Sas-4 proteins are required during basal body duplication in Paramecium

The evolutionary conserved proteins CEP90, FOPNL, and OFD1 recruit centriolar distal appendage proteins to initiate their assembly

In metazoa, cilia assembly is a cellular process that starts with centriole to basal body maturation, migration to the cell surface, and docking to the plasma membrane. Basal body docking involves the interaction of both the distal end of the basal body and the transition fibers/distal appendages, with the plasma membrane. Mutations in numerous genes involved in basal body docking and transition zone assembly are associated with the most severe ciliopathies, highlighting the importance of these events in cilium biogenesis. In this context, the ciliate Paramecium has been widely used as a model system to study basal body and cilia assembly. However, despite the evolutionary conservation of cilia assembly events across phyla, whether the same molecular players are functionally conserved, is not fully known. Here, we demonstrated that CEP90, FOPNL, and OFD1 are evolutionary conserved proteins crucial for ciliogenesis. Using ultrastructure expansion microscopy, we unveiled that these proteins localize at the distal end of both centrioles/basal bodies in Paramecium and mammalian cells. Moreover, we found that these proteins are recruited early during centriole duplication on the external surface of the procentriole. Functional analysis performed both in Paramecium and mammalian cells demonstrate the requirement of these proteins for distal appendage assembly and basal body docking. Finally, we show that mammalian centrioles require another component, Moonraker (MNR), to recruit OFD1, FOPNL, and CEP90, which will then recruit the distal appendage proteins CEP83, CEP89, and CEP164. Altogether, we propose that this OFD1, FOPNL, and CEP90 functional module is required to determine in mammalian cells the future position of distal appendage proteins.

CEP90, FOPNL and OFD1 form an evolutionary conserved module which promotes the assembly of centriolar distal appendages. This study uses ultrastructure expansion microscopy to reveal the recruitment of this module on early-born procentrioles to in turn recruit centriolar distal appendage proteins, proposing that this dictates the future location of distal appendages.

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.

Anterior-posterior pattern formation in ciliates

As single cells, ciliates build, duplicate, and even regenerate complex cortical patterns by largely unknown mechanisms that precisely position organelles along two cell‐wide axes: anterior–posterior and circumferential (left–right). We review our current understanding of intracellular patterning along the anterior–posterior axis in ciliates, with emphasis on how the new pattern emerges during cell division. We focus on the recent progress at the molecular level that has been driven by the discovery of genes whose mutations cause organelle positioning defects in the model ciliate Tetrahymena thermophila. These investigations have revealed a network of highly conserved kinases that are confined to either anterior or posterior domains in the cell cortex. These pattern‐regulating kinases create zones of cortical inhibition that by exclusion determine the precise placement of organelles. We discuss observations and models derived from classical microsurgical experiments in large ciliates (including Stentor) and interpret them in light of recent molecular findings in Tetrahymena. In particular, we address the involvement of intracellular gradients as vehicles for positioning organelles along the anterior‐posterior axis.

Characterisation of centriole biogenesis during multiciliation in planarians

BACKGROUND INFORMATION

Dense multicilia in protozoa and metazoa generate a strong force important for locomotion and extracellular fluid flow. During ciliogenesis, multiciliated cells produce hundreds of centrioles to serve as basal bodies through various pathways including deuterosome-dependent (DD), hyper-activated mother centriole-dependent (MCD) and basal bodydependent (BBD) pathways. The centrosome-free planarian Schmidtea mediterranea is widely used for regeneration studies because its neoblasts are capable of regenerating any body part after injury. However, it is currently unclear how the flatworms generate massive centrioles for multiciliated cells in the pharynx and body epidermis when their cells are initially centriole-free.

RESULTS

In this study, we investigate the progress of centriole amplification during the pharynx regeneration. We observe that the planarian pharyngeal epithelial cells generate their centrioles asynchronously through a de novo pathway. Most of the de novo centrioles are formed individually, whereas the remaining ones are assembled in pairs, possibly by sharing a cartwheel, or in small clusters lacking a nucleation center. Further RNAi experiments show that the known key factors of centriole duplication, including Cep152, Plk4 and Sas6, are crucial for the centriole amplification.

CONCLUSIONS AND SIGNIFICANCE

Our study demonstrates the distinct process of massive centriole biogenesis in S. mediterranea and helps to understand the diversity of centriole biogenesis during evolution.

AEE788 Inhibits Basal Body Assembly and Blocks DNA Replication in the African Trypanosome

Trypanosoma brucei causes human African trypanosomiasis (HAT). The pyrrolopyrimidine AEE788 (a hit for anti-HAT drug discovery) associates with three trypanosome protein kinases. Herein we delineate the effects of AEE788 on T. brucei using chemical biology strategies. AEE788 treatment inhibits DNA replication in the kinetoplast (mitochondrial nucleoid) and nucleus. In addition, AEE788 blocks duplication of the basal body and the bilobe without affecting mitosis. Thus, AEE788 prevents entry into the S-phase of the cell division cycle. To study the kinetics of early events in trypanosome division, we employed an "AEE788 block and release" protocol to stage entry into the S-phase. A time-course of DNA synthesis (nuclear and kinetoplast DNA), duplication of organelles (basal body, bilobe, kinetoplast, nucleus), and cytokinesis was obtained. Unexpected findings include the following: 1) basal body and bilobe duplication are concurrent; 2) maturation of probasal bodies, marked by TbRP2 recruitment, is coupled with nascent basal body assembly, monitored by localization of TbSAS6 at newly forming basal bodies; and 3) kinetoplast division is observed in G2 after completion of nuclear DNA synthesis. Prolonged exposure of trypanosomes to AEE788 inhibited transferrin endocytosis, altered cell morphology, and decreased cell viability. To discover putative effectors for the pleiotropic effects of AEE788, proteome-wide changes in protein phosphorylation induced by the drug were determined. Putative effectors include an SR protein kinase, bilobe proteins, TbSAS4, TbRP2, and BILBO-1. Loss of function of one or more of these effectors can, from published literature, explain the polypharmacology of AEE788 on trypanosome biology.

Parthenogenesis in Insects: The Centriole Renaissance

Building a new organism usually requires the contribution of two differently shaped haploid cells, the male and female gametes, each providing its genetic material to restore diploidy of the new born zygote. The successful execution of this process requires defined sequential steps that must be completed in space and time. Otherwise, development fails. Relevant among the earlier steps are pronuclear migration and formation of the first mitotic spindle that promote the mixing of parental chromosomes and the formation of the zygotic nucleus. A complex microtubule network ensures the proper execution of these processes. Instrumental to microtubule organization and bipolar spindle assembly is a distinct non-membranous organelle, the centrosome. Centrosome inheritance during fertilization is biparental, since both gametes provide essential components to build a functional centrosome. This model does not explain, however, centrosome formation during parthenogenetic development, a special mode of sexual reproduction in which the unfertilized egg develops without the contribution of the male gamete. Moreover, whereas fertilization is a relevant example in which the cells actively check the presence of only one centrosome, to avoid multipolar spindle formation, the development of parthenogenetic eggs is ensured, at least in insects, by the de novo assembly of multiple centrosomes.Here, we will focus our attention on the assembly of functional centrosomes following fertilization and during parthenogenetic development in insects. Parthenogenetic development in which unfertilized eggs are naturally depleted of centrosomes would provide a useful experimental system to investigate centriole assembly and duplication together with centrosome formation and maturation.

SAS-4 Protein in Trypanosoma brucei Controls Life Cycle Transitions by Modulating the Length of the Flagellum Attachment Zone Filament

The evolutionarily conserved centriole/basal body protein SAS-4 regulates centriole duplication in metazoa and basal body duplication in flagellated and ciliated organisms. Here, we report that the SAS-4 homolog in the flagellated protozoan Trypanosoma brucei, TbSAS-4, plays an unusual role in controlling life cycle transitions by regulating the length of the flagellum attachment zone (FAZ) filament, a specialized cytoskeletal structure required for flagellum adhesion and cell morphogenesis. TbSAS-4 is concentrated at the distal tip of the FAZ filament, and depletion of TbSAS-4 in the trypomastigote form disrupts the elongation of the new FAZ filament, generating cells with a shorter FAZ associated with a longer unattached flagellum and repositioned kinetoplast and basal body, reminiscent of epimastigote-like morphology. Further, we show that TbSAS-4 associates with six additional FAZ tip proteins, and depletion of TbSAS-4 disrupts the enrichment of these FAZ tip proteins at the new FAZ tip, suggesting a role of TbSAS-4 in maintaining the integrity of this FAZ tip protein complex. Together, these results uncover a novel function of TbSAS-4 in regulating the length of the FAZ filament to control basal body positioning and life cycle transitions in T. brucei.

Molecular evolution and functional divergence of tubulin superfamily in the fungal tree of life

Microtubules are essential for various cellular activities and β-tubulins are the target of benzimidazole fungicides. However, the evolution and molecular mechanisms driving functional diversification in fungal tubulins are not clear. In this study, we systematically identified tubulin genes from 59 representative fungi across the fungal kingdom. Phylogenetic analysis showed that α-/β-tubulin genes underwent multiple independent duplications and losses in different fungal lineages and formed distinct paralogous/orthologous clades. The last common ancestor of basidiomycetes and ascomycetes likely possessed two paralogs of α-tubulin (α₁/α₂) and β-tubulin (β₁/β₂) genes but α₂-tubulin genes were lost in basidiomycetes and β₂-tubulin genes were lost in most ascomycetes. Molecular evolutionary analysis indicated that α₁, α₂, and β₂-tubulins have been under strong divergent selection and adaptive positive selection. Many positively selected sites are at or adjacent to important functional sites and likely contribute to functional diversification. We further experimentally confirmed functional divergence of two β-tubulins in Fusarium and identified type II variations in FgTub2 responsible for function shifts. In this study, we also identified δ-/ε-/η-tubulins in Chytridiomycetes. Overall, our results illustrated that different evolutionary mechanisms drive functional diversification of α-/β-tubulin genes in different fungal lineages, and residues under positive selection could provide targets for further experimental study.

Multiciliated cells

Cilia are microtubule-based projections that serve a wide variety of essential functions in animal cells. Defects in cilia structure or function have recently been found to underlie diverse human diseases. While many eukaryotic cells possess only one or two cilia, some cells, including those of many unicellular organisms, exhibit many cilia. In vertebrates, multiciliated cells are a specialized population of post-mitotic cells decorated with dozens of motile cilia that beat in a polarized and synchronized fashion to drive directed fluid flow across an epithelium. Dysfunction of human multiciliated cells is associated with diseases of the brain, airway and reproductive tracts. Despite their importance, multiciliated cells are relatively poorly studied and we are only beginning to understand the mechanisms underlying their development and function. Here, we review the general phylogeny and physiology of multiciliation and detail our current understanding of the developmental and cellular events underlying the specification, differentiation and function of multiciliated cells in vertebrates.

The neomuran revolution and phagotrophic origin of eukaryotes and cilia in the light of intracellular coevolution and a revised tree of life

Three kinds of cells exist with increasingly complex membrane-protein targeting: Unibacteria (Archaebacteria, Posibacteria) with one cytoplasmic membrane (CM); Negibacteria with a two-membrane envelope (inner CM; outer membrane [OM]); eukaryotes with a plasma membrane and topologically distinct endomembranes and peroxisomes. I combine evidence from multigene trees, palaeontology, and cell biology to show that eukaryotes and archaebacteria are sisters, forming the clade neomura that evolved ~1.2 Gy ago from a posibacterium, whose DNA segregation and cell division were destabilized by murein wall loss and rescued by the evolving novel neomuran endoskeleton, histones, cytokinesis, and glycoproteins. Phagotrophy then induced coevolving serial major changes making eukaryote cells, culminating in two dissimilar cilia via a novel gliding-fishing-swimming scenario. I transfer Chloroflexi to Posibacteria, root the universal tree between them and Heliobacteria, and argue that Negibacteria are a clade whose OM, evolving in a green posibacterium, was never lost.

Mapping molecules to structure: unveiling secrets of centriole and cilia assembly with near-atomic resolution

Centrioles are microtubule (MT)-based cylinders that form centrosomes and can be modified into basal bodies that template the axoneme, the ciliary MT skeleton. These MT-based structures are present in all branches of the eukaryotic tree of life, where they have important sensing, motility and cellular architecture-organizing functions. Moreover, they are altered in several human conditions and diseases, including sterility, ciliopathies and cancer. Although the ultrastructure of centrioles and derived organelles has been known for over 50 years, the molecular basis of their remarkably conserved properties, such as their 9-fold symmetry, has only now started to be unveiled. Recent advances in imaging, proteomics and crystallography, allowed the building of 3D models of centrioles and derived structures with unprecedented molecular details, leading to a much better understanding of their assembly and function. Here, we cover progress in this field, focusing on the mechanisms of centriole and cilia assembly.

Towards a molecular architecture of centriole assembly

The centriole is an evolutionarily conserved macromolecular structure that is crucial for the formation of flagella, cilia and centrosomes. The ultrastructure of the centriole was first characterized decades ago with the advent of electron microscopy, revealing a striking ninefold radial arrangement of microtubules. However, it is only recently that the molecular mechanisms governing centriole assembly have begun to emerge, including the elucidation of the crucial role of spindle assembly abnormal 6 (SAS-6) proteins in imparting the ninefold symmetry. These advances have brought the field to an exciting era in which architecture meets function.