Regulation of epidermal bullous pemphigoid antigen 1 (BPAG1) synthesis by homeoprotein transcription factors

Sweat gland development requires an eccrine dermal niche and couples two epidermal programs

Eccrine sweat glands are indispensable for human thermoregulation and, similar to other mammalian skin appendages, form from multipotent epidermal progenitors. Limited understanding of how epidermal progenitors specialize to form these vital organs has precluded therapeutic efforts toward their regeneration. Herein, we applied single-nucleus transcriptomics to compare the expression content of wild-type, eccrine-forming mouse skin to that of mice harboring a skin-specific disruption of Engrailed 1 (En1), a transcription factor that promotes eccrine gland formation in humans and mice. We identify two concurrent but disproportionate epidermal transcriptomes in the early eccrine anlagen: one that is shared with hair follicles and one that is En1 dependent and eccrine specific. We demonstrate that eccrine development requires the induction of a dermal niche proximal to each developing gland in humans and mice. Our study defines the signatures of eccrine identity and uncovers the eccrine dermal niche, setting the stage for targeted regeneration and comprehensive skin repair.

Differential modularity of the mammalian Engrailed 1 enhancer network directs sweat gland development

Enhancers are context-specific regulators of expression that drive biological complexity and variation through the redeployment of conserved genes. An example of this is the enhancer-mediated control of Engrailed 1 (EN1), a pleiotropic gene whose expression is required for the formation of mammalian eccrine sweat glands. We previously identified the En1 candidate enhancer (ECE) 18 cis-regulatory element that has been highly and repeatedly derived on the human lineage to potentiate ectodermal EN1 and induce our species' uniquely high eccrine gland density. Intriguingly, ECE18 quantitative activity is negligible outside of primates and ECE18 is not required for En1 regulation and eccrine gland formation in mice, raising the possibility that distinct enhancers have evolved to modulate the same trait. Here we report the identification of the ECE20 enhancer and show it has conserved functionality in mouse and human developing skin ectoderm. Unlike ECE18, knock-out of ECE20 in mice reduces ectodermal En1 and eccrine gland number. Notably, we find ECE20, but not ECE18, is also required for En1 expression in the embryonic mouse brain, demonstrating that ECE20 is a pleiotropic En1 enhancer. Finally, that ECE18 deletion does not potentiate the eccrine phenotype of ECE20 knock-out mice supports the secondary incorporation of ECE18 into the regulation of this trait in primates. Our findings reveal that the mammalian En1 regulatory machinery diversified to incorporate both shared and lineage-restricted enhancers to regulate the same phenotype, and also have implications for understanding the forces that shape the robustness and evolvability of developmental traits.

Differential distribution and genetic determination of eccrine sweat glands and hair follicles in the volar skin of C57BL/6 mice and SD rats

BACKGROUND

Eccrine sweat glands (ESGs) and hair follicles (HFs) are the prominent skin appendages regulating human body temperature. C57BL/6 mice and Sprague-Dawley (SD) rats are the most commonly used model animals for studying ESGs and HFs. Previous studies have shown the distribution of ESGs and HFs in volar hindfeet of C57BL/6 mice, but there are few or no reports on the distribution of ESGs and HFs in volar forefeet of C57BL/6 mice and volar feet of SD rats. Here, we investigated the differential distribution and genetic determination of ESGs and HFs in the volar skin of C57BL/6 mice and SD rats through gross observation, iodine-starch sweat test, double staining with Nile Blue A and Oil Red O, hematoxylin and eosin (HE) staining, double immunofluorescence staining of LIM Homeobox 2 (LHX2)/Na⁺-K⁺-ATPase α1(NKA) or LHX2/Na⁺-K⁺-2Cl^－ cotransporter 1 (NKCC1), and qRT-PCR detection of ESG-related gene Engrailed 1 (En1) and HF-related gene LHX2.

RESULTS

The results showed ESGs but no HFs in the footpads of C57BL/6 mice and SD rats, both ESGs and HFs in the inter-footpads (IFPs) of C57BL/6 mice, and neither ESGs nor HFs in the IFPs of SD rats. The relative quantitative change in En1 was consistent with the differential distribution of ESGs, and the relative quantitative change of LHX2 was consistent with the differential distribution of HFs.

CONCLUSION

C57BL/6 mice and SD rats had their own characteristics in the distribution of ESGs and HFs in the volar skin, and researchers should choose mice or rats, and even forefeet or hindfeet as their research object according to different purposes. The study provides a basis for selection of optimal animal models to study development, wound healing and regeneration of skin appendages.

Repeated mutation of a developmental enhancer contributed to human thermoregulatory evolution

Humans sweat to cool their bodies and have by far the highest eccrine sweat gland density among primates. Humans' high eccrine gland density has long been recognized as a hallmark human evolutionary adaptation, but its genetic basis has been unknown. In humans, expression of the Engrailed 1 (EN1) transcription factor correlates with the onset of eccrine gland formation. In mice, regulation of ectodermal En1 expression is a major determinant of natural variation in eccrine gland density between strains, and increased En1 expression promotes the specification of more eccrine glands. Here, we show that regulation of EN1 has evolved specifically on the human lineage to promote eccrine gland formation. Using comparative genomics and validation of ectodermal enhancer activity in mice, we identified a human EN1 skin enhancer, hECE18. We showed that multiple epistatically interacting derived substitutions in the human ECE18 enhancer increased its activity compared with nonhuman ape orthologs in cultured keratinocytes. Repression of hECE18 in human cultured keratinocytes specifically attenuated EN1 expression, indicating this element positively regulates EN1 in this context. In a humanized enhancer knock-in mouse, hECE18 increased developmental En1 expression in the skin to induce the formation of more eccrine glands. Our study uncovers a genetic basis contributing to the evolution of one of the most singular human adaptations and implicates multiple interacting mutations in a single enhancer as a mechanism for human evolutionary change.

A genetic basis of variation in eccrine sweat gland and hair follicle density

Among the unique features of humans, one of the most salient is the ability to effectively cool the body during extreme prolonged activity through the evapotranspiration of water on the skin's surface. The evolution of this novel physiological ability required a dramatic increase in the density and distribution of eccrine sweat glands relative to other mammals and a concomitant reduction of body hair cover. Elucidation of the genetic underpinnings for these adaptive changes is confounded by a lack of knowledge about how eccrine gland fate and density are specified during development. Moreover, although reciprocal changes in hair cover and eccrine gland density are required for efficient thermoregulation, it is unclear if these changes are linked by a common genetic regulation. To identify pathways controlling the relative patterning of eccrine glands and hair follicles, we exploited natural variation in the density of these organs between different strains of mice. Quantitative trait locus mapping identified a large region on mouse Chromosome 1 that controls both hair and eccrine gland densities. Differential and allelic expression analysis of the genes within this interval coupled with subsequent functional studies demonstrated that the level of En1 activity directs the relative numbers of eccrine glands and hair follicles. These findings implicate En1 as a newly identified and reciprocal determinant of hair follicle and eccrine gland density and identify a pathway that could have contributed to the evolution of the unique features of human skin.

Regionalisation of the skin

The skin displays marked anatomical variation in thickness, colour and in the appendages that it carries. These regional distinctions arise in the embryo, likely founded on a combinatorial positional code of transcription factor expression. Throughout adult life, the skin's distinct anatomy is maintained through both cell autonomous epigenetic processes and by mesenchymal-epithelial induction. Despite the readily apparent anatomical differences in skin characteristics across the body, several fundamental questions regarding how such regional differences first arise and then persist are unresolved. However, it is clear that the skin's positional code is at the molecular level far more detailed than that discernible at the phenotypic level. This provides a latent reservoir of anatomical complexity ready to surface if perturbed by mutation, hormonal changes, ageing or experiment.

Msx2 controls ameloblast terminal differentiation

Late tooth morphogenesis is characterized by a series of events that determine cusp morphogenesis and the histodifferentiation of epithelial cells into enamel-secreting ameloblasts. Mice lacking the homeobox gene Msx2 exhibit defects in cusp morphogenesis and in the process of amelogenesis. To better understand the basis of the Msx2 mutant tooth defects, we have investigated the function of Msx2 during late stages of tooth morphogenesis. Cusp formation is thought to be under the control of the enamel knot, which has been proposed to act as an organizing center during this process (Vaahtokari et al. [ 1996] Mech. Dev. 54:39-43). Bone morphogenetic protein-4 (BMP4) has been suggested to mediate termination of enamel knot signaling by means of regulation of programmed cell death (Jernvall et al. [ 1998] Development 125:161-169). Here, we show that Bmp4 expression in the enamel knot is Msx2-dependent. We further show that during amelogenesis Msx2 is required for the expression of the extracellular matrix gene Laminin 5 alpha 3, which is known to play an essential role during ameloblast differentiation. This result thus provides a paradigm for understanding how transcription factors and extracellular matrix can be integrated into a developmental pathway controlling cell differentiation.

Dorsal versus ventral scales and the dorsoventral patterning of chick foot epidermis

The dorsal and ventral scales of the chick foot can be distinguished morphologically and molecularly: the dorsal oblong overlapping scuta expressing both alpha and beta keratins, and the ventral roundish nonprotruding reticula expressing only alpha keratins. The question arises how En-1 and Lmx1, whose role in dorsoventral limb patterning has been well established, can affect skin morphogenesis, which occurs 8 to 12 days later. Forced expression of En-1 or of Lmx1 in the hindlimb have, respectively, as expected, a ventralizing or a dorsalizing effect on skin, leading to the formation of either reticula-type or scuta-type scales on both faces. In both cases, however, the scales are abnormal and even glabrous skin without any scales at all may form. The normal inductive interactions between dermis and epidermis are disturbed after En-1 or Lmx1 misexpression. Effectively, while Lmx1 endows the dermal precursors of the ventral region with scuta inducing ability, En-1 blocks the competence of the dorsal epidermis to build scuta.

Keratinocyte responsive element 3: analysis of a keratinocyte-specific regulatory sequence in the 230-kDa bullous pemphigoid antigen gene promoter

The 230-kDa bullous pemphigoid antigen gene is expressed primarily, if not exclusively, in basal keratinocytes of the epidermis. Keratinocyte responsive element 3, a cis-element at position -216 to -197 of the human 230-kDa bullous pemphigoid antigen gene promoter, confers tissue-specific expression to this gene (Tamai et al: J Biol Chem 270:7609-7614, 1995). In this study, we investigated the functional characteristics of keratinocyte responsive element 3 on the 230-kDa bullous pemphigoid antigen gene core promoter by transient transfections of cultured normal human keratinocytes and normal human fibroblasts, as well as of lung carcinoma (A549), osteosarcoma (OST), and gastric adenocarcinoma (GT3TKB) cell lines. A 230-kDa bullous pemphigoid antigen gene core promoter/luciferase reporter gene plasmid construct, pBPL, was modified to develop a series of constructs (pKBPL-p4KBPL), which have insertions of one, two, three, or four tandem repeats of keratinocyte responsive element 3, and these plasmids were used in transient transfections of the cultured cells. The promoter activities of pKBPL-p4KBPL constructs, relative to pBPL, in normal human keratinocytes were 7.6-, 15.5-, 4.6-, and 2.7-fold higher, respectively, whereas no upregulatory effect by keratinocyte responsive element 3 insertion was observed in other cell lines tested. prKBPL, a plasmid constructed with keratinocyte responsive element 3 in reverse orientation, showed essentially no activity in normal human keratinocytes. Insertion of a random 20 bp sequence between keratinocyte responsive element 3 and the 230-kDa bullous pemphigoid antigen gene core promoter resulted in about 40% reduction of luciferase activity in normal human keratinocytes. These data suggest that keratinocyte responsive element 3 functions as a position-, copy number-, and orientation-dependent cis-element contributing to tissue-specific regulation of the 230-kDa bullous pemphigoid antigen gene.

The neuronal microtubule-associated protein 1B is under homeoprotein transcriptional control

To identify genes regulated by homeoprotein transcription factors in postnatal neurons, the DNA-binding domain (homeodomain) of Engrailed homeoprotein was internalized into rat cerebellum neurons. The internalized homeodomain (EnHD) acts as a competitive inhibitor of Engrailed and of several homeoproteins (Mainguy et al., 2000). Analysis by differential display revealed that microtubule-associated protein 1B (MAP1B) mRNA is upregulated by EnHD. This upregulation does not require protein synthesis, suggesting a direct effect of the homeodomain on MAP1B transcription. The promoter region of MAP1B was cut into several subdomains, and each subdomain was tested for its ability to bind Engrailed and EnHD and to associate with Engrailed-containing cerebellum nuclear extracts. In addition, the activity, and regulation by Engrailed, of each subdomain and of the entire promoter were evaluated in vivo by electroporation in the chick embryo neural tube. These experiments demonstrate that MAP1B promoter is regulated by Engrailed in vivo. Moreover, they show that one promoter domain that contains all ATTA homeoprotein cognate binding sites common to the rat and human genes is an essential element of this regulation. It is thus proposed that MAP1B, a cytoskeleton protein involved in neuronal growth and regeneration, is under homeoprotein transcriptional regulation.

An induction gene trap for identifying a homeoprotein-regulated locus

An important issue in developmental biology is the identification of homeoprotein target genes. We have developed a strategy based on the internalization and nuclear addressing of exogenous homeodomains, using an engrailed homeodomain (EnHD) to screen an embryonic stem (ES) cell gene trap library. Eight integrated gene trap loci responded to EnHD. One is within the bullous pemphigoid antigen 1 (BPAG1) locus, in a region that interrupts two neural isoforms. By combining in vivo electroporation with organotypic cultures, we show that an already identified BPAG1 enhancer/promoter is differentially regulated by homeoproteins Hoxc-8 and Engrailed in the embryonic spinal cord and mesencephalon. This strategy can therefore be used for identifying and mutating homeoprotein targets. Because homeodomain third helices can internalize proteins, peptides, phosphopeptides, and antisense oligonucleotides, this strategy should be applicable to other intracellular targets for characterizing genetic networks involved in a large number of physiopathological states.

Homeodomain-derived peptides. In and out of the cells

The internalization of homeodomains and of homeopeptides derived from the third helix of the homeodomain of Antennapedia, a Drosophila transcription factor, is used by some investigators to target exogenous hydrophilic compounds into live cells. In addition to this very practical aspect of drug delivery, translocation across biologic membranes of peptides subsequently addressed to the cell cytoplasm and nucleus raises several questions. A first series of questions pertains to the mechanism of translocation. Thanks to the synthesis of several peptides derived from the third helix of the Antennapedia homeodomain, we began to investigate the mechanism of translocation and we have shown that it is not dependent upon the presence of a chiral receptor and probably involves the formation of inverted micelles. A second series of questions is related to the physiologic significance of the phenomenon. In a first approach, we demonstrated that some full-length homeoproteins are internalized and secreted in vitro. The mechanism of internalization is probably similar to that of the homeodomain or of its third helix, but secretion involves a different mechanism which requires an association with specialized intracellular membranous structures. The existence of specific mechanisms for homeoprotein internalization and secretion suggests that this class of transcription factors may have important signaling properties.