Identification of 14-3-3sigma mutation causing cutaneous abnormality in repeated-epilation mutant mouse

Skin Development and Disease: A Molecular Perspective

Skin, the largest organ in the human body, is a crucial protective barrier that plays essential roles in thermoregulation, sensation, and immune defence. This complex organ undergoes intricate processes of development. Skin development initiates during the embryonic stage, orchestrated by molecular cues that control epidermal specification, commitment, stratification, terminal differentiation, and appendage growth. Key signalling pathways are integral in coordinating the development of the epidermis, hair follicles, and sweat glands. The complex interplay among these pathways is vital for the appropriate formation and functionality of the skin. Disruptions in multiple molecular pathways can give rise to a spectrum of skin diseases, from congenital skin disorders to cancers. By delving into the molecular mechanisms implicated in developmental processes, as well as in the pathogenesis of diseases, this narrative review aims to present a comprehensive understanding of these aspects. Such knowledge paves the way for developing innovative targeted therapies and personalised treatment approaches for various skin conditions.

14-3-3 Proteins stabilize actin and vimentin filaments to maintain processes in renal glomerular podocyte

14-3-3 proteins are a ubiquitously expressed family of adaptor proteins. Despite exhibiting high sequence homology, several 14-3-3 isoforms have isoform-specific binding partners and roles. We reported that 14-3-3β interacts with FKBP12 and synaptopodin to maintain the structure of actin fibers in podocytes. However, the precise localization and differential role of 14-3-3 isoforms in kidneys are unclear. Herein, we showed that 14-3-3β in glomeruli was restricted in podocytes, and 14-3-3σ in glomeruli was expressed in podocytes and mesangial cells. Although 14-3-3β was dominantly co-localized with FKBP12 in the foot processes, a part of 14-3-3β was co-localized with Par3 at the slit diaphragm. 14-3-3β interacted with Par3, and FKBP12 bound to 14-3-3β competitively with Par3. Deletion of 14-3-3β enhanced the interaction of Par3 with Par6 in podocytes. Gene silencing for 14-3-3β altered the structure of actin fibers and process formation. 14-3-3β and synaptopodin expression was decreased in podocyte injury models. In contrast, 14-3-3σ in podocytes was expressed in the primary processes. 14-3-3σ interacted with vimentin but not with the actin-associated proteins FKBP12 and synaptopodin. Gene silencing for 14-3-3σ altered the structure of vimentin fibers and process formation. 14-3-3σ and vimentin expression was increased in the early phase of podocyte injury models but was decreased in the late stage. Together, the localization of 14-3-3β at actin cytoskeleton plays a role in maintaining the foot processes and the Par complex in podocytes. In contrast, 14-3-3σ at vimentin cytoskeleton is essential for maintaining primary processes.

Functions of the RIP kinase family members in the skin

The receptor interacting protein kinases (RIPK) are a family of serine/threonine kinases that are involved in the integration of various stress signals. In response to several extracellular and/or intracellular stimuli, RIP kinases engage signaling cascades leading to the activation of NF-κB and mitogen-activated protein kinases, cell death, inflammation, differentiation and Wnt signaling and can have kinase-dependent and kinase-independent functions. Although it was previously suggested that seven RIPKs are part of the RIPK family, phylogenetic analysis indicates that there are only five genuine RIPKs. RIPK1 and RIPK3 are mainly involved in controlling and executing necroptosis in keratinocytes, while RIPK4 controls proliferation and differentiation of keratinocytes and thereby can act as a tumor suppressor in skin. Therefore, in this review we summarize and discuss the functions of RIPKs in skin homeostasis as well as the signaling pathways involved.

Protein profiling of forehead epidermal corneocytes distinguishes frontal fibrosing from androgenetic alopecia

Protein profiling offers an effective approach to characterizing how far epidermis departs from normal in disease states. The present pilot investigation tested the hypothesis that protein expression in epidermal corneocytes is perturbed in the forehead of subjects exhibiting frontal fibrosing alopecia. To this end, samples were collected by tape stripping from subjects diagnosed with this condition and compared to those from asymptomatic control subjects and from those exhibiting androgenetic alopecia. Unlike the latter, which exhibited only 3 proteins significantly different from controls in expression level, forehead samples from frontal fibrosing alopecia subjects displayed 72 proteins significantly different from controls, nearly two-thirds having lower expression. The results demonstrate frontal fibrosing alopecia exhibits altered corneocyte protein expression in epidermis beyond the scalp, indicative of a systemic condition. They also provide a basis for quantitative measures of departure from normal by assaying forehead epidermis, useful in monitoring response to treatment while avoiding invasive biopsy.

Mouse models in palate development and orofacial cleft research: Understanding the crucial role and regulation of epithelial integrity in facial and palate morphogenesis

Cleft lip and cleft palate are common birth defects resulting from genetic and/or environmental perturbations of facial development in utero. Facial morphogenesis commences during early embryogenesis, with cranial neural crest cells interacting with the surface ectoderm to form initially partly separate facial primordia consisting of the medial and lateral nasal prominences, and paired maxillary and mandibular processes. As these facial primordia grow around the primitive oral cavity and merge toward the ventral midline, the surface ectoderm undergoes a critical differentiation step to form an outer layer of flattened and tightly connected periderm cells with a non-stick apical surface that prevents epithelial adhesion. Formation of the upper lip and palate requires spatiotemporally regulated inter-epithelial adhesions and subsequent dissolution of the intervening epithelial seam between the maxillary and medial/lateral nasal processes and between the palatal shelves. Proper regulation of epithelial integrity plays a paramount role during human facial development, as mutations in genes encoding epithelial adhesion molecules and their regulators have been associated with syndromic and non-syndromic orofacial clefts. In this chapter, we summarize mouse genetic studies that have been instrumental in unraveling the mechanisms regulating epithelial integrity and periderm differentiation during facial and palate development. Since proper epithelial integrity also plays crucial roles in wound healing and cancer, understanding the mechanisms regulating epithelial integrity during facial development have direct implications for improvement in clinical care of craniofacial patients.

Keratin 14-dependent disulfides regulate epidermal homeostasis and barrier function via 14-3-3σ and YAP1

The intermediate filament protein keratin 14 (K14) provides vital structural support in basal keratinocytes of epidermis. Recent studies evidenced a role for K14-dependent disulfide bonding in the organization and dynamics of keratin IFs in skin keratinocytes. Here we report that knock-in mice harboring a cysteine-to-alanine substitution at Krt14's codon 373 (C373A) exhibit alterations in disulfide-bonded K14 species and a barrier defect secondary to enhanced proliferation, faster transit time and altered differentiation in epidermis. A proteomics screen identified 14-3-3 as K14 interacting proteins. Follow-up studies showed that YAP1, a transcriptional effector of Hippo signaling regulated by 14-3-3sigma in skin keratinocytes, shows aberrant subcellular partitioning and function in differentiating Krt14 C373A keratinocytes. Residue C373 in K14, which is conserved in a subset of keratins, is revealed as a novel regulator of keratin organization and YAP function in early differentiating keratinocytes, with an impact on cell mechanics, homeostasis and barrier function in epidermis.

14-3-3 proteins regulate desmosomal adhesion via plakophilins

Desmosomes are essential for strong intercellular adhesion and are abundant in tissues exposed to mechanical strain. At the same time, desmosomes need to be dynamic to allow for remodeling of epithelia during differentiation or wound healing. Phosphorylation of desmosomal plaque proteins appears essential for desmosome dynamics. However, the mechanisms how context-dependent post-translational modifications regulate desmosome formation, dynamics or stability are incompletely understood. Here, we show that growth factor signaling regulates the phosphorylation-dependent association of plakophilins 1 and 3 with 14-3-3 protein isoforms and uncover unique and partially antagonistic functions of members of the 14-3-3 family in the regulation of desmosomes. 14-3-3γ associated primarily with cytoplasmic plakophilin 1 phosphorylated at S155 and destabilized intercellular cohesion of keratinocytes by reducing its incorporation into desmosomes. In contrast, stratifin/14-3-3σ interacted preferentially with S285-phosphorylated plakophilin 3 to promote its accumulation at tricellular contact sites, leading to stable desmosomes. Taken together, our study identifies a new layer of regulation of intercellular adhesion by 14-3-3 proteins.

14-3-3ζ loss leads to neonatal lethality by microRNA-126 downregulation-mediated developmental defects in lung vasculature

Background

The 14-3-3 family of proteins have been reported to play an important role in development in various mouse models, but the context specific developmental functions of 14-3-3ζ remain to be determined. In this study, we identified a context specific developmental function of 14-3-3ζ.

Results

Targeted deletion of 14-3-3ζ in the C57Bl/6J murine genetic background led to neonatal lethality due to respiratory distress and could be rescued by out-breeding to the CD-1 or backcrossing to the FVB/NJ congenic background. Histological analysis of lung sections from 18.5 days post coitum embryos (dpc) showed that 14-3-3ζ−/− lung development is arrested at the pseudoglandular stage and exhibits vascular defects. The expression of miR-126, an endothelial-specific miRNA known to regulate lung vascular integrity was down-regulated in the lungs of the 14-3-3ζ−/− embryos in the C57Bl/6J background as compared to their wild-type counterparts. Loss of 14-3-3ζ in endothelial cells inhibited the angiogenic capability of the endothelial cells as determined by both trans-well migration assays and tube formation assays and these defects could be rescued by re-expressing miR-126. Mechanistically, loss of 14-3-3ζ led to reduced Erk1/2 phosphorylation resulting in attenuated binding of the transcription factor Ets2 on the miR-126 promoter which ultimately reduced expression of miR-126.

Conclusion

Our data demonstrates that miR-126 is an important angiogenesis regulator that functions downstream of 14-3-3ζ and downregulation of miR-126 plays a critical role in 14-3-3ζ-loss induced defects in lung vasculature in the C57Bl/6J genetic background.

Electronic supplementary material

The online version of this article (10.1186/s13578-017-0186-y) contains supplementary material, which is available to authorized users.

Periderm: Life-cycle and function during orofacial and epidermal development

Development of the secondary palate involves a complex series of embryonic events which, if disrupted, result in the common congenital anomaly cleft palate. The secondary palate forms from paired palatal shelves which grow initially vertically before elevating to a horizontal position above the tongue and fusing together in the midline via the medial edge epithelia. As the epithelia of the vertical palatal shelves are in contact with the mandibular and lingual epithelia, pathological fusions between the palate and the mandible and/or the tongue must be prevented. This function is mediated by the single cell layered periderm which forms in a distinct and reproducible pattern early in embryogenesis, exhibits highly polarised expression of adhesion complexes, and is shed from the outer surface as the epidermis acquires its barrier function. Disruption of periderm formation and/or function underlies a series of birth defects that exhibit multiple inter-epithelial adhesions including the autosomal dominant popliteal pterygium syndrome and the autosomal recessive cocoon syndrome and Bartsocas Papas syndrome. Genetic analyses of these conditions have shown that IRF6, IKKA, SFN, RIPK4 and GRHL3, all of which are under the transcriptional control of p63, play a key role in periderm formation. Despite these observations, the medial edge epithelia must rapidly acquire the capability to fuse if the palatal shelves are not to remain cleft. This process is driven by TGFβ3-mediated, down-regulation of p63 in the medial edge epithelia which allows periderm migration out of the midline epithelial seam and reduces the proliferative potential of the midline epithelial seam thereby preventing cleft palate. Together, these findings indicate that periderm plays a transient but fundamental role during embryogenesis in preventing pathological adhesion between intimately apposed, adhesion-competent epithelia.

KDF1, encoding keratinocyte differentiation factor 1, is mutated in a multigenerational family with ectodermal dysplasia

Ectodermal dysplasia is a highly heterogeneous group of disorders that variably affect the derivatives of the ectoderm, primarily skin, hair, nails and teeth. TP63, itself mutated in ectodermal dysplasia, links many other ectodermal dysplasia disease genes through a regulatory network that maintains the balance between proliferation and differentiation of the epidermis and other ectodermal derivatives. The ectodermal knockout phenotype of five mouse genes that regulate and/or are regulated by TP63 (Irf6, Ikkα, Ripk4, Stratifin, and Kdf1) is strikingly similar and involves abnormal balance towards proliferation at the expense of differentiation, but only the first three have corresponding ectodermal phenotypes in humans. We describe a multigenerational Saudi family with an autosomal dominant form of hypohidrotic ectodermal dysplasia in which positional mapping and exome sequencing identified a novel variant in KDF1 that fully segregates with the phenotype. The recapitulation of the phenotype we observe in this family by the Kdf1-/- mouse suggests a causal role played by the KDF1 variant.

Meet the neighbors: Mapping local protein interactomes by proximity-dependent labeling with BioID

Proximity-dependent biotin identification (BioID) is a recently developed method that allows the identification of proteins in the close vicinity of a protein of interest in living cells. BioID relies on fusion of the protein of interest with a mutant form of the biotin ligase enzyme BirA (BirA*) that is capable of promiscuously biotinylating proximal proteins irrespective of whether these interact directly or indirectly with the fusion protein or are merely located in the same subcellular neighborhood. The covalent addition of biotin allows the labeled proteins to be purified from cell extracts on the basis of their affinity for streptavidin and identified by mass spectrometry. To date, BioID has been successfully applied to study a variety of proteins and processes in mammalian cells and unicellular eukaryotes and has been shown to be particularly suited to the study of insoluble or inaccessible cellular structures and for detecting weak or transient protein associations. Here, we provide an introduction to BioID, together with a detailed summary of where and how the method has been applied to date, and briefly discuss technical aspects involved in the planning and execution of a BioID study.

Deletion of 14-3-3σ sensitizes mice to DMBA/TPA-induced papillomatosis

The p53-inducible cell cycle regulator 14-3-3σ exhibits tumor suppressive functions and is highly expressed in differentiating layers of the epidermis and hair follicles. 14-3-3σ/SFN/stratifin is frequently silenced in human epithelial cancers, and experimental down-regulation of 14-3-3σ expression immortalizes primary human keratinocytes. In the repeated-epilation (ER) mouse model, a heterozygous nonsense mutation of 14-3-3σ causes repeated hair-loss, hyper-proliferative epidermis, and spontaneous development of papillomas and squamous cell carcinomas in aging mice. Therefore, loss of 14-3-3σ function might contribute to epithelial tumor development. Here, we generated mice with loxP sites surrounding the single 14-3-3σ exon which allowed Cre-mediated deletion of the gene. 14-3-3σ-deficient mice are viable, but demonstrate a permanently disheveled fur. However, histological analyses of the skin did not reveal obvious defects in the hair follicles or the epidermis. Deletion of 14-3-3σ did not enhance spontaneous epidermal tumor development, whereas it increased the frequency and size of DMBA/TPA-induced papillomas. In conclusion, 14-3-3σ is dispensable for normal epidermal homeostasis but critical for suppression of chemically-induced skin carcinogenesis. In addition, these results suggest that the ER mutation of 14-3-3σ is not equivalent to loss of 14-3-3σ, but may represent a gain-of-function variant, which does not reflect the organismal function of wild-type 14-3-3σ.

Deficiency of 14-3-3ε and 14-3-3ζ by the Wnt1 promoter-driven Cre recombinase results in pigmentation defects

Background

The seven 14-3-3 protein isoforms bind to numerous proteins and are involved in a wide variety of cellular events, including the cell cycle, cell division, apoptosis and cancer. We previously found the importance of 14-3-3 proteins in neuronal migration of pyramidal neurons in the developing cortex. Here, we test the function of 14-3-3 proteins in the development of neural crest cells in vivo using mouse genetic approaches.

Results

We found that 14-3-3 proteins are important for the development of neural crest cells, in particular for the pigmentation of the fur on the ventral region of mice.

Conclusions

Our data obtained from the 14-3-3ε/14-3-3ζ/Wnt1-Cre mice strongly indicate the importance of 14-3-3 proteins in the development of melanocyte lineages.

CALML5 is a ZNF750- and TINCR-induced protein that binds stratifin to regulate epidermal differentiation

Outward migration of epidermal progenitors occurs with induction of hundreds of differentiation genes, but the identities of all regulators required for this process are unknown. We used laser capture microdissection followed by RNA sequencing to identify calmodulin-like 5 (CALML5) as the most enriched gene in differentiating outer epidermis. CALML5 mRNA was up-regulated by the ZNF750 transcription factor and then stabilized by the long noncoding RNA TINCR. CALML5 knockout impaired differentiation, abolished keratohyalin granules, and disrupted epidermal barrier function. Mass spectrometry identified SFN (stratifin/14-3-3σ) as a CALML5-binding protein. CALML5 interacts with SFN in suprabasal epidermis, cocontrols 13% of late differentiation genes, and modulates interaction of SFN to some of its binding partners. A ZNF750-TINCR-CALML5-SFN network is thus essential for epidermal differentiation.

14-3-3σ regulates keratinocyte proliferation and differentiation by modulating Yap1 cellular localization

The homozygous repeated epilation (Er/Er) mouse mutant of the gene encoding 14-3-3σ displays an epidermal phenotype characterized by hyperproliferative keratinocytes and undifferentiated epidermis. Heterozygous Er/+ mice develop spontaneous skin tumors and are highly sensitive to tumor-promoting DMBA/TPA induction. The molecular mechanisms underlying 14-3-3σ regulation of epidermal proliferation, differentiation, and tumor formation have not been well elucidated. In the present study, we found that Er/Er keratinocytes failed to sequester Yap1 in the cytoplasm, leading to its nuclear localization during epidermal development in vivo and under differentiation-inducing culture conditions in vitro. In addition, enhanced Yap1 nuclear localization was also evident in DMBA/TPA-induced tumors from Er/+ skin. Furthermore, shRNA knockdown of Yap1 expression in Er/Er keratinocytes inhibited their proliferation, suggesting that YAP1 functions as a downstream effector of 14-3-3σ controlling epidermal proliferation. We then demonstrated that keratinocytes express all seven 14-3-3 protein isoforms, some of which form heterodimers with 14-3-3σ, either full-length WT or the mutant form found in Er/Er mice. However Er 14-3-3σ does not interact with Yap1, as demonstrated by co-immunoprecipitation. We conclude that Er 14-3-3σ disrupts the interaction between 14-3-3 and Yap1, thus fails to block Yap1 nuclear transcriptional function, causing continued progenitor expansion and inhibition of differentiation in Er/Er epidermis.

Palatogenesis and cutaneous repair: A two-headed coin

BACKGROUND

The reparative mechanism that operates following post-natal cutaneous injury is a fundamental survival function that requires a well-orchestrated series of molecular and cellular events. At the end, the body will have closed the hole using processes like cellular proliferation, migration, differentiation and fusion.

RESULTS

These processes are similar to those occurring during embryogenesis and tissue morphogenesis. Palatogenesis, the formation of the palate from two independent palatal shelves growing towards each other and fusing, intuitively, shares many similarities with the closure of a cutaneous wound from the two migrating epithelial fronts.

CONCLUSIONS

In this review, we summarize the current information on cutaneous development, wound healing, palatogenesis and orofacial clefting and propose that orofacial clefting and wound healing are conserved processes that share common pathways and gene regulatory networks.

Periderm prevents pathological epithelial adhesions during embryogenesis

Appropriate development of stratified, squamous, keratinizing epithelia, such as the epidermis and oral epithelia, generates an outer protective permeability barrier that prevents water loss, entry of toxins, and microbial invasion. During embryogenesis, the immature ectoderm initially consists of a single layer of undifferentiated, cuboidal epithelial cells that stratifies to produce an outer layer of flattened periderm cells of unknown function. Here, we determined that periderm cells form in a distinct pattern early in embryogenesis, exhibit highly polarized expression of adhesion complexes, and are shed from the outer surface of the embryo late in development. Mice carrying loss-of-function mutations in the genes encoding IFN regulatory factor 6 (IRF6), IκB kinase-α (IKKα), and stratifin (SFN) exhibit abnormal epidermal development, and we determined that mutant animals exhibit dysfunctional periderm formation, resulting in abnormal intracellular adhesions. Furthermore, tissue from a fetus with cocoon syndrome, a lethal disorder that results from a nonsense mutation in IKKA, revealed an absence of periderm. Together, these data indicate that periderm plays a transient but fundamental role during embryogenesis by acting as a protective barrier that prevents pathological adhesion between immature, adhesion-competent epithelia. Furthermore, this study suggests that failure of periderm formation underlies a series of devastating birth defects, including popliteal pterygium syndrome, cocoon syndrome, and Bartsocas-Papas syndrome.

Stratifin (14-3-3 σ) limits plakophilin-3 exchange with the desmosomal plaque

Desmosomes are prominent cell-cell adhesive junctions in stratified squamous epithelia and disruption of desmosomal adhesion has been shown to have dramatic effects on the function and integrity of these tissues. During normal physiologic processes, such as tissue development and wound healing, intercellular adhesion must be modified locally to allow coordinated cell movements. The mechanisms that control junction integrity and adhesive strength under these conditions are poorly understood. We utilized a proteomics approach to identify plakophilin-3 associated proteins and identified the 14-3-3 family member stratifin. Stratifin interacts specifically with plakophilin-3 and not with other plakophilin isoforms and mutation analysis demonstrated the binding site includes serine 285 in the amino terminal head domain of plakophilin-3. Stratifin interacts with a cytoplasmic pool of plakophilin-3 and is not associated with the desmosome in cultured cells. FRAP analysis revealed that decreased stratifin expression leads to an increase in the exchange rate of cytoplasmic plakophilin-3/GFP with the pool of plakophilin-3/GFP in the desmosome resulting in decreased desmosomal adhesion and increased cell migration. We propose a model by which stratifin plays a role in regulating plakophilin-3 incorporation into the desmosomal plaque by forming a plakophilin-3 stratifin complex in the cytosol and thereby affecting desmosome dynamics in squamous epithelial cells.

Forward genetics identifies Kdf1/1810019J16Rik as an essential regulator of the proliferation-differentiation decision in epidermal progenitor cells

Cell fate decisions during embryogenesis and adult life govern tissue formation, homeostasis and repair. Two key decisions that must be tightly coordinated are proliferation and differentiation. Overproliferation can lead to hyperplasia or tumor formation while premature differentiation can result in a depletion of proliferating cells and organ failure. Maintaining this balance is especially important in tissues that undergo rapid turnover like skin however, despite recent advances, the genetic mechanisms that balance cell differentiation and proliferation are still unclear. In an unbiased genetic screen to identify genes affecting early development, we identified an essential regulator of the proliferation-differentiation balance in epidermal progenitor cells, the Keratinocyte differentiation factor 1 (Kdf1; 1810019J16Rik) gene. Kdf1 is expressed in epidermal cells from early stages of epidermis formation through adulthood. Specifically, Kdf1 is expressed both in epidermal progenitor cells where it acts to curb the rate of proliferation as well as in their progeny where it is required to block proliferation and promote differentiation. Consequently, Kdf1 mutants display both uncontrolled cell proliferation in the epidermis and failure to develop terminal fates. Our findings reveal a dual role for the novel gene Kdf1 both as a repressive signal for progenitor cell proliferation through its inhibition of p63 and a strong inductive signal for terminal differentiation through its interaction with the cell cycle regulator Stratifin.

14-3-3σ stabilizes a complex of soluble actin and intermediate filament to enable breast tumor invasion

The protein 14-3-3σ (stratifin) is frequently described as a tumor suppressor silenced in about 80% of breast tumors. Intriguingly, we show that 14-3-3σ expression, which in normal breast is localized to the myoepithelial cells, tracks with malignant phenotype in two models of basal-like breast cancer progression, and in patients, it is associated with basal-like subtype and poor clinical outcome. We characterized a mechanism by which 14-3-3σ guides breast tumor invasion by integrating cytoskeletal dynamics: it stabilizes a complex of solubilized actin and intermediate filaments to maintain a pool of "bioavailable" complexes for polarized assembly during migration. We show that formation of the actin/cytokeratin/14-3-3σ complex and cellular migration are regulated by PKCζ-dependent phosphorylation, a finding that could form the basis for intervention in aggressive breast carcinomas expressing 14-3-3σ. Our data suggest that the biology of this protein is important in cellular movement and is contingent on breast cancer subtype.

Epidermal development in mammals: key regulators, signals from beneath, and stem cells

Epidermis is one of the best-studied tissues in mammals that contain types of stem cells. Outstanding works in recent years have shed great light on behaviors of different epidermal stem cell populations in the homeostasis and regeneration of the epidermis as well as hair follicles. Also, the molecular mechanisms governing these stem cells are being elucidated, from genetic to epigenetic levels. Compared with the explicit knowledge about adult skin, embryonic development of the epidermis, especially the early period, still needs exploration. Furthermore, stem cells in the embryonic epidermis are largely unstudied or ambiguously depicted. In this review, we will summarize and discuss the process of embryonic epidermal development, with focuses on some key molecular regulators and the role of the sub-epidermal mesenchyme. We will also try to trace adult epidermal stem cell populations back to embryonic development. In addition, we will comment on in vitro derivation of epidermal lineages from ES cells and iPS cells.

Mouse Genetic Models Reveal Surprising Functions of IkB Kinase Alpha in Skin Development and Skin Carcinogenesis

Gene knockout studies unexpectedly reveal a pivotal role for IκB kinase alpha (IKKα) in mouse embryonic skin development. Skin carcinogenesis experiments show that Ikkα heterozygous mice are highly susceptible to chemical carcinogen or ultraviolet B light (UVB) induced benign and malignant skin tumors in comparison to wild-type mice. IKKα deletion mediated by keratin 5 (K5).Cre or K15.Cre in keratinocytes induces epidermal hyperplasia and spontaneous skin squamous cell carcinomas (SCCs) in Ikkα floxed mice. On the other hand, transgenic mice overexpressing IKKα in the epidermis, under the control of a truncated loricrin promoter or K5 promoter, develop normal skin and show no defects in the formation of the epidermis and other epithelial organs, and the transgenic IKKα represses chemical carcinogen or UVB induced skin carcinogenesis. Moreover, IKKα deletion mediated by a mutation, which generates a stop codon in the Ikkα gene, has been reported in a human autosomal recessive lethal syndrome. Downregulated IKKα and Ikkα mutations and deletions are found in human skin SCCs. The collective evidence not only highlights the importance of IKKα in skin development, maintaining skin homeostasis, and preventing skin carcinogenesis, but also demonstrates that mouse models are extremely valuable tools for revealing the mechanisms underlying these biological events, leading our studies from bench side to bedside.

IKK biology

The inhibitor of nuclear factor-κB (IκB) kinase (IKK) complex is the master regulator of the NF-κB signaling pathway. The activation of the IKK complex is a tightly regulated, highly stimulus-specific, and target-specific event that is essential for the plethora of functions attributed to NF-κB. More recently, NF-κB-independent roles of IKK members have brought increased complexity to its biological function. This review highlights some of the major advances in the studies of the process of IKK activation and the biological roles of IKK family members, with a focus on NF-κB-independent functions. Understanding these complex processes is essential for targeting IKK for therapeutics.

A pair of transmembrane receptors essential for the retention and pigmentation of hair

Hair follicles are simple, accessible models for many developmental processes. Here, using mutant mice, we show that Bmpr2, a known receptor for bone morphogenetic proteins (Bmps), and Acvr2a, a known receptor for Bmps and activins, are individually redundant but together essential for multiple follicular traits. When Bmpr2/Acvr2a function is reduced in cutaneous epithelium, hair follicles undergo rapid cycles of hair generation and loss. Alopecia results from a failure to terminate hair development properly, as hair clubs never form, and follicular retraction is slowed. Hair regeneration is rapid due to premature activation of new hair-production programs. Hair shafts differentiate aberrantly due to impaired arrest of medullary-cell proliferation. When Bmpr2/Acvr2a function is reduced in melanocytes, gray hair develops, as melanosomes differentiate but fail to grow, resulting in organelle miniaturization. We conclude that Bmpr2 and Acvr2a normally play cell-type-specific, necessary roles in organelle biogenesis and the shutdown of developmental programs and cell division.

The cell cycle regulator protein 14-3-3σ is essential for hair follicle integrity and epidermal homeostasis

The 14-3-3σ (Stratifin; Sfn) is a cell cycle regulator intimately involved in the program of epithelial keratinization. 14-3-3σ is unique in that it is expressed primarily in epithelial cells and is frequently silenced in epithelial cancers. Despite its well-documented role as a cell cycle regulator and as a tumor suppressor, the function of 14-3-3σ in the intricate balance of proliferation and differentiation in epithelial development is poorly understood. A mutation in 14-3-3σ was found to be responsible for the repeated epilation (Er) phenotype. It has previously been shown that Sfn(+/Er) mice are characterized by repeated hair loss and regrowth, whereas Sfn(Er/Er) mice die at birth displaying severe oral fusions and limb abnormalities as a result of defects in keratinizing epithelia. Here we show that mice heterozygous for the 14-3-3σ mutation have severe defects in hair shaft differentiation, resulting in destruction of the hair shaft during morphogenesis. Furthermore, we report that the interfollicular epidermis and sebaceous glands are hyperproliferative, coincident with expanded nuclear Yap1 (Yes-associated protein 1)--a critical modulator of epidermal stem cell proliferation. We also report that hair follicle stem cells in the bulge cycle abnormally, raising important questions as to the role of 14-3-3σ in the bulge.

14-3-3 regulates the LNK/JAK2 pathway in mouse hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cell (HSPC) functions are governed by intricate signaling networks. The tyrosine kinase JAK2 plays an essential role in cytokine signaling during hematopoiesis. The adaptor protein LNK is a critical determinant of this process through its inhibitory interaction with JAK2, thereby limiting HSPC self-renewal. LNK deficiency promotes myeloproliferative neoplasm (MPN) development in mice, and LNK loss-of-function mutations are found in human MPNs, emphasizing its pivotal role in normal and malignant HSPCs. Here, we report the identification of 14-3-3 proteins as LNK binding partners. 14-3-3 interfered with the LNK-JAK2 interaction, thereby alleviating LNK inhibition of JAK2 signaling and cell proliferation. Binding of 14-3-3 required 2 previously unappreciated serine phosphorylation sites in LNK, and we found that their phosphorylation is mediated by glycogen synthase kinase 3 and PKA kinases. Mutations of these residues abrogated the interaction and augmented the growth inhibitory function of LNK. Conversely, forced 14-3-3 binding constrained LNK function. Furthermore, interaction with 14-3-3 sequestered LNK in the cytoplasm away from the plasma membrane-proximal JAK2. Importantly, bone marrow transplantation studies revealed an essential role for 14-3-3 in HSPC reconstitution that can be partially mitigated by LNK deficiency. We believe that, together, this work implicates 14-3-3 proteins as novel and positive HSPC regulators by impinging on the LNK/JAK2 pathway.

Exome sequence identifies RIPK4 as the Bartsocas-Papas syndrome locus

Pterygium syndromes are complex congenital disorders that encompass several distinct clinical conditions characterized by multiple skin webs affecting the flexural surfaces often accompanied by craniofacial anomalies. In severe forms, such as in the autosomal-recessive Bartsocas-Papas syndrome, early lethality is common, complicating the identification of causative mutations. Using exome sequencing in a consanguineous family, we identified the homozygous mutation c.1127C>A in exon 7 of RIPK4 that resulted in the introduction of the nonsense mutation p.Ser376X into the encoded ankyrin repeat-containing kinase, a protein that is essential for keratinocyte differentiation. Subsequently, we identified a second mutation in exon 2 of RIPK4 (c.242T>A) that resulted in the missense variant p.Ile81Asn in the kinase domain of the protein. We have further demonstrated that RIPK4 is a direct transcriptional target of the protein p63, a master regulator of stratified epithelial development, which acts as a nodal point in the cascade of molecular events that prevent pterygium syndromes.

Loss of the 14-3-3σ tumor suppressor is a critical event in ErbB2-mediated tumor progression

14-3-3σ is a putative tumor suppressor involved in cell-cycle progression and epithelial polarity. We demonstrate that loss of one or both copies of the conditional 14-3-3σ allele results in accelerated mammary and salivary tumorigenesis in mice expressing an activated erbB2 oncogene under the endogenous erbB2 promoter. Significantly, the majority of tumors bearing a single conditional 14-3-3σ allele lose expression of the remaining 14-3-3σ allele, which is associated with epigenetic methylation of the 14-3-3σ locus. In addition to accelerated tumor onset, in a mouse mammary tumor virus-driven ErbB2 tumor model, loss of 14-3-3σ results in enhanced metastatic phenotype that is correlated with loss of cellular junctions. Taken together, these results provide compelling evidence that 14-3-3σ is a potent tumor suppressor involved in ErbB2-driven breast cancer initiation and metastasis.

SIGNIFICANCE

14-3-3σ has been identified as a normal mammary epithelial cell marker frequently downregulated during neoplastic development. Consistent with its potential role as a tumor suppressor, we demonstrate that targeted disruption of 14-3-3σ in a number of epithelial tissues can profoundly impact both the initiation and metastatic phases of ErbB2-mediated tumor progression through modulation of a number of distinct signaling networks.

Role of IKKα in skin squamous cell carcinomas

Squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) are two major types of skin cancer derived from keratinocytes. SCC is a more aggressive type of cancer than BCC in humans. One significant difference between SCC and BCC is that SCC development is generally associated with cell dedifferentiation and morphological changes. When SCC is converted to spindle cell carcinoma, the latest stage of cancer, the tumor cells change to a fibroblastic cell morphology (epithelial-to-mesenchymal transition) and lose their differentiation markers. Recently, several laboratories have reported altered IκB kinase α (IKKα) protein localization, downregulated IKKα, and IKKα gene deletions and mutations in human SCCs of the skin, lung, esophagus, and neck and head. In addition, IKKα reduction promotes chemical carcinogen- and ultraviolet B-induced skin carcinogenesis, and IKKα deletion in keratinocytes causes spontaneous skin SCCs, but not BCCs, in mice. Thus, IKKα emerges as a bona fide skin tumor suppressor. In this article, we will discuss the role of IKKα in skin SCC development.

14-3-3σ and p63 play opposing roles in epidermal tumorigenesis

14-3-3σ plays a regulatory role in epidermal epithelial differentiation and loss of 14-3-3σ leads to increased proliferation and impaired differentiation. A tumor suppressor function for 14-3-3σ has been proposed based on the fact that some epithelial-derived tumors lose 14-3-3σ expression. p63, a p53 family member, is a master regulator of epidermal epithelial proliferation and differentiation and is necessary for the epidermal development. The function of p63 in tumorigenesis is still controversial and poorly defined as multiple isoforms have been found to play either collaborative or opposing roles. By using 'repeated epilation' heterozygous (Er/+) mice containing a dominant-negative 14-3-3σ mutation, the functional relationship of p63 with 14-3-3σ in epidermal proliferation, differentiation and tumorigenesis was investigated. It was found that p63, particularly the ΔNp63α isoform, was strongly expressed in 14-3-3σ-deficient keratinocytes and knockdown of p63 remarkably inhibited proliferation in these cells. To study the functional roles of 14-3-3σ and p63 in epidermal tumorigenesis, we adopted a 7,12-dimethylbenzanthracene/12-O-tetradecanoyl-phorbol-13-acetate (DMBA/TPA) two-stage tumorigenesis procedure to induce formation of skin papillomas and squamous cell carcinomas in Er/+ mice and identified strong p63 expression in resultant tumors. The loss of one allele of p63 caused by the generation of Er/+/p63(+/-) double compound mice decreased the sensitivity to DMBA-/TPA-induced tumorigenesis as compared with Er/+ mice. This study shows that p63 and 14-3-3σ play opposing roles in the development of skin tumors and that the accumulation of p63 is essential for Ras/14-3-3σ mutation-induced papilloma formation and squamous cell carcinoma carcinogenesis.

Yap1 acts downstream of α-catenin to control epidermal proliferation

During development and regeneration, proliferation of tissue-specific stem cells is tightly controlled to produce organs of a predetermined size. The molecular determinants of this process remain poorly understood. Here, we investigate the function of Yap1, the transcriptional effector of the Hippo signaling pathway, in skin biology. Using gain- and loss-of-function studies, we show that Yap1 is a critical modulator of epidermal stem cell proliferation and tissue expansion. Yap1 mediates this effect through interaction with TEAD transcription factors. Additionally, our studies reveal that α-catenin, a molecule previously implicated in tumor suppression and cell density sensing in the skin, is an upstream negative regulator of Yap1. α-catenin controls Yap1 activity and phosphorylation by modulating its interaction with 14-3-3 and the PP2A phosphatase. Together, these data identify Yap1 as a determinant of the proliferative capacity of epidermal stem cells and as an important effector of a "crowd control" molecular circuitry in mammalian skin.

14-3-3σ controls corneal epithelium homeostasis and wound healing

PURPOSE

To investigate the functional role of 14-3-3σ in regulation of the corneal epithelial proliferation, differentiation, and wound-healing response.

METHODS

Corneal phenotypes were investigated in heterozygous repeated epilation (Er) mice carrying mutations in the sfn (14-3-3σ) gene. Immunohistochemistry was used to study the corneal morphogenesis of the Er/Er embryos at embryonic day (E)18.5. Corneal homeostasis and the wound-healing response were investigated macroscopically and microscopically in the adult heterozygous Er mice. Corneal epithelial cell proliferation and differentiation were assessed by BrdU incorporation and immunohistochemistry with specific antibodies for differentiation markers. Furthermore, corneal stroma neovascularization and meibomian gland degeneration were examined by immunohistochemistry. The healing of corneal wounds after debridement was monitored and visualized by fluorescent staining.

RESULTS

Homozygous mutation of 14-3-3σ led to defects in embryonic corneal epithelial development and differentiation, whereas young heterozygotes showed normal corneal development and homeostasis. However, older heterozygotes displayed a dramatic corneal wound-healing defect characterized by hyperplastic basal progenitor cells (some of which undergo a differentiation switch to express markers of keratinized epidermis); cornea stroma changes including neovascularization; and corneal opacity, leading to plaque formation. Aged heterozygotes also showed meibomian gland atrophy.

CONCLUSIONS

14-3-3σ is essential for corneal epithelium differentiation, and plays an important role in corneal epithelium development and daily renewal of the adult corneal epithelium.

A novel role for 14-3-3sigma in regulating epithelial cell polarity

The loss of epithelial polarity is thought to play an important role during mammary tumor progression. Using a unique transgenic mouse model of ErbB2-induced mammary tumorigenesis, we demonstrated previously that amplification of ErbB2 is frequently accompanied by loss of the 14-3-3sigma gene. Here, we demonstrate that ectopic expression of 14-3-3sigma results in restoration of epithelial polarity in ErbB2-transformed mammary tumor cells. We further demonstrate that targeted deletion of 14-3-3sigma within primary mammary epithelial cells increases their proliferative capacity and adversely affects their ability to form polarized structures. Finally, we show that 14-3-3sigma can specifically form complexes with Par3, a protein that is essential for the maintenance of a polarized epithelial state. Taken together, these observations suggest that 14-3-3sigma plays a critical role in retaining epithelial polarity.

Transcriptome profiles of carcinoma-in-situ and invasive non-small cell lung cancer as revealed by SAGE

Background

Non-small cell lung cancer (NSCLC) presents as a progressive disease spanning precancerous, preinvasive, locally invasive, and metastatic lesions. Identification of biological pathways reflective of these progressive stages, and aberrantly expressed genes associated with these pathways, would conceivably enhance therapeutic approaches to this devastating disease.

Methodology/Principal Findings

Through the construction and analysis of SAGE libraries, we have determined transcriptome profiles for preinvasive carcinoma-in-situ (CIS) and invasive squamous cell carcinoma (SCC) of the lung, and compared these with expression profiles generated from both bronchial epithelium, and precancerous metaplastic and dysplastic lesions using Ingenuity Pathway Analysis. Expression of genes associated with epidermal development, and loss of expression of genes associated with mucociliary biology, are predominant features of CIS, largely shared with precancerous lesions. Additionally, expression of genes associated with xenobiotic metabolism/detoxification is a notable feature of CIS, and is largely maintained in invasive cancer. Genes related to tissue fibrosis and acute phase immune response are characteristic of the invasive SCC phenotype. Moreover, the data presented here suggests that tissue remodeling/fibrosis is initiated at the early stages of CIS. Additionally, this study indicates that alteration in copy-number status represents a plausible mechanism for differential gene expression in CIS and invasive SCC.

Conclusions/Significance

This study is the first report of large-scale expression profiling of CIS of the lung. Unbiased expression profiling of these preinvasive and invasive lesions provides a platform for further investigations into the molecular genetic events relevant to early stages of squamous NSCLC development. Additionally, up-regulated genes detected at extreme differences between CIS and invasive cancer may have potential to serve as biomarkers for early detection.

RIP4 regulates epidermal differentiation and cutaneous inflammation

The receptor-interacting protein (RIP) family kinase RIP4 interacts with protein kinase C (PKC) isoforms and is implicated in PKC-dependent signaling pathways. RIP4(-/-) mice die at birth with epidermal differentiation defects, causing fusions of all external orifices and loss of the esophageal lumen. To further understand RIP4 function in the skin, we generated transgenic mice with epidermal-specific expression of RIP4 using the human keratin-14 promoter (K14-RIP4). The K14-RIP4 transgene rescued the epidermal phenotype of RIP4(-/-) mice, showing that RIP4 acts autonomously in the epidermis to regulate differentiation. Although RIP4(-/-) mice share many phenotypic similarities with inhibitor kappaB kinase (IKK)alpha(-/-) mice and stratifin repeated epilation (Sfn(Er/Er)) mice, the K14-RIP4 transgene failed to promote epidermal differentiation in these mutant backgrounds. Unexpectedly, topical treatment of K14-RIP4 mice with 12-O-tetradecanoylphorbol-13-acetate (TPA) induced dramatic, neutrophilic inflammation, an effect that was independent of tumor necrosis factor type 1 receptor (TNFR1/p55) function. Despite their enhanced sensitivity to TPA, K14-RIP4 mice did not have an altered frequency of tumor formation in TPA-promoted skin cancer initiated with 7,12-dimethylbenz[a]anthracene (DMBA). These data suggest that RIP4 functions in the epidermis through PKC-specific signaling pathways to regulate differentiation and inflammation.

14-3-3sigma controls corneal epithelial cell proliferation and differentiation through the Notch signaling pathway

14-3-3sigma (also called stratifin) is specifically expressed in the stratified squamous epithelium and its function was recently shown to be linked to epidermal stratification and differentiation in the skin. In this study, we investigated its role in corneal epithelium cell proliferation and differentiation. We showed that the 14-3-3sigma mutation in repeated epilation (Er) mutant mice results in a dominant negative truncated protein. Primary corneal epithelial cells expressing the dominant negative protein failed to undergo high calcium-induced cell cycle arrest and differentiation. We further demonstrated that blocking endogenous 14-3-3sigma activity in corneal epithelial cells by overexpressing dominative negative 14-3-3sigma led to reduced Notch activity and Notch1/2 transcription. Significantly, expression of the active Notch intracellular domain overcame the block in epithelial cell differentiation in 14-3-3sigma mutant-expressing corneal epithelial cells. We conclude that 14-3-3sigma is critical for regulating corneal epithelial proliferation and differentiation by regulating Notch signaling activity.

Recent advances on skin-resident stem/progenitor cell functions in skin regeneration, aging and cancers and novel anti-aging and cancer therapies

Recent advances in skin-resident adult stem/progenitor cell research have revealed that these immature and regenerative cells with a high longevity provide critical functions in maintaining skin homeostasis and repair after severe injuries along the lifespan of individuals. The establishment of the functional properties of distinct adult stem/progenitor cells found in skin epidermis and hair follicles and extrinsic signals from their niches, which are deregulated during their aging and malignant transformation, has significantly improved our understanding on the etiopathogenesis of diverse human skin disorders and cancers. Particularly, enhanced ultraviolet radiation exposure, inflammation and oxidative stress and telomere attrition during chronological aging may induce severe DNA damages and genomic instability in the skin-resident stem/progenitor cells and their progenies. These molecular events may result in the alterations in key signalling components controlling their self-renewal and/or regenerative capacities as well as the activation of tumour suppressor gene products that trigger their growth arrest and senescence or apoptotic death. The progressive decline in the regenerative functions and/or number of skin-resident adult stem/progenitor cells may cause diverse skin diseases with advancing age. Moreover, the photoaging, telomerase re-activation and occurrence of different oncogenic events in skin-resident adult stem/progenitor cells may also culminate in their malignant transformation into cancer stem/progenitor cells and skin cancer initiation and progression. Therefore, the anti-inflammatory and anti-oxidant treatments and stem cell-replacement and gene therapies as well as the molecular targeting of their malignant counterpart, skin cancer-initiating cells offer great promise to treat diverse skin disorders and cancers.

14-3-3sigma, the double-edged sword of human cancers

14-3-3sigma is a member of a highly conserved family of 14-3-3 proteins that are present in all eukaryotic organisms. 14-3-3sigma has been considered as a tumor suppressor with reduced expression in some human cancers while its increased expression causes resistance to anticancer agents and radiation that cause DNA damages. The increased expression of 14-3-3sigma may also predict poor prognosis in some human cancers. Thus, 14-3-3sigma may play an important role as a double-edged sword in human cancers, which may attribute to its property as a molecular chaperone by binding to various protein ligands important to many cellular processes such as cell cycle checkpoint regulation and apoptosis in response to DNA damages. In this article, we will review recent studies and progresses in understanding 14-3-3sigma as a double-edged sword in human cancers.

From telogen to exogen: mechanisms underlying formation and subsequent loss of the hair club fiber

The hair follicle has the unique capacity to undergo periods of growth, regression, and rest before regenerating itself to restart the cycle. This dynamic cycling capacity enables mammals to change their coats, and for hair length to be controlled on different body sites. More recently, the process of club fiber shedding has been described as a distinct cycle phase known as exogen, and proposed to be an active phase of the hair cycle. This review focuses on the importance of the shedding phase of the hair cycle and, in the context of current literature, analyzes the processes of club fiber formation, retention, and release, which may influence progression through exogen, particularly in relation to human hair.

A tale of terminal differentiation: IKKalpha, the master keratinocyte regulator

Keratinocyte differentiation is the process of cellular maturation from a mitotic state to a terminally differentiated state during which skin builds up a tough yet soft skin barrier to protect the body. Its irreversibility also allows the shedding of excessive keratinocytes, thereby maintaining skin homeostasis and preventing skin diseases. Although the entire journey of keratinocyte differentiation is intricate and not well understood, it is known that Ras is able to block keratinocyte terminal differentiation and instead induce keratinocyte proliferation and transformation. It appears that uncontrolled proliferation actually interrupts differentiation. However, it has been unclear whether there are any innate surveillants that would be able to induce terminal differentiation by antagonizing excessive mitotic activities. Inhibitor of nuclear factor kappaB kinase-alpha (IKKalpha, previously known as Chuk) emerges as a master regulator in the coordinative control of keratinocyte differentiation and proliferation and as a major tumor suppressor in human and mouse skin squamous cell carcinomas. IKKalpha does so largely by integrating into the epidermal growth factor receptor (EGFR)/Ras/extracellular signal-regulated kinase (Erk)/EGFR ligand pathways during mitosis and differentiation. We discuss these findings herein to extend our understanding of how IKKalpha-mediated terminal differentiation serves as an innate surveillant in skin.

Critical role of IkappaB kinase alpha in embryonic skin development and skin carcinogenesis

IkappaB kinase alpha (IKKalpha), IKKbeta, and IKKgamma/NEMO form the IKK complex, which is essential for NF-kappaB activation. However, genetic studies have shown that the role of IKKalpha is distinct from that of IKKbeta or IKKgamma in the development of the mouse embryonic skin. Loss of IKKalpha has been shown to cause epidermal hyperplasia, prevent keratinocyte terminal differentiation, and impair the formation of the skin, resulting in the deaths of IKKalpha-deficient (Ikkalpha-/-) mice soon after birth. Recent experimental data from several laboratories have revealed that IKKalpha functions as a tumor suppressor in human squamous cell carcinomas (SCCs) of skin, lungs, and head and neck. Chemical carcinogenesis studies using mice have shown that reduction in IKKalpha expression increases the number and size of Ras-initiated skin tumors and promotes their progression, indicating that reduced IKKalpha expression provides a selective growth advantage that cooperates with Ras activity to promote skin carcinogenesis. In this review, we will summarize these findings from our and other studies on the role that IKKalpha plays in development of the mouse embryonic skin and skin carcinogenesis.

Presence and distribution of 14-3-3 proteins in human ocular surface tissues

PURPOSE

14-3-3 is a highly conserved, ubiquitously expressed family of proteins. At least seven mammalian isoforms (beta, epsilon, gamma, eta, theta, sigma, and zeta) are known. These proteins associate with over 200 different target molecules and activate several downstream signaling cascades involved in the regulation of metabolism, cell cycle, apoptosis, protein trafficking, transcription, stress responses, and malignant transformations. We are interested in the role of these proteins in the mechanisms regulating homeostasis and the pathologies of the human ocular surface. Therefore, our purpose is to determine the expression of the 14-3-3 proteins in the human cornea, the conjunctiva, and the primary cells comprising these tissues.

METHODS

Using immunofluorescence, we determined the expression of 14-3-3 beta, epsilon, gamma, eta, theta, sigma, and zeta in paraffin sections of the human cornea and conjunctiva. Using indirect immunofluorescence and western blot analysis, we also determined the expression of these isoforms in primary corneal epithelial cells, keratocytes, endothelial cells, and primary conjunctival epithelial cells. The expressions of these isoforms in primary epithelial and endothelial cells were compared with the same expressions in several corneal cell lines. Western blot analysis was used to determine the presence of 14-3-3 isoforms in the culture medium from corneal epithelial cells, cell lines, and the tear fluid.

RESULTS

All the 14-3-3 isoforms were expressed in the corneal and conjunctival epithelia as well as primary epithelial cells and cell lines. Expression of 14-3-3 sigma was confined to epithelial cells and was secreted into the culture medium of primary cells and cell lines. We also report for the first time that two of the secreted isoforms, 14-3-3 gamma and zeta, are also present in the human tear fluid.

CONCLUSIONS

We have determined that all the mammalian 14-3-3 isoforms are expressed in the human cornea, conjunctiva, and the component cells and that the 14-3-3 sigma isoform was found to be epithelial cell specific. We propose that the intracellular and extracellular presence of 14-3-3 sigma suggest its involvement in the epithelia specific signaling pathways.

Maternal Interferon Regulatory Factor 6 is required for the differentiation of primary superficial epithelia in Danio and Xenopus embryos

Early in the development of animal embryos, superficial cells of the blastula form a distinct lineage and adopt an epithelial morphology. In different animals, the fate of these primary superficial epithelial (PSE) cells varies, and it is unclear whether pathways governing segregation of blastomeres into the PSE lineage are conserved. Mutations in the gene encoding Interferon Regulatory Factor 6 (IRF6) are associated with syndromic and non-syndromic forms of cleft lip and palate, consistent with a role for Irf6 in development of oral epithelia, and mouse Irf6 targeted null mutant embryos display abnormal differentiation of oral epithelia and skin. In Danio rerio (zebrafish) and Xenopus laevis (African clawed frog) embryos, zygotic irf6 transcripts are present in many epithelial tissues including the presumptive PSE cells and maternal irf6 transcripts are present throughout all cells at the blastula stage. Injection of antisense oligonucleotides with ability to disrupt translation of irf6 transcripts caused little or no effect on development. By contrast, injection of RNA encoding a putative dominant negative Irf6 caused epiboly arrest, loss of gene expression characteristic of the EVL, and rupture of the embryo at late gastrula stage. The dominant negative Irf6 disrupted EVL gene expression in a cell autonomous fashion. These results suggest that Irf6 translated in the oocyte or unfertilized egg suffices for early development. Supporting the importance of maternal Irf6, we show that depletion of maternal irf6 transcripts in X. laevis embryos leads to gastrulation defects and rupture of the superficial epithelium. These experiments reveal a conserved role for maternally-encoded Irf6 in differentiation of a simple epithelium in X. laevis and D. rerio. This epithelium constitutes a novel model tissue in which to explore the Irf6 regulatory pathway.

IKKalpha, a critical regulator of epidermal differentiation and a suppressor of skin cancer

IκB kinase α (IKKα), one of the two catalytic subunits of the IKK complex involved in nuclear factor κB (NF-κB) activation, also functions as a molecular switch that controls epidermal differentiation. This unexpected function requires IKKα nuclear translocation but does not depend on its kinase activity, and is independent of NF-κB signalling. Ikkα^–/– mice present with a hyperproliferative and undifferentiated epidermis characterized by complete absence of a granular layer and stratum corneum. Ikkα-deficient keratinocytes do not express terminal differentiation markers and continue to proliferate even when subjected to differentiation-inducing stimuli. This antiproliferative function of IKKα is also important for the suppression of squamous cell carcinogenesis. The exact mechanisms by which nuclear IKKα controls keratinocyte proliferation and differentiation remained mysterious for some time. Recent studies, however, have revealed that IKKα is a major cofactor in a TGFβ–Smad2/3 signalling pathway that is Smad4 independent. This pathway controls cell cycle withdrawal during keratinocyte terminal differentiation. Although these are not the only functions of nuclear IKKα, this multifunctional protein is a key regulator of keratinocyte and epidermal differentiation and a critical suppressor of skin cancer.

Primer: strategies for identifying genes involved in renal disease

The globally increasing number of patients with end-stage renal disease urges the identification of molecular pathways involved in renal pathophysiology, to serve as targets for intervention. Moreover, the identification of genetic risk factors or protective genes can aid tailored therapy. Tools that can be used to identify genes involved in renal disease include gene expression arrays, linkage analysis and association studies. Arrays are a powerful and widely used approach to the analysis of gene transcription and protein expression, whereas linkage analysis and association studies link disease susceptibility to particular genetic regions. Animal models are available to pinpoint the disease-associated genes. Candidate genes so far identified in renal disease include those encoding the podocyte proteins nephrin and podocin, the transcription factor WT1, the calcium channel TRPC6 and the enzyme phospholipase C-epsilon-1 (in congenital nephrotic syndrome and focal segmental glomerulosclerosis), and carnosinase (in diabetic nephropathy). In addition, linkage studies have identified chromosomal regions implicated in systemic lupus erythematosus, diabetic nephropathy and familial IgA nephropathy. Future studies will elucidate the emerging role of epigenetic regulation of gene expression in renal disease.

IKKalpha shields 14-3-3sigma, a G(2)/M cell cycle checkpoint gene, from hypermethylation, preventing its silencing

We recently reported that a large proportion of aggressive squamous cell carcinomas of humans and mice express markedly reduced IKKalpha. However, the role of IKKalpha in maintaining genomic stability is unknown. Here we reported that IKKalpha-deficient keratinocytes had a defect in the G(2)/M cell-cycle arrest in response to DNA damage due to downregulated 14-3-3sigma, a cell cycle checkpoint protein. Trimethylated histone H3 lysine 9 (H3-K9) was found to associate with the histone trimethyltransferase Suv39h1 and DNA methyltransferase Dnmt3a in the methylated 14-3-3sigma locus. Reintroduction of IKKalpha restored the expression of 14-3-3sigma. IKKalpha was found to associate with H3 in 14-3-3sigma, which prevented access of Suv39h1 to H3, thereby preventing hypermethylation of 14-3-3sigma. IKKalpha mutants that failed to bind to H3 did not restore the expression of 14-3-3sigma. Thus, IKKalpha protects the 14-3-3sigma locus from hypermethylation, which serves as a mechanism of maintaining genomic stability in keratinocytes.