ARHGAP18-ezrin functions as an autoregulatory module for RhoA in the assembly of distinct actin-based structures.

The location of different actin-based structures is largely regulated by Rho GTPases through specific effectors. We use the apical aspect of epithelial cells as a model system to investigate how RhoA is locally regulated to contribute to two distinct adjacent actin-based structures. Assembly of the non-muscle myosin-2 filaments in the terminal web is dependent on RhoA activity, and assembly of the microvilli also requires active RhoA for phosphorylation and activation of ezrin.

The RabGAPs EPI64A and EPI64B regulate the apical structure of epithelial cells .

Here we report on the related TBC/RabGAPs EPI64A and EPI64B and show that they function to organize the apical aspect of epithelial cells. EPI64A binds the scaffolding protein EBP50/NHERF1, which itself binds active ezrin in epithelial cell microvilli. Epithelial cells additionally express EPI64B that also localizes to microvilli.

Effector-mediated ERM activation locally inhibits RhoA activity to shape the apical cell domain.

Activated ezrin-radixin-moesin (ERM) proteins link the plasma membrane to the actin cytoskeleton to generate apical structures, including microvilli. Among many kinases implicated in ERM activation are the homologues LOK and SLK. CRISPR/Cas9 was used to knock out all ERM proteins or LOK/SLK in human cells.

High Runx1 levels promote a reversible, more-differentiated cell state in hair-follicle stem cells during quiescence.

Quiescent hair follicle (HF) bulge stem cells (SCs) differentiate to early progenitor (EP) hair germ (HG) cells, which divide to produce transit-amplifying matrix cells. EPs can revert to SCs upon injury, but whether this dedifferentiation occurs in normal HF homeostasis (hair cycle) and the mechanisms regulating both differentiation and dedifferentiation are unclear. Here, we use lineage tracing, gain of function, transcriptional profiling, and functional assays to examine the role of observed endogenous Runx1 level changes in the hair cycle.

Defining a tissue stem cell-driven Runx1/Stat3 signalling axis in epithelial cancer.

Cancers and tissue stem cells (SCs) share similar molecular pathways for their self-renewal and differentiation. The race is on to identify unique pathways to specifically target the cancer, while sparing normal SCs. Here, we uncover the transcription factor Runx1/AML1, a known haematopoietic and leukaemia factor, albeit dispensable for normal adult SC homeostasis, as being important for some mouse and human epithelial cancers.

Runx1 directly promotes proliferation of hair follicle stem cells and epithelial tumor formation in mouse skin.

Runx1/AML1 is a transcription factor implicated in tissue stem cell regulation and belongs to the small Runx family of cancer genes. In the hair follicle (HF), Runx1 epithelial deletion in morphogenesis impairs normal adult hair homeostasis (cycle) and blocks adult hair follicle stem cells (HFSCs) in quiescence. Here, we show that these effects are overcome later in adulthood.

Distinct self-renewal and differentiation phases in the niche of infrequently dividing hair follicle stem cells.

In homeostasis of adult vertebrate tissues, stem cells are thought to self-renew by infrequent and asymmetric divisions that generate another stem cell daughter and a progenitor daughter cell committed to differentiate. This model is based largely on in vivo invertebrate or in vitro mammal studies. Here, we examine the dynamic behavior of adult hair follicle stem cells in their normal setting by employing mice with repressible H2B-GFP expression to track cell divisions and Cre-inducible mice to perform long-term single-cell lineage tracing.

Quantitative proliferation dynamics and random chromosome segregation of hair follicle stem cells.

Regulation of stem cell (SC) proliferation is central to tissue homoeostasis, injury repair, and cancer development. Accumulation of replication errors in SCs is limited by either infrequent division and/or by chromosome sorting to retain preferentially the oldest 'immortal' DNA strand. The frequency of SC divisions and the chromosome-sorting phenomenon are difficult to examine accurately with existing methods.

Runx1 modulates developmental, but not injury-driven, hair follicle stem cell activation.

Aml1/Runx1 controls developmental aspects of several tissues, is a master regulator of blood stem cells, and plays a role in leukemia. However, it is unclear whether it functions in tissue stem cells other than blood. Here, we have investigated the role of Runx1 in mouse hair follicle stem cells by conditional ablation in epithelial cells.