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Journal List > Mol Biol Cell > v.19(1); Jan 2008

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Mol Biol Cell. 2008 January; 19(1): 137–149.
doi: 10.1091/mbc.E07-08-0764.
PMCID: PMC2174171
Copyright © 2007 by The American Society for Cell Biology
Constitutively Active Akt Induces Ectodermal Defects and Impaired Bone Morphogenetic Protein Signaling
Carmen Segrelles,*† Marta Moral,*† Corina Lorz,*† Mirentxu Santos,* Jerry Lu,‡ José Luis Cascallana,§ M. Fernanda Lara,* Steve Carbajal,‡ Ana Belén Martínez-Cruz,* Ramón García-Escudero,* Linda Beltran,‡ José C. Segovia,‖ Ana Bravo,§ John DiGiovanni,corresponding author‡ and Jesús M. Paramiocorresponding author*
*Molecular Oncology Unit, Division of Biomedicine, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, E-28040 Madrid, Spain;
‡Department of Carcinogenesis, Science Park-Research Division, University of Texas M.D. Anderson Cancer Center, Smithville, TX 78957;
§Department of Veterinary Clinical Sciences, Veterinary Pathology Unit, Veterinary Faculty, University of Santiago de Compostela, E-27002 Lugo, Spain; and
‖Flow Cytometry Unit, Division of Hematopoiesis, CIEMAT, E-28040 Madrid, Spain
M. Bishr Omary, Monitoring Editor
corresponding authorCorresponding author.
Address correspondence to: John DiGiovanni (Email: jdigiovanni/at/mdanderson.org) or Jesús M. Paramio (Email: jesusm.paramio/at/ciemat.es)
† These authors contributed equally to this work.
Received August 7, 2007; Revised September 21, 2007; Accepted October 17, 2007.
Abstract
Aberrant activation of the Akt pathway has been implicated in several human pathologies including cancer. However, current knowledge on the involvement of Akt signaling in development is limited. Previous data have suggested that Akt-mediated signaling may be an essential mediator of epidermal homeostasis through cell autonomous and noncell autonomous mechanisms. Here we report the developmental consequences of deregulated Akt activity in the basal layer of stratified epithelia, mediated by the expression of a constitutively active Akt1 (myrAkt) in transgenic mice. Contrary to mice overexpressing wild-type Akt1 (Aktwt), these myrAkt mice display, in a dose-dependent manner, altered development of ectodermally derived organs such as hair, teeth, nails, and epidermal glands. To identify the possible molecular mechanisms underlying these alterations, gene profiling approaches were used. We demonstrate that constitutive Akt activity disturbs the bone morphogenetic protein-dependent signaling pathway. In addition, these mice also display alterations in adult epidermal stem cells. Collectively, we show that epithelial tissue development and homeostasis is dependent on proper regulation of Akt expression and activity.
INTRODUCTION
A large number of processes in the cell are modulated by the protein kinase Akt, also known as protein kinase B (PKB). In particular, this kinase has been widely involved in the control of cell survival and apoptosis, proliferation, cell cycle progression, glucose metabolism, and protein translation (Brazil et al., 2004 blue right-pointing triangle). Three Akt isoforms sharing common structural features (Hanada et al., 2004 blue right-pointing triangle) have been found in mammals (Akt1, Akt2, Akt3). Although the relevance of Akt signaling in cancer is widely recognized (Bellacosa et al., 2005 blue right-pointing triangle; Manning and Cantley, 2007 blue right-pointing triangle), its involvement in development has only recently been highlighted. Akt1 and Akt2 knockout (KO) mice are viable and exhibit mild phenotypes characterized by growth retardation and diabetes (Chen et al., 2001 blue right-pointing triangle; Cho et al., 2001 blue right-pointing triangle), suggesting functional redundancy among Akt isoforms. Further proof of these overlapping roles comes from analysis of compound mutant mice. Akt1 and Akt2 double KO mice displayed a much more severe phenotype: dwarfism, impaired skin development, delayed bone development, reduced adipogenesis, and early lethality after birth (Peng et al., 2003 blue right-pointing triangle). More recently, mice with combined mutant alleles of Akt1 and Akt3 have also been generated. Double KO of Akt1 and Akt3 causes embryonic lethality at around embryonic days 11 and 12, and Akt1−/−;Akt3−/− mice have severe developmental defects in the cardiovascular and nervous systems (Yang et al., 2005 blue right-pointing triangle). Akt1−/−;Akt3+/− mice display multiple defects in the thymus, heart, and skin and die within several days after birth, whereas Akt1+/−;Akt3−/− mice are viable (Yang et al., 2005 blue right-pointing triangle). These data demonstrate that the three Akt isoforms have overlapping functions but also may have unique functions in specific organs.
Besides the skin phenotype observed in different compound-deficient mice, several lines of evidence have highlighted the importance of the phophoinositide 3 kinase (PI3K)/Akt pathway in epidermis. Of particular interest is the relevant role of Akt during mouse skin carcinogenesis. We have demonstrated that Akt is a key molecule in insulin growth factor 1 (IGF-1)-mediated mouse skin tumor promotion (Wilker et al., 2005 blue right-pointing triangle). In addition, Akt exerts essential roles in two-stage carcinogenesis protocols affecting tumor proliferation and apoptosis (Segrelles et al., 2002 blue right-pointing triangle) and also modulates the tumor-stroma cross-talk leading to an increase in angiogenesis (Segrelles et al., 2004 blue right-pointing triangle). Recently, using cultured cell systems, we provided evidence indicating that the functions of Akt in epidermal tumors are exerted by transcriptional and posttranscriptional mechanisms and show several parallels with human head and neck squamous cell carcinomas (Segrelles et al., 2006 blue right-pointing triangle). Of relevance, we observed that increased Akt expression modulates β-catenin and ΔNp63 expression, two essential modulators of epidermal development, in this system (Segrelles et al., 2006 blue right-pointing triangle).
To further explore the role of Akt in skin, we have generated transgenic mice expressing either a wild-type form of Akt1 (Aktwt) or a form of Akt1 that is constitutively activated by means of a myristoylation sequence (myrAkt), directed to the basal layer of the stratified epithelia by the K5 promoter (Segrelles et al., 2007 blue right-pointing triangle). These approaches, together with the tissue-specific knock out of PTEN tumor suppressor gene, have been widely used to explore the functions of the Akt signaling pathway in vivo (Yang et al., 2004 blue right-pointing triangle). Notably, there are functional differences among these three approaches. Because PTEN is a negative modulator of PI3K signaling, elimination of PTEN leads to the permanent up-regulation of the PI3K pathway, whereas increased expression of wild-type Akt leads to the amplification of PI3K signaling, and the expression of constitutively active Akt generates a permanent, PI3K-independent signal. Although performing our previous analysis, we observed that many of the founders expressing K5myrAkt, but not those expressing K5Aktwt, displayed developmental defects in multiple ectodermal organs in parallel with the level of transgene expression and Akt activity. Here we have characterized these developmental alterations. We provide evidence that deregulated Akt activity in stratified epithelia mediated by myrAkt expression leads to aberrant bone morphogenetic protein 4 (BMP4) signaling and altered adult epidermal stem cell homeostasis.
MATERIALS AND METHODS
Transgenic Mouse Production and Maintenance
Wild-type mouse Akt cDNA (obtained from Dr. A. Bellacosa, Fox Chase Cancer Center) or myrAkt (obtained from Dr. S. Gutkind, National Institute of Dental and Craniofacial Research, NIH) were placed under a 5.2-kb fragment of the BK5 promoter into the SnaBI site between the rabbit β-globin intron and polyadenylation sequences from a vector previously described (Bol et al., 1998 blue right-pointing triangle). Orientation and integrity of the inserts were confirmed by restriction digests. Purified fragments were used in microinjection. Mice were genotyped using primers specific for β-globin on genomic DNA isolated from tails, as previously described (Bol et al., 1998 blue right-pointing triangle). Transgenic and control mice in a C57BL/DBA/FVB mixed background were housed in a 12/12 light/dark cycle at 24°C and given standard mouse chow and water ad libitum. In some cases, where alterations in oral cavity or teeth were observed, food was supplemented with jelly. In addition to the founders, at least 40 mice of each transgenic line or controls were analyzed in search for developmental defects.
Histology and Akt Kinase Analyses
For histological analysis, samples were fixed in Formalin and embedded in paraffin before sectioning. Sections of 5 μm were cut and stained with H&E. At least five different samples were analyzed for each time point. The expression of BMP2 (1/200 diluted mAb; Abcam, Cambridge, UK), BMP4 (1/50 diluted goat polyclonal; AbCam) and ΔNp63 (1/200 diluted mAb 4A4; Santa Cruz Biotechnology, Santa Cruz, CA) was monitored in deparaffined sections using conventional protocols. For expression of active Akt (phospho Ser 473, 1/50 diluted; Cell Signaling, Beverly, MA), phospho Smad1/5/8 (phosphorylated in Ser463 and 465, 1/100 diluted; Cell Signaling), Foxo3a (1/200 diluted; Upstate, Lake Placid, NY), and BMPRIA (1/100; Santa Cruz Biotechnology), the slides were microwaved for 10 min after deparaffinization to enhance the staining. Keratin 15 (mouse mAb LHK15; Neomarkers, Lab Vision, Fremont, CA; diluted 1/50), and CD34 (rat mAb; eBioscience, San Diego, CA; diluted 1/50) were monitored in 70% ethanol-fixed tissues by double immunofluorescence. Before incubation overnight at 4°C with primary antibodies diluted in bovine serum albumin (BSA)/phosphate-buffered saline (PBS), sections were incubated with 5% horse serum for 45 min to block the Fc receptor in tissue and then washed three times with sterile PBS (pH 7.5). Horseradish peroxidase–, Texas red–, or fluorescein isothiocyanate–conjugated secondary antibodies were purchased from Jackson ImmunoResearch (West Grove, PA) and used at dilutions of 1/4000, 1/500, and 1/100, respectively. Peroxidase was visualized using a DAB kit (Vector, Burlingame, CA). Control slides were obtained by replacing primary antibodies with PBS (data not shown). Akt activity in mouse skin was determined after immunoprecipitation with anti-Akt (Santa Cruz Biotechnology, C-20 antibody 1 ml/25 mg protein) essentially as previously described (Segrelles et al., 2002 blue right-pointing triangle), using histone 2B (H2B; Roche Molecular Biochemicals, Indianapolis, IN) as the substrate for Akt or by Western blot (see below). Autoradiograms were scanned and subsequently quantified using a Phosphorimager (Bio-Rad, Richmond, CA).
Scanning Electron Microscope Studies of Hair Shafts
Hair shafts from the dorsal skin of 30-d-old transgenic and nontransgenic littermates were fixed in 2.5% glutaraldehyde in 0.1 M phosphate buffer (pH 7.5). After dehydration, samples were dried by critical point drying (Balzers, Hudson, NH; CPD020), coated with gold palladium by a Jeol JFC-1100 Ion Sputter, and examined with a Jeol JSM-T220A scanning electron microscope (SEM; Peabody, MA). At least 10 samples from each genotype were analyzed.
RNA Purification and Affymetrix Mouse Gene Chip 430A Analysis
Mouse skin tissue, obtained 30 d after birth, was preserved in RNAlater (Ambion, Austin, TX) and disrupted and homogenized using Mixer Mill MM301 (Retsch). Total RNA was extracted and purified from 30 mg of skin using RNeasy Fibrous Tissue Mini kit (Qiagen, Chatsworth, CA) following the manufacturer's recommendations. The integrity of the RNA populations was tested in the Bioanalyzer (Agilent, Wilmington, DE). Two (transgenic mouse lines L60, L84 and LA) or three pools (control mice) from RNA whole skin extracts of same genotype were done and analyzed, individually, in mouse microarrays. We exported CEL files from Affymetrix GCOS software (Santa Clara, CA), and using the web-based tool Gene Expression Profile Analysis Suite (GEPAS, http://www.gepas.org; Vaquerizas et al., 2005 blue right-pointing triangle), we subtracted the background intensity values with RMA (Irizarry et al., 2003 blue right-pointing triangle), normalized the chips using quantile method (Bolstad et al., 2003 blue right-pointing triangle), and finally log2-transformed and mean-centered the intensity values. Differentially expressed genes with normal variability were extracted between the four mouse genotypes with CLEAR test (2183 Affymetrix probes, p < 0.05; Valls et al., 2007 blue right-pointing triangle). Further analyses were performed using the MeV software (Saeed et al., 2003 blue right-pointing triangle). The selection of genes belonging to clusters 1–4 was made using Template Matching (Pearson R > 0.8, p < 0.01; Pavlidis and Noble, 2001 blue right-pointing triangle). Genes selected in more than one cluster were included in the cluster in which they displayed the best R and p values. The cluster Gene Ontology (GO) analysis of Biological Processes using DAVID software (Dennis et al., 2003 blue right-pointing triangle; http://david.abcc.ncifcrf.gov/home.jsp) from the National Institute of Allergy and Infectious Diseases/NIH was used to identify functional categories for each cluster. The search for GO terms was made using all themes, listed by p value based on EASE score (Hosack et al., 2003 blue right-pointing triangle) and manually curated. EASE score identifies functional categories overrepresented in a gene list relative to the representation within the proteome of a given species.(Hosack et al., 2003 blue right-pointing triangle). Pathway Architect software (Stratagene, La Jolla, CA) was used to identify and characterize possible pathways affected by specific alterations in gene expression. Gene expression microarray dataset has been submitted in GEO database under the accession number GSE9054.
Western Blot Analysis
Extracts, prepared from skin of 30-d-old mice, were ground with a mortar on liquid nitrogen and homogenized in buffer P (Tris, pH 7.5, NaCl 150 mM, EDTA 1 mM, EGTA 1 mM, β-glycerophosphate 40 mM, sodium orthovanadate 1 mM, PMSF 0.1 mM, aprotinin 2 mg/ml, leupeptin 2 mg/ml, NP-40 1%). Total protein (35 μg) from at least three age- and genotype-matched pooled skin samples was used for NuPAGE 4–12% Bis-Tris Gel (Invitrogen, Carlsbad, CA), transferred to nitrocellulose membrane (Invitrogen), and probed with primary antibodies against Akt1/2 (Santa Cruz Biotechnology; 1/500), phosphorylated Akt (Ser 473, diluted 1/50 and Thr 308, diluted 1/10; Cell Signaling), BMPR1 (Santa Cruz Biotechnology; 1/100), BMP4 (Abcam; 1/300), p63 (Santa Cruz Biotechnology; 1/500), and phospho-Smad1/5/8 (recognizing Smad1/5/8 phosphorylated in Ser463 and 465; Cell Signaling; 1/500). Actin (Santa Cruz Biotechnology; 1/1000) was used to normalize protein loading. Secondary antibodies (anti-rabbit, anti-mouse, or anti-goat IgG) were purchased from Jackson ImmunoResearch. Chemiluminescence was performed using manufacturer's recommendations (Pierce, Rockford, IL).
Label-retaining Cell Analysis
Ten-day-old nontransgenic and myrAkt pups (six mice of each class) were injected with BrdU (20 μl of a 12.5 mg/ml dilution in 0.9% NaCl every 12 h for a total of four injections). Skin sections were collected at 30 d after the last injection and bromodeoxyuridine (BrdU) incorporation was measured as percentage of hair follicles (HF) containing positive cells as previously reported (Ruiz et al., 2004 blue right-pointing triangle). Experiments were performed in triplicate (n ≥ 3 per group) and at least 100 follicles were scored in each section.
Fluorescence-activated Cell Sorting Analysis
For each experiment, dorsal skin of four adult animals of each genotype was shaved and treated with trypsin to separate dermis from epidermis. Cell suspensions were strained (40 -μm nylon, Falcon, Oxnard, CA) and stained in PBS containing 2% fetal bovine serum with anti-CD34 coupled to biotin (BD-PharMingen, San Diego, CA) and anti-α6 integrin (CD49f) coupled to PE (PharMingen). Cells were washed and stained with streptavidin-Tricolor (Caltag Laboratories, Burlingame, CA) and washed again. Cells were resuspended in PBS containing 2 μg/ml propidium iodide to exclude dead cells and analyzed in an EPICS XL flow cytometer (Coulter Electronics, Hialeah, FL).
RESULTS
Ectodermal Alterations in myrAkt Transgenic Mice
The K5 promoter was used to target the expression of wild-type Akt1 (Aktwt) or permanently activated Akt1 (myrAkt) to the basal layer of the mouse stratified epithelia. The overall epidermal phenotype and tumor susceptibility of these mice has been reported elsewhere (Segrelles et al., 2007 blue right-pointing triangle). It is worth mentioning that, because of the deleterious effects of myrAkt or wtAkt expression, it was impossible to establish lines from many of the founders. Eventually, we were able to establish two lines with different expression levels of the myrAkt transgene (lines 60 and 84, hereafter L60 and L84) and one line expressing the wtAkt transgene (Line A, hereafter LA). Our analysis was then focused on these three lines and six myrAkt founders.
MyrAkt founders and L84 transgenic mice showing high levels of Akt kinase activity (Figure 1Figure 1., A and A′) displayed developmental defects that affected multiple ectodermally derived organs such as hair, teeth, nails and several ectodermal glands. Of note, these alterations were not observed in mice expressing wtAkt or low levels of myrAkt, associated with lower Akt activity (Figure 1Figure 1., A′ and B). This suggests that a certain threshold of Akt activity is necessary to promote the deregulated development of ectodermal organs. As expected, the increased kinase activity was restricted to those tissues expressing the transgene, such as epidermis (Figure 1Figure 1.C′ and data not shown).
Figure 1.
Figure 1.
Figure 1.
Akt Kinase activity in transgenic mice. (A) Top panel, representative kinase assay using skin extracts and histone H2B as substrate; middle and bottom panels, Western blot of the same extracts blotted against Akt and Actin. (A′) Western blot of (more ...)
With respect to hair, we observed alopecia in the snout, face, and back skin and loss of vibrissae in myrAkt founders with high levels of Akt activity, in myrAkt L84 mice, and sporadically in L60 mice (Figure 2Figure 2.A). Compared with control mice (Figure 2Figure 2.B), the vibrissae of myrAkt mice were structurally distorted in L60 (Figure 2Figure 2.B′) and overall degeneration was evident in L84 mice (Figure 2Figure 2.B″). In parallel, the back skin pelage showed alterations in awl hair shafts (the predominant type of overhairs in the mouse) consisting of an enlarged cortex with the medulla cells arranged in a single row of septa, instead of the four to five septules observed in the controls (compare Figure 2Figure 2.C, control nontransgenic, with C′, L84 transgenic mice). Examination of awl hair shafts with SEM showed that the cuticle is made up of more densely compacted overlapping scales (compare Figure 2Figure 2.D, nontransgenic with D′, L84 transgenic mice). Finally, we also analyzed the distribution of different hair types. We found a decrease in zig-zag hairs, whereas all the other hair types (awl, auchene, and guard) were increased in transgenic mice compared with nontransgenic mice (Table 1). These alterations prompted us to analyze possible hair cycle defects; however, this study had to be limited to those mice that allowed the establishment of transgenic lines, namely L60 and L84, expressing myrAkt, and LA expressing wtAkt. By 28–30 d after birth (the time at which the first postnatal hair anagen is evident) we observed that HFs in control (Figure 2Figure 2.E), L60 (not shown), and LA (Figure 2Figure 2.F) displayed clear anagen features. On the contrary, the HFs of L84 mice showed clear signs of outer root sheet hyperplasia and remained in telogen (Figure 2Figure 2.G), thus demonstrating that an increase in Akt activity above a certain level produces hair cycle defects. By 45 d after birth most of control, L60, and LA hairs are in telogen (not shown, see also Supplementary Figure 1), whereas L84 HFs displayed clear signs of anagen (Figure 2Figure 2.H) with enlarged HF structures (denoted by arrows in Figure 2Figure 2.H). Subsequent analysis of multiple skin sections of different transgenic mice at different time points after birth indicated a loss of synchrony in the first postnatal hair cycle in the L84 mice (Supplementary Figure 1).
Figure 2.
Figure 2.
Figure 2.
Hair phenotype of myrAkt transgenic mice. (A) Example of alopecia in L60 mice affecting the snout and some patches of dorsal skin. (B) Histology of vibrissae from control (B), L60 (B′), and L84 (B″) mice. Note the progressive degeneration. (more ...)
Table 1.
Table 1.
Hair type distribution
Another characteristic displayed by L84 transgenic mice and some founders were long and fragile nails (Figure 3Figure 3.A). Histological analysis revealed that the nails of these transgenic mice (Figure 3Figure 3.B) had normal eponychium (e) and hyponychium (h), but an enlarged nail root (denoted by r in Figure 3Figure 3.B) and hyperplasia of the nail bed epithelium (nb, see insets in Figure 3Figure 3., B and B′) compared with controls (Figure 3Figure 3.B′).
Figure 3.
Figure 3.
Figure 3.
Nail phenotype of myrAkt transgenic mice. (A) Example of nail overgrowth in founder 23. Also the fragility of nails is depicted (inset). Similar observations were obtained in L84 mice. Histology analysis of nails from L84 transgenic (B) and nontransgenic (more ...)
With respect to teeth, most of L84 transgenic mice and some founders showed overgrowth and thinning of the incisors (Figure 4Figure 4.A), which in a few cases was also accompanied by the development of supernumerary teeth located in the palate (arrow in Figure 4Figure 4.B). Additionally, in most cases the second or third molar primarily in the lower jaw, was missing (arrow in Figure 4Figure 4.C).
Figure 4.
Figure 4.
Figure 4.
Tooth phenotype of myrAkt transgenic mice. Representative examples of transgenic mice showing overgrowth and fragility of the incisors (A), supernumerary teeth located in the palate (B), and lack of the second molar in the lower jaw (C).
We also characterized alterations in other ectodermal-derived structures. We observed a significant hyperplasia of the meibomian glands (denoted by mb in Figure 5Figure 5., A and A′). These are a special kind of sebaceous glands at the rim of the eyelids that have no association with adjacent hair follicles and produce a lipid secretion that forms a component of the tear film. We also observed a marked dysplasia of salivary glands (denoted by arrows in Figure 5Figure 5., B and B′) and a marked hypoplasia of the sweat glands of the foot pads in the transgenic L84 mice (denoted by sg in Figure 5Figure 5., C and C′). Collectively, these data demonstrate that the expression of constitutively active Akt in the basal layer of stratified epithelia leads to altered development of multiple ectodermally derived organs in a dose-dependent manner.
Figure 5.
Figure 5.
Figure 5.
Ectodermal gland phenotype of myrAkt transgenic mice. Representative histology sections from meibomian (denoted by mb in A and A′), salivary (denoted by arrows in B and B′), and sweat (denoted by sg in C and C′) glands from control (more ...)
Alterations in Gene Expression Mediated by De-regulated Akt Activity in Skin
The development of ectodermal organs is controlled by multiple pathways involving numerous genes. Most of these pathways modulate the process of hair cycling. The ectodermal and hair cycle defects observed in the transgenic mice suggested that increased and deregulated Akt activity leads to altered expression of genes involved in these developmental pathways. To gain a deeper insight into this aspect, we performed global expression profiling of paired RNA samples from whole skin extracts of transgenic (L60, L84, and LA) and nontransgenic mice at 30 d after birth. The LA mice did not display the ectodermal alterations observed in L84, and in L60 mice only sporadic alopecia was detected in the snout, face, and back skin; none of these transgenic lines displayed hair cycle defects (Figure 2Figure 2.F; see also Supplementary Figure 1). Consequently, the microarray data were first processed to find specific clusters that included genes associated with the modest changes observed in L60 and the more dramatic alterations observed in L84 (Figure 6Figure 6., A and B; clusters 1 and 2, down- and up-regulated, respectively). Subsequently, we also searched for genes only deregulated in L84 compared with LA, controls, and L60 (Figure 6Figure 6., A and B; clusters 3 and 4, down- and up-regulated, respectively). It is worth mentioning that, from the total 173 genes identified, the largest number of changes correspond to those showing alterations only in L84 skin (clusters 3 and 4), which includes 127 genes (see Supplementary Tables 1–4 for the complete list of identified genes).
Figure 6.
Figure 6.
Figure 6.
Summary of microarray data and Foxo3a expression. (A) Heatmap showing the relative expression of different genes selected for the analysis. (B) Clusters of genes that were regulated in transgenic mice according to the pattern denoted by red (Up) or green (more ...)
To identify possible functional alterations in the different skin samples, we analyzed the biological functions of the 173 genes mentioned above using DAVID software (Dennis et al., 2003 blue right-pointing triangle; Table 2). Interestingly, the concept “development” appears as significantly represented in the clusters including those genes that only showed altered expression in L84 skin samples (clusters 3 and 4). Utilizing the Entrez gene database, we narrowed the dataset to those genes that were involved in skin or ectodermal morphogenesis (Table 3). Overall, these data indicate that deregulated Akt activity results in gene expression changes that can modulate epithelial development.
Table 2.
Table 2.
Functional annotation (GOBP) of genes selected from microarray analyses
Table 3.
Table 3.
Representative genes involved in ectoderm development found in clusters
Microarray data were further analyzed using Pathway Architect (Stratagene) to identify and characterize possible signaling pathways affected by specific alterations in gene expression. This analysis revealed that the global expression changes modify BMP4 signaling. In this regard, not only was the Bmp4 gene specifically down-regulated, but also negative regulators of BMP signaling such as Egf (Nonaka et al., 1999 blue right-pointing triangle) and Twsg1 (Scott et al., 2001 blue right-pointing triangle) were up-regulated. In addition, several genes found in these clusters are involved in BMP signaling (Figure 6Figure 6.C and highlighted in bold in Table 3) either as integral pathway effectors or as downstream targets (Figure 6Figure 6.C), thus suggesting that this developmental pathway is altered by myrAkt expression.
The forkhead transcription factor family has been widely described as a target of Akt (Burgering and Kops, 2002 blue right-pointing triangle). Direct phosphorylation of these factors by Akt results in their cytoplasmic retention and inactivation, inhibiting the expression of forkhead transcription factor-regulated genes (Burgering and Kops, 2002 blue right-pointing triangle). We thus studied whether the expression and localization of Foxo3a, a representative member of this family, is affected in the epidermis of the different transgenic mice by postnatal day (pnd) 28. In control, nontransgenic mice (Figure 6Figure 6.D), we observed a positive staining throughout the epithelium, and a number of positive nuclei were detected in the hair follicle and the basal layer of the interfollicular epidermis (denoted by arrows in Figure 6Figure 6.D). A similar staining pattern was observed in L60 (Figure 6Figure 6.E) and LA (Figure 6Figure 6.F) transgenic mice epidermis, although a minor decrease in the interfollicular epidermal positive nuclei was observed in L60 samples. On the contrary, in L84 skin (Figure 6Figure 6.G), the overall staining was decreased and very few positive nuclei were observed (denoted by arrows in Figure 6Figure 6.G). Therefore, in agreement with our previous data (Segrelles et al., 2006 blue right-pointing triangle), the expression of Akt, upon a certain threshold, leads to the reduction and cytoplasmic localization of Foxo3a in keratinocytes (Segrelles et al., 2006 blue right-pointing triangle). This might help to explain, at least in part, some of the changes observed in microarray analyses.
Altered BMP Signaling Mediated by myrAkt in Skin
BMP signaling is thought to perform multiple functions in the regulation of skin appendage morphogenesis and the postnatal growth of HFs (Botchkarev, 2003 blue right-pointing triangle). BMPs function by binding type 1 (BMPR1A and BMPR1B) and type 2 (BMPR2) transmembrane serine/threonine kinase receptors, resulting in phosphorylation of the intracellular proteins Smad 1, 5, and 8. Therefore, to further confirm the possible alterations in BMP signaling, we carried out a detailed histology analysis of several components of this pathway.
As shown in Figure 7Figure 7., expression of BMP4 (Figure 7Figure 7.A) and BMP2 (Figure 7Figure 7.B) was predominant in the HF matrix and outer and inner root sheath (denoted by ORS and IRS, respectively) in nontransgenic mice with very little expression observed in the interfollicular epidermis (denoted by IE). With respect to myrAkt transgenic mice, we observed altered distribution and localization of both morphogens (Figure 7Figure 7., C and D, respectively), because scattered expression was observed in only a few cells. Western blot analysis of whole skin extracts (Figure 7Figure 7.E) confirmed the dramatic decrease in the level of BMP4.
Figure 7.
Figure 7.
Figure 7.
Altered BMP expression and localization in hair follicles of myrAkt transgenic mice. Representative examples of the expression of BMP4 (A and C) and BMP2 (B and D) in control (A and B) and L84 (C and D) transgenic mice. Bars, 250 μm. IE, interfollicular (more ...)
We also examined the expression and localization of BMPRIA and phospho-Smad1 (Ser463 and Ser465). By full anagen (pnd 28), phospho-Smad staining was present in cells of the medulla, cortex, and cuticle of the hair shaft in nontransgenic and LA mice (Figure 8Figure 8.m A and B, respectively). These results are similar to previous reports (Kobielak et al., 2003 blue right-pointing triangle; Andl et al., 2004 blue right-pointing triangle; Yuhki et al., 2004 blue right-pointing triangle; Sharov et al., 2005 blue right-pointing triangle). In contrast, phospho-Smad staining in myrAkt transgenic mice was faint, and only a few cells expressed very low amounts at the tip of the hair (Figure 8Figure 8.C). The apparent decrease in phospho-Smad was confirmed by Western blot of whole skin extracts (Figure 7Figure 7.E). In nontransgenic mice and mice expressing the wtAkt transgene (LA), BMPRIA expression was restricted to a subset of cells in the outer root sheath and hair matrix (Figure 8Figure 8., D and E), as previously reported (Kobielak et al., 2003 blue right-pointing triangle; Andl et al., 2004 blue right-pointing triangle; Yuhki et al., 2004 blue right-pointing triangle; Sharov et al., 2005 blue right-pointing triangle), whereas in myrAkt L84 mice (Figure 8Figure 8.F), BMPRIA expression was observed in the outer and inner root sheath (denoted by ORS and IRS, respectively in Figure 8Figure 8.F) and almost absent in hair tips. Moreover, immunohistochemistry also suggested a possible increased expression of BMPRIA that was confirmed by Western blotting (Figure 7Figure 7.E).
Figure 8.
Figure 8.
Figure 8.
Altered BMP signaling in hair follicles of myrAkt transgenic mice. Representative examples of the expression of phospho Smad1/5/8 (A–C) and BMPRIA (D–F) in control (A and D), LA (B and E), and L84 (C and F) transgenic mice. (G and H) Expression (more ...)
Finally, we also studied the possible changes in the expression and localization of ΔNp63. This protein is an ectoderm-specific direct transcriptional target of BMP signaling (Bakkers et al., 2002 blue right-pointing triangle) and is essential for skin development (Mills et al., 1999 blue right-pointing triangle; Yang et al., 1999 blue right-pointing triangle), and it can be induced by increased Akt activity (Barbieri et al., 2003 blue right-pointing triangle; Segrelles et al., 2006 blue right-pointing triangle). Compared with nontransgenic mice (Figure 8Figure 8.G) and LA mice (not shown), L84 mice did not display any obvious difference in localization, although L84 mice also displayed some ΔNp63-positive cells located in the inner root sheath (denoted by IRS in Figure 8Figure 8.H) in addition to the outer root sheath and the interfollicular epidermis (denoted by ORS and IE in Figure 8Figure 8.H), or overall expression level (Figure 7Figure 7.E). Notably, we found that in control mice ΔNp63 expression correlated with increased phospho-Akt in the bulge areas (Figure 8Figure 8.I), but not in the hair bulb (Figure 8Figure 8.I′), whereas in myrAkt L84 mice, ΔNp63 was coexpressed with phospho-Akt throughout the HF (Figure 8Figure 8.J). These data indicated that, despite the alterations in BMP signaling observed in mice with high expression of the myrAkt transgene the overall expression of ΔNp63 was not affected and discounted the possibility that the phenotypic alterations were primarily mediated by altered ΔNp63 expression.
Deregulated Akt Activity Alters Epidermal Stem Cell Homeostasis
There are several lines of evidence indicating that BMP signaling may influence the behavior of epidermal stem cells (Sharov et al., 2006 blue right-pointing triangle; Zhang et al., 2006b blue right-pointing triangle; Kobielak et al., 2007 blue right-pointing triangle) It has also been shown that other adult stem cell populations, for example, as in hematopoietic tissue (Rossi and Weissman, 2006 blue right-pointing triangle) may be impacted by PTEN/Akt pathway signaling. Because the K5 promoter is also active in these cells, we have thus studied possible alterations in the stem cell population in the skin of L84 transgenic mice. Initially we determined the expression of two putative epidermal stem cell markers, K15 and CD34 (Liu et al., 2003 blue right-pointing triangle; Cotsarelis, 2006 blue right-pointing triangle). The pattern of double immunofluorescence staining suggested an increase in the population of cells in the hair follicle of L84 (Figure 9Figure 9.A′) compared with control mice (Figure 9Figure 9.A). We next determined whether the epidermal stem cell compartment was affected by myrAkt expression using a label-retaining (LR) technique (Cotsarelis et al., 1990 blue right-pointing triangle; Taylor et al., 2000 blue right-pointing triangle). In L84 skin we found a consistent increase in the number of labeled cells 30 d after BrdU administration (Figure 9Figure 9., B′ and C) compared with control mice (Figure 9Figure 9., B and C). To further substantiate these observations, we also performed fluorescence-activated cell sorting (FACS) analysis to determine the proportion of cells expressing integrin α6 and CD34, which is considered characteristic of epidermal bulge stem cells (Blanpain et al., 2004 blue right-pointing triangle; Blanpain and Fuchs, 2006 blue right-pointing triangle). These analyses demonstrate that the proportion of α6+CD34+ cells is similar between control and L84 skin samples; however, L84 epidermis had a dramatic increase in the population characterized as α6low CD34+ at the expense of a partial reduction in the α6high CD34+ cell population (Figure 9Figure 9., D and E). This result is in agreement with the presence of BrdU positive cells with clear suprabasal localization observed during the LR experiments (denoted by arrows in Figure 9Figure 9.B′). Finally, we also performed a colony-forming efficiency experiment using adult epidermis as the source of primary keratinocytes. Five days after plating, multiple abortive colonies were observed in cultures from control mice (Figure 9Figure 9.F), and very few rendered productive colonies after long-term culture (Figure 9Figure 9.G). In contrast, cells from L84 epidermis displayed undifferentiated morphology (Figure 9Figure 9.F′) and produced multiple colonies (Figure 9Figure 9.G). Collectively, these data indicate that constitutively active Akt expression results in an expansion of the stem cell population.
Figure 9.
Figure 9.
Figure 9.
Alterations in L84 epidermal stem cell population. (A and A′) Skin sections from control (A) and L84 (A′) mice by pnd 28 stained with color-coded antibodies showing the expression and localization of keratin K15 (red) and CD34 (green). (more ...)
DISCUSSION
In the current study we present data that strongly support the hypothesis that increased and deregulated Akt activity, triggered by myrAkt expression, provokes dramatic alterations in ectodermal organ development. Collectively, these findings support the conclusions that Akt is an important mediator of epithelial homeostasis. The fact that increased wild-type Akt expression (as in wtAkt mice), which requires upstream control elements to become fully activated (Segrelles et al., 2007 blue right-pointing triangle), does not lead to a similar phenotype points to a finely regulated mechanism of control affecting Akt in the development of these organs. Our results are in agreement with the reported alterations found in different mouse models of Akt deficiency (Di-Poi et al., 2002 blue right-pointing triangle; Peng et al., 2003 blue right-pointing triangle; Yang et al., 2005 blue right-pointing triangle) and the keratinocyte-specific null mutation of epidermal Pten (Suzuki et al., 2003 blue right-pointing triangle). Of note, data obtained from double Akt KO mice, besides arguing for partially overlapping functions of Akt isoforms in vivo, also revealed that some functions of Akt are only discernible when total Akt levels are below a critical threshold in specific cell types and tissues (Dummler et al., 2006 blue right-pointing triangle). Further evaluation of ectodermal derived organs in compound Akt KO mice may reveal additional roles.
The Akt signaling pathway has been widely studied in the context of carcinogenesis (Manning and Cantley, 2007 blue right-pointing triangle) and is associated with increased cell proliferation and survival. Consistently, these functions are altered in L84 mice and also in LA and in L60 (Segrelles et al., 2007 blue right-pointing triangle); however, the major developmental defects are primarily found in L84 mice and several founders. This would indicate that the observed development defects due to increased Akt activity can be delineated from proliferative or antiapoptotic effects. We have taken this aspect as a starting hypothesis for the analysis of the microarray data. Indeed, the consideration of genes that do not display altered expression in LA epidermis certainly restricts the analyses. The list of genes found is relatively small and includes multiple genes previously associated with ectoderm or skin development. Furthermore, in unconstrained analysis of possible pathways involved, we detected the BMP-dependent pathway, indicating that this may be a major mediator in the skin phenotypic alterations found in myrAkt mice. The possibility that this pathway is also a target mediating other ectodermal alterations seems plausible but undoubtedly will require further investigations.
It is widely recognized that BMP signaling is required for proper development of several ectodermal structures (reviewed in Jernvall and Thesleff, 2000 blue right-pointing triangle; Botchkarev, 2003 blue right-pointing triangle). Of note, cre-mediated mutation of the BMPR1A gene causes altered tooth morphogenesis, defective postnatal development of HFs, and abnormal nail growth (Andl et al., 2004 blue right-pointing triangle) and leads to the formation of epidermal tumors (Kobielak et al., 2003 blue right-pointing triangle; Andl et al., 2004 blue right-pointing triangle; Sharov et al., 2006 blue right-pointing triangle; Zhang et al., 2006b blue right-pointing triangle), probably through the altered homeostasis of epidermal stem cells (Sharov et al., 2006 blue right-pointing triangle; Zhang et al., 2006b blue right-pointing triangle; Kobielak et al., 2007 blue right-pointing triangle). In agreement, we also observed the development of epithelial tumors in Akt transgenic mice that in many cases were associated with hair follicle structures (Segrelles et al., 2007 blue right-pointing triangle). Although epidermal-specific deletion of the Bmpr1a gene or Noggin overexpression caused severe alterations in the expression of several genes associated with development and cell cycle (Kobielak et al., 2003 blue right-pointing triangle; Andl et al., 2004 blue right-pointing triangle; Sharov et al., 2006 blue right-pointing triangle; Zhang et al., 2006b blue right-pointing triangle), our microarray analysis did not produce similar results. This difference may be due to the fact that ablation of BmprIa or Noggin overexpression completely abrogates BMP signaling, whereas our data support a possible deregulation together with decreased signaling rather than complete inhibition. As an alternative explanation, searching for genes displaying a selected pattern of expression may obscure or lead to an incomplete analysis of the data. Indeed, as mentioned above, the genes found through the unrestricted analysis of the microarray data also included most of the genes reportedly altered in BmprIa conditional KO and Noggin transgenic mice. Nevertheless this group of genes was also altered in wtAkt transgenic mice that do not display ectodermal defects.
Several lines of evidence have previously shown an association between BMP and PI3K/Akt signaling pathways (Waite and Eng, 2003 blue right-pointing triangle; He et al., 2004 blue right-pointing triangle; Tian et al., 2005 blue right-pointing triangle). In particular it has been shown that altered BMP signaling can, through the modulation of PTEN expression and activity, control the activity of the PI3K/Akt signaling pathway (Tian et al., 2005 blue right-pointing triangle; Zhang et al., 2006b blue right-pointing triangle; Kobielak et al., 2007 blue right-pointing triangle). The present data complement these observations and show through microarray and histology analyses that deregulated Akt activity affects BMP signaling and, as an overall consequence, the BMP pathway is at least partially inhibited. Furthermore, the existence of an autoregulatory loop between BMP and PI3K/PTEN/Akt signaling may exist such that each element is subject to the control of the other, and importantly, disruption of this balance may lead to altered ectodermal development and tumor formation. The molecular bases of this alteration in Bmp4 expression are not known at present; however, among the multiple regulators of Bmp4 gene expression there are several putative candidates that can be modulated by Akt activity. In this regard, p65 RelA, which can be activated by Akt (Madrid et al., 2000 blue right-pointing triangle, 2001 blue right-pointing triangle), is a transcriptional repressor of Bmp4 gene expression in vivo (Zhu et al., 2007 blue right-pointing triangle), whereas Bmp4 transcription is activated by forkhead and NKX2 transcription factors (Zhu et al., 2004 blue right-pointing triangle; Begum et al., 2005 blue right-pointing triangle), which are inactivated by Akt (Burgering and Kops, 2002 blue right-pointing triangle; Naito et al., 2003 blue right-pointing triangle). In this regard, the altered expression and distribution of Foxo3a observed in L84 epidermis might explain the reduced expression of Bmp4. Further studies will help to clarify the functional impact of Akt activity on Bmp4 expression.
Many of the alterations observed in the ectodermal organs of myrAkt mice were similar to the defects present in mice resembling human ectodermal dysplasia syndromes (Thesleff, 2006 blue right-pointing triangle). The possibility that Akt may be involved in these disorders is very intriguing and would certainly merit further investigations. In some cases this group of diseases is associated with altered ΔNp63 expression (Koster and Roop, 2004 blue right-pointing triangle). The finding that expression level of this protein was not altered in myrAkt transgenic mice relative to nontransgenic mice indicates that other targets of Akt kinase may be responsible for the observed phenotype. In this regard, the previously reported cross talk between the glucocorticoid receptor and Akt (Leis et al., 2004 blue right-pointing triangle) and the involvement of the glucocorticoid receptor in certain ectodermal dysplasia syndromes (Perez et al., 2001 blue right-pointing triangle; Cascallana et al., 2005 blue right-pointing triangle) reinforces the possibility that Akt may also be involved in some of these disorders.
In the current study we observed that deregulated Akt activity results in altered homeostasis of adult epidermal stem cells. This result is in agreement with the reported involvement of the PTEN/Akt pathway in the maintenance of other adult stem cells (Li et al., 2002 blue right-pointing triangle, 2003 blue right-pointing triangle; Cheung and Mak, 2006 blue right-pointing triangle; Rossi and Weissman, 2006 blue right-pointing triangle; Yilmaz et al., 2006 blue right-pointing triangle; Zhang et al., 2006a blue right-pointing triangle). On the other hand, the alterations in stem cells observed in the epidermis of myrAkt mice also agree with the reported modulation of these cells by BMP signaling in epidermis and other tissues (Kobielak et al., 2003 blue right-pointing triangle; Rajan et al., 2003 blue right-pointing triangle; He et al., 2004 blue right-pointing triangle; Zhang et al., 2006b blue right-pointing triangle; Kobielak et al., 2007 blue right-pointing triangle). Unexpectedly, we also observed that expression of myrAkt specifically affects the subpopulation of epidermal stem cells characterized by low integrin α6 expression. It has been reported that this suprabasal cell population is derived from that which maintains basal lamina contact and arises only after the start of the first postnatal hair cycle (Blanpain et al., 2004 blue right-pointing triangle). Our data implicate that Akt may affect the transition between these two cell populations and would suggest that Akt may control the cross talk between stem cells and the niche microenvironment.
Collectively, we present evidence that ectodermal organ development is dependent on accurate Akt signaling and that deregulation of this activity results in altered development of these organs, which in the case of skin proceeds through altered BMP signaling and affects epidermal stem cell population.
Supplementary Material
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ACKNOWLEDGMENTS
We express our gratitude to Jesús Martínez and the personnel of the animal facility of CIEMAT for the excellent care of the animals and to Pilar Hernández (CIEMAT) for the histological preparations. This work is partially supported by Grants SAF2002–01037 (MCYT), Oncocycle (CAM), ISCIII-RETIC RD06/0020 (MSC), SAF2005–00033 (MCYT) and Oncology Program from La Caixa Foundation to J.M.P. and by National Institutes of Health Grant CA 37111, National Institute of Environmental Health Sciences Center Grant ES00784, and Cancer Center Support Grant CA16672 to J.D. M.M. is recipient of a predoctoral fellowship form FIS-BEFI (BF03-00201).
Abbreviations used:
BMPbone morphogenetic protein
HFhair follicle.

Footnotes
This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E07-08-0764) on October 24, 2007.
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