Slug deficiency enhances self-renewal of hematopoietic stem cells during hematopoietic regeneration

Epithelial-to-mesenchymal transition transcription factors: New strategies for mesenchymal tissue regeneration

The epithelial-mesenchymal transition transcription factors (EMT-TFs)-ZEB, SNAI, and TWIST families-have been extensively studied in embryonic development and tumor metastasis, providing valuable insight into their roles in cell behavior and transformation. These EMT-TFs have garnered increasing attention in the context of mesenchymal tissue regeneration, potentially contributing an approach for cell therapy. Given that dysregulated EMT-TF expression can impair cell survival and lineage differentiation, controlled regulation of their expression could offer significant advantages for tissue regeneration. However, there is a lack of comprehensive reviews to summarize the influence of the EMT-TFs on mesenchymal tissue regeneration and potential molecular mechanisms. This review explores the regulatory roles of ZEB, SNAI, and TWIST in the regeneration of bone, adipose, cartilage, muscle, and other mesenchymal tissues, with a focus on the underlying molecular signaling mechanisms. Gaining a deeper understanding of how EMT-TFs regulate cell proliferation, apoptosis, migration, and differentiation may offer new insights into the management of mesenchymal tissue repair and open novel avenues for enhancing tissue regeneration.

A review of the role of zinc finger proteins on hematopoiesis

The bone marrow is responsible for producing an incredible number of cells daily in order to maintain blood homeostasis through a process called hematopoiesis. Hematopoiesis is a greatly demanding process and one entirely dependent on complex interactions between the hematopoietic stem cell (HSC) and its surrounding microenvironment. Zinc (Zn2+) is considered an important trace element, playing diverse roles in different tissues and cell types, and zinc finger proteins (ZNF) are proteins that use Zn2+ as a structural cofactor. In this way, the ZNF structure is supported by a Zn2+ that coordinates many possible combinations of cysteine and histidine, with the most common ZNF being of the Cys2His2 (C2H2) type, which forms a family of transcriptional activators that play an important role in different cellular processes such as development, differentiation, and suppression, all of these being essential processes for an adequate hematopoiesis. This review aims to shed light on the relationship between ZNF and the regulation of the hematopoietic tissue. We include works with different designs, including both in vitro and in vivo studies, detailing how ZNF might regulate hematopoiesis.

An "unexpected" role for EMT transcription factors in hematological development and malignancy

The epithelial to mesenchymal transition (EMT) is a fundamental developmental process essential for normal embryonic development. It is also important during various pathogenic processes including fibrosis, wound healing and epithelial cancer cell metastasis and invasion. EMT is regulated by a variety of cell signalling pathways, cell-cell interactions and microenvironmental cues, however the key drivers of EMT are transcription factors of the ZEB, TWIST and SNAIL families. Recently, novel and unexpected roles for these EMT transcription factors (EMT-TFs) during normal blood cell development have emerged, which appear to be largely independent of classical EMT processes. Furthermore, EMT-TFs have also begun to be implicated in the development and pathogenesis of malignant hematological diseases such as leukemia and lymphoma, and now present themselves or the pathways they regulate as possible new therapeutic targets within these malignancies. In this review, we discuss the ZEB, TWIST and SNAIL families of EMT-TFs, focusing on what is known about their normal roles during hematopoiesis as well as the emerging and "unexpected" contribution they play during development and progression of blood cancers.

EMT/MET plasticity in cancer and Go-or-Grow decisions in quiescence: the two sides of the same coin?

Epithelial mesenchymal transition (EMT) and mesenchymal epithelial transition (MET) are genetic determinants of cellular plasticity. These programs operate in physiological (embryonic development, wound healing) and pathological (organ fibrosis, cancer) conditions. In cancer, EMT and MET interfere with various signalling pathways at different levels. This results in gross alterations in the gene expression programs, which affect most, if not all hallmarks of cancer, such as response to proliferative and death-inducing signals, tumorigenicity, and cell stemness. EMT in cancer cells involves large scale reorganisation of the cytoskeleton, loss of epithelial integrity, and gain of mesenchymal traits, such as mesenchymal type of cell migration. In this regard, EMT/MET plasticity is highly relevant to the Go-or-Grow concept, which postulates the dichotomous relationship between cell motility and proliferation. The Go-or-Grow decisions are critically important in the processes in which EMT/MET plasticity takes the central stage, mobilisation of stem cells during wound healing, cancer relapse, and metastasis. Here we outline the maintenance of quiescence in stem cell and metastatic niches, focusing on the implication of EMT/MET regulatory networks in Go-or-Grow switches. In particular, we discuss the analogy between cells residing in hybrid quasi-mesenchymal states and G_Alert, an intermediate phase allowing quiescent stem cells to enter the cell cycle rapidly.

SNAI2 Attenuated the Stem-like Phenotype by Reducing the Expansion of EPCAMhigh Cells in Cervical Cancer Cells

SNAI2 (Snai2) is a zinc-finger transcriptional repressor that belongs to the Snail family. The accumulated evidence suggests that SNAI2 exhibits biphasic effects on regulating a stem-like phenotype in various types of cells, both normal and malignant. In this study, by exogenously expressing SNAI2 in SiHa cells, SNAI2 exhibited the capacity to inhibit a stem-like phenotype in cervical cancer cells. The SNAI2-overexpressing cells inhibited cell growth, tumorsphere formation, tumor growth, enhanced sensitivity to cisplatin, reduced stem cell-related factors' expression, and lowered tumor initiating frequency. In addition, the EPCAM^high cells sorted from SiHa cells exhibited an enhanced capacity to maintain a stem-like phenotype. Further study demonstrated that the trans-suppression of EPCAM expression by SNAI2 led to blockage of the nuclear translocation of β-catenin, as well as reduction in SOX2 and c-Myc expression in SiHa and HeLa cells, but induction in SNAI2 knockdown cells (CaSki), which would be responsible for the attenuation of the stem-like phenotype in cervical cancer cells mediated by SNAI2. All of these results demonstrated that SNAI2 could attenuate the stem-like phenotype in cervical cancer cells through the EPCAM/β-catenin axis.

TRIB3 promotes pulmonary fibrosis through inhibiting SLUG degradation by physically interacting with MDM2

Pulmonary fibrosis (PF) is the pathological structure of incurable fibroproliferative lung diseases that are attributed to the repeated lung injury-caused failure of lung alveolar regeneration (LAR). Here, we report that repetitive lung damage results in a progressive accumulation of the transcriptional repressor SLUG in alveolar epithelial type II cells (AEC2s). The abnormal increased SLUG inhibits AEC2s from self-renewal and differentiation into alveolar epithelial type I cells (AEC1s). We found that the elevated SLUG represses the expression of the phosphate transporter SLC34A2 in AEC2s, which reduces intracellular phosphate and represses the phosphorylation of JNK and P38 MAPK, two critical kinases supporting LAR, leading to LAR failure. TRIB3, a stress sensor, interacts with the E3 ligase MDM2 to suppress SLUG degradation in AEC2s by impeding MDM2-catalyzed SLUG ubiquitination. Targeting SLUG degradation by disturbing the TRIB3/MDM2 interaction using a new synthetic staple peptide restores LAR capacity and exhibits potent therapeutic efficacy against experimental PF. Our study reveals a mechanism of the TRIB3-MDM2-SLUG-SLC34A2 axis causing the LAR failure in PF, which confers a potential strategy for treating patients with fibroproliferative lung diseases.

"In medio stat virtus": Insights into hybrid E/M phenotype attitudes

Epithelial-mesenchymal plasticity (EMP) refers to the ability of cells to dynamically interconvert between epithelial (E) and mesenchymal (M) phenotypes, thus generating an array of hybrid E/M intermediates with mixed E and M features. Recent findings have demonstrated how these hybrid E/M rather than fully M cells play key roles in most of physiological and pathological processes involving EMT. To this regard, the onset of hybrid E/M state coincides with the highest stemness gene expression and is involved in differentiation of either normal and cancer stem cells. Moreover, hybrid E/M cells are responsible for wound healing and create a favorable immunosuppressive environment for tissue regeneration. Nevertheless, hybrid state is responsible of metastatic process and of the increasing of survival, apoptosis and therapy resistance in cancer cells. The present review aims to describe the main features and the emerging concepts regulating EMP and the formation of E/M hybrid intermediates by describing differences and similarities between cancer and normal hybrid stem cells. In particular, the comprehension of hybrid E/M cells biology will surely advance our understanding of their features and how they could be exploited to improve tissue regeneration and repair.

Hemgn Protects Hematopoietic Stem and Progenitor Cells Against Transplantation Stress Through Negatively Regulating IFN-γ Signaling

Hematopoietic stem and progenitor cells (HSPCs) possess the remarkable ability to regenerate the whole blood system in response to ablated stress demands. Delineating the mechanisms that maintain HSPCs during regenerative stresses is increasingly important. Here, it is shown that Hemgn is significantly induced by hematopoietic stresses including irradiation and bone marrow transplantation (BMT). Hemgn deficiency does not disturb steady-state hematopoiesis in young mice. Hemgn^-/- HSPCs display defective engraftment activity during BMT with reduced homing and survival and increased apoptosis. Transcriptome profiling analysis reveals that upregulated genes in transplanted Hemgn^-/- HSPCs are enriched for gene sets related to interferon gamma (IFN-γ) signaling. Hemgn^-/- HSPCs show enhanced responses to IFN-γ treatment and increased aging over time. Blocking IFN-γ signaling in irradiated recipients either pharmacologically or genetically rescues Hemgn^-/- HSPCs engraftment defect. Mechanistical studies reveal that Hemgn deficiency sustain nuclear Stat1 tyrosine phosphorylation via suppressing T-cell protein tyrosine phosphatase TC45 activity. Spermidine, a selective activator of TC45, rescues exacerbated phenotype of HSPCs in IFN-γ-treated Hemgn^-/- mice. Collectively, these results identify that Hemgn is a critical regulator for successful engraftment and reconstitution of HSPCs in mice through negatively regulating IFN-γ signaling. Targeted Hemgn may be used to improve conditioning regimens and engraftment during HSPCs transplantation.

Loss of ATF4 leads to functional aging-like attrition of adult hematopoietic stem cells

Aging of hematopoietic stem cells (HSCs) directly contributes to dysfunction of hematopoietic and immune systems due to aging-associated alterations in HSC features. How the function of adult HSCs is regulated during aging so that relevant pathologic abnormalities may occur, however, remains incompletely understood. Here, we report that ATF4 deficiency provokes severe HSC defects with multifaceted aging-like phenotype via cell-autonomous mechanisms. ATF4 deletion caused expansion of phenotypical HSCs with functional attrition, characterized by defective repopulating and self-renewal capacities and myeloid bias. Moreover, the ATF4^−/− HSC defects were associated with elevated mitochondrial ROS production by targeting HIF1α. In addition, loss of ATF4 significantly delayed leukemogenesis in the MLL-AF9–induced leukemia model. Mechanistically, ATF4 deficiency impaired HSC function with aging-like phenotype and alleviated leukemogenesis by regulating HIF1α and p16^Ink4a. Together, our findings suggest a possibility of developing new strategies for the prevention and management of HSC aging and related pathological conditions.

Linking EMT programmes to normal and neoplastic epithelial stem cells

Epithelial stem cells serve critical physiological functions in the generation, maintenance and repair of diverse tissues through their ability to self-renew and spawn more specialized, differentiated cell types. In an analogous fashion, cancer stem cells have been proposed to fuel the growth, progression and recurrence of many carcinomas. Activation of an epithelial-mesenchymal transition (EMT), a latent cell-biological programme involved in development and wound healing, has been linked to the formation of both normal and neoplastic stem cells, but the mechanistic basis underlying this connection remains unclear. In this Perspective, we outline the instances where aspects of an EMT have been implicated in normal and neoplastic epithelial stem cells and consider the involvement of this programme during tissue regeneration and repair. We also discuss emerging concepts and evidence related to the heterogeneous and plastic cell states generated by EMT programmes and how these bear on our understanding of cancer stem cell biology and cancer metastasis. A more comprehensive accounting of the still-elusive links between EMT programmes and the stem cell state will surely advance our understanding of both normal stem cell biology and cancer pathogenesis.

Proliferation Genes Repressed by TGF-β Are Downstream of Slug/Snail2 in Normal Bronchial Epithelial Progenitors and Are Deregulated in COPD

Slug/Snail2 belongs to the Epithelial-Mesenchymal Transition (EMT)-inducing transcription factors involved in development and diseases. Slug is expressed in adult stem/progenitor cells of several epithelia, making it unique among these transcription factors. To investigate Slug role in human bronchial epithelium progenitors, we studied primary bronchial basal/progenitor cells in an air-liquid interface culture system that allows regenerating a bronchial epithelium. To identify Slug downstream genes we knocked down Slug in basal/progenitor cells from normal subjects and subjects with COPD, a respiratory disease presenting anomalies in the bronchial epithelium and high levels of TGF-β in the lungs. We show that normal and COPD bronchial basal/progenitors, even when treated with TGF-β, express both epithelial and mesenchymal markers, and that the epithelial marker E-cadherin is not a target of Slug and, moreover, positively correlates with Slug. We reveal that Slug downstream genes responding to both differentiation and TGF-β are different in normal and COPD progenitors, with in particular a set of proliferation-related genes that are among the genes repressed downstream of Slug in normal but not COPD. In COPD progenitors at the onset of differentiation in presence of TGF-β,we show that there is positive correlations between the effect of differentiation and TGF-β on proliferation-related genes and on Slug protein, and that their expression levels are higher than in normal cells. As well, the expression of Smad3 and β-Catenin, two molecules from TGF-βsignaling pathways, are higher in COPD progenitors, and our results indicate that proliferation-related genes and Slug protein are increased by different TGF-β-induced mechanisms.

Inhibition of Slug effectively targets leukemia stem cells via the Slc13a3/ROS signaling pathway

Leukemia stem cells (LSCs) are the rare populations of acute myeloid leukemia (AML) cells that are able to initiate, maintain, and propagate AML. Targeting LSCs is a promising approach for preventing AML relapse and improving long-term outcomes. While Slug, a zinc-finger transcription repressor, negatively regulates the self-renewal of normal hematopoietic stem cells, its functions in AML are still unknown. We report here that Slug promotes leukemogenesis and its loss impairs LSC self-renewal and delays leukemia progression. Mechanistically, Slc13a3, a direct target of Slug in LSCs, restricts the self-renewal of LSCs and markedly prolongs recipient survival. Genetic or pharmacological inhibition of SLUG or forced expression of Slc13a3 suppresses the growth of human AML cells. In conclusion, our studies demonstrate that Slug differentially regulates self-renewal of LSCs and normal HSCs, and both Slug and Slc13a3 are potential therapeutic targets of LSCs.

Molecular regulation of Snai2 in development and disease

The transcription factor Snai2, encoded by the SNAI2 gene, is an evolutionarily conserved C2H2 zinc finger protein that orchestrates biological processes critical to tissue development and tumorigenesis. Initially characterized as a prototypical epithelial-to-mesenchymal transition (EMT) transcription factor, Snai2 has been shown more recently to participate in a wider variety of biological processes, including tumor metastasis, stem and/or progenitor cell biology, cellular differentiation, vascular remodeling and DNA damage repair. The main role of Snai2 in controlling such processes involves facilitating the epigenetic regulation of transcriptional programs, and, as such, its dysregulation manifests in developmental defects, disruption of tissue homeostasis, and other disease conditions. Here, we discuss our current understanding of the molecular mechanisms regulating Snai2 expression, abundance and activity. In addition, we outline how these mechanisms contribute to disease phenotypes or how they may impact rational therapeutic targeting of Snai2 dysregulation in human disease.

The transcription factor Slug represses p16Ink4a and regulates murine muscle stem cell aging

Activation of the p16^Ink4a-associated senescence pathway during aging breaks muscle homeostasis and causes degenerative muscle disease by irreversibly dampening satellite cell (SC) self-renewal capacity. Here, we report that the zinc-finger transcription factor Slug is highly expressed in quiescent SCs of mice and functions as a direct transcriptional repressor of p16^Ink4a. Loss of Slug promotes derepression of p16^Ink4a in SCs and accelerates the entry of SCs into a fully senescent state upon damage-induced stress. p16^Ink4a depletion partially rescues defects in Slug-deficient SCs. Furthermore, reduced Slug expression is accompanied by p16^Ink4a accumulation in aged SCs. Slug overexpression ameliorates aged muscle regeneration by enhancing SC self-renewal through active repression of p16^Ink4a transcription. Our results identify a cell-autonomous mechanism underlying functional defects of SCs at advanced age. As p16^Ink4a dysregulation is the chief cause for regenerative defects of human geriatric SCs, these findings highlight Slug as a potential therapeutic target for aging-associated degenerative muscle disease.

Muscle regeneration depends on self-renewal of muscle stem cells but how this is regulated on aging is unclear. Here, the authors identify Slug as regulating p16^Ink4a in quiescent muscle stem cells, and when Slug expression reduces in aged stem cells, p16^Ink4a accumulates, causing regenerative defects.

Epithelial-mesenchymal transition in haematopoietic stem cell development and homeostasis

Epithelial-mesenchymal transition (EMT) is a morphogenetic process of cells that adopt an epithelial organization in their developmental ontogeny or homeostatic maintenance. Abnormalities in EMT regulation result in many malignant tumours in the human body. Tumours associated with the haematopoietic system, however, are traditionally not considered to involve EMT and haematopoietic stem cells (HSCs) are generally not associated with epithelial characteristics. In this review, we discuss the ontogeny and homeostasis of adult HSCs in the context of EMT intermediate states. We provide evidence that cell polarity regulation is critical for both HSC formation from embryonic dorsal aorta and HSC self-renewal and differentiation in adult bone marrow. HSC polarity is controlled by the same set of surface and transcriptional regulators as those described in canonical EMT processes. With an emphasis on partial EMT, we propose that the concept of EMT can be similarly applied in the study of HSC generation, maintenance and pathogenesis.

Zebrafish snai2 mutants fail to phenocopy morphant phenotypes

Snail2 is a zinc-finger transcription factor best known to repress expression of genes encoding cell adherence proteins to facilitate induction of the epithelial-to-mesenchymal transition. While this role has been best documented in the developmental migration of the neural crest and mesoderm, here we expand on previously reported preliminary findings that morpholino knock-down of snai2 impairs the generation of hematopoietic stem cells (HSCs) during zebrafish development. We demonstrate that snai2 morphants fail to initiate HSC specification and show defects in the somitic niche of migrating HSC precursors. These defects include a reduction in sclerotome markers as well as in the Notch ligands dlc and dld, which are known to be essential components of HSC specification. Accordingly, enforced expression of the Notch1-intracellular domain was capable of rescuing HSC specification in snai2 morphants. To parallel our approach, we obtained two mutant alleles of snai2. In contrast to the morphants, homozygous mutant embryos displayed no defects in HSC specification or in sclerotome development, and mutant fish survive into adulthood. However, when these homozygous mutants were injected with snai2 morpholino, HSCs were improperly specified. In summary, our morpholino data support a role for Snai2 in HSC development, whereas our mutant data suggest that Snai2 is dispensable for this process. Together, these findings further support the need for careful consideration of both morpholino and mutant phenotypes in studies of gene function.

A novel slug-containing negative-feedback loop regulates SCF/c-Kit-mediated hematopoietic stem cell self-renewal

The stem cell factor (SCF)/c-Kit pathway has crucial roles in controlling hematopoietic stem cell (HSC) renewal. However, little is known about the intracellular regulation of the SCF/c-Kit pathway in HSCs. We report here that Slug, a zinc-finger transcription repressor, functions as a direct transcriptional repressor of c-Kit in HSCs. Conversely, SCF/c-Kit signaling positively regulates Slug through downstream c-Myc and FoxM1 transcription factors. Intriguingly, c-Kit expression is induced by SCF/c-Kit signaling in Slug-deficient HSCs. The balance between Slug and c-Kit is critical for maintaining HSC repopulating potential in vivo. Together, our studies demonstrate that Slug functions in a novel negative-feedback regulatory loop in the SCF/c-Kit signaling pathway in HSCs.

Loss of Snail2 favors skin tumor progression by promoting the recruitment of myeloid progenitors

Snail2 is a zinc finger transcription factor involved in driving epithelial to mesenchymal transitions. Snail2 null mice are viable, but display defects in melanogenesis, gametogenesis and hematopoiesis, and are markedly radiosensitive. Here, using mouse genetics, we have studied the contributions of Snail2 to epidermal homeostasis and skin carcinogenesis. Snail2 (-/-) mice presented a defective epidermal terminal differentiation and, unexpectedly, an increase in number, size and malignancy of tumor lesions when subjected to the two-stage mouse skin chemical carcinogenesis protocol, compared with controls. Additionally, tumor lesions from Snail2 (-/-) mice presented a high inflammatory component with an elevated percentage of myeloid precursors in tumor lesions that was further increased in the presence of the anti-inflammatory agent dexamethasone. In vitro studies in Snail2 null keratinocytes showed that loss of Snail2 leads to a decrease in proliferation indicating a non-cell autonomous role for Snail2 in the skin carcinogenic response observed in vivo. Bone marrow (BM) cross-reconstitution assays between Snail2 wild-type and null mice showed that Snail2 absence in the hematopoietic system fully reproduces the tumor behavior of the Snail2 null mice and triggers the accumulation of myeloid precursors in the BM, blood and tumor lesions. These results indicate a new role for Snail2 in preventing myeloid precursors recruitment impairing skin chemical carcinogenesis progression.

Epithelial-mesenchymal transitions: from cell plasticity to concept elasticity

Epithelial-mesenchymal transition (EMT) is a developmental cellular process occurring during early embryo development, including gastrulation and neural crest cell migration. It can be broken down in distinct functional steps: (1) loss of baso-apical polarization characterized by cytoskeleton, tight junctions, and hemidesmosomes remodeling; (2) individualization of cells, including a decrease in cell-cell adhesion forces, (3) emergence of motility, and (4) invasive properties, including passing through the subepithelial basement membrane. These phases occur in an uninterrupted process, without requiring mitosis, in an order and with a degree of completion dictated by the microenvironment. The whole process reflects the activation of specific transcription factor families, called EMT transcription factors. Several mechanisms can combine to induce EMT. Some are reversible, involving growth factors and cytokines and/or environmental signals including extracellular matrix and local physical conditions. Others are irreversible, such as genomic alterations during carcinoma progression, along a selective and irreversible clonal drift. In carcinomas, these signals can converge to initiate a metastable phenotype. In this state, similarly to activated keratinocytes during re-epithelialization, cells can initiate a cohort migration and engage into a transient and reversible EMT controlled by the local environment prior to efficient intravasation and metastasis. EMT transcription factors also participate in cancer progression by inducing apoptosis resistance and maintaining stem-like properties exposed in tumor recurrences. These properties, very important on a clinical point of view, are not intrinsically linked to EMT, but can share common pathways.

A multicolor panel of TALE-KRAB based transcriptional repressor vectors enabling knockdown of multiple gene targets

Stable and efficient knockdown of multiple gene targets is highly desirable for dissection of molecular pathways. Because it allows sequence-specific DNA binding, transcription activator-like effector (TALE) offers a new genetic perturbation technique that allows for gene-specific repression. Here, we constructed a multicolor lentiviral TALE-Kruppel-associated box (KRAB) expression vector platform that enables knockdown of multiple gene targets. This platform is fully compatible with the Golden Gate TALEN and TAL Effector Kit 2.0, a widely used and efficient method for TALE assembly. We showed that this multicolor TALE-KRAB vector system when combined together with bone marrow transplantation could quickly knock down c-kit and PU.1 genes in hematopoietic stem and progenitor cells of recipient mice. Furthermore, our data demonstrated that this platform simultaneously knocked down both c-Kit and PU.1 genes in the same primary cell populations. Together, our results suggest that this multicolor TALE-KRAB vector platform is a promising and versatile tool for knockdown of multiple gene targets and could greatly facilitate dissection of molecular pathways.

Cdk4 and Cdk6 cooperate in counteracting the INK4 family of inhibitors during murine leukemogenesis

Cdk4 and Cdk6 are related protein kinases that bind d-type cyclins and regulate cell-cycle progression. Cdk4/6 inhibitors are currently being used in advanced clinical trials and show great promise against many types of tumors. Cdk4 and Cdk6 are inhibited by INK4 proteins, which exert tumor-suppressing functions. To test the significance of this inhibitory mechanism, we generated knock-in mice that express a Cdk6 mutant (Cdk6 R31C) insensitive to INK4-mediated inhibition. Cdk6(R/R) mice display altered development of the hematopoietic system without enhanced tumor susceptibility, either in the presence or absence of p53. Unexpectedly, Cdk6 R31C impairs the potential of hematopoietic progenitors to repopulate upon adoptive transfer or after 5-fluorouracil-induced damage. The defects are overcome by eliminating sensitivity of cells to INK4 inhibitors by introducing the INK4-insensitive Cdk4 R24C allele, and INK4-resistant mice are more susceptible to hematopoietic and endocrine tumors. In BCR-ABL-transformed hematopoietic cells, Cdk6 R31C causes increased binding of p16(INK4a) to wild-type Cdk4, whereas cells harboring Cdk4 R24C and Cdk6 R31C are fully insensitive to INK4 inhibitors, resulting in accelerated disease onset. Our observations reveal that Cdk4 and Cdk6 cooperate in hematopoietic tumor development and suggest a role for Cdk6 in sequestering INK4 proteins away from Cdk4.

Snail transcription factors in hematopoietic cell development: a model of functional redundancy

Coordinated gene expression is crucial in facilitating proper lymphoid cell development and function. The precise patterns of gene expression during B- and T-cell development are regulated through a complex interplay between a multitude of transcriptional regulators, both activators and repressors. We have recently identified the Snail family of transcription factors as playing significant and overlapping roles in lymphoid cell development, in that deletion of both SNAI2 and SNAI3 was required to fully impact the generation of mature T and B cells. Analyses using compound heterozygote animals further demonstrated that SNAI2 and SNAI3 were partially haplosufficient and relatively equivalent in their ability to preserve B-cell generation in the bone marrow. In this review, we summarize studies elucidating the role of the Snail family in hematopoiesis, with a focus on lymphoid cell development. Using the Snail family as an example, we discuss the concepts of functional redundancy and strategies employed to assay transcription factor families for intramember compensation.

Deletion of Snai2 and Snai3 results in impaired physical development compounded by lymphocyte deficiency

The Snail family of transcriptional regulators consists of three highly conserved members. These proteins regulate (repress) transcription via the recruitment of histone deacetylases to target gene promoters that possess the appropriate E-box binding sequences. Murine Snai1 is required for mouse development while Snai2 deficient animals survive with some anomalies. Less is known about the third member of the family, Snai3. To investigate the function of Snai3, we generated a conditional knockin mouse. Utilizing Cre-mediated deletion to facilitate the ablation of Snai3 in T cells or the entire animal, we found little to no effect of the loss of Snai3 in the entire animal or in T cell lineages. This finding provided the hypothesis that absence of Snai3 was mitigated, in part, by the presence of Snai2. To test this hypothesis we created Snai2/Snai3 double deficient mice. The developmental consequences of lacking both of these proteins was manifested in stunted growth, a paucity of offspring including a dramatic deficiency of female mice, and impaired immune cell development within the lymphoid lineages.

Pim1 serine/threonine kinase regulates the number and functions of murine hematopoietic stem cells

The genes and pathways that govern the functions and expansion of hematopoietic stem cells (HSC) are not completely understood. In this study, we investigated the roles of serine/threonine Pim kinases in hematopoiesis in mice. We generated PIM1 transgenic mice (Pim1-Tx) overexpressing human PIM1 driven by vav hematopoietic promoter/regulatory elements. Compared to wild-type littermates, Pim1-Tx mice showed enhanced hematopoiesis as demonstrated by increased numbers of Lin(-) Sca-1 (+) c-Kit (+) (LSK) hematopoietic stem/progenitor cells and cobblestone area forming cells, higher BrdU incorporation in long-term HSC population, and a better ability to reconstitute lethally irradiated mice. We then extended our study using Pim1(-/-), Pim2(-/-), Pim3(-/-) single knockout (KO) mice. HSCs from Pim1(-/-) KO mice showed impaired long-term hematopoietic repopulating capacity in secondary and competitive transplantations. Interestingly, these defects were not observed in HSCs from Pim2(-/-) or Pim3(-/-) KO mice. Limiting dilution competitive transplantation assay estimated that the frequency of LSKCD34(-) HSCs was reduced by approximately 28-fold in Pim1(-/-) KO mice compared to wild-type littermates. Mechanistic studies demonstrated an important role of Pim1 kinase in regulating HSC cell proliferation and survival. Finally, our polymerase chain reaction (PCR) array and confirmatory real-time PCR (RT-PCR) studies identified several genes including Lef-1, Pax5, and Gata1 in HSCs that were affected by Pim1 deletion. Our data provide the first direct evidence for the important role of Pim1 kinase in the regulation of HSCs. Our study also dissects out the relative role of individual Pim kinase in HSC functions and regulation.

Ataxin1L is a regulator of HSC function highlighting the utility of cross-tissue comparisons for gene discovery

Hematopoietic stem cells (HSCs) are rare quiescent cells that continuously replenish the cellular components of the peripheral blood. Observing that the ataxia-associated gene Ataxin-1-like (Atxn1L) was highly expressed in HSCs, we examined its role in HSC function through in vitro and in vivo assays. Mice lacking Atxn1L had greater numbers of HSCs that regenerated the blood more quickly than their wild-type counterparts. Molecular analyses indicated Atxn1L null HSCs had gene expression changes that regulate a program consistent with their higher level of proliferation, suggesting that Atxn1L is a novel regulator of HSC quiescence. To determine if additional brain-associated genes were candidates for hematologic regulation, we examined genes encoding proteins from autism- and ataxia-associated protein–protein interaction networks for their representation in hematopoietic cell populations. The interactomes were found to be highly enriched for proteins encoded by genes specifically expressed in HSCs relative to their differentiated progeny. Our data suggest a heretofore unappreciated similarity between regulatory modules in the brain and HSCs, offering a new strategy for novel gene discovery in both systems.

Our labs, working separately on brain function and blood stem cells, noticed that a particular gene involved in movement disorders was also expressed in the blood system. We discovered through bone marrow transplantation experiments that this gene, called Ataxin-1-like, normally plays a role in restricting the number of blood-forming stem cells; stem cells lacking this gene were more numerous and more active. We wondered if this brain-blood similarity would hold for a larger number of genes, so we used bioinformatics approaches to compare large datasets our labs had generated from each system. We found that a surprising number of genes implicated in autism and ataxia by molecular studies were also highly expressed in blood-forming stem cells. We suggest that such cross-system comparisons could be used more widely to discover genes with important functions in brain and blood, but also perhaps other systems.

Slug controls stem/progenitor cell growth dynamics during mammary gland morphogenesis

Background

Morphogenesis results from the coordination of distinct cell signaling pathways controlling migration, differentiation, apoptosis, and proliferation, along stem/progenitor cell dynamics. To decipher this puzzle, we focused on epithelial-mesenchymal transition (EMT) “master genes”. EMT has emerged as a unifying concept, involving cell-cell adhesion, migration and apoptotic pathways. EMT also appears to mingle with stemness. However, very little is known on the physiological role and relevance of EMT master-genes. We addressed this question during mammary morphogenesis. Recently, a link between Slug/Snai2 and stemness has been described in mammary epithelial cells, but EMT master genes actual localization, role and targets during mammary gland morphogenesis are not known and we focused on this basic question.

Methodology/Principal Findings

Using a Slug–lacZ transgenic model and immunolocalization, we located Slug in a distinct subpopulation covering about 10–20% basal cap and duct cells, mostly cycling cells, coexpressed with basal markers P-cadherin, CK5 and CD49f. During puberty, Slug-deficient mammary epithelium exhibited a delayed development after transplantation, contained less cycling cells, and overexpressed CK8/18, ER, GATA3 and BMI1 genes, linked to luminal lineage. Other EMT master genes were overexpressed, suggesting compensation mechanisms. Gain/loss-of-function in vitro experiments confirmed Slug control of mammary epithelial cell luminal differentiation and proliferation. In addition, they showed that Slug enhances specifically clonal mammosphere emergence and growth, cell motility, and represses apoptosis. Strikingly, Slug-deprived mammary epithelial cells lost their potential to generate secondary clonal mammospheres.

Conclusions/Significance

We conclude that Slug pathway controls the growth dynamics of a subpopulation of cycling progenitor basal cells during mammary morphogenesis. Overall, our data better define a key mechanism coordinating cell lineage dynamics and morphogenesis, and provide physiological relevance to broadening EMT pathways.

Epithelial and mesenchymal subpopulations within normal basal breast cell lines exhibit distinct stem cell/progenitor properties

It has been proposed that epithelial-mesenchymal transition (EMT) in mammary epithelial cells and breast cancer cells generates stem cell features, and that the presence of EMT characteristics in claudin-low breast tumors reveals their origin in basal stem cells. It remains to be determined, however, whether EMT is an inherent property of normal basal stem cells, and if the presence of a mesenchymal-like phenotype is required for the maintenance of all their stem cell properties. We used nontumorigenic basal cell lines as models of normal stem cells/progenitors and demonstrate that these cell lines contain an epithelial subpopulation ("EpCAM+," epithelial cell adhesion molecule positive [EpCAM(pos)]/CD49f(high)) that spontaneously generates mesenchymal-like cells ("Fibros," EpCAM(neg)/CD49f(med/low)) through EMT. Importantly, stem cell/progenitor properties such as regenerative potential, high aldehyde dehydrogenase 1 activity, and formation of three-dimensional acini-like structures predominantly reside within EpCAM+ cells, while Fibros exhibit invasive behavior and mammosphere-forming ability. A gene expression profiling meta-analysis established that EpCAM+ cells show a luminal progenitor-like expression pattern, while Fibros most closely resemble stromal fibroblasts but not stem cells. Moreover, Fibros exhibit partial myoepithelial traits and strong similarities with claudin-low breast cancer cells. Finally, we demonstrate that Slug and Zeb1 EMT-inducers control the progenitor and mesenchymal-like phenotype in EpCAM+ cells and Fibros, respectively, by inhibiting luminal differentiation. In conclusion, nontumorigenic basal cell lines have intrinsic capacity for EMT, but a mesenchymal-like phenotype does not correlate with the acquisition of global stem cell/progenitor features. Based on our findings, we propose that EMT in normal basal cells and claudin-low breast cancers reflects aberrant/incomplete myoepithelial differentiation.

Erg is required for self-renewal of hematopoietic stem cells during stress hematopoiesis in mice

Hematopoietic stem cells (HSCs) are rare residents of the bone marrow responsible for the lifelong production of blood cells. Regulation of the balance between HSC self-renewal and differentiation is central to hematopoiesis, allowing precisely regulated generation of mature blood cells at steady state and expanded production at times of rapid need, as well as maintaining ongoing stem cell capacity. Erg, a member of the Ets family of transcription factors, is deregulated in cancers; and although Erg is known to be required for regulation of adult HSCs, its precise role has not been defined. We show here that, although heterozygosity for functional Erg is sufficient for adequate steady-state HSC maintenance, Erg(+/Mld2) mutant mice exhibit impaired HSC self-renewal after bone marrow transplantation or during recovery from myelotoxic stress. Moreover, although mice functionally compromised for either Erg or Mpl, the receptor for thrombopoietin, a key regulator of HSC quiescence, maintained sufficient HSC activity to sustain hematopoiesis, Mpl(-/-) Erg(+/Mld2) compound mutant mice displayed exacerbated stem cell deficiencies and bone marrow failure. Thus, Erg is a critical regulator of adult HSCs, essential for maintaining self-renewal at times of high HSC cycling.