Biology subjects

Bourdais, A.

Publications and source records attributed to Bourdais, A..

6 recordsLinked to original sources

bioRxiv2023

The clathrin adaptor AP-1B independently controls proliferation and differentiation in the mammalian intestine.

Maintenance of the polarity of the epithelial cells facing the lumen of the small intestine is crucial to ensure the vectorial absorption of nutrients as well as the integrity of the apical brush border and the intestinal barrier. Polarized vesicular trafficking plays a key role in this process, and defective transport due to mutations in apical trafficking-related genes has been shown to affect nutrient absorption. Interestingly, it has been demonstrated that downregulation of the polarized sorting clathrin adaptor AP-1B led to both epithelial polarity and proliferation defects in the mouse intestine. This enlightened a new function of polarized trafficking in the gut epithelium and a novel link between trafficking, polarity, and proliferation. Here, using CRISPR-Cas9-mediated mutation of the AP-1B coding gene Ap1m2 in mouse intestinal organoids, we uncovered a novel proliferation pathway controlled by AP-1B. We showed that the polarity defects induced by Ap1m2 mutations led to a defective apical targeting of both Rab11+ apical recycling endosomes and of the polarity determinant Cdc42. Moreover, we showed that these polarity defects were accompanied by an induction of YAP and EGFR/mTOR-dependent proliferation pathways. Finally, we showed that AP-1B additionally controlled a proliferation-independent differentiation pathway towards the secretory lineage. Overall, our results highlighted the pleiotropic roles played by AP-1B in the homeostasis of the gut epithelium.

cell biology↗

bioRxiv2022

Ectopic activation of the polar body extrusion pathway triggers cell fragmentation in preimplantation embryos

Cell fragmentation occurs during physiological processes, such as apoptosis, migration, or germ cell development. Fragmentation is also commonly observed during preimplantation development of human embryos and is associated with poor implantation prognosis during Assisted Reproductive Technology (ART) procedures. Despite its biological and clinical relevance, the mechanisms leading to cell fragmentation are unclear. Light sheet microscopy imaging of mouse embryos reveals that compromised spindle anchoring, due to Myo1c knockout or dynein inhibition, leads to fragmentation. We further show that defective spindle anchoring brings DNA in close proximity to the cell cortex, which, in stark contrast to previous reports in mitotic cells, locally triggers actomyosin contractility and pinches off cell fragments. The activation of actomyosin contractility by DNA in preimplantation embryos is reminiscent of the signals mediated by small GTPases throughout polar body extrusion (PBE) during meiosis. By interfering with the signals driving PBE, we find that this meiotic signaling pathway remains active during cleavage stages and is both required and sufficient to trigger fragmentation. Together, we find that fragmentation happens in mitosis after ectopic activation of actomyosin contractility by signals emanating from DNA, similar to those observed during meiosis. Our study uncovers the mechanisms underlying fragmentation in preimplantation embryos and, more generally, offers insight into the regulation of mitosis during the maternal-zygotic transition.

developmental biology↗

bioRxiv2022

V0-ATPase downregulation induces MVID-like brush border defects independently of apical trafficking in the mammalian intestine

SummaryIntestinal microvillus atrophy is a major cause of enteropathies such as idiopathic or congenital diarrhea that are often associated with severe morbidity. It can be caused by genetic disorders, inflammatory diseases, toxins or pathogens. In particular, Microvillus inclusion disease (MVID) is characterized by a chronic intractable diarrhea and a severe microvillus atrophy. It is triggered by mutations in MYO5B, STX3, MUNC18.2 or UNC45A which alter epithelial polarity by affecting apical trafficking in intestinal epithelial cells. Furthermore, we recently established that the depletion of the V0 sector of the V-ATPase complex induces an MVID-like phenotype in C. elegans. In this study we investigated the function of the V0-ATPase complex in mouse intestinal organoids. We found that its depletion also triggers a very severe microvillus atrophy in this model. Furthermore, we established that the polarity of intestinal cells is affected in a patient carrying mutations in TCIRG1 which encodes a V0-ATPase subunit. However, V0- ATPase depletion does not recapitulate other MVID-specific phenotypes such as subapical vesicle accumulation and Rab11+ endosomes mislocalization. Finally, we found that the apical localization of the V0-ATPase is disrupted in MVID patients. Altogether these results suggest a role for the V0-ATPase in microvillus atrophy which might be independent from apical trafficking.

cell biology↗

bioRxiv2022

MRCK controls myosin II activation in the polarized cortex of mouse oocytes and promotes spindle rotation and male pronucleus centration

Asymmetric meiotic divisions in oocytes rely on spindle positioning in close vicinity to the cortex. In mouse oocytes arrested at metaphase II, eccentric spindle positioning is associated with a chromatin-induced remodeling of the overlying cortex, including the build-up of an actin cap surrounded by a ring of activated myosin II. While the role of the actin cap in promoting polar body formation was demonstrated, the role of ring myosin II, and its mechanism of activation, have remained elusive. Here, we show that ring myosin II activation requires Myotonic dystrophy kinase-Related Cdc42-binding Kinase (MRCK), downstream of polarized Cdc42. During anaphase-II, inhibition of MRCK resulted in spindle rotation defects and a decreased rate of polar body emission. Remarkably, some oocytes eventually achieved spindle rotation by disengaging one cluster of chromatids from the anaphase spindle. We show that the MRCK/myosin II pathway also regulates the flattening of the fertilization cone to initiate male pronucleus centration. These findings provide novel insights into mammalian oocyte polarization and the role of cortical myosin II in orchestrating asymmetric division.

cell biology↗

bioRxiv2021

Cofilin regulates actin network homeostasis and microvilli length in mouse oocytes

How multiple actin networks coexist in a common cytoplasm, while competing for a shared pool of monomers, is still an ongoing question. This is exemplified by meiotic maturation in the mouse oocyte, which relies on the dynamic remodeling of distinct cortical and cytoplasmic F-actin networks. Here we show that the conserved actin-depolymerizing factor cofilin is activated in a switch-like manner at meiosis resumption from prophase arrest. Interfering with cofilin activation during maturation resulted in widespread microvilli elongation, while cytoplasmic F-actin was depleted, leading to defects in spindle migration and polar body extrusion. In contrast, cofilin inactivation in metaphase II-arrested oocytes resulted in a shutdown of F-actin dynamics, along with a dramatic overgrowth of the polarized actin cap. However, inhibition of the Arp2/3 complex to promote actin cap disassembly elicited ectopic microvilli outgrowth in the polarized cortex. These data establish cofilin as a key player in actin network homeostasis in oocytes, and reveal that microvilli can act as a sink for monomers upon disassembly of a competing network.

cell biology↗

bioRxiv2020

RhoA- and Ran-induced antagonistic forces underlie symmetry breaking and spindle rotation in mouse oocytes.

Mammalian oocyte meiotic divisions are highly asymmetric and produce a large haploid gamete and two small polar bodies. This relies on the ability of the cell to break symmetry and position its spindle close to the cortex before the anaphase occurs. In metaphase II arrested mouse oocytes, the spindle is actively maintained close and parallel to the cortex, until the fertilization triggers the sister chromatids segregation and the rotation of the spindle. The latter must indeed reorient perpendicular to the cortex to enable the cytokinesis ring closure at the base of the polar body. However, the mechanisms underlying symmetry breaking and spindle rotation have remained elusive. In this study, we show that the spindle rotation results from two antagonistic forces. First, an inward contraction of the cytokinesis furrow dependent on RhoA signaling and second, an outward attraction exerted on both lots of chromatids by a RanGTP dependent polarization of the actomyosin cortex. By combining live segmentation and tracking with numerical modelling, we demonstrate that this configuration becomes unstable as the ingression progresses. This leads to spontaneous symmetry breaking, which implies that neither the rotation direction nor the lot of chromatids that eventually gets discarded are biologically predetermined.

developmental biology↗