Monte Carlo Aggregation Code (MCAC) Part 2: Application to soot agglomeration, highlighting the importance of primary particles

During the agglomeration of nanoparticles and in particular, soot, a change in both the flow regime (from free molecular to near continuum) as well as the change of agglomeration regime (from ballistic to diffusive) is expected. However, these effects are rarely taken into account in numerical simulations of particle agglomeration and yet, they are suspected to have an important impact on the agglomeration kinetics, particle morphologies, and size distributions. This work intends to study these properties by using the Monte Carlo Aggregation Code (MCAC) presented in the preceding work (part 1), focusing on the physical impacts of varying the particle volume fraction and monomers size and polydispersity. The results show an important sensitivity of the kinetics of agglomeration, coagulation homogeneity, and agglomerate morphology to the size of monomers. First, for smaller monomer diameters, the agglomeration kinetic is enhanced and agglomerates are characterized by larger fractal dimensions. Second, for large monomer diameters, fractal dimensions down to 1.67 can be found being smaller than the classical 1.78 for Diffusion Limited Cluster Agglomeration (DLCA) mechanism. One important conclusion is that variation in time of both regimes has to be considered for a more accurate simulation of the agglomerate size distribution and morphology.

The genome of Ectocarpus subulatus - A highly stress-tolerant brown alga

Brown algae are multicellular photosynthetic stramenopiles that colonize marine rocky shores worldwide. Ectocarpus sp. Ec32 has been established as a genomic model for brown algae. Here we present the genome and metabolic network of the closely related species, Ectocarpus subulatus Kützing, which is characterized by high abiotic stress tolerance. Since their separation, both strains show new traces of viral sequences and the activity of large retrotransposons, which may also be related to the expansion of a family of chlorophyll-binding proteins. Further features suspected to contribute to stress tolerance include an expanded family of heat shock proteins, the reduction of genes involved in the production of halogenated defence compounds, and the presence of fewer cell wall polysaccharide-modifying enzymes. Overall, E. subulatus has mainly lost members of gene families down-regulated in low salinities, and conserved those that were up-regulated in the same condition. However, 96% of genes that differed between the two examined Ectocarpus species, as well as all genes under positive selection, were found to encode proteins of unknown function. This underlines the uniqueness of brown algal stress tolerance mechanisms as well as the significance of establishing E. subulatus as a comparative model for future functional studies.

Presence of Exogenous Sulfate Is Mandatory for Tip Growth in the Brown Alga Ectocarpus subulatus

Brown algae (Phaeophyceae) are multicellular photoautrophic organisms and the largest biomass producers in coastal regions. A variety of observations indicate that their extracellular matrix (ECM) is involved with screening of salts, development, cell fate selection, and defense responses. It is likely that these functionalities are related to its constitutive structures. The major components of the ECM of brown algae are β-glucans, alginates, and fucose-containing sulfated polysaccharides. The genus Ectocarpus comprises a wide range of species that have adapted to different environments, including isolates of Ectocarpus subulatus, a species highly resistant to low salinity. Previous studies on a freshwater strain of E. subulatus indicated that the sulfate remodeling of fucans is related to the external salt concentration. Here we show that the sulfate content of the surrounding medium is a key parameter influencing both the patterning of the alga and the occurrence of the BAM4 sulfated fucan epitope in walls of apical cells. These results indicate that sulfate uptake and incorporation in the sulfated fucans from apical cells is an essential parameter to sustain tip growth, and we discuss its influence on the architectural plasticity of Ectocarpus.

Isolation of Fucus serratus Gametes and Cultivation of the Zygotes

Zygotes of the Fucale species are a powerful model system to study cell polarization and asymmetrical cell division (Bisgrove and Kropf, 2008). The Fucale species of brown algae grow in the intertidal zone where they reproduce by releasing large female eggs and mobile sperm in the surrounding seawater. The gamete release can be induced from sexually mature fronds in the laboratory and thousands of synchronously developing zygotes are easily obtained. In contrast to other eukaryotic models, such as land plants (Brownlee and Berger, 1995), the embryo is free of maternal tissues and therefore readily amenable to pharmacological approaches. The zygotes are relatively large (up to 100 µm in diameter), facilitating manipulations and imaging studies. During the first hours of zygote development, the alignment of the axis to external cues such as light is labile and can be reversed by light gradients from different directions. A few hours before rhizoid emergence, the alignment of the axis and the polarity are fixed and the cells germinate accordingly. At this stage the zygotes are naturally attached to the substratum through the secretion of cell wall adhesive materials ( Kropf et al., 1988 ; Hervé et al., 2016 ). The first cell division occurs about 24 h after fertilisation and the early embryo is composed of only two cell types that differ in size, shape and developmental fates (i.e., thallus cells and rhizoid cells) ( Bouget et al., 1998 ). The embryo can be successfully cultivated in the laboratory for a few more days (4 weeks maximum) and has an invariant division pattern during the early stages, which allows cell lineages to be traced histologically.

Dynamics of cell wall assembly during early embryogenesis in the brown alga Fucus

Zygotes from Fucus species have been used extensively to study cell polarization and rhizoid outgrowth, and in this model system cell wall deposition aligns with the establishment of polarity. Monoclonal antibodies are essential tools for the in situ analysis of cell wall glycans, and here we report the characteristics of six monoclonal antibodies to alginates (BAM6-BAM11). The use of these, in conjunction with monoclonal antibodies to brown algal sulfated fucans, has enabled the study of the developmental dynamics of the Fucus zygote cell walls. Young zygotes are spherical and all alginate epitopes are deposited uniformly following cellulose deposition. At germination, sulfated fucans are secreted in the growing rhizoid wall. The redistribution of cell wall epitopes was investigated during treatments that cause reorientation of the growth axis (change in light direction) or disrupt rhizoid development (arabinogalactan-protein-reactive Yariv reagent). Alginate modeling was drastically impaired in the latter, and both treatments cause a redistribution of highly sulfated fucan epitopes. The dynamics of cell wall glycans in this system have been visualized in situ for the first time, leading to an enhanced understanding of the early developmental mechanisms of Fucus species. These sets of monoclonal antibodies significantly extend the available molecular tools for brown algal cell wall studies.

Arabinogalactan proteins have deep roots in eukaryotes: identification of genes and epitopes in brown algae and their role in Fucus serratus embryo development

Arabinogalactan proteins (AGPs) are highly glycosylated, hydroxyproline-rich proteins found at the cell surface of plants, where they play key roles in developmental processes. Brown algae are marine, multicellular, photosynthetic eukaryotes. They belong to the phylum Stramenopiles, which is unrelated to land plants and green algae (Chloroplastida). Brown algae share common evolutionary features with other multicellular organisms, including a carbohydrate-rich cell wall. They differ markedly from plants in their cell wall composition, and AGPs have not been reported in brown algae. Here we investigated the presence of chimeric AGP-like core proteins in this lineage. We report that the genome sequence of the brown algal model Ectocarpus siliculosus encodes AGP protein backbone motifs, in a gene context that differs considerably from what is known in land plants. We showed the occurrence of AGP glycan epitopes in a range of brown algal cell wall extracts. We demonstrated that these chimeric AGP-like core proteins are developmentally regulated in embryos of the order Fucales and showed that AGP loss of function seriously impairs the course of early embryogenesis. Our findings shine a new light on the role of AGPs in cell wall sensing and raise questions about the origin and evolution of AGPs in eukaryotes.

Expression of glycosaminoglycans and small proteoglycans in wounds: modulation by the tripeptide-copper complex glycyl-L-histidyl-L-lysine-Cu(2+)

Glycyl-histidyl-lysine-Cu(2+) is a tripeptide-copper complex previously shown to be an activator of wound healing. We have investigated the effects of glycyl-histidyl-lysine-Cu(2+) on the synthesis of glycosaminoglycans and small proteoglycans in a model of rat experimental wounds and in rat dermal fibroblast cultures. Repeated injections of glycyl-histidyl-lysine-Cu(2+) (2 mg per injection) stimulated the wound tissue production, as appreciated by dry weight and total protein measurements. This stimulation was accompanied by an increased production of type I collagen and glycosaminoglycans (assessed, respectively, by hydroxyproline and uronic acid contents of the chamber). Electrophoretic analysis of wound tissue glycosaminoglycans showed an accumulation of chondroitin sulfate and dermatan sulfate in control wound chambers, whereas the proportion of hyaluronic acid decreased with time. The accumulation of chondroitin sulfate and dermatan sulfate was enhanced by glycyl-histidyl-lysine-Cu(2+) treatment. The expression of two small proteoglycans of the dermis, decorin and biglycan, was analyzed by northern blot. The biglycan mRNA steady-state level in the chamber was maximal at day 12, whereas the decorin mRNA increased progressively until the end of the experiment (day 22). Glycyl-histidyl-lysine-Cu(2+) treatment increased the mRNA level of decorin and decreased those of biglycan. In dermal fibroblast cultures, the stimulation of decorin expression by glycyl-histidyl-lysine-Cu(2+) was also found. In contrast, biglycan expression was not modified. These results show that the expression of different proteoglycans in wound tissue are regulated in a different manner during wound healing. The glycyl-histidyl-lysine-Cu(2+) complex is able to modulate the expression of the extracellular matrix macromolecules differently during the wound repair process.

The tripeptide-copper complex glycyl-L-histidyl-L-lysine-Cu2+ stimulates matrix metalloproteinase-2 expression by fibroblast cultures

Glycyl-histidyl-lysine-Cu2+ (GHK-Cu) is a tripeptide-copper complex known to be a potent wound healing agent. We previously showed its ability to stimulate in vitro and in vivo the synthesis of extracellular matrix components. The aim of this study was to determine the effects of GHK-Cu on MMP-2 synthesis by dermal fibroblasts in culture. We showed that GHK-Cu increased MMP-2 levels in conditioned media of cultured fibroblasts. This effect was reproduced by copper ions but not by the tripeptide GHK alone. This stimulation was accompanied by an increase of MMP-2 mRNA level. We also showed that GHK-Cu increased the secretion of the tissue inhibitors of metalloproteinases, TIMP-1 and TIMP-2. Taken together, our results underline that GHK-Cu is not only an activator of connective tissue production but also of the remodeling of the extracellular matrix. It is able to modulate MMP expression by acting directly on wound fibroblasts.

[Regulation of cell activity by the extracellular matrix: the concept of matrikines]

The activity of connective tissue cells is modulated by a number of factors present in their environment. In addition to the soluble factors such as hormones, cytokines or growth factors, cells also receive signals from the surrounding extracellular matrix (ECM) macromolecules. Moreover, they may degrade the ECM proteins and liberate peptides which may by themselves constitute new signals for the surrounding cells. Therefore, an actual regulation loop exists in connective tissue, constituted by peptides generated by ECM degradation and connective tissue cells. The term of "matrikine" has been proposed to designate such ECM-derived peptides able to regulate cell activity. In this review, we summarize some data obtained in our laboratory with two different matrikines: the tripeptide glycyl-histidyl-lysine (GHK) and the heptapeptide cysteinyl-asparaginyl-tyrosyl-tyrosyl-seryl-asparaginyl-serine (CNYYSNS). GHK is a potent activator of ECM synthesis and remodeling, whereas CNYYSNS is able to inhibit polymorphonuclear leukocytes activation and decrease the invasive capacities of cancer cells.

Expression and activation of matrix metalloproteinases in wounds: modulation by the tripeptide-copper complex glycyl-L-histidyl-L-lysine-Cu2+

We investigated the expression and activation of matrix metalloproteinases in a model of experimental wounds in rats, and their modulation by glycyl-L-histidyl-L-lysine-Cu(II), a potent activator of wound repair. Wound chambers were inserted under the skin of Sprague-Dawley rats and received serial injections of either 2 mg glycyl-L-histidyl-L-lysine-Cu(II) or the same volume of saline. The wound fluid and the neosynthetized connective tissue deposited in the chambers were collected and analyzed for matrix metalloproteinase expression and/or activity. Interstitial collagenase increased progressively in the wound fluid throughout the experiment. Glycyl-L-histidyl-L-lysine-Cu(II) treatment did not alter its activity. Matrix metalloproteinase-9 (gelatinase B) and matrix metalloproteinase-2 (gelatinase A) were the two main gelatinolytic activities expressed during the healing process. Pro-matrix metalloproteinase (pro-form of matrix metalloproteinase)-9 was strongly expressed during the early stages of wound healing (day 3). In the wound fluid, it decreased rapidly and disappeared after day 18, whereas in the wound tissue, matrix metalloproteinase-9 expression persisted in the glycyl-L-histidyl-L-lysine-Cu(II) injected chamber until day 22. Pro-matrix metalloproteinase-2 was expressed at low levels at the beginning of the healing process, increased progressively until day 7, then decreased until day 18. Activated matrix metalloproteinase-2 was present in wound fluid and wound tissue. It increased until day 12, then decreased progressively. Glycyl-L-histidyl-L-lysine-Cu(II) injections increased pro-matrix metalloproteinase-2 and activated matrix metalloproteinase-2 during the later stages of healing (days 18 and/or 22). These results demonstrate that various types of matrix metalloproteinases are selectively expressed or activated at the various periods of wound healing. Glycyl-L-histidyl-L-lysine-Cu(II) is able to modulate their expression and might significantly alter wound remodeling.