β1 Integrin-Mediated Adhesion Signalling Is Essential for Epidermal Progenitor Cell Expansion

Background: There is a major discrepancy between the in vitro and in vivo results regarding the role of b1 integrins in the maintenance of epidermal stem/progenitor cells. Studies of mice with skin-specific ablation of b1 integrins suggested that epidermis can form and be maintained in their absence, while in vitro data have shown a fundamental role for these adhesion receptors in stem/progenitor cell expansion and differentiation. Methodology/Principal Findings: To elucidate this discrepancy we generated hypomorphic mice expressing reduced b1 integrin levels on keratinocytes that developed similar, but less severe defects than mice with b1-deficient keratinocytes. Surprisingly we found that upon aging these abnormalities attenuated due to a rapid expansion of cells, which escaped or compensated for the down-regulation of b1 integrin expression. A similar phenomenon was observed in aged mice with a complete, skin-specific ablation of the b1 integrin gene, where cells that escaped Cre-mediated recombination repopulated the mutant skin in a very short time period. The expansion of b1 integrin expressing keratinocytes was even further accelerated in situations of increased keratinocyte proliferation such as wound healing. Conclusions/Significance: These data demonstrate that expression of b1 integrins is critically important for the expansion of epidermal progenitor cells to maintain epidermal homeostasis

High-Throughput Proteomics Detection of Novel Splice Isoforms in Human Platelets

Alternative splicing (AS) is an intrinsic regulatory mechanism of all metazoans. Recent findings suggest that 100% of multiexonic human genes give rise to splice isoforms. AS can be specific to tissue type, environment or developmentally regulated. Splice variants have also been implicated in various diseases including cancer. Detection of these variants will enhance our understanding of the complexity of the human genome and provide disease-specific and prognostic biomarkers. We adopted a proteomics approach to identify exon skip events - the most common form of AS. We constructed a database harboring the peptide sequences derived from all hypothetical exon skip junctions in the human genome. Searching tandem mass spectrometry (MS/MS) data against the database allows the detection of exon skip events, directly at the protein level. Here we describe the application of this approach to human platelets, including the mRNA-based verification of novel splice isoforms of ITGA2, NPEPPS and FH. This methodology is applicable to all new or existing MS/MS datasets

Induction of initial steps of angiogenic differentiation and maturation of endothelial cells by pericytes in vitro and the role of collagen IV

Activation of endothelial cells and recruitment of mural cells define critical steps during the formation of stable vascular elements. Both events are reflected by cocultures of endothelial cells and isolated murine pericyte-like cells and define a versatile platform for the analysis of distinct steps during the angiogenic process in vitro. Isolated pericyte-like cells promote the survival of endothelial cells, induce the assembly of endothelial cells as well as establish direct contacts with forming endothelial alignments. More importantly, they also induce characteristic steps of maturation including the assembly of stable cell-cell junctions, deposition of basement membrane-like matrices and local formation of a central lumen. The presence of pericyte-like cells induces the secretion of extracellular matrices enriched in collagen IV by endothelial cells, which improves endothelial tube formation and provides the adhesive substrate for mural cell recruitment. Collagen-binding integrins contribute differentially to the process, with α1β1 involved in the adhesion of pericyte-like cells to collagen IV and α2β1 mainly involved in endothelial cord formation. These data indicate that pericyte-like cells are essential for the survival of endothelial cells, the efficient formation of endothelial alignments as well as initial steps of maturation of capillary-like structures