Expression and interaction of desmosomal components and keratins provide stable cell cohesion and protect the epidermis against various types of stress. The differentiation-specific isotype composition of the keratin cytoskeleton and desmosomes is regarded as major determinant of adhesive strength. However, the functional significance of individual keratins for the composition and adhesion of desmosomes has not been addressed in full. To overcome keratin redundancy following deletion of individual keratin genes, the entire type II or type I keratin cluster was deleted, resulting in the absence of keratin filaments in epidermal keratinocytes. The comparison of mouse keratinocyte cell lines lacking all keratins or re-expressing distinct keratin isotypes provides an excellent model to examine keratin contribution to the formation and stability of desmosomes. In support with the reported phenotype in vivo, desmosomes assemble in the absence of keratins but are endocytosed at accelerated rates. The internalization of desmosomes is regulated by PKCα-mediated desmoplakin phosphorylation, rendering epithelial sheets highly susceptible to mechanical stress in cell culture. Re-expression of the keratin pair K5/K14, inhibition of PKCα activity, or blocking of endocytosis reconstituted both desmosome localization at the plasma membrane and epithelial adhesion. The data support a model whereby K5/K14 sequesters RACK1, which can bind PKCα and thereby limits DP phosphorylation, promoting desmosome stability/maintenance and intercellular adhesive strength. To investigate the isotype-specific function of keratins, the respective contribution of K5/K14 or K6/K17 to desmosome adhesion, upon their stable re-expression in keratinocytes lacking all keratins was analyzed. This revealed that K5/K14 support stable desmosomes, whereas expression of “wound healing” keratins K6/K17 induce PKCα-mediated desmosome disassembly and subsequent destabilization of epithelial sheets accompanied by faster wound closure. Furthermore, analysis of adherens junctions and actin organization in keratin-free keratinocytes demonstrated a role of keratins in reorganization of the actin cytoskeleton and maturation of adherens junctions. This study identified a hitherto unknown mechanism by which keratins control intercellular adhesion, with potential implications for wound healing, tumor invasion and keratinopathies, settings in which diminished cell adhesion facilitates tissue fragility and neoplastic growth.

Expression and interaction of desmosomal components and keratins provide stable cell cohesion and protect the epidermis against various types of stress. The differentiation-specific isotype composition of the keratin cytoskeleton and desmosomes is regarded as major determinant of adhesive strength. However, the functional significance of individual keratins for the composition and adhesion of desmosomes has not been addressed in full. To overcome keratin redundancy following deletion of individual keratin genes, the entire type II or type I keratin cluster was deleted, resulting in the absence of keratin filaments in epidermal keratinocytes. The comparison of mouse keratinocyte cell lines lacking all keratins or re-expressing distinct keratin isotypes provides an excellent model to examine keratin contribution to the formation and stability of desmosomes. In support with the reported phenotype in vivo, desmosomes assemble in the absence of keratins but are endocytosed at accelerated rates. The internalization of desmosomes is regulated by PKCα-mediated desmoplakin phosphorylation, rendering epithelial sheets highly susceptible to mechanical stress in cell culture. Re-expression of the keratin pair K5/K14, inhibition of PKCα activity, or blocking of endocytosis reconstituted both desmosome localization at the plasma membrane and epithelial adhesion. The data support a model whereby K5/K14 sequesters RACK1, which can bind PKCα and thereby limits DP phosphorylation, promoting desmosome stability/maintenance and intercellular adhesive strength. To investigate the isotype-specific function of keratins, the respective contribution of K5/K14 or K6/K17 to desmosome adhesion, upon their stable re-expression in keratinocytes lacking all keratins was analyzed. This revealed that K5/K14 support stable desmosomes, whereas expression of “wound healing” keratins K6/K17 induce PKCα-mediated desmosome disassembly and subsequent destabilization of epithelial sheets accompanied by faster wound closure. Furthermore, analysis of adherens junctions and actin organization in keratin-free keratinocytes demonstrated a role of keratins in reorganization of the actin cytoskeleton and maturation of adherens junctions. This study identified a hitherto unknown mechanism by which keratins control intercellular adhesion, with potential implications for wound healing, tumor invasion and keratinopathies, settings in which diminished cell adhesion facilitates tissue fragility and neoplastic growth.

Desmosomen sind hochorganisierte Zell-Zell-Verbindungen. Aufgrund ihrer hohen Adhäsivität sind sie für die mechanische Kopplung und strukturelle Stabilität stark beanspruchter Gewebe von essentieller Bedeutung. Die adhäsive interzelluläre Kernstruktur der Desmosomen wird durch die transmembranen Cadherine des Desmocollin und Desmoglein-Typs gebildet. Deren extrazelluläre Domänen stellen den Kontakt zwischen zwei benachbarten Zellen her. Dsc2 ist neben Dsg2 die prädominante Isoform und wird in allen Desmosomen-bildenden Geweben wie auch dem Herzen exprimiert. Ziel der Arbeit war es, die Rolle von Dsc2 bei der Etablierung und Aufrechterhaltung der desmosomalen Adhäsivität und Gewebeintegrität zu untersuchen. Hierfür wurde ein klassisches Knockout-Mausmodell für Dsc2 etabliert und sowohl basal als auch unter Belastungsbedingungen charakterisiert. Unsere Daten demonstrieren, dass der ubiquitäre Knockout keinen Einfluss auf die Morphogenese des Embryos und die postnatale Entwicklung hat. Dsc2-/- -Mäuse waren lebensfähig und wiesen keinen pathologischen Phänotyp auf. Zudem ließen sich in den Herzen adulter Tiere strukturell unveränderte Desmosomen nachweisen. Dahingegen konnte eine verminderte mechanische Beanspruchbarkeit der Dsc2-/- -Herzen aufgezeigt werden. Unter erhöhter Belastung zeigte sich bereits nach wenigen Tagen eine signifikante Reduktion der kardialen Funktion.Die vorliegende Arbeit zeigt somit erstmalig in vivo, dass Dsc2 entgegen der Lehrmeinung nicht essentiell für die Embryonalentwicklung und die Bildung strukturell intakter Desmosomen ist. Anhand des Funktionsverlustes der adulten Knockout-Herzen unter Belastungsbedingungen lässt sich jedoch eine mögliche Rolle von Dsc2 für die Adhäsivität der Desmosomen postulieren. / Desmosomes are highly organized adhesive intercellular junctions providing mechanical strength and structural stability to several tissues such as skin and heart. The adhesive core of desmosomes is formed by the transmembrane glycoproteins desmocollins (Dsc) and desmogleins (Dsg) which link neighbouring cells via interaction with their extracellular cadherin domains. Dsc2 and Dsg2 are the predominant isoforms ubiquitously expressed in all desmosome bearing tissues including the heart. To elucidate the role of Dsc2 for establishment and maintenance of desmosome adhesion and tissue integrity, we generated a constitutive knockout model of the mouse. The effect of gene inactivation was characterized under basal as well as under stress conditions, using two different stress models. Our data demonstrate that Dsc2 is not required for pre- and postnatal development. Dsc2-/- mice were viable and showed no pathological alterations at embryonic or adult stages. Consistently, Dsc2 deficient cardiomyocytes exhibited distinct and ultrastructural well organized desmosomes. However, mutant hearts displayed a decreased stress resistance. Increased mechanical pressure led to a significant reduction of cardiac function in Dsc2-/- animals. In summary, our results demonstrate for the first time in vivo that Dsc2 is not essential for embryonic development and for the establishment and maintenance of distinct and well organized desmosomes. However, the reduced cardiac function in stressed knockout-mice suggests a crucial importance of Dsc2 for desmosomal adhesive strength.