Christine Alberti-Segui

Bio: Christine Alberti-Segui is an academic researcher from BioMérieux. The author has contributed to research in topics: Vacuole & Listeriolysin O. The author has an hindex of 7, co-authored 9 publications receiving 486 citations. Previous affiliations of Christine Alberti-Segui include The Forsyth Institute & Harvard University.

Cytolysin‐dependent delay of vacuole maturation in macrophages infected with Listeria monocytogenes

Abstract: The bacterial pathogen Listeria monocytogenes (Lm) evades the antimicrobial mechanisms of macrophages by escaping from vacuoles to the cytosol, through the action of the cytolysin listeriolysin O (LLO). Because of heterogeneities in the timing and efficiency of escape, important questions about the contributions of LLO to Lm vacuole identity and trafficking have been inaccessible. Expression of cyan fluorescent protein (CFP)-labelled endocytic membrane markers in macrophages along with a yellow fluorescent protein (YFP)-labelled indicator of Lm entry to the cytosol identified compartments lysed by bacteria. Lm escaped from Rab5a-negative, lysosome-associated membrane protein-1 (LAMP1)-negative, Rab7-positive, phosphatidylinositol 3-phosphate [PI(3)P]-positive vacuoles. Lm vacuoles did not label with YFP-Rab5a unless the bacteria were first opsonized with IgG. Wild-type Lm delayed vacuole fusion with LAMP1-positive lysosomes, relative to LLO-deficient Lm. Bacteria prevented from expressing LLO until their arrival into LAMP1-positive lysosomes escaped inefficiently. Thus, the LLO-dependent delay of Lm vacuole fusion with lysosomes affords Lm a competitive edge against macrophage defences by providing bacteria more time in organelles they can penetrate.

Differential function of Listeria monocytogenes listeriolysin O and phospholipases C in vacuolar dissolution following cell-to-cell spread.

Abstract: We investigated the role of listeriolysin O (LLO) and the bacterial phospholipases PI-PLC and PC-PLC in cell-to-cell spread of Listeria monocytogenes. We showed that LLO is essential for cell-to-cell spread in primary murine macrophages. Electron micrographs revealed that in the absence of continued LLO expression, bacteria remain trapped in secondary spreading vacuoles having either a double or single membrane. In bacteria lacking PI-PLC and PC-PLC, cessation of LLO expression after initiation of infection resulted in a significant increase in the proportion of bacteria trapped in double-membrane compartments. We propose that the bacterial phospholipases are involved in the dissolution of the inner membrane of the spreading vacuole, yet are not sufficient for disruption of the outer membrane. As a consequence, we identified LLO as a key factor in the disruption of the outer membrane. This model is consistent with the observation that LLO is dispensable for cell-to-cell spread from human macrophages into a cell type in which LLO is not required for vacuolar escape. These data suggest that during human infection, spreading of L. monocytogenes to distant organs is likely to occur even in the absence of LLO expression, and that the bacterial phospholipases may be sufficient to mediate continued cell-to-cell spread.

Cytoplasmic dynein is required to oppose the force that moves nuclei towards the hyphal tip in the filamentous ascomycete Ashbya gossypii.

Abstract: We have followed the migration of GFP-labelled nuclei in multinucleate hyphae of Ashbya gossypii. For the first time we could demonstrate that the mode of long range nuclear migration consists of oscillatory movements of nuclei with, on average, higher amplitudes in the direction of the growing tip. We could also show that mitotic division proceeds at a constant rate of 0. 64 microm/minute which differs from the biphasic kinetics described for the yeast Saccharomyces cerevisiae. Furthermore we were able to identify the microtubule-based motor dynein as a key element in the control of long range nuclear migration. For other filamentous fungi it had already been demonstrated that inactivating mutations in dynein led to severe problems in nuclear migration, i.e. generation of long nuclei-free hyphal tips and clusters of nuclei throughout the hyphae. This phenotype supported the view that dynein is important for the movement of nuclei towards the tip. In A. gossypii the opposite seems to be the case. A complete deletion of the dynein heavy chain gene leads to nuclear clusters exclusively at the hyphal tips and to an essentially nucleus-free network of hyphal tubes and branches. Anucleate hyphae and branches in the vicinity of nuclear clusters show actin cables and polarized actin patches, as well as microtubules. The slow growth of this dynein null mutant could be completely reverted to wild-type-like growth in the presence of benomyl, which can be explained by the observed redistribution of nuclei in the hyphal network.

Identification of potential cell-surface proteins in Candida albicans and investigation of the role of a putative cell-surface glycosidase in adhesion and virulence

Abstract: Cell-surface proteins are attractive targets for the development of novel antifungals as they are more accessible to drugs than are intracellular targets. By using a computational biology approach, we identified 180 potential cell-surface proteins in Candida albicans, including the known cell-surface adhesin Als1 and other cell-surface antigens, such as Pra1 and Csa1. Six proteins (named Csf1-6 for cell-surface factors) were selected for further biological characterization. First, we verified that the selected CSF genes are expressed in the yeast and/or hyphal form and then we investigated the effect of the loss of each CSF gene on cell-wall integrity, filamentation, adhesion to mammalian cells and virulence. As a result, we identified Csf4, a putative glycosidase with an apparent orthologue in Saccharomyces cerevisiae (Utr2), as an important factor for cell-wall integrity and maintenance. Interestingly, deletion of CSF4 also resulted in a defect in filamentation, a reduction in adherence to mammalian cells in an in vitro adhesion assay, and a prolongation of survival in an immunocompetent mouse model of disseminated candidiasis. A delay in colonization of key organs (e.g. kidney) was also observed, which is consistent with a reduction in virulence of the csf4-deletion strain. These data indicate a key role for extracellular glycosidases in fungal pathogenesis and represent a new site for therapeutic intervention to cure and prevent fungal disease. Copyright © 2004 John Wiley & Sons, Ltd.

HU Protein Affects Transcription of Surface Polysaccharide Synthesis Genes in Porphyromonas gingivalis

Abstract: K-antigen capsule synthesis is an important virulence determinant of the oral anaerobe Porphyromonas gingivalis. We previously reported that the locus required for synthesis of this surface polysaccharide in strain W83 (TIGR identification PG0106 to PG0120) is transcribed as a large (∼16.7-kb) polycistronic message. Through sequence analysis, we have now identified a 77-bp inverted repeat located upstream (206 bp) of the start codon of PG0106 that is capable of forming a large hairpin structure. Further sequence analysis just upstream and downstream of the capsule synthesis genes revealed the presence of two genes oriented in the same direction as the operon that are predicted to encode DNA binding proteins: PG0104, which is highly similar (57%) to DNA topoisomerase III, and PG0121, which has high similarity (72%) to DNA binding protein HU (β-subunit). In this report, we show that these two genes, as well as the 77-bp inverted repeat region, are cotranscribed with the capsule synthesis genes, resulting in a large transcript that is ∼19.4 kb (based on annotation). We also show that a PG0121 recombinant protein is a nonspecific DNA binding protein with strong affinity to the hairpin structure, in vitro, and that transcript levels of the capsule synthesis genes are downregulated in a PG0121 deletion mutant. Furthermore, we show that this decrease in transcript levels corresponds to a decrease in the amount of polysaccharide produced. Interestingly, expression analysis of another polysaccharide synthesis locus (PG1136 to PG1143) encoding genes involved in synthesis of a surface-associated phosphorylated branched mannan (APS) indicated that this locus is also downregulated in the PG0121 mutant. Altogether our data indicate that HU protein modulates expression of surface polysaccharides in P. gingivalis strain W83.

Sepsis-induced immunosuppression: from cellular dysfunctions to immunotherapy

Abstract: Sepsis - which is a severe life-threatening infection with organ dysfunction - initiates a complex interplay of host pro-inflammatory and anti-inflammatory processes. Sepsis can be considered a race to the death between the pathogens and the host immune system, and it is the proper balance between the often competing pro- and anti-inflammatory pathways that determines the fate of the individual. Although the field of sepsis research has witnessed the failure of many highly touted clinical trials, a better understanding of the pathophysiological basis of the disorder and the mechanisms responsible for the associated pro- and anti-inflammatory responses provides a novel approach for treating this highly lethal condition. Biomarker-guided immunotherapy that is administered to patients at the proper immune phase of sepsis is potentially a major advance in the treatment of sepsis and in the field of infectious disease.

Lysosomes: fusion and function

Abstract: Far from being a static organelle at the end of the endocytic pathway, the lysosome is capable of dynamically fusing with many organelles as well as the plasma membrane. The lysosome provides hydrolytic enzymes for the degradation of macromolecules, has secretory functions and is important for plasma membrane repair. Lysosomes are dynamic organelles that receive and degrade macromolecules from the secretory, endocytic, autophagic and phagocytic membrane-trafficking pathways. Live-cell imaging has shown that fusion with lysosomes occurs by both transient and full fusion events, and yeast genetics and mammalian cell-free systems have identified much of the protein machinery that coordinates these fusion events. Many pathogens that hijack the endocytic pathways to enter cells have evolved mechanisms to avoid being degraded by the lysosome. However, the function of lysosomes is not restricted to protein degradation: they also fuse with the plasma membrane during cell injury, as well as having more specialized secretory functions in some cell types.

Antimicrobial mechanisms of phagocytes and bacterial evasion strategies

Abstract: Professional phagocytes have a vast and sophisticated arsenal of microbicidal features. They are capable of ingesting and destroying invading organisms, and can present microbial antigens on their surface, eliciting acquired immune responses. To survive this hostile response, certain bacterial species have developed evasive strategies that often involve the secretion of effectors to co-opt the cellular machinery of the host. In this Review, we present an overview of the antimicrobial defences of the host cell, with emphasis on macrophages, for which phagocytosis has been studied most extensively. In addition, using Mycobacterium tuberculosis, Listeria monocytogenes, Legionella pneumophila and Coxiella burnetii as examples, we describe some of the evasive strategies used by bacteria.

The Ashbya gossypii Genome as a Tool for Mapping the Ancient Saccharomyces cerevisiae Genome

Abstract: We have sequenced and annotated the genome of the filamentous ascomycete Ashbya gossypii. With a size of only 9.2 megabases, encoding 4718 protein-coding genes, it is the smallest genome of a free-living eukaryote yet characterized. More than 90% of A. gossypii genes show both homology and a particular pattern of synteny with Saccharomyces cerevisiae. Analysis of this pattern revealed 300 inversions and translocations that have occurred since divergence of these two species. It also provided compelling evidence that the evolution of S. cerevisiae included a whole genome duplication or fusion of two related species and showed, through inferred ancient gene orders, which of the duplicated genes lost one copy and which retained both copies.

The Cell Wall

Abstract: Secondary thickening in pteridophytes.-The known cases of secondary thickening in recent Pteridophyta have been brought together by HILL23 in a useful resume. After stating the criteria for secondary growth, Botrychium, which has a distinct cambium, and Ophioglossum, which lacks a definite layer, are described, followed by Angiopteris and Marattia, in which a cambium forms a few xylem elements. CORMACK'S observations on the secondary wood in the nodes of Equisetum are cited, though no reference is made to the cambium in the young cone as reported by J]FFREY.24 The other cases of secondary growth include Psilotum, Selaginella spinulosa, and several species of Isoetes, especially I. hystrix, which may show a cambium outside the vascular cylinder.-M. A.