Francis Harper

Regulation of autophagy by cytoplasmic p53

Multiple cellular stressors, including activation of the tumour suppressor p53, can stimulate autophagy. Here we show that deletion, depletion or inhibition of p53 can induce autophagy in human, mouse and nematode cells subjected to knockout, knockdown or pharmacological inhibition of p53. 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.

Syncytin-A knockout mice demonstrate the critical role in placentation of a fusogenic, endogenous retrovirus-derived, envelope gene

In most mammalian species, a key process of placenta development is the fusion of trophoblast cells into a highly specialized, multinucleated syncytiotrophoblast layer, through which most of the maternofetal exchanges take place. Little is known about this process, despite the recent identification of 2 pairs of envelope genes of retroviral origin, independently acquired by the human (syncytin-1 and syncytin-2) and mouse (syncytin-A and syncytin-B) genomes, specifically expressed in the placenta, and with in vitro cell–cell fusion activity. By generating knockout mice, we show here that homozygous syncytin-A null mouse embryos die in utero between 11.5 and 13.5 days of gestation. Refined cellular and subcellular analyses of the syncytin-A-deficient placentae disclose specific disruption of the architecture of the syncytiotrophoblast-containing labyrinth, with the trophoblast cells failing to fuse into an interhemal syncytial layer. Lack of syncytin-A-mediated trophoblast cell fusion is associated with cell overexpansion at the expense of fetal blood vessel spaces and with apoptosis, adding to the observed maternofetal interface structural defects to provoke decreased vascularization, inhibition of placental transport, and fetal growth retardation, ultimately resulting in death of the embryo. These results demonstrate that syncytin-A is essential for trophoblast cell differentiation and syncytiotrophoblast morphogenesis during placenta development, and they provide evidence that genes captured from ancestral retroviruses have been pivotal in the acquisition of new, important functions in mammalian evolution.

The IKK complex contributes to the induction of autophagy

In response to stress, cells start transcriptional and transcription‐independent programs that can lead to adaptation or death. Here, we show that multiple inducers of autophagy, including nutrient depletion, trigger the activation of the IKK (IκB kinase) complex that is best known for its essential role in the activation of the transcription factor NF‐κB by stress. Constitutively active IKK subunits stimulated autophagy and transduced multiple signals that operate in starvation‐induced autophagy, including the phosphorylation of AMPK and JNK1. Genetic inhibition of the nuclear translocation of NF‐κB or ablation of the p65/RelA NF‐κB subunit failed to suppress IKK‐induced autophagy, indicating that IKK can promote the autophagic pathway in an NF‐κB‐independent manner. In murine and human cells, knockout and/or knockdown of IKK subunits (but not that of p65) prevented the induction of autophagy in response to multiple stimuli. Moreover, the knockout of IKK‐β suppressed the activation of autophagy by food deprivation or rapamycin injections in vivo, in mice. Altogether, these results indicate that IKK has a cardinal role in the stimulation of autophagy by physiological and pharmacological stimuli.

Rabies virus P and small P products interact directly with PML and reorganize PML nuclear bodies.

The interferon-induced promyelocytic leukaemia (PML) protein localizes both in the nucleoplasm and in matrix-associated multi-protein complexes known as nuclear bodies (NBs). NBs are disorganized in acute promyelocytic leukaemia or during some viral infections, suggesting that PML NBs could be a part of cellular defense mechanism. Rabies virus, a member of the rhabdoviridae family, replicates in the cytoplasm. Rabies phosphoprotein P and four other amino-terminally truncated products (P2, P3, P4, P5) are all translated from P mRNA. P and P2 are located in the cytoplasm, whereas P3, P4 and P5 are found mostly in the nucleus. Infection with rabies virus reorganized PML NBs. PML NBs became larger and appeared as dense aggregates when analysed by confocal or electron microscopy, respectively. The expression of P sequesters PML in the cytoplasm where both proteins colocalize, whereas that of P3 results in an increase in PML body size, as observed in infected cells. The P and P3 interacted directly in vivo and in vitro with PML. The C-terminal domain of P and the PML RING finger seem to be involved in this binding. Moreover, PML−/− primary mouse embryonic fibroblasts expressed viral proteins at a higher level and produced 20 times more virus than wild-type cells, suggesting that the absence of all PML isoforms resulted in an increase in rabies virus replication.

Functional Characterization of Negri Bodies (NBs) in Rabies Virus-Infected Cells: Evidence that NBs Are Sites of Viral Transcription and Replication

ABSTRACT Rabies virus infection induces the formation of cytoplasmic inclusion bodies that resemble Negri bodies found in the cytoplasm of some infected nerve cells. We have studied the morphogenesis and the role of these Negri body-like structures (NBLs) during viral infection. The results indicate that these spherical structures (one or two per cell in the initial stage of infection), composed of the viral N and P proteins, grow during the virus cycle before appearing as smaller structures at late stages of infection. We have shown that the microtubule network is not necessary for the formation of these inclusion bodies but is involved in their dynamics. In contrast, the actin network does not play any detectable role in these processes. These inclusion bodies contain Hsp70 and ubiquitinylated proteins, but they are not misfolded protein aggregates. NBLs, in fact, appear to be functional structures involved in the viral life cycle. Specifically, using in situ fluorescent hybridization techniques, we show that all viral RNAs (genome, antigenome, and every mRNA) are located inside the inclusion bodies. Significantly, short-term RNA labeling in the presence of BrUTP strongly suggests that the NBLs are the sites where viral transcription and replication take place.

A Novel Epidermal Growth Factor Receptor Inhibitor Promotes Apoptosis in Non-Small Cell Lung Cancer Cells Resistant to Erlotinib

Non-small cell lung cancer (NSCLC) with activating mutations in the epidermal growth factor receptor (EGFR) responds to EGFR tyrosine kinase inhibitors such as erlotinib. However, secondary somatic EGFR mutations (e.g., T790M) confer resistance to erlotinib. BMS-690514, a novel panHER/vascular endothelial growth factor receptor (VEGFR) inhibitor described here, exerted antiproliferative and proapoptotic effects on NSCLC cell lines, with prominent efficacy on H1975 cells expressing the T790M mutation. In this model, BMS-690514 induced a G(1) cell cycle arrest, as well as ultrastructural hallmarks of apoptosis, mitochondrial release of cytochrome c, and activation of caspases involved in the intrinsic (e.g., caspase-2, caspase-3, caspase-7, and caspase-9), but not in the extrinsic (e.g., caspase-8), pathway. Caspase inhibition conferred partial protection against BMS-690514 cytotoxicity, pointing to the involvement of both caspase-dependent and caspase-independent effector mechanisms. Transcriptome analyses revealed the up-regulation of proapoptotic (e.g., Bim, Puma) and cell cycle inhibitory (e.g., p27(Kip1), p57(Kip2)) factors, as well as the down-regulation of antiapoptotic (e.g., Mcl1), heat shock (e.g., HSP40, HSP70, HSP90), and cell cycle promoting [e.g., cyclins B1, D1, and D3; cyclin-dependent kinase 1 (CDK1); MCM family proteins; proliferating cell nuclear antigen (PCNA)] proteins. BMS-690514-induced death of H1975 cells was modified in a unique fashion by a panel of small interfering RNAs targeting apoptosis modulators. Down-regulation of components of the nuclear factor-kappaB survival pathway (e.g., p65, Nemo/IKK gamma, TAB2) sensitized cells to BMS-690514, whereas knockdown of proapoptotic factors (e.g., Puma, Bax, Bak, caspase-2, etc.) and DNA damage-related proteins (e.g., ERCC1, hTERT) exerted cytoprotective effects. BMS-690514 is a new pan-HER/VEGFR inhibitor that may become an alternative to erlotinib for the treatment of NSCLC.

A pair of co-opted retroviral envelope syncytin genes is required for formation of the two-layered murine placental syncytiotrophoblast

In most mammalian species, a critical step of placenta development is the fusion of trophoblast cells into a multinucleated syncytiotrophoblast layer fulfilling essential fetomaternal exchange functions. Key insights into this process came from the discovery of envelope genes of retroviral origin, the syncytins, independently acquired by the human (syncytin-1 and -2), mouse (syncytin-A and -B), and rabbit (syncytin-Ory1) genomes, with fusogenic properties and placenta-specific expression. We previously showed that mouse syncytin-A is essential for the formation of one of the two syncytiotrophoblast layers and for embryo survival. Here, we have generated syncytin-B KO mice and demonstrate that syncytin-B null placenta displays impaired formation of syncytiotrophoblast layer II (ST-II), with evidence of unfused apposed cells, and enlargement of maternal lacunae disrupting the placenta architecture. Unexpectedly, syncytin-B null embryos are viable, with only limited late-onset growth retardation and reduced neonate number. Microarray analyses identified up-regulation of the connexin 30 gene in mutant placentae, with the protein localized at the fetomaternal interface, suggesting gap junction-mediated compensatory mechanisms. Finally, double-KO mice demonstrate premature death of syncytin-A null embryos if syncytin-B is deleted, indicating cooperation between ST-I and ST-II. These findings establish that both endogenous retrovirus-derived syncytin genes contribute independently to the formation of the two syncytiotrophoblast layers during placenta formation, demonstrating a major role of retroviral gene capture, through convergent evolution, to generate multiple placental structures. Although some are absolutely required for completion of pregnancy, others are still amenable to “epigenetic” compensations, thus illustrating the complexity of the molecular machinery that developed during placental evolution.

Highly Ordered Spatial Organization of the Structural Long Noncoding NEAT1 RNAs within Paraspeckle Nuclear Bodies

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.

Identification of autonomous IAP LTR retrotransposons mobile in mammalian cells

Mammalian genomes contain two main classes of retrotransposons, the well-characterized long and short interspersed nuclear elements, which account for ∼30% of the genome, and the long terminal repeat (LTR) retrotransposons, which resemble the proviral integrated form of retroviruses, except for the absence of an envelope gene in some cases. Genetic studies confirmed mobility of the latter class of elements in mice, with a high proportion of phenotypic mutations consequent to transposition of the intracisternal A particle (IAP) family of LTR retrotransposons. Using the mouse genome sequence and an efficient ex vivo retrotransposition assay, we identified functional, master IAP copies that encode all the enzymatic and structural proteins necessary for their autonomous transposition in heterologous cells. By introducing mutations, we found that the three genes gag, prt and pol are all required for retrotransposition and identified the IAP gene products by electron microscopy in the form of intracellular A-type particles in the transfected cells. These prototypic elements, devoid of an envelope gene, are the first LTR retrotransposons autonomous for transposition to be identified in mammals. Their high rates of retrotransposition indicate that they are potent insertional mutagens that could serve as safe (noninfectious) genetic tools in a large panel of cells.

Inhibition of autophagy by TAB2 and TAB3

Autophagic responses are coupled to the activation of the inhibitor of NF‐κB kinase (IKK). Here, we report that the essential autophagy mediator Beclin 1 and TGFβ‐activated kinase 1 (TAK1)‐binding proteins 2 and 3 (TAB2 and TAB3), two upstream activators of the TAK1‐IKK signalling axis, constitutively interact with each other via their coiled‐coil domains (CCDs). Upon autophagy induction, TAB2 and TAB3 dissociate from Beclin 1 and bind TAK1. Moreover, overexpression of TAB2 and TAB3 suppresses, while their depletion triggers, autophagy. The expression of the C‐terminal domain of TAB2 or TAB3 or that of the CCD of Beclin 1 competitively disrupts the interaction between endogenous Beclin 1, TAB2 and TAB3, hence stimulating autophagy through a pathway that requires endogenous Beclin 1, TAK1 and IKK to be optimally efficient. These results point to the existence of an autophagy‐stimulatory ‘switch’ whereby TAB2 and TAB3 abandon inhibitory interactions with Beclin 1 to engage in a stimulatory liaison with TAK1.