Modelling atopic dermatitis during the morphogenetic process involved in reconstruction of a human epidermis

Most crucial role of epidermis is to maintain efficient barrier between the organism and its environment. This barrier is however perturbed in inflammatory skin conditions like atopic dermatitis (AD), one common chronic disease. This review depicts characteristics of a model intending to reproduce epidermal features of AD in vitro. Firstly, methyl-β-cyclodextrin (MβCD) during reconstruction of epidermis was used to deplete cholesterol from plasma membrane because this condition reproduces characteristics of AD at transcriptomic level in monolayer cultures. Major changes are confirmed after same treatment inside reconstructed human epidermis (RHE). However, since early treatment do not reveal impairment to reconstruct a functional epidermal barrier and given the importance of the Th2 dysregulated immune response in AD, cholesterol-depleted RHE at day 11 of reconstruction were then incubated with three Th2-related cytokines (IL-4, IL-13 and IL-25) previously reported as playing important roles in the development of AD, as well as altering overall function of epidermal barrier. When combining both treatments, essential epidermal features of AD are observed. Indeed, RHE then exhibit spongiosis, disappearing granular layer, alteration of barrier function, as well as dysregulated expression levels for genes involved in AD pathogenesis. Moreover, while trying to identify individual roles for each component used to create AD-like alterations, incubation with IL-4 following cholesterol depletion from plasma membrane was found inducing most of the reported alterations. This model suggests potential for better investigations of epidermal AD features and may be considered for eventual in vitro screening of cosmetics or therapeutic compounds.

The role of reelin in the development and evolution of the cerebral cortex

Reelin is an extracellular matrix protein that is defective in reeler mutant mice and plays a key role in the organization of architectonic patterns, particularly in the cerebral cortex. In mammals, a "reelin signal" is activated when reelin, secreted by Cajal-Retzius neurons, binds to receptors of the lipoprotein receptor family on the surface of cortical plate cells, and triggers Dab1 phosphorylation. As reelin is a key component of cortical development in mammals, comparative embryological studies of reelin expression were carried out during cortical development in non-mammalian amniotes (turtles, squamates, birds and crocodiles) in order to assess the putative role of reelin during cortical evolution. The data show that reelin is present in the cortical marginal zone in all amniotes, and suggest that reelin has been implicated in the evolution of the radial organization of the cortical plate in the synapsid lineage leading from stem amniotes to mammals, as well as in the lineage leading to squamates, thus providing an example of homoplastic evolution (evolutionary convergence). The mechanisms by which reelin instructs radial cortical organization in these two lineages seem different: in the synapsid lineage, a drastic amplification of reelin production occurred in Cajal-Retzius cells, whereas in squamates, in addition to reelin-secreting cells in the marginal zone, a second layer of reelin-producing cells developed in the subcortex. Altogether, our results suggest that the reelin-signaling pathway has played a significant role in shaping the evolution of cortical development.

Developmental expression profiles of Celsr (Flamingo) genes in the mouse

Celsr, also called Flamingo (Fmi) genes encode proteins of the cadherin superfamily. Celsr cadherins are seven-pass transmembrane proteins with nine cadherin repeats in the extracellular domain, and an anonymous intracellular C-terminus. The Drosophila Fmi gene regulates epithelial planar cell polarity and dendritic field deployment. The three Flamingo gene orthologs in man and rodents are named, respectively, CELSR1-3 and Celsr1-3. Celsr1 and 2 are expressed during early development, in the brain and epithelia. In this report, we characterized further Celsr genes in the mouse, and examined their developmental pattern of expression. Each Celsr is expressed prominently in the developing brain following a specific pattern, suggesting that they serve distinct functions.

Neuronal migration

Like other motile cells, neurons migrate in three schematic steps, namely leading edge extension, nuclear translocation or nucleokinesis, and retraction of the trailing process. In addition, neurons are ordered into architectonic patterns at the end of migration. Leading edge extension can proceed at the extremity of the axon, by growth cone formation, or from the dendrites, by formation of dendritic tips. Among both categories of leading edges, variation seems to be related to the rate of extension of the leading process. Leading edge extension is directed by microfilament polymerization following integration of extracellular cues and is regulated by Rho-type small GTPases. In humans, mutations of filamin, an actin-associated protein, result in heterotopic neurons, probably due to defective leading edge extension. The second event in neuron migration is nucleokinesis, a process which is critically dependent on the microtubule network, as shown in many cell types, from slime molds to vertebrates. In humans, mutations in the PAFAH1B1 gene (more commonly called LIS1) or in the doublecortin (DCX) gene result in type 1 lissencephalies that are most probably due to defective nucleokinesis. Both the Lis1 and doublecortin proteins interact with microtubules, and two Lis1-interacting proteins, Nudel and mammalian NudE, are components of the dynein motor complex and of microtubule organizing centers. In mice, mutations of Cdk5 or of its activators p35 and p39 result in a migration phenotype compatible with defective nucleokinesis, although an effect on leading edge formation is also likely. The formation of architectonic patterns at the end of migration requires the integrity of the Reelin signalling pathway. Other known components of the pathway include members of the lipoprotein receptor family, the intracellular adaptor Dab1, and possibly integrin alpha 3 beta 1. Defective Reelin leads to poor lamination and, in humans, to a lissencephaly phenotype different from type 1 lissencephaly. Although the action of Reelin is unknown, it may trigger some recognition-adhesion among target neurons. Finally, pattern formation requires the integrity of the external limiting membrane, defects of which lead to overmigration of neurons in meninges and to human type 2 lissencephaly.

The evolution of cortical development. An hypothesis based on the role of the Reelin signaling pathway

Expression of the genes encoding Reelin and Dab1 during cortical development in turtle, lizard, chick and mammals correlates with architectonic patterns. In all species, Reelin is secreted by marginal zone cells, whereas Dab1, which mediates the response to Reelin, is synthesized by cortical plate neurons. This pattern was presumably present in stem amniotes. In mammals, the cortical plate is radially organized and develops from inside to outside, these features depend on amplification of reelin synthesis in the marginal zone. In lizards, the cortical plate develops from outside to inside, similar to other non-mammals, but is radially organized, with an additional layer of Reelin added in the subcortex. Thus, the Reelin pathway played a key role in cortical architectonic evolution in mammalian and squamate lineages.

The Reelin signaling pathway in mouse cortical development

Most of the cerebral cortex derives from the cortical plate which, in all mammals, is radially organized and develops from inside to outside. Several genes involved in the organization and inside-outside development of the embryonic cortical plate in the mouse form the so-called Reelin signaling pathway. Biochemical and genetic arguments show that the extracellular matrix protein Reelin binds to two lipoprotein receptors (VLDLR and ApoER2), which relay the Reelin signal inside target neurons by docking the tyrosine kinase adapter disabled-1 (Dab1). In addition, biochemical evidence suggests that the integrins alpha 3/beta 1 and protocadherins of the CNR family may also modulate the Reelin signal. The mechanisms by which the presence of Reelin stops migration and instructs the radial organization of cortical plate cells remains unknown.

Evolutionarily conserved, alternative splicing of reelin during brain development

Reelin is the protein defective in reeler mutant mice and plays a pivotal role in brain development. However, some uncertainties remain about the relationship between reelin and the reeler phenotype. It is generally believed that reelin, secreted by specific neuronal types such as Cajal-Retzius cells, acts at short distance via the extracellular matrix on target neurons, the response of which requires the Dab1 gene product. However, the pattern of reelin expression in some structures such as olfactory bulb, retina, and spinal cord suggests that the protein might be endowed with different functions. In the present study, we identify two uncommon, evolutionarily conserved splicing events in the 3' part of the transcript that result in different forms of the protein. First, a 6-nucleotide, brain-specific microexon is skipped in about 10% of reelin RNA. In addition, an alternative polyadenylation event involving 10-25% of reelin mRNA results in secretion of a truncated protein lacking the terminal, highly basic stretch. This alternative reelin is generally expressed in the same cells as the major form, but is almost undetectable in retina and spinal cord. Both alternative splicing events are present in mouse, rat, and man, suggesting that the corresponding reelin forms are functionally important.

Reelin, the extracellular matrix protein deficient in reeler mutant mice, is processed by a metalloproteinase

Reelin is the extracellular protein defective in reeler mice. It is believed that reelin acts via the extracellular matrix to influence the development of nearby neurons, but the mechanism remains thus far unknown. In the present work, we present in vivo and in vitro evidence that reelin is cleaved. This processing did not occur in Relnrl-Orl mutant mice in which reelin is not secreted and was prevented in explant cultures by brefeldin treatment, suggesting that it takes place extracellularly or in a postendoplasmic reticulum compartment. Reelin cleavage was inhibited by zinc chelators known to inhibit metalloproteinases but was unaffected by inhibitors of serine, cysteine, or aspartate proteinases. Furthermore, reelin cleavage was insensitive to inhibitors of matrixins, neprilysin, meprin, and peptidyl dipeptidase A, suggesting that the processing enzyme belongs to a different enzyme family. This enzyme and the physiological meaning of reelin processing remain to be characterized further.

A new view of early cortical development

Recently, several genes that regulate the development of the cerebral cortex and are potential pharmacological targets have been cloned. Reelin, an extracellular matrix glycoprotein secreted by Cajal-Retzius cells in the marginal zone, instructs the radial organization of the cortical plate. The response of cortical plate cells to reelin requires the tyrosine kinase adaptor disabled-1 (Dab1). Cyclin-dependent kinase 5 and its activator p35 are necessary for the development of the cortical plate, probably at a later stage than reelin/Dab1. The transcription factor Tbr-1 is essential for differentiation of preplate and Cajal-Retzius cells and for formation of thalamocortical connections, while D1x-1/2 are required for tangential migration. Some neurotrophin systems such as neurotrophin 4, brain-derived neurotrophic factor, and neuregulin and its receptor ErbB are also thought to assist in the regulation of cortical development. In addition, a few genes implicated in human cortical dysplasias have been characterized. LIS1 encodes a protein related to platelet-activating factor acetyl hydrolase that is defective in lissencephaly-1 of the Miller-Dieker type, while the double cortex malformation is related to mutations of a new gene dubbed doublecortn.

A panel of monoclonal antibodies against reelin, the extracellular matrix protein defective in reeler mutant mice

Reelin, the extracellular matrix protein defective in reeler mutant mice, plays a key role during brain development. We therefore raised antibodies directed against various reelin epitopes in order to facilitate biochemical and cell biological studies of this important molecule. Homozygous reeler mice with a large deletion of most of the reelin gene were immunized with fusion proteins and carrier-coupled peptides corresponding to parts of the reelin sequence. Monoclonal antibodies were produced using classical procedures, screened using ELISA and-or western blot prepared with the antigen, and tested by immunohistochemistry and immunoprecipitation assays to detect endogenous reelin. The labeling of Cajal-Retzius cells in the embryonic mouse telencephalon was selected as criterion for positivity in immunohistochemistry. A total of 11 monoclonal antibodies were obtained, providing useful additions to the widely used antibody CR-50. Five are directed against the N-terminal part of reelin, among which three recognize the region that has significant similarity with F-spondin, and two are specific for hinge region located downstream from the F-spondin similarity region and upstream from the reelin repeats. Six antibodies are directed against the C-terminal part of reelin, among which one anti-peptide antibody recognizes the highly basic C-terminal segment. Antibodies against the N-terminal region stain well in immunohistochemistry. By comparison, the labeling of embryonic Cajal-Retzius cells with antibodies directed against the C-terminal region is weaker, suggesting that this part of the molecule might be modified or not be as readily accessible in the tissue as the N-terminus.

Genomic organization of the mouse reelin gene

Reelin is the protein defective in reeler mice, an extensively studied model of brain development. The reelin gene (symbol Reln) codes for a protein of the extracellular matrix that contains eight successive repeats of 350 to 390 amino acids. In this work, we describe the genomic structure of the mouse reelin gene and the 5'-flanking genomic DNA sequences. The reelin gene is composed of 65 exons spread over approximately 450 kb of genomic DNA. We identified different reelin transcripts, formed by alternative splicing of a microexon as well as by use of two different polyadenylation sites. All splice sites conform to the GT-AG rule, except for the splice donor site of intron 30, which is GC instead of GT. A processed pseudogene is present in intron 42. Its nucleotide sequence is 86% identical to the sequence of the rat RDJ1 cDNA, which codes for a DnaJ-like protein of the Hsp40 family. Comparison of 8 intron positions in mouse and human reelin genes reveals a highly conserved genomic structure, suggesting a similar structure of the whole gene in both species. We identified two transcription start sites embedded within a CpG. The promoter region contains putative recognition sites for the transcription factors Sp1 and AP2 but lacks TATA and CAAT boxes. The presence of tandemly repeated regions in the Reelin protein suggests that gene duplication events occurred during evolution. By comparison of the amino acid sequences of the eight repeats and the positions of introns, we suggest a model for the evolution of the repeat coding portion of the reelin gene from a putative ancestral minigene.

A truncated Reelin protein is produced but not secreted in the 'Orleans' reeler mutation (Reln[rl-Orl])

Reelin is the protein defective in reeler mutant mice [I. Bar, C. Lambert de Rouvroit, I. Royaux, D.B. Krizman, C. Dernoncourt, D. Ruelle, M.C. Beckers, A.M. Goffinet, A YAC contig containing the reeler locus with preliminary characterization of candidate gene fragments, Genomics 26 (1995) 543-549; G. D'Arcangelo, G.G. Miao, S.C. Chen, H.D. Soares, J.I. Morgan, T. Curran, A protein related to extracellular matrix proteins deleted in the mouse mutant reeler, Nature 374 (1995) 719-723; S. Hirotsune, T. Takahara, N. Sasaki, K. Hirose, A. Yoshiki, T. Ohashi, M. Kusakabe, Y. Murakami, M. Muramatsu, S. Watanabe, K. Nakao, M. Katsuki, Y. Hayashizaki, The reeler gene encodes a protein with an EGF-like motif expressed by pioneer neurons, Nature Genet. 10 (1995) 77-83]. In the Orleans allele of reeler (symbol: Reln[rl-Orl]), a 220 nucleotide deletion is present in the 3' region of the Reelin message, resulting in a frame shift with production of a predicted protein amputated from its C-terminal amino acids. In this study, we first show that the predicted truncated protein indeed exists in Orleans reeler mice, using several anti-Reelin antibodies. Three antibodies are directed against epitopes located in the N-terminal region of the protein, namely: monoclonal antibody CR-50 [M. Ogawa, T. Miyata, K. Nakajima, K. Yagyu, M. Seike, K. Ikenaka, H. Yamamoto, K. Mikoshiba, The reeler gene-associated antigen on Cajal-Retzius neurons is a crucial molecule for laminar organization of cortical neurons, Neuron 14 (1995) 899-912] (epitope region between Reelin residues 251-407), monoclonal antibody G10 (epitope located between amino acids 199 and 244) and the polyclonal antipeptide rp4 (positions 381-399). A fourth antibody, antipeptide rp5, reacts with the C-terminal (3443-3461) Reelin sequence. In normal embryos, all four antibodies stained cells in the marginal zone with features of Cajal-Retzius cells. While N-terminal specific antibodies detected Reelin immunoreactivity in mouse embryos homozygous for the reeler-Orleans mutation, no staining was obtained with the rp5 antibody, showing the presence of a truncated protein. Moreover, although Reelin could be detected at the surface of living Cajal-Retzius cells of normal mice, it was not revealed after vital staining of embryonic cortex from Orleans reeler mice. These results indicate that the C-terminal region of Reelin is essential for its secretion and suggest that the Orleans reeler phenotype is due to defective Reelin secretion rather than to secretion of an inactive protein.

Aberrant splicing of a mouse disabled homolog, mdab1, in the scrambler mouse

Although accurate long-distance neuronal migration is a cardinal feature of cerebral cortical development, little is known about control of this migration. The scrambler (scm) mouse shows abnormal cortical lamination that is indistinguishable from reeler. Genetic and physical mapping of scm identified yeast artificial chromosomes containing an exon of mdab1, a homolog of Drosophila disabled, which encodes a phosphoprotein that binds nonreceptor tyrosine kinases. mdab1 transcripts showed abnormal splicing in scm homozygotes, with 1.5 kb of intracisternal A particle retrotransposon sequence inserted into the mdab1 coding region in antisense orientation, producing a mutated and truncated predicted protein. Therefore, mdab1 is most likely the scm gene, thus implicating nonreceptor tyrosine kinases in neuronal migration and lamination in developing cerebral cortex.

VirG, a Yersinia enterocolitica lipoprotein involved in Ca2+ dependency, is related to exsB of Pseudomonas aeruginosa

Pathogenic yersiniae require Ca2+ for growth at 37 degrees C. They harbor closely related plasmids of about 70 kb that are essential for virulence. At 37 degrees C and in the absence of Ca2+ ions, these plasmids cause a decrease in growth rate and the release of large amounts of proteins called Yops. Here we describe the virG gene of Yersinia enterocolitica; virG is located just upstream of the virF gene, which encodes the transcriptional activator of some plasmid virulence factors. Analysis of the VirG amino acid sequence suggested that virG encodes a lipoprotein, which was confirmed by [3H]palmitate labeling of VirG-PhoA fusion proteins. A nonpolar virG mutant was constructed and found to be Ca2+ independent for growth at 37 degrees C but to still secrete Yops. This phenotype was complemented by the introduction of a plasmid harboring an intact virG gene. VirG was found to be homologous to ExsB, a protein encoded by a Pseudomonas aeruginosa gene located in the locus controlling exoenzyme S synthesis. Interestingly, the exsA gene, located just downstream of exsB, is also homologous to virF.

Role of the transcriptional activator, VirF, and temperature in the expression of the pYV plasmid genes of Yersinia enterocolitica

Upon incubation at 37 degrees C in the absence of Ca2+ ions, pathogenic yersiniae release high amounts of pYV plasmid-encoded proteins called Yops, involved in pathogenesis. Yersinia enterocolitica also express two outer membrane proteins, an adhesin called YadA and a lipoprotein called YlpA. The production of the Yops is co-ordinately regulated by a 20 kb region of the plasmid referred to as the 'Ca2+ dependence region' and containing at least four loci called virA, virB, virC, and virF. The 8.5 kb virC region, involved in the specific transport of the Yops, is a single operon containing 13 open reading frames called yscA to yscM. Gene virF encodes a key transcriptional activator of the yop, yadA and ylpA genes. It is only transcribed at 37 degrees C and its expression is modulated by a chromosome-encoded histone-like protein called YmoA. We show here that virF also controls the virC operon. By contrast, virF is not essential for the induction of virA and virB. The VirF protein binds specifically to yop promoters. In particular, it protects the region spanning nucleotides -64 to -34 of yopH. In order to analyse the role of temperature in the induction of the yop regulon, we constructed Y. enterocolitica strains expressing virF from the tac promoter. In spite of the fact that virF was transcribed at 25 degrees C, neither the Yops nor YadA were expressed at that temperature. This poor response to VirF at 25 degrees C was at least partially due to a weak and slow transcription of the genes controlled by virF. Surprisingly, when cloned on pACYC184, gene yadA was expressed even in absence of VirF, but remained thermodependent. Hence temperature and virF are both required for the induction of the yop regulon. Among other possible roles, temperature could modify the structure of either the activator itself or the yop promoter. The fact that VirF binds in vitro to yop promoters at 25 degrees C rules out the first hypothesis. In order to test the second hypothesis, we studied, in vivo, the activity of the yopH promoter in ymoA mutants. The yopH promoter became active in the absence of VirF, indicating that yop promoter activity depends upon chromatin structure. We conclude from these two observations that, in vivo, temperature is required to modify the DNA structure of the yop promoters in order to allow the action of the transcriptional activator.

Analysis of virC, an operon involved in the secretion of Yop proteins by Yersinia enterocolitica

Upon incubation at 37 degrees C in the absence of Ca2+ ions, pathogenic yersiniae release large amounts of pYV plasmid-encoded proteins called Yops that are involved in pathogenesis. Yersinia enterocolitica also expresses an outer membrane protein that is considered an adhesin and called YadA (previously called P1 or YopA). The production of Yops is coordinately regulated by a 20-kb region of the plasmid referred to as the Ca2+ dependence region and containing at least four loci called virA, virB, virC, and virF. The virF gene encodes a key transcriptional activator of yop genes. We have shown here that virF is also required for transcription of yadA and that virB is necessary for full transcription of the yop and yadA genes. In contrast, mutations in genes virA and virC had only a weak influence on the transcription of yop and yadA genes. These mutations did not affect the production of YadA but they completely inhibited the translocation of Yops from the intracellular compartment to the extracellular milieu. We inferred from these data that virA and virC are involved in the specific transport of Yops. We analyzed the 8.5-kb virC region and showed that it is most probably a single operon containing 13 open reading frames called yscA to yscM (for Yop secretion). Protein YscC has a putative signal sequence and shares significant homology with outer membrane proteins involved in the secretion of pullulanase by Klebsiella pneumoniae (PulD) or in the assembly of filamentous bacteriophages (gene IV product). At least the putative products of yscD, yscJ, and yscL were shown to be required for the export of Yops. YscJ turned out to be YlpB, a lipoprotein that we had detected previously. The yscM gene shares homology with yopH, the adjacent gene on the pYV plasmid. Its product does not appear to be necessary for the production of Yops. Transcription of the virC operon was subjected to the same regulation as the yop genes.

ymoA, a Yersinia enterocolitica chromosomal gene modulating the expression of virulence functions

The virulence functions of Yersinia enterocolitica include the pYV-encoded Yop proteins and YadA adhesin as well as the chromosome-encoded enterotoxin, Yst. The yop and yadA genes form a temperature-activated regulon controlled by the transcriptional activator VirF. Gene virF, also localized on pYV, is itself thermoinduced in the absence of other pYV genes. The enterotoxin yst gene is silent in some collection strains including strain W22703. This paper describes two Tn5-Tc1 chromosomal insertion mutants of W22703 transcribing virF, and hence the yop and yadA genes, at low temperature. These mutants also resumed their production of Yst, with its typical temperature dependence. Both mutations were insertions in the same gene called ymoA for 'Yersinia modulator'. The cloned ymoA gene fully complemented the two mutations. Several properties of the mutants suggest that ymoA encodes a histone-like protein. According to the nucleic acid sequence, the product of ymoA is an 8064 Da protein rich in aspartic acid (9%), glutamic acid (9%) and lysine (10.5%), but the predicted amino acid sequence shows no similarity with any described histone-like protein. This work supports recent reports which propose a role for DNA topology and bacterial chromatin structure in thermoregulation of virulence functions.

The Yersinia yop regulon

Growth of yersiniae is restricted at 37 degrees C in the absence of calcium ions. This phenomenon correlates with the massive release of a set of proteins called Yops. Growth restriction and Yops production are governed by a 70 kb plasmid called pYV. yop genes are distributed throughout pYV and constitute a thermoactivated regulon controlled by the gene virF. The transcription activator VirF is a member of a new family of regulators including those of the arabinose and rhamnose operons as well as a regulator of enteric colonization pili. The role of calcium ions on the release of Yops remains largely unknown.