IPIP27 coordinates PtdIns(4,5)P2 homeostasis for successful cytokinesis

During cytokinesis, an actomyosin contractile ring drives the separation of the two daughter cells. A key molecule in this process is the inositol lipid PtdIns(4,5)P 2 , which recruits numerous factors to the equatorial region for contractile ring assembly. Despite the importance of PtdIns(4,5)P 2 in cytokinesis, the regulation of this lipid in cell division remains poorly understood. Here, we identify a role for IPIP27 in mediating cellular PtdIns(4,5)P 2 homeostasis. IPIP27 scaffolds the inositol phosphatase oculocerebrorenal syndrome of Lowe (OCRL) by coupling it to endocytic BAR domain proteins. Loss of IPIP27 causes accumulation of PtdIns(4,5)P 2 on aberrant endomembrane vacuoles, mislocalization of the cytokinetic machinery, and extensive cortical membrane blebbing. This phenotype is observed in Drosophila and human cells and can result in cytokinesis failure. We have therefore identified IPIP27 as a key modulator of cellular PtdIns(4,5)P 2 homeostasis required for normal cytokinesis. The results indicate that scaffolding of inositol phosphatase activity is critical for maintaining PtdIns(4,5)P 2 homeostasis and highlight a critical role for this process in cell division. Carim et al. reveal that the IPIP27 protein, which physically couples the inositol phosphatase OCRL to endocytic BAR domain proteins, is required for cellular phosphoinositide homeostasis and normal cell division. The results show the importance of physically scaffolding inositol phosphatase activity within cells

PTEN reduces endosomal PtdIns(4,5)P2 in a phosphatase-independent manner via a PLC pathway

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The actin-binding ERM protein Moesin binds to and stabilizes microtubules at the cell cortex

Ezrin, Radixin, and Moesin (ERM) proteins play important roles in many cellular processes including cell division. Recent studies have highlighted the implications of their metastatic potential in cancers. ERM’s role in these processes is largely attributed to their ability to link actin filaments to the plasma membrane. In this paper, we show that the ERM protein Moesin directly binds to microtubules in vitro and stabilizes microtubules at the cell cortex in vivo. We identified two evolutionarily conserved residues in the FERM (4.1 protein and ERM) domains of ERMs that mediated the association with microtubules. This ERM–microtubule interaction was required for regulating spindle organization in metaphase and cell shape transformation after anaphase onset but was dispensable for bridging actin filaments to the metaphase cortex. These findings provide a molecular framework for understanding the complex functional interplay between the microtubule and actin cytoskeletons mediated by ERM proteins in mitosis and have broad implications in both physiological and pathological processes that require ERMs

Caractérisation du nouveau rôle de la phosphatase dOCRL durant la division cellulaire

La cytocinèse est un processus dynamique qui requiert un couplage précis des forces générées par l’actomyosine avec la membrane plasmique. En métaphase, l’association uniforme de ces forces avec la membrane plasmique permet l’arrondissement cellulaire caractéristique de cette étape. En revanche, durant l’anaphase l’enrichissement de la machinerie de cytocinèse au niveau de l’équateur cellulaire permet la formation du sillon de clivage. Les mécanismes régulant le recrutement équatorial de la machinerie de cytocinèse ne sont pas encore totalement élucidés. Notre travail a démontré que le phosphoinositide PI(4,5)P2 agit comme un repère spatial pour le recrutement de la machinerie de cytocinèse. En effet, lorsque la 5-phosphatase dOCRL est déplétée dans les cellules S2 de drosophile, l’accumulation de PI(4,5)P2 au niveau des endomembranes recrute ectopiquement la machinerie de cytocinèse. dOCRL est l’orthologue de la protéine OCRL1 humaine. Des mutations sur le gène OCRL sont responsables du syndrome de Lowe. Ce dernier est une maladie génétique rare liée au chromosome X. Des recherches plus poussées nous ont permis de décrire une nouvelle fonction de la phosphatase PTEN qui sauve les manifestations cellulaires du syndrome de Lowe. Notre travail pourrait représenter une potentielle avenue thérapeutique pour traiter cette maladie.Cytokinesis is a dynamic process that requires a precise coupling of actomyosin forces with the plasma membrane. In metaphase, a uniform distribution of actomyosin forces through the plasma membrane allows the characteristic cell rounding. During anaphase, the cytokinetic machinery becomes concentrated at the equatorial cortex to allow the cleavage furrow formation. The mechanisms underlying the localization of the cleavage furrow are not fully understood. Here we demonstrate that the phosphoinositide PI(4,5)P2 acts as a spatial cue for cytokinetic machinery recruitment. We have shown in Drosophila S2 cells that when the 5- phosphatase dOCRL is knocked down, ectopic accumulation of PI(4,5)P2 on endomembranes recruits the cytokinetic machinery. dOCRL is the ortholog of the human OCRL1 protein. Mutation in OCRL gene is responsible for Lowe syndrome, a rare, multisystemic X-linked disorder. Our further investigations demonstrated a new and unexpected function of the phosphatase PTEN that rescued the cellular manifestation of the Lowe syndrome. This work allowed us to find a small molecule capable of rescuing the cellular and in vivo manifestations of Lowe syndrome. Our results could represent a new therapeutic avenue to treat this disease

DHTP is an allosteric inhibitor of the kinesin-13 family of microtubule depolymerases

AbstractThe kinesin-13 family of microtubule depolymerases is a major regulator of microtubule dynamics. RNA interference-induced knockdown studies have highlighted their importance in many cell division processes including spindle assembly and chromosome segregation. Since microtubule turnovers and most mitotic events are relatively rapid (in minutes or seconds), developing tools that offer faster control over protein functions is therefore essential to more effectively interrogate kinesin-13 activities in living cells. Here, we report the identification and characterization of a selective allosteric kinesin-13 inhibitor, DHTP. Using high resolution microscopy, we show that DHTP is cell permeable and can modulate microtubule dynamics in cells

NF45 and NF90 Regulate Mitotic Gene Expression by Competing with Staufen-Mediated mRNA Decay.

In human cells, the expression of ∼1,000 genes is modulated throughout the cell cycle. Although some of these genes are controlled by specific transcriptional programs, very little is known about their post-transcriptional regulation. Here, we analyze the expression signature associated with all 687 RNA-binding proteins (RBPs) and identify 39 that significantly correlate with cell cycle mRNAs. We find that NF45 and NF90 play essential roles in mitosis, and transcriptome analysis reveals that they are necessary for the expression of a subset of mitotic mRNAs. Using proteomics, we identify protein clusters associated with the NF45-NF90 complex, including components of Staufen-mediated mRNA decay (SMD). We show that depletion of SMD components increases the binding of mitotic mRNAs to the NF45-NF90 complex and rescues cells from mitotic defects. Together, our results indicate that the NF45-NF90 complex plays essential roles in mitosis by competing with the SMD machinery for a common set of mRNAs