The retinoblastoma gene is involved in multiple aspects of stem cell biology

Biomolecular Evaluation of Piceatannol's Effects in Counteracting the Senescence of Mesenchymal Stromal Cells: A New Candidate for Senotherapeutics?

Several investigations on senescence and its causative role in aging have underscored the importance of developing senotherapeutics, a field focused on killing senescent cells and/or preventing their accumulation within tissues. Using polyphenols in counteracting senescence may facilitate the development of senotherapeutics given their presence in the human diet, their confirmed tolerability and absence of severe side effects, and their role in preventing senescence and inducing the death of senescent cells. Against that background, we evaluated the effect of piceatannol, a natural polyphenol, on the senescence of mesenchymal stromal cells (MSCs), which play a key role in the body's homeostasis. Among our results, piceatannol reduced the number of senescent cells both after genotoxic stress that induced acute senescence and in senescent replicative cultures. Such senotherapeutics activity, moreover, promoted the recovery of cell proliferation and the stemness properties of MSCs. Altogether, our findings demonstrate piceatannol's effectiveness in counteracting senescence by targeting its associated pathways and detecting and affecting P53-dependent and P53-independent senescence. Our study thus suggests that, given piceatannol's various mechanisms to accomplish its pleiotropic activities, it may be able to counteract any senescent phenotypes.

Mutation of SPINOPHILIN (PPP1R9B) found in human tumors promotes the tumorigenic and stemness properties of cells

Rationale: SPINOPHILIN (SPN, PPP1R9B) is an important tumor suppressor involved in the progression and malignancy of different tumors depending on its association with protein phosphatase 1 (PP1) and the ability of the PP1-SPN holoenzyme to dephosphorylate retinoblastoma (pRB).

Methods: We performed a mutational analysis of SPN in human tumors, focusing on the region of interaction with PP1 and pRB. We explored the effect of the SPN-A566V mutation in an immortalized non-tumorigenic cell line of epithelial breast tissue, MCF10A, and in two different p53-mutated breast cancer cells lines, T47D and MDA-MB-468.

Results: We characterized an oncogenic mutation of SPN found in human tumor samples, SPN-A566V, that affects both the SPN-PP1 interaction and its phosphatase activity. The SPN-A566V mutation does not affect the interaction of the PP1-SPN holoenzyme with pocket proteins pRB, p107 and p130, but it affects its ability to dephosphorylate them during G0/G1 and G1, indicating that the PP1-SPN holoenzyme regulates cell cycle progression. SPN-A566V also promoted stemness, establishing a connection between the cell cycle and stem cell biology via pocket proteins and PP1-SPN regulation. However, only cells with both SPN-A566V and mutant p53 have increased tumorigenic and stemness properties.

Conclusions: SPN-A566V, or other equivalent mutations, could be late events that promote tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Cell stemness is maintained upon concurrent expression of RB and the mitochondrial ribosomal protein S18-2

Stemness encompasses the capability of a cell for self-renewal and differentiation. The stem cell maintains a balance between proliferation, quiescence, and regeneration via interactions with the microenvironment. Previously, we showed that ectopic expression of the mitochondrial ribosomal protein S18-2 (MRPS18-2) led to immortalization of primary fibroblasts, accompanied by induction of an embryonic stem cell (ESC) phenotype. Moreover, we demonstrated interaction between S18-2 and the retinoblastoma-associated protein (RB) and hypothesized that the simultaneous expression of RB and S18-2 is essential for maintaining cell stemness. Here, we experimentally investigated the role of S18-2 in cell stemness and differentiation. Concurrent expression of RB and S18-2 resulted in immortalization of Rb1 ^-/- primary mouse embryonic fibroblasts and in aggressive tumor growth in severe combined immunodeficiency mice. These cells, which express both RB and S18-2 at high levels, exhibited the potential to differentiate into various lineages in vitro, including osteogenic, chondrogenic, and adipogenic lineages. Mechanistically, S18-2 formed a multimeric protein complex with prohibitin and the ring finger protein 2 (RNF2). This molecular complex increased the monoubiquitination of histone H2A^Lys119, a characteristic trait of ESCs, by enhanced E3-ligase activity of RNF2. Furthermore, we found enrichment of KLF4 at the S18-2 promoter region and that the S18-2 expression is positively correlated with KLF4 levels. Importantly, knockdown of S18-2 in zebrafish larvae led to embryonic lethality. Collectively, our findings suggest an important role for S18-2 in cell stemness and differentiation and potentially also in cancerogenesis.

Aging: A cell source limiting factor in tissue engineering

Tissue engineering has yet to reach its ideal goal, i.e. creating profitable off-the-shelf tissues and organs, designing scaffolds and three-dimensional tissue architectures that can maintain the blood supply, proper biomaterial selection, and identifying the most efficient cell source for use in cell therapy and tissue engineering. These are still the major challenges in this field. Regarding the identification of the most appropriate cell source, aging as a factor that affects both somatic and stem cells and limits their function and applications is a preventable and, at least to some extents, a reversible phenomenon. Here, we reviewed different stem cell types, namely embryonic stem cells, adult stem cells, induced pluripotent stem cells, and genetically modified stem cells, as well as their sources, i.e. autologous, allogeneic, and xenogeneic sources. Afterward, we approached aging by discussing the functional decline of aged stem cells and different intrinsic and extrinsic factors that are involved in stem cell aging including replicative senescence and Hayflick limit, autophagy, epigenetic changes, miRNAs, mTOR and AMPK pathways, and the role of mitochondria in stem cell senescence. Finally, various interventions for rejuvenation and geroprotection of stem cells are discussed. These interventions can be applied in cell therapy and tissue engineering methods to conquer aging as a limiting factor, both in original cell source and in the in vitro proliferated cells.

Adipose-Derived Mesenchymal Stem Cells from the Elderly Exhibit Decreased Migration and Differentiation Abilities with Senescent Properties

Adipose-derived stem cells (ASCs) can be applied extensively in the clinic because they can be easily isolated and cause less donor-site morbidity; however, their application can be complicated by patient-specific factors, such as age and harvest site. In this study, we systematically evaluated the effects of age on the quantity and quality of human adipose-derived mesenchymal stem cells (hASCs) isolated from excised chest subcutaneous adipose tissue and investigated the underlying molecular mechanism. hASCs were isolated from donors of 3 different age-groups (i.e., child, young adult, and elderly). hASCs are available from individuals across all age-groups and maintain mesenchymal stem cell (MSC) characteristics. However, the increased age of the donors was found to have a significant negative effect on hASCs frequency base on colony-forming unit fibroblasts assay. Moreover, there is a decline in both stromal vascular fraction (SVF) cell yield and the proliferation rate of hASCs with increasing age, although this relationship is not significant. Aging increases cellular senescence, which is manifested as an increase in SA-β-gal-positive cells, increased mitochondrial-specific reactive oxygen species (ROS) production, and the expression of p21 in the elderly. Further, advancing age was found to have a significant negative effect on the adipogenic and osteogenic differentiation potentials of hASCs, particularly at the early and mid-stages of induction, suggesting a slower response to the inducing factors of hASCs from elderly donors. Finally, impaired migration ability was also observed in the elderly group and was determined to be associated with decreased expression of chemokine receptors, such as CXCR4 and CXCR7. Taken together, these results suggest that, while hASCs from different age populations are phenotypically similar, they present major differences at the functional level. When considering potential applications of hASCs in cell-based therapeutic strategies, the negative influence of age on hASC differentiation potential and migration abilities should be taken seriously.

Role of cell cycle regulators in adipose tissue and whole body energy homeostasis

In the course of the last decades, metabolism research has demonstrated that adipose tissue is not an inactive tissue. Rather, adipocytes are key actors of whole body energy homeostasis. Numerous novel regulators of adipose tissue differentiation and function have been identified. With the constant increase of obesity and associated disorders, the interest in adipose tissue function alterations in the XXIst century has become of paramount importance. Recent data suggest that adipocyte differentiation, adipose tissue browning and mitochondrial function, lipogenesis and lipolysis are strongly modulated by the cell division machinery. This review will focus on the function of cell cycle regulators in adipocyte differentiation, adipose tissue function and whole body energy homeostasis; with particular attention in mouse studies.

Coordinated downregulation of Spinophilin and the catalytic subunits of PP1, PPP1CA/B/C, contributes to a worse prognosis in lung cancer

The scaffold protein Spinophilin (Spinophilin, PPP1R9B) is one of the regulatory subunits of phosphatase-1 (PP1), directing it to distinct subcellular locations and targets. The loss of Spinophilin reduces PP1 targeting to pRb, thereby maintaining higher levels of phosphorylated pRb. Spinophilin is absent or reduced in approximately 40% of human lung tumors, correlating with the malignant grade. However, little is known about the relevance of the coordinated activity or presence of Spinophilin and its reported catalytic partners in the prognosis of lung cancer. In the present work, we show that the downregulation of Spinophilin, either by protein or mRNA, is related to a worse prognosis in lung tumors. This effect is more relevant in squamous cell carcinoma, SCC, than in adenocarcinoma. Downregulation of Spinophilin is related to a decrease in the levels of its partners PPP1CA/B/C, the catalytic subunits of PP1. A decrease in these subunits is also related to prognosis in SCC and, in combination with a decrease in Spinophilin, are markers of a poor prognosis in these tumors. The analysis of the genes that correlate to Spinophilin in lung tumors showed clear enrichment in ATP biosynthesis and protein degradation GO pathways. The analysis of the response to several common and pathway-related drugs indicates a direct correlation between the Spinophilin/PPP1Cs ratio and the response to oxaliplatin and bortezomib. This finding indicates that this ratio may be a good predictive biomarker for the activity of the drugs in these tumors with a poor prognosis.

Misidentified Human Gene Functions with Mouse Models: The Case of the Retinoblastoma Gene Family in Senescence

Although mice models rank among the most widely used tools for understanding human genetics, biology, and diseases, differences between orthologous genes among species as close as mammals are possible, particularly in orthologous gene pairs in which one or more paralogous (i.e., duplicated) genes appear in the genomes of the species. Duplicated genes can possess overlapping functions and compensate for each other. The retinoblastoma gene family demonstrates typical composite functionality in its three member genes (i.e., RB1, RB2/P130, and P107), all of which participate in controlling the cell cycle and associated phenomena, including proliferation, quiescence, apoptosis, senescence, and cell differentiation. We analyzed the role of the retinoblastoma gene family in regulating senescence in mice and humans. Silencing experiments with each member of the gene family in mesenchymal stromal cells (MSCs) and fibroblasts from mouse and human tissues demonstrated that RB1 may be indispensable for senescence in mouse cells, but not in human ones, as an example of species specificity. Furthermore, although RB2/P130 seems to be implicated in maintaining human cell senescence, the function of RB1 within any given species might differ by cell type, as an example of cell specificity. For instance, silencing RB1 in mouse fibroblasts induced a reduced senescence not observed in mouse MSCs. Our findings could be useful as a general paradigm of cautions to take when inferring the role of human genes analyzed in animal studies and when examining the role of the retinoblastoma gene family in detail.

Molecular connections of obesity and aging: a focus on adipose protein 53 and retinoblastoma protein

Obesity is an induced health problem that human beings have been facing with non-optimal treatment so far. Humans are on average getting fatter with age, and obesity and aging interact each other to shorten lifetime and decrease life quality. Obesity also causes several aging related-disorders such as cancer, strokes, cardiovascular disease, high blood pressure and type 2 diabetes. So, the molecular connections between aging and obesity are promising targets for bio-medical researches and innovative therapies of many health problems. In this review, we discuss the findings of adipose p53 and Rb-two central molecular linkages between aging and obesity-on lipid metabolism and obesity.

Senescence of mesenchymal stem cells (Review)

Mesenchymal stem cells (MSCs) have been used in cell-based therapy for various diseases, due to their immunomodulatory and inflammatory effects. However, the function of MSCs is known to decline with age, a process that is called senescence. To date, the process of MSC senescence remains unknown as in-depth understanding of the mechanisms involved in cellular senescence is lacking. First, senescent MSCs are so heterogeneous that not all of them express the same phenotypic markers. In addition, the genes and signaling pathways which regulate this process in MSCs are still unknown. Thus, an understanding of the molecular processes controlling MSC senescence is crucial to determining the drivers and effectors of age-associated MSC dysfunction. Moreover, the proper use of MSCs for clinical application requires a general understanding of the MSC aging process. Furthermore, such knowledge is essential for the development of therapeutic interventions that can slow or reverse age-related degenerative changes to enhance repair processes and maintain healthy function in aging tissues. To further clarify the properties of senescent cells, as well as to present significant findings from studies on the mechanisms of cellular aging, we summarize these biological features in the senescence of MSCs in this scenario. This review summarizes recent advances in our understanding of the markers and differentiation potential indicating MSC senescence, as well as factors affecting MSC senescence with particular emphasis on the roles of oxidative stress, intrinsic changes in telomere shortening, histone deacetylase and DNA methyltransferase, genes and signaling pathways and immunological properties.

Impact of lysosomal storage disorders on biology of mesenchymal stem cells: Evidences from in vitro silencing of glucocerebrosidase (GBA) and alpha-galactosidase A (GLA) enzymes

Lysosomal storage disorders (LDS) comprise a group of rare multisystemic diseases resulting from inherited gene mutations that impair lysosomal homeostasis. The most common LSDs, Gaucher disease (GD), and Fabry disease (FD) are caused by deficiencies in the lysosomal glucocerebrosidase (GBA) and alpha-galactosidase A (GLA) enzymes, respectively. Given the systemic nature of enzyme deficiency, we hypothesized that the stem cell compartment of GD and FD patients might be also affected. Among stem cells, mesenchymal stem cells (MSCs) are a commonly investigated population given their role in hematopoiesis and the homeostatic maintenance of many organs and tissues. Since the impairment of MSC functions could pose profound consequences on body physiology, we evaluated whether GBA and GLA silencing could affect the biology of MSCs isolated from bone marrow and amniotic fluid. Those cell populations were chosen given the former's key role in organ physiology and the latter's intriguing potential as an alternative stem cell model for human genetic disease. Our results revealed that GBA and GLA deficiencies prompted cell cycle arrest along with the impairment of autophagic flux and an increase of apoptotic and senescent cell percentages. Moreover, an increase in ataxia-telangiectasia-mutated staining 1 hr after oxidative stress induction and a return to basal level at 48 hr, along with persistent gamma-H2AX staining, indicated that MSCs properly activated DNA repair signaling, though some damages remained unrepaired. Our data therefore suggest that MSCs with reduced GBA or GLA activity are prone to apoptosis and senescence due to impaired autophagy and DNA repair capacity.

Role of the RB-Interacting Proteins in Stem Cell Biology

Human retinoblastoma gene RB1 is the first tumor suppressor gene (TSG) isolated by positional cloning in 1986. RB is extensively studied for its ability to regulate cell cycle by binding to E2F1 and inhibiting the transcriptional activity of the latter. In human embryonic stem cells (ESCs), only a minute trace of RB is found in complex with E2F1. Increased activity of RB triggers differentiation, cell cycle arrest, and cell death. On the other hand, inactivation of the entire RB family (RB1, RBL1, and RBL2) in human ESC induces G2/M arrest and cell death. These observations indicate that both loss and overactivity of RB could be lethal for the stemness of cells. A question arises why inactive RB is required for the survival and stemness of cells? To shed some light on this question, we analyzed the RB-binding proteins. In this review we have focused on 27 RB-binding partners that may have potential roles in different aspects of stem cell biology.

Mesenchymal stromal cells having inactivated RB1 survive following low irradiation and accumulate damaged DNA: Hints for side effects following radiotherapy

Following radiotherapy, bone sarcomas account for a significant percentage of recurring tumors. This risk is further increased in patients with hereditary retinoblastoma that undergo radiotherapy. We analyzed the effect of low and medium dose radiation on mesenchymal stromal cells (MSCs) with inactivated RB1 gene to gain insights on the molecular mechanisms that can induce second malignant neoplasm in cancer survivors. MSC cultures contain subpopulations of mesenchymal stem cells and committed progenitors that can differentiate into mesodermal derivatives: adipocytes, chondrocytes, and osteocytes. These stem cells and committed osteoblast precursors are the cell of origin in osteosarcoma, and RB1 gene mutations have a strong role in its pathogenesis. Following 40 and 2000 mGy X-ray exposure, MSCs with inactivated RB1 do not proliferate and accumulate high levels of unrepaired DNA as detected by persistence of gamma-H2AX foci. In samples with inactivated RB1 the radiation treatment did not increase apoptosis, necrosis or senescence versus untreated cells. Following radiation, CFU analysis showed a discrete number of cells with clonogenic capacity in cultures with silenced RB1. We extended our analysis to the other members of retinoblastoma gene family: RB2/P130 and P107. Also in the MSCs with silenced RB2/P130 and P107 we detected the presence of cells with unrepaired DNA following X-ray irradiation. Cells with unrepaired DNA may represent a reservoir of cells that may undergo neoplastic transformation. Our study suggests that, following radiotherapy, cancer patients with mutations of retinoblastoma genes may be under strict controls to evaluate onset of secondary neoplasms following radiotherapy.

De-regulated expression of the BRG1 chromatin remodeling factor in bone marrow mesenchymal stromal cells induces senescence associated with the silencing of NANOG and changes in the levels of chromatin proteins

Stem cells have a peculiar chromatin architecture that contributes to their unique properties, including uncommitted status, multi/pluripotency and self-renewal. We analyzed the effect of the de-regulation of the SWI/SNF chromatin remodeling complex in mesenchymal stromal cells (MSC) through the silencing and up-regulation of BRG1, which is the ATPase subunit of the complex. The altered expression of BRG1 promoted the senescence of MSC with suppression of the NANOG transcription, which is part of the transcriptional circuitry governing stem cell functions. To gain insight on the way NANOG was silenced, we evaluated how the de-regulated BRG1 expression affect the binding of activators and repressors on the NANOG promoter. We found 4 E2F binding motifs on NANOG promoter, which can be occupied by RB1 and RB2/P130. These are members of the retinoblastoma gene family. In MSC with a silenced BRG1, the relative binding of the 2 retinoblastoma proteins increased, and this was associated with the recruitment of DNMT1. This induced the methylation of CpG on the NANOG promoter. Opposingly, when a high level of BRG1 was present, the same E2F binding motifs were docking sites for BRG1, which induced chromatin compaction without CpG methylation but with increased histone deacetylation, associated with the presence of HDAC1 on E2F binding sites. Besides the sharp regulation of the NANOG expression, we evidenced, through proteomic analysis, that the de-regulation of the SWI/SNF function affected the expression of histones and other nuclear proteins involved in "nuclear architecture," suggesting that BRG1 may act as global regulator of gene expression.

The Rb-E2F transcriptional regulatory pathway in tumor angiogenesis and metastasis

The retinoblastoma tumor suppressor protein Rb plays a major role in regulating G1/S transition and is a critical regulator of cell proliferation. Rb protein exerts its growth regulatory properties mainly by physically interacting with the transcriptionally active members of the E2F transcription factor family, especially E2Fs 1, 2, and 3. Given its critical role in regulating cell proliferation, it is not surprising that Rb is inactivated in almost all tumors, either through the mutation of Rb gene itself or through the mutations of its upstream regulators including K-Ras and INK4. Recent studies have revealed a significant role for Rb and its downstream effectors, especially E2Fs, in regulating various aspects of tumor progression, angiogenesis, and metastasis. Thus, components of the Rb-E2F pathway have been shown to regulate the expression of genes involved in angiogenesis, including VEGF and VEGFR, genes involved in epithelial-mesenchymal transition including E-cadherin and ZEB proteins, and genes involved in invasion and migration like matrix metalloproteinases. Rb has also been shown to play a major role in the functioning of normal and cancer stem cells; further, Rb and E2F appear to play a regulatory role in the energy metabolism of cancer cells. These findings raise the possibility that mutational events that initiate tumorigenesis by inducing uncontrolled cell proliferation might also contribute to the progression and metastasis of cancers through the mediation of the Rb-E2F transcriptional regulatory pathway. This review highlights these recent studies on tumor promoting functions of the Rb-E2F pathway.

Nanotopography alters nuclear protein expression, proliferation and differentiation of human mesenchymal stem/stromal cells

Mesenchymal stem/stromal cells respond to physical cues present in their microenvironment such as substrate elasticity, geometry, or topography with respect to morphology, proliferation, and differentiation. Although studies have demonstrated the role of focal adhesions in topography-mediated changes of gene expression, information linking substrate topography to the nucleus remains scarce. Here we show by two-dimensional gel electrophoresis and western blotting that A-type lamins and retinoblastoma protein are downregulated in mesenchymal stem/stromal cells cultured on 350 nm gratings compared to planar substrates; these changes lead to a decrease in proliferation and changes in differentiation potential.

RB maintains quiescence and prevents premature senescence through upregulation of DNMT1 in mesenchymal stromal cells

Many cell therapies currently being tested are based on mesenchymal stromal cells (MSCs). However, MSCs start to enter the senescent state upon long-term expansion. The role of retinoblastoma (RB) protein in regulating MSC properties is not well studied. Here, we show that RB levels are higher in early-passage MSCs compared with late-passage MSCs. RB knockdown induces premature senescence and reduced differentiation potentials in early-passage MSCs. RB overexpression inhibits senescence and increases differentiation potentials in late-passage MSCs. Expression of DNMT1, but not DNMT3A or DNMT3B, is also higher in early-passage MSCs than in late-passage MSCs. Furthermore, DNMT1 knockdown in early-passage MSCs induces senescence and reduces differentiation potentials, whereas DNMT1 overexpression in late-passage MSCs has the opposite effect. These results demonstrate that RB expressed in early-passage MSCs upregulates DNMT1 expression and inhibits senescence in MSCs. Therefore, genetic modification of RB could be a way to improve the efficiency of MSCs in clinical use.

High grade glioblastoma is associated with aberrant expression of ZFP57, a protein involved in gene imprinting, and of CPT1A and CPT1C that regulate fatty acid metabolism

The diagnosis of glioblastoma is still based on tumor histology, but emerging molecular diagnosis is becoming an important part of glioblastoma classification. Besides the well-known cell cycle-related circuitries that are associated with glioblastoma onset and development, new insights may be derived by looking at pathways involved in regulation of epigenetic phenomena and cellular metabolism, which may both be highly deregulated in cancer cells. We evaluated if in glioblastoma patients the high grade of malignancy could be associated with aberrant expression of some genes involved in regulation of epigenetic phenomena and lipid metabolism. We measured the mRNA levels of ZFP57, TRIM28, CPT1A, CPT1B, and CPT1C in a cohort of 80 patients divided in two groups: grade II and grade IV. We evidenced that high grade glioblastoma is associated with increased level of ZFP57, a protein involved in gene imprinting, and aberrant expression of CPT1A and CPT1C, regulators of fatty acid oxidation. Our study may pave the way to identify new markers that could be potentially useful for diagnosis and/or prognosis of glioblastoma.

Silencing of RB1 and RB2/P130 during adipogenesis of bone marrow stromal cells results in dysregulated differentiation

Bone marrow adipose tissue (BMAT) is different from fat found elsewhere in the body, and only recently have some of its functions been investigated. BMAT may regulate bone marrow stem cell niche and plays a role in energy storage and thermogenesis. BMAT may be involved also in obesity and osteoporosis onset. Given the paramount functions of BMAT, we decided to better clarify the human bone marrow adipogenesis by analyzing the role of the retinoblastoma gene family, which are key players in cell cycle regulation. Our data provide evidence that the inactivation of RB1 or RB2/P130 in uncommitted bone marrow stromal cells (BMSC) facilitates the first steps of adipogenesis. In cultures with silenced RB1 or RB2/P130, we observed an increase of clones with adipogenic potential and a higher percentage of cells accumulating lipid droplets. Nevertheless, the absence of RB1 or RB2/P130 impaired the terminal adipocyte differentiation and gave rise to dysregulated adipose cells, with alteration in lipid uptake and release. For the first time, we evidenced that RB2/P130 plays a role in bone marrow adipogenesis. Our data suggest that while the inactivation of retinoblastoma proteins may delay the onset of last cell division and allow more BMSC to be committed to adipocyte, it did not allow a permanent cell cycle exit, which is a prerequisite for adipocyte terminal maturation.

Silencing of RB1 but not of RB2/P130 induces cellular senescence and impairs the differentiation potential of human mesenchymal stem cells

Stem cell senescence is considered deleterious because it may impair tissue renewal and function. On the other hand, senescence may arrest the uncontrolled growth of transformed stem cells and protect organisms from cancer. This double function of senescence is strictly linked to the activity of genes that the control cell cycle such as the retinoblastoma proteins RB1, RB2/P130, and P107. We took advantage of the RNA interference technique to analyze the role of these proteins in the biology of mesenchymal stem cells (MSC). Cells lacking RB1 were prone to DNA damage. They showed elevated levels of p53 and p21(cip1) and increased regulation of RB2/P130 and P107 expression. These cells gradually adopted a senescent phenotype with impairment of self-renewal properties. No significant modification of cell growth was observed as it occurs in other cell types or systems. In cells with silenced RB2/P130, we detected a reduction of DNA damage along with a higher proliferation rate, an increase in clonogenic ability, and the diminution of apoptosis and senescence. Cells with silenced RB2/P130 were cultivated for extended periods of time without adopting a transformed phenotype. Of note, acute lowering of P107 did not induce relevant changes in the in vitro behavior of MSC. We also analyzed cell commitment and the osteo-chondro-adipogenic differentiation process of clones derived by MSC cultures. In all clones obtained from cells with silenced retinoblastoma genes, we observed a reduction in the ability to differentiate compared with the control clones. In summary, our data show evidence that the silencing of the expression of RB1 or RB2/P130 is not compensated by other gene family members, and this profoundly affects MSC functions.

Opposing regulation of Sox2 by cell-cycle effectors E2f3a and E2f3b in neural stem cells

The mechanisms through which cell-cycle control and cell-fate decisions are coordinated in proliferating stem cell populations are largely unknown. Here, we show that E2f3 isoforms, which control cell-cycle progression in cooperation with the retinoblastoma protein (pRb), have critical effects during developmental and adult neurogenesis. Loss of either E2f3 isoform disrupts Sox2 gene regulation and the balance between precursor maintenance and differentiation in the developing cortex. Both isoforms target the Sox2 locus to maintain baseline levels of Sox2 expression but antagonistically regulate Sox2 levels to instruct fate choices. E2f3-mediated regulation of Sox2 and precursor cell fate extends to the adult brain, where E2f3a loss results in defects in hippocampal neurogenesis and memory formation. Our results demonstrate a mechanism by which E2f3a and E2f3b differentially regulate Sox2 dosage in neural precursors, a finding that may have broad implications for the regulation of diverse stem cell populations.

The abbreviated pluripotent cell cycle

Human embryonic stem cells (hESCs) and induced pluripotent stem cells proliferate rapidly and divide symmetrically producing equivalent progeny cells. In contrast, lineage committed cells acquire an extended symmetrical cell cycle. Self-renewal of tissue-specific stem cells is sustained by asymmetric cell division where one progeny cell remains a progenitor while the partner progeny cell exits the cell cycle and differentiates. There are three principal contexts for considering the operation and regulation of the pluripotent cell cycle: temporal, regulatory, and structural. The primary temporal context that the pluripotent self-renewal cell cycle of hESCs is a short G1 period without reducing periods of time allocated to S phase, G2, and mitosis. The rules that govern proliferation in hESCs remain to be comprehensively established. However, several lines of evidence suggest a key role for the naïve transcriptome of hESCs, which is competent to stringently regulate the embryonic stem cell (ESC) cell cycle. This supports the requirements of pluripotent cells to self-propagate while suppressing expression of genes that confer lineage commitment and/or tissue specificity. However, for the first time, we consider unique dimensions to the architectural organization and assembly of regulatory machinery for gene expression in nuclear microenviornments that define parameters of pluripotency. From both fundamental biological and clinical perspectives, understanding control of the abbreviated ESC cycle can provide options to coordinate control of proliferation versus differentiation. Wound healing, tissue engineering, and cell-based therapy to mitigate developmental aberrations illustrate applications that benefit from knowledge of the biology of the pluripotent cell cycle.

Efficient cultivation of neural stem cells with controlled delivery of FGF-2

Neural stem cells (NSCs) raised the hope for cell-based therapies in human neurodevelopmental and neurodegenerative diseases. Current research strategies aim to isolate, enrich, and propagate homogeneous populations of neural stem cells. Unfortunately, several concerns with NSC cultures currently may limit their therapeutic promise. Exhaustion of growth factors and/or their uncontrolled release with burst and fall in their concentration may greatly affect the in vitro behavior of NSCs. In this context, we investigate whether a device containing heparan sulfate (HS), which is a co-factor in growth factor-mediated cell proliferation and differentiation, could potentiate and prolong the delivery of fibroblast growth factor-2 (FGF-2) and thus improve in vitro NSC cultivation. We demonstrated that NSCs cultivated in media with a controlled release of FGF-2 from a polyelectrolyte polymer showed a higher proliferation rate, and reduced apoptosis and senescence. In these experimental conditions NSCs preserve their stemness properties for a longer period of time compared with controls. Also of interest is that cell fate properties are conserved as well. The controlled release of FGF-2 reduced the level of oxidative stress and this is associated with a lower level of damaged DNA. This result may explain the reduced level of senescence and apoptosis in NSCs cultivated in the presence of hydrogel-releasing FGF-2.

The architectural organization of human stem cell cycle regulatory machinery

Two striking features of human embryonic stem cells that support biological activity are an abbreviated cell cycle and reduced complexity to nuclear organization. The potential implications for rapid proliferation of human embryonic stem cells within the context of sustaining pluripotency, suppressing phenotypic gene expression and linkage to simplicity in the architectural compartmentalization of regulatory machinery in nuclear microenvironments is explored. Characterization of the molecular and architectural commitment steps that license human embryonic stem cells to initiate histone gene expression is providing understanding of the principal regulatory mechanisms that control the G1/S phase transition in primitive pluripotent cells. From both fundamental regulatory and clinical perspectives, further understanding of the pluripotent cell cycle in relation to compartmentalization of regulatory machinery in nuclear microenvironments is relevant to applications of stem cells for regenerative medicine and new dimensions to therapy where traditional drug discovery strategies have been minimally effective.

Dose-dependent effects of R-sulforaphane isothiocyanate on the biology of human mesenchymal stem cells, at dietary amounts, it promotes cell proliferation and reduces senescence and apoptosis, while at anti-cancer drug doses, it has a cytotoxic effect

Brassica vegetables are attracting a great deal of attention as healthy foods because of the fact that they contain substantial amounts of secondary metabolite glucosinolates that are converted into isothiocyanates, such as sulforaphane [(-)1-isothiocyanato-4R-(methylsulfinyl)-butane] (R-SFN), through the actions of chopping or chewing the vegetables. Several studies have analyzed the biological and molecular mechanisms of the anti-cancer activity of synthetic R,S-sulforaphane, which is thought to be a result of its antioxidant properties and its ability to inhibit histone deacetylase enzymes (HDAC). Few studies have addressed the possible antioxidant effects of R-SFN, which could protect cells from the free radical damage that strongly contribute to aging. Moreover, little is known about the effect of R-SFN on stem cells whose longevity is implicated in human aging. We evaluated the effects of R-SFN on the biology on human mesenchymal stem cells (MSCs), which, in addition to their ability to differentiate into mesenchymal tissues, support hematopoiesis, and contribute to the homeostatic maintenance of many organs and tissues. Our investigation found evidence that low doses of R-SFN promote MSCs proliferation and protect them from apoptosis and senescence, while higher doses have a cytotoxic effect, leading to the induction of cell cycle arrest, programmed cell death and senescence. The beneficial effects of R-SFN may be ascribed to its antioxidant properties, which were observed when MSC cultures were incubated with low doses of R-SFN. Its cytotoxic effects, which were observed after treating MSCs with high doses of R-SFN, could be attributed to its HDAC inhibitory activity. In summary, we found that R-SFN, like many other dietary supplements, exhibits a hormetic behavior; it is able to induce biologically opposite effects at different doses.

DelK32-lamin A/C has abnormal location and induces incomplete tissue maturation and severe metabolic defects leading to premature death

The LMNA gene encodes lamin A/C intermediate filaments that polymerize beneath the nuclear membrane, and are also found in the nucleoplasm in an uncharacterized assembly state. They are thought to have structural functions and regulatory roles in signaling pathways via interaction with transcription factors. Mutations in LMNA have been involved in numerous inherited human diseases, including severe congenital muscular dystrophy (L-CMD). We created the Lmna(ΔK32) knock-in mouse harboring a L-CMD mutation. Lmna(ΔK32/ΔK32) mice exhibited striated muscle maturation delay and metabolic defects, including reduced adipose tissue and hypoglycemia leading to premature death. The level of mutant proteins was markedly lower in Lmna(ΔK32/ΔK32), and while wild-type lamin A/C proteins were progressively relocated from nucleoplasmic foci to the nuclear rim during embryonic development, mutant proteins were maintained in nucleoplasmic foci. In the liver and during adipocyte differentiation, expression of ΔK32-lamin A/C altered sterol regulatory element binding protein 1 (SREBP-1) transcriptional activities. Taken together, our results suggest that lamin A/C relocation at the nuclear lamina seems important for tissue maturation potentially by releasing its inhibitory function on transcriptional factors, including but not restricted to SREBP-1. And importantly, L-CMD patients should be investigated for putative metabolic disorders.

Integrated transcriptome and binding sites analysis implicates E2F in the regulation of self-renewal in human pluripotent stem cells

Rapid cellular growth and multiplication, limited replicative senescence, calibrated sensitivity to apoptosis, and a capacity to differentiate into almost any cell type are major properties that underline the self-renewal capabilities of human pluripotent stem cells (hPSCs). We developed an integrated bioinformatics pipeline to understand the gene regulation and functions involved in maintaining such self-renewal properties of hPSCs compared to matched fibroblasts. An initial genome-wide screening of transcription factor activity using in silico binding-site and gene expression microarray data newly identified E2F as one of major candidate factors, revealing their significant regulation of the transcriptome. This is underscored by an elevated level of its transcription factor activity and expression in all tested pluripotent stem cell lines. Subsequent analysis of functional gene groups demonstrated the importance of the TFs to self-renewal in the pluripotency-coupled context; E2F directly targets the global signaling (e.g. self-renewal associated WNT and FGF pathways) and metabolic network (e.g. energy generation pathways, molecular transports and fatty acid metabolism) to promote its canonical functions that are driving the self-renewal of hPSCs. In addition, we proposed a core self-renewal module of regulatory interplay between E2F and, WNT and FGF pathways in these cells. Thus, we conclude that E2F plays a significant role in influencing the self-renewal capabilities of hPSCs.

An integrated approach to dissecting oncogene addiction implicates a Myb-coordinated self-renewal program as essential for leukemia maintenance

Although human cancers have complex genotypes and are genomically unstable, they often remain dependent on the continued presence of single-driver mutations-a phenomenon dubbed "oncogene addiction." Such dependencies have been demonstrated in mouse models, where conditional expression systems have revealed that oncogenes able to initiate cancer are often required for tumor maintenance and progression, thus validating the pathways they control as therapeutic targets. Here, we implement an integrative approach that combines genetically defined mouse models, transcriptional profiling, and a novel inducible RNAi platform to characterize cellular programs that underlie addiction to MLL-AF9-a fusion oncoprotein involved in aggressive forms of acute myeloid leukemia (AML). We show that MLL-AF9 contributes to leukemia maintenance by enforcing a Myb-coordinated program of aberrant self-renewal involving genes linked to leukemia stem cell potential and poor prognosis in human AML. Accordingly, partial and transient Myb suppression precisely phenocopies MLL-AF9 withdrawal and eradicates aggressive AML in vivo without preventing normal myelopoiesis, indicating that strategies to inhibit Myb-dependent aberrant self-renewal programs hold promise as effective and cancer-specific therapeutics. Together, our results identify Myb as a critical mediator of oncogene addiction in AML, delineate relevant Myb target genes that are amenable to pharmacologic inhibition, and establish a general approach for dissecting oncogene addiction in vivo.

Rb and nucleolin antagonize in controlling human CD34 gene expression

Retinoblastoma protein (Rb) controls cell proliferation, differentiation, survival and gene expression and it has a central role in the signaling network that provides a cell cycle checkpoint in the G1 phase of the cell cycle. Studies in mice have shown that Rb regulates interactions between hematopoietic stem cells and their bone marrow microenvironment and it acts as a critical regulator of hematopoietic stem and progenitor cells under stress. In human hematopoiesis, the CD34 protein is expressed on a subset of progenitor cells capable of self-renewal, multilineage differentiation, and hematopoietic reconstitution, and CD34 has a role in the differentiation of hematopoietic cells. Here we find that, in CD34-positive hematopoietic cells, Rb controls the human CD34 promoter region by antagonizing the CD34 promoter factor nucleolin to provide a mechanism that links expression of endogenous CD34 to cell cycle progression. Our study suggests a direct involvement of Rb in the transcriptional program of human CD34-positive hematopoietic stem/progenitor cells, thus providing further insights into the molecular network relevant to the features of these cells.

ZNF703 gene amplification at 8p12 specifies luminal B breast cancer

Luminal B breast cancers represent a fraction of oestrogen receptor (ER)-positive tumours associated with poor recurrence-free and disease-specific survival in all adjuvant systemic treatment categories including hormone therapy alone. Identification of specific signalling pathways driving luminal B biology is paramount to improve treatment. We have studied 100 luminal breast tumours by combined analysis of genome copy number aberrations and gene expression. We show that amplification of the ZNF703 gene, located in chromosomal region 8p12, preferentially occurs in luminal B tumours. We explored the functional role of ZNF703 in luminal B tumours by overexpressing ZNF703 in the MCF7 luminal cell line. Using mass spectrometry, we identified ZNF703 as a co-factor of a nuclear complex comprising DCAF7, PHB2 and NCOR2. ZNF703 expression results in the activation of stem cell-related gene expression leading to an increase in cancer stem cells. Moreover, we show that ZNF703 is implicated in the regulation of ER and E2F1 transcription factor. These findings point out the prominent role of ZNF703 in transcription modulation, stem cell regulation and luminal B oncogenesis.

p16(INK4a) -mediated suppression of telomerase in normal and malignant human breast cells

The cyclin-dependent kinase inhibitor p16(INK4a) (CDKN2A) is an important tumor suppressor gene frequently inactivated in human tumors. p16 suppresses the development of cancer by triggering an irreversible arrest of cell proliferation termed cellular senescence. Here, we describe another anti-oncogenic function of p16 in addition to its ability to halt cell cycle progression. We show that transient expression of p16 stably represses the hTERT gene, encoding the catalytic subunit of telomerase, in both normal and malignant breast epithelial cells. Short-term p16 expression increases the amount of histone H3 trimethylated on lysine 27 (H3K27) bound to the hTERT promoter, resulting in transcriptional silencing, likely mediated by polycomb complexes. Our results indicate that transient p16 exposure may prevent malignant progression in dividing cells by irreversible repression of genes, such as hTERT, whose activity is necessary for extensive self-renewal.

Local mesenchymal stem/progenitor cells are a preferential target for initiation of adult soft tissue sarcomas associated with p53 and Rb deficiency

The cell of origin and pathogenesis of the majority of adult soft tissue sarcomas (STS) remains poorly understood. Because mutations in both the P53 and RB tumor suppressor genes are frequent in STS in humans, we inactivated these genes by Cre-loxP-mediated recombination in mice with floxed p53 and Rb. Ninety-three percent of mice developed spindle cell/pleomorphic sarcomas after a single subcutaneous injection of adenovirus carrying Cre-recombinase. Similar to human STS, these sarcomas overexpress Cxcr4, which contributes to their invasive properties. Using irradiation chimeras generated by transplanting bone marrow cells from mice carrying either the Rosa26StoploxPLacZ or the Z/EG reporter, as well as the floxed p53 and Rb genes, into irradiated p53loxP/loxPRbloxP/loxP mice, it was determined that sarcomas do not originate from bone marrow-derived cells, such as macrophages, but arise from the local resident cells. At the same time, dermal mesenchymal stem cells isolated by strict plastic adherence and low levels of Sca-1 expression (Sca-1low, CD31negCD45neg) have shown enhanced potential for malignant transformation according to soft agar, invasion, and tumorigenicity assays, after the conditional inactivation of both p53 and Rb. Sarcomas formed after transplantation of these cells have features typical for undifferentiated high-grade pleomorphic sarcomas. Taken together, our studies indicate that local Sca-1low dermal mesenchymal stem/progenitor cells are preferential targets for malignant transformation associated with deficiencies in both p53 and Rb.

A functional role of RB-dependent pathway in the control of quiescence in adult epidermal stem cells revealed by genomic profiling

Continuous cell renewal in mouse epidermis is at the expense of a pool of pluripotent cells that lie in a well defined niche in the hair follicle known as the bulge. To identify mechanisms controlling hair follicle stem cell homeostasis, we developed a strategy to isolate adult bulge stem cells in mice and to define their transcriptional profile. We observed that a large number of transcripts are underexpressed in hair follicle stem cells when compared to non-stem cells. Importantly, the majority of these downregulated genes are involved in cell cycle. Using bioinformatics tools, we identified the E2F transcription factor family as a potential element involved in the regulation of these transcripts. To determine their functional role, we used engineered mice lacking Rb gene in epidermis, which showed increased expression of most E2F family members and increased E2F transcriptional activity. Experiments designed to analyze epidermal stem cell functionality (i.e.: hair regrowth and wound healing) imply a role of the Rb-E2F axis in the control of stem cell quiescence in epidermis.

The BRG1 ATPase of chromatin remodeling complexes is involved in modulation of mesenchymal stem cell senescence through RB-P53 pathways

We focused our attention on brahma-related gene 1 (BRG1), the ATPase subunit of the SWItch/Sucrose NonFermentable (SWI/SNF) chromatin remodeling complex, and analyzed its role in mesenchymal stem cell (MSC) biology. We hypothesized that deviation from the correct concentration of these proteins, which act at the highest level of gene regulation, may be deleterious for cells. We wanted to know what would happen if a cell had to cope with altered regulation of gene expression, either by upregulation or downregulation of BRG1. We assumed that cells would try to restore homeostasis or, alternatively, that the event could trigger senescence/apoptosis phenomena. To this end, in MSCs, we silenced BRG1gene. Knockdown of BRG1 expression induced a significant increase in senescent cells and decrease in apoptotic cells. It is interesting that BRG1 downregulation also induced an increase in heterochromatin. At the molecular level, these phenomena were associated with activation of retinoblastoma-like protein 2 (RB2)/P130- and P53-related pathways. Senescence was accompanied by reduced expression of some stemness-related genes. This is consistent with our previous research, which showed that BRG1 upregulation by ectopic expression also induced senescence processes. Together, these data suggest that BRG1 belongs to a class of genes whose expression is tightly regulated; hence, subtle alterations in BRG1 activity seem to negatively affect mechanisms regulating chromatin status and, in turn, impair cellular physiology.

Dual role of parathyroid hormone in endothelial progenitor cells and marrow stromal mesenchymal stem cells

Hematopoietic stem cells derive regulatory information also from parathyroid hormone (PTH). To explore the possibility that PTH may have a role in regulation of other stem cells residing in bone marrow, such as mesenchymal stem cells (MSCs) and endothelial progenitor cells (EPCs) we assessed the effect of this hormone on the in vitro behavior of MSCs and EPCs. We evidenced that MSCs were much more responsive to PTH than EPCs. PTH increased the proliferation rate of MSCs with a diminution of senescence and apoptosis. Taken together, our results may suggest a protective effect of PTH on MSCs that reduces stress phenomena and preserve genome integrity. At the opposite, PTH did not modify the fate of EPCs in culture.

In vitro senescence of rat mesenchymal stem cells is accompanied by downregulation of stemness-related and DNA damage repair genes

Mesenchymal stem cells (MSCs) are of particular interest because they are being tested using cell and gene therapies for a number of human diseases. MSCs represent a rare population in tissues. Therefore, it is essential to grow MSCs in vitro before putting them into therapeutic use. This is compromised by senescence, limiting the proliferative capacity of MSCs. We analyzed the in vitro senescence of rat MSCs, because this animal is a widespread model for preclinical cell therapy studies. After initial expansion, MSCs showed an increased growth doubling time, lost telomerase activity, and expressed senescence-associated beta-galactosidase. Senescence was accompanied by downregulation of several genes involved in stem cell self-renewal. Of interest, several genes involved in DNA repair also showed a significant downregulation. Entry into senescence occurred with characteristic changes in Retinoblastoma (RB) expression patterns. Rb1 and p107 genes expression decreased during in vitro cultivation. In contrast, pRb2/p130 became the prominent RB protein. This suggests that RB2/P130 could be a marker of senescence or that it even plays a role in triggering the process in MSCs.

HIC1 regulates tumor cell responses to endocrine therapies

An intractable problem impeding breast cancer treatment by the most frequently prescribed endocrine therapy tamoxifen is the inevitable development of resistance, and the molecular mechanisms underlying this loss of responsiveness by breast cancers have been under intense investigation but are not yet fully elucidated. Our recent reports demonstrated that the tumor suppressor heavily methylated in cancers 1 (HIC1) plays an essential role in growth suppression mediated by external stimuli. We report here that novel tumor suppressor HIC1 is required for growth suppression by estrogen antagonists in breast cancer cells. We also find that HIC1 expression is dramatically induced by exposure to estrogen antagonists in sensitive cells, via a c-Jun N-terminal kinase 1 (JNK1) and prohibitin-mediated signaling pathway. This induction is lost in spontaneously antagonist-resistant breast cancer cells. Furthermore, reintroducing HIC1 into resistant breast cancer cells restored their sensitivity to the estrogen antagonists, indicating the existence of a novel regulatory mechanism for growth control of breast cancer cells.

Aging of the inceptive cellular population: the relationship between stem cells and aging

The average life expectancy worldwide has about doubled and the global population has increased six fold over the past century. With improving health care in the developed world there is a proportional augmentation in the treatment necessary for elderly patients occasioning the call for increased research in the area of aging and age-related diseases. The manifestation of this research has been focalized on the causative cellular processes and molecular mechanisms involved. Here we will discuss the efforts of this research in the area of stem cells, delving into the regulatory mechanisms and how their de-regulation could be attributed to aging and age-related diseases.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.

G1 to S phase cell cycle transition in somatic and embryonic stem cells

It is well known that G1 to S phase transition is tightly regulated by the expression and phosphorylation of a number of well-characterized cyclins, cyclin-dependent kinases and members of the retinoblastoma gene family. In this review we discuss the role of these components in regulation of G1 to S phase transition in somatic cells and human embryonic stem cells. Most importantly, we discuss some new tenable links between maintenance of pluripotency and cell cycle regulation in embryonic stem cells by describing the role that master transcription factors play in this process. Finally, the differences in cell cycle regulation between murine and human embryonic stem cells are highlighted, raising interesting questions regarding their biology and stages of embryonic development from which they have been derived.

Adult stem cell maintenance and tissue regeneration in the ageing context: the role for A-type lamins as intrinsic modulators of ageing in adult stem cells and their niches

Adult stem cells have been identified in most mammalian tissues of the adult body and are known to support the continuous repair and regeneration of tissues. A generalized decline in tissue regenerative responses associated with age is believed to result from a depletion and/or a loss of function of adult stem cells, which itself may be a driving cause of many age-related disease pathologies. Here we review the striking similarities between tissue phenotypes seen in many degenerative conditions associated with old age and those reported in age-related nuclear envelope disorders caused by mutations in the LMNA gene. The concept is beginning to emerge that nuclear filament proteins, A-type lamins, may act as signalling receptors in the nucleus required for receiving and/or transducing upstream cytosolic signals in a number of pathways central to adult stem cell maintenance as well as adaptive responses to stress. We propose that during ageing and in diseases caused by lamin A mutations, dysfunction of the A-type lamin stress-resistant signalling network in adult stem cells, their progenitors and/or stem cell niches leads to a loss of protection against growth-related stress. This in turn triggers an inappropriate activation or a complete failure of self-renewal pathways with the consequent initiation of stress-induced senescence. As such, A-type lamins should be regarded as intrinsic modulators of ageing within adult stem cells and their niches that are essential for survival to old age.

Transcriptional networks in root cell fate specification

Cell fate in the Arabidopsis root is determined by positional information mediated by plant hormones and interpreted by transcriptional networks. In this review, we summarize recent advances in our understanding of the regulatory networks that control cell fate within the root meristem, and in the interplay of these networks with phytohormones. Recent work describing the importance of chromatin organization in tissue patterning is also highlighted. A new, high resolution root expression map detailing the transciptome of nearly all cell types in the Arabidopsis root across developmental timepoints will provide a framework for understanding these networks.

Retinoblastoma protein and the leukemia-associated PLZF transcription factor interact to repress target gene promoters

Translocations of the retinoic acid receptor-alpha (RARalpha) locus with the promyelocytic leukemia zinc-finger (PLZF) or PML genes lead to expression of oncogenic PLZF-RARalpha or PML-RARalpha fusion proteins, respectively. These fusion oncoproteins constitutively repress RARalpha target genes, in large part through aberrant recruitment of multiprotein co-repressor complexes. PML and PML-RARalpha have previously been shown to associate with the retinoblastoma (Rb) tumour suppressor protein in its hypophosphorylated state. Here, we demonstrate that PLZF also interacts with Rb in vitro and in vivo. The interaction between PLZF and Rb is mediated through the Rb pocket and the region of PLZF that lies between its transcriptional repression (poxvirus and zinc-finger, POZ) and DNA-binding (zinc-finger) domains. In addition, Rb can simultaneously interact with PLZF and the E2F1 S phase-inducing transcription factor, suggesting that these proteins can exist in the same multiprotein complex. In contrast to the interaction of Rb with PML or E2F1, the PLZF-Rb interaction is not dependent on hypophosphorylation of Rb. These data are supported by chromatin immunoprecipitation analysis, which indicates that PLZF associates with the promoter region of CDC6, a known E2F/Rb target gene. Co-expression of PLZF and Rb results in enhancement of transcriptional repression of PLZF and E2F/Rb target genes, indicating functional co-operation between the two proteins. Both PLZF and Rb have been shown to function in stem cells and taken together these data suggest that interactions between PLZF and Rb could be important in stem cell biology.

HGPS and related premature aging disorders: from genomic identification to the first therapeutic approaches

Progeroid syndromes are heritable human disorders displaying features that recall premature ageing. In these syndromes, premature aging is defined as "segmental" since only some of its features are accelerated. A number of cellular biological pathways have been linked to aging, including regulation of the insulin/growth hormone axis, pathways involving ROS metabolism, caloric restriction, and DNA repair. The number of identified genes associated with progeroid syndromes has increased in recent years, possibly shedding light as well on mechanisms underlying ageing in general. Among these, premature aging syndromes related to alterations of the LMNA gene have recently been identified. This review focuses on Hutchinson-Gilford Progeria syndrome and Restrictive Dermopathy, two well-characterized Lamin-associated premature aging syndromes, pointing out the current knowledge concerning their pathophysiology and the development of possible therapeutic approaches.

Transcription factor epiprofin is essential for tooth morphogenesis by regulating epithelial cell fate and tooth number

In tooth morphogenesis, the dental epithelium and mesenchyme interact reciprocally for growth and differentiation to form the proper number and shapes of teeth. We previously identified epiprofin (Epfn), a gene preferentially expressed in dental epithelia, differentiated ameloblasts, and certain ectodermal organs. To identify the role of Epfn in tooth development, we created Epfn-deficient mice (Epfn-/-). Epfn-/- mice developed an excess number of teeth, enamel deficiency, defects in cusp and root formation, and abnormal dentin structure. Mutant tooth germs formed multiple dental epithelial buds into the mesenchyme. In Epfn-/- molars, rapid proliferation and differentiation of the inner dental epithelium were inhibited, and the dental epithelium retained the progenitor phenotype. Formation of the enamel knot, a signaling center for cusps, whose cells differentiate from the dental epithelium, was also inhibited. However, multiple premature nonproliferating enamel knot-like structures were formed ectopically. These dental epithelial abnormalities were accompanied by dysregulation of Lef-1, which is required for the normal transition from the bud to cap stage. Transfection of an Epfn vector promoted dental epithelial cell differentiation into ameloblasts and activated promoter activity of the enamel matrix ameloblastin gene. Our results suggest that in Epfn-deficient teeth, ectopic nonproliferating regions likely bud off from the self-renewable dental epithelium, form multiple branches, and eventually develop into supernumerary teeth. Thus, Epfn has multiple functions for cell fate determination of the dental epithelium by regulating both proliferation and differentiation, preventing continuous tooth budding and generation.