Bundles of Joy

Breaking a cell exactly in half requires huge feats of tiny engineering. Hundreds of proteins oversee the process of mitosis – but arguably the most crucial job, separating the precious DNA, still holds a few mysteries. Here scientists using super-resolution microscopy watch as microtubules – somewhere between fishing lines and microscopic scaffolding – (highlighted in green) first reach out from opposite sides of the cell towards the DNA (purple). The tubules meet in a criss-cross network seen from two different angles (top and bottom rows, left). Later in mitosis (middle and right) the tubules form thick bundles that bridge the gap between the different sides of the cell as the DNA lines up in the centre. Researchers believe kinetochores, linkers between DNA and microtubules, help to coordinate these overlap bundles,, which in turn guide the overall machinery as it begins the careful job of pulling the DNA apart.

Written by John Ankers

Image adapted from work by Jurica Matković and colleagues

Division of Molecular Biology, Ruđer Bošković Institute, Zagreb, Croatia

Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)

Published in Nature Communications, November 2022

Bend and Snap

Breaking up is hard to do – especially in a dividing cell. During mitosis, a cell must separate its pairs of chromosomes – regions of genetic information – by pulling each half towards opposite ‘poles’ before it cleaves in two. This is a carefully timed process, helped by thousands of mechanical parts including these microtubules. In this computer reconstruction, they connect to unseen chromosomes in the centre, then, like fishing lines, help to yank the chromosomes toward poles on the left and right. Researchers studying the bendiness or tortuosity of the tubules, find those at the centre are straighter (highlighted in red) and curvier (yellow) around the edge. Individual tubules also curve more towards the pole ends, and straighten as they reach the kinetochores (central junctions with the chromosomes). In the future, these techniques may spot weaknesses which could be exploited to halt cell division in certain cancers.

Written by John Ankers

Video from work by Robert Kiewisz and colleagues

Experimental Center, Faculty of Medicine Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany

Video originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)

Published in eLife, July 2022

Tomorrow is my boss’ birthday and my lab mate and I made him a chromosome shaped cake, inclusive of kinetochores, microtubules and various kinetochore proteins (we study cell division, so it’s perfect for him). I had a great time making it, and never saw myself making a frosting paste-covered cake, but oh well!! 

A kymograph, displaying the passage of time on the x axis, shows how chromosome separation is regulated in cell division, or more specifically, mitotic anaphase. From the lab of Hélder Maiato at the University of Porto, Portugal, multiple kinetochore pairs (CID-mCherry, red) and spindle microtubules (GFP–α-tubulin, blue above/green below) are visible as a cell progresses through mitosis.

The lab uses RNAi to knock out specific gene expression, along with laser microsurgery and fluorescent-speckle microscopy to study kinetochore function in living animal cells.

Our present research interests are focused in understanding how mitotic fidelity is regulated in space and time, paying particular attention to how the spindle matrix confines SAC signaling and how error-correction mechanisms at the kinetochore-microtubule interface are regulated throughout mitosis.

Maiato began studying +TIPs as a graduate student, focusing his early efforts on the importance of a protein family called CLASPs in mitosis. He has shown that CLASPs shape the mitotic spindle by working both at kinetochores and centrosomes.

“Not satisfied with narrowly studying one protein,” he has since broadened his inquiries into other +TIPs and into mitosis more generally.

Recently, Maiato curated an issue of Chromosome Research on the topic of biased chromosome segregation, an area of study based on an idea from 1975 that non-random chromosome segregation is one way in which stem cells retain their pluripotency, through passing on the newly synthesised (i.e. more probably mutated) strand to the differentiating somatic cell. This came to be known as the immortal DNA strand hypothesis, and seems to still be much under dispute.

In the base of this controversy over the years lie a number of experimental limitations, such as the difficulty of unequivocally distinguishing and selecting stem cells, of correlating these cells with selective labeling of older or newer DNA strands, and of labeling DNA in a nontoxic manner. The debate is also motivated by the near absence of a clear working model for how the mitotic apparatus distinguishes and segregates sister chromatids, such as to separate them according to the age of their respective DNA strands. Finally, discussions have also concerned what the function of biased chromosome segregation might be, questioning its role in conserving sequence information but suggesting that it could play an important role in the segregation of epigenetic states. In any case, these discussions made clear that here, as in many other fields of cell biology, the demonstration of the molecular mechanisms underlying a given process is paramount to establishing its true nature and allowing discussions about its possible meaning

The issue contains a particularly interesting proposal from Arnold Bendich, that uniparental inheritance of organelle DNA may be seen as “a mechanism of DNA abandonment in organelles that have been exposed to extensive DNA damaging agents,” e.g. reactive oxygen species.

This week's Journal of Cell Biology features an interview with Maiato, providing a glimpse into his life both in and out of the lab. Read it here.