Novel features of centriole polarity and cartwheel stacking revealed by cryo-tomography

Time-series reconstruction of the molecular architecture of human centriole assembly

Centriole biogenesis, as in most organelle assemblies, involves the sequential recruitment of sub-structural elements that will support its function. To uncover this process, we correlated the spatial location of 24 centriolar proteins with structural features using expansion microscopy. A time-series reconstruction of protein distributions throughout human procentriole assembly unveiled the molecular architecture of the centriole biogenesis steps. We found that the process initiates with the formation of a naked cartwheel devoid of microtubules. Next, the bloom phase progresses with microtubule blade assembly, concomitantly with radial separation and rapid cartwheel growth. In the subsequent elongation phase, the tubulin backbone grows linearly with the recruitment of the A-C linker, followed by proteins of the inner scaffold (IS). By following six structural modules, we modeled 4D assembly of the human centriole. Collectively, this work provides a framework to investigate the spatial and temporal assembly of large macromolecules.

Centriole and transition zone structures in photoreceptor cilia revealed by cryo-electron tomography

Primary cilia mediate sensory signaling in multiple organisms and cell types but have structures adapted for specific roles. Structural defects in them lead to devastating diseases known as ciliopathies in humans. Key to their functions are structures at their base: the basal body, the transition zone, the "Y-shaped links," and the "ciliary necklace." We have used cryo-electron tomography with subtomogram averaging and conventional transmission electron microscopy to elucidate the structures associated with the basal region of the "connecting cilia" of rod outer segments in mouse retina. The longitudinal variations in microtubule (MT) structures and the lumenal scaffold complexes connecting them have been determined, as well as membrane-associated transition zone structures: Y-shaped links connecting MT to the membrane, and ciliary beads connected to them that protrude from the cell surface and form a necklace-like structure. These results represent a clearer structural scaffold onto which molecules identified by genetics, proteomics, and superresolution fluorescence can be placed in our emerging model of photoreceptor sensory cilia.

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.

ELI trifocal microscope: a precise system to prepare target cryo-lamellae for in situ cryo-ET study

Cryo-electron tomography (cryo-ET) has become a powerful approach to study the high-resolution structure of cellular macromolecular machines in situ. However, the current correlative cryo-fluorescence and electron microscopy lacks sufficient accuracy and efficiency to precisely prepare cryo-lamellae of target locations for subsequent cryo-ET. Here we describe a precise cryogenic fabrication system, ELI-TriScope, which sets electron (E), light (L) and ion (I) beams at the same focal point to achieve accurate and efficient preparation of a target cryo-lamella. ELI-TriScope uses a commercial dual-beam scanning electron microscope modified to incorporate a cryo-holder-based transfer system and embed an optical imaging system just underneath the vitrified specimen. Cryo-focused ion beam milling can be accurately navigated by monitoring the real-time fluorescence signal of the target molecule. Using ELI-TriScope, we prepared a batch of cryo-lamellae of HeLa cells targeting the centrosome with a success rate of ~91% and discovered new in situ structural features of the human centrosome by cryo-ET.

Ciliate cortical organization and dynamics for cell motility: Comparing ciliates and vertebrates

The generation of efficient fluid flow is crucial for organismal development and homeostasis, sexual reproduction, and motility. Multi-ciliated cells possess fields of motile cilia that beat in synchrony to propel fluid. Ciliary arrays are remarkably conserved in their organization and function. Ciliates have polarized multi-ciliary arrays (MCAs) to promote fluid flow for cell motility. The ciliate cortex is decorated with hundreds of basal bodies (BB) forming linear rows along the cell's anterior-posterior axis. BBs scaffold and position cilia to form the organized ciliary array. Nascent BBs assemble at the base of BBs. As nascent BBs mature, they integrate into the cortical BB and cytoskeletal network and nucleate their own cilium. The organization of MCAs is balanced between cortical stability and cortical dynamism. The cortical cytoskeletal network both establishes and maintains a stable organization of the MCA in the face of mechanical forces exerted by ciliary beating. At the same time, MCA organization is plastic, such that it remodels for optimal ciliary mobility during development and in response to environmental conditions. Such plasticity promotes effective feeding and ecological behavior required for these organisms. Together, these properties allow an organism to effectively sense, adapt to, and move through its environment.

Atypical and distinct microtubule radial symmetries in the centriole and the axoneme of Lecudina tuzetae

The centriole is a minute cylindrical organelle present in a wide range of eukaryotic species. Most centrioles have a signature 9-fold radial symmetry of microtubules that is imparted onto the axoneme of the cilia and flagella they template, with 9 centriolar microtubule doublets growing into 9 axonemal microtubule doublets. There are exceptions to the 9-fold symmetrical arrangement of axonemal microtubules in some species, with lower or higher fold symmetries. In the few cases where this has been examined, such alterations in axonemal symmetries are grounded in likewise alterations in centriolar symmetries. Here, we examine the question of microtubule number continuity between centriole and axoneme in flagellated gametes of the gregarine Lecudina tuzetae, which have been reported to exhibit a 6-fold radial symmetry of axonemal microtubules. We used time-lapse differential interference microscopy to identify the stage at which flagellated gametes are present. Thereafter, using electron microscopy and ultrastructure-expansion microscopy coupled to STimulated Emission Depletion (STED) super-resolution imaging, we uncover that a 6- or 5-fold radial symmetry in the axoneme is accompanied by an 8-fold radial symmetry in the centriole. We conclude that the transition between centriolar and axonemal microtubules can be characterized by unexpected plasticity. [Media: see text] [Media: see text] [Media: see text].

Cartwheels create symmetry and asymmetry

In this issue of Structure, Kantsadi et al. (2022) present the crystal structures of coiled-coil bundles from SAS-6, a core component of the centriole cartwheel, and reveal that two coiled-coil domains interact asymmetrically. This work provides insights into how the polarity of centrioles is established.

CPAP insufficiency leads to incomplete centrioles that duplicate but fragment

Centrioles are structures that assemble centrosomes. CPAP is critical for centrosome assembly, and its mutations are found in patients with diseases such as primary microcephaly. CPAP’s centrosomal localization, its dynamics, and the consequences of its insufficiency in human cells are poorly understood. Here we use human cells genetically engineered for fast degradation of CPAP, in combination with superresolution microscopy, to address these uncertainties. We show that three independent centrosomal CPAP populations are dynamically regulated during the cell cycle. We confirm that CPAP is critical for assembly of human centrioles, but not for recruitment of pericentriolar material on already assembled centrioles. Further, we reveal that CPAP insufficiency leads to centrioles with incomplete microtubule triplets that can convert to centrosomes, duplicate, and form mitotic spindle poles, but fragment owing to loss of cohesion between microtubule blades. These findings further our basic understanding of the role of CPAP in centrosome biogenesis and help understand how CPAP aberrations can lead to human diseases.

Paramecium, a Model to Study Ciliary Beating and Ciliogenesis: Insights From Cutting-Edge Approaches

Cilia are ubiquitous and highly conserved extensions that endow the cell with motility and sensory functions. They were present in the first eukaryotes and conserved throughout evolution (Carvalho-Santos et al., 2011). Paramecium has around 4,000 motile cilia on its surface arranged in longitudinal rows, beating in waves to ensure movement and feeding. As with cilia in other model organisms, direction and speed of Paramecium ciliary beating is under bioelectric control of ciliary ion channels. In multiciliated cells of metazoans as well as paramecia, the cilia become physically entrained to beat in metachronal waves. This ciliated organism, Paramecium, is an attractive model for multidisciplinary approaches to dissect the location, structure and function of ciliary ion channels and other proteins involved in ciliary beating. Swimming behavior also can be a read-out of the role of cilia in sensory signal transduction. A cilium emanates from a BB, structurally equivalent to the centriole anchored at the cell surface, and elongates an axoneme composed of microtubule doublets enclosed in a ciliary membrane contiguous with the plasma membrane. The connection between the BB and the axoneme constitutes the transition zone, which serves as a diffusion barrier between the intracellular space and the cilium, defining the ciliary compartment. Human pathologies affecting cilia structure or function, are called ciliopathies, which are caused by gene mutations. For that reason, the molecular mechanisms and structural aspects of cilia assembly and function are actively studied using a variety of model systems, ranging from unicellular organisms to metazoa. In this review, we will highlight the use of Paramecium as a model to decipher ciliary beating mechanisms as well as high resolution insights into BB structure and anchoring. We will show that study of cilia in Paramecium promotes our understanding of cilia formation and function. In addition, we demonstrate that Paramecium could be a useful tool to validate candidate genes for ciliopathies.

Structures of SAS-6 coiled coil hold implications for the polarity of the centriolar cartwheel

Centrioles are eukaryotic organelles that template the formation of cilia and flagella, as well as organize the microtubule network and the mitotic spindle in animal cells. Centrioles have proximal-distal polarity and a 9-fold radial symmetry imparted by a likewise symmetrical central scaffold, the cartwheel. The spindle assembly abnormal protein 6 (SAS-6) self-assembles into 9-fold radially symmetric ring-shaped oligomers that stack via an unknown mechanism to form the cartwheel. Here, we uncover a homo-oligomerization interaction mediated by the coiled-coil domain of SAS-6. Crystallographic structures of Chlamydomonas reinhardtii SAS-6 coiled-coil complexes suggest this interaction is asymmetric, thereby imparting polarity to the cartwheel. Using a cryoelectron microscopy (cryo-EM) reconstitution assay, we demonstrate that amino acid substitutions disrupting this asymmetric association also impair SAS-6 ring stacking. Our work raises the possibility that the asymmetric interaction inherent to SAS-6 coiled-coil provides a polar element for cartwheel assembly, which may assist the establishment of the centriolar proximal-distal axis.

Nine-fold symmetry of centriole: The joint efforts of its core proteins

The centriole is a widely conserved organelle required for the assembly of centrosomes, cilia, and flagella. Its striking feature - the nine-fold symmetrical structure, was discovered over 70 years ago by transmission electron microscopy, and since elaborated mostly by cryo-electron microscopy and super-resolution microscopy. Here, we review the discoveries that led to the current understanding of how the nine-fold symmetrical structure is built. We focus on the recent findings of the centriole structure in high resolution, its assembly pathways, and its nine-fold distributed components. We propose a model that the assembly of the nine-fold symmetrical centriole depends on the concerted efforts of its core proteins.

The centriolar tubulin code

Centrioles are microtubule-based cell organelles present in most eukaryotes. They participate in the control of cell division as part of the centrosome, the major microtubule-organizing center of the cell, and are also essential for the formation of primary and motile cilia. During centriole assembly as well as across its lifetime, centriolar tubulin display marks defined by post-translational modifications (PTMs), such as glutamylation or acetylation. To date, the functions of these PTMs at centrioles are not well understood, although pioneering experiments suggest a role in the stability of this organelle. Here, we review the current knowledge regarding PTMs at centrioles with a particular focus on a possible link between these modifications and centriole's architecture, and propose possible hypothesis regarding centriolar tubulin PTMs's function.

Kinetic and structural roles for the surface in guiding SAS-6 self-assembly to direct centriole architecture

Discovering mechanisms governing organelle assembly is a fundamental pursuit in biology. The centriole is an evolutionarily conserved organelle with a signature 9-fold symmetrical chiral arrangement of microtubules imparted onto the cilium it templates. The first structure in nascent centrioles is a cartwheel, which comprises stacked 9-fold symmetrical SAS-6 ring polymers emerging orthogonal to a surface surrounding each resident centriole. The mechanisms through which SAS-6 polymerization ensures centriole organelle architecture remain elusive. We deploy photothermally-actuated off-resonance tapping high-speed atomic force microscopy to decipher surface SAS-6 self-assembly mechanisms. We show that the surface shifts the reaction equilibrium by ~10⁴ compared to solution. Moreover, coarse-grained molecular dynamics and atomic force microscopy reveal that the surface converts the inherent helical propensity of SAS-6 polymers into 9-fold rings with residual asymmetry, which may guide ring stacking and impart chiral features to centrioles and cilia. Overall, our work reveals fundamental design principles governing centriole assembly.

The centriole exhibits an evolutionarily conserved 9-fold radial symmetry that stems from a cartwheel containing vertically stacked ring polymers that harbor 9 homodimers of the protein SAS-6. Here the authors show how dual properties inherent to surface-guided SAS-6 self-assembly possess spatial information that dictates correct scaffolding of centriole architecture.

Biophysical and Quantitative Principles of Centrosome Biogenesis and Structure

The centrosome is a main orchestrator of the animal cellular microtubule cytoskeleton. Dissecting its structure and assembly mechanisms has been a goal of cell biologists for over a century. In the last two decades, a good understanding of the molecular constituents of centrosomes has been achieved. Moreover, recent breakthroughs in electron and light microscopy techniques have enabled the inspection of the centrosome and the mapping of its components with unprecedented detail. However, we now need a profound and dynamic understanding of how these constituents interact in space and time. Here, we review the latest findings on the structural and molecular architecture of the centrosome and how its biogenesis is regulated, highlighting how biophysical techniques and principles as well as quantitative modeling are changing our understanding of this enigmatic cellular organelle. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Tuning SAS-6 architecture with monobodies impairs distinct steps of centriole assembly

Centrioles are evolutionarily conserved multi-protein organelles essential for forming cilia and centrosomes. Centriole biogenesis begins with self-assembly of SAS-6 proteins into 9-fold symmetrical ring polymers, which then stack into a cartwheel that scaffolds organelle formation. The importance of this architecture has been difficult to decipher notably because of the lack of precise tools to modulate the underlying assembly reaction. Here, we developed monobodies against Chlamydomonas reinhardtii SAS-6, characterizing three in detail with X-ray crystallography, atomic force microscopy and cryo-electron microscopy. This revealed distinct monobody-target interaction modes, as well as specific consequences on ring assembly and stacking. Of particular interest, monobody MB_CRS6-15 induces a conformational change in CrSAS-6, resulting in the formation of a helix instead of a ring. Furthermore, we show that this alteration impairs centriole biogenesis in human cells. Overall, our findings identify monobodies as powerful molecular levers to alter the architecture of multi-protein complexes and tune centriole assembly.

A first-takes-all model of centriole copy number control based on cartwheel elongation

How cells control the numbers of subcellular components is a fundamental question in biology. Given that biosynthetic processes are fundamentally stochastic it is utterly puzzling that some structures display no copy number variation within a cell population. Centriole biogenesis, with each centriole being duplicated once and only once per cell cycle, stands out due to its remarkable fidelity. This is a highly controlled process, which depends on low-abundance rate-limiting factors. How can exactly one centriole copy be produced given the variation in the concentration of these key factors? Hitherto, tentative explanations of this control evoked lateral inhibition- or phase separation-like mechanisms emerging from the dynamics of these rate-limiting factors but how strict centriole number is regulated remains unsolved. Here, a novel solution to centriole copy number control is proposed based on the assembly of a centriolar scaffold, the cartwheel. We assume that cartwheel building blocks accumulate around the mother centriole at supercritical concentrations, sufficient to assemble one or more cartwheels. Our key postulate is that once the first cartwheel is formed it continues to elongate by stacking the intermediate building blocks that would otherwise form supernumerary cartwheels. Using stochastic models and simulations, we show that this mechanism may ensure formation of one and only one cartwheel robustly over a wide range of parameter values. By comparison to alternative models, we conclude that the distinctive signatures of this novel mechanism are an increasing assembly time with cartwheel numbers and the translation of stochasticity in building block concentrations into variation in cartwheel numbers or length.

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.

Overview of the centriole architecture

The centriole is a magnificent molecular assembly of several giga-daltons, one of the largest of the eukaryotic cell, and whose atomic structure remains unsolved to date. However, numerous electron microscopy, cryo-tomography, and super-resolution studies now make it possible to establish a global architectural view of it with its different sub-regions. These analyses broaden our understanding by providing additional informations to cell biology and structural biology approaches. In this review, we describe current knowledge on the overall organization of the centriole. We will highlight each sub-structural element, their differences between species and their putative protein composition. We will conclude on the current limitations that still take us away from a complete atomic view of the centriole architecture.

Architecture of the centriole cartwheel-containing region revealed by cryo-electron tomography

Centrioles are evolutionarily conserved barrels of microtubule triplets that form the core of the centrosome and the base of the cilium. While the crucial role of the proximal region in centriole biogenesis has been well documented, its native architecture and evolutionary conservation remain relatively unexplored. Here, using cryo-electron tomography of centrioles from four evolutionarily distant species, we report on the architectural diversity of the centriole's proximal cartwheel-bearing region. Our work reveals that the cartwheel central hub is constructed from a stack of paired rings with cartwheel inner densities inside. In both Paramecium and Chlamydomonas, the repeating structural unit of the cartwheel has a periodicity of 25 nm and consists of three ring pairs, with 6 radial spokes emanating and merging into a single bundle that connects to the microtubule triplet via the D2-rod and the pinhead. Finally, we identified that the cartwheel is indirectly connected to the A-C linker through the triplet base structure extending from the pinhead. Together, our work provides unprecedented evolutionary insights into the architecture of the centriole proximal region, which underlies centriole biogenesis.

Novel features of centriole polarity and cartwheel stacking revealed by cryo-tomography

Centrioles are polarized microtubule‐based organelles that seed the formation of cilia, and which assemble from a cartwheel containing stacked ring oligomers of SAS‐6 proteins. A cryo‐tomography map of centrioles from the termite flagellate Trichonympha spp. was obtained previously, but higher resolution analysis is likely to reveal novel features. Using sub‐tomogram averaging (STA) in T. spp. and Trichonympha agilis, we delineate the architecture of centriolar microtubules, pinhead, and A‐C linker. Moreover, we report ~25 Å resolution maps of the central cartwheel, revealing notably polarized cartwheel inner densities (CID). Furthermore, STA of centrioles from the distant flagellate Teranympha mirabilis uncovers similar cartwheel architecture and a distinct filamentous CID. Fitting the CrSAS‐6 crystal structure into the flagellate maps and analyzing cartwheels generated in vitro indicate that SAS‐6 rings can directly stack onto one another in two alternating configurations: with a slight rotational offset and in register. Overall, improved STA maps in three flagellates enabled us to unravel novel architectural features, including of centriole polarity and cartwheel stacking, thus setting the stage for an accelerated elucidation of underlying assembly mechanisms.