MEI-1/MEI-2 katanin-like microtubule severing activity is required for Caenorhabditis elegans meiosis

Mechanisms of Meiotic Spindle Initiation in Caenorhabditis elegans Oocytes

Microtubule-based spindle formation is essential to faithful chromosome segregation during cell division. In many animal species, the oocyte meiotic spindle forms without centrosomes, unlike most mitotic cells. Even in mitotic cells, centrosomes are sometimes dispensable for bipolar spindle formation. In some systems, Ran-GEF on chromatin initiates spindle assembly. We found that in C. elegans oocytes, endogenously-tagged Ran-GEF dissociates from chromatin during spindle assembly but re-associates during meiotic anaphase. Meiotic spindle assembly was normal after auxin-induced degradation of Ran-GEF but anaphase I was faster than controls and extrusion of the first polar body frequently failed. In search of a possible alternative pathway for spindle assembly, we found that soluble tubulin concentrates in the nuclear volume during germinal vesicle breakdown as well as in the spindle region during metaphase I and metaphase II. Through light and electron microscopy we found that the concentration of soluble tubulin in the metaphase II spindle region is enclosed by ER sheets which exclude cytoplasmic organelles including mitochondria and yolk granules from the meiotic spindle. We suggest that this concentration of soluble tubulin may be a redundant mechanism promoting spindle assembly near chromosomes. We present data supporting a model in which cytoplasmic organelles exclude cytoplasmic volume to drive concentration of tubulin within the nuclear/spindle envelope.

Microtubule-binding domains in Katanin p80 subunit are essential for severing activity in C. elegans

Microtubule-severing enzymes (MSEs), such as Katanin, Spastin, and Fidgetin play essential roles in cell division and neurogenesis. They damage the microtubule (MT) lattice, which can either destroy or amplify the MT cytoskeleton, depending on the cellular context. However, little is known about how they interact with their substrates. We have identified the microtubule-binding domains (MTBD) required for Katanin function in C. elegans. Katanin is a heterohexamer of dimers containing a catalytic subunit p60 and a regulatory subunit p80, both of which are essential for female meiotic spindle assembly. Here, we report that p80-like(MEI-2) dictates Katanin binding to MTs via two MTBDs composed of basic patches. Substituting these patches reduces Katanin binding to MTs, compromising its function in female meiotic-spindle assembly. Structural alignments of p80-like(MEI-2) with p80s from different species revealed that the MTBDs are evolutionarily conserved, even if the specific amino acids involved vary. Our findings highlight the critical importance of the regulatory subunit (p80) in providing MT binding to the Katanin complex.

Microtubule length correlates with spindle length in C. elegans meiosis

The accurate segregation of chromosomes during female meiosis relies on the precise assembly and function of the meiotic spindle, a dynamic structure primarily composed of microtubules. Despite the crucial role of microtubule dynamics in this process, the relationship between microtubule length and spindle size remains elusive. Leveraging Caenorhabditis elegans as a model system, we combined electron tomography and live imaging to investigate this correlation. Our analysis revealed significant changes in spindle length throughout meiosis, coupled with alterations in microtubule length. Surprisingly, while spindle size decreases during the initial stages of anaphase, the size of antiparallel microtubule overlap decreased as well. Detailed electron tomography shows a positive correlation between microtubule length and spindle size, indicating a role of microtubule length in determining spindle dimensions. Notably, microtubule numbers displayed no significant association with spindle length, highlighting the dominance of microtubule length regulation in spindle size determination. Depletion of the microtubule depolymerase KLP-7 led to elongated metaphase spindles with increased microtubule length, supporting the link between microtubule length and spindle size. These findings underscore the pivotal role of regulating microtubule dynamics, and thus microtubule length, in governing spindle rearrangements during meiotic division, shedding light on fundamental mechanisms dictating spindle architecture.

CRMP5 participates in oocyte meiosis by regulating spastin to correct microtubule-kinetochore misconnection

Our previous studies have suggested that spastin, which aggregates on spindle microtubules in oocytes, may promote the assembly of mouse oocyte spindles by cutting microtubules. This action may be related to CRMP5, as knocking down CRMP5 results in reduced spindle microtubule density and maturation defects in oocytes. In this study, we found that, after knocking down CRMP5 in oocytes, spastin distribution shifted from the spindle to the spindle poles and errors in microtubule-kinetochore attachment appeared in oocyte spindles. However, CRMP5 did not interact with the other two microtubule-severing proteins, katanin-like-1 (KATNAL1) and fidgetin-like-1 (FIGNL1), which aggregate at the spindle poles. We speculate that, in oocytes, due to the reduction of spastin distribution on chromosomes after knocking down CRMP5, microtubule-kinetochore errors cannot be corrected through severing, resulting in meiotic division abnormalities and maturation defects in oocytes. This finding provides new insights into the regulatory mechanisms of spastin in oocytes and important opportunities for the study of meiotic division mechanisms.

Combinatorial and antagonistic effects of tubulin glutamylation and glycylation on katanin microtubule severing

Microtubules have spatiotemporally complex posttranslational modification patterns. How cells interpret this tubulin modification code is largely unknown. We show that C. elegans katanin, a microtubule severing AAA ATPase mutated in microcephaly and critical for cell division, axonal elongation, and cilia biogenesis, responds precisely, differentially, and combinatorially to three chemically distinct tubulin modifications-glycylation, glutamylation, and tyrosination-but is insensitive to acetylation. Glutamylation and glycylation are antagonistic rheostats with glycylation protecting microtubules from severing. Katanin exhibits graded and divergent responses to glutamylation on the α- and β-tubulin tails, and these act combinatorially. The katanin hexamer central pore constrains the polyglutamate chain patterns on β-tails recognized productively. Elements distal to the katanin AAA core sense α-tubulin tyrosination, and detyrosination downregulates severing. The multivalent microtubule recognition that enables katanin to read multiple tubulin modification inputs explains in vivo observations and illustrates how effectors can integrate tubulin code signals to produce diverse functional outcomes.

Cohesin is required for meiotic spindle assembly independent of its role in cohesion in C. elegans

Accurate chromosome segregation requires a cohesin-mediated physical attachment between chromosomes that are to be segregated apart, and a bipolar spindle with microtubule plus ends emanating from exactly two poles toward the paired chromosomes. We asked whether the striking bipolar structure of C. elegans meiotic chromosomes is required for bipolarity of acentriolar female meiotic spindles by time-lapse imaging of mutants that lack cohesion between chromosomes. Both a spo-11 rec-8 coh-4 coh-3 quadruple mutant and a spo-11 rec-8 double mutant entered M phase with separated sister chromatids lacking any cohesion. However, the quadruple mutant formed an apolar spindle whereas the double mutant formed a bipolar spindle that segregated chromatids into two roughly equal masses. Residual non-cohesive COH-3/4-dependent cohesin on separated sister chromatids of the double mutant was sufficient to recruit haspin-dependent Aurora B kinase, which mediated bipolar spindle assembly in the apparent absence of chromosomal bipolarity. We hypothesized that cohesin-dependent Aurora B might activate or inhibit spindle assembly factors in a manner that would affect their localization on chromosomes and found that the chromosomal localization patterns of KLP-7 and CLS-2 correlated with Aurora B loading on chromosomes. These results demonstrate that cohesin is essential for spindle assembly and chromosome segregation independent of its role in sister chromatid cohesion.

Microtubule-severing protein Fidgetin-like 1 promotes spindle organization during meiosis of mouse oocytes

Microtubule-severing proteins (MTSPs) play important roles in mitosis and interphase. However, to the best of our knowledge, no previous studies have evaluated the role of MTSPs in female meiosis in mammals. It was found that FIGNL1, a member of MTSPs, was predominantly expressed in mouse oocytes and distributed at the spindle poles during meiosis in the present study. FIGNL1 was co-localized and interacted with γ-tubulin, an important component of the microtubule tissue centre (MTOC). Fignl1 knockdown by specific small interfering RNA caused spindle defects characterized by an abnormal length:width ratio and decreased microtubule density, which consequently led to aberrant chromosome arrangement, oocyte maturation and fertilization obstacles. In conclusion, the present results suggested that FIGNL1 may be an essential factor in oocyte maturation by influencing the meiosis process via the formation of spindles.

STRIPAK regulation of katanin microtubule severing in the Caenorhabditis elegans embryo

Microtubule severing plays important role in cell structure and cell division. The microtubule severing protein katanin, composed of the MEI-1/MEI-2 subunits in Caenorhabditis elegans, is required for oocyte meiotic spindle formation; however, it must be inactivated for mitosis to proceed as continued katanin expression is lethal. Katanin activity is regulated by 2 ubiquitin-based protein degradation pathways. Another ubiquitin ligase, HECD-1, the homolog of human HECTD1/HECT domain E3 ubiquitin protein ligase 1, regulates katanin activity without affecting katanin levels. In other organisms, HECD-1 is a component of the striatin-interacting kinase phosphatase complex, which affects cell proliferation and a variety of signaling pathways. Here we conducted a systematic screen of how mutations in striatin-interacting kinase phosphatase components affect katanin function in C. elegans. Striatin-interacting kinase phosphatase core components (FARL-11, CASH-1, LET-92, and GCK-1) were katanin inhibitors in mitosis and activators in meiosis, much like HECD-1. By contrast, variable components (SLMP-1, OTUB-2) functioned as activators of katanin activity in mitosis, indicating they may function to alter striatin-interacting kinase phosphatase core function. The core component CCM-3 acted as an inhibitor at both divisions, while other components (MOB-4, C49H3.6) showed weak interactions with katanin mutants. Additional experiments indicate that katanin may be involved with the centralspindlin complex and a tubulin chaperone. HECD-1 shows ubiquitous expression in the cytoplasm throughout meiosis and early development. The differing functions of the different subunits could contribute to the diverse functions of the striatin-interacting kinase phosphatase complex in C. elegans and other organisms.

Multiple motors cooperate to establish and maintain acentrosomal spindle bipolarity in elegans oocyte meiosis

While centrosomes organize spindle poles during mitosis, oocyte meiosis can occur in their absence. Spindles in human oocytes frequently fail to maintain bipolarity and consequently undergo chromosome segregation errors, making it important to understand the mechanisms that promote acentrosomal spindle stability. To this end, we have optimized the auxin-inducible degron system in Caenorhabditis elegans to remove the factors from pre-formed oocyte spindles within minutes and assess the effects on spindle structure. This approach revealed that dynein is required to maintain the integrity of acentrosomal poles; removal of dynein from bipolar spindles caused pole splaying, and when coupled with a monopolar spindle induced by depletion of the kinesin-12 motor KLP-18, dynein depletion led to a complete dissolution of the monopole. Surprisingly, we went on to discover that following monopole disruption, individual chromosomes were able to reorganize local microtubules and re-establish a miniature bipolar spindle that mediated chromosome segregation. This revealed the existence of redundant microtubule sorting forces that are undetectable when KLP-18 and dynein are active. We found that the kinesin-5 family motor BMK-1 provides this force, uncovering the first evidence that kinesin-5 contributes to C. elegans meiotic spindle organization. Altogether, our studies have revealed how multiple motors are working synchronously to establish and maintain bipolarity in the absence of centrosomes.

Meiosis is a specialized form of cell division that produces the gametes required for sexual reproduction, such as egg and sperm cells. Before the cell splits, it copies its genome so that it has four sets of chromosomes. Genetic information is then shuffled between the chromosomes, and the cell undergoes two rounds of division, resulting in four gametes that are genetically distinct.

Prior to division, the duplicated chromosomes are separated by rope-like protein polymers called microtubules. In most cells, structures called centrosomes organize these fibers into a spindle shape that emanates from two ‘poles’ on opposite ends of the cell: the microtubules then attach to the chromosomes and pull them apart. Despite not having centrosomes, egg cells, or ‘oocytes’, are still able to arrange their microtubules into a similar bipolar shape. However, how oocytes form these ‘acentrosomal’ spindles is poorly understood.

Centrosomes do not organize the spindle alone, and receive help from various motor proteins such as dynein. Previous work showed that dynein is involved in arranging acentrosomal poles, but it was not known if it was required to hold the poles together after they initially formed. To investigate, Cavin-Meza et al. developed a strategy that can rapidly remove dynein from oocytes of the roundworm Caenorhabditis elegans.

The experiment showed that dynein is required both to assemble and stabilize acentrosomal spindles in C. elegans. When dynein and an additional motor protein, KLP-18, were both removed from oocytes simultaneously, the poles blew apart, completely disrupting spindle organization. Surprisingly, Cavin-Meza et al. found that the spindles were able to reform and separate the chromosomes. Further probing revealed, for the first time, that a third motor protein (called BMK-1) also helps to organize the spindle into its bipolar structure.

These findings reveal the important role motor proteins play in stabilizing spindles and separating chromosomes in oocytes. Meiosis is prone to mistakes, and these errors are a major cause of miscarriages and birth defects in humans. Therefore, understanding the underlying mechanisms of how oocyte spindles form and remain stable could shed light on why chromosomes sometimes fail to segregate. This may eventually lead to new strategies for combating infertility.

Multiple roles of endocytosis and autophagy in intracellular remodeling during oocyte-to-embryo transition

Fertilization is the starting point for creating new progeny. At this time, the highly differentiated oocyte and sperm fuse to form one zygote, which is then converted into a pluripotent early embryo. Recent studies have shown that the lysosomal degradation system via autophagy and endocytosis plays important roles in the remodeling of intracellular components during oocyte-to-embryo transition. For example, in Caenorhabditis elegans, zygotes show high endocytic activity, and some populations of maternal membrane proteins are selectively internalized and delivered to lysosomes for degradation. Furthermore, fertilization triggers selective autophagy of sperm-derived paternal mitochondria, which establishes maternal inheritance of mitochondrial DNA. In addition, it has been shown that autophagy via liquid-liquid phase separation results in the selective degradation of some germ granule components, which are distributed to somatic cells of early embryos. This review outlines the physiological functions of the lysosomal degradation system and its molecular mechanisms in C. elegans and mouse embryos.

KATNB1 is a master regulator of multiple katanin enzymes in male meiosis and haploid germ cell development

Katanin microtubule-severing enzymes are crucial executers of microtubule regulation. Here, we have created an allelic loss-of-function series of the katanin regulatory B-subunit KATNB1 in mice. We reveal that KATNB1 is the master regulator of all katanin enzymatic A-subunits during mammalian spermatogenesis, wherein it is required to maintain katanin A-subunit abundance. Our data shows that complete loss of KATNB1 from germ cells is incompatible with sperm production, and we reveal multiple new spermatogenesis functions for KATNB1, including essential roles in male meiosis, acrosome formation, sperm tail assembly, regulation of both the Sertoli and germ cell cytoskeletons during sperm nuclear remodelling, and maintenance of seminiferous epithelium integrity. Collectively, our findings reveal that katanins are able to differentially regulate almost all key microtubule-based structures during mammalian male germ cell development, through the complexing of one master controller, KATNB1, with a 'toolbox' of neofunctionalised katanin A-subunits.

Spastin interacts with CRMP5 to promote spindle organization in mouse oocytes by severing microtubules

Microtubule-severing protein (MTSP) is critical for the survival of both mitotic and postmitotic cells. However, the study of MTSP during meiosis of mammalian oocytes has not been reported. We found that spastin, a member of the MTSP family, was highly expressed in oocytes and aggregated in spindle microtubules. After knocking down spastin by specific siRNA, the spindle microtubule density of meiotic oocytes decreased significantly. When the oocytes were cultured in vitro, the oocytes lacking spastin showed an obvious maturation disorder. Considering the microtubule-severing activity of spastin, we speculate that spastin on spindles may increase the number of microtubule broken ends by severing the microtubules, therefore playing a nucleating role, promoting spindle assembly and ensuring normal meiosis. In addition, we found the colocalization and interaction of collapsin response mediator protein 5 (CRMP5) and spastin in oocytes. CRMP5 can provide structural support and promote microtubule aggregation, creating transportation routes, and can interact with spastin in the microtubule activity of nerve cells (30). Knocking down CRMP5 may lead to spindle abnormalities and developmental disorders in oocytes. Overexpression of spastin may reverse the abnormal phenotype caused by the deletion of CRMP5. In summary, our data support a model in which the interaction between spastin and CRMP5 promotes the assembly of spindle microtubules in oocytes by controlling microtubule dynamics, therefore ensuring normal meiosis.

ABNORMAL SHOOT 6 interacts with KATANIN 1 and SHADE AVOIDANCE 4 to promote cortical microtubule severing and ordering in Arabidopsis

Plant interphase cortical microtubules (cMTs) mediate anisotropic cell expansion in response to environmental and developmental cues. In Arabidopsis thaliana, KATANIN 1 (KTN1), the p60 catalytic subunit of the conserved MT-severing enzyme katanin, is essential for cMT ordering and anisotropic cell expansion. However, the regulation of KTN1-mediated cMT severing and ordering remains unclear. In this work, we report that the Arabidopsis IQ67 DOMAIN (IQD) family gene ABNORMAL SHOOT 6 (ABS6) encodes a MT-associated protein. Overexpression of ABS6 leads to elongated cotyledons, directional pavement cell expansion, and highly ordered transverse cMT arrays. Genetic suppressor analysis revealed that ABS6-mediated cMT ordering is dependent on KTN1 and SHADE AVOIDANCE 4 (SAV4). Live imaging of cMT dynamics showed that both ABS6 and SAV4 function as positive regulators of cMT severing. Furthermore, ABS6 directly interacts with KTN1 and SAV4 and promotes their recruitment to the cMTs. Finally, analysis of loss-of-function mutant combinations showed that ABS6, SAV4, and KTN1 work together to ensure the robust ethylene response in the apical hook of dark-grown seedlings. Together, our findings establish ABS6 and SAV4 as positive regulators of cMT severing and ordering, and highlight the role of cMT dynamics in fine-tuning differential growth in plants.

Multiple functions of DYRK2 in cancer and tissue development

Dual-specificity tyrosine-regulated kinases (DYRKs) are evolutionarily conserved from yeast to mammals. Accumulating studies have revealed that DYRKs have important roles in regulation of the cell cycle and survival. DYRK2, a member of the class II DYRK family protein, is a key regulator of p53, and phosphorylates it at Ser46 to induce apoptosis in response to DNA damage. Moreover, recent studies have uncovered that DYRK2 regulates G1/S transition, epithelial-mesenchymal-transition, and stemness in human cancer cells. DYRK2 also appears to have roles in tissue development in lower eukaryotes. Thus, the elucidation of mechanisms for DYRK2 during mammalian tissue development will promote the understanding of cell differentiation, tissue homeostasis, and congenital diseases as well as cancer. In this review, we discuss the roles of DYRK2 in tumor cells. Moreover, we focus on DYRK2-dependent developmental mechanisms in several species including fly (Drosophila), worm (Caenorhabditis elegans), zebrafish (Danio rerio), and mammals.

Crystal Structure of a Heterotetrameric Katanin p60:p80 Complex

Katanin is a microtubule-severing enzyme that is crucial for many cellular processes. Katanin consists of two subunits, p60 and p80, that form a stable complex. The interaction between subunits is mediated by the p60 N-terminal microtubule-interacting and -trafficking domain (p60-MIT) and the p80 C-terminal domain (p80-CTD). Here, we performed a biophysical characterization of the mouse p60-MIT:p80-CTD heterodimer and show that this complex can assemble into heterotetramers. We identified two mutations that enhance heterotetramer formation and determined the X-ray crystal structure of this mutant complex. The structure revealed a domain-swapped heterotetramer consisting of two p60-MIT:p80-CTD heterodimers. Structure-based sequence alignments suggest that heterotetramerization of katanin might be a common feature of various species. Furthermore, we show that enhanced heterotetramerization of katanin impairs its microtubule end-binding properties and increases the enzyme's microtubule lattice binding and severing activities. Therefore, our findings suggest the existence of different katanin oligomers that possess distinct functional properties.

Short-term starvation stress at young adult stages enhances meiotic activity of germ cells to maintain spermatogenesis in aged male Caenorhabditis elegans

To survive and reproduce, living organisms must evolve numerous mechanisms to re‐adjust their physiology when encountering adverse conditions that subject them to severe stress. We found that short‐term starvation (STS) stress in young adult male Caenorhabditis elegans can significantly improve their vitality (relative to nonstressed males) when they are aged. In addition, we found that stress‐treated aged males maintained reproductive activity equivalent to young males, whereas nonstressed aged males quickly lost reproductive ability. STS stress can preserve sperm number and quality in aged male worms. Spermatogenesis involves germ cell mitosis and meiosis. We found that germ cell meiotic activity is more sensitive to aging than mitotic activity and is declining rapidly with age. We examined the role of numerous factors important for spermatogenesis on STS‐preserved spermatogenesis during aging. Our results show that mutant strains deficient in anaphase‐promoting complex/cyclosome (APC/C) function fail to exhibit the STS stress‐enhanced spermatogenesis found in wild‐type N2 worms, suggesting that the mechanism underlying starvation‐induced spermatogenesis involves the APC/C complex, a conserved ubiquitin‐protein ligase E3 complex. Furthermore, transgenic expression of FZY‐1/CDC‐20, a coactivator of APC/C, ameliorated the age‐associated decline of meiosis, similar to the hormetic effect of STS.

Spindle assembly and chromosome dynamics during oocyte meiosis

Organisms that reproduce sexually utilize a specialized form of cell division called meiosis to reduce their chromosome number by half to generate haploid gametes. Meiosis in females is especially error-prone, and this vulnerability has a profound impact on human health: it is estimated that 10-25% of human embryos are chromosomally abnormal, and the vast majority of these defects arise from problems with the female reproductive cells (oocytes). Here, we highlight recent studies that explore how these important cells divide. Although we focus on work in the model organism Caenorhabditis elegans, we also discuss complementary studies in other organisms that together provide new insights into this crucial form of cell division.

Katanin catalyzes microtubule depolymerization independently of tubulin C-terminal tails

Microtubule network remodeling is an essential process for cell development, maintenance, cell division, and motility. Microtubule‐severing enzymes are key players in the remodeling of the microtubule network; however, there are still open questions about their fundamental biochemical and biophysical mechanisms. Here, we explored the ability of the microtubule‐severing enzyme katanin to depolymerize stabilized microtubules. Interestingly, we found that the tubulin C‐terminal tail (CTT), which is required for severing, is not required for katanin‐catalyzed depolymerization. We also found that the depolymerization of microtubules lacking the CTT does not require ATP or katanin's ATPase activity, although the ATP turnover enhanced depolymerization. We also observed that the depolymerization rate depended on the katanin concentration and was best described by a hyperbolic function. Finally, we demonstrate that katanin can bind to filaments that lack the CTT, contrary to previous reports. The results of our work indicate that microtubule depolymerization likely involves a mechanism in which binding, but not enzymatic activity, is required for tubulin dimer removal from the filament ends.

Identification of piRNAs and piRNA clusters in the testes of the Mongolian horse

P-element induced wimpy testis-interacting RNAs (piRNAs) are essential for testicular development and spermatogenesis in mammals. Comparative analyses of the molecular mechanisms of spermatogenesis among different organisms are therefore dependent on accurate characterizations of piRNAs. At present, little is known of piRNAs in non-model organisms. Here, we characterize piRNAs in the Mongolian horse, a hardy breed that reproduces under extreme circumstances. A thorough understanding of spermatogenesis and reproduction in this breed may provide insights for the improvement of fecundity and reproductive success in other breeds. We identified 4,936,717 piRNAs and 7,890 piRNA clusters across both testicular developmental stages. Of these, 2,236,377 putative piRNAs were expressed in the mature samples only, and 2,391,271 putative piRNAs were expressed in the immature samples only. Approximately 3,016 piRNA clusters were upregulated in the mature testes as compared to the immature testes, and 4,874 piRNA clusters were downregulated. Functional and pathway analyses indicated that the candidate generating genes of the predicted piRNAs were likely involved in testicular development and spermatogenesis. Our results thus provide information about differential expression patterns in genes associated with testicular development and spermatogenesis in a non-model animal.

A computational model of the early stages of acentriolar meiotic spindle assembly

The mitotic spindle is an ensemble of microtubules responsible for the repartition of the chromosomal content between the two daughter cells during division. In metazoans, spindle assembly is a gradual process involving dynamic microtubules and recruitment of numerous associated proteins and motors. During mitosis, centrosomes organize and nucleate the majority of spindle microtubules. In contrast, oocytes lack canonical centrosomes but are still able to form bipolar spindles, starting from an initial ball that self-organizes in several hours. Interfering with early steps of meiotic spindle assembly can lead to erroneous chromosome segregation. Although not fully elucidated, this process is known to rely on antagonistic activities of plus end– and minus end–directed motors. We developed a model of early meiotic spindle assembly in mouse oocytes, including key factors such as microtubule dynamics and chromosome movement. We explored how the balance between plus end– and minus end–directed motors, as well as the influence of microtubule nucleation, impacts spindle morphology. In a refined model, we added spatial regulation of microtubule stability and minus-end clustering. We could reproduce the features of early stages of spindle assembly from 12 different experimental perturbations and predict eight additional perturbations. With its ability to characterize and predict chromosome individualization, this model can help deepen our understanding of spindle assembly.

Causative Mutations and Mechanism of Androgenetic Hydatidiform Moles

Androgenetic complete hydatidiform moles are human pregnancies with no embryos and affect 1 in every 1,400 pregnancies. They have mostly androgenetic monospermic genomes with all the chromosomes originating from a haploid sperm and no maternal chromosomes. Androgenetic complete hydatidiform moles were described in 1977, but how they occur has remained an open question. We identified bi-allelic deleterious mutations in MEI1, TOP6BL/C11orf80, and REC114, with roles in meiotic double-strand breaks formation in women with recurrent androgenetic complete hydatidiform moles. We investigated the occurrence of androgenesis in Mei1-deficient female mice and discovered that 8% of their oocytes lose all their chromosomes by extruding them with the spindles into the first polar body. We demonstrate that Mei1^-/- oocytes are capable of fertilization and 5% produce androgenetic zygotes. Thus, we uncover a meiotic abnormality in mammals and a mechanism for the genesis of androgenetic zygotes that is the extrusion of all maternal chromosomes and their spindles into the first polar body.

A Switch in Microtubule Orientation during C. elegans Meiosis

In oocytes of many organisms, meiotic spindles form in the absence of centrosomes [1-5]. Such female meiotic spindles have a pointed appearance in metaphase with microtubules focused at acentrosomal spindle poles. At anaphase, the microtubules of acentrosomal spindles then transition to an inter-chromosomal array, while the spindle poles disappear. This transition is currently not understood. Previous studies have focused on this inter-chromosomal microtubule array and proposed a pushing model to drive chromosome segregation [6, 7]. This model includes an end-on orientation of microtubules with chromosomes. Alternatively, chromosomes were thought to associate along bundles of microtubules [8, 9]. Starting with metaphase, this second model proposed a pure lateral chromosome-to-microtubule association up to the final meiotic stages of anaphase. Here, we applied large-scale electron tomography [10] of staged C. elegans oocytes in meiosis to analyze the orientation of microtubules in respect to chromosomes. We show that microtubules at metaphase I are primarily oriented laterally to the chromosomes and that microtubules switch to an end-on orientation during progression through anaphase. We further show that this switch in microtubule orientation involves a kinesin-13 microtubule depolymerase, KLP-7, which removes laterally associated microtubules around chromosomes. From this, we conclude that both lateral and end-on modes of microtubule-to-chromosome orientations are successively used in C. elegans oocytes to segregate meiotic chromosomes.

Microtubule Dynamics Scale with Cell Size to Set Spindle Length and Assembly Timing

Successive cell divisions during embryonic cleavage create increasingly smaller cells, so intracellular structures must adapt accordingly. Mitotic spindle size correlates with cell size, but the mechanisms for this scaling remain unclear. Using live cell imaging, we analyzed spindle scaling during embryo cleavage in the nematode Caenorhabditis elegans and sea urchin Paracentrotus lividus. We reveal a common scaling mechanism, where the growth rate of spindle microtubules scales with cell volume, which explains spindle shortening. Spindle assembly timing is, however, constant throughout successive divisions. Analyses in silico suggest that controlling the microtubule growth rate is sufficient to scale spindle length and maintain a constant assembly timing. We tested our in silico predictions to demonstrate that modulating cell volume or microtubule growth rate in vivo induces a proportional spindle size change. Our results suggest that scalability of the microtubule growth rate when cell size varies adapts spindle length to cell volume.

Structural basis for disassembly of katanin heterododecamers

The reorganization of microtubules in mitosis, meiosis, and development requires the microtubule-severing activity of katanin. Katanin is a heterodimer composed of an ATPase associated with diverse cellular activities (AAA) subunit and a regulatory subunit. Microtubule severing requires ATP hydrolysis by katanin's conserved AAA ATPase domains. Whereas other AAA ATPases form stable hexamers, we show that katanin forms only a monomer or dimers of heterodimers in solution. Katanin oligomers consistent with hexamers of heterodimers or heterododecamers were only observed for an ATP hydrolysis-deficient mutant in the presence of ATP. X-ray structures of katanin's AAA ATPase in monomeric nucleotide-free and pseudo-oligomeric ADP-bound states revealed conformational changes in the AAA subdomains that explained the structural basis for the instability of the katanin heterododecamer. We propose that the rapid dissociation of katanin AAA oligomers may lead to an autoinhibited state that prevents inappropriate microtubule severing or that cyclical disassembly into heterodimers may critically contribute to the microtubule-severing mechanism.

Post-translational regulation of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is essential for the developmental control handed from maternal products to newly synthesized zygotic genome in the earliest stages of embryogenesis, including maternal component (mRNAs and proteins) degradation and zygotic genome activation (ZGA). Various protein post-translational modifications have been identified during the MZT, such as phosphorylation, methylation and ubiquitination. Precise post-translational regulation mechanisms are essential for the timely transition of early embryonic development. In this review, we summarize recent progress regarding the molecular mechanisms underlying post-translational regulation of maternal component degradation and ZGA during the MZT and discuss some important issues in the field.

Structural basis of katanin p60:p80 complex formation

Interactions between microtubule (MT) interacting and trafficking (MIT) domains and their binding proteins are important for the accurate progression of many cellular processes that require the AAA+ ATPase machinery. Therefore, knowledge on the structural basis of MIT domain interactions is crucial for understanding the molecular mechanisms underlying AAA+ ATPase function. Katanin is a MT-severing AAA+ ATPase that consists of p60 and p80 subunits. Although, the hexameric p60 subunit is active alone, its association with the p80 subunit greatly enhances both the MT-binding and -severing activities of katanin. However, the molecular mechanism of how the p80 subunit contributes to katanin function is currently unknown. Here, we structurally and functionally characterized the interaction between the two katanin subunits that is mediated by the p60-MIT domain and the p80 C-terminal domain (p80-CTD). We show that p60-MIT and p80-CTD form a tight heterodimeric complex, whose high-resolution structure we determined by X-ray crystallography. Based on the crystal structure, we identified two conserved charged residues that are important for p60-MIT:p80-CTD complex formation and katanin function. Moreover, p60-MIT was compared with other MIT domain structures and similarities are discussed.

Microtubule minus-end regulation at spindle poles by an ASPM-katanin complex

ASPM (known as Asp in fly and ASPM-1 in worm) is a microcephaly-associated protein family that regulates spindle architecture, but the underlying mechanism is poorly understood. Here, we show that ASPM forms a complex with another protein linked to microcephaly, the microtubule-severing ATPase katanin. ASPM and katanin localize to spindle poles in a mutually dependent manner and regulate spindle flux. X-ray crystallography revealed that the heterodimer formed by the N- and C-terminal domains of the katanin subunits p60 and p80, respectively, binds conserved motifs in ASPM. Reconstitution experiments demonstrated that ASPM autonomously tracks growing microtubule minus ends and inhibits their growth, while katanin decorates and bends both ends of dynamic microtubules and potentiates the minus-end blocking activity of ASPM. ASPM also binds along microtubules, recruits katanin and promotes katanin-mediated severing of dynamic microtubules. We propose that the ASPM-katanin complex controls microtubule disassembly at spindle poles and that misregulation of this process can lead to microcephaly.

Caenorhabditis elegans oocytes detect meiotic errors in the absence of canonical end-on kinetochore attachments

During mitosis, cells monitor kinetochore–microtubule attachments as a means of detecting errors. Although end-on attachments have not been observed in Caenorhabditis elegans oocytes, Davis-Roca et al. now report that these cells alter key aspects of anaphase progression in the presence of meiotic defects, revealing a new strategy for error detection during cell division.

Mitotically dividing cells use a surveillance mechanism, the spindle assembly checkpoint, that monitors the attachment of spindle microtubules to kinetochores as a means of detecting errors. However, end-on kinetochore attachments have not been observed in Caenorhabditis elegans oocytes and chromosomes instead associate with lateral microtubule bundles; whether errors can be sensed in this context is not known. Here, we show that C. elegans oocytes delay key events in anaphase, including AIR-2/Aurora B relocalization to the microtubules, in response to a variety of meiotic defects, demonstrating that errors can be detected in these cells and revealing a mechanism that regulates anaphase progression. This mechanism does not appear to rely on several components of the spindle assembly checkpoint but does require the kinetochore, as depleting kinetochore components prevents the error-induced anaphase delays. These findings therefore suggest that in this system, kinetochores could be involved in sensing meiotic errors using an unconventional mechanism that does not use canonical end-on attachments.

Endoplasmic-reticulum-mediated microtubule alignment governs cytoplasmic streaming

Cytoplasmic streaming refers to a collective movement of cytoplasm observed in many cell types. The mechanism of meiotic cytoplasmic streaming (MeiCS) in Caenorhabditis elegans zygotes is puzzling as the direction of the flow is not predefined by cell polarity and occasionally reverses. Here, we demonstrate that the endoplasmic reticulum (ER) network structure is required for the collective flow. Using a combination of RNAi, microscopy and image processing of C. elegans zygotes, we devise a theoretical model, which reproduces and predicts the emergence and reversal of the flow. We propose a positive-feedback mechanism, where a local flow generated along a microtubule is transmitted to neighbouring regions through the ER. This, in turn, aligns microtubules over a broader area to self-organize the collective flow. The proposed model could be applicable to various cytoplasmic streaming phenomena in the absence of predefined polarity. The increased mobility of cortical granules by MeiCS correlates with the efficient exocytosis of the granules to protect the zygotes from osmotic and mechanical stresses.

Regulation of the MEI-1/MEI-2 Microtubule-Severing Katanin Complex in Early Caenorhabditis elegans Development

After fertilization, rapid changes of the Caenorhabditis elegans cytoskeleton occur in the transition from meiosis to mitosis, requiring precise regulation. The MEI-1/MEI-2 katanin microtubule-severing complex is essential for meiotic spindle formation but must be quickly inactivated to allow for proper formation of the mitotic spindle. MEI-1/MEI-2 inactivation is dependent on multiple redundant pathways. The primary pathway employs the MEL-26 substrate adaptor for the CUL-3/cullin-based E3 ubiquitin ligase, which targets MEI-1 for proteosomal degradation. Here, we used quantitative antibody staining to measure MEI-1 levels to determine how other genes implicated in MEI-1 regulation act relative to CUL-3/MEL-26. The anaphase-promoting complex/cyclosome, APC/C, the DYRK (Dual-specificity tyrosine-regulated kinase), MBK-2, and the CUL-2-based E3 ubiquitin ligase act together to degrade MEI-1, in parallel to MEL-26/CUL-3. CUL-2 is known to keep MEL-26 low during meiosis, so CUL-2 apparently changes its target from MEL-26 in meiosis to MEI-1 in mitosis. RFL-1, an activator of cullin E3 ubiquitin ligases, activates CUL-2 but not CUL-3 for MEI-1 elimination. HECD-1 (HECT/Homologous to the E6AP carboxyl terminus domain) E3 ligase acts as a MEI-1 activator in meiosis but functions as an inhibitor during mitosis, without affecting levels of MEI-1 or MEI-2. Our results highlight the multiple layers of MEI-1 regulation that are required during the switch from the meiotic to mitotic modes of cell division.

Microtubule-severing activity of the AAA+ ATPase Katanin is essential for female meiotic spindle assembly

In most animals, female meiotic spindles are assembled in the absence of centrosomes. How microtubules (MTs) are organized into acentrosomal meiotic spindles is poorly understood. In Caenorhabditis elegans, assembly of female meiotic spindles requires MEI-1 and MEI-2, which constitute the microtubule-severing AAA+ ATPase Katanin. However, the role of MEI-2 is not known and whether MT severing is required for meiotic spindle assembly is unclear. Here, we show that the essential role of MEI-2 is to confer MT binding to Katanin, which in turn stimulates the ATPase activity of MEI-1, leading to MT severing. To test directly the contribution of MT severing to meiotic spindle assembly, we engineered Katanin variants that retained MT binding and MT bundling activities but that were inactive for MT severing. In vivo analysis of these variants showed disorganized microtubules that lacked focused spindle poles reminiscent of the Katanin loss-of-function phenotype, demonstrating that the MT-severing activity is essential for meiotic spindle assembly in C. elegans Overall, our results reveal the essential role of MEI-2 and provide the first direct evidence supporting an essential role of MT severing in meiotic spindle assembly in C. elegans.

A novel chromosome segregation mechanism during female meiosis

During conventional anaphase A, chromosomes move outward toward spindle poles. Caenorhabditis elegans meiotic spindle poles move inward toward chromosomes to achieve the same end.

In a wide range of eukaryotes, chromosome segregation occurs through anaphase A, in which chromosomes move toward stationary spindle poles, anaphase B, in which chromosomes move at the same velocity as outwardly moving spindle poles, or both. In contrast, Caenorhabditis elegans female meiotic spindles initially shorten in the pole-to-pole axis such that spindle poles contact the outer kinetochore before the start of anaphase chromosome separation. Once the spindle pole-to-kinetochore contact has been made, the homologues of a 4-μm-long bivalent begin to separate. The spindle shortens an additional 0.5 μm until the chromosomes are embedded in the spindle poles. Chromosomes then separate at the same velocity as the spindle poles in an anaphase B–like movement. We conclude that the majority of meiotic chromosome movement is caused by shortening of the spindle to bring poles in contact with the chromosomes, followed by separation of chromosome-bound poles by outward sliding.

Mother centrioles are kicked out so that starfish zygote can grow

Most oocytes eliminate their centrioles during meiotic divisions through unclear mechanisms. In this issue, Borrego-Pinto et al. (2016. J Cell. Biol. http://dx.doi.org/10.1083/jcb.201510083) show that mother centrioles need to be eliminated from starfish oocytes by extrusion into the polar bodies for successful embryo development.

Oocyte Maturation and Development

Sexual reproduction is essential for many organisms to propagate themselves. It requires the formation of haploid female and male gametes: oocytes and sperms. These specialized cells are generated through meiosis, a particular type of cell division that produces cells with recombined genomes that differ from their parental origin. In this review, we highlight the end process of female meiosis, the divisions per se, and how they can give rise to a functional female gamete preparing itself for the ensuing zygotic development. In particular, we discuss why such an essential process in the propagation of species is so poorly controlled, producing a strong percentage of abnormal female gametes in the end. Eventually, we examine aspects related to the lack of centrosomes in female oocytes, the asymmetry in size of the mammalian oocyte upon division, and in mammals the direct consequences of these long-lived cells in the ovary.

Proteomic Analysis of the Mammalian Katanin Family of Microtubule-severing Enzymes Defines Katanin p80 subunit B-like 1 (KATNBL1) as a Regulator of Mammalian Katanin Microtubule-severing

The Katanin family of microtubule-severing enzymes is critical for remodeling microtubule-based structures that influence cell division, motility, morphogenesis and signaling. Katanin is composed of a catalytic p60 subunit (A subunit, KATNA1) and a regulatory p80 subunit (B subunit, KATNB1). The mammalian genome also encodes two additional A-like subunits (KATNAL1 and KATNAL2) and one additional B-like subunit (KATNBL1) that have remained poorly characterized. To better understand the factors and mechanisms controlling mammalian microtubule-severing, we have taken a mass proteomic approach to define the protein interaction module for each mammalian Katanin subunit and to generate the mammalian Katanin family interaction network (Katan-ome). Further, we have analyzed the function of the KATNBL1 subunit and determined that it associates with KATNA1 and KATNAL1, it localizes to the spindle poles only during mitosis and it regulates Katanin A subunit microtubule-severing activity in vitro. Interestingly, during interphase, KATNBL1 is sequestered in the nucleus through an N-terminal nuclear localization signal. Finally KATNB1 was able to compete the interaction of KATNBL1 with KATNA1 and KATNAL1. These data indicate that KATNBL1 functions as a regulator of Katanin A subunit microtubule-severing activity during mitosis and that it likely coordinates with KATNB1 to perform this function.

Oocyte Meiotic Spindle Assembly and Function

Gametogenesis in animal oocytes reduces the diploid genome content of germline precursors to a haploid state in gametes by discarding ¾ of the duplicated chromosomes through a sequence of two meiotic cell divisions called meiosis I and II. The assembly of the microtubule-based spindle structure that mediates this reduction in genome content remains poorly understood compared to our knowledge of mitotic spindle assembly and function. In this review, we consider the diversity of oocyte meiotic spindle assembly and structure across animal phylogeny, review recent advances in our understanding of how animal oocytes assemble spindles in the absence of the centriole-based microtubule-organizing centers that dominate mitotic spindle assembly, and discuss different models for how chromosomes are captured and moved to achieve chromosome segregation during oocyte meiotic cell division.

A novel family of katanin-like 2 protein isoforms (KATNAL2), interacting with nucleotide-binding proteins Nubp1 and Nubp2, are key regulators of different MT-based processes in mammalian cells

Katanins are microtubule (MT)-severing AAA proteins with high phylogenetic conservation throughout the eukaryotes. They have been functionally implicated in processes requiring MT remodeling, such as spindle assembly in mitosis and meiosis, assembly/disassembly of flagella and cilia and neuronal morphogenesis. Here, we uncover a novel family of katanin-like 2 proteins (KATNAL2) in mouse, consisting of five alternatively spliced isoforms encoded by the Katnal2 genomic locus. We further demonstrate that in vivo these isoforms are able to interact with themselves, with each other and moreover directly and independently with MRP/MinD-type P-loop NTPases Nubp1 and Nubp2, which are integral components of centrioles, negative regulators of ciliogenesis and implicated in centriole duplication in mammalian cells. We find KATNAL2 localized on interphase MTs, centrioles, mitotic spindle, midbody and the axoneme and basal body of sensory cilia in cultured murine cells. shRNAi of Katnal2 results in inefficient cytokinesis and severe phenotypes of enlarged cells and nuclei, increased numbers of centrioles and the manifestation of aberrant multipolar mitotic spindles, mitotic defects, chromosome bridges, multinuclearity, increased MT acetylation and an altered cell cycle pattern. Silencing or stable overexpression of KATNAL2 isoforms drastically reduces ciliogenesis. In conclusion, KATNAL2s are multitasking enzymes involved in the same cell type in critically important processes affecting cytokinesis, MT dynamics, and ciliogenesis and are also implicated in cell cycle progression.

KIAA0556 is a novel ciliary basal body component mutated in Joubert syndrome

BACKGROUND

Joubert syndrome (JBTS) and related disorders are defined by cerebellar malformation (molar tooth sign), together with neurological symptoms of variable expressivity. The ciliary basis of Joubert syndrome related disorders frequently extends the phenotype to tissues such as the eye, kidney, skeleton and craniofacial structures.

RESULTS

Using autozygome and exome analyses, we identified a null mutation in KIAA0556 in a multiplex consanguineous family with hallmark features of mild Joubert syndrome. Patient-derived fibroblasts displayed reduced ciliogenesis potential and abnormally elongated cilia. Investigation of disease pathophysiology revealed that Kiaa0556 (-/-) null mice possess a Joubert syndrome-associated brain-restricted phenotype. Functional studies in Caenorhabditis elegans nematodes and cultured human cells support a conserved ciliary role for KIAA0556 linked to microtubule regulation. First, nematode KIAA0556 is expressed almost exclusively in ciliated cells, and the worm and human KIAA0556 proteins are enriched at the ciliary base. Second, C. elegans KIAA0056 regulates ciliary A-tubule number and genetically interacts with an ARL13B (JBTS8) orthologue to control cilium integrity. Third, human KIAA0556 binds to microtubules in vitro and appears to stabilise microtubule networks when overexpressed. Finally, human KIAA0556 biochemically interacts with ciliary proteins and p60/p80 katanins. The latter form a microtubule-severing enzyme complex that regulates microtubule dynamics as well as ciliary functions.

CONCLUSIONS

We have identified KIAA0556 as a novel microtubule-associated ciliary base protein mutated in Joubert syndrome. Consistent with the mild patient phenotype, our nematode, mice and human cell data support the notion that KIAA0556 has a relatively subtle and variable cilia-related function, which we propose is related to microtubule regulation.

KLP-7 acts through the Ndc80 complex to limit pole number in C. elegans oocyte meiotic spindle assembly

KLP-7/MCAK regulates kinetochore–microtubule attachment and spindle tension to promote the coalescence of early spindle pole foci, which produces a bipolar structure during the acentrosomal process of oocyte meiotic spindle assembly in C. elegans.

During oocyte meiotic cell division in many animals, bipolar spindles assemble in the absence of centrosomes, but the mechanisms that restrict pole assembly to a bipolar state are unknown. We show that KLP-7, the single mitotic centromere–associated kinesin (MCAK)/kinesin-13 in Caenorhabditis elegans, is required for bipolar oocyte meiotic spindle assembly. In klp-7(−) mutants, extra microtubules accumulated, extra functional spindle poles assembled, and chromosomes frequently segregated as three distinct masses during meiosis I anaphase. Moreover, reducing KLP-7 function in monopolar klp-18(−) mutants often restored spindle bipolarity and chromosome segregation. MCAKs act at kinetochores to correct improper kinetochore–microtubule (k–MT) attachments, and depletion of the Ndc-80 kinetochore complex, which binds microtubules to mediate kinetochore attachment, restored bipolarity in klp-7(−) mutant oocytes. We propose a model in which KLP-7/MCAK regulates k–MT attachment and spindle tension to promote the coalescence of early spindle pole foci that produces a bipolar structure during the acentrosomal process of oocyte meiotic spindle assembly.

Separase Cleaves the N-Tail of the CENP-A Related Protein CPAR-1 at the Meiosis I Metaphase-Anaphase Transition in C. elegans

Centromeres are defined epigenetically in the majority of eukaryotes by the presence of chromatin containing the centromeric histone H3 variant CENP-A. Most species have a single gene encoding a centromeric histone variant whereas C. elegans has two: HCP-3 (also known as CeCENP-A) and CPAR-1. Prior RNAi replacement experiments showed that HCP-3 is the functionally dominant isoform, consistent with CPAR-1 not being detectable in embryos. GFP::CPAR-1 is loaded onto meiotic chromosomes in diakinesis and is enriched on bivalents until meiosis I. Here we show that GFP::CPAR-1 signal loss from chromosomes precisely coincides with homolog segregation during anaphase I. This loss of GFP::CPAR-1 signal reflects proteolytic cleavage between GFP and the histone fold of CPAR-1, as CPAR-1::GFP, in which GFP is fused to the C-terminus of CPAR-1, does not exhibit any loss of GFP signal. A focused candidate screen implicated separase, the protease that initiates anaphase by cleaving the kleisin subunit of cohesin, in this cleavage reaction. Examination of the N-terminal tail sequence of CPAR-1 revealed a putative separase cleavage site and mutation of the signature residues in this site eliminated the cleavage reaction, as visualized by retention of GFP::CPAR-1 signal on separating homologous chromosomes at the metaphase-anaphase transition of meiosis I. Neither cleaved nor uncleavable CPAR-1 were centromere-localized in mitosis and instead localized throughout chromatin, indicating that centromere activity has not been retained in CPAR-1. Although the functions of CPAR-1 and of its separase-dependent cleavage remain to be elucidated, this effort reveals a new substrate of separase and provides an in vivo biosensor to monitor separase activity at the onset of meiosis I anaphase.

The non-catalytic domains of Drosophila katanin regulate its abundance and microtubule-disassembly activity

Microtubule severing is a biochemical reaction that generates an internal break in a microtubule and regulation of microtubule severing is critical for cellular processes such as ciliogenesis, morphogenesis, and meiosis and mitosis. Katanin is a conserved heterodimeric ATPase that severs and disassembles microtubules, but the molecular determinants for regulation of microtubule severing by katanin remain poorly defined. Here we show that the non-catalytic domains of Drosophila katanin regulate its abundance and activity in living cells. Our data indicate that the microtubule-interacting and trafficking (MIT) domain and adjacent linker region of the Drosophila katanin catalytic subunit Kat60 cooperate to regulate microtubule severing in two distinct ways. First, the MIT domain and linker region of Kat60 decrease its abundance by enhancing its proteasome-dependent degradation. The Drosophila katanin regulatory subunit Kat80, which is required to stabilize Kat60 in cells, conversely reduces the proteasome-dependent degradation of Kat60. Second, the MIT domain and linker region of Kat60 augment its microtubule-disassembly activity by enhancing its association with microtubules. On the basis of our data, we propose that the non-catalytic domains of Drosophila katanin serve as the principal sites of integration of regulatory inputs, thereby controlling its ability to sever and disassemble microtubules.

Microtubule severing by katanin p60 AAA+ ATPase requires the C-terminal acidic tails of both α- and β-tubulins and basic amino acid residues in the AAA+ ring pore

The microtubule (MT) network is highly dynamic and undergoes dramatic reorganizations during the cell cycle. Dimers of α- and β-tubulins rapidly polymerize to and depolymerize from the end of MT fibrils in an intrinsic GTP-dependent manner. MT severing by ATP-driven enzymes such as katanin and spastin contributes significantly to microtubule dynamics, and it has been shown that katanin p60, a AAA+ family protein, has ATPase and MT-severing activities. The mechanism of MT severing by katanin p60 is poorly understood, and the residues in katanin p60 and tubulins important for severing activity were therefore explored in this study. MT-severing activity, but not ATPase activity, was inhibited by mutations of the conserved aromatic residue and the flanking basic residues in the pore region of the katanin p60 hexameric ring. When the acidic residue-rich C-terminal unstructured segment of either α- or β-tubulin was removed, polymerized MTs were resistant to katanin p60 treatment. Interactions between katanin p60 and the mutant MTs, on the other hand, were unaffected. Taken together, these findings led us to propose that the interactions between the positively charged residues of katanin p60 and the acidic tails of both tubulins are essential for efficient severing of MTs.

The disease-associated formin INF2/EXC-6 organizes lumen and cell outgrowth during tubulogenesis by regulating F-actin and microtubule cytoskeletons

We investigate how outgrowth at the basolateral cell membrane is coordinated with apical lumen formation in the development of a biological tube by characterizing exc-6, a gene required for C. elegans excretory cell (EC) tubulogenesis. We show that EXC-6 is orthologous to the human formin INF2, which polymerizes filamentous actin (F-actin) and binds microtubules (MTs) in vitro. Dominant INF2 mutations cause focal segmental glomerulosclerosis (FSGS), a kidney disease, and FSGS+Charcot-Marie-Tooth neuropathy. We show that activated INF2 can substitute for EXC-6 in C. elegans and that disease-associated mutations cause constitutive activity. Using genetic analysis and live imaging, we show that exc-6 regulates MT and F-actin accumulation at EC tips and dynamics of basolateral-localized MTs, indicating that EXC-6 organizes F-actin and MT cytoskeletons during tubulogenesis. The pathology associated with INF2 mutations is believed to reflect misregulation of F-actin, but our results suggest alternative or additional mechanisms via effects on MT dynamics.

Translational control of the oogenic program by components of OMA ribonucleoprotein particles in Caenorhabditis elegans

The oocytes of most sexually reproducing animals arrest in meiotic prophase I. Oocyte growth, which occurs during this period of arrest, enables oocytes to acquire the cytoplasmic components needed to produce healthy progeny and to gain competence to complete meiosis. In the nematode Caenorhabditis elegans, the major sperm protein hormone promotes meiotic resumption (also called meiotic maturation) and the cytoplasmic flows that drive oocyte growth. Prior work established that two related TIS11 zinc-finger RNA-binding proteins, OMA-1 and OMA-2, are redundantly required for normal oocyte growth and meiotic maturation. We affinity purified OMA-1 and identified associated mRNAs and proteins using genome-wide expression data and mass spectrometry, respectively. As a class, mRNAs enriched in OMA-1 ribonucleoprotein particles (OMA RNPs) have reproductive functions. Several of these mRNAs were tested and found to be targets of OMA-1/2-mediated translational repression, dependent on sequences in their 3'-untranslated regions (3'-UTRs). Consistent with a major role for OMA-1 and OMA-2 in regulating translation, OMA-1-associated proteins include translational repressors and activators, and some of these proteins bind directly to OMA-1 in yeast two-hybrid assays, including OMA-2. We show that the highly conserved TRIM-NHL protein LIN-41 is an OMA-1-associated protein, which also represses the translation of several OMA-1/2 target mRNAs. In the accompanying article in this issue, we show that LIN-41 prevents meiotic maturation and promotes oocyte growth in opposition to OMA-1/2. Taken together, these data support a model in which the conserved regulators of mRNA translation LIN-41 and OMA-1/2 coordinately control oocyte growth and the proper spatial and temporal execution of the meiotic maturation decision.

Overexpression of OsKTN80a, a katanin P80 ortholog, caused the repressed cell elongation and stalled cell division mediated by microtubule apparatus defects in primary root in Oryza sativa

Katanin, a microtubule-severing enzyme, consists of two subunits: the catalytic subunit P60, and the regulatory subunit P80. In several species, P80 functions in meiotic spindle organization, the flagella biogenesis, the neuronal development, and the male gamete production. However, the P80 function in higher plants remains elusive. In this study, we found that there are three katanin P80 orthologs (OsKTN80a, OsKTN80b, and OsKTN80c) in Oryza sativa L. Overexpression of OsKTN80a caused the retarded root growth of rice seedlings. Further investigation indicates that the retained root growth was caused by the repressed cell elongation in the elongation zone and the stalled cytokinesis in the division zone in the root tip. The in vivo examination suggests that OsKTN80a acts as a microtubule stabilizer. We prove that OsKTN80a, possibly associated with OsKTN60, is involved in root growth via regulating the cell elongation and division.

Global characterization of the oocyte-to-embryo transition in Caenorhabditis elegans uncovers a novel mRNA clearance mechanism

The oocyte-to-embryo transition (OET) is thought to be mainly driven by post-transcriptional gene regulation. However, expression of both RNAs and proteins during the OET has not been comprehensively assayed. Furthermore, specific molecular mechanisms that regulate gene expression during OET are largely unknown. Here, we quantify and analyze transcriptome-wide, expression of mRNAs and thousands of proteins in Caenorhabditis elegans oocytes, 1-cell, and 2-cell embryos. This represents a first comprehensive gene expression atlas during the OET in animals. We discovered a first wave of degradation in which thousands of mRNAs are cleared shortly after fertilization. Sequence analysis revealed a statistically highly significant presence of a polyC motif in the 3' untranslated regions of most of these degraded mRNAs. Transgenic reporter assays demonstrated that this polyC motif is required and sufficient for mRNA degradation after fertilization. We show that orthologs of human polyC-binding protein specifically bind this motif. Our data suggest a mechanism in which the polyC motif and binding partners direct degradation of maternal mRNAs. Our data also indicate that endogenous siRNAs but not miRNAs promote mRNA clearance during the OET.

Lack of association of KATNAL1 gene sequence variants and azoospermia in humans

PURPOSE

A recent experiment indicated that a loss of function mutation in the murine Katnal1 gene resulted in male factor infertility due to premature exfoliation of spermatids. This study investigated the relevance of this gene to infertility in humans.

METHODS

Multiple methods of genetic analysis were employed to investigate whether mutations in human KATNAL1 have a causative role in male infertility. This was a genetic association study, which included DNA samples from 105 men with non-obstructive azoospermia (NOA) and 242 anonymous sperm donor controls. 28 commercially available TaqMan SNP assays were used to haplotype samples from both groups and genetically tag regions of interest across the entire gene. AmpliSeq primers were then designed for identified regions so that targeted next-generation sequencing (NGS) could be used to identify causative variants.

RESULTS

Four SNPs in the 3'UTR demonstrated a putative association with NOA. The AmpliSeq primers designed for the 3'UTR provided 83 % coverage of the 7,202 basepairs within the regions of interest. Variant sites were analyzed against genetic models to identify sequence polymorphisms which associated with NOA. No variants met standard criteria for significance when tested between the groups.

CONCLUSIONS

This study suggests a lack of association of KATNAL1 gene sequence variants and azoospermia in humans.

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.