INTERCELLULAR AND SYSTEM-LEVEL ASPECTS OF THE METABOLIC INTERACTIONS BETWEEN NEURONS AND GLIA

Quand on parle de l'acide lactique (aussi connu sous le nom de lactate) une des premières choses qui vient à l'esprit, c'est son implication en cas d'intense activité musculaire. Sa production pendant une activité physique prolongée est associée avec la sensation de fatigue. Il n'est donc pas étonnant que cette molécule ait été longtemps considérée comme un résidu du métabolisme, possiblement toxique et donc à éliminer. En fait, il a été découvert que le lactate joue un rôle prépondérant dans le métabolisme grâce à son fort potentiel énergétique. Le cerveau, en particulier les neurones qui le composent, est un organe très gourmand en énergie. Récemment, il a été démontré que les astrocytes, cellules du cerveau faisant partie de la famille des cellules gliales, utilisent le glucose pour produire du lactate comme source d'énergie et le distribue aux neurones de manière adaptée à leur activité. Cette découverte a renouvelé l'intérêt scientifique pour le lactate. Aujourd'hui, plusieurs études ont démontré l'implication du lactate dans d'autres fonctions de la physiologie cérébrale. Dans le cadre de notre étude, nous nous sommes intéressés au rapport entre neurones et astrocytes avec une attention particulière pour le rôle du lactate. Nous avons découvert que le lactate possède la capacité de modifier la communication entre les neurones. Nous avons aussi décrypté le mécanisme grâce auquel le lactate agit, qui est basé sur un récepteur présent à la surface des neurones. Cette étude montre une fonction jusque-là insoupçonnée du lactate qui a un fort impact sur la compréhension de la relation entre neurones et astrocytes. - Relatively to its volume, the brain uses a large amount of glucose as energy source. Furthermore, a tight link exists between the level of synaptic activity and the consumption of energy equivalents. Astrocytes have been shown to play a central role in the regulation of this so-called neurometabolic coupling. They are thought to deliver the metabolic substrate lactate to neurons in register to glutamatergic activity. The astrocytic uptake of glutamate, released in the synaptic cleft, is the trigger signal that activates an intracellular cascade of events that leads to the production and release of lactate from astrocytes. The main goal of this thesis work was to obtain detailed information on the metabolic and functional interplay between neurons and astrocytes, in particular on the influence of lactate besides its metabolic effects. To gain access to both spatial and temporal aspects of these dynamic interactions, we used optical microscopy associated with specific fluorescent indicators, as well as electrophysiology. In the first part of this thesis, we show that lactate decreases spontaneous neuronal, activity in a concentration-dependent manner and independently of its metabolism. We further identified a receptor-mediated pathway underlying this modulatory action of lactate. This finding constituted a novel mechanism for the modulation of neuronal transmission by lactate. In the second part, we have undergone a characterization of a new pharmacological tool, a high affinity glutamate transporter inhibitor. The finality of this study was to investigate the detailed pharmacological properties of the compound to optimize its use as a suppressor of glutamate signal from neuron to astrocytes. In conclusion, both studies have implications not only for the understanding of the metabolic cooperation between neurons and astrocytes, but also in the context of the glial modulation of neuronal activity. - Par rapport à son volume, le cerveau utilise une quantité massive de glucose comme source d'énergie. De plus, la consommation d'équivalents énergétiques est étroitement liée au niveau d'activité synaptique. Il a été montré que dans ce couplage neurométabolique, un rôle central est joué par les astrocytes. Ces cellules fournissent le lactate, un substrat métabolique, aux neurones de manière adaptée à leur activité glutamatergique. Plus précisément, le glutamate libéré dans la fente synaptique par les neurones, est récupéré par les astrocytes et déclenche ainsi une cascade d'événements intracellulaires qui conduit à la production et libération de lactate. Les travaux de cette thèse ont visé à étudier la relation métabolique et fonctionnelle entre neurones et astrocytes, avec une attention particulière pour des rôles que pourrait avoir le lactate au-delà de sa fonction métabolique. Pour étudier les aspects spatio-temporels de ces interactions dynamiques, nous avons utilisé à la fois la microscopie optique associée à des indicateurs fluorescents spécifiques, ainsi que l'électrophysiologie. Dans la première partie de cette thèse, nous montrons que le lactate diminue l'activité neuronale spontanée de façon concentration-dépendante et indépendamment de son métabolisme. Nous avons identifié l'implication d'un récepteur neuronal au lactate qui sous-tend ce mécanisme de régulation. La découverte de cette signalisation via le lactate constitue un mode d'interaction supplémentaire et nouveau entre neurones et astrocytes. Dans la deuxième partie, nous avons caractérisé un outil pharmacologique, un inhibiteur des transporteurs du glutamate à haute affinité. Le but de cette étude était d'obtenir un agent pharmacologique capable d'interrompre spécifiquement le signal médié par le glutamate entre neurones et astrocytes pouvant permettre de mieux comprendre leur relation. En conclusion, ces études ont une implication non seulement pour la compréhension de la coopération entre neurones et astrocytes mais aussi dans le contexte de la modulation de l'activité neuronale par les cellules gliales.

Understanding the low-temperature limitations to forest growth through calibration of a forest dynamics model with tree-ring data

The sensitivity of altitudinal and latitudinal tree-line ecotones to climate change, particularly that of temperature, has received much attention. To improve our understanding of the factors affecting tree-line position, we used the spatially explicit dynamic forest model TreeMig. Although well-suited because of its landscape dynamics functions, TreeMig features a parabolic temperature growth response curve, which has recently been questioned. and the species parameters are not specifically calibrated for cold temperatures. Our main goals were to improve the theoretical basis of the temperature growth response curve in the model and develop a method for deriving that curve's parameters from tree-ring data. We replaced the parabola with an asymptotic curve, calibrated for the main species at the subalpine (Swiss Alps: Pinus cembra, Larix decidua, Picea abies) and boreal (Fennoscandia: Pinus sylvestris, Betula pubescens, P. abies) tree-lines. After fitting new parameters, the growth curve matched observed tree-ring widths better. For the subalpine species, the minimum degree-day sum allowing, growth (kDDMin) was lowered by around 100 degree-days; in the case of Larix, the maximum potential ring-width was increased to 5.19 mm. At the boreal tree-line, the kDDMin for P. sylvestris was lowered by 210 degree-days and its maximum ring-width increased to 2.943 mm; for Betula (new in the model) kDDMin was set to 325 degree-days and the maximum ring-width to 2.51 mm; the values from the only boreal sample site for Picea were similar to the subalpine ones, so the same parameters were used. However, adjusting the growth response alone did not improve the model's output concerning species' distributions and their relative importance at tree-line. Minimum winter temperature (MinWiT, mean of the coldest winter month), which controls seedling establishment in TreeMig, proved more important for determining distribution. Picea, P. sylvestris and Betula did not previously have minimum winter temperature limits, so these values were set to the 95th percentile of each species' coldest MinWiT site (respectively -7, -11, -13). In a case study for the Alps, the original and newly calibrated versions of TreeMig were compared with biomass data from the National Forest Inventor), (NFI). Both models gave similar, reasonably realistic results. In conclusion, this method of deriving temperature responses from tree-rings works well. However, regeneration and its underlying factors seem more important for controlling species' distributions than previously thought. More research on regeneration ecology, especially at the upper limit of forests. is needed to improve predictions of tree-line responses to climate change further.

Involvement of PPARβ/δ in mesenchymal-epidermal interactions during skin wound healing

SUMMARY : Skin wound repair is a complex and highly coordinated process, where a variety of cell types unite to regenerate the damaged tissue. Several works have elucidated cellular and molecular mechanisms, in which mesenchymal-epidermal interactions play an essential role for the regulation of skin homeostasis and repair. Peroxisome Proliferator-Activated Receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear receptor superfamily. Three related isotypes (PPARα, PPARß/δ and PPARγ) have been found, which exhibit distinct tissue distribution and specific physiological functions. PPARß/δ was identified as a crucial player of skin homeostasis. In the mouse skin, PPARß/δ has been described to control proliferation-differentiation state, adhesion and migration, and survival of the keratinocytes during healing. PPARß/δ has been implicated as well in the development of the hair follicles, in which mesenchymal-secreted hepatocyte growth factor (HGF) is involved. These data suggest that the biological activity of PPARß/δ is modulated by mesenchymal-epidermal interactions and that, in turn, PPARß/δ also modulates some of these signals. The aim of the present work was to elucidate the nature of the signals exchanged between the epidermis and dermis compartments, and more particularly those which are under the control of PPARß/δ. In the first part of the study, we showed that PPARß/8 in dermal fibroblasts down-regulates the mitotic activity of keratinocytes by inhibiting the IL-1 signalling pathway via the production of secreted IL-1 receptor antagonist (sIL-1Ra), a natural antagonist of this signalling. The regulation of IL-1 signalling by PPARß/δ is required for anon-pathological skin wound repair. These findings provide evidence for a novel homeostatic control of keratinocyte proliferation and differentiation mediated by the regulation of IL-1 signalling via dermal PPARß/δ fibroblasts. Proteolysis of the extracellular matrix (ECM) is a key process involved in wound repair and modifications in its activity are often associated with an alteration óf the wound closure. This process implies specific proteinases, as matrix metalloproteinases (MMPs), which are finely modulated by IL-1 signalling. In line with the first results, the second part of the work showed that MMP8 and MMP13, which are two important collagenases involved in mouse skin wound repair, are regulated by PPARß/δ. Their expression is indirectly down-regulated by dermal PPARß/δ, via the production of sIL-1Ra, resulting in the inhibition of IL-1 signalling, known to regulate the expression of numerous MMPs. We suggest that, in absence of PPARß/δ, the positive regulation of these two collagenases could participate to the delay of skin wound healing, which has been observed in mice deleted for PPARßlS. The potential therapeutic role of PPARß/b could be as well extending to inflammatory and hyperproliferative skin diseases involving IL-1 signalling, such as psoriasis or skin cancers. Quite interestingly, MMP1 (analogue of mouse MMP13) plays an essential role in human photoaging, suggesting that PPARß/δ could as well be an attractive target for photoprotection. RESUME : La cicatrisation est un processus complexe et extrêmement organisé, impliquant un grand nombre de cellules qui s'unissent pour régénérer le tissu endommagé. De nombreux travaux nous ont éclairés sur les mécanismes cellulaires et moléculaires, dans lesquels les interactions épidermo-mésenchymateuses détiennent un rôle capital à la fois dans la régulation de l'homéostasie et dans la réparation de la peau. PPAR (Peroxisome proliferatar-activated receptor), qui appartient à la superfamille des récepteurs nucléaires, se définit comme un facteur de transcription activé par des ligands très spécifiques. Trois isotypes (PPARa, PPARß/δ et PPARy) ont été décrits et sont caractérisés par une distribution tissulaire et des fonctions physiologiques clairement définies. PPARß/δ a été identifié comme étant un important acteur dans l'homéostasie de la peau. Chez la souris, il a été décrit comme contrôlant l'état de prolifération et de différenciation, le processus d'adhésion et de migration, ainsi que la survie des kératinocytes au cours de la cicatrisation. PPARßIS a également été défini comme contrôlant le développement des follicules pileux, impliquant la sécrétion par le mésenchyme du facteur de croissance HGF. Ces données suggèrent que l'activité biologique de PPARß/δ est modulée par des interactions épidermo-mésenchymateuses, et qu'en retour, il possède la capacité de moduler certains de ces signaux. L`objectif de ce travail a été d'élucider la nature des signaux échangés entre les compartiments épidermique et dermique, et plus particulièrement ceux qui sont sous le contrôle de PPARß/δ. Dans la première partie de l'étude, nous avons montré que les fibroblastes exprimant PPARß/δ réduisent l'activité mitotique des kératinocytes en inhibant la voie de signalisation IL-1, via la production de sIL-1Ra (secreted IL-1 receptor antagonist), défini comme un antagoniste naturel de cette voie de signalisation. La régulation de cette dernière par PPARß/δ est donc nécessaire pour une cicatrisation de type non pathologique. Ces résultats offrent donc une nouvelle preuve du contrôle de l'homéostasie et de l'état de prolifération/différenciation des kératinocytes par les fibroblastes exprimant PPARß/δ, en régulant la voie de signalisation IL-1. Le mécanisme de dégradation de la matrice extracellulaire (MEC) est une étape essentielle lors du processus de cicatrisation. Ainsi des modifications de cette activité protéolytïque sont souvent associées à une altération de la fermeture de la plaie. Ce processus implique des protéinases, comme les MMPs, qui sont finement modulés par la voie de signalisation IL-1. En accord avec les premiers résultats, la seconde partie des nos travaux a montré que les collagénases MMP8 et MMP13, connues pour être d'importantes molécules impliquées lors de la réparation tissulaire chez la souris, sont modulées par l'activité de PPARß/δ. Leurs expressions sont indirectement régulées par PPARß/δ, via la production. de sIL-1 Ra, entraînant ainsi l'inhibition de la voie de signalisation IL-1, décrite pour réguler l'expression de nombreuses MMPs, Nous suggérons donc qu'en absence de PPARß/δ, la régulation de ces deux collagénases pourrait être impliquée dans le retard de cicatrisation, observé chez les souris déficientes pour PPARß/δ. L'activité biologique de PPARß/δ pourrait être ainsi étendue à des maladies hyperproliferatives et inflammatoires de la peau, impliquant la voie de signalisation IL-1, comme le psoriasis ou certains cancers de la peau, et ce à des fins thérapeutiques. Il est aussi intéressant de relever que chez l'homme, MMP1 (présenté comme l'analogue de MMP13 de la souris} joue un rôle primordial dans le photo-vieillissement, nous suggérons donc que PPARß/δ pourrait ainsi être une cible attrayante concernant la photoprotection.

High-resolution numerical modelling of flow-vegetation interactions

In this paper, we present and apply a new three-dimensional model for the prediction of canopy-flow and turbulence dynamics in open-channel flow. The approach uses a dynamic immersed boundary technique that is coupled in a sequentially staggered manner to a large eddy simulation. Two different biomechanical models are developed depending on whether the vegetation is dominated by bending or tensile forces. For bending plants, a model structured on the Euler-Bernoulli beam equation has been developed, whilst for tensile plants, an N-pendula model has been developed. Validation against flume data shows good agreement and demonstrates that for a given stem density, the models are able to simulate the extraction of energy from the mean flow at the stem-scale which leads to the drag discontinuity and associated mixing layer.

Interactions of tumor necrosis factor (TNF) and TNF receptor family members in the mouse and human.

Ligands of the tumor necrosis factor superfamily (TNFSF) (4-1BBL, APRIL, BAFF, CD27L, CD30L, CD40L, EDA1, EDA2, FasL, GITRL, LIGHT, lymphotoxin alpha, lymphotoxin alphabeta, OX40L, RANKL, TL1A, TNF, TWEAK, and TRAIL) bind members of the TNF receptor superfamily (TNFRSF). A comprehensive survey of ligand-receptor interactions was performed using a flow cytometry-based assay. All ligands engaged between one and five receptors, whereas most receptors only bound one to three ligands. The receptors DR6, RELT, TROY, NGFR, and mouse TNFRH3 did not interact with any of the known TNFSF ligands, suggesting that they either bind other types of ligands, function in a ligand-independent manner, or bind ligands that remain to be identified. The study revealed that ligand-receptor pairs are either cross-reactive between human and mouse (e.g. Tweak/Fn14, RANK/RANKL), strictly species-specific (GITR/GITRL), or partially species-specific (e.g. OX40/OX40L, CD40/CD40L). Interestingly, the receptor binding patterns of lymphotoxin alpha and alphabeta are redundant in the human but not in the mouse system. Ligand oligomerization allowed detection of weak interactions, such as that of human TNF with mouse TNFR2. In addition, mouse APRIL exists as two different splice variants differing by a single amino acid. Although human APRIL does not interact with BAFF-R, the shorter variant of mouse APRIL exhibits weak but detectable binding to mouse BAFF-R.

Genetic differentiation in two European tree frog (Hyla arborea) metapopulations in contrasted landscapes of Western Switzerland

The survival of threatened species as the European tree frog (Hyla arborea) is strongly dependent on the genetic variability within populations, as well as gene flow between them. In Switzerland, only two sectors in its western part still harbour metapopulations. The first is characterised by a very heterogeneous and urbanized landscape, while the second is characterised by a uninterrupted array of suitable habitats. In this study, six microsatellite loci were used to establish levels of genetic differentiation among the populations from the two different locations. The results show that the metapopulations have: (i) weak levels of genetic differentiation (FST within metapopulation ≈ 0.04), (ii) no difference in levels of genetic structuring between them, (iii) significant (p = 0.019) differences in terms of genetic diversity (Hs) and observed heterozygozity (Ho), the metapopulation located in a disturbed landscape showing lower values. Our results suggest that even if the dispersal of H. arborea among contiguous ponds seems to be efficient in areas of heterogeneous landscape, a loss of genetic diversity can occur.

Warming experiments underpredict plant phenological responses to climate change.

Warming experiments are increasingly relied on to estimate plant responses to global climate change. For experiments to provide meaningful predictions of future responses, they should reflect the empirical record of responses to temperature variability and recent warming, including advances in the timing of flowering and leafing. We compared phenology (the timing of recurring life history events) in observational studies and warming experiments spanning four continents and 1,634 plant species using a common measure of temperature sensitivity (change in days per degree Celsius). We show that warming experiments underpredict advances in the timing of flowering and leafing by 8.5-fold and 4.0-fold, respectively, compared with long-term observations. For species that were common to both study types, the experimental results did not match the observational data in sign or magnitude. The observational data also showed that species that flower earliest in the spring have the highest temperature sensitivities, but this trend was not reflected in the experimental data. These significant mismatches seem to be unrelated to the study length or to the degree of manipulated warming in experiments. The discrepancy between experiments and observations, however, could arise from complex interactions among multiple drivers in the observational data, or it could arise from remediable artefacts in the experiments that result in lower irradiance and drier soils, thus dampening the phenological responses to manipulated warming. Our results introduce uncertainty into ecosystem models that are informed solely by experiments and suggest that responses to climate change that are predicted using such models should be re-evaluated.

The complex history of the olive tree: from Late Quaternary diversification of Mediterranean lineages to primary domestication in the northern Levant.

The location and timing of domestication of the olive tree, a key crop in Early Mediterranean societies, remain hotly debated. Here, we unravel the history of wild olives (oleasters), and then infer the primary origins of the domesticated olive. Phylogeography and Bayesian molecular dating analyses based on plastid genome profiling of 1263 oleasters and 534 cultivated genotypes reveal three main lineages of pre-Quaternary origin. Regional hotspots of plastid diversity, species distribution modelling and macrofossils support the existence of three long-term refugia; namely the Near East (including Cyprus), the Aegean area and the Strait of Gibraltar. These ancestral wild gene pools have provided the essential foundations for cultivated olive breeding. Comparison of the geographical pattern of plastid diversity between wild and cultivated olives indicates the cradle of first domestication in the northern Levant followed by dispersals across the Mediterranean basin in parallel with the expansion of civilizations and human exchanges in this part of the world.