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1.  Selective killing of p53-deficient cancer cells by SP600125 
EMBO Molecular Medicine  2012;4(6):500-514.
The genetic or functional inactivation of p53 is highly prevalent in human cancers. Using high-content videomicroscopy based on fluorescent TP53+/+ and TP53−/− human colon carcinoma cells, we discovered that SP600125, a broad-spectrum serine/threonine kinase inhibitor, kills p53-deficient cells more efficiently than their p53-proficient counterparts, in vitro. Similar observations were obtained in vivo, in mice carrying p53-deficient and -proficient human xenografts. Such a preferential cytotoxicity could be attributed to the failure of p53-deficient cells to undergo cell cycle arrest in response to SP600125. TP53−/− (but not TP53+/+) cells treated with SP600125 became polyploid upon mitotic abortion and progressively succumbed to mitochondrial apoptosis. The expression of an SP600125-resistant variant of the mitotic kinase MPS1 in TP53−/− cells reduced SP600125-induced polyploidization. Thus, by targeting MPS1, SP600125 triggers a polyploidization program that cannot be sustained by TP53−/− cells, resulting in the activation of mitotic catastrophe, an oncosuppressive mechanism for the eradication of mitosis-incompetent cells.
PMCID: PMC3443949  PMID: 22438244
caspases; HCT 116; high-throughput screening; mitochondrial outer membrane permeabilization; MPS1
3.  Highly Ordered Spatial Organization of the Structural Long Noncoding NEAT1 RNAs within Paraspeckle Nuclear Bodies 
Molecular Biology of the Cell  2010;21(22):4020-4027.
We describe the spatial organization of the two NEAT1 noncoding (nc)RNAs required for the integrity of the paraspeckle nuclear bodies. The central sequences of the long transcript are internal when its extremities and the short isoform are peripheral, indicating how RNA can contribute to the architecture of nuclear bodies.
Paraspeckles (PSPs) are nuclear bodies associated with the retention in the nucleus of specific mRNAs. Two isoforms of a long noncoding RNA (NEAT1_v1/Menε and NEAT1_v2/Menβ) are required for the integrity of PSPs. Here, we analyzed the molecular organization of PSPs by immuno- and in situ hybridization electron microscopy. Detection of the paraspeckle markers PSPC1 and P54NRB/NONO confirm the identity between PSPs and the previously described interchromatin granule-associated zones (IGAZs). High-resolution in situ hybridization of NEAT1 transcripts revealed a highly ordered organization of IGAZ/PSPs. Although the 3.7-kb NEAT1_v1 and the identical 5′ end of the 22.7-kb NEAT1_v2 transcripts are confined to the periphery, central sequences of NEAT1_v2 are found within the electron-dense core of the bodies. Moreover, the 3′ end of NEAT1_v2 also localize to the periphery, indicating possible architectures for IGAZ/PSPs. These results further suggest that the organization of NEAT1 transcripts constrains the geometry of these bodies. Accordingly, we observed in HeLa and NIH 3T3 cells that IGAZ/PSPs are elongated structures with a well-defined diameter. Our results provide new insight on the ability of noncoding RNAs to form subcellular structures.
PMCID: PMC2982136  PMID: 20881053
6.  Regulation of autophagy by cytoplasmic p53 
Nature cell biology  2008;10(6):676-687.
Multiple cellular stressors, including activation of the tumour suppressor p53, can stimulate autophagy. Here we show that knockout, knockdown or pharmacological inhibition of p53 can induce autophagy in human, mouse and nematode cells. Enhanced autophagy improved the survival of p53-deficient cancer cells under conditions of hypoxia and nutrient depletion, allowing them to maintain high ATP levels. Inhibition of p53 led to autophagy in enucleated cells, and cytoplasmic, not nuclear, p53 was able to repress the enhanced autophagy of p53-/- cells. Many different inducers of autophagy (for example, starvation, rapamycin and toxins affecting the endoplasmic reticulum) stimulated proteasome-mediated degradation of p53 through a pathway relying on the E3 ubiquitin ligase HDM2. Inhibition of p53 degradation prevented the activation of autophagy in several cell lines, in response to several distinct stimuli. These results provide evidence of a key signalling pathway that links autophagy to the cancer-associated dysregulation of p53.
PMCID: PMC2676564  PMID: 18454141
7.  The GLN Family of Murine Endogenous Retroviruses Contains an Element Competent for Infectious Viral Particle Formation▿  
Journal of Virology  2008;82(9):4413-4419.
Several families of endogenous retroviruses (ERVs) have been identified in the mouse genome, in several instances by in silico searches, but for many of them it remains to be determined whether there are elements that can still encode functional retroviral particles. Here, we identify, within the GLN family of highly reiterated ERVs, one, and only one, copy that encodes retroviral particles prone to infection of mouse cells. We show that its envelope protein confers an ecotropic host range and recognizes a receptor different from mCAT1 and mSMIT1, the two previously identified receptors for other ecotropic mouse retroviruses. Electron microscopy disclosed viral particle assembly and budding at the cell membrane, as well as release of mature particles into the extracellular space. These particles are closely related to murine leukemia virus (MLV) particles, with which they have most probably been confused in the past. This study, therefore, identifies a new class of infectious mouse ERVs belonging to the family Gammaretroviridae, with one family member still functional today. This family is in addition to the two MLV and mouse mammary tumor virus families of active mouse ERVs with an extracellular life cycle.
PMCID: PMC2293071  PMID: 18287236
8.  Murine Endogenous Retrovirus MuERV-L Is the Progenitor of the “Orphan” Epsilon Viruslike Particles of the Early Mouse Embryo▿  
Journal of Virology  2007;82(3):1622-1625.
Viruslike particles which displayed a peculiar wheellike appearance that distinguished them from A-, B- or C-type particles had previously been described in the early mouse embryo. The maximum expression of these so-called epsilon particles was observed in two-cell-stage embryos, followed by their rapid decline at later stages of development and no particles detected at the zygote one-cell stage. Here, we show that these particles are in fact produced by a newly discovered murine endogenous retrovirus (ERV) belonging to the widespread family of mammalian ERV-L elements and named MuERV-L. Using antibodies that we raised against the Gag protein of these elements, Western blot analysis and in toto immunofluorescence studies of the embryos at various stages disclosed the same developmental expression profile as that observed for epsilon particles. Using expression vectors for cloned, full-length, entirely coding MuERV-L copies and cell transfection, direct identification of the epsilon particles was finally achieved by high-resolution electron microscopy.
PMCID: PMC2224431  PMID: 18045933
9.  Murine MusD Retrotransposon: Structure and Molecular Evolution of an “Intracellularized” Retrovirus▿  
Journal of Virology  2006;81(4):1888-1898.
We had previously identified active autonomous copies of the MusD long terminal repeat-retrotransposon family, which have retained transpositional activity. These elements are closely related to betaretroviruses but lack an envelope (env) gene. Here we show that these elements encode strictly intracellular virus-like particles that can unambiguously be identified by electron microscopy. We demonstrate intracellular maturation of the particles, with a significant proportion of densely packed cores for wild-type MusD but not for a protease mutant. We show that the molecular origin of this unexpected intracellular localization is solely dependent on the N-terminal part of the Gag protein, which lacks a functional sequence for myristoylation and plasma membrane targeting: replacement of the N-terminal domain of the MusD matrix protein by that of its closest relative—the Mason-Pfizer monkey virus—led to targeting of the MusD Gag to the plasma membrane, with viral particles budding and being released into the cell supernatant. These particles can further be pseudotyped with a heterologous envelope protein and become infectious, thus “reconstituting” a functional retrovirus prone to proviral insertions. Consistent with its retroviral origin, a sequence with a constitutive transport element-like activity can further be identified at the MusD 3′ untranslated region. A molecular scenario is proposed that accounts for the transition, during evolution, from an ancestral infectious betaretrovirus to the strictly intracellular MusD retrotransposon, involving not only the loss of the env gene but also an inability to escape the cell—via altered targeting of the Gag protein—resulting de facto in the generation of a very successful “intracellularized” insertional mutagen.
PMCID: PMC1797557  PMID: 17151128

Results 1-9 (9)