Results

Snail mRNA and Protein Are Expressed Transiently at the Hair Bud Stage of Follicle Morphogenesis
Although Snail family members are most frequently associated with EMTs, they also participate in many malignant processes involving a down-regulation but not a quantitative abrogation of intercellular junctions [18]. The range of developmental processes in which Snail family members have been implicated thus includes the type of epithelial remodeling that is observed in hair follicle bud formation. Given our prior observation that exogenously added Snail can participate with LEF-1/β-catenin in down-regulating E-cadherin expression in keratinocytes [4], coupled with the established requirement for LEF-1/β-catenin in hair follicle morphogenesis [4,19], we turned to addressing whether Snail/Slug family members might also participate in the process.
PCR analyses identified transient Snail mRNA expression during a period of skin embryogenesis when waves of hair follicles are forming (unpublished data).To pinpoint specifically where Snail mRNA is expressed in the developing skin, we conducted in situ hybridization using a cRNA probe unique to the Snail 3′ untranslated region (UTR). Embryonic day 17.5 (E17.5) was chosen, since the multiple waves of follicle morphogenesis occurring at this time enabled us to evaluate Snail expression at different stages of the process. As shown in Figure 1A, specific hybridization was detected within the epithelium of nascent hair buds. By contrast, as follicles progressed further through their development (e.g., germ and peg stages), they exhibited no signs of hybridization (Figure 1A). The transient nature of Snail mRNA expression during follicle development was most apparent in hybridized skin sections containing follicles from two different waves of morphogenesis (as shown in Figure 1). Hybridizing hair buds from a later wave appeared juxtaposed with nonhybridizing follicles from an earlier wave.
Figure 1  Snail Is Expressed Exclusively in the Hair Bud during Morphogenesis
Embryos were either frozen in OCT embedding compound (A, F, and H) or embedded in paraffin (C, D, E, and G), and then sectioned (8 μm).
(A) In situ hybridizations with Snail sense or antisense cRNA probes. Black dotted lines demarcate the basement membrane that separates the epidermis (epi) from dermis (der). Arrows point to Snail RNA expression, restricted to the hair bud stage of follicle morphogenesis. It was not seen in later hair germ or peg stages.
(B) Expression of Snail protein coincides with hair development. Protein extracts were prepared from keratinocytes transfected with empty expression vector (K14), containing the K14 promoter or with the vector driving HA-tagged Snail (K14-Snail); or from whole skin from E13.5 to P5 animals, including newborn (nb). Equal amounts of proteins were then resolved by SDS-PAGE through 12% gels and subjected to Western blotting using either an affinity-purified Snail polyclonal antiserum, which we generated, or anti-tubulin (loading control).
(C–E) Immunohistochemistry shows expression of Snail protein in the nuclei of cells within the hair and skin. (C) E13.5 skin with a single layered epidermis (epi) shows no Snail expression. (D) The first morphological sign that cells have adopted a hair follicle fate is a cluster of cells called a placode in E16.5 skin. Snail is not expressed at this stage of development. (E) Snail is expressed in the hair bud of E17.5 skin but not in later stages of development such as the germ or peg.
(F) Immunofluorescence with anti-Ki67 (green) identifies the proliferating cells of the skin, restricted to the basal layer of the epidermis and developing hair follicles. Anti-β4 int labeling reveals the presence of the hemidesmosomal integrin β4, restricted to the base of cells adhering to the underlying basement membrane. The white dotted line marks the outermost surface of the skin.
(G) Immunohistochemistry with pMAPK marks a subset of proliferating cells within the epidermis and hair bud. Anti-pMAPK labeling was consistently robust within the hair bud.
(H) Immunofluorescence with anti-laminin 5 (lam5), which demarcates the basement membrane, and anti-E-cadherin (E-cad), a component of AJs. At the leading edge of the growing bud, cell-cell borders show markedly diminished anti-E-cadherin labeling (arrowheads). To determine whether this transient nature of Snail mRNA expression is reflected at the protein level, we generated an antibody against the N-terminal sequence that resides upstream of the more conserved zinc finger domains. As judged by Western blot analysis, the antibody did not detect endogenous proteins from cultured keratinocytes, but it did yield a band of the expected size from keratinocytes transiently expressing a hemagglutinin (HA)-tagged Snail protein (Figure 1B). The antibody also recognized a band corresponding to the size of endogenous Snail (approximately 28 kDa) in lysates from embryonic mouse skin, the temporal appearance of which corresponded to the waves of hair follicle morphogenesis from E15.5 to newborn when over 90% of the hair on the mouse is formed (Figure 1B). Consistent with the Western blot data, immunohistochemical analysis did not detect Snail in single-layered E13.5 epidermis (Figure 1C) nor in the placode, which is the earliest morphological sign of the commitment of multipotent cells of the embryonic ectoderm to a hair cell fate (Figure 1D). Consistent with the in situ hybridization results, anti-Snail antibody labeled only hair buds and not follicles at more mature stages of development (Figure 1E). Taken together, the anti-Snail antibody appeared to be specific for its target protein. It did not detect other Snail family members known to be expressed in keratinocytes and/or skin (unpublished data). Furthermore, the immunohistochemical data paralleled our Snail in situ hybridization data revealing transient Snail expression at the hair bud stage (Figure 1A).
As judged by immunohistochemistry, Snail protein was localized to the nuclei of the hair bud cells (Figure 1E). This feature was consistent with Snail's known function as a transcriptional repressor [12,13]. Additionally, anti-Snail labeling was detected in only three of the four major waves of follicle morphogenesis. Snail was not found in the buds of guard hairs that are the earliest of all hairs to form (at E13.5), and which constitute less than 5% of the mouse coat (unpublished data).
As judged by immunofluorescence with antibodies against the proliferating nuclear antigen Ki67, the timing of Snail expression coincided with the stage at which the developing follicle enhanced its proliferation and down-growth (Figure 1F). Immunohistochemistry with antibodies against the active (phosphorylated) form of MAPK (pMAPK) marked a subset of the proliferating (Ki67-positive) cells, and pMAPK-positive cells were enriched in the hair bud (Figure 1G). The timing of Snail induction and Ki67 and pMAPK enrichment in the hair bud appeared to follow closely the induction of LEF-1/β-catenin activity, known to initiate in the hair placode stage [20]. However, like placodes, hair buds exhibited down-regulation in E-cadherin expression (Figure 1H; see also [4]).

Sustained Expression of Snail Results in Epidermal Hyperproliferation and Differentiation Defects in Tg Mouse Skin
The striking spike of Snail expression coincident with hair bud formation and enhanced proliferation prompted us to examine the consequences of ectopically expressing Snail elsewhere in mouse skin epidermis. To distinguish Tg from endogenous Snail, we used the HA-epitope, shown previously not to alter Snail's transcriptional activity [12]. Of 20 K14-Snail[HA] Tg animals generated, three expressed the transgene and all exhibited analogous phenotypes. Mice that integrated the transgene at the single-cell stage died at or shortly after birth. The three surviving full-Tg founder mice harbored transgene integrations that gave stable transmission of mosaic Snail gene expression through the germline. Progressively poor health necessitated our sacrificing most offspring from these lines within a year of birth.
As Snail Tg animals grew, they became distinguished by their small size, short tails, and flaky skin (Figure 2A). Histological analyses of 3-d old (P3) mice revealed mosaic patches marked by epidermal thickening (Figure 2B). The mosaic morphology was reflected at the level of Tg Snail protein, with only the hyperthickened regions expressing nuclear HA-tagged Snail (Figure 2C). These hyperthickened areas were marked by excessive proliferation, as revealed by antibodies against the proliferating nuclear antigen Ki67 (Figure 2D and 2E). Activated, pMAPK-positive cells were also prevalent in these areas (Figure 2F and 2G), as were cells expressing keratin 6, a keratin induced in the suprabasal layers of hyperproliferative skin (Figure 2H and 2I).
Figure 2  Misexpression of Snail in Mouse Skin Epidermis Results in Hyperproliferation
Three different surviving Tg founder mice harbored a K14-Snail transgene that was integrated into a locus that resulted in inheritable, mosaic expression of the transgene in skin epidermis. All displayed similar abnormalities, as did their offspring.
(A) P16 WT and K14-Snail Tg mice. Insets denote magnified tail segments, which displayed a mosaic, flaky appearance in Tg mice. Size differences appeared with age, and are likely due to K14-promoter activity in the tongue and oral epithelium, resulting in progressive defects and reduced food intake. Hence, skin sections from young (P3) mice were analyzed (B–I).
(B) Hematoxylin- and eosin-stained Tg skin section. Double arrows demarcate the border of mosaic histology, with seemingly normal epidermis (epi) and a mature hair follicle (hf) at left and hyperthickened epidermis at right.
(C) Immunofluorescence of Tg skin section labeled with antibodies as color-coded on frame. Double arrows demarcate the border of mosaic anti-Snail (green), revealing Snail expression coincident with regions of hyperthickened epidermis (at left) and absent in regions of normal epidermis (at right).
(D–I) Sections of P3 WT or Tg skin (affected region) subjected to either immunofluorescence (D, E, H, and I) or immunohistochemistry (F and G) with antibodies as indicated on the panel. Anti-keratin 5 indicates K5, normally restricted to the basal layer of the epidermis; anti-keratin 6 detects keratin 6, expressed in postnatal epidermis under conditions such as wounding, in which hyperproliferation occurs. All other antibodies are as in the legend to Figure 2. Comparison of D and E provide representative examples that illustrate that pMAPK is found in only a subset of all proliferating (Ki67-positive) cells. Note also the presence of Ki67- (E) and pMAPK-positive (G) cells in some suprabasal areas; Ki67-positive cells colabeled with anti-Snail (E). Expression of the Snail transgene did not block terminal differentiation in the hyperproliferative epidermis, but it distorted it markedly (Figure 3A–3H). Typical of most hyperproliferating conditions, Snail expression led to a large expansion in layers with spinous and granular morphology. Additionally, however, was a marked and variable expansion of keratin 5 (K5), normally restricted to the innermost basal layer (see Figure 3). Although the failure of Snail-null mice to develop past gastrulation [21] precluded our ability to study the loss of Snail function in skin development, a good correlation emerged between the expression of Snail protein and the extension of K5, Ki67, and pMAPK suprabasally (compare data in Figures 2 and 3).
Figure 3  Alterations in the Differentiation Program and Basement Membrane Organization in Snail-Expressing Tg Epidermis
(A–H) Immunofluorescence of skin sections from P3 WT and Tg mice. Shown are affected areas of Tg skin; in areas where Snail protein was not expressed, stainings were normal. Sections were labeled with antibodies as indicated and color-coded on each frame. Antibodies are against markers of normal epidermal differentiation, and include K5 (a basally expressed keratin), K1 (a suprabasal keratin, expressed in spinous layer cells), involucrin (Inv; a suprabasally expressed cornified envelope protein found in upper spinous and granular layer cells), loricrin (Lor; a cornified envelope protein expressed in the granular layer), and filaggrin (Fil; a protein that bundles keratin filaments in the granular layer and stratum corneum). Note abnormal extension of anti-K5 suprabasally, often present in anti-K1 positive suprabasal Tg cells.
(I–N) Immunohistochemistry (I and J) or immunofluorescence (K–N) of sections of P30 Wt (I, K, and M) and Tg (J, L, and N) (affected areas) skins using the antibodies indicated. Note that with age, affected areas of the Tg epidermis became increasingly undulating, often exhibiting papilloma-like invaginations (J). Insets in I and J are magnified views of the boxed areas, illustrating the absence (Wt) or presence (Tg) of nuclear anti-cyclin D staining. With age, affected areas of the Tg epidermis also displayed perturbations within the basement membrane, as judged by antibody labeling against either basement membrane (K and L) or hemidesmosomal (M and N) components. Double arrows in L demarcate mosaic zones, revealing that perturbations were restricted to hyperthickened, i.e., Snail-positive zones (to left of double arrows). Other abbreviations are as noted in the legend to Figure 2. The changes in hyperproliferation and differentiation were not initially accompanied by gross signs of epithelial invaginations. With age, however, epidermal folds and undulations developed in areas where Snail was expressed, and proliferative markers persisted in these regions (Figure 3I and 3J; anti-cyclin D staining). The undulations were accompanied by partial dissolution of the underlying basement membrane (Figure 3K and 3L). Aberrant staining was also observed with antibodies against components of the hemidesmosomes, which provide strong adhesion of basal epidermal cells to the underlying basal lamina (Figure 3M and 3N). Interestingly, similar types of alterations occur in the basement membrane in the hair bud of embryonic and newborn mice when Snail is normally expressed. The fact that the basement membrane separating the epidermis from the dermis is altered only in the adult Tg animals suggests the involvement of intermediary factors not as readily available in the epidermis as they are in the follicle.

Possible Links between Epidermal Hyperproliferation and Down-regulation of AJ Proteins in Snail Tg Mice
Given that the E-cadherin promoter is a direct target for Snail-mediated repression in vitro [4,12,13], and that E-cadherin was down-regulated in Snail-expressing hair buds, we examined the status of E-cadherin and other AJ proteins within regions of hyperproliferative epidermis where Tg Snail was present (Figure 4A). In these regions, immunofluorescence staining of E-cadherin and α-catenin were markedly diminished. In contrast, the intensity of antibody staining for two other AJ proteins, β-catenin and Ajuba, was still strong. Interestingly, however, despite appreciable immunofluorescence, localization of β-catenin and Ajuba appeared to be largely cytoplasmic rather than at cell-cell borders (Figure 4A insets).
Figure 4  Snail-Mediated Remodeling of AJs Contributes to Hyperproliferation
(A) Immunofluorescence of skin sections from P30 Wt and Tg mice. Shown are affected areas of Tg skin; in areas where Snail protein was not expressed, stainings were normal. Antibodies used are against AJ proteins and include E-cadherin (E-cad), the transmembrane core protein; β-catenin (β-cat), which binds E-cadherin at AJs and which can also participate as a transcription cofactor when associated with LEF-1/TCF proteins in the nucleus; α-catenin (α-cat) which binds to both β-catenin and Ajuba, a close relative of zyxin; and Ajuba, which can associate with proteins that bind to the actin cytoskeleton, as well as with Grb-2, a mediator of the GTP nucleotide-exchange protein Sos, involved in activation of the Ras-MAPK signaling cascade. In Snail-expressing Tg regions, there was a reduced staining with anti-E-cad and anti-α-cat and a more diffuse staining with anti-Ajuba. Insets in the panels for β-catenin and Ajuba staining are magnified views of the boxed areas. Arrows mark membrane localization of the protein and asterisks mark cells with elevated levels of cytoplasmic β-catenin or Ajuba.
(B) Western blot analyses of protein extracts from P30 Wt and Tg back and ear skins. Antibodies are as in (A) except anti-P-cad, which detects P-cadherin, whose expression in the hair follicle was not affected, and anti-tubulin, which detects tubulin, a control for equal protein loadings. Note that the reductions seen in E-cadherin and α-catenin are likely to be underestimates of the actual differences in affected regions, since the Tg skin expressed Snail mosaically.
(C) In the presence of elevated Snail, α-catenin levels can be restored by overexpression of E-cadherin. Keratinocytes were transfected with either HA-tagged Snail (Snail[HA]; images on the left) or Snail(HA) and Ecad(HA) (images on the right). 2 d after transfection, cells were switched from low-calcium growth medium to high-calcium medium for 6 h to induce AJ formation. Cells were stained with antibodies as indicated on the panels. Arrowheads point to sites of intercellular contact between a Snail-transfected keratinocyte and its neighboring untransfected cell.
(D) Reintroduction of E-cadherin in keratinocytes expressing Snail returns pMAPK to basal levels. Keratinocytes were transfected with control vector (K14), or Snail(HA), or Snail(HA) + E-cad(HA). After 2 d, cells were serum starved for 4 h and whole cell lysates were made and Western blotted with antibodies to pMAPK, HA to recognize the HA-tagged Snail and E-cadherin protein, 20or tubulin as a loading control.
(E) Ajuba interacts with Grb-2 under conditions where α-catenin levels are reduced. Protein extracts were made from skins of P30 Wt and K14-Snail Tg P30 mice (blots on the left) and of newborn Wt and K14-Cre/α-catenin (fl/fl) conditionally null animals (blots on the right) [7]. Equal amounts of protein extracts were treated with anti-Grb-2 antibody (+) or control isotype antibody (–), and following centrifugation, immunoprecipitates were subjected to SDS-PAGE and Western blot analysis with anti-Ajuba and anti-Grb-2 antibodies. Note the presence of Ajuba only under conditions where levels of α-catenin and other AJ proteins were aberrantly low or absent.
(F) Transgene expression of excess Ajuba or the Grb-2-interacting domain (pre-LIM) of Ajuba in keratinocytes results in the activation of the Ras-MAPK pathway. Primary newborn mouse keratinocytes were transfected with either the empty K14 expression vector (K14), or the expression vector driving Snail, full length Ajuba, or the pre-LIM domain of Ajuba in the absence or presence of a peptide inhibitor (inh) that disrupts the interaction between Grb-2 and Sos. 48 h posttransfection, protein extracts were prepared and subjected to SDS-PAGE and Western blot analyses with antibodies against pMAPK, total MAPK, Ajuba (also recognizing the smaller, pre-LIM domain), and Snail. Architectural differences in the epidermis made Western blot analyses somewhat difficult to gauge. However, in regions such as ear skin, where the highest levels of Snail protein were expressed, the effects were accentuated. In both back skin and ear skin, overall levels of E-cadherin and α-catenin were reduced, under conditions where β-catenin and Ajuba levels remained unchanged relative to controls (Figure 4B). Taken together, these data were consistent with our results obtained from immunofluorescence microscopy.
A priori, the decrease in α-catenin levels could be due to either direct transcriptional repression by Snail or perturbations in AJ formation caused by the decrease in E-cadherin gene expression. To distinguish between these possibilities, we tested whether α-catenin levels could be restored by exogenous expression of E-cadherin in Snail-expressing keratinocytes. As shown in Figure 4C, transiently transfected keratinocytes expressing HA-tagged Snail displayed a loss of E-cadherin and α-catenin at cell-cell borders. Coexpression of exogenous HA-tagged E-cadherin not only enabled cell-cell border localization of E-cadherin protein, but also rescued the cell-cell border staining of α-catenin (Figure 4C). The ability to restore α-catenin expression and localization under these conditions argues against the notion that Snail transcriptionally represses α-catenin. Rather, the findings are consistent with a previous report that E-cadherin is required for the translation of α-catenin mRNA [22].
Despite the reductions in AJ markers, Tg skin still displayed sealed membranes and intercellular junctions that were largely intact, as judged by ultrastructural analyses (unpublished data). In this respect, the skin epithelium resembled that of the hair bud, where the down-regulation in junction proteins is permissive for cell-cell remodeling without abrogating intercellular adhesion.
The similarities between Snail Tg epidermis and hair buds extended to the hyperproliferative state, leading us to wonder whether the down-regulation of AJ proteins might contribute to this condition. Given the increase in pMAPK staining in Snail Tg epidermis (see Figure 2G), we used pMAPK levels as our assay to test whether the loss of E-cadherin contributed to the Snail-mediated increase in proliferation. Consistent with our in vivo observations, transfected keratinocytes expressing Snail exhibited a substantial increase in pMAPK levels relative to control cells (Figure 4D). Coexpression of E-cadherin with Snail appeared to abrogate this effect. Together, these findings raised the possibility that an AJ-associated protein that is normally sequestered at the plasma membrane may participate in a proliferation signaling pathway when AJs are deconstructed.
Numerous studies have correlated a down-regulation of E-cadherin with a translocation of β-catenin to the nucleus and a transactivation of genes that are regulated by the LEF-1/T cell factor (TCF) family of DNA binding proteins [23,24,25]. The presence of nuclear cyclin D in hyperproliferative Snail Tg epidermis was particularly intriguing since prior studies have reported cyclin D gene as a direct target of TCF/β-catenin transcription [26]. This said, we did not detect nuclear β-catenin in our Tg epidermis, and mating the Snail Tg mice against the TOPGal reporter mouse [20] gave no signs of ectopic LEF-1/Tcf/β-catenin activity (unpublished data).
We next turned to the presence of cytoplasmic Ajuba for a possible mechanistic link to the proliferative increase in our Snail Tg epidermis. In addition to its documented ability to bind α-catenin [10], Ajuba can also associate with growth factor receptor-bound protein-2 (Grb-2)/son of sevenless (Sos), the nucleotide exchange factor for Ras, which is upstream from activation of MAPK [9]. Given the increase in pMAPK staining in Tg skin, we examined the possibility that Ajuba might have changed its binding partner in Snail-expressing epidermis. Interestingly, Ajuba was readily detected in anti-Grb-2 immunoprecipitates of protein lysates from skins of Snail Tg mice but not from the corresponding wild-type (WT) animals (Figure 4E). When these experiments were repeated with α-catenin-null epidermis, a similar Grb-2-Ajuba association was detected, and again, this interaction was not detected in the protein extracts from control littermate skin (Figure 4E). Together, these data demonstrate that the reduction in α-catenin levels, either by Snail-mediated down-regulation of E-cadherin or by α-catenin conditional targeting, allows Ajuba to interact with Grb-2/Sos.
If the competition between Grb-2/Sos and α-catenin for Ajuba is functionally relevant to the hyperproliferative state of a keratinocyte, then overexpression of Ajuba would be expected to bypass the competition and promote activation of the Ras-MAPK pathway in WT keratinocytes. Indeed, when serum-starved keratinocytes were transiently transfected with an Ajuba expression vector, the levels of pMAPK were not only elevated but also comparable to those transfected with the K14-HASnail transgene (Figure 4F). This activation was abolished when cells were treated with a small peptide inhibitor that specifically interrupts the Grb-2/Sos interaction (Figure 4F; see lanes marked “inh”) [27].
Ajuba's pre-LIM domain is the segment that associates with Grb-2's Src-homology 3 domain [9]. When this domain was overexpressed in serum-starved keratinocytes, a comparable elevation in pMAPK was observed (Figure 4F). As expected, the small peptide inhibitor that interrupts the Grb-2/Sos association blocked the effects. These data suggested that by elevating cytosolic Ajuba levels, Ajuba's pre-LIM domain may associate with Grb-2/Sos in a manner that stimulates its nucleotide exchange activity and leads to activation of the Ras-MAPK pathway. Although this pathway provides one mechanism by which Snail expression and proliferation may be coupled in skin epithelium, proliferative circuitries involving AJs are known to be complex and often interwoven. Future studies will be needed to systematically dissect these putative intricacies at a molecular level.

Probing the Regulation of Snail Gene Expression Reveals an Essential Link to TGF-β2 Signaling in the Developing Hair Bud
The temporal spike of Snail mRNA expression in the hair bud prompted us to consider what factor(s) may be regulating the Snail gene. A variety of extracellular signals have an impact on the cell type-specific expression of different Snail family members, and many of them, including Wnts, BMPs, FGFs, and TGF-βs, also affect hair bud development [2,16,28]. Since Snail is not expressed in cultured skin keratinocytes that secrete active BMPs and FGFs (see Figure 1B), we focused our attention on Wnt and TGF-β signaling as more likely candidates for Snail induction in this cell type.
Previously, we showed that effective transmission of a Wnt-3a signal in cultured keratinocytes can be achieved through their exposure to the BMP inhibitor noggin, which induces LEF-1 expression [4]. In vitro, these conditions down-regulated the E-cadherin promoter and induced a LEF-1/β-catenin-sensitive reporter gene, TOPFLASH [4]. In contrast, Snail expression was not induced by these conditions (Figure 5A). Thus, despite essential roles for Wnts and noggin in hair follicle specification [4,29,30], our studies did not support an essential role for these signals in governing Snail expression in keratinocytes.
Figure 5  TGF-β2, but Not Wnt/noggin, Transiently Induces Snail Expression In Vitro
(A) Failure of Wnt and noggin signaling to induce Snail in cultured keratinocytes. Primary mouse keratinocytes were treated with Wnt- and/or noggin-conditioned medium (+) or the corresponding control medium (–). These conditions are known to activate the LEF-1/β-catenin reporter TOPGal and down-regulate the E-cadherin promoter (see [4] for details). Using Western blot analyses, cellular proteins were then analyzed for Snail, LEF-1, β-catenin, and tubulin. Proteins from keratinocytes transfected with K14-Snail were used as a positive control for Snail expression.
(B) TGF-β2 can induce Snail protein. Primary keratinocytes were treated for the indicated times with recombinant TGF-β2 (+) or heat inactivated TGF-β2 (–).Total cellular proteins were then isolated and analyzed by Western blot for Snail, pSMAD2 (reflective of activated TGF- signaling), and tubulin. Note the activation of Snail expression, peaking at 2 h post-TGF-β2 treatment and then disappearing thereafter.
(C) Snail mRNA expression is transiently induced by TGF-β2. The experiment in (B) was repeated, and this time, total RNAs were isolated from keratinocytes treated with TGF-β2 for the indicated times. RT-PCR was then used with (+) or without (–) reverse transcriptase (RT) and with primer sets specific for Snail and GAPDH mRNAs. Note that Snail mRNA expression also peaked at 2 h, paralleling Snail protein.
(D) TGF-β2 treatment results in enhanced activity of a Snail promoter-β-galactosidase reporter. Keratinocytes were transfected with a β-galactosidase reporter driven by a Snail promoter that is either WT (wt prom) or harbors a mutation in a putative pSMAD2/pSMAD4 binding site (mt prom). At 2 d posttransfection, cells were treated with either TGF-β or heat-inactivated TGF-β2 (inact) for the times indicated. β-galactosidase assays were then conducted, and results are reported as fold increase over a basal level of activity of 1. The experiment was repeated three times in triplicate, and error bars reflect variations in the results. TGF-β1 has been shown to induce Snail family members in hepatocytes and heart [15, 31]. In keratinocytes, however, TGF-β1 inhibits keratinocyte growth and seems to be involved in triggering the destructive phase of the cycling hair follicle [32]. Of the loss-of-function mutations generated in each of the TGF-β genes, only the TGF-β2 null state blocked follicle development at the hair bud stage [32]. Thus, we turned towards addressing whether TGF-β2 might be involved in regulating Snail expression in keratinocytes isolated from the basal layer of the epidermis. Though there is no cell culture system available to specifically study placodal cells, these keratinocytes are their progenitors and are the closest approximation available to study the behavior of epithelial cells of the placode.
Interestingly, treatment of cultured keratinocytes with as little as 5 ng/ml of TGF-β2 caused a rapid and transient induction of Snail (Figure 5B). Following this treatment, Snail protein was detected within 30 min, peaked at 2 h, and then declined thereafter. The induction of Snail appeared to be specific for the active form of the growth factor, as pretreatment of TGF-β2 for 10 min at 100 °C obliterated the response [Figure 5B, lanes marked (–)]. By contrast, although TGF-β receptor activation remained elevated during the duration of the experiment (as measured by the sustained phosphorylation of the downstream effector SMAD2) Snail expression could not be maintained (Figure 5B). Thus, although Snail expression correlated with phosphorylated SMAD2 (pSMAD2) induction, its decline seemed to rely on secondary downstream events.
The rapid kinetics of Snail expression were reflected at the mRNA level, suggesting that Snail promoter activity in keratinocytes might be sensitive to TGF-β2 signaling (Figure 5C). To test this possibility, we engineered a transgene driving the β-galactosidase reporter under the control of approximately 2.2 kb of promoter sequence located 5′ from the transcription initiation site of the mouse Snail gene. At 2 d after transient transfection, keratinocytes were treated with TGF-β2 (t = 0) and then assayed for transgene activity over the same time course in which we had observed Snail protein induction. The results of this experiment are presented in Figure 5D.
Within 0.5 h of TGF-β2 treatment, Snail promoter activity had increased 3-fold, and by 2 h, it peaked to approximately 10-fold over control levels (Figure 5D). Thereafter, Snail promoter activity rapidly returned to the basal levels seen in unstimulated keratinocytes. The kinetics of Snail promoter activity closely paralleled those observed for Snail protein induction. Moreover, the stimulatory effects appeared to be specific to TGF-β2, since they were abrogated either by heat inactivation of the TGF-β2 protein or by mutation of a putative SMAD binding element located about 1.8 kb 5′ from the Snail transcription start site (Figure 5D). Taken together, these results suggested that in keratinocytes, TGF-β2 signaling results in a pSMAD2-dependent transient activation of the Snail gene, and that maintenance of Snail protein relies, in part, upon sustained promoter activity.
The brevity of Snail gene and protein induction in TGF-β2 treated cultured keratinocytes resembled the temporal appearance of Snail mRNA and protein at the initiation of hair follicle morphogenesis in embryonic mouse skin. To test whether TGF-β2 might be required for Snail induction in hair bud formation in vivo, we first analyzed whether TGF-β2 was expressed in or around the hair bud. Consistent with previous observations [33], an anti-TGF-β2 antibody labeled developing hair buds (Figure 6A). This labeling appeared to be specific as judged by the lack of staining in follicle buds from mice homozygous for a TGF-β2 null mutation (Figure 6A; [34]). Moreover, the downstream effector of TGF-β2 signaling, pSMAD2, was also expressed in WT, but not TGF-β2-null, hair buds (Figure 6B). Together, these data underscore the importance of the TGF-β2 isoform despite expression of both TGF-β1 and TGF-β2 in developing hair buds at this stage.
Figure 6  TGF-β2 Is Necessary to Induce Snail Expression and Regulate Proliferation and E-Cadherin in the Hair Bud
(A–D) Skins from TGF-β2 WT or KO E17.5 embryos were analyzed for expression of TGF-β2 protein (A), which is present in the epidermis and dermis as previously described [33] and in the hair bud, pSMAD2 (B), Snail (C), and Snail mRNA (D). Arrows point to the hair buds.
(E) Western blot analyses of Snail expression in the skins of 2-wk-old K14-Smad2 transgenic (SMAD2 TG) and WT littermate (WT) mice. Antibody to tubulin was used as a control for equal protein loadings. The K14-Smad2 Tg mouse was previously shown to possess activated TGF-β signaling [35].
(F–G) Proliferation markers Ki67 (F) and pMAPK (G) are diminished in TGF-β2-null hair relative to its WT counterpart.
(H–J) TGF-β2-null hair fails to down-regulate E-cadherin (H). Wnt and noggin signaling pathways are still intact in the TGF-β2 null hair as nuclear LEF-1 (I) and nuclear β-catenin (J) are still expressed. To further explore the possible relation between Snail and TGF-β2, we examined the status of Snail expression in TGF-β2-null hair buds. As judged by immunohistochemistry, Snail protein was absent from E17.5 skin of TGF-β2-null embryos but not from that of control littermates (Figure 6C). This effect appeared to be exerted at the transcriptional level, since Snail mRNAs were also not found in TGF-β2 null hair buds under conditions in which the signal was readily detected in the hair buds of littermate skin (Figure 6D).
Conversely, in 2-wk-old K14-Smad2 Tg mice, which display elevated TGF-β signaling in skin [35], Snail protein was readily detected by Western blot analyses, where it was not found in postnatal skin (Figure 6E). Taken together, these results provide compelling evidence that TGF-β2 is functionally important for inducing Snail gene expression in a pSMAD-dependent manner in developing hair buds. Whether pMARK activity also contributes to Snail induction was not addressed in the present study [15].
Although some hair buds still formed in TGF-β2 null skin, their number was reduced by approximately 50% [32]. Thus, although the pathway mediated by TGF-β2 signaling impacts the earliest step of epithelial invagination, it does not appear to be essential for bud morphogenesis. Consistent with this notion, basement membrane remodeling still took place in the TGF-β2-null buds, as judged by immunofluorescence with antibodies against β4 integrin, an integral component of keratinocyte-mediated adhesion to its underlying basement membrane (Figure 6F). In contrast, TGF-β2 signaling appeared to be an important factor for the early proliferation that occurs in the developing hair buds, as judged by anti-Ki67 and anti-pMAPK immunofluorescence (Figure 6F and 6G).
If TGF-β2 stimulates Snail expression in developing buds, loss of this morphogen would be expected to affect the expression of genes that are typically repressed by Snail. Since a major target for Snail-mediated repression is the E-cadherin gene [12,13], we investigated the status of E-cadherin in TGF-β2-null buds. As shown in Figure 6H, hair buds in TGF-β2 null skin displayed elevated immunofluorescence staining relative to their WT counterparts.
Previously we demonstrated that the concerted action of the extracellular signals Wnt and noggin are required for the generation of a LEF-1/β-catenin transcription complex to repress E-cadherin transcription at the onset of hair fate specification. As shown in Figure 6I and 6J, both WT and TGF-β2 null buds exhibited nuclear LEF-1 and β-catenin localization, signs that the Wnt-noggin signaling pathway was intact. These data suggest that during hair follicle morphogenesis, TGF-β2 functions subsequently to Wnt/noggin-mediated determination of hair fate. Moreover, through activation of Snail gene expression, TGF-β2 appears to work in tandem with these other morphogens to down-regulate E-cadherin levels, which contributes to the activation of proliferative circuitries.