centrosome

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Impaired cell division leads to neuronal disorder

Prof. Erich Nigg and his research group at the Biozentrum of the University of Basel have discovered an amino acid signal essential for error-free cell division. This signal regulates the number of centrosomes in the cell, and its absence results in the development of pathologically altered cells. Remarkably, such altered cells are found in people with a neurodevelopmental disorder, called autosomal recessive primary microcephaly. The results of these investigations have been published in the current issue of the US journal “Current Biology”.

Cell division is the basis of all life. Of central importance is the error-free segregation of genetic material, the chromosomes. A flawless division process is a prerequisite for the development of healthy, new cells, whilst errors in cell division can cause illnesses such as cancer. The centrosome, a tiny cell organelle, plays a decisive role in this process.

Prof. Erich Nigg’s research group at the Biozentrum of the University of Basel has investigated an important step in cell division: the duplication of the centrosome and its role in the correct segregation of the chromosomes into two daughter cells. The protein STIL has an essential function in this process. It ensures that centrosome duplicate before one half of the genetic material is transported into each of the two daughter cells.

KEN-Box important for protein breakdown

During cell division, the protein STIL is degraded. If this does not occur, the protein accumulates in the cell, which then causes an overproduction of centrosomes. As a consequence, mis-segregated chromosomes are incorporated into the daughter cells, which then represent cells with faulty genetic material. The scientists discovered an amino acid signal on the STIL protein, a so-called KEN-Box, and showed that this is critical for the breakdown of the protein: “The Ken-Box is the signal that orders the protein degradation machinery to break down the STIL protein,” explains Christian Arquint, the first author of this publication. In the absence of the KEN-Box, the protein is not degraded.

Absence of the KEN-Box causes microcephaly

In some patients with microcephaly, a neuronal disorder that leads to a reduced number of nerve cells being produced and, therefore, a smaller brain, the KEN-box is lacking from the STIL protein. The scientists were thus able to demonstrate a tantalizing connection between the absence of this particular amino acid signal and an illness. “When during our investigations of cell division and centrosome duplication we came across a connection to the disorder microcephaly, we were particularly pleased, as this helps us to better understand how this disorder develops“, says Christian Arquint.

In the future, the research group led by Erich Nigg plans to uncover other connections between errors of cell division and the illness microcephaly. They also want to focus on the investigation of other proteins that play important roles in the process of cell division, in particular those involved in centrosome duplication.

Centrosomes and Cancer: Settling an Old Debate

Early last century, German biologist Theodor Boveri observed that cancer cells often harbor multiple copies of a cellular structure known as the centrosome. He was also the first to suggest that the extra centrosomes drive cancer. Researchers have since learned a great deal about the structure and many functions of Boveri’s “special organ of cell division.” But why cancer cells harbor multiple copies of this organelle — and whether they are “addicted” to having so many — has remained unanswered. So has the question of whether healthy human cells even require centrosomes to divide, making more cells. Now, 101 years after Boveri first aired his suspicions, researchers may have some answers.

A new study, published April 30 in Science, shows that while cancer cells are not addicted to multiple centrosomes, healthy cells absolutely require them to proceed with cell division. In the absence of centrosomes, healthy cells don’t divide, while malignant cells continue dividing and multiplying.

“Our results have settled a long-running debate in cell biology,” said co-senior author Karen Oegema, PhD, professor of cellular and molecular medicine at University of California, San Diego School of Medicine and member of the Ludwig Institute for Cancer Research. “Centrosomes make things so much better for healthy dividing cells, that cells have a protective mechanism that halts their division if they lose centrosomes.”

Ordinarily, the resting cell’s single centrosome serves as an organizing center for the cell’s skeleton. When a cell divides, however, the centrosome takes on another function. The centrosome duplicates and helps ensure chromosomes are distributed equally between the two daughter cells. Many cancer cells contain multiple centrosomes, and this error contributes to the misdistribution and abnormal numbers of chromosomes in daughter cells.

Still, it wasn’t clear that centrosomes are absolutely needed for cell division. Biologists have long known that other mechanisms exist to separate chromosomes. “The growing feeling among a number of cell biologists is that the centrosome is like the appendix of the cell,” said co-senior author Andrew Shiau, PhD, director of the Ludwig Institute’s Small Molecule Discovery Program in San Diego and visiting scientist at UC San Diego.

Earlier studies had sought to resolve the issue by cutting centrosomes out of cells or destroying them with lasers. But both normal and cancer cells treated this way simply remade their lost centrosomes, and then continued dividing.

To get around this limitation, in this study the researchers designed and synthesized a molecule that specifically and reversibly inhibits an enzyme named Plk4, which controls the assembly of centrioles — barrel-like protein structures from which centrosomes are made. They then showed that exposure to this inhibitor, called centrinone, eliminates centrosomes from both healthy cells and cancerous ones. When the compound was removed, cancer cells reverted to precisely the number of centrosomes they had before exposure to the molecule.

“This was in marked contrast to what normal cells would do when we persistently removed centrosomes,” Oegema said. “Normal cells arrested their growth when their centrosomes were absent. This suggests that they absolutely require centrosomes for division, which was not at all the thinking in the field.”

The researchers show that the pause in the division of healthy cells is governed by a protein named p53, which is mutated in about half of all cancers. Levels of p53 were elevated in cells treated with centrinone. When the protein was temporarily inactivated in normal cells, they too failed to arrest upon exposure to centrinone.

This new ability to reversibly eliminate centrosomes using centrinone is likely to benefit research in a wide variety of biomedical fields, given the organelle’s multiple roles — from organizing the cytoskeleton to sprouting hair-like structures known as cilia on certain cells. The findings might also have applications for cancer therapy, even if cancer cells aren’t addicted to centrosomes.

“The idea,” said Shiau, “is that you trigger p53 in normal cells and have them stop multiplying — and then introduce another agent that only kills continuously dividing cells.”

Researchers are now developing more drug-like variants of centrinone. Their goal is to identify combination cancer therapies that can be tested in clinical studies.

Pictured: Centrosomes (green nodes) gather at the poles of a cell as it prepares to divide

ROBO-SPERM
SOURCE: Wellcome Images, B0007805 Sperm cell
Digital artwork/Computer graphic by Anna Tanczos

THE STRUCTURE OF A SPERM CELL
The male gamete

Each single spermatozoan (plural spermatozoa) contains a haploid number of chromosomes (one copy of each) in the nucleus situated in the head region.

Surrounding the head of the sperm cell is the acrosome cap [shown in blue] - this contains special enzymes that digest the zona pellucida (the membrane surrounding the egg cell) to assist in fertilization.

The mid piece of the sperm cell contains two centrosomes [in green] and coiled mitochondria [gold]. The sperm will contribute both centrosomes to the embryo upon fertilization, which are essential for organising microtubules and thus subsequent cell divisions of the embryo.

The large coiled mitochondria [gold] provide the sperm with the energy supplied to the long flagella (or tail) required for it motility.

A Living Cage

The typical eukaryotic, which basically covers every cell found in animals, plants, fungi, slime molds, protozoa and algae, is a packed place, crammed with a nucleus (itself containing six feet of tightly wound DNA), mitochondria, centrosomes, peroxisomes, lysosomes, ribosomes, endoplasmic reticula (both smooth and rough), actin filaments, Golgi vesicles and more.

All of these cellular elements are in constant action, buzzing with communication and the movement of molecules. The image above, produced by Maria Voigt at the RCSB Protein Data Bank, depicts a clathrin cage, which is essentially a little basket for carrying and moving things around inside cells. Clathrin derives from the Latin clatratus, which means lattice.

Cells have a lot of them. They’re used to transfer nutrients, import signaling receptors, mediate an immune response after sampling the world outside the cell and the clean-up of cellular debris. When not in use, the cages are broken up, to be reassembled when next needed.

The cage above is roughly 50 nanometers wide, a size almost too small to imagine. By comparison, there are 25.4 million nanometers in an inch. A sheet of paper is roughly 100,000 nanometers thick. A single strand of human DNA is 2.5 nanometers in diameter.

The image is one of this year’s winners of the Wellcome Image Awards.