REGULATION (part 1/2)

Lamin Targets

The nuclear lamina binds chromatin in vitro and is thought to function in its organization, but genes that interact with it are unknown. Using an in vivo approach, ~500 Drosophila genes were identified that interact with B-type lamin (Lam). These genes are transcriptionally silent and late replicating, lack active histone marks and are widely spaced. These factors collectively predict lamin binding behavior, indicating that the nuclear lamina integrates variant and invariant chromatin features. Consistently, proximity of genomic regions to the nuclear lamina is partly conserved between cell types, and induction of gene expression or active histone marks reduces Lam binding. Lam target genes cluster in the genome, and these clusters are coordinately expressed during development. This genome-wide analysis gives clear insight into the nature and dynamic behavior of the genome at the nuclear lamina, and implies that intergenic DNA functions in the global organization of chromatin in the nucleus (Pickersgill, 2006).

In Drosophila, the essential nuclear lamina component lamin (Lam) is a B-type lamin, encoded by the ubiquitously expressed Dm0 gene. To obtain a genome-wide map of Lam binding, DamID was used. DamID has been successfully used to identify in vivo genome binding sites of a variety of DNA-binding and chromatin proteins. DamID is based on the in vivo expression of a chimeric protein consisting of a chromatin protein of interest fused to E. coli DNA adenine methyltransferase (Dam). Expression of low amounts of this fusion protein leads to preferential adenine methylation of DNA in the vicinity of native binding sites of the chromatin protein. Subsequently, adenine-methylated DNA fragments are isolated, labeled with a fluorescent dye and hybridized to a microarray. Genomic binding sites of the protein can then be identified based on the methylation pattern (Pickersgill, 2006).

FISH analysis indicates that Lam-target loci are preferentially, but not exclusively, located at the nuclear envelope. There are several possible biological explanations for this. First, a sub-fraction of Lam present in the nuclear interior may specifically interact with target loci. However, an intranuclear version of Lam (LamDeltaCaax) does not bind detectably to targets of wild-type (nuclear lamina-bound) Lam, and TSA-induced loss of Lam binding was visible by FISH as relocation towards the nuclear interior. It is therefore considered more likely that the partial association of Lam targets with the nuclear lamina reflects the dynamics of the interaction. Chromatin in the nucleus has been shown to undergo rapid constrained brownian motion and can move over several microns on a longer time scale. Thus, genes may move back and forth between the lamina and interior in each individual nucleus over time. In addition, positioning of chromosomes inside the interphase nucleus may be determined during the previous mitosis and is likely to be subject to a certain degree of randomness. As a consequence, different chromosomal regions may have a specific probability of being located at the nuclear periphery, both at the individual and population level (Pickersgill, 2006).

In many species, genes that are coexpressed are often found to be clustered along chromosomes. Several models suggest how the linear proximity of genes can facilitate their coordinated regulation: (1) neighboring genes may share a regulatory element such as an enhancer or silencer; (2) neighboring genes may together adopt a certain chromatin conformation, such as a decondensed loop or a chromosomal domain marked by specific histone modifications; (3) a cluster of genes may be located in a nuclear compartment that is either repressive or permissive to transcription. Although examples supporting the first and second model have been described, evidence that nuclear compartmentalization of gene clusters is linked to coregulation has not been reported. This study provides genome-wide molecular evidence that large chromosomal domains containing multiple genes dynamically associate with the nuclear lamina and that these domains are units of developmental regulation (Pickersgill, 2006).

This analysis suggests that Lam-binding genes are identified by a specific combination of DNA and chromatin features. Four features were identified that correlate with Lam binding: low levels of transcription, absence of active histone marks, mid- to late replication timing and large intergenic regions. These data are in agreement with relatively low-resolution characterization of mammalian peripheral chromatin or single-gene observations: low levels of transcription are consistent with certain repressed loci that have a preferential location at the nuclear periphery. Absence of active histone marks is consistent with mass spectrometric analysis of biochemically purified human peripheral chromatin and with data suggesting that the histone deacetylase HDAC3 interacts with the Lamin-associated protein LAP2β. Mid- to late replication timing of Lam targets is in agreement with microscopic evidence that in vertebrates, mid- to late-replicating DNA is located preferentially near the nuclear envelope, and the presence of large intergenic regions is consistent with low-resolution microscopic mapping of gene-poor regions of the human genome to the nuclear periphery. Although even in the current high-resolution analysis each of these factors by itself has limited power to predict Lam binding, a multiple regression model combining these factors can explain nearly 50% of all variance in observed Lam binding. This cooperation of different factors suggests that the nuclear lamina acts as an integrator of different types of genomic and chromatin features (Pickersgill, 2006).

Notably, it was observed that Lam target genes are frequently flanked by very long intergenic regions. Because gene spacing is invariant between cell types, it may explain the similarity in chromosome organization that was observed between cultured embryonic cells and larval polytene tissues, which have different gene expression profiles. Consistently, ecdysone treatment does not significantly alter the Lam binding features of most of the genome. By contributing to Lam binding, intergenic ('junk') DNA may exert an important general function in higher-order chromatin organization. An interesting question is how these long intergenic regions might facilitate Lam interaction. Intergenic regions are generally poor in active histone marks, and as such may be very similar to inactive genes. Efficient interaction with Lam may therefore require long regions of transcriptionally inactive chromatin. Another important question is whether the chromatin characteristics of Lam-interacting genes are the cause or consequence of their Lam-association. A causative role for inactive chromatin in Lam targeting is strongly supported by the observation that the histone deacetylase (HDAC) inhibitor TSA globally reduces Lam association, suggesting that low levels of histone acetylation are a requirement for Lam interaction. However, the nuclear lamina may also have a role in keeping chromatin in an inactive state. In mammalian cells the nuclear lamina harbors histone deacetylase activity, and consistent with this, TSA treatment causes preferential hyperacetylation of chromatin at the nuclear periphery. Thus, nuclear lamina-associated HDACs could help to maintain the repressed state of peripheral chromatin. Future studies should be aimed at deciphering the functional interactions between the nuclear lamina and the genome. This genome-wide study of Lam association provides a firm basis for this task (Pickersgill, 2006).

Protein Interactions

Disassembly of Lamin during oogenesis

Stage 14 Drosophila oocytes are arrested in first meiotic metaphase. A cell-free extract of these oocytes catalyzes apparent disassembly of purified Drosophila nuclei as well as of nuclear Lamin polymers formed in vitro from isolated interphase lamins. Biochemically, the oocyte extract catalyzes Lamin solubilization and phosphorylation as well as characteristic changes in one- and two-dimensional gel mobility. A previously unidentified soluble lamin isoform is easily seen after in vitro disassembly. This isoform is detectable but present only in very small quantities in vivo and is apparently derived specifically from one of the two interphase lamin isoforms. Cell-free nuclear lamina disassembly is ATP-dependent: the addition of calcium to extracts blocks disassembly, as judged both morphologically and biochemically. This system will allow enzymological characterization of cell-free lamina disassembly as well as molecular analysis of specific Drosophila mutants (Maus, 1995).

During Drosophila oogenesis, nurse cells transfer their cytoplasmic contents to developing oocytes and then die. Loss of function for the dcp-1 gene, which encodes a caspase, causes female sterility by inhibiting this transfer. dcp-1- nurse cells are defective in the cytoskeletal reorganization and nuclear breakdown that normally accompany this process. Breakdown of the nuclear envelope is a central event during apoptosis and is accompanied by caspase-mediated cleavage of nuclear lamins. To address whether nuclear lamins are degraded as nurse cells become permeable, egg chambers were examined for the distribution of Lamin Dm0. Loss of lamin Dm0 signal in control nurse cells takes place by stage 11, at which time lamin staining appears to be a diffuse cytoplasmic cloud around the nuclei. Mutant nurse cells continue to show distinct nuclear envelope staining as late as stage 14. Thus dcp-1 mutants are defective in the cleavage or dissociation, or both, of nuclear lamins. This failure in lamin breakdown is a likely cause of the defect in nuclear permeability revealed by a beta-Gal marker. Lamin breakdown is likely to be directly due to DCP-1 protease activity. Actin is localized to the plasma membrane during early stages in control and mutant egg chambers. During stage 10B in control egg chambers, actin bundles form throughout the cytoplasm, connecting the nuclei and plasma membrane. In contrast, actin in many dcp-1 mutant egg chambers remains associated with the plasma membrane, even in stage 14 egg chambers. Therefore, dcp-1 activity is required for the proper formation of cytoplasmic actin bundles in nurse cells. The dcp-1- phenotype suggests that the cytoskeletal and nuclear events in the nurse cells make use of the machinery normally associated with apoptosis and that apoptosis of the nurse cells is a necessary event for oocyte development (McCall, 1998).

Lamin assembly, disassembly, transformation and phosphorylation

Two major immunocross-reactive polypeptides of the Drosophila nuclear envelope, distinguishable in interphase cells on the basis of one-dimensional electrophoretic mobility, have been localized to the nuclear lamina by immunoelectron microscopy. These have been designated Lamins Dm1 and Dm2. Both lamins are apparently derived posttranslationally from a single, primary translation product, Lamin Dm0. A pathway has been established whereby Lamin Dm0 is processed almost immediately upon synthesis in the cytoplasm to Lamin Dm1. Processing occurs posttranslationally, is apparently proteolytic, and has been reconstituted from cell-free extracts in vitro. Processing in vitro is ATP dependent. Once assembled into the nuclear envelope, a portion of Lamin Dm1 is converted into Lamin Dm2 by differential phosphorylation. Throughout most stages of development and in Schneider 2 tissue culture cells, both lamin isoforms are present in approximately equal abundance. However, during heat shock, nearly all Lamin Dm2 is converted into Lamin Dm1 (Smith, 1987).

Two isoforms of a single nuclear Lamin, distinguishable on one-dimensional SDS-polyacrylamide gels, have previously been identified in Drosophila nuclei during interphase. A third species, designated Lamin Dmmit, has now been identified as soluble in extracts of Drosophila tissue culture cells blocked in mitosis by drugs. An apparently identical form is the only lamin species detectable in late-stage egg chambers and early embryos. Phosphoamino acid analyses suggest that the conversion of Lamins Dm1 and Dm2 to Lamin Dmmit is brought about by a specific rearrangement of phosphate groups rather than by dramatic net changes in the levels of Lamin phosphorylation. The residues involved in these phosphorylation/dephosphorylation reactions have been tentatively mapped to a 17.8-kD cyanogen bromide fragment containing amino acids 385-547. This represents a potential "hinge" domain in the Lamin structure between the end of coil 2 and the globular COOH terminus. These results have implications for understanding the regulation of nuclear envelope breakdown during mitosis and karyoskeletal dynamics during oogenesis and early embryogenesis (Smith, 1989).

The Drosophila nuclear Lamin is highly phosphorylated during interphase. Two interphase isoforms, differing in degree of phosphorylation, can be distinguished by one-dimensional SDS-polyacrylamide gel electrophoresis. One migrates with an apparent mass of 74 kDa (Lamin Dm1); the other is more highly phosphorylated and migrates as a 76 kDa protein (Lamin Dm2). A monoclonal antibody was generated, ADL84, which binds to Lamin Dm1 but not Lamin Dm2. Binding of ADL84 to Lamin Dm2 is restored by phosphatase treatment of immunoblots containing lamins. Immunoprecipitation with ADL84 demonstrates that purified Drosophila nuclear Lamins Dm1 and Dm2 are present as a random mixture of homo- and heterodimers. Indirect immunofluorescence experiments suggest that Lamin Dm1 is present in all Drosophila cell types. The epitope for ADL84 was mapped by analyzing binding to bacterially expressed lamin deletion mutants and subsequently by screening for point mutants (randomly generated by polymerase chain reaction) which are not recognized by ADL84. The ADL84-epitope encompasses amino acids R22PPSAGP (arginine 22-proline 28). Peptide competition experiments demonstrate directly that phosphorylation of serine 25 impedes Lamin binding by ADL84. This suggests that serine 25 is the Lamin Dm2-specific phosphorylation site (Stuurman, 1995).

Polymerization of intermediate filament proteins results from interactions among several distinct binding sites on the constituent proteins. Nuclear Lamin head-to-tail polymers arise from one such interaction. Drosophila Lamin-derived fragments were studied containing either the NH2-terminal or COOH-terminal binding site with a combination of co-immunoprecipitation, yeast two-hybrid, analytical ultracentrifugation, and electron microscopic assays. Fragment binding and full-length Lamin head-to-tail polymerization are similar to each other in morphology, buffer requirements, and inhibition after phosphorylation with cdc2 kinase. Deletion analysis localizes the binding sites to the ends of the rod domain that are highly conserved among all intermediate filament proteins. Point mutants, defective in binding, were isolated. Two are identical to point mutations in specific human keratin genes known to affect keratin assembly and to cause genetic skin diseases. Results further indicate that the binding sites only function in specific sequence contexts and that binding can be modulated by elements outside the binding sites (like the cdc2 kinase phosphorylation site). These data indicate that one type of interaction in intermediate filament protein polymerization is the longitudinal binding of dimers via the conserved end segments of the coiled-coil rod domain (Stuurman, 1996).

Time-resolved, two-component, three-dimensional fluorescence light microscopy imaging in living Drosophila early embryos is used to demonstrate that a large fraction of the nuclear envelope lamins remain localized to a rim in the nuclear periphery until well into metaphase. The process of Lamin delocalization and dispersal, typical of 'open' forms of mitosis, does not begin until about the time the final, metaphase geometry of the mitotic spindle is attained. Lamin dispersal is completed about the time that the chromosomal movements of anaphase begin. This pattern of nuclear lamina breakdown appears to be intermediate between traditional designations of 'open' and 'closed' mitoses. These results thus clarify earlier observations of lamins in mitosis in fixed Drosophila early embryos, clearly showing that the observed lamin localization does not result from a structurally defined 'spindle envelope' that persists throughout mitosis. During this extended time interval of lamin localization in the nuclear periphery, the lamina undergoes an extensive series of structural rearrangements that are closely coupled to, and likely driven by, the movements of the centrosomes and microtubules that produce the mitotic spindle. Throughout this time the nuclear envelope structure is permeable to large macromolecules, which are excluded in interphase. While the functional significance of these structural dynamics is not yet clear, it is consistent with a functional role for the lamina in mitotic spindle formation (Paddy, 1996).

Mitotic Lamin disassembly results from phosphorylation at specific sites. In vitro, lamins can form head-to-tail polymers that disassemble upon phosphorylation by cdc2 kinase. A co-immunoprecipitation assay, employing Drosophila nuclear Lamin fragments was used to study the effect of phosphorylation on head-to-tail binding. Phosphorylation of serine-50 by cAMP-dependent kinase inhibits head-to-tail binding in the same manner as phosphorylation of serine-42 by cdc2 kinase. Results suggest that multiple pathways may be employed to disassemble nuclear Lamins in vivo (Stuurman, 1997).

Lamin and nuclear envelope assembly

The role of the Drosophila Lamin protein in nuclear envelope assembly was studied using a Drosophila in vitro assembly system that reconstitutes nuclei from added sperm chromatin or naked DNA. Upon incubation of the embryonic assembly extract with anti-Drosophila Lamin antibodies, the attachment of nuclear membrane vesicles to chromatin surface and nuclear envelope formation does not occur. Lamina assembly and nuclear membrane vesicle attachment to the chromatin are inhibited only when the activity of the 75-kD Lamin isoform is inhibited in both soluble and membrane-vesicles fractions. Incubation of decondensed sperm chromatin with an extract that is depleted of nuclear membranes reveals the presence of Lamin molecules on the chromatin periphery. High concentrations of bacterially expressed Lamin molecules added to the extract are able to associate with the chromatin periphery and do not inhibit nuclear envelope assembly. After nuclear reconstitution, a fraction of the Lamin pool is converted into the typical 74- and 76-kD isoforms. Together, these data strongly support an essential role of the lamina in nuclear envelope assembly (Ulitzur, 1992).

Scaffold attachment regions: Interactions of specific DNA sequences with the laminar matrix

Histone-depleted nuclei maintain sequence-specific interactions with genomic DNA at sites known as scaffold attachment regions (SARs) or matrix attachment regions. In the budding yeast S. cerevisiae, autonomously replicating sequence elements bind the nuclear scaffold. These observations are extended to the fission yeast S. pombe. Four SARs previously mapped in the genomic DNA of Drosophila bind in vitro to nuclear scaffolds from both yeast species. In view of these results, the ability of the Drosophila SARs to promote autonomous replication of plasmids in the two yeast species have been analyzed. Two of the Drosophila SARs have autonomously replicating sequence activity in budding yeast, and three function in fission yeast, while four flanking non-SAR sequences are totally inactive in both (Amati, 1990a).

Nuclei isolated from eukaryotic cells can be depleted of histones and most soluble nuclear proteins to isolate a structural framework called the nuclear scaffold. This structure maintains specific interactions with genomic DNA at sites known as scaffold attached regions (SARs), which are thought to be the bases of DNA loops. In both S. cerevisiae and S. pombe, genomic ARS elements are recovered as SARs. In addition, SARs from Drosophila melanogaster bind to yeast nuclear scaffolds in vitro; a subclass of these promotes autonomous replication of plasmids in yeast. Fine mapping studies were carried out on the Drosophila fushi tarazu SAR, which has both SAR and ARS activities in yeast. The data establish a close relationship between the sequences involved in ARS activity and scaffold binding: ARS elements that can bind the nuclear scaffold in vitro promote more efficient plasmid replication in vivo, but scaffold association is not a strict prerequisite for ARS function. Efficient interaction with nuclear scaffolds from both yeast and Drosophila requires a minimal length of SAR DNA that contains reiteration of a narrow minor groove structure of the double helix. This M/SAR is found in a 2.57 kb ftz upstream regulatory element. A 189 base pair minimal fragment has ARS function. However, based on growth rates and mitotic stability, its activity is lower than that of the entire SAR. The addition of flanking sequences, including as little as 100 bp of AT-rich DNA to the left of the minimal sequence, can enhance the replicative ability of the ARS. These results implicate lamins in initiation of DNA replication (Amati, 1990b).

A 32P-labeling strategy was developed to study the interaction(s) in tissue culture cells between proteins and nucleic acids. Interphase and mitotic nuclear lamins were studied in Drosophila Kc cells. After bromodeoxyuridine incorporation and in vivo photo-crosslinking with 366 nm light, interphase lamins are found to be associated with nucleic acid. Interactions with DNA as well as RNA are detected. In contrast, interaction of nucleic acids with mitotic lamin is not observed. Photo-crosslinking in the presence of antibiotics distamycin and/or chromomycin suggests that interphase lamins interact with both A-T-rich DNA and G-C-rich DNA; interactions with G-C-rich DNA predominated. These results have implications for understanding the interphase organization of the higher eukaryotic cell nucleus as well as the transition of cells from interphase to mitosis (Rzepecki, 1998).

Lamin interaction with Bicaudal-D

In a yeast two-hybrid screen an interaction was identified between Drosophila lamin Dm0, a structural nuclear protein, and BICD, a protein involved in oocyte development. The interaction can be reconstituted in vitro and takes place between segments of both proteins predicted to form coiled coils. The affinity for lamin Dm0 of the minimal binding site on BICD is modulated in a complex fashion by other BICD segments. A point mutation, F684I, that causes the dominant, bicaudal, Bic-D phenotype inhibits lamin binding in the context of the minimal lamin-binding site, but not in a larger BICD fragment. The minimal lamin-binding site of BICD binds to a few other coiled-coil proteins, but binding to these proteins is not influenced by the F684I point mutation, suggesting that the interaction with lamin may play a role in Bic-D function. Structural studies demonstrated that BICD is 60%-70% alpha-helical, is a dimer, and consists of two parts: a thin rod-shaped part of about 32 nm, and a thicker rod-shaped part of about 26 nm. Likely, the thinner rod-shaped part of full-length BICD consists of the N-terminal half of the protein, and the lamin-binding site is located within the thicker rod-shaped part (Stuurman, 1999).

Lamin and Caspase

While Caenorhabditis elegans has only a single identified caspase, CED-3, whose activity is absolutely required for all developmental programmed cell deaths, most mammalian cell types express multiple caspases with varying specificities. The fruit fly possesses two known caspases: DCP-1 and drICE. The role of drICE was examined in in vitro apoptosis of the D. melanogaster cell line S2. Cytoplasmic lysates made from S2 cells undergoing apoptosis induced by either reaper expression or cycloheximide treatment contain a caspase activity with DEVD specificity that can cleave p35, lamin DmO, drICE and DCP-1 in vitro, one that can trigger chromatin condensation in isolated nuclei. Immunodepletion of drICE from lysates is sufficient to remove most measurable in vitro apoptotic activity; re-addition of exogenous drICE to such immunodepleted lysates restores apoptotic activity. It is concluded that, at least in S2 cells, drICE can be the sole caspase effector of apoptosis (Fraser 1997).

The JIL-1 kinase interacts with lamin Dm0 and regulates nuclear lamina morphology of Drosophila nurse cells

A yeast two-hybrid screen was used to identify lamin Dmo as an interaction partner for the nuclear JIL-1 kinase. This molecular interaction was confirmed by GST-fusion protein pull-down assays and by co-immunoprecipitation experiments. Using deletion construct analysis a predicted globular domain of the basic region of the COOH-terminal domain of JIL-1 was shown to be sufficient for mediating the molecular interactions with lamin Dmo. A reciprocal analysis with truncated lamin Dmo constructs showed that the interaction with JIL-1 required sequences in the tail domain of lamin Dmo that include the Ig-like fold. Further support for a molecular interaction between JIL-1 and lamin Dmo in vivo was provided by genetic interaction assays. Nuclear positioning and lamina morphology were abnormal in JIL-1 mutant egg chambers. The most common phenotypes observed were abnormal nurse cell nuclear lamina protrusions through the ring canals near the oocyte, as well as dispersed and mislocalized lamin throughout the egg chamber. These phenotypes were completely rescued by a full-length JIL-1 transgenic construct. Thus, these results suggest that the JIL-1 kinase is required to maintain nuclear morphology and integrity of nurse cells during oogenesis and that this function may be linked to molecular interactions with lamin Dmo (Bao, 2005).

The results from the yeast two-hybrid interaction assays suggest that the interaction between JIL-1 and lamin Dmo is direct. However, JIL-1 is localized to euchromatic regions of chromosomes and lamin Dmo is mainly a component of the inner nuclear membrane raising the question of how this interaction occurs. Recently it has become clear that lamins and associated proteins in the nuclear envelope are involved in several nuclear activities apart from providing a barrier between the nucleoplasm and the cytoplasm. One of these functions of the nuclear lamina is to serve as a scaffold that provides attachment sites for interphase chromatin directly or indirectly regulating many nuclear activities such as DNA replication and transcription, nuclear and chromatin organization, cell development and differentiation, nuclear migration, and apoptosis. In Drosophila, it has been shown that direct interactions between the tail domain of lamin Dmo and histone H2A and H2B may mediate the attachment of chromosomes to the nuclear lamina. Interestingly, the early embryonic nuclear lamina protein YA (Young Arrest), which is a lamin Dmo binding protein, when ectopically expressed in larval salivary gland cells, associates with interband regions of polytene chromosomes. Thus, there is considerable evidence for direct interactions of lamins with chromatin associated proteins such as JIL-1. Furthermore, lamins have also been found in the nuclear interior and the possibility remains that there may be a hitherto undetected soluble pool of JIL-1 that potentially could provide additional avenues for direct interactions (Bao, 2005).

To determine whether disruptions in nuclear lamina organization could be detected in JIL-1 mutant backgrounds, fixed ovaries, embryos, imaginal discs and polytene salivary glands labeled with lamin Dmo antibody were examined. Abnormal lamin Dmo distribution was observed only in ovaries of JIL-1z2/JIL-1h9 flies. One phenotype which was found in about 5% of mutant egg chambers was dispersed and mislocalized lamin was found throughout the egg chamber. This is not likely to be a consequence of apoptotic events because lamins are degraded by proteolysis during apoptosis and do not show accumulation. The phenotype may therefore reflect a destabilization of the integrity of the nuclear lamina leading to lamin Dmo dispersal. Thus, these experiments may provide evidence that the stability of the nuclear lamina in Drosophila egg chambers depends on JIL-1 kinase activity and phosphorylation of lamin Dmo. Unfortunately, this hypothesis cannot be tested at the present time because of a lack of a functional in vitro JIL-1 kinase assay. The other phenotype observed in JIL-1 mutant egg chambers with high penetrance (42.8%) is abnormally positioned nurse cell nuclei which extended nuclear lamina protrusions through the ring canals near the oocyte. It is not clear how this phenotype arises. However, several morphogenetic processes such as anterior-posterior/dorso-ventral axis formation as well as cell and nuclear migration during oogenesis require reciprocal cell signaling between germline, oocyte and nurse cells, and somatic follicle cells. In JIL-1 mutant backgrounds cell signaling pathways that normally prevent nurse cell nuclei from responding to posterior migration signals may be downregulated, resulting in a posterior dislocalization towards the oocyte. It has been shown that the nuclear lamina is involved in regulating nuclear migration in the developing eye through interactions of the lamin Dmo-binding protein Klarsicht with the microtubule organizing center. Furthermore, Bicaudal-D, a dynein-interacting protein required for control of nuclear migration and cytoskeletal organization in oogenesis has been shown to interact with lamin Dmo in yeast two-hybrid assays. Thus, dynamic local interactions of cytoskeleton-associated motor proteins linked to lamin Dmo may be capable of providing the forces necessary for generating the observed deformations of the nuclear lamina. Nuclear lamins are generally considered to provide stiffness and incompressibility to the nuclear envelope suggesting that the aberrations in nuclear morphology observed here may be linked to a weakening of the nuclear lamina. However, the present experiments cannot distinguish between the possibilities that JIL-1 may be involved in nuclear deformation by regulating nuclear lamina cytoskeletal interactions via direct modulation of lamin Dmo or indirectly by modulating a signal transduction pathway, or both (Bao, 2005).

These results suggest that JIL-1 kinase is required to maintain nuclear morphology and integrity of nurse cells during oogenesis. It has recently been shown that some lamin Dmo interactions occur only during early development, indicating that special properties of the nuclear lamina may be required for regulating nuclear processes and morphology at specific developmental stages. For example, the lamin Dmo binding protein YA is expressed only in ovaries and pre-gastrulation embryos and is required for the interaction between chromatin and the nuclear envelope during early embryogenesis. Previously, it has been shown that the interaction between JIL-1 and Lola zf5, a splice variant of the complex lola locus encoding multiple different transcription factors, is developmentally regulated and restricted to early embryogenesis as well. Thus, it will be informative in future experiments to further explore the interaction between JIL-1 and lamin Dmo to clarify how this interaction contributes to nuclear lamina function in development (Bao, 2005).

Mapping of regions of Lamin that are required for its binding to chromosomes

Continued: see Regulation part 2/2

Lamin: Biological Overview | Evolutionary Homologs | Developmental Biology | Effects of Mutation | References

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