Aberrant cortex contractions impact mammalian oocyte quality

The cortex controls cell shape. In mouse oocytes, the cortex thickens in an Arp2/3-complex-dependent manner, ensuring chromosome positioning and segregation. Surprisingly, we identify that mouse oocytes lacking the Arp2/3 complex undergo cortical actin remodeling upon division, followed by cortical contractions that are unprecedented in mammalian oocytes. Using genetics, imaging, and machine learning, we show that these contractions stir the cytoplasm, resulting in impaired organelle organization and activity. Oocyte capacity to avoid polyspermy is impacted, leading to a reduced female fertility. We could diminish contractions and rescue cytoplasmic anomalies. Similar contractions were observed in human oocytes collected as byproducts during IVF (in vitro fertilization) procedures. These contractions correlate with increased cytoplasmic motion, but not with defects in spindle assembly or aneuploidy in mice or humans. Our study highlights a multiscale effect connecting cortical F-actin, contractions, and cytoplasmic organization and affecting oocyte quality, with implications for female fertility.

Extraocular muscle stem cells exhibit distinct cellular properties associated with non-muscle molecular signatures

Skeletal muscle stem cells (MuSCs) are recognised as functionally heterogeneous. Cranial MuSCs are reported to have greater proliferative and regenerative capacity when compared with those in the limb. A comprehensive understanding of the mechanisms underlying this functional heterogeneity is lacking. Here, we have used clonal analysis, live imaging and single cell transcriptomic analysis to identify crucial features that distinguish extraocular muscle (EOM) from limb muscle stem cell populations. A MyogeninntdTom reporter showed that the increased proliferation capacity of EOM MuSCs correlates with deferred differentiation and lower expression of the myogenic commitment gene Myod. Unexpectedly, EOM MuSCs activated in vitro expressed a large array of extracellular matrix components typical of mesenchymal non-muscle cells. Computational analysis underscored a distinct co-regulatory module, which is absent in limb MuSCs, as driver of these features. The EOM transcription factor network, with Foxc1 as key player, appears to be hardwired to EOM identity as it persists during growth, disease and in vitro after several passages. Our findings shed light on how high-performing MuSCs regulate myogenic commitment by remodelling their local environment and adopting properties not generally associated with myogenic cells.

Capturing Cytoskeleton-Based Agitation of The Mouse Oocyte Nucleus Across Spatial Scales

A major challenge in understanding the causes of female infertility is to elucidate mechanisms governing the development of female germ cells, named oocytes. Their development is marked by cell growth and subsequent divisions, two critical phases that prepare the oocyte for fusion with sperm to initiate embryogenesis. During growth, oocytes reorganize their cytoplasm to position the nucleus at the cell center, an event predictive of successful oocyte development in mice and humans and, thus, their embryogenic potential. In mouse oocytes, this cytoplasmic reorganization was shown to be driven by the cytoskeleton, the activity of which generates mechanical forces that agitate, reposition, and penetrate the nucleus. Consequently, this cytoplasmic-to-nucleoplasmic force transmission tunes the dynamics of nuclear RNA-processing organelles known as biomolecular condensates. This protocol provides an experimental framework to document, with high temporal resolution, the impact of the cytoskeleton on the nucleus across spatial scales in mouse oocytes. It details the imaging and image analysis steps and tools necessary to evaluate i) cytoskeletal activity in the oocyte cytoplasm, ii) cytoskeleton-based agitation of the oocyte nucleus, and iii) its effects on biomolecular condensate dynamics in the oocyte nucleoplasm. Beyond oocyte biology, the methods elaborated here can be adapted for use in somatic cells to similarly address cytoskeleton-based tuning of nuclear dynamics across scales.

DeXtrusion: Automatic recognition of epithelial cell extrusion through machine learning in vivo

Accurately counting and localising cellular events from movies is an important bottleneck of high content tissue/embryo live imaging. Here, we propose a new methodology based on deep learning allowing automatic detection of cellular events and their precise x-y-t localisation on live fluorescent imaging movies without segmentation. We focused on the detection of cell extrusion, the expulsion of dying cells from the epithelial layer, and devised DeXtrusion: a pipeline based on recurrent neural networks for automatic detection of cell extrusion/cell death events in large movies of epithelia marked with cell contour. The pipeline, initially trained on movies of the Drosophila pupal notum marked with fluorescent E-cadherin, is easily trainable, provides fast and accurate extrusion predictions in a large range of imaging conditions, and can also detect other cellular events such as cell division or cell differentiation. It also performs well on other epithelial tissues with reasonable retraining. Our methodology could easily be applied for other cellular events detected in live fluorescent microscopy and help to democratise the use of deep learning for automatic event detections in developing tissues.

Filopodia-like protrusions of adjacent somatic cells shape the developmental potential of oocytes

The oocyte must grow and mature before fertilization, thanks to a close dialogue with the somatic cells that surround it. Part of this communication is through filopodia-like protrusions, called transzonal projections (TZPs), sent by the somatic cells to the oocyte membrane. To investigate the contribution of TZPs to oocyte quality, we impaired their structure by generating a full knockout mouse of the TZP structural component myosin-X (MYO10). Using spinning disk and super-resolution microscopy combined with a machine-learning approach to phenotype oocyte morphology, we show that the lack of Myo10 decreases TZP density during oocyte growth. Reduction in TZPs does not prevent oocyte growth but impairs oocyte-matrix integrity. Importantly, we reveal by transcriptomic analysis that gene expression is altered in TZP-deprived oocytes and that oocyte maturation and subsequent early embryonic development are partially affected, effectively reducing mouse fertility. We propose that TZPs play a role in the structural integrity of the germline-somatic complex, which is essential for regulating gene expression in the oocyte and thus its developmental potential.

In mice and humans, brain microvascular contractility matures postnatally

Although great efforts to characterize the embryonic phase of brain microvascular system development have been made, its postnatal maturation has barely been described. Here, we compared the molecular and functional properties of brain vascular cells on postnatal day (P)5 vs. P15, via a transcriptomic analysis of purified mouse cortical microvessels (MVs) and the identification of vascular-cell-type-specific or -preferentially expressed transcripts. We found that endothelial cells (EC), vascular smooth muscle cells (VSMC) and fibroblasts (FB) follow specific molecular maturation programs over this time period. Focusing on VSMCs, we showed that the arteriolar VSMC network expands and becomes contractile resulting in a greater cerebral blood flow (CBF), with heterogenous developmental trajectories within cortical regions. Samples of the human brain cortex showed the same postnatal maturation process. Thus, the postnatal phase is a critical period during which arteriolar VSMC contractility required for vessel tone and brain perfusion is acquired and mature.

Active diffusion in oocytes nonspecifically centers large objects during prophase I and meiosis I

Nucleus centering in mouse oocytes results from a gradient of actin-positive vesicle activity and is essential for developmental success. Here, we analyze 3D model simulations to demonstrate how a gradient in the persistence of actin-positive vesicles can center objects of different sizes. We test model predictions by tracking the transport of exogenous passive tracers. The gradient of activity induces a centering force, akin to an effective pressure gradient, leading to the centering of oil droplets with velocities comparable to nuclear ones. Simulations and experimental measurements show that passive particles subjected to the gradient exhibit biased diffusion toward the center. Strikingly, we observe that the centering mechanism is maintained in meiosis I despite chromosome movement in the opposite direction; thus, it can counteract a process that specifically off-centers the spindle. In conclusion, our findings reconcile how common molecular players can participate in the two opposing functions of chromosome centering versus off-centering.

Chromosomes function as a barrier to mitotic spindle bipolarity in polyploid cells

Ploidy variations such as genome doubling are frequent in human tumors and have been associated with genetic instability favoring tumor progression. How polyploid cells deal with increased centrosome numbers and DNA content remains unknown. Using Drosophila neuroblasts and human cancer cells to study mitotic spindle assembly in polyploid cells, we found that most polyploid cells divide in a multipolar manner. We show that even if an initial centrosome clustering step can occur at mitotic entry, the establishment of kinetochore-microtubule attachments leads to spatial chromosome configurations, whereby the final coalescence of supernumerary poles into a bipolar array is inhibited. Using in silico approaches and various spindle and DNA perturbations, we show that chromosomes act as a physical barrier blocking spindle pole coalescence and bipolarity. Importantly, microtubule stabilization suppressed multipolarity by improving both centrosome clustering and pole coalescence. This work identifies inhibitors of bipolar division in polyploid cells and provides a rationale to understand chromosome instability typical of polyploid cancer cells.

Microtubule Dynamics Scale with Cell Size to Set Spindle Length and Assembly Timing

Successive cell divisions during embryonic cleavage create increasingly smaller cells, so intracellular structures must adapt accordingly. Mitotic spindle size correlates with cell size, but the mechanisms for this scaling remain unclear. Using live cell imaging, we analyzed spindle scaling during embryo cleavage in the nematode Caenorhabditis elegans and sea urchin Paracentrotus lividus. We reveal a common scaling mechanism, where the growth rate of spindle microtubules scales with cell volume, which explains spindle shortening. Spindle assembly timing is, however, constant throughout successive divisions. Analyses in silico suggest that controlling the microtubule growth rate is sufficient to scale spindle length and maintain a constant assembly timing. We tested our in silico predictions to demonstrate that modulating cell volume or microtubule growth rate in vivo induces a proportional spindle size change. Our results suggest that scalability of the microtubule growth rate when cell size varies adapts spindle length to cell volume.

Microtubule stabilization drives 3D centrosome migration to initiate primary ciliogenesis

The classical view of centrosome decentering and migration to the cell periphery during ciliogenesis is that it is pulled toward its final destination. Here, Pitaval et al. argue that microtubule stabilization in the early stages of ciliogenesis generates pushing forces that propel the centrosome toward the apical pole.

Primary cilia are sensory organelles located at the cell surface. Their assembly is primed by centrosome migration to the apical surface, yet surprisingly little is known about this initiating step. To gain insight into the mechanisms driving centrosome migration, we exploited the reproducibility of cell architecture on adhesive micropatterns to investigate the cytoskeletal remodeling supporting it. Microtubule network densification and bundling, with the transient formation of an array of cold-stable microtubules, and actin cytoskeleton asymmetrical contraction participate in concert to drive apical centrosome migration. The distal appendage protein Cep164 appears to be a key actor involved in the cytoskeleton remodeling and centrosome migration, whereas intraflagellar transport 88’s role seems to be restricted to axoneme elongation. Together, our data elucidate the hitherto unexplored mechanism of centrosome migration and show that it is driven by the increase and clustering of mechanical forces to push the centrosome toward the cell apical pole.

Centrosome centering and decentering by microtubule network rearrangement

Numerical simulations are used to investigate the role of microtubule network architecture in centrosome positioning. Microtubule gliding along cell edges and pivoting around the centrosome are key regulators of the orientation of pushing forces, the magnitude of which depends on the number, dynamics, and stiffness of microtubules.

The centrosome is positioned at the cell center by pushing and pulling forces transmitted by microtubules (MTs). Centrosome decentering is often considered to result from asymmetric, cortical pulling forces exerted in particular by molecular motors on MTs and controlled by external cues affecting the cell cortex locally. Here we used numerical simulations to investigate the possibility that it could equally result from the redistribution of pushing forces due to a reorientation of MTs. We first showed that MT gliding along cell edges and pivoting around the centrosome regulate MT rearrangement and thereby direct the spatial distribution of pushing forces, whereas the number, dynamics, and stiffness of MTs determine the magnitude of these forces. By modulating these parameters, we identified different regimes, involving both pushing and pulling forces, characterized by robust centrosome centering, robust off-centering, or “reactive” positioning. In the last-named conditions, weak asymmetric cues can induce a misbalance of pushing and pulling forces, resulting in an abrupt transition from a centered to an off-centered position. Taken together, these results point to the central role played by the configuration of the MTs on the distribution of pushing forces that position the centrosome. We suggest that asymmetric external cues should not be seen as direct driver of centrosome decentering and cell polarization but instead as inducers of an effective reorganization of the MT network, fostering centrosome motion to the cell periphery.

Architecture and Connectivity Govern Actin Network Contractility

Actomyosin contractility plays a central role in a wide range of cellular processes, including the establishment of cell polarity, cell migration, tissue integrity, and morphogenesis during development. The contractile response is variable and depends on actomyosin network architecture and biochemical composition. To determine how this coupling regulates actomyosin-driven contraction, we used a micropatterning method that enables the spatial control of actin assembly. We generated a variety of actin templates and measured how defined actin structures respond to myosin-induced forces. We found that the same actin filament crosslinkers either enhance or inhibit the contractility of a network, depending on the organization of actin within the network. Numerical simulations unified the roles of actin filament branching and crosslinking during actomyosin contraction. Specifically, we introduce the concept of "network connectivity" and show that the contractions of distinct actin architectures are described by the same master curve when considering their degree of connectivity. This makes it possible to predict the dynamic response of defined actin structures to transient changes in connectivity. We propose that, depending on the connectivity and the architecture, network contraction is dominated by either sarcomeric-like or buckling mechanisms. More generally, this study reveals how actin network contractility depends on its architecture under a defined set of biochemical conditions.

Dynamic reorganization of the actin cytoskeleton

Cellular processes, including morphogenesis, polarization, and motility, rely on a variety of actin-based structures. Although the biochemical composition and filament organization of these structures are different, they often emerge from a common origin. This is possible because the actin structures are highly dynamic. Indeed, they assemble, grow, and disassemble in a time scale of a second to a minute. Therefore, the reorganization of a given actin structure can promote the formation of another. Here, we discuss such transitions and illustrate them with computer simulations.

Geometrical and mechanical properties control actin filament organization

The different actin structures governing eukaryotic cell shape and movement are not only determined by the properties of the actin filaments and associated proteins, but also by geometrical constraints. We recently demonstrated that limiting nucleation to specific regions was sufficient to obtain actin networks with different organization. To further investigate how spatially constrained actin nucleation determines the emergent actin organization, we performed detailed simulations of the actin filament system using Cytosim. We first calibrated the steric interaction between filaments, by matching, in simulations and experiments, the bundled actin organization observed with a rectangular bar of nucleating factor. We then studied the overall organization of actin filaments generated by more complex pattern geometries used experimentally. We found that the fraction of parallel versus antiparallel bundles is determined by the mechanical properties of actin filament or bundles and the efficiency of nucleation. Thus nucleation geometry, actin filaments local interactions, bundle rigidity, and nucleation efficiency are the key parameters controlling the emergent actin architecture. We finally simulated more complex nucleation patterns and performed the corresponding experiments to confirm the predictive capabilities of the model.