Search

Peeling skin syndrome is an autosomal recessive genodermatosis characterized by the shedding of the outer epidermis. In the acral form, the dorsa of the hands and feet are predominantly affected. Ultrastructural analysis has revealed tissue separation at the junction between the granular cells and the stratum corneum in the outer epidermis. Genomewide linkage analysis in a consanguineous Dutch kindred mapped the gene to 15q15.2 in the interval between markers D15S1040 and D15S1016. Two homozygous missense mutations, T109M and G113C, were found in TGM5, which encodes transglutaminase 5 (TG5), in all affected persons in two unrelated families. The mutation was present on the same haplotype in both kindreds, indicating a probable ancestral mutation. TG5 is strongly expressed in the epidermal granular cells, where it cross-links a variety of structural proteins in the terminal differentiation of the epidermis to form the cornified cell envelope. An established, in vitro, biochemical cross-linking assay revealed that, although T109M is not pathogenic, G113C completely abolishes TG5 activity. Three-dimensional modeling of TG5 showed that G113C lies close to the catalytic domain, and, furthermore, that this glycine residue is conserved in all known transglutaminases, which is consistent with pathogenicity. Other families with more-widespread peeling skin phenotypes lacked TGM5 mutations. This study identifies the first causative gene in this heterogeneous group of skin disorders and demonstrates that the protein cross-linking function performed by TG5 is vital for maintaining cell-cell adhesion between the outermost layers of the epidermis.

TP63, an important epithelial developmental gene, has significant homology to p53. Unlike p53, the expression of p63 is regulated by two different promoters resulting in proteins with opposite functions: the full-length transcriptionally active TAp63 and the dominant-negative DeltaNp63. We investigated the downstream mechanisms by which TAp63alpha elicits apoptosis. TAp63alpha directly transactivates the CD95 gene via the p53 binding site in the first intron resulting in upregulation of a functional CD95 death receptor. Stimulation and blocking experiments of the CD95, TNF-R and TRAIL-R death receptor systems revealed that TAp63alpha can trigger expression of each of these death receptors. Furthermore, our findings demonstrate a link between TAp63alpha and the mitochondrial apoptosis pathway. TAp63alpha upregulates expression of proapoptotic Bcl-2 family members like Bax and BCL2L11 and the expression of RAD9, DAP3 and APAF1. Of clinical relevance is the fact that TAp63alpha is induced by many chemotherapeutic drugs and that inhibiting TAp63 function leads to chemoresistance. Thus, beyond its importance in development and differentiation, we describe an important role for TAp63alpha in the induction of apoptosis and chemosensitivity.

The epidermis functions as a barrier against the environment by means of several layers of terminally differentiated, dead keratinocytes - the cornified layer, which forms the endpoint of epidermal differentiation and death. The cornified envelope replaces the plasma membrane of differentiating keratinocytes and consists of keratins that are enclosed within an insoluble amalgam of proteins, which are crosslinked by transglutaminases and surrounded by a lipid envelope. New insights into the molecular mechanisms and the physiological endpoints of cornification are increasing our understanding of the pathological defects of this unique form of programmed cell death, which is associated with barrier malfunctions and ichthyosis.

p73, like its homologue, the tumor suppressor p53, is able to induce apoptosis in several cell types. This property is important for the involvement of p73 in cancer development and therapy. However, in contrast with p53, the TAp73 gene has two distinct promoters coding for two protein isoforms with opposite effects: while the transactivation proficient TAp73 shows pro-apoptotic effects, the amino-terminal-deleted DeltaNp73 has an anti-apoptotic function. Indeed, the relative expression of these two proteins is related to the prognosis of several cancers. Here we discuss recent developments in the control of p73-induced apoptosis. First, TAp73 induces ER stress via the direct transactivation of Scotin. Second, TAp73 induces the mitochondrial pathway by directly transactivating both Bax and the BH3 only protein PUMA promoters. While the first transactivation is weak, and not sufficient to trigger apoptosis (at least in the in vitro cellular models so far evaluated), the induction of PUMA is strong and lethal. Third, the promoter of the death receptor CD95 contains a p53 responsive element and preliminary experiments suggest that TAp73 also activates the death receptor pathway. In addition, TAp73 is able to transactivate its own second promoter, thus inducing the expression of the anti-apoptotic DeltaNp73 isoform. Therefore, the balance between TAp73 and DeltaNp73 finely regulates cellular sensitivity to death.

Epidermolytic palmoplantar keratoderma (EPPK) is an autosomal dominant skin disorder characterized by hyperkeratosis of the palms and soles associated with histologic findings of hyperkeratosis and epidermolysis. Ultrastructurally, there is vacuolization of the cytoplasm and abnormal keratin filament network with tonofilament clumping. EPPK is caused by mutations in the keratin 9 gene (KRT9), which is expressed exclusively in suprabasal keratinocytes of palmoplantar epidermis. The mutation R162W is the most frequent keratin 9 alteration reported in patients from different geographical areas. We present three unrelated Italian families affected by EPPK in which we confirmed the presence of the R162W mutation, by RT-PCR analysis followed by sequencing of the KRT9 gene, in all affected members. The finding of the same mutation in all patients, together with the previous reports of the disease, strongly suggest that position 162 of the KRT9 gene represents a mutation "hot-spot", probably due to the peculiarity of the sequence.

Transglutaminases (TGases) are seven enzymes, cross-linking proteins by gamma-glutamil-epsilon-lysine bonds, four of which are expressed in the skin. A new member of the TGase family, TGase 5, has been identified recently, and in the present study we evaluated its role in keratinocyte differentiation in vitro. In addition to the previously described isoforms, full-length TGase 5 and Delta3 (deletion of exon 3), we identified two new splicing variants, Delta11 and Delta3Delta11 (deletion of exons 11 or 3, 11). We expressed full-length TGase 5, Delta3, Delta11, and Delta3Delta11 isoforms in the keratinocyte and baculovirus systems. The results indicate that both full-length TGase 5 and Delta11 are active, whereas Delta3 and Delta3Delta11 have very low activity. Expression studies show that full-length TGase 5 is induced during the early stages of keratinocyte differentiation and is differently regulated in comparison with the other epidermal TGases. Kinetic and in vitro cross-linking experiments indicate that full-length TGase 5 is very efficient in using specific epidermal substrates (loricrin, involucrin, and SPR3). In keratinocyte expression system, TGase 5 isoforms are retained in an intermediate filament-enriched fraction, suggesting its association with insoluble proteins. Indeed, TGase 5 co-localize with vimentin and it is able to cross-link vimentin in vitro.

White Sponge Nevus (WSN) is a rare, autosomal dominant disorder that predominantly affects noncornified stratified squamous epithelia. Clinically, it is characterized by the presence of soft, white, and ‘‘spongy’’ plaques in the oral mucosa. The characteristic histopathologic features are epithelial thickening, parakeratosis, and vacuolization of the suprabasal layer of oral epithelial keratinocytes. Mutations in keratin 4 (K4) and keratin 13 (K13) genes have already been demonstrated to be responsible for WSN; the identification of new keratin mutations in a stratified squamous epithelia closely related to epidermis is of relevance for the understanding of the biochemistry of intermediate filaments, and for genotype phenotype correlations. In this study we investigated a 27-y-old, female Italian patient, affected by white asymptomatic oral plaques. Sequence analysis revealed a 3 bp (ACA) heterozygous insertion localized in the helix initiation motif of the 1A alpha helical domain of K4. We report this new K4 gene mutation and describe an amino acid insertion, in the 1A domain, responsible for a keratin disease.

Several in vivo and in vitro experimental model systems demonstrate a direct relationship between the expression and activity of tissue transglutaminase [tTG; also called transglutaminase type 2 (TGase 2)] and programmed cell death or apoptosis. This is based on mRNA and protein studies, sense and antisense transfection, identification of N epsilon-(gamma-glutamyl)-lysine cross-links in extracted apoptotic bodies, and in blue mouse experiments. In the epidermis, apoptosis occurs under particular conditions in the proliferative basal layer with the involvement of the tTG enzyme. However, in epidermal keratinocytes other TGases (TGase 1, TGase 3, and perhaps TGase X) are normally activated in a terminal differentiation program, called cornification, that leads to cell death. These cells perform their functions after death, providing an elastic physical and permeability barrier to the skin. In fact, TGase 1 mutations cause the skin disease lamellar ichthyosis. Because all TGases share strong similarities in structure and function, being involved in mechanisms of cell death, this chapter describes the current assays for TGases at the mRNA, protein, and enzymatic levels. We also describe procedures to produce, purify, and characterize recombinant TGases, to identify mutation in disease, to isolate cross-linked bodies, and to analyze the N epsilon-(gamma-glutamyl)-lysine isopeptide cross-links. Finally, we discuss general rules for the interpretation and comparison of these events in cell death.

The cell envelope (CE) is a vital structure for barrier function in terminally differentiated dead stratified squamous epithelia. It is assembled by transglutaminase (TGase) cross-linking of several proteins, including hSPR3 in certain specialized epithelia normally subjected to mechanical trauma. Biochemical studies show that hSPR3 serves as a complete substrate for TGase1, TGase2, and TGase3. Multiple adjacent glutamines and lysines of only head-and-tail domain sequences are used by each enzyme for cross-linking. Structural data suggest that the hSPR3 central repeats, as well as hSPR1 and hSPR 2, are highly flexible and mobile; thus, the TGases might not be able to recognize the residues localized on the repeats as adequate substrate. To investigate this hypothesis further and to complete the structural investigation of hSPR3, we performed circular dichroism (CD) studies on peptides corresponding to the N- and C-terminal domain. CD spectra have also been carried out in the presence of different concentrations of the structure-promoting agent cosolvent trifluoroethanol (TFE), which mimics a partial hydrophobic environment found in vivo in or next to the membrane. In fact, this agent increases the dielectric constant of water proportionally, depending on its concentration, and confers structuring properties to the solution, to peptides and proteins that have a structuring propensity. The results indicate that in both the N-terminal and C-terminal, peptides acquire a more ordered structure as a function of the TFE concentration in water. This ability of both N- and C-terminal domain to acquire a more stable ordered conformation might be relevant for SPR3 to act as substrate of TGases. Indeed, only the N- and C-terminus is cross-linked by TGase1 and 3.

The cell envelope (CE) is a vital structure for barrier function in terminally differentiated dead stratified squamous epithelia. It is assembled by transglutaminase (TGase) cross-linking of several proteins, including SPR3 in certain specialized epithelia normally subjected to mechanical trauma. We have expressed recombinant human SPR3 in order to study its cross-linking properties. It serves as a complete substrate for, and is cross-linked at similar efficiencies by, the three enzymes (TGases 1, 2 and 3) that are widely expressed in many epithelia. Multiple adjacent glutamines (4, 5, 16, 17, 18, 19 and 167) and lysines (6, 21, 164, 166 and 168) of only head and tail domain sequences are used for cross-linking. However, each enzyme preferentially uses certain residues on the head domain. Moreover, our in vitro data suggest a defined temporal order of cross-linking of SPR3 in vivo: It is first cross-linked by TGase 3 into short intra- and inter-chain oligomers which are later further cross-linked to the CE by TGase 1. To investigate the absence of cross-linking in the central domain (e.g. lysine in position 2 of each of the 16 repeats) we performed structural studies on recombinant SPR3 and on a synthetic peptide containing three repeats of the central domain. 2D H-1 NMR spectroscopy, TOCSY and ROESY, shows strong and medium intensity NOEs connectivities along the amino acid sequence with one weak long range NOE contact between Thr and Cys of subsequent repeats. Distance geometry computation on the basis of intensities of NOEs found generated 50 compatible structures grouped in three main families differing by the number of H-bonds. These measurements were repeated at different concentrations of trifluoroethanol (TFE)-water mixture, an alpha-helical promoting solvent, in order to check the stability of the conformations determined; no changes were observed up to 50% TFE in solution. Also temperature changes did not produce any variation in the ROESY spectrum in the same condition as above. The NMR and circular dichroism data strongly indicate the presence of an ordered (not alpha-helix nor beta-sheet) highly flexible structure in the eight amino acids repetitive units of SPR3, confirming the prediction of one possible beta-turn per each repeating unit. Thus, biochemical and biophysical data, strongly support SPR3 to function as a flexible cross-bridging protein to provide tensile strength or rigidity to the CE of the stratified squamous epithelia in which it is expressed.

The cornified cell envelope (CE) is a crucial structure for barrier function in terminally differentiated dead stratified squamous epithelia. It is assembled by transglutaminase enzymes (TGases) that cross-link several proteins such as loricrin and the small proline rich (SPR) proteins. Human SPR2 protein is cross-linked with widely differing efficiencies by TGases 1, 2, and 3 using exclusively residues in the N- and C-terminal domains. In order to understand if the absence of the cross-linking catalyzed by TGases in the central domain is due to the conformation adopted, we have investigated the structural properties in solution of three peptides that correspond to the N-terminal domain, to three repeats of the central domain, and to the C-terminal domain. Together, the NMR and CD data strongly indicate the presence of a highly flexible non alpha-helix, non beta-sheet structure in SPR2. Thus, SPR2 appears to function as a flexible cross-bridging protein to provide tensile strength or rigidity to the CE of the stratified squamous epithelia in which it is expressed.

The small proline-rich (SPR) proteins are components of the cornified cell envelope of stratified squamous epithelia and become cross-linked to other proteins by transglutaminases (TGases). The SPR2 family is the most complex, as it consists of several differentially expressed members of the same size. To explore their physical and cross-linking properties, we have expressed in bacteria a human SPR2 family member, and purified it to homogeneity. By circular dichroism, it possesses no alpha or beta structure but has some organized structure associated with the central peptide repeat domain. The TGase 1, 2, and 3 enzymes expressed in epithelia use the recombinant SPR2 protein as a complete substrate in vitro, but with widely differing kinetic efficiencies, and in different ways. With TGase 1, only one glutamine on the head domain and one lysine on the tail domain were used for limited interchain cross-linking. With TGase 3, multiple head and tail domain residues were used for extensive interchain cross-linking. The total usage of glutamine and lysine residues in vitro by TGase 3 was similar to that seen in earlier in vivo studies. We conclude that SPR2 proteins are cross-linked in epithelia primarily by the TGase 3 enzyme, a minor extent by TGase 1, and probably not by TGase 2.

We have addressed the question of how keratin intermediate filaments are associated with the cell envelope at the periphery of cornified epidermal cells. Many peptides from human epidermal cell envelopes containing isopeptide crosslinks inserted by transglutaminases in vivo have been characterized. A major subset involves the type II keratin chains keratin 1, 2e, 5, or 6 crosslinked to several protein partners through a lysine residue located in a conserved region of the V1 subdomain of their head domains. This sequence specificity was confirmed in in vitro crosslinking experiments. Previously the causative mutation in a family with diffuse nonepidermolytic palmar-plantar keratoderma was shown to be the loss in one allele of the same lysine residue of the keratin 1 chain. Ultrastructural studies of affected palm epidermis have revealed abnormalities in the organization of keratin filaments subjacent to the cell envelope and in the shape of the cornified cells. Together, these data suggest a mechanism for the coordination of cornified cell structure by permanent covalent attachment of the keratin intermediate filament cytoskeleton to the cell envelope by transglutaminase crosslinking. Furthermore, these studies identify the essential role of a conserved lysine residue on the head domains of type II keratins in the supramolecular organization of keratin filaments in cells.

The transcription factor p63 is expressed as at least six different isoforms, of which two have been assigned critical biological roles within ectodermal development and skin stem cell biology on the one hand and supervision of the genetic stability of oocytes on the other hand. These two isoforms contain a C-terminal inhibitory domain that negatively regulates their transcriptional activity. This inhibitory domain contains two individual components: one that uses an internal binding mechanism to interact with and mask the transactivation domain and one that is based on sumoylation. We have carried out an extensive alanine scanning study to identify critical regions within the inhibitory domain. These experiments show that a stretch of ~13 amino acids is crucial for the binding function. Further, investigation of transcriptional activity and the intracellular level of mutants that cannot be sumoylated suggests that sumoylation reduces the concentration of p63. We therefore propose that the inhibitory function of the C-terminal domain is in part due to direct inhibition of the transcriptional activity of the protein and in part due to indirect inhibition by controlling the concentration of p63. Keywords: p63, transcriptional regulation, auto-inhibition, sumoylation

In this report, we show that the overexpression of tissue transglutaminase (tTG) in the human neuroblastoma cell line SK-N-BE(2) renders these neural crest-derived cells highly susceptible to death by apoptosis. Cells transfected with a full-length tTG cDNA, under the control of a constitutive promoter, show a drastic reduction in proliferative capacity paralleled by a large increase in cell death rate. The dying tTG-transfected cells exhibit both cytoplasmic and nuclear changes characteristic of cells undergoing apoptosis. The tTG-transfected cells express high Bcl-2 protein levels as well as phenotypic neural cell adhesion molecule markers (NCAM and neurofilaments) of cells differentiating along the neuronal pathway. In keeping with these findings, transfection of neuroblastoma cells with an expression vector containing segments of the human tTG cDNA in antisense orientation resulted in a pronounced decrease of both spontaneous and retinoic acid (RA)-induced apoptosis. We also present evidence that (i) the apoptotic program of these neuroectodermal cells is strictly regulated by RA and (ii) cell death by apoptosis in the human neuroblastoma SK-N-BE(2) cells preferentially occurs in the substrate-adherent phenotype. For the first time, we report here a direct effect of tTG in the phenotypic maturation toward apoptosis. These results indicate that the tTG-dependent irreversible cross-linking of intracellular protein represents an important biochemical event in the induction of the structural changes featuring cells dying by apoptosis.