An interkinetic envelope surrounds chromosomes between meiosis I and II in C. elegans oocytes

At the end of cell division, the nuclear envelope reassembles around the decondensing chromosomes. Female meiosis culminates in two consecutive cell divisions of the oocyte, meiosis I and II, which are separated by a brief transition phase known as interkinesis. Due to the absence of chromosome decondensation and the suppression of genome replication during interkinesis, it has been widely assumed that the nuclear envelope does not reassemble between meiosis I and II. By analyzing interkinesis in C. elegans oocytes, we instead show that an atypical structure made of two lipid bilayers, which we termed the interkinetic envelope, surrounds the surface of the segregating chromosomes. The interkinetic envelope shares common features with the nuclear envelope but also exhibits specific characteristics that distinguish it, including its lack of continuity with the endoplasmic reticulum, unique protein composition, assembly mechanism, and function in chromosome segregation. These distinct attributes collectively define the interkinetic envelope as a unique and specialized structure that has been previously overlooked.

Nuclear envelope assembly relies on CHMP-7 in the absence of BAF-LEM-mediated hole closure

Barrier-to-autointegration factor (BAF) protein is a DNA-binding protein that crosslinks chromatin to allow mitotic nuclear envelope (NE) assembly. The LAP2-emerin-MAN1 (LEM)-domain protein LEMD2 and ESCRT-II/III hybrid protein CHMP7 close NE holes surrounding spindle microtubules (MTs). BAF binds LEM-domain family proteins to repair NE ruptures in interphase, but whether BAF-LEM binding participates in NE hole closure around spindle MTs is not known. Here, we took advantage of the stereotypical event of NE formation in fertilized Caenorhabditis elegans oocytes to show that BAF-LEM binding and LEM-2-CHMP-7 have distinct roles in NE closure around spindle MTs. LEM-2 and EMR-1 (homologs of LEMD2 and emerin) function redundantly with BAF-1 (the C. elegans BAF protein) in NE closure. Compromising BAF-LEM binding revealed an additional role for EMR-1 in the maintenance of the NE permeability barrier. In the absence of BAF-LEM binding, LEM-2-CHMP-7 was required for NE assembly and embryo survival. The winged helix domain of LEM-2 recruits CHMP-7 to the NE in C. elegans and a LEM-2-independent nucleoplasmic pool of CHMP-7 also contributes to NE stability. Thus, NE hole closure surrounding spindle MTs requires redundant mechanisms that safeguard against failure in NE assembly to support embryogenesis.

Nuclear envelope assembly relies on CHMP-7 in the absence of BAF-LEM-mediated hole closure

Barrier-to-autointegration factor (BAF) is a DNA binding protein that crosslinks chromatin to assemble the nuclear envelope (NE) after mitosis. BAF also binds the Lap2b-Emerin-Man1 (LEM) domain family of NE proteins to repair interphase ruptures. The NE adaptors to ESCRTs, LEMD2-CHMP7, seal NE holes surrounding mitotic spindle microtubules (MTs), but whether NE hole closure in mitosis involves BAF-LEM binding is not known. Here, we analyze NE sealing after meiosis II in C. elegans oocytes to show that BAF-LEM binding and LEM-2 LEMD2 -CHMP-7 have distinct roles in hole closure around spindle MTs. LEM-2/EMR-1 emerin function redundantly with BAF-1 to seal the NE. Compromising BAF-LEM binding revealed an additional role for EMR-1 in maintenance of the NE permeability barrier and an essential role for LEM-2-CHMP-7 in preventing NE assembly failure. The WH domain of LEM-2 recruits the majority of CHMP-7 to the NE in C. elegans and a LEM-2 -independent pool of CHMP-7, which is mostly enriched in the nucleoplasm, also contributes to NE stability. Thus, NE hole closure surrounding spindle MTs requires redundant mechanisms that safeguard against failure in NE assembly to support embryogenesis.

Mechanistic Insights of the LEMD2 p.L13R Mutation and Its Role in Cardiomyopathy

BACKGROUND

Nuclear envelope proteins play an important role in the pathogenesis of hereditary cardiomyopathies. Recently, a new form of arrhythmic cardiomyopathy caused by a homozygous mutation (p.L13R) in the inner nuclear membrane protein LEMD2 was discovered. The aim was to unravel the molecular mechanisms of mutant LEMD2 in the pathogenesis of cardiomyopathy.

METHODS

We generated a Lemd2 p.L13R knock-in mouse model and a corresponding cell model via CRISPR/Cas9 technology and investigated the cardiac phenotype as well as cellular and subcellular mechanisms of nuclear membrane rupture and repair.

RESULTS

Knock-in mice developed a cardiomyopathy with predominantly endocardial fibrosis, left ventricular dilatation, and systolic dysfunction. Electrocardiograms displayed pronounced ventricular arrhythmias and conduction disease. A key finding of knock-in cardiomyocytes on ultrastructural level was a significant increase in nuclear membrane invaginations and decreased nuclear circularity. Furthermore, increased DNA damage and premature senescence were detected as the underlying cause of fibrotic and inflammatory remodeling. As the p.L13R mutation is located in the Lap2/Emerin/Man1 (LEM)-domain, we observed a disrupted interaction between mutant LEMD2 and BAF (barrier-to-autointegration factor), which is required to initiate the nuclear envelope rupture repair process. To mimic increased mechanical stress with subsequent nuclear envelope ruptures, we investigated mutant HeLa-cells upon electrical stimulation and increased stiffness. Here, we demonstrated impaired nuclear envelope rupture repair capacity, subsequent cytoplasmic leakage of the DNA repair factor KU80 along with increased DNA damage, and recruitment of the cGAS (cyclic GMP-AMP synthase) to the nuclear membrane and micronuclei.

CONCLUSIONS

We show for the first time that the Lemd2 p.L13R mutation in mice recapitulates human dilated cardiomyopathy with fibrosis and severe ventricular arrhythmias. Impaired nuclear envelope rupture repair capacity resulted in increased DNA damage and activation of the cGAS/STING/IFN pathway, promoting premature senescence. Hence, LEMD2 is a new player inthe disease group of laminopathies.

Cortical microtubule pulling forces contribute to the union of the parental genomes in the C. elegans zygote

Previously, we reported that the Polo-like kinase PLK-1 phosphorylates the single Caenorhabditis elegans lamin (LMN-1) to trigger lamina depolymerization during mitosis. We showed that this event is required to form a pronuclear envelope scission event that removes membranes on the juxtaposed oocyte and sperm pronuclear envelopes in the zygote, allowing the parental chromosomes to merge in a single nucleus after segregation (Velez-Aguilera et al., 2020). Here, we show that cortical microtubule pulling forces contribute to pronuclear envelopes scission by promoting mitotic spindle elongation, and conversely, nuclear envelopes remodeling facilitates spindle elongation. We also demonstrate that weakening the pronuclear envelopes via PLK-1-mediated lamina depolymerization, is a prerequisite for the astral microtubule pulling forces to trigger pronuclear membranes scission. Finally, we provide evidence that PLK-1 mainly acts via lamina depolymerization in this process. These observations thus indicate that temporal coordination between lamina depolymerization and mitotic spindle elongation facilitates pronuclear envelopes scission and parental genomes unification.

Regulation of diverse nuclear shapes: pathways working independently, together

Membrane-bound organelles provide physical and functional compartmentalization of biological processes in eukaryotic cells. The characteristic shape and internal organization of these organelles is determined by a combination of multiple internal and external factors. The maintenance of the shape of nucleus, which houses the genetic material within a double membrane bilayer, is crucial for a seamless spatio-temporal control over nuclear and cellular functions. Dynamic morphological changes in the shape of nucleus facilitate various biological processes. Chromatin packaging, nuclear and cytosolic protein organization, and nuclear membrane lipid homeostasis are critical determinants of overall nuclear morphology. As such, a multitude of molecular players and pathways act together to regulate the nuclear shape. Here, we review the known mechanisms governing nuclear shape in various unicellular and multicellular organisms, including the non-spherical nuclei and non-lamin-related structural determinants. The review also touches upon cellular consequences of aberrant nuclear morphologies.

The Nesprin-1/-2 ortholog ANC-1 regulates organelle positioning in C. elegans independently from its KASH or actin-binding domains

KASH proteins in the outer nuclear membrane comprise the cytoplasmic half of linker of nucleoskeleton and cytoskeleton (LINC) complexes that connect nuclei to the cytoskeleton. Caenorhabditis elegans ANC-1, an ortholog of Nesprin-1/2, contains actin-binding and KASH domains at opposite ends of a long spectrin-like region. Deletion of either the KASH or calponin homology (CH) domains does not completely disrupt nuclear positioning, suggesting neither KASH nor CH domains are essential. Deletions in the spectrin-like region of ANC-1 led to significant defects, but only recapitulated the null phenotype in combination with mutations in the transmembrane (TM) span. In anc-1 mutants, the endoplasmic reticulum ER, mitochondria, and lipid droplets were unanchored, moving throughout the cytoplasm. The data presented here support a cytoplasmic integrity model where ANC-1 localizes to the ER membrane and extends into the cytoplasm to position nuclei, ER, mitochondria, and other organelles in place.

Exploring the nuclear lamina in health and pathology using C. elegans

The eukaryotic genome inside the nucleus is enveloped by two membranes, the Outer Nuclear Membrane (ONM) and the Inner Nuclear Membrane (INM). Tethered to the INM is the nuclear lamina, a fibrillar network composed of lamins-the nuclear intermediate filaments, and membrane associated proteins. The nuclear lamina interacts with several nuclear structures, including chromatin. As most nuclear functions, including regulation of gene expression, chromosome segregation and duplication as well as nuclear structure, are highly conserved in metazoans, the Caenorhabditis elegans nematode serves as a powerful model organism to study nuclear processes and architecture. This translucent organism can easily be observed under a microscope as a live embryo, larvae and even adult. Here we will review the data on nuclear lamina composition and functions gathered from studies using C. elegans model organisms: We will discuss genome spatial organization and its contribution to gene expression. We will review both the interaction between the cytoplasm and the nucleus and mechanotransduction mechanism. Finally, we will discuss disease causing mutation in nuclear lamins, including the use of this animal model in diseases research.

d-Serine and d-Alanine Regulate Adaptive Foraging Behavior in Caenorhabditis elegans via the NMDA Receptor

d-Serine (d-Ser) is a coagonist for NMDA-type glutamate receptors and is thus important for higher brain function. d-Ser is synthesized by serine racemase and degraded by d-amino acid oxidase. However, the significance of these enzymes and the relevant functions of d-amino acids remain unclear. Here, we show that in the nematode Caenorhabditis elegans, the serine racemase homolog SERR-1 and d-amino acid oxidase DAAO-1 control an adaptive foraging behavior. Similar to many organisms, C. elegans immediately initiates local search for food when transferred to a new environment. With prolonged food deprivation, the worms exhibit a long-range dispersal behavior as the adaptive foraging strategy. We found that serr-1 deletion mutants did not display this behavior, whereas daao-1 deletion mutants immediately engaged in long-range dispersal after food removal. A quantitative analysis of d-amino acids indicated that d-Ser and d-alanine (d-Ala) are both synthesized and suppressed during food deprivation. A behavioral pharmacological analysis showed that the long-range dispersal behavior requires NMDA receptor desensitization. Long-term pretreatment with d-Ala, as well as with an NMDA receptor agonist, expanded the area searched by wild-type worms immediately after food removal, whereas pretreatment with d-Ser did not. We propose that d-Ser and d-Ala are endogenous regulators that cooperatively induce the long-range dispersal behavior in C. elegans through actions on the NMDA receptor.SIGNIFICANCE STATEMENT In mammals, d-serine (d-Ser) functions as an important neuromodulator of the NMDA-type glutamate receptor, which regulates higher brain functions. In Caenorhabditis elegans, previous studies failed to clearly define the physiological significance of d-Ser, d-alanine (d-Ala), and their metabolic enzymes. In this study, we found that these d-amino acids and their associated enzymes are active during food deprivation, leading to an adaptive foraging behavior. We also found that this behavior involved NMDA receptor desensitization.

Nuclear Organization in Stress and Aging

The eukaryotic nucleus controls most cellular processes. It is isolated from the cytoplasm by the nuclear envelope, which plays a prominent role in the structural organization of the cell, including nucleocytoplasmic communication, chromatin positioning, and gene expression. Alterations in nuclear composition and function are eminently pronounced upon stress and during premature and physiological aging. These alterations are often accompanied by epigenetic changes in histone modifications. We review, here, the role of nuclear envelope proteins and histone modifiers in the 3-dimensional organization of the genome and the implications for gene expression. In particular, we focus on the nuclear lamins and the chromatin-associated protein BAF, which are linked to Hutchinson–Gilford and Nestor–Guillermo progeria syndromes, respectively. We also discuss alterations in nuclear organization and the epigenetic landscapes during normal aging and various stress conditions, ranging from yeast to humans.

Characterization of a Unique Form of Arrhythmic Cardiomyopathy Caused by Recessive Mutation in LEMD2

Nuclear envelope proteins have been shown to play an important role in the pathogenesis of inherited dilated cardiomyopathy. Here, we present a remarkable cardiac phenotype caused by a homozygous LEMD2 mutation in patients of the Hutterite population with juvenile cataract. Mutation carriers develop arrhythmic cardiomyopathy with mild impairment of left ventricular systolic function but severe ventricular arrhythmias leading to sudden cardiac death. Affected cardiac tissue from a deceased patient and fibroblasts exhibit elongated nuclei with abnormal condensed heterochromatin at the periphery. The patient fibroblasts demonstrate cellular senescence and reduced proliferation capacity, which may suggest an involvement of LEM domain containing protein 2 in chromatin remodeling processes and premature aging.

Heterochromatic foci and transcriptional repression by an unstructured MET-2/SETDB1 co-factor LIN-65

The segregation of the genome into accessible euchromatin and histone H3K9-methylated heterochromatin helps silence repetitive elements and tissue-specific genes. In Caenorhabditis elegans, MET-2, the homologue of mammalian SETDB1, catalyzes H3K9me1 and me2, yet like SETDB1, its regulation is enigmatic. Contrary to the cytosolic enrichment of overexpressed MET-2, we show that endogenous MET-2 is nuclear throughout development, forming perinuclear foci in a cell cycle-dependent manner. Mass spectrometry identified two cofactors that bind MET-2: LIN-65, a highly unstructured protein, and ARLE-14, a conserved GTPase effector. All three factors colocalize in heterochromatic foci. Ablation of lin-65, but not arle-14, mislocalizes and destabilizes MET-2, resulting in decreased H3K9 dimethylation, dispersion of heterochromatic foci, and derepression of MET-2 targets. Mutation of met-2 or lin-65 also disrupts the perinuclear anchoring of genomic heterochromatin. Loss of LIN-65, like that of MET-2, compromises temperature stress resistance and germline integrity, which are both linked to promiscuous repeat transcription and gene expression.

The CSR-1 endogenous RNAi pathway ensures accurate transcriptional reprogramming during the oocyte-to-embryo transition in Caenorhabditis elegans

Endogenous RNAi (endoRNAi) is a conserved mechanism for fine-tuning gene expression. In the nematode Caenorhabditis elegans, several endoRNAi pathways are required for the successful development of reproductive cells. The CSR-1 endoRNAi pathway promotes germ cell development, primarily by facilitating the expression of germline genes. In this study, we report a novel function for the CSR-1 pathway in preventing premature activation of embryonic transcription in the developing oocytes, which is accompanied by a general Pol II activation. This CSR-1 function requires its RNase activity, suggesting that, by controlling the levels of maternal mRNAs, CSR-1-dependent endoRNAi contributes to an orderly reprogramming of transcription during the oocyte-to-embryo transition.

During the oocyte-to-embryo transition, the control of development is transferred from the mother to the embryo. A key event during this transition is the transcriptional activation of the embryonic genome, which is tightly controlled. Here, by using the nematode C. elegans, we uncover a role for endogenous RNA interference in this process. We demonstrate that a specific endoRNAi pathway, employing the Argonaute protein CSR-1, functions as a break on gene-specific, and potentially global, activation of embryonic transcription in the developing oocytes. Our findings reveal a new layer of control over the transcriptional reprogramming during the oocyte-to-embryo transition, raising questions about its potential conservation in mammalian development.

Nuclear Lamins: Thin Filaments with Major Functions

The nuclear lamina is a nuclear peripheral meshwork that is mainly composed of nuclear lamins, although a small fraction of lamins also localizes throughout the nucleoplasm. Lamins are classified as type V intermediate filament (IF) proteins. Mutations in lamin genes cause at least 15 distinct human diseases, collectively termed laminopathies, including muscle, metabolic, and neuronal diseases, and can cause accelerated aging. Most of these mutations are in the LMNA gene encoding A-type lamins. A growing number of nuclear proteins are known to bind lamins and are implicated in both nuclear and cytoskeletal organization, mechanical stability, chromatin organization, signaling, gene regulation, genome stability, and cell differentiation. Recent studies reveal the organization of the lamin filament meshwork in somatic cells where they assemble as tetramers in cross-section of the filaments.

An Efficient FLP-Based Toolkit for Spatiotemporal Control of Gene Expression in Caenorhabditis elegans

Site-specific recombinases are potent tools to regulate gene expression. In particular, the Cre (cyclization recombination) and FLP (flipase) enzymes are widely used to either activate or inactivate genes in a precise spatiotemporal manner. Both recombinases work efficiently in the popular model organism Caenorhabditis elegans, but their use in this nematode is still only sporadic. To increase the utility of the FLP system in C. elegans, we have generated a series of single-copy transgenic strains that stably express an optimized version of FLP in specific tissues or by heat induction. We show that recombination efficiencies reach 100% in several cell types, such as muscles, intestine, and serotonin-producing neurons. Moreover, we demonstrate that most promoters drive recombination exclusively in the expected tissues. As examples of the potentials of the FLP lines, we describe novel tools for induced cell ablation by expression of the PEEL-1 toxin and a versatile FLP-out cassette for generation of GFP-tagged conditional knockout alleles. Together with other recombinase-based reagents created by the C. elegans community, this toolkit increases the possibilities for detailed analyses of specific biological processes at developmental stages inside intact animals.

Nuclear envelope localization of LEMD2 is developmentally dynamic and lamin A/C dependent yet insufficient for heterochromatin tethering

Peripheral heterochromatin in mammalian nuclei is tethered to the nuclear envelope by at least two mechanisms here referred to as the A- and B-tethers. The A-tether includes lamins A/C and additional unknown components presumably INM protein(s) interacting with both lamins A/C and chromatin. The B-tether includes the inner nuclear membrane (INM) protein Lamin B-receptor, which binds B-type lamins and chromatin. Generally, at least one of the tethers is always present in the nuclear envelope of mammalian cells. Deletion of both causes the loss of peripheral heterochromatin and consequently inversion of the entire nuclear architecture, with this occurring naturally in rod photoreceptors of nocturnal mammals. The tethers are differentially utilized during development, regulate gene expression in opposite manners, and play an important role during cell differentiation. Here we aimed to identify the unknown chromatin binding component(s) of the A-tether. We analyzed 10 mouse tissues by immunostaining with antibodies against 7 INM proteins and found that every cell type has specific, although differentially and developmentally regulated, sets of these proteins. In particular, we found that INM protein LEMD2 is concomitantly expressed with A-type lamins in various cell types but is lacking in inverted nuclei of rod cells. Truncation or deletion of Lmna resulted in the downregulation and mislocalization of LEMD2, suggesting that the two proteins interact and pointing at LEMD2 as a potential chromatin binding mediator of the A-tether. Using nuclei of mouse rods as an experimental model lacking peripheral heterochromatin, we expressed a LEMD2 transgene alone or in combination with lamin C in these cells and observed no restoration of peripheral heterochromatin in either case. We conclude that in contrary to the B-tether, the A-tether has a more intricate composition and consists of multiple components that presumably vary, at differing degrees of redundancy, between cell types and differentiation stages.

Cell Biology of the Caenorhabditis elegans Nucleus

Studies on the Caenorhabditis elegans nucleus have provided fascinating insight to the organization and activities of eukaryotic cells. Being the organelle that holds the genetic blueprint of the cell, the nucleus is critical for basically every aspect of cell biology. The stereotypical development of C. elegans from a one cell-stage embryo to a fertile hermaphrodite with 959 somatic nuclei has allowed the identification of mutants with specific alterations in gene expression programs, nuclear morphology, or nuclear positioning. Moreover, the early C. elegans embryo is an excellent model to dissect the mitotic processes of nuclear disassembly and reformation with high spatiotemporal resolution. We review here several features of the C. elegans nucleus, including its composition, structure, and dynamics. We also discuss the spatial organization of chromatin and regulation of gene expression and how this depends on tight control of nucleocytoplasmic transport. Finally, the extensive connections of the nucleus with the cytoskeleton and their implications during development are described. Most processes of the C. elegans nucleus are evolutionarily conserved, highlighting the relevance of this powerful and versatile model organism to human biology.

Glutathione reductase gsr-1 is an essential gene required for Caenorhabditis elegans early embryonic development

Glutathione is the most abundant thiol in the vast majority of organisms and is maintained in its reduced form by the flavoenzyme glutathione reductase. In this work, we describe the genetic and functional analysis of the Caenorhabditis elegans gsr-1 gene that encodes the only glutathione reductase protein in this model organism. By using green fluorescent protein reporters we demonstrate that gsr-1 produces two GSR-1 isoforms, one located in the cytoplasm and one in the mitochondria. gsr-1 loss of function mutants display a fully penetrant embryonic lethal phenotype characterized by a progressive and robust cell division delay accompanied by an aberrant distribution of interphasic chromatin in the periphery of the cell nucleus. Maternally expressed GSR-1 is sufficient to support embryonic development but these animals are short-lived, sensitized to chemical stress, have increased mitochondrial fragmentation and lower mitochondrial DNA content. Furthermore, the embryonic lethality of gsr-1 worms is prevented by restoring GSR-1 activity in the cytoplasm but not in mitochondria. Given the fact that the thioredoxin redox systems are dispensable in C. elegans, our data support a prominent role of the glutathione reductase/glutathione pathway in maintaining redox homeostasis in the nematode.

The nuclear lamina in health and disease

The nuclear lamina (NL) is a structural component of the nuclear envelope and makes extensive contacts with integral nuclear membrane proteins and chromatin. These interactions are critical for many cellular processes, such as nuclear positioning, perception of mechanical stimuli from the cell surface, nuclear stability, 3-dimensional organization of chromatin and regulation of chromatin-binding proteins, including transcription factors. The NL is present in all nucleated metazoan cells but its composition and interactome differ between tissues. Most likely, this contributes to the broad spectrum of disease manifestations in humans with mutations in NL-related genes, ranging from muscle dystrophies to neurological disorders, lipodystrophies and progeria syndromes. We review here exciting novel insight into NL function at the cellular level, in particular in chromatin organization and mechanosensation. We also present recent observations on the relation between the NL and metabolism and the special relevance of the NL in muscle tissues. Finally, we discuss new therapeutic approaches to treat NL-related diseases.

Matefin/SUN-1 Phosphorylation on Serine 43 Is Mediated by CDK-1 and Required for Its Localization to Centrosomes and Normal Mitosis in C. elegans Embryos

Matefin/SUN-1 is an evolutionary conserved C. elegans inner nuclear membrane SUN-domain protein. By creating a bridge with the KASH-domain protein ZYG-12, it connects the nucleus to cytoplasmic filaments and organelles. Matefin/SUN-1 is expressed in the germline where it undergoes specific phosphorylation at its N-terminal domain, which is required for germline development and homologous chromosome pairing. The maternally deposited matefin/SUN-1 is then essential for embryonic development. Here, we show that in embryos, serine 43 of matefin/SUN-1 (S43) is phosphorylated in a CDK-1 dependent manner and is localized throughout the cell cycle mostly to centrosomes. By generating animals expressing phosphodead S43A and phosphomimetic S43E mutations, we show that phosphorylation of S43 is required to maintain centrosome integrity and function, as well as for the localization of ZYG-12 and lamin. Expression of S43E in early embryos also leads to an increase in chromatin structural changes, decreased progeny and to almost complete embryonic lethality. Down regulation of emerin further increases the occurrence of chromatin organization abnormalities, indicating possible collaborative roles for these proteins that is regulated by S43 phosphorylation. Taken together, these results support a role for phosphorylation of serine 43 in matefin/SUN-1 in mitosis.

New Insights into Mechanisms and Functions of Nuclear Size Regulation

Nuclear size is generally maintained within a defined range in a given cell type. Changes in cell size that occur during cell growth, development, and differentiation are accompanied by dynamic nuclear size adjustments in order to establish appropriate nuclear-to-cytoplasmic volume relationships. It has long been recognized that aberrations in nuclear size are associated with certain disease states, most notably cancer. Nuclear size and morphology must impact nuclear and cellular functions. Understanding these functional implications requires an understanding of the mechanisms that control nuclear size. In this review, we first provide a general overview of the diverse cellular structures and activities that contribute to nuclear size control, including structural components of the nucleus, effects of DNA amount and chromatin compaction, signaling, and transport pathways that impinge on the nucleus, extranuclear structures, and cell cycle state. We then detail some of the key mechanistic findings about nuclear size regulation that have been gleaned from a variety of model organisms. Lastly, we review studies that have implicated nuclear size in the regulation of cell and nuclear function and speculate on the potential functional significance of nuclear size in chromatin organization, gene expression, nuclear mechanics, and disease. With many fundamental cell biological questions remaining to be answered, the field of nuclear size regulation is still wide open.

Caenorhabditis elegans polo-like kinase PLK-1 is required for merging parental genomes into a single nucleus

Before the first zygotic division, the nuclear envelopes of the maternal and paternal pronuclei disassemble, allowing both sets of chromosomes to be incorporated into a single nucleus in daughter cells after mitosis. We found that in Caenorhabditis elegans, partial inactivation of the polo-like kinase PLK-1 causes the formation of two nuclei, containing either the maternal or paternal chromosomes, in each daughter cell. These two nuclei gave rise to paired nuclei in all subsequent cell divisions. The paired-nuclei phenotype was caused by a defect in forming a gap in the nuclear envelopes at the interface between the two pronuclei during the first mitotic division. This was accompanied by defects in chromosome congression and alignment of the maternal and paternal metaphase plates relative to each other. Perturbing chromosome congression by other means also resulted in failure to disassemble the nuclear envelope between the two pronuclei. Our data further show that PLK-1 is needed for nuclear envelope breakdown during early embryogenesis. We propose that during the first zygotic division, PLK-1-dependent chromosome congression and metaphase plate alignment are necessary for the disassembly of the nuclear envelope between the two pronuclei, ultimately allowing intermingling of the maternal and paternal chromosomes.

Lamin-Binding Proteins in Caenorhabditis elegans

The nuclear lamina, composed of lamins and numerous lamin-associated proteins, is required for mechanical stability, mechanosensing, chromatin organization, developmental gene regulation, mRNA transcription, DNA replication, nuclear assembly, and nuclear positioning. Mutations in lamins or lamin-binding proteins cause at least 18 distinct human diseases that affect specific tissues such as muscle, adipose, bone, nerve, or skin, and range from muscular dystrophies to lipodystrophy, peripheral neuropathy, or accelerated aging. Caenorhabditis elegans has unique advantages in studying lamin-binding proteins. These advantages include the low complexity of genes encoding lamin and lamin-binding proteins, advanced transgenic techniques, simple application of RNA interference, sophisticated genetic strategies, and a large collection of mutant lines. This chapter provides detailed and comprehensive protocols for the genetic and phenotypic analysis of lamin-binding proteins in C. elegans.