Implication of Pectic Components in Cell Surface Interactions between Tomato Root Cells and Fusarium oxysporum f. sp. radicis-lycopersici: A Cytochemical Study by Means of a Lectin with Polygalacturonic Acid-Binding Specificity

Aplysia gonad lectin, a polygalacturonic acid-binding lectin isolated from the sea mollusc Aplysia depilans, was complexed to colloidal gold and used for localizing polygalacturonic-acid-containing molecules in tomato root tissues infected with Fusarium oxysporum f. sp. radicis-lycopersici (FORL). Colonization of host tissues by FORL was associated with striking wall modifications including disruption and even loss of middle lamellae. According to the labeling pattern observed in host wall areas adjacent to fungal penetration channels, it is likely that FORL pectolytic enzymes act through localized wall degradation. The release of polygalacturonic acid-rich wall fragments and the accumulation of polygalacturonic acid-containing molecules in some altered phloem cells were frequently observed and considered to be specific host reactions to fungal attack. The heavy deposition of such molecules at strategic sites such as wall oppositions and intercellular spaces provides support to their implication in the plant defense system. The possible interrelation between polygalacturonic acid-containing molecules and other polymers such as lignin and phenolic compounds remains to be investigated further. The role of these molecules in host-pathogen interactions is discussed in relation to plant defense.     

Ultrastructural localization ofl -fucose residues in nuclei of root primordia of the green peaPisum sativum

Summary Sugars have been demonstrated in animal cell nuclei, but only a few studies have mentioned their presence in plant cell nuclei. In this studyl-fucose residues were localized at the ultrastructural level, usingUlex europeaus agglutinin I lectin, during the early stages of germination ofPisum sativum and in mature root tip cells. This sugar was present after 1 h of germination, and its concentration was found to vary during 3 to 6 h imbition; after 72 h of imbition its concentration had more than doubled. Furthermore, labelling was particularly abundant in the nucleolus, nucleolus-associated bodies and dense nuclear bodies. The possibility that some of thel-fucose residues are associated with proteins is discussed.     

In vivo studies on the nuclear behavior of the arbuscular mycorrhizal fungusGigaspora rosea grown under axenic conditions

Summary The distribution and fate of nuclei of the arbuscular-my-corrhizal fungusGigaspora rosea during late stages of axenic cultures were studied in fixed cultures by transmitted light, conventional and confocal laser scanning microscopy, and in live cultures with two-photon fluorescence microscopy. Mature specimens not yet showing apical septation displayed oval-shaped nuclei localized in lateral positions of the hypha all along the germ-tube length. Beside these, round-shaped nuclei were found to migrate along the central germ-tube core. Some (rare) germ-tube areas, delimited by septa and containing irregularly shaped, much brighter fluorescent nuclei were also found. Specimens that had just initiated the septation process after germ-tube growth arrest displayed round or oval-shaped nuclei in several portions of the germ tubes. These hyphal areas often alternated with other septa-delimited cytoplasmic clusters which contained distorted, brightly fluorescent nuclei. Completely septated specimens mostly lacked nuclei along their germ tubes. However, highly fluorescent chromatin masses appeared within remnants of cytoplasmic material, often compressed between close septa. Our results provide a first clear picture of the in vivo distribution of nuclei along arbuscular mycorrhizal fungal germ tubes issued from resting spores, and suggest that selective areas of their coenocytic hyphae are under specific, single nuclear control. They indicate as well that random autolytic processes occur along senescingG. rosea germ tubes, probably as a consequence of the absence of a host root signal for mycorrhizal formation. Finally, the data presented here allow us to envisage the fate of nuclei released by the germinating spore after nonsymbiotic fungal growth arrest.Abbreviations AM fungi arbuscular-mycorrhizal fungi - DAPI 4, 6-diamidino-2-phenylindole - FM fluorescence microscopy - CLSM confocal laser scanning microscopy - 2PM two-photon microscopy - PI propidium iodide - PMT photomultiplier tube     

Seeing More by Showing Less: Orientation-Dependent Transparency Rendering for Fiber Tractography Visualization

Fiber tractography plays an important role in exploring the architectural organization of fiber trajectories, both in fundamental neuroscience and in clinical applications. With the advent of diffusion MRI (dMRI) approaches that can also model “crossing fibers”, the complexity of the fiber network as reconstructed with tractography has increased tremendously. Many pathways interdigitate and overlap, which hampers an unequivocal 3D visualization of the network and impedes an efficient study of its organization. We propose a novel fiber tractography visualization approach that interactively and selectively adapts the transparency rendering of fiber trajectories as a function of their orientation to enhance the visibility of the spatial context. More specifically, pathways that are oriented (locally or globally) along a user-specified opacity axis can be made more transparent or opaque. This substantially improves the 3D visualization of the fiber network and the exploration of tissue configurations that would otherwise be largely covered by other pathways. We present examples of fiber bundle extraction and neurosurgical planning cases where the added benefit of our new visualization scheme is demonstrated over conventional fiber visualization approaches.     

The active and the inactive form of the hAT1 receptor have an identical ligand-binding environment: an MPA study on a constitutively active angiotensin II receptor mutant

Several models of activation mechanisms were proposed for G protein-coupled receptors (GPCRs), yet no direct methods exist for their elucidation. The availability of constitutively active mutants has given an opportunity to study active receptor conformations within acceptable limits using models such as the angiotensin II type 1 (AT1)1 receptor mutant N111G-hAT1 which displays an important constitutive activity. Recently, by using methionine proximity assay, we showed for the hAT1 receptor that TMD III, VI, and VII form the ligand-binding pocket of the C-terminal amino acid of an antagonistic AngII analogue. In the present contribution, we investigated whether the same residues would also constitute the ligand-binding contacts in constitutively activated mutant (CAM) receptors. For this purpose, the same Met mutagenesis strategy was carried out on the N111G double mutants. Analysis of 43 receptors mutants in the N111G-hAT1 series, photolabeled and CNBr digested, showed that there were only subtle structural changes between the wt-receptor and its constitutively active form.     

Identification of a Novel HtrA1-susceptible Cleavage Site in Human Aggrecan: EVIDENCE FOR THE INVOLVEMENT OF HtrA1 IN AGGRECAN PROTEOLYSIS IN VIVO

Mass spectrometry-based proteomic analyses performed on cartilage tissue extracts identified the serine protease HtrA1/PRSS11 as a major protein component of human articular cartilage, with elevated levels occurring in association with osteoarthritis. Overexpression of a catalytically active form of HtrA1, but not an active site mutant (S328A), caused a marked reduction in proteoglycan content in chondrocyte-seeded alginate cultures. Aggrecan degradation fragments were detected in conditioned media from the alginate cultures overexpressing active HtrA1. Incubation of native or recombinant aggrecan with wild type HtrA1 resulted in distinct cleavage of these substrates. Cleavage of aggrecan by HtrA1 was strongly enhanced by HtrA1 agonists such as CPII, a C-terminal hexapeptide derived from the C-propeptide of procollagen IIα1 (i.e. chondrocalcin). A novel HtrA1-susceptible cleavage site within the interglobular domain (IGD) of aggrecan was identified, and an antibody that specifically recognizes the neoepitope sequence (VQTV³⁵⁶) generated at the HtrA1 cleavage site was developed. Western blot analysis demonstrated that HtrA1-generated aggrecan fragments containing the VQTV³⁵⁶ neoepitope were significantly more abundant in osteoarthritic cartilage compared with cartilage from healthy joints, implicating HtrA1 as a critical protease involved in proteoglycan turnover and cartilage degradation during degenerative joint disease.The mammalian high-temperature requirement A (HtrA) family of serine proteases is defined by a characteristic trypsin-like serine protease domain and one or two C-terminal PDZ domains. Four mammalian HtrA proteins have been identified to date, HtrA1–4. HtrA1 (also called PRSS11) is a ubiquitously expressed extracellular serine protease which contains a signal sequence for secretion, an insulin-like growth factor (IGF)²-binding protein domain, and a Kazal-type serine protease inhibitor domain in addition to the serine protease domain and one C-terminal PDZ domain (1). HtrA1 has been implicated in the progression of several pathologies including age-related macular degeneration, cancer, Alzheimer disease, rheumatoid arthritis, and osteoarthritis (OA) (2–10). HtrA1 has also been shown to inhibit osteoblast mineralization (11).Expression of HtrA1 has been found to be elevated in articular cartilage in association with OA (5). In addition, HtrA1 levels are up-regulated in murine cartilage after experimentally induced joint damage (6). The physiological role of HtrA1 in OA disease progression as well as in other pathologies is unclear. Preliminary studies using in vitro digestion assays suggest that HtrA1 might be capable of digesting cartilage extracellular matrix (ECM) proteins such as fibromodulin, cartilage oligomeric matrix protein (COMP), fibronectin, decorin, and aggrecan (6, 12, 13). Furthermore, it was recently reported that elevated levels of HtrA1 protein (∼7-fold above normal) are present in synovial fluids obtained from OA patients and that fibronectin fragments generated by HtrA1 cleavage induced the expression of catabolic enzymes such as matrix metalloproteinases-1 (MMP-1) and MMP-3 in synovial fibroblasts (4). HtrA1 has also been shown to modulate multiple signaling pathways in vitro. It binds to transforming growth factor-β family proteins including transforming growth factor-β1 and bone morphogenetic proteins 2 and 4 and inhibits signaling mediated by these factors (14, 15). In addition, HtrA1 has been shown to cleave IGF-binding protein-5 and possibly regulate signaling mediated by IGF (16). These findings suggest that the protease HtrA1 may play a physiological role in cartilage during OA.Articular cartilage is made up of chondrocytes surrounded by the ECM comprised mainly of the proteoglycan, aggrecan, and type II collagen. During normal homeostasis there is a dynamic balance between anabolic activities such as proteoglycan synthesis as well as catabolic activities in which the ECM is destroyed. When the catabolic activities of proteases, such as MMPs and aggrecanases, offset new matrix synthesis, focal degradation and loss of articular cartilage occurs, resulting in the development of OA. In some in vitro digestion studies, we and others have shown degradation of aggrecan by recombinant HtrA1 (6, 12, 13). In the present study we set out to examine the physiological relevance of aggrecan cleavage by HtrA1 in OA disease progression.