Adhesion molecules and homeoproteins in the phenotypic determination of skin appendages

Adhesion Molecules in Non-melanoma Skin Cancers: A Comprehensive Review

Basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) are the most frequently diagnosed cancers, generating significant medical and financial problems. Cutaneous carcinogenesis is a very complex process characterized by genetic and molecular alterations, and mediated by various proteins and pathways. Cell adhesion molecules (CAMs) are transmembrane proteins responsible for cell-to-cell and cell-to-extracellular matrix adhesion, engaged in all steps of tumor progression. Based on their structures they are divided into five major groups: cadherins, integrins, selectins, immunoglobulins and CD44 family. Cadherins, integrins and CD44 are the most studied in the context of non-melanoma skin cancers. The differences in expression of adhesion molecules may be related to the invasiveness of these tumors, through the loss of tissue integrity, neovascularization and alterations in intercellular signaling processes. In this article, each group of CAMs is briefly described and the present knowledge on their role in the development of non-melanoma skin cancers is summarized.

Mesenchymal-epithelial interactions in the skin: aiming for site-specific tissue regeneration

Since trunk skin (or non-palmoplantar skin) is less durable under mechanical stress than sole skin (palm, plantar or palmoplantar skin), conventional trunk-derived skin grafts (including the trunk dermis) commonly result in erosion and ulceration when transplanted on to plantar wounds caused by various injuries including, diabetes mellitus or collagen diseases (including systemic sclerosis, polyarthritis nodosa and rheumatoid arthritis). However, trunk-derived epidermis can adopt a plantar phenotype, characterized by keratin 9 expression, hypopigmentation and thick suprabasal layers, through factors derived from plantar dermal fibroblasts in the wounds. Thus, intractable plantar wounds with exposed bones can be treated with the combination of bone marrow exposure, occlusive dressing and epidermal grafting. The higher expression of dickkopf 1 (DKK1), an inhibitor of canonical Wnt/beta-catenin signals, in the plantar dermis partly explains these phenomena. Thus, mesenchymal-epithelial interactions play important roles not only in embryogenesis (the embryonic development) but also in maintaining the homeostasis of adult tissue. The topographical (site-specific) interactions of growth factors and substances, including DKKs, fibroblast growth factors (FGFs) and transforming growth factor-beta (TGF-beta) family proteins including bone morphogenetic proteins (BMPs), may explain the site-specific differences in the skin in addition to the expression patterns of HOX genes and sonic hedgehogs (Shhs). We review the importance of dermal-epidermal interactions in tissue homeostasis and regeneration, especially in palms and soles.

HOXB4 homeodomain protein is expressed in developing epidermis and skin disorders and modulates keratinocyte proliferation

The HOX homeodomain proteins are fundamental regulators of organ and tissue development, where they are thought to function as transcription factors, and HOX gene expression has been associated with numerous types of cancers. Previous studies have demonstrated that enforced expression of the HOXB4 protein transforms cultured fibroblasts and leads to a selective expansion of the hematopoietic stem cell pool, suggesting that this protein might play a role in cellular proliferation. In support of this concept, we now show that enforced expression of HOXB4 in human neonatal keratinocytes results in increased cellular proliferation and colony formation as well as decreased expression of the alpha-2-integrin and CD44 cell surface adhesion molecules. We previously have reported HOXB4 gene expression in the basal and suprabasal layers of developing human skin and now show extensive HOXB4 mRNA in psoriatic skin and basal cell carcinoma. In fetal human skin HOXB4 protein expression was both nuclear and cytoplasmic within epidermal basal cells and in hair follicle inner and outer root sheath cells, whereas strong nuclear signals were observed in the bulge region. In adult skin, HOXB4 protein expression was both nuclear and cytoplasmic, but was predominantly localized to the intermediate and differentiated cell layers. In contrast to the striking gradient patterns of HOX gene and protein expression previously described in developing spinal cord and limb, HOXB4 protein was uniformly detected in all regions of the fetal and adult skin. Although little HOXB4 signal localized to proliferative cell layers, as marked by proliferating cell nuclear antigen (PCNA) staining, in normal adult epidermis, nuclear HOXB4 protein expression substantially overlapped with PCNA-positive cell in a series of samples of hyperproliferative skin. Taken together, these data suggest that nuclear HOXB4 protein may play a role in the regulation of cellular proliferation/adhesion in developing fetal human epidermis and in hyperproliferation conditions, including cancers, in adult epidermis. Published 2002 Wiley-Liss, Inc.

Development and evolution of the mammalian limb: adaptive diversification of nails, hooves, and claws

Paleontological evidence indicates that the evolutionary diversification of mammals early in the Cenozoic era was characterized by an adaptive radiation of distal limb structures. Likewise, neontological data show that morphological variation in distal limb integumentary appendages (e.g., nails, hooves, and claws) can be observed not only among distantly related mammalian taxa but also among closely related species within the same clade. Comparative analysis of nail, claw, and hoof morphogenesis reveals relatively subtle differences in mesenchymal and epithelial patterning underlying these adult differences in distal limb appendage morphology. Furthermore, studies of regulatory gene expression during vertebrate claw development demonstrate that many of the signaling molecules involved in patterning ectodermal derivatives such as teeth, hair, and feathers are also involved in organizing mammalian distal limb appendages. For example, Bmp4 signaling plays an important role during the recruitment of mesenchymal cells into the condensations forming the terminal phalanges, whereas Msx2 affects the length of nails and claws by suppressing proliferation of germinal epidermal cells. Evolutionary changes in the form of distal integumentary appendages may therefore result from changes in gene expression during formation of mesenchymal condensations (Bmp4, posterior Hox genes), induction of the claw fold and germinal matrix (shh), and/or proliferation of epidermal cells in the claw matrix (Msx1, Msx2). The prevalence of convergences and parallelisms in nail and claw structure among mammals underscores the existence of multiple morphogenetic pathways for evolutionary change in distal limb appendages.

Localization of HB9 homeobox gene mRNA and protein during the early stages of chick feather development

We performed in situ hybridization and immunohistochemical analysis of HB9 homeobox gene mRNA and protein, respectively, during chick feather development. HB9 mRNA was highly expressed in epidermal basal cells and dermal cells of the placodes and feather buds, but not in those of the interplacodes and interbud regions. HB9 protein was predominantly expressed in dermal cells of the symmetric short buds and decreased after the asymmetric bud stage when the feather bud had become elongated along the anterior-posterior (A-P) and proximal-distal (P-D) axis. These results suggest that HB9 gene is regulated in a spatiotemporal manner during feather development, and may be involved in early feather bud morphogenesis.

Distinct patterns of NCAM expression are associated with defined stages of murine hair follicle morphogenesis and regression

Hair follicle development, growth (anagen), and regression (catagen) largely result from bidirectional epithelial-mesenchymal interactions whose molecular basis is still unclear. Because adhesion molecules are critically involved in pattern formation and because the fundamental importance of neural cell adhesion molecule (NCAM) for feather development has been demonstrated, we studied the protein expression patterns of NCAM during hair follicle development and regression in the C57BL/6 mouse model. During murine hair follicle development, NCAM immunoreactivity (IR) was first detected on epithelial hair placodes and later on selected keratinocytes in the distal outer root sheath. Mesenchymal NCAM immunoreactivity (IR) was noted on fibroblasts of the future dermal papilla (DP) and the perifollicular connective tissue sheath. Fetal hair follicle elongation coincided with strong, ubiquitous dermal NCAM IR, which remained strong until the follicles entered into their first neonatal catagen. At this time, the strong interfollicular dermal NCAM IR decreased substantially. During consecutive hair cycles, mesenchymal NCAM IR was seen exclusively on DP and perifollicular connective tissue sheath fibroblasts and on the trailing cells of regressing catagen hair follicles. These highly restricted and developmentally controlled expression patterns suggest an important role for NCAM in hair follicle topobiology during morphogenesis and cyclic remodeling of this miniorgan.

The gene defective in anhidrotic ectodermal dysplasia is expressed in the developing epithelium, neuroectoderm, thymus, and bone

Anhidrotic ectodermal dysplasia (EDA) is characterized by defects in the development of teeth, hair, and sweat glands. To study the expression of the human gene defective in EDA in human fetal development (Weeks 6-23 of gestational age) and in adult tissues, in situ hybridization and immunohistochemistry were used. First signs of expression were detected at Week 8 in epidermis and in neuroectodermal cells. Starting at Week 12, osteoblasts and thymus were positive for EDA mRNA. Hair follicles expressed EDA mRNA from 18 weeks. The presence of the EDA protein coincided with mRNA expression in the tissues examined. The expression pattern of the EDA gene is consistent with typical involvement of the skin in the syndrome. However, the expression is not limited to the ectodermal tissues and many sites of expression are not obviously reflected in the clinical features of the syndrome.

CHOXC-8 and CHOXD-13 expression in embryonic chick skin and cutaneous appendage specification

We studied the expression of two distantly clustered Hox genes which could, respectively, be involved in specification of dorsal feather- and foot scale-forming skin in the chick embryo: cHoxc-8, a median paralog, and cHoxd-13, located at the 5' extremity of the HoxD cluster. The cHoxc-8 transcripts are present at embryonic day 3.5 (E3.5) in the somitic cells, which give rise to the dorsal dermis by E5, and at E6.5-8.5 in the dorsal dermal and epidermal cells during the first stages of feather morphogenesis. The cHoxd-13 transcripts are present at E4.5-9.5 in the autopodial mesenchyme and at E10.5-12.5 in the plantar dermis during the initiation of reticulate scale morphogenesis. Both the cHoxc-8 and cHoxd-13 transcripts are no longer detectable after the anlagen stage of cutaneous appendage morphogenesis. Furthermore, heterotopic dermal-epidermal recombinations of dorsal, plantar, and apteric tissues revealed that the epidermal ability or inability to form feathers is already established by the time of skin formation. Retinoic acid (RA) treatment at E11 induces after 12 hr an inhibition of cHoxd-13 expression in the plantar dermis, followed by the formation of feather filaments on the reticulate scales. When E7.5 dorsal explants are treated with RA for 6 days, they form scale-like structures where the Hox transcripts are no more detectable. Protein analysis revealed that the plantar filaments, made up of feather beta-keratins, corresponded to a homeotic transformation, whereas the scale-like structures, composed also of feather beta-keratins, were teratoid. These results strengthen the hypothesis that different homeobox genes play a significant role in specifying the regional identity of the different epidermal territories.

Molecular histology in skin appendage morphogenesis

Classical histological studies have demonstrated the cellular organization of skin appendages and helped us appreciate the intricate structures and function of skin appendages. At this juncture, questions can be directed to determine how these cellular organizations are achieved. How do cells rearrange themselves to form the complex cyto-architecture of skin appendages? What are the molecular bases of the morphogenesis and histogenesis of skin appendages? Recently, many new molecules expressed in a spatial and temporal specific manner during the formation of skin appendages were identified by molecular biological approaches. In this review, novel molecular techniques that are useful in skin appendage research are discussed. The distribution of exemplary molecules from different categories including growth factors, intracellular signaling molecules, homeobox genes, adhesion molecules, and extracellular matrix molecules are summarized in a diagram using feather and hair as models. We hope that these results will serve as the ground work for completing the molecular mapping of skin appendages which will refine and re-define our understanding of the developmental process beyond relying on morphological criteria. We also hope that the listed protocols will help those who are interested in this venture. This new molecular histology of skin appendages is the foundation for forming new hypotheses on how molecules are mechanistically involved in skin appendage development and for designing experiments to test them. This may also lead to the modulation of healing and regeneration processes in future treatment modalities.

Developmentally programmed expression of hyaluronan in human skin and its appendages

The expression of hyaluronan (HA) in fetal human skin was studied by using a biotinylated HA-binding probe. The uniform expression of HA in primitive skin was changed after the 9th week, when differentiation of the basement membrane zone increased HA in the subepidermal mesenchyme. Maturation of the papillary dermis at the 12-20th weeks led to the thickening of this HA-enriched zone; the underlying reticular layer was less intensely stained. In epidermis the number of cell layers rapidly increased after the 9th week. At first all epidermal layers were HA-positive. A complete loss of HA from the upper intermediate cells on the 18th week preceded the formation of mature granular and cornified layers. Peridermal cells remained HA-positive even when the underlying stratum corneum turned negative. The tightly apposed basal epithelial cells, the first stage of hair follicle and eccrine sweat gland formation, became almost completely depleted of HA. With advancing bulb development HA returned in the epithelial compartment, until maturation of the hair follicles restricted its expression to the outer root sheath and hair matrix. Maturation of the sebaceous glands led to the expression of HA pericellularly in the germinative cells and intracellularly in the mature sebocytes. Marked changes thus occur in the distribution of HA during fetal skin development; the primitive tissues exhibited a uniform widespread expression of HA, and maturing tissues showed distinct locally regulated patterns. The loss of epithelial HA in the hair follicle anlagen and upper intermediate cells turned out to be early differentiation markers.

FGF induces new feather buds from developing avian skin

Induction of skin appendages involves a cascade of molecular events. The fibroblast growth factor (FGF) family of peptide growth factors is involved in cell proliferation and morphogenesis. We explored the role of the FGFs during skin appendage induction using developing chicken feather buds as a model. FGF-1, FGF-2, or FGF-4 was added directly to the culture medium or was released from pre-soaked Affigel blue beads. Near the midline, FGFs led to fusion of developing feather buds, representing FGFs' ability to expand feather bud domains in developing skin. In lateral regions of the explant where feather placodes have not formed, FGF treatment produces a zone of condensation and a region with an increased number of feather buds. In ventral epidermis that is normally apteric (without feathers), FGFs can also induce new feather buds. Like normal feather buds, the newly induced buds express Shh. The expression of Grb, Ras, Raf, and Erk, intracellular signaling molecules known to be downstream to tyrosine kinase receptors such as the FGF receptor, was enriched in feather bud domains. Genistein, an inhibitor of tyrosine kinase, suppressed feather bud formation and the effect of FGF. These results indicate that there are varied responses to FGFs depending on epithelial competence. All the phenotypic responses, however, show that FGFs facilitate the formation of skin appendage domains.

Early events during avian skin appendage regeneration: dependence on epithelial-mesenchymal interaction and order of molecular reappearance

Early molecular events during the development and regeneration of skin appendages were studied using cultured chicken skin explants with epithelial-mesenchymal recombination. The explant epithelium was separated from the mesenchyme, rotated 90 degrees or 180 degrees, recombined with the mesenchyme, and cultured. After this procedure, existing feather buds disappeared and new buds were regenerated. The location of the new buds is determined by the original dermal condensations, whereas the orientation is dictated by the original epithelium. The temporal expression of key morphogenetic molecules was examined 3, 6, and 20 h after recombination by whole-mount in situ hybridization and immunostaining. The results showed the following. (i) Placode formation and the expression of wingless-int (Wnt) 7a and Msx-1 in the placode epithelium are mesenchyme dependent. (ii) Hox C6 and neural cell adhesion molecule (NCAM) expression in the anterior mesenchyme is placode epithelium dependent. (iii) Bone morphogenetic protein (BMP)-2, BMP-4, and fibroblast growth factor (FGF)-4 expression in the original dermal condensations was unaffected by recombination. (iv) Old dermal condensations can induce new placodes with new Wnt 7a, sonic hedgehog (Shh), and Msx-1 and -2 expression. (v) The new placode epithelium can then induce new Hox C6 and NCAM microgradients in the feather bud mesenchyme. (vi) The order of appearance can be classified into four groups in the following order: BMP-2, BMP-4, and FGF-4 (peptide growth factors); Wnt 7a and Shh (Drosophila segment polarity gene homologs); Msx-1 and Msx-2 (Msx class homeobox genes); and then Hox C6 (Hox class homeobox genes) and NCAM (adhesion molecules). These results suggest an order for the molecular cascade during the inductive phase of skin appendage development.

Homeobox genes Msx-1 and Msx-2 are associated with induction and growth of skin appendages

The mechanism involved in the morphogenesis of skin appendages is a fundamental issue underlying the development and healing of skin. To identify molecules involved in the induction and growth of skin appendages, we studied the expression of two homeobox genes, Msx-1 and Msx-2, during embryonic chicken skin development. We found that i) both Msx-1 and Msx-2 are early markers of epithelial placodes for skin appendages; ii) both Msx-1 and Msx-2 are expressed in the growing feather bud epithelia but not in the interbud epithelia; iii) although mostly overlapping, there are differences between the expression of the two Msx genes, Msx-1 being expressed more toward the anterior whereas Msx-2 is expressed more toward the distal feather bud; iv) there is no body-position-specific expression pattern as was observed for members of the Hox A-D clusters; v) in the feather follicle, Msx-1 and 2 are expressed in the collar and barb ridge epithelia, both regions of continuous cell proliferation; vi) when feather-bud growth was inhibited by forskolin, an activator of adenylyl cyclase, the expression of both genes was reduced. These results showed that Msx genes are specifically expressed in epithelial domains destined to become skin appendages. Its function in skin-appendage morphogenesis may be twofold, first in making epithelial cells competent to become skin appendages and, second, in making epithelial cells maintain their potential for continuous growth.

Role of androgens in the developmental biology of the pilosebaceous unit

The growth and development of pilosebaceous units in their characteristic pattern depends on the interaction of androgens and diverse biologic factors. Stromal-epithelial interactions are essential features. Considerable evidence suggests that androgens stimulate the growth of sensitive pilosebaceous units primarily by acting on specific stromal cells and that androgens and retinoic acid interact to regulate specific stages of sebocyte differentiation.

The 5'-sequence of the murine Hox-b3 (Hox-2.7) gene and its intron contain multiple transcription-regulatory elements

We sought to clone and characterize the murine Hox-b3 gene. In Xenopus embryos, the homologous gene has been shown to be responsive to retinoic acid, an agent which has profound effects on tissue growth and development. By plaque hybridization, using a partial, murine Hox-b3 cDNA as a probe, we screened a genomic library and isolated a series of overlapping clones. Restriction fragments from positive clones were sequenced by the dideoxy method on an automated DNA sequencer. We report the genomic sequence of the murine Hox-b3 gene. The sequence has been submitted to the GenBank database (accession number U02278). Our sequence extends from the P1 promoter through the coding sequence of the gene to the 3'-untranslated region. In common with other homeobox genes, there is an intron between the conserved hexapeptide and the homeobox. It is 866 bp long and has 3'- and 5'-splice sites very similar to the consensus, a long polypyrimidine tract and a potential branch point near the 3'-splice site. We have analyzed the sequence 5' to the initiation codon and the intron for putative control elements, and have identified a series of putative transcription factor binding sites in the P1 promoter and intron, including two for the retinoid X receptor-beta. Their possible significance is discussed. The sequences we have identified may be responsible for the observed pattern of expression of the gene. This sequence and the clones from which it is derived will enable a molecular dissection of the P1 promoter region.

Expression of the homeobox gene HOXC4 in keratinocytes of normal skin and epithelial skin tumors is correlated with differentiation

Homeobox (HOX) genes share a highly conserved 183-bp sequence. The encoded proteins are capable of binding to specific DNA sequences and functioning as transcription factors. HOX genes play a critical role in the temporal and spatial differentiation of cells during embryogenesis. In several adult tissues, HOX genes are expressed in a constant, tissue-specific pattern, whereas in malignant tumors of these tissues an altered expression pattern was found. We investigated the expression of HOXC4 in adult normal skin by reverse-transcription polymerase chain reaction and non-radioactive RNA in situ hybridization. Moreover, HOXC4 expression was studied in various epidermal neoplasms (solar keratosis, six specimens; Bowen's disease, four; squamous cell carcinoma, nine; basal cell carcinoma, three) by RNA in situ hybridization. HOXC4 was found to be expressed in the suprabasal layers of the epidermis in normal skin specimens and the adjacent non-lesional epidermis of all other specimens. Atypical keratinocytes of solar keratoses and Bowen's disease as well as basaloid cells of basal cell carcinomas were negative. In squamous cell carcinoma, well differentiated areas with keratinization showed HOXC4 expression, whereas poorly differentiated areas were negative. Immunostaining with an antibody against cytokeratin 10, a marker of epidermal differentiation, was performed. A good correlation between the distribution pattern of HOXC4 and cytokeratin 10 in the lesions examined was found. These results suggest that HOXC4 is expressed mainly in differentiated keratinocytes. Lack of differentiation (as in neoplastic cells) is accompanied by downregulation of HOXC4 expression.