Photosynthesis of a temperate fallow C<Subscript>3</Subscript> herbaceous ecosystem: measurements and model simulations at the leaf and canopy levels

The objectives of the study were to characterize photosynthesis of temperate fallow C₃herbaceous species and examine the performance of a simple photosynthesis model (based on the Farquhar’s equations) to simulate carbon fluxes at the leaf and canopy levels. The maximum rate of carboxylation at 25°C (V _m0) was estimated for sunlit leaves using in situ gas exchange data under saturating irradiance. Throughout the seasons, leaf measurements indicate that values of V _m0 were similar for the four major species of the fallow. The rate declined from March (100 μmol m⁻² s⁻¹) to July (50 μmol m⁻² s⁻¹) and remained almost constant until November. The maximum quantum yield estimated for Potentilla reptans L. (dominant species) was 0.082 mol(CO₂) mol⁻¹(photon absorbed), similar to values already published for C₃ species. Leaf area index (LAI) increased from winter (less than 0.2 m² m⁻²) to spring (up to 4 m² m⁻²). Rates of canopy photosynthesis (measured with a canopy chamber) strongly depended on LAI and temperature, in addition to irradiance. They reached a maximum of 25 μmol m⁻² s⁻¹ and were intermediate between those published for C₄ grassland or cultivated species, and on woody species. At leaf level, simulations gave realistic predictions. At canopy level, the model had the ability to reproduce the effects of environmental and seasonal conditions. However, simulations underestimated the photosynthetic activity of the fallow canopy.     

LRP-1 Promotes Cancer Cell Invasion by Supporting ERK and Inhibiting JNK Signaling Pathways

Background

The low-density lipoprotein receptor-related protein-1 (LRP-1) is an endocytic receptor mediating the clearance of various extracellular molecules involved in the dissemination of cancer cells. LRP-1 thus appeared as an attractive receptor for targeting the invasive behavior of malignant cells. However, recent results suggest that LRP-1 may facilitate the development and growth of cancer metastases in vivo, but the precise contribution of the receptor during cancer progression remains to be elucidated. The lack of mechanistic insights into the intracellular signaling networks downstream of LRP-1 has prevented the understanding of its contribution towards cancer.

Methodology/Principal Findings

Through a short-hairpin RNA-mediated silencing approach, we identified LRP-1 as a main regulator of ERK and JNK signaling in a tumor cell context. Co-immunoprecipitation experiments revealed that LRP-1 constitutes an intracellular docking site for MAPK containing complexes. By using pharmacological agents, constitutively active and dominant-negative kinases, we demonstrated that LRP-1 maintains malignant cells in an adhesive state that is favorable for invasion by activating ERK and inhibiting JNK. We further demonstrated that the LRP-1-dependent regulation of MAPK signaling organizes the cytoskeletal architecture and mediates adhesive complex turnover in cancer cells. Moreover, we found that LRP-1 is tethered to the actin network and to focal adhesion sites and controls ERK and JNK targeting to talin-rich structures.

Conclusions

We identified ERK and JNK as the main molecular relays by which LRP-1 regulates focal adhesion disassembly of malignant cells to support invasion.     

Characterization of the calcium-dependent proteolytic system in a mouse muscle cell line

Many studies have demonstrated that the calcium-dependent proteolytic system (calpains and calpastatin) is involved in myoblast differentiation. It is also known that myogenic differentiation can be studied in vitro. In the present experiments, using a mouse muscle cell line (C₂C₁₂) we have analyzed both the sequences of appearance and the expression profiles of calpains 1, 2, 3 and calpastatin during the course of myoblast differentiation. Our results mainly show that the expression of ubiquitous calpains (calpain 1 and 2) and muscle-specific calpain (calpain 3) at the mRNAs level as well as at the protein level do not change significantly all along this biological process. In the same time, the specific inhibitor of ubiquitous calpains, calpastatin, presents a stable expression at mRNAs level as well as protein level, all along myoblast to myotube transition. A comparison with other myogenic cells is presented.     

Molecular systematics and paleobiogeography of the South American sigmodontine rodents [published erratum appears in Mol Biol Evol 1998 Feb;15(2):224]

The murid rodent subfamily Sigmodontinae contains 79 genera which are distributed throughout the New World. The time of arrival of the first sigmodontines in South America and the estimated divergence time(s) of the different lineages of South American sigmodontines have been controversial due to the lack of a good fossil record and the immense number of extant species. The "early-arrival hypothesis" states that the sigmodontines must have arrived in South America no later than the early Miocene, at least 20 MYA, in order to account for their vast present-day diversity, whereas the "late-arrival hypothesis" includes the sigmodontines as part of the Plio-Pleistocene Great American Interchange, which occurred approximately 3.5 MYA. The phylogenetic relationships among 33 of these genera were reconstructed using mitochondrial DNA (mtDNA) sequence data from the ND3, ND4L, arginine tRNA, and ND4 genes, which we show to be evolving at the same rate. A molecular clock was calibrated for these genes using published fossil dates, and the genetic distances were estimated from the DNA sequences in this study. The molecular clock was used to estimate the dates of the South American sigmodontine origin and the main sigmodontine radiation in order to evaluate the "early-" and "late-arrival" scenarios. We estimate the time of the sigmodontine invasion of South America as between approximately 5 and 9 MYA, supporting neither of the scenarios but suggesting two possible models in which the invading lineage was either (1) ancestral to the oryzomyines, akodonts, and phyllotines or (2) ancestral to the akodonts and phyllotines and accompanied by the oryzomyines. The sigmodontine invasion of South America provides an example of the advantage afforded to a lineage by the fortuitous invasion of a previously unexploited habitat, in this case an entire continent.      

The abundance of additional N-acetyllactosamine units in N-linked tetraantennary oligosaccharides of human Tamm-Horsfall glycoprotein is a donor-specific feature

Previously, treatment of Tamm-Horsfall glycoprotein (THp) from different donors with endo-beta-galactosidase has been shown to liberate a tetra- and a Sd(a)-active pentasaccharide, concluding the presence of N-linked carbohydrate chains containing additional N - acetyllactosamine units. These type of oligosaccharides were not found in a detailed structure elucidation of the carbohydrate moiety of THp of one male donor, suggesting a donor-specific feature for these type of structures. Therefore, THp was isolated from four healthy male donors and each subjected to endo-beta-galactosidase treatment in order to release these tetra- and Sd(a)-active pentasaccharide. Differences were observed in the total amount of released tetra- and Sda-active pentasaccharide of the used donors (42, 470, 478, 718 microg/100 mg THp), indicating that the presence of repeating N-acetyllactosamine units incorporated into the N-glycan moiety of THp is donor specific. Furthermore, a higher expression of the Sd(a) determinant on antennae which display N-acetyllactosamine elongation was observed, suggesting a better accessibility for the beta-N-acetylgalactosaminyltransferase. In order to characterize the N-glycans containing repeating N- acetyllactosamine units, carbohydrate chains were enzymatically released from THp and isolated. The tetraantennary fraction, which accounts for more than 33% of the total carbohydrate moiety of THp, was used to isolate oligosaccharides containing additional N - acetyllactosamine units. Five N-linked tetraantennary oligosaccharides containing a repeating N-acetyllactosamine unit were identified, varying from structures bearing four Sd(a) determinants to structures containing no Sd(a) determinant (see below). One compound was used in order to specify the branch location of the additional N- acetyllactosamine unit, and it appeared that only the Gal-6' and Gal-8' residues were occupied by a repeating N -acetyllactosamine unit.      

Ribosomal RNA sequence phylogeny is not congruent with ascospore morphology among species in Ceratocystis sensu stricto

The genus Ceratocystis sensu stricto includes important fungal pathogens of woody and herbaceous plants. This genus is distinguished from species in Ceratocystis sensu lato by the presence of Chalara anamorphs. Ascospore shape has been used extensively in delineating Ceratocystis taxa, which show a large variety of ascospore shapes. Sequence analysis of one region of he 18S ribosomal RNA subunit and two regions of the 28S ribosomal RNA subunit showed that there was a majority of multiple substitutions at nucleotide sites and that there was a low transition/transversion ratio, T = 0.72. Both of these results suggest that these are well established, old species. Ascospore morphology, for the most part, was not congruent with the molecular phylogeny, and the use of morphological characters may be misleading in the taxonomy of these species.      

Membrane compartmentalized ATP and its preferential use by the Na, K-ATPase of human red cell ghosts

This paper describes work which begins to define the molecular organization in the region of the membrane that comprises the functional domain of the Na:K pump. The membrane-bound phosphoglycerate kinase (PGK) and Na, K-ATPase appear to be directly linked via a compartmentalized form of ATP. Evidence for the membrane pool of ATP is based on the labeling characteristics of the phosphoproteins by [γ-(32)P]ATP of ghosts incubated under various conditions. Preincubation of ghosts in the presence of ATP at 37 degrees C, but not at 0 degrees C, completely obscures the formation of the Na-phosphoprotein in ghosts washed and subsequently incubated in the presence of [gamma-(32)P]ATP. In contrast to the Na component, the Mg component of phosphorylation is only slightly altered by preincubation with ATP. ATPase activity measured as (32)P(i) liberated during the subsequent incubation at 0 degrees C, reflects completely the differential effects of preincubation with ATP on (32)P incorporation into phosphoprotein. ATP placed within the pool by preincubation can be removed by operating the Na, K-ATPase or the PGK reaction in the reverse direction by use of exogenous substrates. Alternatively, the membrane pool of ATP can be formed also from exogenous substrates by running the PGK reaction in the forward direction. These results, while providing direct support for a membrane compartment of ATP, also indicate the location of this compartment in relation to the PGK and the Na, K-ATPase. In addition, these results also imply that the Mg and Na components are different enzymatic entities since substrate ATP can be derived from separate sources.     

Mitogen-activated protein kinase activity during goat oocyte maturation and the acquisition of meiotic competence

Changes in MPF and MAPK activities during meiotic maturation of goat oocytes were investigated. Detection of MPF activity occurred concomitantly with GVBD, increased at MI, decreased during anaphase-telophase I transition, and increased thereafter in MII oocytes. The appearance of MAPK activity was delayed compared to MPF activity. MAPK activity increased after GVBD and persisted during the MI-MII transition. Whether MAPK was implicated in goat oocyte meiotic competence was also investigated by using oocytes from different follicle size categories that arrest at specific stages of the maturation process (GV, GVBD, MI, and MII). Results indicate that the ability of goat oocytes to resume meiosis is not directly related to the presence of Erk2. The ability to phosphorylate MAPK is acquired by the oocyte during follicular growth after the ability to resume meiosis. GVBD-arrested oocytes exhibited a high level of MPF activity after 27 hr of culture. However, 28% of oocytes from this group contained inactive MAPK, and 72% exhibited high MAPK activity. In addition, 29% of GVBD-arrested oocytes contained a residual interphasic network without recruitment of microtubules around the condensed chromosomes; 71% of GVBD-arrested oocytes displayed recruitment of microtubules near the condensed chromosomes and contained asters of microtubules distributed throughout the cytoplasm. These results indicate that oocytes arrested at GVBD were not exactly at the same point in the meiotic cell cycle progression, and suggest that MAPK could be implicated in the regulation of microtubule organization. The data presented here suggest that in goat oocytes, MAPK is not implicated in the early events of meiosis resumption, but rather in post-GVBD events such as spindle formation and MII arrest. © 1996 Wiley-Liss Inc.