Inner nuclear membrane and regulation of Smad-mediated signaling

Nuclear envelope protein MAN1 regulates clock through BMAL1

Circadian clocks serve as internal pacemakers that influence many basic homeostatic processes; consequently, the expression and function of their components are tightly regulated by intricate networks of feedback loops that fine-tune circadian processes. Our knowledge of these components and pathways is far from exhaustive. In recent decades, the nuclear envelope has emerged as a global gene regulatory machine, although its role in circadian regulation has not been explored. We report that transcription of the core clock component BMAL1 is positively modulated by the inner nuclear membrane protein MAN1, which directly binds the BMAL1 promoter and enhances its transcription. Our results establish a novel connection between the nuclear periphery and circadian rhythmicity, therefore bridging two global regulatory systems that modulate all aspects of bodily functions.

The cellular mastermind(?)-mechanotransduction and the nucleus

Cells respond to mechanical stimulation by activation of specific signaling pathways and genes that allow the cell to adapt to its dynamic physical environment. How cells sense the various mechanical inputs and translate them into biochemical signals remains an area of active investigation. Recent reports suggest that the cell nucleus may be directly implicated in this cellular mechanotransduction process. Taken together, these findings paint a picture of the nucleus as a central hub in cellular mechanotransduction-both structurally and biochemically-with important implications in physiology and disease.

LAP2 is widely overexpressed in diverse digestive tract cancers and regulates motility of cancer cells

Background

Lamina-associated polypeptides 2 (LAP2) is a nuclear protein that connects the nuclear lamina with chromatin. Although its critical roles in genetic disorders and hematopoietic malignancies have been described, its expression and roles in digestive tract cancers have been poorly characterized.

Methods

To examine the expression of LAP2 in patient tissues, we performed immunohistochemistry and real-time PCR. To examine motility of cancer cells, we employed Boyden chamber, wound healing and Matrigel invasion assays. To reveal its roles in metastasis in vivo, we used a liver metastasis xenograft model. To investigate the underlying mechanism, a cDNA microarray was conducted.

Results

Immunohistochemistry in patient tissues showed widespread expression of LAP2 in diverse digestive tract cancers including stomach, pancreas, liver, and bile duct cancers. Real-time PCR confirmed that LAP2β is over-expressed in gastric cancer tissues. Knockdown of LAP2β did not affect proliferation of most digestive tract cancer cells except pancreatic cancer cells. However, knockdown of LAP2β decreased motility of all tested cancer cells. Moreover, overexpression of LAP2β increased motility of gastric and pancreatic cancer cells. In the liver metastasis xenograft model, LAP2β increased metastatic efficacy of gastric cancer cells and mortality in tested mice. cDNA microarrays showed the possibility that myristoylated alanine-rich C kinase substrate (MARCKS) and interleukin6 (IL6) may mediate LAP2β-regulated motility of cancer cells.

Conclusions

From the above results, we conclude that LAP2 is widely overexpressed in diverse digestive tract cancers and LAP2β regulates motility of cancer cells and suggest that LAP2β may have utility for diagnostics and therapeutics in digestive tract cancers.

Uncoordinated transcription and compromised muscle function in the lmna-null mouse model of Emery- Emery-Dreyfuss muscular dystrophy

LMNA encodes both lamin A and C: major components of the nuclear lamina. Mutations in LMNA underlie a range of tissue-specific degenerative diseases, including those that affect skeletal muscle, such as autosomal-Emery-Dreifuss muscular dystrophy (A-EDMD) and limb girdle muscular dystrophy 1B. Here, we examine the morphology and transcriptional activity of myonuclei, the structure of the myotendinous junction and the muscle contraction dynamics in the lmna-null mouse model of A-EDMD. We found that there were fewer myonuclei in lmna-null mice, of which ∼50% had morphological abnormalities. Assaying transcriptional activity by examining acetylated histone H3 and PABPN1 levels indicated that there was a lack of coordinated transcription between myonuclei lacking lamin A/C. Myonuclei with abnormal morphology and transcriptional activity were distributed along the length of the myofibre, but accumulated at the myotendinous junction. Indeed, in addition to the presence of abnormal myonuclei, the structure of the myotendinous junction was perturbed, with disorganised sarcomeres and reduced interdigitation with the tendon, together with lipid and collagen deposition. Functionally, muscle contraction became severely affected within weeks of birth, with specific force generation dropping as low as ∼65% and ∼27% of control values in the extensor digitorum longus and soleus muscles respectively. These observations illustrate the importance of lamin A/C for correct myonuclear function, which likely acts synergistically with myotendinous junction disorganisation in the development of A-EDMD, and the consequential reduction in force generation and muscle wasting.

Cell-specific and lamin-dependent targeting of novel transmembrane proteins in the nuclear envelope

Nuclear envelope complexity is expanding with respect to identification of protein components. Here we test the validity of proteomics results that identified 67 novel predicted nuclear envelope transmembrane proteins (NETs) from liver by directly comparing 30 as tagged fusions using targeting assays. This confirmed 21 as NETs, but 4 only targeted in certain cell types, underscoring the complexity of interactions that tether NETs to the nuclear envelope. Four NETs accumulated at the nuclear rim in normal fibroblasts but not in fibroblasts lacking lamin A, suggesting involvement of lamin A in tethering them in the nucleus. However, intriguingly, for the NETs tested alternative mechanisms for nuclear envelope retention could be found in Jurkat cells that normally lack lamin A. This study expands by a factor of three the number of liver NETs analyzed, bringing the total confirmed to 31, and shows that several have multiple mechanisms for nuclear envelope retention.

Overlapping functions of nuclear envelope proteins NET25 (Lem2) and emerin in regulation of extracellular signal-regulated kinase signaling in myoblast differentiation

Mutations in certain nuclear envelope (NE) proteins cause muscular dystrophies and other disorders, but the disease mechanisms remain unclear. The nuclear envelope transmembrane protein NET25 (Lem2) is a truncated paralog of MAN1, an NE component linked to bone disorders. NET25 and MAN1 share an approximately 40-residue LEM homology domain with emerin, the protein mutated in X-linked Emery-Dreifuss muscular dystrophy. However, roles for NET25 and MAN1 in myogenesis have not yet been described. Using RNA interference in C2C12 myoblasts, we show for the first time that both NET25 and MAN1 are required for myogenic differentiation. NET25 depletion causes hyperactivation of extracellular signal-regulated kinase 1/2 at the onset of differentiation, and pharmacological inhibition of this transient overactivation rescues myogenesis. In contrast, pharmacological inhibition of both mitogen-activated protein kinase and transforming growth factor beta signaling is required to rescue differentiation after MAN1 depletion. Ectopic expression of silencing-resistant NET25 rescues myogenesis after depletion of emerin but not after MAN1 silencing. Thus, NET25 and emerin have at least partially overlapping functions during myogenic differentiation, which are distinct from those of MAN1. Our work supports the hypothesis that deregulation of cell signaling contributes to NE-linked disorders and suggests that mutations in NET25 and MAN1 may cause muscle diseases.

A-type lamins and signaling: the PI 3-kinase/Akt pathway moves forward

Lamin A/C is a nuclear lamina constituent mutated in a number of human inherited disorders collectively referred to as laminopathies. The occurrence and significance of lamin A/C interplay with signaling molecules is an old question, suggested by pioneer studies performed in vitro. However, this relevant question has remained substantially unanswered, until data obtained in cellular and organismal models of laminopathies have indicated two main aspects of lamin A function. The first aspect is that lamins establish functional interactions with different protein platforms, the second aspect is that lamin A/C activity and altered function may elicit different effects in different cells and tissue types and even in different districts of the same tissue. Both these observations strongly suggest that signaling mechanisms targeting lamin A/C or its binding partners may regulate such a plastic behavior. A number of very recent data show involvement of kinases, as Akt and Erk, or phosphatases, as PP1 and PP2, in lamin A-linked cellular mechanisms. Moreover, altered activation of signaling in laminopathies and rescue of the pathological phenotype in animal models by inhibitors of signaling pathways, strongly suggest that signaling effectors related to lamin A/C may be implicated in the pathogenesis of laminopathies and may represent targets of therapeutic intervention. In face of such an open perspective of basic and applied research, we review current evidence of lamin A/C interplay with signaling molecules, with particular emphasis on the lamin A-Akt interaction and on the biological significance of their relationship.

Nuclear lamins: key regulators of nuclear structure and activities

The nuclear lamina is a proteinaceous structure located underneath the inner nuclear membrane (INM), where it associates with the peripheral chromatin. It contains lamins and lamin-associated proteins, including many integral proteins of the INM, chromatin modifying proteins, transcriptional repressors and structural proteins. A fraction of lamins is also present in the nucleoplasm, where it forms stable complexes and is associated with specific nucleoplasmic proteins. The lamins and their associated proteins are required for most nuclear activities, mitosis and for linking the nucleoplasm to all major cytoskeletal networks in the cytoplasm. Mutations in nuclear lamins and their associated proteins cause about 20 different diseases that are collectively called laminopathies'. This review concentrates mainly on lamins, their structure and their roles in DNA replication, chromatin organization, adult stem cell differentiation, aging, tumorogenesis and the lamin mutations leading to laminopathic diseases.

Cell cycle-dependent phosphorylation of MAN1

The LEM (LAP2beta, Emerin, and MAN1) proteins are essential for nuclear membrane targeting to chromatin via an association with barrier-to-autointegration factor (BAF). Herein, we focused on the mitotic phosphorylation of MAN1 and its biological role. MAN1 was phosphorylated in a cell cycle-dependent manner in the Xenopus egg cell-free system, and the mitotic phosphorylation at the N-terminal region of MAN1 suppressed the binding of MAN1 to BAF. Titansphere column chromatography followed by MS/MS sequencing identified at least three M-phase-specific phosphorylation sites, Thr-209, Ser-351, and Ser-402, and one cell cycle-independent phosphorylation site, Ser-463. An in vitro BAF binding assay involving mutants S402A and S402E suggested that the phosphorylation of Ser-402 was important for regulation of the binding of MAN1 to BAF.

The inner nuclear envelope as a transcription factor resting place

Just as people head to the beaches for a well-deserved rest, accumulating evidence suggests that transcription factors take similar 'vacations' at the nuclear envelope. Recent studies indicate that the periphery of the nucleus provides a platform for sequestering transcription factors away from chromatin. Several transcriptional regulators, operating in different signal-transduction pathways, have been found to interact physically with components of the inner nuclear membrane. In general, this association seems to restrict access to their target genes and limit their transactivation or transrepression abilities. The mechanisms of inner nuclear membrane association are diverse, and include regulated associations with the nuclear lamina and integral membrane proteins. Together, these findings indicate that the inside of the nuclear envelope functions as a resting place for transcription factors and suggest a more direct role for the nuclear envelope in gene regulation than previously anticipated.

Lamins and lamin-associated proteins in aging and disease

Lamins, together with the lamin-associated proteins of the inner nuclear membrane, are structural proteins in the nucleus that mediate mechanical stress resistance. Novel findings show that lamin complexes also have scaffolding functions in the formation and regulation of higher order chromatin and in epigenetic regulatory pathways. Furthermore, lamins serve as scavenging complexes and regulators of signaling molecules in diverse pathways. Lamin complexes in the nuclear interior contribute to retinoblastoma-mediated cell cycle regulation. Because of their multiple and diverse roles, lamins are linked to an increasing number of human diseases. The molecular mechanisms of these diseases, which are just beginning to emerge, may involve cell cycle and differentiation defects in adult stem cells and genomic instability.

What MAN1 does to the Smads

The inner nuclear membrane protein MAN1 has been identified as an important factor in transforming growth factor beta/bone morphogenic protein (TGFbeta/BMP) signaling. Loss of MAN1 results in three autosomal dominant diseases in humans; all three characterized by increased bone density. Xenopus embryos lacking MAN1 develop severe morphological defects. Both in humans and in Xenopus embryos the defects originate from deregulation of TGFbeta/BMP signaling. Several independent studies have shown that MAN1 is antagonizing TGFbeta/BMP signaling through binding to regulatory Smads. Here, recent progress in understanding MAN1 functions is summarized and a model for MAN1-dependent regulation of TGFbeta/BMP signaling is proposed.

Inner nuclear membrane proteins Asi1, Asi2, and Asi3 function in concert to maintain the latent properties of transcription factors Stp1 and Stp2

In yeast the homologous transcription factors Stp1 and Stp2 are synthesized as latent cytoplasmic precursors with N-terminal regulatory domains. In response to extracellular amino acids the regulatory domains are endoproteolytically excised by the plasma membrane-localized SPS sensor. The processed forms of Stp1 and Stp2 efficiently enter the nucleus and induce expression of amino acid permease genes. We recently reported that the inner nuclear membrane protein Asi1 is required to prevent unprocessed forms of Stp1 and Stp2, which ectopically enter the nucleus, from binding SPS sensor-regulated promoters. Here we show that Asi3, an Asi1 homolog, and Asi2 are integral proteins of the inner nuclear membrane that function in concert with Asi1. In cells lacking any of the three Asi proteins, unprocessed full-length forms of Stp1 and Stp2 constitutively induce SPS sensor-regulated genes. Our results demonstrate that the Asi proteins ensure the fidelity of SPS sensor signaling by maintaining the dormant, or repressed state, of gene expression in the absence of inducing signals. This study documents additional components of a novel mechanism controlling transcription in eukaryotic cells.

Nuclear envelope transmembrane proteins (NETs) that are up-regulated during myogenesis

BACKGROUND

The nuclear lamina is a protein meshwork lining the inner nuclear membrane, which contains a polymer of nuclear lamins associated with transmembrane proteins of the inner nuclear membrane. The lamina is involved in nuclear structure, gene expression, and association of the cytoplasmic cytoskeleton with the nucleus. We previously identified a group of 67 novel putative nuclear envelope transmembrane proteins (NETs) in a large-scale proteomics analysis. Because mutations in lamina proteins have been linked to several human diseases affecting skeletal muscle, we examined NET expression during differentiation of C2C12 myoblasts. Our goal was to identify new nuclear envelope and lamina components whose expression is coordinated with muscle differentiation.

RESULTS

Using transcriptional microarray analysis, we found that expression of 6 of the NETs significantly increases during myoblast differentiation. We confirmed these results using quantitative RT-PCR, and furthermore, found that all 6 NETs are expressed at high levels in adult mouse skeletal muscle relative to 9 other tissues examined. Using epitope-tagged cDNAs, we determined that the 5 NETs we could analyze (NETs 9, 25, 32, 37 and 39) all target to the nuclear envelope in C2C12 cells. Furthermore, the 3 NETs that we could analyze by immunoblotting were highly enriched in nuclear envelopes relative to microsomal membranes purified from mouse liver. Database searches showed that 4 of the 6 up-regulated NETs contain regions of homology to proteins previously linked to signaling.

CONCLUSION

This work identified 6 NETs that are predicted to have important functions in muscle development and/or maintenance from their expression patterns during myoblast differentiation and in mouse tissues. We confirmed that 5 of these NETs are authentic nuclear envelope proteins. Four members of this group have potential signaling functions at the NE, based on their sequence homologies.

Altered subcellular location of phosphorylated Smads in Alzheimer's disease

A number of growth factors and cytokines, such as transforming growth factor beta 1 (TGF-beta1), is elevated in Alzheimer's disease (AD), giving rise to activated intracellular mitogenic signaling cascades. Activated mitogenic signaling involving the mitogen-activated protein kinases (MAPKs) and other protein kinases might alter the phosphorylation states of structural proteins such as tau, resulting in hyperphosphorylated deposits. Many intracellular signaling proteins are potential targets of misregulated phosphorylation and dephosphorylation. Recently, a crosstalk between MAPKs and Smad proteins, both involved in mediating TGF-beta1 signaling, has been reported. Although TGF-beta1 has previously been shown to be involved in the pathogenesis of AD, the role of Smad proteins has not been investigated. In this study we thus analysed the subcellular distribution of phosphorylated Smad2 and Smad3 in the hippocampus of both normal and AD brains. Here we report on strong nuclear detection of phosphorylated Smad2 and Smad3 in neurons of control brains. In AD brains these phosphorylated proteins were additionally found in cytoplasmic granules in hippocampal neurons, within amyloid plaques and attached to neurofibrillary tangles. Our data suggest a critical role of Smad proteins in the pathogenesis of AD.