Adult somatic stem cells fuel tissue renewal, repair, and remodeling to maintain organ structure and function by tuning their proliferation and differentiation rates to match the changing needs of their resident tissues. Given their potency, even incremental alterations in stem cell behavior could lead to substantial changes in tissue size and architecture. Yet, these types of effects are strikingly rare, strongly implying that stem cells are under tight homeostatic regulation that allows the system to react rapidly to disturbances and to efficiently restore proper functions. The main results of this project were identification of such mechanisms.
We had previously established an organoid system that, for the first time, allows expansion and long-term maintenance of multipotent hair follicle SCs (HFSCs). Strikingly, the system promotes de novo generation of HFSCs from non-HFSCs, and vice versa, in a dynamic self-organizing process (Chacón-Martínez et al., EMBO J 2017). Using this system, we discovered that ability of progenitors to return to the HFSC state requires capability to suppress a metabolic switch from glycolysis to oxidative phosphorylation and glutamine metabolism that occurs during early HFSC differentiation. Mechanistically, HFSC fate reversibility and modulation of mitochondrial metabolism are regulated by the mTORC2-Akt signaling axis that is active in the HFSC niche (Kim et al., 2020 Cell Metabolism). Recently, we have upscaled the organoid cultures to allow drug screening to identify compounds that promote stem cell fate (Biggs et al., in preparation).
To understand the role of tissue architecture, we first focused on studying how the hair follicle stem cell compartment forms. Here, we identified a key role for coordinated mechanical forces stemming from contractile, proliferative, and proteolytic activities across the epithelial and mesenchymal compartments in generating the initial hair follicle invagination, the placode (Villeneuve, et al., BiorXiv 2021; in press Nature Cell Biology)
In parallel we focused on understanding how mechanical forces control global patterns of gene expression to mediate lineage decisions, but also more generally explores the role of mechanical force in regulating nuclear and genome architecture. We discovered covered how mechanical deformation induces calcium-dependent nuclear softening driven by loss of H3K9me3-marked heterochromatin proximal to the nuclear lamina in epidermal stem cells. The resulting changes in chromatin mobility, viscoelasticity, and architecture are required to insulate genetic material from mechanical force. Failure to mount this nuclear mechanoresponse results in DNA damage (Nava et al., 2020 Cell). Importantly, persisting mechanical signals to the nucleus are also relevant for ageing-related decline of stem cell function, since ageing-induced changes in extracellular matrix deposition and crosslinking lead to increased tissue stiffening. The resulting mechanical stress on stem cells results in transcriptional repression, compromising the ability of stem cells to become rapidly activated in response to regenerative signals, thus attenuating tissue renewal (Koester et al., 2021 Nat Cell Biol). Collectively our results thus reveal how mechanical signals integrate nuclear architecture, chromatin organization and transcriptional regulation to control lineage commitment and facilitate generation of specific tissue patterns.
