An epigenetic regulator emerges as microtubule minus-end binding and stabilizing factor in mitosis

The Arabidopsis homolog of microspherule protein 1 is essential for embryogenesis and interacts with the Myb-like transcription factor DRMY1

The evolutionarily conserved microspherule protein 1 (MCRS1) has diverse functions, ranging from transcriptional regulation to stabilization of microtubule minus ends in acentrosomal spindles in mammals. A previous study suggested that in the model plant Arabidopsis thaliana, inactivation of an MCRS1 homolog gene led to aborted embryogenesis. To test whether this lethality was caused solely by sporophytic defects, we used the heterozygous emb1967-1/mcrs1-1 mutant for reciprocal crosses with the wild-type plant and found that the MCRS1 gene was dispensable for the development of both male and female gametophytes. An MCRS1-GFP fusion protein was expressed in the mcrs1 mutant and suppressed the mutation as evidenced by restored growth. This functional fusion protein exclusively localized to interphase nuclei and became unnoticeable during mitosis before reappearing in the reforming daughter nuclei. Affinity purification of the MCRS1-GFP protein specifically recovered the Myb-like transcription factor DRMY1 (Development Regulated Myb-like 1) but not microtubule-associated factors. Direct MCRS1-DRMY1 interaction was also demonstrated by a localization-based assay in living cells. Thus, we hypothesized that MCRS1's function was perhaps linked to transcription factors like DRMY1 and its paralog DP1 for regulation of gene expression during sporophyte development.

The role of chromatin-related epigenetic modulations in CAKUT

Congenital anomalies of the kidney and urinary tract (CAKUT) represent a major health burden in humans. Phenotypes range from renal hypoplasia or renal agenesis, cystic renal dysplasia, duplicated or horseshoe kidneys to obstruction of the ureteropelvic junction, megaureters, duplicated ureters, urethral valves or bladder malformations. Over the past decade, next-generation sequencing has identified numerous causative genes; however, the genetic basis of most cases remains unexplained. It is assumed that environmental factors have a significant impact on the phenotype, but, overall, the pathogenesis has remained poorly understood. Interestingly however, CAKUT is a common phenotypic feature in two human syndromes, Kabuki and Koolen-de Vries syndrome, caused by dysfunction of genes encoding for KMT2D and KANSL1, both members of protein complexes playing an important role in histone modifications. In this chapter, we discuss current knowledge regarding epigenetic modulation in renal development and a putatively under-recognized role of epigenetics in CAKUT.

Genetic Diversity Analysis and Identification of Candidate Genes for Growth Traits in Chengkou Mountain Chicken

Growth traits constitute critical factors in the breeding program of broiler chickens. The Chengkou mountain chicken A-lineage (CMC-A) represents a breed specifically bred for meat production. To further explore the growth performance of the CMC-A population, this study conducted whole-genome sequencing on 464 CMC-A roosters to systematically evaluate their genetic diversity. Additionally, runs of homozygosity (ROH) islands and genome-wide association studies (GWASs) were employed to identify the loci and functional genes influencing the growth traits in Chengkou mountain chickens. The results revealed a high level of genetic diversity and low levels of inbreeding in Chengkou mountain chickens. Several genes associated with stress resistance, muscle growth, and fat deposition were pinpointed through ROH island identification. Moreover, 52 SNP loci were detected, along with 71 candidate genes. These findings enhance our understanding of the genetic architecture underlying the growth traits in Chengkou mountain chickens and provide a theoretical foundation for subsequent breeding endeavors.

CAMSAP3 forms dimers via its α-helix domain that directly stabilize non-centrosomal microtubule minus ends

Microtubules are vital components of the cytoskeleton. Their plus ends are dynamic and respond to changes in cell morphology, whereas the minus ends are stable and serve a crucial role in microtubule seeding and maintaining spatial organization. In mammalian cells, the calmodulin-regulated spectrin-associated proteins (CAMSAPs), play a key role in directly regulating the dynamics of non-centrosomal microtubules minus ends. However, the molecular mechanisms are not yet fully understood. Our study reveals that CAMSAP3 forms dimers through its C-terminal α-helix; this dimerization not only enhances the microtubule-binding affinity of the CKK domain but also enables the CKK domain to regulate the dynamics of microtubules. Furthermore, CAMSAP3 also specializes in decorating at the minus end of microtubules through the combined action of the microtubule-binding domain (MBD) and the C-terminal α-helix, thereby achieving dynamic regulation of the minus ends of microtubules. These findings are crucial for advancing our understanding and treatment of diseases associated with non-centrosomal microtubules.

KAT8 beyond Acetylation: A Survey of Its Epigenetic Regulation, Genetic Variability, and Implications for Human Health

Lysine acetyltransferase 8, also known as KAT8, is an enzyme involved in epigenetic regulation, primarily recognized for its ability to modulate histone acetylation. This review presents an overview of KAT8, emphasizing its biological functions, which impact many cellular processes and range from chromatin remodeling to genetic and epigenetic regulation. In many model systems, KAT8's acetylation of histone H4 lysine 16 (H4K16) is critical for chromatin structure modification, which influences gene expression, cell proliferation, differentiation, and apoptosis. Furthermore, this review summarizes the observed genetic variability within the KAT8 gene, underscoring the implications of various single nucleotide polymorphisms (SNPs) that affect its functional efficacy and are linked to diverse phenotypic outcomes, ranging from metabolic traits to neurological disorders. Advanced insights into the structural biology of KAT8 reveal its interaction with multiprotein assemblies, such as the male-specific lethal (MSL) and non-specific lethal (NSL) complexes, which regulate a wide range of transcriptional activities and developmental functions. Additionally, this review focuses on KAT8's roles in cellular homeostasis, stem cell identity, DNA damage repair, and immune response, highlighting its potential as a therapeutic target. The implications of KAT8 in health and disease, as evidenced by recent studies, affirm its importance in cellular physiology and human pathology.

Loss of KANSL3 leads to defective inner cell mass and early embryonic lethality

e-Lysine acetylation is a prominent histone mark found at transcriptionally active loci. Among many lysine acetyl transferases, nonspecific lethal complex (NSL) members are known to mediate the modification of histone H4. In addition to histone modifications, the KAT8 regulatory complex subunit 3 gene (Kansl3), a core member of NSL complex, has been shown to be involved in several other cellular processes such as mitosis and mitochondrial activity. Although functional studies have been performed on NSL complex members, none of the four core proteins, including Kansl3, have been studied during early mouse development. Here we show that homozygous knockout Kansl3 embryos are lethal at peri-implantation stages, failing to hatch out of the zona pellucida. When the zona pellucida is removed in vitro, Kansl3 null embryos form an abnormal outgrowth with significantly disrupted inner cell mass (ICM) morphology. We document lineage-specific defects at the blastocyst stage with significantly reduced ICM cell number but no difference in trophectoderm cell numbers. Both epiblast and primitive endoderm lineages are altered with reduced cell numbers in null mutants. These results show that Kansl3 is indispensable during early mouse embryonic development and with defects in both ICM and trophectoderm lineages.

Late relapse of chronic myeloid leukemia after allogeneic bone marrow transplantation points to KANSARL (KANSL1::ARL17A) alteration: a case report with insights on the molecular landscape

Chronic myeloid leukemia is a myeloproliferative neoplasm characterized by the presence of the Philadelphia chromosome and the consequent BCR::ABL1 oncoprotein. In the era before the introduction of tyrosine kinase inhibitors (TKIs), the only potentially curative treatment was allogeneic hematopoietic stem cell transplantation (HSCT). Here, we present the case of a patient affected by CML who experienced a relapse 20 years after allogeneic HSCT. Following relapse, the patient was treated with imatinib and bosutinib, resulting in a deep molecular response and successfully discontinued treatment. Additional analysis including whole-exome sequencing and RNA sequencing provided some insights on the molecular mechanisms of the relapse: the identification of the fusion transcript KANSL1::ARL17A (KANSARL), a cancer predisposition fusion gene, could justify a condition of genomic instability which may be associated with the onset and/or probably the late relapse of his CML.

Transcriptional regulation by the NSL complex enables diversification of IFT functions in ciliated versus nonciliated cells

Members of the NSL histone acetyltransferase complex are involved in multiorgan developmental syndromes. While the NSL complex is known for its importance in early development, its role in fully differentiated cells remains enigmatic. Using a kidney-specific model, we discovered that deletion of NSL complex members KANSL2 or KANSL3 in postmitotic podocytes led to catastrophic kidney dysfunction. Systematic comparison of two primary differentiated cell types reveals the NSL complex as a master regulator of intraciliary transport genes in both dividing and nondividing cells. NSL complex ablation led to loss of cilia and impaired sonic hedgehog pathway in ciliated fibroblasts. By contrast, nonciliated podocytes responded with altered microtubule dynamics and obliterated podocyte functions. Finally, overexpression of wild-type but not a double zinc finger (ZF-ZF) domain mutant of KANSL2 rescued the transcriptional defects, revealing a critical function of this domain in NSL complex assembly and function. Thus, the NSL complex exhibits bifurcation of functions to enable diversity of specialized outcomes in differentiated cells.

Effects of urban living environments on mental health in adults

Urban-living individuals are exposed to many environmental factors that may combine and interact to influence mental health. While individual factors of an urban environment have been investigated in isolation, no attempt has been made to model how complex, real-life exposure to living in the city relates to brain and mental health, and how this is moderated by genetic factors. Using the data of 156,075 participants from the UK Biobank, we carried out sparse canonical correlation analyses to investigate the relationships between urban environments and psychiatric symptoms. We found an environmental profile of social deprivation, air pollution, street network and urban land-use density that was positively correlated with an affective symptom group (r = 0.22, Pperm < 0.001), mediated by brain volume differences consistent with reward processing, and moderated by genes enriched for stress response, including CRHR1, explaining 2.01% of the variance in brain volume differences. Protective factors such as greenness and generous destination accessibility were negatively correlated with an anxiety symptom group (r = 0.10, Pperm < 0.001), mediated by brain regions necessary for emotion regulation and moderated by EXD3, explaining 1.65% of the variance. The third urban environmental profile was correlated with an emotional instability symptom group (r = 0.03, Pperm < 0.001). Our findings suggest that different environmental profiles of urban living may influence specific psychiatric symptom groups through distinct neurobiological pathways.

Mcrs1 regulates G2/M transition and spindle assembly during mouse oocyte meiosis

Microspherule protein 1 (Mcrs1) is a component of the nonspecific lethal (NSL) complex and the chromatin remodeling INO80 complex, which participates in transcriptional regulation during mitosis. Here, we investigate the roles of Mcrs1 during female meiosis in mice. We demonstrate that Mcrs1 is a novel regulator of the meiotic G2/M transition and spindle assembly in mouse oocytes. Mcrs1 is present in the nucleus and associates with spindle poles and chromosomes of oocytes during meiosis I. Depletion of Mcrs1 alters HDAC2-mediated H4K16ac, H3K4me2, and H3K9me2 levels in nonsurrounded nucleolus (NSN)-type oocytes, and reduces CDK1 activity and cyclin B1 accumulation, leading to G2/M transition delay. Furthermore, Mcrs1 depletion results in abnormal spindle assembly due to reduced Aurora kinase (Aurka and Aurkc) and Kif2A activities, suggesting that Mcrs1 also plays a transcription-independent role in regulation of metaphase I oocytes. Taken together, our results demonstrate that the transcription factor Mcrs1 has important roles in cell cycle regulation and spindle assembly in mouse oocyte meiosis.

MCRS1 modulates the heterogeneity of microtubule minus-end morphologies in mitotic spindles

Faithful chromosome segregation requires the assembly of a bipolar spindle, consisting of two antiparallel microtubule (MT) arrays having most of their minus ends focused at the spindle poles and their plus ends overlapping in the spindle midzone. Spindle assembly, chromosome alignment, and segregation require highly dynamic MTs. The plus ends of MTs have been extensively investigated but their minus-end structure remains poorly characterized. Here, we used large-scale electron tomography to study the morphology of the MT minus ends in three dimensionally reconstructed metaphase spindles in HeLa cells. In contrast to the homogeneous open morphology of the MT plus ends at the kinetochores, we found that MT minus ends are heterogeneous, showing either open or closed morphologies. Silencing the minus end-specific stabilizer, MCRS1 increased the proportion of open MT minus ends. Altogether, these data suggest a correlation between the morphology and the dynamic state of the MT ends. Taking this heterogeneity of the MT minus-end morphologies into account, our work indicates an unsynchronized behavior of MTs at the spindle poles, thus laying the groundwork for further studies on the complexity of MT dynamics regulation.

The genomic stability regulator PTIP is required for proper chromosome segregation in mitosis

Background

The Pax transcription activation domain-interacting protein (PTIP) is a nuclear protein that is an essential component of H3K4 methylation for gene activation in vascular, kidney, B cell, and adipocyte development. Furthermore, it plays a key role in genomic stability in higher eukaryotic cells. It binds to 53BP1 and antagonizes inappropriate homologous recombination for a proper DNA damage response. Interestingly, an early study reported mitotic defects after PTIP inactivation, but it is not clear whether PTIP directly facilitates mitotic processes.

Results

Here, we showed that PTIP is essential for the mitotic integrity of HeLa cells. PTIP inactivation increases cell death during mitotic exit, which appears to result from direct mitotic defects. PTIP inactivation did not affect the G2M DNA damage checkpoint during interphase upon etoposide treatment. However, in mitosis, PTIP inactivation results in prolonged mitotic time, inefficient chromosome alignment, and increased cell death. Furthermore, PTIP localizes to the mitotic centrosome via BRCT domains at the C-terminus.

Conclusion

This study reveals a novel function of PTIP in maintaining the genomic stability of higher eukaryotes during mitosis. Therefore, its deregulation, which occurs in various tumors, may destabilize the genome by introducing an abnormal DNA damage response, as well as erroneous chromosome segregation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13008-022-00081-4.

Genomic architecture and functional effects of potential human inversion supergenes

Supergenes are involved in adaptation in multiple organisms, but they are little known in humans. Genomic inversions are the most common mechanism of supergene generation and maintenance. Here, we review the information about two large inversions that are the best examples of potential human supergenes. In addition, we do an integrative analysis of the newest data to understand better their functional effects and underlying genetic changes. We have found that the highly divergent haplotypes of the 17q21.31 inversion of approximately 1.5 Mb have multiple phenotypic associations, with consistent effects in brain-related traits, red and white blood cells, lung function, male and female characteristics and disease risk. By combining gene expression and nucleotide variation data, we also analysed the molecular differences between haplotypes, including gene duplications, amino acid substitutions and regulatory changes, and identify CRHR1, KANLS1 and MAPT as good candidates to be responsible for these phenotypes. The situation is more complex for the 8p23.1 inversion, where there is no clear genetic differentiation. However, the inversion is associated with several related phenotypes and gene expression differences that could be linked to haplotypes specific of one orientation. Our work, therefore, contributes to the characterization of both exceptional variants and illustrates the important role of inversions. This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.

Genetic Control of Kinetochore-Driven Microtubule Growth in Drosophila Mitosis

Centrosome-containing cells assemble their spindles exploiting three main classes of microtubules (MTs): MTs nucleated by the centrosomes, MTs generated near the chromosomes/kinetochores, and MTs nucleated within the spindle by the augmin-dependent pathway. Mammalian and Drosophila cells lacking the centrosomes generate MTs at kinetochores and eventually form functional bipolar spindles. However, the mechanisms underlying kinetochore-driven MT formation are poorly understood. One of the ways to elucidate these mechanisms is the analysis of spindle reassembly following MT depolymerization. Here, we used an RNA interference (RNAi)-based reverse genetics approach to dissect the process of kinetochore-driven MT regrowth (KDMTR) after colcemid-induced MT depolymerization. This MT depolymerization procedure allows a clear assessment of KDMTR, as colcemid disrupts centrosome-driven MT regrowth but not KDMTR. We examined KDMTR in normal Drosophila S2 cells and in S2 cells subjected to RNAi against conserved genes involved in mitotic spindle assembly: mast/orbit/chb (CLASP1), mei-38 (TPX2), mars (HURP), dgt6 (HAUS6), Eb1 (MAPRE1/EB1), Patronin (CAMSAP2), asp (ASPM), and Klp10A (KIF2A). RNAi-mediated depletion of Mast/Orbit, Mei-38, Mars, Dgt6, and Eb1 caused a significant delay in KDMTR, while loss of Patronin had a milder negative effect on this process. In contrast, Asp or Klp10A deficiency increased the rate of KDMTR. These results coupled with the analysis of GFP-tagged proteins (Mast/Orbit, Mei-38, Mars, Eb1, Patronin, and Asp) localization during KDMTR suggested a model for kinetochore-dependent spindle reassembly. We propose that kinetochores capture the plus ends of MTs nucleated in their vicinity and that these MTs elongate at kinetochores through the action of Mast/Orbit. The Asp protein binds the MT minus ends since the beginning of KDMTR, preventing excessive and disorganized MT regrowth. Mei-38, Mars, Dgt6, Eb1, and Patronin positively regulate polymerization, bundling, and stabilization of regrowing MTs until a bipolar spindle is reformed.

Two distinct males absent on the first (MOF)-containing histone acetyltransferases are involved in the epithelial-mesenchymal transition in different ways in human cells

Human males absent on the first (MOF), a histone acetyltransferase (HAT), forms male-specific lethal (MSL) and non-specific lethal (NSL), two multiprotein HATs, in cells. MSL was originally discovered in dosage compensation study in Drosophila that can specifically acetylate H4K16, while NSL can simultaneously catalyze the H4 at K5, K8, and K16 sites. However, comparative studies of the two HATs in regulating specific biological functions are rarely reported. Here, we present evidence to argue that MSL and NSL function in different ways in the epithelial-to-mesenchymal transition (EMT) process. At first, CRISPR/Cas9-mediated MSL1 (a key subunit of the MSL)-knockout (KO) and NSL3 (a key subunit of the NSL)-KO cells seem to prefer to grow in clusters. Interestingly, the former promotes cell survival and clonal formation, while the latter has the opposite effect on it. Cell staining revealed that MSL1-KO leads to multipolarized spindles, while NSL3-KO causes more lumen-like cells. Furthermore, in Transwell experiments, silencing of MSL1 promotes cell invasion in 293 T, MCF-7, and MDA-MB-231 cells. In contrast, the inhibitory effects on cell invasion are observed in the same NSL3-silenced cells. Consistent with this, mesenchymal biomarkers, like N-cadherin, vimentin, and snail, are negatively correlated with the expression level of MSL1; however, a positive relationship between these proteins and NSL3 in cells has been found. Further studies have clarified that MSL1, but not NSL3, can specifically bind to the E-box-containing Snail promoter region and thereby negatively regulate Snail transactivation. Also, silencing of MSL1 promotes the lung metastasis of B16F10 melanoma cells in mice. Finally, ChIP-Seq analysis indicated that the NSL may be mainly involved in phosphoinositide-mediated signaling pathways. Taken together, the MOF-containing MSL and NSL HATs may regulate the EMT process in different ways in order to respond to different stimuli.

Kansl1 haploinsufficiency impairs autophagosome-lysosome fusion and links autophagic dysfunction with Koolen-de Vries syndrome in mice

Koolen-de Vries syndrome (KdVS) is a rare disorder caused by haploinsufficiency of KAT8 regulatory NSL complex subunit 1 (KANSL1), which is characterized by intellectual disability, heart failure, hypotonia, and congenital malformations. To date, no effective treatment has been found for KdVS, largely due to its unknown pathogenesis. Using siRNA screening, we identified KANSL1 as an essential gene for autophagy. Mechanistic study shows that KANSL1 modulates autophagosome-lysosome fusion for cargo degradation via transcriptional regulation of autophagosomal gene, STX17. Kansl1^+/− mice exhibit impairment in the autophagic clearance of damaged mitochondria and accumulation of reactive oxygen species, thereby resulting in defective neuronal and cardiac functions. Moreover, we discovered that the FDA-approved drug 13-cis retinoic acid can reverse these mitophagic defects and neurobehavioral abnormalities in Kansl1^+/− mice by promoting autophagosome-lysosome fusion. Hence, these findings demonstrate a critical role for KANSL1 in autophagy and indicate a potentially viable therapeutic strategy for KdVS.

Here the authors show that the Koolen-de Vries syndrome associated gene KANSL1 modulates autophagosome-lysosome fusion via transcriptional regulation of autophagosomal gene Syntaxin17, and that 13-cis retinoic acid can reverses mitophagic defects and neurobehavioural abnormalities of mice lacking Kansl1.

Modulation of cellular processes by histone and non-histone protein acetylation

Lysine acetylation is a widespread and versatile protein post-translational modification. Lysine acetyltransferases and lysine deacetylases catalyse the addition or removal, respectively, of acetyl groups at both histone and non-histone targets. In this Review, we discuss several features of acetylation and deacetylation, including their diversity of targets, rapid turnover, exquisite sensitivity to the concentrations of the cofactors acetyl-CoA, acyl-CoA and NAD⁺, and tight interplay with metabolism. Histone acetylation and non-histone protein acetylation influence a myriad of cellular and physiological processes, including transcription, phase separation, autophagy, mitosis, differentiation and neural function. The activity of lysine acetyltransferases and lysine deacetylases can, in turn, be regulated by metabolic states, diet and specific small molecules. Histone acetylation has also recently been shown to mediate cellular memory. These features enable acetylation to integrate the cellular state with transcriptional output and cell-fate decisions.

Various effects of two types of kinesin-5 inhibitors on mitosis and cell proliferation

Kinesin-5 has received considerable attention as a new target for mitosis. Various small-molecule compounds targeting kinesin-5 have been developed in the last few decades. However, the differences in the cellular effects of kinesin-5 inhibitors remain poorly understood. Here, we used two different kinesin-5 inhibitors, biphenyl-type PVZB1194 and S-trityl-L-cysteine-type PVEI0021, to examine their effects on molecular events involving kinesin-5. Our biochemical study of kinesin-5 protein-protein interactions showed that PVZB1194-treated kinesin-5 interacted with TPX2 microtubule nucleation factor, Aurora-A kinase, receptor for hyaluronan-mediated motility, and γ-tubulin, as did untreated mitotic kinesin-5. However, PVEI0021 prevented kinesin-5 from binding to these proteins. In mitotic HeLa cells recovered from nocodazole inhibition, kinesin-5 colocalized with these binding proteins, along with microtubules nucleated near kinetochores. By acting on kinesin-5 interactions with chromatin-associated microtubules, PVZB1194, rather than PVEI0021, not only affected the formation of dispersed microtubule clusters but also enhanced the stability of microtubules. In addition, screening for mitotic inhibitors working synergistically with the kinesin-5 inhibitors revealed that paclitaxel synergistically inhibited HeLa cell proliferation only with PVZB1194. In contrast, the Aurora-A inhibitor MLN8237 exerted a synergistic anti-cell proliferation effect when combined with either inhibitor. Together, these results have provided a better understanding of the molecular action of kinesin-5 inhibitors and indicate their usefulness as molecular tools for the study of mitosis and the development of anticancer agents.

Imbalanced autophagy causes synaptic deficits in a human model for neurodevelopmental disorders

Macroautophagy (hereafter referred to as autophagy) is a finely tuned process of programmed degradation and recycling of proteins and cellular components, which is crucial in neuronal function and synaptic integrity. Mounting evidence implicates chromatin remodeling in fine-tuning autophagy pathways. However, this epigenetic regulation is poorly understood in neurons. Here, we investigate the role in autophagy of KANSL1, a member of the nonspecific lethal complex, which acetylates histone H4 on lysine 16 (H4K16ac) to facilitate transcriptional activation. Loss-of-function of KANSL1 is strongly associated with the neurodevelopmental disorder Koolen-de Vries Syndrome (KdVS). Starting from KANSL1-deficient human induced-pluripotent stem cells, both from KdVS patients and genome-edited lines, we identified SOD1 (superoxide dismutase 1), an antioxidant enzyme, to be significantly decreased, leading to a subsequent increase in oxidative stress and autophagosome accumulation. In KANSL1-deficient neurons, autophagosome accumulation at excitatory synapses resulted in reduced synaptic density, reduced GRIA/AMPA receptor-mediated transmission and impaired neuronal network activity. Furthermore, we found that increased oxidative stress-mediated autophagosome accumulation leads to increased MTOR activation and decreased lysosome function, further preventing the clearing of autophagosomes. Finally, by pharmacologically reducing oxidative stress, we could rescue the aberrant autophagosome formation as well as synaptic and neuronal network activity in KANSL1-deficient neurons. Our findings thus point toward an important relation between oxidative stress-induced autophagy and synapse function, and demonstrate the importance of H4K16ac-mediated changes in chromatin structure to balance reactive oxygen species- and MTOR-dependent autophagy.

Abbreviations: APO: apocynin; ATG: autophagy related; BAF: bafilomycin A₁; BSO: buthionine sulfoximine; CV: coefficient of variation; DIV: days in vitro; H4K16ac: histone 4 lysine 16 acetylation; iPSC: induced-pluripotent stem cell; KANSL1: KAT8 regulatory NSL complex subunit 1; KdVS: Koolen-de Vries Syndrome; LAMP1: lysosomal associated membrane protein 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MEA: micro-electrode array; MTOR: mechanistic target of rapamycin kinase; NSL complex: nonspecific lethal complex; 8-oxo-dG: 8-hydroxydesoxyguanosine; RAP: rapamycin; ROS: reactive oxygen species; sEPSCs: spontaneous excitatory postsynaptic currents; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; SYN: synapsin; WRT: wortmannin.

The metaphase spindle at steady state - Mechanism and functions of microtubule poleward flux

The mitotic spindle is a bipolar cellular structure, built from tubulin polymers, called microtubules, and interacting proteins. This macromolecular machine orchestrates chromosome segregation, thereby ensuring accurate distribution of genetic material into the two daughter cells during cell division. Powered by GTP hydrolysis upon tubulin polymerization, the microtubule ends exhibit a metastable behavior known as the dynamic instability, during which they stochastically switch between the growth and shrinkage phases. In the context of the mitotic spindle, dynamic instability is furthermore regulated by microtubule-associated proteins and motor proteins, which enables the spindle to undergo profound changes during mitosis. This highly dynamic behavior is essential for chromosome capture and congression in prometaphase, as well as for chromosome alignment to the spindle equator in metaphase and their segregation in anaphase. In this review we focus on the mechanisms underlying microtubule dynamics and sliding and their importance for the maintenance of shape, structure and dynamics of the metaphase spindle. We discuss how these spindle properties are related to the phenomenon of microtubule poleward flux, highlighting its highly cooperative molecular basis and role in keeping the metaphase spindle at a steady state.

Moonlighting in Mitosis: Analysis of the Mitotic Functions of Transcription and Splicing Factors

Moonlighting proteins can perform one or more additional functions besides their primary role. It has been posited that a protein can acquire a moonlighting function through a gradual evolutionary process, which is favored when the primary and secondary functions are exerted in different cellular compartments. Transcription factors (TFs) and splicing factors (SFs) control processes that occur in interphase nuclei and are strongly reduced during cell division, and are therefore in a favorable situation to evolve moonlighting mitotic functions. However, recently published moonlighting protein databases, which comprise almost 400 proteins, do not include TFs and SFs with secondary mitotic functions. We searched the literature and found several TFs and SFs with bona fide moonlighting mitotic functions, namely they localize to specific mitotic structure(s), interact with proteins enriched in the same structure(s), and are required for proper morphology and functioning of the structure(s). In addition, we describe TFs and SFs that localize to mitotic structures but cannot be classified as moonlighting proteins due to insufficient data on their biochemical interactions and mitotic roles. Nevertheless, we hypothesize that most TFs and SFs with specific mitotic localizations have either minor or redundant moonlighting functions, or are evolving towards the acquisition of these functions.

A liquid-like spindle domain promotes acentrosomal spindle assembly in mammalian oocytes

Mammalian oocytes segregate chromosomes with a microtubule spindle that lacks centrosomes, but the mechanisms by which acentrosomal spindles are organized and function are largely unclear. In this study, we identify a conserved subcellular structure in mammalian oocytes that forms by phase separation. This structure, which we term the liquid-like meiotic spindle domain (LISD), permeates the spindle poles and forms dynamic protrusions that extend well beyond the spindle. The LISD selectively concentrates multiple microtubule regulatory factors and allows them to diffuse rapidly within the spindle volume. Disruption of the LISD via different means disperses these factors and leads to severe spindle assembly defects. Our data suggest a model whereby the LISD promotes meiotic spindle assembly by serving as a reservoir that sequesters and mobilizes microtubule regulatory factors in proximity to spindle microtubules.

RNAi-mediated depletion of the NSL complex subunits leads to abnormal chromosome segregation and defective centrosome duplication in Drosophila mitosis

The Drosophila Nonspecific Lethal (NSL) complex is a major transcriptional regulator of housekeeping genes. It contains at least seven subunits that are conserved in the human KANSL complex: Nsl1/Wah (KANSL1), Dgt1/Nsl2 (KANSL2), Rcd1/Nsl3 (KANSL3), Rcd5 (MCRS1), MBD-R2 (PHF20), Wds (WDR5) and Mof (MOF/KAT8). Previous studies have shown that Dgt1, Rcd1 and Rcd5 are implicated in centrosome maintenance. Here, we analyzed the mitotic phenotypes caused by RNAi-mediated depletion of Rcd1, Rcd5, MBD-R2 or Wds in greater detail. Depletion of any of these proteins in Drosophila S2 cells led to defects in chromosome segregation. Consistent with these findings, Rcd1, Rcd5 and MBD-R2 RNAi cells showed reduced levels of both Cid/CENP-A and the kinetochore component Ndc80. In addition, RNAi against any of the four genes negatively affected centriole duplication. In Wds-depleted cells, the mitotic phenotypes were similar but milder than those observed in Rcd1-, Rcd5- or MBD-R2-deficient cells. RT-qPCR experiments and interrogation of published datasets revealed that transcription of many genes encoding centromere/kinetochore proteins (e.g., cid, Mis12 and Nnf1b), or involved in centriole duplication (e.g., Sas-6, Sas-4 and asl) is substantially reduced in Rcd1, Rcd5 and MBD-R2 RNAi cells, and to a lesser extent in wds RNAi cells. During mitosis, both Rcd1-GFP and Rcd5-GFP accumulate at the centrosomes and the telophase midbody, MBD-R2-GFP is enriched only at the chromosomes, while Wds-GFP accumulates at the centrosomes, the kinetochores, the midbody, and on a specific chromosome region. Collectively, our results suggest that the mitotic phenotypes caused by Rcd1, Rcd5, MBD-R2 or Wds depletion are primarily due to reduced transcription of genes involved in kinetochore assembly and centriole duplication. The differences in the subcellular localizations of the NSL components may reflect direct mitotic functions that are difficult to detect at the phenotypic level, because they are masked by the transcription-dependent deficiency of kinetochore and centriolar proteins.

The Drosophila Nonspecific Lethal (NSL) complex is a conserved protein assembly that controls transcription of more than 4,000 housekeeping genes. We analyzed the mitotic functions of four genes, Rcd1, Rcd5, MBD-R2 and wds, encoding NSL subunits. Inactivation of these genes by RNA interference (RNAi) resulted in defects in both chromosome segregation and centrosome duplication. Our analyses indicate that RNAi against Rcd1, Rcd5 or MBD-R2 reduces transcription of genes involved in centromere/kinetochore assembly and centriole replication. During interphase, Rcd1, Rcd5, MBD-R2 and Wds are confined to the nucleus, as expected for transcription factors. However, during mitosis each of these proteins relocates to specific mitotic structures. Our results suggest that the four NSL components work together as a complex to stimulate transcription of genes encoding important mitotic determinants. However, the different localization of the proteins during mitosis suggests that they might have acquired secondary “moonlighting” functions that directly contribute to the mitotic process.

The non-specific lethal (NSL) complex at the crossroads of transcriptional control and cellular homeostasis

The functionality of chromatin is tightly regulated by post-translational modifications that modulate transcriptional output from target loci. Among the post-translational modifications of chromatin, reversible ε-lysine acetylation of histone proteins is prominent at transcriptionally active genes. Lysine acetylation is catalyzed by lysine acetyltransferases (KATs), which utilize the central cellular metabolite acetyl-CoA as their substrate. Among the KATs that mediate lysine acetylation, males absent on the first (MOF/KAT8) is particularly notable for its ability to acetylate histone 4 lysine 16 (H4K16ac), a modification that decompacts chromatin structure. MOF and its non-specific lethal (NSL) complex members have been shown to localize to gene promoters and enhancers in the nucleus, as well as to microtubules and mitochondria to regulate key cellular processes. Highlighting their importance, mutations or deregulation of NSL complex members has been reported in both human neurodevelopmental disorders and cancer. Based on insight gained from studies in human, mouse, and Drosophila model systems, this review discusses the role of NSL-mediated lysine acetylation in a myriad of cellular functions in both health and disease. Through these studies, the importance of the NSL complex in regulating core transcriptional and signaling networks required for normal development and cellular homeostasis is beginning to emerge.

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.

Detection of novel fusion-transcripts by RNA-Seq in T-cell lymphoblastic lymphoma

Fusions transcripts have been proven to be strong drivers for neoplasia-associated mutations, although their incidence in T-cell lymphoblastic lymphoma needs to be determined yet. Using RNA-Seq we have selected 55 fusion transcripts identified by at least two of three detection methods in the same tumour. We confirmed the existence of 24 predicted novel fusions that had not been described in cancer or normal tissues yet, indicating the accuracy of the prediction. Of note, one of them involves the proto oncogene TAL1. Other confirmed fusions could explain the overexpression of driver genes such as COMMD3-BMI1, LMO1 or JAK3. Five fusions found exclusively in tumour samples could be considered pathogenic (NFYG-TAL1, RIC3-TCRBC2, SLC35A3-HIAT1, PICALM MLLT10 and MLLT10-PICALM). However, other fusions detected simultaneously in normal and tumour samples (JAK3-INSL3, KANSL1-ARL17A/B and TFG-ADGRG7) could be germ-line fusions genes involved in tumour-maintaining tasks. Notably, some fusions were confirmed in more tumour samples than predicted, indicating that the detection methods underestimated the real number of existing fusions. Our results highlight the potential of RNA-Seq to identify new cryptic fusions, which could be drivers or tumour-maintaining passenger genes. Such novel findings shed light on the searching for new T-LBL biomarkers in these haematological disorders.

Mps1 regulates spindle morphology through MCRS1 to promote chromosome alignment

Accurate partitioning of chromosomes during mitosis is essential for genetic stability and requires the assembly of the dynamic mitotic spindle and proper kinetochore–microtubule attachment. The spindle assembly checkpoint (SAC) monitors the incompleteness and errors in kinetochore–microtubule attachment and delays anaphase. The SAC kinase Mps1 regulates the recruitment of downstream effectors to unattached kinetochores. Mps1 also actively promotes chromosome alignment during metaphase, but the underlying mechanism is not completely understood. Here, we show that Mps1 regulates chromosome alignment through MCRS1, a spindle assembly factor that controls the dynamics of the minus end of kinetochore microtubules. Mps1 binds and phosphorylates MCRS1. This mechanism enables KIF2A localization to the minus end of spindle microtubules. Thus, our study reveals a novel role of Mps1 in regulating the dynamics of the minus end of microtubules and expands the functions of Mps1 in genome maintenance.

Risk gene-set and pathways in 22q11.2 deletion-related schizophrenia: a genealogical molecular approach

The 22q11.2 deletion is a strong, but insufficient, "first hit" genetic risk factor for schizophrenia (SZ). We attempted to identify "second hits" from the entire genome in a unique multiplex 22q11.2 deletion syndrome (DS) family. Bioinformatic analysis of whole-exome sequencing and comparative-genomic hybridization array identified de novo and inherited, rare and damaging variants, including copy number variations, outside the 22q11.2 region. A specific 22q11.2-haplotype was associated with psychosis. The interaction of the identified "second hits" with the 22q11.2 haploinsufficiency may affect neurodevelopmental processes, including neuron projection, cytoskeleton activity, and histone modification in 22q11.2DS-ralated psychosis. A larger load of variants, involved in neurodevelopment, in combination with additional molecular events that affect sensory perception, olfactory transduction and G-protein-coupled receptor signaling may account for the development of 22q11.2DS-related SZ. Comprehensive analysis of multiplex families is a promising approach to the elucidation of the molecular pathophysiology of 22q11.2DS-related SZ with potential relevance to treatment.

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.

Animal Female Meiosis: The Challenges of Eliminating Centrosomes

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

Genomic convergence of locus-based GWAS meta-analysis identifies AXIN1 as a novel Parkinson's gene

Parkinson's disease (PD) is a common, disabling neurodegenerative disorder with significant genetic underpinnings. Multiple genome-wide association studies (GWAS) have been conducted with identification of several PD loci. However, these only explain about 25% of PD genetic risk indicating that additional loci of modest effect remain to be discovered. Association clustering methods such as gene-based tests are more powerful than single-variant analysis for identifying modest genetic effects. Combined with the locus-based algorithm, OASIS, the most significant association signals can be homed in. Here, two dbGAP GWAS datasets (7415 subjects (2750 PD and 4845 controls) genotyped for 0.78 million SNPs) were analyzed using combined clustering algorithms to identify 88 PD candidate genes in 24 loci. These were further investigated for gene expression in substantia nigra (SN) of PD and control subjects on GEO datasets. Expression differences were also assessed in normal brains SN versus white matter on BRAINEAC datasets. This genetic and functional analysis identified AXIN1, a key regulator of Wnt/β-catenin signaling, as a novel PD gene. This finding links PD with inflammation. Other significantly associated genes were CSMD1, CLDN1, ZNF141, ZNF721, RHOT2, RICTOR, KANSL1, and ARHGAP27. Novel PD genes were identified using genomic convergence of gene-wide and locus-based tests and expression studies on archived datasets.

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.

Moonlighting with WDR5: A Cellular Multitasker

WDR5 is a highly conserved WD40 repeat-containing protein that is essential for proper regulation of multiple cellular processes. WDR5 is best characterized as a core scaffolding component of histone methyltransferase complexes, but emerging evidence demonstrates that it does much more, ranging from expanded functions in the nucleus through to controlling the integrity of cell division. The purpose of this review is to describe the current molecular understandings of WDR5, discuss how it participates in diverse cellular processes, and highlight drug discovery efforts around WDR5 that may form the basis of new anti-cancer therapies.

NuMA recruits dynein activity to microtubule minus-ends at mitosis

To build the spindle at mitosis, motors exert spatially regulated forces on microtubules. We know that dynein pulls on mammalian spindle microtubule minus-ends, and this localized activity at ends is predicted to allow dynein to cluster microtubules into poles. How dynein becomes enriched at minus-ends is not known. Here, we use quantitative imaging and laser ablation to show that NuMA targets dynactin to minus-ends, localizing dynein activity there. NuMA is recruited to new minus-ends independently of dynein and more quickly than dynactin; both NuMA and dynactin display specific, steady-state binding at minus-ends. NuMA localization to minus-ends involves a C-terminal region outside NuMA’s canonical microtubule-binding domain and is independent of minus-end binders γ-TuRC, CAMSAP1, and KANSL1/3. Both NuMA’s minus-end-binding and dynein-dynactin-binding modules are required to rescue focused, bipolar spindle organization. Thus, NuMA may serve as a mitosis-specific minus-end cargo adaptor, targeting dynein activity to minus-ends to cluster spindle microtubules into poles.

Every time a cell divides, it needs to duplicate its DNA and evenly distribute it between the two new ‘daughter’ cells. To move and distribute DNA, the cell builds a large machine called a spindle, which is made of stiff cables called microtubules.

Many proteins, including a motor called dynein, help to organize the spindle’s microtubules. One of dynein’s jobs is to cluster all microtubules at the two tips of the spindle, pulling them into shape. Without this clustering, spindles have the wrong shape and structure and can make mistakes when moving DNA.

Microtubules have a ‘plus’ end and a ‘minus’ end, and motor proteins usually only travel in one specified direction. Dynein, for example, moves towards the minus end of microtubules, which is where most of the clustering happens. It can form a complex with other proteins that help clustering, one of which is called NuMA. Until now, it was thought that dynein transports NuMA to the minus ends.

To test this model, Hueschen et al. studied dividing human cells under a microscope and isolated minus ends with the help of a laser. The experiments showed that, in fact, NuMA gets to minus ends independently of dynein. Once it is on the minus ends, NuMA grabs hold of another protein complex called dynactin, which then gathers dynein. Dynein then pulls the spindles into shape from the minus ends. When NuMA was experimentally removed from the cells, dynein-dynactin complexes were scattered along the entire length of the microtubule instead of pulling specifically on minus-ends, which resulted in disorganized spindles. Thus, where dynein complexes pull determines what spindle shape they build.

Hueschen et al. also showed that dynein complexes only pull on minus-ends while the cell divides, which makes sense, because NuMA remains hidden in the cell nucleus for the rest of the time. Together, these results suggest that NuMA makes sure dynein pulls specifically on the minus-ends of the microtubules to tighten the spindle at the right time.

A next step will be to test how the mechanical properties of the spindle are changed without dynein pulling on minus-ends. A better knowledge of how different proteins work together to build the spindle and help cells divide can help us understand what goes wrong when cells divide abnormally, as in the case of cancer cells.

QuaDMutEx: quadratic driver mutation explorer

BACKGROUND

Somatic mutations accumulate in human cells throughout life. Some may have no adverse consequences, but some of them may lead to cancer. A cancer genome is typically unstable, and thus more mutations can accumulate in the DNA of cancer cells. An ongoing problem is to figure out which mutations are drivers - play a role in oncogenesis, and which are passengers - do not play a role. One way of addressing this question is through inspection of somatic mutations in DNA of cancer samples from a cohort of patients and detection of patterns that differentiate driver from passenger mutations.

RESULTS

We propose QuaDMutEx, a method that incorporates three novel elements: a new gene set penalty that includes non-linear penalization of multiple mutations in putative sets of driver genes, an ability to adjust the method to handle slow- and fast-evolving tumors, and a computationally efficient method for finding gene sets that minimize the penalty, through a combination of heuristic Monte Carlo optimization and exact binary quadratic programming. Compared to existing methods, the proposed algorithm finds sets of putative driver genes that show higher coverage and lower excess coverage in eight sets of cancer samples coming from brain, ovarian, lung, and breast tumors.

CONCLUSIONS

Superior ability to improve on both coverage and excess coverage on different types of cancer shows that QuaDMutEx is a tool that should be part of a state-of-the-art toolbox in the driver gene discovery pipeline. It can detect genes harboring rare driver mutations that may be missed by existing methods. QuaDMutEx is available for download from https://github.com/bokhariy/QuaDMutEx under the GNU GPLv3 license.

MLL/WDR5 Complex Regulates Kif2A Localization to Ensure Chromosome Congression and Proper Spindle Assembly during Mitosis

Mixed-lineage leukemia (MLL), along with multisubunit (WDR5, RbBP5, ASH2L, and DPY30) complex catalyzes the trimethylation of H3K4, leading to gene activation. Here, we characterize a chromatin-independent role for MLL during mitosis. MLL and WDR5 localize to the mitotic spindle apparatus, and loss of function of MLL complex by RNAi results in defects in chromosome congression and compromised spindle formation. We report interaction of MLL complex with several kinesin and dynein motors. We further show that the MLL complex associates with Kif2A, a member of the Kinesin-13 family of microtubule depolymerase, and regulates the spindle localization of Kif2A during mitosis. We have identified a conserved WDR5 interaction (Win) motif, so far unique to the MLL family, in Kif2A. The Win motif of Kif2A engages in direct interactions with WDR5 for its spindle localization. Our findings highlight a non-canonical mitotic function of MLL complex, which may have a direct impact on chromosomal stability, frequently compromised in cancer.

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.

O-Linked N-acetylglucosamine transferase 1 regulates global histone H4 acetylation via stabilization of the nonspecific lethal protein NSL3

The human males absent on the first (MOF)-containing histone acetyltransferase nonspecific lethal (NSL) complex comprises nine subunits including the O-linked N-acetylglucosamine (O-GlcNAc) transferase, isoform 1 (OGT1). However, whether the O-GlcNAc transferase activity of OGT1 controls histone acetyltransferase activity of the NSL complex and whether OGT1 physically interacts with the other NSL complex subunits remain unclear. Here, we demonstrate that OGT1 regulates the activity of the NSL complex by mainly acetylating histone H4 Lys-16, Lys-5, and Lys-8 via O-GlcNAcylation and stabilization of the NSL complex subunit NSL3. Knocking down or overexpressing OGT1 in human cells remarkably affected the global acetylation of histone H4 residues Lys-16, Lys-5, and Lys-8. Because OGT1 is a subunit of the NSL complex, we also investigated the function of OGT1 in this complex. Co-transfection/co-immunoprecipitation experiments combined with in vitro O-GlcNAc transferase assays confirmed that OGT1 specifically binds to and O-GlcNAcylates NSL3. In addition, wheat germ agglutinin affinity purification verified the occurrence of O-GlcNAc modification on NSL3 in cells. Moreover, O-GlcNAcylation of NSL3 by wild-type OGT1 (OGT1-WT) stabilized NSL3. This stabilization was lost after co-transfection of NSL3 with an OGT1 mutant, OGT1^C964A, that lacks O-GlcNAc transferase activity. Furthermore, stabilization of NSL3 by OGT1-WT significantly increased the global acetylation levels of H4 Lys-5, Lys-8, and Lys-16 in cells. These results suggest that OGT1 regulates the activity of the NSL complex by stabilizing NSL3.

Identification of KANSARL as the first cancer predisposition fusion gene specific to the population of European ancestry origin

Gene fusion is one of the hallmarks of cancer. Recent advances in RNA-seq of cancer transcriptomes have facilitated the discovery of fusion transcripts. In this study, we report identification of a surprisingly large number of fusion transcripts, including six KANSARL (KANSL1-ARL17A) transcripts that resulted from the fusion between the KANSL1 and ARL17A genes using a RNA splicingcode model. Five of these six KANSARL fusion transcripts are novel. By systematic analysis of RNA-seq data of glioblastoma, prostate cancer, lung cancer, breast cancer, and lymphoma from different regions of the World, we have found that KANSARL fusion transcripts were rarely detected in the tumors of individuals from Asia or Africa. In contrast, they exist in 30 - 52% of the tumors from North Americans cancer patients. Analysis of CEPH/Utah Pedigree 1463 has revealed that KANSARL is a familially-inherited fusion gene. Further analysis of RNA-seq datasets of the 1000 Genome Project has indicated that KANSARL fusion gene is specific to 28.9% of the population of European ancestry origin. In summary, we demonstrated that KANSARL is the first cancer predisposition fusion gene associated with genetic backgrounds of European ancestry origin.

Mitotic spindle assembly in animal cells: a fine balancing act

The mitotic spindle has a crucial role in ensuring the accurate segregation of chromosomes into the two daughter cells during cell division, which is paramount for maintaining genome integrity. It is a self-organized and dynamic macromolecular structure that is constructed from microtubules, microtubule-associated proteins and motor proteins. Thirty years of research have led to the identification of centrosome-, chromatin- and microtubule-mediated microtubule nucleation pathways that each contribute to mitotic spindle assembly. Far from being redundant pathways, data are now emerging regarding how they function together to ensure the timely completion of mitosis. We are also beginning to comprehend the multiple mechanisms by which cells regulate spindle scaling. Together, this research has increased our understanding of how cells coordinate hundreds of proteins to assemble the dynamic, precise and robust structure that is the mitotic spindle.

Aurora-A regulates MCRS1 function during mitosis

The mitotic spindle is made of microtubules (MTs) nucleated through different pathways involving the centrosomes, the chromosomes or the walls of pre-existing MTs. MCRS1 is a RanGTP target that specifically associates with the chromosome-driven MTs protecting them from MT depolymerases. MCRS1 is also needed for the control of kinetochore fiber (K-fiber) MT minus-ends dynamics in metaphase. Here, we investigated the regulation of MCRS1 activity in M-phase. We show that MCRS1 is phosphorylated by the Aurora-A kinase in mitosis on Ser35/36. Although this phosphorylation has no role on MCRS1 localization to chromosomal MTs and K-fiber minus-ends, we show that it regulates MCRS1 activity in mitosis. We conclude that Aurora-A activity is particularly important in the tuning of K-fiber minus-ends dynamics in mitosis.

Chromatin-Binding Proteins Moonlight as Mitotic Microtubule Regulators

Some microtubule (MT)-associated proteins bind to MTs and chromatin simultaneously to fulfill their mitotic spindle function. By contrast, a growing number of chromatin-binding proteins leave mitotic chromatin and interact with MTs via their chromatin-binding domains. I discuss this switch from chromatin to MT binding as a key regulatory principle of spindle formation.

The RanGTP Pathway: From Nucleo-Cytoplasmic Transport to Spindle Assembly and Beyond

The small GTPase Ran regulates the interaction of transport receptors with a number of cellular cargo proteins. The high affinity binding of the GTP-bound form of Ran to import receptors promotes cargo release, whereas its binding to export receptors stabilizes their interaction with the cargo. This basic mechanism linked to the asymmetric distribution of the two nucleotide-bound forms of Ran between the nucleus and the cytoplasm generates a switch like mechanism controlling nucleo-cytoplasmic transport. Since 1999, we have known that after nuclear envelope breakdown (NEBD) Ran and the above transport receptors also provide a local control over the activity of factors driving spindle assembly and regulating other aspects of cell division. The identification and functional characterization of RanGTP mitotic targets is providing novel insights into mechanisms essential for cell division. Here we review our current knowledge on the RanGTP system and its regulation and we focus on the recent advances made through the characterization of its mitotic targets. We then briefly review the novel functions of the pathway that were recently described. Altogether, the RanGTP system has moonlighting functions exerting a spatial control over protein interactions that drive specific functions depending on the cellular context.

Thirty years of search and capture: The complex simplicity of mitotic spindle assembly

Cell division is enacted by a microtubule-based, self-assembling macromolecular machine known as the mitotic spindle. In 1986, Kirschner and Mitchison proposed that by undergoing dynamic cycles of growth and disassembly, microtubules search for chromosomes. Capture of microtubules by the kinetochores progressively connects chromosomes to the bipolar spindle. 30 years later, “search and capture” remains the cornerstone of spindle assembly. However, a variety of facilitating mechanisms such as regulation of microtubule dynamics by diffusible gradients, spatially selective motor activities, and adaptive changes in chromosome architecture have been discovered. We discuss how these mechanisms ensure that the spindle assembles rapidly and with a minimal number of errors.