Bridging centrioles and PCM in proper space and time

Asymmetry of centrosomes in Drosophila neural stem cells requires protein phosphatase 4

Asymmetric cell division is used by stem cells to create diverse cell types while self-renewing the stem cell population. Biased segregation of molecularly distinct centrosomes could provide a mechanism to maintain stem cell fate, induce cell differentiation or both. However, the molecular mechanisms generating molecular and functional asymmetric centrosomes remain incompletely understood. Here, we show that in asymmetrically dividing fly neural stem cells, protein phosphatase 4 (Pp4) is necessary for correct centrosome asymmetry establishment during mitosis, and microtubule organizing center (MTOC) maintenance in interphase. Using in vivo live-cell imaging, we show that while wild-type neural stem cells always maintain one active MTOC, Pp4 mutant neuroblasts contain two inactive centrioles in interphase. Furthermore, centrosomes of Pp4 mutant neural stem cells mature in mitosis but fail to correctly transfer the centriolar protein Centrobin (Cnb) from the mother to the daughter centriole. Using superresolution imaging, we find that phosphomimetic Centrobin fails to accurately relocalize in mitosis. We propose that Pp4 regulates the timely relocalization of Cnb in mitosis to establish two molecularly distinct centrosomes. In addition, Pp4 is also necessary to maintain MTOC activity in interphase, ensuring biased centrosome segregation. Mechanistically, Pp4 could regulate centrosome asymmetry by dephosphorylating both Cnb and gamma-Tubulin.

Drosophila Alms1 proteins regulate centriolar cartwheel assembly by enabling Plk4-Ana2 amplification loop

Centrioles play a central role in cell division by recruiting pericentriolar material (PCM) to form the centrosome. Alterations in centriole number or function lead to various diseases including cancer or microcephaly. Centriole duplication is a highly conserved mechanism in eukaryotes. Here, we show that the two Drosophila orthologs of the Alström syndrome protein 1 (Alms1a and Alms1b) are unexpected novel players of centriole duplication in fly. Using Ultrastructure Expansion Microscopy, we reveal that Alms1a is a PCM protein that is loaded proximally on centrioles at the onset of procentriole formation, whereas Alms1b caps the base of mature centrioles. We demonstrate that chronic loss of Alms1 proteins (with RNA null alleles) affects PCM maturation, whereas their acute loss (in RNAi KD) completely disrupts procentriole formation before Sas-6 cartwheel assembly. We establish that Alms1 proteins are required for the amplification of the Plk4-Ana2 pool at the duplication site and the subsequent Sas-6 recruitment. Thus, Alms1 proteins are novel critical but highly buffered regulators of PCM and cartwheel assembly in flies.

Protein phosphatase 4 is required for centrosome asymmetry in fly neural stem cells

Asymmetric cell division is used by stem cells to create diverse cell types while self-renewing the stem cell population. Biased segregation of molecularly distinct centrosomes could provide a mechanism to maintain stem cell fate, induce cell differentiation or both. However, the molecular mechanisms generating molecular and functional asymmetric centrosomes remain incompletely understood. Here, we show that in asymmetrically dividing fly neural stem cells, Protein phosphatase 4 (Pp4) is necessary for correct centrosome asymmetry establishment during mitosis, and microtubule organizing center (MTOC) maintenance in interphase. Using in-vivo live cell imaging we show that while wild type neural stem cells always maintain one active MTOC, Pp4 mutant neuroblasts contain two inactive centrioles in interphase. Furthermore, centrosomes of Pp4 mutant neural stem cells mature in mitosis but fail to correctly transfer the centriolar protein Centrobin (Cnb) from the mother to the daughter centriole. Using superresolution imaging, we find that phosphomimetic Centrobin fails to accurately relocalize in mitosis. We propose that Pp4 regulates the timely relocalization of Cnb in mitosis to establish two molecularly distinct centrosomes. In addition, Pp4 is also necessary to maintain MTOC activity in interphase, ensuring biased centrosome segregation. Mechanistically, Pp4 could regulate centrosome asymmetry by dephosphorylating both Cnb and gamma-Tubulin.

SIGNIFICANCE STATEMENT

Asymmetric centrosome segregation occurs in stem cells and has been linked with cell fate decisions. Protein phosphatase 4 (Pp4), a conserved Serine/Threonine phosphatase, regulates centrosome asymmetry in Drosophila neural stem cells by acting upon gamma tubulin and Centrobin. Pp4 regulates centrosome asymmetry establishment in mitosis and interphase, necessary for biased centrosome segregation.

Emerin preserves stem cell survival through maintenance of centrosome and nuclear lamina structure

Drosophila female germline stem cells (GSCs) complete asymmetric mitosis in the presence of an intact, but permeable, nuclear envelope and nuclear lamina (NL). This asymmetric division requires a modified centrosome cycle, wherein mitotic centrosomes with mature pericentriolar material (PCM) embed in the NL and interphase centrosomes with reduced PCM leave the NL. This centrosome cycle requires Emerin, an NL protein required for GSC survival and germ cell differentiation. In emerin mutants, interphase GSC centrosomes retain excess PCM, remain embedded in the NL and nucleate microtubule asters at positions of NL distortion. Here, we investigate the contributions of abnormal interphase centrosomes to GSC loss. Remarkably, reducing interphase PCM in emerin mutants rescues GSC survival and partially restores germ cell differentiation. Direct tests of the effects of abnormal centrosomes were achieved by expression of constitutively active Polo kinase to drive enlargement of interphase centrosomes in wild-type GSCs. Notably, these conditions failed to alter NL structure or decrease GSC survival. However, coupling enlarged interphase centrosomes with nuclear distortion promoted GSC loss. These studies establish that Emerin maintains centrosome structure to preserve stem cell survival.

Cep57 regulates human centrosomes through multivalent interactions

Human Cep57 is a coiled-coil scaffold at the pericentriolar matrix (PCM), controlling centriole duplication and centrosome maturation for faithful cell division. Genetic truncation mutations of Cep57 are associated with the mosaic-variegated aneuploidy (MVA) syndrome. During interphase, Cep57 forms a complex with Cep63 and Cep152, serving as regulators for centrosome maturation. However, the molecular interplay of Cep57 with these essential scaffolding proteins remains unclear. Here, we demonstrate that Cep57 undergoes liquid-liquid phase separation (LLPS) driven by three critical domains (NTD, CTD, and polybasic LMN). In vitro Cep57 condensates catalyze microtubule nucleation via the LMN motif-mediated tubulin concentration. In cells, the LMN motif is required for centrosomal microtubule aster formation. Moreover, Cep63 restricts Cep57 assembly, expansion, and microtubule polymerization activity. Overexpression of competitive constructs for multivalent interactions, including an MVA mutation, leads to excessive centrosome duplication. In Cep57-depleted cells, self-assembly mutants failed to rescue centriole disengagement and PCM disorganization. Thus, Cep57's multivalent interactions are pivotal for maintaining the accurate structural and functional integrity of human centrosomes.

Mechanisms underlying Myosin 10's contribution to the maintenance of mitotic spindle bipolarity

Myosin 10 (Myo10) couples microtubules and integrin-based adhesions to movement along actin filaments via its microtubule-binding MyTH4 domain and integrin-binding FERM domain, respectively. Here we show that Myo10-depleted HeLa cells and mouse embryo fibroblasts (MEFs) both exhibit a pronounced increase in the frequency of multipolar spindles. Staining of unsynchronized metaphase cells showed that the primary driver of spindle multipolarity in Myo10-depleted MEFs and in Myo10-depleted HeLa cells lacking supernumerary centrosomes is pericentriolar material (PCM) fragmentation, which creates y-tubulin-positive acentriolar foci that serve as extra spindle poles. For HeLa cells possessing supernumerary centrosomes, Myo10 depletion further accentuates spindle multipolarity by impairing the clustering of the extra spindle poles. Complementation experiments show that Myo10 must interact with both microtubules and integrins to promote PCM/pole integrity. Conversely, Myo10 only needs interact with integrins to promote supernumerary centrosome clustering. Importantly, images of metaphase Halo-Myo10 knockin cells show that the myosin localizes exclusively to the spindle and the tips of adhesive retraction fibers. We conclude that Myo10 promotes PCM/pole integrity in part by interacting with spindle microtubules, and that it promotes supernumerary centrosome clustering by supporting retraction fiber-based cell adhesion, which likely serves to anchor the microtubule-based forces driving pole focusing.

The E3 ligase Poe promotes Pericentrin degradation

Centrosomes are essential parts of diverse cellular processes, and precise regulation of the levels of their constituent proteins is critical for their function. One such protein is Pericentrin (PCNT) in humans and Pericentrin-like protein (PLP) in Drosophila. Increased PCNT expression and its protein accumulation are linked to clinical conditions including cancer, mental disorders, and ciliopathies. However, the mechanisms by which PCNT levels are regulated remain underexplored. Our previous study demonstrated that PLP levels are sharply down-regulated during early spermatogenesis and this regulation is essential to spatially position PLP on the proximal end of centrioles. We hypothesized that the sharp drop in PLP protein was a result of rapid protein degradation during the male germ line premeiotic G2 phase. Here, we show that PLP is subject to ubiquitin-mediated degradation and identify multiple proteins that promote the reduction of PLP levels in spermatocytes, including the UBR box containing E3 ligase Poe (UBR4), which we show binds to PLP. Although protein sequences governing posttranslational regulation of PLP are not restricted to a single region of the protein, we identify a region that is required for Poe-mediated degradation. Experimentally stabilizing PLP, via internal PLP deletions or loss of Poe, leads to PLP accumulation in spermatocytes, its mispositioning along centrioles, and defects in centriole docking in spermatids.

Architectural basis for cylindrical self-assembly governing Plk4-mediated centriole duplication in human cells

Proper organization of intracellular assemblies is fundamental for efficient promotion of biochemical processes and optimal assembly functionality. Although advances in imaging technologies have shed light on how the centrosome is organized, how its constituent proteins are coherently architected to elicit downstream events remains poorly understood. Using multidisciplinary approaches, we showed that two long coiled-coil proteins, Cep63 and Cep152, form a heterotetrameric building block that undergoes a stepwise formation into higher molecular weight complexes, ultimately generating a cylindrical architecture around a centriole. Mutants defective in Cep63•Cep152 heterotetramer formation displayed crippled pericentriolar Cep152 organization, polo-like kinase 4 (Plk4) relocalization to the procentriole assembly site, and Plk4-mediated centriole duplication. Given that the organization of pericentriolar materials (PCM) is evolutionarily conserved, this work could serve as a model for investigating the structure and function of PCM in other species, while offering a new direction in probing the organizational defects of PCM-related human diseases.

Myosin 10 uses its MyTH4 and FERM domains differentially to support two aspects of spindle pole biology required for mitotic spindle bipolarity

Myosin 10 (Myo10) has the ability to link actin filaments to integrin-based adhesions and to microtubules by virtue of its integrin-binding FERM domain and microtubule-binding MyTH4 domain, respectively. Here we used Myo10 knockout cells to define Myo10's contribution to the maintenance of spindle bipolarity, and complementation to quantitate the relative contributions of its MyTH4 and FERM domains. Myo10 knockout HeLa cells and mouse embryo fibroblasts (MEFs) both exhibit a pronounced increase in the frequency of multipolar spindles. Staining of unsynchronized metaphase cells showed that the primary driver of spindle multipolarity in knockout MEFs and knockout HeLa cells lacking supernumerary centrosomes is pericentriolar material (PCM) fragmentation, which creates γ-tubulin-positive acentriolar foci that serve as additional spindle poles. For HeLa cells possessing supernumerary centrosomes, Myo10 depletion further accentuates spindle multipolarity by impairing the clustering of the extra spindle poles. Complementation experiments show that Myo10 must interact with both integrins and microtubules to promote PCM/pole integrity. Conversely, Myo10's ability to promote the clustering of supernumerary centrosomes only requires that it interact with integrins. Importantly, images of Halo-Myo10 knock-in cells show that the myosin localizes exclusively within adhesive retraction fibers during mitosis. Based on these and other results, we conclude that Myo10 promotes PCM/pole integrity at a distance, and that it facilitates supernumerary centrosome clustering by promoting retraction fiber-based cell adhesion, which likely provides an anchor for the microtubule-based forces driving pole focusing.

Zika virus alters centrosome organization to suppress the innate immune response

Zika virus (ZIKV) is a flavivirus transmitted via mosquitoes and sex to cause congenital neurodevelopmental defects, including microcephaly. Inherited forms of microcephaly (MCPH) are associated with disrupted centrosome organization. Similarly, we found that ZIKV infection disrupted centrosome organization. ZIKV infection disrupted the organization of centrosomal proteins including CEP63, a MCPH-associated protein. The ZIKV nonstructural protein NS3 bound CEP63, and expression of NS3 was sufficient to alter centrosome architecture and CEP63 localization. Loss of CEP63 suppressed ZIKV-induced centrosome disorganization, indicating that ZIKV requires CEP63 to disrupt centrosome organization. ZIKV infection or CEP63 loss decreased the centrosomal localization and stability of TANK-binding kinase 1 (TBK1), a regulator of the innate immune response. ZIKV infection also increased the centrosomal accumulation of the CEP63 interactor DTX4, a ubiquitin ligase that degrades TBK1. Therefore, we propose that ZIKV disrupts CEP63 function to increase centrosomal DTX4 localization and destabilization of TBK1, thereby tempering the innate immune response.

Pericentrin interacts with Kinesin-1 to drive centriole motility

Centrosome positioning is essential for their function. Typically, centrosomes are transported to various cellular locations through the interaction of centrosomal microtubules (MTs) with motor proteins anchored at the cortex or the nuclear surface. However, it remains unknown how centrioles migrate in cellular contexts in which they do not nucleate MTs. Here, we demonstrate that during interphase, inactive centrioles move directly along the interphase MT network as Kinesin-1 cargo. We identify Pericentrin-Like-Protein (PLP) as a novel Kinesin-1 interacting molecule essential for centriole motility. In vitro assays show that PLP directly interacts with the cargo binding domain of Kinesin-1, allowing PLP to migrate on MTs. Binding assays using purified proteins revealed that relief of Kinesin-1 autoinhibition is critical for its interaction with PLP. Finally, our studies of neural stem cell asymmetric divisions in the Drosophila brain show that the PLP-Kinesin-1 interaction is essential for the timely separation of centrioles, the asymmetry of centrosome activity, and the age-dependent centrosome inheritance.

Mitotic Maturation Compensates for Premature Centrosome Splitting and PCM Loss in Human cep135 Knockout Cells

Centrosomes represent main microtubule organizing centers (MTOCs) in animal cells. Their duplication in S-phase enables the establishment of two MTOCs in M-phase that define the poles of the spindle and ensure equal distribution of chromosomes and centrosomes to the two daughter cells. While key functions of many centrosomal proteins have been addressed in RNAi experiments and chronic knockdown, knockout experiments with complete loss of function in all cells enable quantitative analysis of cellular phenotypes at all cell-cycle stages. Here, we show that the centriolar satellite proteins SSX2IP and WDR8 and the centriolar protein CEP135 form a complex before centrosome assembly in vertebrate oocytes and further functionally interact in somatic cells with established centrosomes. We present stable knockouts of SSX2IP, WDR8, and CEP135 in human cells. While loss of SSX2IP and WDR8 are compensated for, cep135 knockout cells display compromised PCM recruitment, reduced MTOC function, and premature centrosome splitting with imbalanced PCMs. Defective cep135 knockout centrosomes, however, manage to establish balanced spindle poles, allowing unperturbed mitosis and regular cell proliferation. Our data show essential functions of CEP135 in interphase MTOCs and demonstrate that loss of individual functions of SSX2IP, WDR8, and CEP135 are fully compensated for in mitosis.

Expression analyses of Cep152, a responsible gene product for autosomal recessive primary microcephaly, during mouse brain development

Centrosomal protein 152 (Cep152) regulates centriole duplication as a molecular scaffold during the cell cycle. Its gene abnormalities are responsible for autosomal recessive primary microcephaly 9 (MCPH9) and Seckel syndrome. In this study, we prepared an antibody against mouse Cep152, anti-Cep152, and performed expression analyses focusing on mouse brain development. Western blotting analyses revealed that Cep152 with a molecular mass of ~150 kDa was expressed strongly at embryonic day (E) 13 and then gradually decreased during the brain development process. Instead, protein bands of ~80 kDa and ~60 kDa came to be recognized after postnatal day (P)15 and P30, respectively. In immunohistochemical analyses, Cep152 was enriched in the centrosome of neuronal progenitors in the ventricular zone at E14, whereas it was diffusely distributed mainly in the cytoplasm of cortical neurons at P18. In developing cerebellum at P7, Cep152 was localized at the centrosome in the external granular layer, where neurogenesis takes place. Notably, biochemical analysis revealed that Cep152 was also present in the post-synaptic density fraction. Subsequent immunofluorescent analyses showed co-localization of Cep152 with excitatory synaptic markers, PSD-95 and synaptophysin, but not with an inhibitory synaptic marker gephyrin in differentiated primary cultured hippocampal neurons. The obtained results suggest that Cep152 takes part not only in neurogenesis during corticogenesis but also in the regulation of synaptic function in differentiated neurons.

Centrosome instability: when good centrosomes go bad

The centrosome is a tiny cytoplasmic organelle that organizes and constructs massive molecular machines to coordinate diverse cellular processes. Due to its many roles during both interphase and mitosis, maintaining centrosome homeostasis is essential to normal health and development. Centrosome instability, divergence from normal centrosome number and structure, is a common pathognomonic cellular state tightly associated with cancers and other genetic diseases. As novel connections are investigated linking the centrosome to disease, it is critical to understand the breadth of centrosome functions to inspire discovery. In this review, we provide an introduction to normal centrosome function and highlight recent discoveries that link centrosome instability to specific disease states.

PCMD-1 bridges the centrioles and the pericentriolar material scaffold in C. elegans

Correct cell division relies on the formation of a bipolar spindle. In animal cells, microtubule nucleation at the spindle poles is facilitated by the pericentriolar material (PCM), which assembles around a pair of centrioles. Although centrioles are essential for PCM assembly, the proteins that anchor the PCM to the centrioles are less known. Here, we investigate the molecular function of PCMD-1 in bridging the PCM and the centrioles in Caenorhabditis elegans. We demonstrate that the centrosomal recruitment of PCMD-1 is dependent on the outer centriolar protein SAS-7. The most C-terminal part of PCMD-1 is sufficient to target it to the centrosome, and the coiled-coil domain promotes its accumulation by facilitating self-interaction. We reveal that PCMD-1 interacts with the PCM scaffold protein SPD-5, the mitotic kinase PLK-1 and the centriolar protein SAS-4. Using an ectopic translocation assay, we show that PCMD-1 can selectively recruit downstream PCM scaffold components to an ectopic location in the cell, indicating that PCMD-1 is able to anchor the PCM scaffold proteins at the centrioles. Our work suggests that PCMD-1 is an essential functional bridge between the centrioles and the PCM.

The careful control of Polo kinase by APC/C-Ube2C ensures the intercellular transport of germline centrosomes during Drosophila oogenesis

A feature of metazoan reproduction is the elimination of maternal centrosomes from the oocyte. In animals that form syncytial cysts during oogenesis, including Drosophila and human, all centrosomes within the cyst migrate to the oocyte where they are subsequently degenerated. The importance and the underlying mechanism of this event remain unclear. Here, we show that, during early Drosophila oogenesis, control of the Anaphase Promoting Complex/Cyclosome (APC/C), the ubiquitin ligase complex essential for cell cycle control, ensures proper transport of centrosomes into the oocyte through the regulation of Polo/Plk1 kinase, a critical regulator of the integrity and activity of the centrosome. We show that novel mutations in the APC/C-specific E2, Vihar/Ube2c, that affect its inhibitory regulation on APC/C cause precocious Polo degradation and impedes centrosome transport, through destabilization of centrosomes. The failure of centrosome migration correlates with weakened microtubule polarization in the cyst and allows ectopic microtubule nucleation in nurse cells, leading to the loss of oocyte identity. These results suggest a role for centrosome migration in oocyte fate maintenance through the concentration and confinement of microtubule nucleation activity into the oocyte. Considering the conserved roles of APC/C and Polo throughout the animal kingdom, our findings may be translated into other animals.

Human centrosome organization and function in interphase and mitosis

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

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.

Constructing PCM with architecturally distinct higher-order assemblies

Pericentriolar material (PCM) present around a pair of centrioles functions as a platform for various cellular processes, including microtubule (MT) assembly. While PCM is known to be an electron-dense proteinaceous matrix made of long coiled-coil proteins and their client molecules, the molecular mechanism underlying PCM organization remains largely elusive. A growing body of evidence suggests that PCM is constructed in part by an interphase cylindrical self-assembly and the mitotic mesh-like architectures surrounding it. In this review, we will discuss how these higher-order structures are constructed to achieve the functional proficiency of the centrosome.

A molecular mechanism for the procentriole recruitment of Ana2

McLamarrah et al. characterize an early step in centriole duplication. They show that Plk4 hyperphosphorylates Ana2, which increases the affinity of Ana2 for the G-box domain of Sas4, promoting Ana2’s accumulation at the procentriole and, consequently, daughter centriole formation.

During centriole duplication, a preprocentriole forms at a single site on the mother centriole through a process that includes the hierarchical recruitment of a conserved set of proteins, including the Polo-like kinase 4 (Plk4), Ana2/STIL, and the cartwheel protein Sas6. Ana2/STIL is critical for procentriole assembly, and its recruitment is controlled by the kinase activity of Plk4, but how this works remains poorly understood. A structural motif called the G-box in the centriole outer wall protein Sas4 interacts with a short region in the N terminus of Ana2/STIL. Here, we show that binding of Ana2 to the Sas4 G-box enables hyperphosphorylation of the Ana2 N terminus by Plk4. Hyperphosphorylation increases the affinity of the Ana2–G-box interaction, and, consequently, promotes the accumulation of Ana2 at the procentriole to induce daughter centriole formation.

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

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

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

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

Sperm Head-Tail Linkage Requires Restriction of Pericentriolar Material to the Proximal Centriole End

The centriole, or basal body, is the center of attachment between the sperm head and tail. While the distal end of the centriole templates the cilia, the proximal end associates with the nucleus. Using Drosophila, we identify a centriole-centric mechanism that ensures proper proximal end docking to the nucleus. This mechanism relies on the restriction of pericentrin-like protein (PLP) and the pericentriolar material (PCM) to the proximal end of the centriole. PLP is restricted proximally by limiting its mRNA and protein to the earliest stages of centriole elongation. Ectopic positioning of PLP to more distal portions of the centriole is sufficient to redistribute PCM and microtubules along the entire centriole length. This results in erroneous, lateral centriole docking to the nucleus, leading to spermatid decapitation as a result of a failure to form a stable head-tail linkage.

The Microtubule-Depolymerizing Kinesin-13 Klp10A Is Enriched in the Transition Zone of the Ciliary Structures of Drosophila melanogaster

The precursor of the flagellar axoneme is already present in the primary spermatocytes of Drosophila melanogaster. During spermatogenesis each primary spermatocyte shows a centriole pair that moves to the cell membrane and organizes an axoneme-based structure, the cilium-like region (CLR). The CLRs persist through the meiotic divisions and are inherited by young spermatids. During spermatid differentiation the ciliary caps elongate giving rise to the sperm axoneme. Mutations in Klp10A, a kinesin-13 of Drosophila, results in defects of centriole/CLR organization in spermatocytes and of ciliary cap assembly in elongating spermatids. Reduced Klp10A expression also results in strong structural defects of sensory type I neurons. We show, here, that this protein displays a peculiar localization during male gametogenesis. The Klp10A signal is first detected at the distal ends of the centrioles when they dock to the plasma membrane of young primary spermatocytes. At the onset of the first meiotic prometaphase, when the CLRs reach their full size, Klp10A is enriched in a distinct narrow area at the distal end of the centrioles and persists in elongating spermatids at the base of the ciliary cap. We conclude that Klp10A could be a core component of the ciliary transition zone in Drosophila.

Microtubule minus-end regulation at a glance

Microtubules are cytoskeletal filaments essential for numerous aspects of cell physiology. They are polarized polymeric tubes with a fast growing plus end and a slow growing minus end. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge on the dynamics and organization of microtubule minus ends. Several factors, including the γ-tubulin ring complex, CAMSAP/Patronin, ASPM/Asp, SPIRAL2 (in plants) and the KANSL complex recognize microtubule minus ends and regulate their nucleation, stability and interactions with partners, such as microtubule severing enzymes, microtubule depolymerases and protein scaffolds. Together with minus-end-directed motors, these microtubule minus-end targeting proteins (-TIPs) also control the formation of microtubule-organizing centers, such as centrosomes and spindle poles, and mediate microtubule attachment to cellular membrane structures, including the cell cortex, Golgi complex and the cell nucleus. Structural and functional studies are starting to reveal the molecular mechanisms by which dynamic -TIP networks control microtubule minus ends.