Cellular senescence in mechano-sensation and ECM organization
Aging is an irreversible, progressive process resulting in a decline of tissue functionality and its regenerative capacity. With an increasingly aged population in developed countries, understanding deviations in healing processes after injury in aged is key for developing novel treatment strategies. During life the body accumulates with cells which lack a proliferative capacity due to an irreversible cell cycle arrest which is termed cellular senescence. Senescent cells secrete a variety of pro-inflammatory cytokines which recruit immune cells for tissue clearance during development and to prevent cancer. Although this feature confers early-life benefits, the increasing pro-inflammatory environment contributes to late-life debility (Campisi 2013). This antagonistic function is even further emphasized by recent observations which not only show the accumulation of senescent cells within an injured tissue but that elimination of senescent cells delays wound healing in vivo (Demaria et al. 2014). While this investigation related the effect to the secretory phenotype, little is known how senescent cells directly contribute to tissue formation through ECM formation.
We investigated how the senescence program affects extracellular matrix deposition and tissue tension using a recently published in vitro wound healing model system (Brauer et al. 2019). We observed that cellular senescence strongly affected macroscopic tissue tension and fibrillar collagen secretion and assembly in an antagonistic manner. While senescence due to over-expression of cell cycle inhibitors (p16INK4/p21CIP) resulted in an increased contraction, DNA-damage-induced (Mitomycin C-treated) senescence strongly reduced tissue contraction. This indicates a direct and antagonistic role of cellular senescence in establishing tissue tension. Aside of the tensile state of the resulting ECM, cellular senescence also affected the biochemical composition via differential regulation of ECM proteins, e.g. collagen, fibronectin, decorin, and tenascin C. Furthermore, we found evidence that these macroscopic perturbations might be influenced by an altered mechanotransduction of senescent cells already on the single cell level (2D). The expression of integrins was altered correlating with an enlarged cell morphology and higher abundance of large focal adhesions particularly in cells that were driven into senescence by selective over-expression (p16/p21). How these two observations are linked remains so far unknown.
Based on these preliminary data, we propose a novel research question which we believe to be relevant and of high interest for a PhD project. Based on the understanding that senescent cells seem to strongly alter the extracellular niche we want to better understand the relevance of this finding for the in vivo tissue regeneration process. While the in vitro experiments focused on the behavior of a homogeneous population, senescent cells in vivo are found in relatively low abundance. We believe that senescent cells nevertheless impact the regeneration process and hypothesize that the ECM formed by senescent cells exhibits specific cell-instructive cues which influence cellular processes, e.g. migration, survival but also differentiation of surrounding stromal cells. A distinct matrix phenotype thereby might contribute to the impact senescent cells exert during tissue regeneration aside of their secretory phenotype. The PhD student will optimize a recently established procedure for decellularization in order to obtain cell-free ECM generated by senescent cells. The matrix will be used as a template for studying the cellular response both of fibroblasts and MSCs. Differences in ECM-dependent cellular behavior would support the hypothesis that an ECM created by senescent cells affects behavior of surrounding stromal cells. The student will further focus on the consequences of senescence-modulated cellular mechano-sensation on tissue formation. Starting from a basic description of cellular mechano-sensation, cellular organization will be studied on simplified geometries to describe differences in multi-scale cell organization (Werner et al. 2016). Finally collective cell organization, which leads to ECM formation, will be studied in more detail inside the established scaffold-based 3D tissue model to further understand the impact of senescent cells on early tissue regeneration.
Together, the results will contribute to a principle understanding of (i) how the resulting matrix properties steer the behavior of non-senescent through a distinct cell-instructive matrix code and (ii) how senescent cells sense and respond to their environment with regard of cell organization and tissue formation.
Brauer, Erik, Evi Lippens, Oliver Klein, Grit Nebrich, Sophie Schreivogel, Gabriela Korus, Georg N Duda, and Ansgar Petersen. 2019. “Collagen Fibrils Mechanically Contribute to Tissue Contraction in an In Vitro Wound Healing Scenario.” Advanced Science (Weinheim, Baden-Wurttemberg, Germany) 6 (9): 1801780. doi:10.1002/advs.201801780. [PubMed]
Campisi, Judith. 2013. “Aging, Cellular Senescence, and Cancer.” Annual Review of Physiology 75 (1): 685–705. doi:10.1146/annurev-physiol-030212-183653. [PubMed]
Demaria, Marco, Naoko Ohtani, Sameh A. Youssef, Francis Rodier, Wendy Toussaint, James R. Mitchell, Remi-Martin Laberge, et al. 2014. “An Essential Role for Senescent Cells in Optimal Wound Healing through Secretion of PDGF-AA.” Developmental Cell 31 (6): 722–33. doi:10.1016/j.devcel.2014.11.012. [PubMed]
Werner, Maike, Sébastien B. G. Blanquer, Suvi P. Haimi, Gabriela Korus, John W. C. Dunlop, Georg N. Duda, Dirk. W. Grijpma, and Ansgar Petersen. 2016. “Surface Curvature Differentially Regulates Stem Cell Migration and Differentiation via Altered Attachment Morphology and Nuclear Deformation.” Advanced Science 201600347: 1600347. doi:10.1002/advs.201600347. [PubMed]