How plastic can phenotypic plasticity be? The branching coral Stylophora pistillata as a model system

Phenotypic plasticity enables multicellular organisms to adjust morphologies and various life history traits to variable environmental challenges. Here, we elucidate fixed and plastic architectural rules for colony astogeny in multiple types of colonial ramets, propagated by cutting from genets of the branching coral Stylophora pistillata from Eilat, the Red Sea. We examined 16 morphometric parameters on 136 one-year old S. pistillata colonies (of seven genotypes), originating from small fragments belonging, each, to one of three single-branch types (single tips, start-up, and advanced bifurcating tips) or to structural preparative manipulations (representing a single or two growth axes). Experiments were guided by the rationale that in colonial forms, complexity of evolving phenotypic plasticity can be associated with a degree of structural modularity, where shapes are approached by erecting iterative growth patterns at different levels of coral-colony organization. Analyses revealed plastic morphometric characters at branch level, and predetermined morphometric traits at colony level (only single trait exhibited plasticity under extreme manipulation state). Therefore, under the experimental manipulations of this study, phenotypic plasticity in S. pistillata appears to be related to branch level of organization, whereas colony traits are controlled by predetermined genetic architectural rules. Each level of organization undergoes its own mode of astogeny. However, depending on the original ramet structure, the spherical 3-D colonial architecture in this species is orchestrated and assembled by both developmental trajectories at the branch level, and traits at the colony level of organization. In nature, branching colonial forms are often subjected to harsh environmental conditions that cause fragmentation of colony into ramets of different sizes and structures. Developmental traits that are plastic, responding to fragment structure and are not predetermine in controlling astogeny, allow formation of species-specific architecture product through integrated but variable developmental routes. This adaptive plasticity or regeneration is an efficient mechanism by which isolated fragments of branching coral species cope with external environmental forces.     

'Cup cell disease' in the colonial tunicate Botryllus schlosseri

A new progressive, fatal disease called 'cup cell disease' was characterized in ex situ cultures of Botryllus schlosseri, a colonial tunicate. The disease originated as a few dark spots growing within zooids. The infected colonies then started to deteriorate, morphologically diagnosed by ampullar retraction, lethargic blood circulation and by a swollen and soft tunic matrix. In later stages of the disease, developed buds were also affected. Many large black dots were scattered within the tunic matrix, and zooids were transformed to opaque, dilated, sac-like structures, signaling impending death. Colonies were infected periodically, even without direct tissue contact. The time course from first appearance to colony death ranged between 30 and 45 d. Histological studies, in vitro culturing of blood cells and blood smears revealed the existence of numerous cup-like cells (up to 4.8 microm diameter on average) with a yellowish cell wall and transparent cytoplasm that was not stained by various dyes (except azocarmine-G). Cells were refractive under bright-field illumination and revealed a flattened wall with flanges, characteristic of species of the phylum Haplosporidia. Cup cells aggregated in blood vessels and in internal parts of zooids and buds and were phagocytosed by blood cells. In a single case, plasmodia-like structures were found only in the tunic matrix of an infected colony. This is the first record in botryllid ascidians of an infectious lethal disease associated with haplosporidian protists.     

Enzyme activity--it's all about image

Unraveling the functional roles of proteins is a major challenge facing the postgenome researcher. Advances towards this goal have been made through the development of both chemical and biochemical tools for monitoring protein activity. Recently, a myriad of fluorescence-based imaging tools have emerged for in vitro, in vivo and whole animal applications. These tools have provided methods to monitor the spatial and temporal distribution of proteins and bioorganic molecules dynamically. Here, recent advances in chemical and biochemical techniques that allow the detection of enzymatic activity within intact cells and in vivo are reviewed. Such technologies have the potential to be integrated into drug-development programs to facilitate both the functional validation of pharmaceutical targets and the treatment of human disease.     

VEGF162, a new heparin-binding vascular endothelial growth factor splice form that is expressed in transformed human cells

The splice forms of vascular endothelial growth factor (VEGF) differ in biological properties such as the receptor types that they recognize and their interaction with heparan sulfate proteoglycans. We have identified a new VEGF mRNA splice form encoding a VEGF species containing 162 amino acids (VEGF(162)) in human A431 ovarian carcinoma cells. This novel mRNA contains the peptides encoded by exons 1-5, 6A, 6B, and 8 of the VEGF gene. Recombinant VEGF(162) is biologically active. It induces proliferation of endothelial cells in vitro and angiogenesis in vivo as determined by the alginate bead assay. VEGF(162) binds less efficiently than VEGF(145) but more efficiently than VEGF(165) to a natural basement membrane produced by corneal endothelial cells. VEGF(138), an artificial VEGF form that contains exon 6B but lacks exons 6A and 7, did not bind to this basement membrane at all, indicating that exon 6B probably interferes with the interaction of exon 6A with heparin and heparan sulfate proteoglycans.     

Role of electrostatic interactions in PDZ domain ligand recognition

PDZ domains are protein-protein interaction modules that normally recognize short C-terminal peptides. The apparent requirement for a ligand with a free terminal carboxylate group has led to the proposal that electrostatic interactions with the terminus play a significant role in recognition. However, this model has been called into question by the more recent finding that PDZ domains can recognize some internal peptide motifs that occur within a specific secondary structure context. Although these motifs bind at the same interface, they lack a terminal charge. Here we have investigated the role of electrostatics in PDZ-mediated recognition in the mouse alpha1-syntrophin PDZ domain by examining the salt dependence of binding to both terminal and internal ligands and the effects of mutating a conserved basic residue previously proposed to play a role in electrostatic recognition. These studies indicate that direct electrostatic interactions with the peptide terminus do not play a significant energetic role in binding. Additional chemical modification studies of the peptide terminus support a model in which steric and hydrogen bonding complementarity play a primary role in recognition specificity. Peptides with a free carboxy terminus, or presented within a specific structural context, can satisfy these requirements.     

Nitric oxide ameliorates hydrophobic bile acid-induced apoptosis in isolated rat hepatocytes by non-mitochondrial pathways

Hydrophobic bile acids are toxic to isolated rat hepatocytes by mechanisms involving mitochondrial dysfunction and oxidative stress. In the current study we examined the role of nitric oxide (NO), a potential mediator of apoptosis, during bile acid-induced apoptosis. Freshly isolated rat hepatocytes and hepatic mitochondria generated NO and peroxynitrite (ONOO(-)) in a concentration- and time-dependent manner when exposed to the toxic bile salt glycochenodeoxycholate (GCDC) (25-500 microm), which was prevented by the nitric-oxide synthase (NOS) inhibitors N(G)-monomethyl-N-arginine monoacetate (l-NMMA) and 1400W. Relationships between hepatocyte NO production and apoptosis were examined by comparing the effects of NOS inhibitors with other inhibitors of GCDC-induced apoptosis. Inhibitors of caspases 8 and 9, the mitochondrial permeability transition blocker cyclosporin A, and the antioxidant idebenone reduced NO generation and apoptosis in GCDC-treated hepatocytes. In contrast, NOS inhibitors had no effect on GCDC-induced apoptosis despite marked reduction of NO and ONOO(-). However, treatment with the NO donors S-nitroso-N-acetylpenicillamine and spermine NONOate [N-(-aminoethyl)N-(2-hydroxy-2-nitrohydrazino)-1,2-ethylenediamine) inhibited apoptosis and caspase 3 activity while significantly elevating NO levels above GCDC-stimulated levels. Neither NO donors nor NOS inhibitors affected GCDC-induced mitochondrial permeability transition or cytochrome c release from liver mitochondria or GCDC-induced mitochondrial depolarization from isolated hepatocytes, suggesting that NO inhibits bile acid-induced hepatocyte apoptosis by a non-mitochondrial-dependent pathway. In conclusion, whereas NO produced from GCDC-treated hepatocytes neither mediates nor protects against bile acid-induced apoptosis, higher levels of NO inhibit GCDC-induced hepatocyte apoptosis by caspase-dependent pathways.     

Two clusters of charged residues located in the electropositive face of the von Willebrand factor A1 domain are essential for heparin binding

The VWF A1 domain seems to possess two heparin binding regions (residues 565-587 and 633-648) displaying positively charged amino acids, but the overall polyanion-A1 domain interaction scheme remains essentially elusive. To probe this molecular reaction as well as the role of electrostatic forces in VWF-heparin interaction, we performed mutagenesis and molecular modeling experiments. Fifteen mutated rVWFs were expressed [R571A, K572A, R573A, K585A, R571A/K572A/R573A, R578A/R579A, R578A/R579A/K585A, R571A/K572A/R573A/R578A/R579A/K585A (6A), K642G, K643G, K644G, K645G, K642G/K645G, K643G/K644G, and K642G/K643G/K644G/K645G (4G)]. Experimental results indicate that the multimeric structure of the mutants was similar to that of wild-type (WT) rVWF and that all rVWFs displayed normal binding to four conformation-dependent mAbs directed against the A1 domain. Three variants displayed significant reductions in the level of heparin binding. The 6A variant showed 39.2 +/- 1.3% of the WT rVWF level (p < 0.005), while mutants K643G/K644G and 4G showed 63.6 +/- 3.2 and 53.3 +/- 5% of the WT rVWF level, respectively (p < 0.005). Computational investigations showed that one face of the A1 domain is strongly electropositive, indicating that electrostatic forces should be essential in steering heparin onto its binding site. In agreement with our experimental data, the most striking alterations of the electrostatic potential contours were seen for mutants 4G, K643G/K644G, and 6A. Our data suggest that two clusters, one at positions 571-573, 578, 579, and 585 and the other at positions 642-645, act in concert for the recognition of heparin, forming a single extended binding surface across the electropositive face of the VWF A1 domain. A structural model of the VWF A1 domain-heparin complex is proposed, taking into account both experimental and computer modeling data.     

Regulation of light-dependent Gqalpha translocation and morphological changes in fly photoreceptors

Heterotrimeric G-proteins relay signals between membrane-bound receptors and downstream effectors. Little is known, however, about the regulation of Galpha subunit localization within the natural endogenous environment of a specialized signaling cell. Here we show, using live Drosophila flies, that light causes massive and reversible translocation of the visual Gqalpha to the cytosol, associated with marked architectural changes in the signaling compartment. Molecular genetic dissection together with detailed kinetic analysis enabled us to characterize the translocation cycle and to unravel how signaling molecules that interact with Gqalpha affect these processes. Epistatic analysis showed that Gqalpha is necessary but not sufficient to bring about the morphological changes in the signaling organelle. Furthermore, mutant analysis indicated that Gqbeta is essential for targeting of Gqalpha to the membrane and suggested that Gqbeta is also needed for efficient activation of Gqalpha by rhodopsin. Our results support the 'two-signal model' hypothesis for membrane targeting in a living organism and characterize the regulation of both the activity-dependent Gq localization and the cellular architectural changes in Drosophila photoreceptors.     

The aqueous accessibility in the external half of transmembrane domain I of the GABA transporter GAT-1 Is modulated by its ligands

The sodium- and chloride-dependent gamma-aminobutyric acid (GABA) transporter GAT-1 is the first identified member of a family of transporters, which maintain low synaptic neurotransmitter levels and thereby enable efficient synaptic transmission. To obtain evidence for the idea that the highly conserved transmembrane domain I (TMD I) participates in the permeation pathway, we have determined the impact of impermeant methanethiosulfonate (MTS) reagents on cysteine residues engineered into this domain. As a background the essentially insensitive but fully active C74A mutant has been used. Transport activity of mutants with a cysteine introduced cytoplasmic to glycine 63 is largely unaffected and is resistant to the impermeant MTS reagents. Conversely, transport activity in mutants extracellular to glycine 63 is strongly impacted. Nevertheless, transport activity could be measured in all but three mutants: G65C, N66C, and R69C. In each of the six active cysteine mutants the activity is highly sensitive to the impermeant MTS reagents. This sensitivity is potentiated by sodium in L64C, F70C, and Y72C, but is protected in V67C and P71C. GABA protects in L64C, W68C, F70C, and P71C. The non-transportable GABA analogue SKF100330A also protects in L64C, W68C, and P71C as well as V67C, but strikingly potentiates inhibition in F70C. Although cysteine substitution in this region may have perturbed the native structure of GAT-1, our observations, taken together with the recently published accessibility study on the related serotonin transporter (Henry, L. K., Adkins, E. M., Han, Q., and Blakely, R. D. (2003) J. Biol. Chem. 278, 37052-37063), suggest that the extracellular part of TMD I is conformationally sensitive, lines the permeation pathway, and forms a more extended structure than expected from a membrane-embedded alpha-helix.     

A Conserved Methionine Residue Controls the Substrate Selectivity of a Neuronal Glutamate Transporter

Glutamate transporters located in the brain maintain low synaptic concentrations of the neurotransmitter by coupling its flux to that of sodium and other cations. In the binding pocket of the archeal homologue Glt_Ph, a conserved methionine residue has been implicated in the binding of the benzyl moiety of the nontransportable substrate analogue threo-β-benzyloxyaspartate. To determine whether the corresponding methionine residue of the neuronal glutamate transporter EAAC1, Met-367, fulfills a similar role, M367L, M367C, and M367S mutants were expressed in HeLa cells and Xenopus laevis oocytes to monitor radioactive transport and transport currents, respectively. The apparent affinity of the Met-367 mutants for d-aspartate and l-glutamate, but not for l-aspartate, was 10–20-fold reduced as compared with wild type. Unlike wild type, the magnitude of I_max was different for each of the three substrates. d-Glutamate, which is also a transportable substrate of EAAC1, did not elicit any detectable response with M367C and M367S but acted as a nontransportable substrate analogue in M367L. In the mutants, substrates inhibited the anion conductance as opposed to the stimulation observed with wild type. Remarkably, the apparent affinity of the blocker d,l-threo-β-benzyloxyaspartate in the mutants was similar to that of wild type EAAC1. Our results are consistent with the idea that the side chain of Met-367 fulfills a steric role in the positioning of the substrate in the binding pocket in a step subsequent to its initial binding.