Cortical branched actin determines cell cycle progression

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Barhl2 maintient les facteurs de transcription T comme répresseurs et désactive ainsi la réponse de Wnt/β-Catenin entraînant la formation de l'organisateur de Spemann.

International audienceA hallmark of Wnt/β-Catenin signaling is the extreme diversity of its transcriptional response, which varies depending on the cell and developmental context. What controls this diversity is poorly understood. In all cases, the switch from transcriptional repression to activation depends on a nuclear increase in β-Catenin, which detaches the transcription factor T cell factor 7 like 1 (Tcf7l1) bound to Groucho (Gro) transcriptional co-repressors from its DNA-binding sites and transiently converts Tcf7/Lymphoid enhancer binding factor 1 (Lef1) into a transcriptional activator. One of the earliest and evolutionarily conserved functions of Wnt/β-Catenin signaling is the induction of the blastopore lip organizer. Here, we demonstrate that the evolutionarily conserved BarH-like homeobox-2 (Barhl2) protein stabilizes the Tcf7l1-Gro complex and maintains the repressed expression of Tcf target genes by a mechanism that depends on histone deacetylase 1 (Hdac-1) activity. In this way, Barhl2 switches off the Wnt/β-Catenin-dependent early transcriptional response, thereby limiting the formation of the organizer in time and/or space. This study reveals a novel nuclear inhibitory mechanism of Wnt/Tcf signaling that switches off organizer fate determination.L'une des caractéristiques de la signalisation de Wnt/β-Catenin est l'extrême diversité de sa réponse transcriptionnelle, qui varie en fonction de la cellule et du contexte de développement. Ce qui contrôle cette diversité est mal compris. Dans tous les cas, le passage de la répression transcriptionnelle à l'activation dépend d'une augmentation nucléaire de la β-Caténine, qui détache le facteur de transcription T (Tcf7l1) lié aux co-répresseurs transcriptionnels de Groucho (Gro) de ses sites de liaison ADN et transforme temporairement Tcf7 en un activateur de la transcription. L'une des premières fonctions de signalisation de Wnt/β-Catenin, conservée au cours de l'évolution, est l'induction de l'organiseur de lSpemann. Nous démontrons ici que la protéine BarH-like Homeobox-2 (Barhl2), conservée au cours de l'évolution, stabilise le complexe Tcf7l1-Gro et maintient l'expression réprimée des gènes cibles du Tcf par un mécanisme qui dépend de l'activité des histone désacétylases 1 (Hdac-1). De cette façon, Barhl2 désactive la réponse transcriptionnelle précoce dépendante de Wnt/β-Catenin, limitant ainsi la formation de l'organisateur dans le temps et/ou l'espace. Cette étude révèle un nouveau mécanisme d'inhibition nucléaire de la signalisation Wnt/Tcf qui désactive la détermination du sort de l'organisateur

AtREC8 and AtSCC3 are essential to the monopolar orientation of the kinetochores during meiosis

International audienceThe success of the first meiotic division relies (among other factors) on the formation of bivalents between homologous chromosomes, the monopolar orientation of the sister kinetochores at metaphase I and the maintenance of centromeric cohesion until the onset of anaphase II. The meiotic cohesin subunit, Rec8 has been reported to be one of the key players in these processes, but its precise role in kinetochore orientation is still under debate. By contrast, much less is known about the other non-SMC cohesin subunit, Scc3. We report the identification and the characterisation of AtSCC3, the sole Arabidopsis homologue of Scc3. The detection of AtSCC3 in mitotic cells, the embryo lethality of a null allele Atscc3-2, and the mitotic defects of the weak allele Atscc3-1 suggest that AtSCC3 is required for mitosis. AtSCC3 was also detected in meiotic nuclei as early as interphase, and bound to the chromosome axis from early leptotene through to anaphase I. We show here that both AtREC8 and AtSCC3 are necessary not only to maintain centromere cohesion at anaphase I, but also for the monopolar orientation of the kinetochores during the first meiotic division. We also found that AtREC8 is involved in chromosome axis formation in an AtSPO11-1-independent manner. Finally, we provide evidence for a role of AtSPO11-1 in the stability of the cohesin complex

CYFIP2 containing WAVE complexes inhibit cell migration

Branched actin networks polymerized by the Arp2/3 complex are critical for cell migration. The WAVE complex is the major Arp2/3 activator at the leading edge of migrating cells. However, multiple distinct WAVE complexes can be assembled in a cell, due to the combinatorial complexity of paralogous subunits. When systematically analyzing the contribution of each WAVE complex subunit to the metastasis-free survival of breast cancer patients, we found that overexpression of the CYFIP2 subunit was surprisingly associated with good prognosis. Gain and loss of function experiments in transformed and untransformed mammary epithelial cells revealed that cell migration was always inversely related to CYFIP2 levels. The role of CYFIP2 was systematically opposite to the role of the paralogous subunit CYFIP1 or of the NCKAP1 subunit. The specific CYFIP2 function in inhibiting cell migration was related to its unique ability to down-regulate classical pro-migratory WAVE complexes. The anti-migratory function of CYFIP2 was also revealed in migration of prechordal plate cells during gastrulation of the zebrafish embryo, indicating that the unique function of CYFIP2 is critically important in both physiological and pathophysiological migrations

Mafa-dependent GABAergic activity promotes mouse neonatal apneas

International audienceAbstract While apneas are associated with multiple pathological and fatal conditions, the underlying molecular mechanisms remain elusive. We report that a mutated form of the transcription factor Mafa (Mafa 4A ) that prevents phosphorylation of the Mafa protein leads to an abnormally high incidence of breath holding apneas and death in newborn Mafa 4A/4A mutant mice. This apneic breathing is phenocopied by restricting the mutation to central GABAergic inhibitory neurons and by activation of inhibitory Mafa neurons while reversed by inhibiting GABAergic transmission centrally. We find that Mafa activates the Gad2 promoter in vitro and that this activation is enhanced by the mutation that likely results in increased inhibitory drives onto target neurons. We also find that Mafa inhibitory neurons are absent from respiratory, sensory (primary and secondary) and pontine structures but are present in the vicinity of the hypoglossal motor nucleus including premotor neurons that innervate the geniohyoid muscle, to control upper airway patency. Altogether, our data reveal a role for Mafa phosphorylation in regulation of GABAergic drives and suggest a mechanism whereby reduced premotor drives to upper airway muscles may cause apneic breathing at birth

Restrained activation of CYFIP2-containing WAVE complexes controls membrane protrusions and cell migration

Abstract Branched actin networks polymerized by the Arp2/3 complex are critical for cell migration. The WAVE complex is the machinery that activates Arp2/3 in a RAC1-dependent manner at the leading edge of migrating cells. Multiple WAVE complexes are assembled in a cell through various combinations of paralogous subunits. Here we report the surprising phenotype associated with loss-of-function of CYFIP2, a subunit of the WAVE complex. In three different human mammary cell lines and in prechordal plate cells of gastrulating zebrafish embryos, CYFIP2 depletion promoted, rather than impaired, membrane protrusions and migration persistence. CYFIP2, however, assembled WAVE complexes that polymerize branched actin at the cell cortex and rescued membrane protrusions of CYFIP1/2 double knock-out cells, although less efficiently than CYFIP1. Point mutations of CYFIP2 associated with intellectual disability in children were gain-of-function, as they made CYFIP2 as active as CYFIP1 in this rescue experiment. Biochemical reconstitutions of CYFIP2-containing WAVE complexes showed that they bound equally well to active RAC1 as CYFIP1-containing WAVE complexes, yet they were poorly activated in response to RAC1 binding. Together these results suggest that CYFIP2-containing WAVE complexes titrate active RAC1 and thereby prevent efficient CYFIP1-containing complexes from being activated. In this context, where cell migration is governed by the balance of CYFIP1/2 expression, releasing the restrained activity of CYFIP2-containing WAVE complexes leads to pathology

The Arp1/11 minifilament of dynactin primes the endosomal Arp2/3 complex

Dendritic actin networks develop from a first actin filament through branching by the Arp2/3 complex. At the surface of endosomes, the WASH complex activates the Arp2/3 complex and interacts with the capping protein for unclear reasons. Here, we show that the WASH complex interacts with dynactin and uncaps it through its FAM21 subunit. In vitro, the uncapped Arp1/11 minifilament elongates an actin filament, which then primes the WASH-induced Arp2/3 branching reaction. In dynactin-depleted cells or in cells where the WASH complex is reconstituted with a FAM21 mutant that cannot uncap dynactin, formation of branched actin at the endosomal surface is impaired. Our results reveal the importance of the WASH complex in coordinating two complexes containing actin-related proteins

NRL and CRX Define Photoreceptor Identity and Reveal Subgroup-Specific Dependencies in Medulloblastoma

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