Ayelet Margalit

The nuclear lamina comes of age

Many nuclear proteins form lamin-dependent complexes, including LEM-domain proteins, nesprins and SUN-domain proteins. These complexes have roles in chromatin organization, gene regulation and signal transduction. Some link the nucleoskeleton to cytoskeletal structures, ensuring that the nucleus and centrosome assume appropriate intracellular positions. These complexes provide new insights into cell architecture, as well as a foundation for the understanding of the molecular mechanisms that underlie the human laminopathies — clinical disorders that range from Emery–Dreifuss muscular dystrophy to the accelerated ageing seen in Hutchinson–Gilford progeria syndrome.

The nuclear lamina and its functions in the nucleus.

The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of lamins, which are also present in the nuclear interior, and lamin-associated proteins. The increasing number of proteins that interact with lamins and the compound interactions between these proteins and chromatin-associated proteins make the nuclear lamina a highly complex but also a very exciting structure. The nuclear lamina is an essential component of metazoan cells. It is involved in most nuclear activities including DNA replication, RNA transcription, nuclear and chromatin organization, cell cycle regulation, cell development and differentiation, nuclear migration, and apoptosis. Specific mutations in nuclear lamina genes cause a wide range of heritable human diseases. These diseases include Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy, dilated cardiomyopathy (DCM) with conduction system disease, familial partial lipodystrophy (FPLD), autosomal recessive axonal neuropathy (Charcot-Marie-Tooth disorder type 2, CMT2), mandibuloacral dysplasia (MAD), Hutchison Gilford Progeria syndrome (HGS), Greenberg Skeletal Dysplasia, and Pelger-Huet anomaly (PHA). Genetic analyses in Caenorhabditis elegans, Drosophila, and mice show new insights into the functions of the nuclear lamina, and recent structural analyses have begun to unravel the molecular structure and assembly of lamins and their associated proteins.

Breaking and making of the nuclear envelope

During mitosis, a single nucleus gives rise to two nuclei that are identical to the parent nucleus. Mitosis consists of a continuous sequence of events that must be carried out once and only once. Two such important events are the disassembly of the nuclear envelope (NE) during the first stages of mitosis, and its accurate reassembly during the last stages of mitosis. NE breakdown (NEBD) is initiated when maturation‐promoting factor (MPF) enters the nucleus and starts phosphorylating nuclear pore complexes (NPCs) and nuclear lamina proteins, followed by NPC and lamina breakdown. Nuclear reassembly starts when nuclear membranes assemble onto the chromatin. This article focuses on the different models of NEBD and reassembly with emphasis on recent data.

Specific and conserved sequences in D. melanogaster and C. elegans lamins and histone H2A mediate the attachment of lamins to chromosomes

The intimate association between nuclear lamins and chromatin is thought to regulate higher order chromatin organization. Previous studies have mapped a region between the rod domain and the Ig fold in the tail domain of Drosophila melanogaster lamin Dm0, which binds chromatin in vitro via the histone H2A/H2B dimer. This region contains an evolutionarily conserved nuclear localization signal (NLS) KRKR, and a sequence composed of the amino acids TRAT. Here we show that binding of lamin Dm0 to chromatin requires both NLS and TRAT sequences. Substituting either of the threonine residues in the TRAT sequence with negatively charged residues decreases the binding of lamin Dm0 to chromatin, indicating that this binding could be regulated by phosphorylation. Both lamin Dm0 and C. elegans Ce-lamin bind directly to histone H2A in vitro and this binding requires the NLS. The amino and carboxyl tail domains of histone H2A are each essential, but not sufficient, for binding to lamin Dm0; only a polypeptide containing both histone H2A tail domains binds efficiently to lamin Dm0. Taken together, these results suggest that specific residues in lamin Dm0 and histone H2A mediate the attachment of the nuclear lamina to chromosomes in vivo, which could have implications on the understanding of laminopathic diseases.

Barrier to autointegration factor blocks premature cell fusion and maintains adult muscle integrity in C. elegans

Barrier to autointegration factor (BAF) binds double-stranded DNA, selected histones, transcription regulators, lamins, and LAP2–emerin–MAN1 (LEM) domain proteins. During early Caenorhabditis elegans embryogenesis, BAF-1 is required to organize chromatin, capture segregated chromosomes within the nascent nuclear envelope, and assemble lamin and LEM domain proteins in reforming nuclei. In this study, we used C. elegans with a homozygous deletion of the baf-1 gene, which survives embryogenesis and larval stages, to report that BAF-1 regulates maturation and survival of the germline, cell migration, vulva formation, and the timing of seam cell fusion. In the seam cells, BAF-1 represses the expression of the EFF-1 fusogen protein, but fusion still occurs in C. elegans lacking both baf-1 and eff-1. This suggests the existence of an eff-1–independent mechanism for cell fusion. BAF-1 is also required to maintain the integrity of specific body wall muscles in adult animals, directly implicating BAF in the mechanism of human muscular dystrophies (laminopathies) caused by mutations in the BAF-binding proteins emerin and lamin A.

Matefin/SUN-1 is a nuclear envelope receptor for CED-4 during Caenorhabditis elegans apoptosis.

In Caenorhabditis elegans, the antiapoptotic protein CED-9 is localized at the mitochondria, where it binds the proapoptotic protein CED-4. Induction of apoptosis begins when the proapoptotic protein EGL-1 is expressed and binds CED-9. The binding of EGL-1 to CED-9 releases CED-4 from CED-9 and causes the activation of the caspase CED-3. Upon its release from CED-9, CED-4 rapidly translocates to the nuclear envelope (NE) in a CED-3-independent manner. However, the identity of the NE receptor for CED-4 and its possible role in the execution of apoptosis has remained unknown. Here, we show that the inner nuclear membrane SUN-domain protein matefin/SUN-1 is the NE receptor for CED-4. Our data demonstrate that matefin/SUN-1 binds CED-4 and is specifically required for CED-4 translocation and maintenance at the NE. The role of matefin/SUN-1 in the execution of apoptosis is further suggested by the significant reduction in the number of apoptotic cells in the organism after matefin/SUN-1 down-regulation by RNAi. The finding that matefin/SUN-1 is required for the execution of apoptosis adds an important link between cytoplasmic and nuclear apoptotic events.