Nitrogen dynamics and phytoplankton community structure: the role of organic nutrients

Dissolved organic nitrogen (DON) is recognised as an important N source for phytoplankton. However, its relative importance for phytoplankton nutrition and community composition has not been studied comprehensively. This study, conducted in a typical Scottish fjord, representative of near-pristine coastal environments, evaluates the utilisation of DON and dissolved inorganic nitrogen (DIN) by different microbial size fractions and the relationship of phytoplankton community composition with DON and other parameters. The study demonstrated that DON was important in supporting phytoplankton throughout the yearly production cycle. The higher-than-expected urea uptake rates and large fraction of the spring bloom production supported by DON suggested that organic N not only contributes to regenerated production and to the nutrition of the small phytoplankton fraction, but can also contribute substantially to new production of the larger phytoplankton in coastal waters. Multivariate statistical techniques revealed two phytoplankton assemblages with peaks in abundance at different times of the year: a spring group dominated by Skeletonema spp., Thalassiosira spp., and Pseudo-nitzschia spp. group delicatissima; and a summer/autumn group dominated by Chaetoceros spp., Scrippsiella spp., and Pseudo-nitzschia spp. group seriata. The multivariate pattern in community composition and abundance of these taxa was significantly correlated with the multivariate pattern of DON, urea, dissolved free amino acids, DIN, temperature, salinity, and daylength, with daylength and urea being particularly important, suggesting both physical and chemical controls on community composition.     

Collagen: the organic matrix of bone

Collagen is the principal organic matrix in bone. The triple helical region of the molecule is 1014 amino acids long. In fibrils these molecules are staggered axially by integers of 234 residues or 68 nm (D). This axial shift occurs by self-assembly and can be understood in terms of a periodicity in the occurrence of apolar and polar residues in the amino acid sequence. Because the molecular length L = 4.47 D, there are gaps 1.5 X 36.5 nm regularly arrayed throughout the fibrils. The three-dimensional molecular arrangement is a quasi-hexagonal lattice with three distinct values for the principal interplanar spacings. Analysis of the intensity distribution in the medium-angle X-ray diffraction patterns from tendons has produced the following picture of the molecular arrangement in fibrils (Fraser et al. 1983). The molecular helices have a coherent length of 32 nm and are tilted parallel to a specific place within the lattice. A regular azimuthal interaction exists between these helices. This crystalline region could be the overlap region with a non-crystalline gap region. However, the gap is still regular axially and the molecular helices retain their structure; their lateral packing is perturbed although they retain a 'gap'. Neutron and X-ray scattering experiments have shown that calcium hydroxyapatite crystals occur in the gap and are nucleated at a specific though unknown location within the gap. The c-axis of the apatite crystals is parallel to the fibril axis and its length c = 0.688 nm is close to the axial periodicity in a protein with an extended beta-conformation. If the telopeptides at the end of a collagen molecule do have this conformation they would either have a highly heterogeneous conformation or exist in a folded manner because the overall length of the telopeptides is shorter than a regular collagen repeat of 0.029 nm would allow.     

Retinol-induced modification of the extracellular matrix of endothelial cells: Its role in growth control

Summary The growth of the endothelial cell (EC) is tightly regulated throughout the body. Many factors have been implicated in modulating EC growth including diffusible compounds, cell-to-cell interactions, and the extracellular matrix (ECM). Retinol, or vitamin A alcohol, has recently been shown to inhibit the growth of bovine capillary ECs, in vitro. Retinoids are known to modify ECM in other cell systems, and pure ECM components have been shown to effect EC growth rates. We, therefore, examined the role of the matrix in the retinol-induced inhibition of ECs. Cell-free matrices from control and vitamin A-treated ECs were prepared by removing cells with EGTA treatment after 7 d of culture. Matrix proteins were analyzed by solubilizing the matrices in 5M quanidine-HCl and performing Western blot analysis using specific antibodies to matrix proteins. In isolating the ECM, we observed that retinol-treated cultures of ECs were resistant to EGTA removal; retinol-treated ECs required twice the exposure time to EGTA to detach from their matrix than did controls cells. Western blot analysis of matrix proteins derived from control and retinol-treated EC cultures demonstrated a 1.6-fold increase in lamininβ chains and a 2.5-fold increase in fibronectin in the ECM of retinol-treated EC compared to control cell matrix. Functional properties of these matrices were assessed by plating control and Day 6 retinol-treated ECs onto the matrices and measuring attachment and growth by determining cell numbers at 24, 72, and 144 h. These studies revealed that control cells attached in greatest numbers to a control matrix whereas retinol-treated ECs preferentially attached to a matrix derived from retinol-treated cells. Furthermore, control ECs which grew rapidly on a control matrix were growth inhibited on a retinol-derived matrix. These data indicate that vitamin A treatment of ECs effects both their phenotype and influences the composition and the functional properties of their underlying ECM. These studies also demonstrate that alterations of the matrix are at least in part responsible for the growth inhibition of EC by retinol.     

Modulation of differentiation markers in human choriocarcinoma cells by extracellular matrix: on the role of a three-dimensional matrix structure

Abstract. During spontaneous or chemically induced differentiation human choriocarcinoma cells express typical characteristics of the normal differentiating trophoblast: 1) increased production of peptide and steroid hormones (chorionic gonadotropin, placental lactogen, estrogens, progesterone); 2) increased activity of cellular alkaline phosphatase; 3) morphological transition from cytotrophoblast to syncytiotrophoblast-like cells; and 4) arrested cell proliferation. Since the extracellular matrix is known to control gene expression we have examined the effects of different substrates composed of matrix macromolecules on the differentiation of BeWo choriocarcinoma cells. Matrices tested were; fibronectin, laminin, collagens type I and type IV, the basement membrane-like complex matrix Matrigel, and a complex matrix extracted from human term placenta. Irrespective of the type of molecule(s), it was consistently found that, whenever the matrix molecules were presented as threedimensional structures (as opposed to protein coatings on tissue culture plastic) the response of affected differentiation markers monitored was highly pronounced. Morphology was changed from monolayers to rounded colonies, cell proliferation was reduced, and the secretion of chorionic gonadotropin was increased up to tenfold. Heterogeneous effects were observed on progesterone secretion and on the activity of cellular alkaline phosphatase. Cell adhesion to matrix molecules, however, did not depend on the structure of the matrix. This study demonstrates that gene expression in these tumor cells can be modified by extracellular matrix and highlights that not only the presence of effector molecules in the matrix but also the three-dimensional structure of the matrix is important for the induction of differentiation.     

The role of the lipid matrix for structure and function of the GPCR rhodopsin

Photoactivation of rhodopsin in lipid bilayers results within milliseconds in a metarhodopsin I (MI)-metarhodopsin II (MII) equilibrium that is very sensitive to the lipid composition. It has been well established that lipid bilayers that are under negative curvature elastic stress from incorporation of lipids like phosphatidylethanolamines (PE) favor formation of MII, the rhodopsin photointermediate that is capable of activating G protein. Furthermore, formation of the MII state is favored by negatively charged lipids like phosphatidylserine and by lipids with longer hydrocarbon chains that yield bilayers with larger membrane hydrophobic thickness. Cholesterol and rhodopsin-rhodopsin interactions from crowding of rhodopsin molecules in lipid bilayers shift the MI-MII equilibrium towards MI. A variety of mechanisms seems to be responsible for the large, lipid-induced shifts between MI and MII: adjustment of the thickness of lipid bilayers to rhodopsin and adjustment of rhodopsin helicity to the thickness of bilayers, curvature elastic deformations in the lipid matrix surrounding the protein, direct interactions of PE headgroups and polyunsaturated hydrocarbon chains with rhodopsin, and direct or lipid-mediated interactions between rhodopsin molecules. This article is part of a Special Issue entitled: Membrane protein structure and function.     

Gene targeting reveals the role of Oc90 as the essential organizer of the otoconial organic matrix

A critical part of the functional development of our peripheral balance system is the embryonic formation of otoconia, composite crystals that overlie and provide optimal stimulus input to the sensory epithelium of the gravity receptor in the inner ear. To date neither the functions of otoconial proteins nor the processes of crystal formation are clearly defined. Using gene targeting and protein analysis strategies, we demonstrate that the predominant mammalian otoconin, otoconin-90/95 (Oc90), is essential for formation of the organic matrix of otoconia by specifically recruiting other matrix components, which includes otolin, a novel mammalian otoconin that we identified to be in wildtype murine otoconia. We show that this matrix controls otoconia growth and morphology by embedding the crystallites during seeding and growth. During otoconia development, the organic matrix forms prior to CaCO3 deposition and provides optimal calcification efficiency. Histological and ultrastructural examinations show normal inner ear epithelial morphology but reduced acellular matrices, including otoconial, cupular and tectorial membranes, in Oc90 null mice, likely due to an absence of Oc90 and a profound reduction of otolin. Our data demonstrate the critical roles of otoconins in otoconia seeding, growth and anchoring and suggest mechanistic similarities and differences between otoconia and bone calcification.     

The role of matrix proteins in the control of nacreous layer deposition during pearl formation

To study the function of pearl oyster matrix proteins in nacreous layer biomineralization in vivo, we examined the deposition on pearl nuclei and the expression of matrix protein genes in the pearl sac during the early stage of pearl formation. We found that the process of pearl formation involves two consecutive stages: (i) irregular calcium carbonate (CaCO(3)) deposition on the bare nucleus and (ii) CaCO(3) deposition that becomes more and more regular until the mature nacreous layer has formed on the nucleus. The low-expression level of matrix proteins in the pearl sac during periods of irregular CaCO(3) deposition suggests that deposition may not be controlled by the organic matrix during this stage of the process. However, significant expression of matrix proteins in the pearl sac was detected by day 30-35 after implantation. On day 30, a thin layer of CaCO(3), which we believe was amorphous CaCO(3), covered large aragonites. By day 35, the nacreous layer had formed. The whole process is similar to that observed in shells, and the temporal expression of matrix protein genes indicated that their bioactivities were crucial for pearl development. Matrix proteins controlled the crystal phase, shape, size, nucleation and aggregation of CaCO(3) crystals.     

The role of polymer matrix structure and interparticle interactions in diffusion-limited drug release.

A lattice random-walk model is used to simulate diffusion in a porous polymer. This model may be useful for the practical design of drug-release systems. Both interacting and noninteracting particles (random walkers) were allowed to diffuse through a pore with a single exit hole. It was found that the specific interactions among the diffusing particles have little influence on the overall release rate. Diffusion through more complicated structures was investigated by simulating the diffusion of particles through two pores connected by a constricted channel whose length and width were varied. The overall rate of release was found to be proportional to the width of the constricted channel. When the length of the channel was greater than or equal to the length of the pore, the rate of release was also inversely proportional to the channel length. From a practical standpoint, release rates can be decreased (and times for release increased) by one or two orders of magnitude by decreasing the width and expanding the length of the interconnecting channels in the polymer matrix.     

Vertebrate gastrulation: polarity genes control the matrix

The PCP signaling cascade controls polarized cell behaviors in various organisms. New evidence suggests that this signaling cascade also controls the deposition of extracellular matrix during vertebrate gastrulation.     

The role of the acrosomal matrix in fertilization

Mammalian sperm must have properly formed acrosomes to be fully functional in the process of binding and penetrating the zona pellucida (ZP), the extracellular matrix surrounding the egg. There is much evidence to raise doubts about the old "bag of enzymes" paradigm of acrosomal function, although this is the model that seems to prevail. We concur with other scientists that acrosomal exocytosis is not an all or none event where the acrosome is either "intact" or "reacted". As determined by transmission electron microscopy of human sperm undergoing acrosomal exocytosis, six stages can be identified, with the intermediate ones involving loss of acrosomal matrix material. In the mouse, there is a temporal relationship among four stages of acrosomal exocytosis. Numerous evidences suggest a more complex role for the acrosome in fertilization in which the acrosomal matrix is a scaffold for sperm-ZP interactions that self-regulates by a controlled disassembly mechanism.