Blood pressure-independent cardiac hypertrophy induced by locally activated renin-angiotensin system.

Cardiac hypertrophy is frequent in chronic hypertension. The renin-angiotensin system, via its effector angiotensin II (Ang II), regulates blood pressure and participates in sustaining hypertension. In addition, a growing body of evidence indicates that Ang II acts also as a growth factor. However, it is still a matter of debate whether the trophic effect of Ang II can trigger cardiac hypertrophy in the absence of elevated blood pressure. To address this question, transgenic mice overexpressing the rat angiotensinogen gene, specifically in the heart, were generated to increase the local activity of the renin-angiotensin system and therefore Ang II production. These mice develop myocardial hypertrophy without signs of fibrosis independently from the presence of hypertension, demonstrating that local Ang II production is important in mediating the hypertrophic response in vivo.

Active strain and activation models in cardiac electromechanics

We present a model for mechanical activation of the cardiac tissue depending on the evolution of the transmembrane electrical potential and certain gating/ionic variables that are available in most of electrophysiological descriptions of the cardiac membrane. The basic idea consists in adding to the chosen ionic model one ordinary differential equation for the kinetics of the mechanical activation function. A relevant example illustrates the desired properties of the proposed model, such as delayed muscle contraction and correct magnitude of the muscle fibers' shortening.

Genetic analysis of sudden cardiac death victims: a survey of current forensic autopsy practices.

Autopsy-negative sudden cardiac deaths (SCD) seen in forensic practice are most often thought to be the result of sudden arrhythmic death syndrome. Postmortem genetic analysis is recommended in such cases, but is currently performed in only a few academic centers. In order to determine actual current practice, an on-line questionnaire was sent by e-mail to members of various forensic medical associations. The questions addressed routine procedures employed in cases of sudden cardiac death (autopsy ordering, macroscopic and microscopic cardiac examination, conduction tissue examination, immunohistochemistry and electron microscopy, biochemical markers, sampling and storage of material for genetic analyses, toxicological analyses, and molecular autopsy). Some questions concerned the legal and ethical aspects of genetic analyses in postmortem examinations, as well as any existing multidisciplinary collaborations in SCD cases. There were 97 respondents, mostly from European countries. Genetic testing in cases of sudden cardiac death is rarely practiced in routine forensic investigation. Approximately 60% of respondents reported not having the means to perform genetic postmortem testing and 40% do not collect adequate material to perform these investigations at a later date, despite working at university hospitals. The survey demonstrated that many of the problems involved in the adequate investigation of SCD cases are often financial in origin, due to the fact that activities in forensic medicine are often paid by and dependent on the judicial authorities. Problems also exist concerning the contact with family members and/or the family doctor, as well as the often-nonexistent collaboration with others clinicians with special expertise beneficial in the investigation of SCD cases, such as cardiologists and geneticists. This study highlights the importance in establishing guidelines for molecular autopsies in forensic medicine.

Predictors of mortality from pump failure and sudden cardiac death in patients with systolic heart failure and left ventricular dyssynchrony: results of the CARE-HF trial.

BACKGROUND: Determining a specific death cause may facilitate individualized therapy in patients with heart failure (HF). Cardiac resynchronization therapy (CRT) decreased mortality in the Cardiac Resynchronization in Heart Failure trial by reducing pump failure and sudden cardiac death (SCD). This study analyzes predictors of specific causes of death. METHODS AND RESULTS: Univariate and multivariate analyses used 8 baseline and 3-month post-randomization variables to predict pump failure and SCD (categorized as "definite," "probable," and "possible"). Of 255 deaths, 197 were cardiovascular. There were 71 SCDs with a risk reduction by CRT of 0.47 (95% confidence interval 0.29-0.76; P = .002) with similar reductions in SCD classified as definite, probable, and possible. Univariate SCD predictors were 3-month HF status (mitral regurgitation [MR] severity, plasma brain natriuretic peptide [BNP], end-diastolic volume, and systolic blood pressure), whereas randomization to CRT decreased risk. Multivariate SCD predictors were randomization to CRT 0.56 (0.53-0.96, P = .035) and 3-month MR severity 1.82 (1.77-2.60, P = .0012). Univariate pump failure death predictors related to baseline HF state (quality of life score, interventricular mechanical delay, end-diastolic volume, plasma BNP, MR severity, and systolic pressure), whereas randomization to CRT and nonischemic cardiomyopathy decreased risk; multivariate predictors of pump failure death were baseline plasma BNP and systolic pressure and randomization to CRT. CONCLUSION: CRT decreased SCD in patients with systolic HF and ventricular dyssynchrony. SCD risk was increased with increased severity of MR (including the 3-month value for MR as a time-dependent covariate) and reduced by randomization to CRT. HF death was increased related to the level of systolic blood pressure, log BNP, and randomization to CRT. These results emphasize the importance and interdependence of HF severity to mortality from pump failure and SCD.

A spatially explicit life cycle inventory of the global textile chain

Life cycle analyses (LCA) approaches require adaptation to reflect the increasing delocalization of production to emerging countries. This work addresses this challenge by establishing a country-level, spatially explicit life cycle inventory (LCI). This study comprises three separate dimensions. The first dimension is spatial: processes and emissions are allocated to the country in which they take place and modeled to take into account local factors. Emerging economies China and India are the location of production, the consumption occurs in Germany, an Organisation for Economic Cooperation and Development country. The second dimension is the product level: we consider two distinct textile garments, a cotton T-shirt and a polyester jacket, in order to highlight potential differences in the production and use phases. The third dimension is the inventory composition: we track CO2, SO2, NO (x), and particulates, four major atmospheric pollutants, as well as energy use. This third dimension enriches the analysis of the spatial differentiation (first dimension) and distinct products (second dimension). We describe the textile production and use processes and define a functional unit for a garment. We then model important processes using a hierarchy of preferential data sources. We place special emphasis on the modeling of the principal local energy processes: electricity and transport in emerging countries. The spatially explicit inventory is disaggregated by country of location of the emissions and analyzed according to the dimensions of the study: location, product, and pollutant. The inventory shows striking differences between the two products considered as well as between the different pollutants considered. For the T-shirt, over 70% of the energy use and CO2 emissions occur in the consuming country, whereas for the jacket, more than 70% occur in the producing country. This reversal of proportions is due to differences in the use phase of the garments. For SO2, in contrast, over two thirds of the emissions occur in the country of production for both T-shirt and jacket. The difference in emission patterns between CO2 and SO2 is due to local electricity processes, justifying our emphasis on local energy infrastructure. The complexity of considering differences in location, product, and pollutant is rewarded by a much richer understanding of a global production-consumption chain. The inclusion of two different products in the LCI highlights the importance of the definition of a product's functional unit in the analysis and implications of results. Several use-phase scenarios demonstrate the importance of consumer behavior over equipment efficiency. The spatial emission patterns of the different pollutants allow us to understand the role of various energy infrastructure elements. The emission patterns furthermore inform the debate on the Environmental Kuznets Curve, which applies only to pollutants which can be easily filtered and does not take into account the effects of production displacement. We also discuss the appropriateness and limitations of applying the LCA methodology in a global context, especially in developing countries. Our spatial LCI method yields important insights in the quantity and pattern of emissions due to different product life cycle stages, dependent on the local technology, emphasizing the importance of consumer behavior. From a life cycle perspective, consumer education promoting air-drying and cool washing is more important than efficient appliances. Spatial LCI with country-specific data is a promising method, necessary for the challenges of globalized production-consumption chains. We recommend inventory reporting of final energy forms, such as electricity, and modular LCA databases, which would allow the easy modification of underlying energy infrastructure.

Integrating life cycle costs and environmental impacts of composite rail car-bodies for a Korean train

Background, aim, and scope A coupled Life Cycle Costing and life cycle assessment has been performed for car-bodies of the Korean Tilting Train eXpress (TTX) project using European and Korean databases, with the objective of assessing environmental and cost performance to aid materials and process selection. More specifically, the potential of polymer composite car-body structures for the Korean Tilting Train eXpress (TTX) has been investigated. Materials and methods This assessment includes the cost of both carriage manufacturing and use phases, coupled with the life cycle environmental impacts of all stages from raw material production, through carriage manufacture and use, to end-of-life scenarios. Metallic carriages were compared with two composite options: hybrid steel-composite and full-composite carriages. The total planned production for this regional Korean train was 440 cars, with an annual production volume of 80 cars. Results and discussion The coupled analyses were used to generate plots of cost versus energy consumption and environmental impacts. The results show that the raw material and manufacturing phase costs are approximately half of the total life cycle costs, whilst their environmental impact is relatively insignificant (3-8%). The use phase of the car-body has the largest environmental impact for all scenarios, with near negligible contributions from the other phases. Since steel rail carriages weigh more (27-51%), the use phase cost is correspondingly higher, resulting in both the greatest environmental impact and the highest life cycle cost. Compared to the steel scenario, the hybrid composite variant has a lower life cycle cost (16%) and a lower environmental impact (26%). Though the full composite rail carriage may have the highest manufacturing cost, it results in the lowest total life cycle costs and lowest environmental impacts. Conclusions and recommendations This coupled cost and life cycle assessment showed that the full composite variant was the optimum solution. This case study showed that coupling of technical cost models with life cycle assessment offers an efficient route to accurately evaluate economic and environmental performance in a consistent way.

Multiplex targeted high-throughput sequencing for Mendelian cardiac disorders.

Mendelian cardiomyopathies and arrhythmias are characterized by an important genetic heterogeneity, rendering Sanger sequencing very laborious and expensive. As a proof of concept, we explored multiplex targeted high-throughput sequencing (HTS) as a fast and cost-efficient diagnostic method for individuals suffering from Mendelian cardiac disorders. We designed a DNA capture assay including all exons from 130 genes involved in cardiovascular Mendelian disorders and analysed simultaneously four samples by multiplexing. Two patients had familial hypertrophic cardiomyopathy (HCM) and two patients suffered from long QT syndrome (LQTS). In patient 1 with HCM, we identified two known pathogenic missense variants in the two most frequently mutated sarcomeric genes MYH7 and MYBPC. In patient 2 with HCM, a known acceptor splice site variant in MYBPC3 was found. In patient 3 with LQTS, two missense variants in the genes SCN5A and KCNQ were identified. Finally, in patient 4 with LQTS a known missense variant was found in MYBPC3, which is usually mutated in patients with cardiomyopathy. Our results showed that multiplex targeted HTS works as an efficient and cost-effective tool for molecular diagnosis of heterogeneous disorders in clinical practice and offers new insights in the pathogenesis of these complex diseases.

O2 consumption by the fick method. Methodologic factors.

This study aimed to compare O2 consumption (VO2) determination by the gas-exchange (VO2GE) and Fick (VO2F) methods in cardiac surgical patients. A total of 10 mechanically ventilated postoperative patients were studied prospectively. Thermodilution was performed using three randomly applied techniques: room temperature saline injected at end expiration, room temperature saline randomly injected in the respiratory cycle, and iced saline injected at end expiration. The influence of the number of thermodilution determinations was assessed by comparing results from 2 and 10 injections. The variability of VO2F was greater than that of VO2GE. There was no bias between VO2GE and VO2F values using injectate at room temperature. Accuracy and precision were not improved by increasing the number of cardiac output determinations from 2 to 10. A significant bias was observed using ice-cold injectate, VO2F being 18.0 +/- 15.4 ml/min/m2 lower than VO2GE (p = 0.001). Published results when comparing VO2F and VO2GE are discrepant. However, a significant bias was found in all studies using cold injectate, with lower VO2F values. We conclude that iced injectate should not be used to assess VO2 in critically ill patients.

Histone H1 expression varies during the Leishmania major life cycle.

The deduced amino acid sequence of Leishmania major sw3 cDNA reveals the presence of characteristic histone H1 amino acid motifs. However, the open reading frame is of an unusually small size for histone H1 (105 amino acids) because it lacks the coding potential for the central hydrophobic globular domain of linker histones present in other eukaryotes. Here, we provide biochemical evidence that the SW3 protein is indeed a L. major nuclear histone H1, and that it is differentially expressed during the life cycle of the parasite. Due to its high lysine content, the SW3 protein can be purified to a high degree from L. major nuclear lysates with 5% perchloric acid, a histone H1 preparative method. Using an anti-SW3 antibody, this protein is detected as a 17 kDa or as a 17/19 kDa doublet in the nuclear subfraction in different L. major strains. The nuclear localization of the SW3 protein is further supported by immunofluorescence studies. During in vitro promastigote growth, both the sw3 cytoplasmic mRNA and its protein progressively accumulate within parasites from early log phase to stationary phase. Within amastigotes, the high level of H1 expression is maintained but decreases when amastigotes differentiate into promastigotes. Together, these observations suggest that the different levels of this histone H1 protein could influence the varying degrees of chromatin condensation during the life-cycle of the parasite, and provide us with tools to study this mechanism.

Persistent left superior vena cava in cardiac congenital surgery.

Persistent left superior vena cava (LSVC) is a relatively frequent finding in congenital cardiac malformation. The scope of the study was to analyze the timing of diagnosis of persistent LSVC, the timing of diagnosis of associated anomalies of the coronary sinus, and the global impact on morbidity and mortality of persistent LSVC in children with congenital heart disease after cardiac surgery. Retrospective analysis of a cohort of children after cardiac surgery on bypass for congenital heart disease. Three hundred seventy-one patients were included in the study, and their median age was 2.75 years (IQR 0.65-6.63). Forty-seven children had persistent LSVC (12.7 %), and persistent LSVC was identified on echocardiography before surgery in 39 patients (83 %). In three patients (6.4 %) with persistent LSVC, significant inflow obstruction of the left ventricle developed after surgery leading to low output syndrome or secondary pulmonary hypertension. In eight patients (17 %), persistent LSVC was associated with a partially or completely unroofed coronary sinus and in two cases (4 %) with coronary sinus ostial atresia. Duration of mechanical ventilation was significantly shorter in the control group (1.2 vs. 3.0 days, p = 0.04), whereas length of stay in intensive care did not differ. Mortality was also significantly lower in the control group (2.5 vs. 10.6 %, p = 0.004). The results of study show that persistent LSVC in association with congenital cardiac malformation increases the risk of mortality in children with cardiac surgery on cardiopulmonary bypass. Recognition of a persistent LSVC and its associated anomalies is mandatory to avoid complications during or after cardiac surgery.

Gene transfer of a soluble IL-1 type 2 receptor-Ig fusion protein improves cardiac allograft survival in rats.

OBJECTIVE: Interleukin-1 (IL-1) mediates ischemia-reperfusion injury and graft inflammation after heart transplantation. IL-1 affects target cells through two distinct types of transmembrane receptors, type-1 receptor (IL-1R1), which transduces the signal, and the non-signaling type-2 receptor (IL-1R2), which acts as a ligand sink that subtracts IL-1beta from IL-1R1. We analyzed the efficacy of adenovirus (Ad)-mediated gene transfer of a soluble IL-1R2-Ig fusion protein in delaying cardiac allograft rejection and the mechanisms underlying the protective effect. METHODS: IL-1 inhibition by IL-1R2-Ig was tested using an in vitro functional assay whereby endothelial cells preincubated with AdIL-1R2-Ig or control virus were stimulated with recombinant IL-1beta or tumor necrosis factor-alpha (TNF-alpha), and urokinase-type plasminogen activator (u-PA) induction was measured by zymography. AdIL-1R2-Ig was delivered to F344 rat donor hearts ex vivo, which were placed in the abdominal position in LEW hosts. Intragraft inflammatory cell infiltrates and proinflammatory cytokine expression were analyzed by immunohistochemistry and real-time reverse transcriptase-polymerase chain reaction (RT-PCR), respectively. RESULTS: IL-1R2-Ig specifically inhibited IL-1beta-induced u-PA responses in vitro. IL-1R2-Ig gene transfer reduced intragraft monocytes/macrophages and CD4(+) cell infiltrates (p<0.05), TNF-alpha and transforming growth factor-beta (TGF-beta) expression (p<0.05), and prolonged graft survival (15.6+/-5.7 vs 10.3+/-2.5 days with control vector and 10.1+/-2.1 days with buffer alone; p<0.01). AdIL-1R2-Ig combined with a subtherapeutic regimen of cyclosporin A (CsA) was superior to CsA alone (19.4+/-3.0 vs 15.9+/-1.8 days; p<0.05). CONCLUSIONS: Soluble IL-1 type-2 receptor gene transfer attenuates cardiac allograft rejection in a rat model. IL-1 inhibition may be useful as an adjuvant therapy in heart transplantation.

Early predictors of outcome in comatose survivors of ventricular fibrillation and non-ventricular fibrillation cardiac arrest treated with hypothermia: a prospective study

OBJECTIVES: Current indications for therapeutic hypothermia (TH) are restricted to comatose patients with cardiac arrest (CA) due to ventricular fibrillation (VF) and without circulatory shock. Additional studies are needed to evaluate the benefit of this treatment in more heterogeneous groups of patients, including those with non-VF rhythms and/or shock and to identify early predictors of outcome in this setting. DESIGN: Prospective study, from December 2004 to October 2006. SETTING: 32-bed medico-surgical intensive care unit, university hospital. PATIENTS: Comatose patients with out-of-hospital CA. INTERVENTIONS: TH to 33 +/- 1 degrees C (external cooling, 24 hrs) was administered to patients resuscitated from CA due to VF and non-VF (including asystole or pulseless electrical activity), independently from the presence of shock. MEASUREMENTS AND MAIN RESULTS: We hypothesized that simple clinical criteria available on hospital admission (initial arrest rhythm, duration of CA, and presence of shock) might help to identify patients who eventually survive and might most benefit from TH. For this purpose, outcome was related to these predefined variables. Seventy-four patients (VF 38, non-VF 36) were included; 46% had circulatory shock. Median duration of CA (time from collapse to return of spontaneous circulation [ROSC]) was 25 mins. Overall survival was 39.2%. However, only 3.1% of patients with time to ROSC > 25 mins survived, as compared to 65.7% with time to ROSC < or = 25 mins. Using a logistic regression analysis, time from collapse to ROSC, but not initial arrest rhythm or presence of shock, independently predicted survival at hospital discharge. CONCLUSIONS: Time from collapse to ROSC is strongly associated with outcome following VF and non-VF cardiac arrest treated with therapeutic hypothermia and could therefore be helpful to identify patients who benefit most from active induced cooling.

Induction of cardiac Angptl4 by dietary fatty acids is mediated by peroxisome proliferator-activated receptor beta/delta and protects against fatty acid-induced oxidative stress.

RATIONALE: Although dietary fatty acids are a major fuel for the heart, little is known about the direct effects of dietary fatty acids on gene regulation in the intact heart. OBJECTIVE: To study the effect of dietary fatty acids on cardiac gene expression and explore the functional consequences. METHODS AND RESULTS: Oral administration of synthetic triglycerides composed of one single fatty acid altered cardiac expression of numerous genes, many of which are involved in the oxidative stress response. The gene most significantly and consistently upregulated by dietary fatty acids encoded Angiopoietin-like protein (Angptl)4, a circulating inhibitor of lipoprotein lipase expressed by cardiomyocytes. Induction of Angptl4 by the fatty acid linolenic acid was specifically abolished in peroxisome proliferator-activated receptor (PPAR)beta/delta(-/-) and not PPARalpha(-/-) mice and was blunted on siRNA-mediated PPARbeta/delta knockdown in cultured cardiomyocytes. Consistent with these data, linolenic acid stimulated binding of PPARbeta/delta but not PPARalpha to the Angptl4 gene. Upregulation of Angptl4 resulted in decreased cardiac uptake of plasma triglyceride-derived fatty acids and decreased fatty acid-induced oxidative stress and lipid peroxidation. In contrast, Angptl4 deletion led to enhanced oxidative stress in the heart, both after an acute oral fat load and after prolonged high fat feeding. CONCLUSIONS: Stimulation of cardiac Angptl4 gene expression by dietary fatty acids and via PPARbeta/delta is part of a feedback mechanism aimed at protecting the heart against lipid overload and consequently fatty acid-induced oxidative stress.

Regulation of the activity of DnaA and its role during the cell cycle of caulobacter crescentus

The initiation of chromosomal replication must be tightly regulated so that the genome is replicated only once per cell cycle. In most bacteria, DnaA binds to the origin of replication and initiates chromosomal replication. DnaA is a dual-function protein that also acts as an important transcription factor that regulates the expression of many genes in bacteria. Thus, understanding how this protein is regulated during the bacterial cell cycle is of major importance. The α-proteobacterium Caulobacter crescentus is an excellent model to study the bacterial cell cycle, mainly because it is possible to isolate synchronized cell cultures and because it initiates the replication of its chromosome once per cell cycle and at a specific time of the cell cycle. This latest feature is of special interest for the major aim of my thesis work, which focused on the temporal and spatial regulation of the activity of the essential DnaA protein in C. crescentus. In Escherichia coli, the Hda protein converts ATP-DnaA into ADP- DnaA by stimulating the ATPase activity of DnaA, to prevent over-initiation of chromosome replication. We propose that there exists a similar mechanism in C. crescentus, which is not only involved in the temporal control of chromosome replication, but also in the control of gene expression. First, we provided evidences indicating that the hydrolysis of the ATP bound to DnaA is essential for the viability of C. crescentus. Our results suggest that ATP-DnaA promotes the initiation of chromosome replication, since we found that cells over-expressing a DnaA protein with a mutated ATPase domain, DnaA(R357A), over-initiated chromosome replication, unlike cells expressing the wild-type DnaA protein at similar levels. By contrast, the DnaA(R357A) protein was less active than DnaA in promoting the transcription of three essential genes, suggesting that these may be more efficiently activated by ADP-DnaA than ATP-DnaA. We propose that the ATP-DnaA to ADP-DnaA switch down-regulates the initiation of DNA replication while activating the transcription of several essential genes involved in subsequent cell cycle events. Second, we studied the role of the HdaA protein, homologous to Hda, in promoting the ATP- DnaA to ADP-DnaA switch in C. crescentus. HdaA is essential for viability and its depletion in the cell leads to an over-replication of the chromosome, indicating that HdaA is a negative regulator of DNA replication. HdaA dynamically co-localizes with the replisome. In this work, we identified DnaN, the β-clamp of the DNA polymerase, as the replisome component that interacts directly with HdaA and that recruits HdaA to the replisome in live C. crescentus cells. We also showed that a mutant HdaA protein that cannot interact or co-localize with DnaN is not functional, indicating that HdaA is probably activated by DnaN. However, we found that another non-functional HdaA protein, mutated in the conserved Arginine finger of its AAA+ domain, was able to localize at the replisome, suggesting that the AAA+ domain of HdaA exerts its essential function after the recruitment of HdaA to the replisome. We propose that HdaA stimulates the ATPase activity of DnaA once DNA replication is ongoing, via its interaction with DnaN and the activity of the two conserved R fingers of DnaA and HdaA. Finally, we created different strains in which HdaA, DnaN or DnaA were over-produced. We observed that the over-production of HdaA seems to lead to a delay in chromosome replication, while the over-production of DnaN had an opposite effect. Our results also indicate that the over-production of DnaA may intensify the over-initiation phenotype of cells depleted for HdaA. We conclude that the dynamic interplay of HdaA and DnaN in the cell contributes to regulating the ATP-DnaA/ADP-DnaA ratio in the cell, to ensure once per cell cycle initiation of chromosomal replication in C. crescentus. Altogether, our work provided important information on the regulation of the activity of DnaA in C. crescentus. Since DnaA, HdaA and DnaN are well-conserved proteins, most of our findings are useful to understand how chromosome replication and gene expression are controlled by DnaA in many other bacterial species. - L'initiation de la réplication des chromosomes doit être précisément régulée de telle sorte que le génome ne soit répliqué qu'une seule fois par cycle cellulaire. Chez la plupart des bactéries, DnaA se lie à l'origine de réplication du chromosome et en initie sa réplication. DnaA est aussi un facteur de transcription qui régule l'expression de nombreux gènes bactériens. De ce fait, il est très important de comprendre comment DnaA est régulée au cours du cycle cellulaire bactérien. L'a-protéobactérie Caulobacter crescentus est un excellent modèle pour étudier le cycle cellulaire bactérien, essentiellement parce qu'il est aisé d'isoler des populations de cellules synchronisées à la même étape du cycle cellulaire et parce que cette bactérie n'initie la réplication de son chromosome qu'une seule fois et à un moment précis de son cycle. Cette dernière caractéristique est particulièrement pertinente pour l'objectif de mon travail doctoral, qui consistait à comprendre comment l'activité de la protéine essentielle DnaA est régulée dans l'espace et dans le temps chez C. crescentus. Chez Escherichia coli, la protéine Hda convertie DnaA-ATP en DnaA-ADP en stimulant l'activité ATPasique de DnaA, ce qui empêche la sur-initiation de la réplication du chromosome. Nous proposons qu'un mécanisme similaire existe chez C. crescentus. Il serait non seulement nécessaire au contrôle de la réplication du chromosome, mais aussi au contrôle de l'expression de certains gènes. Dans un premier temps, nous avons mis en évidence le fait que l'hydrolyse de l'ATP lié à DnaA est un processus essentiel à la viabilité de C. crescentus. Nos résultats suggèrent que DnaA-ATP initie la réplication du chromosome, comme nous avons observé que des cellules qui sur-expriment une protéine DnaA(R357A) mutée sans domaine ATPasique fonctionnel, sur-initie la réplication de leur chromosome, contrairement aux cellules qui sur-expriment la protéine DnaA sauvage à des niveaux équivalents. Au contraire, la protéine DnaA(R357A) était moins active que la protéine DnaA sauvage pour promouvoir la transcription de trois gènes essentiels, ce qui suggère que ces derniers sont peut-être plus efficacement activés par DnaA-ADP que DnaA-ATP. Nous proposons que la conversion de DnaA-ATP en DnaA-ADP réprime l'initiation de la réplication, tandis qu'elle active la transcription de plusieurs gènes impliqués dans des étapes plus tardives du cycle cellulaire. Dans un deuxième temps, nous avons étudié le rôle de la protéine HdaA, homologue à Hda, dans la conversion de DnaA-ATP en DnaA-ADP chez C. crescentus. Cette protéine est essentielle à la viabilité de C. crescentus et sa déplétion donne des cellules qui sur-initient la réplication de leur chromosome, suggérant que HdaA est un répresseur de la réplication du chromosome. HdaA co-localise de manière dynamique avec le réplisome. Lors de mon travail doctoral, nous avons démontré que DnaN, le β-clamp de l'ADN polymérase, est l'élément qui recrute HdaA au réplisome in vivo. Nous avons aussi montré qu'une protéine HdaA mutante qui ne peut pas interagir ou co-localiser avec DnaN, n'est pas fonctionnelle, ce qui suggère que HdaA est activée par DnaN. Nous avons néanmoins aussi isolé une autre protéine HdaA non fonctionnelle, dont une arginine conservée de son domaine AAA+ était mutée, mais qui pouvait toujours co-localiser avec le réplisome, ce qui suggère que le domaine AAA+ de HdaA est nécessaire après le recrutement de HdaA au réplisome. Nous proposons que HdaA stimule l'activité ATPasique de DnaA qu'une fois que la réplication a commencé, grâce à son interaction avec DnaN et aux deux arginines conservées des protéines HdaA et DnaA. Finalement, nous avons construit différentes souches sur-exprimant HdaA, DnaN ou DnaA. Nous avons observé que la sur-production de HdaA retarde la réplication du chromosome, tandis que la sur-production de DnaN a un effet opposé. Nos observations suggèrent aussi que la sur-expression de DnaA dans des cellules déplétées pour HdaA aggrave leur phénotype de sur-initiation. Nous en concluons que HdaA et DnaN collaborent étroitement et de manière dynamique pour réguler le rapport DnaA-ATP/DnaA-ADP dans la cellule, pour s'assurer que la réplication du chromosome ne soit initiée qu'une seule fois par cycle cellulaire chez C. crescentus. Globalement, notre travail a mis en évidence des informations importantes sur la régulation de l'activité de DnaA chez C. crescentus. Comme DnaA, HdaA et DnaN sont des protéines très conservées, la plupart de nos découvertes sont utiles pour mieux comprendre comment la réplication du chromosome bactérien et l'expression des gènes sont contrôlées par DnaA chez de nombreuses autres espèces bactériennes.