PP2A-B55SUR-6 promotes nuclear envelope breakdown in C. elegans embryos.

Nuclear envelope (NE) disassembly during mitosis is critical to ensure faithful segregation of the genetic material. NE disassembly is a phosphorylation-dependent process wherein mitotic kinases hyper-phosphorylate lamina and nucleoporins to initiate nuclear envelope breakdown (NEBD). In this study, we uncover an unexpected role of the PP2A phosphatase B55SUR-6 in NEBD during the first embryonic division of Caenorhabditis elegans embryo. B55SUR-6 depletion delays NE permeabilization and stabilizes lamina and nucleoporins. As a result, the merging of parental genomes and chromosome segregation is impaired. NEBD defect upon B55SUR-6 depletion is not due to delayed mitotic onset or mislocalization of mitotic kinases. Importantly, we demonstrate that microtubule-dependent mechanical forces synergize with B55SUR-6 for efficient NEBD. Finally, our data suggest that the lamin LMN-1 is likely a bona fide target of PP2A-B55SUR-6. These findings establish a model highlighting biochemical crosstalk between kinases, PP2A-B55SUR-6 phosphatase, and microtubule-generated mechanical forces in timely NE dissolution.

Membrane compartmentalization of Ect2/Cyk4/Mklp1 and NuMA/dynein regulates cleavage furrow formation.

In animal cells, spindle elongation during anaphase is temporally coupled with cleavage furrow formation. Spindle elongation during anaphase is regulated by NuMA/dynein/dynactin complexes that occupy the polar region of the cell membrane and are excluded from the equatorial membrane. How NuMA/dynein/dynactin are excluded from the equatorial membrane and the biological significance of this exclusion remains unknown. Here, we show that the centralspindlin (Cyk4/Mklp1) and its interacting partner RhoGEF Ect2 are required for NuMA/dynein/dynactin exclusion from the equatorial cell membrane. The Ect2-based (Ect2/Cyk4/Mklp1) and NuMA-based (NuMA/dynein/dynactin) complexes occupy mutually exclusive membrane surfaces during anaphase. The equatorial membrane enrichment of Ect2-based complexes is essential for NuMA/dynein/dynactin exclusion and proper spindle elongation. Conversely, NuMA-based complexes at the polar region of the cell membrane ensure spatially confined localization of Ect2-based complexes and thus RhoA. Overall, our work establishes that membrane compartmentalization of NuMA-based and Ect2-based complexes at the two distinct cell surfaces restricts dynein/dynactin and RhoA for coordinating spindle elongation with cleavage furrow formation.

NuMA interaction with chromatin is vital for proper chromosome decondensation at the mitotic exit.

NuMA is an abundant long coiled-coil protein that plays a prominent role in spindle organization during mitosis. In interphase, NuMA is localized to the nucleus and hypothesized to control gene expression and chromatin organization. However, because of the prominent mitotic phenotype upon NuMA loss, its precise function in the interphase nucleus remains elusive. Here, we report that NuMA is associated with chromatin in interphase and prophase but released upon nuclear envelope breakdown (NEBD) by the action of Cdk1. We uncover that NuMA directly interacts with DNA via evolutionarily conserved sequences in its C-terminus. Notably, the expression of the DNA-binding-deficient mutant of NuMA affects chromatin decondensation at the mitotic exit, and nuclear shape in interphase. We show that the nuclear shape defects observed upon mutant NuMA expression are due to its potential to polymerize into higher-order fibrillar structures. Overall, this work establishes the spindle-independent function of NuMA in choreographing proper chromatin decompaction and nuclear shape by directly associating with the DNA.

PP2A--B55γ counteracts Cdk1 and regulates proper spindle orientation through the cortical dynein adaptor NuMA.

Proper orientation of the mitotic spindle is critical for accurate development and morphogenesis. In human cells, spindle orientation is regulated by the evolutionarily conserved protein NuMA, which interacts with dynein and enriches it at the cell cortex. Pulling forces generated by cortical dynein orient the mitotic spindle. Cdk1-mediated phosphorylation of NuMA at threonine 2055 (T2055) negatively regulates its cortical localization. Thus, only NuMA not phosphorylated at T2055 localizes at the cell cortex. However, the identity and the mechanism of action of the phosphatase complex involved in T2055 dephosphorylation remains elusive. Here, we characterized the PPP2CA-B55γ (PPP2R2C)-PPP2R1B complex that counteracts Cdk1 to orchestrate cortical NuMA for proper spindle orientation. In vitro reconstitution experiments revealed that this complex is sufficient for T2055 dephosphorylation. Importantly, we identified polybasic residues in NuMA that are critical for T2055 dephosphorylation, and for maintaining appropriate cortical NuMA levels for accurate spindle elongation. Furthermore, we found that Cdk1-mediated phosphorylation and PP2A-B55γ-mediated dephosphorylation at T2055 are reversible events. Altogether, this study uncovers a novel mechanism by which Cdk1 and its counteracting PP2A-B55γ complex orchestrate spatiotemporal levels of cortical force generators for flawless mitosis.

Centrosome Aurora A gradient ensures single polarity axis in C. elegans embryos.

Cellular asymmetries are vital for generating cell fate diversity during development and in stem cells. In the newly fertilized Caenorhabditis elegans embryo, centrosomes are responsible for polarity establishment, i.e. anterior-posterior body axis formation. The signal for polarity originates from the centrosomes and is transmitted to the cell cortex, where it disassembles the actomyosin network. This event leads to symmetry breaking and the establishment of distinct domains of evolutionarily conserved PAR proteins. However, the identity of an essential component that localizes to the centrosomes and promotes symmetry breaking was unknown. Recent work has uncovered that the loss of Aurora A kinase (AIR-1 in C. elegans and hereafter referred to as Aurora A) in the one-cell embryo disrupts stereotypical actomyosin-based cortical flows that occur at the time of polarity establishment. This misregulation of actomyosin flow dynamics results in the occurrence of two polarity axes. Notably, the role of Aurora A in ensuring a single polarity axis is independent of its well-established function in centrosome maturation. The mechanism by which Aurora A directs symmetry breaking is likely through direct regulation of Rho-dependent contractility. In this mini-review, we will discuss the unconventional role of Aurora A kinase in polarity establishment in C. elegans embryos and propose a refined model of centrosome-dependent symmetry breaking.

Silencing of class I small heat shock proteins affects seed-related attributes and thermotolerance in rice seedlings.

MAIN CONCLUSION: Silencing of CI-sHsps by RNAi negatively affected the seed germination process and heat stress response of rice seedlings. Seed size of RNAiCI-sHsp was reduced as compared to wild-type plants. Small heat shock proteins (sHsps) are the ATP-independent chaperones ubiquitously expressed in response to diverse environmental and developmental cues. Cytosolic sHsps constitute the major repertoire of sHsp family. Rice cytosolic class I (CI)-sHsps consists of seven members (Hsp16.9A, Hsp16.9B, Hsp16.9C, Hsp17.4, Hsp17.7, Hsp17.9A and Hsp18). Purified OsHsp17.4 and OsHsp17.9A proteins exhibited chaperone activity by preventing formation of large aggregates with model substrate citrate synthase. OsHsp16.9A and OsHsp17.4 showed nucleo-cytoplasmic localization, while the localization of OsHsp17.9A was preferentially in the nucleus. Transgenic tobacco plants expressing OsHsp17.4 and OsHsp17.9A proteins and Arabidopsis plants ectopically expressing OsHsp17.4 protein showed improved thermotolerance to the respective trans-hosts during the post-stress recovery process. Single hairpin construct was designed to generate all CI-sHsp silenced (RNAiCI-sHsp) rice lines. The major vegetative and reproductive attributes of the RNAiCI-sHsp plants were comparable to the wild-type rice plants. Basal and acquired thermotolerance response of RNAiCI-sHsp seedlings of rice was mildly affected. The seed length of RNAiCI-sHsp rice plants was significantly reduced. The seed germination process was delayed and seed thermotolerance of RNAiCI-sHsp was negatively affected than the non-transgenic seeds. We, thus, implicate that sHsp genes are critical in seedling thermotolerance and seed physiology.

Centrosome Aurora A regulates RhoGEF ECT-2 localisation and ensures a single PAR-2 polarity axis in C. elegans embryos.

Proper establishment of cell polarity is essential for development. In the one-cell C. elegans embryo, a centrosome-localised signal provides spatial information for polarity establishment. It is hypothesised that this signal causes local inhibition of the cortical actomyosin network, and breaks symmetry to direct partitioning of the PAR proteins. However, the molecular nature of the centrosomal signal that triggers cortical anisotropy in the actomyosin network to promote polarity establishment remains elusive. Here, we discover that depletion of Aurora A kinase (AIR-1 in C. elegans) causes pronounced cortical contractions on the embryo surface, and this creates more than one PAR-2 polarity axis. This function of AIR-1 appears to be independent of its role in microtubule nucleation. Importantly, upon AIR-1 depletion, centrosome positioning becomes dispensable in dictating the PAR-2 axis. Moreover, we uncovered that a Rho GEF, ECT-2, acts downstream of AIR-1 in regulating contractility and PAR-2 localisation, and, notably, AIR-1 depletion influences ECT-2 cortical localisation. Overall, this study provides a novel insight into how an evolutionarily conserved centrosome Aurora A kinase inhibits promiscuous PAR-2 domain formation to ensure singularity in the polarity establishment axis.

Mechanisms of Spindle Positioning: Lessons from Worms and Mammalian Cells.

Proper positioning of the mitotic spindle is fundamental for specifying the site for cleavage furrow, and thus regulates the appropriate sizes and accurate distribution of the cell fate determinants in the resulting daughter cells during development and in the stem cells. The past couple of years have witnessed tremendous work accomplished in the area of spindle positioning, and this has led to the emergence of a working model unravelling in-depth mechanistic insight of the underlying process orchestrating spindle positioning. It is evident now that the correct positioning of the mitotic spindle is not only guided by the chemical cues (protein⁻protein interactions) but also influenced by the physical nature of the cellular environment. In metazoans, the key players that regulate proper spindle positioning are the actin-rich cell cortex and associated proteins, the ternary complex (Gα/GPR-1/2/LIN-5 in Caenorhabditis elegans, Gαi/Pins/Mud in Drosophila and Gαi1-3/LGN/NuMA in humans), minus-end-directed motor protein dynein and the cortical machinery containing myosin. In this review, I will mainly discuss how the abovementioned components precisely and spatiotemporally regulate spindle positioning by sensing the physicochemical environment for execution of flawless mitosis.

Plk1 regulates spindle orientation by phosphorylating NuMA in human cells.

Proper orientation of the mitotic spindle defines the correct division plane and is essential for accurate cell division and development. In metazoans, an evolutionarily conserved complex comprising of NuMA/LGN/Gαi regulates proper orientation of the mitotic spindle by orchestrating cortical dynein levels during metaphase. However, the molecular mechanisms that modulate the spatiotemporal dynamics of this complex during mitosis remain elusive. Here, we report that acute inactivation of Polo-like kinase 1 (Plk1) during metaphase enriches cortical levels of dynein/NuMA/LGN and thus influences spindle orientation. We establish that this impact of Plk1 on cortical levels of dynein/NuMA/LGN is through NuMA, but not via dynein/LGN. Moreover, we reveal that Plk1 inhibition alters the dynamic behavior of NuMA at the cell cortex. We further show that Plk1 directly interacts and phosphorylates NuMA. Notably, NuMA-phosphorylation by Plk1 impacts its cortical localization, and this is needed for precise spindle orientation during metaphase. Overall, our finding connects spindle-pole pool of Plk1 with cortical NuMA and answers a long-standing puzzle about how spindle-pole Plk1 gradient dictates proper spindle orientation for error-free mitosis.

Mitotic Spindle: Illuminating Spindle Positioning with a Biological Lightsaber.

In metazoans, positioning of the mitotic spindle is controlled by the microtubule-dependent motor protein dynein, which associates with the cell cortex. Using optogenetic tools, two new studies examine how the levels and activity of dynein are regulated at the cortex to ensure proper positioning of the mitotic spindle.

Discovery of a Selective Aurora A Kinase Inhibitor by Virtual Screening.

Here we report the discovery of a selective inhibitor of Aurora A, a key regulator of cell division and potential anticancer target. We used the atom category extended ligand overlap score (xLOS), a 3D ligand-based virtual screening method recently developed in our group, to select 437 shape and pharmacophore analogs of reference kinase inhibitors. Biochemical screening uncovered two inhibitor series with scaffolds unprecedented among kinase inhibitors. One of them was successfully optimized by structure-based design to a potent Aurora A inhibitor (IC50 = 2 nM) with very high kinome selectivity for Aurora kinases. This inhibitor locks Aurora A in an inactive conformation and disrupts binding to its activator protein TPX2, which impairs Aurora A localization at the mitotic spindle and induces cell division defects. This phenotype can be rescued by inhibitor-resistant Aurora A mutants. The inhibitor furthermore does not induce Aurora B specific effects in cells.

Aurora A kinase regulates proper spindle positioning in C. elegans and in human cells.

Accurate spindle positioning is essential for error-free cell division. The one-cell Caenorhabditis elegans embryo has proven instrumental for dissecting mechanisms governing spindle positioning. Despite important progress, how the cortical forces that act on astral microtubules to properly position the spindle are modulated is incompletely understood. Here, we report that the PP6 phosphatase PPH-6 and its associated subunit SAPS-1, which positively regulate pulling forces acting on spindle poles, associate with the Aurora A kinase AIR-1 in C. elegans embryos. We show that acute inactivation of AIR-1 during mitosis results in excess pulling forces on astral microtubules. Furthermore, we uncover that AIR-1 acts downstream of PPH-6-SAPS-1 in modulating spindle positioning, and that PPH-6-SAPS-1 negatively regulates AIR-1 localization at the cell cortex. Moreover, we show that Aurora A and the PP6 phosphatase subunit PPP6C are also necessary for spindle positioning in human cells. There, Aurora A is needed for the cortical localization of NuMA and dynein during mitosis. Overall, our work demonstrates that Aurora A kinases and PP6 phosphatases have an ancient function in modulating spindle positioning, thus contributing to faithful cell division.

NuMA interacts with phosphoinositides and links the mitotic spindle with the plasma membrane.

The positioning and the elongation of the mitotic spindle must be carefully regulated. In human cells, the evolutionary conserved proteins LGN/Gαi1-3 anchor the coiled-coil protein NuMA and dynein to the cell cortex during metaphase, thus ensuring proper spindle positioning. The mechanisms governing cortical localization of NuMA and dynein during anaphase remain more elusive. Here, we report that LGN/Gαi1-3 are dispensable for NuMA-dependent cortical dynein enrichment during anaphase. We further establish that NuMA is excluded from the equatorial region of the cell cortex in a manner that depends on the centralspindlin components CYK4 and MKLP1. Importantly, we reveal that NuMA can directly associate with PtdInsP (PIP) and PtdInsP2 (PIP2) phosphoinositides in vitro. Furthermore, chemical or enzymatic depletion of PIP/PIP2 prevents NuMA cortical localization during mitosis, and conversely, increasing PIP2 levels augments mitotic cortical NuMA. Overall, our study uncovers a novel function for plasma membrane phospholipids in governing cortical NuMA distribution and thus the proper execution of mitosis.

NuMA phosphorylation by CDK1 couples mitotic progression with cortical dynein function.

Spindle positioning and spindle elongation are critical for proper cell division. In human cells, an evolutionary conserved ternary complex (NuMA/LGN/Gαi) anchors dynein at the cortex during metaphase, thus ensuring correct spindle positioning. Whether this complex contributes to anaphase spindle elongation is not known. More generally, the mechanisms coupling mitotic progression with spindle behaviour remain elusive. Here, we uncover that levels of cortical dynein markedly increase during anaphase in a NuMA-dependent manner. We demonstrate that during metaphase, CDK1-mediated phosphorylation at T2055 negatively regulates NuMA cortical localization and that this phosphorylation is counteracted by PPP2CA phosphatase activity. We establish that this tug of war is essential for proper levels of cortical dynein and thus spindle positioning during metaphase. Moreover, we find that upon CDK1 inactivation in anaphase, the rise in dephosphorylated NuMA at the cell cortex leads to cortical dynein enrichment, and thus to robust spindle elongation. Our findings uncover a mechanism whereby the status of NuMA phosphorylation coordinates mitotic progression with proper spindle function.

Mechanisms of spindle positioning: cortical force generators in the limelight.

Correct positioning of the spindle governs placement of the cytokinesis furrow and thus plays a crucial role in the partitioning of fate determinants and the disposition of daughter cells in a tissue. Converging evidence indicates that spindle positioning is often dictated by interactions between the plus-end of astral microtubules that emanate from the spindle poles and an evolutionary conserved cortical machinery that serves to pull on them. At the heart of this machinery lies a ternary complex (LIN-5/GPR-1/2/Gα in Caenorhabditis elegans and NuMA/LGN/Gαi in Homo sapiens) that promotes the presence of the motor protein dynein at the cell cortex. In this review, we discuss how the above components contribute to spindle positioning and how the underlying mechanisms are precisely regulated to ensure the proper execution of this crucial process in metazoan organisms.

MISP is a novel Plk1 substrate required for proper spindle orientation and mitotic progression.

Precise positioning of the mitotic spindle determines the correct cell division axis and is crucial for organism development. Spindle positioning is mediated through a cortical machinery by capturing astral microtubules, thereby generating pushing/pulling forces at the cell cortex. However, the molecular link between these two structures remains elusive. Here we describe a previously uncharacterized protein, MISP (C19orf21), as a substrate of Plk1 that is required for correct mitotic spindle positioning. MISP is an actin-associated protein throughout the cell cycle. MISP depletion led to an impaired metaphase-to-anaphase transition, which depended on phosphorylation by Plk1. Loss of MISP induced mitotic defects including spindle misorientation accompanied by shortened astral microtubules. Furthermore, we find that MISP formed a complex with and regulated the cortical distribution of the +TIP binding protein p150(glued), a subunit of the dynein-dynactin complex. We propose that Plk1 phosphorylates MISP, thus stabilizing cortical and astral microtubule attachments required for proper mitotic spindle positioning.

Cortical dynein is critical for proper spindle positioning in human cells.

Correct spindle positioning is fundamental for proper cell division during development and in stem cell lineages. Dynein and an evolutionarily conserved ternary complex (nuclear mitotic apparatus protein [NuMA]-LGN-Gα in human cells and LIN-5-GPR-1/2-Gα in Caenorhabditis elegans) are required for correct spindle positioning, but their relationship remains incompletely understood. By analyzing fixed specimens and conducting live-imaging experiments, we uncovered that appropriate levels of ternary complex components are critical for dynein-dependent spindle positioning in HeLa cells and C. elegans embryos. Moreover, using mutant versions of Gα in both systems, we established that dynein acts at the membrane to direct spindle positioning. Importantly, we identified a region within NuMA that mediates association with dynein. By using this region to target dynein to the plasma membrane, we demonstrated that the mere presence of dynein at that location is sufficient to direct spindle positioning in HeLa cells. Overall, we propose a model in which the ternary complex serves to anchor dynein at the plasma membrane to ensure correct spindle positioning.

Complexity of the heat stress response in plants.

Plants have evolved a variety of responses to elevated temperatures that minimize damage and ensure protection of cellular homeostasis. New information about the structure and function of heat stress proteins and molecular chaperones has become available. At the same time, transcriptome analysis of Arabidopsis has revealed the involvement of factors other than classical heat stress responsive genes in thermotolerance. Recent reports suggest that both plant hormones and reactive oxygen species also contribute to heat stress signaling. Additionally, an increasing number of mutants that have altered thermotolerance have extended our understanding of the complexity of the heat stress response in plants.

A novel transcriptional cascade regulating expression of heat stress proteins during seed development of Arabidopsis.

Within the Arabidopsis thaliana family of 21 heat stress transcription factors (Hsfs), HsfA9 is exclusively expressed in late stages of seed development. Here, we present evidence that developmental expression of HsfA9 is regulated by the seed-specific transcription factor ABSCISIC ACID-INSENSITIVE3 (ABI3). Intriguingly, ABI3 knockout lines lack detectable levels of HsfA9 transcript and protein, and further ectopic expression of ABI3 conferred the ability to accumulate HsfA9 in response to abscisic acid in transgenic plantlets. Consequently, the most abundant heat stress proteins (Hsps) in seeds (Hsp17.4-CI, Hsp17.7-CII, and Hsp101) were not detectable in the ABI3 knockout lines, but their expression could be detected in plants ectopically expressing HsfA9 in vegetative tissues. Furthermore, this seed-specific transcription factor cascade was reconstructed in transient beta-glucuronidase reporter assays in mesophyll protoplasts by showing that ABI3 could activate the HsfA9 promoter, whereas HsfA9 in turn was shown to be a potent activator on the promoters of Hsp genes. Thus, our study establishes a genetic framework in which HsfA9 operates as a specialized Hsf for the developmental expression of Hsp genes during seed maturation.