Msd1/SSX2IP-dependent microtubule anchorage ensures spindle orientation and primary cilia formation

Plant microtubule nucleating apparatus and its potential signaling pathway

Plant cell cortical microtubules are located beneath the plasma membrane and direct the location of cellulose synthases during interphase, influencing cell morphology. Microtubule-associated proteins (MAPs) regulate these microtubules in response to growth and environmental stimuli. This review focuses on recent advances in understanding microtubule nucleation mechanisms in plants and the spatiotemporal regulation of cortical arrays via phytohormone signaling. Emphasis is placed on the conserved nature of the gamma-tubulin ring complex (γTuRC) and plant-specific components. The discussion includes the role of the Augmin complex and the distinct function of the Msd1-Wdr8 complex in plants. We also explore the effects of hormone signaling, particularly brassinosteroids, on the microtubule regulatory apparatus. The interplay between hormone signaling pathways and microtubule dynamics, including phosphorylation events and post-translational modifications, is also addressed. Finally, the impact of environmental signals and the role of protein post-translational modifications in regulating microtubule organization are suggested for future research.

Defining a cohort of anemia-activated cis elements reveals a mechanism promoting erythroid precursor function

Acute anemia elicits broad transcriptional changes in erythroid progenitors and precursors. We previously discovered a cis-regulatory transcriptional enhancer at the sterile alpha motif domain-14 enhancer locus (S14E), defined by a CANNTG-spacer-AGATAA composite motif and occupied by GATA1 and TAL1 transcription factors, is required for survival in severe anemia. However, S14E is only 1 of dozens of anemia-activated genes containing similar motifs. In a mouse model of acute anemia, we identified populations of expanding erythroid precursors, which increased expression of genes that contain S14E-like cis elements. We reveal that several S14E-like cis elements provide important transcriptional control of newly identified anemia-inducing genes, including the Ssx-2 interacting protein (Ssx2ip). Ssx2ip expression was determined to play an important role in erythroid progenitor/precursor cell activities, cell cycle regulation, and cell proliferation. Over a weeklong course of acute anemia recovery, we observed that erythroid gene activation mediated by S14E-like cis elements occurs during a phase coincident with low hematocrit and high progenitor activities, with distinct transcriptional programs activated at earlier and later time points. Our results define a genome-wide mechanism in which S14E-like enhancers control transcriptional responses during erythroid regeneration. These findings provide a framework to understand anemia-specific transcriptional mechanisms, ineffective erythropoiesis, anemia recovery, and phenotypic variability within human populations.

Structure and function of distal and subdistal appendages of the mother centriole

Centrosomes are composed of centrioles surrounded by pericentriolar material. The two centrioles in G1 phase are distinguished by the localization of their appendages in the distal and subdistal regions; the centriole possessing both types of appendage is older and referred to as the mother centriole, whereas the other centriole lacking appendages is the daughter centriole. Both distal and subdistal appendages in vertebrate cells consist of multiple proteins assembled in a hierarchical manner. Distal appendages function mainly in the initial process of ciliogenesis, and subdistal appendages are involved in microtubule anchoring, mitotic spindle regulation and maintenance of ciliary signaling. Mutations in genes encoding components of both appendage types are implicated in ciliopathies and developmental defects. In this Review, we discuss recent advances in knowledge regarding the composition and assembly of centriolar appendages, as well as their roles in development and disease.

Microtubule Anchoring: Attaching Dynamic Polymers to Cellular Structures

Microtubules are dynamic, filamentous polymers composed of α- and β-tubulin. Arrays of microtubules that have a specific polarity and distribution mediate essential processes such as intracellular transport and mitotic chromosome segregation. Microtubule arrays are generated with the help of microtubule organizing centers (MTOC). MTOCs typically combine two principal activities, the de novo formation of microtubules, termed nucleation, and the immobilization of one of the two ends of microtubules, termed anchoring. Nucleation is mediated by the γ-tubulin ring complex (γTuRC), which, in cooperation with its recruitment and activation factors, provides a template for α- and β-tubulin assembly, facilitating formation of microtubule polymer. In contrast, the molecules and mechanisms that anchor newly formed microtubules at MTOCs are less well characterized. Here we discuss the mechanistic challenges underlying microtubule anchoring, how this is linked with the molecular activities of known and proposed anchoring factors, and what consequences defective microtubule anchoring has at the cellular and organismal level.

Regulators of tubulin polyglutamylation control nuclear shape and cilium disassembly by balancing microtubule and actin assembly

Cytoskeletal networks play an important role in regulating nuclear morphology and ciliogenesis. However, the role of microtubule (MT) post-translational modifications in nuclear shape regulation and cilium disassembly has not been explored. Here we identified a novel regulator of the tubulin polyglutamylase complex (TPGC), C11ORF49/CSTPP1, that regulates cytoskeletal organization, nuclear shape, and cilium disassembly. Mechanistically, loss of C11ORF49/CSTPP1 impacts the assembly and stability of the TPGC, which modulates long-chain polyglutamylation levels on microtubules (MTs) and thereby balances the binding of MT-associated proteins and actin nucleators. As a result, loss of TPGC leads to aberrant, enhanced assembly of MTs that penetrate the nucleus, which in turn leads to defects in nuclear shape, and disorganization of cytoplasmic actin that disrupts the YAP/TAZ pathway and cilium disassembly. Further, we showed that C11ORF49/CSTPP1-TPGC plays mechanistically distinct roles in the regulation of nuclear shape and cilium disassembly. Remarkably, disruption of C11ORF49/CSTPP1-TPGC also leads to developmental defects in vivo. Our findings point to an unanticipated nexus that links tubulin polyglutamylation with nuclear shape and ciliogenesis.

Function of Centriolar Satellites and Regulation by Post-Translational Modifications

Centriolar satellites are small membrane-less granules that gravitate around the centrosome. Recent advances in defining the satellite proteome and interactome have unveiled hundreds of new satellite components thus illustrating the complex nature of these particles. Although initially linked to the homeostasis of centrosome and the formation of primary cilia, these composite and highly dynamic structures appear to participate in additional cellular processes, such as proteostasis, autophagy, and cellular stress. In this review, we first outline the main features and many roles of centriolar satellites. We then discuss how post-translational modifications, such as phosphorylation and ubiquitination, shape their composition and functions. This is of particular interest as interfering with these processes may provide ways to manipulate these structures.

Identification of the centrosomal maturation factor SSX2IP as a Wtip-binding partner by targeted proximity biotinylation

Wilms tumor-1-interacting protein (Wtip) is a LIM-domain-containing adaptor that links cell junctions with actomyosin complexes and modulates actomyosin contractility and ciliogenesis in Xenopus embryos. The Wtip C-terminus with three LIM domains associates with the actin-binding protein Shroom3 and modulates Shroom3-induced apical constriction in ectoderm cells. By contrast, the N-terminal domain localizes to apical junctions in the ectoderm and basal bodies in skin multiciliated cells, but its interacting partners remain largely unknown. Targeted proximity biotinylation (TPB) using anti-GFP antibody fused to the biotin ligase BirA identified SSX2IP as a candidate protein that binds GFP-WtipN. SSX2IP, also known as Msd1 or ADIP, is a component of cell junctions, centriolar satellite protein and a targeting factor for ciliary membrane proteins. WtipN physically associated with SSX2IP and the two proteins readily formed mixed aggregates in overexpressing cells. By contrast, we observed only partial colocalization of full length Wtip and SSX2IP, suggesting that Wtip adopts a ‘closed’ conformation in the cell. Furthermore, the double depletion of Wtip and SSX2IP in early embryos uncovered the functional interaction of the two proteins during neural tube closure. Our results suggest that the association of SSX2IP and Wtip is essential for cell junction remodeling and morphogenetic processes that accompany neurulation. We propose that TPB can be a general approach that is applicable to other GFP-tagged proteins.

An anchoring complex recruits katanin for microtubule severing at the plant cortical nucleation sites

Microtubules are severed by katanin at distinct cellular locations to facilitate reorientation or amplification of dynamic microtubule arrays, but katanin targeting mechanisms are poorly understood. Here we show that a centrosomal microtubule-anchoring complex is used to recruit katanin in acentrosomal plant cells. The conserved protein complex of Msd1 (also known as SSX2IP) and Wdr8 is localized at microtubule nucleation sites along the microtubule lattice in interphase Arabidopsis cells. Katanin is recruited to these sites for efficient release of newly formed daughter microtubules. Our cell biological and genetic studies demonstrate that Msd1-Wdr8 acts as a specific katanin recruitment factor to cortical nucleation sites (but not to microtubule crossover sites) and stabilizes the association of daughter microtubule minus ends to their nucleation sites until they become severed by katanin. Molecular coupling of sequential anchoring and severing events by the evolutionarily conserved complex renders microtubule release under tight control of katanin activity.

OCRL regulates lysosome positioning and mTORC1 activity through SSX2IP-mediated microtubule anchoring

Lysosomal positioning and mTOR (mammalian target of rapamycin) signaling coordinate cellular responses to nutrient levels. Inadequate nutrient sensing can result in growth delays, a hallmark of Lowe syndrome. OCRL mutations cause Lowe syndrome, but the role of OCRL in nutrient sensing is unknown. Here, we show that OCRL is localized to the centrosome by its ASH domain and that it recruits microtubule-anchoring factor SSX2IP to the centrosome, which is important in the formation of the microtubule-organizing center. Deficiency of OCRL in human and mouse cells results in loss of microtubule-organizing centers and impaired microtubule-based lysosome movement, which in turn leads to mTORC1 inactivation and abnormal nutrient sensing. Centrosome-targeted PACT-SSX2IP can restore microtubule anchoring and mTOR activity. Importantly, boosting the activity of mTORC1 restores the nutrient sensing ability of Lowe patients' cells. Our findings highlight mTORC1 as a novel therapeutic target for Lowe syndrome.

Rpgrip1l controls ciliary gating by ensuring the proper amount of Cep290 at the vertebrate transition zone

A range of severe human diseases called ciliopathies is caused by the dysfunction of primary cilia. Primary cilia are cytoplasmic protrusions consisting of the basal body (BB), the axoneme, and the transition zone (TZ). The BB is a modified mother centriole from which the axoneme, the microtubule-based ciliary scaffold, is formed. At the proximal end of the axoneme, the TZ functions as the ciliary gate governing ciliary protein entry and exit. Since ciliopathies often develop due to mutations in genes encoding proteins that localize to the TZ, the understanding of the mechanisms underlying TZ function is of eminent importance. Here, we show that the ciliopathy protein Rpgrip1l governs ciliary gating by ensuring the proper amount of Cep290 at the vertebrate TZ. Further, we identified the flavonoid eupatilin as a potential agent to tackle ciliopathies caused by mutations in RPGRIP1L as it rescues ciliary gating in the absence of Rpgrip1l.

Prominin-1 Modulates Rho/ROCK-Mediated Membrane Morphology and Calcium-Dependent Intracellular Chloride Flux

Membrane morphology is an important structural determinant as it reflects cellular functions. The pentaspan membrane protein Prominin-1 (Prom1/CD133) is known to be localised to protrusions and plays a pivotal role in migration and the determination of cellular morphology; however, the underlying mechanism of its action have been elusive. Here, we performed molecular characterisation of Prom1, focussing primarily on its effects on cell morphology. Overexpression of Prom1 in RPE-1 cells triggers multiple, long, cholesterol-enriched fibres, independently of actin and microtubule polymerisation. A five amino acid stretch located at the carboxyl cytosolic region is essential for fibre formation. The small GTPase Rho and its downstream Rho-associated coiled-coil-containing protein kinase (ROCK) are also essential for this process, and active Rho colocalises with Prom1 at the site of initialisation of fibre formation. In mouse embryonic fibroblast (MEF) cells we show that Prom1 is required for chloride ion efflux induced by calcium ion uptake, and demonstrate that fibre formation is closely associated with chloride efflux activity. Collectively, these findings suggest that Prom1 affects cell morphology and contributes to chloride conductance.

Spatial and proteomic profiling reveals centrosome-independent features of centriolar satellites

Centriolar satellites are small electron-dense granules that cluster in the vicinity of centrosomes. Satellites have been implicated in multiple critical cellular functions including centriole duplication, centrosome maturation, and ciliogenesis, but their precise composition and assembly properties have remained poorly explored. Here, we perform in vivo proximity-dependent biotin identification (BioID) on 22 human satellite proteins, to identify 2,113 high-confidence interactions among 660 unique polypeptides. Mining this network, we validate six additional satellite components. Analysis of the satellite interactome, combined with subdiffraction imaging, reveals the existence of multiple unique microscopically resolvable satellite populations that display distinct protein interaction profiles. We further show that loss of satellites in PCM1-depleted cells results in a dramatic change in the satellite interaction landscape. Finally, we demonstrate that satellite composition is largely unaffected by centriole depletion or disruption of microtubules, indicating that satellite assembly is centrosome-independent. Together, our work offers the first systematic spatial and proteomic profiling of human centriolar satellites and paves the way for future studies aimed at better understanding the biogenesis and function(s) of these enigmatic structures.

Microtubule asters anchored by FSD1 control axoneme assembly and ciliogenesis

Defective ciliogenesis causes human developmental diseases termed ciliopathies. Microtubule (MT) asters originating from centrosomes in mitosis ensure the fidelity of cell division by positioning the spindle apparatus. However, the function of microtubule asters in interphase remains largely unknown. Here, we reveal an essential role of MT asters in transition zone (TZ) assembly during ciliogenesis. We demonstrate that the centrosome protein FSD1, whose biological function is largely unknown, anchors MT asters to interphase centrosomes by binding to microtubules. FSD1 knockdown causes defective ciliogenesis and affects embryonic development in vertebrates. We further show that disruption of MT aster anchorage by depleting FSD1 or other known anchoring proteins delocalizes the TZ assembly factor Cep290 from centriolar satellites, and causes TZ assembly defects. Thus, our study establishes FSD1 as a MT aster anchorage protein and reveals an important function of MT asters anchored by FSD1 in TZ assembly during ciliogenesis.

Microtubule asters originate from centrosomes but their role during interphase remains largely unknown. Here, the authors find that microtubule asters anchored by previously-uncharacterized FSD1 play a role in ciliogenesis by maintaining the dynamic localization of centriolar satellites.

Ccdc61 controls centrosomal localization of Cep170 and is required for spindle assembly and symmetry

Microtubule nucleation was uncovered as a key principle of spindle assembly. However, the mechanistic details about microtubule nucleation and the organization of spindle formation and symmetry are currently being revealed. Here we describe the function of coiled-coil domain containing 61 (Ccdc61), a so far uncharacterized centrosomal protein, in spindle assembly and symmetry. Our data describe that Ccdc61 is required for spindle assembly and precise chromosome alignments in mitosis. Microtubule tip-tracking experiments in the absence of Ccdc61 reveal a clear loss of the intrinsic symmetry of microtubule tracks within the spindle. Furthermore, we show that Ccdc61 controls the centrosomal localization of centrosomal protein of 170 kDa (Cep170), a protein that was shown previously to localize to centrosomes as well as spindle microtubules and promotes microtubule organization and microtubule assembly. Interestingly, selective disruption of Ccdc61 impairs the binding between Cep170 and TANK binding kinase 1, an interaction that is required for microtubule stability. In summary, we have discovered Ccdc61 as a centrosomal protein with an important function in mitotic microtubule organization.

LRRC45 contributes to early steps of axoneme extension

Cilia perform essential signalling functions during development and tissue homeostasis. A key event in ciliogenesis occurs when the distal appendages of the mother centriole form a platform that docks ciliary vesicles and removes CP110-Cep97 inhibitory complexes. Here, we analysed the role of LRRC45 in appendage formation and ciliogenesis. We show that the core appendage proteins Cep83 and SCLT1 recruit LRRC45 to the mother centriole. Once there, LRRC45 recruits the keratin-binding protein FBF1. The association of LRRC45 with the basal body of primary and motile cilia in both differentiated and stem cells reveals a broad function in ciliogenesis. In contrast to the appendage components Cep164 and Cep123, LRRC45 was not essential for either docking of early ciliary vesicles or for removal of CP110. Rather, LRRC45 promotes cilia biogenesis in CP110-uncapped centrioles by organising centriolar satellites, establishing the transition zone and promoting the docking of Rab8 GTPase-positive vesicles. We propose that, instead of acting solely as a platform to recruit early vesicles, centriole appendages form discrete scaffolds of cooperating proteins that execute specific functions that promote the initial steps of ciliogenesis.

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.

Microtubule-Organizing Centers: Towards a Minimal Parts List

Despite decades of molecular analysis of the centrosome, an important microtubule-organizing center (MTOC) of animal cells, the molecular basis of microtubule organization remains obscure. A major challenge is the sheer complexity of the interplay of the hundreds of proteins that constitute the centrosome. However, this complexity owes not only to the centrosome's role as a MTOC but also to the requirements of its duplication cycle and to various other functions such as the formation of cilia, the integration of various signaling pathways, and the organization of actin filaments. Thus, rather than using the parts lists to reconstruct the centrosome, we propose to identify the subset of proteins minimally needed to assemble a MTOC and to study this process at non-centrosomal sites.

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.

Regulation of spindle integrity and mitotic fidelity by BCCIP

Centrosomes together with the mitotic spindle ensure the faithful distribution of chromosomes between daughter cells, and spindle orientation is a major determinant of cell fate during tissue regeneration. Spindle defects are not only an impetus of chromosome instability but are also a cause of developmental disorders involving defective asymmetric cell division. In this work, we demonstrate BCCIP, especially BCCIPα, as a previously unidentified component of the mitotic spindle pole and the centrosome. We demonstrate that BCCIP localizes proximal to the mother centriole and participates in microtubule organization and then redistributes to the spindle pole to ensure faithful spindle architecture. We find that BCCIP depletion leads to morphological defects, disoriented mitotic spindles, chromosome congression defects and delayed mitotic progression. Our study identifies BCCIP as a novel factor critical for microtubule regulation and explicates a mechanism utilized by BCCIP in tumor suppression.

Regulation of centriolar satellite integrity and its physiology

Centriolar satellites comprise cytoplasmic granules that are located around the centrosome. Their molecular identification was first reported more than a quarter of a century ago. These particles are not static in the cell but instead constantly move around the centrosome. Over the last decade, significant advances in their molecular compositions and biological functions have been achieved due to comprehensive proteomics and genomics, super-resolution microscopy analyses and elegant genetic manipulations. Centriolar satellites play pivotal roles in centrosome assembly and primary cilium formation through the delivery of centriolar/centrosomal components from the cytoplasm to the centrosome. Their importance is further underscored by the fact that mutations in genes encoding satellite components and regulators lead to various human disorders such as ciliopathies. Moreover, the most recent findings highlight dynamic structural remodelling in response to internal and external cues and unexpected positive feedback control that is exerted from the centrosome for centriolar satellite integrity.

A non-canonical function of Plk4 in centriolar satellite integrity and ciliogenesis through PCM1 phosphorylation

Centrioles are the major constituents of the animal centrosome, in which Plk4 kinase serves as a master regulator of the duplication cycle. Many eukaryotes also contain numerous peripheral particles known as centriolar satellites. While centriolar satellites aid centriole assembly and primary cilium formation, it is unknown whether Plk4 plays any regulatory roles in centriolar satellite integrity. Here we show that Plk4 is a critical determinant of centriolar satellite organisation. Plk4 depletion leads to the dispersion of centriolar satellites and perturbed ciliogenesis. Plk4 interacts with the satellite component PCM1, and its kinase activity is required for phosphorylation of the conserved S372. The nonphosphorylatable PCM1 mutant recapitulates phenotypes of Plk4 depletion, while the phosphomimetic mutant partially rescues the dispersed centriolar satellite patterns and ciliogenesis in cells depleted of PCM1. We show that S372 phosphorylation occurs during the G1 phase of the cell cycle and is important for PCM1 dimerisation and interaction with other satellite components. Our findings reveal that Plk4 is required for centriolar satellite function, which may underlie the ciliogenesis defects caused by Plk4 dysfunction.

WDR8 is a centriolar satellite and centriole-associated protein that promotes ciliary vesicle docking during ciliogenesis

Ciliogenesis initiates at the mother centriole through a series of events that include membrane docking, displacement of cilia-inhibitory proteins and axoneme elongation. Centriolar proteins, in particular at distal and subdistal appendages, carry out these functions. Recently, cytoplasmic complexes named centriolar satellites have also been shown to promote ciliogenesis. Little is known about the functional and molecular relationship between appendage proteins, satellites and cilia biogenesis. Here, we identified the WD-repeat protein 8 (WDR8, also known as WRAP73) as a satellite and centriolar component. We show that WDR8 interacts with the satellite proteins SSX2IP and PCM1 as well as the centriolar proximal end component Cep135. Cep135 is required for the recruitment of WDR8 to centrioles. Depletion experiments revealed that WDR8 and Cep135 have strongly overlapping functions in ciliogenesis. Both are indispensable for ciliary vesicle docking to the mother centriole and for unlocking the distal end of the mother centriole from the ciliary inhibitory complex CP110-Cep97. Our data thus point to an important function of centriolar proximal end proteins in ciliary membrane biogenesis, and establish WDR8 and Cep135 as two factors that are essential for the initial steps of ciliation.

The conserved Wdr8-hMsd1/SSX2IP complex localises to the centrosome and ensures proper spindle length and orientation

The centrosome plays a pivotal role in a wide range of cellular processes and its dysfunction is causally linked to many human diseases including cancer and developmental and neurological disorders. This organelle contains more than one hundred components, and yet many of them remain uncharacterised. Here we identified a novel centrosome protein Wdr8, based upon the structural conservation of the fission yeast counterpart. We showed that Wdr8 constitutively localises to the centrosome and super resolution microscopy uncovered that this protein is enriched at the proximal end of the mother centriole. Furthermore, we identified hMsd1/SSX2IP, a conserved spindle anchoring protein, as one of Wdr8 interactors by mass spectrometry. Wdr8 formed a complex and partially colocalised with hMsd1/SSX2IP. Intriguingly, knockdown of Wdr8 or hMsd1/SSX2IP displayed very similar mitotic defects, in which spindle microtubules became shortened and misoriented. Indeed, Wdr8 depletion resulted in the reduced recruitment of hMsd1/SSX2IP to the mitotic centrosome, though the converse is not true. Together, we propose that the conserved Wdr8-hMsd1/SSX2IP complex plays a critical role in controlling proper spindle length and orientation.

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Human Wdr8 is a centrosomal protein enriched in the proximal end of the centriole.

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Wdr8 and hMsd1/SSX2IP form a complex conserved in fungi.

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Depletion of Wdr8 results in shorter, tilted spindle microtubules.

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Depletion phenotypes of Wdr8 are very similar to those of hMsd1/SSX2IP knockdown.

The Msd1-Wdr8-Pkl1 complex anchors microtubule minus ends to fission yeast spindle pole bodies

Msd1–Wdr8 are delivered by Pkl1 to mitotic spindle pole bodies, where the Msd1–Wdr8–Pkl1 complex anchors the minus ends of spindle microtubules and antagonizes the outward-pushing forces generated by Cut7/kinesin-5 in fission yeast.

The minus ends of spindle microtubules are anchored to a microtubule-organizing center. The conserved Msd1/SSX2IP proteins are localized to the spindle pole body (SPB) and the centrosome in fission yeast and humans, respectively, and play a critical role in microtubule anchoring. In this paper, we show that fission yeast Msd1 forms a ternary complex with another conserved protein, Wdr8, and the minus end–directed Pkl1/kinesin-14. Individual deletion mutants displayed the identical spindle-protrusion phenotypes. Msd1 and Wdr8 were delivered by Pkl1 to mitotic SPBs, where Pkl1 was tethered through Msd1–Wdr8. The spindle-anchoring defect imposed by msd1/wdr8/pkl1 deletions was suppressed by a mutation of the plus end–directed Cut7/kinesin-5, which was shown to be mutual. Intriguingly, Pkl1 motor activity was not required for its anchoring role once targeted to the SPB. Therefore, spindle anchoring through Msd1–Wdr8–Pkl1 is crucial for balancing the Cut7/kinesin-5–mediated outward force at the SPB. Our analysis provides mechanistic insight into the spatiotemporal regulation of two opposing kinesins to ensure mitotic spindle bipolarity.

Centriolar satellite- and hMsd1/SSX2IP-dependent microtubule anchoring is critical for centriole assembly

Human Msd1/SSX2IP is a component of centriolar satellites and essential for microtubule anchoring to the centrosome. Anchoring defects lead to abnormal accumulation of centriolar components at centriolar satellites, which interferes with centriole assembly. Loss of Msd1/SSX2IP produces supernumerary centriole precursors specifically in tumor cells.

Centriolar satellites are numerous electron-dense granules dispersed around the centrosome. Mutations in their components are linked to various human diseases, but their molecular roles remain elusive. In particular, the significance of spatial communication between centriolar satellites and the centrosome is unknown. hMsd1/SSX2IP localizes to both the centrosome and centriolar satellites and is required for tethering microtubules to the centrosome. Here we show that hMsd1/SSX2IP-mediated microtubule anchoring is essential for proper centriole assembly and duplication. On hMsd1/SSX2IP knockdown, the centriolar satellites become stuck at the microtubule minus end near the centrosome. Intriguingly, these satellites contain many proteins that normally localize to the centrosome. Of importance, microtubule structures, albeit not being anchored properly, are still required for the emergence of abnormal satellites, as complete microtubule depolymerization results in the disappearance of these aggregates from the vicinity of the centrosome. We highlighted, using superresolution and electron microscopy, that under these conditions, centriole structures are faulty. Remarkably, these cells are insensitive to Plk4 overproduction–induced ectopic centriole formation, yet they accelerate centrosome reduplication upon hydroxyurea arrest. Finally, the appearance of satellite aggregates is cancer cell specific. Together our findings provide novel insights into the mechanism of centriole assembly and microtubule anchoring.

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.

FAK transduces extracellular forces that orient the mitotic spindle and control tissue morphogenesis

Spindle orientation is critical for proper morphogenesis of organs and tissues as well as for the maintenance of tissue morphology. Although significant progress has been made in understanding the mechanisms linking the cell cortex to the spindle and the well-documented role that extracellular forces play in spindle orientation, how such forces are transduced to the cortex remains poorly understood. Here we report that focal adhesion kinase (FAK) is necessary for correct spindle orientation and as a result, indispensable for proper epithelial morphogenesis in the vertebrate embryo. We show that FAK's role in spindle orientation is dependent on its ability to localize at focal adhesions and its interaction with paxillin, but is kinase activity independent. Finally, we present evidence that FAK is required for external force-induced spindle reorientation, suggesting that FAK's involvement in this process stems from a role in the transduction of external forces to the cell cortex.

Primary cilium-associated genes mediate bone marrow stromal cell response to hypoxia

Currently there is intense interest in using mesenchymal stem cells (MSC) for therapeutic interventions in many diseases and conditions. To accelerate the therapeutic use of stem cells we must understand how they sense their environment. Primary cilia are an extracellular sensory organelle present on most growth arrested cells that transduce information about the cellular environment into cells, triggering signaling cascades that have profound effects on development, cell cycle, proliferation, differentiation and migration. Migrating cells likely encounter differing oxygen tensions, therefore we investigated the effect of oxygen tension on cilia. Using bone marrow stromal cells (BMSCs, also known as bone marrow-derived mesenchymal stem cells) we found that oxygen tension significantly affected the length of cilia in primary BMSCs. Chronic exposure to hypoxia specifically down-regulated genes involved in hedgehog signaling and re-localized the Smo and Gli2 proteins to cilia. Investigating the effects of chemotactic migration on cilia, we observed significantly longer cilia in migrating cells which was again, strongly influenced by oxygen tension. Finally, using computational modeling we identified links between migration and ciliation signaling pathways, characterizing the novel role of HSP90 and PI3K signaling in regulating BMSC cilia length. These findings enhance our current understanding of BMSC adaptions to hypoxia and advance our knowledge of BMSC biology and cilia regulation.

Are there conserved roles for the extracellular matrix, cilia, and junctional complexes in left-right patterning?

Many different types of molecules have essential roles in patterning the left-right axis and directing asymmetric morphogenesis. In particular, the relationship between signaling molecules and transcription factors has been explored extensively. Another group of proteins implicated in left-right patterning are components of the extracellular matrix, apical junctions, and cilia. These structural molecules have the potential to participate in the conversion of morphogenetic cues from the extracellular environment into morphogenetic patterning via their interactions with the actin cytoskeleton. Although it has been relatively easy to temporally position these proteins within the hierarchy of the left-right patterning pathway, it has been more difficult to define how they mechanistically fit into these pathways. Consequently, our understanding of how these factors impart patterning information to influence the establishment of the left-right axis remains limited. In this review, we will discuss those structural molecules that have been implicated in early phases of left-right axis development.