CSHL scientists discover link between control of chromosome duplication and segregation

February 06, 2009

Team finds that Orc1, part of machinery that initiates DNA replication, prevents excess centrosome duplication

Cold Spring Harbor, N.Y. - Before a cell can divide into two, first it must duplicate its genetic material--the DNA packed in its chromosomes. The two new sets of chromosomes then have to be separated from one another and correctly distributed to the resulting "daughter" cells, so that both daughter cells are genetically identical to the original, or "parent," cell.

During cell division, a cellular organ called the centrosome, and a copy of the centrosome, position themselves at opposite ends of the dividing cell. Each centrosome serves as an anchor for a spindle, a complex structure of filament-like tubules that radiates out from each centrosome and connects with special sites called centromeres on the chromosomes. By pulling on the chromosomes, the spindles separate them into two sets, each divided equally into the two emerging daughter cells.

It's crucial that cells duplicate their centrosome only once during each division cycle. Extra copies can result in incorrect distribution of chromosomes, which can lead to genomic instability and cancer. Hence the importance of new research by Professor Bruce Stillman, Ph.D., and his lab group at Cold Spring Harbor Laboratory (CSHL). They have identified a protein molecule that controls the copying of the centrosome in human cells and prevents it from being re-duplicated. Their findings will appear in the February 6th issue of the journal Science.

Double duty for Orc1

The molecule shown by Dr. Stillman and his colleagues to control centrosome duplication is Orc1, one of six proteins that comprise the Origin Recognition Complex. ORC, as it is called, is an assembly that attaches to particular sequences within all the DNA in the cell and prepares it for duplication. Recently it had become clear that some ORC proteins might be doing more than jump-starting DNA duplication; the accumulation of extra centrosome copies in cells that lack ORC suggested that some or all ORC proteins might play a role in centrosome duplication as well.

To investigate which of the ORC proteins limit centrosome copying, Stillman and co-investigators Adriana Hemerly, Supriya Prasanth and Khalid Siddiqui, used RNA interference, or RNAi, a technique that uses small pieces of RNA to shut off specific genes. They blocked the production of each of the proteins that combine to form ORC in human cells. Loss of Orc1 alone, the scientists found, spurred cells to accumulate excess centrosomes.

Cells that were induced to produce more Orc1, on the other hand, had the normal amount of centrosomes, even when centrosomes were induced to re-duplicate via drug treatment of cells. It was thus deduced that Orc1 allows cells to duplicate centrosomes once per division cycle, but prevents centrosomes from being re-duplicated.

This new role for Orc1 seems to be separate from its duties in helping cells copy DNA. The CSHL team found that a shortened version of Orc1 that lacked the ability to start DNA duplication was still able to limit centrosome copying to once per cell-division cycle.

Orc1 forces new centrosomes to stay in touch

Within each centrosome are a pair of tiny machines called centrioles. These duplicate during cell division to produce two centriole pairs. Stillman's laboratory found that Orc1 also controls the number of centrioles in a cell. Before a pair is copied, the two centrioles normally stay connected to each other. Upon the cell's commitment to cell division, however, the centriole pair is duplicated to produce two new centriole pairs; this occurs precisely as copying of the chromosomes gets under way.

Stillman's team hypothesizes that it is this "engagement" of the paired centrioles that stops the original centriole pair from duplicating. In cells that lacked Orc1, the CSHL scientists found that the centrioles were "disengaged" from the original, suggesting that Orc1 might prevent re-duplication by helping the new centrosomes to stay connected to the old.

This function of Orc1 depends on its ability to physically associate with the centrosomes, the researchers showed. They suggest that Orc1 is ferried to the centrosomes by the action of a protein known as Cyclin A. This protein is found at high levels in cells at the start of the division cycle and helps cells make one copy of their DNA.

But a related protein called Cyclin E may be the target of Orc1. Cyclin E, which was also found to associate with Orc1, is known to be required for centriole and centrosome duplication and also stimulates the duplication of DNA in chromosomes. Orc1 antagonizes Cyclin E activity so that it duplicates centrosomes but cannot re-duplicate them.

The scientists thus propose that Orc1 enforces the number of centrosome copies by moving to centrosomes during the short temporal window in the cell division cycle when Cyclin E is still present in the cell. "During this time, if the effects of Cyclin E activity aren't counteracted by Orc1, centrosome re-duplication can occur," explains Stillman.

"I also think that this discovery suggests an ancient link between the processes that duplicate DNA and the processes that separate the DNA in cells before cell division," he added.

"Orc1 controls centriole and centrosome copy number in human cells" will appear in the February 6th issue of Science. The full citation is: Adriana S. Hemerly, Supriya G. Prasanth, Khalid Siddiqui and Bruce Stillman. This article is available online at www.sciencemag.org. Bruce Stillman is President of the Laboratory in addition to running his own research laboratory.

Cold Spring Harbor Laboratory

The Centrosome in Cell Replication and Early Development, Volume 49 (Current Topics in Developmental Biology)
by Robert E. Palazzo (Editor), Gerald P. Schatten (Editor)

Centrosomes play an integral role in the growth of cells and the ultimate development of many animals, and sometimes plants. In addition to the normal growth of cells, centrosomes can also play a key role in the spread of cancer and are of increased interest to both the genetics and oncology communities. Volume 49 of Current Topics in Developmental Biology will present all known research surrounding the centrosome, across a variety of systems, will be well referenced, and speculate where the research is headed.

Key Features
* Discusses centrosomes and cancer, centrosomes and early development, and molecular biology of the centrosome
* Heavily illustrated, with many color figures
* Chapters written by international leaders in the...

The Centrosome (Cell Biology)
by Vitauts I. Kalnins (Editor)

The centrosome is increasingly recognized as an essential structure in cells, having multiple functions. This book aims to present a comprehensive treatment of this organelle, ranging from molecular to behavioural levels of cellular activities.

The Ovarian Egg of Limulus: A Contribution to the Problem of the Centrosome and Yolk-Nucleus.
by John P. Munson (Author)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated.1899 Excerpt: ... egg of the frog. He was unable to state whether this was accompanied by a division of the nucleus. Balbiani ('93) found that the ovarian eggs of spiders also degenerate; and he figures follicles filled with cells. I am unable to make any positive statements in regard to the immediate causes of metamorphosis. I believe, however, that the following statement can be made: the cause of the disturbance lies in the egg itself. In the present case there is no true follicle epithelium surrounding the egg. The cells of the stalk, which correspond to the follicle epithelium in other eggs, and...

Centrosomes and Spindle Pole Bodies, Volume 67 (Methods in Cell Biology)
by Robert E. Palazzo (Editor), Trisha N. Davis (Editor)

Containing a comprehensive collection of convenient and quantitative methods for studying centrosomes, spindle pole bodies and related organelles, this text is a valuable resource for researchers and others interested in studying the role of these organelles in cell replication. Chapters outlining the role of these organelles in other cell functions are also included, and a wide variety of experimental systems for analyzing these organelles are presented. Detailed protocols for experiments are contained in each chapter for researchers to perform in their own labs. This volume outlines key methodologies used to analyze centrosomes and spindle pole bodies, their replication, and reproduction in the clear, well-illustrated style of the METHODS IN CELL BIOLOGY series.

The Behavior Of The Centrosomes In The Fertilized Egg Of Myzostoma Glabrum, Leuckart (1895)
by William Morton Wheeler (Author)

This book is a facsimile reprint and may contain imperfections such as marks, notations, marginalia and flawed pages.

The Origin And Behavior Of The Centrosomes In The Annelid Egg...
by Albert Davis Mead (Author)

This is a reproduction of a book published before 1923. This book may have occasional imperfections
such as missing or blurred pages, poor pictures, errant marks, etc. that were either part of the original artifact,
or were introduced by the scanning process. We believe this work is culturally important, and despite the imperfections,
have elected to bring it back into print as part of our continuing commitment to the preservation of printed works
worldwide. We appreciate your understanding of the imperfections in the preservation process, and hope you enjoy this valuable book.
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The Regenerating Nervous System of Lumbricidae and the Centrosome of Its Nerve Cells From the Bulletin of the Museum of Comparative Zoology at Harvard College, Vol. XXXVII, No. 3
by Herbert W. Rand (Author)

Centrosomes: Webster's Facts and Phrases
by Icon Group (Author)

Ever need a fact or quotation on centrosomes? Designed for speechwriters, journalists, writers, researchers, students, professors, teachers, historians, academics, scrapbookers, trivia buffs and word lovers, this is the largest book ever created for this single word. It represents a compilation from a variety of sources with a linguistic emphasis on anything relating to the term "centrosomes," including non-conventional usage and alternative meanings which capture ambiguities. The entries cover all parts of speech (noun, verb, adverb or adjective usage) as well as use in modern slang, pop culture, social sciences (linguistics, history, geography, economics, sociology, political science), business, computer science, literature, law, medicine, psychology, mathematics, chemistry, physics,...

The Centrosome: Cell and Molecular Mechanisms of Functions and Dysfunctions in Disease
by Heide Schatten (Editor)

The Centrosome: Cell and Molecular Mechanisms of Functions and Dysfunctions in Disease includes chapters on classic and modern aspects of centrosome research to cover topics of current interest that have not been covered in depth in most books on the market so far. It extends on previous topics and includes new exciting aspects of centrosome research focused on primary cilia and their dysfunctions that are implicated in numerous diseases. Each chapter will be written by experts in their fields who will contribute their unique expertise in specific research fields and include cell and molecular details that are important for the specific subtopics. The book will be comprehensive, concise and will include reviews of key topics in the field. Cutting edge new information will be balanced with...

Centrosomes in Development and Disease
by Erich A. Nigg (Editor)

Discovered over a century ago, the centrosome is the major microtubule organizing center of the animal cell. It is a tiny organelle of surprising structural complexity. Over the last few years our understanding of the structure and composition of centrosomes has greatly advanced, and the demonstration of frequent centrosome anomalies in most common human tumors has sparked additional interest in the role of this organelle in a broader scientific community.The centrosome controls the number and distribution of microtubules—a major element of the cell cytoskeleton—and hence influences many important cellular functions and properties. These include cell shape, polarity, and motility, as well as the intracellular transport and positioning of various organelles. Of particular interest,...