The retinoblastoma gene product is a cell cycle-dependent, nuclear matrix-associated protein

Progeria of stem cells: stem cell exhaustion in Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a rare, fatal genetic disorder that is characterized by segmental accelerated aging. The major causal mutation associated with HGPS triggers abnormal messenger RNA splicing of the lamin A gene leading to changes in the nuclear architecture. To date, two models have been proposed to explain how mutations in the lamin A gene could lead to HGPS, structural fragility and altered gene expression. We favor a compatible model that links HGPS to stem cell-driven tissue regeneration. In this model, nuclear fragility of lamin A-deficient cells increases apoptotic cell death to levels that exhaust tissues' ability for stem cell-driven regeneration. Tissue-specific differences in cell death or regenerative potential, or both, result in the tissue-specific segmental aging pattern seen in HGPS. We propose that the pattern of aging-related conditions present or absent in HGPS can provide insight into the genetic and environmental factors that contribute to normal aging.

Primary laminopathy fibroblasts display altered genome organization and apoptosis

A number of diseases associated with specific tissue degeneration and premature aging have mutations in the nuclear envelope proteins A-type lamins or emerin. Those diseases with A-type lamin mutation are inclusively termed laminopathies. Due to various hypothetical roles of nuclear envelope proteins in genome function we investigated whether alterations to normal genomic behaviour are apparent in cells with mutations in A-type lamins and emerin. Even though the distributions of these proteins in proliferating laminopathy fibroblasts appear normal, there is abnormal nuclear positioning of both chromosome 18 and 13 territories, from the nuclear periphery to the interior. This genomic organization mimics that found in normal nonproliferating quiescent or senescent cells. This finding is supported by distributions of modified pRb in the laminopathy cells. All laminopathy cell lines tested and an X-linked Emery-Dreifuss muscular dystrophy cell line also demonstrate increased incidences of apoptosis. The most extreme cases of apoptosis occur in cells derived from diseases with mutations in the tail region of the LMNA gene, such as Dunningan-type familial partial lipodystrophy and mandibuloacral dysplasia, and this correlates with a significant level of micronucleation in these cells.

Gene silencing at the nuclear periphery

The nuclear envelope (NE) is composed of inner and outer nuclear membranes (INM and ONM, respectively), nuclear pore complexes and an underlying mesh like supportive structure--the lamina. It has long been known that heterochromatin clusters at the nuclear periphery adjacent to the nuclear lamina, hinting that proteins of the lamina may participate in regulation of gene expression. Recent studies on the molecular mechanisms involved show that proteins of the nuclear envelope participate in regulation of transcription on several levels, from direct binding to transcription factors to induction of epigenetic histone modifications. Three INM proteins; lamin B receptor, lamina-associated polypeptide 2beta and emerin, were shown to bind chromatin modifiers and/or transcriptional repressors inducing, at least in one case, histone deacetylation. Emerin and another INM protein, MAN1, have been linked to down-regulation of specific signaling pathways, the retino blastoma 1/E2F MyoD and transforming growth factor beta/bone morphogenic protein, respectively. Therefore, cumulative data suggests that proteins of the nuclear lamina regulate transcription by recruiting chromatin modifiers and transcription factors to the nuclear periphery. In this minireview we describe the recent literature concerning mechanisms of gene repression by proteins of the NE and suggest the hypothesis that the epigenetic "histone code", dictating transcriptional repression, is "written" in part, at the NE by its proteins. Finally, as aberrant gene expression is one of the mechanisms speculated to underlie the newly discovered group of genetic diseases termed nuclear envelopathies/laminopathies, elucidating the repressive role of NE proteins is a major challenge to both researchers and clinicians.

Nucleoplasmic LAP2alpha-lamin A complexes are required to maintain a proliferative state in human fibroblasts

In human diploid fibroblasts (HDFs), expression of lamina-associated polypeptide 2 alpha (LAP2alpha) upon entry and exit from G(0) is tightly correlated with phosphorylation and subnuclear localization of retinoblastoma protein (Rb). Phosphoisoforms of Rb and LAP2alpha are down-regulated in G(0). Although RbS780 phosphoform and LAP2alpha are up-regulated upon reentry into G(1) and colocalize in the nucleoplasm, RbS795 migrates between nucleoplasmic and speckle compartments. In HDFs, which are null for lamins A/C, LAP2alpha is mislocalized within nuclear aggregates, and this is correlated with cell cycle arrest and accumulation of Rb within speckles. Nuclear retention of nucleoplasmic Rb during G(1) phase but not of speckle-associated Rb depends on lamin A/C. siRNA knock down of LAP2alpha or lamin A/C in HDFs leads to accumulation of Rb in speckles and G(1) arrest, probably because of activation of a cell cycle checkpoint. Our results suggest that LAP2alpha and lamin A/C are involved in controlling Rb localization and phosphorylation, and a lack or mislocalization of either protein leads to cell cycle arrest in HDFs.

Lamins A and C are present in the nuclei of early porcine embryos, with lamin A being distributed in large intranuclear foci

Gametogenesis and embryogenesis are dynamic developmental stages marked by extensive modifications in the organization of the genome and nuclear architecture. In the literature it is conveyed that only B-type lamins are required in these early stages of development and that A-type lamins are not present or required until differentiation of specific cell types associated with specialized tissue is initiated. To assess the presence of nuclear structures that are putatively involved in genome regulation, we investigated the distribution of lamin proteins throughout the early stages of porcine embryonic development, using testes tissue sections, oocytes and in-vitro fertilized (IVF) porcine embryos and employing anti-lamin antibodies. We have shown that anti-lamin A staining is present at the one-cell, two-cell, four-cell, and six- to eight-cell stages of early porcine embryo development, but diminishes at the morulae and blastocyst stages. Large intranuclear anti-lamin A foci are prominent in the early preimplantation stages. Both anti-lamin A/C and anti-lamin B staining were clearly present in all embryonic stages. Immature porcine oocytes revealed lamin rings using the monoclonal anti-lamin A/C antibody and many immature oocytes exhibited a pale rim staining pattern with anti-lamin A antibody. A-type lamins were not observed in sperm precursor cells. Thus, we have shown that A-type lamins and B-type lamins are present at the nuclear envelope in very early porcine embryos and that lamin A is also found in large intranuclear aggregates in two-cell to eight-cell embryos but is lacking from later embryonic stages.

Laminopathies: multiple disorders arising from defects in nuclear architecture

Lamins are the major structural proteins of the nucleus in an animal cell. In addition to being essential for nuclear integrity and assembly, lamins are involved in the organization of nuclear processes such as DNA replication, transcription and repair. Mutations in the human lamin A gene lead to highly debilitating genetic disorders that primarily affect muscle, adipose, bone or neuronal tissues and also cause premature ageing syndromes. Mutant lamins alter nuclear integrity and hinder signalling pathways involved in muscle differentiation and adipocyte differentiation, suggesting tissue-specific roles for lamins. Furthermore, cells expressing mutant lamins are impaired in their response to DNA damaging agents. Recent reports indicate that certain lamin mutations act in a dominant negative manner to cause nuclear defects and cellular toxicity, and suggest a possible role for aberrant lamins in normal ageing processes.

Guilt by Association

The discovery that many inherited diseases are linked to interacting nuclear envelope proteins has raised the possibility that human genetic studies could be assisted by a fusion with proteomics. Two principles could be applied. In the first, the proteome of an organelle associated with a genetically variable disease is determined. The chromosomal locations of the genes encoding the organellar proteins are then determined. If a related disease is linked to a large chromosomal region that includes a gene identified in the organelle, then that gene has an increased likelihood of causing the disease. Directly sequencing this allele from patient samples might speed identification compared with further genetic linkage studies as has been demonstrated for multiple diseases associated with the nuclear envelope. The second principle is that if an organelle has been implicated in the pathology of a particular disorder, then comparison of the organelle proteome from control and patient cells might highlight differences that could indicate the causative protein. The distinct, tissue-specific pathologies associated with nuclear envelope diseases suggest that many tissues will have a set of disorders linked to this organelle, and there are numerous as yet unmapped or partially mapped syndromes that could benefit from such an approach.

Stabilization of the retinoblastoma protein by A-type nuclear lamins is required for INK4A-mediated cell cycle arrest

Mutations in the LMNA gene, which encodes all A-type lamins, including lamin A and lamin C, cause a variety of tissue-specific degenerative diseases termed laminopathies. Little is known about the pathogenesis of these disorders. Previous studies have indicated that A-type lamins interact with the retinoblastoma protein (pRB). Here we probe the functional consequences of this association and further examine links between nuclear structure and cell cycle control. Since pRB is required for cell cycle arrest by p16(ink4a), we tested the responsiveness of multiple lamin A/C-depleted cell lines to overexpression of this CDK inhibitor and tumor suppressor. We find that the loss of A-type lamin expression results in marked destabilization of pRB. This reduction in pRB renders cells resistant to p16(ink4a)-mediated G(1) arrest. Reintroduction of lamin A, lamin C, or pRB restores p16(ink4a)-responsiveness to Lmna(-/-) cells. An array of lamin A mutants, representing a variety of pathologies as well as lamin A processing mutants, was introduced into Lmna(-/-) cells. Of these, a mutant associated with mandibuloacral dysplasia (MAD R527H), as well as two lamin A processing mutants, but not other disease-associated mutants, failed to restore p16(ink4a) responsiveness. Although our findings do not rule out links between altered pRB function and laminopathies, they fail to support such an assertion. These findings do link lamin A/C to the functional activation of a critical tumor suppressor pathway and further the possibility that somatic mutations in LMNA contribute to tumor progression.

Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

It has been demonstrated that nuclear lamins are important proteins in maintaining cellular as well as nuclear integrity, and in maintaining chromatin organization in the nucleus. Moreover, there is growing evidence that lamins play a prominent role in transcriptional control. The family of laminopathies is a fast-growing group of diseases caused by abnormalities in the structure or processing of the lamin A/C ( LMNA) gene. Mutations or incorrect processing cause more than a dozen different inherited diseases, ranging from striated muscular diseases, via fat- and peripheral nerve cell diseases, to progeria. This broad spectrum of diseases can only be explained if the responsible A-type lamin proteins perform multiple functions in normal cells. This review gives an overview of current knowledge on lamin structure and function and all known diseases associated with LMNA abnormalities. Based on the knowledge of the different functions of A-type lamins and associated proteins, explanations for the observed phenotypes are postulated. It is concluded that lamins seem to be key players in, among others, controlling the process of cellular ageing, since disturbance in lamin protein structure gives rise to several forms of premature ageing.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Involvement of the N-terminal unique domain of Chk tyrosine kinase in Chk-induced tyrosine phosphorylation in the nucleus

Chk tyrosine kinase phosphorylates Src-family kinases and suppresses their kinase activity. We recently showed that Chk localizes to the nucleus as well as the cytoplasm and inhibits cell proliferation. In this study, we explored the role of the N-terminal unique domain of Chk in nuclear localization and Chk-induced tyrosine phosphorylation in the nucleus. In situ binding experiments showed that the N-terminal domain of Chk was associated with the nucleus and the nuclear matrix. The presence of the N-terminal domain of Chk led to a fourfold increase in cell population exhibiting Chk-induced tyrosine phosphorylation in the nucleus. Expression of Chk but not kinase-deficient Chk induced tyrosine phosphorylation of a variety of proteins ranging from 23 kDa to approximately 200 kDa, especially in Triton X-100-insoluble fraction that included chromatin and the nuclear matrix. Intriguingly, in situ subnuclear fractionations revealed that Chk induced tyrosine phosphorylation of proteins that were associated with the nuclear matrix. These results suggest that various unidentified substrates of Chk, besides Src-family kinases, may be present in the nucleus. Thus, our findings indicate that the importance of the N-terminal domain to Chk-induced tyrosine phosphorylation in the nucleus, implicating that these nuclear tyrosine-phosphorylated proteins may contribute to inhibition of cell proliferation.

A mechanism of AP-1 suppression through interaction of c-Fos with lamin A/C

AP-1 (Activating Protein 1) transcription factor activity is tightly regulated at multiple levels, including dimer formation (i.e., Fos/Jun). Here we show that the intermediate filament protein lamin A/C suppresses AP-1 function through direct interaction with c-Fos, and that both proteins can interact and colocalize at the nuclear envelope (NE) in mammalian cells. Perinuclear localization of c-Fos is absent in Lmna-null cells but can be restored by lamin A overexpression. In vitro, preincubation of c-Fos with lamin A prior to the addition of c-Jun inhibits AP-1 DNA-binding activity. In vivo, overexpression of lamin A reduces the formation of c-Fos/c-Jun heterodimers, and suppresses AP-1 DNA-binding and transcriptional activity. Notably, c-Fos colocalizes with lamin A/C at the NE in starvation-synchronized quiescent cells lacking detectable AP-1 DNA binding. In contrast, serum-induced AP-1 DNA-binding activity coincides with abundant nucleoplasmic c-Fos expression without changes in lamin A/C localization. We also found that Lmna-null cells display enhanced proliferation. In contrast, lamin A overexpression causes growth arrest, and ectopic c-Fos partially overcomes lamin A/C-induced cell cycle alterations. We propose lamin A/C-mediated c-Fos sequestration at the NE as a novel mechanism of transcriptional and cell cycle control.

Nuclear envelope dystrophies show a transcriptional fingerprint suggesting disruption of Rb–MyoD pathways in muscle regeneration

Mutations of lamin A/C (LMNA) cause a wide range of human disorders, including progeria, lipodystrophy, neuropathies and autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD). EDMD is also caused by X-linked recessive loss-of-function mutations of emerin, another component of the inner nuclear lamina that directly interacts with LMNA. One model for disease pathogenesis of LMNA and emerin mutations is cell-specific perturbations of the mRNA transcriptome in terminally differentiated cells. To test this model, we studied 125 human muscle biopsies from 13 diagnostic groups (125 U133A, 125 U133B microarrays), including EDMD patients with LMNA and emerin mutations. A Visual and Statistical Data Analyzer (VISDA) algorithm was used to statistically model cluster hierarchy, resulting in a tree of phenotypic classifications. Validations of the diagnostic tree included permutations of U133A and U133B arrays, and use of two probe set algorithms (MAS5.0 and MBEI). This showed that the two nuclear envelope defects (EDMD LMNA, EDMD emerin) were highly related disorders and were also related to fascioscapulohumeral muscular dystrophy (FSHD). FSHD has recently been hypothesized to involve abnormal interactions of chromatin with the nuclear envelope. To identify disease-specific transcripts for EDMD, we applied a leave-one-out (LOO) cross-validation approach using LMNA patient muscle as a test data set, with reverse transcription-polymerase chain reaction (RT-PCR) validations in both LMNA and emerin patient muscle. A high proportion of top-ranked and validated transcripts were components of the same transcriptional regulatory pathway involving Rb1 and MyoD during muscle regeneration (CRI-1, CREBBP, Nap1L1, ECREBBP/p300), where each was specifically upregulated in EDMD. Using a muscle regeneration time series (27 time points) we develop a transcriptional model for downstream consequences of LMNA and emerin mutations. We propose that key interactions between the nuclear envelope and Rb and MyoD fail in EDMD at the point of myoblast exit from the cell cycle, leading to poorly coordinated phosphorylation and acetylation steps. Our data is consistent with mutations of nuclear lamina components leading to destabilization of the transcriptome in differentiated cells.

Homozygous and compound heterozygous mutations in ZMPSTE24 cause the laminopathy restrictive dermopathy

Restrictive dermopathy (RD) is a lethal human genetic disorder characterized by very tight, thin, easily eroded skin, rocker bottom feet, and joint contractures. This disease was recently reported to be associated with a single heterozygous mutation in ZMPSTE24 and hypothesized to be a digenic disorder (Navarro et al, Lamin A and ZMPSTE24 (FACE-1) defects cause nuclear disorganization and identify restrictive dermopathy as a lethal neonatal laminopathy. Hum Mol Genet 13:2493-2503, 2004). ZMPSTE24 encodes an enzyme necessary for the correct processing and maturation of lamin A, an intermediate filament component of the nuclear envelope. Here we present four unrelated patients with homozygous mutations in ZMPSTE24 and a fifth patient with compound heterozygous mutations in ZMPSTE24. Two of the three different mutations we found are novel, and all are single base insertions that result in messenger RNA frameshifts. As a consequence of the presumed lack of ZMPSTE24 activity, prelamin A, the unprocessed toxic form of lamin A, was detected in the nuclei of both cultured cells and tissue from RD patients, but not in control nuclei. Abnormally aggregated lamin A/C was also observed. These results indicate that RD is an autosomal recessive laminopathy caused by inactivating ZMPSTE24 mutations that result in defective processing and nuclear accumulation of prelamin A.

Nuclear titin interacts with A- and B-type lamins in vitro and in vivo

Lamins form structural filaments in the nucleus. Mutations in A-type lamins cause muscular dystrophy, cardiomyopathy and other diseases, including progeroid syndromes. To identify new binding partners for lamin A, we carried out a two-hybrid screen with a human skeletal-muscle cDNA library, using the Ig-fold domain of lamin A as bait. The C-terminal region of titin was recovered twice. Previous investigators showed that nuclear isoforms of titin are essential for chromosome condensation during mitosis. Our titin fragment, which includes two regions unique to titin (M-is6 and M-is7), bound directly to both A- and B-type lamins in vitro. Titin binding to disease-causing lamin A mutants R527P and R482Q was reduced 50%. Studies in living cells suggested lamin-titin interactions were physiologically relevant. In Caenorhabditis elegans embryos, two independent C. elegans (Ce)-titin antibodies colocalized with Ce-lamin at the nuclear envelope. In lamin-downregulated [lmn-1(RNAi)] embryos, Ce-titin was undetectable at the nuclear envelope suggesting its localization or stability requires Ce-lamin. In human cells (HeLa), antibodies against the titin-specific domain M-is6 gave both diffuse and punctate intranuclear staining by indirect immunofluorescence, and recognized at least three bands larger than 1 MDa in immunoblots of isolated HeLa nuclei. In HeLa cells that transiently overexpressed a lamin-binding fragment of titin, nuclei became grossly misshapen and herniated at sites lacking lamin B. We conclude that the C-terminus of nuclear titin binds lamins in vivo and might contribute to nuclear organization during interphase.

CK2 phosphorylation weakens 90 kDa MFP1 association to the nuclear matrix in Allium cepa

MFP1 is a conserved plant coiled-coil protein located on the stroma side of the chloroplast thylakoids, as well as in the nuclear matrix. It displays species-specific variability in the number of genes, proteins, and expression. Allium cepa has two nuclear proteins antigenically related to MFP1 with different M(r), pI, distribution, and expression, but only the 90 kDa MFP1 protein is a nuclear matrix component that associates with both the nucleoskeletal filaments and a new category of nuclear bodies. The 90 kDa AcMFP1 migrates in two-dimensional blots as two sets of spots. The hypo-phosphorylated forms (pI approximately 9.5) are tightly bound to the nuclear matrix, while high ionic strength buffers release the more acidic hyper-phosphorylated ones (pI approximately 8.5), suggesting that the protein is post-translationally modified, and that these modifications control its attachment to the nuclear matrix. Dephosphorylation by exogenous alkaline phosphatase and phosphorylation by exogenous CK2, as well as specific inhibition and stimulation of endogenous CK2 with heparin and spermine and spermidine, respectively, revealed that the protein is an in vitro and in vivo substrate of this enzyme, and that CK2 phosphorylation weakens the strength of its binding to the nuclear matrix. In synchronized cells, the nuclear 90 kDa AcMFP1 phosphorylation levels vary during the cell cycle with a moderate peak in G2. These results provide the first evidence for AcMFP1 in vivo phosphorylation, and open up further research on its nuclear functions.

Altered splicing in prelamin A-associated premature aging phenotypes

Hutchinson-Gilford progeria (HGPS), a rare and severe developmental disorder characterized by features recalling premature aging, and restrictive dermopathy (RD), a neonatal lethal genodermatosis, have recently been identified as being primary or secondary "laminopathies." These are heterogeneous disorders due to altered function of lamins A/C or related proteins. In physiological conditions, mature lamin A is obtained through a series of post-translational processing steps performed on a protein precursor, prelamin A. The major pathophysiological mechanism involved in progeria is an aberrant splicing of pre-mRNAs issued from the LMNA gene, due to a de novo heterozygous point mutation, leading to the production and accumulation of truncated lamin A precursors. Aberrant splicing of prelamin A pre-mRNAs causing the production of more extensively truncated precursors is involved in the allelic disease restrictive dermopathy. Other restrictive dermopathy cases are due to the inactivation of a key enzyme involved in the maturation of lamin A precursors (ZMPSTE24). In functional terms, all these conditions share the same pathophysiological basis: intranuclear accumulation of lamin A precursors, which cannot be fully processed (due to primary or secondary events) and exert toxic, dominant negative effects on nuclear homeostasis. Most other laminopathies are due to autosomal dominant LMNA point mutations inferred to cause single amino acid substitutions. In any case, the impact of these mutations on pre-mRNA splicing has rarely been assessed. These disorders affect different tissues and organs, mainly including bone, skin, striated muscles, adipose tissue, vessels, and peripheral nerves in isolated or combined fashions, giving rise to syndromes whose severity ranges from mild to perinatally lethal. In this chapter we review the structure and functions of lamins A/C in physiological and pathological conditions, describe their known or putative roles, namely, in the pathogenesis of HGPS and RD in relation to existing animal models, and envisage possible targeted therapeutic strategies on the basis of recent research results.

The BTB domain of the nuclear matrix protein NRP/B is required for neurite outgrowth

The neuronal nuclear matrix protein, NRP/B, contains a BTB domain and kelch repeats and is expressed in primary neurons but not in primary glial cells. To examine the function of NRP/B in neurons, we analyzed the structure/function of the NRP/B-BTB domain and its role in neurite outgrowth. Based on three-dimensional modeling of NRP/B, we generated an NRP/B-BTB mutant containing three mutations in the conserved amino acids D47A, H60A and R61D that was termed BTB mutant A. BTB mutant A significantly reduced the dimerization of NRP/B compared to wild-type NRP/B. The NRP/B-BTB domain was required for nuclear localization and mediated the association of NRP/B with p110RB through the TR subdomain within the B pocket of p110RB. Overexpression of wild-type NRP/B and NRP/B-BTB domain significantly induced neurite outgrowth in PC12 cells and enhanced the G0-G1 cell population by ∼23% compared to the control cells, whereas NRP/B-BTB mutant A reduced neurite outgrowth by 70-80%, and inhibited NRP/B-p110RB association. Single cell microinjection of NRP/B-specific antibodies also blocked the neurite outgrowth of PC12 cells upon NGF stimulation. Interference of NRP/B expression by small interfering RNA (NRP/B-siRNA) inhibited neurite outgrowth and suppressed the NGF-induced outgrowth of neurites in PC12 cells. Additionally, p110RB phosphorylation at serine residue 795 was significantly reduced in PC12 cells treated with NRP/B siRNA compared to those treated with control GFP-siRNA, indicating that p110RB is a downstream target of NRP/B. Thus, the BTB domain of NRP/B regulates neurite outgrowth through its interaction with the TR subdomain within the B pocket of p110RB, and the conserved amino acids D47A, H60A and R61D within this domain of NRP/B are crucial residues for neurite extension in neuronal cells. These findings support a role for the BTB-domain of NRP/B as an important regulator of neuronal differentiation.

Subnuclear compartmentalization of sequence-specific transcription factors and regulation of eukaryotic gene expression

To date, the majority of the research regarding eukaryotic transcription factors has focused on characterizing their function primarily through in vitro methods. These studies have revealed that transcription factors are essentially modular structures, containing separate regions that participate in such activities as DNA binding, protein-protein interaction, and transcriptional activation or repression. To fully comprehend the behavior of a given transcription factor, however, these domains must be analyzed in the context of the entire protein, and in certain cases the context of a multiprotein complex. Furthermore, it must be appreciated that transcription factors function in the nucleus, where they must contend with a variety of factors, including the nuclear architecture, chromatin domains, chromosome territories, and cell-cycle-associated processes. Recent examinations of transcription factors in the nucleus have clarified the behavior of these proteins in vivo and have increased our understanding of how gene expression is regulated in eukaryotes. Here, we review the current knowledge regarding sequence-specific transcription factor compartmentalization within the nucleus and discuss its impact on the regulation of such processes as activation or repression of gene expression and interaction with coregulatory factors.

Nuclear tumor suppressors in space and time

Numerous studies have identified key binding partners and functional activities of nuclear tumor-suppressor proteins such as the retinoblastoma protein, p53 and BRCA1. Historically, less attention has been given to the subnuclear locations of these proteins. Here, we describe several recent studies that promote the view that regulated association with subcompartments of the nucleus is inherent to tumor-suppressor function.

Lamin A/C expression is a marker of mouse and human embryonic stem cell differentiation

Nuclear lamins comprise the nuclear lamina, a scaffold-like structure that lines the inner nuclear membrane. B-type lamins are present in almost all cell types, but A-type lamins are expressed predominantly in differentiated cells, suggesting a role in maintenance of the differentiated state. Previous studies have shown that lamin A/C is not expressed during mouse development before day 9, nor in undifferentiated mouse embryonic carcinoma cells. To further investigate the role of lamins in cell phenotype maintenance and differentiation, we examined lamin expression in undifferentiated mouse and human embryonic stem (ES) cells. Wide-field and confocal immunofluorescence microscopy and semiquantitative reverse transcription-polymerase chain reaction analysis revealed that undifferentiated mouse and human ES cells express lamins B1 and B2 but not lamin A/C. Mouse ES cells display high levels of lamins B1 and B2 localized both at the nuclear periphery and throughout the nucleoplasm, but in human ES cells, B1 and B2 expression is dimmer and localized primarily at the nuclear periphery. Lamin A/C expression is activated during human ES cell differentiation before downregulation of the pluripotency marker Oct-3/4 but not before the downregulation of the pluripotency markers Tra-1-60, Tra-1-81, and SSEA-4. Our results identify the absence of A-type lamin expression as a novel marker for undifferentiated ES cells and further support a role for nuclear lamins in cell maintenance and differentiation.

Hairless contains a novel nuclear matrix targeting signal and associates with histone deacetylase 3 in nuclear speckles

Hair follicle cycling is a highly regulated and dynamic cellular process consisting of phases of growth, regression, and quiescence. The hairless (hr) gene encodes a nuclear factor that is highly expressed in the skin, where it appears to be an essential regulator during the regression in the catagen hair follicle. In hairless mice, as well as humans with congenital atrichia, the absence of hr protein initiates a premature and abnormal catagen due to defects in the signaling required for hair follicle remodeling. Here, we report that hr protein is a nuclear protein that is tightly associated with the nuclear matrix scaffold. Using a series of deletion constructs of the mouse hr gene, we monitored the sub-cellular localization of the recombinant protein by in situ immunolocalization and biochemical fractionation after nuclear matrix extraction of transiently transfected cells. We identified a novel nuclear matrix-targeting signal (NMTS) in the hr protein and mapped the domain to amino acid residues 111-186 of the mouse hr sequence. Furthermore, we provide evidence that this region not only mediates the interaction of hr with components of the nuclear architecture, but also specifies the sub-nuclear location of the hr protein to nuclear domains containing deacetylase activity. The N-terminal region directs hr to a speckled nuclear pattern that co-localizes with the histone deacetylase 3 (HDAC), but not with HDAC1 or HDAC7. Based on our findings, we propose that hr protein is part of a specific multi-protein repressor complex and that hr may be involved in chromatin remodeling.

MrgX is not essential for cell growth and development in the mouse

MRGX is one of the members of MORF4/MRG family of transcriptional regulators, which are involved in cell growth regulation and cellular senescence. We have shown that MRGX and MRG15 associate with Rb in nucleoprotein complexes and regulate B-myb promoter activity. To elucidate the functions of MRGX and to explore its potential role in modulating cell growth in vivo, we have generated MrgX-deficient mice. Characterization of the expression pattern of mouse MrgX demonstrated it was ubiquitously expressed in all tissues of adult mice and also during embryogenesis and overlapped with its homolog Mrg15. MRGX and MRG15 proteins localize predominantly to the chromatin fraction in the nucleus, although a small amount of both proteins localized to the nuclear matrix. Whereas disruption of Mrg15 results in embryonic lethality, absence of MrgX did not impair mouse development and MrgX null mice are healthy and fertile. MrgX-deficient and wild-type mouse embryonic fibroblasts (MEFs) also had similar growth rates and showed no differences in cell cycle-related gene expression in response to serum stimulation. Mrg15 expression in MrgX-deficient tissues and MEFs was not upregulated compared with wild-type tissues and MEFs. MRG15 is highly conserved with orthologs present from humans to yeast and is essential for survival of mice. In contrast, MRGX, which evolved later, is expressed only in vertebrates, suggesting that the lack of phenotype of MrgX-deficient mice is secondary to a compensatory effect by the evolutionarily conserved MRG15 protein but not vice versa.

Altered pre-lamin A processing is a common mechanism leading to lipodystrophy

Lipodystrophies are a heterogeneous group of human disorders characterized by the anomalous distribution of body fat associated with insulin resistance and altered lipid metabolism. The pathogenetic mechanism of inherited lipodystrophies is not yet clear; at the molecular level they have been linked to mutations of lamin A/C, peroxisome proliferator-activated receptor (PPARgamma) and other seemingly unrelated proteins. In this study, we examined lamin A/C processing in three laminopathies characterized by lipodystrophic phenotypes: Dunnigan type familial partial lipodystrophy, mandibuloacral dysplasia and atypical Werner's syndrome. We found that the lamin A precursor was specifically accumulated in lipodystrophy cells. Pre-lamin A was located at the nuclear envelope and co-localized with the adipocyte transcription factor sterol regulatory element binding protein 1 (SREBP1). Using co-immunoprecipitation experiments, we obtained the first demonstration of an in vivo interaction between SREBP1 and pre-lamin A. Binding of SREBP1 to the lamin A precursor was detected in patient fibroblasts as well as in control fibroblasts forced to accumulate pre-lamin A by farnesylation inhibitors. In contrast, SREBP1 did not interact in vivo with mature lamin A or C in cultured fibroblasts. To gain insights into the effect of pre-lamin A accumulation in adipose tissue, we inhibited lamin A precursor processing in 3T3-L1 pre-adipocytes. Our results show that pre-lamin A sequesters SREBP1 at the nuclear rim, thus decreasing the pool of active SREBP1 that normally activates PPARgamma and causing impairment of pre-adipocyte differentiation. This defect can be rescued by treatment with troglitazone, a known PPARgamma ligand activating the adipogenic program.

Poly(ADP-ribose) polymerase-1: association with nuclear lamins in rodent liver cells

The distribution of poly(ADP-ribose) polymerase-1 (PARP-1) over different nuclear compartments was studied by nuclear fractionation procedures and Western analysis revealing a prominent role of the nuclear matrix. This structure is operationally defined by the solubility properties of the A- and B-type lamins under defined experimental conditions. We consistently observed that most of the nuclear matrix-associated PARP-1 partitioned, in an active form, with the insoluble, lamin-enriched protein fractions that were prepared by a variety of established biochemical procedures. These PARP-1-protein interactions resisted salt extraction, disulfide reduction, RNase and DNase digestion. An inherent ability of PARP-1 to reassemble with the lamins became evident after a cycle of solubilization/dialysis using either urea or Triton X-100 and disulfide reduction, indicating that these interactions were dominated by hydrophobic forces. Together with in vivo crosslinking and co-immunoprecipitation experiments our results show that the lamins are prominent PARP-1-binding partners which could contribute to the functional sequestration of the enzyme on the nuclear matrix.

A-type nuclear lamins, progerias and other degenerative disorders

Nuclear lamins were identified as core nuclear matrix constituents over 20 years ago. They have been ascribed structural roles such as maintaining nuclear integrity and assisting in nuclear envelope formation after mitosis, and have also been linked to nuclear activities including DNA replication and transcription. Recently, A-type lamin mutations have been linked to a variety of rare human diseases including muscular dystrophy, lipodystrophy, cardiomyopathy, neuropathy and progeroid syndromes (collectively termed laminopathies). Most diseases arise from dominant, missense mutations, leading to speculation as to how different mutations in the same gene can give rise to such a diverse set of diseases, some of which share little phenotypic overlap. Understanding the cellular dysfunctions that lead to laminopathies will almost certainly provide insight into specific roles of A-type lamins in nuclear organization. Here, we compare and contrast the LMNA mutations leading to laminopathies with emphasis on progerias, and discuss possible functional roles for A-type lamins in the maintenance of healthy tissues.

Both lamin A and lamin C mutations cause lamina instability as well as loss of internal nuclear lamin organization

We have applied the fluorescence loss of intensity after photobleaching (FLIP) technique to study the molecular dynamics and organization of nuclear lamin proteins in cell lines stably transfected with green fluorescent protein (GFP)-tagged A-type lamin cDNA. Normal lamin A and C proteins show abundant decoration of the inner layer of the nuclear membrane, the nuclear lamina, and a generally diffuse localization in the nuclear interior. Bleaching studies revealed that, while the GFP-tagged lamins in the lamina were virtually immobile, the intranuclear fraction of these molecules was partially mobile. Intranuclear lamin C was significantly more mobile than intranuclear lamina A. In search of a structural cause for the variety of inherited diseases caused by A-type lamin mutations, we have studied the molecular organization of GFP-tagged lamin A and lamin C mutants R453W and R386K, found in Emery-Dreifuss muscular dystrophy (EDMD), and lamin A and lamin C mutant R482W, found in patients with Dunnigan-type familial partial lipodystrophy (FPLD). In all mutants, a prominent increase in lamin mobility was observed, indicating loss of structural stability of lamin polymers, both at the perinuclear lamina and in the intranuclear lamin organization. While the lamin rod domain mutant showed overall increased mobility, the tail domain mutants showed mainly intranuclear destabilization, possibly as a result of loss of interaction with chromatin. Decreased stability of lamin mutant polymers was confirmed by flow cytometric analyses and immunoblotting of nuclear extracts. Our findings suggest a loss of function of A-type lamin mutant proteins in the organization of intranuclear chromatin and predict the loss of gene regulatory function in laminopathies.

Sequestration of pRb by cyclin D3 causes intranuclear reorganization of lamin A/C during muscle cell differentiation

The A-type lamins that localize in nuclear domains termed lamin speckles are reorganized and antigenically masked specifically during myoblast differentiation. This rearrangement was observed to be linked to the myogenic program as lamin speckles, stained with monoclonal antibody (mAb) LA-2H10, were reorganized in MyoD-transfected fibroblasts induced to transdifferentiate to muscle cells. In C2C12 myoblasts, speckles were reorganized early during differentiation in cyclin D3-expressing cells. Ectopic cyclin D3 induced lamin reorganization in C2C12 myoblasts but not in other cell types. Experiments with adenovirus E1A protein that can bind to and segregate the retinoblastoma protein (pRb) indicated that pRb was essential for the cyclin D3-mediated reorganization of lamin speckles. Cyclin D3-expressing myoblasts displayed site-specific reduction of pRb phosphorylation. Furthermore, disruption of lamin structures by overexpression of lamins inhibited expression of the muscle regulatory factor myogenin. Our results suggest that the reorganization of internal lamins in muscle cells is mediated by key regulators of the muscle differentiation program.

Dynamics of DNA repair suggested by the subcellular localization of Brca1 and Brca2 proteins

The localization of proteins to specific subcellular compartments often reveals clues regarding their biological functions. Although significant progress has been made towards understanding how damaged DNA is repaired, experiments to date have primarily focused on signal transduction pathways that activate DNA repair protein complexes and on how these complexes are assembled. Current evidence suggests that certain DNA repair processes are spatially organized such that aberrant DNA structures can be brought into proximity with DNA repair proteins at fixed sites. Since biochemical evidence suggests that the tumor suppressor proteins, Brca1 and Brca2, may mediate the assembly of protein complexes involved in the repair of damaged DNA, we have performed subcellular fractionation experiments to determine the subnuclear localization of these proteins. The majority of Brca1 and Brca2 proteins were found to interact tightly with the nuclear matrix. Furthermore, within the limits of detection, localization of Brca1 and Brca2 to the nuclear matrix was not altered following treatment of cells with DNA damaging agents that activate homology-mediated double-stranded DNA break and transcription-coupled repair pathways. Our findings suggest that Brca1 and Brca2 may perform their DNA repair-related functions from positions that are anchored to the nuclear matrix. These data are consistent with proposed models that suggest that components of specific repair complexes residing on the nuclear matrix function to recruit damaged DNA.

Nuclear envelope, nuclear lamina, and inherited disease

The nuclear envelope is composed of the nuclear membranes, nuclear lamina, and nuclear pore complexes. In recent years, mutations in nuclear-envelope proteins have been shown to cause a surprisingly wide array of inherited diseases. While the mutant proteins are generally expressed in most or all differentiated somatic cells, many mutations cause fairly tissue-specific disorders. Perhaps the most dramatic case is that of mutations in A-type lamins, intermediate filament proteins associated with the inner nuclear membrane. Different mutations in the same lamin proteins have been shown to cause striated muscle diseases, partial lipodystrophy syndromes, a peripheral neuropathy, and disorders with features of severe premature aging. In this review, we summarize fundamental aspects of nuclear envelope structure and function, the inherited diseases caused by mutations in lamins and other nuclear envelope proteins, and possible pathogenic mechanisms.

MAN1, an integral protein of the inner nuclear membrane, binds Smad2 and Smad3 and antagonizes transforming growth factor-beta signaling

MAN1 (also known as LEMD3) is an integral protein of the inner nuclear membrane. Recently, mutations in MAN1 have been shown to result in osteopoikilosis, Buschke-Ollendorff syndrome and melorheostosis. We show that the nucleoplasmic, C-terminal domain of human MAN1 binds to Smad2 and Smad3 and antagonizes signaling by transforming growth factor-beta (TGF-beta). In a yeast two-hybrid screen using the C-terminal domain of MAN1 as bait, eight positive clones were obtained that encoded Smad3. In direct two-hybrid assays, this portion of MAN1 bound to Smad2 and Smad3. In glutathione-S-transferase precipitation assays, the C-terminal domain of MAN1 bound to Smad2 and Smad3 under stringent conditions. Antibodies against MAN1 were able to co-immunoprecipiate Smad2 from cells, demonstrating that they reside in the same complex in vivo. TGF-beta treatment stimulated transcription from a reporter gene in control cells, but reporter gene stimulation was significantly inhibited in cells overexpressing MAN1 or its C-terminal domain but not its N-terminal domain. TGF-beta-induced cell proliferation arrest was also inhibited in stable cell lines overexpressing MAN1. These results show that the nuclear envelope regulates a signal transduction pathway and have implications for how mutations in nuclear envelope proteins cause different human diseases.

The Function of Nuclear Architecture: A Genetic Approach

Eukaryotic genomes are distributed on linear chromosomes that are grouped together in the nucleus, an organelle separated from the cytoplasm by a characteristic double membrane studded with large proteinaceous pores. The chromatin within chromosomes has an as yet poorly characterized higher-order structure, but in addition to this, chromosomes and specific subchromosomal domains are nonrandomly positioned in nuclei. This review examines functional implications of the long-range organization of the genome in interphase nuclei. A rigorous test of the physiological importance of nuclear architecture is achieved by introducing mutations that compromise both structure and function. Focussing on such genetic approaches, we address general concepts of interphase nuclear order, the role of the nuclear envelope (NE) and lamins, and finally the importance of spatial organization for DNA replication and heritable gene expression.

Visualization of a highly organized intranuclear network of filaments in living mammalian cells

For 30 years, the mammalian cell nucleus has been hypothesized to contain a filamentous framework, the nuclear matrix or karyoskeleton, which regulates nuclear structure and function. However, such an organized network of filaments has never been observed in living cells. Here we show that human Cdc14B phosphatase in living cells tightly associates with long filaments that begin at the nucleolar periphery and extend to the nuclear envelope, frequently making close connections with nuclear pore complexes. We demonstrate that Cdc14B contains a bipartite signal that directs it to the intranuclear filaments, and we also detect a small amount of Cdc14B on interphase and mitotic centrosomes. Furthermore, we show that Cdc14B is critical for the maintenance of proper nuclear structure together with polo-like kinase Plk1. This work provides the first direct evidence for the existence of an intranuclear filamentous framework in living mammalian cells and implicates Cdc14B in the control of mammalian nuclear architecture.

Decreased mechanical stiffness in LMNA-/- cells is caused by defective nucleo-cytoskeletal integrity: implications for the development of laminopathies

Laminopathies comprise a group of inherited diseases with variable clinical phenotypes, caused by mutations in the lamin A/C gene (LMNA). A prominent feature in several of these diseases is muscle wasting, as seen in Emery-Dreifuss muscle dystrophy, dilated cardiomyopathy and limb-girdle muscular dystrophy. Although the mechanisms underlying this phenotype remain largely obscure, two major working hypotheses are currently being investigated, namely, defects in gene regulation and/or abnormalities in nuclear architecture causing cellular fragility. In this study, using a newly developed cell compression device we have tested the latter hypothesis. The device allows controlled application of mechanical load onto single living cells, with simultaneous visualization of cellular deformation and quantitation of resistance. With the device, we have compared wild-type (MEF+/+) and LMNA knockout (MEF-/-) mouse embryonic fibroblasts (MEFs), and found that MEF-/- cells show a significantly decreased mechanical stiffness and a significantly lower bursting force. Partial rescue of the phenotype by transfection with either lamin A or lamin C prevented gross nuclear disruption, as seen in MEF-/- cells, but was unable to fully restore mechanical stiffness in these cells. Our studies show a direct correlation between absence of LMNA proteins and nuclear fragility in living cells. Simultaneous recordings by confocal microscopy revealed that the nuclei in MEF-/- cells, in contrast to MEF+/+ cells, exhibited an isotropic deformation upon indentation, despite an anisotropic deformation of the cell as a whole. This nuclear behaviour is indicative for a loss of interaction of the disturbed nucleus with the surrounding cytoskeleton. In addition, careful investigation of the three-dimensional organization of actin-, vimentin- and tubulin-based filaments showed a disturbed interaction of these structures in MEF-/- cells. Therefore, we suggest that in addition to the loss of nuclear stiffness, the loss of a physical interaction between nuclear structures (i.e. lamins) and the cytoskeleton is causing more general cellular weakness and emphasizes a potential key function for lamins in maintaining cellular tensegrity.

Lco1 is a novel widely expressed lamin-binding protein in the nuclear interior

A-type lamins are localized at the nuclear envelope and in the nucleoplasm, and are implicated in human diseases called laminopathies. In a yeast two-hybrid screen with lamin C, we identified a novel widely expressed 171-kDa protein that we named Lamin companion 1 (Lco1). Three independent biochemical assays showed direct binding of Lco1 to the C-terminal tail of A-type lamins with an affinity of 700 nM. Lco1 also bound the lamin B1 tail with lower affinity (2 microM). Ectopic Lco1 was found primarily in the nucleoplasm and colocalized with endogenous intranuclear A-type lamins in HeLa cells. Overexpression of prelamin A caused redistribution of ectopic Lco1 to the nuclear rim together with ectopic lamin A, confirming association of Lco1 with lamin A in vivo. Whereas the major C-terminal lamin-binding fragment of Lco1 was cytoplasmic, the N-terminal Lco1 fragment localized in the nucleoplasm upon expression in cells. Furthermore, full-length Lco1 was nuclear in cells lacking A-type lamins, showing that A-type lamins are not required for nuclear targeting of Lco1. We conclude that Lco1 is a novel intranuclear lamin-binding protein. We hypothesize that Lco1 is involved in organizing the internal lamin network and potentially relevant as a laminopathy disease gene or modifier.

Abnormal cell cycle regulation in primary human uveal melanoma cultures

Uveal malignant melanoma is the most frequent primary intraocular tumor in adult humans. The cellular events leading to neoplasic transformation of normal uveal melanocytes are not well known when compared to other cancers. In this study, we investigated the role of G1 and G1/S regulatory proteins of the cell cycle in human uveal melanoma (UM) primary cell cultures, since these proteins are common targets in tumor development. Further, freshly established and characterized tumor cells are a better model for in vitro studies when compared to cell lines established long ago. Human primary cell cultures from eight different UM were established, as well as one primary culture from rhesus uveal normal melanocytes (UNM). Primary human UM cultures were characterized by a low establishment and growing rate. From four successful cultures, three showed a high expression of cyclin D1, cyclin E, p16NK4A, and p27KIP1 with no variations in cyclin A, cyclin-dependent kinase 2 (CDK2), and CDK4. Interestingly, in one of the cultured tumors, tumor suppressor protein retinoblastoma (Rb) did not bind E2F despite the fact that Rb was found in its hypophosphorylated form. No mutations in either RB1 or the Rb-binding pocket of E2F-1 were detected. Furthermore, we identified seven proteins co-immunoprecipitating with Rb in this tumor, including Lamin A/C and six proteins not previously reported to bind Rb: Hsc70, high mobility group protein 1 (HMG-1), hnRPN, glyceraldehyde 3 phosphate dehydrogenase (G3PDH), EF-1, and EF-2. Our results indicate that the overexpression of cyclins D1/E and CDKIs p16 and p27, together with a deregulation of the Rb/E2F pathway, may be implicated in the development of human UM.

Form follows function: The genomic organization of cellular differentiation

The extent to which the nucleus is functionally organized has broad biological implications. Evidence supports the idea that basic nuclear functions, such as transcription, are structurally integrated within the nucleus. Moreover, recent studies indicate that the linear arrangement of genes within eukaryotic genomes is nonrandom. We suggest that determining the relationship between nuclear organization and the linear arrangement of genes will lead to a greater understanding of how transcriptomes, dedicated to a particular cellular function or fate, are coordinately regulated. Current network theories may provide a useful framework for modeling the inherent complexity the functional organization of the nucleus.

A-type lamins regulate retinoblastoma protein function by promoting subnuclear localization and preventing proteasomal degradation

The retinoblastoma protein (pRB) is a critical regulator of cell proliferation and differentiation and an important tumor suppressor. In the G(1) phase of the cell cycle, pRB localizes to perinucleolar sites associated with lamin A/C intranuclear foci. Here, we examine pRB function in cells lacking lamin A/C, finding that pRB levels are dramatically decreased and that the remaining pRB is mislocalized. We demonstrate that A-type lamins protect pRB from proteasomal degradation. Both pRB levels and localization are restored upon reintroduction of lamin A. Lmna(-/-) cells resemble Rb(-/-) cells, exhibiting altered cell-cycle properties and reduced capacity to undergo cell-cycle arrest in response to DNA damage. These findings establish a functional link between a core nuclear structural component and an important cell-cycle regulator. They further raise the possibility that altered pRB function may be a contributing factor in dystrophic syndromes arising from LMNA mutation.

Runx2 control of organization, assembly and activity of the regulatory machinery for skeletal gene expression

We present an overview of Runx involvement in regulatory mechanisms that are requisite for fidelity of bone cell growth and differentiation, as well as for skeletal homeostasis and the structural and functional integrity of skeletal tissue. Runx-mediated control is addressed from the perspective of support for biological parameters of skeletal gene expression. We review recent findings that are consistent with an active role for Runx proteins as scaffolds for integration, organization and combinatorial assembly of nucleic acids and regulatory factors within the three-dimensional context of nuclear architecture.

Accumulation of mutant lamin A causes progressive changes in nuclear architecture in Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder, commonly caused by a point mutation in the lamin A gene that results in a protein lacking 50 aa near the C terminus, denoted LADelta50. Here we show by light and electron microscopy that HGPS is associated with significant changes in nuclear shape, including lobulation of the nuclear envelope, thickening of the nuclear lamina, loss of peripheral heterochromatin, and clustering of nuclear pores. These structural defects worsen as HGPS cells age in culture, and their severity correlates with an apparent increase in LADelta50. Introduction of LADelta50 into normal cells by transfection or protein injection induces the same changes. We hypothesize that these alterations in nuclear structure are due to a concentration-dependent dominant-negative effect of LADelta50, leading to the disruption of lamin-related functions ranging from the maintenance of nuclear shape to regulation of gene expression and DNA replication.

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.

Proteins that bind A-type lamins: integrating isolated clues

What do such diverse molecules as DNA, actin, retinoblastoma protein and protein kinase Cα all have in common? They and additional partners bind `A-type' lamins, which form stable filaments in animal cell nuclei. Mutations in A-type lamins cause a bewildering range of tissue-specific diseases, termed `laminopathies', including Emery-Dreifuss muscular dystrophy and the devastating Hutchinson-Gilford progeria syndrome, which mimics premature aging. Considered individually and collectively, partners for A-type lamins form four loose groups: architectural partners, chromatin partners, gene-regulatory partners and signaling partners. We describe 16 partners in detail, summarize their binding sites in A-type lamins, and sketch portraits of ternary complexes and functional pathways that might depend on lamins in vivo. On the basis of our limited current knowledge, we propose lamin-associated complexes with multiple components relevant to nuclear structure (e.g. emerin, nesprin 1α, actin) or signaling and gene regulation (e.g. LAP2α, retinoblastoma, E2F-DP heterodimers, genes) as `food for thought'. Testing these ideas will deepen our understanding of nuclear function and human disease.

Nuclear microenvironments support assembly and organization of the transcriptional regulatory machinery for cell proliferation and differentiation

The temporal and spatial organization of transcriptional regulatory machinery provides microenvironments within the nucleus where threshold concentrations of genes and cognate factors facilitate functional interactions. Conventional biochemical, molecular, and in vivo genetic approaches, together with high throughput genomic and proteomic analysis are rapidly expanding our database of regulatory macromolecules and signaling pathways that are requisite for control of genes that govern proliferation and differentiation. There is accruing insight into the architectural organization of regulatory machinery for gene expression that suggests signatures for biological control. Localized scaffolding of regulatory macromolecules at strategic promoter sites and focal compartmentalization of genes, transcripts, and regulatory factors within intranuclear microenvironments provides an infrastructure for combinatorial control of transcription that is operative within the three dimensional context of nuclear architecture.

Mitotic partitioning and selective reorganization of tissue-specific transcription factors in progeny cells

Postmitotic gene expression requires restoration of nuclear organization and assembly of regulatory complexes. The hematopoietic and osteogenic Runx (Cbfa/AML) transcription factors are punctately organized in the interphase nucleus and provide a model for understanding the subnuclear organization of tissue-specific regulatory proteins after mitosis. Here we have used quantitative in situ immunofluorescence microscopy and quantitative image analysis to show that Runx factors undergo progressive changes in cellular localization during mitosis while retaining a punctate distribution. In comparison, the acetyl transferase p300 and acetylated histone H4 remain localized with DNA throughout mitosis while the RNA processing factor SC35 is excluded from mitotic chromatin. Subnuclear organization of Runx foci is completely restored in telophase, and Runx proteins are equally partitioned into progeny nuclei. In contrast, subnuclear organization of SC35 is restored subsequent to telophase. Our results show a sequential reorganization of Runx and its coregulatory proteins that precedes restoration of RNA processing speckles. Thus, mitotic partitioning and spatiotemporal reorganization of regulatory proteins together render progeny cells equivalently competent to support phenotypic gene expression.

A new look at the cellular scaffold by embedment-free electron microscopy method

The basic scaffold of most cells is afforded by the cytoskeleton (comprising microfilaments, intermediate filaments and the microtubules). The conventional methods of electron microscopy fail to visualize filamentous cell structure. They can show only these filaments lying at the section surface. Heavy metal staining (I), and the optical properties of the resins used for embedding are similar to those of proteins hence most proteinaceous structures remain unresolved and the cytoplasm seems to be quite homogenous (II). Aldehyde fixation could cross-link proteins and lead to the emergence of artificial structures (III). These limitations may be overcome by the use of the embedment-free electron microscopy (EF-EM). This technique present cellular scaffold as a purified, isolated, three-dimensional network with various thickness of filaments. Our study on the dynamic aspect of cellular scaffold indicate that the thickness and arrangement of filaments depend on cell type and both physiological or pathological environments. Thank also to the adaptation of immunocytochemistry to EF-EM it was possible to understand the nuclear matrix and cytomatrix structure in relation to function. Thus, combination these methods revealed findings suggesting the nuclear homing of proapoptotic proteins and their association with intermediate filaments.