially normal, while SDS-PAGE analysis found that some low molecular weight keratin-associated proteins (KAP) were present at much lower levels in FL wool. Examination of transcript prevalence revealed that the KAP6.1, KAP7 and KAP8 genes in FL mutant follicles are downregulated, while the KAP2.12 and KAP4.2 genes are upregulated. TEM analysis indicated that there is only one type of cortical cell, the paracortical cell, in the fibre of FL mutants, while there are paracortical and orthocortical cells in fibres of normal Merino sheep. In contrast, SEM suggested the surface topography of FL wool fibres is normal. The evidence presented here strongly suggests that the properties of FL wool can be ascribed, at least in part, to the lower content of high glycine/tyrosine proteins and the reduction in orthocortical cells in mutant wool fibres.