Skin Homeostasis and Ageing
Adult somatic stem cells fuel tissue renewal, repair, and remodeling to maintain organ structure and function. 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, allowing the system to react rapidly to disturbances and to efficiently restore proper functions. However, mechanisms of such population-level regulation are poorly understood.
Stem cells reside in distinct niches that mediate the balanced response of stem cells to the needs of the tissue, prevent stem cell depletion, and restrict excessive stem cell expansion. Although the importance of niches in stem cell regulation has been established, the complexity of mammalian stem cell niches has prevented identification of the precise nature of the niche-derived signals and hindered mechanistic studies of adult stem cell regulation.
As a self-renewing organ maintained by multiple distinct stem cell populations, the epidermis represents an outstanding paradigm to study stem cells and their interactions with the niche. The mammalian epidermis is composed of a pilosebaceous unit that consists of the hair follicle (HF), the sebaceous gland, surrounded by the interfollicular epidermis (IFE). Each unit contains distinct stem cell populations that fuel constant renewal of the IFE and HFs during postnatal tissue homeostasis and regeneration. Interestingly, when restricted by tissue architecture, the specific stem cell pools within the skin epidermis remain strictly compartmentalized. However, upon tissue injury or when transplanted, the specialized stem cells exhibit broader potency in their new microenvironment. This suggests that the molecular composition, and perhaps the distinct topology and/or mechanics of the HF and IFE niches guide stem cell behavior. Uncovering these mechanisms and establishing their relevance in physiology, aging and cancer is the focus of our research.
Recent research highlights include:
- Identifying a chaperone-dependent mechanism by which the integrin-actin linkage regulates cellular force generation, extracellular matrix remodeling and skin fibrosis (Radovanac et al, EMBOJ 2013)
- Uncovering a key role for the local remodeling of the extracellular matrix within the stem cell niche as a driver of SC activation and tissue homeostasis in the epidermis (Morgner et al, Nat Commun 2015)
- Discovery of a novel mechanism by which mechanical force regulates nuclear architecture and chromatin structure, thereby affecting epigenetic silencing of epidermal stem cell lineage commitment genes (Le et al. Nat Cell Biol, 2016)
- Establishing a ex vivo niche that allows enrichment and dynamic, bidirectional reprogramming of hair follicle stem cells (HFSCs) (Chacón-Martínez et al, EMBOJ 2016) as well as cancer stem cells (tumor initiating cells) from squamous cell carcinomas, and which can now be used as a high throughput discovery tool for adult stem cell and cancer biology (patent EP15188393.1 pending)
Niches are critical for stem cell (SC) function, but it is not clear how they are established and how the niche architecture impacts the organization and fate of resident SCs and their progeny. Murine hair follicle stem cells (HFSCs) represent one of the most successful genetic model systems used to uncover fundamental biology of adult tissue-resident SCs. However, the lack of a system that recapitulates their native niche, enabling maintenance of HFSCs in the absence of other heterologous cell types, and allowing precise manipulation and monitoring of HFSC fate decisions has been one of the major obstacles in understanding HFSC regulation and function. We have now broken through this barrier by deconstructing the essential components of the niche, enabling us to develop an ex vivo culture system that, for the first time, allows to enrich and maintain HFSCs without loss of their multipotency (Chacón-Martínez et al., EMBOJ 2016).
Intriguingly, studies using this system have shown that epidermal cell mixtures self-evolve into a population equilibrium state of HFSCs and differentiated progeny. Strikingly, we further observe that dynamic, bidirectional interconversion of HFSCs and differentiated cells drives this self-organizing process. Moreover, HFSCs can be derived completely de novo even from purified populations of non-HFSCs. The unique tunable, defined nature of the culture system allows us to:
a) Delineate how niche composition, mechanics, and topology regulate SC fate and reprogramming using niche bioengineering in combination with imaging, lineage tracing and molecular biology
b) Dissect the genetic and epigenetic requirements of the observed phenotypic plasticity using time-resolved next generation sequencing of the niche self-organization process
c) Identify druggable pathways that regulate the plasticity of SC fate on the population level tool using chemical inhibitor library drug screens
How precise, dynamic coordination of cell position and fate are achieved and maintained in mammalian organs is a fundamental open question. We address this in the mammalian epidermis, a highly stereotypically organized stratified epithelium where self-renewal is maintained by SCs that pass through defined stages of differentiation while transiting upwards through the cell layers. We hypothesize that biomechanical signaling integrates single cell behavior to couple proliferation, cell fate and positioning to generate and maintain global patterns of a multicellular tissue.
Our current work aims to:
- Establish quantitative principles of the stratification process by combining biomechanical analyses, in vivo imaging, and mathematical modeling
- Delineate the in vivo role of cortical tension and actomyosin contractility in stratification and skin barrier function using NMIIA-deficient mice
- Discover the epigenetic mechanisms by which age-related changes in tissue mechanics contribute to the decline of stemness during aging
Tissue mechanics and cellular interactions are a driving force of morphogenesis4, but little is known about the mechanisms that sense physical forces and how they control organ growth and patterning through SC fate and self-organization. To decipher how mechanical forces regulate SC identity, we have sought to identify pathways that respond to force and establish their functional significance in SC fate determination.
We show that a mechanosensory complex of emerin (Emd), non-muscle myosin IIA (NMIIA) and actin relays extrinsic mechanical forces by controlling gene silencing and chromatin compaction, thereby regulating the kinetics of lineage commitment. Force-driven enrichment of Emd at the outer nuclear membrane of epidermal stem cells leads to defective heterochromatin anchoring to the nuclear lamina and a switch from H3K9me2,3 to H3K27me3occupancy at constitutive heterochromatin. Emd enrichment at the outer nuclear membrane is also accompanied by the recruitment of NMIIA to promote local actin polymerization that reduces nuclear actin levels, which results in attenuation of transcription and subsequent accumulation of H3K27me3 at facultative heterochromatin. Restoring nuclear actin levels in the presence of mechanical stress counteracts PRC2-mediated silencing of transcribed genes (Le et al., Nat Cell Biol 2016).
Taken together, our results reveal how mechanical signals integrate transcriptional regulation, chromatin organization and nuclear architecture to control lineage commitment and tissue morphogenesis.
Our future work will:
- Characterize the effect of extrinsic force on 3D chromatin organization at high spatiotemporal resolution using Hi-C technologies
- Identify the molecular mechanisms by which actin dynamics and nuclear actin regulate gene expression and SC fate both in vitro and in vivo
- Uncover the molecular mechanisms by which nuclear envelope transmits mechanical signals to chromatin and characterize the functional relevance of this signaling during mechanical stress and aging
- Decipher how different forms of heterochromatin act as rheological elements of the nucleus and function in the mechanical response of the nucleus using atomic force microscopy and related biophysical methods in combination with biochemistry and cell biology