Multiple novel functions of lamina associated polypeptide 2α in striated muscle

Physalin B attenuates liver fibrosis via suppressing LAP2α-HDAC1-mediated deacetylation of the transcription factor GLI1 and hepatic stellate cell activation

BACKGROUND AND PURPOSE

Liver fibrosis is one of the leading causes of morbidity and mortality worldwide but lacks any acceptable therapy. The transcription factor glioma-associated oncogene homologue 1 (GLI1) is a potentially important therapeutic target in liver fibrosis. This study investigates the anti-fibrotic activities and potential mechanisms of the phytochemical, physalin B.

EXPERIMENTAL APPROACH

Two mouse models (CCl₄ challenge and bile duct ligation) were used to assess antifibrotic effects of physalin B in vivo. Mouse primary hepatic stellate cells (pHSCs) and human HSC line LX-2 also served as in vitro liver fibrosis models. Liver fibrogenic genes, GLI1 and GLI1 downstream genes were examined using Western blot and quantitative real-time PCR (qRT-PCR). GLI1 acetylation and LAP2α-HDAC1 interaction were analysed by co-immunoprecipitation.

KEY RESULTS

In vivo, physalin B administration attenuated hepatic histopathological injury and collagen accumulation and decreased expression of fibrogenic genes. Physalin B dose-dependently suppressed fibrotic marker expression in LX-2 cells and mouse pHSCs. Mechanistic studies showed that physalin B inhibited GLI activity by non-canonical Hedgehog signalling. Physalin B blocked formation of lamina-associated polypeptide 2α (LAP2α)/histone deacetylase 1 (HDAC1) complexes, thus inhibiting HDAC1-mediated GLI1 deacetylation. Physalin B up-regulated acetylation of GLI1, down-regulated expression of GLI1 and subsequently inhibited HSC activation.

CONCLUSION AND IMPLICATIONS

Physalin B exerted potent antifibrotic effects in vitro and in vivo by disrupting LAP2α/HDAC1 complexes, increasing GLI1 acetylation and inactivating GLI1. This indicates that the phytochemical physalin B may be a potential therapeutic candidate for the treatment of liver fibrosis.

Laminopathy-causing lamin A mutations reconfigure lamina-associated domains and local spatial chromatin conformation

The nuclear lamina contributes to the regulation of gene expression and to chromatin organization. Mutations in A-type nuclear lamins cause laminopathies, some of which are associated with a loss of heterochromatin at the nuclear periphery. Until recently however, little if any information has been provided on where and how lamin A interacts with the genome and on how disease-causing lamin A mutations may rearrange genome conformation. Here, we review aspects of nuclear lamin association with the genome. We highlight recent evidence of reorganization of lamin A-chromatin interactions in cellular models of laminopathies, and implications on the 3-dimensional rearrangement of chromatin in these models, including patient cells. We discuss how a hot-spot lipodystrophic lamin A mutation alters chromatin conformation and epigenetic patterns at an anti-adipogenic locus, and conclude with remarks on links between lamin A, Polycomb and the pathophysiology of laminopathies. The recent findings presented here collectively argue towards a deregulation of large-scale and local spatial genome organization by a subset of lamin A mutations causing laminopathies.

Chromatin dynamics governed by a set of nuclear structural proteins

During the past three decades, the study of nuclear and chromatin organization has become of great interest. The organization and dynamics of chromatin are directly responsible for many functions including gene regulation, genome replication, and maintenance. In order to better understand the details of these mechanisms, we need to understand the role of specific proteins that take part in these processes. The genome in the nucleus is organized in different length scales, ranging from the bead-like nucleosomes through topological associated domains up to chromosome territories. The mechanisms that maintain these structures, however, remain to be fully elucidated. Previous works highlighted the significance of lamin A, an important nucleoplasmic protein; however, there are other nuclear structural proteins that are also important for chromatin organization. Studying the organizational aspects of the nucleus is a complex task, and different methods have been developed and adopted for this purpose, including molecular and imaging methods. Here we describe the use of the live-cell imaging method and demonstrate that the dynamics of the nucleus is strongly related to its organizational mechanisms. We labeled different genomic sites in the nucleus and measured the effect of nuclear structural proteins on their dynamics. We studied lamin A, BAF, Emerin, lamin B, CTCF, and Cohesin and discuss how each of them affect chromatin dynamics. Our findings indicate that lamin A and BAF have a significant effect on chromosomes dynamics, while other proteins mildly affect the type of the diffusion while the volume of motion is not affected.

LAP2 Proteins Chaperone GLI1 Movement between the Lamina and Chromatin to Regulate Transcription

Understanding transcription factor navigation through the nucleus remains critical for developing targeted therapeutics. The GLI1 transcription factor must maintain maximal Hedgehog pathway output in basal cell carcinomas (BCCs), and we have previously shown that resistant BCCs increase GLI1 deacetylation through atypical protein kinase Cι/λ (aPKC) and HDAC1. Here we identify a lamina-associated polypeptide 2 (LAP2) isoform-dependent nuclear chaperoning system that regulates GLI1 movement between the nuclear lamina and nucleoplasm to achieve maximal activation. LAP2β forms a two-site interaction with the GLI1 zinc-finger domain and acetylation site, stabilizing an acetylation-dependent reserve on the inner nuclear membrane (INM). By contrast, the nucleoplasmic LAP2α competes with LAP2β for GLI1 while scaffolding HDAC1 to deacetylate the secondary binding site. aPKC functions to promote GLI1 association with LAP2α, promoting egress off the INM. GLI1 intranuclear trafficking by LAP2 isoforms represents a powerful signal amplifier in BCCs with implications for zinc finger-based signal transduction and therapeutics.

Physiological and Pathological Aging Affects Chromatin Dynamics, Structure and Function at the Nuclear Edge

Lamins are intermediate filaments that form a complex meshwork at the inner nuclear membrane. Mammalian cells express two types of Lamins, Lamins A/C and Lamins B, encoded by three different genes, LMNA, LMNB1, and LMNB2. Mutations in the LMNA gene are associated with a group of phenotypically diverse diseases referred to as laminopathies. Lamins interact with a large number of binding partners including proteins of the nuclear envelope but also chromatin-associated factors. Lamins not only constitute a scaffold for nuclear shape, rigidity and resistance to stress but also contribute to the organization of chromatin and chromosomal domains. We will discuss here the impact of A-type Lamins loss on alterations of chromatin organization and formation of chromatin domains and how disorganization of the lamina contributes to the patho-physiology of premature aging syndromes.

Expression of Leukemia-Associated Nup98 Fusion Proteins Generates an Aberrant Nuclear Envelope Phenotype

Chromosomal translocations involving the nucleoporin NUP98 have been described in several hematopoietic malignancies, in particular acute myeloid leukemia (AML). In the resulting chimeric proteins, Nup98's N-terminal region is fused to the C-terminal region of about 30 different partners, including homeodomain (HD) transcription factors. While transcriptional targets of distinct Nup98 chimeras related to immortalization are relatively well described, little is known about other potential cellular effects of these fusion proteins. By comparing the sub-nuclear localization of a large number of Nup98 fusions with HD and non-HD partners throughout the cell cycle we found that while all Nup98 chimeras were nuclear during interphase, only Nup98-HD fusion proteins exhibited a characteristic speckled appearance. During mitosis, only Nup98-HD fusions were concentrated on chromosomes. Despite the difference in localization, all tested Nup98 chimera provoked morphological alterations in the nuclear envelope (NE), in particular affecting the nuclear lamina and the lamina-associated polypeptide 2α (LAP2α). Importantly, such aberrations were not only observed in transiently transfected HeLa cells but also in mouse bone marrow cells immortalized by Nup98 fusions and in cells derived from leukemia patients harboring Nup98 fusions. Our findings unravel Nup98 fusion-associated NE alterations that may contribute to leukemogenesis.

Prepatterning of differentiation-driven nuclear lamin A/C-associated chromatin domains by GlcNAcylated histone H2B

Dynamic interactions of nuclear lamins with chromatin through lamin-associated domains (LADs) contribute to spatial arrangement of the genome. Here, we provide evidence for prepatterning of differentiation-driven formation of lamin A/C LADs by domains of histone H2B modified on serine 112 by the nutrient sensor O-linked N-acetylglucosamine (H2BS112GlcNAc), which we term GADs. We demonstrate a two-step process of lamin A/C LAD formation during in vitro adipogenesis, involving spreading of lamin A/C–chromatin interactions in the transition from progenitor cell proliferation to cell-cycle arrest, and genome-scale redistribution of these interactions through a process of LAD exchange within hours of adipogenic induction. Lamin A/C LADs are found both in active and repressive chromatin contexts that can be influenced by cell differentiation status. De novo formation of adipogenic lamin A/C LADs occurs nonrandomly on GADs, which consist of megabase-size intergenic and repressive chromatin domains. Accordingly, whereas predifferentiation lamin A/C LADs are gene-rich, post-differentiation LADs harbor repressive features reminiscent of lamin B1 LADs. Release of lamin A/C from genes directly involved in glycolysis concurs with their transcriptional up-regulation after adipogenic induction, and with downstream elevations in H2BS112GlcNAc levels and O-GlcNAc cycling. Our results unveil an epigenetic prepatterning of adipogenic LADs by GADs, suggesting a coupling of developmentally regulated lamin A/C-genome interactions to a metabolically sensitive chromatin modification.

Depletion of lamina-associated polypeptide 1 from cardiomyocytes causes cardiac dysfunction in mice

We previously showed that striated muscle-selective depletion of lamina-associated polypeptide 1 (LAP1), an integral inner nuclear membrane protein, leads to profound muscular dystrophy with premature death in mice. As LAP1 is also depleted in hearts of these mice, we examined their cardiac phenotype. Striated muscle-selective LAP1 knockout mice display ventricular systolic dysfunction with abnormal induction of genes encoding cardiomyopathy related proteins. To eliminate possible confounding effects due to skeletal muscle pathology, we generated a new mouse line in which LAP1 is deleted in a cardiomyocyte-selective manner. These mice had no skeletal muscle pathology and appeared overtly normal at 20 weeks of age. However, cardiac echocardiography revealed that they developed left ventricular systolic dysfunction and cardiac gene expression analysis revealed abnormal induction of cardiomyopathy-related genes. Our results demonstrate that LAP1 expression in cardiomyocytes is required for normal left ventricular function, consistent with a report of cardiomyopathy in a human subject with mutation in the gene encoding LAP1.

Lamina-associated polypeptide (LAP)2α and nucleoplasmic lamins in adult stem cell regulation and disease

A-type lamins are components of the lamina network at the nuclear envelope, which mediates nuclear stiffness and anchors chromatin to the nuclear periphery. However, A-type lamins are also found in the nuclear interior. Here we review the roles of the chromatin-associated, nucleoplasmic LEM protein, lamina-associated polypeptide 2α (LAP2α) in the regulation of A-type lamins in the nuclear interior. The lamin A/C-LAP2α complex may be involved in the regulation of the retinoblastoma protein-mediated pathway and other signaling pathways balancing proliferation and differentiation, and in the stabilization of higher-order chromatin organization throughout the nucleus. Loss of LAP2α in mice leads to selective depletion of the nucleoplasmic A-type lamin pool, promotes the proliferative stem cell phenotype of tissue progenitor cells, and delays stem cell differentiation. These findings support the hypothesis that LAP2α and nucleoplasmic lamins are regulators of adult stem cell function and tissue homeostasis. Finally, we discuss potential implications of this concept for defining the molecular disease mechanisms of lamin-linked diseases such as muscular dystrophy and premature aging syndromes.

Evolution: functional evolution of nuclear structure

The evolution of the nucleus, the defining feature of eukaryotic cells, was long shrouded in speculation and mystery. There is now strong evidence that nuclear pore complexes (NPCs) and nuclear membranes coevolved with the endomembrane system, and that the last eukaryotic common ancestor (LECA) had fully functional NPCs. Recent studies have identified many components of the nuclear envelope in living Opisthokonts, the eukaryotic supergroup that includes fungi and metazoan animals. These components include diverse chromatin-binding membrane proteins, and membrane proteins with adhesive lumenal domains that may have contributed to the evolution of nuclear membrane architecture. Further discoveries about the nucleoskeleton suggest that the evolution of nuclear structure was tightly coupled to genome partitioning during mitosis.

Lamin A/C protein is overexpressed in tissue-invading prostate cancer and promotes prostate cancer cell growth, migration and invasion through the PI3K/AKT/PTEN pathway

Prostate cancer (PC) remains the second most common cause of cancer-related death in Western countries. A previous proteomics study suggested that the nuclear membrane protein lamin A/C to be a maker to discriminate low- and high-Gleason score tumors and to identify high-risk cancers. To characterize its function in PC cells, we performed a detailed expression analysis in PC tissue and explored the consequences of down or upregulation of lamin A/C in PC cells. Our results confirm an increased lamin A/C protein expression in high-risk cancers and show association of expression with tumor cell formations at the invasion fronts of tumors and in invasion 'spearheading' tumor cell clusters. In the prostate tumor cell lines, LNCaP, DU145, and PC3 small hairpin RNA knockdown or overexpression of lamin A/C resulted in inhibition or stimulation, respectively, of cell growth, colony formation, migration and invasion. Further mechanism studies suggested that the lamin A/C-related malignant behavior is regulated through modulation of the phosphoinositide 3-kinase (PI3K)/AKT/PTEN signaling pathway. Western blot results indicated that knockdown or overexpression of lamin A/C decreased or increased, respectively, protein levels of the PI3K subunits p110 and p85 in all three cell lines; phosphor-AKT in the PTEN-negative cell lines LNCaP and PC3, and, increased or decreased, respectively, PTEN protein levels in PTEN-positive DU145 cells. Together, our data suggest that lamin A/C proteins are positively involved in malignant behavior of PC cells through the PI3K/AKT/PTEN pathway. Lamin A/C may represent a new oncogenic factor and a novel therapeutic target for PC.