Otx2 promotes granule cell precursor proliferation and Shh-dependent medulloblastoma maintenance in vivo.

The developmental gene OTX2 is expressed by cerebellar granule cell precursors (GCPs), a cell population which undergoes massive expansion during the early postnatal period in response to sonic hedgehog (Shh). GCPs are thought to be at the origin of most medulloblastomas, a devastating paediatric cancer that arises in the developing cerebellum. OTX2 is overexpressed in all types of medulloblastomas, except in Shh-dependent type 2 medulloblastomas, although it has GCPs as cell-of-origin. This has led to the current view that OTX2 is not involved in tumorigenesis of this subgroup. How OTX2 might contribute to normal or tumoral GCP development in vivo remains unresolved. Here, we have investigated, for the first time, the physiological function of this factor in regulating proliferation and tumorigenesis in the developing mouse cerebellum. We first characterized Otx2-expressing cells in the early postnatal cerebellum and showed that they represent a unique subpopulation of highly proliferative GCPs. We next performed in vivo loss-of-function analysis to dissect out the role of Otx2 in these cells and identified a novel, Shh-independent, function for this factor in controlling postnatal GCP proliferation and cerebellum morphogenesis. Finally, we addressed the function of Otx2 in the context of type 2 medulloblastomas by directing Shh-dependent tumour formation in Otx2+ cells of the developing cerebellum and assessing the effects of Otx2 ablation in this context. We unravel an unexpected, mandatory function for Otx2 in sustaining cell proliferation and long-term maintenance of these tumours in vivo, therefore bringing unpredicted insight into the mechanisms of type 2 medulloblastoma subsistence. Together, these data pinpoint, for the first time, a crucial Shh-independent role for Otx2 in the control of proliferation of normal and tumoral granule cell precursors in vivo and make it an attractive candidate for targeted therapy in Shh-dependent medulloblastomas.

A new genetically engineered mouse model of choroid plexus carcinoma.

Choroid plexus carcinomas (CPCs) are highly malignant brain tumours predominantly found in children and associated to poor prognosis. Improved therapy for these cancers would benefit from the generation of animal models. Here we have created a novel mouse CPC model by expressing a stabilised form of c-Myc (MycT58A) and inactivating Trp53 in the choroid plexus of newborn mice. This induced aberrant proliferation of choroid plexus epithelial cells, leading to aggressive tumour development and death within 150 days. Choroid plexus tumours occurred with a complete penetrance in all brain ventricles, with prevalence in the lateral and fourth ventricles. Histological and cellular analysis indicated that these tumours were CPCs resembling their human counterparts. Comparison of gene expression profiles of CPCs and non-neoplastic tissues revealed profound alterations in cell cycle regulation and DNA damage responses, suggesting that dysregulation of cell division and DNA checkpoint pathways may represent key vulnerabilities. This novel animal model of CPC provides an invaluable tool to elucidate the mechanism of CPC formation and to develop successful therapies against this devastating paediatric cancer.

Comprehensive interactome of Otx2 in the adult mouse neural retina.

The Otx2 homeodomain transcription factor exerts multiple functions in specific developmental contexts, probably through the regulation of different sets of genes. Protein partners of Otx2 have been shown to modulate its activity. Therefore, the Otx2 interactome may play a key role in selecting a precise target-gene repertoire, hence determining its function in a specific tissue. To address the nature of Otx2 interactome, we generated a new recombinant Otx2(CTAP-tag) mouse line, designed for protein complexes purification. We validated this mouse line by establishing the Otx2 interactome in the adult neural retina. In this tissue, Otx2 is thought to have overlapping function with its paralog Crx. Our analysis revealed that, in contrary to Crx, Otx2 did not develop interactions with proteins that are known to regulate phototransduction genes but showed specific partnership with factors associated with retinal development. The relationship between Otx2 and Crx in the neural retina should therefore be considered as complementarity rather than redundancy. Furthermore, study of the Otx2 interactome revealed strong associations with RNA processing and translation machineries, suggesting unexpected roles for Otx2 in the regulation of selected target genes all along the transcription/translation pathway. The Otx2(CTAP-tag) line, therefore, appears suitable for a systematic approach to Otx2 protein-protein interactions. genesis 53:685-694, 2015. © 2015 Wiley Periodicals, Inc.

Early results from a multi-component French public-private partnership initiative to improve participation in clinical research - CeNGEPS: a prospective before-after study.

BACKGROUND: A public-private (51/49 %) partnership was initiated in 2007 in France to improve the attractiveness of French sites in industry-sponsored international clinical trials. This initiative developed and implemented a combination of structuring actions and support actions. Here we report the assessment of the impact after 6 years on participation of French study sites in industry-sponsored clinical trials. METHODS: We performed a prospective before-after study of clinical research activities in French public hospitals to assess the impact of actions developed and implemented by CeNGEPS. The programme involved a combination of structuring actions (establishment of sites of excellence, national networks and dedicated clinical research assistants (CRAs)), support actions (tools, templates and training) and competitive budget allocation for sites or networks based on performance. The impact was assessed using the following performance criteria: 1) reduction of the delay to contract signature to ≤ 60 days for 80 % of the trial sites; 2) inclusion of ≥80 % of the planned number of patients by at least 80 % of trial sites; 3) closure of <15 % of trials sites without patients enrolled. RESULTS: In 2013, the median delay to contract signature was: 55 days, compared with 76 days in 2008 (27.6 % reduction), 50.5 % of all sites and 58 % of sites with a dedicated CRA included ≥80 % of the planned number of patients compared with 44.8 % in 2008 (12.7 % increase) and 21.3 % of all sites and 9 % of sites with a dedicated CRA closed with no patients included, compared with 26.4 % in 2008 (19.3 and 65.9 %, respectively). CONCLUSIONS: These results provide evidence that it is possible to improve a country's attractiveness for industry-sponsored clinical research. The two main actions, i.e. establishing sites of excellence throughout the country with well-trained, dedicated staff and establishing a national network of clinical investigators, could be adapted to other countries in Western Europe to improve Europe's attractiveness to industry-funded trials.

Off-label prescriptions: how to identify them, frame them, announce them and monitor them in practice?

Following the Mediator crisis and the passage of the Health and Safety Law of December 2011, off-label prescriptions are a real concern shared by all those involved in healthcare system. Off-label, in the strictest sense of the term, is defined as all prescriptions that do not correspond to the summary of product characteristics (SPC), particularly those that fail to comply with the indications and dosage regimens defined by the marketing authorization (MA) for clear safety reasons. There are various rasons for off-label prescriptions, both conscious and unconscious. They are intended to respond to unmet medical needs, the needs of poorly studied populations or not studied at all in trials, but in relation to whom it is reasonable to extrapolate that MA would be given (common-sense prescriptions) and, additionally, to urgent public health needs (such as baclofen, pregnant women, and HIV drugs). All these prescriptions would deserve to be studied for a potential MA. However, there are off-label prescriptions that need to be restricted or even penalized in the case of compassionate prescriptions or unjustified prescriptions or prescriptions not based on any scientific grounds. Off-label prescriptions are not easy to track down because if the prescriber has to write "off-label" on his prescription, then clearly, in practice, he will only do so in exceptional cases. Neither the pharmacists who dispense the drug nor the Social Security that reimburses it, have access to the diagnosis (or targeted indication). Thus, in order to identify the off-label prescription, we must be able to cross reference the available databases (such as pharmacovigilance database, medicalized information system program [programme de médicalisation des systèmes d'information, PMSI], hospital drug formularies, general sample of beneficiaries [échantillon généraliste de bénéficiaires, EGB] or national inter-regional Health Insurance Information System [système national d'informations inter-régions d'Assurance maladie, SNIIRAM], sales data, and data from market surveys). The shared computerized patient file may resolve this problem. The temporary use recommendation (TUR) proposed by the Drug Safety Law will only partially deal with this problem for recently marketed molecules. This temporary and exceptional mechanism will authorize a recognized off-label prescription, which may be reimbursed and monitored for 3 years. These TURs will only concern a small portion of "off-label" drugs having yet a positive risk/benefit ratio (conditional MA) but this is far from matching with majority of off-label prescriptions. As such, and in order to improve the use of drugs, it is important to propose a control system for all "off-label" prescriptions with a dedicated committee: the "off-label" committee which would determine the frame of the "off-label" prescriptions.

Loss of Otx2 in the adult retina disrupts retinal pigment epithelium function, causing photoreceptor degeneration.

Photoreceptors are specialized neurons of the retina that receive nursing from the adjacent retinal pigment epithelium (RPE). Frequent in the elderly, photoreceptor loss can originate from primary dysfunction of either cell type. Despite intense interest in the etiology of these diseases, early molecular actors of late-onset photoreceptor degeneration remain elusive, mostly because of the lack of dedicated models. Conditional Otx2 ablation in the adult mouse retina elicits photoreceptor degeneration, providing a new model of late-onset neuronal disease. Here, we use this model to identify the earliest events after Otx2 ablation. Electroretinography and gene expression analyses suggest a nonautonomous, RPE-dependent origin for photoreceptor degeneration. This is confirmed by RPE-specific ablation of Otx2, which results in similar photoreceptor degeneration. In contrast, constitutive Otx2 expression in RPE cells prevents degeneration of photoreceptors in Otx2-ablated retinas. We use chromatin immunoprecipitation followed by massive sequencing (ChIP-seq) analysis to identify the molecular network controlled in vivo by Otx2 in RPE cells. We uncover four RPE-specific functions coordinated by Otx2 that underpin the cognate photoreceptor degeneration. Many direct Otx2 target genes are associated with human retinopathies, emphasizing the significance of the model. Importantly, we report a secondary genetic response after Otx2 ablation, which largely precedes apoptosis of photoreceptors, involving inflammation and stress genes. These findings thus provide novel general markers for clinical detection and prevention of neuronal cell death.

Developmental origins of the adipocyte lineage: new insights from genetics and genomics studies.

The current epidemic of obesity and overweight has caused a surge of interest in the study of adipose tissue formation. Much progress has been made in defining the transcriptional networks controlling the terminal differentiation of adipocyte progenitors into mature adipocytes. However, the early steps of adipocyte development and the embryonic origin of this lineage have been largely disregarded until recently. In mammals, two functionally different types of adipose tissues coexist, which are both involved in energy balance but assume opposite functions. White adipose tissue (WAT) stores energy, while brown adipose tissue (BAT) is specialized in energy expenditure. WAT and BAT can be found as several depots located in various sites of the body. Individual fat depots exhibit different timing of appearance during development, as well as distinct functional properties, suggesting possible differences in their developmental origin. This hypothesis has recently been revisited through large-scale genomics studies and in vivo lineage tracing approaches, which are reviewed in this report. These studies have provided novel fundamental insights into adipocyte biology, pointing out distinct developmental origins for WAT and BAT, as well as for individual WAT depots. They suggest that the adipose tissue is composed of distinct mini-organs, exhibiting developmental and functional differences, as well as variable contribution to obesity-related metabolic diseases.

PBX1: a novel stage-specific regulator of adipocyte development.

Although adipocyte terminal differentiation has been extensively studied, the early steps of adipocyte development and the embryonic origin of this lineage remain largely unknown. Here we describe a novel role for the pre-B-cell leukemia transcription factor one (PBX1) in adipocyte development using both mouse embryonic stem cells (mESCs) and human multipotent adipose-derived stem (hMADS) cells. We show that Pbx1(-/-) mESCs are unable to generate adipocytes, despite normal expression of neuroectoderm and neural crest (NC) markers. Early adipocyte lineage markers are not induced in Pbx1(-/-) mESCs, suggesting that Pbx1 controls the generation and/or the maintenance of adipocyte progenitors (APs) from the NC. We further characterize the function of PBX1 in postnatal adipogenesis and show that silencing of PBX1 expression in hMADS cells reduces their proliferation by preventing their entry in the S phase of the cell cycle. Furthermore, it promotes differentiation of hMADS cells into adipocytes and partially substitutes for glucocorticoids and rosiglitazone, two key proadipogenic agents. These effects involve direct modulation of PPARγ activity, most likely through regulation of the biosynthesis of PPARγ natural endogenous ligand(s). Together, our data suggest that PBX1 regulates adipocyte development at multiple levels, promoting the generation of NC-derived APs during embryogenesis, while favoring APs proliferation and preventing their commitment to the adipocyte lineage in postnatal life.

Comprehensive transcriptome analysis of mouse embryonic stem cell adipogenesis unravels new processes of adipocyte development.

BACKGROUND: The current epidemic of obesity has caused a surge of interest in the study of adipose tissue formation. While major progress has been made in defining the molecular networks that control adipocyte terminal differentiation, the early steps of adipocyte development and the embryonic origin of this lineage remain largely unknown. RESULTS: Here we performed genome-wide analysis of gene expression during adipogenesis of mouse embryonic stem cells (ESCs). We then pursued comprehensive bioinformatic analyses, including de novo functional annotation and curation of the generated data within the context of biological pathways, to uncover novel biological functions associated with the early steps of adipocyte development. By combining in-depth gene regulation studies and in silico analysis of transcription factor binding site enrichment, we also provide insights into the transcriptional networks that might govern these early steps. CONCLUSIONS: This study supports several biological findings: firstly, adipocyte development in mouse ESCs is coupled to blood vessel morphogenesis and neural development, just as it is during mouse development. Secondly, the early steps of adipocyte formation involve major changes in signaling and transcriptional networks. A large proportion of the transcription factors that we uncovered in mouse ESCs are also expressed in the mouse embryonic mesenchyme and in adipose tissues, demonstrating the power of our approach to probe for genes associated with early developmental processes on a genome-wide scale. Finally, we reveal a plethora of novel candidate genes for adipocyte development and present a unique resource that can be further explored in functional assays.

The FunGenES database: a genomics resource for mouse embryonic stem cell differentiation.

Embryonic stem (ES) cells have high self-renewal capacity and the potential to differentiate into a large variety of cell types. To investigate gene networks operating in pluripotent ES cells and their derivatives, the "Functional Genomics in Embryonic Stem Cells" consortium (FunGenES) has analyzed the transcriptome of mouse ES cells in eleven diverse settings representing sixty-seven experimental conditions. To better illustrate gene expression profiles in mouse ES cells, we have organized the results in an interactive database with a number of features and tools. Specifically, we have generated clusters of transcripts that behave the same way under the entire spectrum of the sixty-seven experimental conditions; we have assembled genes in groups according to their time of expression during successive days of ES cell differentiation; we have included expression profiles of specific gene classes such as transcription regulatory factors and Expressed Sequence Tags; transcripts have been arranged in "Expression Waves" and juxtaposed to genes with opposite or complementary expression patterns; we have designed search engines to display the expression profile of any transcript during ES cell differentiation; gene expression data have been organized in animated graphs of KEGG signaling and metabolic pathways; and finally, we have incorporated advanced functional annotations for individual genes or gene clusters of interest and links to microarray and genomic resources. The FunGenES database provides a comprehensive resource for studies into the biology of ES cells.

Information sheets and informed consent forms for clinical study participants: towards standardised recommendations?

Subjects taking part in biomedical research must be provided with legible and intelligible information enabling them to freely give their informed consent. At present, sponsors tend to provide many different types of information, not all of which is directly connected with or indeed really informative about studies for those taking part.As a result of this observation, a round table was convened during the Clinical Pharmacology meetings to deliberate on the creation of a charter concerning the drafting of information documents for biomedical research participants as well as a code of good practice for the preparation of such documents.Recommendations were made based on the efforts of the various working groups concerned, such as the French National Conference of Ethics Committees (CNCP), users' representatives, patients associations and the French industrial and institutional sponsors association (CPI), together with proposals contained in the literature.The deliberations of the round table may be subsumed under the following 3 categories and 14 points: 1) Format: design, drafting rules, layout, table of contents, glossary. 2) CONTENT: introductory page, description of the study, risks, benefits. 3) Regulatory aspects: legal aspects, CNIL message (data protection), financial aspects, conflict of interests, model and varied signatures. This document should help make research in France more attractive and it was decided after the Clinical Pharmacology meeting to submit the charter and related documents for approval by the various actors involved: DGS (Direction Générale de la Santé), Afssaps (Agence Française de Sécurité Sanitaire des Produits de Santé), CNCP, CPI and LEEM (Les entreprises du médicament). Once the charter has been validated, it will be made available to Sponsors and Ethics Committees in order to ensure greater uniformity and legibility regarding information given to study subjects.

Commitment of mouse embryonic stem cells to the adipocyte lineage requires retinoic acid receptor beta and active GSK3.

Key events leading to terminal differentiation of preadipocytes into adipocytes have been identified in recent years. However, signaling pathways involved in the decision of stem cells to follow the adipogenic lineage have not yet been characterized. We have previously shown that differentiating mouse embryonic stem (mES) cells give rise to functional adipocytes upon an early treatment with retinoic acid (RA). The goal of this work was to identify regulators of RA-induced commitment of mES cells to the adipocyte lineage. First, we investigated the role of RA receptor (RAR) isotypes in the induction of mES cell adipogenesis. Using synthetic retinoids selective of RAR isotypes, we show that RARbeta activation is both sufficient and necessary to trigger commitment of mES cells to adipocytes. Then, we performed a small-scale drug screening to find signaling pathways involved in RARbeta-induced mES cell adipogenesis. We show that pharmacological inhibitors of glycogen synthase kinase (GSK) 3, completely inhibit RARbeta-induced adipogenesis in mES cells. This finding uncovers the requirement of active GSK3 in RARbeta-induced commitment of mES cells toward the adipocyte lineage. Finally, we investigated the role of the Wnt pathway, in which GSK3 is a critical negative regulator, in adipocyte commitment by analyzing Wnt pathway activity in RA- and RARbeta-induced mES cell adipogenesis. Our results suggest that although RARbeta and active GSK3 are required for RA-induced adipogenesis, they might be acting through a Wnt pathway-independent mechanism.

Developmental origin of adipocytes: new insights into a pending question.

The current epidemic of obesity has caused a surge of interest in the study of adipose tissue formation. Much progress has been made in defining the transcriptional networks controlling the terminal differentiation of preadipocytes into mature adipocytes. However, the mechanisms that direct MSCs (mesenchymal stem cells) down the adipocyte lineage remain largely unknown. Similarly, although adipocytes are generally described to derive from mesoderm, the study of the developmental origin of MSCs and adipose tissues has been largely disregarded until now. Functional variations do exist between different adipose tissues, which suggest possible differences in their developmental origin and might explain why some depots are more associated than other to metabolic disorders. This review summarizes the surprising findings that have recently emerged from both embryonic stem cells and lineage-tracing studies in vivo, unravelling an unsuspected developmental origin for MSCs and adipocytes in the neural crest.

The generation of adipocytes by the neural crest.

Fat cells (adipocytes) develop from adipocyte precursor cells (preadipocytes) that themselves derive from mesenchymal progenitors. Although the events controlling preadipocyte differentiation into mature adipocytes have been largely explored, the mechanisms that direct mesenchymal progenitors down the adipocyte pathway remain unknown. Similarly, although adipocytes are generally thought to derive from mesoderm, key information is lacking regarding the origin and the development of the adipose tissue during embryogenesis. The aim of this study was to gain insight into the ontogeny of fat cells, both in mouse embryonic stem (mES) cell-derived cultures and during normal development. We first used genetically engineered mES cells to produce and select ES cell-derived neuroepithelial progenitors and showed that neuroectoderm, rather than mesoderm, may be a source of adipocytes in mES cell-derived cultures. We then used primary and secondary cultures of developing quail neural crest (NC) cells to demonstrate that NC cells are able, upon stimulation with defined factors, to differentiate into adipocytes, thus providing a powerful system to study the earliest stages of adipocyte differentiation. Finally, we mapped NC derivatives in vivo using Cre-mediated recombination in transgenic mice and demonstrated that a subset of adipocytes originates from the NC during normal development.

Nucleofection is a valuable transfection method for transient and stable transgene expression in adipose tissue-derived stem cells.

Adipose tissue-derived stem cells are a powerful tool for in vitro study of adult stem cell biology. So far, they have not been extensively used for gain or loss of function studies since they are resistant to most common transfection methods. Herein, we tested several classic transfection methods on human multipotent adipose tissue-derived stem (hMADS) cells. Our results showed that lipofectants and calcium phosphate were poorly efficient for transgene delivery in hMADS cells. In contrast, nucleofection, an electroporation-based method that is assumed to target plasmid DNA directly to the cell nucleus, led to a significant transient transgene expression in hMADS cells (up to 76% enhanced green fluorescent protein [EGFP]-positive cells were detected). Furthermore, after selection of hMADS cells that were nucleofected with a selectable plasmid coding for EGFP, stable EGFP expressing clones could be propagated in culture and efficiently induced to differentiate into EGFP-positive adipocytes and osteoblasts. Finally, we verified that nucleofected hMADS cells could produce a functional, transgene-encoded, secreted protein. To this aim, hMADS cells were nucleofected with a plasmid coding for leukemia inhibitory factor (LIF). This protein was detected at high concentrations in supernatants from pCAG-LIF transfected hMADS cells. Moreover, supernatants were able to maintain mouse embryonic stem cells' undifferentiated phenotype, indicating that hMADS cells could secrete a functional LIF protein. Taken together, our data demonstrate that nucleofection allows both transient and stable gene expression in adipose tissue-derived stem cells, without impairing their differentiation potential.

[State of unmet medical needs in France in 2006: necessity of reinforcing research effort]. / Besoins médicaux non couverts en France en 2006: l'effort de recherche doit être poursuivi.

Leem (French Pharmaceutical Companies) realized an inventory of unmet medical needs in 2006 in France for 12 pathologies. All of them are considered as national public health priorities by the law of August 9th, 2004. Allied to the epidemiological projections, analyses concerned various stages and/or pathology forms, impact of guidelines in clinical practice, therapeutic strategies, marketed therapeutics and pharmacological products in an advanced phase of clinical development. With more than 100 products listed in clinical phase III or pre-registration/marketed for those pathologies, French Pharmaceutical Companies contribute, quasi exclusively, to the development of innovative pharmaceutical products to answer unmet medical needs. This study illustrates the necessity of French Government to support therapeutic innovation led by Pharmaceutical Companies in France.

Generation and characterization of oligodendrocytes from lineage-selectable embryonic stem cells in vitro.

Oligodendrocytes develop from proliferating oligodendrocyte precursor cells (OPCs), which arise in germinal zones, migrate throughout the developing white matter and divide a limited number of times before they terminally differentiate. Thus far, it has been possible to purify OPCs only from the rat optic nerve, but the purified cells cannot be obtained in large enough numbers for conventional biochemical analyses. Moreover, the central nervous system stem cells that give rise to OPCs have not been purified, limiting the ability to study the earliest stages of commitment to the oligodendrocyte lineage. Pluripotent mouse embryonic stem (ES) cells can be propagated indefinitely in culture and induced to differentiate into various cell types. We describe protocols for culture conditions in which neural precursor cells, OPCs, and oligodendrocytes can be efficiently produced from genetically modified ES cells. This strategy should be useful for study of the intracellular and extracellular factors that direct central nervous system stem cells down the oligodendrocyte pathway and influence subsequent oligodendrocyte differentiation. It may also be useful for producing OPCs and oligodendrocytes from human ES cells for cell therapy and drug screening.

A dynamic switch in the replication timing of key regulator genes in embryonic stem cells upon neural induction.

Mammalian embryonic stem (ES) cells can either self-renew or generate progenitor cells that have a more restricted developmental potential. This provides an important model system to ask how pluripotency, cell commitment and differentiation are regulated at the level of chromatin-based changes that distinguish stem cells from their differentiated progeny. Here we show that the differentiation of ES cells to neural progenitors results in dynamic changes in the epigenetic status of multiple genes that encode transcription factors critical for early embryonic development or lineage specification. In particular, we demonstrate that DNA replication at a subset of neural-associated genes including Pax3, Pax6, Irx3, Nkx2.9 and Mash1 is advanced upon neural induction, consistent with increased locus accessibility. Conversely, many ES-associated genes including Oct4, Nanog, Utf1, Foxd3, Cripto and Rex1 that replicate early in ES cells switch their replication timing to later in S-phase in response to differentiation. Detailed analysis of the Rex1 locus reveals that delayed replication extends to a 2.8 Mb region surrounding the gene and is associated with substantial reductions in the level of histone H3K9 and H4 acetylation at the promoter. These results show that loss of pluripotency (and lineage choice) is associated with extensive and predictable changes in the replication timing of key regulator genes.

Roles for p53 and p73 during oligodendrocyte development.

Oligodendrocytes make myelin in the vertebrate central nervous system (CNS). They develop from oligodendrocyte precursor cells (OPCs), most of which divide a limited number of times before they stop and differentiate. OPCs can be purified from the developing rat optic nerve and stimulated to proliferate in serum-free culture by PDGF. They can be induced to differentiate in vitro by either thyroid hormone (TH) or PDGF withdrawal. It was shown previously that a dominant-negative form of p53 could inhibit OPC differentiation induced by TH but not by PDGF withdrawal, suggesting that the p53 family of proteins might play a part in TH-induced differentiation. As the dominant-negative p53 used inhibited all three known p53 family members - p53, p63 and p73 - it was uncertain which family members are important for this process. Here, we provide evidence that both p53 and p73, but not p63, are involved in TH-induced OPC differentiation and that p73 also plays a crucial part in PDGF-withdrawal-induced differentiation. This is the first evidence for a role of p73 in the differentiation of a normal mammalian cell.