A requirement for the Abnormal Spindle protein to organise microtubules of the central spindle for cytokinesis inDrosophila

Fly Fam161 is an essential centriole and cilium transition zone protein with unique and diverse cell type-specific localizations

Family with sequence similarity 161 (Fam161) is an ancient family of microtubule-binding proteins located at the centriole and cilium transition zone (TZ) lumen that exhibit rapid evolution in mice. However, their adaptive role is unclear. Here, we used flies to gain insight into their cell type-specific adaptations. Fam161 is the sole orthologue of FAM161A and FAM161B found in flies. Mutating Fam161 results in reduced male reproduction and abnormal geotaxis behaviour. Fam161 localizes to sensory neuron centrioles and their specialized TZ (the connecting cilium) in a cell type-specific manner, sometimes labelling only the centrioles, sometimes labelling the centrioles and cilium TZ and sometimes labelling the TZ with varying lengths that are longer than other TZ proteins, defining a new ciliary compartment, the extra distal TZ. These findings suggest that Fam161 is an essential centriole and TZ protein with a unique cell type-specific localization in fruit flies that can produce cell type-specific adaptations.

Autosomal Recessive Primary Microcephaly: Not Just a Small Brain

Microcephaly or reduced head circumference results from a multitude of abnormal developmental processes affecting brain growth and/or leading to brain atrophy. Autosomal recessive primary microcephaly (MCPH) is the prototype of isolated primary (congenital) microcephaly, affecting predominantly the cerebral cortex. For MCPH, an accelerating number of mutated genes emerge annually, and they are involved in crucial steps of neurogenesis. In this review article, we provide a deeper look into the microcephalic MCPH brain. We explore cytoarchitecture focusing on the cerebral cortex and discuss diverse processes occurring at the level of neural progenitors, early generated and mature neurons, and glial cells. We aim to thereby give an overview of current knowledge in MCPH phenotype and normal brain growth.

Two novel truncating variants of the ASPM gene identified in a nonconsanguineous Chinese family associated with primary microcephaly

Primary autosomal recessive microcephaly 5 (MCPH5) is a rare neurodevelopmental disorder with a relatively high incidence in regions where consanguineous marriage is widely practiced; So far, only a few MCPH5 cases have been reported from China. Here, we report clinical and molecular characteristics of two Chinese MCPH5 patients, a 24-year-old woman proband and her brother, a 19-year-old man, from a nonconsanguineous family. Main manifestations in the proband were small head circumference, premature closure of fontanelles, impaired concentration and moderate intellectual disability. The proband's brother had similar symptoms, but he was hyperactive and had a more severe sloping forehead. Brain imaging revealed global reduction in brain size, especially in the frontal lobes bilaterally and anterior horns of lateral ventricles. Sequencing results revealed that both patients carried a novel nonsense variant p.Tyr2004* (c.6012_6013delTA) and a novel frameshift variant p.Arg2005Serfs*48 (c.6015_6016delGG) in the ASPM gene. These variants were interpreted to be pathogenic in the in-silico analysis. Our findings help to expand the mutation spectrum of ASPM and provide new opportunities for assisting the traditional clinical diagnosis on the cases with atypical characteristics.

Multiple pools of PP2A regulate spindle assembly, kinetochore attachments and cohesion in Drosophila oocytes

Meiosis in female oocytes lacks centrosomes, the microtubule-organizing centers. In Drosophila oocytes, meiotic spindle assembly depends on the chromosomal passenger complex (CPC). To investigate the mechanisms that regulate Aurora B activity, we examined the role of protein phosphatase 2A (PP2A) in Drosophila oocyte meiosis. We found that both forms of PP2A, B55 and B56, antagonize the Aurora B spindle assembly function, suggesting that a balance between Aurora B and PP2A activity maintains the oocyte spindle during meiosis I. PP2A-B56, which has a B subunit encoded by two partially redundant paralogs, wdb and wrd, is also required for maintenance of sister chromatid cohesion, establishment of end-on microtubule attachments, and metaphase I arrest in oocytes. WDB recruitment to the centromeres depends on BUBR1, MEI-S332 and kinetochore protein SPC105R. Although BUBR1 stabilizes microtubule attachments in Drosophila oocytes, it is not required for cohesion maintenance during meiosis I. We propose at least three populations of PP2A-B56 regulate meiosis, two of which depend on SPC105R and a third that is associated with the spindle.

The UFM1 cascade times mitosis entry associated with microcephaly

Posttranslational modifications enhance the functional diversity of the proteome by modifying the substrates. The UFM1 cascade is a novel ubiquitin-like modification system. The mutations in UFM1, its E1 (UBA5) and E2 (UFC1), have been identified in patients with microcephaly. However, its pathological mechanisms remain unclear. Herein, we observed the disruption of the UFM1 cascade in Drosophila neuroblasts (NBs) decreased the number of NBs, leading to a smaller brain size. The lack of ufmylation in NBs resulted in an increased mitotic index and an extended G2/M phase, indicating a defect in mitotic progression. In addition, live imaging of the embryos revealed an impaired E3 ligase (Ufl1) function resulted in premature entry into mitosis and failed cellularization. Even worse, the embryonic lethality occurred as early as within the first few mitotic cycles following the depletion of Ufm1. Knockdown of ufmylation in the fixed embryos exhibited severe phenotypes, including detached centrosomes, defective microtubules, and DNA bridge. Furthermore, we observed that the UFM1 cascade could alter the level of phosphorylation on tyrosine-15 of CDK1 (pY15-CDK1), which is a negative regulator of the G2 to M transition. These findings yield unambiguous evidence suggesting that the UFM1 cascade is a microcephaly-causing factor that regulates the progression of the cell cycle at mitosis phase entry.

Pelota-interacting G protein Hbs1 is required for spermatogenesis in Drosophila

Hbs1, which is homologous to the GTPase eRF3, is a small G protein implicated in mRNA quality control. It interacts with a translation-release factor 1-like protein Dom34/Pelota to direct decay of mRNAs with ribosomal stalls. Although both proteins are evolutionarily conserved in eukaryotes, the biological function of Hbs1 in multicellular organisms is yet to be characterized. In Drosophila, pelota is essential for the progression through meiosis during spermatogenesis and germline stem cell maintenance. Here we show that homozygous Hbs1 mutant flies are viable, female-fertile, but male-sterile, which is due to defects in meiosis and spermatid individualization, phenotypes that are also observed in pelota hypomorphic mutants. In contrast, Hbs1 mutants have no obvious defects in germline stem cell maintenance. We show that Hbs1 genetically interacts with pelota during spermatid individualization. Furthermore, Pelota with a point mutation on the putative Hbs1-binding site cannot substitute the wild type protein for normal spermatogenesis. These data suggest that Pelota forms a complex with Hbs1 to regulate multiple processes during spermatogenesis. Our results reveal a specific requirement of Hbs1 in male gametogenesis in Drosophila and indicate an essential role for the RNA surveillance complex Pelota-Hbs1 in spermatogenesis, a function that could be conserved in mammals.

Fascetto interacting protein ensures proper cytokinesis and ploidy

Cell division is critical for development, organ growth, and tissue repair. The later stages of cell division include the formation of the microtubule (MT)-rich central spindle in anaphase, which is required to properly define the cell equator, guide the assembly of the acto-myosin contractile ring and ultimately ensure complete separation and isolation of the two daughter cells via abscission. Much is known about the molecular machinery that forms the central spindle, including proteins needed to generate the antiparallel overlapping interzonal MTs. One critical protein that has garnered great attention is the protein regulator of cytokinesis 1, or Fascetto (Feo) in Drosophila, which forms a homodimer to cross-link interzonal MTs, ensuring proper central spindle formation and cytokinesis. Here, we report on a new direct protein interactor and regulator of Feo we named Feo interacting protein (FIP). Loss of FIP results in a reduction in Feo localization, rapid disassembly of interzonal MTs, and several defects related to cytokinesis failure, including polyploidization of neural stem cells. Simultaneous reduction in Feo and FIP results in very large, tumorlike DNA-filled masses in the brain that contain hundreds of centrosomes. In aggregate, our data show that FIP acts directly on Feo to ensure fully accurate cell division.

Coordination of Embryogenesis by the Centrosome in Drosophila melanogaster

The first 3 h of Drosophila melanogaster embryo development are exemplified by rapid nuclear divisions within a large syncytium, transforming the zygote to the cellular blastoderm after 13 successive cleavage divisions. As the syncytial embryo develops, it relies on centrosomes and cytoskeletal dynamics to transport nuclei, maintain uniform nuclear distribution throughout cleavage cycles, ensure generation of germ cells, and coordinate cellularization. For the sake of this review, we classify six early embryo stages that rely on processes coordinated by the centrosome and its regulation of the cytoskeleton. The first stage features migration of one of the female pronuclei toward the male pronucleus following maturation of the first embryonic centrosomes. Two subsequent stages distribute the nuclei first axially and then radially in the embryo. The remaining three stages involve centrosome-actin dynamics that control cortical plasma membrane morphogenesis. In this review, we highlight the dynamics of the centrosome and its role in controlling the six stages that culminate in the cellularization of the blastoderm embryo.

Female Meiosis: Synapsis, Recombination, and Segregation in Drosophila melanogaster

A century of genetic studies of the meiotic process in Drosophila melanogaster females has been greatly augmented by both modern molecular biology and major advances in cytology. These approaches, and the findings they have allowed, are the subject of this review. Specifically, these efforts have revealed that meiotic pairing in Drosophila females is not an extension of somatic pairing, but rather occurs by a poorly understood process during premeiotic mitoses. This process of meiotic pairing requires the function of several components of the synaptonemal complex (SC). When fully assembled, the SC also plays a critical role in maintaining homolog synapsis and in facilitating the maturation of double-strand breaks (DSBs) into mature crossover (CO) events. Considerable progress has been made in elucidating not only the structure, function, and assembly of the SC, but also the proteins that facilitate the formation and repair of DSBs into both COs and noncrossovers (NCOs). The events that control the decision to mature a DSB as either a CO or an NCO, as well as determining which of the two CO pathways (class I or class II) might be employed, are also being characterized by genetic and genomic approaches. These advances allow a reconsideration of meiotic phenomena such as interference and the centromere effect, which were previously described only by genetic studies. In delineating the mechanisms by which the oocyte controls the number and position of COs, it becomes possible to understand the role of CO position in ensuring the proper orientation of homologs on the first meiotic spindle. Studies of bivalent orientation have occurred in the context of numerous investigations into the assembly, structure, and function of the first meiotic spindle. Additionally, studies have examined the mechanisms ensuring the segregation of chromosomes that have failed to undergo crossing over.

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.

Identification of a Novel Nonsense ASPM Mutation in a Large Consanguineous Pakistani Family Using Targeted Next-Generation Sequencing

AIMS

To identify the pathogenic mutation underlying microcephaly primary hereditary (MCPH) in a large consanguineous Pakistani family.

METHODS

A five-generation family with an autosomal recessive transmission of MCPH was recruited. Targeted next-generation DNA sequencing was carried out to analyze the genomic DNA sample from the proband with MCPH using a previously designed panel targeting 46 known microcephaly-causing genes. Sanger sequencing was performed to verify all identified variants.

RESULTS

We found a novel homozygous nonsense mutation, c.7543C>T, in the ASPM gene. This mutation led to the substitution of an arginine with a stop codon at amino acid residue 2515 (p.Arg2515Ter). The mutation cosegregated with the MCPH phenotype in all affected and obligate carrier family members, but was not present in public databases (dbSNP147, Exome Variant Server, the 1000 Genomes Project, Exome Aggregation Consortium, Human Gene Mutation Database, and ClinVar) or 200 control individuals. The c.7543C>T mutation in ASPM may activate nonsense-mediated mRNA decay pathways and could underlie the pathogenesis of MCPH through a loss-of-function mechanism.

CONCLUSIONS

The c.7543C>T (p.Arg2515Ter) mutation in ASPM is a novel pathogenic mutation for the typical MCPH phenotype in this family.

Zika virus: An emerging challenge for obstetrics and gynecology

Microcephaly is a rare birth defect, however, the re-emerging mosquito and sexual transmitted flavivirus, Zika virus (ZIKV), had changed the situation and caused an urgent challenge for the obstetrics and gynecology. This review will brief summarize the epidemiology and virology of ZIKV. And compared the animal models that had developed for the ZIKV infections. These animal models will be benefit for the development of vaccines and anti-ZIKV drugs. Furthermore, the genes that are involved in the causation of microcephaly were also summarized. Finally, the Wnt signal is critical for the brain development as well as innate immune response. Based on previous literatures, we proposed that ZIKV-induced microcephaly might result from the influence of Wnt/β-catenin signaling pathway through the regulation of miRNA-34.

Parthenogenesis in Insects: The Centriole Renaissance

Building a new organism usually requires the contribution of two differently shaped haploid cells, the male and female gametes, each providing its genetic material to restore diploidy of the new born zygote. The successful execution of this process requires defined sequential steps that must be completed in space and time. Otherwise, development fails. Relevant among the earlier steps are pronuclear migration and formation of the first mitotic spindle that promote the mixing of parental chromosomes and the formation of the zygotic nucleus. A complex microtubule network ensures the proper execution of these processes. Instrumental to microtubule organization and bipolar spindle assembly is a distinct non-membranous organelle, the centrosome. Centrosome inheritance during fertilization is biparental, since both gametes provide essential components to build a functional centrosome. This model does not explain, however, centrosome formation during parthenogenetic development, a special mode of sexual reproduction in which the unfertilized egg develops without the contribution of the male gamete. Moreover, whereas fertilization is a relevant example in which the cells actively check the presence of only one centrosome, to avoid multipolar spindle formation, the development of parthenogenetic eggs is ensured, at least in insects, by the de novo assembly of multiple centrosomes.Here, we will focus our attention on the assembly of functional centrosomes following fertilization and during parthenogenetic development in insects. Parthenogenetic development in which unfertilized eggs are naturally depleted of centrosomes would provide a useful experimental system to investigate centriole assembly and duplication together with centrosome formation and maturation.

Cooperation Between Kinesin Motors Promotes Spindle Symmetry and Chromosome Organization in Oocytes

The oocyte spindle in most animal species is assembled in the absence of the microtubule-organizing centers called centrosomes. Without the organization provided by centrosomes, acentrosomal meiotic spindle organization may rely heavily on the bundling of microtubules by kinesin motor proteins. Indeed, the minus-end directed kinesin-14 NCD, and the plus-end directed kinesin-6 Subito are known to be required for oocyte spindle organization in Drosophila melanogaster How multiple microtubule-bundling kinesins interact to produce a functional acentrosomal spindle is not known. In addition, there have been few studies on the meiotic function of one of the most important microtubule-bundlers in mitotic cells, the kinesin-5 KLP61F. We have found that the kinesin-5 KLP61F is required for spindle and centromere symmetry in oocytes. The asymmetry observed in the absence of KLP61F depends on NCD, the kinesin-12 KLP54D, and the microcephaly protein ASP. In contrast, KLP61F and Subito work together in maintaining a bipolar spindle. We propose that the prominent central spindle, stabilized by Subito, provides the framework for the coordination of multiple microtubule-bundling activities. The activities of several proteins, including NCD, KLP54D, and ASP, generate asymmetries within the acentrosomal spindle, while KLP61F and Subito balance these forces, resulting in the capacity to accurately segregate chromosomes.

The chromosomal basis of meiotic acentrosomal spindle assembly and function in oocytes

Several aspects of meiosis are impacted by the absence of centrosomes in oocytes. Here, we review four aspects of meiosis I that are significantly affected by the absence of centrosomes in oocyte spindles. One, microtubules tend to assemble around the chromosomes. Two, the organization of these microtubules into a bipolar spindle is directed by the chromosomes. Three, chromosome bi-orientation and attachment to microtubules from the correct pole require modification of the mechanisms used in mitotic cells. Four, chromosome movement to the poles at anaphase cannot rely on polar anchoring of spindle microtubules by centrosomes. Overall, the chromosomes are more active participants during acentrosomal spindle assembly in oocytes, compared to mitotic and male meiotic divisions where centrosomes are present. The chromosomes are endowed with information that can direct the meiotic divisions and dictate their own behavior in oocytes. Processes beyond those known from mitosis appear to be required for their bi-orientation at meiosis I. As mitosis occurs without centrosomes in many systems other than oocytes, including all plants, the concepts discussed here may not be limited to oocytes. The study of meiosis in oocytes has revealed mechanisms that are operating in mitosis and will probably continue to do so.

B-type nuclear lamin and the nuclear pore complex Nup107-160 influences maintenance of the spindle envelope required for cytokinesis in Drosophila male meiosis

In higher eukaryotes, nuclear envelope (NE) disassembly allows chromatin to condense and spindle microtubules to access kinetochores. The nuclear lamina, which strengthens the NE, is composed of a polymer meshwork made of A- and B-type lamins. We found that the B-type lamin (Lam) is not fully disassembled and continues to localize along the spindle envelope structure during Drosophila male meiosis I, while the A-type lamin (LamC) is completely dispersed throughout the cytoplasm. Among the nuclear pore complex proteins, Nup107 co-localized with Lam during this meiotic division. Surprisingly, Lam depletion resulted in a higher frequency of cytokinesis failure in male meiosis. We also observed the similar meiotic phenotype in Nup107-depleted cells. Abnormal localization of Lam was found in the Nup-depleted cells at premeiotic and meiotic stages. The central spindle microtubules became abnormal and recruitment of a contractile ring component to the cleavage sites was disrupted in Lam-depleted cells and Nup107-depleted cells. Therefore, we speculate that both proteins are required for a reinforcement of the spindle envelope, which supports the formation of central spindle microtubules essential for cytokinesis in Drosophila male meiosis.

The Postpharyngeal Gland: Specialized Organ for Lipid Nutrition in Leaf-Cutting Ants

There are several hypotheses about the possible functions of the postpharyngeal gland (PPG) in ants. The proposed functions include roles as cephalic or gastric caeca and diverticulum of the digestive tract, mixing of hydrocarbons, nestmate recognition, feeding larvae, and the accumulation of lipids inside this gland, whose origin is contradictory. The current study aimed to investigate the functions of these glands by examining the protein expression profile of the PPGs of Atta sexdens rubropilosa (Hymenoptera, Formicidae). Mated females received lipid supplementation and their glands were extracted and analyzed using a proteomic approach. The protocol used combined two-dimensional electrophoresis and shotgun strategies, followed by mass spectrometry. We also detected lipid β-oxidation by immunofluorescent marking of acyl-CoA dehydrogenase. Supplying ants with lipids elicited responses in the glandular cells of the PPG; these included increased expression of proteins related to defense mechanisms and signal transduction and reorganization of the cytoskeleton due to cell expansion. In addition, some proteins in PPG were overexpressed, especially those involved in lipid and energy metabolism. Part of the lipids may be reduced, used for the synthesis of fatty alcohol, transported to the hemolymph, or may be used as substrate for the synthesis of acetyl-CoA, which is oxidized to form molecules that drive oxidative phosphorylation and produce energy for cellular metabolic processes. These findings suggest that this organ is specialized for lipid nutrition of adult leaf-cutting ants and characterized like a of diverticulum foregut, with the ability to absorb, store, metabolize, and mobilize lipids to the hemolymph. However, we do not rule out that the PPG may have other functions in other species of ants.

The intimate genetics of Drosophila fertilization

The union of haploid gametes at fertilization initiates the formation of the diploid zygote in sexually reproducing animals. This founding event of embryogenesis includes several fascinating cellular and nuclear processes, such as sperm-egg cellular interactions, sperm chromatin remodelling, centrosome formation or pronuclear migration. In comparison with other aspects of development, the exploration of animal fertilization at the functional level has remained so far relatively limited, even in classical model organisms. Here, we have reviewed our current knowledge of fertilization in Drosophila melanogaster, with a special emphasis on the genes involved in the complex transformation of the fertilizing sperm nucleus into a replicated set of paternal chromosomes.

An Asp-CaM complex is required for centrosome-pole cohesion and centrosome inheritance in neural stem cells

Calmodulin is required for abnormal spindle’s (Asp’s) ability to cross-link microtubules and ensure proper centrosome inheritance in neural stem cells, but it is dispensable for Asp’s role in brain size determination.

The interaction between centrosomes and mitotic spindle poles is important for efficient spindle formation, orientation, and cell polarity. However, our understanding of the dynamics of this relationship and implications for tissue homeostasis remains poorly understood. Here we report that Drosophila melanogaster calmodulin (CaM) regulates the ability of the microcephaly-associated protein, abnormal spindle (Asp), to cross-link spindle microtubules. Both proteins colocalize on spindles and move toward spindle poles, suggesting that they form a complex. Our binding and structure–function analysis support this hypothesis. Disruption of the Asp–CaM interaction alone leads to unfocused spindle poles and centrosome detachment. This behavior leads to randomly inherited centrosomes after neuroblast division. We further show that spindle polarity is maintained in neuroblasts despite centrosome detachment, with the poles remaining stably associated with the cell cortex. Finally, we provide evidence that CaM is required for Asp’s spindle function; however, it is completely dispensable for Asp’s role in microcephaly suppression.

Molekulare Grundlagen der autosomal-rezessiven primären Mikrozephalie

Die primäre autosomal-rezessive Mikrozephalie (MCPH) ist eine genetisch sehr heterogene Erkrankung, die klinisch definiert wird durch das Vorliegen einer kongenitalen, nicht progressiven Mikrozephalie, einer mentalen Retardierung variablen Ausmaßes bei weitgehend normaler Körpergröße und das Fehlen von zusätzlichen Fehlbildungen und weiteren neurologischen Befunden. Bislang konnten Mutationen in 14 verschiedenen Genen identifiziert werden, deren Produkte auf zellulärer Ebene insbesondere bei Vorgängen der Zellteilung, der Zellzyklusregulierung und bei der Aktivierung von DNA-Reparaturmechanismen nach DNA-Schädigungen eine wichtige Rolle spielen. Darüber hinaus sind auch syndromale Formen der Mikrozephalie bekannt, zu denen u. a. das Seckel-Syndrom sowie der mikrozephale osteodysplastische primordiale Kleinwuchs Typ II (MOPD II) zählen.

Aspm sustains postnatal cerebellar neurogenesis and medulloblastoma growth in mice

Alterations in genes that regulate brain size may contribute to both microcephaly and brain tumor formation. Here, we report that Aspm, a gene that is mutated in familial microcephaly, regulates postnatal neurogenesis in the cerebellum and supports the growth of medulloblastoma, the most common malignant pediatric brain tumor. Cerebellar granule neuron progenitors (CGNPs) express Aspm when maintained in a proliferative state by sonic hedgehog (Shh) signaling, and Aspm is expressed in Shh-driven medulloblastoma in mice. Genetic deletion of Aspm reduces cerebellar growth, while paradoxically increasing the mitotic rate of CGNPs. Aspm-deficient CGNPs show impaired mitotic progression, altered patterns of division orientation and differentiation, and increased DNA damage, which causes progenitor attrition through apoptosis. Deletion of Aspm in mice with Smo-induced medulloblastoma reduces tumor growth and increases DNA damage. Co-deletion of Aspm and either of the apoptosis regulators Bax or Trp53 (also known as p53) rescues the survival of neural progenitors and reduces the growth restriction imposed by Aspm deletion. Our data show that Aspm functions to regulate mitosis and to mitigate DNA damage during CGNP cell division, causes microcephaly through progenitor apoptosis when mutated, and sustains tumor growth in medulloblastoma.

Caenorhabditis elegans Aurora A kinase is required for the formation of spindle microtubules in female meiosis

Female meiotic spindles are organized in the absence of centrosomes. Caenorhabditis elegans Aurora A (AIR-1) is dispensable for the initial assembly of meiotic microtubules within the oocyte nuclei, but its kinase activity is continuously required for the stabilization of meiotic spindle microtubules after germinal vesicle breakdown.

In many animals, female meiotic spindles are assembled in the absence of centrosomes, the major microtubule (MT)-organizing centers. How MTs are formed and organized into meiotic spindles is poorly understood. Here we report that, in Caenorhabditis elegans, Aurora A kinase/AIR-1 is required for the formation of spindle microtubules during female meiosis. When AIR-1 was depleted or its kinase activity was inhibited in C. elegans oocytes, although MTs were formed around chromosomes at germinal vesicle breakdown (GVBD), they were decreased during meiotic prometaphase and failed to form a bipolar spindle, and chromosomes were not separated into two masses. Whereas AIR-1 protein was detected on and around meiotic spindles, its kinase-active form was concentrated on chromosomes at prometaphase and on interchromosomal MTs during late anaphase and telophase. We also found that AIR-1 is involved in the assembly of short, dynamic MTs in the meiotic cytoplasm, and these short MTs were actively incorporated into meiotic spindles. Collectively our results suggest that, after GVBD, the kinase activity of AIR-1 is continuously required for the assembly and/or stabilization of female meiotic spindle MTs.

Molecular genetics of human primary microcephaly: an overview

Autosomal recessive primary microcephaly (MCPH) is a neurodevelopmental disorder that is characterised by microcephaly present at birth and non-progressive mental retardation. Microcephaly is the outcome of a smaller but architecturally normal brain; the cerebral cortex exhibits a significant decrease in size. MCPH is a neurogenic mitotic disorder, though affected patients demonstrate normal neuronal migration, neuronal apoptosis and neural function. Twelve MCPH loci (MCPH1-MCPH12) 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 and CDK6. It is predicted that MCPH gene mutations may lead to the disease phenotype due to a disturbed mitotic spindle orientation, premature chromosomal condensation, signalling response as a result of damaged DNA, microtubule dynamics, transcriptional control or a few other hidden centrosomal mechanisms that can regulate the number of neurons produced by neuronal precursor cells. Additional findings have further elucidated the microcephaly aetiology and pathophysiology, which has informed the clinical management of families suffering from MCPH. The provision of molecular diagnosis and genetic counselling may help to decrease the frequency of this disorder.

Molecular and cellular basis of autosomal recessive primary microcephaly

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

Cytokinesis in Drosophila male meiosis

Cytokinesis separates the cytoplasm and the duplicated genome into two daughter cells at the end of cell division. This process must be finely regulated to maintain ploidy and prevent tumor formation. Drosophila male meiosis provides an excellent cell system for investigating cytokinesis. Mutants affecting this process can be easily identified and spermatocytes are large cells particularly suitable for cytological analysis of cytokinetic structures. Over the past decade, the powerful tools of Drosophila genetics and the unique characteristics of this cell system have led researchers to identify molecular players of the cell cleavage machinery and to address important open questions. Although spermatocyte cytokinesis is incomplete, resulting in formation of stable intercellular bridges, the molecular mechanisms are largely conserved in somatic cells. Thus, studies of Drosophila male meiosis will shed new light on the complex cell circuits regulating furrow ingression and substantially further our knowledge of cancer and other human diseases.

Caenorhabditis elegans oocyte meiotic spindle pole assembly requires microtubule severing and the calponin homology domain protein ASPM-1

Oocyte meiotic spindles are bipolar but assemble without centrosomes. Three Caenorhabditis elegans genes that contribute are that for the calponin homology domain protein, aspm-1; the katanin mei-1; and the kinesin-12 family member klp-18. The results indicate that both microtubule severing and ASPM-1 promote pole assembly, whereas KLP-18 promotes bipolarity.

In many animals, including vertebrates, oocyte meiotic spindles are bipolar but assemble in the absence of centrosomes. Although meiotic spindle positioning in oocytes has been investigated extensively, much less is known about their assembly. In Caenorhabditis elegans, three genes previously shown to contribute to oocyte meiotic spindle assembly are the calponin homology domain protein encoded by aspm-1, the katanin family member mei-1, and the kinesin-12 family member klp-18. We isolated temperature-sensitive alleles of all three and investigated their requirements using live-cell imaging to reveal previously undocumented requirements for aspm-1 and mei-1. Our results indicate that bipolar but abnormal oocyte meiotic spindles assemble in aspm-1(-) embryos, whereas klp-18(-) and mei-1(-) mutants assemble monopolar and apolar spindles, respectively. Furthermore, two MEI-1 functions—ASPM-1 recruitment to the spindle and microtubule severing—both contribute to monopolar spindle assembly in klp-18(-) mutants. We conclude that microtubule severing and ASPM-1 both promote meiotic spindle pole assembly in C. elegans oocytes, whereas the kinesin 12 family member KLP-18 promotes spindle bipolarity.

KIFC3 promotes mitotic progression and integrity of the central spindle in cytokinesis

Kinesin-14 motor proteins play a variety of roles during metaphase and anaphase. However, it is not known whether members of this family of motors also participate in the dramatic changes in mitotic spindle organization during the transition from telophase to cytokinesis. We have identified the minus-end-directed motor, KIFC3, as an important contributor to central bridge morphology at this stage. KIFC3's unique motor-dependent localization at the central bridge allows it to congress microtubules, promoting efficient progress through cytokinesis. Conversely, when KIFC3 function is perturbed, abscission is delayed, and the central bridge is both widened and extended. Examination of KIFC3 on growing microtubules in interphase indicates that it caps microtubules released from the centrosome, both in the region of the centrosome and in the cell periphery. In line with other kinesin-14 family members, KIFC3 may guide free microtubules to their destination at the bridge and/or may slide and crosslink central bridge microtubules in order to stage the cells for abscission.

The microcephaly protein Asp regulates neuroepithelium morphogenesis by controlling the spatial distribution of myosin II

Mutations in ASPM are the most frequent cause of microcephaly, a disorder characterized by reduced brain size at birth. ASPM is recognized as a major regulator of brain size, yet its role during neural development remains poorly understood. Moreover, the role of ASPM proteins in invertebrate brain morphogenesis has never been investigated. Here, we characterized the function of the Drosophila ASPM orthologue, Asp, and found that asp mutants present severe defects in brain size and neuroepithelium morphogenesis. We show that size reduction depends on the mitotic function of Asp, whereas regulation of tissue shape depends on an uncharacterized function. Asp interacts with myosin II regulating its polarized distribution along the apico-basal axis. In the absence of Asp, mislocalization of myosin II results in interkinetic nuclear migration and tissue architecture defects. We propose that Asp regulates neuroepithelium morphogenesis through myosin-II-mediated structural and mechanical processes to maintain force balance and tissue cohesiveness.

What Drosophila spermatocytes tell us about the mechanisms underlying cytokinesis

Cytokinesis separates the genomic material and organelles of a dividing cell equitably into two physically distinct daughter cells at the end of cell division. This highly choreographed process involves coordinated reorganization and regulated action of the actin and microtubule cytoskeletal systems, an assortment of motor proteins, and membrane trafficking components. Due to their large size, the ease with which exquisite cytological analysis may be performed on them, and the availability of numerous mutants and other genetic tools, Drosophila spermatocytes have provided an excellent system for exploring the mechanistic basis for the temporally programmed and precise spatially localized events of cytokinesis. Mutants defective in male meiotic cytokinesis can be easily identified in forward genetic screens by the production of multinucleate spermatids. In addition, the weak spindle assembly checkpoint in spermatocytes, which causes only a small delay of anaphase onset in the presence of unattached chromosomes, allows investigation of whether gene products required for spindle assembly and chromosome segregation are also involved in cytokinesis. Perhaps due to the large size of spermatocytes and the requirement for two rapid-fire rounds of division without intervening S or growth phases during meiosis, male meiotic mutants have also revealed much about molecular mechanisms underlying new membrane addition during cytokinesis. Finally, cell type-specific differences in the events that set up and complete cytokinesis are emerging from comparison of spermatocytes with cells undergoing mitosis either elsewhere in the organism or in tissue culture.

Autosomal Recessive Primary Microcephaly (MCPH): clinical manifestations, genetic heterogeneity and mutation continuum

Autosomal Recessive Primary Microcephaly (MCPH) is a rare disorder of neurogenic mitosis characterized by reduced head circumference at birth with variable degree of mental retardation. In MCPH patients, brain size reduced to almost one-third of its original volume due to reduced number of generated cerebral cortical neurons during embryonic neurogensis. So far, seven genetic loci (MCPH1-7) for this condition have been mapped with seven corresponding genes (MCPH1, WDR62, CDK5RAP2, CEP152, ASPM, CENPJ, and STIL) identified from different world populations. Contribution of ASPM and WDR62 gene mutations in MCPH World wide is more than 50%. By and large, primary microcephaly patients are phenotypically indistinguishable, however, recent studies in patients with mutations in MCPH1, WDR62 and ASPM genes showed a broader clinical and/or cellular phenotype. It has been proposed that mutations in MCPH genes can cause the disease phenotype by disturbing: 1) orientation of mitotic spindles, 2) chromosome condensation mechanism during embryonic neurogenesis, 3) DNA damage-response signaling, 4) transcriptional regulations and microtubule dynamics, 5) certain unknown centrosomal mechanisms that control the number of neurons generated by neural precursor cells. Recent discoveries of mammalian models for MCPH have open up horizons for researchers to add more knowledge regarding the etiology and pathophysiology of MCPH. High incidence of MCPH in Pakistani population reflects the most probable involvement of consanguinity. Genetic counseling and clinical management through carrier detection/prenatal diagnosis in MCPH families can help reducing the incidence of this autosomal recessive disorder.

The relevance of symmetric and asymmetric cell divisions to human central nervous system diseases

During development of the embryonic central nervous system (CNS), large numbers of neurons and glia are generated from the neuroepithelium and its progenitor derivatives as a result of symmetric and asymmetric cell divisions. We describe the biology of symmetric and asymmetric cell divisions in the CNS as gleaned from animal models, and discuss the relevance of these processes to human CNS development and disease.

ASPM gene expression in medulloblastoma

PURPOSE

Medulloblastomas are the most common malignant tumors of the central nervous system in childhood. The incidence is about 19-20% between children younger than 16 years old with peak incidence between 4 and 7 years. Despite its sensibility to no specific therapeutic means like chemotherapy and radiotherapy, the treatment is very aggressive and frequently results in regression, growth deficit, and endocrine dysfunction. From this point of view, new treatment approaches are needed such as molecular targeted therapies. Studies in glioblastoma demonstrated that ASPM gene was overexpressed when compared to normal brain and ASPM inhibition by siRNA-mediated inhibits tumor cell proliferation and neural stem cell proliferation, supporting ASPM gene as a potential molecular target in glioblastoma. The aim of this work was to evaluate ASPM expression in medulloblastoma fragment samples, and to compare the results with the patient clinical features.

METHODS

Analysis of gene expression was performed by quantitative PCR real time using SYBR Green system in tumor samples from 37 children. The t test was used to analyze the gene expression, and Mann-Whitney test was performed to analyze the relationship between gene expressions and clinical characteristics. Kaplan-Meier test evaluated curve survival.

RESULTS

All samples overexpressed ASPM gene more than 40-fold. However, we did not find any association between the overexpressed samples and the clinical parameters.

CONCLUSION

ASPM overexpression may modify the ability of stem cells to differentiate during the development of the central nervous system, contributing to the development of medulloblastoma, a tumor of embryonic origin from cerebellar progenitor cells.

Human ASPM participates in spindle organisation, spindle orientation and cytokinesis

Background

Mutations in the Abnormal Spindle Microcephaly related gene (ASPM) are the commonest cause of autosomal recessive primary microcephaly (MCPH) a disorder characterised by a small brain and associated mental retardation. ASPM encodes a mitotic spindle pole associated protein. It is suggested that the MCPH phenotype arises from proliferation defects in neural progenitor cells (NPC).

Results

We show that ASPM is a microtubule minus end-associated protein that is recruited in a microtubule-dependent manner to the pericentriolar matrix (PCM) at the spindle poles during mitosis. ASPM siRNA reduces ASPM protein at the spindle poles in cultured U2OS cells and severely perturbs a number of aspects of mitosis, including the orientation of the mitotic spindle, the main determinant of developmental asymmetrical cell division. The majority of ASPM depleted mitotic cells fail to complete cytokinesis. In MCPH patient fibroblasts we show that a pathogenic ASPM splice site mutation results in the expression of a novel variant protein lacking a tripeptide motif, a minimal alteration that correlates with a dramatic decrease in ASPM spindle pole localisation. Moreover, expression of dominant-negative ASPM C-terminal fragments cause severe spindle assembly defects and cytokinesis failure in cultured cells.

Conclusions

These observations indicate that ASPM participates in spindle organisation, spindle positioning and cytokinesis in all dividing cells and that the extreme C-terminus of the protein is required for ASPM localisation and function. Our data supports the hypothesis that the MCPH phenotype caused by ASPM mutation is a consequence of mitotic aberrations during neurogenesis. We propose the effects of ASPM mutation are tolerated in somatic cells but have profound consequences for the symmetrical division of NPCs, due to the unusual morphology of these cells. This antagonises the early expansion of the progenitor pool that underpins cortical neurogenesis, causing the MCPH phenotype.

Still entangled: assembly of the central spindle by multiple microtubule modulators

The central spindle is a microtubule-based structure that assembles during anaphase of mitosis in animal cells and is essential for multiple steps of cytokinesis. Central spindle assembly occurs by the cooperative action of multiple microtubule motors and modulators. Here, we review the mechanism by which the central spindle is formed, the role of several key proteins in this process and how central spindle assembly is temporally and spatially coordinated with mitosis.

Proper organization of microtubule minus ends is needed for midzone stability and cytokinesis

Successful cytokinesis is critical for maintaining genome stability and requires the assembly of a robust central spindle to specify the cleavage furrow position, to prevent separated chromosomes from coming back together, and to contribute to midbody abscission. A proper central spindle is assembled and maintained by a number of microtubule-associated and molecular motor proteins that sort microtubules into bundles with their plus ends overlapping at the center. The mechanisms by which different factors organize the central spindle microtubules remain unclear. We found that perturbation of the minus-end-directed Kinesin-14 HSET increased the frequency of polyploid cells, which resulted from a failure in cytokinesis. In addition, HSET knockdown resulted in severe midzone microtubule organization, most notably at microtubule minus ends, as well as mislocalization of several midbody-associated proteins. Biochemical analysis showed that both human HSET and Xenopus XCTK2 cofractionated with the gamma-tubulin ring complexes on sucrose gradients and that XCTK2 associated with gamma-tubulin and Xgrip109 by immunoprecipitation. Our data reveal the novel finding that a minus-end-directed motor contributes to the organization and stability of the central spindle, which is needed for proper cytokinesis.

YA is needed for proper nuclear organization to transition between meiosis and mitosis in Drosophila

BACKGROUND

The Drosophila YA protein is required to initiate the embryonic cleavage divisions. After egg activation, YA enters nuclei and interacts with chromatin and the nuclear lamina. This study was designed to define more precisely the events prior to the first cleavage division that are dependent upon YA.

RESULTS

We find that meiosis is completed normally in the absence of YA function. The first defects in embryos and eggs from mutant mothers first appear just after the completion of meiosis, and are seen as abnormal associations among the resultant haploid nuclei. These defects are associated with asynchronies in the cell cycle-dependent chromatin condensation state of the haploid nuclei. However, we find evidence of DNA replication in the absence of YA function.

CONCLUSION

Our data suggest YA function is needed at a control point, following meiosis II and the initiation of the first postmeiotic S phase, which is sensitive to the chromatin condensation state of the haploid meiotic products.

Cytological analysis of meiosis in fixed Drosophila ovaries

Methods are described to analyze two different parts of the Drosophila ovary, which correspond to early stages (pachytene) and late stages (metaphase I and beyond) of meiosis. In addition to taking into account morphology, the techniques differ by fixation conditions and the method to isolate the tissue. Most of these methods are whole mounts, which preserve the three-dimensional structure.

Girds 'n' cleeks o' cytokinesis: microtubule sticks and contractile hoops in cell division

Microtubules maintain an intimate relationship with the rings of anillin, septins and actomyosin filaments throughout cytokinesis. In Drosophila, peripheral microtubules emanating from the spindle poles contact the equatorial cell cortex to deliver the signal that initiates formation of the cytokinetic furrow. Mutations that affect microtubule stability lead to ectopic furrowing because peripheral microtubules contact inappropriate cortical sites. The PAV-KLP (Pavarotti-kinesin-like protein)/RacGAP50C (where GAP is GTPase-activating protein) centralspindlin complex moves towards the plus ends of microtubules to reach the cell equator. When RacGAP50C is tethered to the cell membrane, furrowing initiates at multiple non-equatorial sites, indicating that mis-localization of this single molecule is sufficient to promote furrowing. Furrow formation and ingression requires RhoA activation by the RhoGEF (guanine-nucleotide-exchange factor) Pebble, which interacts with RacGAP50C. RacGAP50C also binds anillin, which associates with actin, myosin and septins. Thus RacGAP50C plays a pivotal role during furrow formation by activating RhoA and linking the peripheral microtubules with the nascent rings through its interaction with anillin.

Wispy, the Drosophila homolog of GLD-2, is required during oogenesis and egg activation

Egg activation is the process that modifies mature, arrested oocytes so that embryo development can proceed. One key aspect of egg activation is the cytoplasmic polyadenylation of certain maternal mRNAs to permit or enhance their translation. wispy (wisp) maternal-effect mutations in Drosophila block development during the egg-to-embryo transition. We show here that the wisp gene encodes a member of the GLD-2 family of cytoplasmic poly(A) polymerases (PAPs). The WISP protein is required for poly(A) tail elongation of bicoid, Toll, and torso mRNAs upon egg activation. In Drosophila, WISP and Smaug (SMG) have previously been reported to be required to trigger the destabilization of maternal mRNAs during egg activation. SMG is the major regulator of this activity. We report here that SMG is still translated in activated eggs from wisp mutant mothers, indicating that WISP does not regulate mRNA stability by controlling the translation of smg mRNA. We have also analyzed in detail the very early developmental arrest associated with wisp mutations. Pronuclear migration does not occur in activated eggs laid by wisp mutant females. Finally, we find that WISP function is also needed during oogenesis to regulate the poly(A) tail length of dmos during oocyte maturation and to maintain a high level of active (phospho-) mitogen-activated protein kinases (MAPKs).

Novel protein-truncating mutations in the ASPM gene in families with autosomal recessive primary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a neurodevelopmental disorder that causes reduction in brain size. Individuals affected with the disorder show a small but architecturally normal cerebral cortex and are associated with mental retardation of mild-to severe form. MCPH is genetically heterogeneous with six loci, and four genes have been identified so far. Homozygous mutations in the ASPM gene, located at MCPH5 locus on chromosome 1q31, are the most common cause of MCPH particularly in the Pakistani population. In the present study, we have ascertained ten Pakistani and one Kashmiri family with primary microcephaly. We screened for potential mutations of the ASPM gene in seven consanguineous families (six Pakistani and one Kashmiri) linked to MCPH5 locus. Two previously reported (8508delGA, W1326X) and four novel sequence variants (Y1712X, I1717X, Y3353X, R3244X) were detected and all were predicted to be protein truncating. The degree of mental retardation in the affected individuals of the seven families varied from mild to moderate, and was not dependent on the location of mutations in the ASPM gene.

Spindle assembly in the oocytes of mouse and Drosophila--similar solutions to a problem

In the oocytes of many organisms a bipolar spindle is assembled in the absence of centrosomes. In this article we review how this occurs in two model organisms, Drosophila melanogaster and Mus musculus. Common themes include an important role for the chromosomes but paradoxically, organization of a bipolar spindle may not involve kinetochore microtubules. Some comparisons are not yet possible, however, since the same genes have usually not been studied in both systems.

Microcephalin coordinates mitosis in the syncytialDrosophilaembryo

Microcephalin (MCPH1) is mutated in primary microcephaly, an autosomal recessive human disorder of reduced brain size. It encodes a protein with three BRCT domains that has established roles in DNA damage signalling and the cell cycle, regulating chromosome condensation. Significant adaptive evolutionary changes in primate MCPH1 sequence suggest that changes in this gene could have contributed to the evolution of the human brain. To understand the developmental role of microcephalin we have studied its function in Drosophila. We report here that Drosophila MCPH1 is cyclically localised during the cell cycle, co-localising with DNA during interphase, but not with mitotic chromosomes. mcph1 mutant flies have a maternal effect lethal phenotype, due to mitotic arrest occurring in early syncytial cell cycles. Mitotic entry is slowed from the very first mitosis in such embryos, with prolonged prophase and metaphase stages; and frequent premature separation as well as detachment of centrosomes. As a consequence, centrosome and nuclear cycles become uncoordinated, resulting in arrested embryonic development. Phenotypic similarities with abnormal spindle (asp) and centrosomin (cnn) mutants (whose human orthologues are also mutated in primary microcephaly), suggest that further studies in the Drosophila embryo may establish a common developmental and cellular pathway underlying the human primary microcephaly phenotype.

Native nonmuscle myosin II stability and light chain binding inDrosophila melanogaster

Native nonmuscle myosin IIs play essential roles in cellular and developmental processes throughout phylogeny. Individual motor molecules consist of a heterohexameric complex of three polypeptides which, when properly assembled, are capable of force generation. Here, we more completely characterize the properties, relationships and associations that each subunit has with one another in Drosophila melanogaster. All three native nonmuscle myosin II polypeptide subunits are expressed in close to constant stoichiometry to each other throughout development. We find that the stability of two subunits, the heavy chain and the regulatory light chain, depend on one another whereas the stability of the third subunit, the essential light chain, does not depend on either the heavy chain or regulatory light chain. We demonstrate that heavy chain aggregates, which form when regulatory light chain is lacking, associate with the essential light chain in vivo-thus showing that regulatory light chain association is required for heavy chain solubility. By immunodepletion we find that the majority of both light chains are associated with the nonmuscle myosin II heavy chain but pools of free light chain and/or light chain bound to other proteins are present. We identify four myosins (myosin II, myosin V, myosin VI and myosin VIIA) and a microtubule-associated protein (asp/Abnormal spindle) as binding partners for the essential light chain (but not the regulatory light chain) through mass spectrometry and co-precipitation. Using an in silico approach we identify six previously uncharacterized genes that contain IQ-motifs and may be essential light chain binding partners.

The gammaTuRC components Grip75 and Grip128 have an essential microtubule-anchoring function in the Drosophila germline

The gamma-tubulin ring complex (gammaTuRC) forms an essential template for microtubule nucleation in animal cells. The molecular composition of the gammaTuRC has been described; however, the functions of the subunits proposed to form the cap structure remain to be characterized in vivo. In Drosophila, the core components of the gammaTuRC are essential for mitosis, whereas the cap component Grip75 is not required for viability but functions in bicoid RNA localization during oogenesis. The other cap components have not been analyzed in vivo. We report the functional characterization of the cap components Grip128 and Grip75. Animals with mutations in Dgrip128 or Dgrip75 are viable, but both males and females are sterile. Both proteins are required for the formation of distinct sets of microtubules, which facilitate bicoid RNA localization during oogenesis, the formation of the central microtubule aster connecting the meiosis II spindles in oocytes and cytokinesis in male meiosis. Grip75 and Grip128 anchor the axoneme at the nucleus during sperm elongation. We propose that Grip75 and Grip128 are required to tether microtubules at specific microtubule-organizing centers, instead of being required for general microtubule nucleation. The gammaTuRC cap structure may be essential only for non-centrosome-based microtubule functions.

What primary microcephaly can tell us about brain growth

Autosomal recessive primary microcephaly (MCPH) is a neuro-developmental disorder that causes a great reduction in brain growth in utero. MCPH is hypothesized to be a primary disorder of neurogenic mitosis, leading to reduced neuron number. Hence, MCPH proteins are likely to be important components of cellular pathways regulating human brain size. At least six genes can cause this disorder and four of these have recently been identified: autosomal recessive primary microcephaly 1 (MCPH1), abnormal spindle-like, microcephaly associated (ASPM), cyclin-dependent kinase 5 regulatory subunit-associated protein 2 (CDK5RAP2) and centromere protein J (CENPJ). Whereas aberration of ASPM is the most common cause of MCPH, MCPH1 patients can be more readily diagnosed by the finding of increased numbers of "prophase-like cells" on routine cytogenetic investigation. Three MCPH proteins are centrosomal components but have apparently diverse roles that affect mitosis. There is accumulating evidence that evolutionary changes to the MCPH genes have contributed to the large brain size seen in primates, particularly humans. The aim of this article is to review what has been learnt about the rare condition primary microcephaly and the information this provides about normal brain growth.

Deadlock, a novel protein of Drosophila, is required for germline maintenance, fusome morphogenesis and axial patterning in oogenesis and associates with centrosomes in the early embryo

The deadlock gene is required for a number of key developmental events in Drosophila oogenesis. Females homozygous for mutations in the deadlock gene lay few eggs and those exhibit severe patterning defects along both the anterior-posterior and dorsal-ventral axis. In this study, we analyzed eggs and ovaries from deadlock mutants and determined that deadlock is required for germline maintenance, stability of mitotic spindles, localization of patterning determinants, oocyte growth and fusome biogenesis in males and females. Deadlock encodes a novel protein which colocalizes with the oocyte nucleus at midstages of oogenesis and with the centrosomes of early embryos. Our genetic and immunohistological experiments point to a role for Deadlock in microtubule function during oogenesis.

The meiotic spindle of the Drosophila oocyte: the role of centrosomin and the central aster

We provide here the first evidence that a distinct midzone is present in the Drosophila melanogaster female meiosis I spindle. This region has the ability to bind the Pavarotti kinesin-like (PAV-KLP) and Abnormal spindle (Asp) proteins, indicating a correct organization of the central spindle microtubules. We also identified the core component centrosomal protein centrosomin (CNN) at an unexpected site within the anaphase I spindle, indicating a role for CNN during the biogenesis of the female meiotic apparatus. However, there are no apparent defects in the midzone organization of cnn oocytes, whereas defects occur later when the central aster forms. The primary mutant phenotype of cnn oocytes is the failure to form a developed central microtubule organizing center (MTOC), although twin meiosis II spindles usually do form. Thus the central MTOC may not be essential for the formation of the inner poles of twin meiosis II spindles, as generally proposed, but it might be involved in maintaining their proper spacing. We discuss the proposal that, in the presence of a central MTOC, a chromatin-driven mechanism of spindle assembly like that described during meiosis I may control the morphogenesis of the twin meiosis II spindles.

Evolution of primary microcephaly genes and the enlargement of primate brains

Brain size, in relation to body size, has varied markedly during the evolution of mammals. In particular, a large cerebral cortex is a feature that distinguishes humans from our fellow primates. Such anatomical changes must have a basis in genetic alterations, but the molecular processes involved have yet to be defined. However, recent advances from the cloning of two human disease genes promise to make inroads in this important area. Microcephalin (MCPH1) and Abnormal spindle-like microcephaly associated (ASPM) are genes mutated in primary microcephaly, a human neurodevelopmental disorder. In this 'atavistic' condition, brain size is reduced in volume to a size comparable with that of early hominids. Hence, it has been proposed that these genes evolved adaptively with increasing primate brain size. Subsequent studies have lent weight to this hypothesis by showing that both genes have undergone positive selection during great ape evolution. Further functional characterisation of their proteins will contribute to an understanding of the molecular and evolutionary processes that have determined human brain size.

The Drosophila gamma-tubulin small complex subunit Dgrip84 is required for structural and functional integrity of the spindle apparatus

Gamma-tubulin, a protein critical for microtubule assembly, functions within multiprotein complexes. However, little is known about the respective role of gamma-tubulin partners in metazoans. For the first time in a multicellular organism, we have investigated the function of Dgrip84, the Drosophila orthologue of the Saccharomyces cerevisiae gamma-tubulin-associated protein Spc97p. Mutant analysis shows that Dgrip84 is essential for viability. Its depletion promotes a moderate increase in the mitotic index, correlated with the appearance of monopolar or unpolarized spindles, impairment of centrosome maturation, and increase of polyploid nuclei. This in vivo study is strengthened by an RNA interference approach in cultured S2 cells. Electron microscopy analysis suggests that monopolar spindles might result from a failure of centrosome separation and an unusual microtubule assembly pathway via centriolar triplets. Moreover, we point to an involvement of Dgrip84 in the spindle checkpoint regulation and in the maintenance of interphase microtubule dynamics. Dgrip84 also seems essential for male meiosis, ensuring spindle bipolarity and correct completion of cytokinesis. These data sustain that Dgrip84 is required in some aspects of microtubule dynamics and organization both in interphase and mitosis. The nature of a minimal gamma-tubulin complex necessary for proper microtubule organization in the metazoans is discussed.