Acid-sensing ion channels : modulation by serine-proteases and involvement in acid-induced action potential generation in hippocampal neurons

SUMMARY Acid-sensing ion channels (ASICs) are non-voltage gated sodium channels. They are activated by rapid extracellular acidification and generate an inactivating inward current. Four ASIC genes have been cloned: ASIC1, 2, 3 and 4, with variants a and b for ASIC1and AS1C2. ASICs are expressed in neurons of the central (CNS) and peripheral nervous system (PNS). In the CNS, ASICs have a role in learning, memory, as well as in neuronal death in ischemia. In the PNS, ASICs are involved in the perception of acid-induced pain, as well as in mechanoperception. In one part of my thesis project, we addressed the question of the mechanism of regulation of ASIC1 a by the serine protease trypsin at the molecular level. Trypsin modifies the function of ASIC1 a but not of ASIC1b. In order to identify the channel region responsible for this effect, we created chimeras between ASIC1 a and 1b. Subsequently, to identify the exact trypsin target(s), we mutated predicted trypsin sites in the region identified by the chimera. In the second part of a project, we investigated the role of ASICs at the cellular level, in neuronal signaling. Using the whole-cell patch clamp in hippocampal neuronal culture, we studied the potential involvement of ASICs in action potential (AP) generation. In the first part of the thesis work, we showed that trypsin modifies ASIC1a function: it shifts the pH activation and the steady-state inactivation curve towards more acidic values and accelerates the time course of the channel recovery from inactivation. We also showed that trypsin cleaves ASIC1a and that the functional effect and a channel cleavage correlate. In the inactivated state, channels cannot be modified by trypsin. Cleavage occurs in a channel region that is also important for inactivation of all ASICs; a part of this region is critical for the inhibition of ASIC1 a by the spider toxin Psalmotoxin1. In the second part of the thesis work, we showed that ASIC activity can modulate AP generation. ASIC activity by itself can induce trains of APs. In situations in which this activity by itself is not sufficient to induce APs, it can contribute to AP generation. During high neuronal activity, ASIC activity can block already existing trains of APs. In conclusion, depending on the activity of neuron in a particular moment, ASICs can differently modulate AP generation; they can induce, facilitate or inhibit APs. We also showed that trypsin changes the capability of ASICs to modulate AP generation by shifting the pH dependence to more acidic values, which adapts channel gating to pH conditions which may occur in pathological conditions such as ischemia. Our finding that trypsin modifies ASIC1 a function identifies a novel pharmacological tool, and proposes a mechanism of ASIC1a regulation that may have a physiological importance. The identification of the exact site of trypsin action gives insight to the molecular mechanisms of ASIC regulation. This work proposes a role in modulation of AP generation for ASICs in the CNS. RESUME Les canaux ASIC sont les canaux ioniques activés par l'acidification rapide extracellulaire. Activés, ils génèrent un courant entrant qui inactive en présence de stimulus acide. Quatre gènes ASIC ont été clonés, ASIC1, 2, 3 et 4, avec les variants a et b pour ASIC1 et 2. Les ASICs sont exprimés dans les neurones du système nerveux central (SNC) et périphérique (SNP). Dans le SNC, les ASIC ont un rôle dans le mémoire, apprentissage et la mort neuronale dans t'ischémie. Dans le SNP, ils ont un rôle dans la perception de la douleur et méchanosensation. Dans une partie de mon projet de thèse, nous avons étudié les mécanismes de la régulation d'ASIC1a par la sérine-protéase trypsine au niveau moléculaire. La trypsine modifie la fonction d'ASIC1a et pas ASIC1b. Nous avons créé les chimères entre ASIC1 a et 1 b, afin d'identifier la région du canal responsable pour l'effet. Pour identifier le(s) site(s) exactes de l'action de la trypsine, nous avons muté les sites potentiels de la trypsine dans la région identifiée par les chimères. Dans la deuxième partie du projet, nous avons étudié le rôle des ASICs au niveau cellulaire. En utilisant la technique du patch clamp dans les cultures des neurones de l'hippocampe, nous avons étudié l'implication des ASICs dans la génération des potentiels d'action (PA). Nous avons montré que la trypsine agit sur le canal ASIC1a ; elle décale l'activation et « steady-state » inactivation vers les valeurs plus acides, et elle raccourcit le temps du « recovery » du canal. La trypsine coupe ASIC1a sur le résidu K145 et l'effet fonctionnel et la coupure corrèlent. Nous avons identifié la région du canal responsable pour l'inactivation de tous les ASICs ; une partie de cette région est responsable pour ['inhibition d'ASIC1 a par la Psalmotoxinel . Nous avons montré que les ASICs peuvent moduler la génération des PAs. L'activité des ASICs peut induire les trains des PAs. Quand l'activité des ASICs n'est pas suffisante pour induire le PA, elle peut contribuer à sa génération. Pendant l'activité neuronale forte, l'activité des ASICs peut bloquer les trains des PAs qui existent déjà. En conclusion, dépendant de l'activité neuronale, les ASICs peuvent moduler la génération des PAs différemment ; ils peuvent induire, faciliter ou inhiber les PAs. La trypsine change la capacité des ASICs de moduler les PAs. Après l'action de la trypsine, les ASICs peuvent moduler la génération des PAs dans les conditions légèrement acides, suivies par les fluctuations du pH acide, qui peuvent exister dans l'ischémie. Le fait que la trypsine agit sur ASIC1a définit l'outil pharmacologique et propose le mécanisme de la régulation d'ASICI a qui pourrait avoir l'importance physiologique. L'identification du site de l'action de la trypsine éclaircit les mécanismes moléculaires de la régulation des ASICs. Cette étude propose un rôle des ASICs dans la modulation de la génération des PAs. Résumé pour le public large Les neurones sont les cellules de système nerveux dont la fonction est la signalisation. Comme toutes les autres cellules, les neurones ont une membrane qui sépare l'intérieur du milieu extérieur. Cette membrane est imperméable pour des particules chargées (ions). Dans cette membrane existent les protéines spécifiques, « canaux », qui permettent le transport des ions d'un côté de la membrane à l'autre, comme réponse aux stimuli différents. Ce transport des ions à travers la membrane génère un courant, qu'on peut mesurer. Ce courant est la base de la communication entre les neurones, ou, ce qu'on appelle la signalisation neuronale. Quand ce courant est suffisamment grand, il permet la génération du potentiel d'action, qui est le message principal de communication neuronale. Les canaux ASIC (acid-sensing ion channel), que nous étudions dans le laboratoire, sont activés par les acides. Les acides sont relâchés dans beaucoup de situations dans le système nerveux. Les ASIC ont été découverts récemment (en 1996), et nous ne connaissons pas encore très bien toutes les fonctions de ces canaux. Nous savons qu'ils ont un rôle dans le mémoire, apprentissage, la sensation de la douleur et l'infarctus cérébral. Dans la première partie de ce projet de thèse, nous avons voulu mieux comprendre comment fonctionnent ces canaux. Pour faire ça, nous avons étudié la régulation des ASICs par une protéine, trypsine, qui coupe le canal ASIC. Nous avons étudié ou exactement la trypsine coupe le canal et quels effets ça produit sur la fonction du canal. Dans la deuxième partie du projet de thèse, nous avons voulu mieux connaître comment le canal fonctionne au niveau de la cellule, comment il interagit avec les autres canaux et si il a un rôle dans la génération des potentiels d'action. Nous avons pu montrer que la trypsine change la fonction du canal, ce qui lui permet de fonctionner différemment. Nous avons aussi déterminé ou exactement ta trypsine coupe le canal. Au niveau de la cellule, nous avons montré que les ASIC peuvent moduler la génération des potentiels d'action, étant, dépendant de l'activité du neurone, soit activateurs, soit inhibiteurs. La trypsine est une molécule qui peut être libérée dans le système nerveux pendant certaines conditions, comme l'infarctus cérébral. A cause de ça, les connaissances que la trypsine agit sur le anal ASIC pourraient être important physiologiquement. La connaissance de l'endroit exacte ou la trypsine coupe le canal nous aide à mieux comprendre la relation structure-fonction du canal. La modulation de la génération des potentiels d'actions par les ASIC indique que ces canaux peuvent avoir un rôle important dans la signalisation neuronale.

Combined simulation and mutagenesis analyses reveal the involvement of key residues for peroxisome proliferator-activated receptor alpha helix 12 dynamic behavior.

The dynamic properties of helix 12 in the ligand binding domain of nuclear receptors are a major determinant of AF-2 domain activity. We investigated the molecular and structural basis of helix 12 mobility, as well as the involvement of individual residues with regard to peroxisome proliferator-activated receptor alpha (PPARalpha) constitutive and ligand-dependent transcriptional activity. Functional assays of the activity of PPARalpha helix 12 mutants were combined with free energy molecular dynamics simulations. The agreement between the results from these approaches allows us to make robust claims concerning the mechanisms that govern helix 12 functions. Our data support a model in which PPARalpha helix 12 transiently adopts a relatively stable active conformation even in the absence of a ligand. This conformation provides the interface for the recruitment of a coactivator and results in constitutive activity. The receptor agonists stabilize this conformation and increase PPARalpha transcription activation potential. Finally, we disclose important functions of residues in PPARalpha AF-2, which determine the positioning of helix 12 in the active conformation in the absence of a ligand. Substitution of these residues suppresses PPARalpha constitutive activity, without changing PPARalpha ligand-dependent activation potential.

Primary motor cortex involvement in Alzheimer disease.

In Alzheimer disease (AD) the involvement of entorhinal cortex, hippocampus, and associative cortical areas is well established. Regarding the involvement of the primary motor cortex the reported data are contradictory. In order to determine whether the primary motor cortex is involved in AD, the brains of 29 autopsy cases were studied, including, 17 cases with severe cortical AD-type changes with definite diagnoses of AD, 7 age-matched cases with discrete to moderate cortical AD-type changes, and 5 control cases without any AD-type cortical changes. Morphometric analysis of the cortical surface occupied by senile plaques (SPs) on beta-amyloid-immunostained sections and quantitative analysis of neurofibrillary tangles (NFTs) on Gallyas-stained sections was performed in 5 different cortical areas including the primary motor cortex. The percentage of cortical surface occupied by SPs was similar in all cortical areas, without significant difference and corresponded to 16.7% in entorhinal cortex, 21.3% in frontal associative, 16% in parietal associative, and 15.8% in primary motor cortex. The number of NFTs in the entorhinal cortex was significantly higher (41 per 0.4 mm2), compared with those in other cortical areas (20.5 in frontal, 17.9 in parietal and 11.5 in the primary motor cortex). Our findings indicate that the primary motor cortex is significantly involved in AD and suggest the appearance of motor dysfunction in late and terminal stages of the disease.

Exome sequencing identifies DYNC2H1 mutations as a common cause of asphyxiating thoracic dystrophy (Jeune syndrome) without major polydactyly, renal or retinal involvement.

BACKGROUND: Jeune asphyxiating thoracic dystrophy (JATD) is a rare, often lethal, recessively inherited chondrodysplasia characterised by shortened ribs and long bones, sometimes accompanied by polydactyly, and renal, liver and retinal disease. Mutations in intraflagellar transport (IFT) genes cause JATD, including the IFT dynein-2 motor subunit gene DYNC2H1. Genetic heterogeneity and the large DYNC2H1 gene size have hindered JATD genetic diagnosis. AIMS AND METHODS: To determine the contribution to JATD we screened DYNC2H1 in 71 JATD patients JATD patients combining SNP mapping, Sanger sequencing and exome sequencing. RESULTS AND CONCLUSIONS: We detected 34 DYNC2H1 mutations in 29/71 (41%) patients from 19/57 families (33%), showing it as a major cause of JATD especially in Northern European patients. This included 13 early protein termination mutations (nonsense/frameshift, deletion, splice site) but no patients carried these in combination, suggesting the human phenotype is at least partly hypomorphic. In addition, 21 missense mutations were distributed across DYNC2H1 and these showed some clustering to functional domains, especially the ATP motor domain. DYNC2H1 patients largely lacked significant extra-skeletal involvement, demonstrating an important genotype-phenotype correlation in JATD. Significant variability exists in the course and severity of the thoracic phenotype, both between affected siblings with identical DYNC2H1 alleles and among individuals with different alleles, which suggests the DYNC2H1 phenotype might be subject to modifier alleles, non-genetic or epigenetic factors. Assessment of fibroblasts from patients showed accumulation of anterograde IFT proteins in the ciliary tips, confirming defects similar to patients with other retrograde IFT machinery mutations, which may be of undervalued potential for diagnostic purposes.

Involvement of connexin 43 in meiotic maturation of bovine oocytes.

In ovarian follicles, cumulus cells provide the oocyte with small molecules that permit growth and control maturation. These nutrients reach the germinal cell through gap junction channels, which are present between the cumulus cells and the oocyte, and between the cumulus cells. In this study the involvement of intercellular communication mediated by gap junction channels on oocyte maturation of in vitro cultured bovine cumulus-oocyte complexes (COCs) was investigated. The stages of oocyte maturation were determined by Hoechst 33342 staining, which showed that 90% of COCs placed in the maturation medium for 24 h progress to the metaphase II stage. Bovine COC gap junction communication was disrupted initially using n-alkanols, which inhibit any passage through gap junctions. In the presence of 1-heptanol (3 mmol l(-1)) or octanol (3.0 mmol l(-1) and 0.3 mmol l(-1)), only 29% of the COCs reached metaphase II. Removal of the uncoupling agent was associated with restoration of oocyte maturation, indicating that treatment with n-alkanols was neither cytotoxic nor irreversible. Concentrations of connexin 43 (Cx43), the major gap junction protein expressed in the COCs, were decreased specifically using a recombinant adenovirus expressing the antisense Cx43 cDNA (Ad-asCx43). The efficacy of adenoviral infection was > 95% in cumulus cells evaluated after infection with recombinant adenoviruses expressing the green fluorescence protein. RT-PCR performed on total RNA isolated from Ad-asCx43-infected COCs showed that the rat Cx43 cDNA was transcribed. Western blot analysis revealed a three-fold decrease in Cx43 expression in COCs expressing the antisense RNA for Cx43. Injection of cumulus cells with Lucifer yellow demonstrated further that the resulting lower amount of Cx43 in infected COCs is associated with a two-fold decrease in the extent of coupling between cumulus cells. In addition, oocyte maturation was decreased by 50% in the infected COC cultures. These results indicate that Cx43-mediated communication between cumulus cells plays a crucial role in maturation of bovine oocytes.

Altered growth in male peroxisome proliferator-activated receptor gamma (PPARgamma) heterozygous mice: involvement of PPARgamma in a negative feedback regulation of growth hormone action.

The peroxisome proliferator-activated receptor gamma (PPARgamma) plays a major role in fat tissue development and physiology. Mutations in the gene encoding this receptor have been associated to disorders in lipid metabolism. A thorough investigation of mice in which one PPARgamma allele has been mutated reveals that male PPARgamma heterozygous (PPARgamma +/-) mice exhibit a reduced body size associated with decreased body weight, reflecting lean mass reduction. This phenotype is reproduced when treating the mice with a PPARgamma- specific antagonist. Monosodium glutamate treatment, which induces weight gain and alters body growth in wild-type mice, further aggravates the growth defect of PPARgamma +/- mice. The levels of circulating GH and that of its downstream effector, IGF-I, are not altered in mutant mice. However, the IGF-I mRNA level is decreased in white adipose tissue (WAT) of PPARgamma +/- mice and is not changed by acute administration of recombinant human GH, suggesting an altered GH action in the mutant animals. Importantly, expression of the gene encoding the suppressor of cytokine signaling-2, which is an essential negative regulator of GH signaling, is strongly increased in the WAT of PPARgamma +/- mice. Although the relationship between the altered GH signaling in WAT and reduced body size remains unclear, our results suggest a novel role of PPARgamma in GH signaling, which might contribute to the metabolic disorder affecting insulin signaling in PPARgamma mutant mice.

Sentinel lymph node involvement and a high Breslow index are independent factors of risk for early relapse of melanoma

Rapport de synthèse : Le ganglion sentinelle (GS) se défini comme le premier ganglion de la chaîne ganglionnaire qui draine le territoire anatomique où siège une tumeur et, par conséquent, celui ayant le plus de possibilités de recevoir des métastases. La combinaison des deux techniques de détection du GS existantes, lymphoscintigraphie et coloration, permettent de déceler le GS dans 95-100% des cas. Le taux d'attente métastatique du GS varie entre 16 et 21 % des patients. Dans 50 à 87% des cas, le GS est le seul site de métastase et la probabilité de trouver des micro-métastases dans des ganglions appartenant aux relais supérieurs sans atteinte du GS est estimée à moins de 2%. Ces chiffres relèvent l'importance de la détection du GS. L'emploi de cette technique offre de nombreux avantages par rapport à la lymphadénéctomie élective que nous décrirons. Selon Rousseau et al., il existe une probable association entre le statut du GS et la survie de la maladie. Cette interprétation et celles d'autres auteurs soulignent la pertinence clinique du statut du GS dans le mélanome. En ce qui concerne la survie sans maladie (DFS) et la survie globale (OS), aucune différence significative n'a été observée entre les patients ayant subi une résection complète immédiate des ganglions lymphatiques et ceux qui d'abord ont subi une résection chirurgicale et analyse du GS secondaire, suivies par une dissection élective en cas de positivité. L'objectif de cette étude prospective était d'évaluer la pertinence de la positivité tumorale du GS dans l'évaluation des risques de rechute du mélanome. Cette étude a confirmé l'intérêt de la scintigraphie des ganglions lymphatiques (associée à la technique de coloration par bleu et celle de détection par sonde portable) dans l'identification du GS comme approche thérapeutique au stade précoce du mélanome. Elle a montré, en autre, que le statut du GS et l'indice de Breslow sont des facteurs de risque indépendants importants de rechute chez des patients atteints d'un mélanome au stade précoce. La combinaison de ces deux paramètres a permis de créer des groupes de patients à risque de rechute différents qui pourraient conduire à l'adaptation des protocoles de thérapie en fonction de ces risques.

Paternal involvement in child caregiving and infant sociability

In this study, the influence of paternal involvement in caregiving on infant sociability was assessed using a strange situation paradigm adapted from the work of Ainsworth, Blehar, Waters, and Wall (1978). Thirty-seven families participated with their firstborn child (aged 12-14 months). According to the questionnaire, 20 nontraditional fathers were very involved in caretaking, and 17 fathers were less or not at all involved. Infant sociability was assessed using three of Ainsworth et al.'s rating scales: proximity or contact-seeking, avoidance, and distance interaction. Results indicated that infants of nontraditional fathers were globally more sociable with all their partners (father, mother, and stranger) than infants of traditional fathers. Furthermore, results suggested that it was not only the father-infant relationship or infant development which were affected by the amount of paternal involvement in daily caretaking but the family system as a whole. Indeed, infants from nontraditional families appeared to interact equally with their fathers and mothers in direct interaction. In addition, these infants interacted at a distance with a stranger as much in the presence of their fathers as in the presence of their mothers, thereby suggesting that both parents represented an equally secure base.

Conditional Involvement of CONSTITUTIVE PHOTOMORPHOGENIC1 in the Degradation of Phytochrome A.

All higher plants possess multiple phytochrome photoreceptors, with phytochrome A (phyA) being light labile and other members of the family being relatively light stable (phyB-phyE in Arabidopsis [Arabidopsis thaliana]). phyA also differs from other members of the family because it enables plants to deetiolate in far-red light-rich environments typical of dense vegetational cover. Later in development, phyA counteracts the shade avoidance syndrome. Light-induced degradation of phyA favors the establishment of a robust shade avoidance syndrome and was proposed to be important for phyA-mediated deetiolation in far-red light. phyA is ubiquitylated and targeted for proteasome-mediated degradation in response to light. Cullin1 and the ubiquitin E3 ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) have been implicated in this process. Here, we systematically analyze the requirement of cullins in this process and show that only CULLIN1 plays an important role in light-induced phyA degradation. In addition, the role of COP1 in this process is conditional and depends on the presence of metabolizable sugar in the growth medium. COP1 acts with SUPPRESSOR OF PHYTOCHROME A (SPA) proteins. Unexpectedly, the light-induced decline of phyA levels is reduced in spa mutants irrespective of the growth medium, suggesting a COP1-independent role for SPA proteins.

Involvement of glial cells in the neurotoxicity of parathion and chlorpyrifos.

An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to investigate the influence of glial cells on the neurotoxicity of two organophosphorus pesticides (OPs), chlorpyrifos and parathion. Mixed-cell aggregate cultures were treated continuously for 10 days between DIV 5 and 15. Parathion induced astrogliosis at concentration at which MAP-2 immunostaining, found here to be more sensitive than neuron-specific enzyme activities, was not affected. In contrast, chlorpyrifos induced a comparatively weak gliotic reaction, and only at concentrations at which neurons were already affected. After similar treatments, increased neurotoxicity of parathion and chlorpyrifos was found in aggregate cultures deprived of glial cells. These results suggest that glial cells provide neuroprotection against OPs toxicity. To address the question of the difference in toxicity between parathion and chlorpyrifos, the toxic effects of their leaving groups, p-nitrophenol and trichloropyridinol, were studied in mixed-cell aggregates. General cytotoxicity was more pronounced for trichloropyridinol and both compounds had similar toxic effects on neuron-specific enzyme activities. In contrast, trichloropyridinol induced a much stronger decrease in glutamine synthetase activity, the enzymatic marker of astrocytes. Trichloropyridinol may exert a toxic effect on astrocytes, compromising their neuroprotective function, thus exacerbating the neurotoxicity of chlorpyrifos. This is in line with the suggestion that glial cells may contribute to OPs neurotoxicity, and with the view that OPs may exert their neurotoxic effects through different mechanisms.