Evolution of bilaterally asymmetrical larvae in freshwater mussels (Bivalvia: Unionoida: Unionidae)

Bilaterally asymmetrical glochidia (i.e. bivalved parasitic larvae bearing a large marginal appendage on a single valve) have been reported from five Asian freshwater mussel genera belonging to two separate subfamilies, the Gonideinae (i.e. Pseudodon, Solenaia, and Physunio) and Rectidentinae (i.e. Contradens and Trapezoideus). This classification requires that the bilaterally asymmetrical glochidium‐bearing mussels are not monophyletic, and suggests that this atypical larval morphology evolved twice in the same geographic region. Although homoplastic glochidium characters are known (e.g. marginal appendages and size), we hypothesized that bilaterally asymmetrical glochidia represent a novel morphological synapomorphy. We tested the monophyly of the mussels bearing bilaterally asymmetrical glochidia using a molecular matrix consisting of representatives from all six freshwater mussel families and three molecular markers (28S, 16S, and COI). Bayesian inference, maximum likelihood, and ancestral state reconstruction were employed to estimate the phylogeny and larval trait transformations. The reconstructed phylogeny rejects the monophyly of the asymmetrical glochidium‐bearing mussels and resolves two putative origins of asymmetrical glochidia; however, ancestral state reconstruction supports asymmetrical glochidia as a synapomorphy of only one supraspecific taxon of the Rectidentinae. In the Gonideinae, asymmetrical glochidia were autapomorphic of Pseudodon cambodjensis (Petit, 1865). That is, no other taxa resolved among the Gonideinae had bilaterally asymmetrical glochidia, including other Pseudodon species. We describe how the alleged intrageneric glochidial variation in Pseudodon, and in the other genera of the Gonideinae reported to have asymmetrical glochidia (i.e. Solenaia and Physunio), challenge the resolved convergence of asymmetrical glochidia. Our results are discussed in the context of freshwater mussel larval evolution, patterns in life‐history traits, and the classification of freshwater mussels generally. © 2015 The Linnean Society of London     

Recombinant functional multidomain hemoglobin from the gastropod Biomphalaria glabrata

The extracellular hemoglobin multimer of the planorbid snail Biomphalaria glabrata, intermediate host of the human parasite Schistosoma mansoni, is presumed to be a 1.44 MDa complex of six 240 kDa polypeptide subunits, arranged as three disulfide-bridged dimers. The complete amino acid sequence of two subunit types (BgHb1 and BgHb2), and the partial sequence of a third type (BgHb3) are known. Each subunit encompasses 13 paralogus heme domains, and N-terminally a smaller plug domain responsible for subunit dimerization. We report here the recombinant expression of different functional fragments of BgHb2 in Escherichia coli, and of the complete functional subunits BgHb1 and BgHb2 in insect cells; BgHb1 was also expressed as disulfide-bridged dimer (480 kDa). Oxygen-binding measurements of the recombinant products show a P(50) of about 7 mmHg and the absence of a significant cooperativity or Bohr effect. The covalently linked dimer of BgHb1, but not the monomer, is capable to form aggregates closely resembling native BgHb molecules in the electron microscope.     

Interplay between structure and dynamics in chitosan films investigated with solid-state NMR, dynamic mechanical analysis, and X-ray diffraction

Modern solid-state NMR techniques, combined with X-ray diffraction, revealed the molecular origin of the difference in mechanical properties of self-associated chitosan films. Films cast from acidic aqueous solutions were compared before and after neutralization, and the role of the counterion (acetate vs Cl(-)) was investigated. There is a competition between local structure and long-range order. Hydrogen bonding gives good mechanical strength to neutralized films, which lack long-range organization. The long-range structure is better defined in films cast from acidic solutions in which strong electrostatic interactions cause rotational distortion around the chitosan chains. Plasticization by acetate counterions enhances long-range molecular organization and film flexibility. In contrast, Cl(-) counterions act as a defect and impair the long-range organization by immobilizing hydration water. Molecular motion and proton exchange are restricted, resulting in brittle films despite the high moisture content.     

Starch and the clock: the dark side of plant productivity

Efforts to improve photosynthetic efficiency should result in increased rates of carbon assimilation in crop plants in the next few decades. Translation of increased assimilation into higher productivity will require a greater understanding of the relationship between assimilation and growth. In this review, we discuss new progress in understanding how carbon is provided for metabolism and growth at night. In Arabidopsis leaves, the circadian clock controls the rate of degradation of starch to ensure an optimal carbon supply and hence continued growth during the night. These discoveries shed new light on the integration of carbon assimilation and growth over the light-dark cycle. They reveal the importance of considering the carbon economy of the whole plant in attempting to increase crop productivity.     

Cytoplasmic thioredoxin reductase is essential for embryogenesis but dispensable for cardiac development

Two distinct thioredoxin/thioredoxin reductase systems are present in the cytosol and the mitochondria of mammalian cells. Thioredoxins (Txn), the main substrates of thioredoxin reductases (Txnrd), are involved in numerous physiological processes, including cell-cell communication, redox metabolism, proliferation, and apoptosis. To investigate the individual contribution of mitochondrial (Txnrd2) and cytoplasmic (Txnrd1) thioredoxin reductases in vivo, we generated a mouse strain with a conditionally targeted deletion of Txnrd1. We show here that the ubiquitous Cre-mediated inactivation of Txnrd1 leads to early embryonic lethality. Homozygous mutant embryos display severe growth retardation and fail to turn. In accordance with the observed growth impairment in vivo, Txnrd1-deficient embryonic fibroblasts do not proliferate in vitro. In contrast, ex vivo-cultured embryonic Txnrd1-deficient cardiomyocytes are not affected, and mice with a heart-specific inactivation of Txnrd1 develop normally and appear healthy. Our results indicate that Txnrd1 plays an essential role during embryogenesis in most developing tissues except the heart.     

Calcitriol inhibits growth response to Platelet-Derived Growth Factor-BB in human prostate cells

Calcitriol, a hormonal form of Vitamin D, regulates growth of normal and cancer cells of various origins by modulation of peptide growth factors signaling. Platelet-Derived Growth Factor (PDGF) signaling pathway is involved in prostate cancer progression. We studied the expression of PDGF receptors in human prostate primary stromal cells and cancer epithelial cell lines and growth response to PDGF-BB isoform. We found that the expression of PDGF receptors and PDGF-BB-mediated cell growth are regulated by calcitriol in prostate cells. Quantitative RT-PCR analysis revealed a lower level of mRNA for PDGF receptors in LNCaP and PC-3 cells than in primary stromal cells. Western blotting showed a high amount of PDGFRalpha and beta proteins in primary stromal cells that could not be detected in LNCaP, which may explain the resistance of LNCaP cells to growth-promoting effect of PDGF-BB. Addition of Epidermal Growth Factor (EGF) to the culture medium induces the expression of PDGFRbeta and restores responsiveness of LNCaP to PDGF-BB to some extent. Calcitriol down-regulates PDGFRbeta expression and negatively regulates PDGF-mediated cell growth. Calcitriol does not affect PDGFRalpha and PDGF-B mRNA expression. We suggest that inhibition of PDGFRbeta expression by calcitriol might reduce responsiveness of prostate cells to mitogenic action of PDGF-BB.     

Molecular basis of the complex formation between the two calcium-binding proteins S100A8 (MRP8) and S100A9 (MRP14)

S100 proteins form characteristic homo- and/or heterodimers that play a role in calcium-mediated signaling. We characterized the formation of the human S100A8/S100A9 heterodimer using the yeast two-hybrid system. Employing site-directed mutagenesis we found that distinct hydrophobic amino acids of helix I/I' are located at a crucial site of the S100A8/S100A9 dimer interface, whereas conserved residues within helix IV/IV' are not important for heterodimerization. Furthermore, amino acids Y16 and F68 prevent homodimerization of human S100A8. These data demonstrate for the first time the functional relevance of distinct hydrophobic amino acids for human S100A8/S100A9 complex formation in vivo.     

Quaternary structure and functional properties of Penaeus monodon hemocyanin

The hemocyanin of the tiger shrimp, Penaeus monodon, was investigated with respect to stability and oxygen binding. While hexamers occur as a major component, dodecamers and traces of higher aggregates are also found. Both the hexamers and dodecamers were found to be extremely stable against dissociation at high pH, independently of the presence of calcium ions, in contrast to the known crustacean hemocyanins. This could be caused by only a few additional noncovalent interactions between amino acids located at the subunit-subunit interfaces. Based on X-ray structures and sequence alignments of related hemocyanins, the particular amino acids are identified. At all pH values, the p50 and Bohr coefficients of the hexamers are twice as high as those of dodecamers. While the oxygen binding of hexamers from crustaceans can normally be described by a simple two-state model, an additional conformational state is needed to describe the oxygen-binding behaviour of Penaeus monodon hemocyanin within the pH range of 7.0 to 8.5. The dodecamers bind oxygen according to the nested Monod-Whyman-Changeaux (MWC) model, as observed for the same aggregation states of other hemocyanins. The oxygen-binding properties of both the hexameric and dodecameric hemocyanins guarantee an efficient supply of the animal with oxygen, with respect to the ratio between their concentrations. It seems that under normoxic conditions, hexamers play the major role. Under hypoxic conditions, the hexamers are expected not to be completely loaded with oxygen. Here, the dodecamers are supposed to be responsible for the oxygen supply.