RNA methyltransferase SPOUT1/CENP-32 links mitotic spindle organization with the neurodevelopmental disorder SpADMiSS

SPOUT1/CENP-32 encodes a putative SPOUT RNA methyltransferase previously identified as a mitotic chromosome associated protein. SPOUT1/CENP-32 depletion leads to centrosome detachment from the spindle poles and chromosome misalignment. Aided by gene matching platforms, we identified 24 individuals with neurodevelopmental delays from 18 families with bi-allelic variants in SPOUT1/CENP-32 detected by exome/genome sequencing. Zebrafish spout1/cenp-32 mutants showed reduction in larval head size with concomitant apoptosis likely associated with altered cell cycle progression. In vivo complementation assays in zebrafish indicated that SPOUT1/CENP-32 missense variants identified in humans are pathogenic. Crystal structure analysis of SPOUT1/CENP-32 revealed that most disease-associated missense variants mapped to the catalytic domain. Additionally, SPOUT1/CENP-32 recurrent missense variants had reduced methyltransferase activity in vitro and compromised centrosome tethering to the spindle poles in human cells. Thus, SPOUT1/CENP-32 pathogenic variants cause an autosomal recessive neurodevelopmental disorder: SpADMiSS ( SPOUT1 Associated Development delay Microcephaly Seizures Short stature) underpinned by mitotic spindle organization defects and consequent chromosome segregation errors.

The neurological and non-neurological roles of the primary microcephaly-associated protein ASPM

Primary microcephaly (MCPH), is a neurological disorder characterized by small brain size that results in numerous developmental problems, including intellectual disability, motor and speech delays, and seizures. Hitherto, over 30 MCPH causing genes (MCPHs) have been identified. Among these MCPHs, MCPH5, which encodes abnormal spindle-like microcephaly-associated protein (ASPM), is the most frequently mutated gene. ASPM regulates mitotic events, cell proliferation, replication stress response, DNA repair, and tumorigenesis. Moreover, using a data mining approach, we have confirmed that high levels of expression of ASPM correlate with poor prognosis in several types of tumors. Here, we summarize the neurological and non-neurological functions of ASPM and provide insight into its implications for the diagnosis and treatment of MCPH and cancer.

The impact of TP53 activation and apoptosis in primary hereditary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a constellation of disorders that share significant brain size reduction and mild to moderate intellectual disability, which may be accompanied by a large variety of more invalidating clinical signs. Extensive neural progenitor cells (NPC) proliferation and differentiation are essential to determine brain final size. Accordingly, the 30 MCPH loci mapped so far (MCPH1-MCPH30) encode for proteins involved in microtubule and spindle organization, centriole biogenesis, nuclear envelope, DNA replication and repair, underscoring that a wide variety of cellular processes is required for sustaining NPC expansion during development. Current models propose that altered balance between symmetric and asymmetric division, as well as premature differentiation, are the main mechanisms leading to MCPH. Although studies of cellular alterations in microcephaly models have constantly shown the co-existence of high DNA damage and apoptosis levels, these mechanisms are less considered as primary factors. In this review we highlight how the molecular and cellular events produced by mutation of the majority of MCPH genes may converge on apoptotic death of NPCs and neurons, via TP53 activation. We propose that these mechanisms should be more carefully considered in the alterations of the sophisticated equilibrium between proliferation, differentiation and death produced by MCPH gene mutations. In consideration of the potential druggability of cell apoptotic pathways, a better understanding of their role in MCPH may significantly facilitate the development of translational approaches.

A Zebrafish/Drosophila Dual System Model for Investigating Human Microcephaly

Microcephaly presents in neurodevelopmental disorders with multiple aetiologies, including bi-allelic mutation in TUBGCP2, a component of the biologically fundamental and conserved microtubule-nucleation complex, γ-TuRC. Elucidating underlying principles driving microcephaly requires clear phenotype recapitulation and assay reproducibility, areas where go-to experimental models fall short. We present an alternative simple vertebrate/invertebrate dual system to investigate fundamental TUBGCP2-related processes driving human microcephaly and associated developmental traits. We show that antisense morpholino knockdown (KD) of the Danio rerio homolog, tubgcp2, recapitulates human TUBGCP2-associated microcephaly. Co-injection of wild type mRNA pre-empts microcephaly in 55% of KD zebrafish larvae, confirming causality. Body shortening observed in morphants is also rescued. Mitotic marker (pH3) staining further reveals aberrantly accumulated dividing brain cells in microcephalic tubgcp2 KD morphants, indicating that tubgcp2 depletion disrupts normal mitosis and/or proliferation in zebrafish neural progenitor brain cells. Drosophila melanogaster double knockouts (KO) for TUBGCP2 homologs Grip84/cg7716 also develop microcephalic brains with general microsomia. Exacerbated Grip84/cg7716-linked developmental aberration versus single mutations strongly suggests interactive or coinciding gene functions. We infer that tubgcp2 and Grip84/cg7716 affect brain size similarly to TUBGCP2 and recapitulate both microcephaly and microcephaly-associated developmental impact, validating the zebrafish/fly research model for human microcephaly. Given the conserved cross-phyla homolog function, the data also strongly support mitotic and/or proliferative disruption linked to aberrant microtubule nucleation in progenitor brain cells as key mechanistic defects for human microcephaly.

Overexpression of abnormal spindle-like microcephaly-associated (ASPM) increases tumor aggressiveness and predicts poor outcome in patients with lung adenocarcinoma

Background

Cumulative evidence points to abnormal spindle-like microcephaly-associated (ASPM) protein being overexpressed in various cancers, and the aberrant expression of ASPM has been shown to promote cancer tumorigenicity and progression. However, its role and clinical significance in lung adenocarcinoma (LUAD) remains unclear. This study aimed to determine the expression patterns of ASPM and its clinical significance in LUAD.

Methods

In total, 4 original worldwide LUAD microarray mRNA expression datasets (N=1,116) with clinical and follow-up annotations were downloaded from The Cancer Genome Atlas (TCGA) and the Gene Expression Omnibus (GEO) databases. The expression of ASPM protein in LUAD patients was detected by immunohistochemistry. Survival analysis and Cox regression analysis were used to examine the prognostic value of ASPM expression. Gene set enrichment analysis (GSEA) was performed to investigate the relationship between ASPM and LUAD.

Results

Dataset analyses and immunohistochemistry revealed that ASPM expression was significantly higher in the LUAD tissues compared with normal lung tissues, especially in the advanced tumor stage. Additionally, overexpression of ASPM was significantly correlated with shorter overall survival (OS) and relapse-free survival (RFS) in LUAD. Univariate and multivariate Cox regression analyses revealed that the overexpression of ASPM was a potential independent predictor of poor OS and RFS. However, ASPM overexpression was not significantly associated with predicting OS in lung squamous cell carcinoma. GSEA analysis demonstrated that ASPM was significantly enriched in the cell cycle, DNA replication, homologous recombination, RNA degradation, mismatch repair, and p53 signaling pathways.

Conclusions

These findings demonstrate the important role of ASPM in the tumorigenesis and progression of LUAD.

Microtubule-associated proteins promote microtubule generation in the absence of γ-tubulin in human colon cancer cells

The γ-tubulin complex acts as the predominant microtubule (MT) nucleator that initiates MT formation and is therefore an essential factor for cell proliferation. Nonetheless, cellular MTs are formed after experimental depletion of the γ-tubulin complex, suggesting that cells possess other factors that drive MT nucleation. Here, by combining gene knockout, auxin-inducible degron, RNA interference, MT depolymerization/regrowth assay, and live microscopy, we identified four microtubule-associated proteins (MAPs), ch-TOG, CLASP1, CAMSAPs, and TPX2, which are involved in γ-tubulin-independent MT generation in human colon cancer cells. In the mitotic MT regrowth assay, nucleated MTs organized noncentriolar MT organizing centers (ncMTOCs) in the absence of γ-tubulin. Depletion of CLASP1 or TPX2 substantially delayed ncMTOC formation, suggesting that these proteins might promote MT nucleation in the absence of γ-tubulin. In contrast, depletion of ch-TOG or CAMSAPs did not affect the timing of ncMTOC appearance. CLASP1 also accelerates γ-tubulin-independent MT regrowth during interphase. Thus, MT generation can be promoted by MAPs without the γ-tubulin template.

Loss of acentriolar MTOCs disrupts spindle pole Aurora A and assembly of the liquid-like meiotic spindle domain in oocytes

Oocyte-specific knockdown of pericentrin (PCNT) in transgenic (Tg) mice disrupts acentriolar microtubule-organizing center (aMTOC) formation, leading to spindle instability and error-prone meiotic division. Here, we show that PCNT-depleted oocytes lack phosphorylated Aurora A (pAURKA) at spindle poles, while overall levels are unaltered. To test aMTOC-associated AURKA function, metaphase II (MII) control (WT) and Tg oocytes were briefly exposed to a specific AURKA inhibitor (MLN8237). Similar defects were observed in Tg and MLN8237-treated WT oocytes, including altered spindle structure, increased chromosome misalignment and impaired microtubule regrowth. Yet, AURKA inhibition had a limited effect on Tg oocytes, revealing a critical role for aMTOC-associated AURKA in regulating spindle stability. Notably, spindle instability was associated with disrupted γ-tubulin and lack of the liquid-like meiotic spindle domain (LISD) in Tg oocytes. Analysis of this Tg model provides the first evidence that LISD assembly depends expressly on aMTOC-associated AURKA, and that Ran-mediated spindle formation ensues without the LISD. These data support that loss of aMTOC-associated AURKA and failure of LISD assembly contribute to error-prone meiotic division in PCNT-depleted oocytes, underscoring the essential role of aMTOCs for spindle stability.

ASPM promotes homologous recombination-mediated DNA repair by safeguarding BRCA1 stability

DNA double-strand break (DSB) repair by homologous recombination (HR) is essential for ensuring genome stability. Abnormal spindle-like microcephaly-associated (ASPM) gene encodes a spindle protein that is commonly implicated in primary microcephaly. We found that ASPM is recruited to sites of DNA damage in a PARP2-dependent manner. ASPM interacts with BRCA1 and its E3 ligase HERC2, preventing HERC2 from accessing to BRCA1 and ensuring BRCA1 stability. Inhibition of ASPM expression promotes HERC2-mediated BRCA1 degradation, compromises HR repair efficiency and chromosome stability, and sensitizes cancer cells to ionizing radiation. Moreover, we observed a synergistic effect between ASPM and PARP inhibition in killing cancer cells. This research has uncovered a novel function for ASPM in facilitating HR-mediated repair of DSBs by ensuring BRCA1 stability. ASPM might constitute a promising target for synthetic lethality-based cancer therapy.

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ASPM is recruited to sites of DNA damage in a PARP2-dependent manner.

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ASPM promotes DSB-end resection to facilitate HR repair.

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ASPM prevents HERC2 from accessing to BRCA1 and ensuring BRCA1 stability.

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Inhibition of ASPM sensitizes cancer cells to ionizing radiation and PARP inhibitor.

CEP215 and AURKA regulate spindle pole focusing and aMTOC organization in mouse oocytes

Acentriolar microtubule-organizing centers (aMTOCs) play a critical role in stable meiotic spindle assembly in oocytes, necessary for accurate chromosome segregation. Yet, there is a limited understanding of the essential regulatory components of these unique MTOCs. In somatic cells, CEP215 (Centrosomal Protein 215) serves as an important regulator of centrosome maturation and spindle organization. Here, we assessed whether it has a similar function in mouse oocytes. CEP215 was detected in oocyte lysates and specifically localized to aMTOCs throughout the progression of meiosis in a pericentrin-dependent manner. Super-resolution microscopy revealed CEP215 co-localization with pericentrin and a unique pore/ring-like structural organization of aMTOCs. Interestingly, inhibition of Aurora Kinase A in either MI or MII-stage oocytes resulted in a striking loss of the ring-like aMTOC organization and pronounced CEP215 clustering at spindle poles, as well as shorter spindles with highly focused poles. In vitro siRNA-mediated transcript knockdown effectively reduced CEP215 in approximately 85% of the oocytes. Maturation rates to MII were similar in the Cep215 siRNA and injected controls; however, a high percentage (~40%) of the Cep215-knockdown oocytes showed notable variations in spindle pole focusing. Surprisingly, pericentrin and γ-tubulin localization and fluorescence intensity at aMTOCs were unaltered in knockdown oocytes, contrasting with mitotic cells where CEP215 depletion reduced γ-tubulin at centrosomes. Our results demonstrate that CEP215 is a functional component of oocyte aMTOCs and participates in the regulation of meiotic spindle pole focusing. Moreover, these studies reveal a vital role for Aurora Kinase A activity in the maintenance of aMTOC organization in oocytes.

Applications of brain organoids in neurodevelopment and neurological diseases

A brain organoid is a self-organizing three-dimensional tissue derived from human embryonic stem cells or pluripotent stem cells and is able to simulate the architecture and functionality of the human brain. Brain organoid generation methods are abundant and continue to improve, and now, an in vivo vascularized brain organoid has been encouragingly reported. The combination of brain organoids with immune-staining and single-cell sequencing technology facilitates our understanding of brain organoids, including the structural organization and the diversity of cell types. Recent publications have reported that brain organoids can mimic the dynamic spatiotemporal process of early brain development, model various human brain disorders, and serve as an effective preclinical platform to test and guide personalized treatment. In this review, we introduce the current state of brain organoid differentiation strategies, summarize current progress and applications in the medical domain, and discuss the challenges and prospects of this promising technology.

Centriole and PCM cooperatively recruit CEP192 to spindle poles to promote bipolar spindle assembly

The pericentriolar material (PCM) that accumulates around the centriole expands during mitosis and nucleates microtubules. Here, we show the cooperative roles of the centriole and PCM scaffold proteins, pericentrin and CDK5RAP2, in the recruitment of CEP192 to spindle poles during mitosis. Systematic depletion of PCM proteins revealed that CEP192, but not pericentrin and/or CDK5RAP2, was crucial for bipolar spindle assembly in HeLa, RPE1, and A549 cells with centrioles. Upon double depletion of pericentrin and CDK5RAP2, CEP192 that remained at centriole walls was sufficient for bipolar spindle formation. In contrast, through centriole removal, we found that pericentrin and CDK5RAP2 recruited CEP192 at the acentriolar spindle pole and facilitated bipolar spindle formation in mitotic cells with one centrosome. Furthermore, the perturbation of PLK1, a critical kinase for PCM assembly, efficiently suppressed bipolar spindle formation in mitotic cells with one centrosome. Overall, these data suggest that the centriole and PCM scaffold proteins cooperatively recruit CEP192 to spindle poles and facilitate bipolar spindle formation.

Genome editing in stem cells for genetic neurodisorders

The recent advent of genome editing techniques and their rapid improvement paved the way in establishing innovative human neurological disease models and in developing new therapeutic opportunities. Human pluripotent (both induced or naive) stem cells and neural stem cells represent versatile tools to be applied to multiple research needs and, together with genomic snip and fix tools, have recently made possible the creation of unique platforms to directly investigate several human neural affections. In this chapter, we will discuss genome engineering tools, and their recent improvements, applied to the stem cell field, focusing on how these two technologies may be pivotal instruments to deeply unravel molecular mechanisms underlying development and function, as well as disorders, of the human brain. We will review how these frontier technologies may be exploited to investigate or treat severe neurodevelopmental disorders, such as microcephaly, autism spectrum disorder, schizophrenia, as well as neurodegenerative conditions, including Parkinson's disease, Huntington's disease, Alzheimer's disease, and spinal muscular atrophy.

Identification of Abnormal Spindle Microtubule Assembly as a Promising Therapeutic Target for Osteosarcoma

Objective

To demonstrate the expression of abnormal spindle microtubule assembly (ASPM) in clinical osteosarcoma tissue specimens collected in our hospital, and to explore the function of ASPM in osteosarcoma in vitro and in vivo.

Methods

Tissue specimens from 82 cases of osteosarcoma were collected and analyzed by immunohistochemistry assay. We also investigated the relationship between ASPM expression and clinicopathological characteristics in the patients. We transfected shASPM plasmid and the empty control plasmid, respectively, and then used quantitative polymerase chain reaction and western blot analysis to detect ASPM expression. Cell colony assay and MTT were used to observe the proliferation ability. In vivo study was undertaken to explore the ASPM function further.

Results

In this study, ASPM showed high expression in osteosarcoma tissue samples compared with non‐tumor normal tissues. ASPM was positively correlated with clinical pathological characteristics, including tumor size (P = 0.024) and clinical stage (P = 0.045). Our results further showed that ASPM depletion dramatically inhibited the proliferation of osteosarcoma cells (with fewer cells in the sh‐RNA‐ASPM group compared with the control group(P < 0.05, respectively), and the in vivo assays further confirmed that ASPM ablation markedly blocked tumor growth compared with control (P < 0.05).

Conclusion

Our data provides strong evidence that the high expression of ASPM in osteosarcoma promotes proliferation in vitro and in vivo, indicating its potential role as an osteosarcoma therapeutic target.

Abnormal spindle-like microcephaly-associated protein (ASPM) contributes to the progression of Lung Squamous Cell Carcinoma (LSCC) by regulating CDK4

Lung cancer is a type of malignant tumor with high morbidity and mortality. Due to its complicated etiology and clinical manifestations, no significant therapeutic advance has been made. Lung squamous cell carcinoma (LSCC) is the most common type of lung cancer. To combat this disease, novel therapeutic targets are badly requirement. ASPM (Abnormal spindle-like microcephaly-associated protein) is involved in multiple cellular or developmental processes, such as neurogenesis and brain growth. ASPM is also reported widely expressed in multiple tumor tissues and involved in the development and progression of several cancers including lung cancer. However, the potential role on ASPM on LSCC is still unclear. In this study, we reported that ASPM was related to the poor prognosis of patients with lung squamous cell carcinoma. Our results further showed that ASPM depletion dramatically inhibited the proliferation of LSCC cells, consistent with the obviously decreased of cyclin D1(CCND1) and cyclin dependent kinases 4 (CDK4) expression. In vivo assays further confirmed ASPM ablation markedly blocked tumor growth in vivo compared with control. In addition, a co-expression was found between ASPM and CDK4 in human tumor tissues. Taken together, our data provides strong evidence that ASPM promotes lung squamous cell carcinoma proliferation in vitro and in vivo, and indicates its potential role as a LSCC therapeutic target.

Cytoskeletal Associated Filamin A and RhoA Affect Neural Progenitor Specification During Mitosis

Neural progenitor proliferation and cell fate decision from self-renewal to differentiation are crucial factors in determining brain size and morphology. The cytoskeletal dependent regulation of these processes is not entirely known. The actin-binding filamin A (FlnA) was shown to regulate proliferation of progenitors by directing changes in cell cycles proteins such as Cdk1 during G2/M phase. Here we report that functional loss of FlnA not only affects the rate of proliferation by altering cell cycle length but also causes a defect in early differentiation through changes in cell fate specification. FlnA interacts with Rho GTPase RhoA, and FlnA loss impairs RhoA activation. Disruption of either of these cytoskeletal associated proteins delays neurogenesis and promotes neural progenitors to remain in proliferative states. Aurora kinase B (Aurkb) has been implicated in cytokinesis, and peaks in expression during the G2/M phase. Inhibition of FlnA or RhoA impairs Aurkb degradation and alters its localization during mitosis. Overexpression of Aurkb replicates the same delay in neurogenesis seen with loss of FlnA or RhoA. Our findings suggest that shared cytoskeletal processes can direct neural progenitor proliferation by regulating the expression and localization of proteins that are implicated in the cell cycle progression and cell fate specification.

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.

Tubgcp3 Is Required for Retinal Progenitor Cell Proliferation During Zebrafish Development

The centrosomal protein γ-tubulin complex protein 3 (Tubgcp3/GCP3) is required for the assembly of γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs), which play critical roles in mitotic spindle formation during mitosis. However, its function in vertebrate embryonic development is unknown. Here, we generated the zebrafish tubgcp3 mutants using the CRISPR/Cas9 system and found that the tubgcp3 mutants exhibited the small eye phenotype. Tubgcp3 is required for the cell cycle progression of retinal progenitor cells (RPCs), and its depletion caused cell cycle arrest in the mitotic (M) phase. The M-phase arrested RPCs exhibited aberrant monopolar spindles and abnormal distributed centrioles and γ-tubulin. Moreover, these RPCs underwent apoptosis finally. Our study provides the in vivo model for the functional study of Tubgcp3 and sheds light on the roles of centrosomal γ-tubulin complexes in vertebrate development.

Context-dependent spindle pole focusing

The formation of a robust, bi-polar spindle apparatus, capable of accurate chromosome segregation, is a complex process requiring the co-ordinated nucleation, sorting, stabilization and organization of microtubules (MTs). Work over the last 25 years has identified protein complexes that act as functional modules to nucleate spindle MTs at distinct cellular sites such as centrosomes, kinetochores, chromatin and pre-existing MTs themselves. There is clear evidence that the extent to which these different MT nucleating pathways contribute to spindle mass both during mitosis and meiosis differs not only between organisms, but also in different cell types within an organism. This plasticity contributes the robustness of spindle formation; however, whether such plasticity is present in other aspects of spindle formation is less well understood. Here, we review the known roles of the protein complexes responsible for spindle pole focusing, investigating the evidence that these, too, act co-ordinately and differentially, depending on cellular context. We describe relationships between MT minus-end directed motors dynein and HSET/Ncd, depolymerases including katanin and MCAK, and direct minus-end binding proteins such as nuclear-mitotic apparatus protein, ASPM and Patronin/CAMSAP. We further explore the idea that the focused spindle pole acts as a non-membrane bound condensate and suggest that the metaphase spindle pole be treated as a transient organelle with context-dependent requirements for function.

Comprehensive review on the molecular genetics of autosomal recessive primary microcephaly (MCPH)

Primary microcephaly (MCPH) is an autosomal recessive sporadic neurodevelopmental ailment with a trivial head size characteristic that is below 3-4 standard deviations. MCPH is the smaller upshot of an architecturally normal brain; a significant decrease in size is seen in the cerebral cortex. At birth MCPH presents with non-progressive mental retardation, while secondary microcephaly (onset after birth) presents with and without other syndromic features. MCPH is a neurogenic mitotic syndrome nevertheless pretentious patients demonstrate normal neuronal migration, neuronal apoptosis and neural function. Eighteen MCPH loci (MCPH1-MCPH18) have been mapped to date from various populations around the world and contain the following genes: Microcephalin, WDR62, CDK5RAP2, CASC5, ASPM, CENPJ, STIL, CEP135, CEP152, ZNF335, PHC1, CDK6, CENPE, SASS6, MFSD2A, ANKLE2, CIT and WDFY3, clarifying our understanding about the molecular basis of microcephaly genetic disorder. It has previously been reported that phenotype disease is caused by MCB gene mutations and the causes of this phenotype are disarrangement of positions and organization of chromosomes during the cell cycle as a result of mutated DNA, centriole duplication, neurogenesis, neuronal migration, microtubule dynamics, transcriptional control and the cell cycle checkpoint having some invisible centrosomal process that can manage the number of neurons that are produced by neuronal precursor cells. Furthermore, researchers inform us about the clinical management of families that are suffering from MCPH. Establishment of both molecular understanding and genetic advocating may help to decrease the rate of this ailment. This current review study examines newly identified genes along with previously identified genes involved in autosomal recessive MCPH.

Dynein-Dynactin-NuMA clusters generate cortical spindle-pulling forces as a multi-arm ensemble

To position the mitotic spindle within the cell, dynamic plus ends of astral microtubules are pulled by membrane-associated cortical force-generating machinery. However, in contrast to the chromosome-bound kinetochore structure, how the diffusion-prone cortical machinery is organized to generate large spindle-pulling forces remains poorly understood. Here, we develop a light-induced reconstitution system in human cells. We find that induced cortical targeting of NuMA, but not dynein, is sufficient for spindle pulling. This spindle-pulling activity requires dynein-dynactin recruitment by NuMA’s N-terminal long arm, dynein-based astral microtubule gliding, and NuMA’s direct microtubule-binding activities. Importantly, we demonstrate that cortical NuMA assembles specialized focal structures that cluster multiple force-generating modules to generate cooperative spindle-pulling forces. This clustering activity of NuMA is required for spindle positioning, but not for spindle-pole focusing. We propose that cortical Dynein-Dynactin-NuMA (DDN) clusters act as the core force-generating machinery that organizes a multi-arm ensemble reminiscent of the kinetochore.

Almost every time a cell divides, it must share copies of its genetic material between two new daughter cells. A large molecular machine called the mitotic spindle makes this happen. The spindle is made of protein filaments known as microtubules that radiate out from two points at opposite ends of the cell. Some of these filaments attach to the genetic material in the center of the cell; some extend in the other direction and anchor the spindle to the cell membrane.

The anchoring filaments – also known as astral microtubules – can position the mitotic spindle, which controls whether the cell splits straight down the middle (to give two identically sized cells) or off-center (which gives cells of different sizes). The force required to move the spindle comes from complexes of proteins under the cell membrane that contain a molecular motor called dynein, its partner dynactin, and three other proteins – including one called NuMA. The astral microtubules interact with this force-generating machinery, but it was unclear how these proteins are arranged at the membrane.

One way to explore interactions in a protein complex is to use a light-induced reconstitution system. This technique involves molecules that will bind together whenever a light shines on them. Fusing these molecules with different proteins means that experimenters can control exactly where, and when, those proteins interact.

Okumura et al. have now used a light-induced reconstitution system to understand how the force-generating machinery positions the spindle in human cells. One of the system’s molecules was fused to a protein located at the cell membrane; the other was fused to either the dynein motor or NuMA protein. Using light to move dynein around on the membrane did not move the spindle. Yet, changing the position of NuMA, by moving the light, was enough to rotate the spindle inside the cell.

Understanding how these complexes of proteins work increases our understanding of how cells divide. Using the light-induced system to move the spindle could also reveal more about the role of symmetric and asymmetric cell division in organizing tissues. In particular, being able to manipulate the position and size of daughter cells will provide insight into how cell division shapes and maintains tissues during animal development.

Co-translational protein targeting facilitates centrosomal recruitment of PCNT during centrosome maturation in vertebrates

As microtubule-organizing centers of animal cells, centrosomes guide the formation of the bipolar spindle that segregates chromosomes during mitosis. At mitosis onset, centrosomes maximize microtubule-organizing activity by rapidly expanding the pericentriolar material (PCM). This process is in part driven by the large PCM protein pericentrin (PCNT), as its level increases at the PCM and helps recruit additional PCM components. However, the mechanism underlying the timely centrosomal enrichment of PCNT remains unclear. Here, we show that PCNT is delivered co-translationally to centrosomes during early mitosis by cytoplasmic dynein, as evidenced by centrosomal enrichment of PCNT mRNA, its translation near centrosomes, and requirement of intact polysomes for PCNT mRNA localization. Additionally, the microtubule minus-end regulator, ASPM, is also targeted co-translationally to mitotic spindle poles. Together, these findings suggest that co-translational targeting of cytoplasmic proteins to specific subcellular destinations may be a generalized protein targeting mechanism.

Before a cell divides, it creates a copy of its genetic material (DNA) and evenly distributes it between the new ‘daughter’ cells with the help of a complex called the mitotic spindle. This complex is made of long cable-like protein chains called microtubules.

To ensure that each daughter cell receives an equal amount of DNA, structures known as centrosomes organize the microtubules during the division process. Centrosomes have two rigid cores, called centrioles, which are surrounded by a matrix of proteins called the pericentriolar material. It is from this material that the microtubules are organized.

The pericentriolar material is a dynamic structure and changes its size by assembling and disassembling its protein components. The larger the pericentriolar material, the more microtubules can form. Before a cell divides, it rapidly expands in a process called centrosome maturation. A protein called pericentrin initiates the maturation by helping to recruit other proteins to the centrosome. Pericentrin molecules are large, and it takes the cell between 10 and 20 minutes to make each one. Nevertheless, the cell can produce and deliver large quantities of pericentrin to the centrosome in a matter of minutes. We do not yet know how this happens.

To investigate this further, Sepulveda, Antkowiak, Brust-Mascher et al. used advanced microscopy to study zebrafish embryos and human cells grown in the laboratory. The results showed that cells build and transport pericentrin at the same time. Cells use messenger RNA molecules as templates to build proteins. These feed into protein factories called ribosomes, which assemble the building blocks in the correct order. Rather than waiting for the pericentrin production to finish, the cell moves the active factories to the centrosome with the help of a molecular motor called dynein. By the time the pericentrin molecules are completely made by ribosomes, they are already at the centrosome, ready to help with the recruitment of other proteins during centrosome maturation.

These findings improve our understanding of centrosome maturation. The next step is to find out how the cell coordinates this process with the recruitment of other proteins to the centrosome. It is also possible that the cell uses similar processes to deliver other proteins to different parts of the cell.

Coming into Focus: Mechanisms of Microtubule Minus-End Organization

Microtubule organization has a crucial role in regulating cell architecture. The geometry of microtubule arrays strongly depends on the distribution of sites responsible for microtubule nucleation and minus-end attachment. In cycling animal cells, the centrosome often represents a dominant microtubule-organizing center (MTOC). However, even in cells with a radial microtubule system, many microtubules are not anchored at the centrosome, but are instead linked to the Golgi apparatus or other structures. Non-centrosomal microtubules predominate in many types of differentiated cell and in mitotic spindles. In this review, we discuss recent advances in understanding how the organization of centrosomal and non-centrosomal microtubule networks is controlled by proteins involved in microtubule nucleation and specific factors that recognize free microtubule minus ends and regulate their localization and dynamics.