Roles for p53 and p73 during oligodendrocyte development

Novel Perspectives on p53 Function in Neural Stem Cells and Brain Tumors

Malignant glioma is the most common brain tumor in adults and is associated with a very poor prognosis. Mutations in the p53 tumor suppressor gene are frequently detected in gliomas. p53 is well-known for its ability to induce cell cycle arrest, apoptosis, senescence, or differentiation following cellular stress. That the guardian of the genome also controls stem cell self-renewal and suppresses pluripotency adds a novel level of complexity to p53. Exactly how p53 works in order to prevent malignant transformation of cells in the central nervous system remains unclear, and despite being one of the most studied proteins, there is a need to acquire further knowledge about p53 in neural stem cells. Importantly, the characterization of glioma cells with stem-like properties, also known as brain tumor stem cells, has opened up for the development of novel targeted therapies. Here, we give an overview of what is currently known about p53 in brain tumors and neural stem cells. Specifically, we review the literature regarding transformation of adult neural stem cells and, we discuss how the loss of p53 and deregulation of growth factor signaling pathways, such as increased PDGF signaling, lead to brain tumor development. Reactivation of p53 in brain tumor stem cell populations in combination with current treatments for glioma should be further explored and may become a viable future therapeutic approach.

p73 regulates maintenance of neural stem cell

p73, a member of the p53 family, is a transcription factor that plays a key role in many biological processes. In the present study, we show that TAp73 is expressed in neural stem cells (NSC) and its expression increases following their differentiation. NSC from p73 null mice have a reduced proliferative potential, together with reduced expression of members of the Sox-2 and Notch gene families known to be important for NSC proliferation. In parallel with this in vitro data, the width of the neurogenic areas was reduced in the brains of embryonic and adult p73−/− mice. These data suggest that p73, and in particular TAp73, is important for maintenance of the NSC pool.

Analysis of the intracellular localization of p73 N-terminal protein isoforms TAp73 and ∆Np73 in medulloblastoma cell lines

The protein homologous to the tumor suppressor p53, p73, has essential roles in development and tumorigenesis. This protein exists in a wide range of isoforms with different, even antagonistic, functions. However, there are virtually no detailed morphological studies analyzing the endogenous expression of p73 isoforms at the cellular level in cancer cells. In this study, we investigated the expression and subcellular distribution of two N-terminal isoforms, TAp73 and ΔNp73, in medulloblastoma cells using immunofluorescence microscopy. Both proteins were observed in all cell lines examined, but differences were noted in their intracellular localization between the reference Daoy cell line and four newly established medulloblastoma cell lines (MBL-03, MBL-06, MBL-07 and MBL-10). In the new cell lines, TAp73 and ΔNp73 were located predominantly in cell nuclei. However, there was heterogeneity in TAp73 distribution in the cells of all MBL cell lines, with the protein located in the nucleus and also in a limited non-random area in the cytoplasm. In a small percentage of cells, we detected cytoplasmic localization of TAp73 only, i.e., nuclear exclusion was observed. Our results provide a basis for future studies on the causes and function of distinct intracellular localization of p73 protein isoforms with respect to different protein-protein interactions in medulloblastoma cells.

Implication of TAp73 in the p53-independent pathway of Puma induction and Puma-dependent apoptosis in primary cortical neurons

Puma (p53 up-regulated modulator of apoptosis) is a BH3-only protein member of the Bcl-2 family that controls apoptosis by regulating the release of pro-apoptotic factors from mitochondria. Previously, we reported that sodium arsenite (NaAsO(2)) induces Puma-dependent apoptosis in cortical neurons in a p53-independent manner. The following evidence shows that p53-independent Puma activation by NaAsO(2) is mediated by the p53-related protein TAp73: (i) NaAsO(2) causes TAp73alpha accumulation and increases p53-independent expression of p73 target genes; (ii) two p53 response elements in the Puma promoter are required for NaAsO(2)-mediated activation of a Puma reporter construct; (iii) expression of the inhibitory DeltaNp73alpha and DeltaNp73beta isoforms decreases NaAsO(2)-mediated induction of Puma and other p53-family target genes in a p53-null background; (iv) DeltaNp73alpha and DeltaNp73beta expression protects the neurons from NaAsO(2)-dependent apoptosis. Interestingly, although ER stressors also induce p53-independent, Puma-dependent apoptosis, they do not increase TAp73 expression while NaAsO(2) does not induce notable endoplasmic reticulum (ER) stress. In contrast, DNA damaging agents, okadaic acid, and H(2)O(2) all induce apoptosis in a strictly Puma- and p53-dependent manner. Hence, the pivotal position of Puma as mediator of apoptosis in cortical neurons is because of the availability of at least three independent signalling pathways that ensure its activation.

p63 and p73, the ancestors of p53

p73 and p63 are two homologs of the tumor suppressive transcription factor p53. Given the high degree of structural similarity shared by the p53 family members, p73 and p63 can bind and activate transcription from the majority of the p53-responsive promoters. Besides overlapping functions shared with p53 (i.e., induction of apoptosis in response to cellular stress), the existence of extensive structural variability within the family determines unique roles for p63 and p73. Their crucial and specific functions in controlling development and differentiation are well exemplified by the p63 and p73 knockout mouse phenotypes. Here, we describe the contribution of p63 and p73 to human pathology with emphasis on their roles in tumorigenesis and development.

Molecular control of brain size: regulators of neural stem cell life, death and beyond

The proper development of the brain and other organs depends on multiple parameters, including strictly controlled expansion of specific progenitor pools. The regulation of such expansion events includes enzymatic activities that govern the correct number of specific cells to be generated via an orchestrated control of cell proliferation, cell cycle exit, differentiation, cell death etc. Certain proteins in turn exert direct control of these enzymatic activities and thus progenitor pool expansion and organ size. The members of the Cip/Kip family (p21Cip1/p27Kip1/p57Kip2) are well-known regulators of cell cycle exit that interact with and inhibit the activity of cyclin-CDK complexes, whereas members of the p53/p63/p73 family are traditionally associated with regulation of cell death. It has however become clear that the roles for these proteins are not as clear-cut as initially thought. In this review, we discuss the roles for proteins of the Cip/Kip and p53/p63/p73 families in the regulation of cell cycle control, differentiation, and death of neural stem cells. We suggest that these proteins act as molecular interfaces, or "pilots", to assure the correct assembly of protein complexes with enzymatic activities at the right place at the right time, thereby regulating essential decisions in multiple cellular events.

Differences in the transactivation domains of p53 family members: a computational study

The N terminal transactivation domain of p53 is regulated by ligases and coactivator proteins. The functional conformation of this region appears to be an alpha helix which is necessary for its appropriate interactions with several proteins including MDM2 and p300. Folding simulation studies have been carried out to examine the propensity and stability of this region and are used to understand the differences between the family members with the ease of helix formation following the order p53 > p73 > p63. It is clear that hydrophobic clusters control the kinetics of helix formation, while electrostatic interactions control the thermodynamic stability of the helix. Differences in these interactions between the family members may partially account for the differential binding to, and regulation by, MDM2 (and MDMX). Phosphorylations of the peptides further modulate the stability of the helix and control associations with partner proteins.

Effects of extracellular matrix molecules on the growth properties of oligodendrocyte progenitor cells in vitro

The extracellular matrix (ECM) is a component of neural cell niches and regulates multiple functions of diverse cell types. To date, limited information is available concerning its biological effects on the growth properties of oligodendrocyte progenitor cells (OPCs). In the present study, we examined effects of several ECM components, i.e., fibronectin, laminin, and Matrigel, on the survival, proliferation, migration, process extension, and purity of OPCs isolated from embryonic day 15 rat spinal cords. All three ECM components enhanced these biological properties of the OPCs compared with a non-ECM substrate, poly-D-lysine. However, the extents of their effects were somewhat different. Among these ECMs, fibronectin showed the strongest effect on almost all aspects of the growth properties of OPCs, implying that this molecule is a better substrate for the growth of OPCs in vitro. Because of its survival- and growth-promoting effects on OPCs, fibronectin may be considered as a candidate substrate for enhancing OPC-mediated repair under conditions when exogenous delivery or endogenous stimulation of OPCs is applied.

Two-tier transcriptional control of oligodendrocyte differentiation

Oligodendrocytes (OLs) are the myelin-forming cells of the central nervous system (CNS). They differentiate from proliferative OL precursor cells that migrate from the embryonic neuroepithelium throughout the developing CNS before associating with axons and elaborating myelin. Recent research into the regulation of OL differentiation has uncovered a two-stage mechanism of transcriptional control that combines epigenetic repression of transcriptional inhibitors with direct transcriptional activation of myelin genes. This 'two-pronged' approach creates a fail-safe system of genetic control to ensure orderly and unambiguous expression of the myelination program during development and during repair of demyelinated lesions.

p57kip2 is dynamically regulated in experimental autoimmune encephalomyelitis and interferes with oligodendroglial maturation

The mechanisms preventing efficient remyelination in the adult mammalian central nervous system after demyelinating inflammatory diseases, such as multiple sclerosis, are largely unknown. Partial remyelination occurs in early disease stages, but repair capacity diminishes over time and with disease progression. We describe a potent candidate for the negative regulation of oligodendroglial differentiation that may underlie failure to remyelinate. The p57kip2 gene is dynamically regulated in the spinal cord during MOG-induced experimental autoimmune encephalomyelitis. Transient down-regulation indicated that it is a negative regulator of post-mitotic oligodendroglial differentiation. We then applied short hairpin RNA-mediated gene suppression to cultured oligodendroglial precursor cells and demonstrated that down-regulation of p57kip2 accelerates morphological maturation and promotes myelin expression. We also provide evidence that p57kip2 interacts with LIMK-1, implying that p57kip2 affects cytoskeletal dynamics during oligodendroglial maturation. These data suggest that sustained down-regulation of p57kip2 is important for oligodendroglial maturation and open perspectives for future therapeutic approaches to overcome the endogenous remyelination blockade in multiple sclerosis.

Inhibition of p53 transcriptional activity: a potential target for future development of therapeutic strategies for primary demyelination

Oligodendrogliopathy, microglial infiltration, and lack of remyelination are detected in the brains of patients with multiple sclerosis and are accompanied by high levels of the transcription factor p53. In this study, we used the cuprizone model of demyelination, characterized by oligodendrogliopathy and microglial infiltration, to define the effect of p53 inhibition. Myelin preservation, decreased microglial recruitment, and gene expression were observed in mice lacking p53 or receiving systemic administration of the p53 inhibitor pifithrin-alpha, compared with untreated controls. Decreased levels of lypopolysaccharide-induced gene expression were also observed in vitro, in p53(-/-) primary microglial cultures or in pifithrin-alpha-treated microglial BV2 cells. An additional beneficial effect of lack or inhibition of p53 was observed in Sox2+ multipotential progenitors of the subventricular zone that responded with increased proliferation and oligodendrogliogenesis. Based on these results, we propose transient inhibition of p53 as a potential therapeutic target for demyelinating conditions primarily characterized by oligodendrogliopathy.

DNp73 a matter of cancer: mechanisms and clinical implications

The p53 family proteins carry on a wide spectrum of biological functions from differentiation, cell cycle arrest, apoptosis, and chemosensitivity of tumors. NH2-terminally truncated p73 (referred to as DNp73) acts as a potent inhibitor of all these tumor suppressor properties, implying that it has oncogenic functions in human tumorigenesis. This was favored by the observation that high DNp73 expression levels in a variety of cancers are associated with adverse clinico-pathological characteristics and the response failure to chemotherapy. The actual challenge is the deciphering of the molecular mechanisms by which DNp73 promotes malignancy and to unravel the regulatory pathways for controlling TP73 isoform expression. This review is focused on recent findings leaving no doubt that N-terminally truncated p73 proteins are operative during oncogenesis, thus underscoring its significance as a marker for disease severity in patients and as target for cancer therapy.

Transcriptional control of oligodendrogenesis

Oligodendrocytes (OGs) assemble the myelin sheath around axons in the central nervous system. Specification of cells into the OG lineage is largely the result of interplay between bone morphogenetic protein, sonic hedgehog and Notch signaling pathways, which regulate expression of transcription factors (TFs) dictating spatial and temporal aspects of oligodendrogenesis. Many of these TFs and others then direct OG development through to a mature myelinating OG. Here we describe signaling pathways and TFs that are inductive, inhibitory, and/or permissive to OG specification and maturation. We develop a basic transcriptional network and identify similarities and differences between regulation of oligodendrogenesis in the spinal cord and brain.

Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH

Mutations in the XPD subunit of the DNA repair/transcription factor TFIIH yield the rare genetic disorder trichothiodystrophy (TTD). Although this syndrome was initially associated with a DNA repair defect, individuals with TTD develop neurological features, such as microcephaly and hypomyelination that could be connected to transcriptional defects. Here we show that an XPD mutation in TTD mice results in a spatial and selective deregulation of thyroid hormone target genes in the brain. Molecular analyses performed on the mice brain tissue demonstrate that TFIIH is required for the stabilization of thyroid hormone receptors (TR) to their DNA-responsive elements. The limiting amounts of TFIIH found in individuals with TTD thus contribute to the deregulation of TR-responsive genes. The discovery of an unexpected stabilizing function for TFIIH deepens our understanding of the pathogenesis and neurological manifestations observed in TTD individuals.

The glial response to CNS HIV infection includes p53 activation and increased expression of p53 target genes

HIV-associated dementia (HAD) is a chronic neuroinflammatory disease that remains an important clinical problem without available rational treatment. As HIV does not infect neurons, the pathogenesis of HAD is thought to be secondary to the impact of infected leukocytes, including parenchymal microglia, which can secrete inflammatory mediators and viral products that alter the function of surrounding uninfected cells. We previously reported that the transcription factor p53 accumulates in neurons, microglia, and astrocytes of HAD patients. We have also shown that microglia from p53-deficient mice fail to induce neurotoxicity in response to the HIV coat protein gp120 in a coculture system, supporting the hypothesis that p53 plays a pathogenic role in the chronic neuroinflammatory component of HIV-associated neurodegeneration. We analyzed the extent and cell type specificity of p53 accumulation in subcortical white matter of ten AIDS patients that had previously been shown to demonstrate white matter p53 accumulation. To determine if p53 activation functioned to alter gene expression in HAD, cortical tissue sections were also immunolabeled for the p53 target genes Bax and p21(WAF1). These studies reveal that microglia, astrocytes, and oligodendrocytes all demonstrate p53 activation in response to HIV infection. We observed immunoreactivity for both Bax and p21(WAF1) in neurons and glia from patients demonstrating elevated p53 immunoreactivity. Our findings demonstrate that widespread increased p53 expression is present in HAD. Activation of p53 mediated pathways in the glia of HAD patients may contribute to the neuroinflammatory processes that promote neurodegeneration by inhibiting glial proliferation and/or promoting glial cell dysfunction.

A crucial role for p57(Kip2) in the intracellular timer that controls oligodendrocyte differentiation

The intracellular molecular mechanism that controls the timing of oligodendrocyte differentiation remains unknown. Temple and Raff (1986) previously showed that an oligodendrocyte precursor cell (OPC) can divide a maximum of approximately eight times before its daughter cells simultaneously cease proliferating and differentiate into oligodendrocytes. They postulated that over time the level of an intracellular molecule might synchronously change in each daughter cell, ultimately reaching a level that prohibited additional proliferation. Here, we report the discovery of such a molecule, the cyclin-dependent kinase inhibitor p57(Kip2) (Cdkn1c). We show in vitro that all daughters of a clone of OPCs express similar levels of p57(Kip2), that p57(Kip2) levels increase over time in proliferating OPCs, and that p57(Kip2) levels regulate how many times an OPC can divide before differentiating. These findings reveal a novel part of the mechanism by which OPCs measure time and are likely to extend to similar timers in many other precursor cell types.

DeltaNp73 modulates nerve growth factor-mediated neuronal differentiation through repression of TrkA

p73, a member of the p53 family, expresses two classes of proteins: the full-length TAp73 and the N-terminally truncated DeltaNp73. While TAp73 possesses many p53-like features, DeltaNp73 is dominant negative towards TAp73 and p53 and appears to have distinct functions in tumorigenesis and neuronal development. Given its biological importance, we investigated the role of DeltaNp73 in nerve growth factor (NGF)-mediated neuronal differentiation in PC12 cells. We show that overexpression of DeltaNp73alpha or DeltaNp73beta inhibits NGF-mediated neuronal differentiation in both p53-dependent and -independent manners. In line with this, we showed that the level of endogenous DeltaNp73 is progressively diminished in differentiating PC12 cells upon NGF treatment and knockdown of DeltaNp73 promotes NGF-mediated neuronal differentiation. Interestingly, we found that the ability of DeltaNp73 to suppress NGF-mediated neuronal differentiation is correlated with its ability to regulate the expression of TrkA, the high-affinity NGF receptor. Specifically, we found that DeltaNp73 directly binds to the TrkA promoter and transcriptionally represses TrkA expression, which in turn attenuates the NGF-mediated mitogen-activated protein kinase pathway. Conversely, the steady-state level of TrkA is increased upon knockdown of DeltaNp73. Furthermore, we found that histone deacetylase 1 (HDAC1) and HDAC2 are recruited by DeltaNp73 to the TrkA promoter and act as corepressors to suppress TrkA expression, which can be relieved by trichostatin A, an HDAC inhibitor. Taken together, we conclude that DeltaNp73 negatively regulates NGF-mediated neuronal differentiation by transrepressing TrkA.

The mystery of intracellular developmental programmes and timers

There has been a revolution in understanding animal development in the last 25 years or so, but there is at least one area of development that has been relatively neglected and therefore remains largely mysterious. This is the intracellular programmes and timers that run in developing precursor cells and change the cells over time. The molecular mechanisms underlying these programmes are largely unknown. My colleagues and I have studied such programmes in two types of rodent neural precursor cells: those that give rise to oligodendrocytes, which make myelin in the CNS (central nervous system), and those that give rise to the various cell types in the retina.

The Yin and Yang of cell cycle progression and differentiation in the oligodendroglial lineage

In white matter disorders such as leukodystrophies (LD), periventricular leucomalacia (PVL), or multiple sclerosis (MS), the hypomyelination or the remyelination failure by oligodendrocyte progenitor cells involves errors in the sequence of events that normally occur during development when progenitors proliferate, migrate through the white matter, contact the axon, and differentiate into myelin-forming oligodendrocytes. Multiple mechanisms underlie the eventual progressive deterioration that typifies the natural history of developmental demyelination in LD and PVL and of adult-onset demyelination in MS. Over the past few years, pathophysiological studies have mostly focused on seeking abnormalities that impede oligodendroglial maturation at the level of migration, myelination, and survival. In contrast, there has been a strikingly lower interest for early proliferative and differentiation events that are likely to be equally critical for white matter development and myelin repair. This review highlights the Yin and Yang principles of interactions between intrinsic factors that coordinately regulate progenitor cell division and the onset of differentiation, i.e. the initial steps of oligodendrocyte lineage progression that are obviously crucial in health and diseases.

TAp73 isoforms antagonize Notch signalling in SH-SY5Y neuroblastomas and in primary neurones

p73, like Notch, has been implicated in neurodevelopment and in the maintenance of the mature central nervous system. In this study, by the use of reporter-gene assays, we demonstrate that C-promoter binding factor-1 (CBF-1)-dependent gene transcription driven by the Notch-1 intracellular domain (N1(ICD)) is potently antagonized by exogenously expressed transactivating (TA) p73 splice variants in SH-SY5Y neuroblastomas and in primary neurones. Time course analysis indicated that the inhibitory effects of TAp73 are direct and are not mediated via the product of a downstream target gene. We found that endogenous TAp73 stabilized by either c-Abl or cisplatin treatment also potently antagonized N1(ICD)/CBF-1-dependent gene transcription. Furthermore, western blotting revealed that exogenous TAp73 suppressed endogenous hairy and enhancer of split-1 (HES-1) protein levels and antagonized the increase in HES-1 protein induced by exogenous N1(ICD) expression. Evidence of a direct physical interaction between N1(ICD) and TAp73alpha was demonstrated by co-immunoprecipitation. Using Notch deletion constructs, we demonstrate that TAp73alpha binds the N1(ICD) in a region C-terminal of aa 2094. Interestingly, DeltaNp73alpha and TAp73alpha(R292H) also co-purified with N1(ICD), but neither inhibited N1(ICD)/CBF-1-dependent transcription. This suggests that an intact transactivation (TA) domain and the ability to bind DNA are necessary for TAp73 to antagonize Notch signalling. Finally we found that TAp73alpha reversed the N1(ICD)-mediated repression of retinoic acid-induced differentiation of SH-SY5Y neuroblastomas, providing functional evidence for an inhibitory effect of TAp73alpha on notch signalling. Collectively, these findings may have ramifications for neurodevelopment, neurodegeneration and oncogenesis.

Acting via a cell surface receptor, thyroid hormone is a growth factor for glioma cells

Recent evidence suggests that the thyroid hormone L-thyroxine (T4) stimulates growth of cancer cells via a plasma membrane receptor on integrin alphaVbeta3. The contribution of this recently described receptor for thyroid hormone and receptor-based stimulation of cellular mitogen-activated protein kinase [MAPK; extracellular signal-regulated kinase 1/2 (ERK1/2)] activity, to enhancement of cell proliferation by thyroid hormone was quantitated functionally and by immunologic means in three glioma cell lines exposed to T4. At concentrations of 1 to 100 nmol/L, T4 caused proliferation of C6, F98, and GL261 cells, measured by accumulation of proliferating cell nuclear antigen (PCNA) and radiolabeled thymidine incorporation. This effect was inhibited by the T4 analogue, tetraiodothyroacetic acid, and by an alphaVbeta3 RGD recognition site peptide, both of which block T4 binding to integrin alphaVbeta3 but are not agonists. Activation of MAPK by T4 was similarly inhibited by tetraiodothyroacetic acid and the RGD peptide. The thyroid hormone 3,5,3'-triiodo-L-thyronine (T3) and T4 were equipotent stimulators of PCNA accumulation in C6, F98, and GL261 cells, but physiologic concentrations of T3 are 50-fold lower than those of T4. In conclusion, our studies suggest that glioblastoma cells are thyroid hormone dependent and provide a molecular basis for recent clinical observations that induction of mild hypothyroidism may improve duration of survival in glioblastoma patients. The present experiments infer a novel cell membrane receptor-mediated basis for the growth-promoting activity of thyroid hormone in such tumors and suggest new therapeutic approaches to the treatment of patients with glioblastoma.

DNp73α protects myogenic cells from apoptosis

The P73 gene is transcribed from two promoters, P1 and P2, that direct the expression of multiple transactivation competent (TA) and dominant negative (DN) isoforms. TAp73 transcription factors mediate cell cycle arrest and/or apoptosis in response to DNA damage and are involved in developmental processes. P73 mRNA levels increase and the P1p73 promoter is upregulated during myogenic differentiation of C2C12 skeletal muscle satellite cells. The DNp73 proteins act as trans-repressors of p53- and p73-dependent transcription, and possess both antiapoptotic and pro-proliferative potential. Here, we show that DNp73alpha is expressed in proliferating C2C12 myoblasts, rapidly accumulates in differentiating myocytes and remains elevated in C2C12 myotubes. By combining transactivation assays and chromatin immunoprecipitation analysis, we could show that the upregulation of the P2p73 promoter during myogenic differentiation is mediated by the coordinated recruitment and activity of MyoD and p53/p73. Abrogation of DNp73 expression by specific siRNA led to a strong potentiation of the spontaneous apoptosis of C2C12 myoblasts induced to differentiate. Finally, unlike TAp73 that contributes to DNA damage-induced apoptosis of myotubes, endogenous DNp73 mediates the relative resistance of differentiated myotubes to DNA damage. Altogether, our findings identify DNp73alpha as an important target in designing strategies aimed at the potentiation of the regenerative potential of skeletal satellite cells.

Increased DNA damage sensitivity and apoptosis in cells lacking the Snf5/Ini1 subunit of the SWI/SNF chromatin remodeling complex

The gene encoding the SNF5/Ini1 core subunit of the SWI/SNF chromatin remodeling complex is a tumor suppressor in humans and mice, with an essential role in early embryonic development. To investigate further the function of this gene, we have generated a Cre/lox-conditional mouse line. We demonstrate that Snf5 deletion in primary fibroblasts impairs cell proliferation and survival without the expected derepression of most retinoblastoma protein-controlled, E2F-responsive genes. Furthermore, Snf5-deficient cells are hypersensitive to genotoxic stress, display increased aberrant mitotic features, and accumulate phosphorylated p53, leading to elevated expression of a specific subset of p53 target genes, suggesting a role for Snf5 in the DNA damage response. p53 inactivation does not rescue the proliferation defect caused by Snf5 deficiency but reduces apoptosis and strongly accelerates tumor formation in Snf5-heterozygous mice.

Neurodevelopment on route p63

All known members of the p53 gene family, including the two homologs p73 and p63, have multiple biological functions. In neurons, p53 and p73 are known to regulate cell death in the developing and adult nervous system. A report by Jacobs et al. in this issue of Neuron shows that the more ancestral member of this gene family, p63, is an essential proapoptotic protein during neuronal development.

Over-expression of p73β results in apoptotic death of post-mitotic hNT neurons

The p53-related p73 protein is an important mediator of apoptosis, development and tumorigenesis. Previously, we showed that over-expression of the p73beta isoform induced apoptosis in proliferating neuronal cells; however, the study did not address the effect of p73 in post-mitotic neurons. To address this question, we used post-mitotic hNT neurons, which have been used as a model of human central nervous system neurons. We found that over-expression of p73beta in hNT neurons resulted in apoptosis and an increase in the expression of p57Kip2 and Bax, but no increase in p53 expression. These results suggest that apoptosis of post-mitotic neurons induced by p73beta may involve these mediators. Understanding the regulation of p73 expression will be important for understanding the development of the nervous system and may have implications for the treatment of neurological diseases.

Multipotency of FBD-103a, a neural progenitor cell line from the p53-deficient mouse

We previously established cell lines from brains of p53-deficient embryos, and have now characterized one of these lines, FBD-103a, in detail. Recloning FBD-103a yielded 3 types of subclones: 5 generated the neuronal lineage (Type 1), 3 generated the glial lineage (Type 2), and 10 gave rise to both lineages as the parental line (Type 3), indicating that FBD-103a is a multipotent neural progenitor cell line indistinguishable from true neural stem cells. RT-PCR analyses of transcription factor expression indicated that the transition of multipotent Type 3 clones to either neuronally or glially differentiated progeny was marked by down-regulation of Ascl1/Mash1 and Olig1 and up-regulation of Nrsf/Rest. As expected for neural stem cells, FBD-103a and Type 3 clones formed neurospheres when cultured on a non-adhesive substrate in a serum-free medium containing fibroblast growth factor-2 (FGF2). Interestingly, the transition between Type 3 and Type 1 neuronal- or Type 2 glial-specified cells proved to be reversible; Type 1 and Type 2 subclones could also form neurospheres, from which both neuron-generating and glia-generating progenies could be derived. Moreover, when maintained on an adherent substratum that prevented neurosphere formation, but with FGF2 and without serum, Type 2 clones could generate Type 3 multipotent cells. Thus, at least in the absence of p53, specialized cell-cell interactions within neurospheres are not essential for persistence or recapture of the capacity for self-renewal and multipotency by cells differentiating along glial pathways, a result of possible significance to the pathogenesis of malignant brain tumors.

p73, a sophisticated p53 family member in the cancer world

p73 belongs to a family of p53-related nuclear transcription factors that includes p53, p73 and p63. The overall structure and sequence homology indicates that a p63/p73-like protogene is the ancestral gene, whereas p53 evolved later in higher organisms. In accordance with their structural similarity, p73 functions in a manner analogous to p53 by inducing tumor cell apoptosis and participating in the cell cycle checkpoint control through transactivating an overlapping set of p53/p73-target genes. In sharp contrast to p53, however, p73 is expressed as two NH(2)-terminally distinct isoforms including transcriptionally active (TA) and transcriptionally inactive (DeltaN) forms. DeltaNp73, which has oncogenic potential, acts in a dominant negative manner against TAp73 as well as p53. p73 is induced to be stabilized in response to a subset of DNA-damaging agents in a way that is distinct from that of p53, and exerts its pro-apoptotic activity. Several lines of evidence suggest that p73 can induce tumor cell apoptosis in a p53-dependent and p53-independent manner. Some tumors exhibit resistance to the p53-dependent apoptotic program, therefore p73, which can induce apoptotic cell death by p53-independent mechanisms, is particularly useful. In this review, we discuss the regulatory mechanisms of p73 activity, and also the functional significance of p73 in the regulation of cellular processes including tumorigenesis, apoptosis and neurogenesis.

Redox regulation of precursor cell function: insights and paradoxes

Studies on oligodendrocytes, the myelin-forming cells of the central nervous system, and on the progenitor cells from which they are derived, have provided several novel insights into the role of intracellular redox state in cell function. This review discusses our findings indicating that intracellular redox state is utilized by the organism as a means of regulating the balance between progenitor cell division and differentiation. This regulation is achieved in part through cell-intrinsic differences that modify the response of cells to extracellular signaling molecules, such that cells that are slightly more reduced are more responsive to inducers of cell survival and division and less responsive to inducers of differentiation or cell death. Cells that are slightly more oxidized, in contrast, show a greater response to inducers of differentiation or cell death, but less response to inducers of proliferation or survival. Regulation is also achieved by the ability of exogenous signaling molecules to modify intracellular redox state in a highly predictable manner, such that signaling molecules that promote self-renewal make progenitor cells more reduced and those that promote differentiation make cells more oxidized. In both cases, the redox changes induced by exposure to exogenous signaling molecules are a necessary component of their mode of action. Paradoxically, the results obtained through studies on the oligodendrocyte lineage are precisely the opposite of what might be predicted from a large number of studies demonstrating the ability of reactive oxidative species to enhance the effects of signaling through receptor tyrosine kinase receptors and to promote cell proliferation. Taken in sum, available data demonstrate clearly the existence of two distinct programs of cellular responses to changes in oxidative status. In one of these, becoming even slightly more oxidized is sufficient to inhibit proliferation and induce differentiation. In the second program, similar changes enhance proliferation. It is not yet clear how cells can interpret putatively identical signals in such opposite manners, but it does already seem clear that resolving this paradox will provide insights of considerable relevance to the understanding of normal development, tissue repair, and tumorigenesis.

Full-length p73α Represses Drug-induced Apoptosis in Small Cell Lung Carcinoma Cells

The p73 gene, a member of the p53 family, encodes several variants through differential splicing and use of alternative promoters. At the NH2 terminus, two different promoters generate the full-length and the DeltaN isoforms, with or without the transactivating domain. At the COOH terminus, seven isoforms generated through alternative splicing have been cloned. Previous studies have demonstrated that DeltaNp73 isoforms exert a dominant-negative effect on p73 by blocking their transactivation activity and hence the ability to induce apoptosis. Considerable efforts are made to identify the functional diversity of the COOH-terminal p73 variants. In this study, we found that p73alpha inhibited drug-induced apoptosis in small cell lung carcinoma cells, whereas p73beta promoted it. p73alpha prevented Bax activation, mitochondrial dysfunction, and caspase activation. In addition, p73alpha was also able to reduce apoptosis induced by the BH3-only protein PUMA (p53 up-regulated modulator of apoptosis). Furthermore, we discovered that p73alpha is able to inhibit the pro-apoptotic effect of p73beta, demonstrating the existence of equilibrium between these two p73 isoforms. In conclusion, the reported overexpression of p73alpha in certain tumor types, and our findings that p73alpha exerts anti-apoptotic functions, indicate a potential oncogenic activity for p73.

P53 and beta-catenin activity during estrogen treatment of osteoblasts

Background

This study was undertaken to examine the relationship between the tumor suppressor gene p53 and the nuclear signaling protein beta-catenin during bone differentiation. Cross talk between p53 and beta-catenin pathways has been demonstrated and is important during tumorigenesis and DNA damage, where deregulation of beta catenin activates p53. In this study, we used estrogen treatment of osteoblasts as a paradigm to study the relationship between the two proteins during osteoblast differentiation.

Results

We exposed osteoblast-like ROS17/2.8 cells to 17-beta estradiol (E2), in a short term assay, and studied the cellular distribution and expression of beta-catenin. We found beta-catenin to be up regulated several fold following E2 treatment. Levels of p53 and its functional activity mirrored the quantitative changes seen in beta-catenin. Alkaline phosphatase, an early marker of osteoblast differentiation, was increased in a manner similar to beta-catenin and p53. In order to determine if there was a direct relationship between alkaline phosphatase expression and beta-catenin, we used two different approaches. In the first approach, treatment with LiCl, which is known to activate beta-catenin, caused a several fold increase in alkaline phosphatase activity. In the second approach, transient transfection of wild type beta-catenin into osteoblasts increased alkaline phosphatase activity two fold over basal levels, showing that beta catenin expression can directly affect alkaline phosphatase expression. However increase in beta catenin activity was not associated with an increase in its signaling activity through TCF/LEF mediated transcription. Immunofluorescence analyses of p53 and beta-catenin localization showed that E2 first caused an increase in cytosolic beta-catenin followed by the accumulation of beta-catenin in the nucleus. Nuclear p53 localization was detected in several cells.

Expression of p53 was accompanied by distribution of beta-catenin to the cytoplasm and cell borders. A sub population of cells staining strongly for both proteins appeared to be apoptotic.

Conclusion

These results suggest that interactions between p53 and beta-catenin signaling pathways may play a key role in osteoblast differentiation and maintenance of tissue homeostasis.

Spatiotemporal Switch from ΔNp73 to TAp73 Isoforms during Nephrogenesis

p73 is a member of the p53 gene family, which also includes p53 and p63. These proteins share sequence similarity and target genes but also have divergent roles in cancer and development. Unlike p53, transcription of the p73 gene yields multiple full-length (transactivation (TA) domain) and amino terminus-truncated (DeltaN) isoforms. DeltaNp73 acts in a dominant negative fashion to inhibit the actions of TAp73 and p53 on their target genes, promoting cell survival and proliferation and suppressing apoptosis. The balance between TAp73 and its negative regulator, DeltaNp73, may therefore represent an important determinant of developmental cell fate. There is little if anything known regarding the developmental regulation of the p73 gene. In this study, we showed that TAp73 and DeltaNp73 exhibit reciprocal spatiotemporal expression and functions during nephrogenesis. TAp73 was predominantly expressed in the differentiation domain of the renal cortex in an overlapping manner with the vasopressin-sensitive water channel aquaporin-2 (AQP-2). Chromatin immunoprecipitation assays demonstrated that the endogenous AQP-2 promoter was occupied by TAp73 in a developmentally regulated manner. Furthermore TAp73 stimulated AQP-2 promoter-driven reporter expression. TAp73 also activated the bradykinin B2 receptor (B2R) promoter, a developmentally regulated gene involved in regulation of sodium excretion. The transcriptional effects of TAp73 on AQP-2 and B2R were independent of p53. In marked contrast to TAp73, DeltaNp73 isoforms were induced early in development and were preferentially expressed in proliferating nephron precursors. Moreover DeltaNp73 was a potent repressor of B2R gene transcription. We conclude that the p73 gene is developmentally regulated during kidney organogenesis. The spatiotemporal switch from DeltaNp73 to TAp73 may play an important role in the terminal differentiation program of the developing nephron.

A bipotent neural progenitor cell line cloned from a cerebellum of an adultp53-deficient mouse generates both neurons and oligodendrocytes

Here we report developmental characteristics of a clonal cell line 2Y-3t established from a multifocal neoplasm that arose in a cerebellum of an adult p53-deficient mouse. The tumorigenicity of the line was not observed in soft agar assay or in nude mouse assay. In serum-containing medium, 2Y-3t cells were epithelial-like in morphology and were mitotic. When they were cultured in serum-free medium, the expressions of neural stem and/or progenitor cell markers were decreased. Concomitantly, the expressions of neuronal and oligodendrocyte markers were increased in concert with morphological differentiation, and DNA synthesis ceased. None of astrocyte markers were detected under these culture conditions. Double-labelling studies revealed that two cell populations coexisted, expressing neuronal or oligodendrocyte markers. Triiodothyronine (T3) increased the oligodendrocyte population when 2Y-3t cells were cultured in serum-free medium. Recloning of the line gave rise to three types of subclones. Sixteen subclones were capable of generating both neurons and oligodendrocytes, four subclones were capable of generating only neurons and one subclone was capable of generating only oligodendrocytes. Thus, 2Y-3t cells have characteristics of bipotent neural progenitor cells capable of generating both neurons and oligodendrocytes. In addition, the line expressed mRNA for Pax-2 and had GAD67-positive cells when cultured in serum-free medium. However, none of the mRNAs for Zic-1, Math1, zebrin or Calbindin-D28k were detected, suggesting that the 2Y-3t line might generate the GABAergic interneuron lineage of the mouse cerebellum.

Identification of pigment epithelium-derived factor as a direct target of the p53 family member genes

p63 and p73 show a high degree of structural homology to p53 and are members of a family of transcriptional factors that can activate transcription of p53-responsive genes. p53 is mutated in more than 50% of human cancers, whereas p63 and p73 are rarely mutated. Studies of knockout mice also revealed an unexpected functional diversity among the p53 family. To determine how p63 and p73 are involved in tumorigenesis and normal development, we used cDNA microarray to examine 9216 genes in human colorectal cancer cells. We discovered that the expression of pigment epithelium-derived factor (PEDF) was specifically induced by either p63 or p73, but not by p53. We also report here that the PEDF gene contains a response element specific for p63 and p73 in its promoter region and is a direct target of p63 and p73. Collectively, p63 and p73 may be involved in cell fate by inducing PEDF expression.

New antibodies recognizing p73: Comparison with commercial antibodies

p73, unlike p53, is expressed as a number of isomeric forms. Alternative splicing at the 3' end of p73 transcript, together with the usage of a second promoter downstream of exon 3, can generate up to 24 p73 isoforms. Variants lacking the TA domain (DeltaN isoforms) are induced by TAp73 and by p53, and inhibit their transcriptional activity. However, understanding the complex biology of p73 has been handicapped by the lack of high affinity specific antibodies for the different isoforms. Here, we report the characterization, by Western blotting and immunoprecipitation, of three new polyclonal antisera recognizing all p73 isoforms, only DeltaN isoforms or only p73alpha, and which have advantages of affinity and specificity over previously available antibodies.

Ectopic Expression of p73α, but Not p73β, Suppresses Myogenic Differentiation

The TRP73 gene, a member of the p53 family, encodes several variants through differential splicing and use of alternative promoters. At the N terminus, two different promoters generate the full-length and the DeltaN isoforms, with or without the transactivating domain. At the C terminus, seven isoforms generated through alternative splicing have been cloned. Previous studies have demonstrated that DeltaN-p73 interferes with p73-induced apoptosis. However, there has been no evidence for functional diversity of the C-terminal p73 variants. In this study, we found that p73alpha and p73beta exerted differential effect on the differentiation of C2C12 myoblasts. Although p73beta lacked any detectable effect on differentiation, p73alpha caused a substantial delay in the expression of muscle-specific genes. In co-transfection experiments p73alpha, but not p73beta, attenuated the transcriptional activity of MyoD. Microarray-based gene profiling confirmed the protraction of MyoD-dependent gene expression in C2C12 cells stably expressing p73alpha. Notwithstanding the differential effect on differentiation, p73alpha and p73beta showed similar activity in sensitizing C2C12 myoblasts to cisplatin-induced cell death. These results demonstrated a functional diversity between the two C-terminal variants of p73 and suggested that p73alpha can regulate cellular differentiation in addition to its role in stimulating cell death.