Nucleoporins NPP-1, NPP-3, NPP-4, NPP-11 and NPP-13 are required for proper spindle orientation in C. elegans

Ultrastructure Expansion Microscopy applied to C. elegans embryos

Visualization of organelles using expansion microscopy has been previously applied to Caenorhadbitis elegans adult gonads or worms. However, its application to embryos has remained a challenge due to the protective eggshell barrier. Here, by combining freeze-cracking and ultrastructure expansion microscopy (U-ExM), we demonstrate a four-time isotropic expansion of C. elegans embryos. As an example structure, we chose the nuclear pore and demonstrate that we achieve sufficient resolution to distinguish them individually. Our work provides proof of principle for U-ExM in C. elegans embryos, which will be applicable for imaging a wide range of cellular structures in this model system.

Structure, Maintenance, and Regulation of Nuclear Pore Complexes: The Gatekeepers of the Eukaryotic Genome

In eukaryotic cells, the genetic material is segregated inside the nucleus. This compartmentalization of the genome requires a transport system that allows cells to move molecules across the nuclear envelope, the membrane-based barrier that surrounds the chromosomes. Nuclear pore complexes (NPCs) are the central component of the nuclear transport machinery. These large protein channels penetrate the nuclear envelope, creating a passage between the nucleus and the cytoplasm through which nucleocytoplasmic molecule exchange occurs. NPCs are one of the largest protein assemblies of eukaryotic cells and, in addition to their critical function in nuclear transport, these structures also play key roles in many cellular processes in a transport-independent manner. Here we will review the current knowledge of the NPC structure, the cellular mechanisms that regulate their formation and maintenance, and we will provide a brief description of a variety of processes that NPCs regulate.

Imaging Fluorescent Nuclear Pore Complex Proteins in C. elegans

C. elegans is a well-characterized and relatively simple model organism, making it attractive for studying nuclear pore complex proteins in cell and developmental biology. C. elegans is transparent and highly amendable to genetic manipulation. Therefore, it is possible to generate fluorescently tagged proteins and combine this with various light microscopy techniques to study protein behavior in space and time. Here, we provide protocols to prepare both fixed and live C. elegans for confocal and light sheet microscopy. This enables the analysis of nuclear pore complex proteins from embryonic stages to the aging adult.

Analysis of Nuclear Pore Complexes in Caenorhabditis elegans by Live Imaging and Functional Genomics

Nuclear pore complexes (NPCs) are essential to communication of macromolecules between the cell nucleus and the surrounding cytoplasm. RNA synthesized in the nucleus is exported through NPCs to function in the cytoplasm, whereas transcription factors and other proteins are selectively and actively imported. In addition, many NPC constituents, known as nuclear pore proteins (nucleoporins or nups), also play critical roles in other processes, such as genome organization, gene expression, and kinetochore function. Thanks to its genetic amenability and transparent body, the nematode Caenorhabditis elegans is an attractive model to study NPC dynamics. We provide here an overview of available genome engineered strains and FLP/Frt-based tools to study tissue-specific functions of individual nucleoporins. We also present protocols for live imaging of fluorescently tagged nucleoporins in intact tissues of embryos, larvae, and adult and for analysis of interactions between nucleoporins and chromatin by DamID.

The first steps in the life of a worm: Themes and variations in asymmetric division in C. elegans and other nematodes

Starting with Boveri in the 1870s, microscopic investigation of early embryogenesis in a broad swath of nematode species revealed the central role of asymmetric cell division in embryonic axis specification, blastomere positioning, and cell fate specification. Notably, across the class Chromadorea, a conserved theme emerges-asymmetry is first established in the zygote and specifies its asymmetric division, giving rise to an anterior somatic daughter cell and a posterior germline daughter cell. Beginning in the 1980s, the emergence of Caenorhabditis elegans as a model organism saw the advent of genetic tools that enabled rapid progress in our understanding of the molecular mechanisms underlying asymmetric division, in many cases defining key paradigms that turn out to regulate asymmetric division in a wide range of systems. Yet, the consequence of this focus on C. elegans came at the expense of exploring the extant diversity of developmental variation exhibited across nematode species. Given the resurgent interest in evolutionary studies facilitated in part by new tools, here we revisit the diversity in this asymmetric first division, juxtaposing molecular insight into mechanisms of symmetry-breaking, spindle positioning and fate specification, with a consideration of plasticity and variability within and between species. In the process, we hope to highlight questions of evolutionary forces and molecular variation that may have shaped the extant diversity of developmental mechanisms observed across Nematoda.

Decreased mechanotransduction prevents nuclear collapse in a Caenorhabditis elegans laminopathy

Nuclear envelopathies are a complex group of human diseases caused by mutations in nuclear envelope proteins, including progeria, myopathy, and dystonia. Here, we used the Caenorhabditis elegans germline as a model system to investigate the function of the OOC-5/torsinA AAA+ ATPase, which localizes to the nuclear envelope and is mutated in early-onset DYT1 dystonia in humans. We show that OOC-5/torsinA promotes the function of the LINC complex, which spans the nuclear envelope and transmits forces to the nuclear lamina. Remarkably, decreasing the function of OOC-5/torsinA or the LINC complex prevents nuclear collapse in the absence of a functional nuclear lamina. Therapeutics targeting torsinA or the LINC complex might prevent nuclear damage from endogenous forces in certain nuclear envelopathies.

The function of the nucleus depends on the integrity of the nuclear lamina, an intermediate filament network associated with the linker of nucleoskeleton and cytoskeleton (LINC) complex. The LINC complex spans the nuclear envelope and mediates nuclear mechanotransduction, the process by which mechanical signals and forces are transmitted across the nuclear envelope. In turn, the AAA+ ATPase torsinA is thought to regulate force transmission from the cytoskeleton to the nucleus. In humans, mutations affecting nuclear envelope-associated proteins cause laminopathies, including progeria, myopathy, and dystonia, though the extent to which endogenous mechanical stresses contribute to these pathologies is unclear. Here, we use the Caenorhabditis elegans germline as a model to investigate mechanisms that maintain nuclear integrity as germ cell nuclei progress through meiotic development and migrate for gametogenesis—processes that require LINC complex function. We report that decreasing the function of the C. elegans torsinA homolog, OOC-5, rescues the sterility and premature aging caused by a null mutation in the single worm lamin homolog. We show that decreasing OOC-5/torsinA activity prevents nuclear collapse in lamin mutants by disrupting the function of the LINC complex. At a mechanistic level, OOC-5/torsinA promotes the assembly or maintenance of the lamin-associated LINC complex and this activity is also important for interphase nuclear pore complex insertion into growing germline nuclei. These results demonstrate that LINC complex-transmitted forces damage nuclei with a compromised nuclear lamina. Thus, the torsinA–LINC complex nexus might comprise a therapeutic target for certain laminopathies by preventing damage from endogenous cellular forces.

Dissecting the eQTL Micro-Architecture in Caenorhabditis elegans

The study of expression quantitative trait loci (eQTL) using natural variation in inbred populations has yielded detailed information about the transcriptional regulation of complex traits. Studies on eQTL using recombinant inbred lines (RILs) led to insights on cis and trans regulatory loci of transcript abundance. However, determining the underlying causal polymorphic genes or variants is difficult, but ultimately essential for the understanding of regulatory networks of complex traits. This requires insight into whether associated loci are single eQTL or a combination of closely linked eQTL, and how this QTL micro-architecture depends on the environment. We addressed these questions by testing for independent replication of previously mapped eQTL in Caenorhabditis elegans using new data from introgression lines (ILs). Both populations indicate that the overall heritability of gene expression, number, and position of eQTL differed among environments. Across environments we were able to replicate 70% of the cis- and 40% of the trans-eQTL using the ILs. Testing eight different simulation models, we suggest that additive effects explain up to 60-93% of RIL/IL heritability for all three environments. Closely linked eQTL explained up to 40% of RIL/IL heritability in the control environment whereas only 7% in the heat-stress and recovery environments. In conclusion, we show that reproducibility of eQTL was higher for cis vs. trans eQTL and that the environment affects the eQTL micro-architecture.

C. elegans pronuclei fuse after fertilization through a novel membrane structure

After fertilization, parental genomes are enclosed in two separate pronuclei. In Caenorhabditis elegans, and possibly other organisms, when the two pronuclei first meet, the parental genomes are separated by four pronuclear membranes. To understand how these membranes are breached to allow merging of parental genomes we used focused ion beam scanning electron microscopy (FIB-SEM) to study the architecture of the pronuclear membranes at nanometer-scale resolution. We find that at metaphase, the interface between the two pronuclei is composed of two membranes perforated by fenestrations ranging from tens of nanometers to several microns in diameter. The parental chromosomes come in contact through one of the large fenestrations. Surrounding this fenestrated, two-membrane region is a novel membrane structure, a three-way sheet junction, where the four membranes of the two pronuclei fuse and become two. In the plk-1 mutant, where parental genomes fail to merge, these junctions are absent, suggesting that three-way sheet junctions are needed for formation of a diploid genome.

Channel Nucleoporins Recruit PLK-1 to Nuclear Pore Complexes to Direct Nuclear Envelope Breakdown in C. elegans

In animal cells, nuclear envelope breakdown (NEBD) is required for proper chromosome segregation. Whereas mitotic kinases have been implicated in NEBD, how they coordinate their activity to trigger this event is unclear. Here, we show that both in human cells and Caenorhabditis elegans, the Polo-like kinase 1 (PLK-1) is recruited to the nuclear pore complexes, just prior to NEBD, through its Polo-box domain (PBD). We provide evidence that PLK-1 localization to the nuclear envelope (NE) is required for efficient NEBD. We identify the central channel nucleoporins NPP-1/Nup58, NPP-4/Nup54, and NPP-11/Nup62 as the critical factors anchoring PLK-1 to the NE in C. elegans. In particular, NPP-1, NPP-4, and NPP-11 primed at multiple Polo-docking sites by Cdk1 and PLK-1 itself physically interact with the PLK-1 PBD. We conclude that nucleoporins play an unanticipated regulatory role in NEBD, by recruiting PLK-1 to the NE thereby facilitating phosphorylation of critical downstream targets.

Nucleoporins NPP-10, NPP-13 and NPP-20 are required for HCP-4 nuclear import to establish correct centromere assembly

Centromeres form a chromosomal platform for the assembly of the kinetochores, which are required for orderly chromosome segregation. Assembly of both centromeres and kinetochores proceeds by a step-by-step mechanism that is regulated in time and space. It has been suggested that the regulated nuclear import of centromeric proteins is involved in this process. We show that the knockdown of nucleoporins NPP-10, NPP-13 and NPP-20 in Caenorhabditiselegans affects early steps in centromere formation and sister centromere resolution, and results in severe chromosomal defects in the early embryo. These phenotypes mirror the knockdown phenotype of HCP-4 (an ortholog of mammalian CENP-C), a key factor for centromere formation and inner kinetochore assembly. HCP-4 is present in the cytoplasm during interphase. It is imported into nuclei and assembled in centromeres during prophase. Following the knockdown of NPP-10, NPP-13 and NPP-20, HCP-4 remains in the cytosol throughout prophase due to stalled import. In prometaphase and later mitotic stages after breakdown of the nuclear envelope, HCP-4 is not incorporated into centromeres. These results indicate that correct timing of the availability of HCP-4 by nuclear import is essential.

A novel function for the Caenorhabditis elegans torsin OOC-5 in nucleoporin localization and nuclear import

Mutation in the human AAA+ protein torsinA leads to DYT1 dystonia. Loss of a Caenorhabditis elegans torsin, OOC-5, leads to defects in nucleoporin localization and nuclear import, a novel phenotype for a torsin mutant. NE ultrastructural defects similar to those in mouse and fly torsin mutants are also found, showing conservation of function.

Torsin proteins are AAA+ ATPases that localize to the endoplasmic reticular/nuclear envelope (ER/NE) lumen. A mutation that markedly impairs torsinA function causes the CNS disorder DYT1 dystonia. Abnormalities of NE membranes have been linked to torsinA loss of function and the pathogenesis of DYT1 dystonia, leading us to investigate the role of the Caenorhabditis elegans torsinA homologue OOC-5 at the NE. We report a novel role for torsin in nuclear pore biology. In ooc-5–mutant germ cell nuclei, nucleoporins (Nups) were mislocalized in large plaques beginning at meiotic entry and persisted throughout meiosis. Moreover, the KASH protein ZYG-12 was mislocalized in ooc-5 gonads. Nups were mislocalized in adult intestinal nuclei and in embryos from mutant mothers. EM analysis revealed vesicle-like structures in the perinuclear space of intestinal and germ cell nuclei, similar to defects reported in torsin-mutant flies and mice. Consistent with a functional disruption of Nups, ooc-5–mutant embryos displayed impaired nuclear import kinetics, although the nuclear pore-size exclusion barrier was maintained. Our data are the first to demonstrate a requirement for a torsin for normal Nup localization and function and suggest that these functions are likely conserved.

An RNAi-based suppressor screen identifies interactors of the Myt1 ortholog of Caenorhabditis elegans

Oocyte maturation in all species is controlled by a protein complex termed the maturation promoting factor (MPF). MPF comprises a cyclin-dependent kinase (CDK) and its partner cyclin, and it is regulated by dueling regulatory phosphorylation events on the CDK. In Caenorhabditis elegans, the Wee1/Myt1 ortholog WEE-1.3 provides the inhibitory phosphorylations on CDK-1 that keep MPF inactive and halt meiosis. Prior work has shown that depletion of WEE-1.3 in C. elegans results in precocious oocyte maturation in vivo and a highly penetrant infertility phenotype. This study sought to further define the precocious maturation phenotype and to identify novel interactors with WEE-1.3. We found that WEE-1.3 is expressed throughout the germline and in developing embryos in a perinuclear pattern, and demonstrated that oocytes in WEE-1.3–depleted germlines have begun to transcribe embryonic genes and exhibit inappropriate expression of proteins normally restricted to fertilized eggs. In addition, we performed an RNAi suppressor screen of the infertile phenotype to identify novel factors that, when co-depleted with WEE-1.3, restore fertility to these animals. We screened ∼1900 essential genes by RNAi feeding and identified 44 (∼2% of the tested genes) that are suppressors of the WEE-1.3 depletion phenotype. The suppressors include many previously unidentified players in the meiotic cell cycle and represent a pool of potential WEE-1.3 interacting proteins that function during C. elegans oocyte maturation and zygotic development.

The nucleoporin Nup205/NPP-3 is lost near centrosomes at mitotic onset and can modulate the timing of this process in Caenorhabditis elegans embryos

Through an RNAi-based modifier screen, we identified the nucleoporin Nup205/NPP-3 as a negative regulator of mitotic onset in Caenorhabditis elegans. Strikingly, NPP-3 is lost from the nuclear envelope at mitotic onset in an AIR-1– and centrosome-dependent manner. We propose a model whereby centrosomes and AIR-1 promote timely mitosis by locally removing NPP-3.

Regulation of mitosis in time and space is critical for proper cell division. We conducted an RNA interference–based modifier screen to identify novel regulators of mitosis in Caenorhabditis elegans embryos. Of particular interest, this screen revealed that the Nup205 nucleoporin NPP-3 can negatively modulate the timing of mitotic onset. Furthermore, we discovered that NPP-3 and nucleoporins that are associated with it are lost from the nuclear envelope (NE) in the vicinity of centrosomes at the onset of mitosis. We demonstrate that centrosomes are both necessary and sufficient for NPP-3 local loss, which also requires the activity of the Aurora-A kinase AIR-1. Our findings taken together support a model in which centrosomes and AIR-1 promote timely onset of mitosis by locally removing NPP-3 and associated nucleoporins from the NE.

Dissecting the NUP107 complex: multiple components and even more functions

The Nuclear Pore Complex (NPC) is a fascinating structure whose functional relevance and complexity attract significant interest. Within the NPC, several different subcomplexes interact with each other to form a highly conserved and stable structure. One of these subcomplexes is the NUP107 complex, constituted by 7-9 members. A wide variety of functions have been ascribed to the NUP107 complex, ranging from NPC assembly to mRNA export to cell differentiation. Recently, genetic dissection of the NUP107 complex has provided novel insight to the assembly of the complex and has, moreover, revealed an unexpected connection with the mitotic spindle assembly checkpoint protein MAD1.

Time-lapse microscopy of early embryogenesis in Caenorhabditis elegans

Caenorhabditis elegans has often been used as a model system in studies of early developmental processes. The transparency of the embryos, the genetic resources, and the relative ease of transformation are qualities that make C. elegans an excellent model for early embryogenesis. Laser-based confocal microscopy and fluorescently labeled tags allow researchers to follow specific cellular structures and proteins in the developing embryo. For example, one can follow specific organelles, such as lysosomes or mitochondria, using fluorescently labeled dyes. These dyes can be delivered to the early embryo by means of microinjection into the adult gonad. Also, the localization of specific proteins can be followed using fluorescent protein tags. Examples are presented here demonstrating the use of a fluorescent lysosomal dye as well as fluorescently tagged histone and ubiquitin proteins. The labeled histone is used to visualize the DNA and thus identify the stage of the cell cycle. GFP-tagged ubiquitin reveals the dynamics of ubiquitinated vesicles in the early embryo. Observations of labeled lysosomes and GFP:: ubiquitin can be used to determine if there is colocalization between ubiquitinated vesicles and lysosomes. A technique for the microinjection of the lysosomal dye is presented. Techniques for generating transgenenic strains are presented elsewhere. For imaging, embryos are cut out of adult hermaphrodite nematodes and mounted onto 2% agarose pads followed by time-lapse microscopy on a standard laser scanning confocal microscope or a spinning disk confocal microscope. This methodology provides for the high resolution visualization of early embryogenesis.

The gsdf gene locus harbors evolutionary conserved and clustered genes preferentially expressed in fish previtellogenic oocytes

The gonadal soma-derived factor (GSDF) belongs to the transforming growth factor-β superfamily and is conserved in teleostean fish species. Gsdf is specifically expressed in the gonads, and gene expression is restricted to the granulosa and Sertoli cells in trout and medaka. The gsdf gene expression is correlated to early testis differentiation in medaka and was shown to stimulate primordial germ cell and spermatogonia proliferation in trout. In the present study, we show that the gsdf gene localizes to a syntenic chromosomal fragment conserved among vertebrates although no gsdf-related gene is detected on the corresponding genomic region in tetrapods. We demonstrate using quantitative RT-PCR that most of the genes localized in the synteny are specifically expressed in medaka gonads. Gsdf is the only gene of the synteny with a much higher expression in the testis compared to the ovary. In contrast, gene expression pattern analysis of the gsdf surrounding genes (nup54, aff1, klhl8, sdad1, and ptpn13) indicates that these genes are preferentially expressed in the female gonads. The tissue distribution of these genes is highly similar in medaka and zebrafish, two teleostean species that have diverged more than 110 million years ago. The cellular localization of these genes was determined in medaka gonads using the whole-mount in situ hybridization technique. We confirm that gsdf gene expression is restricted to Sertoli and granulosa cells in contact with the premeiotic and meiotic cells. The nup54 gene is expressed in spermatocytes and previtellogenic oocytes. Transcripts corresponding to the ovary-specific genes (aff1, klhl8, and sdad1) are detected only in previtellogenic oocytes. No expression was detected in the gonocytes in 10 dpf embryos. In conclusion, we show that the gsdf gene localizes to a syntenic chromosomal fragment harboring evolutionary conserved genes in vertebrates. These genes are preferentially expressed in previtelloogenic oocytes, and thus, they display a different cellular localization compared to that of the gsdf gene indicating that the later gene is not co-regulated. Interestingly, our study identifies new clustered genes that are specifically expressed in previtellogenic oocytes (nup54, aff1, klhl8, sdad1).

NPP-16/Nup50 function and CDK-1 inactivation are associated with anoxia-induced prophase arrest in Caenorhabditis elegans

Cellular and genetic analysis supports the notion that NPP-16/NUP50 and CDK-1 function to reversibly arrest prophase blastomeres in Caenorhabditis elegans embryos exposed to anoxia. The anoxia-induced shift of cells from an actively dividing state to an arrested state reveals a previously uncharacterized prophase checkpoint in the C. elegans embryo.

Oxygen, an essential nutrient, is sensed by a multiple of cellular pathways that facilitate the responses to and survival of oxygen deprivation. The Caenorhabditis elegans embryo exposed to severe oxygen deprivation (anoxia) enters a state of suspended animation in which cell cycle progression reversibly arrests at specific stages. The mechanisms regulating interphase, prophase, or metaphase arrest in response to anoxia are not completely understood. Characteristics of arrested prophase blastomeres and oocytes are the alignment of condensed chromosomes at the nuclear periphery and an arrest of nuclear envelope breakdown. Notably, anoxia-induced prophase arrest is suppressed in mutant embryos lacking nucleoporin NPP-16/NUP50 function, indicating that this nucleoporin plays an important role in prophase arrest in wild-type embryos. Although the inactive form of cyclin-dependent kinase (CDK-1) is detected in wild-type–arrested prophase blastomeres, the inactive state is not detected in the anoxia exposed npp-16 mutant. Furthermore, we found that CDK-1 localizes near chromosomes in anoxia-exposed embryos. These data support the notion that NPP-16 and CDK-1 function to arrest prophase blastomeres in C. elegans embryos. The anoxia-induced shift of cells from an actively dividing state to an arrested state reveals a previously uncharacterized prophase checkpoint in the C. elegans embryo.

The nuclear envelope as a signaling node in development and disease

The development of a membrane-bound structure separating DNA from other cellular components was the epochal evolutionary event that gave rise to eukaryotes, possibly occurring up to 2 billion years ago. Yet, this view of the nuclear envelope as a physical barrier greatly underestimates its fundamental impact on cellular organization and complexity, much of which is only beginning to be understood. Indeed, alterations of nuclear envelope structure and protein composition are essential to many aspects of metazoan development and cellular differentiation. Mutations in genes encoding nuclear envelope proteins cause a fascinating array of diseases referred to as "nuclear envelopathies" or "laminopathies" that affect different tissues and organ systems. We review recent work on the nuclear envelope, including insights derived from the study of nuclear envelopathies. These studies are uncovering new functions for nuclear envelope proteins and underlie an emerging view of the nuclear envelope as a critical signaling node in development and disease.

Nup358 interacts with APC and plays a role in cell polarization

Asymmetric localization of adenomatous polyposis coli (APC) to the ends of a subset of microtubules located in the leading edges is essential for the establishment of front-rear polarity during cell migration. APC is known to associate with microtubules in three ways: through interaction with the plus-end tracking protein EB1, direct binding through a C-terminal basic region, and through interaction with the plus-end motor kinesin-2. Here we report that the middle region of APC has a previously unidentified microtubule plus-end-targeting function, suggesting an additional microtubule-binding mode for APC. Through the same region, APC interacts with Nup358 (also called RanBP2), a microtubule-binding nucleoporin. Ectopic expression of the middle region of APC is sufficient to recruit endogenous Nup358 to the plus ends of microtubules. Furthermore, our results indicate that Nup358 cooperates with kinesin-2 to regulate the localization of APC to the cell cortex through a nuclear-transport-independent mechanism. Using RNA interference and a scratch-induced wound-healing assay we demonstrate that Nup358 functions in polarized cell migration. These results reveal a more active role for structural nucleoporins in regulating fundamental cellular processes than previously anticipated.

Inactivation of the C. elegans lipin homolog leads to ER disorganization and to defects in the breakdown and reassembly of the nuclear envelope

The nuclear envelope (NE) is a dynamic structure, undergoing periods of growth, breakdown and reassembly during the cell cycle. In yeast, altering lipid synthesis by inactivating the yeast homolog of lipin, a phosphatidic acid phosphohydrolase, leads to disorganization of the peripheral ER and abnormal nuclear shape. These results suggest that lipid metabolism contributes to NE dynamics; however, since yeast undergo closed mitosis, the relevance of these observations to higher eukaryotes is unclear. In mammals, lipin has been implicated in adipose tissue differentiation, insulin resistance, lipid storage and obesity, but the underlying cellular defects caused by altering lipin levels are not known. Here, we identify the Caenorhabditis elegans lipin homolog (LPIN-1) and examine its affect on NE dynamics. We find that downregulating LPIN-1 by RNAi results in the appearance of membrane sheets and other abnormal structures in the peripheral ER. Moreover, lpin-1 RNAi causes defects in NE breakdown, abnormal chromosome segregation and irregular nuclear morphology. These results uncover cellular processes affected by lipin in metazoa, and suggest that lipid synthesis has a role in NE dynamics.

Actin microfilaments guide the polarized transport of nuclear pore complexes and the cytoplasmic dispersal of Vasa mRNA during GVBD in the ascidian Halocynthia roretzi

Meiosis reinitiation starts with the germinal vesicle breakdown (GVBD) within the gonad before spawning. Here, we have extended our previous observations and identified the formation of conspicuous actin bundles emanating from the germinal vesicle (GV) during its breakdown in the ascidian Halocynthia roretzi. Time-lapse video recordings and fluorescent labelling of microfilaments (MFs) indicate that these microfilamentous structures invariantly elongate towards the vegetal hemisphere at the estimated speed of 20 mum/min. Interestingly, the nuclear pore complex protein Nup153 accumulates at the vegetal tip of actin bundles. To determine if these structures play a role in the formation of the germ plasm, we have analyzed the localization pattern of Vasa transcript in maturing oocytes and early embryos. We found that Hr-Vasa mRNA, one of Type II postplasmic/PEM mRNAs, changes from a granular and perinuclear localization to an apparent uniform cytoplasmic distribution during oocyte maturation, and then concentrate in the centrosome-attracting body (CAB) by the eight-cell stage. In addition, treatments with Latrunculin B, but not with Nocodazole, blocked the redistribution of Nup153 and Hr-Vasa mRNA, suggesting that these mechanisms are both actin-dependant. We discuss the pleiotropic role played by MFs, and the relationship between nuclear pores, maternal Vasa mRNA and germ plasm in maturing ascidian oocytes.

The nuclear transport machinery in Caenorhabditis elegans: A central role in morphogenesis

Our full understanding of the various roles for the nuclear transport machinery has come from a variety of model organisms including yeast, nematodes, fruit flies and vertebrates. Using the nematode Caenorhabditis elegans, it has been shown that the karyopherin family of nuclear transporters and the components of the Ran cycle have roles not only in nuclear protein transport, but also in mitotic spindle formation and regulation, and in nuclear envelope assembly. These studies have also demonstrated a role for nuclear transport factors in cellular differentiation and development, particularly for the formation of germ cells. This review highlights the small number of studies in C. elegans that have been critical to our understanding of this important cellular process.

Structure, dynamics and function of nuclear pore complexes

Nuclear pore complexes are large aqueous channels that penetrate the nuclear envelope, thereby connecting the nuclear interior with the cytoplasm. Until recently, these macromolecular complexes were viewed as static structures, the only function of which was to control the molecular trafficking between the two compartments. It has now become evident that this simplistic scenario is inaccurate and that nuclear pore complexes are highly dynamic multiprotein assemblies involved in diverse cellular processes ranging from the organization of the cytoskeleton to gene expression. In this review, we discuss the most recent developments in the nuclear-pore-complex field, focusing on the assembly, disassembly, maintenance and function of this macromolecular structure.

Expression and imaging of fluorescent proteins in the C. elegans gonad and early embryo

The Caenorhabditis elegans gonad and early embryo have recently emerged as an attractive metazoan model system for studying cell and developmental biology. The success of this system is attributable to the stereotypical architecture and reproducible cell divisions of the gonad/early embryo, coupled with penetrant RNAi-mediated protein depletion. These features have facilitated the development of visual assays with high spatiotemporal resolution to monitor specific subcellular processes. Assay development has relied heavily on the emergence of methods to circumvent germline silencing to allow the expression of transgenes encoding fluorescent fusion proteins. In this chapter, we discuss methods for the expression and imaging of fluorescent proteins in the C. elegans germline, including the design of transgenes for optimal expression, the generation of transgenic worm lines by ballistic bombardment, the construction of multimarker lines by mating, and methods for live imaging of the gonad and early embryo.

SPD-3 is required for spindle alignment in Caenorhabditis elegans embryos and localizes to mitochondria

During the development of multicellular organisms, cellular diversity is often achieved through asymmetric cell divisions that produce two daughter cells having different developmental potentials. Prior to an asymmetric cell division, cellular components segregate to opposite ends of the cell defining an axis of polarity. The mitotic spindle rotationally aligns along this axis of polarity, thereby ensuring that the cleavage plane is positioned such that segregated components end up in individual daughter cells. Here we report our characterization of a novel gene required for spindle alignment in Caenorhabditis elegans. During the first mitosis in spd-3(oj35) embryos the spindle failed to align along the anterior/posterior axis, leading to abnormal cleavage configurations. spd-3(oj35) embryos had additional defects reminiscent of dynein/dynactin loss-of-function possibly caused by the mislocalization of dynactin. Surprisingly, we found that SPD-3GFP localized to mitochondria. Consistent with this localization, spd-3(oj35) worms exhibited slow growth and increased ATP concentrations, which are phenotypes similar to those described for other mitochondrial mutants in C. elegans. To our knowledge, SPD-3 is the first example of a link between mitochondria and spindle alignment in C. elegans.

What can Caenorhabditis elegans tell us about the nuclear envelope?

The nuclear envelope (NE) of the eukaryotic cell provides an essential barrier that separates the nuclear compartment from the cytoplasm. In addition, the NE is involved in essential functions such as nuclear stability, regulation of gene expression, centrosome separation and nuclear migration and positioning. In metazoa the NE breaks down and re-assembles around the segregated chromatids during each cell division. In this review we discuss the molecular constituents of the Caenorhabditis elegans NE and describe their role in post-mitotic NE re-formation, as well as the usefulness of C. elegans as an in vivo system for analyzing NE dynamics.

Cell and Molecular Biology of the Spindle Matrix

The concept of a spindle matrix has long been proposed to account for incompletely understood features of microtubule spindle dynamics and force production during mitosis. In its simplest formulation, the spindle matrix is hypothesized to provide a stationary or elastic molecular matrix that can provide a substrate for motor molecules to interact with during microtubule sliding and which can stabilize the spindle during force production. Although this is an attractive concept with the potential to greatly simplify current models of microtubule spindle behavior, definitive evidence for the molecular nature of a spindle matrix or for its direct role in microtubule spindle function has been lagging. However, as reviewed here multiple studies spanning the evolutionary spectrum from lower eukaryotes to vertebrates have provided new and intriguing evidence that a spindle matrix may be a general feature of mitosis.

MEL-28 is downstream of the Ran cycle and is required for nuclear-envelope function and chromatin maintenance

Early embryonic development depends on the faithful execution of basic cell biological processes whose coordination remains largely unknown. With a global network analysis, we found MEL-28 to be associated with two types of complexes, one implicated in nuclear-envelope function and the other in chromatin organization. Here, we show that MEL-28, a protein that shuttles between the nucleus and the kinetochore during the cell cycle, is required for the structural and functional integrity of the nuclear envelope. In addition, mel-28(RNAi) embryos exhibit defects in chromosome condensation, pronuclear migration, kinetochore assembly, and spindle assembly. This combination of mel-28(RNAi) phenotypes resemble those caused by depleting members of the Ran cycle in C. elegans, a conserved cellular signaling pathway that is required for mitotic spindle assembly, nuclear-envelope reformation after mitosis, and nucleocytoplasmic exchange (reviewed in). Although MEL-28 localization to the nuclear periphery is not dependent on nuclear pore components, it is dependent on RAN-1 and other key components of the Ran cycle. Thus, MEL-28 is downstream of the Ran cycle and is required for both proper nuclear-envelope function and chromatin maintenance.