Ultrastructural diversity between centrioles of eukaryotes

Life-cycle-coupled evolution of mitosis in close relatives of animals

Eukaryotes have evolved towards one of two extremes along a spectrum of strategies for remodelling the nuclear envelope during cell division: disassembling the nuclear envelope in an open mitosis or constructing an intranuclear spindle in a closed mitosis1,2. Both classes of mitotic remodelling involve key differences in the core division machinery but the evolutionary reasons for adopting a specific mechanism are unclear. Here we use an integrated comparative genomics and ultrastructural imaging approach to investigate mitotic strategies in Ichthyosporea, close relatives of animals and fungi. We show that species in this clade have diverged towards either a fungal-like closed mitosis or an animal-like open mitosis, probably to support distinct multinucleated or uninucleated states. Our results indicate that multinucleated life cycles favour the evolution of closed mitosis.

Towards understanding centriole elimination

Centrioles are microtubule-based structures crucial for forming flagella, cilia and centrosomes. Through these roles, centrioles are critical notably for proper cell motility, signalling and division. Recent years have advanced significantly our understanding of the mechanisms governing centriole assembly and architecture. Although centrioles are typically very stable organelles, persisting over many cell cycles, they can also be eliminated in some cases. Here, we review instances of centriole elimination in a range of species and cell types. Moreover, we discuss potential mechanisms that enable the switch from a stable organelle to a vanishing one. Further work is expected to provide novel insights into centriole elimination mechanisms in health and disease, thereby also enabling scientists to readily manipulate organelle fate.

The Endothelial Centrosome: Specific Features and Functional Significance for Endothelial Cell Activity and Barrier Maintenance

This review summarizes information about the specific features that are characteristic of the centrosome and its relationship with the cell function of highly specialized cells, such as endotheliocytes. It is based on data from other researchers and our own long-term experience. The participation of the centrosome in the functional activity of these cells, including its involvement in the performance of the main barrier function of the endothelium, is discussed. According to modern concepts, the centrosome is a multifunctional complex and an integral element of a living cell; the functions of which are not limited only to the ability to polymerize microtubules. The location of the centrosome near the center of the interphase cell, the concentration of various regulatory proteins in it, the organization of the centrosome radial system of microtubules through which intracellular transport is carried out by motor proteins and the involvement of the centrosome in the process of the perception of the external signals and their transmission make this cellular structure a universal regulatory and distribution center, controlling the entire dynamic morphology of an animal cell. Drawing from modern data on the tissue-specific features of the centrosome's structure, we discuss the direct involvement of the centrosome in the performance of functions by specialized cells.

The 3D architecture and molecular foundations of de novo centriole assembly via bicentrioles

Centrioles are structurally conserved organelles, composing both centrosomes and cilia. In animal cycling cells, centrioles often form through a highly characterized process termed canonical duplication. However, a large diversity of eukaryotes assemble centrioles de novo through uncharacterized pathways. This unexplored diversity is key to understanding centriole assembly mechanisms and how they evolved to assist specific cellular functions. Here, we show that, during spermatogenesis of the bryophyte Physcomitrium patens, centrioles are born as a co-axially oriented centriole pair united by a cartwheel. Interestingly, we observe that these centrioles are twisted in opposite orientations. Microtubules emanate from the bicentrioles, which localize to the spindle poles during cell division. After their separation, the two resulting sister centrioles mature asymmetrically, elongating specific microtubule triplets and a naked cartwheel. Subsequently, two motile cilia are assembled that appear to alternate between different motility patterns. We further show that centriolar components SAS6, Bld10, and POC1, which are conserved across eukaryotes, are expressed during spermatogenesis and required for this de novo biogenesis pathway. Our work supports a scenario where centriole biogenesis, while driven by conserved molecular modules, is more diverse than previously thought.

Analyzing Centrioles and Cilia by Expansion Microscopy

Expansion microscopy is an imaging method based on isotropic physical expansion of biological samples, which improves optical resolution and allows imaging of subresolutional cellular components by conventional microscopes. Centrioles are small microtubule-based cylindrical structures that build centrosomes and cilia, two organelles essential for vertebrates. Due to a centriole's small size, electron microscopy has traditionally been used to study centriole length and ultrastructural features. Recently, expansion microscopy has been successfully used as an affordable and accessible alternative to electron microscopy in the analysis of centriole and cilia length and structural features. Here, we describe an expansion microscopy approach for the analysis of centrioles and cilia in large populations of mammalian adherent and nonadherent cells and multiciliated cultures.

Casein kinase TbCK1.2 regulates division of kinetoplast DNA, and movement of basal bodies in the African trypanosome

The single mitochondrial nucleoid (kinetoplast) of Trypanosoma brucei is found proximal to a basal body (mature (mBB)/probasal body (pBB) pair). Kinetoplast inheritance requires synthesis of, and scission of kinetoplast DNA (kDNA) generating two kinetoplasts that segregate with basal bodies into daughter cells. Molecular details of kinetoplast scission and the extent to which basal body separation influences the process are unavailable. To address this topic, we followed basal body movements in bloodstream trypanosomes following depletion of protein kinase TbCK1.2 which promotes kinetoplast division. In control cells we found that pBBs are positioned 0.4 um from mBBs in G1, and they mature after separating from mBBs by at least 0.8 um: mBB separation reaches ~2.2 um. These data indicate that current models of basal body biogenesis in which pBBs mature in close proximity to mBBs may need to be revisited. Knockdown of TbCK1.2 produced trypanosomes containing one kinetoplast and two nuclei (1K2N), increased the percentage of cells with uncleaved kDNA 400%, decreased mBB spacing by 15%, and inhibited cytokinesis 300%. We conclude that (a) separation of mBBs beyond a threshold of 1.8 um correlates with division of kDNA, and (b) TbCK1.2 regulates kDNA scission. We propose a Kinetoplast Division Factor hypothesis that integrates these data into a pathway for biogenesis of two daughter mitochondrial nucleoids.

Human centrosome organization and function in interphase and mitosis

Centrosomes were first described by Edouard Van Beneden and named and linked to chromosome segregation by Theodor Boveri around 1870. In the 1960-1980s, electron microscopy studies have revealed the remarkable ultrastructure of a centriole -- a nine-fold symmetrical microtubular assembly that resides within a centrosome and organizes it. Less than two decades ago, proteomics and genomic screens conducted in multiple species identified hundreds of centriole and centrosome core proteins and revealed the evolutionarily conserved nature of the centriole assembly pathway. And now, super resolution microscopy approaches and improvements in cryo-tomography are bringing an unparalleled nanoscale-detailed picture of the centriole and centrosome architecture. In this chapter, we summarize the current knowledge about the architecture of human centrioles. We discuss the structured organization of centrosome components in interphase, focusing on localization/function relationship. We discuss the process of centrosome maturation and mitotic spindle pole assembly in centriolar and acentriolar cells, emphasizing recent literature.

Centrosome structure and biogenesis: Variations on a theme?

Centrosomes are composed of two orthogonally arranged centrioles surrounded by an electron-dense matrix called the pericentriolar material (PCM). Centrioles are cylinders with diameters of ~250 nm, are several hundred nanometres in length and consist of 9-fold symmetrically arranged microtubules (MT). In dividing animal cells, centrosomes act as the principal MT-organising centres and they also organise actin, which tunes cytoplasmic MT nucleation. In some specialised cells, the centrosome acquires additional critical structures and converts into the base of a cilium with diverse functions including signalling and motility. These structures are found in most eukaryotes and are essential for development and homoeostasis at both cellular and organism levels. The ultrastructure of centrosomes and their derived organelles have been known for more than half a century. However, recent advances in a number of techniques have revealed the high-resolution structures (at Å-to-nm scale resolution) of centrioles and have begun to uncover the molecular principles underlying their properties, including: protein components; structural elements; and biogenesis in various model organisms. This review covers advances in our understanding of the features and processes that are critical for the biogenesis of the evolutionarily conserved structures of the centrosomes. Furthermore, it discusses how variations of these aspects can generate diversity in centrosome structure and function among different species and even between cell types within a multicellular organism.

Centriole length control

Centrioles are microtubule-based structures involved in cell division and ciliogenesis. Centriole formation is a highly regulated cellular process and aberrations in centriole structure, size or numbers have implications in multiple human pathologies. In this review, we propose that the proteins that control centriole length can be subdivided into two classes based on their antagonistic activities on centriolar microtubules, which we refer to as 'centriole elongation activators' (CEAs) and 'centriole elongation inhibitors' (CEIs). We discuss and illustrate the structure-function relationship of CEAs and CEIs as well as their interaction networks. Based on our current knowledge, we formulate some outstanding open questions in the field and present possible routes for future studies.

Expansion microscopy on Drosophila spermatocyte centrioles

Drosophila spermatocyte centrioles are ideal for imaging studies. Their large, characteristic V conformation is both easy to identify and measure using standard imaging techniques. However, certain detailed features, such as their ninefold symmetry, are only visible below the diffraction limit of light. This is therefore a system that can benefit from the increased effective resolution potentially achievable by expansion microscopy. Here, I provide detailed protocols of two types of expansion microscopy methodologies applied to Drosophila spermatocyte centrioles, and discuss which is able to achieve the highest effective resolution in this system. I describe how to precisely measure these organelles post-expansion, and discuss how they can therefore be used as "molecular rulers" to troubleshoot and compare expansion techniques. I also provide protocols to combine expansion microscopy with super-resolution imaging in this tissue, discussing potential pitfalls. I conclude that expansion microscopy provides an effective alternative for thick tissues that are not amenable for traditional super-resolution techniques.

With Age Comes Maturity: Biochemical and Structural Transformation of a Human Centriole in the Making

Centrioles are microtubule-based cellular structures present in most human cells that build centrosomes and cilia. Proliferating cells have only two centrosomes and this number is stringently maintained through the temporally and spatially controlled processes of centriole assembly and segregation. The assembly of new centrioles begins in early S phase and ends in the third G1 phase from their initiation. This lengthy process of centriole assembly from their initiation to their maturation is characterized by numerous structural and still poorly understood biochemical changes, which occur in synchrony with the progression of cells through three consecutive cell cycles. As a result, proliferating cells contain three structurally, biochemically, and functionally distinct types of centrioles: procentrioles, daughter centrioles, and mother centrioles. This age difference is critical for proper centrosome and cilia function. Here we discuss the centriole assembly process as it occurs in somatic cycling human cells with a focus on the structural, biochemical, and functional characteristics of centrioles of different ages.

A Proximity Mapping Journey into the Biology of the Mammalian Centrosome/Cilium Complex

The mammalian centrosome/cilium complex is composed of the centrosome, the primary cilium and the centriolar satellites, which together regulate cell polarity, signaling, proliferation and motility in cells and thereby development and homeostasis in organisms. Accordingly, deregulation of its structure and functions is implicated in various human diseases including cancer, developmental disorders and neurodegenerative diseases. To better understand these disease connections, the molecular underpinnings of the assembly, maintenance and dynamic adaptations of the centrosome/cilium complex need to be uncovered with exquisite detail. Application of proximity-based labeling methods to the centrosome/cilium complex generated spatial and temporal interaction maps for its components and provided key insights into these questions. In this review, we first describe the structure and cell cycle-linked regulation of the centrosome/cilium complex. Next, we explain the inherent biochemical and temporal limitations in probing the structure and function of the centrosome/cilium complex and describe how proximity-based labeling approaches have addressed them. Finally, we explore current insights into the knowledge we gained from the proximity mapping studies as it pertains to centrosome and cilium biogenesis and systematic characterization of the centrosome, cilium and centriolar satellite interactomes.

Centrioles and Ciliary Structures during Male Gametogenesis in Hexapoda: Discovery of New Models

Centrioles are-widely conserved barrel-shaped organelles present in most organisms. They are indirectly involved in the organization of the cytoplasmic microtubules both in interphase and during the cell division by recruiting the molecules needed for microtubule nucleation. Moreover, the centrioles are required to assemble cilia and flagella by the direct elongation of their microtubule wall. Due to the importance of the cytoplasmic microtubules in several aspects of the cell life, any defect in centriole structure can lead to cell abnormalities that in humans may result in significant diseases. Many aspects of the centriole dynamics and function have been clarified in the last years, but little attention has been paid to the exceptions in centriole structure that occasionally appeared within the animal kingdom. Here, we focused our attention on non-canonical aspects of centriole architecture within the Hexapoda. The Hexapoda is one of the major animal groups and represents a good laboratory in which to examine the evolution and the organization of the centrioles. Although these findings represent obvious exceptions to the established rules of centriole organization, they may contribute to advance our understanding of the formation and the function of these organelles.

The peculiar structure of the flagellar axoneme in Coccinellidae (Insecta-Coleoptera)

The ultrastructure of the complex organisation of the spermatozoa in Harmonia axyridis and Adalia decempunctata (Coccinellidae) was studied, with particular emphasis on the origin of the anterior shifting of the axonemal structure, which becomes parallel to the nucleus in the sperm flagellum. In studying the spermiogenesis, a centriolar remodelling was observed with the long centriole, present in the early spermatids, transformed in the spermatozoa into an exceptionally long and narrowed basal body (about 0.16 × 3.5-4.0 μm long) displaying a 9 + 0 microtubular pattern in the proximal part and a 9 + 2 pattern in the following part; this is a characteristic not observed in any other pterygotan insect. The sperm also have a very long acrosome surrounded by a dense layer of material extending along the whole basal body. These two uncommon features were discussed in the light of sperm movement.

Coordination of eukaryotic cilia and flagella

Propulsion by slender cellular appendages called cilia and flagella is an ancient means of locomotion. Unicellular organisms evolved myriad strategies to propel themselves in fluid environments, often involving significant differences in flagella number, localisation and modes of actuation. Remarkably, these appendages are highly conserved, occurring in many complex organisms such as humans, where they may be found generating physiological flows when attached to surfaces (e.g. airway epithelial cilia), or else conferring motility to male gametes (e.g. undulations of sperm flagella). Where multiple cilia arise, their movements are often observed to be highly coordinated. Here I review the two main mechanisms for motile cilia coordination, namely, intracellular and hydrodynamic, and discuss their relative importance in different ciliary systems.

Centrioles without microtubules: a new morphological type of centriole

The centrosome is the organizing center of microtubules in the cell, the basis for the origin of cilia and flagella and a site for the concentration of a regulatory proteins multitude. The centrosome comprises two centrioles surrounded by pericentriolar material. Centrioles in the cells of different organisms can contain nine triplets, doublets or singlets of microtubules. Here, we show that in somatic cells of male wasp larvae Anisopteromalus calandrae, centrioles do not contain microtubules and are composed of nine electron-dense prongs, which together form a cogwheel structure. These microtubule-free centrioles can be the platform for procentriole formation and form microtubule-free cilia-like structures. In nymph and imago cells centrioles have a microtubule triplet structure. Our study describes how centriole structure differs in a development-stage-dependent and a cell-type-dependent manner. The discovery of a centriole without microtubules casts a new light on the centriole formation process and the evolution of this organelle.

Revisiting Centrioles in Nematodes-Historic Findings and Current Topics

Theodor Boveri is considered as the “father” of centrosome biology. Boveri’s fundamental findings have laid the groundwork for decades of research on centrosomes. Here, we briefly review his early work on centrosomes and his first description of the centriole. Mainly focusing on centriole structure, duplication, and centriole assembly factors in C. elegans, we will highlight the role of this model in studying germ line centrosomes in nematodes. Last but not least, we will point to future directions of the C. elegans centrosome field.

Cilium structure, assembly, and disassembly regulated by the cytoskeleton

The cilium, once considered a vestigial structure, is a conserved, microtubule-based organelle critical for transducing extracellular chemical and mechanical signals that control cell polarity, differentiation, and proliferation. The cilium undergoes cycles of assembly and disassembly that are controlled by complex inter-relationships with the cytoskeleton. Microtubules form the core of the cilium, the axoneme, and are regulated by post-translational modifications, associated proteins, and microtubule dynamics. Although actin and septin cytoskeletons are not major components of the axoneme, they also regulate cilium organization and assembly state. Here, we discuss recent advances on how these different cytoskeletal systems­ affect cilium function, structure, and organization.