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

Gata6 promotes hair follicle progenitor cell renewal by genome maintenance during proliferation

Cell proliferation is essential to rapid tissue growth and repair, but can result in replication-associated genome damage. Here, we implicate the transcription factor Gata6 in adult mouse hair follicle regeneration where it controls the renewal of rapidly proliferating epithelial (matrix) progenitors and hence the extent of production of terminally differentiated lineages. We find that Gata6 protects against DNA damage associated with proliferation, thus preventing cell cycle arrest and apoptosis. Furthermore, we show that in vivo Gata6 stimulates EDA-receptor signaling adaptor Edaradd level and NF-κB pathway activation, known to be important for DNA damage repair and stress response in general and for hair follicle growth in particular. In cultured keratinocytes, Edaradd rescues DNA damage, cell survival, and proliferation of Gata6 knockout cells and restores MCM10 expression. Our data add to recent evidence in embryonic stem and neural progenitor cells, suggesting a model whereby developmentally regulated transcription factors protect from DNA damage associated with proliferation at key stages of rapid tissue growth. Our data may add to understanding why Gata6 is a frequent target of amplification in cancers.

Transient RUNX1 Expression during Early Mesendodermal Differentiation of hESCs Promotes Epithelial to Mesenchymal Transition through TGFB2 Signaling

The transition of human embryonic stem cells (hESCs) from pluripotency to lineage commitment is not fully understood, and a role for phenotypic transcription factors in the initial stages of hESC differentiation remains to be explored. From a screen of candidate factors, we found that RUNX1 is selectively and transiently upregulated early in hESC differentiation to mesendodermal lineages. Transcriptome profiling and functional analyses upon RUNX1 depletion established a role for RUNX1 in promoting cell motility. In parallel, we discovered a loss of repression for several epithelial genes, indicating that loss of RUNX1 impaired an epithelial to mesenchymal transition during differentiation. Cell biological and biochemical approaches revealed that RUNX1 depletion specifically compromised TGFB2 signaling. Both the decrease in motility and deregulated epithelial marker expression upon RUNX1 depletion were rescued by reintroduction of TGFB2, but not TGFB1. These findings identify roles for RUNX1-TGFB2 signaling in early events of mesendodermal lineage commitment.

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RUNX1 is transiently upregulated during early mesendoderm differentiation of hESCs

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RUNX1 promotes motility and the EMT process during mesendodermal differentiation

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RUNX1 knockdown specifically inhibits TGFB2 signaling

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Reintroduction of TGFB2, but not TGFB1, rescues the phenotype of RUNX1 depletion

sPLA2 -IIA Overexpression in Mice Epidermis Depletes Hair Follicle Stem Cells and Induces Differentiation Mediated Through Enhanced JNK/c-Jun Activation

Secretory phospholipase A2 Group-IIA (sPLA2 -IIA) catalyzes the hydrolysis of the sn-2 position of glycerophospholipids to yield fatty acids and lysophospholipids. sPLA2 -IIA is deregulated in various cancers; however, its role in hair follicle stem cell (HFSC) regulation is obscure. Here we report a transgenic mice overexpressing sPLA2 -IIA (K14-sPLA2 -IIA) showed depletion of HFSC pool. This was accompanied with increased differentiation, loss of ortho-parakeratotic organization and enlargement of sebaceous gland, infundibulum and junctional zone. The colony forming efficiency of keratinocytes was significantly reduced. Microarray profiling of HFSCs revealed enhanced level of epithelial mitogens and transcription factors, c-Jun and FosB that may be involved in proliferation and differentiation. Moreover, K14-sPLA2 -IIA keratinocytes showed enhanced activation of EGFR and JNK1/2 that led to c-Jun activation, which co-related with enhanced differentiation. Further, depletion of stem cells in bulge is associated with high levels of chromatin silencing mark, H3K27me3 and low levels of an activator mark, H3K9ac suggestive of alteration in gene expression contributing toward stem cells differentiation. Our results, first time uncovered that overexpression of sPLA2 -IIA lead to depletion of HFSCs and differentiation associated with altered histone modification. Thus involvement of sPLA2 -IIA in stem cells regulation and disease pathogenesis suggest its prospective clinical implications. Stem Cells 2016;34:2407-2417.

Foxi3 Deficiency Compromises Hair Follicle Stem Cell Specification and Activation

The hair follicle is an ideal system to study stem cell specification and homeostasis due to its well characterized morphogenesis and stereotypic cycles of stem cell activation upon each hair cycle to produce a new hair shaft. The adult hair follicle stem cell niche consists of two distinct populations, the bulge and the more activation-prone secondary hair germ (HG). Hair follicle stem cells are set aside during early stages of morphogenesis. This process is known to depend on the Sox9 transcription factor, but otherwise the establishment of the hair follicle stem cell niche is poorly understood. Here, we show that that mutation of Foxi3, a Forkhead family transcription factor mutated in several hairless dog breeds, compromises stem cell specification. Further, loss of Foxi3 impedes hair follicle downgrowth and progression of the hair cycle. Genome-wide profiling revealed a number of downstream effectors of Foxi3 including transcription factors with a recognized function in hair follicle stem cells such as Lhx2, Runx1, and Nfatc1, suggesting that the Foxi3 mutant phenotype results from simultaneous downregulation of several stem cell signature genes. We show that Foxi3 displays a highly dynamic expression pattern during hair morphogenesis and cycling, and identify Foxi3 as a novel secondary HG marker. Absence of Foxi3 results in poor hair regeneration upon hair plucking, and a sparse fur phenotype in unperturbed mice that exacerbates with age, caused by impaired secondary HG activation leading to progressive depletion of stem cells. Thus, Foxi3 regulates multiple aspects of hair follicle development and homeostasis. Stem Cells 2016;34:1896-1908.

The RUNX family: developmental regulators in cancer

RUNX proteins belong to a family of metazoan transcription factors that serve as master regulators of development. They are frequently deregulated in human cancers, indicating a prominent and, at times, paradoxical role in cancer pathogenesis. The contextual cues that direct RUNX function represent a fast-growing field in cancer research and could provide insights that are applicable to early cancer detection and treatment. This Review describes how RUNX proteins communicate with key signalling pathways during the multistep progression to malignancy; in particular, we highlight the emerging partnership of RUNX with p53 in cancer suppression.

A transcriptional mechanism integrating inputs from extracellular signals to activate hippocampal stem cells

The activity of adult stem cells is regulated by signals emanating from the surrounding tissue. Many niche signals have been identified, but it is unclear how they influence the choice of stem cells to remain quiescent or divide. Here we show that when stem cells of the adult hippocampus receive activating signals, they first induce the expression of the transcription factor Ascl1 and only subsequently exit quiescence. Moreover, lowering Ascl1 expression reduces the proliferation rate of hippocampal stem cells, and inactivating Ascl1 blocks quiescence exit completely, rendering them unresponsive to activating stimuli. Ascl1 promotes the proliferation of hippocampal stem cells by directly regulating the expression of cell-cycle regulatory genes. Ascl1 is similarly required for stem cell activation in the adult subventricular zone. Our results support a model whereby Ascl1 integrates inputs from both stimulatory and inhibitory signals and converts them into a transcriptional program activating adult neural stem cells.

Proteomic profiling of cardiac tissue by isolation of nuclei tagged in specific cell types (INTACT)

The proper dissection of the molecular mechanisms governing the specification and differentiation of specific cell types requires isolation of pure cell populations from heterogeneous tissues and whole organisms. Here, we describe a method for purification of nuclei from defined cell or tissue types in vertebrate embryos using INTACT (isolation of nuclei tagged in specific cell types). This method, previously developed in plants, flies and worms, utilizes in vivo tagging of the nuclear envelope with biotin and the subsequent affinity purification of the labeled nuclei. In this study we successfully purified nuclei of cardiac and skeletal muscle from Xenopus using this strategy. We went on to demonstrate the utility of this approach by coupling the INTACT approach with liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomic methodologies to profile proteins expressed in the nuclei of developing hearts. From these studies we have identified the Xenopus orthologs of 12 human proteins encoded by genes, which when mutated in human lead to congenital heart disease. Thus, by combining these technologies we are able to identify tissue-specific proteins that are expressed and required for normal vertebrate organ development.

Runx family genes in a cartilaginous fish, the elephant shark (Callorhinchus milii)

The Runx family genes encode transcription factors that play key roles in hematopoiesis, skeletogenesis and neurogenesis and are often implicated in diseases. We describe here the cloning and characterization of Runx1, Runx2, Runx3 and Runxb genes in the elephant shark (Callorhinchus milii), a member of Chondrichthyes, the oldest living group of jawed vertebrates. Through the use of alternative promoters and/or alternative splicing, each of the elephant shark Runx genes expresses multiple isoforms similar to their orthologs in human and other bony vertebrates. The expression profiles of elephant shark Runx genes are similar to those of mammalian Runx genes. The syntenic blocks of genes at the elephant shark Runx gene loci are highly conserved in human, but represented by shorter conserved blocks in zebrafish indicating a higher degree of rearrangements in this teleost fish. Analysis of promoter regions revealed conservation of binding sites for transcription factors, including two tandem binding sites for Runx that are totally conserved in the distal promoter regions of elephant shark Runx1-3. Several conserved noncoding elements (CNEs), which are putative cis-regulatory elements, and miRNA binding sites were identified in the elephant shark and human Runx gene loci. Some of these CNEs and miRNA binding sites are absent in teleost fishes such as zebrafish and fugu. In summary, our analysis reveals that the genomic organization and expression profiles of Runx genes were already complex in the common ancestor of jawed vertebrates.

Adult hair follicle stem cells do not retain the older DNA strands in vivo during normal tissue homeostasis

Tissue stem cells have been proposed to segregate the chromosomes asymmetrically (in a non-random manner), thereby retaining preferentially the older "immortal" DNA strands bearing the stemness characteristics into one daughter cell, whereas the newly synthesized strands are segregated to the other daughter cell that will commit to differentiation. Moreover, this non-random segregation would protect the stem cell genome from accumulating multiple mutations during repeated DNA replication. This long-standing hypothesis remains an active subject of study due to conflicting results for some systems and lack of consistency among different tissue stem cell populations. In this review, we will focus on work done in the hair follicle, which is one of the best-understood vertebrate tissue stem cell system to date. In cell culture analysis of paired cultured keratinocytes derived from hair follicle, stem cells suggested a non-random segregation of chromosome with respect to the older DNA strand. In vivo, the hair follicle stem cells appear to self-renew and differentiate at different phases of their homeostatic cycle. The fate decisions occur in quiescence when some stem cells migrate out of their niche and commit to differentiation without self-renewal. The stem cells left behind in the niche self-renew symmetrically and randomly segregate the chromosomes at each division, making more stem cells. This model seems to apply to at least a few other vertebrate tissue stem cells in vivo.

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. We find that forced Runx1 expression induces hair degeneration (catagen) and simultaneously promotes changes in the quiescent bulge SC transcriptome toward a cell state resembling the EP HG fate. This cell-state transition is functionally reversible. We propose that SC differentiation and dedifferentiation are likely to occur during normal HF degeneration and niche restructuring in response to changes in endogenous Runx1 levels associated with SC location with respect to the niche.

New insights into the role of Runx1 in epithelial stem cell biology and pathology

The transcription factor Runx1 has been studied in leukemia and blood for decades, but recently it has been also implicated in epithelial biology and pathology. Particularly in mouse skin Runx1 modulates Wnt signaling levels thereby regulating timely induction of hair follicle specification, proper maturation of the emerging adult hair follicle stem cells in embryogenesis, and timely stem cell (SC) activation during adult homeostasis. Moreover, Runx1 acts as a tumor promoter in mouse skin squamous tumor formation and maintenance, likely by repressing p21 and promoting Stat3 activation. Similarly, Runx1 is essential for oral epithelium tumorigenesis mediated in mice by Ras, and for growth of three kinds of human epithelial cancer cells. In contrast, Runx1 has a tumor suppressor function in the mouse intestine and shows tumor subtype specific behavior in human breast cancer. Multiple studies revealed Runx1 SNPs to be associated with human cancers and autoimmune disease. With this information as background, the field is poised for functional and mechanistic studies to elucidate the role of Runx1 in formation and/or progression of epithelial-based human disease.

Transcription factors Runx1 to 3 are expressed in the lacrimal gland epithelium and are involved in regulation of gland morphogenesis and regeneration

PURPOSE

Lacrimal gland (LG) morphogenesis and repair are regulated by a complex interplay of intrinsic factors (e.g., transcription factors) and extrinsic signals (e.g., soluble growth/signaling factors). Many of these interconnections remain poorly characterized. Runt-related (Runx) factors belong to a small family of heterodimeric transcription factors known to regulate lineage-specific proliferation and differentiation of stem cells. The purpose of this study was to define the expression pattern and the role of Runx proteins in LG development and regeneration.

METHODS

Expression of epithelial-restricted transcription factors in murine LG was examined using immunostaining, qRT-PCR, and RT(2)Profiler PCR microarrays. The role of Runx transcription factors in LG morphogenesis was studied using siRNA and ex vivo LG cultures. Expression of Runx transcription factors during LG regeneration was assessed using in vivo model of LG regeneration.

RESULTS

We found that Runx factors are expressed in the epithelial compartment of the LG; in particular, Runx1 was restricted to the epithelium with highest level of expression in ductal and centroacinar cells. Downregulation of Runx1 to 3 expression using Runx-specific siRNAs abolished LG growth and branching and our data suggest that Runx1, 2, and 3 are partially redundant in LG development. In siRNA-treated LG, reduction of branching correlated with reduction of epithelial proliferation, as well as expression of cyclin D1 and the putative epithelial progenitor cell marker cytokeratin-5. Runx1, Runx3, and cytokeratin-5 expression increased significantly in regenerating LG and there was modest increase in Runx2 expression during LG differentiation.

CONCLUSIONS

Runx1 and 2 are new markers of the LG epithelial lineage and Runx factors are important for normal LG morphogenesis and regeneration.

Runx1 and p21 synergistically limit the extent of hair follicle stem cell quiescence in vivo

Mechanisms of tissue stem cell (SC) quiescence control are important for normal homeostasis and for preventing cancer. Cyclin-dependent kinase inhibitors (CDKis) are known inhibitors of cell cycle progression. We document CDKis expression in vivo during hair follicle stem cell (HFSC) homeostasis and find p21 (cyclin-dependent kinase inhibitor 1a, Cdkn1a), p57, and p15 up-regulated at quiescence onset. p21 appears important for HFSC timely onset of quiescence. Conversely, we find that Runx1 (runt related transcription factor 1), which is known for promoting HFSC proliferation, represses p21, p27, p57, and p15 transcription in HFSC in vivo. Intriguingly, in cell culture, tumors, and normal homeostasis, Runx1 and p21 interplay modulates proliferation in opposing directions under the different conditions. Unexpectedly, Runx1 and p21 synergistically limit the extent of HFSC quiescence in vivo, which antagonizes the role of p21 as a cell cycle inhibitor. Importantly, we find in cultured keratinocytes that Runx1 and p21 bind to the p15 promoter and synergistically repress p15 mRNA transcription, thereby restraining cell cycle arrest. This documents a surprising ability of a CDKi (p21) to act as a direct transcriptional repressor of another CDKi (p15). We unveil a robust in vivo mechanism that enforces quiescence of HFSCs, and a context-dependent role of a CDKi (p21) to limit quiescence of SCs, potentially by directly down-regulating mRNA levels of (an)other CDKi(s).

Transcriptional control of epidermal stem cells

Transcriptional regulation is fundamentally important for the progression of tissue stem cells through different stages of development and differentiation. Mammalian skin epidermis is an excellent model system to study such regulatory mechanisms due to its easy accessibility, stereotypic spatial arrangement, and availability of well-established cell type/lineage differentiation markers. Moreover, epidermis is one of the few mammalian tissues the stem cells of which can be maintained and propagated in culture to generate mature cell types and a functional tissue (reviewed in [1]), offering in vitro and ex vivo platforms to probe deep into the underlying cell and molecular mechanisms of biological functions.

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. We implicate Runx1 as a SC-intrinsic gene in mouse hair follicle and oral epithelia by genetic lineage tracing in adulthood. Runx1-expressing SCs, but not other cells that ectopically upregulate Runx1 by injury and inflammation, are at the skin tumour origin. Runx1 loss impairs tumour initiation and maintenance and the growth of oral, skin, and ovarian epithelial human cancer cells. Runx1 stimulates Stat3 signalling via direct transcriptional repression of SOCS3 and SOCS4 and this is essential for cancer cell growth. Thus, Runx1 is a broader epithelial SC and cancer factor than previously recognized, and qualifies as an attractive potential target for both prevention and therapy of several epithelial cancers.

Mesenchymal-epithelial interactions during hair follicle morphogenesis and cycling

Embryonic hair follicle induction and formation are regulated by mesenchymal-epithelial interactions between specialized dermal cells and epidermal stem cells that switch to a hair fate. Similarly, during postnatal hair growth, communication between mesenchymal dermal papilla cells and surrounding epithelial matrix cells coordinates hair shaft production. Adult hair follicle regeneration in the hair cycle again is thought to be controlled by activating signals originating from the mesenchymal compartment and acting on hair follicle stem cells. Although many signaling pathways are implicated in hair follicle formation and growth, the precise nature, timing, and intersection of these inductive and regulatory signals remains elusive. The goal of this review is to summarize our current understanding and to discuss recent new insights into mesenchymal-epithelial interactions during hair follicle morphogenesis and cycling.

Hairy tale of signaling in hair follicle development and cycling

Hair follicles (HFs) is an appendage from the vertebrate skin epithelium, and is critical for environmental sensing, animal appearance, and body heat maintenance. HFs arise from the embryonic ectoderm and regenerate cyclically during adult life. Distinct morphological and functional stages from development through homeostasis have been extensively studied for the past decades to dissect the critical molecular mechanisms. Accumulating work suggests that different signaling cascades, such as Wnt, Bmp, Shh, and Notch, together with specific combinations of transcription factors are at work at different stages. Here we provide a comprehensive review of mouse genetics studies, which include lineage tracing along with knockout and over-expression of core genes from key signaling pathways, to paint an updated view of the molecular regulatory network that govern each stage of hair follicle development and adult cycling.

DNA methylation dynamics during in vivo differentiation of blood and skin stem cells

DNA methylation is a mechanism of epigenetic regulation that is common to all vertebrates. Functional studies underscore its relevance for tissue homeostasis, but the global dynamics of DNA methylation during in vivo differentiation remain underexplored. Here we report high-resolution DNA methylation maps of adult stem cell differentiation in mouse, focusing on 19 purified cell populations of the blood and skin lineages. DNA methylation changes were locus specific and relatively modest in magnitude. They frequently overlapped with lineage-associated transcription factors and their binding sites, suggesting that DNA methylation may protect cells from aberrant transcription factor activation. DNA methylation and gene expression provided complementary information, and combining the two enabled us to infer the cellular differentiation hierarchy of the blood lineage directly from genome-scale data. In summary, these results demonstrate that in vivo differentiation of adult stem cells is associated with small but informative changes in the genomic distribution of DNA methylation.

Development and homeostasis of the skin epidermis

The skin epidermis is a stratified epithelium that forms a barrier that protects animals from dehydration, mechanical stress, and infections. The epidermis encompasses different appendages, such as the hair follicle (HF), the sebaceous gland (SG), the sweat gland, and the touch dome, that are essential for thermoregulation, sensing the environment, and influencing social behavior. The epidermis undergoes a constant turnover and distinct stem cells (SCs) are responsible for the homeostasis of the different epidermal compartments. Deregulation of the signaling pathways controlling the balance between renewal and differentiation often leads to cancer formation.

Functional interplay between p63 and p53 controls RUNX1 function in the transition from proliferation to differentiation in human keratinocytes

The interfollicular epidermis is continuously renewed, thanks to a regulated balance between proliferation and differentiation. The ΔNp63 transcription factor has a key role in the control of this process. It has been shown that ΔNp63 directly regulates Runt-related transcription factor 1 (RUNX1) transcription factor expression in mouse keratinocytes. The present study showed for the first time that RUNX1 is expressed in normal human interfollicular epidermis and that its expression is tightly regulated during the transition from proliferation to differentiation. It demonstrated that ΔNp63 directly binds two different RUNX1 regulatory DNA sequences and modulates RUNX1 expression differentially in proliferative or differentiated human keratinocytes. It also showed that the regulation of RUNX1 expression by ΔNp63 is dependent on p53 and that this coregulation relies on differential binding and activation of RUNX1 regulatory sequences by ΔNp63 and p53. We also found that RUNX1 inhibits keratinocyte proliferation and activates directly the expression of KRT1, a critical actor in early keratinocyte differentiation. Finally, we described that RUNX1 expression, similar to ΔNp63 and p53, was strongly expressed and downregulated in basal cell carcinomas and squamous cell carcinomas respectively. Taken together, these data shed light on the importance of tight control of the functional interplay between ΔNp63 and p53 in regulating RUNX1 transcription factor expression for proper regulation of interfollicular epidermal homeostasis.

Core binding factor beta functions in the maintenance of stem cells and orchestrates continuous proliferation and differentiation in mouse incisors

Rodent incisors grow continuously throughout life, and epithelial progenitor cells are supplied from stem cells in the cervical loop. We report that epithelial Runx genes are involved in the maintenance of epithelial stem cells and their subsequent continuous differentiation and therefore growth of the incisors. Core binding factor β (Cbfb) acts as a binding partner for all Runx proteins, and targeted inactivation of this molecule abrogates the activity of all Runx complexes. Mice deficient in epithelial Cbfb produce short incisors and display marked underdevelopment of the cervical loop and suppressed epithelial Fgf9 expression and mesenchymal Fgf3 and Fgf10 expression in the cervical loop. In culture, FGF9 protein rescues these phenotypes. These findings indicate that epithelial Runx functions to maintain epithelial stem cells and that Fgf9 may be a target gene of Runx signaling. Cbfb mutants also lack enamel formation and display downregulated Shh mRNA expression in cells differentiating into ameloblasts. Furthermore, Fgf9 deficiency results in a proximal shift of the Shh expressing cell population and ectopic FGF9 protein suppresses Shh expression. These findings indicate that Shh as well as Fgf9 expression is maintained by Runx/Cbfb but that Fgf9 antagonizes Shh expression. The present results provide the first genetic evidence that Runx/Cbfb genes function in the maintenance of stem cells in developing incisors by activating Fgf signaling loops between the epithelium and mesenchyme. In addition, Runx genes also orchestrate continuous proliferation and differentiation by maintaining the expression of Fgf9 and Shh mRNA.

An RNA interference screen uncovers a new molecule in stem cell self-renewal and long-term regeneration

Adult stem cells sustain tissue maintenance and regeneration throughout the lifetime of an animal. These cells often reside in specific signalling niches that orchestrate the stem cell's balancing act between quiescence and cell-cycle re-entry based on the demand for tissue regeneration. How stem cells maintain their capacity to replenish themselves after tissue regeneration is poorly understood. Here we use RNA-interference-based loss-of-function screening as a powerful approach to uncover transcriptional regulators that govern the self-renewal capacity and regenerative potential of stem cells. Hair follicle stem cells provide an ideal model. These cells have been purified and characterized from their native niche in vivo and, in contrast to their rapidly dividing progeny, they can be maintained and passaged long-term in vitro. Focusing on the nuclear proteins and/or transcription factors that are enriched in stem cells compared with their progeny, we screened ∼2,000 short hairpin RNAs for their effect on long-term, but not short-term, stem cell self-renewal in vitro. To address the physiological relevance of our findings, we selected one candidate that was uncovered in the screen: TBX1. This transcription factor is expressed in many tissues but has not been studied in the context of stem cell biology. By conditionally ablating Tbx1 in vivo, we showed that during homeostasis, tissue regeneration occurs normally but is markedly delayed. We then devised an in vivo assay for stem cell replenishment and found that when challenged with repetitive rounds of regeneration, the Tbx1-deficient stem cell niche becomes progressively depleted. Addressing the mechanism of TBX1 action, we discovered that TBX1 acts as an intrinsic rheostat of BMP signalling: it is a gatekeeper that governs the transition between stem cell quiescence and proliferation in hair follicles. Our results validate the RNA interference screen and underscore its power in unearthing new molecules that govern stem cell self-renewal and tissue-regenerative potential.

Home sweet home: skin stem cell niches

The epidermis and its appendages, such as the hair follicle (HF), continually regenerate throughout postnatal mammalian life due to the activity of resident epithelial stem cells (SCs). The follicular SC niche, or the bulge, is composed of a heterogeneous population of self-renewing multipotent cells. Multiple intrinsic molecular mechanisms promote the transition of follicular SCs from quiescence to activation. In addition, numerous extrinsic cell types influence the activity and characteristics of bulge cells. Ultimately, the balance between these intrinsic and extrinsic mechanisms influences the function of bulge cells during homeostasis and tissue regeneration and likely contributes to skin tumorigenesis. Here, we review both the intrinsic and extrinsic factors that contribute to the skin SC niche.

Ontogeny and Homeostasis of Adult Epithelial Skin Stem Cells

Mouse epithelial skin stem cells constitute an important model system for understanding the dynamics of stem cell emergence and behavior in an intact vertebrate tissue. Recent published work defined discrete populations of epithelial stem cells in the adult skin epithelium, which reside in the hair follicle bulge and germ, isthmus, sebaceous gland and inter-follicular epidermis. Adult epidermal and hair follicle stem cells seem to adopt mostly symmetric or unidirectional fate decisions of either one of two possible fates: (1) differentiate and be lost from the tissue or (2) expand symmetrically to self-renew. Asymmetric divisions appear to be mostly implicated in differentiation and stratification of the epidermis. While mechanisms of adult stem cell homeostasis begin to be unraveled, the embryonic origin of the adult epithelial skin stem cells is poorly understood. Recent studies reported Sox9, Lgr6, and Runx1 expression in subpopulations of cells in the embryonic hair placode. These subpopulations seem to act as precursors of different classes of adult epithelial stem cells. In particular, Runx1 regulates a Wnt-mediated cross-talk between the nascent adult-type hair follicle stem cells and their environment, which is essential for timely stem cell emergence, proper maturation, long-term differentiation potential, and maintenance. The new data begin to define the basic dynamics and regulatory pathways governing the ontogeny of adult epithelial stem cells.

Runx1 loss minimally impacts long-term hematopoietic stem cells

RUNX1 encodes a DNA binding subunit of the core-binding transcription factors and is frequently mutated in acute leukemia, therapy-related leukemia, myelodysplastic syndrome, and chronic myelomonocytic leukemia. Mutations in RUNX1 are thought to confer upon hematopoietic stem cells (HSCs) a pre-leukemic state, but the fundamental properties of Runx1 deficient pre-leukemic HSCs are not well defined. Here we show that Runx1 deficiency decreases both apoptosis and proliferation, but only minimally impacts the frequency of long term repopulating HSCs (LT-HSCs). It has been variously reported that Runx1 loss increases LT-HSC numbers, decreases LT-HSC numbers, or causes age-related HSC exhaustion. We attempt to resolve these discrepancies by showing that Runx1 deficiency alters the expression of several key HSC markers, and that the number of functional LT-HSCs varies depending on the criteria used to score them. Finally, we identify genes and pathways, including the cell cycle and p53 pathways that are dysregulated in Runx1 deficient HSCs.

RUNX1 and its understudied role in breast cancer

The transcription factor Runt-related transcription factor 1 (RUNX1) is critical for the earliest steps of hematopoiesis. RUNX1 was originally identified as a gene fusion in acute myeloid leukemia (AML) and thus has garnered heavy attention as a tumor suppressor in hematopoietic malignancies. However, RUNX1 is also strongly expressed in breast epithelia and may be misregulated during tumorigenesis. Here, I discuss our recent work implicating RUNX1 in proliferation control during breast epithelial-acinar morphogenesis. My goal is to place these findings in the context of a handful of other reports, which together argue that RUNX1 could act as a tumor suppressor gene in breast cancer. Testing this hypothesis requires focused in vivo studies, because the major commercial platform for global mRNA expression profiling does not reliably reflect RUNX1 levels. Our in vitro results indicate that hyperproliferation in RUNX1-deficient breast epithelia relies on another family of transcription factors, the Forkhead box O (FOXO) proteins. FOXOs could, therefore, represent a synthetic-lethal target for RUNX1-deficient tumors if the hypothesized link to breast cancer is correct.

Runx1 modulates adult hair follicle stem cell emergence and maintenance from distinct embryonic skin compartments

Runx1 controls timing of adult HSFC and short-term progenitor emergence in the embryonic epithelium and HFSC differentiation and long-term skin integrity in embryonic mesenchyme.

Runx1 controls hematopoietic stem cell emergence and hair follicle stem cell (HFSC) activation and proliferation in adult skin. Here we use lineage tracing and mouse genetic manipulation to address the role of Runx1 in the embryonic development of HFSCs. We find Runx1 is expressed in distinct classes of embryonic skin precursors for short-term HF progenitors, adult HFSCs, and mesenchymal progenitors. Runx1 acts in the embryonic epithelium for timely emergence of adult HFSCs and short-term progenitors, but is dispensable for both of them. In contrast, Runx1 is strictly needed in the embryonic mesenchyme for proper adult HFSC differentiation and long-term skin integrity. Our data implicate Runx1 in epithelial cell adhesion and migration and in regulation of paracrine epithelial–mesenchymal cross talk. The latter involves Lef1 and Wnt signaling modulation in opposing directions from two distinct skin compartments. Thus, a master regulator of hematopoiesis also controls HFSC emergence and maintenance via modulation of bidirectional cross talking between nascent stem cells and their niche.

Stem cell dynamics in mouse hair follicles: a story from cell division counting and single cell lineage tracing

Understanding tissue stem cells behavior is a prerequisite for elucidating the mechanisms that govern their self-renewal and differentiation. Previously, we provided single cell lineage tracing and proliferation history data (based on H2B-GFP label dilution over time) in mouse hair follicles. We proposed a population deterministic model with symmetric stem cell fate decisions throughout life. Here we provide data suggesting that in hair follicle stem cells the self-renewing divisions within the niche (bulge) are symmetric with respect to localization of daughter cells near the basement membrane, an important niche component. In contrast, when cells migrate from the niche to the differentiating zone where they become short-lived progenitors, their daughter cells can orient themselves asymmetrically relative to the basement membrane. Furthermore, we document the dynamic re-localization of cells within the bulge to accommodate the hair follicle morphological changes through hair cycle. In addition, we provide a method to compute the change in number of cells generated by division from H2B-GFP pulse-chase data, and to estimate the minimum cell loss encountered when the fold change can be experimentally determined. We computed a minimum of 42% of bulge cell loss during one hair cycle, a massive rate of loss previously unrecognized. Finally, we showed that a multipotent population of cells found at the junction zone between hair follicle and epidermis, known to express Lrig1, cycle more rapidly than some other hair follicle compartments.

Abnormal hair follicle development and altered cell fate of follicular keratinocytes in transgenic mice expressing DeltaNp63alpha

The transcription factor p63 plays an essential role in epidermal morphogenesis. Animals lacking p63 fail to form many ectodermal organs, including the skin and hair follicles. Although the indispensable role of p63 in stratified epithelial skin development is well established, relatively little is known about this transcriptional regulator in directing hair follicle morphogenesis. Here, using specific antibodies, we have established the expression pattern of DeltaNp63 in hair follicle development and cycling. DeltaNp63 is expressed in the developing hair placode, whereas in mature hair its expression is restricted to the outer root sheath (ORS), matrix cells and to the stem cells of the hair follicle bulge. To investigate the role of DeltaNp63 in hair follicle morphogenesis and cycling, we have utilized a Tet-inducible mouse model system with targeted expression of this isoform to the ORS of the hair follicle. DeltaNp63 transgenic animals display dramatic defects in hair follicle development and cycling, eventually leading to severe hair loss. Strikingly, expression of DeltaNp63 leads to a switch in cell fate of hair follicle keratinocytes, causing them to adopt an interfollicular epidermal (IFE) cell identity. Moreover, DeltaNp63 transgenic animals exhibit a depleted hair follicle stem-cell niche, which further contributes to the overall cycling defects observed in the mutant animals. Finally, global transcriptome analysis of transgenic skin identified altered expression levels of crucial mediators of hair morphogenesis, including key members of the Wnt/beta-catenin signaling pathway, which, in part, account for these effects. Our data provide evidence supporting a role for DeltaNp63alpha in actively suppressing hair follicle differentiation and directing IFE cell lineage commitment.

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. By deleting Runx1 after the end of morphogenesis, we demonstrate its direct role in promoting anagen onset and HFSC proliferation. Runx1 deletion resulted in cyclin-dependent kinase inhibitor Cdkn1a (p21) upregulation. Interfering with Runx1 function in cultured HFSCs impaired their proliferation and normal G(0)/G1 and G(1)/S cell cycle progression. The proliferation defect could be rescued by Runx1 readdition or by p21 deletion. Chemically induced skin tumorigenesis in mice turned on broad Runx1 expression in regions of the skin epithelium, papillomas, and squamous cell carcinomas. In addition, it revealed reduced rates of tumor formation in the absence of Runx1 that were accompanied by decreased epithelial levels of phospho-Stat3. Runx1 protein expression was similar in normal human and mouse hair cycles. We propose that Runx1 may act as a skin oncogene by directly promoting proliferation of the epithelial cells.

The core binding factor CBF negatively regulates skeletal muscle terminal differentiation

BACKGROUND

Core Binding Factor or CBF is a transcription factor composed of two subunits, Runx1/AML-1 and CBF beta or CBFbeta. CBF was originally described as a regulator of hematopoiesis.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that CBF is involved in the control of skeletal muscle terminal differentiation. Indeed, downregulation of either Runx1 or CBFbeta protein level accelerates cell cycle exit and muscle terminal differentiation. Conversely, overexpression of CBFbeta in myoblasts slows terminal differentiation. CBF interacts directly with the master myogenic transcription factor MyoD, preferentially in proliferating myoblasts, via Runx1 subunit. In addition, we show a preferential recruitment of Runx1 protein to MyoD target genes in proliferating myoblasts. The MyoD/CBF complex contains several chromatin modifying enzymes that inhibits MyoD activity, such as HDACs, Suv39h1 and HP1beta. When overexpressed, CBFbeta induced an inhibition of activating histone modification marks concomitant with an increase in repressive modifications at MyoD target promoters.

CONCLUSIONS/SIGNIFICANCE

Taken together, our data show a new role for Runx1/CBFbeta in the control of the proliferation/differentiation in skeletal myoblasts.

Runx family genes, niche, and stem cell quiescence

In multicellular organisms, terminally differentiated cells of most tissues are short-lived and therefore require constant replenishment from rapidly dividing stem cells for homeostasis and tissue repair. For the stem cells to last throughout the lifetime of the organism, however, a small subset of stem cells, which are maintained in a hibernation-like state known as stem cell quiescence, is required. Such dormant stem cells reside in the niche and are activated into proliferation only when necessary. A multitude of factors are required for the maintenance of stem cell quiescence and niche. In particular, the Runx family genes have been implicated in stem cell quiescence in various organisms and tissues. In this review, we discuss the maintenance of stem cell quiescence in various tissues, mainly in the context of the Runx family genes, and with special focus on the hematopoietic system.

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. We provide direct evidence for the infrequent stem cell division model in intact tissue. Moreover, we find that differentiation of progenitor cells occurs at different times and tissue locations than self-renewal of stem cells. Distinct fates of differentiation or self-renewal are assigned to individual cells in a temporal-spatial manner. We propose that large clusters of tissue stem cells behave as populations whose maintenance involves unidirectional daughter-cell-fate decisions.

Krüppel-like factor 4, a "pluripotency transcription factor" highly expressed in male postmeiotic germ cells, is dispensable for spermatogenesis in the mouse

Krüppel-like factor 4 (Klf4, GKLF) was originally characterized as a zinc finger transcription factor essential for terminal differentiation and cell lineage allocation of several cell types in the mouse. Mice lacking Klf4 die postnatally within hours due to impaired skin barrier function and subsequent dehydration. Recently, KLF4 was also used in cooperation with other transcription factors to reprogram differentiated cells to pluripotent embryonic stem cell-like cells. Moreover, involvement in oncogenesis was also ascribed to KLF4, which is aberrantly expressed in some types of tumors such as breast, gastric and colon cancer. We previously have shown that Klf4 is strongly expressed in postmeiotic germ cells of mouse and human testes suggesting a role for Klf4 also during spermiogenesis. In order to analyze its function we deleted Klf4 in germ cells using the Cre-loxP system. Homologous recombination of the Klf4 locus has been confirmed by genomic southern blotting and the absence of the protein in germ cells was demonstrated by Western blotting and immunofluorescence. Despite its important roles in several significant biological settings, deletion of Klf4 in germ cells did not impair spermiogenesis. Histologically, the mutant testes appeared normal and the mice were fertile. In order to identify genes that were regulated by KLF4 in male germ cells we performed microarray analyses using a whole genome array. We identified many genes exhibiting changed expression in mutants even including the telomerase reverse transcriptase mRNA, which is a stem cell marker. However, in summary, the lack of KLF4 alone does not prevent complete spermatogenesis.

The hair follicle as a dynamic miniorgan

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. The hair shaft consists of terminally differentiated keratinocytes that are produced by the hair follicle. Hair follicle development takes place during fetal skin development and relies on tightly regulated ectodermal-mesodermal interactions. After birth, mature and actively growing hair follicles eventually become anchored in the subcutis, and periodically regenerate by spontaneously undergoing repetitive cycles of growth (anagen), apoptosis-driven regression (catagen), and relative quiescence (telogen). Our molecular understanding of hair follicle biology relies heavily on mouse mutants with abnormalities in hair structure, growth, and/or pigmentation. These mice have allowed novel insights into important general molecular and cellular processes beyond skin and hair biology, ranging from organ induction, morphogenesis and regeneration, to pigment and stem cell biology, cell proliferation, migration and apoptosis. In this review, we present basic concepts of hair follicle biology and summarize important recent advances in the field.

Epidermal homeostasis: a balancing act of stem cells in the skin

The skin epidermis and its array of appendages undergo ongoing renewal by a process called homeostasis. Stem cells in the epidermis have a crucial role in maintaining tissue homeostasis by providing new cells to replace those that are constantly lost during tissue turnover or following injury. Different resident skin stem cell pools contribute to the maintenance and repair of the various epidermal tissues of the skin, including interfollicular epidermis, hair follicles and sebaceous glands. Interestingly, the basic mechanisms and signalling pathways that orchestrate epithelial morphogenesis in the skin are reused during adult life to regulate skin homeostasis.

Runx1 is involved in the fusion of the primary and the secondary palatal shelves

Runx1 is expressed in medial edge epithelial (MEE) cells of the palatal shelf. Conditionally rescued Runx1(-/-) mice showed limited clefting in the anterior junction between the primary and the secondary palatal shelves, but not in the junction between the secondary palates. In wild type mice, the fusing epithelial surface exhibited a rounded cobblestone-like appearance, while such cellular prominence was less evident in the Runx1 mutants. We also found that Fgf18 was expressed in the mesenchyme underlying the MEE and that locally applied FGF18 induced ectopic Runx1 expression in the epithelium of the palatal explants, indicating that Runx1 was induced by mesenchymal Fgf18 signaling. On the other hand, unpaired palatal explant cultures revealed the presence of anterior-posterior (A-P) differences in the MEE fates and fusion mechanism. Interestingly, the location of anterior clefting in Runx1 mutants corresponded to the region with different MEE behavior. These data showed a novel function of Runx1 in morphological changes in the MEE cells in palatal fusion, which is, at least in part, regulated by the mesenchymal Fgf signaling via an epithelial-mesenchymal interaction.

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. To address this question, we developed a strategy to count divisions of hair follicle (HF) SCs over time, and provide the first quantitative proliferation history of a tissue SC during its normal homoeostasis. We uncovered an unexpectedly high cellular turnover in the SC compartment in one round of activation. Our study provides quantitative data in support of the long-standing infrequent SC division model, and shows that HF SCs do not retain the older DNA strands or sort their chromosome. This new ability to count divisions in vivo has relevance for obtaining basic knowledge of tissue kinetics.