SUMMARY Hair follicles (HFs) undergo cyclic bouts of degeneration, rest, and regeneration. During rest (telogen), the hair germ (HG) appears as a small cell cluster between the slow-cycling bulge and dermal papilla (DP). Here we show that HG cells are derived from bulge stem cells (SCs) but become responsive quicker to DP-promoting signals. In vitro, HG cells also proliferate sooner but display shorter-lived potential than bulge cells. Molecularly, they more closely resemble activated bulge rather than transit-amplifying (matrix) cells. Transcriptional profiling reveals precocious activity of both HG and DP in late telogen, accompanied by Wnt signaling in HG and elevated FGFs and BMP inhibitors in DP. FGFs and BMP inhibitors participate with Wnts in exerting selective and potent stimuli to the HG both in vivo and in vitro. Our findings suggest a model where HG cells fuel initial steps in hair regeneration, while the bulge is the engine maintaining the process.

Summary Here, we exploit the hair follicle to define the point at which stem cells become irreversibly committed along a differentiation lineage. Employing histone and nucleotide double-pulse-chase and lineage tracing, we show that the early SC descendents en route to becoming transit-amplifying cells retain stemness and slow-cycling properties and home back to the bulge niche when hair growth stops. These become the primary SCs for the next hair cycle, while initial bulge SCs become reserves for injury. Proliferating descendents further en route irreversibly lose their stemness, although they retain many SC markers and survive, unlike their transit-amplifying progeny. Remarkably, these progeny also home back to the bulge. Combining purification and gene expression analysis with differential ablation and functional experiments, we define critical functions for these non-SC niche residents, and unveil the intriguing concept that an irreversibly committed cell in an SC lineage can become an essential contributor to the niche microenvironment.

SUMMARY Although in vitro studies of embryonic stem cells have identified polycomb repressor complexes (PRCs) as key regulators of differentiation, it remains unclear as to how PRC-mediated mechanisms control fates of multipotent progenitors in developing tissues. Here, we show that an essential PRC component, Ezh2, is expressed in epidermal progenitors but diminishes concomitant with embryonic differentiation and with postnatal decline in proliferative activity. We show that Ezh2 controls proliferative potential of basal progenitors by repressing the Ink4A-Ink4B locus and tempers the developmental rate of differentiation by preventing premature recruitment of AP1 transcriptional activator to the structural genes that are required for epidermal differentiation. Together, our studies reveal that PRCs control epigenetic modifications temporally and spatially in tissue-restricted stem cells. They maintain their proliferative potential and globally repressing undesirable differentiation programs while selectively establishing a specific terminal differentiation program in a stepwise fashion.

SUMMARY In adult skin, epithelial hair follicle stem cells (SCs) reside in a quiescent niche and are essential for cyclic bouts of hair growth. Niche architecture becomes pronounced postnatally at the start of the first hair cycle. Whether SCs exist or function earlier is unknown. Here we show that quiescent cells appear early in skin development, express SC markers, and later give rise to the adult SC population. To test whether early quiescent cells function as SCs, we use Sox9-Cre for genetic marking and K14-Cre to embryonically ablate Sox9, an essential adult SC gene. We find that the progeny of Sox9-expressing cells contribute to all skin epithelial lineages and Sox9 is required for SC specification. In the absence of early SCs, hair follicle and sebaceous gland morphogenesis is blocked and epidermal wound repair is compromised. These findings establish the existence of early hair follicle SCs and reveal their physiological importance in tissue morphogenesis.

SummaryStem and progenitor cells utilize asymmetric cell divisions to balance proliferation and differentiation. Evidence from lower eukaryotes shows that this process is regulated by proteins asymmetrically distributed at the cell cortex during mitosis: (1) Par3-Par6-aPKC, conferring polarity; (2) Gαi-LGN/AGS3-NuMA-p150glued, governing spindle positioning. Here, we focus on developing mouse skin, where progenitors execute a switch from predominantly symmetric to asymmetric divisions concomitant with stratification. Using in vivo skin-specific lentiviral RNAi, we investigate spindle orientation regulation and provide direct evidence that LGN, Numa1 and Dctn1 are involved. In compromising asymmetric cell divisions, we uncover profound defects in stratification, differentiation and barrier formation, and implicate Notch signalling as an important effector. Our study demonstrates the efficacy of applying RNAi in vivo to mammalian systems, and the ease of uncovering complex genetic interactions, here to gain insights into how changes in spindle orientation are coupled to establishing proper tissue architecture during skin development.

During embryogenesis, multipotent progenitors within the single-layered surface epithelium differentiate to form the epidermis and its appendages. Here, we show that microRNAs (miRNAs) have an essential role in orchestrating these events. We cloned more than 100 miRNAs from skin and show that epidermis and hair follicles differentially express discrete miRNA families. To explore the functional significance of this finding, we conditionally targeted Dicer1 gene ablation in embryonic skin progenitors. Within the first week after loss of miRNA expression, cell fate specification and differentiation were not markedly impaired, and in the interfollicular epidermis, apoptosis was not markedly increased. Notably, however, developing hair germs evaginate rather than invaginate, thereby perturbing the epidermal organization. Here we characterize miRNAs in skin, the existence of which was hitherto unappreciated, and demonstrate their differential expression and importance in the morphogenesis of epithelial tissues within this vital organ.

Mammalian epidermis consists of a basal layer of proliferative progenitors that gives rise to multiple differentiating layers to provide a waterproof envelope covering the skin surface. To accomplish this, progenitor cells must detach from the basal layer, move upward, and execute a terminal differentiation program consisting of three distinct stages: spinous, granular layer, and stratum corneum. Notch signaling has been implicated in late stages of differentiation, but the commitment switch remains unknown. Here we show with loss and gain-of-function studies that active Notch intracellular domain (NICD) and its obligate canonical signaling partner RBP-J act at the basal/suprabasal juncture to induce spinous and down-regulate basal fate. Spinous layers are absent in RBP-J conditional null epidermis and expanded when Notch1 signaling is elevated transgenically in epidermis. We show that RBP-J is essential for mediating both spinous gene activation and basal gene repression. In contrast, the NICD/RBP-J target gene Hes1 is expressed in spinous layers and mediates spinous gene induction but not basal gene repression. These data uncover an early role for RBP-J and Notch in commitment of epidermal cells to terminally differentiate and reveal that spinous gene induction is mediated by a Hes1-dependent mechanism, while basal gene repression occurs independently of Hes1.[Keywords: RBP-J; Hes1; Notch; epidermis; stem cell fate] Supplemental material is available at http://www.genesdev.org.

Highlights d Hyperactive neurons release excess FAs in lipid particles associated with ApoE d Astrocytes endocytose neuron-derived lipid particles, delivering the FAs to LDs d Astrocytes with LDs upregulate metabolic and detoxification genes d Neural activity triggers astrocytic consumption of FAs by mitochondrial oxidation