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Les astrocytes, ces auxiliaires indispensables des neurones

Posted on March 19, 2019 at 11:40 AM

Une étude de 2017 suggère que les astrocytes (un type de cellules gliales en forme d’étoile) pourraient finalement jouer un rôle bien plus important qu’on ne le pensait dans notre corps, notamment en régulant notre rythme circadien.



Un astrocyte est une cellule gliale du système nerveux central. Nous savions qu’ils jouent des rôles fonctionnels importants en participant notamment à la gliose, le phénomène de cicatrisation des lésions du système nerveux. Ils interviennent aussi au niveau des synapses pour capturer les neurotransmetteurs ou au niveau des capillaires pour assurer la barrière hématoencéphalique. Ils ont aussi un rôle de transport de molécules et d’approvisionnement des neurones en lactate. En revanche, nous pensions que les astrocytes n’avaient qu’un rôle de soutien et de structure du système nerveux central, mais une étude récente menée par des chercheurs de l’Université de Washington à St Louis suggère que les astrocytes joueraient finalement un rôle majeur en régulant notre rythme circadien.

Jusqu’à récemment, la recherche sur les horloges biologiques ne s’est concentrée que sur les neurones. On a longtemps considéré notre horloge interne comme étant contrôlée par les noyaux suprachiasmatiques (SCN), une région du cerveau dans l’hypothalamus composée d’environ 20 000 neurones. Mais nous savons également que près de 6 000 cellules astrocytes siègent aussi dans la même zone même si leur fonction exacte n’a jamais été pleinement comprise. Considérés comme « secondaires », ils seraient en fait plus importants qu’on ne le pensait. En isolant les astrocytes des neurones avec lesquels ils étaient entrelacés, les chercheurs ont aujourd’hui pu modifier les horloges des astrocytes chez des souris et surveiller ensuite leur comportement. Et quelle ne fut pas leur surprise !


Par fluorescence, les scientifiques ont en effet pu constater que les astrocytes expriment le gène d’horloge selon un modèle rythmique : en d’autres termes, les astrocytes gardent la notion du temps dans les tissus vivants où ils interagissent les uns avec les autres et avec les neurones. Enfin, lorsque les scientifiques ont supprimé le gène d’horloge dans les astrocytes via le nouvel outil d’édition de gènes CRISPR-Cas9, ils se sont alors rendu compte que l’horloge interne des souris s’était ralentie et décalée d’environ une heure par jour.

Les scientifiques ne savent toujours pas comment les astrocytes interagissent avec les neurones dans cette fonction de maîtrise du temps. Bien sûr, nous ne pouvons pas encore garantir que les astrocytes réglementent également les horloges du corps de la même manière chez l’Homme. De futures études devraient bientôt nous donner la réponse.

Un rôle nourricier et épurateur

« On s'est intéressé aux neurones parce qu'on pouvait écouter leur activité électrique (l'influx nerveux). Les astrocytes semblaient inertes, dit Pierre Magistratti, considéré comme le spécialiste mondial du sujet. Ce n'est que depuis vingt à vingt-cinq ans, avec de nouvelles techniques de microscopie, qu'on a découvert leur activité et leur biochimie propre. »

Pour résumer, les neurones communiquent rapidement de point à point par leurs synapses, des zones de connexion. Les astrocytes, dont chacun est en contact avec plusieurs neurones et environ 10 000 synapses, sont traversés par des « tsunamis de calcium », qui se diffusent plus lentement d'astrocyte en astrocyte, mais sur une zone étendue. Les chercheurs prennent pour analogie, le téléphone, qui relie précisément deux points, et la radio qui diffuse simultanément son message à des centaines de récepteurs.

Le rôle nourricier et épurateur des astrocytes, à la fois en contact avec les vaisseaux sanguins et les neurones, a été reconnu dans les années 1990. « Ils restaient cependant le petit personnel des neurones », sourit Yves Agid. Depuis dix ans, des études vont beaucoup plus loin. Au-delà de leur rôle de chef d'orchestre de la plasticité neuronale (la capacité de notre cerveau à s'adapter). Les astrocytes joueraient notamment un rôle dans le sommeil, l'apprentissage, la mémorisation, voire dans d'autres fonctions supérieures.

Une étude (presque trop belle) de 2007 a montré que la capacité mémorielle de souriceaux était multipliée si on leur injectait des astrocytes humains (plus complexes que ceux des autres espèces). D'autres études confirment qu'une altération des astrocytes empêche la mémorisation. Les astrocytes, siège de la pensée ? Les scientifiques ne vont pas jusque-là mais avancent que neurones et astrocytes forment un véritable couple, en symbiose.

Au-delà d'un modèle renouvelé du cerveau, le rôle négligé des astrocytes pourrait expliquer pourquoi les essais thérapeutiques de prévention des maladies neurodégénératives (Alzheimer, Parkinson...) ont tous été, jusqu'ici, des échecs. Pierre Magistratti et Yves Agid l'assurent : « Les astrocytes sont une cible pour des thérapeutiques. Une nouvelle voie. »

VIDEO : https://www.youtube.com/watch?v=64MgiEDWyRg

Sources :

L'homme glial, (Ed. Odile Jacob, 190 p., 23 €;)

Site sciencepost.fr

Journal Ouest-France "les Mystères de la Glie" (Philippe Richard, mars 2018 )

USCD Jacobs (School of Engineering)

Anatomy Zone (Video)


Categories: Ce que la science peine à expliquer, Santé et plantes, Insolite mais vrai !

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