Tuesday, February 12, 2019

Variations on a Theme: Mitosis in Fungi and Animals or, a real cluster-%@!*

by Christopher Molenaar

Greetings, budding biologists! Today, we’re talking about *drum roll* cell division!

I can feel your eyes rolling through the internet. Trust me, though–you didn’t cover this in high school, so bear with me until we get to the fun part. First, we’ll run a crash course on human mitosis, before diving into the odd world of mitosis in single-celled fungi, where cell division can get… weird. Two groups of fungi, ascomycetes and basidiomycetes, have unique ways to divide; not only that, but comparing these organisms to animals suggests an evolutionary history for how mitosis evolved in “us”! Once we describe cell division in human cells, we’ll look at C. albicans(an Ascomycete) and C. neoformans (a Basidiomycete) to see that evolutionary history.

Strap yourselves in.

By this point, you’ve probably taken some sort of high school biology; my guess is you’ve been forced to memorize mitosis (cell division that generates two identical daughter cells) to the point that your mnemonic for IPMAT is ingrained in your mind like any old piece of nonsense trivia. Or, maybe you really enjoyed cell biology, have a strong understanding of cell division, but lack an affinity for social cues and norms. Either way, we’re all very familiar with a figure like this:
We had one cell, and now we have two. Classic mitosis. Notice the nuclear membrane breaking down in Prophase and themicrotubule spindles connecting to the chromosomes. We’ll talk about that later…(Figure from here.)
If you need a brief review, diploid cells (cells with two sets of each chromosome) undergoing mitosis need to 1) condense their genetic material into chromosomes 2) properly separate sister chromatids from these chromosomes into the cellular space of the two new cells and 3) pinch off the cellular membrane to produce two viable daughter cells. There’s a lot more happening, of course, but this is all we’ll focus on here.

In high school biology, I only ever learned about traditional animal mitosis (human mitosis, really); it’s etched in my brain, and yet I never thought to ask any questions about it. Mitosis is this incredibly complex series of interactions between genetic material, the nuclear membrane, the cytoskeleton, and a host of other cell proteins–how did we “get here”? How far back, evolutionarily, does this process go? How much diversity is there for mitosis overall?

In case you were wondering, this is the fun part.

Diagram of kinetochore assembled on
centromere based on electron microscopy.8
To understand the difference between animal and fungal methods of cell division, we need to get into the nitty-gritty of mitosis, focusing on one component of the process: the kinetochore. A kinetochore is a protein structure, built on the chromosome Lego-style from several smaller proteins, that facilitates an interaction between the spindle microtubules and the chromosomes (shown in orange in the image below).2Remember, the spindle microtubules stretch across the cell to bind to the chromosome (shown in green). The kinetochore forms on the centromere (a specific location on the chromosome) as a sort of middle-man for this interaction; in animals, this structure is removed when the cell is not dividing. Generally, the kinetochore has trilaminar architecture: i.e., it has three main layers that interact with different components of the mitotic process.2The inner layer, for example, interacts directly with the chromosomal DNA, and outer proteins interact with microtubules.2  This structuring is mostly conserved in eukaryotes (plants, fungi, animals, etc.), meaning that it’s common across these organisms.3Yet the timingvaries greatly, as do surrounding steps in the mitotic process.

Cell division in C.
albicans 
looks a lot like
division in S. cerevisiae,
shown here.2
What do I mean by “surrounding steps”? While the kinetochore is being assembled and the microtubules are extending, the nuclear membrane (or  envelope) needs to undergo some alterations for effective mitosis–in humans, at least. As shown in the image above, the nuclear membrane breaks down during mitosis in almost all animals–this is termed “open mitosis”.3,4However, this isn’t the only possibility! Other eukaryotes, like fungi, undergo closed mitosis, where the nuclear membrane never breaks down.3,4

Take Candida albicans, for example. C. albicans is a fungi in the Ascomycete phyla, a group that contains other budding yeasts like Saccharomyces cerevisiae (the yeast used for beer, wine, bread, and a bunch of other foods). When diploid C. albicans cells divide by mitosis, the nuclear membrane stays completely intact throughout the process, stretching into the daughter cell space before pinching into two nuclei (shown right)5. Additionally, the kinetochore is fully assembled and attached to microtubules for the entire cell cycle–even when cells aren’t dividing!2Pretty weird, right? When these organisms divide, the separation of genetic material into daughter cells relies on microtubules withinthe nucleus, rather than microtubules coming from outside the nucleus (as in animal mitosis). This is an example from the Ascomycetes; what about other fungal phyla, though?

Well, that’s what got me interested in this topic. A fungus that I study, Cryptococcus neoformans, follows some pretty unique rules compared to the Ascomycetes I just mentioned. C. neoformansis a Basidiomycete–this phylum, sometimes called the club fungi, includes the typical mushroom you would put on a salad. C. neoformans, though, is another single-celled organism, like C. albicans. This fungus has a lot of interesting behaviors, but before I go off on a tangent, let’s talk about how it divides and assembles the kinetochore structure.

Remember, in humans and almost all animals, the nuclear membrane is completely degraded to allow the chromosomes to migrate to opposing sides of the dividing cell; the kinetochore is assembled during mitosis, and is removed in non-dividing cells.2,4However, in some ascomycetous fungi, the nuclear membrane never breaks down, and instead gets pinched off like the cellular membrane during telophase. Additionally, the kinetochore can remain on the centromere throughout the cell cycle.2,6But what does C. neoformans do? Well, it’s a little of both.

In C. neoformans, kinetochores assemble just before mitosis, like in animals.5The inner kinetochore proteins remain on the centromere throughout the cell cycle, but the middle and outer proteins only assemble when the chromosomes cluster during mitosis.2Not only do these structures have different timing of assembly compared to ascomycetes–they take up a completely different location! In Ascomycetes, the centromeres of different chromosomes are clustered in a single location throughout the cell cycle (until chromosomes are split into two recipient daughter cells); as mentioned, the microtubule-kinetochore-centromere complex is maintained even when cells are not dividing in most of these fungi.6,7However, in Basidiomyceteslike C. neoformans, the centromeres are not clustered; instead, these regions of the chromosome are spaced out around the periphery of the nucleus, more similar to the arrangement of genetic material in some animals.2
This is C. neoformans undergoing mitosis. Three things are shown here: degradation of the nuclear membrane (red), clustering/declustering of the kinetochores and centromeres, and microtubules originating from outside the nucleus to direct the genetic material.2
Lastly, a major difference between basidiomycete and ascomycete mitosis, and a similarity between the former and animal mitosis: theC. neoformansnuclear membrane is partially degraded during mitosis.2As the microtubule spindle migrates to the daughter cell (shown at right, next to the star), the nuclear membrane is broken to allow this migration. This is a significant variation from conventional closed mitosis in fungi, where the membrane stays completely intact throughout the entire cell cycle.

In a recent study of C. neoformans, it was suggested that this variety of mitosis is highly reminiscent of animal mitosis, and that mitotic events associated with animals (like kinetochore assembly, opening of the nuclear membrane, and clustering/declustering of centromeres) evolved in the fungal kingdom.2These distinctions in mitosis between ascomycete S. cerevisiae, basidiomycete C. neoformans, and humans are laid out nicely in the image below.
Comparing the mitotic process of an ascomycete, the
basidiomycete C. neoformans, and human cells.2
So there it is: some added spice to traditional human cell division! Mitosis isn’t just an acronym that you have to remember–it’s a complex process with a lot of diversity within eukaryotes! Unfortunately, a lot of foundational biological concepts get reduced and packaged so that they’re more manageable for testing, and cell division is certainly one of these. In biology, though, the more you learn about a concept, the more fascinating it is! Mitosis can be achieved via a spectrum of approaches; understanding the connections between these can give a lot of insight into evolutionary relationships. Perhaps next time I see a cell division figure, I’ll give less groan and more glory to a genuinely incredible biological process.



References

2.  Kozubowski L, Yadav V, Chatterjee G, et al. Ordered kinetochore assembly in the human pathogenic basidiomycetous yeast Cryptoccous neoformansmBio2013, 4(5) 1-8. 
3.  Meraldi P, McAinsh A, et al. Phylogenetic and structural analysis of centromeric DNA and kinetochore proteins. Genome Biology2006, 7(23). 
4.  Przewloka M, Zhang W, et al. Molecular analysis of core kinetochore composition and assembly in Drosophila melanogasterPlosOne2007. 
5.  Kozubowski L, Heitman J. Profiling a killer, the development of Cryptococcus neoformansFEMS Microbiology Reviews2012, 36(1), 78-94. 
6.  Thakur J, Sanyal K. The essentiality of the fungus-specific Dam1 complex is correlated with a One-Kinetochore-One-Microtubule interaction present throughout the cell cycle, independent of the nature of a centromere. Eukaryotic Cell2011, 10(10), 1295-1305. 
7.  Jin Q, Fuchs J, Loidl J. Centromere clustering is a major determinant of yeast interphase nuclear organization. Journal of Cell Science2000, 113, 1903-1912. 
8.  McEwen B, Dong Y, VandenBeldt K. Using electron microscopy to understand functional mechanisms of chromosome alignment on the mitotic spindle. Methods in Cell Biology2007, 79, 259-293.

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