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Yixian Zheng - One of the best experts on this subject based on the ideXlab platform.
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Computational analyses reveal spatial relationships between nuclear pore complexes and specific Lamins.
The Journal of cell biology, 2021Co-Authors: Mark Kittisopikul, Yixian Zheng, Stephen A. Adam, Takeshi Shimi, Meltem Tatli, Joseph R. Tran, Ohad Medalia, Khuloud Jaqaman, Robert D. GoldmanAbstract:Nuclear Lamin isoforms form fibrous meshworks associated with nuclear pore complexes (NPCs). Using datasets prepared from subpixel and segmentation analyses of 3D-structured illumination microscopy images of WT and Lamin isoform knockout mouse embryo fibroblasts, we determined with high precision the spatial association of NPCs with specific Lamin isoform fibers. These relationships are retained in the enlarged Lamin meshworks of Lmna-/- and Lmnb1-/- fibroblast nuclei. Cryo-ET observations reveal that the Lamin filaments composing the fibers contact the nucleoplasmic ring of NPCs. Knockdown of the ring-associated nucleoporin ELYS induces NPC clusters that exclude Lamin A/C fibers but include LB1 and LB2 fibers. Knockdown of the nucleoporin TPR or NUP153 alters the arrangement of Lamin fibers and NPCs. Evidence that the number of NPCs is regulated by specific Lamin isoforms is presented. Overall the results demonstrate that Lamin isoforms and nucleoporins act together to maintain the normal organization of Lamin meshworks and NPCs within the nuclear envelope.
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Nuclear Lamins A/C and B1 Provide a Structural Framework That Organizes and Anchors Nuclear Pore Complexes
2020Co-Authors: Mark Kittisopikul, Yixian Zheng, Stephen A. Adam, Takeshi Shimi, Meltem Tatli, Joseph R. Tran, Ohad Medalia, Khuloud Jaqaman, Robert D. GoldmanAbstract:Abstract Nuclear Lamin isoforms form fibrous meshworks associated with nuclear pore complexes (NPCs). Using data sets prepared from sub-pixel and segmentation analyses of 3D-Structured Illumination Microscopy images of WT and Lamin isoform knockout mouse embryo fibroblasts, we determined with high precision the spatial association of NPCs with specific Lamin isoform fibers. These relationships are retained in the enlarged Lamin meshworks of Lmna-/- and Lmnb1-/- fibroblast nuclei. Cryo-ET observations reveal that the Lamin filaments composing the fibers contact the nucleoplasmic ring of NPCs. Knockdown of the ring-associated nucleoporin ELYS induces NPC clusters that exclude Lamin A/C fibers, but include LB1 and LB2 fibers. Knockdown of the nucleoporins TPR or NUP153 alter the arrangement of Lamin fibers and NPCs. Evidence that the number of NPCs is regulated by specific Lamin isoforms is presented. Overall the results demonstrate that Lamin isoforms and nucleoporins act together to maintain the normal organization of Lamin meshworks and NPCs within the nuclear envelope.
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Lamin B Counteracts the Kinesin Eg5 to Restrain Spindle Pole Separation during Spindle Assembly
The Journal of biological chemistry, 2010Co-Authors: Benjamin Goodman, Pablo A. Iglesias, Wilbur E. Channels, Minhua Qiu, Ge Yang, Yixian ZhengAbstract:Lamin B is a component of the membranous spindle matrix isolated from Xenopus egg extracts, and it is required for proper spindle morphogenesis. Besides Lamin B, the spindle matrix contains spindle assembly factors (SAFs) such as Eg5 and dynein which are known to regulate microtubule organization and SAFs known to promote microtubule assembly such as Maskin and XMAP215. Because Lamin B does not bind directly to microtubules, it must affect spindle morphogenesis indirectly by influencing the function of spindle matrix-associated SAFs. Using different assays in Xenopus egg extracts, we found that depleting Lamin B caused formation of elongated and multipolar spindles, which could be reversed by partially inhibiting the kinesin Eg5, revealing an antagonistic relationship between Eg5 and Lamin B. However, Lamin B only very weakly antagonizes Eg5 in mediating poleward microtubule-flux based on fluorescence speckle microscopy. Depleting Lamin B led to a very small but statistically significant increase in flux. Furthermore, flux reduction caused by partial Eg5 inhibition is only slightly reversed by removing Lamin B. Because Lamin B does not bind to Eg5, our studies suggest two nonexclusive mechanisms by which Lamin B can indirectly antagonize Eg5. It could function in a network that restricts Eg5-driven microtubule sliding only when microtubules come into transient contact with the network. Lamin B could also function to sequester microtubule polymerization activities within the spindle. Without Lamin B, increased microtubule assembly caused by the released SAFs would lead to excessive microtubule sliding that results in formation of elongated and multipolar spindles.
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Requirement for Nudel and dynein for assembly of the Lamin B spindle matrix
Nature cell biology, 2009Co-Authors: Ming Ying Tsai, Shusheng Wang, Rong Chen, Jonh R. Yates, Xueliang Zhu, Yixian ZhengAbstract:The small guanosine triphosphatase Ran loaded with GTP (RanGTP) can stimulate assembly of the type V intermediate filament protein Lamin B into a membranous Lamin B spindle matrix, which is required for proper microtubule organization during spindle assembly. Microtubules in turn enhance assembly of the matrix. Here we report that the isolated matrix contains known spindle assembly factors such as dynein and Nudel. Using spindle assembly assays in Xenopus egg extracts, we show that Nudel regulates microtubule organization during spindle assembly independently of its function at kinetochores. Importantly, Nudel interacts directly with Lamin B to facilitate the accumulation and assembly of Lamin-B-containing matrix on microtubules in a dynein-dependent manner. Perturbing either Nudel or dynein inhibited the assembly of Lamin B matrix. However, depleting Lamin B still allowed the formation of matrices containing dynein and Nudel. Therefore, dynein and Nudel regulate assembly of the Lamin B matrix. Interestingly, we found that whereas depleting Lamin B resulted in disorganized spindle and spindle poles, disrupting the function of Nudel or dynein caused a complete lack of spindle pole focusing. We suggest that Nudel regulates microtubule organization in part by facilitating assembly of the Lamin B spindle matrix in a dynein-dependent manner.
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A Mitotic Lamin B Matrix Induced by RanGTP Required for Spindle Assembly
Science (New York N.Y.), 2006Co-Authors: Ming Ying Tsai, Stephen A. Adam, Robert D. Goldman, Shusheng Wang, Jill M. Heidinger, Dale K. Shumaker, Yixian ZhengAbstract:Mitotic spindle morphogenesis is a series of highly coordinated movements that lead to chromosome segregation and cytokinesis. We report that the intermediate filament protein Lamin B, a component of the interphase nuclear Lamina, functions in spindle assembly. Lamin B assembled into a matrix-like network in mitosis through a process that depended on the presence of the guanosine triphosphate–bound form of the small guanosine triphosphatase Ran. Depletion of Lamin B resulted in defects in spindle assembly. Dominant negative mutant Lamin B proteins that disrupt Lamin B assembly in interphase nuclei also disrupted spindle assembly in mitosis. Furthermore, Lamin B was essential for the formation of the mitotic matrix that tethers a number of spindle assembly factors. We propose that Lamin B is a structural component of the long-soughtafter spindle matrix that promotes microtubule assembly and organization in mitosis.
Reimer Stick - One of the best experts on this subject based on the ideXlab platform.
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Evolution of the Lamin protein family at the base of the vertebrate lineage
Cell and Tissue Research, 2020Co-Authors: Reimer Stick, Annette PeterAbstract:Lamin proteins are major constituents of the nuclear Lamina. They are required for fundamental nuclear activities, as evidenced by the large number of Laminopathies. Mutations in the human Lamin A/C gene exhibit a broad spectrum of clinical manifestations. Most non-vertebrates including the nearest relatives of the vertebrates have only a single Lamin gene. In jawed vertebrates (Gnathostomata), four Lamin subtypes (B1, B2, LIII, and A) are found. Lampreys and hagfish form the two orders of jawless vertebrates, Agnatha, which represent the sister group of the Gnathostomata at the base of the vertebrate lineage. Lamin sequence information of lampreys and hagfish sheds light on the evolution of the Lamin protein family at the base of the vertebrate lineage. In the genomes of the lamprey ( Petromyzon marinus ) and the hagfish ( Eptatretus burgeri ), only three Lamin genes are present, a Lamin A gene is lacking. The presence of an LIII gene in both, lampreys and hagfish, proves that the distinguishing features of this gene had been established before the agnathan/gnathostome split. The other two agnathan Lamins, LmnI and LmnII, deviate strongly in their sequences from those of the gnathostome Lamins. For none of these two agnathan Lamins can orthology be established to one of the gnathostome Lamin types. In the direct chromosomal neighbourhood of all three hagfish Lamin genes, a MARCH3 paralog is found. This can be interpreted as further evidence that the vertebrate Lamin genes have arisen in the course of the two rounds of whole genome duplication that took place at the base of the vertebrate lineage.
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Evolutionary changes in Lamin expression in the vertebrate lineage
Nucleus (Austin Tex.), 2017Co-Authors: Reimer Stick, Annette PeterAbstract:ABSTRACTThe nuclear Lamina is involved in fundamental nuclear functions and provides mechanical stability to the nucleus. Lamin filaments form a meshwork closely apposed to the inner nuclear membrane and a small fraction of Lamins exist in the nuclear interior. Mutations in Lamin genes cause severe hereditary diseases, the Laminopathies. During vertebrate evolution the Lamin protein family has expanded. While most vertebrate genomes contain 4 Lamin genes, encoding the Lamins A, B1, B2, and LIII, the majority of non-vertebrate genomes harbor only a single Lamin gene. We have collected Lamin gene and cDNA sequence information for representatives of the major vertebrate lineages. With the help of RNA-seq data we have determined relative Lamin expression levels for representative tissues for species of 9 different gnathostome lineages. Here we report that the level of Lamin A expression is low in cartilaginous fishes and ancient fishes and increases toward the mammals. Lamin B1 expression shows an inverse ten...
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Molecular characterization of Xenopus Lamin LIV reveals differences in the Lamin composition of sperms in amphibians and mammals.
Nucleus (Austin Tex.), 2010Co-Authors: Friederike Von Moeller, Tanja Barendziak, Ketaki Apte, Martin W. Goldberg, Reimer StickAbstract:Lamins are nuclear intermediate filament proteins. They are involved in most nuclear activities and are essential for retaining the mechano-elastic properties of the nucleus. Somatic cells of vertebrates express Lamins A, B1, and B2 while Lamin LIII, a major component of the amphibian oocyte Lamina is absent in mammals. The organization of the Lamina of germ cells differs significantly from that of somatic cells. Mammalian spermatogenic cells express two short Lamins, C2 and B3, that are splice isoforms of Lamin A and B2, respectively. Here we identify the previously described Xenopus Lamin LIV as splice variant of the Lamin LIII gene. LIV contains 40 extra residues in coil 2A of the rod domain, which results in altered assembly properties. Xenopus Lamin LIV and mammalian B3 assemble into short structures rather than into long IF-like filaments. Expression of Lamin LIV is restricted to male germ cells suggesting that it might be the functional equivalent of mammalian Lamin B3. We provide evidence that lam...
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The gene structure of B-type nuclear Lamins ofXenopus laevis: Implications for the evolution of the vertebrate Lamin family
Chromosome Research, 1994Co-Authors: Reimer StickAbstract:The gene structure of the Xenopus laevis Lamin B1 gene is presented, together with a partial analysis of the Lamin B2 gene of the same species. The intron/exon pattern of Lamin B1 is strikingly similar to other vertebrate Lamin genes and is completely identical to that of Xenopus Lamin B3. An additional intron present in the murine Lamin B2 gene has a counterpart in the orthologue gene of Xenopus. These data complete a comparison of the genomic organization of all vertebrate Lamin types known so far. They allow the conclusion that the genes encoding B1 and B3 probably reflect the ancestral Lamin gene organization and that the B2-type Lamins are derived from them. A non-conforming splice junction sequence is found in the Xenopus Lamin B1 gene. A GC is found instead of the canonical GT dinucleotide at the 5′ end of intron IX. Comparison with other unusual splice sites indicates that the Lamin B1 gene represents a functional gene.
Georg Krohne - One of the best experts on this subject based on the ideXlab platform.
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The myristoylation site of meiotic Lamin C2 promotes local nuclear membrane growth and the formation of intranuclear membranes in somatic cultured cells.
European Journal of Cell Biology, 2005Co-Authors: Kristina Prüfert, Manfred Alsheimer, Ricardo Benavente, Georg KrohneAbstract:Lamin C2 is a splice product of the mammalian Lamin A gene and expressed in primary spermatocytes where it is distributed in the form of discontinuous plaques at the nuclear envelope. We have previously shown that the aminoterminal hexapetide GNAEGR of Lamin C2 following the start methionine is essential for its association with the nuclear envelope and that the aminoterminal glycine of the hexapeptide is myristoylated. Here we have analyzed the ultrastructural changes induced in COS-7 and Xenopus A6 cells by overexpressing rat Lamin C2 or a human Lamin C mutant possessing the Lamin C2-specific hexapeptide at its aminoterminus. Both Lamins were targeted to the nuclear envelope of mammalian and amphibian cells and induced the formation of intranuclear membranes, whereas wild-type human Lamin C and a Lamin C2 mutant, that both lack this lipid moiety, did not. Our data indicate that the myristoyl group of Lamin C2 has besides its demonstrated role in nuclear envelope association additional functions during spermatogenesis. Our present study complements previously published results where we have shown that the CxxM motif of Lamins promotes nuclear membrane growth (Prufert et al., 2004. J. Cell Sci. 117, 6105-6116).
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The Lamin CxxM motif promotes nuclear membrane growth.
Journal of Cell Science, 2004Co-Authors: Kristina Prüfert, Annette Vogel, Georg KrohneAbstract:We analyzed the influence of Lamins on nuclear envelope growth in cultured Xenopus A6 cells by the overexpression of human Lamin A, Xenopus and zebrafish Lamins B2 and Drosophila Lamins Dm0 and C as GFP fusion proteins. Lamins containing a CxxM motif in their primary sequence (Lamins A, B2, Dm0) induced the formation of lobulated nuclei with multi-membrane-layered, highly folded nuclear membranes and intranuclear membrane assemblies, as observed by electron microscopy. Such morphological alterations were not observed with Drosophila Lamin C, a Lamin without this motif or with a Lamin B2 mutant (B2-SxxM) where the cysteine of the CxxM motif is replaced by a serine. Drosophila Lamin C mutants containing a CxxM motif behaved like B-type Lamins thus confirming that this tetrapeptide is directly involved in the morphological changes we observed. Nuclear membrane proliferation could also be induced by Lamin B2 in COS-7 cells and in zebrafish embryos but not by human Lamin A in COS-7 cells. We speculate that the human Lamin A is incompletely processed in Xenopus A6 cells and therefore behaves in this cell line like a B-type Lamin. Our results indicate that the CxxM motif of B-type Lamins has a dual function: it mediates Lamin targeting to the inner nuclear membrane thereby promoting nuclear membrane growth.
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characterization and quantitation of three b type Lamins in xenopus oocytes and eggs increase of Lamin li protein synthesis during meiotic maturation
Journal of Cell Science, 1996Co-Authors: David Lourim, Anja Kempf, Georg KrohneAbstract:We have previously shown that Xenopus oocytes, eggs, and early embryos contain Lamins LII and LIII, and that portions of each are associated with distinct egg vesicle populations. We now report that a Lamin similar or identical to the B-type Lamin LI is also present in oocyte nuclei and in egg extracts. We have quantitated the three B-type Lamins per oocyte nucleus, and have calculated relative ratios of LI:LIII = 1:100, and LII:LIII = 1:10. Similar to Lamin LII, 5–15% of Lamin LI is associated with egg membranes in a biochemically stable manner. Egg vesicles absorbed with Lamin isoform-specific antibodies to magnetic beads indicate that Lamin LI-associated egg membranes are of heterogenous morphology, and are independent from the Lamin LII and LIII vesicle populations. Compared to other nuclear envelope proteins, the synthesis of Lamin LI protein is specifically elevated during meiotic maturation, resulting in a 4- to 12-fold higher amount of Lamin LI in eggs than is present in oocyte nuclei. Immunofluorescence and immunoblot analysis demonstrated that Lamins LI, LII, and LIII are associated with the nuclear envelope formed on demembranated sperm when added to activated egg extract. These results strongly suggest that three different Lamin-associated vesicle populations are involved in the formation of a nuclear envelope in egg extracts.
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Membrane-associated Lamins in Xenopus egg extracts: identification of two vesicle populations.
The Journal of cell biology, 1993Co-Authors: D Lourim, Georg KrohneAbstract:Nuclear Lamin isoforms of vertebrates can be divided into two major classes. The B-type Lamins are membrane associated throughout the cell cycle, whereas A-type Lamins are recovered from mitotic cell homogenates in membrane-free fractions. A feature of oogenesis in birds and mammals is the nearly exclusive presence of B-type Lamins in oocyte nuclear envelopes. In contrast, oocytes and early cleavage embryos of the amphibian Xenopus laevis are believed to contain a single Lamin isoform, Lamin LIII, which after nuclear envelope breakdown during meiotic maturation is reported to be completely soluble. Consequently, we have reexamined the Lamin complement of Xenopus oocyte nuclear envelopes, egg extracts, and early embryos. An mAb (X223) specific for the homologous B-type Lamins B2 of mouse and LII of Xenopus somatic cells (Hoger, T., K. Zatloukal, I. Waizenegger, and G. Krohne. 1990. Chromosoma. 99:379-390) recognized a Xenopus oocyte nuclear envelope protein biochemically distinct from Lamin LIII and very similar or identical to somatic cell Lamin LII. Oocyte Lamin LII was detectable in nuclear envelopes of early cleavage embryos. Immunoblotting of fractionated egg extracts revealed that approximately 20-23% of Lamin LII and 5-7% of Lamin LIII were membrane associated. EM immunolocalization demonstrated that membrane-bound Lamins LII and LIII are associated with separate vesicle populations. These findings are relevant to the interpretation of nuclear reconstitution experiments using Xenopus egg extracts.
Loren G. Fong - One of the best experts on this subject based on the ideXlab platform.
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Lamin B1 and Lamin B2 are long-lived proteins with distinct functions in retinal development
Molecular biology of the cell, 2016Co-Authors: David Razafsky, Loren G. Fong, Stephen G. Young, Candace L Ward, Chloe Potter, Wanqiu Zhu, Yunlu Xue, Vladimir J. Kefalov, Didier HodzicAbstract:Lamin B1 and Lamin B2 are essential building blocks of the nuclear Lamina, a filamentous meshwork lining the nucleoplasmic side of the inner nuclear membrane. Deficiencies in Lamin B1 and Lamin B2 impair neurodevelopment, but distinct functions for the two proteins in the development and homeostasis of the CNS have been elusive. Here we show that embryonic depletion of Lamin B1 in retinal progenitors and postmitotic neurons affects nuclear integrity, leads to the collapse of the LaminB2 meshwork, impairs neuronal survival, and markedly reduces the cellularity of adult retinas. In stark contrast, a deficiency of Lamin B2 in the embryonic retina has no obvious effect on Lamin B1 localization or nuclear integrity in embryonic retinas, suggesting that Lamin B1, but not Lamin B2, is strictly required for nucleokinesis during embryonic neurogenesis. However, the absence of Lamin B2 prevents proper Lamination of adult retinal neurons, impairs synaptogenesis, and reduces cone photoreceptor survival. We also show that Lamin B1 and Lamin B2 are extremely long-lived proteins in rod and cone photoreceptors. OF interest, a complete absence of both proteins during postnatal life has little or no effect on the survival and function of cone photoreceptors.
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Reciprocal knock-in mice to investigate the functional redundancy of Lamin B1 and Lamin B2.
Molecular biology of the cell, 2014Co-Authors: John M. Lee, Yuko Yoshinaga, Pieter J. De Jong, Angelica Tatar, Chika Nobumori, Hea-jin Jung, Catherine Coffinier, Loren G. FongAbstract:Lamins B1 and B2 (B-type Lamins) have very similar sequences and are expressed ubiquitously. In addition, both Lmnb1- and Lmnb2-deficient mice die soon after birth with neuronal layering abnormalities in the cerebral cortex, a consequence of defective neuronal migration. The similarities in amino acid sequences, expression patterns, and knockout phenotypes raise the question of whether the two proteins have redundant functions. To investigate this topic, we generated "reciprocal knock-in mice"-mice that make Lamin B2 from the Lmnb1 locus (Lmnb1(B2/B2)) and mice that make Lamin B1 from the Lmnb2 locus (Lmnb2(B1/B1)). Lmnb1(B2/B2) mice produced increased amounts of Lamin B2 but no Lamin B1; they died soon after birth with neuronal layering abnormalities in the cerebral cortex. However, the defects in Lmnb1(B2/B2) mice were less severe than those in Lmnb1-knockout mice, indicating that increased amounts of Lamin B2 partially ameliorate the abnormalities associated with Lamin B1 deficiency. Similarly, increased amounts of Lamin B1 in Lmnb2(B1/B1) mice did not prevent the neurodevelopmental defects elicited by Lamin B2 deficiency. We conclude that Lamins B1 and B2 have unique roles in the developing brain and that increased production of one B-type Lamin does not fully complement loss of the other.
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new lmna knock in mice provide a molecular mechanism for the segmental aging in hutchinson gilford progeria syndrome
Human Molecular Genetics, 2014Co-Authors: Hea-jin Jung, Shao H. Yang, Stephen G. Young, Angelica Tatar, Chika Nobumori, Loren G. FongAbstract:Lamins A and C (products of the LMNA gene) are found in roughly equal amounts in peripheral tissues, but the brain produces mainly Lamin C and little Lamin A. In HeLa cells and fibroblasts, the expression of preLamin A (the precursor to Lamin A) can be reduced by miR-9, but the relevance of those cell culture studies to Lamin A regulation in the brain was unclear. To address this issue, we created two new Lmna knock-in alleles, one (LmnaPLAO-5NT) with a 5-bp mutation in a predicted miR-9 binding site in preLamin A's 3′ UTR, and a second (LmnaPLAO-UTR) in which preLamin A's 3′ UTR was replaced with Lamin C's 3′ UTR. Neither allele had significant effects on Lamin A levels in peripheral tissues; however, both substantially increased preLamin A transcript levels and Lamin A protein levels in the cerebral cortex and the cerebellum. The increase in Lamin A expression in the brain was more pronounced with the LmnaPLAO-UTR allele than with the LmnaPLAO-5NT allele. With both alleles, the increased expression of preLamin A transcripts and Lamin A protein was greater in the cerebral cortex than in the cerebellum. Our studies demonstrate the in vivo importance of preLamin A's 3′ UTR and its miR-9 binding site in regulating Lamin A expression in the brain. The reduced expression of preLamin A in the brain likely explains why children with Hutchinson–Gilford progeria syndrome (a progeroid syndrome caused by a mutant form of preLamin A) are spared from neurodegenerative disease.
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farnesylation of Lamin b1 is important for retention of nuclear chromatin during neuronal migration
Proceedings of the National Academy of Sciences of the United States of America, 2013Co-Authors: Hea-jin Jung, Loren G. Fong, Yuko Yoshinaga, Pieter J. De Jong, John M. Lee, Angelica Tatar, Chika Nobumori, Catherine Coffinier, Chris N Goulbourne, Stephen G. YoungAbstract:The role of protein farnesylation in Lamin A biogenesis and the pathogenesis of progeria has been studied in considerable detail, but the importance of farnesylation for the B-type Lamins, Lamin B1 and Lamin B2, has received little attention. Lamins B1 and B2 are expressed in nearly every cell type from the earliest stages of development, and they have been implicated in a variety of functions within the cell nucleus. To assess the importance of protein farnesylation for B-type Lamins, we created knock-in mice expressing nonfarnesylated versions of Lamin B1 and Lamin B2. Mice expressing nonfarnesylated Lamin B2 developed normally and were free of disease. In contrast, mice expressing nonfarnesylated Lamin B1 died soon after birth, with severe neurodevelopmental defects and striking nuclear abnormalities in neurons. The nuclear Lamina in migrating neurons was pulled away from the chromatin so that the chromatin was left “naked” (free from the nuclear Lamina). Thus, farnesylation of Lamin B1—but not Lamin B2—is crucial for brain development and for retaining chromatin within the bounds of the nuclear Lamina during neuronal migration.
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Are B-type Lamins essential in all mammalian cells?
Nucleus (Austin Tex.), 2011Co-Authors: Shao H. Yang, Loren G. Fong, Hea-jin Jung, Catherine Coffinier, Stephen G. YoungAbstract:The B-type Lamins are widely assumed to be essential for mammalian cells. In part, this assumption is based on a highly cited study that found that RNAi-mediated knockdown of Lamin B1 or Lamin B2 in HeLa cells arrested cell growth and led to apoptosis. Studies indicating that B-type Lamins play roles in DNA replication, the formation of the mitotic spindle, chromatin organization and regulation of gene expression have fueled the notion that B-type Lamins must be essential. But surprisingly, this idea had never been tested with genetic approaches. Earlier this year, a research group from UCLA reported the development of genetically modified mice that lack expression of both Lmnb1 and Lmnb2 in skin keratinocytes (a cell type that proliferates rapidly and participates in complex developmental programs). They reasoned that if Lamins B1 and B2 were truly essential, then keratinocyte-specific Lamin B1/Lamin B2 knockout mice would exhibit severe pathology. Contrary to expectations, the skin and hair of Lamin B1/Lamin B2-deficient mice were quite normal, indicating that the B-type Lamins are dispensable in some cell types. The same UCLA research group has gone on to show that Lamin B1 and Lamin B2 are critical for neuronal migration in the developing brain and for neuronal survival. The absence of either Lamin B1 or Lamin B2, or the absence of both B-type Lamins, results in severe neurodevelopmental abnormalities.
Stephen G. Young - One of the best experts on this subject based on the ideXlab platform.
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Lamin B1 and Lamin B2 are long-lived proteins with distinct functions in retinal development
Molecular biology of the cell, 2016Co-Authors: David Razafsky, Loren G. Fong, Stephen G. Young, Candace L Ward, Chloe Potter, Wanqiu Zhu, Yunlu Xue, Vladimir J. Kefalov, Didier HodzicAbstract:Lamin B1 and Lamin B2 are essential building blocks of the nuclear Lamina, a filamentous meshwork lining the nucleoplasmic side of the inner nuclear membrane. Deficiencies in Lamin B1 and Lamin B2 impair neurodevelopment, but distinct functions for the two proteins in the development and homeostasis of the CNS have been elusive. Here we show that embryonic depletion of Lamin B1 in retinal progenitors and postmitotic neurons affects nuclear integrity, leads to the collapse of the LaminB2 meshwork, impairs neuronal survival, and markedly reduces the cellularity of adult retinas. In stark contrast, a deficiency of Lamin B2 in the embryonic retina has no obvious effect on Lamin B1 localization or nuclear integrity in embryonic retinas, suggesting that Lamin B1, but not Lamin B2, is strictly required for nucleokinesis during embryonic neurogenesis. However, the absence of Lamin B2 prevents proper Lamination of adult retinal neurons, impairs synaptogenesis, and reduces cone photoreceptor survival. We also show that Lamin B1 and Lamin B2 are extremely long-lived proteins in rod and cone photoreceptors. OF interest, a complete absence of both proteins during postnatal life has little or no effect on the survival and function of cone photoreceptors.
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new lmna knock in mice provide a molecular mechanism for the segmental aging in hutchinson gilford progeria syndrome
Human Molecular Genetics, 2014Co-Authors: Hea-jin Jung, Shao H. Yang, Stephen G. Young, Angelica Tatar, Chika Nobumori, Loren G. FongAbstract:Lamins A and C (products of the LMNA gene) are found in roughly equal amounts in peripheral tissues, but the brain produces mainly Lamin C and little Lamin A. In HeLa cells and fibroblasts, the expression of preLamin A (the precursor to Lamin A) can be reduced by miR-9, but the relevance of those cell culture studies to Lamin A regulation in the brain was unclear. To address this issue, we created two new Lmna knock-in alleles, one (LmnaPLAO-5NT) with a 5-bp mutation in a predicted miR-9 binding site in preLamin A's 3′ UTR, and a second (LmnaPLAO-UTR) in which preLamin A's 3′ UTR was replaced with Lamin C's 3′ UTR. Neither allele had significant effects on Lamin A levels in peripheral tissues; however, both substantially increased preLamin A transcript levels and Lamin A protein levels in the cerebral cortex and the cerebellum. The increase in Lamin A expression in the brain was more pronounced with the LmnaPLAO-UTR allele than with the LmnaPLAO-5NT allele. With both alleles, the increased expression of preLamin A transcripts and Lamin A protein was greater in the cerebral cortex than in the cerebellum. Our studies demonstrate the in vivo importance of preLamin A's 3′ UTR and its miR-9 binding site in regulating Lamin A expression in the brain. The reduced expression of preLamin A in the brain likely explains why children with Hutchinson–Gilford progeria syndrome (a progeroid syndrome caused by a mutant form of preLamin A) are spared from neurodegenerative disease.
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farnesylation of Lamin b1 is important for retention of nuclear chromatin during neuronal migration
Proceedings of the National Academy of Sciences of the United States of America, 2013Co-Authors: Hea-jin Jung, Loren G. Fong, Yuko Yoshinaga, Pieter J. De Jong, John M. Lee, Angelica Tatar, Chika Nobumori, Catherine Coffinier, Chris N Goulbourne, Stephen G. YoungAbstract:The role of protein farnesylation in Lamin A biogenesis and the pathogenesis of progeria has been studied in considerable detail, but the importance of farnesylation for the B-type Lamins, Lamin B1 and Lamin B2, has received little attention. Lamins B1 and B2 are expressed in nearly every cell type from the earliest stages of development, and they have been implicated in a variety of functions within the cell nucleus. To assess the importance of protein farnesylation for B-type Lamins, we created knock-in mice expressing nonfarnesylated versions of Lamin B1 and Lamin B2. Mice expressing nonfarnesylated Lamin B2 developed normally and were free of disease. In contrast, mice expressing nonfarnesylated Lamin B1 died soon after birth, with severe neurodevelopmental defects and striking nuclear abnormalities in neurons. The nuclear Lamina in migrating neurons was pulled away from the chromatin so that the chromatin was left “naked” (free from the nuclear Lamina). Thus, farnesylation of Lamin B1—but not Lamin B2—is crucial for brain development and for retaining chromatin within the bounds of the nuclear Lamina during neuronal migration.
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Are B-type Lamins essential in all mammalian cells?
Nucleus (Austin Tex.), 2011Co-Authors: Shao H. Yang, Loren G. Fong, Hea-jin Jung, Catherine Coffinier, Stephen G. YoungAbstract:The B-type Lamins are widely assumed to be essential for mammalian cells. In part, this assumption is based on a highly cited study that found that RNAi-mediated knockdown of Lamin B1 or Lamin B2 in HeLa cells arrested cell growth and led to apoptosis. Studies indicating that B-type Lamins play roles in DNA replication, the formation of the mitotic spindle, chromatin organization and regulation of gene expression have fueled the notion that B-type Lamins must be essential. But surprisingly, this idea had never been tested with genetic approaches. Earlier this year, a research group from UCLA reported the development of genetically modified mice that lack expression of both Lmnb1 and Lmnb2 in skin keratinocytes (a cell type that proliferates rapidly and participates in complex developmental programs). They reasoned that if Lamins B1 and B2 were truly essential, then keratinocyte-specific Lamin B1/Lamin B2 knockout mice would exhibit severe pathology. Contrary to expectations, the skin and hair of Lamin B1/Lamin B2-deficient mice were quite normal, indicating that the B-type Lamins are dispensable in some cell types. The same UCLA research group has gone on to show that Lamin B1 and Lamin B2 are critical for neuronal migration in the developing brain and for neuronal survival. The absence of either Lamin B1 or Lamin B2, or the absence of both B-type Lamins, results in severe neurodevelopmental abnormalities.
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Posttranslational Processing of Nuclear Lamins
Protein Prenylation PART A, 2011Co-Authors: Brandon S. J. Davies, Loren G. Fong, Shao H. Yang, Hea-jin Jung, Catherine Coffinier, Stephen G. YoungAbstract:Publisher Summary This chapter reviews the posttranslational processing of the nuclear Lamins and discusses its importance to health and disease. The nuclear Lamina, a meshwork formed from intermediate filament proteins, provides structural scaffolding for the cell nucleus. The principal proteins of the nuclear Lamina are Lamin A, Lamin B1, Lamin B2, and Lamin C. PreLamin A (the precursor to mature Lamin A), Lamin B1, and Lamin B2 contain a carboxyl-terminal CAAX motif that triggers farnesylation and methylation of a carboxyl-terminal cysteine. Although it has long been recognized that Lamin B1 and Lamin B2 undergo farnesylation and methylation, the purpose of these modifications remains unclear. The purpose of preLamin A processing remains a mystery. The carboxyl terminus of preLamin A is farnesylated and methylated, but that segment is clipped off and degraded during the biogenesis of mature Lamin A. Interfering with the conversion of farnesyl-preLamin A to mature Lamin A leads to severe disease. The purpose of preLamin A processing remains elusive and the ongoing challenge is to understand why this pathway has been preserved in vertebrate evolution.
