Centrosomal ALIX regulates mitotic spindle orientation by modulating astral microtubule dynamics

Arginylation of ⍺-tubulin at E77 regulates microtubule dynamics via MAP1S

Arginylation is the posttranslational addition of arginine to a protein by arginyltransferase-1 (ATE1). Previous studies have found that ATE1 targets multiple cytoskeletal proteins, and Ate1 deletion causes cytoskeletal defects, including reduced cell motility and adhesion. Some of these defects have been linked to actin arginylation, but the role of other arginylated cytoskeletal proteins has not been studied. Here, we characterize tubulin arginylation and its role in the microtubule cytoskeleton. We identify ATE1-dependent arginylation of ⍺-tubulin at E77. Ate1-/- cells and cells overexpressing non-arginylatable ⍺-tubulinE77A both show a reduced microtubule growth rate and increased microtubule stability. Additionally, they show an increase in the fraction of the stabilizing protein MAP1S associated with microtubules, suggesting that E77 arginylation directly regulates MAP1S binding. Knockdown of Map1s is sufficient to rescue microtubule growth rate and stability to wild-type levels. Together, these results demonstrate a new type of tubulin regulation by posttranslational arginylation, which modulates microtubule growth rate and stability through the microtubule-associated protein, MAP1S.

PHLPP1 regulates CFTR activity and lumen expansion through AMPK

Complex organ development depends on single lumen formation and its expansion during tubulogenesis. This can be achieved by correct mitotic spindle orientation during cell division, combined with luminal fluid filling that generates hydrostatic pressure. Using a human 3D cell culture model, we have identified two regulators of these processes. We find that pleckstrin homology leucine-rich repeat protein phosphatase (PHLPP) 2 regulates mitotic spindle orientation, and thereby midbody positioning and maintenance of a single lumen. Silencing the sole PHLPP family phosphatase in Drosophila melanogaster, phlpp, resulted in defective spindle orientation in Drosophila neuroblasts. Importantly, cystic fibrosis transmembrane conductance regulator (CFTR) is the main channel regulating fluid transport in this system, stimulated by phosphorylation by protein kinase A and inhibited by the AMP-activated protein kinase AMPK. During lumen expansion, CFTR remains open through the action of PHLPP1, which stops activated AMPK from inhibiting ion transport through CFTR. In the absence of PHLPP1, the restraint on AMPK activity is lost and this tips the balance in the favour of channel closing, resulting in the lack of lumen expansion and accumulation of mucus.

Understanding the underlying mechanisms governing spindle orientation: How far are we from there?

Proper spindle orientation is essential for cell fate determination and tissue morphogenesis. Recently, accumulating studies have elucidated several factors that regulate spindle orientation, including geometric, internal and external cues. Abnormality in these factors generally leads to defects in the physiological functions of various organs and the development of severe diseases. Herein, we first review models that are commonly used for studying spindle orientation. We then review a conservative heterotrimeric complex critically involved in spindle orientation regulation in different models. Finally, we summarize some cues that affect spindle orientation and explore whether we can establish a model that precisely elucidates the effects of spindle orientation without interfusing other spindle functions. We aim to summarize current models used in spindle orientation studies and discuss whether we can build a model that disturbs spindle orientation alone. This can substantially improve our understanding of how spindle orientation is regulated and provide insights to investigate this complex event.

UVB Inhibits Proliferation, Cell Cycle and Induces Apoptosis via p53, E2F1 and Microtubules System in Cervical Cancer Cell Lines

Ultraviolet (UV) exposure has been linked to skin damage and carcinogenesis, but recently UVB has been proposed as a therapeutic approach for cancer. Herein, we investigated the cellular and molecular effects of UVB in immortal and tumorigenic HPV positive and negative cells. Cells were irradiated with 220.5 to 1102.5 J/m² of UVB and cell proliferation was evaluated by crystal violet, while cell cycle arrest and apoptosis analysis were performed through flow cytometry. UVB effect on cells was recorded at 661.5 J/m² and it was exacerbated at 1102.5 J/m². All cell lines were affected by proliferation inhibition, cell cycle ablation and apoptosis induction, with different degrees depending on tumorigenesis level or HPV type. Analysis of the well-known UV-responsive p53, E2F1 and microtubules system proteins was performed in SiHa cells in response to UVB through Western-blotting assays. E2F1 and the Microtubule-associated protein 2 (MAP2) expression decrease correlated with cellular processes alteration while p53 and Microtubule-associated Protein 1S (MAP1S) expression switch was observed since 882 J/m², suggesting they were required under more severe cellular damage. However, expression transition of α-Tubulin3C and β-Tubulin was abruptly noticed until 1102.5 J/m² and particularly, γ-Tubulin protein expression remained without alteration. This study provides insights into the effect of UVB in cervical cancer cell lines.

Mauve/LYST limits fusion of lysosome-related organelles and promotes centrosomal recruitment of microtubule nucleating proteins

Lysosome-related organelles (LROs) are endosomal compartments carrying tissue-specific proteins, which become enlarged in Chediak-Higashi syndrome (CHS) due to mutations in LYST. Here, we show that Drosophila Mauve, a counterpart of LYST, suppresses vesicle fusion events with lipid droplets (LDs) during the formation of yolk granules (YGs), the LROs of the syncytial embryo, and opposes Rab5, which promotes fusion. Mauve localizes on YGs and at spindle poles, and it co-immunoprecipitates with the LDs' component and microtubule-associated protein Minispindles/Ch-TOG. Minispindles levels are increased at the enlarged YGs and diminished around centrosomes in mauve-derived mutant embryos. This leads to decreased microtubule nucleation from centrosomes, a defect that can be rescued by dominant-negative Rab5. Together, this reveals an unanticipated link between endosomal vesicles and centrosomes. These findings establish Mauve/LYST's role in regulating LRO formation and centrosome behavior, a role that could account for the enlarged LROs and centrosome positioning defects at the immune synapse of CHS patients.

Centrosomes in mitotic spindle assembly and orientation

The centrosome is present in most animal cells and functions as the major microtubule-organizing center to ensure faithful chromosome segregation during cell division. As cells transition from interphase to mitosis, the duplicated centrosomes separate and move to opposite sides of the cell where the spindle assembles. Centrosomes not only nucleate but also organize microtubules of the mitotic spindle. The mitotic spindle is anchored to the cell cortex by the astral microtubules emanating from the centrosomes. Proper orientation of the mitotic spindle is essential for correct cell division. Centrosome-localized polo-like kinase Plk1 has been linked to regulation of proper spindle orientation. A number of proteins including MISP and NuMA have been implicated in the Plk1-mediated spindle orientation pathway.

Centrosomes in asymmetric cell division

Asymmetric cell division (ACD) is a strategy for achieving cell diversity. Research carried out over the last two decades has shown that in some cell types that divide asymmetrically, mother and daughter centrosomes are noticeably different from one another in structure, behaviour, and fate, and that robust ACD depends upon centrosome function. Here, I review the latest advances in this field with special emphasis on the complex structure-function relationship of centrosomes with regards to ACD and on mechanistic insight derived from cell types that divide symmetrically but is likely to be relevant in ACD. I also include a comment arguing for the need to investigate the centrosome cycle in other cell types that divide asymmetrically.

Nuclear Isoforms of Neurofibromin Are Required for Proper Spindle Organization and Chromosome Segregation

Mitotic spindles are highly organized, microtubule (MT)-based, transient structures that serve the fundamental function of unerring chromosome segregation during cell division and thus of genomic stability during tissue morphogenesis and homeostasis. Hence, a multitude of MT-associated proteins (MAPs) regulates the dynamic assembly of MTs in preparation for mitosis. Some tumor suppressors, normally functioning to prevent tumor development, have now emerged as significant MAPs. Among those, neurofibromin, the product of the Neurofibromatosis-1 gene (NF1), a major Ras GTPase activating protein (RasGAP) in neural cells, controls also the critical function of chromosome congression in astrocytic cellular contexts. Cell type- and development-regulated splicings may lead to the inclusion or exclusion of NF1exon51, which bears a nuclear localization sequence (NLS) for nuclear import at G2; yet the functions of the produced NLS and ΔNLS neurofibromin isoforms have not been previously addressed. By using a lentiviral shRNA system, we have generated glioblastoma SF268 cell lines with conditional knockdown of NLS or ΔNLS transcripts. In dissecting the roles of NLS or ΔNLS neurofibromins, we found that NLS-neurofibromin knockdown led to increased density of cytosolic MTs but loss of MT intersections, anastral spindles featuring large hollows and abnormal chromosome positioning, and finally abnormal chromosome segregation and increased micronuclei frequency. Therefore, we propose that NLS neurofibromin isoforms exert prominent mitotic functions.

Regulation of mitotic spindle orientation by phosphorylation of end binding protein 1

End binding protein 1 (EB1) is a key regulator of microtubule dynamics that orchestrates hierarchical interaction networks at microtubule plus ends to control proper cell division. EB1 activity is known to be regulated by serine/threonine phosphorylation; however, how tyrosine phosphorylation affects EB1 activity remains poorly understood. In this study, we mapped the tyrosine phosphorylation pattern of EB1 in synchronized cells and identified two tyrosine phosphorylation sites (Y217 and Y247) in mitotic cells. Using phospho-deficient (Y/F) and phospho-mimic (Y/D) mutants, we revealed that Y247, but not Y217, is critical for astral microtubule stability. The Y247D mutant contributed to increased spindle angle, indicative of defects in spindle orientation. Time-lapse microscopy revealed that the Y247D mutant significantly delayed mitotic progression by increasing the duration times of prometaphase and metaphase. Structural analysis suggests that Y247 mutants lead to instability of the hydrophobic cavity in the EB homology (EBH) domain, thereby affecting its interactions with p150^glued, a protein essential for Gαi/LGN/NuMA complex capture. These findings uncover a crucial role for EB1 phosphorylation in the regulation of mitotic spindle orientation and cell division.

Vps28 Is Involved in the Intracellular Trafficking of Awd, the Drosophila Homolog of NME1/2

The Awd (abnormal wing discs) gene is the Drosophila homolog of human NME1 and NME2 metastasis suppressor genes. These genes play a key role in tumor progression. Extensive studies revealed that intracellular NME1/2 protein levels could be related to either favorable or poor prognosis depending on tissue context. More recently, extracellular activities of NME1/2 proteins have also been reported, including a tumor- promoting function. We used Drosophila as a genetic model to investigate the mechanism controlling intra- and extracellular levels of NME1/2. We examined the role of several components of the ESCRT (endosomal sorting complex required for transport) complex in controlling Awd trafficking. We show that the Vps28 component of the ESCRT-I complex is required for maintenance of normal intracellular level of Awd in larval adipocytes. We already showed that blocking of Shibire (Shi)/Dynamin function strongly- lowers Awd intracellular level. To further investigate this down regulative effect, we analyzed the distribution of endosomal markers in wild type and Shi-defective adipocytes. Our results suggest that Awd does not enter CD63-positive endosomes. Interestingly, we found that in fat body cells, Awd partly- colocalizes with the ESCRT accessory component ALiX, the ALG-2 (apoptosis-linked gene 2)-interacting protein X. Moreover, we show that the intracellular levels of both proteins are downregulated by blocking the function of the Dynamin encoded by the shibire gene.

The quantification and regulation of microtubule dynamics in the mitotic spindle

Microtubules play essential roles in cellular organization, cargo transport, and chromosome segregation during cell division. During mitosis microtubules form a macromolecular structure known as the mitotic spindle that is responsible for the accurate segregation of chromosomes between the two daughter cells. This is accomplished thanks to finely tuned control of microtubule dynamics. Even small changes in microtubule dynamics during spindle formation and/or operation may lead to chromosome mis-segregation, chromosome instability and aneuploidy. These three events are directly correlated with human diseases like cancer and developmental defects. Precise measurements of microtubule dynamics in the spindle will allow us to discover new molecules involved in regulating microtubule dynamics and enable a deeper understanding of the mechanisms that underlie mitosis and cancer emergence and development. Moreover, many chemotherapeutic agents for cancer treatment are targeted to microtubules, so continued investigation of their dynamics with utmost precision will facilitate the development of new drugs. Measuring microtubule dynamics in the spindle has been a difficult task until recently. With the development of new and gentler microscopic techniques, and new computer programs, we can perform better and more accurate measurements of microtubule dynamics during mitosis.