Basal bodies bend in response to ciliary forces

A ternary complex of MIPs in the A-tubule of basal bodies and axonemes depends on RIB22 and the EF-hand domain of RIB72A in Tetrahymena cilia

The lumens of the highly stable microtubules that make up the core of basal bodies, cilia, and flagella are coated with a network of proteins known as MIPs, or microtubule inner proteins. MIPs are hypothesized to enhance the rigidity and stability of these microtubules, but how they assemble and contribute to cilia function is poorly understood. Here we describe a ciliate specific MIP, RIB22, in Tetrahymena thermophila. RIB22 is a calmodulin-like protein found in the A-tubule of doublet and triplet microtubules in cilia and basal bodies. Its localization is dependent on the conserved MIP RIB72. Here we use cryogenic electron tomography (cryoET) to examine RIB22 and its interacting partners in axonemes and basal bodies. RIB22 forms a ternary complex with the C-terminal EF-hand domain of RIB72A and another MIP, FAM166A. Tetrahymena strains lacking RIB22 or the EF-hand domain of RIB72A showed impaired cilia function. CryoET on axonemes from these strains demonstrated an interdependence of the three proteins for stabilization within the structure. Deletion of the RIB72A EF-hand domain resulted in the apparent loss of multiple MIPs in the region. These findings emphasize the intricacy of the MIP network and the importance of understanding MIPs' functions during cilium assembly and regulation.

The structure of basal body inner junctions from Tetrahymena revealed by electron cryo-tomography

The cilium is a microtubule-based eukaryotic organelle critical for many cellular functions. Its assembly initiates at a basal body and continues as an axoneme that projects out of the cell to form a functional cilium. This assembly process is tightly regulated. However, our knowledge of the molecular architecture and the mechanism of assembly is limited. By applying cryo-electron tomography, we obtained structures of the inner junction in three regions of the cilium from Tetrahymena: the proximal, the central core of the basal body, and the axoneme. We identified several protein components in the basal body. While a few proteins are distributed throughout the entire length of the organelle, many are restricted to specific regions, forming intricate local interaction networks in the inner junction and bolstering local structural stability. By examining the inner junction in a POC1 knockout mutant, we found the triplet microtubule was destabilized, resulting in a defective structure. Surprisingly, several axoneme-specific components were found to "infiltrate" into the mutant basal body. Our findings provide molecular insight into cilium assembly at the inner junctions, underscoring its precise spatial regulation.

A delta-tubulin/epsilon-tubulin/Ted protein complex is required for centriole architecture

Centrioles have a unique, conserved architecture formed by three linked, 'triplet', microtubules arranged in ninefold symmetry. The mechanisms by which these triplet microtubules are formed remain unclear but likely involve the noncanonical tubulins delta-tubulin and epsilon-tubulin. Previously, we found that human cells lacking delta-tubulin or epsilon-tubulin form abnormal centrioles, characterized by an absence of triplet microtubules, lack of central core protein POC5, and a futile cycle of centriole formation and disintegration (Wang et al., 2017). Here, we show that human cells lacking either TEDC1 or TEDC2 have similar abnormalities. Using ultrastructure expansion microscopy, we observed that mutant centrioles elongate to the same length as control centrioles in G2 phase and fail to recruit central core scaffold proteins. Remarkably, mutant centrioles also have an expanded proximal region. During mitosis, these mutant centrioles further elongate before fragmenting and disintegrating. All four proteins physically interact and TEDC1 and TEDC2 can form a subcomplex in the absence of the tubulins, supporting an AlphaFold Multimer model of the tetramer. TEDC1 and TEDC2 localize to centrosomes and are mutually dependent on each other and on delta-tubulin and epsilon-tubulin for localization. Our results demonstrate that delta-tubulin, epsilon-tubulin, TEDC1, and TEDC2 function together to promote robust centriole architecture, laying the foundation for future studies on the mechanisms underlying the assembly of triplet microtubules and their interactions with centriole structure.

The ignored structure in female fertility: cilia in the fallopian tubes

Cilia in the fallopian tubes (CFT) play an important role in female infertility, but have not been explored comprehensively. This review reveals the detection techniques for CFT function and morphology, and the related analysis of female infertility and other gynaecological disorders. CFT differentiate from progenitor cells, and develop into primary cilia and motile cilia. Primary cilia coordinate multiple signalling pathways, and motile cilia produce laminar flow through bidirectional intraflagellar transport, which drives the movement of oocytes and gametes. Several methods for quantitative detection and protein analysis have been used to explore the factors contributing to the decrease in ciliary beat frequency (CBF), and the cellular mechanism of ciliary cell death and shedding. In both primary and secondary ciliary disorders associated with reproductive diseases, abnormal alterations in ciliary quantity, ciliary structure, CBF and ciliary signalling pathways result in abnormal tubal laminar flow, and diminished oocyte retrieval and transport capabilities.

An interaction network of inner centriole proteins organised by POC1A-POC1B heterodimer crosslinks ensures centriolar integrity

Centriole integrity, vital for cilia formation and chromosome segregation, is crucial for human health. The inner scaffold within the centriole lumen composed of the proteins POC1B, POC5 and FAM161A is key to this integrity. Here, we provide an understanding of the function of inner scaffold proteins. We demonstrate the importance of an interaction network organised by POC1A-POC1B heterodimers within the centriole lumen, where the WD40 domain of POC1B localises close to the centriole wall, while the POC5-interacting WD40 of POC1A resides in the centriole lumen. The POC1A-POC5 interaction and POC5 tetramerization are essential for inner scaffold formation and centriole stability. The microtubule binding proteins FAM161A and MDM1 by binding to POC1A-POC1B, likely positioning the POC5 tetramer near the centriole wall. Disruption of POC1A or POC1B leads to centriole microtubule defects and deletion of both genes causes centriole disintegration. These findings provide insights into organisation and function of the inner scaffold.

The Structure of Cilium Inner Junctions Revealed by Electron Cryo-tomography

The cilium is a microtubule-based organelle critical for many cellular functions. Its assembly initiates at a basal body and continues as an axoneme that projects out of the cell to form a functional cilium. This assembly process is tightly regulated. However, our knowledge of the molecular architecture and the mechanism of assembly is limited. By applying electron cryotomography and subtomogram averaging, we obtained subnanometer resolution structures of the inner junction in three distinct regions of the cilium: the proximal region of the basal body, the central core of the basal body, and the flagellar axoneme. The structures allowed us to identify several basal body and axoneme components. While a few proteins are distributed throughout the entire length of the organelle, many are restricted to particular regions of the cilium, forming intricate local interaction networks and bolstering local structural stability. Finally, by knocking out a critical basal body inner junction component Poc1, we found the triplet MT was destabilized, resulting in a defective structure. Surprisingly, several axoneme-specific components were found to "infiltrate" into the mutant basal body. Our findings provide molecular insight into cilium assembly at its inner Junctions, underscoring its precise spatial regulation.

Poc1 bridges basal body inner junctions to promote triplet microtubule integrity and connections

Basal bodies (BBs) are conserved eukaryotic structures that organize cilia. They are comprised of nine, cylindrically arranged, triplet microtubules (TMTs) connected to each other by inter-TMT linkages which stabilize the structure. Poc1 is a conserved protein important for BB structural integrity in the face of ciliary forces transmitted to BBs. To understand how Poc1 confers BB stability, we identified the precise position of Poc1 in the Tetrahymena BB and the effect of Poc1 loss on BB structure. Poc1 binds at the TMT inner junctions, stabilizing TMTs directly. From this location, Poc1 also stabilizes inter-TMT linkages throughout the BB, including the cartwheel pinhead and the inner scaffold. The full localization of the inner scaffold protein Fam161A requires Poc1. As ciliary forces are increased, Fam161A is reduced, indicative of a force-dependent molecular remodeling of the inner scaffold. Thus, while not essential for BB assembly, Poc1 promotes BB interconnections that establish an architecture competent to resist ciliary forces.

The molecular architecture of the ciliary transition zones

Cilia and flagella are specialized eukaryotic organelles projecting from the surface of eukaryotic cells that play a central role in various physiological processes, including cell motility, sensory perception, and signal transduction. At the base of these structures lies the ciliary transition zone, a pivotal region that functions as a gatekeeper and communication hub for ciliary activities. Despite its crucial role, the intricacies of its architecture remain poorly understood, especially given the variations in its organization across different cell types and species. In this review, we explore the molecular architecture of the ciliary transition zone, with a particular focus on recent findings obtained using cryotomography and super-resolution imaging techniques.

Time and power dependence of laser-induced photodamage on human sperm revealed by longitudinal rolling measurement using optical tweezers

Lasers are widely applied in assisted reproductive technologies, including sperm fixation, sperm selection and intracytoplasmic sperm injections, to reduce procedure time and improve consistency and reproducibility. However, quantitative studies on laser-induced photodamage of sperm are lacking. In this study, we demonstrated that, by using optical tweezers, the kinematic parameters of freely swimming sperm are correlated with the frequency as well as the percentage of pausing duration of longitudinal rolling of the same sperm head in the optical trap. Furthermore, by trapping individual sperm cells using 1064-nm optical tweezers, we quantitatively characterized the time-dependence of longitudinal rolling frequency and percentage of pausing duration of sperm under different laser powers. Our study revealed that, as trapping time and the laser power time increase, the longitudinal rolling frequency of the optically trapped sperm decreases with an increasing percentage of pausing duration, which characterizes the effect of laser power and duration on the photodamage of individual sperm cells. Our study provides experimental basis for the optimization of laser application in assisted reproductive technology, which may reduce the photodamage-induced biosafety risk in the future.

The evolution of centriole degradation in mouse sperm

Centrioles are subcellular organelles found at the cilia base with an evolutionarily conserved structure and a shock absorber-like function. In sperm, centrioles are found at the flagellum base and are essential for embryo development in basal animals. Yet, sperm centrioles have evolved diverse forms, sometimes acting like a transmission system, as in cattle, and sometimes becoming dispensable, as in house mice. How the essential sperm centriole evolved to become dispensable in some organisms is unclear. Here, we test the hypothesis that this transition occurred through a cascade of evolutionary changes to the proteins, structure, and function of sperm centrioles and was possibly driven by sperm competition. We found that the final steps in this cascade are associated with a change in the primary structure of the centriolar inner scaffold protein FAM161A in rodents. This information provides the first insight into the molecular mechanisms and adaptive evolution underlying a major evolutionary transition within the internal structure of the mammalian sperm neck.

CCDC15 localizes to the centriole inner scaffold and controls centriole length and integrity

Centrioles are microtubule-based organelles responsible for forming centrosomes and cilia, which serve as microtubule-organizing, signaling, and motility centers. Biogenesis and maintenance of centrioles with proper number, size, and architecture are vital for their functions during development and physiology. While centriole number control has been well-studied, less is understood about their maintenance as stable structures with conserved size and architecture during cell division and ciliary motility. Here, we identified CCDC15 as a centriole protein that colocalizes with and interacts with the inner scaffold, a crucial centriolar subcompartment for centriole size control and integrity. Using ultrastructure expansion microscopy, we found that CCDC15 depletion affects centriole length and integrity, leading to defective cilium formation, maintenance, and response to Hedgehog signaling. Moreover, loss-of-function experiments showed CCDC15's role in recruiting both the inner scaffold protein POC1B and the distal SFI1/Centrin-2 complex to centrioles. Our findings reveal players and mechanisms of centriole architectural integrity and insights into diseases linked to centriolar defects.

Poc1 is a basal body inner junction protein that promotes triplet microtubule integrity and interconnections

Basal bodies (BBs) are conserved eukaryotic structures that organize motile and primary cilia. The BB is comprised of nine, cylindrically arranged, triplet microtubules (TMTs) that are connected to each other by inter-TMT linkages which maintain BB structure. During ciliary beating, forces transmitted to the BB must be resisted to prevent BB disassembly. Poc1 is a conserved BB protein important for BBs to resist ciliary forces. To understand how Poc1 confers BB stability, we identified the precise position of Poc1 binding in the Tetrahymena BB and the effect of Poc1 loss on BB structure. Poc1 binds at the TMT inner junctions, stabilizing TMTs directly. From this location, Poc1 also stabilizes inter-TMT linkages throughout the BB, including the cartwheel pinhead and the inner scaffold. Moreover, we identify a molecular response to ciliary forces via a molecular remodeling of the inner scaffold, as determined by differences in Fam161A localization. Thus, while not essential for BB assembly, Poc1 promotes BB interconnections that establish an architecture competent to resist ciliary forces.