Water level drawdown makes boreal peatland vegetation more responsive to weather conditions

Climate warming and projected increase in summer droughts puts northern peatlands under pressure by subjecting them to a combination of gradual drying and extreme weather events. The combined effect of those on peatland functions is poorly known. Here, we studied the impact of long-term water level drawdown (WLD) and contrasting weather conditions on leaf phenology and biomass production of ground level vegetation in boreal peatlands. Data were collected during two contrasting growing seasons from a WLD experiment including a rich and a poor fen and an ombrotrophic bog. Results showed that WLD had a strong effect on both leaf area development and biomass production, and these responses differed between peatland types. In the poor fen and the bog, WLD increased plant growth, while in the rich fen, WLD reduced the growth of ground level vegetation. Plant groups differed in their response, as WLD reduced the growth of graminoids, while shrubs and tree seedlings benefited from it. In addition, the vegetation adjusted to the lower WTs, was more responsive to short-term climatic variations. The warmer summer resulted in a greater maximum and earlier peaking of leaf area index, and greater biomass production by vascular plants and Sphagnum mosses at WLD sites. In particular, graminoids benefitted from the warmer conditions. The change towards greater production in the WLD sites in general and during the warmer weather in particular, was related to the observed transition in plant functional type composition towards arboreal vegetation.     

Ribosomal RNA secondary structure: compensatory mutations and implications for phylogenetic analysis

Using sequence data from the 28S ribosomal RNA (rRNA) genes of selected vertebrates, we investigated the effects that constraints imposed by secondary structure have on the phylogenetic analysis of rRNA sequence data. Our analysis indicates that characters from both base-pairing regions (stems) and non-base-pairing regions (loops) contain phylogenetic information, as judged by the level of support of the phylogenetic results compared with a well-established tree based on both morphological and molecular data. The best results (the greatest level of support of well-accepted nodes) were obtained when the complete data set was used. However, some previously supported nodes were resolved using either the stem or loop bases alone. Stem bases sustain a greater number of compensatory mutations than would be expected at random, but the number is < 40% of that expected under a hypothesis of perfect compensation to maintain secondary structure. Therefore, we suggest that in phylogenetic analyses, the weighting of stem characters be reduced by no more than 20%, relative to that of loop characters. In contrast to previous suggestions, we do not recommend weighting of stem positions by one-half, compared with that of loop positions, because this overcompensates for the constraints that selection imposes on the secondary structure of rRNA.      

Exenatide promotes cognitive enhancement and positive brain metabolic changes in PS1-KI mice but has no effects in 3xTg-AD animals

Recent studies have shown that type 2 diabetes mellitus (T2DM) is a risk factor for cognitive dysfunction or dementia. Insulin resistance is often associated with T2DM and can induce defective insulin signaling in the central nervous system as well as increase the risk of cognitive impairment in the elderly. Glucagone like peptide-1 (GLP-1) is an incretin hormone and, like GLP-1 analogs, stimulates insulin secretion and has been employed in the treatment of T2DM. GLP-1 and GLP-1 analogs also enhance synaptic plasticity and counteract cognitive deficits in mouse models of neuronal dysfunction and/or degeneration. In this study, we investigated the potential neuroprotective effects of long-term treatment with exenatide, a GLP-1 analog, in two animal models of neuronal dysfunction: the PS1-KI and 3xTg-AD mice. We found that exenatide promoted beneficial effects on short- and long-term memory performances in PS1-KI but not in 3xTg-AD animals. In PS1-KI mice, the drug increased brain lactate dehydrogenase activity leading to a net increase in lactate levels, while no effects were observed on mitochondrial respiration. On the contrary, exenatide had no effects on brain metabolism of 3xTg-AD mice. In summary, our data indicate that exenatide improves cognition in PS1-KI mice, an effect likely driven by increasing the brain anaerobic glycolysis rate.     

Gluconeogenesis,an essential metabolic pathway for pathogenic Francisella

Intracellular multiplication and dissemination of the infectious bacterial pathogen Francisella tularensis implies the utilization of multiple host‐derived nutrients. Here, we demonstrate that gluconeogenesis constitutes an essential metabolic pathway in Francisella pathogenesis. Indeed, inactivation of gene glpX, encoding the unique fructose 1,6‐bisphosphatase of Francisella, severely impaired bacterial intracellular multiplication when cells were supplemented by gluconeogenic substrates such as glycerol or pyruvate. The ΔglpX mutant also showed a severe virulence defect in the mouse model, confirming the importance of this pathway during the in vivo life cycle of the pathogen. Isotopic profiling revealed the major role of the Embden–Meyerhof (glycolysis) pathway in glucose catabolism in Francisella and confirmed the importance of glpX in gluconeogenesis. Altogether, the data presented suggest that gluconeogenesis allows Francisella to cope with the limiting glucose availability it encounters during its infectious cycle by relying on host amino acids. Hence, targeting the gluconeogenic pathway might constitute an interesting therapeutic approach against this pathogen.     

Importance of Host Cell Arginine Uptake in Francisella Phagosomal Escape and Ribosomal Protein Amounts

Upon entry into mammalian host cells, the pathogenic bacterium Francisella must import host cell arginine to multiply actively in the host cytoplasm. We identified and functionally characterized an arginine transporter (hereafter designated ArgP) whose inactivation considerably delayed bacterial phagosomal escape and intracellular multiplication. Intramacrophagic growth of the ΔargP mutant was fully restored upon supplementation of the growth medium with excess arginine, in both F. tularensis subsp. novicida and F. tularensis subsp. holarctica LVS, demonstrating the importance of arginine acquisition in these two subspecies. High-resolution mass spectrometry revealed that arginine limitation reduced the amount of most of the ribosomal proteins in the ΔargP mutant. In response to stresses such as nutritional limitation, repression of ribosomal protein synthesis has been observed in all kingdoms of life. Arginine availability may thus contribute to the sensing of the intracellular stage of the pathogen and to trigger phagosomal egress. All MS data have been deposited in the ProteomeXchange database with identifier PXD001584 (http://proteomecentral.proteomexchange.org/dataset/PXD001584).Francisella tularensis is a Gram-negative coccobacillus responsible for the zoonotic disease tularemia. This pathogen is considered as one of the most virulent microorganisms and has been categorized as a Category A select agent by the US Centers for Disease Control and Prevention. It is known to infect a broad range of animal species such as mammals including humans. F. tularensis is a facultative intracellular pathogen able to multiply in a variety of cell types but is thought to multiply in vivo preferentially in macrophages (1). Upon entry into macrophages, Francisella escapes within one hour from Francisella-containing phagosome to reach the cytosolic compartment where it multiplies actively (2). To multiply efficiently in its cytosolic niche, Francisella must evade the immune responses of the host (3) and possess dedicated virulence attributes (4). Francisella also needs to adapt its metabolism to the nutrients available in the infected host (5). It has, therefore, developed sophisticated tools recently designated as “nutritional virulence” attributes (6, 7).Amino acids have been described as the essential nutrient source that Francisella has evolved to scavenge, by different means, during its intracellular life cycle (8). For examples, activation of the host macro-autophagy degradation machinery (9), and exploitation of host polypeptides such as glutathione (10), allow Francisella to obtain its essential amino acids. We recently demonstrated that Francisella possessed amino acid transporters that were required to fulfill some of its metabolic requirements during the different stages of its intracellular progression (11–13). The virulence of the intracellular pathogen Salmonella also depends on the simultaneous exploitation of numerous different host nutrients, including vitamins, carbohydrates, and amino acids (14). More generally, comparisons of the predicted nutrient utilization and biosynthetic pathways of other mammalian pathogens suggest that most of them share the capability to utilize multiple nitrogen and carbon sources.Francisella genomes are predicted to encode 11 amino acid/polyamine/organocation (APC)¹ family members (15), possibly involved in different amino acid uptake functions. Remarkably, only two of them were identified in at least four different genetic screens as potentially involved in bacterial virulence that is, the glutamate transporter GadC and FTN_0848 (in F. novicida). We have shown that GadC promoted bacterial resistance to the oxidative stress generated in the phagosomal compartment by fueling the tricarboxylic acid (TCA) cycle (13). FTN_0848 is the closest paralog of GadC in F. novicida and shares 33.4% amino acid identity with GadC. In addition, the gene FTN_0848 has been identified in multiple in vitro and in vivo screens, in both F. novicida and F. tularensis LVS (16–21), suggesting that it displays functions that could not be replaced by other Francisella transporter.We demonstrate here that FTN-0848, which we have designated argP, encodes a high affinity arginine transporter (hence designated ArgP for simplification). Arginine is an essential amino acid that can only be obtained by intracellular Francisella via import from the host. We show that ArgP-mediated arginine uptake is crucial for efficient phagosomal escape, highlighting for the first time the importance of essential amino acids during early stage infection. The data presented further suggest that arginine limitation influences ribosomal protein synthesis.