Essential Roles forCaenorhabditis elegansLamin Gene in Nuclear Organization, Cell Cycle Progression, and Spatial Organization of Nuclear Pore Complexes

Abstract

Caenorhabditis elegans has a single lamin gene, designated lmn-1 (previously termed CeLam-1). Antibodies raised against the lmn-1 product (Ce-lamin) detected a 64-kDa nuclear envelope protein. Ce-lamin was detected in the nuclear periphery of all cells except sperm and was found in the nuclear interior in embryonic cells and in a fraction of adult cells. Reductions in the amount of Ce-lamin protein produce embryonic lethality. Although the majority of affected embryos survive to produce several hundred nuclei, defects can be detected as early as the first nuclear divisions. Abnormalities include rapid changes in nuclear morphology during interphase, loss of chromosomes, unequal separation of chromosomes into daughter nuclei, abnormal condensation of chromatin, an increase in DNA content, and abnormal distribution of nuclear pore complexes (NPCs). Under conditions of incomplete RNA interference, a fraction of embryos escaped embryonic arrest and continue to develop through larval life. These animals exhibit additional phenotypes including sterility and defective segregation of chromosomes in germ cells. Our observations show that lmn-1 is an essential gene in C. elegans, and that the nuclear lamins are involved in chromatin organization, cell cycle progression, chromosome segregation, and correct spacing of NPCs. INTRODUCTIONThe nuclear lamina is a filamentous meshwork that is present between the inner nuclear membrane and the peripheral chromatin. The inner nuclear membrane and the nuclear lamina are involved in organizing nuclear structure and regulating nuclear events. These include the organization of the higher order structure of chromatin and regulation of nuclear assembly and disassembly. The nuclear lamina is a primary target for caspases in apoptosis (reviewed in Goldberg et al., 1999b). Lamins are the major proteins of the nuclear lamina. They are classified as type-V intermediate filaments and are composed of an ␣-helical rod domain flanked by a short amino (head) and a long carboxy (tail) domains. The rod domain of lamins is 52-nm long and contains four ␣-helices, each composed of heptad repeats. Coiled-coil interactions and head-to-tail associations between lamin monomers form 10-to 200-nm thick lamin filaments (reviewed in Stuurman et al., 1998) In vivo, lamin filaments are closely associated with the chromatin fibers (Belmont et al., 1993). In vitro, lamins can bind interphase chromatin (Hoger et al., 1991;Yuan et al., 1991;Taniura et al., 1995;Ulitzur et al., 1997;Goldberg et al., 1999a), mitotic chromosomes (Glass and Gerace, 1990;Glass et al., 1993), or specific DNA sequences (Shoeman and Traub, 1990;Luderus et al., 1992;Luderus et al., 1994;Baricheva et al., 1996;Zhao et al., 1996). The binding site of vertebrate lamins to chromatin is localized to specific sequences in the tail domain and can be displaced with the core histones H2A and H2B (Taniura et al., 1995;Goldberg et al., 1999a).The composition of the nuclear lamina varies in different cell types and is under developmental regulation ...

Distinct structural and mechanical properties of the nuclear lamina in Hutchinson–Gilford progeria syndrome

“…In fact, whole HGPS nuclei were, if anything, slightly more resistant to mechanical pressure than normal nuclei. One major function of the lamina is to resist deformation of the nucleus (23). In intact tissues, the nucleus and nuclear envelope are constantly exposed to high levels of mechanical stress, particularly in tissues such as bone, skeletal muscle, heart, and blood vessels.…”

Intermediate filaments in Caenorhabditis elegans

“…lmn-1(RNAi) embryos show early embryonic arrest [Liu et al, 2000] (Table I). It has been suggested in this context that LMN-1 is required for nuclear stabilization and assembly, nuclear pore complex distribution, chromosome segregation and nuclear positioning [Riemer et al, 1993;Liu et al, 2000].…”

Proteins that associate with lamins: Many faces, many functions

“…Given the structural roles of cytoplasmic intermediate filament proteins, it is not surprising that their nuclear counterparts, the lamins together with LAPs are to a large extent providing mechanical support for the NE [69,74,75]. The relevance in nuclear architecture has been demonstrated for several LAPs.…”