Nuclear transport and the mitotic apparatus: an evolving relationship

Emerging hypotheses that propose key roles for components of the nuclear transport apparatus in mitotic progression.

Participants:

Ariane Abrieu, Yves Barral, Valérie Doye (Organisateur), Annette Flotho, Vincent Galy, Helder Maiato, Michael Matunis, Andrea Musacchio, Jonathon Pines, Bernd Pulverer, Robert Scott, Jason Swedlow, Jan van Deursen, Karsten Weis, Richard Wozniak (Organisateur), Timothy Yen, Michela Zuccolo

Review

Nuclear transport and the mitotic apparatus: an evolving relationship ?
by Valérie Doye
02-07 July, 2007

In eukaryotes, the nuclear envelope (NE) separates nuclear and cytoplasmic cellular activities. During interphase, bidirectional transport of macromolecules between these two compartments occurs through elaborate structures embedded in the NE, termed nuclear pore complexes (NPCs), which are composed of multiple copies of ~30 different proteins termed nucleoporins (Nups).
During mitosis, the replicated chromosomes are divided and distributed to two daughter cells. The kinetochores, proteinaceous scaffolds that assemble around a specialized region of each chromosome known as the centromere, play a fundamental function in mitosis: they are responsible for the formation of robust connection with microtubules, the molecular “cables” that transport the sister chromatids towards the daughter cells. Kinetochores also host a safety device known as the spindle assembly checkpoint, which monitors the formation of proper microtubule-kinetochore attachment, and prevents the separation of the sister chromatids until each of the kinetochores are attached to microtubules.
As metazoans cells (including those of human origin) enter mitosis the NE dissociates from the chromatin and NPCs disassemble into its component parts. At this time chromosomes condense into visibly definable structures and are exposed to the cytoplasm and the forming mitotic spindle. Following microtubule-dependent chromosome segregation to the daughter cells, NE and NPCs reassemble around the two chromatin masses and the two nuclei are formed.  The spatial and temporal coordination of these events are required for the faithful separation of the daughter chromosomes and the inheritance of genome. A breakdown in the fidelity of these processes has catastrophic consequences including cell death and tumorigenesis.
For many years it was thought that the NPCs and kinetochores played separate and distinct roles in cells, with the former controlling nucleocytoplasmic transport and the latter mediating chromosome segregation during mitosis.  However, in recent years several pieces of data have lead to the conclusion that the functions of these organelles are intertwined.  For example, two proteins that play an important role in the spindle assembly checkpoint have binding sites on both NPCs and kinetochores.  Moreover, several NPC proteins are recruited to kinetochores during mitosis when the NPC is disassembled.  The significances of these interactions to the established functions of these two organelles, however, remain to be defined.
As the number of observations demonstrating links between the NPCs and kinetochores has accumulated, so has the interest in understanding these relationships among researchers in these fields.  The aim of the meeting that took place at Les Treilles was to bring together researchers (well-known experts as well as PhD students) from the nuclear transport and the kinetochore fields and allow them to exchange ideas and concepts.  During this meeting, the participants presented a general introduction to their field of research, which was followed by the most recent data gathered by their laboratory. Besides the specific data and hypothesis presented by each speaker, the format of the sessions and the extended length of each presentation created an environment that allowed extensive interactions between the lecturer and the audience, including in depth discussion of technical issues, emerging technologies (for example new imaging tools) and the benefits of various model systems (including yeast, worm, fly, mouse, and human).
As outlined in the summary scheme, the various and apparently diverse topics of the sessions (regulation of mitotic entry or exit, microtubule dynamics, implication of post-transcriptional modification, including phosphorylation and sumoylation, NPC biogenesis, etc.) were intimately related. As a consequence of this format and ambience of Les Treilles, sessions were followed by very lively and prolonged discussions that lead to the formulation of new ideas and testable hypotheses and, ultimately, the establishment of several new collaborations.
By the end of the week, all participants emphasized that the format of this conference (differing from conventional meeting gathering more than 60 participants) was unique and clearly optimal to stimulate scientific interactions in a newly emerging field. It was thus decided that a meeting in a similar format should be scheduled in 2009.

 

Scheme highlighting the network of interactions between the topics presented by the speakers during the meeting: Protein and complexes localized at both nuclear pore complexes and kinetochores appear in grey.

The presentations are indicated below chronologically:

Timothy YEN (Fox Chase Cancer Center, Philadelphia, USA): Kinetochore assembly in human cells.
Jason SWEDLOW (The University of Dundee, UK): Functional Proteomic Analysis of Replicating and Mitotic Chromatin
Andrea MUSACCHIO (European Institute of Oncology, Milan): Insight into microtubule binding from the crystal structure of a bonsai Ndc80 complex.
Jonathon PINES (University of Cambridge, Wellcome Trust /Cancer Research UK): Getting in and out of mitosis.
Valérie DOYE (Institut Curie, Paris): The Nup107-160 complex dynamics during the cell cycle.
Vincent GALY (Institut Pasteur, Paris). MEL-28/ELYS dual localization and function during cell cycle.
Jan van DEURSEN (Mayo Clinic, Rochester, MN, USA): Nucleoporins in chromosomal instability and cancer.
Annette FLOTHO (University Göttingen, Germany): The RanGAP1-RanBP2 complex in mitosis.
Richard WOZNIAK (University of Alberta, Canada): The role of the NPC and karyopherins in the regulated sumoylation of septins.
Michael MATUNIS (Johns Hopkins University, Baltimore): SUMO-2/3 modification and SUMO-2/3 binding regulate the targeting of CENP-E to kinetochores – Interactions and functions of the SUMO isopeptidase, SENP2, at the NPC.
Yves BARRAL (ETH, Zurich): Nuclear pores in aging and rejuvenation: is there a kinetochore connection?
Andrea MUSACCHIO (European Institute of Oncology, Milano): -Mad1 and Mad2: their function in the spindle assembly checkpoint and modalities of their recruitment to kinetochores.
Robert SCOTT (University of Alberta, Canada): The Nuclear Export Factor Xpo1p Regulates the Turnover of Mad1p at Kinetochores.
Karsten WEIS (University of California, Berkeley, USA): The role of transmembrane proteins in nuclear pores assembly.
Vincent GALY (Institut Pasteur, Paris). Function of the transmembrane nucleoprin gp210 in mitosis.
Michela ZUCCOLO (Institut Curie, Paris): The Nup107-160 complex and NEBD: another role for nucleoporins in cell division.
Helder MAIATO (University of Porto): Synchronizing chromosome segregation.
Timothy YEN (Fox Chase Cancer Center, Philadelphia, USA): Determinants of kinetochore/microtubule attachments.
Ariane ABRIEU (CNRS – CRBM, Montpellier, France): Separating chromosomes during mitosis
Bernd PULVERER (Chief Editor, Nature Cell Biology, London): Editorial challenges at the Nature journals.

Publication en 2010 : Cellular and Molecular Life Sciences , volume 67, n° 13, pp. 2215-2230, July 2010, “Nuclear transport and the mitotic apparatus: an evolving relationship”, Richard Wozniak, Brian Burke, Valérie Doye)

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