Regulation of cytokinesis

At the end of mitosis, daughter cells are separated from each other by cytokinesis. This process involves equal partitioning and segregation of cytoplasm between the two cells. Despite years of study, the mechanism driving cytokinesis in animal cells is not fully understood. Actin and myosin are major components of the contractile ring, the structure at the equator between the dividing cells that provides the force necessary to constrict the cytoplasm. Despite this, there are also tantalizing results suggesting that cytokinesis can occur in the absence of myosin. It is unclear what the roles are of the few other contractile ring components identified to date. While it has been difficult to identify important proteins involved in cytokinesis, it has been even more challenging to pinpoint the regulatory mechanisms that govern this vital process. Cytokinesis must be precisely controlled both spatially and temporally; potential regulators of these parameters are just beginning to be identified. This review discusses the recent progress in our understanding of cytokinesis in animal cells and the mechanisms that may regulate it. Received 24 August 1998; received after revision 9 October 1998; accepted 9 October 1998     

SPIN90 dephosphorylation is required for cofilin-mediated actin depolymerization in NMDA-stimulated hippocampal neurons

Actin plays a fundamental role in the regulation of spine morphology (both shrinkage and enlargement) upon synaptic activation. In particular, actin depolymerization is crucial for the spine shrinkage in NMDAR-mediated synaptic depression. Here, we define the role of SPIN90 phosphorylation/dephosphorylation in regulating actin depolymerization via modulation of cofilin activity. When neurons were treated with NMDA, SPIN90 was dephosphorylated by STEP61 (striatal-enriched protein tyrosine phosphatase) and translocated from the spines to the dendritic shafts. In addition, phosphorylated SPIN90 bound cofilin and then inhibited cofilin activity, suggesting that SPIN90 dephosphorylation is a prerequisite step for releasing cofilin so that cofilin can adequately sever actin filaments into monomeric form. We found that SPIN90 YE, a phosphomimetic mutant, remained in the spines after NMDAR activation where it bound cofilin, thereby effectively preventing actin depolymerization. This led to inhibition of the activity-dependent redistribution of cortactin and drebrin A, as well as of the morphological changes in the spines that underlie synaptic plasticity. These findings indicate that NMDA-induced SPIN90 dephosphorylation and translocation initiates cofilin-mediated actin dynamics and spine shrinkage within dendritic spines, thereby modulating synaptic activity.     

Tyrosine 842 in the activation loop is required for full transformation by the oncogenic mutant FLT3-ITD

The type III receptor tyrosine kinase FLT3 is frequently mutated in acute myeloid leukemia. Oncogenic FLT3 mutants display constitutive activity leading to aberrant cell proliferation and survival. Phosphorylation on several critical tyrosine residues is known to be essential for FLT3 signaling. Among these tyrosine residues, Y842 is located in the so-called activation loop. The position of this tyrosine residue is well conserved in all receptor tyrosine kinases. It has been reported that phosphorylation of the activation loop tyrosine is critical for catalytic activity for some but not all receptor tyrosine kinases. The role of Y842 residue in FLT3 signaling has not yet been studied. In this report, we show that Y842 is not important for FLT3 activation or ubiquitination but plays a critical role in regulating signaling downstream of the receptor as well as controlling receptor stability. We found that mutation of Y842 in the FLT3-ITD oncogenic mutant background reduced cell viability and increased apoptosis. Furthermore, the introduction of the Y842 mutation in the FLT3-ITD background led to a dramatic reduction in in vitro colony forming capacity. Additionally, mice injected with cells expressing FLT3-ITD/Y842F displayed a significant delay in tumor formation, compared to FLT3-ITD expressing cells. Microarray analysis comparing gene expression regulated by FLT3-ITD versus FLT3-ITD/Y842F demonstrated that mutation of Y842 causes suppression of anti-apoptotic genes. Furthermore, we showed that cells expressing FLT3-ITD/Y842F display impaired activity of the RAS/ERK pathway due to reduced interaction between FLT3 and SHP2 leading to reduced SHP2 activation. Thus, we suggest that Y842 is critical for FLT3-mediated RAS/ERK signaling and cellular transformation.     

Importance of G2 for the cytokinesis of plant cells: Specific blocking by deoxyguanosine

Summary The modalities of the deoxyguanosine blocking effect on meristematic root cells of Allium sativum L. reveals that, during G₂ phase, fundamental processes leading to cytokinesis take place.     

A small GTPase, human Rab32, is required for the formation of autophagic vacuoles under basal conditions

Here we show that a small GTPase, Rab32, is a novel protein required for the formation of autophagic vacuoles. We found that the wild-type or GTP-bound form of human Rab32 expressed in HeLa and COS cells is predominantly localized to the endoplasmic reticulum (ER), and overexpression induces the formation of autophagic vacuoles containing an autophagosome marker protein LC3, the ER-resident protein calnexin and endosomal/lysosomal membrane protein LAMP-2, even under nutrient-rich conditions. The recruitment of Rab32 to the ER membrane was necessary for autophagic vacuole formation, suggesting involvement of the ER as a source of autophagosome membranes. In contrast, the expression of the inactive form of, or siRNA-specific for, Rab32 caused the formation of p62/SQSTM1 and ubiquitinated protein-accumulating aggresome-like structures and significantly prevented constitutive autophagy. We postulate that Rab32 facilitates the formation of autophagic vacuoles whose membranes are derived from the ER and regulates the clearance of aggregated proteins by autophagy. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A threshold of transmembrane potential is required for mitochondrial dynamic balance mediated by DRP1 and OMA1

As an organellar network, mitochondria dynamically regulate their organization via opposing fusion and fission pathways to maintain bioenergetic homeostasis and contribute to key cellular pathways. This dynamic balance is directly linked to bioenergetic function: loss of transmembrane potential across the inner membrane (Δψ _m) disrupts mitochondrial fission/fusion balance, causing fragmentation of the network. However, the level of Δψ _m required for mitochondrial dynamic balance, as well as the relative contributions of fission and fusion pathways, have remained unclear. To explore this, mitochondrial morphology and Δψ _m were examined via confocal imaging and tetramethyl rhodamine ester (TMRE) flow cytometry, respectively, in cultured 143B osteosarcoma cells. When normalized to the TMRE value of untreated 143B cells as 100%, both genetic (mtDNA-depleted ρ⁰) and pharmacological [carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-treated] cell models below 34% TMRE fluorescence were unable to maintain mitochondrial interconnection, correlating with loss of fusion-active long OPA1 isoforms (L-OPA1). Mechanistically, this threshold is maintained by mechanistic coordination of DRP1-mediated fission and OPA1-mediated fusion: cells lacking either DRP1 or the OMA1 metalloprotease were insensitive to loss of Δψ _m, instead maintaining an obligately fused morphology. Collectively, these findings demonstrate a mitochondrial ‘tipping point’ threshold mediated by the interaction of Δψ _m with both DRP1 and OMA1; moreover, DRP1 appears to be required for effective OPA1 maintenance and processing, consistent with growing evidence for direct interaction of fission and fusion pathways. These results suggest that Δψ _m below threshold coordinately activates both DRP1-mediated fission and OMA1 cleavage of OPA1, collapsing mitochondrial dynamic balance, with major implications for a range of signaling pathways and cellular life/death events.     

Crustecdysone is without estrogenic or antiestrogenic activity in the rat

Zusammenfassung Es wird mittels Vaginalzytologie bei Ratten gezeigt, dass Ecdyson keine Östrogenaktivität und bei östrogenbehandelten Tieren auch keine Antioestrogen-Aktivität entfaltet.

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A grant from the Ford Foundation and the Indian Council of Medical Research is gratefully acknowledged.     

Biological effects of lithium: Experimental analysis in plant cytokinesis

Summary The biological effects of lithium ions have been studied, using plant cytokinesis in onion root meristems as the experimental model. Lithium induces binucleate cells by inhibiting cell plate formation. Moreover, lithium and caffeine have additive effects on the induction of binucleate cells. Na⁺, K⁺, Ca⁺⁺ and Mg⁺⁺ antagonize lithium-induced inhibition of cytokinesis.     

Recruitment of Pyk2 to SHPS-1 signaling complex is required for IGF-I-dependent mitogenic signaling in vascular smooth muscle cells

In vascular smooth muscle cells, IGF-I stimulates SHPS-1/SHP2/Src complex formation which is required for IGF-I-stimulated cell proliferation. Using SHP2/Src silencing and a Pyk2/Y402F mutant, we showed that Pyk2 was also recruited to the SHPS-1 complex. Pyk2 recruitment to SHPS-1 is mediated via the interaction of Pyk2 Tyr402 and the Src in response to IGF-I. Following Src/Pyk2 association, Src phosphorylates Pyk2 on Tyr881 providing a binding site for Grb2. Cells expressing Pyk2/Y881F showed decreased Grb2 recruitment to SHPS-1 and impaired Shc/Grb2 association. This change led to reduced Erk1/2 (MAP kinase) activation and cell proliferation in response to IGF-I. Our results show that, following its recruitment to the SHPS-1 signaling complex, Pyk2 localizes Grb2 in close proximity to Shc thereby facilitating Shc/Grb2 association which leads to Erk1/2 activation in response to IGF-I. Thus, Pyk2 recruitment to SHPS-1 plays an important role in regulating the IGF-I-stimulated mitogenic response.     

Inhibition of cytokinesis in plant cells by 6-dimethylaminopurine and puromycin

Summary Binucleate cells are produced in garlic root tip cells with puromycin and with 6-dimethylaminopurine (6-DMAP), the purine component of puromycin. The possibility that the effect of puromycin on cytokinesis is due to 6-DMAP-which appears to be without short term effects on protein synthesis-is discussed.This work was supported by AI 031871 from the Centre National de la Recherche Scientifique (M-C.B.) and the University René Descartes (M-C.B. and F.L.), Paris.The authors would like to thank Miss G. Daouse and Miss M. Delage for excellent technical assistance.     

Antitumor activity of the isodon diterpenoids: Structural requirements for the activity

Summary A significant antitumor activity of oridonin (1) and lasiokaurin (2), the kaurene-type diterpenoids ofIsodon species, was shown by their i.p. injection to the test mice inoculated by Ehrlich ascites carcinoma. Enmein (8), compounds9 and3 were also active under larger dose. Subsequently, the relationship between their chemical structure and antitumor activity was investigated, and the activity of oridonin (1) and lasiokaurin (2) was rationalized in terms of their structural feature.     

Antitumor activity of the Isodon diterpenoids: structural requirements for the activity.

A significant antitumor activity of oridonin (1) and lasiokaurin (2), the kaurene-type diterpenoids of Isodon species, was shown by their i.p. injection to the test mice inoculated by Ehrlich ascites carcinoma. Enmein (8), compounds 9 and 3 were also active under larger dose. Subsequently, the relationship between their chemical structure and antitumor activity was investigated, and the activity of oridonin (1) and lasiokaurin (2) was rationalized in terms of their structural feature.     

Nitrogen pretreatment period required for complete elimination of the oxygen effect in X-rayedDrosophila embryos

Zusammenfassung Zur vollständigen Ausschaltung des Sauerstoff-Effektes bei der Bestrahlung vonDrosophila-Embryonen genügt eine 15 Sekunden dauernde Vorbehandlung mit Stickstoff.     

Time required for heterogenous induction in chick embryo ectoderm

Résumé Le foie de l'embryon de poulet de 14–20 jours d'incubation sert d'inducteur pour découvrir la durée nécessaire pour déclencher des inductions neurales dans l'ectoblaste du poulet. Nos résultats ont montré que 4 h de contact entre l'inducteur et l'ectoblaste n'est pas suffisant à l'induction d'une différenciation neurale. Après 6 h de contact l'ectoblaste réagissant se différentie au sens neural.

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Work supported by a grant from the National Research Council of Canada.