LBR, a chromatin and lamin binding protein from the inner nuclear membrane, is proteolyzed at late stages of apoptosis

ANGPTL4 binds to the leptin receptor to regulate ectopic bone formation

Leptin protein was thought to be unique to leptin receptor (LepR), but the phenotypes of mice with mutation in LepR [db/db (diabetes)] and leptin [ob/ob (obese)] are not identical, and the cause remains unclear. Here, we show that db/db, but not ob/ob, mice had defect in tenotomy-induced heterotopic ossification (HO), implicating alternative ligand(s) for LepR might be involved. Ligand screening revealed that ANGPTL4 (angiopoietin-like protein 4), a stress and fasting-induced factor, was elicited from brown adipose tissue after tenotomy, bound to LepR on PRRX1+ mesenchymal cells at the HO site, thus promotes chondrogenesis and HO development. Disruption of LepR in PRRX1+ cells, or lineage ablation of LepR+ cells, or deletion of ANGPTL4 impeded chondrogenesis and HO in mice. Together, these findings identify ANGPTL4 as a ligand for LepR to regulate the formation of acquired HO.

Molecular Mechanisms for the Regulation of Nuclear Membrane Integrity

The nuclear membrane serves a critical role in protecting the contents of the nucleus and facilitating material and signal exchange between the nucleus and cytoplasm. While extensive research has been dedicated to topics such as nuclear membrane assembly and disassembly during cell division, as well as interactions between nuclear transmembrane proteins and both nucleoskeletal and cytoskeletal components, there has been comparatively less emphasis on exploring the regulation of nuclear morphology through nuclear membrane integrity. In particular, the role of type II integral proteins, which also function as transcription factors, within the nuclear membrane remains an area of research that is yet to be fully explored. The integrity of the nuclear membrane is pivotal not only during cell division but also in the regulation of gene expression and the communication between the nucleus and cytoplasm. Importantly, it plays a significant role in the development of various diseases. This review paper seeks to illuminate the biomolecules responsible for maintaining the integrity of the nuclear membrane. It will delve into the mechanisms that influence nuclear membrane integrity and provide insights into the role of type II membrane protein transcription factors in this context. Understanding these aspects is of utmost importance, as it can offer valuable insights into the intricate processes governing nuclear membrane integrity. Such insights have broad-reaching implications for cellular function and our understanding of disease pathogenesis.

A biallelic loss-of-function variant in TMEM147 causes profound intellectual disability and spasticity

Intellectual disability (ID), occurring in syndromic or non-syndromic forms, is the most common neurodevelopmental disorder. Although many cases are caused by single gene defects, ID is highly genetically heterogeneous. Biallelic variants in the transmembrane protein TMEM147 have recently been linked to intellectual disability with dysmorphic facial features. TMEM147 is believed to localize to the endoplasmic reticulum membrane and nuclear envelope and also involved in biogenesis of multi-pass membrane proteins. Here, we report two patients born to a consanguineous family with a novel loss-of-function variant; (NM_001242597.2:c.193-197del) in TMEM147 causing intellectual disability and spasticity. Whole exome sequencing and validating Sanger sequencing were utilized to confirm the identified causal variant. Our findings were in line with the previously described patients with TMEM147 variants manifesting intellectual disability as a major clinical sign but also featured spasticity as a phenotypic expansion. This study provides additional evidence for the pathogenicity of TMEM147 mutations in intellectual disability and expands the phenotypic and variant spectrum linked to this gene.

Nucleus Mechanosensing in Cardiomyocytes

Cardiac muscle contraction is distinct from the contraction of other muscle types. The heart continuously undergoes contraction-relaxation cycles throughout an animal's lifespan. It must respond to constantly varying physical and energetic burdens over the short term on a beat-to-beat basis and relies on different mechanisms over the long term. Muscle contractility is based on actin and myosin interactions that are regulated by cytoplasmic calcium ions. Genetic variants of sarcomeric proteins can lead to the pathophysiological development of cardiac dysfunction. The sarcomere is physically connected to other cytoskeletal components. Actin filaments, microtubules and desmin proteins are responsible for these interactions. Therefore, mechanical as well as biochemical signals from sarcomeric contractions are transmitted to and sensed by other parts of the cardiomyocyte, particularly the nucleus which can respond to these stimuli. Proteins anchored to the nuclear envelope display a broad response which remodels the structure of the nucleus. In this review, we examine the central aspects of mechanotransduction in the cardiomyocyte where the transmission of mechanical signals to the nucleus can result in changes in gene expression and nucleus morphology. The correlation of nucleus sensing and dysfunction of sarcomeric proteins may assist the understanding of a wide range of functional responses in the progress of cardiomyopathic diseases.

Bi-allelic loss-of-function variants in TMEM147 cause moderate to profound intellectual disability with facial dysmorphism and pseudo-Pelger-Huët anomaly

The transmembrane protein TMEM147 has a dual function: first at the nuclear envelope, where it anchors lamin B receptor (LBR) to the inner membrane, and second at the endoplasmic reticulum (ER), where it facilitates the translation of nascent polypeptides within the ribosome-bound TMCO1 translocon complex. Through international data sharing, we identified 23 individuals from 15 unrelated families with bi-allelic TMEM147 loss-of-function variants, including splice-site, nonsense, frameshift, and missense variants. These affected children displayed congruent clinical features including coarse facies, developmental delay, intellectual disability, and behavioral problems. In silico structural analyses predicted disruptive consequences of the identified amino acid substitutions on translocon complex assembly and/or function, and in vitro analyses documented accelerated protein degradation via the autophagy-lysosomal-mediated pathway. Furthermore, TMEM147-deficient cells showed CKAP4 (CLIMP-63) and RTN4 (NOGO) upregulation with a concomitant reorientation of the ER, which was also witnessed in primary fibroblast cell culture. LBR mislocalization and nuclear segmentation was observed in primary fibroblast cells. Abnormal nuclear segmentation and chromatin compaction were also observed in approximately 20% of neutrophils, indicating the presence of a pseudo-Pelger-Huët anomaly. Finally, co-expression analysis revealed significant correlation with neurodevelopmental genes in the brain, further supporting a role of TMEM147 in neurodevelopment. Our findings provide clinical, genetic, and functional evidence that bi-allelic loss-of-function variants in TMEM147 cause syndromic intellectual disability due to ER-translocon and nuclear organization dysfunction.

Cell Senescence and Central Regulators of Immune Response

Pathways regulating cell senescence and cell cycle underlie many processes associated with ageing and age-related pathologies, and they also mediate cellular responses to exposure to stressors. Meanwhile, there are central mechanisms of the regulation of stress responses that induce/enhance or weaken the response of the whole organism, such as hormones of the hypothalamic-pituitary-adrenal (HPA) axis, sympathetic and parasympathetic systems, thymic hormones, and the pineal hormone melatonin. Although there are many analyses considering relationships between the HPA axis and organism ageing, we found no systematic analyses of relationships between the neuroendocrine regulators of stress and inflammation and intracellular mechanisms controlling cell cycle, senescence, and apoptosis. Here, we provide a review of the effects of neuroendocrine regulators on these mechanisms. Our analysis allowed us to postulate a multilevel system of central regulators involving neurotransmitters, glucocorticoids, melatonin, and the thymic hormones. This system finely regulates the cell cycle and metabolic/catabolic processes depending on the level of systemic stress, stage of stress response, and energy capabilities of the body, shifting the balance between cell cycle progression, cell cycle stopping, senescence, and apoptosis. These processes and levels of regulation should be considered when studying the mechanisms of ageing and the proliferation on the level of the whole organism.

Precursors of thymic peptides as stress sensors

INTRODUCTION

A large volume of data indicates that the known thymic hormones, thymulin, thymopoietin, thymosin-α, thymosin-β, and thymic humoral factor-y2, exhibit different spectra of activities. Although large in volume, available data are rather fragmented, resulting in a lack of understanding of the role played by thymic hormones in immune homeostasis.

AREA COVERED

Existing data compartmentalizes the effect of thymic peptides into 2 categories: influence on immune cells and interconnection with neuroendocrine systems. The current study draws attention to a third aspect of the thymic peptide effect that has not been clarified yet, wherein ubiquitous and highly abundant intranuclear precursors of so called 'thymic peptides' play a fundamental role in all somatic cells.

EXPERT OPINION

Our analysis indicated that, under certain stress-related conditions, these precursors are cleaved to form immunologically active peptides that rapidly leave the nucleus and intracellular spaces, to send 'distress signals' to the immune system, thereby acting as stress sensors. We propose that these peptides may form a link between somatic cells and immune as well as neuroendocrine systems. This model may provide a better understanding of the mechanisms underlying immune homeostasis, leading thereby to the development of new therapeutic regimes utilizing the characteristics of thymic peptides.

The nuclear envelope: target and mediator of the apoptotic process

Apoptosis is characterized by the destruction of essential cell organelles, including the cell nucleus. The nuclear envelope (NE) separates the nuclear interior from the cytosol. During apoptosis, the apoptotic machinery, in particular caspases, increases NE permeability by cleaving its proteins, such as those of the nuclear pore complex (NPC) and the nuclear lamina. This in turns leads to passive diffusion of cytosolic apoptogenic proteins, such as caspases and nucleases, through NPCs into the nucleus and the subsequent breakdown of the NE and destruction of the nucleus. However, NE leakiness at early stages of the apoptotic process can also occur in a caspase-independent manner, where Bax, by a non-canonical action, promotes transient and repetitive localized generation and subsequent rupture of nuclear protein-filled nuclear bubbles. This NE rupture leads to discharge of apoptogenic nuclear proteins from the nucleus to the cytosol, a process that can contribute to the death process. Therefore, the NE may play a role as mediator of cell death at early stages of apoptosis. The NE can also serve as a platform for assembly of complexes that regulate the death process. Thus, the NE should be viewed as both a mediator of the cell death process and a target.

MicroRNA-222 reprogrammed cancer-associated fibroblasts enhance growth and metastasis of breast cancer

BACKGROUND

Cancer-associated fibroblasts (CAFs) are known to impact on tumour behaviour, but the mechanisms controlling this are poorly understood.

METHODS

Breast normal fibroblasts (NFs) or CAFs were isolated from cancers by laser microdissection or were cultured. Fibroblasts were transfected to manipulate miR-222 or Lamin B receptor (LBR). The fibroblast-conditioned medium was collected and used to treat epithelial BC lines MDA-MB-231 and MDA-MB-157. Migration, invasion, proliferation or senescence was assessed using transwell, MTT or X-gal assays, respectively.

RESULTS

MiR-222 was upregulated in CAFs as compared with NFs. Ectopic miR-222 expression in NFs induced CAF-like expression profiles, while miR-222 knockdown in CAFs inhibited CAF phenotypes. LBR was identified as a direct miR-222 target, and was functionally relevant since LBR knockdown phenocopied miR-222 overexpression and LBR overexpression phenocopied miR-222 knockdown. MiR-222 overexpression, or LBR knockdown, was sufficient to induce NFs to show the CAF characteristics of enhanced migration, invasion and senescence, and furthermore, the conditioned medium from these fibroblasts induced increased BC cell migration and invasion. The reverse manipulations in CAFs inhibited these behaviours in fibroblasts, and inhibited paracrine influences on BC cells.

CONCLUSION

MiR-222/LBR have key roles in controlling pro-progression influences of CAFs in BC. This pathway may present therapeutic opportunities to inhibit CAF-induced cancer progression.

Comprehensive analysis of metastasis-related genes reveals a gene signature predicting the survival of colon cancer patients

Objective

The mechanism underlying colon cancer metastasis remain unclear. This study aimed to elucidate the genes alteration during the metastasis of colon cancer and identify genes that crucial to the metastasis and survival of colon cancer patients.

Methods

The dataset of primary and metastasis tissue of colon cancer, and dataset of high and low metastasis capability of colon cancer cells were selected as training cohort, and the overlapped differentially expressed genes (DEGs) were screened from the training cohort. The functional enrichment analysis for the overlapped DEGs was performed. The prognostic value of overlapped DEGs were analyzed in The Cancer Genome Atlas dataset, and a gene signature was developed using genes that related to the overall survival (OS). The prognostic value of the gene signature was further confirmed in a validation cohort.

Results

A total of 184 overlapped DEGs were screened from the training cohort. Functional enrichment analysis revealed the significant gene functions and pathways of the overlapped DEGs. Four hub genes (3-oxoacid CoA-transferase 1, actinin alpha 4, interleukin 8, integrin subunit alpha 3) were identified using protein–protein network analysis. Six genes (aldehyde dehydrogenase 2, neural precursor cell expressed, developmentally down-regulated 9, filamin A, lamin B receptor, twinfilin actin binding protein 1, serine and arginine rich splicing factor 1) were closely related to the OS of colon cancer patients. A gene signature was developed using these six genes based on their risk score, and the validation cohort indicated that the prognostic value of this gene signature was high in the prediction of colon cancer patients.

Conclusions

Our study demonstrates a gene profiles related to the metastasis of colon cancer, and identify a six-gene signature that acts as an independent biomarker on the prognosis of colon cancer.

Comprehending a Killer: The Akt/mTOR Signaling Pathways Are Temporally High-Jacked by the Highly Pathogenic 1918 Influenza Virus

Previous transcriptomic analyses suggested that the 1918 influenza A virus (IAV1918), one of the most devastating pandemic viruses of the 20th century, induces a dysfunctional cytokine storm and affects other innate immune response patterns. Because all viruses are obligate parasites that require host cells for replication, we globally assessed how IAV1918 induces host protein dysregulation. We performed quantitative mass spectrometry of IAV1918-infected cells to measure host protein dysregulation. Selected proteins were validated by immunoblotting and phosphorylation levels of members of the PI3K/AKT/mTOR pathway were assessed. Compared to mock-infected controls, >170 proteins in the IAV1918-infected cells were dysregulated. Proteins mapped to amino sugar metabolism, purine metabolism, steroid biosynthesis, transmembrane receptors, phosphatases and transcription regulation. Immunoblotting demonstrated that IAV1918 induced a slight up-regulation of the lamin B receptor whereas all other tested virus strains induced a significant down-regulation. IAV1918 also strongly induced Rab5b expression whereas all other tested viruses induced minor up-regulation or down-regulation. IAV1918 showed early reduced phosphorylation of PI3K/AKT/mTOR pathway members and was especially sensitive to rapamycin. These results suggest the 1918 strain requires mTORC1 activity in early replication events, and may explain the unique pathogenicity of this virus.

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Proteomic analyses of influenza 1918 virus-infected cells identified >170 dysregulated host proteins.

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Dysregulated proteins mapped to numerous important cellular pathways.

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1918 virus infection showed prominent early reduced phosphorylation of PI3K/Akt/mTOR.

The 1918 influenza pandemic was one of the most devastating infectious disease events of the 20th century, resulting in 20–100 million deaths. Gene-based assays showed severe dysregulation of the host's cytokine responses, but little was known about global protein responses to virus infection. This work identifies unique and temporal alterations in phosphorylation of the PI3K/AKT/mTOR signaling pathway, which is important in determining cell death. This work paves the way for further research on how this pathway influences host mechanisms responsible for aiding virus replication and in determining levels and severity of influenza virus-induced patho

Consequences of Lamin B1 and Lamin B Receptor Downregulation in Senescence

Anchoring of heterochromatin to the nuclear envelope appears to be an important process ensuring the spatial organization of the chromatin structure and genome function in eukaryotic nuclei. Proteins of the inner nuclear membrane (INM) mediating these interactions are able to recognize lamina-associated heterochromatin domains (termed LAD) and simultaneously bind either lamin A/C or lamin B1. One of these proteins is the lamin B receptor (LBR) that binds lamin B1 and tethers heterochromatin to the INM in embryonic and undifferentiated cells. It is replaced by lamin A/C with specific lamin A/C binding proteins at the beginning of cell differentiation and in differentiated cells. Our functional experiments in cancer cell lines show that heterochromatin in cancer cells is tethered to the INM by LBR, which is downregulated together with lamin B1 at the onset of cell transition to senescence. The downregulation of these proteins in senescent cells leads to the detachment of centromeric repetitive sequences from INM, their relocation to the nucleoplasm, and distension. In cells, the expression of LBR and LB1 is highly coordinated as evidenced by the reduction of both proteins in LBR shRNA lines. The loss of the constitutive heterochromatin structure containing LADs results in changes in chromatin architecture and genome function and can be the reason for the permanent loss of cell proliferation in senescence.

Caspases rule the intracellular trafficking cartel

During apoptosis, caspases feast on several hundreds of cellular proteins to orchestrate rapid cellular demise. Indeed, caspases are known to get a taste of every cellular process in one way or another, activating some, but most often shutting them down. Thus, it is not surprising that caspases proteolyze proteins involved in intracellular trafficking with particularly devastating consequences for this important process. This review article focuses on how caspases target the machinery responsible for smuggling goods within and outside the cell.

Caspase-mediated proteolysis of the sorting nexin 2 disrupts retromer assembly and potentiates Met/hepatocyte growth factor receptor signaling

The unfolding of apoptosis involves the cleavage of hundreds of proteins by the caspase family of cysteinyl peptidases. Among those substrates are proteins involved in intracellular vesicle trafficking with a net outcome of shutting down the crucial processes governing protein transport to organelles and to the plasma membrane. However, because of the intertwining of receptor trafficking and signaling, cleavage of specific proteins may lead to unintended consequences. Here we show that in apoptosis, sorting nexin 1 and 2 (SNX1 and SNX2), two proteins involved in endosomal sorting, are cleaved by initiator caspases and also by executioner caspase-6 in the case of SNX2. Moreover, SNX1 is cleaved at multiple sites, including following glutamate residues. Cleavage of SNX2 results in a loss of association with the endosome-to-trans-Golgi network transport protein Vps35 and in a delocalization from endosomes of its associated partner Vps26. We also demonstrate that SNX2 depletion causes an increase in hepatocyte growth factor receptor tyrosine phosphorylation and Erk1/2 signaling in cells. Finally, we show that SNX2 mRNA and protein levels are decreased in colorectal carcinoma and that lower SNX2 gene expression correlates with an increase in cancer patient mortality. Our study reveals the importance to characterize the cleavage fragments produced by caspases of specific death substrates given their potential implication in the mechanism of regulation of physiological (signaling/trafficking) pathways or in the dysfunction leading to pathogenesis.

Loss of lamin B receptor is necessary to induce cellular senescence

Cellular transition to senescence is associated with extensive chromatin reorganization and changes in gene expression. Recent studies appear to imply an association of lamin B1 (LB1) reduction with chromatin rearrangement in human fibroblasts promoted to senescence, while the mechanisms and structural features of these relationships have not yet been clarified. In this work, we examined the functions of LB1 and the lamin B receptor (LBR) in human cancer cells. We found that both LB1 and LBR tend to deplete during cancer cell transfer to senescence by γ-irradiation. A functional study employing silencing of LBR by small hairpin ribonucleic acid (shRNA) constructs revealed reduced LB1 levels suggesting that the regulation of both proteins is interrelated. The reduced expression of LBR resulted in the relocation of centromeric heterochromatin (CSH) from the inner nuclear membrane (INM) to the nucleoplasm and is associated with its unfolding. This indicates that LBR tethers heterochromatin to INM in cycling cancer cells and that LB1 is an integral part of this tethering. Down-regulation of LBR and LB1 at the onset of senescence are thus necessary for the release of heterochromatin binding to lamina, resulting in changes in chromatin architecture and gene expression. However, the senescence phenotype was not manifested in cell lines with reduced LBR and LB1 expression suggesting that other factors, such as deoxyribonucleic acid (DNA) damage, are needed to trigger senescence. We conclude that the primary response of cells to various stresses leading to senescence consists of the down-regulation of LBR and LB1 to attain reversal of the chromatin architecture.

DNA fragmentation and membrane damage of bocachico Prochilodus magdalenae (Ostariophysi: Prochilodontidae) sperm following cryopreservation with dimethylsulfoxide and glucose

The endangered bocachico Prochilodus magdalenae is a native freshwater fish of Colombia, the most captured species locally and one of the most important species for ex-situ conservation (germplasm banks). The aim of this study was to examine the effect of three concentrations of Dimethylsulfoxide (DMSO) (5%, 10%, 15%) and three of glucose (305, 333, 361 mM) in the extender on spermatic DNA fragmentation (F-DNA) (by Halomax®, Chromatin dispersion) and membrane damage (D-Me) (by eosin-nigrosin staining). After assessment of sperm quality by computer analysis of motility, one part of semen from males was diluted separately with three parts of extender and filled into 0.5 ml straws. Freezing was carried out in liquid nitrogen vapor dry shipper for 30 minutes and thawed at 60ºC for 8 seconds in a water bath and evaluated for the percentage of cells found with F-DNA and D-Me. The results demonstrated that cryopreservation causes greater F-DNA (13.62 ± 1.6% to 28.91 ± 3.25) and D-Me (24.27 ± 1.1% to 58.33 ± 2.81%) when compared with pre-freezing semen (PFS) (6.71 ± 1.54% and 2.34 ± 0.5%, respectively for F-DNA and D-Me). A significant interaction was found between DMSO and glucose concentration in this experiment. Use of extender: 10% DMSO + 305 mM glucose + 12% chicken egg yolk and, 10% DMSO + 333 mM glucose + 12% chicken egg yolk, allow for lower F-DNA and D-Me during cryopreservation of bocachico semen. A high correlation between F-DNA and D-Me was found (r = 0.771).

Altered dynamics of ubiquitin hybrid proteins during tumor cell apoptosis

The ubiquitin hybrid genes Uba80 and Uba52 encode ubiquitin (Ub), which is fused to the ribosomal proteins S27a (RPS27a) and L40 (RPL40), respectively. Here, we show that these genes are preferentially over-expressed during hepatoma cell apoptosis. Experiments using the tet-inducible transgenic system revealed that over-expression of the ubiquitin hybrid genes sensitized the cells to apoptosis. Further analysis suggested that Ub, and not RPS27a or RPL40, was associated with apoptotic cell death. Cleavage-resistant mutation analysis revealed that the N-terminal portion and the last two amino acids (GG) of Ub are critical for cleavage at the junction between the two protein moieties. An apoptogenic stimulus enhances the nuclear targeting and aggregation of Ub in the nucleus, resulting in histone H2A deubiquitylation followed by abnormal ubiquitylation of the nuclear envelope and the lamina. These events accompany the apoptotic nuclear morphology in the late stage of apoptosis. Each fused RP is localized in the nucleoli. These results suggest a role for Ub hybrid proteins in the altered nuclear dynamics of Ub during tumor cell apoptosis induced by apoptogenic stimuli.

Reduced lymphocyte longevity and homeostatic proliferation in lamin B receptor-deficient mice results in profound and progressive lymphopenia

The lamin B receptor (LBR) is a highly unusual inner nuclear membrane protein with multiple functions. Reduced levels are associated with decreased neutrophil lobularity, whereas complete absence of LBR results in severe skeletal dysplasia and in utero/perinatal lethality. We describe a mouse pedigree, Lym3, with normal bone marrow and thymic development but profound and progressive lymphopenia particularly within the T cell compartment. This defect arises from a point mutation within the Lbr gene with only trace mutant protein detectable in homozygotes, albeit sufficient for normal development. Reduced T cell homeostatic proliferative potential and life span in vivo were found to contribute to lymphopenia. To investigate the role of LBR in gene silencing in hematopoietic cells, we examined gene expression in wild-type and mutant lymph node CD8 T cells and bone marrow neutrophils. Although LBR deficiency had a very mild impact on gene expression overall, for common genes differentially expressed in both LBR-deficient CD8 T cells and neutrophils, gene upregulation prevailed, supporting a role for LBR in their suppression. In summary, this study demonstrates that LBR deficiency affects not only nuclear architecture but also proliferation, cell viability, and gene expression of hematopoietic cells.

Lamin B receptor regulates the growth and maturation of myeloid progenitors via its sterol reductase domain: implications for cholesterol biosynthesis in regulating myelopoiesis

Lamin B receptor (LBR) is a bifunctional nuclear membrane protein with N-terminal lamin B and chromatin-binding domains plus a C-terminal sterol Δ(14) reductase domain. LBR expression increases during neutrophil differentiation, and deficient expression disrupts neutrophil nuclear lobulation characteristic of Pelger-Huët anomaly. Thus, LBR plays a critical role in regulating myeloid differentiation, but how the two functional domains of LBR support this role is currently unclear. We previously identified abnormal proliferation and deficient functional maturation of promyelocytes (erythroid, myeloid, and lymphoid [EML]-derived promyelocytes) derived from EML-ic/ic cells, a myeloid model of ichthyosis (ic) bone marrow that lacks Lbr expression. In this study, we provide new evidence that cholesterol biosynthesis is important to myeloid cell growth and is supported by the sterol reductase domain of Lbr. Cholesterol biosynthesis inhibitors caused growth inhibition of EML cells that increased in EML-derived promyelocytes, whereas cells lacking Lbr exhibited complete growth arrest at both stages. Lipid production increased during wild-type neutrophil maturation, but ic/ic cells exhibited deficient levels of lipid and cholesterol production. Ectopic expression of a full-length Lbr in EML-ic/ic cells rescued both nuclear lobulation and growth arrest in cholesterol starvation conditions. Lipid production also was rescued, and a deficient respiratory burst was corrected. Expression of just the C-terminal sterol reductase domain of Lbr in ic/ic cells also improved each of these phenotypes. Our data support the conclusion that the sterol Δ(14) reductase domain of LBR plays a critical role in cholesterol biosynthesis and that this process is essential to both myeloid cell growth and functional maturation.

Requirement for lamin B receptor and its regulation by importin {beta} and phosphorylation in nuclear envelope assembly during mitotic exit

Lamin B receptor (LBR), a chromatin and lamin B-binding protein in the inner nuclear membrane, has been proposed to target the membrane precursor vesicles to chromatin mediated by importin β during the nuclear envelope (NE) assembly. However, the mechanisms for the binding of LBR with importin β and the membrane targeting by LBR in NE assembly remain largely unknown. In this report, we show that the amino acids (aa) 69-90 of LBR sequences are required to bind with importin β at aa 45-462, and the binding is essential for the NE membrane precursor vesicle targeting to the chromatin during the NE assembly at the end of mitosis. We also show that this binding is cell cycle-regulated and dependent on the phosphorylation of LBR Ser-71 by p34(cdc2) kinase. RNAi knockdown of LBR causes the NE assembly failure and abnormal chromatin decondensation of the daughter cell nuclei, leading to the daughter cell death at early G(1) phase by apoptosis. Perturbation of the interaction of LBR with importin β by deleting the LBR N-terminal spanning region or aa 69-73 also induces the NE assembly failure, the abnormal chromatin decondensation, and the daughter cell death. The first transmembrane domain of LBR promotes the NE production and expansion, because overexpressing this domain is sufficient to induce membrane overproduction of the NE. Thus, these results demonstrate that LBR targets the membrane precursor vesicles to chromatin by interacting with importin β in a LBR phosphorylation-dependent manner during the NE assembly at the end of mitosis and that the first transmembrane domain of LBR promotes the LBR-bearing membrane production and the NE expansion in interphase.

Dual roles of intermediate filaments in apoptosis

New roles have emerged recently for intermediate filaments (IFs), namely in modulating cell adhesion and growth, and providing resistance to various forms of stress and to apoptosis. In this context, we first summarize findings on the IF association with the cell response to mechanical stress and growth stimulation, in light of growth-related signaling events that are relevant to death-receptor engagement. We then address the molecular mechanisms by which IFs can provide cell resistance to apoptosis initiated by death-receptor stimulation and to necrosis triggered by excessive oxidative stress. In the same way, we examine IF involvement, along with cytolinker participation, in sequential caspase-mediated protein cleavages that are part of the overall cell death execution, particularly those that generate new functional IF protein fragments and uncover neoantigen markers. Finally, we report on the usefulness of these markers as diagnostic tools for disease-related aspects of apoptosis in humans. Clearly, the data accumulated in recent years provide new and significant insights into the multiple functions of IFs, particularly their dual roles in cell response to apoptotic insults.

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.

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.

The carboxyl-terminal region common to lamins A and C contains a DNA binding domain

Lamins A and C are intermediate filament proteins which polymerize into the nucleus to form the nuclear lamina network. The lamina is apposed to the inner nuclear membrane and functions in tethering chromatin to the nuclear envelope and in maintaining nuclear shape. We have recently characterized a globular domain that adopts an immunoglobulin fold in the carboxyl-terminal tail common to lamins A and C. Using an electrophoretic mobility shift assay (EMSA), we show that a peptide containing this domain interacts in vitro with DNA after dimerization through a disulfide bond, but does not interact with the core particle or the dinucleosome. The covalent dimer binds a 30-40 bp DNA fragment with a micromolar affinity and no sequence specificity. Using nuclear magnetic resonance (NMR) and an EMSA, we observed that two peptide regions participate in the DNA binding: the unstructured amino-terminal part containing the nuclear localization signal and a large positively charged region centered around amino acid R482 at the surface of the immunoglobulin-like domain. Mutations R482Q and -W, which are responsible for Dunnigan-type partial lipodystrophy, lower the affinity of the peptide for DNA. We conclude that the carboxyl-terminal end of lamins A and C binds DNA and suggest that alterations in lamin-DNA interactions may play a role in the pathophysiology of some lamin-linked diseases.

Many cuts to ruin: a comprehensive update of caspase substrates

Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.

Partial cleavage of A-type lamins concurs with their total disintegration from the nuclear lamina during apoptosis

Although activated caspase 6 is capable of cleaving both A- and B-type lamins during apoptosis, the higher-order structure of the nuclear lamina may cause a differential breakdown of these two types of lamins. In order to obtain a better understanding of the dynamics and the consequences of the rapid, coordinated breakdown of the lamina complex, we applied the green fluorescent protein (GFP) technology in living cells, in which the fate of individual caspase cleavage fragments of A- and B-type lamins was examined. CHO-K1 cells were stably transfected with cDNA constructs encoding N-terminally GFP-labelled hybrids of lamin A, lamin Adelta10, lamin C or lamin B1. The course of the apoptotic process, induced by the kinase inhibitor staurosporine or by the proteasome inhibitor MG132, was monitored by digital imaging microscopy or confocal microscopy. Time-lapse recordings showed that parallel to DNA condensation N-terminally GFP-tagged A-type lamins became diffusely dispersed throughout the nucleoplasm and rapidly translocated to the cytoplasm. In contrast, the majority of GFP-lamin B1 signal remained localised at the nuclear periphery, even after extensive DNA condensation. Comparison of lamin B1-GFP signal with A-type lamin antibody staining in the same apoptotic cells confirmed the temporal differences between A- and B-type lamina dispersal. Immunoblotting revealed only a partial cleavage of A-type lamins and an almost complete cleavage of lamin B1 during apoptosis. In contrast to lamin B1 in normal cells, this cleaved lamin B1, which is apparently still associated with the nuclear membrane, can be completely extracted by methanol or ethanol. Fluorescence loss of intensity after photobleaching experiments showed that in apoptotic cells A-type lamin-GFP molecules diffuse almost freely in both nucleoplasm and cytoplasm, while the lamin B1-GFP fragments remain more stably associated with the nuclear membrane, which is confirmed by co-localisation immunofluorescence studies with a nucleoporin p62 antibody. Our results therefore clearly show a differential behaviour of A- and B-type lamins during apoptosis, suggesting not only distinct differences in the organisation of the lamina filaments, but also that caspase cleavage of only a small fraction of A-type lamins is needed for its complete disintegration.

Fate of the nuclear lamina during Caenorhabditis elegans apoptosis

Invertebrates and in Drosophila, lamins and lamin-associated proteins are primary targets for cleavage by caspases. Eliminating mammalian lamins causes apoptosis, whereas expressing mutant lamins that cannot be cleaved by caspase-6 delay apoptosis. Caenorhabditis elegans has a single lamin protein, Ce-lamin, and a caspase, CED-3, that is responsible for most if not all somatic apoptosis. In this study we show that in C. elegans embryos induced to undergo apoptosis Ce-lamin is degraded surprisingly late. In such embryos CED-4 translocated to the nuclear envelope but the cytological localization of Ce-lamin remained similar to that in wild-type embryos. TUNEL labeling indicated that Ce-lamin was degraded only after DNA is fragmented. Ce-lamin, Ce-emerin, or Ce-MAN1 were not cleaved by recombinant CED-3, showing that these lamina proteins are not substrates for CED-3 cleavage. These results suggest that lamin cleavage probably is not essential for apoptosis in C. elegans.

Staurosporine treatment and serum starvation promote the cleavage of emerin in cultured mouse myoblasts: involvement of a caspase-dependent mechanism

Emerin is a nuclear membrane-anchored protein which is absent or mutated in patients affected by Emery-Dreifuss muscular dystrophy. In this study, we induced apoptosis in cultured mouse myoblasts to evaluate emerin fate during the nuclear destabilization involved in programmed cell death. Emerin proteolysis was observed in myocytes during the apoptotic process. Myoblast apoptosis and emerin degradation were associated with chromatin compaction and detachment from the nuclear lamina, as detected by electron microscopy. In vivo specific inhibition of caspase 3 or caspase 6 activity completely abolished emerin proteolysis. These results show that the process of programmed cell death in muscle cells leads to emerin proteolysis, which appears to be related to caspase 6 activation and to cleavage of other nuclear envelope proteins, that share sequence homologies or functional features with emerin.

Sequential degradation of proteins from the nuclear envelope during apoptosis

We have produced new antibodies specific for the integral pore membrane protein POM121. Using these antibodies we show that during apoptosis POM121 becomes proteolytically degraded in a caspase-dependent manner. The POM121 antibodies and antibodies specific for other proteins of the nuclear envelope were used in a comparative study of nuclear apoptosis in staurosporine-treated buffalo rat liver cells. Nuclei from these cells were classified in three different stages of apoptotic progression: stage I, moderately condensed chromatin surrounded by a smooth nuclear periphery; stage II, compact patches of condensed chromatin collapsing against a smooth nuclear periphery; stage III, round compact chromatin bodies surrounded by grape-shaped nuclear periphery. We have performed double labeling immunofluorescence microscopy of individual apoptotic cells and quantitative immunoblotting analysis of total proteins from apoptotic cell cultures. The results showed that degradation of nuclear envelope marker proteins occurred in a specific order. POM121 degradation occurred surprisingly early and was initiated before nucleosomal DNA degradation could be detected using TUNEL assay and completed before clustering of the nuclear pores. POM121 was eliminated significantly more rapid compared with NUP153 (a peripheral protein located in the nucleoplasmic basket of the nuclear pore complex) and lamin B (a component of the nuclear lamina). Disappearance of NUP153 and lamin B was coincident with onset of DNA fragmentation and clustering of nuclear pores. By contrast, the peripheral NPC protein p62 was degraded much later. The results suggest that degradation of POM121 may be an important early step in propagation of nuclear apoptosis.

Nuclear apoptotic changes: an overview

Apoptosis is a form of active cell death essential for morphogenesis, development, differentiation, and homeostasis of multicellular organisms. The activation of genetically controlled specific pathways that are highly conserved during evolution results in the characteristic morphological features of apoptosis that are mainly evident in the nucleus. These include chromatin condensation, nuclear shrinkage, and the formation of apoptotic bodies. The morphological changes are the result of molecular alterations, such as DNA and RNA cleavage, post-translational modifications of nuclear proteins, and proteolysis of several polypeptides residing in the nucleus. During the last five years our understanding of the process of apoptosis has dramatically increased. However, the mechanisms that lead to apoptotic changes in the nucleus have been only partially clarified. Here, we shall review the most recent findings that may explain why the nucleus displays these striking modifications. Moreover, we shall take into consideration the emerging evidence about apoptotic events as a trigger for the generation of autoantibodies to nuclear components.

HSV and glycoprotein J inhibit caspase activation and apoptosis induced by granzyme B or Fas

HSV-1 inhibits apoptosis of infected cells, presumably to ensure that the infected cell survives long enough to allow completion of viral replication. Because cytotoxic lymphocytes kill their targets via the induction of apoptosis, protection from apoptosis could constitute a mechanism of immune evasion for HSV. Several HSV genes are involved in the inhibition of apoptosis, including Us5, which encodes glycoprotein J (gJ). Viruses deleted for Us5 showed defects in inhibition of caspase activation after Fas ligation or UV irradiation. Transfected cells expressing the Us5 gene product gJ were protected from Fas- or UV-induced apoptosis, as measured by morphology, caspase activation, membrane permeability changes, or mitochondrial transmembrane potential. In contrast, caspase 3 activation in mitochondria-free cell lysates by granzyme (gr)B was inhibited equivalently by Us5 deletion and rescue viruses, suggesting that gJ is not required for HSV to inhibition this process. However, mitochondria-free lysates from transfected cells expressing Us5/gJ were protected from grB-induced caspase activation, suggesting that Us5/gJ is sufficient to inhibit this process. Transfected cells expressing Us5/gJ were also protected from death induced by incubation with purified grB and perforin. These findings suggest that HSV has a comprehensive set of immune evasion functions that antagonize both Fas ligand- and grB-mediated pathways of CTL-induced apoptosis. The understanding of HSV effects on killing by CTL effector mechanisms may shed light on the incomplete control of HSV infections by the immune system and may allow more rational approaches to the development of immune modulatory treatments for HSV infection.

Nuclear changes in necrotic HL-60 cells

Cell death in eukaryotes can occur by either apoptosis or necrosis. Apoptosis is characterized by well-defined nuclear changes which are thought to be the consequence of both proteolysis and DNA fragmentation. On the other hand, the nuclear modifications that occur during necrosis are largely less known. Here, we have investigated whether or not nuclear modifications occur during ethanol-induced necrotic cell death of HL-60 cells. By means of immunofluorescence staining, we demonstrate that the patterns given by antibodies directed against some nuclear proteins (lamin B1, NuMA, topoisomerase IIalpha, SC-35, B23/nucleophosmin) changed in necrotic cells. The changes in the spatial distribution of NuMA strongly resembled those described to occur during apoptosis. On the contrary, the fluorescent pattern characteristic for other nuclear proteins (C23/nucleolin, UBF, fibrillarin, RNA polymerase I) did not change during necrosis. By immunoblotting analysis, we observed that some nuclear proteins (SAF-A, SATB1, NuMA) were cleaved during necrosis, and in the case of SATB1, the apoptotic signature fragment of 70 kDa was also present to the same extent in necrotic samples. Caspase inhibitors did not prevent proteolytic cleavage of the aforementioned polypeptides during necrosis, while they were effective if apoptosis was induced. In contrast, lamin B1 and topoisomerase IIalpha were uncleaved in necrotic cells, whereas they were proteolyzed during apoptosis. Transmission electron microscopy analysis revealed that slight morphological changes were present in the nuclear matrix fraction prepared from necrotic cells. However, these modifications (mainly consisting of a rarefaction of the inner fibrogranular network) were not as striking as those we have previously described in apoptotic HL-60 cells. Taken together, our results indicate that during necrosis marked biochemical and morphological changes do occur at the nuclear level. These alterations are quite distinct from those known to take place during apoptosis. Our results identify additional biochemical and morphological criteria that could be used to discriminate between the two types of cell death. J. Cell. Biochem. Suppl. 36: 19-31, 2001.

Fate of the inner nuclear membrane protein lamin B receptor and nuclear lamins in herpes simplex virus type 1 infection

During herpesvirus egress, capsids bud through the inner nuclear membrane. Underlying this membrane is the nuclear lamina, a meshwork of intermediate filaments with which it is tightly associated. Details of alterations to the lamina and the inner nuclear membrane during infection and the mechanisms involved in capsid transport across these structures remain unclear. Here we describe the fate of key protein components of the nuclear envelope and lamina during herpes simplex virus type 1 (HSV-1) infection. We followed the distribution of the inner nuclear membrane protein lamin B receptor (LBR) and lamins A and B(2) tagged with green fluorescent protein (GFP) in live infected cells. Together with additional results from indirect immunofluorescence, our studies reveal major morphologic distortion of nuclear-rim LBR and lamins A/C, B(1), and B(2). By 8 h p.i., we also observed a significant redistribution of LBR-GFP to the endoplasmic reticulum, where it colocalized with a subpopulation of cytoplasmic glycoprotein B by immunofluorescence. In addition, analysis by fluorescence recovery after photobleaching reveals that LBR-GFP exhibited increased diffusional mobility within the nuclear membrane of infected cells. This is consistent with the disruption of interactions between LBR and the underlying lamina. In addition to studying stably expressed GFP-lamins by fluorescence microscopy, we studied endogenous A- and B-type lamins in infected cells by Western blotting. Both approaches reveal a loss of lamins associated with virus infection. These data indicate major disruption of the nuclear envelope and lamina of HSV-1-infected cells and are consistent with a virus-induced dismantling of the nuclear lamina, possibly in order to gain access to the inner nuclear membrane.

Mistargeting of B-type lamins at the end of mitosis: implications on cell survival and regulation of lamins A/C expression

We previously showed that targeting of protein phosphatase 1 (PP1) to the nuclear envelope (NE) by the A-kinase anchoring protein, AKAP149, correlates with nuclear assembly of B-type lamins in vitro. We demonstrate here that failure of AKAP149-mediated assembly of B-type lamins into the nuclear lamina at the end of mitosis is followed by apoptosis, and induces expression of the gene encoding A-type lamins in cells that normally do not express lamins A/C. In HeLa cells, inhibition of PP1 association with the NE mediated by a peptide containing the PP1-binding domain of AKAP149 results in failure of B-type lamins to assemble, and in their rapid caspase-dependent proteolysis. However, assembly of lamins A/C is not affected. Nonetheless, apoptosis follows within hours of nuclear reformation after mitosis. In lymphoid KE37 cells, which do not express lamins A/C, inhibition of B-type lamin assembly triggers rapid synthesis and nuclear assembly of both lamins A and C before apoptosis takes place. The results indicate that nuclear assembly of B-type lamins is essential for cell survival. They also suggest that mistargeting of B-type lamins at the end of mitosis elicits a tentative rescue process to assemble a nuclear lamina in lymphoid cells that normally do not express lamins A/C.

The gamma-2-herpesvirus bovine herpesvirus 4 causes apoptotic infection in permissive cell lines

Increasing evidence suggests that regulation of apoptosis in infected cells is associated with several viral infections. The gammaherpesvirus bovine herpesvirus 4 (BHV-4) has been shown to harbor genes with antiapoptotic potentialities. However, here we have demonstrated that productive infection of adherent, permissive cell lines by BHV-4 resulted in a cytopathic effect characterized by induction of apoptosis. This phenomenon was confirmed using different techniques to detect apoptosis and using different virus strains and cell targets. Apoptosis induced by BHV-4 was inhibited by (1) treatment with doses of heparin, which completely inhibited virus attachment and infectivity; (2) UV treatment, which completely abrogated infectivity; and (3) treatment with a dose of phosphonoacetic acid, which blocked virus replication. Virus-induced apoptosis was associated with a down-regulation of Bcl-2 expression and was reduced by Z-VAD-FMK, but not by Z-DEVD-FMK (caspase-3-specific) caspase inhibitors. Inhibition of apoptosis by Z-VAD-FMK treatment during infection did not modify virus yield. Therefore, despite the presence of antiapoptotic genes in its genoma, BHV-4 could complete its cycle of productive infection while inducing apoptosis of infected cells. This finding might have implications for the pathobiology of BHV-4 and other gammaherpesviruses in vivo.

Caspase-mediated cleavage of the chromosome-binding domain of lamina-associated polypeptide 2 alpha

Lamina-associated polypeptide 2 alpha (LAP2 alpha) is a non-membrane-bound isoform of the LAP2 family involved in nuclear structure organization. Using various cell systems, including Jurkat, HL-60, and HeLa cells, and different death-inducing agents, such as anti-Fas antibody, topoisomerase inhibitors, and staurosporine, we found that LAP2 alpha was cleaved during apoptosis as rapidly as lamin B in a caspase-dependent manner yielding stable N- and C-terminal fragments of approximately 50 and 28 kDa, respectively. Based on fragment size and localization of immunoreactive epitopes, four potential cleavage sites were mapped between amino acids 403–485. These sites were located within a domain that has previously been described to be essential and sufficient for association of LAP2 alpha with chromosomes, suggesting that LAP2 alpha cleavage impairs its chromatin-binding properties. Immunofluorescence microscopy demonstrated that, unlike full length protein, apoptotic fragments did not colocalize with condensed chromatin, but remained in the nuclear compartment as long as a single nucleus was visible. Subfractionation analyses showed that the N-terminal LAP2 alpha fragment was extracted from intranuclear structures in detergent/salt buffers, whereas the C-terminal fragment remained associated with a residual framework devoid of chromatin. Our data suggest that early cleavage of LAP2 alpha) is important for chromatin reorganization during apoptosis.

Mammalian caspases: structure, activation, substrates, and functions during apoptosis

Apoptosis is a genetically programmed, morphologically distinct form of cell death that can be triggered by a variety of physiological and pathological stimuli. Studies performed over the past 10 years have demonstrated that proteases play critical roles in initiation and execution of this process. The caspases, a family of cysteine-dependent aspartate-directed proteases, are prominent among the death proteases. Caspases are synthesized as relatively inactive zymogens that become activated by scaffold-mediated transactivation or by cleavage via upstream proteases in an intracellular cascade. Regulation of caspase activation and activity occurs at several different levels: (a) Zymogen gene transcription is regulated; (b) antiapoptotic members of the Bcl-2 family and other cellular polypeptides block proximity-induced activation of certain procaspases; and (c) certain cellular inhibitor of apoptosis proteins (cIAPs) can bind to and inhibit active caspases. Once activated, caspases cleave a variety of intracellular polypeptides, including major structural elements of the cytoplasm and nucleus, components of the DNA repair machinery, and a number of protein kinases. Collectively, these scissions disrupt survival pathways and disassemble important architectural components of the cell, contributing to the stereotypic morphological and biochemical changes that characterize apoptotic cell death.

Inner nuclear membrane protein LBR preferentially interacts with DNA secondary structures and nucleosomal linker

The lamin B receptor (LBR) is an integral protein of inner nuclear membrane whose nucleoplasmic amino-terminal domain contributes to the attachment of the membrane to chromatin. Here we analyzed the interactions of a recombinant GST protein containing the amino-terminal domain of the protein with in vitro reconstituted nucleosomes and short DNA fragments. Data show that the LBR amino-terminal domain (AT) binds linker DNA but does not interact with the nucleosome core. Titration and competition studies revealed that the interaction between LBR AT and DNA is saturable, of high affinity (K(D) approximately 4 nM), independent of DNA sequence, and enhanced by DNA curvature and supercoiling. In this respect, LBR amino-terminal domain binding to nucleosomes is similar to that of histone H1 and non histone proteins HMG1/2 which both bind preferentially to linker DNA and present a significant affinity for DNA secondary structures.

The fate of the nuclear matrix-associated-region-binding protein SATB1 during apoptosis

Special AT-rich sequence-binding protein 1 (SATB1), predominantly expressed in thymocytes, was identified as a component of the nuclear matrix protein fraction. Programmed cell death of Jurkat T-cells was induced by various stimuli in Fas-dependent and -independent fashion. During apoptosis, but not during necrosis, SATB1 was cleaved, as rapidly as was lamin B, in a caspase-dependent way yielding a stable 70 kDa fragment. The same result was obtained for apoptotic HL60-cells. We constructed various deletion constructs of SATB1, expressing protein chimeras tagged with green fluorescent protein (GFP). Transient transfection of these into Jurkat or HeLa cells followed by initiation of apoptosis allowed us to map the potential caspase-6 cleavage site VEMD to the N-terminal third of SATB1, leaving an intact DNA-binding domain in the C-terminal part of the protein. Our results suggest that apoptosis-specific breakdown of SATB1, a transcriptional activator of the CD8a gene, might be of physiological relevance during thymic clonal deletion and apoptosis of peripheral T-lymphoid cells.

Review: nuclear lamins--structural proteins with fundamental functions

The nuclear lamina is located between the inner nuclear membrane and the peripheral chromatin. It is composed of both peripheral and integral membrane proteins, including lamins and lamina-associated proteins. Lamins can interact with one another, with lamina-associated proteins, with nuclear scaffold proteins, and with chromatin. Likewise, most of the lamina-associated proteins are likely to interact directly with chromatin. The nuclear lamina is required for proper cell cycle regulation, chromatin organization, DNA replication, cell differentiation, and apoptosis. Mutations in proteins of the nuclear lamina can disrupt these activities and cause genetic diseases. The structure and assembly of the nuclear lamina proteins and their roles in chromatin organization and cell cycle regulation were recently reviewed. In this review, we discuss the roles of the nuclear lamina in DNA replication and apoptosis and analyze how mutations in nuclear lamina proteins might cause genetic diseases.

The A-kinase-anchoring protein AKAP95 is a multivalent protein with a key role in chromatin condensation at mitosis

Protein kinase A (PKA) and the nuclear A-kinase-anchoring protein AKAP95 have previously been shown to localize in separate compartments in interphase but associate at mitosis. We demonstrate here a role for the mitotic AKAP95-PKA complex. In HeLa cells, AKAP95 is associated with the nuclear matrix in interphase and redistributes mostly into a chromatin fraction at mitosis. In a cytosolic extract derived from mitotic cells, AKAP95 recruits the RIIalpha regulatory subunit of PKA onto chromatin. Intranuclear immunoblocking of AKAP95 inhibits chromosome condensation at mitosis and in mitotic extract in a PKA-independent manner. Immunodepletion of AKAP95 from the extract or immunoblocking of AKAP95 at metaphase induces premature chromatin decondensation. Condensation is restored in vitro by a recombinant AKAP95 fragment comprising the 306-carboxy-terminal amino acids of the protein. Maintenance of condensed chromatin requires PKA binding to chromatin-associated AKAP95 and cAMP signaling through PKA. Chromatin-associated AKAP95 interacts with Eg7, the human homologue of Xenopus pEg7, a component of the 13S condensin complex. Moreover, immunoblocking nuclear AKAP95 inhibits the recruitment of Eg7 to chromatin in vitro. We propose that AKAP95 is a multivalent molecule that in addition to anchoring a cAMP/PKA-signaling complex onto chromosomes, plays a role in regulating chromosome structure at mitosis.

Caspase-dependent proteolysis of integral and peripheral proteins of nuclear membranes and nuclear pore complex proteins during apoptosis

We have studied the fate of the nuclear envelope (NE) in different human cells committed to apoptosis by different chemical agents. Using a battery of antibodies against marker proteins of the three domains of the nuclear envelope, namely lamin B (LB) for the lamina, transmembrane proteins LBR and LAP2 for the inner nuclear membrane, and nucleoporins p62, Nup153 and gp210 for the nuclear pore complexes (NPCs), we observed a selective and conserved cleavage of LB, LAP2 and Nup153. In lymphoid cells, the rate of cleavage of these markers was independent of the apoptosis inducing agent, actinomycin D or etoposide, and more rapid than in attached epithelial cells. While lamin B is cleaved by caspase 6, the protease responsible for the cleavage of LAP2 and Nup153 was probably caspase 3, since (1) cleavage of both proteins was specifically prevented by in vivo addition of caspase 3 inhibitor Ac-DEVD-CHO and (2) consensus sites for these caspases are present in both proteins. As LB, LAP2 and Nup153 are exposed at the inner face of the nuclear envelope and all interact with chromatin, we suggest that their cleavage allows both the detachment of NE from chromatin and the clustering of NPCs in the plane of the membrane, two conserved morphological features of apoptosis observed in this study.

Molecular genetics of the Smith-Lemli-Opitz syndrome and postsqualene sterol metabolism

The Smith-Lemli-Opitz syndrome is a disorder of morphogenesis resulting from an enzymatic defect in the last step of cholesterol metabolism (reduction of 7-dehydrocholesterol). Analysis of the defective gene and identification of mutations therein have paved the way for the study of the molecular genetics of the disorder which is caused by numerous different mutations. Future efforts should identify a postulated intracellular signalling activity of sterol intermediates, isolate proteins that govern the sterol traffic between intracellular compartments, structurally characterize the enzyme delta 7-sterol reductase defective in the Smith-Lemli-Opitz syndrome and investigate the pathomechanism of sterol depletion-induced dysmorphogenesis.