Regulation of mitotic spindle orientation: an integrated view

A cortical pool of LIN-5 (NuMA) controls cytokinetic furrow formation and cytokinesis completion

In animal cells, cleavage furrow formation is controlled by localized activation of the GTPase RhoA at the equatorial membrane using cues transmitted from the spindle. Here, we explore the function of LIN-5, a well-studied protein known for its role in aster separation and spindle positioning in cleavage furrow formation. We show that the cortical pool of LIN-5, recruited by GPR-1/2 and important for cortical force generation, regulates cleavage furrow formation independently of its roles in aster separation and spindle positioning. Instead, our data suggest that enrichment of LIN-5/GPR-1/2 at the polar cortical region is essential to ensure the timely accumulation of contractile ring components-myosin II and Anillin at the equatorial cortex. We additionally define a late cytokinesis role of cortical LIN-5/GPR-1/2 in midbody stabilization and abscission. These results indicate that the cortical LIN-5/GPR-1/2 complex contributes to multiple aspects of cytokinesis independently of its roles in spindle positioning and elongation.

The RNA-binding protein YTHDF3 affects gastric cancer cell migration and response to paclitaxel by regulating EZRIN

BACKGROUND

Gastric cancer (GC) is the fourth most common cause of cancer-related mortality and the fifth most common cancer worldwide. Despite efforts, the identification of biomarkers and new therapeutic approaches for GC remains elusive. Recent studies have begun to reveal the role of N6-adenosine methylation (m6A) in the regulation of gene expression.

METHODS

The expression of the reader YT521-B homology domain-containing family 3 (YTHDF3) in GC was assessed in 331 patients using immunohistochemistry. GC cell lines depleted of YTHDF3 using CRISPR-Cas9 were evaluated for migration, metastasis, orientation of the mitotic spindle, and response to paclitaxel. The association between YTHDF3 and EZRIN (EZR) mRNA was shown using RNA sequencing, immunofluorescence, real-time PCR, and RNA immunoprecipitation. The single-base elongation- and ligation-based qPCR amplification (SELECT) method was used to map m6A in the EZR transcript.

RESULTS

YTHDF3 was significantly overexpressed in GC, and high levels of YTHDF3 were predictive of the response to chemotherapy. In GC cell lines, YTHDF3 was the most highly expressed reader protein. YTHDF3 depletion impaired cytoskeleton organization, cell migration and metastasis, and orientation of the mitotic spindle, leading to an increased response to paclitaxel. EZR was one of the downregulated targets in the YTHDF3 knockout cell models and was associated with the observed phenotype.

CONCLUSION

YTHDF3 contributes to cell motility and response to paclitaxel in GC cell lines, at least in part through EZR regulation. The YTHDF3-EZR regulatory axis is a novel molecular player in GC, with clinical relevance and potential therapeutic utility.

Physical limits of natural computation as the biological constraints of morphogenesis, evolution, and consciousness: On the 100th anniversary of Efim Liberman (1925-2011)

Efim Liberman (1925-2011) introduced in 1972 the idea of natural computation realized internally by living systems. He considered the physical principles employed by living systems as essential constraints that limit the computational process occurring in the course of adaptation, morphogenesis, and neural activity. The most general limits determined by the physical fundamental constants are universal for all nature. However, the more specific constraints are intrinsic to each biological system and can be overcome in the course of the evolutionary process. We discuss the roles of biological macromolecules, particularly the cytoskeleton, in shaping the actualization patterns formed in the internal measurement process occurring in living systems. Cytoskeletal rearrangements determine cellular, morphogenetic, and perceptive transformations in living systems and participate in the combinatorial genetic events that lead to evolutionary transformations. The operation of neurons is based on the transmission of signals via the cytoskeleton, where the digital output is generated that can be decoded through a reflexive action of the perceiving agent. It is concluded that the principles of natural computation formulated by Liberman represent the most fundamental feature of living beings and form the basis for the general theory of biological systems, with essential consequences for understanding metabolic closure, morphogenesis, evolution, and consciousness.

Meru co-ordinates spindle orientation with cell polarity and cell cycle progression

Correct mitotic spindle alignment is essential for tissue architecture and plays an important role in cell fate specification through asymmetric cell division. Spindle tethering factors such as Drosophila Mud (NuMA in mammals) are recruited to the cell cortex and capture astral microtubules, pulling the spindle in the correct orientation. However, how spindle tethering complexes read the cell polarity axis and how spindle attachment is coupled to mitotic progression remains poorly understood. We explore these questions in Drosophila sensory organ precursors (SOPs), which divide asymmetrically to give rise to epidermal mechanosensory bristles. We show that the scaffold protein Meru, which is enriched at the posterior cortex by the Frizzled/Dishevelled planar cell polarity complex, in turn recruits Mud, linking the spindle tethering and polarity machineries. Furthermore, Cyclin A/Cdk1 associates with Meru at the posterior cortex, promoting the formation of the Mud/Meru/Dsh complex via Meru and Dsh phosphorylation. Thus, Meru couples spindle orientation with cell polarity and provides a cell cycle-dependent cue for spindle tethering.

The Rac1 homolog CED-10 is a component of the MES-1/SRC-1 pathway for asymmetric division of the Caenorhabditis elegans EMS blastomere

Asymmetric cell division is essential for the creation of cell types with different identities and functions. The endomesodermal precursor cell (EMS) of the 4-cell Caenorhabditis elegans embryo undergoes an asymmetric division in response to partially redundant signaling pathways. One pathway involves a Wnt signal from the neighboring P2 cell, while the other pathway is defined by the receptor-like MES-1 transmembrane protein localized at the EMS-P2 cell contact and the cytoplasmic kinase SRC-1. In response to these signals, the EMS nuclear-centrosome complex rotates, so that the spindle forms on the anterior-posterior axis; after division, the daughter cell contacting P2 becomes the endodermal precursor cell. Here, we identify the Rac1 homolog CED-10 as a new component of the MES-1/SRC-1 pathway. Loss of CED-10 affects both spindle positioning and endoderm specification in the EMS cell. SRC-1 dependent phosphorylation at the EMS-P2 contact is reduced. However, the asymmetric division of the P2 cell, which is also MES-1 and SRC-1 dependent, appears normal in ced-10 mutants. These and other results suggest that CED-10 acts upstream of, or at the level of, SRC-1 activity in the EMS cell. In addition, we find that the branched actin regulator ARX-2 is enriched at the EMS-P2 cell contact site, in a CED-10-dependent manner. Loss of ARX-2 results in EMS spindle orientation defects, suggesting that CED-10 acts through branched actin to promote spindle orientation in the EMS cell.

Early spinal cord development: from neural tube formation to neurogenesis

As one of the simplest and most evolutionarily conserved parts of the vertebrate nervous system, the spinal cord serves as a key model for understanding the principles of nervous system construction. During embryonic development, the spinal cord originates from a population of bipotent stem cells termed neuromesodermal progenitors, which are organized within a transient embryonic structure known as the neural tube. Neural tube morphogenesis differs along its anterior-to-posterior axis: most of the neural tube (including the regions that will develop into the brain and the anterior spinal cord) forms via the bending and dorsal fusion of the neural groove, but the establishment of the posterior region of the neural tube involves de novo formation of a lumen within a solid medullary cord. The early spinal cord primordium consists of highly polarized neural progenitor cells organized into a pseudostratified epithelium. Tight regulation of the cell division modes of these progenitors drives the embryonic growth of the neural tube and initiates primary neurogenesis. A rich history of observational and functional studies across various vertebrate models has advanced our understanding of the cellular events underlying spinal cord development, and these foundational studies are beginning to inform our knowledge of human spinal cord development.

Perturbed cell fate decision by schizophrenia-associated AS3MTd2d3 isoform during corticogenesis

The neurodevelopmental theory of schizophrenia emphasizes early brain development in its etiology. Genome-wide association studies have linked schizophrenia to genetic variations of AS3MT (arsenite methyltransferase) gene, particularly the increased expression of AS3MTd2d3 isoform. To investigate the biological basis of this association with schizophrenia pathophysiology, we established a transgenic mouse model (AS3MTd2d3-Tg) ectopically expressing AS3MTd2d3 at the cortical neural stem cells. AS3MTd2d3-Tg mice exhibited enlarged ventricles and deficits in sensorimotor gating and sociability. Single-cell and single-nucleus RNA sequencing analyses of AS3MTd2d3-Tg brains revealed cell fate imbalances and altered excitatory neuron composition. AS3MTd2d3 localized to centrosome, disrupting mitotic spindle orientation and differentiation in developing neocortex and organoids, in part through NPM1 (Nucleophosmin 1). The structural analysis identified that hydrophobic residues exposed in AS3MTd2d3 are critical for its pathogenic function. Therefore, our findings may help to explain the early pathological features of schizophrenia.

Dynein-driven regulation of postsynaptic membrane architecture and synaptic function

Cytoplasmic dynein is essential in motor neurons for retrograde cargo transport that sustains neuronal connectivity. Little, however, is known about dynein function on the postsynaptic side of the circuit. Here, we report distinct postsynaptic roles for dynein at neuromuscular junctions in Drosophila. Intriguingly, we show that dynein puncta accumulate postsynaptically at glutamatergic synaptic terminals. Moreover, Skittles (Sktl), a phosphatidylinositol 4-phosphate 5-kinase that produces phosphatidylinositol 4,5-bisphosphate (PIP2) to organize the spectrin cytoskeleton, also localizes specifically to glutamatergic synaptic terminals. Depletion of postsynaptic dynein disrupted the accumulation of Skittles and the PIP2 phospholipid, and organization of the spectrin cytoskeleton at the postsynaptic membrane. Coincidental with dynein depletion, we observed an increase in the size of ionotropic glutamate receptor (iGluR) fields and an increase in the amplitude and frequency of miniature excitatory junctional potentials. PIP2 levels did not affect iGluR clustering, nor did dynein affect the levels of iGluR subunits at the neuromuscular junction. Our observations suggest a separate, transport-independent function for dynein in iGluR cluster organization. Based on the close apposition of dynein puncta to the iGluR fields, we speculate that dynein at the postsynaptic membrane contributes to the organization of the receptor fields, hence ensuring proper synaptic transmission.

Canonical Hedgehog Signaling Controls Astral Microtubules and Mitotic Spindle Orientation in Neural Progenitors and iPSCs

Mitotic spindle orientation is crucial for cell fate determination and tissue organization. Although the intracellular machinery governing spindle orientation is well characterized, whether and how secreted factors, such as morphogens, regulate this process remains poorly understood. This study investigated the role of Hedgehog (HH) signaling in modulating mitotic spindle orientation in neural progenitor cells and in induced pluripotent stem cells (iPSCs). Time-lapse microscopy of cerebral organoids and iPSCs revealed that HH signaling increases the angle of the mitotic spindle relative to the apical surface, prolongs mitosis, and enhances spindle rotation. Mechanistically, HH signaling reduces both the number and the length of astral microtubules, key regulators of spindle orientation. This reduction correlates with increased spindle angle in iPSCs. Furthermore, we show that canonical HH signaling, involving GLI-dependent transcriptional regulation, contributes to these effects. RNA sequencing and gene set enrichment analysis (GSEA) revealed that HH signaling upregulates genes associated with microtubule depolymerization, suggesting a transcriptional mechanism by which HH signaling influences astral microtubule dynamics and, consequently, mitotic spindle orientation. These findings highlight a novel link between a morphogen, transcriptional regulation, and the control of mitotic spindle orientation, with implications for development and tissue homeostasis.

Palmitoylated Importin α Regulates Mitotic Spindle Orientation Through Interaction with NuMA

Regulation of cell division orientation is a fundamental process critical to differentiation and tissue homeostasis. Microtubules emanating from the mitotic spindle pole bind a conserved complex of proteins at the cell cortex which orients the spindle and ultimately the cell division plane. Control of spindle orientation is of particular importance in developing tissues, such as the developing brain. Misorientation of the mitotic spindle and thus subsequent division plane misalignment can contribute to improper segregation of cell fate determinants in developing neuroblasts, leading to a rare neurological disorder known as microcephaly. We demonstrate that the nuclear transport protein importin α, when palmitoylated, plays a critical role in mitotic spindle orientation through localizing factors, such as NuMA, to the cell cortex. We also observe craniofacial developmental defects in Xenopus laevis when importin α palmitoylation is abrogated, including smaller head and brains, a hallmark of spindle misorientation and microcephaly. These findings characterize not only a role for importin α in spindle orientation, but also a broader role for importin α palmitoylation which has significance for many cellular processes.

Polarity and migration of cranial and cardiac neural crest cells: underlying molecular mechanisms and disease implications

The Neural Crest cells are multipotent progenitor cells formed at the neural plate border that differentiate and give rise to a wide range of cell types and organs. Directional migration of NC cells and their correct positioning at target sites are essential during embryonic development, and defects in these processes results in congenital diseases. The NC migration begins with the epithelial-mesenchymal transition and extracellular matrix remodeling. The main cellular mechanisms that sustain this migration include contact inhibition of locomotion, co-attraction, chemotaxis and mechanical cues from the surrounding environment, all regulated by proteins that orchestrate cell polarity and motility. In this review we highlight the molecular mechanisms involved in neural crest cell migration and polarity, focusing on the role of small GTPases, Heterotrimeric G proteins and planar cell polarity complex. Here, we also discuss different congenital diseases caused by altered NC cell migration.

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, species diversification. The micromere of the sea urchin embryo may serve as one of those examples: an ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, activator of G-protein signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans. This raises the question of what evolutionary modifications of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

A 3D micro-printed single cell micro-niche with asymmetric niche signals - An in vitro model for asymmetric cell division study

Intricate microenvironment signals orchestrate to affect cell behavior and fate during tissue morphogenesis. However, the underlying mechanisms on how specific local niche signals influence cell behavior and fate are not fully understood, owing to the lack of in vitro platform able to precisely, quantitatively, spatially, and independently manipulate individual niche signals. Here, microarrays of protein-based 3D single cell micro-niche (3D-SCμN), with precisely engineered biophysical and biochemical niche signals, are micro-printed by a multiphoton microfabrication and micropatterning technology. Mouse embryonic stem cell (mESC) is used as the model cell to study how local niche signals affect stem cell behavior and fate. By precisely engineering the internal microstructures of the 3D SCμNs, we demonstrate that the cell division direction can be controlled by the biophysical niche signals, in a cell shape-independent manner. After confining the cell division direction to a dominating axis, single mESCs are exposed to asymmetric biochemical niche signals, specifically, cell-cell adhesion molecule on one side and extracellular matrix on the other side. We demonstrate that, symmetry-breaking (asymmetric) niche signals successfully trigger cell polarity formation and bias the orientation of asymmetric cell division, the mitosis process resulting in two daughter cells with differential fates, in mESCs.

Mechanobiology of the nucleus during the G2-M transition

Cellular behavior is continuously influenced by mechanical forces. These forces span the cytoskeleton and reach the nucleus, where they trigger mechanotransduction pathways that regulate downstream biochemical events. Therefore, the nucleus has emerged as a regulator of cellular response to mechanical stimuli. Cell cycle progression is regulated by cyclin-CDK complexes. Recent studies demonstrated these biochemical pathways are influenced by mechanical signals, highlighting the interdependence of cellular mechanics and cell cycle regulation. In particular, the transition from G2 to mitosis (G2-M) shows significant changes in nuclear structure and organization, ranging from nuclear pore complex (NPC) and nuclear lamina disassembly to chromosome condensation. The remodeling of these mechanically active nuclear components indicates that mitotic entry is particularly sensitive to forces. Here, we address how mechanical forces crosstalk with the nucleus to determine the timing and efficiency of the G2-M transition. Finally, we discuss how the deregulation of nuclear mechanics has consequences for mitosis.

Lateral cell polarization drives organization of epithelia in sea anemone embryos and embryonic cell aggregates

One of the first organizing processes during animal development is the assembly of embryonic cells into epithelia. Common features unite epithelialization across select bilaterians, however, we know less about the molecular and cellular mechanisms that drive epithelial emergence in early branching nonbilaterians. In sea anemones, epithelia emerge both during embryonic development and after cell aggregation of dissociated tissues. Although adhesion is required to keep cells together, it is not clear whether cell polarization plays a role as epithelia emerge from disordered aggregates. Here, we use the embryos of the sea anemone Nematostella vectensis to investigate the evolutionary origins of epithelial development. We demonstrate that lateral cell polarization is essential for epithelial organization in both embryos and aggregates. With disrupted lateral polarization, cell contact in the aggregate is not sufficient to trigger epithelialization and further tissue development. Specifically, knockdown of the conserved lateral polarity and tumor suppressor protein Lethal giant larvae (Lgl) disrupts epithelia in developing embryos and impairs the capacity of dissociated cells to epithelialize from aggregates. In contrast to other systems, cells in Nematostella lgl knockdown embryos do not undergo excessive proliferation. Cells with reduced Lgl levels lose their columnar shape and proper positioning of their mitotic spindles and basal bodies. Due to misoriented divisions and aberrant shapes, cells arrange nonuniformly without forming a monolayer. Together our data show that, in Nematostella, Lgl drives epithelialization in embryos and cell aggregates through its effect on cell shape and organelle localization.

mInsc coordinates Par3 and NuMA condensates for assembly of the spindle orientation machinery in asymmetric cell division

As a fundamental process governing the self-renewal and differentiation of stem cells, asymmetric cell division is controlled by several conserved regulators, including the polarity protein Par3 and the microtubule-associated protein NuMA, which orchestrate the assembly and interplay of the Par3/Par6/mInsc/LGN complex at the apical cortex and the LGN/Gαi/NuMA/Dynein complex at the mitotic spindle to ensure asymmetric segregation of cell fate determinants. However, this model, which is well-supported by genetic studies, has been challenged by evidence of competitive interaction between NuMA and mInsc for LGN. Here, the solved crystal structure of the Par3/mInsc complex reveals that mInsc competes with Par6β for Par3, raising questions about how proteins assemble overlapping targets into functional macromolecular complexes. Unanticipatedly, we discover that Par3 can recruit both Par6β and mInsc by forming a dynamic condensate through phase separation. Similarly, the phase-separated NuMA condensate enables the coexistence of competitive NuMA and mInsc with LGN in the same compartment. Bridge by mInsc, Par3/Par6β and LGN/NuMA condensates coacervate, robustly enriching all five proteins both in vitro and within cells. These findings highlight the pivotal role of protein condensates in assembling multi-component signalosomes that incorporate competitive protein-protein interaction pairs, effectively overcoming stoichiometric constraints encountered in conventional protein complexes.

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, to species diversification. The micromere of the sea urchin embryo may serve as one of those examples: An ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, Activator of G-protein Signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans. This raises the question of what evolutionary modifications of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

Radial glia progenitor polarity in health and disease

Radial glia (RG) are the main progenitor cell type in the developing cortex. These cells are highly polarized, with a long basal process spanning the entire thickness of the cortex and acting as a support for neuronal migration. The RG cell terminates by an endfoot that contacts the pial (basal) surface. A shorter apical process also terminates with an endfoot that faces the ventricle, with a primary cilium protruding in the cerebrospinal fluid. These cell domains have particular subcellular compositions that are critical for the correct functioning of RG. When altered, this can affect proper development of the cortex, ultimately leading to cortical malformations, associated with different pathological outcomes. In this review, we focus on the current knowledge concerning the cell biology of these bipolar stem cells and discuss the role of their polarity in health and disease.

Simvastatin activates the spindle assembly checkpoint and causes abnormal cell division by modifying small GTPases

Simvastatin is an inhibitor of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, which is a rate-limiting enzyme of the cholesterol synthesis pathway. It has been used clinically as a lipid-lowering agent to reduce low-density lipoprotein (LDL) cholesterol levels. In addition, antitumor activity has been demonstrated. Although simvastatin attenuates the prenylation of small GTPases, its effects on cell division in which small GTPases play an important role, have not been examined as a mechanism underlying its cytostatic effects. In this study, we determined its effect on cell division. Cell cycle synchronization experiments revealed a delay in mitotic progression in simvastatin-treated cells at concentrations lower than the IC50. Time-lapse imaging analysis indicated that the duration of mitosis, especially from mitotic entry to anaphase onset, was prolonged. In addition, simvastatin increased the number of cells exhibiting misoriented anaphase/telophase and bleb formation. Inhibition of the spindle assembly checkpoint (SAC) kinase Mps1 canceled the mitotic delay. Additionally, the number of cells exhibiting kinetochore localization of BubR1, an essential component of SAC, was increased, suggesting an involvement of SAC in the mitotic delay. Enhancement of F-actin formation and cell rounding at mitotic entry indicates that cortical actin dynamics were affected by simvastatin. The cholesterol removal agent methyl-β-cyclodextrin (MβCD) accelerated mitotic progression differently from simvastatin, suggesting that cholesterol loss from the plasma membrane is not involved in the mitotic delay. Of note, the small GTPase RhoA, which is a critical factor for cortical actin dynamics, exhibited upregulated expression. In addition, Rap1 was likely not geranylgeranylated. Our results demonstrate that simvastatin affects actin dynamics by modifying small GTPases, thereby activating the spindle assembly checkpoint and causing abnormal cell division.

A cytokinetic ring-driven cell rotation achieves Hertwig's rule in early development

Hertwig's rule states that cells divide along their longest axis, usually driven by forces acting on the mitotic spindle. Here, we show that in contrast to this rule, microtubule-based pulling forces in early Caenorhabditis elegans embryos align the spindle with the short axis of the cell. We combine theory with experiments to reveal that in order to correct this misalignment, inward forces generated by the constricting cytokinetic ring rotate the entire cell until the spindle is aligned with the cell's long axis. Experiments with slightly compressed mouse zygotes indicate that this cytokinetic ring-driven mechanism of ensuring Hertwig's rule is general for cells capable of rotating inside a confining shell, a scenario that applies to early cell divisions of many systems.

Mechanical stress combines with planar polarised patterning during metaphase to orient embryonic epithelial cell divisions

The planar orientation of cell division (OCD) is important for epithelial morphogenesis and homeostasis. Here, we ask how mechanics and antero-posterior (AP) patterning combine to influence the first divisions after gastrulation in the Drosophila embryonic epithelium. We analyse hundreds of cell divisions and show that stress anisotropy, notably from compressive forces, can reorient division directly in metaphase. Stress anisotropy influences the OCD by imposing metaphase cell elongation, despite mitotic rounding, and overrides interphase cell elongation. In strongly elongated cells, the mitotic spindle adapts its length to, and hence its orientation is constrained by, the cell long axis. Alongside mechanical cues, we find a tissue-wide bias of the mitotic spindle orientation towards AP-patterned planar polarised Myosin-II. This spindle bias is lost in an AP-patterning mutant. Thus, a patterning-induced mitotic spindle orientation bias overrides mechanical cues in mildly elongated cells, whereas in strongly elongated cells the spindle is constrained close to the high stress axis.

The Rac1 homolog CED-10 is a component of the MES-1/SRC-1 pathway for asymmetric division of the C. elegans EMS blastomere

Asymmetric cell division is essential for the creation of cell types with different identities and functions. The EMS blastomere of the four-cell Caenorhabditis elegans embryo undergoes an asymmetric division in response to partially redundant signaling pathways. One pathway involves a Wnt signal emanating from the neighboring P2 cell, while the other pathway is defined by the receptor-like MES-1 protein localized at the EMS/P2 cell contact, and the cytoplasmic kinase SRC-1. In response to these pathways, the EMS nuclear-centrosome complex rotates so that the spindle forms on the anterior-posterior axis; after division, the daughter cell contacting P2 becomes the endodermal precursor cell. Here we identify the Rac1 homolog, CED-10, as a new component of the MES-1/SRC-1 pathway. Loss of CED-10 affects both spindle positioning and endoderm specification. Although MES-1 is still present at the EMS/P2 contact in ced-10 embryos, SRC-1 dependent phosphorylation is reduced. These and other results suggest that CED-10 acts downstream of MES-1 and upstream of, or at the level of, SRC-1 activity. In addition, we find that the branched actin regulator ARX-2 is enriched at the EMS/P2 cell contact site, in a CED-10 dependent manner. Loss of ARX-2 results in spindle positioning defects, suggesting that CED-10 acts through branched actin to promote the asymmetric division of the EMS cell.

Lateral cell polarization drives organization of epithelia in sea anemone embryos and embryonic cell aggregates

One of the first organizing processes during animal development is the assembly of embryonic cells into epithelia. In certain animals, including Hydra and sea anemones, epithelia also emerge when cells from dissociated tissues are aggregated back together. Although cell adhesion is required to keep cells together, it is not clear whether cell polarization plays a role as epithelia emerge from disordered aggregates. Here, we demonstrate that lateral cell polarization is essential for epithelial organization in both embryos and aggregates of the sea anemone Nematostella vectensis. Specifically, knock down of the lateral polarity protein Lgl disrupts epithelia in developing embryos and impairs the capacity of dissociated cells to epithelialize from aggregates. Cells in lgl mutant epithelia lose their columnar shape and have mispositioned mitotic spindles and ciliary basal bodies. Together, our data suggest that in Nematostella, Lgl is required to establish lateral cell polarity and position cytoskeletal organelles in cells of embryos and aggregates during de novo epithelial organization.

Reduction of cortical pulling at mitotic entry facilitates aster centration

Equal cell division relies upon astral microtubule-based centering mechanisms, yet how the interplay between mitotic entry, cortical force generation and long astral microtubules leads to symmetric cell division is not resolved. We report that a cortically located sperm aster displaying long astral microtubules that penetrate the whole zygote does not undergo centration until mitotic entry. At mitotic entry, we find that microtubule-based cortical pulling is lost. Quantitative measurements of cortical pulling and cytoplasmic pulling together with physical simulations suggested that a wavelike loss of cortical pulling at mitotic entry leads to aster centration based on cytoplasmic pulling. Cortical actin is lost from the cortex at mitotic entry coincident with a fall in cortical tension from ∼300pN/µm to ∼100pN/µm. Following the loss of cortical force generators at mitotic entry, long microtubule-based cytoplasmic pulling is sufficient to displace the aster towards the cell center. These data reveal how mitotic aster centration is coordinated with mitotic entry in chordate zygotes.

Positioning centrioles and centrosomes

Centrosomes are the primary microtubule organizer in eukaryotic cells. In addition to shaping the intracellular microtubule network and the mitotic spindle, centrosomes are responsible for positioning cilia and flagella. To fulfill these diverse functions, centrosomes must be properly located within cells, which requires that they undergo intracellular transport. Importantly, centrosome mispositioning has been linked to ciliopathies, cancer, and infertility. The mechanisms by which centrosomes migrate are diverse and context dependent. In many cells, centrosomes move via indirect motor transport, whereby centrosomal microtubules engage anchored motor proteins that exert forces on those microtubules, resulting in centrosome movement. However, in some cases, centrosomes move via direct motor transport, whereby the centrosome or centriole functions as cargo that directly binds molecular motors which then walk on stationary microtubules. In this review, we summarize the mechanisms of centrosome motility and the consequences of centrosome mispositioning and identify key questions that remain to be addressed.

Stationary shapes of axisymmetric vesicles beyond lowest-energy configurations

We conduct a systematic exploration of the energy landscape of vesicle morphologies within the framework of the Helfrich model. Vesicle shapes are determined by minimizing the elastic energy subject to constraints of constant area and volume. The results show that pressurized vesicles can adopt higher-energy spindle-like configurations that require the action of point forces at the poles. If the internal pressure is lower than the external one, multilobed shapes are predicted. We utilize our results to rationalize experimentally observed spindle shapes of giant vesicles in a uniform AC electric field.

Dynein directs prophase centrosome migration to control the stem cell division axis in the developing Caenorhabditis elegans epidermis

The microtubule motor dynein is critical for the assembly and positioning of mitotic spindles. In Caenorhabditis elegans, these dynein functions have been extensively studied in the early embryo but remain poorly explored in other developmental contexts. Here, we use a hypomorphic dynein mutant to investigate the motor's contribution to asymmetric stem cell-like divisions in the larval epidermis. Live imaging of seam cell divisions that precede formation of the seam syncytium shows that mutant cells properly assemble but frequently misorient their spindle. Misoriented divisions misplace daughter cells from the seam cell row, generate anucleate compartments due to aberrant cytokinesis, and disrupt asymmetric cell fate inheritance. Consequently, the seam becomes disorganized and populated with extra cells that have lost seam identity, leading to fatal epidermal rupture. We show that dynein orients the spindle through the cortical GOA-1Gα-LIN-5NuMA pathway by directing the migration of prophase centrosomes along the anterior-posterior axis. Spindle misorientation in the dynein mutant can be partially rescued by elongating cells, implying that dynein-dependent force generation and cell shape jointly promote correct asymmetric division of epithelial stem cells.

Interplay between the plasma membrane and cell-cell adhesion maintains epithelial identity for correct polarised cell divisions

Polarised epithelial cell divisions represent a fundamental mechanism for tissue maintenance and morphogenesis. Morphological and mechanical changes in the plasma membrane influence the organisation and crosstalk of microtubules and actin at the cell cortex, thereby regulating the mitotic spindle machinery and chromosome segregation. Yet, the precise mechanisms linking plasma membrane remodelling to cell polarity and cortical cytoskeleton dynamics to ensure accurate execution of mitosis in mammalian epithelial cells remain poorly understood. Here, we manipulated the density of mammary epithelial cells in culture, which led to several mitotic defects. Perturbation of cell-cell adhesion formation impairs the dynamics of the plasma membrane, affecting the shape and size of mitotic cells and resulting in defects in mitotic progression and the generation of daughter cells with aberrant architecture. In these conditions, F- actin-astral microtubule crosstalk is impaired, leading to mitotic spindle misassembly and misorientation, which in turn contributes to chromosome mis-segregation. Mechanistically, we identify S100 Ca2+-binding protein A11 (S100A11) as a key membrane-associated regulator that forms a complex with E-cadherin (CDH1) and the leucine-glycine-asparagine repeat protein LGN (also known as GPSM2) to coordinate plasma membrane remodelling with E-cadherin-mediated cell adhesion and LGN-dependent mitotic spindle machinery. Thus, plasma membrane-mediated maintenance of mammalian epithelial cell identity is crucial for correct execution of polarised cell divisions, genome maintenance and safeguarding tissue integrity.

Asymmetric division of stem cells and its cancer relevance

Asymmetric division is a fundamental process for generating cell diversity and maintaining the stem cell population. During asymmetric division, proteins, organelles, and even RNA are distributed unequally between the two daughter cells, determining their distinct cell fates. The mechanisms orchestrating this process are extremely complex. Dysregulation of asymmetric division can potentially trigger cancer progression. Cancer stem cells, in particular, undergo asymmetric division, leading to intra-tumoral heterogeneity, which contributes to treatment refractoriness. In this review, we delve into the cellular and molecular mechanisms that govern asymmetric division and explore its relevance to tumorigenesis.

LGN loss randomizes spindle orientation and accelerates tumorigenesis in PTEN-deficient epidermis

Loss of cell polarity and disruption of tissue organization are key features of tumorigenesis that are intrinsically linked to spindle orientation. Epithelial tumors are often characterized by spindle orientation defects, but how these defects impact tumor formation driven by common oncogenic mutations is not fully understood. Here, we examine the role of spindle orientation in adult epidermis by deleting a key spindle regulator, LGN, in normal tissue and in a PTEN-deficient mouse model. We report that LGN deficiency in PTEN mutant epidermis leads to a threefold increase in the likelihood of developing tumors on the snout, and an over 10-fold increase in tumor burden. In this tissue, loss of LGN alone increases perpendicular and oblique divisions of epidermal basal cells, at the expense of a planar orientation of division. PTEN loss alone does not significantly affect spindle orientation in these cells, but the combined loss of PTEN and LGN fully randomizes basal spindle orientation. A subset of LGN- and PTEN-deficient animals have increased amounts of proliferative spinous cells, which may be associated with tumorigenesis. These results indicate that loss of LGN impacts spindle orientation and accelerates epidermal tumorigenesis in a PTEN-deficient mouse model.

The AP-2 complex interacts with γ-TuRC and regulates the proliferative capacity of neural progenitors

Centrosomes are organelles that nucleate microtubules via the activity of gamma-tubulin ring complexes (γ-TuRC). In the developing brain, centrosome integrity is central to the progression of the neural progenitor cell cycle, and its loss leads to microcephaly. We show that NPCs maintain centrosome integrity via the endocytic adaptor protein complex-2 (AP-2). NPCs lacking AP-2 exhibit defects in centrosome formation and mitotic progression, accompanied by DNA damage and accumulation of p53. This function of AP-2 in regulating the proliferative capacity of NPCs is independent of its role in clathrin-mediated endocytosis and is coupled to its association with the GCP2, GCP3, and GCP4 components of γ-TuRC. We find that AP-2 maintains γ-TuRC organization and regulates centrosome function at the level of MT nucleation. Taken together, our data reveal a novel, noncanonical function of AP-2 in regulating the proliferative capacity of NPCs and open new avenues for the identification of novel therapeutic strategies for the treatment of neurodevelopmental and neurodegenerative disorders with AP-2 complex dysfunction.

Phase Separation as a Driver of Stem Cell Organization and Function during Development

A properly organized subcellular composition is essential to cell function. The canonical organizing principle within eukaryotic cells involves membrane-bound organelles; yet, such structures do not fully explain cellular complexity. Furthermore, discrete non-membrane-bound structures have been known for over a century. Liquid-liquid phase separation (LLPS) has emerged as a ubiquitous mode of cellular organization without the need for formal lipid membranes, with an ever-expanding and diverse list of cellular functions that appear to be regulated by this process. In comparison to traditional organelles, LLPS can occur across wider spatial and temporal scales and involves more distinct protein and RNA complexes. In this review, we discuss the impacts of LLPS on the organization of stem cells and their function during development. Specifically, the roles of LLPS in developmental signaling pathways, chromatin organization, and gene expression will be detailed, as well as its impacts on essential processes of asymmetric cell division. We will also discuss how the dynamic and regulated nature of LLPS may afford stem cells an adaptable mode of organization throughout the developmental time to control cell fate. Finally, we will discuss how aberrant LLPS in these processes may contribute to developmental defects and disease.

NUSAP1 regulates mouse oocyte meiotic maturation

The correct assembly of the spindle apparatus directly regulates the precise separation of chromosomes in mouse oocytes, which is crucial for obtaining high-quality oocytes capable of successful fertilization. The localization, assembly, migration, and disassembly of the spindle are regulated by a series of spindle-associated proteins, which exhibit unique expression level variations and specific localization in oocytes. Proteomic analysis revealed that among many representative spindle-associated proteins, the expression level of nucleolar and spindle-associated protein 1 (NUSAP1) significantly increased after meiotic resumption, with a magnitude of change higher than that of other proteins. However, the role of NUSAP1 during oocyte meiosis maturation has not been reported. Here, we report that NUSAP1 is distributed within the cell nucleus during the germinal vesicle (GV) oocytes with non-surrounded nucleolus stage and is not enriched in the nucleus during the GV-surrounded nucleolus stage. Interestingly, NUSAP1 forms distinct granular aggregates near the spindle poles during the prophase of the first meiotic division (Pro-MI), metaphase I, and anaphase I/telophase I stages. Nusap1 depletion leads to chromosome misalignment, increased aneuploidy, and abnormal spindle assembly, particularly a decrease in spindle pole width. Correspondingly, RNA-seq analysis revealed significant suppression of the "establishment of spindle orientation" signaling pathway. Additionally, the attenuation of F-actin in NUSAP1-deficient oocytes may affect the asymmetric division process. Gene ontology analysis of NUSAP1 interactomes, identified through mass spectrometry here, revealed significant enrichment for RNA binding. As an RNA-binding protein, NUSAP1 is likely involved in the regulation of messenger RNA homeostasis by influencing the dynamics of processing (P)-body components. Overall, our results demonstrate the critical importance of precise regulation of NUSAP1 expression levels and protein localization for maintaining mouse oocyte meiosis.

Stationary shapes of axisymmetric vesicles beyond lowest-energy configurations

We conduct a systematic exploration of the energy landscape of vesicle morphologies within the framework of the Helfrich model. Vesicle shapes are determined by minimizing the elastic energy subject to constraints of constant area and volume. The results show that pressurized vesicles can adopt higher-energy spindle-like configurations that require the action of point forces at the poles. If the internal pressure is lower than the external one, multilobed shapes are predicted. We utilize our results to rationalize the experimentally observed spindle shapes of giant vesicles in a uniform AC field.

Nrg1 Regulates Cardiomyocyte Migration and Cell Cycle in Ventricular Development

BACKGROUND

Cardiac ventricles provide the contractile force of the beating heart throughout life. How the primitive endocardium-layered myocardial projections called trabeculae form and mature into the adult ventricles is of great interest for biology and regenerative medicine. Trabeculation is dependent on the signaling protein Nrg1 (neuregulin-1). However, the mechanism of action of Nrg1 and its role in ventricular wall maturation are poorly understood.

METHODS

We investigated the functions and downstream mechanisms of Nrg1 signaling during ventricular chamber development using confocal imaging, transcriptomics, and biochemical approaches in mice with cardiac-specific inactivation or overexpression of Nrg1.

RESULTS

Analysis of cardiac-specific Nrg1 mutant mice showed that the transcriptional program underlying cardiomyocyte-oriented cell division and trabeculae formation depends on endocardial Nrg1 to myocardial ErbB2 (erb-b2 receptor tyrosine kinase 2) signaling and phospho-Erk (phosphorylated extracellular signal-regulated kinase; pErk) activation. Early endothelial loss of Nrg1 and reduced pErk activation diminished cardiomyocyte Pard3 and Crumbs2 (Crumbs Cell Polarity Complex Component 2) protein and altered cytoskeletal gene expression and organization. These alterations are associated with abnormal gene expression related to mitotic spindle organization and a shift in cardiomyocyte division orientation. Nrg1 is crucial for trabecular growth and ventricular wall thickening by regulating an epithelial-to-mesenchymal transition-like process in cardiomyocytes involving migration, adhesion, cytoskeletal actin turnover, and timely progression through the cell cycle G2/M phase. Ectopic cardiac Nrg1 overexpression and high pErk signaling caused S-phase arrest, sustained high epithelial-to-mesenchymal transition-like gene expression, and prolonged trabeculation, blocking compact myocardium maturation. Myocardial trabecular patterning alterations resulting from above- or below-normal Nrg1-dependent pErk activation were concomitant with sarcomere actin cytoskeleton disorganization. The Nrg1 loss- and gain-of-function transcriptomes were enriched for Yap1 (yes-associated protein-1) gene signatures, identifying Yap1 as a potential downstream effector. Furthermore, biochemical and imaging data reveal that Nrg1 influences pErk activation and Yap1 nuclear-cytoplasmic distribution during trabeculation.

CONCLUSIONS

These data establish the Nrg1-ErbB2/ErbB4-Erk axis as a crucial regulator of cardiomyocyte cell cycle progression and migration during ventricular development.

A Haspin-ARHGAP11A axis regulates epithelial morphogenesis through Rho-ROCK dependent modulation of LIMK1-Cofilin

Throughout mitosis, a plethora of processes must be efficiently concerted to ensure cell proliferation and tissue functionality. The mitotic spindle does not only mediate chromosome segregation, but also defines the axis of cellular division, thus determining tissue morphology. Functional spindle orientation relies on precise actin dynamics, shaped in mitosis by the LIMK1-Cofilin axis. The kinase Haspin acts as a guardian of faithful chromosome segregation that ensures amphitelic chromosome attachment and prevents unscheduled cohesin cleavage. Here, we report an unprecedented role for Haspin in the determination of spindle orientation in mitosis. We show that, during mitosis, Haspin regulates Rho-ROCK activity through ARHGAP11A, a poorly characterized GAP, and that ROCK is in turn responsible for the mitotic activation of LIMK1 and stabilization of the actin cytoskeleton, thus supporting a functional spindle orientation. By exploiting 3D cell cultures, we show that this pathway is pivotal for the establishment of a morphologically functional tissue.

Length limitation of astral microtubules orients cell divisions in murine intestinal crypts

Planar spindle orientation is critical for epithelial tissue organization and is generally instructed by the long cell-shape axis or cortical polarity domains. We introduced mouse intestinal organoids in order to study spindle orientation in a monolayered mammalian epithelium. Although spindles were planar, mitotic cells remained elongated along the apico-basal (A-B) axis, and polarity complexes were segregated to basal poles, so that spindles oriented in an unconventional manner, orthogonal to both polarity and geometric cues. Using high-resolution 3D imaging, simulations, and cell-shape and cytoskeleton manipulations, we show that planar divisions resulted from a length limitation in astral microtubules (MTs) which precludes them from interacting with basal polarity, and orient spindles from the local geometry of apical domains. Accordingly, lengthening MTs affected spindle planarity, cell positioning, and crypt arrangement. We conclude that MT length regulation may serve as a key mechanism for spindles to sense local cell shapes and tissue forces to preserve mammalian epithelial architecture.

Research Advances in the Role of the Tropomyosin Family in Cancer

Cancer is one of the most difficult diseases for human beings to overcome. Its development is closely related to a variety of factors, and its specific mechanisms have been a hot research topic in the field of scientific research. The tropomyosin family (Tpm) is a group of proteins closely related to the cytoskeleton and actin, and recent studies have shown that they play an important role in various cancers, participating in a variety of biological activities, including cell proliferation, invasion, and migration, and have been used as biomarkers for various cancers. The purpose of this review is to explore the research progress of the Tpm family in tumorigenesis development, focusing on the molecular pathways associated with them and their relevant activities involved in tumors. PubMed and Web of Science databases were searched for relevant studies on the role of Tpms in tumorigenesis and development and the activities of Tpms involved in tumors. Data from the literature suggest that the Tpm family is involved in tumor cell proliferation and growth, tumor cell invasion and migration, tumor angiogenesis, tumor cell apoptosis, and immune infiltration of the tumor microenvironment, among other correlations. It can be used as a potential biomarker for early diagnosis, follow-up, and therapeutic response of some tumors. The Tpm family is involved in cancer in a close relationship with miRNAs and LncRNAs. Tpms are involved in tumor tissue invasion and migration as a key link. On this basis, TPM is frequently used as a biomarker for various cancers. However, the specific molecular mechanism of its involvement in cancer progression has not been explained clearly, which remains an important direction for future research.

Drosophila Adducin facilitates phase separation and function of a conserved spindle orientation complex

Asymmetric cell division (ACD) allows stem cells to generate differentiating progeny while simultaneously maintaining their own pluripotent state. ACD involves coupling mitotic spindle orientation with cortical polarity cues to direct unequal segregation of cell fate determinants. In Drosophila neural stem cells (neuroblasts; NBs), spindles orient along an apical-basal polarity axis through a conserved complex of Partner of Inscuteable (Pins; human LGN) and Mushroom body defect (Mud; human NuMA). While many details of its function are well known, the molecular mechanics that drive assembly of the cortical Pins/Mud complex remain unclear, particularly with respect to the mutually exclusive Pins complex formed with the apical scaffold protein Inscuteable (Insc). Here we identify Hu li tai shao (Hts; human Adducin) as a direct Mud-binding protein, using an aldolase fold within its head domain (HtsHEAD) to bind a short Mud coiled-coil domain (MudCC) that is adjacent to the Pins-binding domain (MudPBD). Hts is expressed throughout the larval central brain and apically polarizes in mitotic NBs where it is required for Mud-dependent spindle orientation. In vitro analyses reveal that Pins undergoes liquid-liquid phase separation with Mud, but not with Insc, suggesting a potential molecular basis for differential assembly mechanics between these two competing apical protein complexes. Furthermore, we find that Hts binds an intact Pins/Mud complex, reduces the concentration threshold for its phase separation, and alters the liquid-like property of the resulting phase separated droplets. Domain mapping and mutational analyses implicate critical roles for both multivalent interactions (via MudCC oligomerization) and protein disorder (via an intrinsically disordered region in Hts; HtsIDR) in phase separation of the Hts/Mud/Pins complex. Our study identifies a new component of the spindle positioning machinery in NBs and suggests that phase separation of specific protein complexes might regulate ordered assembly within the apical domain to ensure proper signaling output.

Mechanisms underlying spindle assembly and robustness

The microtubule-based spindle orchestrates chromosome segregation during cell division. Following more than a century of study, many components and pathways contributing to spindle assembly have been described, but how the spindle robustly assembles remains incompletely understood. This process involves the self-organization of a large number of molecular parts - up to hundreds of thousands in vertebrate cells - whose local interactions give rise to a cellular-scale structure with emergent architecture, mechanics and function. In this Review, we discuss key concepts in our understanding of spindle assembly, focusing on recent advances and the new approaches that enabled them. We describe the pathways that generate the microtubule framework of the spindle by driving microtubule nucleation in a spatially controlled fashion and present recent insights regarding the organization of individual microtubules into structural modules. Finally, we discuss the emergent properties of the spindle that enable robust chromosome segregation.

Deep learning techniques and mathematical modeling allow 3D analysis of mitotic spindle dynamics

Time-lapse microscopy movies have transformed the study of subcellular dynamics. However, manual analysis of movies can introduce bias and variability, obscuring important insights. While automation can overcome such limitations, spatial and temporal discontinuities in time-lapse movies render methods such as 3D object segmentation and tracking difficult. Here, we present SpinX, a framework for reconstructing gaps between successive image frames by combining deep learning and mathematical object modeling. By incorporating expert feedback through selective annotations, SpinX identifies subcellular structures, despite confounding neighbor-cell information, non-uniform illumination, and variable fluorophore marker intensities. The automation and continuity introduced here allows the precise 3D tracking and analysis of spindle movements with respect to the cell cortex for the first time. We demonstrate the utility of SpinX using distinct spindle markers, cell lines, microscopes, and drug treatments. In summary, SpinX provides an exciting opportunity to study spindle dynamics in a sophisticated way, creating a framework for step changes in studies using time-lapse microscopy.

A novel tool for the unbiased characterization of epithelial monolayer development in culture

The function of an epithelial tissue is intertwined with its architecture. Epithelial tissues are often described as pseudo-two-dimensional, but this view may be partly attributed to experimental bias: many model epithelia, including cultured cell lines, are easiest to image from the "top-down." We measured the three-dimensional architecture of epithelial cells in culture and found that it varies dramatically across cultured regions, presenting a challenge for reproducibility and cross-study comparisons. We therefore developed a novel tool (Automated Layer Analysis, "ALAn") to characterize architecture in an unbiased manner. Using ALAn, we find that cultured epithelial cells can organize into four distinct architectures and that architecture correlates with cell density. Cells exhibit distinct biological properties in each architecture. Organization in the apical-basal axis is determined early in monolayer development by substrate availability, while disorganization in the apical-basal axis arises from an inability to form substrate connections. Our work highlights the need to carefully control for three-dimensional architecture when using cell culture as a model system for epithelial cell biology and introduces a novel tool, built on a set of rules that can be widely applied to epithelial cell culture.

The Drosophila mitotic spindle orientation machinery requires activation, not just localization

The orientation of the mitotic spindle at metaphase determines the placement of the daughter cells. Spindle orientation in animals typically relies on an evolutionarily conserved biological machine comprised of at least four proteins - called Pins, Gαi, Mud, and Dynein in flies - that exerts a pulling force on astral microtubules and reels the spindle into alignment. The canonical model for spindle orientation holds that the direction of pulling is determined by asymmetric placement of this machinery at the cell cortex. In most cell types, this placement is thought to be mediated by Pins, and a substantial body of literature is therefore devoted to identifying polarized cues that govern localized cortical enrichment of Pins. In this study we revisit the canonical model and find that it is incomplete. Spindle orientation in the Drosophila follicular epithelium and embryonic ectoderm requires not only Pins localization but also direct interaction between Pins and the multifunctional protein Discs large. This requirement can be over-ridden by interaction with another Pins interacting protein, Inscuteable.

Additive manufacturing of Schwann cell-laden collagen/alginate nerve guidance conduits by freeform reversible embedding regulate neurogenesis via exosomes secretion towards peripheral nerve regeneration

Peripheral nerve injury is a common clinical problem that could be debilitating to one's quality of life. The complex nerve guidance conduits (NGCs) with cells in order to improve nerve regeneration. Therefore, we used freeform reversible embedding of suspended hydrogels to fabricate Schwann cells (SCs)-laden collagen/alginate (Col/Alg) NGCs. First, we evaluated Col influence on the characteristics of NGCs. After which, Wharton's jelly mesenchymal stem cells (WJMSC) are seeded onto the inner channel of NGCs and evaluated neural regeneration behaviors. Results indicated the SCs-laden NGCs with 2.5 % Col found the highest proliferation and secretion of neurotrophic protein. Furthermore, co-culture of SCs promoted differentiation of WJMSC as seen from the increased neurogenic-related protein in NGCs. To determine the molecular mechanism between SCs and WJMSC, we demonstrated the neurotrophic factors secreted by SCs act on tropomyosin receptor kinase A (TrkA) receptors of WJMSC to promote nerve regeneration. In addition, our study demonstrated SCs-derived exosomes had a critical role in regulating neural differentiation of WJMSC. Taken together, this study demonstrates the fabrication of SCs-laden Col/Alg NGCs for nerve regeneration and understanding regarding the synergistic regenerative mechanisms of different cells could bring us a step closer for clinical treatment of large nerve defects.

CDK5RAP2 is a Wnt target gene and promotes stemness and progression of oral squamous cell carcinoma

In oral squamous cell carcinoma (OSCC), a highly aggressive and frequently lethal malignancy, the role and action mechanism of the microtubule regulatory protein CDK5RAP2 have not been fully understood. Here, we show that CDK5RAP2 is highly expressed in OSCC and its expression correlates with clinical stage and lymph node metastasis of the disease. The expression of CDK5RAP2 is regulated by the Wnt signaling pathway. Depletion of CDK5RAP2 inhibits the tumorigenesis and migration of OSCC cells and alters the OSCC cancer stem (-like) cell (CSC) signature. Notably, suppression of CDK5RAP2 expression disrupts spindle orientation during mitosis. Collectively, these results identify CDK5RAP2 as a potential CSC marker and reveal a mechanism that controls the CSC population in OSCC.

Regulation and functions of cell division in the intestinal tissue

In multicellular organisms, epithelial cells are key elements of tissue organization. In developing epithelial tissues, cellular proliferation and differentiation are under the tight regulation of morphogenetic programs to ensure correct organ formation and functioning. In these processes, proliferation rates and division orientation regulate the speed, timing and direction of tissue expansion but also its proper patterning. Moreover, tissue homeostasis relies on spatio-temporal modulations of daughter cell behavior and arrangement. These aspects are particularly crucial in the intestine, which is one of the most proliferative tissues in adults, making it a very attractive adult organ system to study the role of cell division on epithelial morphogenesis and organ function. Although epithelial cell division has been the subject of intense research for many years in multiple models, it still remains in its infancy in the context of the intestinal tissue. In this review, we focus on the current knowledge on cell division and regulatory mechanisms at play in the intestinal epithelial tissue, as well as their importance in developmental biology and physiopathology.

E-cadherin in developing murine T cells controls spindle alignment and progression through β-selection

A critical stage of T cell development is β-selection; at this stage, the T cell receptor β (TCRβ) chain is generated, and the developing T cell starts to acquire antigenic specificity. Progression through β-selection is assisted by low-affinity interactions between the nascent TCRβ chain and peptide presented on stromal major histocompatibility complex and cues provided by the niche. In this study, we identify a cue within the developing T cell niche that is critical for T cell development. E-cadherin mediates cell-cell interactions and influences cell fate in many developmental systems. In developing T cells, E-cadherin contributed to the formation of an immunological synapse and the alignment of the mitotic spindle with the polarity axis during division, which facilitated subsequent T cell development. Collectively, these data suggest that E-cadherin facilitates interactions with the thymic niche to coordinate the β-selection stage of T cell development.

Scribble and E-cadherin cooperate to control symmetric daughter cell positioning by multiple mechanisms

The fate of the two daughter cells is intimately connected to their positioning, which is in turn regulated by cell junction remodelling and orientation of the mitotic spindle. How multiple cues are integrated to dictate the ultimate positioning of daughters is not clear. Here, we identify novel mechanisms of regulation of daughter positioning in single MCF10A cells. The polarity protein, Scribble cooperates with E-cadherin for sequential roles in daughter positioning. First Scribble stabilises E-cadherin at the mitotic cortex as well as the retraction fibres, to mediate spindle orientation. Second, Scribble re-locates to the junction between the two daughters to allow a new E-cadherin-based-interface to form between them, influencing the width of the nascent daughter-daughter junction and subsequent cell positioning. Thus, E-cadherin and Scribble dynamically relocate to different intracellular sites during cell division to orient the mitotic spindle and control placement of the daughter cells after cell division. This article has an associated First Person interview with the first author of the paper.

Annexin A1 is a polarity cue that directs mitotic spindle orientation during mammalian epithelial morphogenesis

Oriented cell divisions are critical for the formation and maintenance of structured epithelia. Proper mitotic spindle orientation relies on polarised anchoring of force generators to the cell cortex by the evolutionarily conserved protein complex formed by the G_αi subunit of heterotrimeric G proteins, the Leucine-Glycine-Asparagine repeat protein (LGN) and the nuclear mitotic apparatus protein. However, the polarity cues that control cortical patterning of this ternary complex remain largely unknown in mammalian epithelia. Here we identify the membrane-associated protein Annexin A1 (ANXA1) as an interactor of LGN in mammary epithelial cells. Annexin A1 acts independently of G_αi to instruct the accumulation of LGN and nuclear mitotic apparatus protein at the lateral cortex to ensure cortical anchoring of Dynein-Dynactin and astral microtubules and thereby planar alignment of the mitotic spindle. Loss of Annexin A1 randomises mitotic spindle orientation, which in turn disrupts epithelial architecture and luminogenesis in three-dimensional cultures of primary mammary epithelial cells. Our findings establish Annexin A1 as an upstream cortical cue that regulates LGN to direct planar cell divisions during mammalian epithelial morphogenesis.

Automated Immunofluorescence Staining for Analysis of Mitotic Stages and Division Orientation in Brain Sections

Microcephaly often results from mitotic defects in neuronal progenitors, frequently by decreasing proliferation rates or shifting cell fates. During neurogenesis, oriented cell division-the molecular control of mitotic spindle positioning to control the axis of division-represents an important mechanism to balance expansion of the progenitor pool with generating cellular diversity. While mostly studied in the context of cortical development, more recently, spindle orientation has emerged as a key player in the formation of other brain regions such as the cerebellum. Here we describe methods to perform automated dual-color fluorescent immunohistochemistry on murine cerebellar sections using the mitotic markers phospho-Histone H3 and Survivin, and detail analytical and statistical approaches to display and compare division orientation datasets.