Building a centriole

Ultrastructure expansion microscopy (U-ExM) of mouse and human kidneys for analysis of subcellular structures

Ultrastructure expansion microscopy (U-ExM) involves the physical magnification of specimens embedded in hydrogels, which allows for super-resolution imaging of subcellular structures using a conventional diffraction-limited microscope. Methods for expansion microscopy exist for several organisms, organs, and cell types, and used to analyze cellular organelles and substructures in nanoscale resolution. Here, we describe a simple step-by-step U-ExM protocol for the expansion, immunostaining, imaging, and analysis of cytoskeletal and organellar structures in kidney tissue. We detail the critical modified steps to optimize isotropic kidney tissue expansion, and preservation of the renal cell structures of interest. We demonstrate the utility of the approach using several markers of renal cell types, centrioles, cilia, the extracellular matrix, and other cytoskeletal elements. Finally, we show that the approach works well on mouse and human kidney samples that were preserved using different fixation and embedding conditions. Overall, this protocol provides a simple and cost-effective approach to analyze both preclinical and clinical renal samples in high detail, using conventional lab supplies and standard widefield or confocal microscopy.

Ultrastructure expansion microscopy (U-ExM) of mouse and human kidneys for analysis of subcellular structures

Ultrastructure expansion microscopy (U-ExM) involves the physical magnification of specimens embedded in hydrogels, which allows for super-resolution imaging of subcellular structures using a conventional diffraction-limited microscope. Methods for expansion microscopy exist for several organisms, organs, and cell types, and used to analyze cellular organelles and substructures in nanoscale resolution. Here, we describe a simple step-by-step U-ExM protocol for the expansion, immunostaining, imaging, and analysis of cytoskeletal and organellar structures in kidney tissue. We detail the critical modified steps to optimize isotropic kidney tissue expansion, and preservation of the renal cell structures of interest. We demonstrate the utility of the approach using several markers of renal cell types, centrioles, cilia, the extracellular matrix, and other cytoskeletal elements. Finally, we show that the approach works well on mouse and human kidney samples that were preserved using different fixation and storage conditions. Overall, this protocol provides a simple and cost-effective approach to analyze both pre-clinical and clinical renal samples in high detail, using conventional lab supplies and standard widefield or confocal microscopy.

The centrosome - diverse functions in fertilization and development across species

The centrosome is a non-membrane-bound organelle that is conserved across most animal cells and serves various functions throughout the cell cycle. In dividing cells, the centrosome is known as the spindle pole and nucleates a robust microtubule spindle to separate genetic material equally into two daughter cells. In non-dividing cells, the mother centriole, a substructure of the centrosome, matures into a basal body and nucleates cilia, which acts as a signal-transducing antenna. The functions of centrosomes and their substructures are important for embryonic development and have been studied extensively using in vitro mammalian cell culture or in vivo using invertebrate models. However, there are considerable differences in the composition and functions of centrosomes during different aspects of vertebrate development, and these are less studied. In this Review, we discuss the roles played by centrosomes, highlighting conserved and divergent features across species, particularly during fertilization and embryonic development.

Genetic Primary Microcephalies: When Centrosome Dysfunction Dictates Brain and Body Size

Primary microcephalies (PMs) are defects in brain growth that are detectable at or before birth and are responsible for neurodevelopmental disorders. Most are caused by biallelic or, more rarely, dominant mutations in one of the likely hundreds of genes encoding PM proteins, i.e., ubiquitous centrosome or microtubule-associated proteins required for the division of neural progenitor cells in the embryonic brain. Here, we provide an overview of the different types of PMs, i.e., isolated PMs with or without malformations of cortical development and PMs associated with short stature (microcephalic dwarfism) or sensorineural disorders. We present an overview of the genetic, developmental, neurological, and cognitive aspects characterizing the most representative PMs. The analysis of phenotypic similarities and differences among patients has led scientists to elucidate the roles of these PM proteins in humans. Phenotypic similarities indicate possible redundant functions of a few of these proteins, such as ASPM and WDR62, which play roles only in determining brain size and structure. However, the protein pericentrin (PCNT) is equally required for determining brain and body size. Other PM proteins perform both functions, albeit to different degrees. Finally, by comparing phenotypes, we considered the interrelationships among these proteins.

The Centriole's Role in Miscarriages

Centrioles are subcellular organelles essential for normal cell function and development; they form the cell’s centrosome (a major cytoplasmic microtubule organization center) and cilium (a sensory and motile hair-like cellular extension). Centrioles with evolutionarily conserved characteristics are found in most animal cell types but are absent in egg cells and exhibit unexpectedly high structural, compositional, and functional diversity in sperm cells. As a result, the centriole’s precise role in fertility and early embryo development is unclear. The centrioles are found in the spermatozoan neck, a strategic location connecting two central functional units: the tail, which propels the sperm to the egg and the head, which holds the paternal genetic material. The spermatozoan neck is an ideal site for evolutionary innovation as it can control tail movement pre-fertilization and the male pronucleus’ behavior post-fertilization. We propose that human, bovine, and most other mammals–which exhibit ancestral centriole-dependent reproduction and two spermatozoan centrioles, where one canonical centriole is maintained, and one atypical centriole is formed–adapted extensive species-specific centriolar features. As a result, these centrioles have a high post-fertilization malfunction rate, resulting in aneuploidy, and miscarriages. In contrast, house mice evolved centriole-independent reproduction, losing the spermatozoan centrioles and overcoming a mechanism that causes miscarriages.

Microtubule and Actin Cytoskeletal Dynamics in Male Meiotic Cells of Drosophila melanogaster

Drosophila dividing spermatocytes offer a highly suitable cell system in which to investigate the coordinated reorganization of microtubule and actin cytoskeleton systems during cell division of animal cells. Like male germ cells of mammals, Drosophila spermatogonia and spermatocytes undergo cleavage furrow ingression during cytokinesis, but abscission does not take place. Thus, clusters of primary and secondary spermatocytes undergo meiotic divisions in synchrony, resulting in cysts of 32 secondary spermatocytes and then 64 spermatids connected by specialized structures called ring canals. The meiotic spindles in Drosophila males are substantially larger than the spindles of mammalian somatic cells and exhibit prominent central spindles and contractile rings during cytokinesis. These characteristics make male meiotic cells particularly amenable to immunofluorescence and live imaging analysis of the spindle microtubules and the actomyosin apparatus during meiotic divisions. Moreover, because the spindle assembly checkpoint is not robust in spermatocytes, Drosophila male meiosis allows investigating of whether gene products required for chromosome segregation play additional roles during cytokinesis. Here, we will review how the research studies on Drosophila male meiotic cells have contributed to our knowledge of the conserved molecular pathways that regulate spindle microtubules and cytokinesis with important implications for the comprehension of cancer and other diseases.

Human pluripotent stem cell-derived brain organoids as in vitro models for studying neural disorders and cancer

The sheer complexities of brain and resource limitation of human brain tissue greatly hamper our understanding of the brain disorders and cancers. Recently developed three-dimensional (3D) brain organoids (BOs) are self-organized and spontaneously differentiated from human pluripotent stem cells (hPSCs) in vitro, which exhibit similar features with cell type diversity, structural organization, and functional connectivity as the developing human brain. Based on these characteristics, hPSC-derived BOs (hPDBOs) provide new opportunities to recapitulate the complicated processes during brain development, neurodegenerative disorders, and brain cancers in vitro. In this review, we will provide an overview of existing BO models and summarize the applications of this technology in modeling the neural disorders and cancers. Furthermore, we will discuss the challenges associated with their use as in vitro models for disease modeling and the potential future direction.

Fluorescence-Based Ratiometric Analysis of Sperm Centrioles (FRAC) Finds Patient Age and Sperm Morphology Are Associated With Centriole Quality

A large proportion of infertility and miscarriage causes are unknown. One potential cause is a defective sperm centriole, a subcellular structure essential for sperm motility and embryonic development. Yet, the extent to which centriolar maladies contribute to male infertility is unknown due to the lack of a convenient way to assess centriole quality. We developed a robust, location-based, ratiometric assay to overcome this roadblock, the Fluorescence-based Ratiometric Assessment of Centrioles (FRAC). We performed a case series study with semen samples from 33 patients, separated using differential gradient centrifugation into higher-grade (pellet) and lower-grade (interface) sperm fractions. Using a reference population of higher-grade sperm from infertile men with morphologically standard sperm, we found that 79% of higher-grade sperm of infertile men with substandard sperm morphology have suboptimal centrioles (P = 0.0005). Moreover, tubulin labeling of the sperm distal centriole correlates negatively with age (P = 0.004, R = -0.66). These findings suggest that FRAC is a sensitive method and that patient age and sperm morphology are associated with centriole quality.

Sas-4 Colocalizes with the Ciliary Rootlets of the Drosophila Sensory Organs

The Drosophila eye displays peculiar sensory organs of unknown function, the mechanosensory bristles, that are intercalated among the adjacent ommatidia. Like the other Drosophila sensory organs, the mechanosensory bristles consist of a bipolar neuron and two tandemly aligned centrioles, the distal of which nucleates the ciliary axoneme and represents the starting point of the ciliary rootlets. We report here that the centriole associated protein Sas-4 colocalizes with the short ciliary rootlets of the mechanosensory bristles and with the elongated rootlets of chordotonal and olfactory neurons. This finding suggests an unexpected cytoplasmic localization of Sas-4 protein and points to a new underscored role for this protein. Moreover, we observed that the sheath cells associated with the sensory neurons also display two tandemly aligned centrioles but lacks ciliary axonemes, suggesting that the dendrites of the sensory neurons are dispensable for the assembly of aligned centrioles and rootlets.

The sperm centrioles

Centrioles are eukaryotic subcellular structures that produce and regulate massive cytoskeleton superstructures. They form centrosomes and cilia, regulate new centriole formation, anchor cilia to the cell, and regulate cilia function. These basic centriolar functions are executed in sperm cells during their amplification from spermatogonial stem cells during their differentiation to spermatozoa, and finally, after fertilization, when the sperm fuses with the egg. However, sperm centrioles exhibit many unique characteristics not commonly observed in other cell types, including structural remodeling, centriole-flagellum transition zone migration, and cell membrane association during meiosis. Here, we discuss five roles of sperm centrioles: orchestrating early spermatogenic cell divisions, forming the spermatozoon flagella, linking the spermatozoon head and tail, controlling sperm tail beating, and organizing the cytoskeleton of the zygote post-fertilization. We present the historic discovery of the centriole as a sperm factor that initiates embryogenesis, and recent genetic studies in humans and other mammals evaluating the current evidence for the five functions of sperm centrioles. We also examine information connecting the various sperm centriole functions to distinct clinical phenotypes. The emerging picture is that centrioles are essential sperm components with remarkable functional diversity and specialization that will require extensive and in-depth future studies.

The Enigma of Centriole Loss in the 1182-4 Cell Line

The Drosophila melanogaster cell line 1182-4, which constitutively lacks centrioles, was established many years ago from haploid embryos laid by females homozygous for the maternal haploid (mh) mutation. This was the first clear example of animal cells regularly dividing in the absence of this organelle. However, the cause of the acentriolar nature of the 1182-4 cell line remained unclear and could not be clearly assigned to a particular genetic event. Here, we detail historically the longstanding mystery of the lack of centrioles in this Drosophila cell line. Recent advances, such as the characterization of the mh gene and the genomic analysis of 1182-4 cells, allow now a better understanding of the physiology of these cells. By combining these new data, we propose three reasonable hypotheses of the genesis of this remarkable phenotype.

Centriole Number and the Accumulation of Microtubules Modulate the Timing of Apical Insertion during Radial Intercalation

Centrioles are microtubule (MT)-based structures that provide important functions during cell migration, cell division, and cell signaling [1]. Modulating centriole number in 3D cell cultures has been shown to influence protrusive behavior [2-5]. Here, we address in vivo the role of centrioles and the accumulation of MTs on the protrusive behavior required during the initiation of radial intercalation. Radial intercalation is an important developmental process whereby cells undergo polarized movements and interdigitate into a more superficial layer [6, 7]. It is commonly employed during metamorphic events, such as the tissue thinning coupled with expansion or during the introduction of different cell types into an epithelium. During radial intercalation, cells emerge from a basal layer by undergoing a process of apical migration, apical insertion, and expansion [8]. In Xenopus skin, multiciliated cells (MCCs), which contain ∼150 centrioles, and ionocytes (ICs), which contain two centrioles, differentiate during the same developmental window, but MCCs complete intercalation prior to ICs. Here, we utilize this difference in timing to create a quantifiable assay for insertion and find that the timing of insertion is modulated by changes in centriole number and the accumulation of acetylated MTs. Additionally, centrioles align between the nucleus and the leading edge creating an axis of migration with apically oriented (+) ends. Using the MT (-) end protein CAMSAP1 fused to the apically positioned Par6 protein, we have artificially reversed the orientation of MTs and find that the accumulation of MTs in either orientation is sufficient to promote apical insertion.

Human Brain Organoids to Decode Mechanisms of Microcephaly

Brain organoids are stem cell-based self-assembling 3D structures that recapitulate early events of human brain development. Recent improvements with patient-specific 3D brain organoids have begun to elucidate unprecedented details of the defective mechanisms that cause neurodevelopmental disorders of congenital and acquired microcephaly. In particular, brain organoids derived from primary microcephaly patients have uncovered mechanisms that deregulate neural stem cell proliferation, maintenance, and differentiation. Not only did brain organoids reveal unknown aspects of neurogenesis but also have illuminated surprising roles of cellular structures of centrosomes and primary cilia in regulating neurogenesis during brain development. Here, we discuss how brain organoids have started contributing to decoding the complexities of microcephaly, which are unlikely to be identified in the existing non-human models. Finally, we discuss the yet unresolved questions and challenges that can be addressed with the use of brain organoids as in vitro models of neurodevelopmental disorders.

The Emerging Role of ncRNAs and RNA-Binding Proteins in Mitotic Apparatus Formation

Mounting experimental evidence shows that non-coding RNAs (ncRNAs) serve a wide variety of biological functions. Recent studies suggest that a part of ncRNAs are critically important for supporting the structure of subcellular architectures. Here, we summarize the current literature demonstrating the role of ncRNAs and RNA-binding proteins in regulating the assembly of mitotic apparatus, especially focusing on centrosomes, kinetochores, and mitotic spindles.

The ubiquitin ligase FBXW7 targets the centriolar assembly protein HsSAS-6 for degradation and thereby regulates centriole duplication

Formation of a single new centriole from a pre-existing centriole is strictly controlled to maintain correct centrosome number and spindle polarity in cells. However, the mechanisms that govern this process are incompletely understood. Here, using several human cell lines, immunofluorescence and structured illumination microscopy methods, and ubiquitination assays, we show that the E3 ubiquitin ligase F-box and WD repeat domain-containing 7 (FBXW7), a subunit of the SCF ubiquitin ligase, down-regulates spindle assembly 6 homolog (HsSAS-6), a key protein required for procentriole cartwheel assembly, and thereby regulates centriole duplication. We found that FBXW7 abrogation stabilizes HsSAS-6 and increases its recruitment to the mother centriole at multiple sites, leading to supernumerary centrioles. Ultrastructural analyses revealed that FBXW7 is broadly localized on the mother centriole and that its presence is reduced at the site where the HsSAS-6-containing procentriole is formed. This observation suggested that FBXW7 restricts procentriole assembly to a specific site to generate a single new centriole. In contrast, during HsSAS-6 overexpression, FBXW7 strongly associated with HsSAS-6 at the centriole. We also found that SCF^FBXW7 interacts with HsSAS-6 and targets it for ubiquitin-mediated degradation. Further, we identified putative phosphodegron sites in HsSAS-6, whose substitutions rendered it insensitive to FBXW7-mediated degradation and control of centriole number. In summary, SCF^FBXW7 targets HsSAS-6 for degradation and thereby controls centriole biogenesis by restraining HsSAS-6 recruitment to the mother centriole, a molecular mechanism that controls supernumerary centrioles/centrosomes and the maintenance of bipolar spindles.

Plk1/Polo Phosphorylates Sas-4 at the Onset of Mitosis for an Efficient Recruitment of Pericentriolar Material to Centrosomes

Centrosomes are the major microtubule-organizing centers, consisting of centrioles surrounded by a pericentriolar material (PCM). Centrosomal PCM is spatiotemporally regulated to be minimal during interphase and expands as cells enter mitosis. It is unclear how PCM expansion is initiated at the onset of mitosis. Here, we identify that, in Drosophila, Plk1/Polo kinase phosphorylates the conserved centrosomal protein Sas-4 in vitro. This phosphorylation appears to occur at the onset of mitosis, enabling Sas-4's localization to expand outward from meiotic and mitotic centrosomes. The Plk1/Polo kinase site of Sas-4 is then required for an efficient recruitment of Cnn and γ-tubulin, bona fide PCM proteins that are essential for PCM expansion and centrosome maturation. Point mutations at Plk1/Polo sites of Sas-4 affect neither centrosome structure nor centriole duplication but specifically reduce the affinity to bind Cnn and γ-tubulin. These observations identify Plk1/Polo kinase regulation of Sas-4 as essential for efficient PCM expansion.

A transient microtubule-based structure uncovers a new intrinsic asymmetry between the mother centrioles in the early Drosophila spermatocytes

Parent centrioles are characterized in most organisms by individual morphological traits and have distinct asymmetries that provide different functional properties. By contrast, mother and daughter centrioles are morphologically undistinguishable during Drosophila male gametogenesis. Here we report the presence of previously unrecognized microtubule-based structures that extend into the peripheral cytoplasm of the Drosophila polar spermatocytes at the onset of the first meiosis and are positive for the typical centriolar protein Sas-4 and for the kinesin-like protein Klp10A. These structures have a short lifespan and are no longer found in early apolar spermatocytes. Remarkably, each polar spermatocyte holds only one microtubule-based structure that is associated with one of the sister centriole pairs and specifically with the mother centriole. These findings reveal an inherent asymmetry between the parent centrioles at the onset of male meiosis and also uncover unexpected functional properties between the mother centrioles of the same cells.

The Role of Sperm Centrioles in Human Reproduction - The Known and the Unknown

Each human spermatozoon contains two remodeled centrioles that it contributes to the zygote. There, the centrioles reconstitute a centrosome that assembles the sperm aster and participate in pronuclei migration and cleavage. Thus, centriole abnormalities may be a cause of male factor infertility and failure to carry pregnancy to term. However, the precise mechanisms by which sperm centrioles contribute to embryonic development in humans are still unclear, making the search for a link between centriole abnormalities and impaired male fecundity particularly difficult. Most previous investigations into the role of mammalian centrioles during fertilization have been completed in murine models; however, because mouse sperm and zygotes appear to lack centrioles, these studies provide information that is limited in its applicability to humans. Here, we review studies that examine the role of the sperm centrioles in the early embryo, with particular emphasis on humans. Available literature includes case studies and case-control studies, with a few retrospective studies and no prospective studies reported. This literature has provided some insight into the morphological characteristics of sperm centrioles in the zygote and has allowed identification of some centriole abnormalities in rare cases. Many of these studies suggest centriole involvement in early embryogenesis based on phenotypes of the embryo with only indirect evidence for centriole abnormality. Overall, these studies suggest that centriole abnormalities are present in some cases of sperm with asthenoteratozoospermia and unexplained infertility. Yet, most previously published studies have been restricted by the laborious techniques (like electron microscopy) and the limited availability of centriolar markers, resulting in small-scale studies and the lack of solid causational evidence. With recent progress in sperm centriole biology, such as the identification of the unique composition of sperm centrioles and the discovery of the atypical centriole, it is now possible to begin to fill the gaps in sperm centriole epidemiology and to identify the etiology of sperm centriole dysfunction in humans.

Centrosomes in Branching Morphogenesis

The centrosome, a major microtubule organizer, has important functions in regulating the cytoskeleton as well as the position of cellular structures and orientation of cells within tissues. The centrosome serves as the main cytoskeleton-organizing centre in the cell and is the classical site of microtubule nucleation and anchoring. For these reasons, centrosomes play a very important role in morphogenesis, not just in the early stages of cell divisions but also in the later stages of organogenesis. Many organs such as lung, kidney and blood vessels develop from epithelial tubes that branch into complex networks. Cells in the nervous system also form highly branched structures in order to build complex neuronal networks. During branching morphogenesis, cells have to rearrange within tissues though multicellular branching or through subcellular branching, also known as single-cell branching. For highly branched structures to be formed during embryonic development, the cytoskeleton needs to be extensively remodelled. The centrosome has been shown to play an important role during these events.

Poc1B and Sas-6 Function Together during the Atypical Centriole Formation in Drosophila melanogaster

Insects and mammals have atypical centrioles in their sperm. However, it is unclear how these atypical centrioles form. Drosophila melanogaster sperm has one typical centriole called the giant centriole (GC) and one atypical centriole called the proximal centriole-like structure (PCL). During early sperm development, centriole duplication factors such as Ana2 and Sas-6 are recruited to the GC base to initiate PCL formation. The centriolar protein, Poc1B, is also recruited at this initiation stage, but its precise role during PCL formation is unclear. Here, we show that Poc1B recruitment was dependent on Sas-6, that Poc1B had effects on cellular and PCL Sas-6, and that Poc1B and Sas-6 were colocalized in the PCL/centriole core. These findings suggest that Sas-6 and Poc1B interact during PCL formation. Co-overexpression of Ana2 and Sas-6 induced the formation of ectopic particles that contained endogenous Poc1 proteins and were composed of PCL-like structures. These structures were disrupted in Poc1 mutant flies, suggesting that Poc1 proteins stabilize the PCL-like structures. Lastly, Poc1B and Sas-6 co-overexpression also induced the formation of PCL-like structures, suggesting that they can function together during the formation of the PCL. Overall, our findings suggest that Poc1B and Sas-6 function together during PCL formation.

Protein Hydroxylation by Hypoxia-Inducible Factor (HIF) Hydroxylases: Unique or Ubiquitous?

All metazoans that utilize molecular oxygen (O₂) for metabolic purposes have the capacity to adapt to hypoxia, the condition that arises when O₂ demand exceeds supply. This is mediated through activation of the hypoxia-inducible factor (HIF) pathway. At physiological oxygen levels (normoxia), HIF-prolyl hydroxylases (PHDs) hydroxylate proline residues on HIF-α subunits leading to their destabilization by promoting ubiquitination by the von-Hippel Lindau (VHL) ubiquitin ligase and subsequent proteasomal degradation. HIF-α transactivation is also repressed in an O₂-dependent way due to asparaginyl hydroxylation by the factor-inhibiting HIF (FIH). In hypoxia, the O₂-dependent hydroxylation of HIF-α subunits by PHDs and FIH is reduced, resulting in HIF-α accumulation, dimerization with HIF-β and migration into the nucleus to induce an adaptive transcriptional response. Although HIFs are the canonical substrates for PHD- and FIH-mediated protein hydroxylation, increasing evidence indicates that these hydroxylases may also have alternative targets. In addition to PHD-conferred alterations in protein stability, there is now evidence that hydroxylation can affect protein activity and protein/protein interactions for alternative substrates. PHDs can be pharmacologically inhibited by a new class of drugs termed prolyl hydroxylase inhibitors which have recently been approved for the treatment of anemia associated with chronic kidney disease. The identification of alternative targets of HIF hydroxylases is important in order to fully elucidate the pharmacology of hydroxylase inhibitors (PHI). Despite significant technical advances, screening, detection and verification of alternative functional targets for PHDs and FIH remain challenging. In this review, we discuss recently proposed non-HIF targets for PHDs and FIH and provide an overview of the techniques used to identify these.

Insights into photoreceptor ciliogenesis revealed by animal models

Photoreceptors are polarized neurons, with very specific subcellular compartmentalization and unique requirements for protein expression and trafficking. Each photoreceptor contains an outer segment, the site of photon capture that initiates vision, an inner segment that houses the biosynthetic machinery and a synaptic terminal for signal transmission to downstream neurons. Outer segments and inner segments are connected by a connecting cilium (CC), the equivalent of a transition zone (TZ) of primary cilia. The connecting cilium is part of the basal body/axoneme backbone that stabilizes the outer segment. This report will update the reader on late developments in photoreceptor ciliogenesis and transition zone formation, specifically in mouse photoreceptors, focusing on early events in photoreceptor ciliogenesis. The connecting cilium, an elongated and narrow structure through which all outer segment proteins and membrane components must traffic, functions as a gate that controls access to the outer segment. Here we will review genes and their protein products essential for basal body maturation and for CC/TZ genesis, sorted by phenotype. Emphasis is given to naturally occurring mouse mutants and gene knockouts that interfere with CC/TZ formation and ciliogenesis.

Cep85 Relays Plk1 Activity to Phosphorylated Nek2A for Its Timely Activation in Centrosome Disjunction

Timely centrosome separation is critical for accurate chromosome separation. It is initiated by Nek2A at the onset of mitosis, but the mechanism for the strict requirement of phosphorylated Nek2A for its own activation remains unclear. In this study, we have found that Plk1 interacts with Cep85 and forms a ternary complex with Cep85-Nek2A. Nek2A binding, but not its kinase activity, is pre-required for Cep85 to be phosphorylated by Plk1. Nek2A-dependent Cep85 phosphorylation, in turn, leads to the dissociation of phosphorylated Cep85 exclusively from phospho-Nek2A, thereby increasing the freed phospho-Nek2A activity. Both kinases are also required for phosphorylating endogenous Cep85 in cells, and timely phosphorylation of Cep85 and Nek2A is crucial for initiating centrosome disjunction at G2/M. Overall, our study has uncovered a previously unrecognized role of Plk1 and Nek2A and identified Cep85 as a missing piece directly relaying Plk1 activity to Nek2A for its activation in centrosome disjunction.

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Cep85 prevents centrosome separation by binding to and inhibiting Nek2A in interphase

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Plk1 binds to Cep85 and forms a ternary Plk1-Cep85-Nek2A complex in late G2

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Nek2A-assisting Cep85 phosphorylation by Plk1 releases phospho-Nek2A from Cep85

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Freed phospho-Nek2A initiates centrosome separation in G2/M

Inhibition of CPAP-tubulin interaction prevents proliferation of centrosome-amplified cancer cells

Centrosome amplification is a hallmark of human cancers that can trigger cancer cell invasion. To survive, cancer cells cluster amplified extra centrosomes and achieve pseudobipolar division. Here, we set out to prevent clustering of extra centrosomes. Tubulin, by interacting with the centrosomal protein CPAP, negatively regulates CPAP‐dependent peri‐centriolar material recruitment, and concurrently microtubule nucleation. Screening for compounds that perturb CPAP–tubulin interaction led to the identification of CCB02, which selectively binds at the CPAP binding site of tubulin. Genetic and chemical perturbation of CPAP–tubulin interaction activates extra centrosomes to nucleate enhanced numbers of microtubules prior to mitosis. This causes cells to undergo centrosome de‐clustering, prolonged multipolar mitosis, and cell death. 3D‐organotypic invasion assays reveal that CCB02 has broad anti‐invasive activity in various cancer models, including tyrosine kinase inhibitor (TKI)‐resistant EGFR‐mutant non‐small‐cell lung cancers. Thus, we have identified a vulnerability of cancer cells to activation of extra centrosomes, which may serve as a global approach to target various tumors, including drug‐resistant cancers exhibiting high incidence of centrosome amplification.

Asterless is a Polo-like kinase 4 substrate that both activates and inhibits kinase activity depending on its phosphorylation state

Centriole assembly initiates when Polo-like kinase 4 (Plk4) interacts with a centriole "targeting-factor." In Drosophila, Asterless/Asl (Cep152 in humans) fulfills the targeting role. Interestingly, Asl also regulates Plk4 levels. The N-terminus of Asl (Asl-A; amino acids 1-374) binds Plk4 and promotes Plk4 self-destruction, although it is unclear how this is achieved. Moreover, Plk4 phosphorylates the Cep152 N-terminus, but the functional consequence is unknown. Here, we show that Plk4 phosphorylates Asl and mapped 13 phospho-residues in Asl-A. Nonphosphorylatable alanine (13A) and phosphomimetic (13PM) mutants did not alter Asl function, presumably because of the dominant role of the Asl C-terminus in Plk4 stabilization and centriolar targeting. To address how Asl-A phosphorylation specifically affects Plk4 regulation, we generated Asl-A fragment phospho-mutants and expressed them in cultured Drosophila cells. Asl-A-13A stimulated kinase activity by relieving Plk4 autoinhibition. In contrast, Asl-A-13PM inhibited Plk4 activity by a novel mechanism involving autophosphorylation of Plk4's kinase domain. Thus, Asl-A's phosphorylation state determines which of Asl-A's two opposing effects are exerted on Plk4. Initially, nonphosphorylated Asl binds Plk4 and stimulates its kinase activity, but after Asl is phosphorylated, a negative-feedback mechanism suppresses Plk4 activity. This dual regulatory effect by Asl-A may limit Plk4 to bursts of activity that modulate centriole duplication.

Animal Female Meiosis: The Challenges of Eliminating Centrosomes

Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation. Female reproductive cells, oocytes, grow up to large sizes when they accumulate energy stocks and store proteins as well as mRNAs to enable rapid cell divisions after fertilisation. At the same time, metazoan oocytes eliminate their centrosomes, i.e., major microtubule-organizing centres (MTOCs), during or right after the long growth phases. Centrosome elimination poses two key questions: first, how can the centrosome be re-established after fertilisation? In general, metazoan oocytes exploit sperm components, i.e., the basal body of the sperm flagellum, as a platform to reinitiate centrosome production. Second, how do most metazoan oocytes manage to build up meiotic spindles without centrosomes? Oocytes have evolved mechanisms to assemble bipolar spindles solely around their chromosomes without the guidance of pre-formed MTOCs. Female animal meiosis involves microtubule nucleation and organisation into bipolar microtubule arrays in regulated self-assembly under the control of the Ran system and nuclear transport receptors. This review summarises our current understanding of the molecular mechanism underlying self-assembly of meiotic spindles, its spatio-temporal regulation, and the key players governing this process in animal oocytes.

Rapid Evolution of Sperm Produces Diverse Centriole Structures that Reveal the Most Rudimentary Structure Needed for Function

Centrioles are ancient subcellular protein-based organelles that maintain a conserved number and structure across many groups of eukaryotes. Centriole number (two per cells) is tightly regulated; each pre-existing centriole nucleates only one centriole as the cell prepares for division. The structure of centrioles is barrel-shaped, with a nine-fold symmetry of microtubules. This organization of microtubules is essential for the ancestral function of centriole–cilium nucleation. In animal cells, centrioles have gained an additional role: recruiting pericentriolar material (PCM) to form a centrosome. Therefore, it is striking that in animal spermatozoa, the centrioles have a remarkable diversity of structures, where some are so anomalous that they are referred to as atypical centrioles and are barely recognizable. The atypical centriole maintains the ability to form a centrosome and nucleate a new centriole, and therefore reveals the most rudimentary structure that is needed for centriole function. However, the atypical centriole appears to be incapable of forming a cilium. Here, we propose that the diversity in sperm centriole structure is due to rapid evolution in the shape of the spermatozoa head and neck. The enhanced diversity may be driven by a combination of direct selection for novel centriole functions and pleiotropy, which eliminates centriole properties that are dispensable in the spermatozoa function.

Osmotic Stress Blocks Mobility and Dynamic Regulation of Centriolar Satellites

Centriolar satellites (CS) are small proteinaceous granules that cluster around the centrosome and serve as cargo vehicles for centrosomal proteins. It is generally accepted that CS support a number of canonical and specialized centrosome functions. Consequently, these highly dynamic structures are the target of regulation by several cellular signalling pathways. Two decades of research have led to the identification of a large number of molecular components and new biological roles of CS. Here, we summarize the latest advances in the continuous efforts to uncover the compositional, functional, dynamic and regulatory aspects of CS. We also report on our discovery that osmotic stress conditions render CS immobile and insensitive to remodelling. Upon a range of p38-activating stimuli, MK2 phosphorylates the CS component CEP131, resulting in 14-3-3 binding and a block to CS formation. This normally manifests as a rapid cellular depletion of satellites. In the case of osmotic stress, a potent inducer of p38 activity, CS translocation and dissolution is blocked, with the net result that satellites persist in an immobile state directly adjacent to the centrosome. Our results highlight a unique scenario where p38 activation and CS depletion is uncoupled, with potential implications for physiological and pathological osmotic stress responses.

An ordered pattern of Ana2 phosphorylation by Plk4 is required for centriole assembly

Polo-like kinase 4 (Plk4) initiates an early step in centriole assembly by phosphorylating Ana2/STIL, a structural component of the procentriole. Here, we show that Plk4 binding to the central coiled-coil (CC) of Ana2 is a conserved event involving Polo-box 3 and a previously unidentified putative CC located adjacent to the kinase domain. Ana2 is then phosphorylated along its length. Previous studies showed that Plk4 phosphorylates the C-terminal STil/ANa2 (STAN) domain of Ana2/STIL, triggering binding and recruitment of the cartwheel protein Sas6 to the procentriole assembly site. However, the physiological relevance of N-terminal phosphorylation was unknown. We found that Plk4 first phosphorylates the extreme N terminus of Ana2, which is critical for subsequent STAN domain modification. Phosphorylation of the central region then breaks the Plk4-Ana2 interaction. This phosphorylation pattern is important for centriole assembly and integrity because replacement of endogenous Ana2 with phospho-Ana2 mutants disrupts distinct steps in Ana2 function and inhibits centriole duplication.

Loss and Rebirth of the Animal Microtubule Organizing Center: How Maternal Expression of Centrosomal Proteins Cooperates with the Sperm Centriole in Zygotic Centrosome Reformation

Centrosomes are the main microtubule organizing centers in animal cells. In particular during embryogenesis, they ensure faithful spindle formation and proper cell divisions. As metazoan centrosomes are eliminated during oogenesis, they have to be reassembled upon fertilization. Most metazoans use the sperm centrioles as templates for new centrosome biogenesis while the egg's cytoplasm re-prepares all components for on-going centrosome duplication in rapidly dividing embryonic cells. We discuss our knowledge and the experimental challenges to analyze zygotic centrosome reformation, which requires genetic experiments to enable scrutinizing respective male and female contributions. Male and female knockout animals and mRNA injection to mimic maternal expression of centrosomal proteins could point a way to the systematic molecular dissection of the process. The most recent data suggest that timely expression of centrosome components in oocytes is the key to zygotic centrosome reformation that uses male sperm as coordinators for de novo centrosome production.

A novel genetic syndrome with STARD9 mutation and abnormal spindle morphology

Intellectual disability (ID) is one of neurodevelopmental disorders characterized by serious defects in both intelligence and adaptive behavior. Although it has been suggested that genetic aberrations associated with the process of cell division underlie ID, the cytological evidence for mitotic defects in actual patient's cells is rarely reported. Here, we report a novel mutation in the STARD9 (also known as KIF16A) gene found in a patient with severe ID, characteristic features, epilepsy, acquired microcephaly, and blindness. Using whole-exome sequence analysis, we sequenced potential candidate genes in the patient. We identified a homozygous single-nucleotide deletion creating a premature stop codon in the STARD9 gene. STARD9 encodes a 4,700 amino acid protein belonging to the kinesin superfamily. Depletion of STARD9 or overexpression of C-terminally truncated STARD9 mutants were known to induce spindle assembly defects in human culture cells. To determine cytological features in the patient cells, we isolated lymphoblast cells from the patient, and performed immunofluorescence analysis. Remarkably, mitotic defects, including multipolar spindle formation, fragmentation of pericentriolar materials and centrosome amplification, were observed in the cells. Taken together, our findings raise the possibility that controlled expression of full-length STARD9 is necessary for proper spindle assembly in cell division during human development. We propose that mutations in STARD9 result in abnormal spindle morphology and cause a novel genetic syndrome with ID.

The PLK4-STIL-SAS-6 module at the core of centriole duplication

Centrioles are microtubule-based core components of centrosomes and cilia. They are duplicated exactly once during S-phase progression. Central to formation of each new (daughter) centriole is the formation of a nine-fold symmetrical cartwheel structure onto which microtubule triplets are deposited. In recent years, a module comprising the protein kinase polo-like kinase 4 (PLK4) and the two proteins STIL and SAS-6 have been shown to stay at the core of centriole duplication. Depletion of any one of these three proteins blocks centriole duplication and, conversely, overexpression causes centriole amplification. In this short review article, we summarize recent insights into how PLK4, STIL and SAS-6 co-operate in space and time to form a new centriole. These advances begin to shed light on the very first steps of centriole biogenesis.

Microtubule-Organizing Centers

The organization of microtubule networks is crucial for controlling chromosome segregation during cell division, for positioning and transport of different organelles, and for cell polarity and morphogenesis. The geometry of microtubule arrays strongly depends on the localization and activity of the sites where microtubules are nucleated and where their minus ends are anchored. Such sites are often clustered into structures known as microtubule-organizing centers, which include the centrosomes in animals and spindle pole bodies in fungi. In addition, other microtubules, as well as membrane compartments such as the cell nucleus, the Golgi apparatus, and the cell cortex, can nucleate, stabilize, and tether microtubule minus ends. These activities depend on microtubule-nucleating factors, such as γ-tubulin-containing complexes and their activators and receptors, and microtubule minus end-stabilizing proteins with their binding partners. Here, we provide an overview of the current knowledge on how such factors work together to control microtubule organization in different systems.

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.

Kinetic analysis of de novo centriole assembly in heat-shocked mammalian cells

Mammalian cells are capable of de novo centriole formation after the removal of existing centrioles. This suggests that de novo centriole assembly is repressed in normally duplicating cells to maintain a constant number of centrioles in the cells. However, neither the mechanism of de novo centriole assembly nor that of its hypothesized repression is understood due to the lack of an experimental system. We found that the heat shock (HS; 42°C, 2 h) of mouse embryonic fibroblasts caused the separation of centriole pairs, a transient increase in polo-like kinase (Plk) 4 expression, and the formation of a complex containing γ-tubulin, pericentrin, HS protein (Hsp) 90, and Plk4, in approximately half of the cells. Subsequently, spindle-assembly abnormal protein (Sas) 6, centrosomal protein (Cep) 135, and centrin localized to the complex, and tubulin consequently became polyglutamylated, indicating de novo centriole assembly in the heat-shocked cells. These results suggested that HS-induced de novo centriole assembly could provide an experimental system for further elucidating the regulation of centrosome number in mammalian cells. © 2016 Wiley Periodicals, Inc.

Cep295 is a conserved scaffold protein required for generation of a bona fide mother centriole

Centrioles surrounded by pericentriolar material (PCM) serve as the core structure of the centrosome. A newly formed daughter centriole grows into a functional mother centriole. However, the underlying mechanisms remain poorly understood. Here we show that Cep295, an evolutionarily conserved protein, is required for generation of a bona fide mother centriole organizing a functional centrosome. We find that Cep295 is recruited to the proximal centriole wall in the early stages of procentriole assembly. Cep295 then acts as a scaffold for the proper assembly of the daughter centriole. We also find that Cep295 binds directly to and recruits Cep192 onto the daughter centriole wall, which presumably endows the function of the new mother centriole for PCM assembly, microtubule-organizing centre activity and the ability for centriole formation. These findings led us to propose that Cep295 acts upstream of the conserved pathway for centriole formation and promotes the daughter-to-mother centriole conversion.

Production of Basal Bodies in bulk for dense multicilia formation

Centriole number is normally under tight control and is directly linked to ciliogenesis. In cells that use centrosomes as mitotic spindle poles, one pre-existing mother centriole is allowed to duplicate only one daughter centriole per cell cycle. In multiciliated cells, however, many centrioles are generated to serve as basal bodies of the cilia. Although deuterosomes were observed more than 40 years ago using electron microscopy and are believed to produce most of the basal bodies in a mother centriole-independent manner, the underlying molecular mechanisms have remained unknown until recently. From these findings arise more questions and a call for clarifications that will require multidisciplinary efforts.

Molecular basis for CPAP-tubulin interaction in controlling centriolar and ciliary length

Centrioles and cilia are microtubule-based structures, whose precise formation requires controlled cytoplasmic tubulin incorporation. How cytoplasmic tubulin is recognized for centriolar/ciliary-microtubule construction remains poorly understood. Centrosomal-P4.1-associated-protein (CPAP) binds tubulin via its PN2-3 domain. Here, we show that a C-terminal loop-helix in PN2-3 targets β-tubulin at the microtubule outer surface, while an N-terminal helical motif caps microtubule's α-β surface of β-tubulin. Through this, PN2-3 forms a high-affinity complex with GTP-tubulin, crucial for defining numbers and lengths of centriolar/ciliary-microtubules. Surprisingly, two distinct mutations in PN2-3 exhibit opposite effects on centriolar/ciliary-microtubule lengths. CPAP(F375A), with strongly reduced tubulin interaction, causes shorter centrioles and cilia exhibiting doublet- instead of triplet-microtubules. CPAP(EE343RR) that unmasks the β-tubulin polymerization surface displays slightly reduced tubulin-binding affinity inducing over-elongation of newly forming centriolar/ciliary-microtubules by enhanced dynamic release of its bound tubulin. Thus CPAP regulates delivery of its bound-tubulin to define the size of microtubule-based cellular structures using a 'clutch-like' mechanism.

Functional specialization of chordate CDK1 paralogs during oogenic meiosis

Cyclin-dependent kinases (CDKs) are central regulators of eukaryotic cell cycle progression. In contrast to interphase CDKs, the mitotic phase CDK1 is the only CDK capable of driving the entire cell cycle and it can do so from yeast to mammals. Interestingly, plants and the marine chordate, Oikopleura dioica, possess paralogs of the highly conserved CDK1 regulator. However, whereas in plants the 2 CDK1 paralogs replace interphase CDK functions, O. dioica has a full complement of interphase CDKs in addition to its 5 odCDK1 paralogs. Here we show specific sub-functionalization of odCDK1 paralogs during oogenesis. Differential spatiotemporal dynamics of the odCDK1a, d and e paralogs and the meiotic polo-like kinase 1 (Plk1) and aurora kinase determine the subset of meiotic nuclei in prophase I arrest that will seed growing oocytes and complete meiosis. Whereas we find odCDK1e to be non-essential, knockdown of the odCDK1a paralog resulted in the spawning of non-viable oocytes of reduced size. Knockdown of odCDK1d also resulted in the spawning of non-viable oocytes. In this case, the oocytes were of normal size, but were unable to extrude polar bodies upon exposure to sperm, because they were unable to resume meiosis from prophase I arrest, a classical function of the sole CDK1 during meiosis in other organisms. Thus, we reveal specific sub-functionalization of CDK1 paralogs, during the meiotic oogenic program.

Structural characterization of procentrioles in Drosophila spermatids

Male gametogenesis in insects is unusual in that the centrioles do not duplicate during the second meiosis and the differentiating spermatids inherit only one centriole. Here it is showed that a distinct procentriole is assembled close to the proximal region of the centriole in early S13 spermatids at the onion stage, confirming previous reports of a proximal centriole-like structure at the proximal end of the spermatid centriole. However, the procentrioles of Drosophila spermatids do not behave like true procentrioles, but their development is blocked at an early stage before the assembly of a complete A-tubule set. Therefore, they may represent early frozen stages of procentriole assembly that do not develop further and eventually disappear in late spermatids.

Two Polo-like kinase 4 binding domains in Asterless perform distinct roles in regulating kinase stability

The Asterless N terminus promotes Plk4 homodimerization and autophosphorylation during interphase, whereas its C terminus stabilizes Plk4 during mitosis.

Plk4 (Polo-like kinase 4) and its binding partner Asterless (Asl) are essential, conserved centriole assembly factors that induce centriole amplification when overexpressed. Previous studies found that Asl acts as a scaffolding protein; its N terminus binds Plk4’s tandem Polo box cassette (PB1-PB2) and targets Plk4 to centrioles to initiate centriole duplication. However, how Asl overexpression drives centriole amplification is unknown. In this paper, we investigated the Asl–Plk4 interaction in Drosophila melanogaster cells. Surprisingly, the N-terminal region of Asl is not required for centriole duplication, but a previously unidentified Plk4-binding domain in the C terminus is required. Mechanistic analyses of the different Asl regions revealed that they act uniquely during the cell cycle: the Asl N terminus promotes Plk4 homodimerization and autophosphorylation during interphase, whereas the Asl C terminus stabilizes Plk4 during mitosis. Therefore, Asl affects Plk4 in multiple ways to regulate centriole duplication. Asl not only targets Plk4 to centrioles but also modulates Plk4 stability and activity, explaining the ability of overexpressed Asl to drive centriole amplification.

Atypical centrioles during sexual reproduction

Centrioles are conserved, self-replicating, microtubule-based, 9-fold symmetric subcellular organelles that are essential for proper cell division and function. Most cells have two centrioles and maintaining this number of centrioles is important for animal development and physiology. However, how animals gain their first two centrioles during reproduction is only partially understood. It is well established that in most animals, the centrioles are contributed to the zygote by the sperm. However, in humans and many animals, the sperm centrioles are modified in their structure and protein composition, or they appear to be missing altogether. In these animals, the origin of the first centrioles is not clear. Here, we review various hypotheses on how centrioles are gained during reproduction and describe specialized functions of the zygotic centrioles. In particular, we discuss a new and atypical centriole found in sperm and zygote, called the proximal centriole-like structure (PCL). We also discuss another type of atypical centriole, the "zombie" centriole, which is degenerated but functional. Together, the presence of centrioles, PCL, and zombie centrioles suggests a universal mechanism of centriole inheritance among animals and new causes of infertility. Since the atypical centrioles of sperm and zygote share similar functions with typical centrioles in somatic cells, they can provide unmatched insight into centriole biology.

Cartwheel assembly

The cartwheel is a subcentriolar structure consisting of a central hub and nine radially arranged spokes, located at the proximal end of the centriole. It appears at the initial stage of the centriole assembly process as the first ninefold symmetrical structure. The cartwheel was first described more than 50 years ago, but it is only recently that its pivotal role in establishing the ninefold symmetry of the centriole was demonstrated. Significant progress has since been made in understanding its fine structure and assembly mechanism. Most importantly, the central part of the cartwheel, from which the ninefold symmetry originates, is shown to form by self-association of nine dimers of the protein SAS-6. This finding, together with emerging data on other components of the cartwheel, has opened new avenues in centrosome biology.

Autoinhibition and relief mechanism for Polo-like kinase 4

Polo-like kinase 4 (Plk4) is a master regulator of centriole duplication, and its hyperactivity induces centriole amplification. Homodimeric Plk4 has been shown to be ubiquitinated as a result of autophosphorylation, thus promoting its own degradation and preventing centriole amplification. Unlike other Plks, Plk4 contains three rather than two Polo box domains, and the function of its third Polo box (PB3) is unclear. Here, we performed a functional analysis of Plk4's structural domains. Like other Plks, Plk4 possesses a previously unidentified autoinhibitory mechanism mediated by a linker (L1) near the kinase domain. Thus, autoinhibition is a conserved feature of Plks. In the case of Plk4, autoinhibition is relieved after homodimerization and is accomplished by PB3 and by autophosphorylation of L1. In contrast, autophosphorylation of the second linker promotes separation of the Plk4 homodimer. Therefore, autoinhibition delays the multiple consequences of activation until Plk4 dimerizes. These findings reveal a complex mechanism of Plk4 regulation and activation which govern the process of centriole duplication.

Centriolar satellites: key mediators of centrosome functions

Centriolar satellites are small, microscopically visible granules that cluster around centrosomes. These structures, which contain numerous proteins directly involved in centrosome maintenance, ciliogenesis, and neurogenesis, have traditionally been viewed as vehicles for protein trafficking towards the centrosome. However, the recent identification of several new centriolar satellite components suggests that this model offers only an incomplete picture of their cellular functions. While the mechanisms controlling centriolar satellite status and function are not yet understood in detail, emerging evidence points to these structures as important hubs for dynamic, multi-faceted regulation in response to a variety of cues. In this review, we summarize the current knowledge of the roles of centriolar satellites in regulating centrosome functions, ciliogenesis, and neurogenesis. We also highlight newly discovered regulatory mechanisms targeting centriolar satellites and their functional status, and we discuss how defects in centriolar satellite components are intimately linked to a wide spectrum of human diseases.

Molecular and cellular basis of autosomal recessive primary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a rare hereditary neurodevelopmental disorder characterized by a marked reduction in brain size and intellectual disability. MCPH is genetically heterogeneous and can exhibit additional clinical features that overlap with related disorders including Seckel syndrome, Meier-Gorlin syndrome, and microcephalic osteodysplastic dwarfism. In this review, we discuss the key proteins mutated in MCPH. To date, MCPH-causing mutations have been identified in twelve different genes, many of which encode proteins that are involved in cell cycle regulation or are present at the centrosome, an organelle crucial for mitotic spindle assembly and cell division. We highlight recent findings on MCPH proteins with regard to their role in cell cycle progression, centrosome function, and early brain development.

Spastin-interacting protein NA14/SSNA1 functions in cytokinesis and axon development

Hereditary spastic paraplegias (HSPs) are a genetically diverse group of inherited neurological disorders (SPG1-72) with the cardinal feature of prominent lower-extremity spasticity due to a length-dependent axonopathy of corticospinal motor neurons. The most frequent form of autosomal dominant HSP results from mutations of the SPG4 gene product spastin. This is an ATPase associated with diverse cellular activities (AAA) protein that binds to and severs microtubules. While spastin participates in crucial cellular processes such as cytokinesis, endosomal tubulation, and axon development, its role in HSP pathogenesis remains unclear. Spastin interacts in cells with the NA14 protein, a major target for auto-antibodies in Sjögren's syndrome (nuclear autoantigen 1; SSNA1). Our analysis of endogenous spastin and NA14 proteins in HeLa cells and rat cortical neurons in primary culture revealed a clear distribution of both proteins to centrosomes, with NA14 localizing specifically to centrioles. Stable NA14 knockdown in cell lines dramatically affected cell division, in particular cytokinesis. Furthermore, overexpression of NA14 in neurons significantly increased axon outgrowth and branching, while also enhancing neuronal differentiation. We postulate that NA14 may act as an adaptor protein regulating spastin localization to centrosomes, temporally and spatially regulating the microtubule-severing activity of spastin that is particularly critical during the cell cycle and neuronal development.