1. Research Background:

Epithelia form mechanical, and innate immune barriers with the outside world that provide a first line of defense for the organism against a range of environmental stresses including mechanical forces and microbes. To ensure life-long protection these barriers are constantly renewed under homeostatic conditions and, quickly repaired upon stress and injury. A dysfunctional barrier results in a range of (sometimes deadly) diseases. In addition, aging is associated with an increasing dysfunctional epithelial barrier. How epithelial tissue barriers coordinate cell and tissue structure with growth and metabolic activity to control morphogenesis and life-long homeostatic renewal is not known.

2. Research questions addresses by the group:

The Niessen laboratory aims at understanding how cell structure controls tissue function. Using the self-renewing epidermis as a paradigm the Niessen laboratory asks how regulators of cell and tissue architecture control the formation and homeostatic regeneration of epithelial barriers. Using amultidisciplinary approach, the group addresses at different scale lengths (organismal, tissue, cell, sub- cellullar, and molecular) how adhesion, polarity and tyrosine kinase signaling interact to control the formation and maintenance of epithelial barriers, and asks how alterations in these biomechanical signaling networks contribute to aging and aging-associated diseases.

3. Possible projects:

1. Regulation of cell fate during division.
   Epithelial tissues have to balance self-renewal of stem cells with differentiation to assure life-long tissue regeneration under homeostatic and injury conditions. Oriented cell division couples cell fate to division and positioning of daughters within the tissue. However, the mechanisms that couple cell division and cell fate are not known. The polarity protein aPKC controls cell fate and mitotic spindle orientation to regulate epidermal stem cell dynamics and tissue homeostasis. Using aPKC as a paradigm we will ask whether cell fate is controlled in cell division. Together with Achim Tresch, we plan to use CRIPS-Cas technology to trace daughter cell fate and combine this with single cell RNA sequencing analysis, epigenetics and cell biology approaches.
2. Biomechanical regulation of cell fate and position in self-renewing epithelia
   In self-renewing epithelial tissues like the epidermis, proliferation and differentiation are in physically different compartments separated by a sharp boundary. Moreover, these boundaries are lost in aging and disease resulting in uncoupling of cell fate, shape, and position within the tissue. Together with our collaborators Lisa Manning (Syracuse University) and Sara Wickström (University of Helsinki) we ask whether and how cell shape controls (stem) cell fate, and whether cell shape changes are sufficient to change position and differentiation status in the tissue. We combine in silico modelling of multilayered epithelia (Manning lab), with 3D cell cultures, in vivo models, and biophysical approaches using loss of signaling, adhesion and polarity molecules as proxies to induce changes in cell shape
3. Regulation of mechanical stress protection
   How cells respond to mechanical stress to maintain tissue integrity is largely unknown. Adhesive junctions coupled to the actin cytoskeleton are key sensors and transducers of mechanical signals. How these mechanosensitive units maintain healthy proteostasis to promote mechanical resistance is notknown. Our data identified the polarity protein aPKC as a central coordinator of junctional and cytoskeletal dynamics while also interacting and/or phosphorylating key autophagy proteins. We thus hypothesize that mechanical force spatially regulates aPKC activity to coordinate autophagy of damaged mechanosensitive proteins to promote junctional dynamics necessary to resist stress and maintain short and long-term tissue function. Takin the AJ as a paradigm for mechano- sensitive units we will askwhether healthy proteostasis of mechanosensitive units is essential for stress resistance and define the pathways by which cells deal with mechanical stress to maintain tissue function, and whether these responses are impaired in aging.
4. Skin-metabolic organ communication in metabolic syndrome
   Epidermal insulin/IGF-1 signaling (IIS) control epidermal morphogenesis and skin barrier function, but whether impaired insulin function in the skin directly contributes to skin-associated pathology in type II diabetes is less clear. We very recently found that high fat diet mimics the phenotype caused by loss ofepidermal IIS, including impaired epidermal barrier function and altered p63 signaling. Moreover, on high fat diet insulin resistance in the skin precedes central insulin resistance. Using epidermal IIS knockouts we will ask how insulin/IGF control the mechanical and innate propertiesof the epidermal barrier and how this results in systemic changes that contribute to metabolic syndrome and its long- term skin pathogenesis. Finally, we are transferring findings to human settings to ask whether barrier dysfunction correlates with human hyperglycemia and development of Type II diabetes, and here also examine the role of the microbiome.

4. Applied Methods and model organisms:

Mouse transgenic models, primary cell culture, and human disease models. High end Imaging and image analysis, proteomics, biomechicanical and biophysical approaches such as traction and atomic force microscopy, controlled strain methods (together with strong collaborators). In collaboration: bioinformatics.

5. Desirable skills and qualifications:

We are looking for enthusiastic candidatesthat are inspired by science and, using a multidisciplinary approach, want to address fundamental questions in the cell biology of aging, with implications for translational medicine. A strong background in mechanical biology, intravital imaging, mouse genetics or computational biology is, depending on the project, of advantage

6. References and key publications:

• Guenschmann, C., Stachelscheid H., Akyüz M.D, Schmitz A., Missero, C., Bruning, J.C. and Niessen CM (2013) IGF-1 controls epidermal morphogenesis via regulation of FoxO-mediated p63 inhibition. Developmental Cell 26(2): 176-87
• Niessen MT, Scott, J, Zielinski, J, Vorhagen S, Blanpain, C, Leitges, M. and Niessen CM. (2103) The cellpolarity protein aPKC/\ couples asymmetric divisions to cell fate decisions in the epidermal lineage. J. Cell Biology 202:887-900
• Rübsam M, Mertz AF, Kubo A, Marg S, Jüngst C, Goranci-Buzhala G, Schauss AC, Horsley V, Dufresne E,Moser M, Ziegler W, Amagai M, Wickström SA and Niessen CM (2017) E-cadherin integrates mechanotransduction and EGFR signaling to control junctional tissue polarization and Tight Junction positioning. Nature Communications, 2017 Nov 1;8(1):1250. doi: 10.1038/s41467-017-01170-7.
• Miroshnikova YA, Le HQ, David Schneider D, Thalheim T, Rübsam M, Bremicker N, Polleux J, Kamprad N, Tarantola M, Wang I, Balland M, Niessen CM, Galle J, Wickström SA. (2018) A biomechanical network of adhesion forces and cell mechanics couples proliferation and differentiation to drive epidermal stratification. Nature Cell Biology, 20:69-80. doi: 10.1038/s41556-017-0005
• Vorhagen S*^*, Kleefisch D*, Persa, O-D, Graband, A., Schwickert A, Saynisch M., Leitges M, Niessen CM#,and Iden S# (2018) Shared and independent functions of aPKCλ and Par3 in skin tumorigenesis. Oncogene, May 23. doi: 10.1038/s41388- 018-0313-
• Wickström, SA and Niessen CM (2018) Cell adhesion and mechanics as drivers of tissue organizationand differentiation: a question of local symmetry breaking. Curr Opin Cell Biol, 54:89-97. doi: 10.1016/j.ceb.2018.05.003. [Epub ahead of print]
• Shafraz O*, Rübsam M*, Stahley SN, Caldara AL, Kowalczyk AP, Niessen CM, Sivasankar S. (2018) E-cadherin binds to desmoglein to facilitate desmosome formation. Elife, Jul 12;7. pii: e37629. doi: 10.7554/eLife.37629
• Persa OD, Niessen CM (2019) Epithelial polarity limits EMT. Nat Cell Biol. 21:299-300. doi: 10.1038/s41556-019-0284-7.
• Peters F*, Tellkamp F*, Brodesser S, Wachmuth E, Tosetti B, Karow U, Bloch W, Utermohlen O, KrönkeM, Niessen CM (2020) Murine epidermal Ceramide Synthase 4 is a key regulator of skin barrier homeostasis. J. Investig. Derm. doi.org/10.1016/j.jid.2020.02.006
• Nava MM, Miroshnikova YA, Biggs LC, Whitefield DB, Metge F, Boucas J, Vihinen H, Jokitalo E, García Arcos JM, Hoffmann B, Merkel R, Niessen CM, Dahl KN, and Wickström SA. (2020) Heterochromatin-driven nuclear softening protects the genome against mechanical stress-induced damage. Cell 181: 800-819 doi.org/10.1016/j.cell.2020.03.052
• Sahu P, Sussman DM, Rübsam M, Mertz A, Horsley V, Dufresne E, Niessen CM, Marchetti MC, ManningML, Schwarz JM. (2020) Small-scale demixing in confluent biological tissues. Soft Matter 6:3325-3337 doi.org/10.1039/C9SM01084J
• Rübsam M, Niessen CM (2020) Stretch exercises of stem cells expand the skin. Nature 584: 196-198, doi: 10.1038/d41586-020-02158-y.
• Niessen CM and Plachta NA. (2020) Emerging mechanisms driving cell differentiation. Curr Opin Cell Biol67:iii-v. doi: 10.1016/j.ceb.2020.10.003. Epub 2020 Nov 10, Co-editor of the issue
• Persa OD, Koester J and Niessen CM (2021) Regulation of cell polarity and tissue architecture inepidermal aging and cancer. J. Invest Dermatol. 141:1017-1023 doi: 10.1016/j.jid.2020.12.012
• Koester J, Miroshnikova Y, Ghatak S, Chacón-Martínez C, Morgner J, Li X, Atanassov I, Altmuller J, BirkDE, Koch M, Bloch W, Bartusel M, Niessen CM, Rada- Iglesias A, and Wickström S. Niche stiffeningcompromises stem cell potential during aging by reducing chromatin accessibility at bivalent promoters. Nature Cell Biology 23(7):771-781. doi: 10.1038/s41556-021-00705-x
• Höhfeld J, Benzing T, Bloch W, Fürst DO, Gehlert S, Hesse M, Hoffmann B, Hoppe T, Huesgen PF, KöhnM, Kolanus W, Merkel R, Niessen CM, Pokryzwa W, Rinschen MM, Wachten D, and Warscheid B (2021) Mechanical Stress Protection. EMBO Reports 22:e52507. doi: 10.15252/embr.202152507.