Niessen, Carien M / Brinkkötter, Paul - C 5

Human diseases often display cytoskeletal rearrangement and altered cell polarity. We propose that alterations in key regulators of cell- and tissue architecture underlie a range of degenerative and inflammatory diseases. To examine this we have generated epidermal specific mouse models for key regulators of cyto-architecture and shown important functions for skin barrier function, integrity, cell fate and inflammation.

Continuous renewal of the interfollicular epidermis (IFE) and its appendages, hair follicles (HF) and sebaceous glands, is crucial for organisms to maintain and restore the skin barrier that protects from dehydration and external challenges, such as UV and microbes. Perturbations of the skin barrier contribute to a range of very common human skin diseases such as atopic dermatitis, psoriasis, impaired wound healing and skin cancer. A common feature of these diseases is an altered cyto-architecture often accompanied by changes in adhesive interactions and barrier properties. The atypical protein kinase C (aPKC)/Par3/Par6 pathway regulates polarity and mediates insulin/IGF1 and NfκB signaling (Moscat et al., 2009). This pathway may thus integrate regulation of cyto-architecture with control of growth, metabolism and inflammation. The overall aim of this proposal is to elucidate how polarity proteins control epithelial barrier formation and maintenance and ask how alterations in cell and tissue architecture contribute to disturbed skin homeostasis and disease.

Previously, we have shown that epidermal loss of aPKCλ results in stem cell exhaustion, cell fate changes, and premature aging (Niessen et al., 2013), accompanied by an increase in asymmetric spindle orientation, which are predicted to promote differentiation. As epidermal loss of aPKCλ promoted ACD and shifted stem cells towards differentiation, we would predict that an increase in SCDs will produce more undifferentiated progenitor cells. To this end we have generated a knock-in mouse model in which a membrane-targeted aPKCλ mutant (aPKCλ^caax), shown in Drosophila to promote stem cell renewal, is expressed in the epidermis. This resulted in opposite phenotypes from loss of aPKCλ, such as stem cell expansion and prolonged stem cell quiescence. Surprisingly, this was also accompanied by increased asymmetric spindle orientation, indicating that increased spindle orientation alone cannot predict cell fate (Vorhagen et al., submitted).  The aPKC interactome/proteome revealed several candidates, such as the polarity protein Lgl and the spindle orientation proteins Numa and Dynactin, as aPKC interaction and/or substrates and first experiments suggest that these proteins control cell fate and spindle orientation downstream of aPKCs.

Using different epidermal knockouts we have uncovered novel roles for key regulators of the cyto-architecture, cadherins and the polarity protein complex atypical kinase C (aPKC)/Par3, in the polarized organization of tension high junctions and the cytoskeleton essential for the restricted formation of functional barrier promoting tight junctions to the upper epidermal layers (Rübsam et al, in revision; Tellkamp et al, in preparation). These studies also revealed a novel and unexpected role for EGFR signalling in the structural barrier that is potentially linked to its role in innate immune signalling (Fig.2), which may potentially explain how cancer patients treated with EGFR develop skin rashes.

Our combined proteomic and functional approaches have identified aPKCs and cadherins as key regulators in a network that controls epidermal differentiation, barrier function and skin homeostasis. Our results provide novel insides into how altered epithelial barrier function results in human disease. The ultimate goal is to use our different models to screen small molecular compounds and characterize novel molecular targets downstream of polarity pathways for the intervention in (skin) barrier related diseases.