Caractérisation de la vulnérabilité sélective des neurones dopaminergiques dans le contexte de la maladie de Parkinson

La maladie de Parkinson (MP) est une maladie neurodégénérative dont les symptômes moteurs caractéristiques sont causés par la mort des neurones dopaminergiques de la substance noire compacte (SNc) dans le mésencéphale. Dans environs 15% des cas, des mutations dans des gènes encodant des protéines telles que Parkin, Pink1, DJ-1, α-synucléine, LRRK2 ou GBA, sont responsable de l’apparition de la maladie. Ces gènes sont impliqués dans des processus physiologiques comme la mitophagie, la fonction lysosomiale, la réponse au stress oxydatif, la présentation antigénique mitochondriale ou le transport axonal. De manière intéressante, ces processus ont tous un impact direct ou indirect sur la capacité des neurones à produire l’énergie nécessaire à leur survie, ainsi que sur leur niveau basal de stress oxydatif. Une des questions centrales dans l’étude de la MP est de savoir pourquoi des altérations dans des processus aussi généraux mènent à la mort sélective de petits groupes de neurones dans le cerveau, tels que les neurones dopaminergiques de la SNc. La comparaison de ces neurones à leurs voisins de l’aire tegmentaire ventrale (VTA) qui sont eux aussi dopaminergiques, mais bien moins affectés dans la MP, est un des exemples les plus flagrants de cette vulnérabilité sélective. Une hypothèse récente suggère que certaines caractéristiques propres aux neurones dopaminergiques de la SNc mettraient une immense pression sur leurs capacités bioénergétiques, ce qui les rendrait sélectivement vulnérables à tout stress supplémentaire. Par exemple, ces neurones auraient tendance à former des dérivés toxiques de la dopamine dans des conditions de stress, ce qui impacterait négativement leur capacité à produire efficacement leur énergie. Ils auraient aussi un patron de décharge de type « pacemaker » impliquant des courants calciques très énergivores. Finalement, ils posséderaient une arborisation axonale particulièrement élaborée, nécessitant une quantité phénoménale d’énergie pour y propager les potentiels d’action et y induire la relâche de neurotransmetteurs. Une de nos hypothèses est que ces caractéristiques rendent les neurones dopaminergiques de la SNc particulièrement actifs au niveau bioénergétique et les poussent à la limite de leurs capacités énergétiques. Tout stress cellulaire supplémentaire, tel que ceux associés aux altérations génétiques énoncées précédemment, à l’exposition à des toxines environnementales ou tout simplement au vieillissement, pourrait alors mener à leur mort. De manière intéressante, il semble que l’élaboration d’un axone à l’arborisation ultra-complexe soit une caractéristique commune aux populations neuronales affectées dans la MP, telles que les neurones dopaminergiques de la SNc, les neurones cholinergiques du noyau pédonculopontin ou les neurones noradrénergiques du locus coeruleus, contrairement à la présence de dopamine ou de courants calciques liés à une décharge ou de type « pacemaker ». Il est donc plus probable que cette caractéristique soit centrale dans la vulnérabilité sélective des populations neuronales dans la MP. Malheureusement, aucune comparaison directe de la taille de l’arborisation axonale de ces neurones avec des populations neuronales épargnées n’est à ce jour disponible, tout comme l’impact de la modulation de la taille de cette arborisation sur les besoins énergétiques et la vulnérabilité des neurones. Pour commencer à évaluer cette possibilité, nous avons comparé, par culture cellulaire, les neurones dopaminergiques de la SNc à ceux de la VTA et montré qu’ils sont particulièrement actifs au point de vue bioénergétique, avec peu de capacité de réserve, et qu’ils possèdent une arborisation axonale de taille bien plus importante. À l’aide de la sémaphorine 7A, un facteur de guidage axonal, nous avons réduit la taille de cette arborisation et réduit du même coup les besoins énergétiques et la vulnérabilité de ces neurones. Dans un deuxième temps, nous avons évalué ces mêmes paramètres chez des neurones de souris KO pour Parkin, Pink1 ou DJ-1 et montré que les neurones de la SNc KO pour Parkin sont plus vulnérables aux conditions de culture et sont moins efficaces pour produire leur énergie. Dans un dernier temps, nous avons démontré, in vivo cette fois, que les neurones dopaminergiques de la SNc ont une arborisation axonale de taille plus importante que ceux de la VTA et avons utilisé le KO conditionnel du récepteur D2 pour augmenter encore plus l’étendue de cette arborisation axonale. Dans ce modèle, nous avons montré que les neurones de la SNc sont plus vulnérables aux lésions à la 6-OHDA, mais pas à la surexpression d’α-synucléine. Dans son ensemble, cette thèse amène pour la première fois des preuves directes que la taille de l’arborisation axonale est un facteur majeur dans la vulnérabilité sélective des neurones dopaminergiques de la SNc dans le contexte de la MP.Parkinson's disease (PD) is a neurodegenerative disease whose characteristic motor symptoms are caused by the death of midbrain dopaminergic neurons of the subtantia nigra pars compacta (SNc). In about 15% of cases, mutations in gene products such as Parkin, Pink1, DJ-1, α-synuclein, LRRK2 or GBA are responsible for the onset of the disease. These genes are involved in physiological processes such as mitophagy, lysosomal function, oxidative stress response, mitochondrial antigen presentation or axonal transport. Interestingly, these processes are all directly or indirectly related to the ability of neurons to produce the energy they need for survival and to their level of basal oxidative stress. One of the central questions in the study of PD is why alterations in such ubiquitous processes lead to the selective death of small groups of neurons in the brain, such as the dopaminergic neurons of the SNc. The comparison of these neurons with their neighbours of the ventral tegmental area (VTA), which are also dopaminergic but much less affected in PD, is one of the most striking examples of this selective vulnerability. One recent hypothesis suggests that certain characteristics specific to SNc dopaminergic neurons induce a tremendous pressure on their bioenergetic capacities and increase basal oxidative stress, which could make them selectively vulnerable to any additional stress. For example, these neurons are thought to form toxic derivatives of dopamine under stress conditions, which could negatively impact their ability to efficiently produce their energy. They also have a pacemaking firing pattern implicating energy intensive calcium currents. Finally, they are thought to have a particularly elaborated axonal arborization, requiring a phenomenal amount of energy to propagate the action potentials and induce release of neurotransmitters. One of our hypotheses is that these characteristics make SNc dopaminergic neurons particularly active bioenergetically and push them to the limit of their energetic capacities. Any subsequent cellular stress, such as those associated with the previously mentioned genetic alterations, exposure to environmental toxins or simply aging, could then lead to their death. Interestingly, the development of an ultra-complex axonal arborization is a potentially shared feature of neuronal populations affected in PD, including dopaminergic neurons of the SNc, cholinergic neurons of the pedunculopontine nucleus or noradrenergic neurons of the locus coeruleus. It is therefore possible that this characteristic is central to the selective vulnerability of certain neuronal populations in PD. Unfortunately, no direct comparison of the axonal arborization size of these neurons with spared neuronal populations is currently available, neither is the impact of the modulation of the axonal arborization size on energy requirements and neuronal vulnerability. To begin to evaluate this possibility, we compared the dopaminergic neurons of the SNc to those of the VTA in culture and showed that they are particularly bioenergetically active, with little spare capacity, and have a much larger axonal arborization size. Using semaphorin 7A, an axon guidance factor, we reduced the size of this axon and at the same time reduced the energy requirements and vulnerability of these neurons. In a second step, we evaluated these same parameters in SNc neurons from Parkin, Pink1 or DJ-1 KO mice and showed that SNc neurons from Parkin KO mice are more vulnerable to culture conditions and are less efficient in producing their energy. Lastly, we demonstrated, in vivo this time, that dopaminergic neurons of the SNc have a larger axonal arborization than those of the VTA and used the conditional KO of the D2 receptor to further increase the size of this axon. In this model, SNc neurons were more vulnerable to 6-OHDA lesions, but not to overexpression of α-synuclein. Overall, this thesis provides for the first-time direct evidence that axonal arborization size is a major factor in the selective vulnerability of dopaminergic neurons of the SNc in the context of Parkinson's disease

Développement de nouvelles formulations de poudres métalliques pour la fabrication de composantes de haute performance

L'industrie automobile demande sans cesse des manufacturiers de pièces fabriquées par métallurgie des poudres des produits présentant des propriétés mécaniques supérieures tout en minimisant les coûts. L'emploi des poudres autotrempantes est particulièrement bien adapté pour répondre à ses demandes, car ces dernières permettent la trempe directe à la fin du cycle de frittage, éliminant ainsi les étapes supplémentaires habituellement nécessaires pour le traitement thermique. Cette thèse présente les résultats de la modélisation de l'influence des éléments préalliés et prémélangés sur l'optimisation de la compressibilité et de l'autotrempabilité. La planification d'expériences a été utilisée afin de minimiser le nombre d'expériences nécessaires afin de caractériser l'effet des différents facteurs étudiés. De petites quantités de poudres atomisées ont été produites par atomisation à l'eau au laboratoire de métallurgie des poudres de l'Université Laval. Un premier plan d'expériences a permis d'étudier l'effet d'éléments préalliés (nickel, chrome, molybdène et manganèse) et prémélangés (nickel, chrome, cuivre et manganèse). Les propriétés mécaniques ont aussi été mesurées. Un deuxième plan d'expériences a été construit à partir des résultats obtenus lors de la première série. Les résultats montrent que de tous les éléments étudiés, seuls le nickel, le chrome et le molybdène préalliés ont un effet significatif sur la compressibilité et l'autotrempabilité. De plus, pour l'intervalle de composition chimique étudié, la composition chimique de la poudre qui optimise la compressibilité et l'autotrempabilité est : 1,5%-pds Ni, 0,40-0,55%-pds Cr et 1,00-1,25%-pds Mo

On Cell Loss and Selective Vulnerability of Neuronal Populations in Parkinson's Disease

Significant advances have been made uncovering the factors that render neurons vulnerable in Parkinson's disease (PD). However, the critical pathogenic events leading to cell loss remain poorly understood, complicating the development of disease-modifying interventions. Given that the cardinal motor symptoms and pathology of PD involve the loss of dopamine (DA) neurons of the substantia nigra pars compacta (SNc), a majority of the work in the PD field has focused on this specific neuronal population. PD however, is not a disease of DA neurons exclusively: pathology, most notably in the form of Lewy bodies and neurites, has been reported in multiple regions of the central and peripheral nervous system, including for example the locus coeruleus, the dorsal raphe nucleus and the dorsal motor nucleus of the vagus. Cell and/or terminal loss of these additional nuclei is likely to contribute to some of the other symptoms of PD and, most notably to the non-motor features. However, exactly which regions show actual, well-documented, cell loss is presently unclear. In this review we will first examine the strength of the evidence describing the regions of cell loss in idiopathic PD, as well as the order in which this loss occurs. Secondly, we will discuss the neurochemical, morphological and physiological characteristics that render SNc DA neurons vulnerable, and will examine the evidence for these characteristics being shared across PD-affected neuronal populations. The insights raised by focusing on the underpinnings of the selective vulnerability of neurons in PD might be helpful to facilitate the development of new disease-modifying strategies and improve animal models of the disease

Elevated Mitochondrial Bioenergetics and Axonal Arborization Size Are Key Contributors to the Vulnerability of Dopamine Neurons

SummaryAlthough the mechanisms underlying the loss of neurons in Parkinson’s disease are not well understood, impaired mitochondrial function and pathological protein aggregation are suspected as playing a major role. Why DA (dopamine) neurons and a select small subset of brain nuclei are particularly vulnerable to such ubiquitous cellular dysfunctions is presently one of the key unanswered questions in Parkinson’s disease research. One intriguing hypothesis is that their heightened vulnerability is a consequence of their elevated bioenergetic requirements. Here, we show for the first time that vulnerable nigral DA neurons differ from less vulnerable DA neurons such as those of the VTA (ventral tegmental area) by having a higher basal rate of mitochondrial OXPHOS (oxidative phosphorylation), a smaller reserve capacity, a higher density of axonal mitochondria, an elevated level of basal oxidative stress, and a considerably more complex axonal arborization. Furthermore, we demonstrate that reducing axonal arborization by acting on axon guidance pathways with Semaphorin 7A reduces in parallel the basal rate of mitochondrial OXPHOS and the vulnerability of nigral DA neurons to the neurotoxic agents MPP+ (1-methyl-4-phenylpyridinium) and rotenone. Blocking L-type calcium channels with isradipine was protective against MPP+ but not rotenone. Our data provide the most direct demonstration to date in favor of the hypothesis that the heightened vulnerability of nigral DA neurons in Parkinson’s disease is directly due to their particular bioenergetic and morphological characteristics

Increased vulnerability of nigral dopamine neurons after expansion of their axonal arborization size through D2 dopamine receptor conditional knockout

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Rare genetic mutations in genes such as Parkin, Pink1, DJ-1, α-synuclein, LRRK2 and GBA are found to be responsible for the disease in about 15% of the cases. A key unanswered question in PD pathophysiology is why would these mutations, impacting basic cellular processes such as mitochondrial function and neurotransmission, lead to selective degeneration of SNc DA neurons? We previously showed in vitro that SNc DA neurons have an extremely high rate of mitochondrial oxidative phosphorylation and ATP production, characteristics that appear to be the result of their highly complex axonal arborization. To test the hypothesis in vivo that axon arborization size is a key determinant of vulnerability, we selectively labeled SNc or VTA DA neurons using floxed YFP viral injections in DAT-cre mice and showed that SNc DA neurons have a much more arborized axon than those of the VTA. To further enhance this difference, which may represent a limiting factor in the basal vulnerability of these neurons, we selectively deleted in mice the DA D2 receptor (D2-cKO), a key negative regulator of the axonal arbour of DA neurons. In these mice, SNc DA neurons have a 2-fold larger axonal arborization, release less DA and are more vulnerable to a 6-OHDA lesion, but not to α-synuclein overexpression when compared to control SNc DA neurons. This work adds to the accumulating evidence that the axonal arborization size of SNc DA neurons plays a key role in their vulnerability in the context of PD

Evidence for Cold Accretion: Primitive Gas Flowing onto a Galaxy at z~0.274

We present UV and optical observations from the Cosmic Origins Spectrograph on the Hubble Space Telescope and Keck of a z= 0.27395 Lyman limit system (LLS) seen in absorption against the QSO PG1630+377. We detect H I absorption with log N(HI)=17.06\pm0.05 as well as Mg II, C III, Si III, and O VI in this system. The column densities are readily explained if this is a multi-phase system, with the intermediate and low ions arising in a very low metallicity ([Mg/ H] =-1.71 \pm 0.06) photoionized gas. We identify via Keck spectroscopy and Large Binocular Telescope imaging a 0.3 L_* star-forming galaxy projected 37 kpc from the QSO at nearly identical redshift (z=0.27406, \Delta v = -26 \kms) with near solar metallicity ([O/ H]=-0.20 \pm 0.15). The presence of very low metallicity gas in the proximity of a near-solar metallicity, sub-L_* galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A search of the literature reveals that such low metallicity LLSs are not uncommon. We found that 50% (4/8) of the well-studied z < 1 LLSs have metallicities similar to the present system and show sub-L_* galaxies with rho < 100 kpc in those fields where redshifts have been surveyed. We argue that the properties of these primitive LLSs and their host galaxies are consistent with those of cold mode accretion streams seen in galaxy simulations.Comment: Accepted for publication in the Astrophysical Journa

A Hubble Space Telescope Study of Lyman Limit Systems: Census and Evolution

We present a survey for optically thick Lyman limit absorbers at z<2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs) increasing the number of catalogued LLSs at z<2.6 by a factor of ~10. We compile a statistical sample of 50 tau_LLS > 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z)=dn/dz, at these redshifts is well described by a single power law, l(z) = C1 (1+z)^gamma, with gamma=1.33 +/- 0.61 at z<2.6, or with gamma=1.83 +/- 0.21 over the redshift range 0.2 < z < 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ~1.5 over the ~0.6 Gyr from z=4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ~8 Gyr from z=2.6 to 0.25. We show that the column density distribution function, f(N(HI)), at low redshift is not well fitted by a single power law index (f(N(HI)) = C2 N(HI)^(-beta)) over the column density range 13 17.2. While low and high redshift f(N(HI)) distributions are consistent for log N(HI)>19.0, there is some evidence that f(N(HI)) evolves with z for log N(HI) < 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ~9 from z~5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L* galaxies.Comment: Accepted by Ap

Genomic Convergence among ERRα, PROX1, and BMAL1 in the Control of Metabolic Clock Outputs

Metabolic homeostasis and circadian rhythms are closely intertwined biological processes. Nuclear receptors, as sensors of hormonal and nutrient status, are actively implicated in maintaining this physiological relationship. Although the orphan nuclear receptor estrogen-related receptor α (ERRα, NR3B1) plays a central role in the control of energy metabolism and its expression is known to be cyclic in the liver, its role in temporal control of metabolic networks is unknown. Here we report that ERRα directly regulates all major components of the molecular clock. ERRα-null mice also display deregulated locomotor activity rhythms and circadian period lengths under free-running conditions, as well as altered circulating diurnal bile acid and lipid profiles. In addition, the ERRα-null mice exhibit time-dependent hypoglycemia and hypoinsulinemia, suggesting a role for ERRα in modulating insulin sensitivity and glucose handling during the 24-hour light/dark cycle. We also provide evidence that the newly identified ERRα corepressor PROX1 is implicated in rhythmic control of metabolic outputs. To help uncover the molecular basis of these phenotypes, we performed genome-wide location analyses of binding events by ERRα, PROX1, and BMAL1, an integral component of the molecular clock. These studies revealed the existence of transcriptional regulatory loops among ERRα, PROX1, and BMAL1, as well as extensive overlaps in their target genes, implicating these three factors in the control of clock and metabolic gene networks in the liver. Genomic convergence of ERRα, PROX1, and BMAL1 transcriptional activity thus identified a novel node in the molecular circuitry controlling the daily timing of metabolic processes

Understanding the circumgalactic medium is critical for understanding galaxy evolution

Galaxies evolve under the influence of gas flows between their interstellar medium and their surrounding gaseous halos known as the circumgalactic medium (CGM). The CGM is a major reservoir of galactic baryons and metals, and plays a key role in the long cycles of accretion, feedback, and recycling of gas that drive star formation. In order to fully understand the physical processes at work within galaxies, it is therefore essential to have a firm understanding of the composition, structure, kinematics, thermodynamics, and evolution of the CGM. In this white paper we outline connections between the CGM and galactic star formation histories, internal kinematics, chemical evolution, quenching, satellite evolution, dark matter halo occupation, and the reionization of the larger-scale intergalactic medium in light of the advances that will be made on these topics in the 2020s. We argue that, in the next decade, fundamental progress on all of these major issues depends critically on improved empirical characterization and theoretical understanding of the CGM. In particular, we discuss how future advances in spatially-resolved CGM observations at high spectral resolution, broader characterization of the CGM across galaxy mass and redshift, and expected breakthroughs in cosmological hydrodynamic simulations will help resolve these major problems in galaxy evolution.Comment: Astro2020 Decadal Science White Pape