HA 1791/10-2: Funktion von Plakophilin 3 bei der Regulation der Proliferation von Keratinozyten

Desmosomes are intercellular adhesive contacts that are especiallv abundant in tissues prone to mechanical strain such as the skin and the heart. Our data suggest that members of the plakophilin familv of desmosomal proteins controi desmosome adhesive strength, size and number. Whereas plakophilin 1 strengthens intercellular adhesion and increases desmosome size in vivo and in vitro, plakophilin 3 renders desmosomes more dvnamic. in agreement, our mouse knockout studies reveal severe skin fragilitv in plakophilin 1 KO mice with postnatallethalitv. Moreover, we have shown that piakophilins 1 and 3 act as switches between desmosomal adhesion, cell proliferation, cell migration and anchorage independent growth. Cvtoplasmic plakophilin 1 stimulates proliferation bVassoeiating with the translation initiation complex to increase protein biosvnthesis. Activation of these regulatory functions depends on plakophilin 1 phosphorylation bVthe Akt2 kinase downstream of IGF1/insulin signaling. Phosphorvlated plakophilin 1 becomes trapped in the cvtoplasm bV 14-3-311w] hich interferes with its incorporation into desmosomes. We found that plakophilin 3 knockout keratinocytes reveal significantlv reduced proliferation rates, in agreement with growth retardation of plakophilin 3 KOmice, whereas its overexpression prornotes proliferation of keratinocytes. Our comparative gene expression analvsis revealed that 30%of all genes downregulated in plakophilin 3 KOcells are cell cycle related, manv of them being E2Ftargets. Moreover, we find that plakophilin 3 translocates to the mitotic spindie. The identification of several putative interacting proteins essential for mitotic spindie formation and dvnamies suggests an additional role in cell CVcleprogression. Basedon these data we propose the hvpothesis that plakophilin 3 is involved in cell CVclecontrol bv (al regulating Sphase entry via control of E2F/Rbactivitv and (bI regulating the mitotic spindie in assoeiation with microtubule binding proteins. Our project aims to verify this hvpothesis bVvalidation of E2Ftarget gene regulation and an anaivsis of the molecular mechanism bVwhich plakophilin 3 might influence E2F/Rbactivitv to promote S-phaseentry. Moreover, plakophilin 3 localization and protein interactions during mitosis and mechanisms controlling its changes in localization and protein interactions in a cell cycle dependent manner will be studied. We expect that our studies will have a major impact on understanding the interplav between desmosomal adhesion and proliferation which has to be tightlV balanced to guarantee epidermal homeostasis. Moreover, we will gain insight into the role of plakophilin 3 in modulating desmosomes to allow for mobilitv without interfering with cohesion during mitosis.