Review coordinated by Life Science Editors.

Reviewed by: Dr. Helen Pickersgill, Life Science Editors

Potential Conflicts of Interest: None

Main point of the paper: By combining multiple stains and antibodies with ultrastructural expansion (light) microscopy in Plasmodium falciparum during the course of mitosis within red blood cells (the asexual blood stage), when it causes the symptoms of malaria, the authors identified new structural features of cell division in this important human parasite.

Why this is interesting: Imaging the dramatic physical events that occur when cells divide tells us a lot about the biology of the process and is insightful to compare between different eukaryotes, but many organisms are too small to visualise by light microscopy, which is the most versatile imaging technique. So, they used an existing preparation technique to enlarge the parasites, a wide array of dyes and antibodies, and sampled at multiple timepoints so that more intracellular structures could be visualised and their behaviour and potential functions in cell division revealed.

Background: Expansion microscopy (ExM) has been around since 2015 (doi: 10.1126/science.1260088) and is a fairly simple and affordable technique. It involves physically magnifying a specimen by embedding it in a polyelectrolyte gel that swells up in water enabling super-resolution imaging. It has been previously applied to Plasmodium and other Apicomplexa, but not with so many different labels across different timepoints at this important life-stage.

Results: • They imaged 13 subcellular structures (including microtubules, microtubule organising centres, apicoplasts, Golgi and the ER) at multiple timepoints covering the entire asexual blood stage. • Among many results were the following: • They found a central role for the nuclear MTOC in coordinating mitosis and likely in establishing apical-basal polarity early during the asexual cycle. o the MTOC is tethered to the parasite plasma membrane (via cytoplasmic extensions) throughout mitosis. o the cytoplasmic extensions of the MTOC were closely associated (in numbers and positions) with several apical structures including the Golgi and the basal complex. o the MTOC is tethered to the mitochondria and apicoplast during fission and may also regulate their copy numbers. • They performed the first detailed characterization of the short-lived interpolar spindles, previously difficult to visualize, which consist of microtubules connecting duplicated MTOCs as they move to opposite sides of the cell. They found a large variation in their size, which may be how they enable the MTOCs to move without detaching from the plasma membrane. • They were able to study the biogenesis of rhoptries, which secrete proteins required for the parasite to invade host red blood cells, and discovered that: o biogenesis begins earlier than thought and that they associate with MTOCs during the remaining rounds of mitosis. o Rhoptry pairs are likely synthesized independently because over 95% are different sizes and densities.

Remaining thoughts: • Clearly written and easy to read paper with stunning images. • Comprehensive (including descriptions of when things didn’t work), but remains largely descriptive (of course). • Could be shortened/sharpened to make it more manageable to read without losing the main messages, which are somewhat lost in the text. • A top-level, specialised paper that opens the door for more targeted studies of organelle functions during mitosis and comparisons of these functions with other (higher) eukaryotes. • By identifying key players in mitosis during the asexual blood stage it may reveal candidate therapeutic targets for treating malaria.