Volume 86, February 2024, 102277Author links open overlay panel, , Abstract

Cytoskeletal dynamics are essential for cellular homeostasis and development for both metazoans and protozoans. The function of cytoskeletal elements in protozoans can diverge from that of metazoan cells, with microtubules being more stable and actin filaments being more dynamic. This is particularly striking in protozoan parasites that evolved to enter metazoan cells. Here, we review recent progress towards understanding cytoskeletal dynamics in protozoan parasites, with a focus on divergent properties compared to classic model organisms.

Section snippetsIntroductory motivation

Parasitism is likely the predominant form of life on earth. We here define parasites as eukaryotic pathogens, and these can come in many shapes and sizes. Some live as single-celled protozoans with diameters of less than a micrometer; others such as intestinal worms, grow meters long. Parasites live in or on another organism and damage their hosts. As parasites pose a major burden on global human health, parasite research has produced significant value from a medical point of view. It, however,

Pronounced functional separation of actin and tubulin-based cytoskeletons

In metazoans, cytoplasmic microtubules usually function to stabilize cellular integrity, while actin filaments generate dynamic force. In single-celled parasites, this functional separation often appears amplified. Importantly for the focus of this review, in most protozoans, actin filaments are rarely detected and appear highly dynamic and unstable, while microtubules can be exceptionally stable and are often not depolymerized by classic drugs known to interfere with metazoan microtubules.

Building different microtubule arrays in parallel

Examples of unique microtubule-based structures in parasites include arrays of subpellicular microtubules (SPMTs) in Apicomplexa, the ventral disc of Giardia, and the conoid fibers of Toxoplasma gondii (Figure 1) [13]. Often, these diverse structures assemble in parallel in a common cytoplasm. To understand how only a few tubulin building blocks (Table 1) lead to this plethora of observed microtubule architectures, we need to explore how intrinsic microtubule properties are diversified by

Microtubules in parasite division

One fascinating feature of parasite division, which is very different from the textbook picture of cell division as it occurs in animal cells, is their sometimes asymmetric microtubule biogenesis and inheritance. Trypanosomes, for example, multiply by binary fission, prior to which they grow a new flagellum alongside the mother flagellum (Figure 1c). As in Giardia, the question is how microtubule dynamics are spatially regulated to allow extension of the new daughter flagellum while maintaining

Microtubule dynamics

At the heart of connecting tubulin biochemistry to larger-scale structures will be the quantitative analysis of microtubule dynamics in living parasites (Figure 3a,b). Apart from a few examples [28, 29, 30, 18], the small size of parasites has so far precluded detailed live imaging in single cells. The intrinsic dynamicity of Plasmodium microtubules has been recently shown by in vitro reconstitution [31]. In this study, tubulin was purified from Plasmodium blood stages that contain very few

Post-translational modifications of parasite microtubules

The recruitment and activity of some MAPs (not yet MIPs) have been shown to be regulated by tubulin post-translational modifications (PTMs). Tubulin polyglutamylation, for example, was shown to promote the activity of microtubule-severing enzymes [39]. Other PTMs, however, directly regulate microtubule dynamics (Figure 2c). Acetylation, for example, confers flexural rigidity and mechanical resistance on microtubules [40]. While most of the mechanistic studies on PTMs have been done on in vitro

The unusual actins and actin-binding proteins of parasites

While actin genes are highly conserved across metazoans, they show surprising sequence diversity in parasitic protozoans. Actin from Giardia lamblia shows less than 60% identity to yeast or human actin [45], and the two actin proteins in Plasmodium show less than 80% identity to yeast and human actin, as well as among each other [46]. Only about a dozen classical ABPs including formins, could be identified in the genome of Plasmodium, with only one being an actin filament-binding protein,

Actin in gliding motility

In Plasmodium and T. gondii, a flattened set of organelles called the inner membrane complex (IMC) subtends the plasma membrane. Actin is polymerized by formin 1 that is localized at the apical (front) end of the parasites [54, 55, 56, 57∗∗]. These actin filaments are linked to plasma membrane-embedded receptors that bind ligands on the substrate of the host cell surface, while myosin motors are anchored in the IMC by a unique set of light chains and associated proteins and move actin filaments

Unique actin dynamics in parasites

Apicomplexan actin itself shows intriguing characteristics. Filaments polymerize in vitro up to 30 μm but rapidly break upon adenosine triphosphate to adenosine diphosphate conversion and reach a ‘steady-state’ length of just 100 nm [71, 72, 73, 74, 75]. Recent work on the actin of Leishmania, parasites as distantly related to Plasmodium as to humans, showed similarly similar dynamics [76]. Interestingly, unlike Plasmodium actin [72,77], Leishmania actin would copolymerize with human actin [76

Host cell actin and intracellular parasites

To invade cells, parasites also interact with host cell actin, for example, T. gondii depolymerizes host cell actin with the use of a unique protein, toxofilin, to facilitate entry [84, 85, 86] and Cryptosporidium induces an actin belt that forms a barrier between the parasite and the host cell cytoplasm while occupying the intestinal wall [87]. Host cell GTPases contribute to the successful invasion of hepatocytes by Plasmodium [88], and once the parasites grow in hepatocytes, large actin

Conclusions

Why study the cytoskeleton of parasites? We hope to have transmitted some of our fascination with the diverse functions of cytoskeletal components in protozoans that allow us to expand the structural and biochemical space that these ubiquitous polymers can occupy across eukaryotic life. We hope that our few examples have highlighted how parasites build intriguing and unique intracellular assemblies and how they move at high speed through tissues. We strongly believe that the study of parasite

Funding

We report no relevant funding sources associated with this article.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this study.

Acknowledgements

Work in the laboratory of SR is supported by the DFG IRTG2290 and a Max Planck Fellowship. Work in the laboratory of FF is supported by the DFG research networks SFB 1129, SPP 2225, SPP 2332, the German Center for Infection Research and a Wellcome Trust Discovery Award. We thank Andrew Kennard and the Fritz-Laylin lab (University of Massachusetts, Amherst) for sharing unpublished N. fowleri tubulin sequence data.

References (92)M.C. Steele-Ogus et al.Identification of actin filament-associated proteins in giardia lamblia

Microbiol Spectr

(2021 Sep 3)

W.R. Hardin et al.The Giardia ventrolateral flange is a lamellar membrane protrusion that supports attachment

PLoS Pathog

(2022 Apr 28)

M. Martinez et al.Origin and arrangement of actin filaments for gliding motility in apicomplexan parasites revealed by cryo-electron tomography

Nat Commun

(2023 Aug 9)

A. Graindorge, K. Frénal, D. Jacot, J. Salamun, J.B. Marq, D. Soldati-Favre, The Conoid Associated Motor MyoH Is...M. Del Rosario et al.Apicomplexan F-actin is required for efficient nuclear entry during host cell invasion

EMBO Rep

(2019 Dec 5)

J. Ripp et al.Phosphorylation of myosin A regulates gliding motility and is essential for Plasmodium transmission

EMBO Rep

(2022 Jul 5)

G. HamoirThe discovery of meiosis by E. Van Beneden, a breakthrough in the morphological phase of heredity

Int J Dev Biol

(1992 Mar)

H. Ngô et al.Double-stranded RNA induces mRNA degradation in Trypanosoma brucei

Proc Natl Acad Sci U S A

(1998 Dec 8)

H. Shi et al.Genetic interference in Trypanosoma brucei by heritable and inducible double-stranded RNA

RNA

(2000 Jul)

K.B. Velle et al.Conserved actin machinery drives microtubule-independent motility and phagocytosis in Naegleria

J Cell Biol

(2020 Nov 2)

A.J. Lopez et al.Structure and function of Plasmodium actin II in the parasite mosquito stages

PLoS Pathog

(2023 Mar 6)

S.M. Prostak et al.The actin networks of chytrid fungi reveal evolutionary loss of cytoskeletal complexity in the fungal kingdom

Curr Biol

(2021)

J.L. Ferreira et al.Parasite microtubule arrays

Curr Biol

(2023)

J.L. Ferreira et al.Variable microtubule architecture in the malaria parasite

Nat Commun

(2023 Mar 3)

S. Yahiya et al.Live-cell fluorescence imaging of microgametogenesis in the human malaria parasite Plasmodium falciparum

PLoS Pathog

(2022 Feb 7)

B. Liffner et al.Atlas of Plasmodium falciparum intraerythrocytic development using expansion microscopy

bioRxiv [Preprint]

(2023 May 15)

J. Li et al.Repurposing the mitotic machinery to drive cellular elongation and chromatin reorganisation in Plasmodium falciparum gametocytes

Nat Commun

(2022 Aug 27)

R. Rashpa et al.Expansion microscopy of Plasmodium gametocytes reveals the molecular architecture of a bipartite microtubule organisation centre coordinating mitosis with axoneme assembly

PLoS Pathog

(2022 Jan 25)

C. Nosala et al.The hyperstability and composition of Giardia's ventral disc highlights the remarkable versatility of microtubule organelles

bioRxiv

(2018)

K.D. Hagen et al.The domed architecture of Giardia's ventral disc is necessary for attachment and host pathogenesis

bioRxiv

(2023)

S.G. McInally et al.Length-dependent disassembly maintains four different flagellar lengths in Giardia

Elife

(2019 Dec 19)

V. Varga, C. Leduc, V. Bormuth, S. Diez, J. Howard, Kinesin-8 motors act cooperatively to mediate length-dependent...E. Bertiaux et al.Bidirectional intraflagellar transport is restricted to two sets of microtubule doublets in the trypanosome flagellum

J Cell Biol

(2018 Dec 3)

E. Calvo-Álvarez et al.Redistribution of FLAgellar Member 8 during the trypanosome life cycle: consequences for cell fate prediction

Cell Microbiol

(2021 Sep)

R. Dubey et al.Differential roles for inner membrane complex proteins across Toxoplasma gondii and Sarcocystis neurona development

mSphere

(2017 Oct 18)

R. Tomasina et al.Separate to operate: the centriole-free inner core of the centrosome regulates the assembly of the intranuclear spindle in Toxoplasma gondii

mBio

(2022 Oct 26)

C.S. Simon et al.An extended DNA-free intranuclear compartment organizes centrosome microtubules in malaria parasites

Life Sci Alliance

(2021 Sep 17)

M. Machado et al.Plasmodium falciparum CRK4 links early mitotic events to the onset of S-phase during schizogony

mBio

(2023 Aug 31)

J.M. Leung et al.A doublecortin-domain protein of Toxoplasma and its orthologs bind to and modify the structure and organization of tubulin polymers

BMC Mol Cell Biol

(2020 Feb 28)

W.G. Hirst et al.Purification of functional Plasmodium falciparum tubulin allows for the identification of parasite-specific microtubule inhibitors

Curr Biol

(2022 Feb 28)

X. Wang et al.Cryo-EM structure of cortical microtubules from human parasite Toxoplasma gondii identifies their microtubule inner proteins

Nat Commun

(2021 May 24)

S.Y. Sun et al.Cryo-ET of Toxoplasma parasites gives subnanometer insight into tubulin-based structures

Proc Natl Acad Sci U S A

(2022 Feb 8)

J. Liu et al.An ensemble of specifically targeted proteins stabilizes cortical microtubules in the human parasite Toxoplasma gondii

Mol Biol Cell

(2016 Feb 1)

C.R. Harding et al.Alveolar proteins stabilize cortical microtubules in Toxoplasma gondii

Nat Commun

(2019)

I.F. Tengganu et al.The cortical microtubules of Toxoplasma gondii underlie the helicity of parasite movement

J Cell Sci

(2023 Sep 1)

C. Cuveillier et al.MAP6 is an intraluminal protein that induces neuronal microtubules to coil

Sci Adv

(2020 Apr 1)

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