Consumption of a polarized membrane reservoir drives asymmetric membrane expansion during the unequal divisions of neural stem cells

Forces Shaping the Blastocyst

The blastocyst forms during the first days of mammalian development. The structure of the blastocyst is conserved among placental mammals and is paramount to the establishment of the first mammalian lineages. The blastocyst is composed of an extraembryonic epithelium, the trophectoderm (TE), that envelopes a fluid-filled lumen and the inner cell mass (ICM). To shape the blastocyst, embryos transit through three stages driven by forces that have been characterized in the mouse embryo over the past decade. The morphogenetically quiescent cleavage stages mask dynamic cytoskeletal remodeling. Then, during the formation of the morula, cells pull themselves together and the strongest ones internalize. Finally, the blastocyst forms after the pressurized lumen breaks the radial symmetry of the embryo before expanding in cycles of collapses and regrowth. In this review, we delineate the force patterns sculpting the blastocyst, based on our knowledge on the mouse and, to some extent, human embryos.

Rewinding the clock: mechanisms of dedifferentiation

Adult stem cells maintain tissue homeostasis through the production of differentiating cells. Considerable recent work has identified that stem cells themselves are replaceable through the process of dedifferentiation. The capacity and mechanisms of dedifferentiation vary widely among species and organ contexts. However, some core features are commonly present. In this review, we summarize 'hallmarks' of dedifferentiation, including mechanisms for maintenance of potency, sensation of loss, and migration, and review the current understanding of dedifferentiation as a true replacement mechanism.

Dynamic Plasma Membrane Topography Linked With Arp2/3 Actin Network Induction During Cell Shape Change

Recent studies show the importance of mesoscale changes to plasma membrane (PM) topography during cell shape change. Local folding and flattening of the cell surface is mechanosensitive, changing in response to both microenvironment structural elements and intracellular cytoskeletal activities. These topography changes elicit local mechanical signaling events that act in conjunction with molecular signal transduction pathways to remodel the cell cortex. Experimental manipulations of local PM curvature show its sufficiency for recruiting Arp2/3 actin network induction pathways. Additionally, studies of diverse cell shape changes-ranging from neutrophil migration to early Drosophila embryo cleavage to neural stem cell asymmetric division-show that local generation of PM folding is linked with local Arp2/3 actin network induction, which then remodels the PM topography during dynamic control of cell structure. These examples are reviewed in detail, together with known and potential causes of PM topography changes, downstream effects, and higher-order feedback.

Multifaceted regulation of asymmetric cell division by the actin cytoskeleton

Asymmetric cell division (ACD) is essential for generating cell diversity in multicellular eukaryotes, yet the underlying mechanisms remain largely unresolved. Well-established models of ACD, such as microtubule-based spindle displacement in Caenorhabditis elegans embryos and preprophase band assembly in plants provide valuable insights but fail to fully explain asymmetry establishment in others. In this article, we discuss how actin-dependent mechanisms govern ACD in model systems, highlighting emerging commonalities and differences. Given its broad impact, the actin cytoskeleton may play a more significant role in ACD than currently recognized, serving as a fundamental component during organismal development across kingdoms.

Lgl resets Par complex membrane loading at mitotic exit to enable asymmetric neural stem cell division

The Par complex regulates cell polarity in diverse animal cells 1-4 , but how its localization is restricted to a specific membrane domain remains unclear. We investigated how the tumor suppressor Lethal giant larvae (Lgl) polarizes the Par complex in Drosophila neural stem cells (NSCs or neuroblasts). In contrast to epithelial cells, where Lgl and the Par complex occupy mutually exclusive membrane domains, Lgl is cytoplasmic when the Par complex is apically polarized in NSCs5. Importantly, we found that Lgl's key function is not in directly regulating metaphase Par polarity, but rather in removing the Par complex from the membrane at the end of mitosis, creating a "polarity reset" for the next cell cycle. Without this Lgl-mediated reset, we found that residual Par complex remains on the basal membrane during subsequent divisions, disrupting fate determinant polarization and proper asymmetric cell division. These findings reveal a novel mechanism of polarity regulation by Lgl and highlight the importance of the prepolarized state in Par-mediated polarity.

Membrane oscillations driven by Arp2/3 constrict the intercellular bridge during neural stem cell divisions

After the first furrowing step of animal cell division, the nascent sibling cells remain connected by a thin intercellular bridge (ICB). In isolated cells nascent siblings migrate away from each other to generate tension and constrict the ICB, but less is known about how cells complete cytokinesis when constrained within tissues. We examined the ICBs formed by Drosophila larval brain neural stem cell (NSC) asymmetric divisions and find that they rely on constriction focused at the central midbody region rather than the flanking arms of isolated cell ICBs. Super-resolution, full volume imaging revealed unexpected oscillatory waves in plasma membrane sheets surrounding the ICB pore during its formation and constriction. We find that these membrane dynamics are driven by Arp2/3-dependent branched actin networks. Inhibition of Arp2/3 complex activity blocks membrane oscillations and prevents ICB formation and constriction. Our results identify a previously unrecognized role for localized membrane oscillations in ICB function when cells cannot generate tension through migration.

Centrosome-organized plasma membrane infoldings linked to growth of a cortical actin domain

Regulated cell shape change requires the induction of cortical cytoskeletal domains. Often, local changes to plasma membrane (PM) topography are involved. Centrosomes organize cortical domains and can affect PM topography by locally pulling the PM inward. Are these centrosome effects coupled? At the syncytial Drosophila embryo cortex, centrosome-induced actin caps grow into dome-like compartments for mitoses. We found the nascent cap to be a collection of PM folds and tubules formed over the astral centrosomal MT array. The localized infoldings require centrosome and dynein activities, and myosin-based surface tension prevents them elsewhere. Centrosome-engaged PM infoldings become specifically enriched with an Arp2/3 induction pathway. Arp2/3 actin network growth between the infoldings counterbalances centrosomal pulling forces and disperses the folds for actin cap expansion. Abnormal domain topography with either centrosome or Arp2/3 disruption correlates with decreased exocytic vesicle association. Together, our data implicate centrosome-organized PM infoldings in coordinating Arp2/3 network growth and exocytosis for cortical domain assembly.

The Drosophila neuroblast polarity cycle at a glance

Drosophila neural stem cells, or neuroblasts, rapidly proliferate during embryonic and larval development to populate the central nervous system. Neuroblasts divide asymmetrically to create cellular diversity, with each division producing one sibling cell that retains the neuroblast fate and another that differentiates into glia or neurons. This asymmetric outcome is mediated by the transient polarization of numerous factors to the cell cortex during mitosis. The powerful genetics and outstanding imaging tractability of the neuroblast make it an excellent model system for studying the mechanisms of cell polarity. This Cell Science at a Glance article and the accompanying poster explore the phases of the neuroblast polarity cycle and the regulatory circuits that control them. We discuss the key features of the cycle - the targeted recruitment of proteins to specific regions of the plasma membrane and multiple phases of highly dynamic actomyosin-dependent cortical flows that pattern both protein distribution and membrane structure.

Cooperative regulation of C1-domain membrane recruitment polarizes atypical protein kinase C

Recruitment of the Par complex protein atypical protein kinase C (aPKC) to a specific membrane domain is a key step in the polarization of animal cells. While numerous proteins and phospholipids interact with aPKC, how these interactions cooperate to control its membrane recruitment has been unknown. Here, we identify aPKC's C1 domain as a phospholipid interaction module that targets aPKC to the membrane of Drosophila neural stem cells (NSCs). The isolated C1 binds the NSC membrane in an unpolarized manner during interphase and mitosis and is uniquely sufficient among aPKC domains for targeting. Other domains, including the catalytic module and those that bind the upstream regulators Par-6 and Bazooka, restrict C1's membrane targeting activity-spatially and temporally-to the apical NSC membrane during mitosis. Our results suggest that aPKC polarity results from cooperative activation of autoinhibited C1-mediated membrane binding activity.

Polarized SCAR and the Arp2/3 complex regulate apical cortical remodeling in asymmetrically dividing neuroblasts

Although the formin-nucleated actomyosin cortex has been shown to drive the changes in cell shape that accompany animal cell division in both symmetric and asymmetric cell divisions, the mitotic role of cortical Arp2/3-nucleated actin networks remain unclear. Here using asymmetrically dividing Drosophila neural stem cells as a model system, we identify a pool of membrane protrusions that form at the apical cortex of neuroblasts as they enter mitosis. Strikingly, these apically localized protrusions are enriched in SCAR, and depend on SCAR and Arp2/3 complexes for their formation. Because compromising SCAR or the Arp2/3 complex delays the apical clearance of Myosin II at the onset of anaphase and induces cortical instability at cytokinesis, these data point to a role for an apical branched actin filament network in fine-tuning the actomyosin cortex to enable the precise control of cell shape changes during an asymmetric cell division.