Cell Biology of theCaenorhabditis elegansNucleus

Abstract

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.KEYWORDS Caenorhabditis elegans; chromatin; gene expression; lamin; LEM-domain proteins; LINC; P granule; nuclear pore complex; nuclear envelope; nucleocytoplasmic transport; nucleolus; WormBook TABLE OF CONTENTSAbstract 25 C. elegans as a model system to study cell biology of the nucleus 26 Structural components of the nucleus 27

Visualizing the metazoan proliferation-terminal differentiation decisionin vivo

“…However, the 2x-mKate2 version of the biosensor substantially increased the dynamic range in both the SM and uterine lineages, with peak values of 2.5 and 3.4, respectively ( Figure S2A-F). Notably, upon inhibiting the nuclear export of DHB::2x-mKate2 through RNAi-induced depletion of xpo-1/Exportin 1 in the uterine lineage, there was a statistically significant retention of DHB::2x-mKate2 in the nucleus relative to control (n = 45, P ≤ 1x10 -7 ; Figure 2F, G), supporting the notion that this version of the biosensor is predominantly exported from the nucleus via active transport (Cohen-Fix and Askjaer, 2017;Timney et al, 2016). Together, these data provide strong evidence that the mass of the DHB::2x-mKate2 fusion protein decreases or limits the rate of diffusion back into the nucleus during the cell cycle, increasing the dynamic range of the biosensor.…”

“…The codon-optimized GFP we used in the generation of DHB::GFP is based on the monomeric sequence of a 26.9 kD eGFP. The synthesized codon-optimized DHB, including a nine amino acid (3xGAS) flexible linker, is predicted to be 11.2 kD (predicted by ExPASy Compute pl/MW (https://web.expasy.org/compute_pi/)) (Artimo et al, 2012), such that the DHB::GFP fusion protein is predicted to be 38.2 kD, which is below the threshold that requires active nuclear import/export machinery (Cohen-Fix and Askjaer, 2017;Timney et al, 2016).…”

Loss of an H3K9me anchor rescues laminopathy-linked changes in nuclear organization and muscle function in an Emery-Dreifuss muscular dystrophy model

“…Moreover, we and others have amply demonstrated that condensed, transcriptionally silent heterochromatin is found adjacent to the INM in contact with the nuclear lamina in worms as in other metazoans (Ikegami et al 2010;Meister et al 2010b;Towbin et al 2012). The peripheral tethering of chromatin depends, at least in part, on the interaction of heterochromatin-specific histone modifications with INM components (Harr et al 2016;Cohen-Fix and Askjaer 2017). In C. elegans embryos this is mediated by CEC-4, a nuclear envelope (NE)-associated protein that has a chromodomain (CD) with 46% identity to the CD of human heterochromatin protein 1α (HP1α) (Gonzalez-Sandoval et al 2015).…”

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

“…The nuclear envelope (NE) is composed of a double membrane that forms a diffusion barrier between the cytoplasm and nucleoplasm. It also includes various proteins and protein complexes, such as nuclear pore complexes (NPCs) and the nuclear lamina (for reviews, see Ungricht and Kutay, 2017; Cohen-Fix and Askjaer, 2017; De Magistris and Antonin, 2018). During mitosis in metazoans, NE breakdown (NEBD) allows microtubules nucleated by cytoplasmic centrosomes to access the chromosomes.…”