Conséquences fonctionnelles, comportementales et adaptatives d'une mutation de la MAO (MonoAmine Oxydase) chez le poisson cavernicole aveugle Astyanax mexicanus.

par Constance Pierre

Projet de thèse en Sciences de la vie et de la santé

Sous la direction de Sylvie Retaux.

Thèses en préparation à Paris Saclay , dans le cadre de Signalisations et Réseaux Intégratifs en Biologie , en partenariat avec Neuro-PSI - Institut des Neurosciences Paris Saclay (laboratoire) et de Université Paris-Sud (établissement de préparation de la thèse) depuis le 01-10-2016 .


  • Résumé

    Le cerveau de vertébrés a subi une énorme diversification au cours de l'évolution. L'équipe utilise le poisson Astyanax mexicanus pour comprendre cette diversification à l'échelle micro évolutive. Ce poisson existe sous deux populations : une population de surface et une population cavernicole. Les poissons cavernicoles présentent différents traits adaptatifs en lien avec leur milieu de vie : ils sont aveugles, dépigmentés, ont une activité de recherche constante de nourriture, ne nagent pas en banc, dorment très peu, etc... une mutation dans le gène de la monoamine oxydase (MAO) a été détectée dans des populations cavernicoles qui pourrait peut-être être à l'origine des différences de comportements entre poisson de surface et cavernicole. Cette enzyme dégrade la sérotonine. Cette mutation cause une réduction de l'activité enzymatique de cette protéine et une hausse des niveaux de sérotonine dans le cerveau des poissons cavernicoles. Quelles sont les conséquences comportementales et fonctionnelles de cette mutation dans la MAO ? Pour répondre à cette question, il sera généré par édition du génome ou par back cross un poisson de surface avec cette mutation. La neurochimie, le comportement et les réseaux neuronaux de ce poisson seront comparés avec ceux des poissons de surface normaux (non mutés dans la MAO), des poissons cavernicoles normaux (mutés), et des poissons cavernicoles sans la mutation. Le caractère adaptatif de cette mutation sera aussi étudié. Ce projet permettra donc d'étudier les relations génotype-phénotype dans un contexte évolutif et de mieux comprendre les mécanismes à l'origine de l'évolution et de la diversification des cerveaux des vertébrés.

  • Titre traduit

    Functional, behavioral and adaptive consequences of the mutation in MAO (Mono Amine Oxidase) in the blind cavefish Astyanax mexicanus


  • Résumé

    We are interested in the molecular and developmental basis of the evolution of brain and behavior. We work on a teleost fish, Astyanax mexicanus (similar to zebrafish in terms of animal model). Within this species, some populations correspond to normal fish living in rivers, while other populations of cavefish have evolved in the complete darkness of caves, and have adapted to this environmental change. Cavefish differ from surface fish in many aspects: they are blind because they lose their eyes during development; they are de-pigmented; and they have evolved a number of adaptive behaviors. In particular, cavefish have evolved a persistent food searching behavior that probably augments their chances to find food in the dark; they have lost the aggressiveness that is a trademark of their surface counterparts; they have lost the schooling behavior; and they sleep very little. Of note, all these changes are not due to their loss of vision, but are rather genetically encoded. Recently, we have found that the MAO enzyme (Mono Amine Oxidase, the serotonin degrading enzyme) carries a point mutation in its coding sequence in cavefish, changing a proline into a leucine, and causing a two-fold reduction of its enzymatic activity. Thus, cavefish show very high levels of serotonin in their brains. Their condition would be considered as strongly pathological in humans, knowing the central role of serotonin in controlling mood, emotions, stress, and social behavior. Quite surprisingly however, we have further found that MAO is mutated in several populations of cavefish (leaving in different, geographically distinct, caves). This suggests that the MAO mutation might actually have been selected in caves, and might therefore be advantageous and adaptive in the cave environment. However we have also found that the MAO mutation is not fixed in the cavefish population that we use in the laboratory, so that a few cavefish individuals carry wild-type MAO alleles. The next questions are: 1) What are the exact consequences, in the cavefish brain, of this MAO mutation? 2) And has this mutation been selected in caves for its adaptive value? To answer the first question, the student will generate a transgenic line of surface Astyanax which carries a leucine to proline mutation in MAO (to mimic the cavefish genotype), using the CRISPER technology for genome editing. Cavefish lines either homozygous or heterozygous for the mutation, or homozygous for the wild-type allele will be obtained by selected crosses between cavefish individuals. On these lines as well on normal surface fish and heterozygote F1 hybrids resulting from crosses between a cavefish and a surface fish, the student will perform: 1) Functional biochemical characterization of these various lines, by measurements of their brain serotonin (and other monoamines and metabolites) levels and MAO enzymatic activity. 2) Behavioral characterization of these various lines, by measurements of their feeding, social, schooling, locomotor, and sleeping behavior. These experiments will tell us whether the MAO mutation in cavefish is, or not, or in part, responsible for the so-called “cavefish behavioral syndrome”. To answer the second question, the student will use surface fish samples (fin clips) collected in the wild and test whether the MAO mutated allele can be found (at very low frequency) in the surface fish populations inhabiting the rivers in the region of Mexico where caves are located. This would suggest that the MAO mutation has been selected in various caves from the standing genetic variation existing in the natural surface population (and did not arise by de novo mutation in cave populations). If time permits, a search for marks of selective sweep on the MAO locus will be performed. This would clearly indicate that the MAO mutation has been recently and repeatedly selected in different caves for its adaptive value.