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Strong, steady forces needed for cell division

Biologists studying cell division have long disagreed about how much force is needed to pull chromosomes apart — in order to form two new cells. A question fundamental to how cells divide.

Says cell biologist Thomas Maresca PhD, and assistant professor in biology at the University of Massachusetts Amherst (UMass Amherst) : "We know we can't fully understand the kinetochore until we understand tension forces and their strength. But, estimates have been all over the map. They differ by orders of magnitude, hundreds of times, and some by a thousand-fold. But now, I think we've finally got the answer."

The correct segregation of chromosomes is fundamental for accurate cell division.

Chromosome mis-segregation leads to aneuploidy — an abnormal number of chromosomes in a cell.

Aneuploidy is the leading cause of miscarriages and chromosome abnormalities — and linked to cancers.

Using two unique sensors to measure opposing forces inside dividing fruit fly cells, Maresca and colleagues at UMass Amherst, propose that kinetochore fibers exert hundreds of pico-tons of pole directed force onto kinetochores. Their work appears in Nature Communications.

In experiments over three years that yielded more than 3,200 data points, Maresca's group found: "In the nano-scale world of molecular motors, the forces we measured are very large. Within cells there are lots of different types of motors — many are like sprinters — but, what we measured is more like a bulldozer producing high force at a slow, steady rate."

In normal cell division, chromosomes line up near the center of the cell, where a structure — the spindle — aligns two copies of each chromosome for separation. This allignment requires the bridge-like protein — the kinetochore — to maintain proper tension on the chromosomes via its spindle filaments called microtubules. At division, molecular engines pull the chromosome copies apart, while the microtubules peel away as division progresses.

For this delicate work, Maresca and graduate students Anna Ye and Stuart Cane, used two different fluorescent light force sensors inserted into each kinetochore. Then — using powerful microscopes — they detected light fluoresced from each molecule within a sensor.

One sensor produced fluorescence with force — the other sensor brightened with tension.

As both sensors were calibrated, changes were measured in fluorescence corresponding to the amount of force used. Researchers could now reach a better estmate of the force applied to kinetochores.

Maresca: "Our data leads us to believe it is actually the microtubule tracks that are capable of producing the most amount of force we measured. It is strong but not fast, so we think it's the road, not the motor, that exerts the most force."

Maresca believes "intrakinetochore stretch" lies at the molecular intersection of both error detection and error correction mechanisms for aneuploidy.

High-fidelity transmission of the genome through cell division requires that all sister kinetochores bind to dynamic microtubules (MTs) from opposite spindle poles. The application of opposing forces to this bioriented configuration produces tension that stabilizes kinetochore–microtubule (kt–MT) attachments. Defining the magnitude of force that is applied to kinetochores is central to understanding the mechano-molecular underpinnings of chromosome segregation; however, existing kinetochore force measurements span orders of magnitude. Here we measure kinetochore forces by engineering two calibrated force sensors into the Drosophila kinetochore protein centromere protein (CENP)-C. Measurements of both reporters indicate that they are, on average, under ∼1–2 piconewtons (pNs) of force at metaphase. Based on estimates of the number of CENP-C molecules and MTs per Drosophila kinetochore and envisioning kinetochore linkages arranged such that they distribute forces across them, we propose that kinetochore fibres (k-fibres) exert hundreds of pNs of poleward-directed force to bioriented kinetochores.

For this work, some data were gathered in the Light Microscopy Core and Nikon Center of Excellence at UMass Amherst's Institute for Applied Life Sciences, with support from the Massachusetts Life Sciences Center. Funding came from UMass Amherst, the March of Dimes Foundation, the Charles H. Hood Foundation and the NIH National Institute of General Medical Sciences.

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Oct 28, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   

How much force is at work when a cell's molecular engines are organizing (BLUE) chromosomes?
Fundamental to understanding how cells divide, scientists at UMass Amherst may now have an answer.
Image Credit: Beata Edyta Mierzwa, beatascienceart.com


Phospholid by Wikipedia