Neighborhood Regulation
Our discoveries have resolved several long-standing debates in the field:
Although epidermal stem cells are equally likely to self-renew or differentiate, sibling stem cells drive the timing of commitments. In fact, the exit of a stem cell from the basal layer – by differentiation – causes the counterbalancing division of a neighboring stem cell within one to two days (Rompolas 2016; Mesa, Kawaguchi & Cockburn 2018).
Representative images showing the segmentation of basal cells based on the cortical K14-actinGFP signal. Representative example of an individual basal cell increasing in area concurrent with a neighbor differentiation, losing the G1 reporter (mKO2-hCdt1(30/120)) signal, and dividing over the course of 48 hr.
Epithelial cell density regulates resident immune cell density within the skin epidermis. These immune cells tile the epithelial space they inhabit to maximize surveillance coverage despite dynamic changes in basal stem cell proliferation (Park & Matte-Martone 2021).
Representative image of epithelial basal stem cells and immune cells in the ear with Voronoi diagrams representing the actual vs a random distribution of Langerhans Cells (LC) demonstrating spatial tiling.
The upregulation of differentiation genes precedes the downregulation of stemness genes, and differentiation-committed cells remain capable of dividing to produce daughter cells fated to further differentiate (Cockburn & Annusver 2022).
Violin plots of stem- and differentiation-associated gene expression within cells grouped into pseudotime bins from a scRNA-seq trajectory analysis of epidermal stem cells.
Recently, we have worked on understanding the local homeostatic conditions stemming from resident immune cells and basal stem cells, the gradual process of differentiation and its transcriptional changes, the role of calcium signaling on stem cell proliferation at the basal layer, and the global chromatin state of individual stem cells and how it changes during differentiation.
Differentiation starts in the stem cell layer and differentiation-committed cells remain capable of dividing
We studied the process of differentiation over time to look at changes in progenitor identity and commitment based on transcriptional trajectories. We found that delamination occurs after three or four days of differentiation, when expression of the differentiation marker K10 (keratin 10) is upregulated while stem cell genes (such as K14) are downregulated. These K10 positive cells are still capable of division, indicating that differentiation is uncoupled from cell cycle exit. However, their proliferative capacity does not impair K10 positive cells from maturing and stratifying upwards. Blocking proliferation does not impact the skin barrier or tissue architecture due to increased proliferation and differentiation of neighboring epithelial cells (Cockburn & Annusver 2022).
Representative revisited basal cells as they induce K10 reporter expression and later exits the basal layer. The second was observed to proliferate after the onset of K10 reporter expression.
Calcium signaling drives cell cycle progression
We further discovered highly coordinated calcium signaling within the epidermal stem cell layer, and this calcium signaling drives cell cycle progression (Moore*, Bhaskar* & Gao* 2023).
Representative max intensity projection of 30-min recording of the basal layer of control and G2-depleted Ca2+-sensor mice. Color scale indicates GCaMP6s signal across time.