Hepatocyte-Specific Deletion of Mouse Lamin A/C Leads to Male-Selective Steatohepatitis

LEMD2-associated progeroid syndrome: Expanding the phenotype of the nuclear envelopathy caused by a defect in LEMD2 gene

Nuclear envelopathies are rare genetic diseases that compromise the integrity of the nuclear envelope. Patients with a defect in LEM domain nuclear envelope protein 2 (LEMD2) leading to LEMD2-associated progeroid syndrome are exceedingly scarce in number, yet they exhibit shared clinical features including skeletal abnormalities and a prematurely-aged appearance. Our study broadens the understanding of LEMD2-associated progeroid syndrome by detailing its phenotypic and molecular characteristics in the first female and fourth reported case, highlighting a distinct impact on metabolic functions. The patient's history revealed growth delay, facial and skeletal abnormalities, and recurrent abdominal pain crises caused by hepatomegaly. Comparisons with the previously documented cases emphasized similarities in skeletal and facial features while showcasing unique variations, notably in cardiac and hepatic manifestations. In vitro experiments conducted on patient-derived peripheral blood and urinary epithelial cells and LEMD2-downregulated HepG2 cells confirmed abnormalities in the structure of the nuclear envelope in all three tissue-types. Overall, our work offers a comprehensive profile of a patient with LEMD2-related syndrome, emphasizing the hepatic involvement in the disease and broadening our understanding of clinical and molecular implications. This study not only contributes specific insights into LEMD2-related conditions but also underscores potential therapeutic paths for disorders affecting nuclear envelope dynamics.

Defective prelamin A processing promotes unconventional necroptosis driven by nuclear RIPK1

Defects in the prelamin A processing enzyme caused by loss-of-function mutations in the ZMPSTE24 gene are responsible for a spectrum of progeroid disorders characterized by the accumulation of farnesylated prelamin A. Here we report that defective prelamin A processing triggers nuclear RIPK1-dependent signalling that leads to necroptosis and inflammation. We show that accumulated prelamin A recruits RIPK1 to the nucleus to facilitate its activation upon tumour necrosis factor stimulation in ZMPSTE24-deficient cells. Kinase-activated RIPK1 then promotes RIPK3-mediated MLKL activation in the nucleus, leading to nuclear envelope disruption and necroptosis. This signalling relies on prelamin A farnesylation, which anchors prelamin A to nuclear envelope to serve as a nucleation platform for necroptosis. Genetic inactivation of necroptosis ameliorates the progeroid phenotypes in Zmpste24-/- mice. Our findings identify an unconventional nuclear necroptosis pathway resulting from ZMPSTE24 deficiency with pathogenic consequences in progeroid disorder and suggest RIPK1 as a feasible target for prelamin A-associated progeroid disorders.

Gene targeting in adult organs using in vivo cleavable donor plasmids for CRISPR-Cas9 and CRISPR-Cas12a

The CRISPR-Cas system for in vivo genome editing is a powerful tool for gene therapy against several diseases. We have previously developed the pCriMGET_9-12a system, an in vivo cleavable donor plasmid for precise targeted knock-in of exogenous DNA by both Cas9 and Cas12a. Here, we show that the pCriMGET_9-12a system can be applied for in vivo in-frame knock-in of exogenous DNA in adult mouse liver by hydrodynamic delivery of the targeting plasmids. The in vivo cleavable pCriMGET_9-12a donor plasmids significantly increased the knock-in efficiency of both CRISPR-Cas9 and CRISPR-Cas12a in the adult mouse liver compared to uncleavable donor plasmids. This strategy also achieved in-frame reporter gene knock-in without indel mutations. Therefore, in vivo gene targeting using the pCriMGET_9-12a system may contribute to the establishment of safer, more precise, versatile and efficient gene therapy methods in adult organs.

Lamin A/C mediates microglial activation by modulating cell proliferation and immune response

Lamin A/C is involved in macrophage activation and premature aging, also known as progeria. As the resident macrophage in brain, overactivation of microglia causes brain inflammation, promoting aging and brain disease. In this study, we investigated the role of Lamin A/C in microglial activation and its impact on progeria using Lmna-/- mice, primary microglia, Lmna knockout (Lmna-KO) and Lmna-knockdown (Lmna-KD) BV2 cell lines. We found that the microglial activation signatures, including cell proliferation, morphology changes, and proinflammatory cytokine secretion (IL-1β, IL-6, and TNF-α), were significantly suppressed in all Lamin A/C-deficient models when stimulated with LPS. TMT-based quantitative proteomic and bioinformatic analysis were further applied to explore the mechanism of Lamin A/C-regulated microglia activation from the proteome level. The results revealed that immune response and phagocytosis were impaired in Lmna-/- microglia. Stat1 was identified as the hub protein in the mechanism by which Lamin A/C regulates microglial activation. Additionally, DNA replication, chromatin organization, and mRNA processing were also altered by Lamin A/C, with Ki67 fulfilling the main hub function. Lamin A/C is a mechanosensitive protein and, the immune- and proliferation-related biological processes are also regulated by mechanotransduction. We speculate that Lamin A/C-mediated mechanotransduction is required for microglial activation. Our study proposes a novel mechanism for microglial activation mediated by Lamin A/C.

A common variant that alters SUN1 degradation associates with hepatic steatosis and metabolic traits in multiple cohorts

BACKGROUND & AIMS

Non-alcoholic fatty liver disease (NAFLD), and its progressive form steatohepatitis (NASH), represent a genetically and phenotypically diverse entity for which there is no approved therapy, making it imperative to define the spectrum of pathways contributing to its pathogenesis. Rare variants in genes encoding nuclear envelope proteins cause lipodystrophy with early-onset NAFLD/NASH; we hypothesized that common variants in nuclear envelope-related genes might also contribute to hepatic steatosis and NAFLD.

METHODS

Using hepatic steatosis as the outcome of interest, we performed an association meta-analysis of nuclear envelope-related coding variants in three large discovery cohorts (N >120,000 participants), followed by phenotype association studies in large validation cohorts (N >600,000) and functional testing of the top steatosis-associated variant in cell culture.

RESULTS

A common protein-coding variant, rs6461378 (SUN1 H118Y), was the top steatosis-associated variant in our association meta-analysis (p <0.001). In ancestrally distinct validation cohorts, rs6461378 associated with histologic NAFLD and with NAFLD-related metabolic traits including increased serum fatty acids, type 2 diabetes, hypertension, cardiovascular disease, and decreased HDL. SUN1 H118Y was subject to increased proteasomal degradation relative to wild-type SUN1 in cells, and SUN1 H118Y-expressing cells exhibited insulin resistance and increased lipid accumulation.

CONCLUSIONS

Collectively, these data support a potential causal role for the common SUN1 variant rs6461378 in NAFLD and metabolic disease.

IMPACT AND IMPLICATIONS

Non-alcoholic fatty liver disease (NAFLD), with an estimated global prevalence of nearly 30%, is a growing cause of morbidity and mortality for which there is no approved pharmacologic therapy. Our data provide a rationale for broadening current concepts of NAFLD genetics and pathophysiology to include the nuclear envelope, and particularly Sad1 and UNC84 domain containing 1 (SUN1), as novel contributors to this common liver disease. Furthermore, if future studies confirm causality of the common SUN1 H118Y variant, it has the potential to become a broadly relevant therapeutic target in NAFLD and metabolic disease.

Hepatocyte-specific loss of LAP2α protects against diet-induced hepatic steatosis, steatohepatitis, and fibrosis in male mice

There is increasing evidence for the importance of the nuclear envelope in lipid metabolism, nonalcoholic fatty liver disease (NAFLD), and nonalcoholic steatohepatitis (NASH). Human mutations in LMNA, encoding A-type nuclear lamins, cause early-onset insulin resistance and NASH, while hepatocyte-specific deletion of Lmna predisposes to NASH with fibrosis in male mice. Given that variants in the gene encoding LAP2α, a nuclear protein that regulates lamin A/C, were previously identified in patients with NAFLD, we sought to determine the role of LAP2α in NAFLD using a mouse genetic model. Hepatocyte-specific Lap2α-knockout (Lap2α(ΔHep)) mice and littermate controls were fed normal chow or high-fat diet (HFD) for 8 wk or 6 mo. Unexpectedly, male Lap2α(ΔHep) mice showed no increase in hepatic steatosis or NASH compared with controls. Rather, Lap2α(ΔHep) mice demonstrated reduced hepatic steatosis, with decreased NASH and fibrosis after long-term HFD. Accordingly, pro-steatotic genes including Cidea, Mogat1, and Cd36 were downregulated in Lap2α(ΔHep) mice, along with concomitant decreases in expression of pro-inflammatory and pro-fibrotic genes. These data indicate that hepatocyte-specific Lap2α deletion protects against hepatic steatosis and NASH in mice and raise the possibility that LAP2α could become a potential therapeutic target in human NASH.NEW & NOTEWORTHY The nuclear envelope and lamina regulate lipid metabolism and susceptibility to nonalcoholic steatohepatitis (NASH), but the role of the nuclear lamin-binding protein LAP2α in NASH has not been explored. Our data demonstrate that hepatocyte-specific loss of LAP2α protects against diet-induced hepatic steatosis, NASH, and fibrosis in male mice, with downregulation of pro-steatotic, pro-inflammatory, and pro-fibrotic lamin-regulated genes. These findings suggest that targeting LAP2α could have future potential as a novel therapeutic avenue in NASH.

N-Methyl Protoporphyrin IX: An Understudied Porphyrin

N-Methyl protoporphyrin IX (NmePPIX) is a derivative of protoporphyrin IX (PPIX) and the lattice of heme. Certain xenobiotics strongly induce NmePPIX production in the liver. The existence of endogenous NmePPIX in untreated animal liver has also been reported. The detailed mechanisms of NmePPIX biosynthesis remain unclear, but cytochrome P450 enzymes are thought to be critical in xenobiotic-induced NmePPIX production. High levels of NmePPIX cause PPIX accumulation because NmePPIX is a potent inhibitor (K_i = 7 nM) of ferrochelatase, the last enzyme in the heme biosynthesis pathway that converts PPIX to heme. NmePPIX is also involved in several other physiological processes, including inhibition of nitric oxide production and promotion of lamin aggregation. Compared to the two well-characterized porphyrins, PPIX and heme, NmePPIX is understudied regarding the mechanism of formation, fate, and physiological functions. This Review summarizes the current understanding of NmePPIX and provides perspectives on areas of future research on NmePPIX.

Redistribution of lamina-associated domains reshapes binding of pioneer factor FOXA2 in development of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is highly prevalent in type 2 diabetes mellitus and the elderly, impacting 40% of individuals over 70. Regulation of heterochromatin at the nuclear lamina has been associated with aging and age-dependent metabolic changes. We previously showed that changes at the lamina in aged hepatocytes and laminopathy models lead to redistribution of lamina-associated domains (LADs), opening of repressed chromatin, and up-regulation of genes regulating lipid synthesis and storage, culminating in fatty liver. Here, we test the hypothesis that change in the expression of lamina-associated proteins and nuclear shape leads to redistribution of LADs, followed by altered binding of pioneer factor FOXA2 and by up-regulation of lipid synthesis and storage, culminating in steatosis in younger NAFLD patients (aged 21-51). Changes in nuclear morphology alter LAD partitioning and reduced lamin B1 signal correlate with increased FOXA2 binding before severe steatosis in young mice placed on a western diet. Nuclear shape is also changed in younger NAFLD patients. LADs are redistrubted and lamin B1 signal decreases similarly in mild and severe steatosis. In contrast, FOXA2 binding is similar in normal and NAFLD patients with moderate steatosis and is repositioned only in NAFLD patients with more severe lipid accumulation. Hence, changes at the nuclear lamina reshape FOXA2 binding with progression of the disease. Our results suggest a role for nuclear lamina in etiology of NAFLD, irrespective of aging, with potential for improved stratification of patients and novel treatments aimed at restoring nuclear lamina function.

Hepatocytes Deficient in Nuclear Envelope Protein Lamina-associated Polypeptide 1 are an Ideal Mammalian System to Study Intranuclear Lipid Droplets

Lipid droplets (LDs) are generally considered to be synthesized in the ER and utilized in the cytoplasm. However, LDs have been observed inside nuclei in some cells, although recent research on nuclear LDs has focused on cultured cell lines. To better understand nuclear LDs that occur in vivo, here we examined LDs in primary hepatocytes from mice following depletion of the nuclear envelope protein lamina-associated polypeptide 1 (LAP1). Microscopic image analysis showed that LAP1-depleted hepatocytes contain frequent nuclear LDs, which differ from cytoplasmic LDs in their associated proteins. We found type 1 nucleoplasmic reticula, which are invaginations of the inner nuclear membrane, are often associated with nuclear LDs in these hepatocytes. Furthermore, in vivo depletion of the nuclear envelope proteins lamin A and C from mouse hepatocytes led to severely abnormal nuclear morphology, but significantly fewer nuclear LDs than were observed upon depletion of LAP1. In addition, we show both high-fat diet feeding and fasting of mice increased cytoplasmic lipids in LAP1-depleted hepatocytes but reduced nuclear LDs, demonstrating a relationship of LD formation with nutritional state. Finally, depletion of microsomal triglyceride transfer protein did not change the frequency of nuclear LDs in LAP1-depleted hepatocytes, suggesting that it is not required for the biogenesis of nuclear LDs in these cells. Together, these data show that LAP1-depleted hepatocytes represent an ideal mammalian system to investigate the biogenesis of nuclear LDs and their partitioning between the nucleus and cytoplasm in response to changes in nutritional state and cellular metabolism in vivo.

Cytoskeleton alterations in non-alcoholic fatty liver disease

BACKGROUND

Due to its extremely high prevalence and severity, non-alcoholic fatty liver disease (NALFD) is a serious health and economic concern worldwide. Developing effective methods of diagnosis and therapy demands a deeper understanding of its molecular basis. One of the strategies in such an endeavor is the analysis of alterations in the morphology of liver cells. Such alterations, widely reported in NAFLD patients and disease models, are related to the cytoskeleton. Therefore, the fate of the cytoskeleton components is useful to uncover the molecular basis of NAFLD, to further design innovative approaches for its diagnosis and therapy.

MAIN FINDINGS

Several cytoskeleton proteins are up-regulated in liver cells of NAFLD patients. Under pathological conditions, keratin 18 is released from hepatocytes and its detection in the blood emerges as a non-invasive diagnosis tool. α-Smooth muscle actin is up-regulated in hepatic stellate cells and its down-regulation has been widely tested as a potential NALFD therapeutic approach. Other cytoskeleton proteins, such as vimentin, are also up-regulated.

CONCLUSIONS

NAFLD progression involves alterations in expression levels of proteins that build the liver cytoskeleton or associate with it. These findings provide a timely opportunity of developing novel approaches for NAFLD diagnosis and therapy.

Regulation of Lipid Metabolism by Lamin in Mutation-Related Diseases

Nuclear lamins, known as type 5 intermediate fibers, are composed of lamin A, lamin C, lamin B1, and lamin B2, which are encoded by LMNA and LMNB genes, respectively. Importantly, mutations in nuclear lamins not only participate in lipid disorders but also in the human diseases, such as lipodystrophy, metabolic-associated fatty liver disease, and dilated cardiomyopathy. Among those diseases, the mechanism of lamin has been widely discussed. Thereby, this review mainly focuses on the regulatory mechanism of the mutations in the lamin gene in lipid alterations and the human diseases. Considering the protean actions, targeting nuclear lamins may be a potent therapeutic avenue for lipid metabolic disorders and human diseases in the future.

Diurnal shift of mouse activity by the deficiency of an aging-related gene Lmna

Nuclear lamina is a fundamental structure of the cell nucleus and regulates a wide range of molecular pathways. Defects of components of the nuclear lamina cause aging-like physiological disorders, called laminopathy. Generally, aging and diseases are often associated with perturbation of various time-of-day-dependent regulations, but it remains still elusive whether laminopathy induces any changes of the circadian clock and physiological rhythms. Here we demonstrated that deficiency of Lmna gene in mice caused an obvious shift of locomotor activities to the daytime. The abnormal activity profile was accompanied by a remarkable change in phase-angle between the central clock in the suprachiasmatic nucleus (SCN) and lung peripheral clocks, leaving the phase of the SCN clock unaffected by the mutation. These observations suggest that Lmna deficiency causes a change of the habitat from nocturnal to diurnal behaviors. On the other hand, molecular oscillation and its phase resetting mechanism were intact in both the Lmna-deficient cells and progeria-mimicking cells. Intriguingly, high-fat diet feeding extended the short lifespan and ameliorated the abnormalities of the behaviors and the phase of the peripheral clock in the Lmna-deficient mice. The present study supports the important contribution of the energy conditions to a shift between the diurnal and nocturnal activities.

Genomic loci mispositioning in Tmem120a knockout mice yields latent lipodystrophy

Little is known about how the observed fat-specific pattern of 3D-spatial genome organisation is established. Here we report that adipocyte-specific knockout of the gene encoding nuclear envelope transmembrane protein Tmem120a disrupts fat genome organisation, thus causing a lipodystrophy syndrome. Tmem120a deficiency broadly suppresses lipid metabolism pathway gene expression and induces myogenic gene expression by repositioning genes, enhancers and miRNA-encoding loci between the nuclear periphery and interior. Tmem120a-/- mice, particularly females, exhibit a lipodystrophy syndrome similar to human familial partial lipodystrophy FPLD2, with profound insulin resistance and metabolic defects that manifest upon exposure to an obesogenic diet. Interestingly, similar genome organisation defects occurred in cells from FPLD2 patients that harbour nuclear envelope protein encoding LMNA mutations. Our data indicate TMEM120A genome organisation functions affect many adipose functions and its loss may yield adiposity spectrum disorders, including a miRNA-based mechanism that could explain muscle hypertrophy in human lipodystrophy.

Disrupting the LINC complex by AAV mediated gene transduction prevents progression of Lamin induced cardiomyopathy

Mutations in the LaminA gene are a common cause of monogenic dilated cardiomyopathy. Here we show that mice with a cardiomyocyte-specific Lmna deletion develop cardiac failure and die within 3-4 weeks after inducing the mutation. When the same Lmna mutations are induced in mice genetically deficient in the LINC complex protein SUN1, life is extended to more than one year. Disruption of SUN1's function is also accomplished by transducing and expressing a dominant-negative SUN1 miniprotein in Lmna deficient cardiomyocytes, using the cardiotrophic Adeno Associated Viral Vector 9. The SUN1 miniprotein disrupts binding between the endogenous LINC complex SUN and KASH domains, displacing the cardiomyocyte KASH complexes from the nuclear periphery, resulting in at least a fivefold extension in lifespan. Cardiomyocyte-specific expression of the SUN1 miniprotein prevents cardiomyopathy progression, potentially avoiding the necessity of developing a specific therapeutic tailored to treating each different LMNA cardiomyopathy-inducing mutation of which there are more than 450.

The Nuclear Envelope in Lipid Metabolism and Pathogenesis of NAFLD

Simple Summary

The liver is a major organ regulating lipid metabolism and a proper liver function is essential to health. Nonalcoholic fatty liver disease (NAFLD) is a condition with abnormal fat accumulation in the liver without heavy alcohol use. NAFLD is becoming one of the most common liver diseases with the increase in obesity in many parts of the world. There is no approved cure for the disease and a better understanding of disease mechanism is needed for effective prevention and treatment. The nuclear envelope, a membranous structure that surrounds the cell nucleus, is connected to the endoplasmic reticulum where the vast majority of cellular lipids are synthesized. Growing evidence indicates that components in the nuclear envelope are involved in cellular lipid metabolism. We review published studies with various cell and animal models, indicating the essential roles of nuclear envelope proteins in lipid metabolism. We also discuss how defects in these proteins affect cellular lipid metabolism and possibly contribute to the pathogenesis of NAFLD.

Abstract

Nonalcoholic fatty liver disease (NAFLD) is a burgeoning public health problem worldwide. Despite its tremendous significance for public health, we lack a comprehensive understanding of the pathogenic mechanisms of NAFLD and its more advanced stage, nonalcoholic steatohepatitis (NASH). Identification of novel pathways or cellular mechanisms that regulate liver lipid metabolism has profound implications for the understanding of the pathology of NAFLD and NASH. The nuclear envelope is topologically connected to the ER, where protein synthesis and lipid synthesis occurs. Emerging evidence points toward that the nuclear lamins and nuclear membrane-associated proteins are involved in lipid metabolism and homeostasis. We review published reports that link these nuclear envelope proteins to lipid metabolism. In particular, we focus on the recent work demonstrating the essential roles for the nuclear envelope-localized torsinA/lamina-associated polypeptide (LAP1) complex in hepatic steatosis, lipid secretion, and NASH development. We also discuss plausible pathogenic mechanisms by which the loss of either protein in hepatocytes leads to hepatic dyslipidemia and NASH development.

Genotype-phenotype analysis of LMNA-related diseases predicts phenotype-selective alterations in lamin phosphorylation

Laminopathies are rare diseases associated with mutations in LMNA, which encodes nuclear lamin A/C. LMNA variants lead to diverse tissue-specific phenotypes including cardiomyopathy, lipodystrophy, myopathy, neuropathy, progeria, bone/skin disorders, and overlap syndromes. The mechanisms underlying these heterogeneous phenotypes remain poorly understood, although post-translational modifications, including phosphorylation, are postulated as regulators of lamin function. We catalogued all known lamin A/C human mutations and their associated phenotypes, and systematically examined the putative role of phosphorylation in laminopathies. In silico prediction of specific LMNA mutant-driven changes to lamin A phosphorylation and protein structure was performed using machine learning methods. Some of the predictions we generated were validated via assessment of ectopically expressed wild-type and mutant LMNA. Our findings indicate phenotype- and mutant-specific alterations in lamin phosphorylation, and that some changes in phosphorylation may occur independently of predicted changes in lamin protein structure. Therefore, therapeutic targeting of phosphorylation in the context of laminopathies will likely require mutant- and kinase-specific approaches.

The Laminopathies and the Insights They Provide into the Structural and Functional Organization of the Nucleus

In recent years, our perspective on the cell nucleus has evolved from the view that it is a passive but permeable storage organelle housing the cell's genetic material to an understanding that it is in fact a highly organized, integrative, and dynamic regulatory hub. In particular, the subcompartment at the nuclear periphery, comprising the nuclear envelope and the underlying lamina, is now known to be a critical nexus in the regulation of chromatin organization, transcriptional output, biochemical and mechanosignaling pathways, and, more recently, cytoskeletal organization. We review the various functional roles of the nuclear periphery and their deregulation in diseases of the nuclear envelope, specifically the laminopathies, which, despite their rarity, provide insights into contemporary health-care issues.

Nuclear envelope-localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis

Deciphering novel pathways regulating liver lipid content has profound implications for understanding the pathophysiology of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Recent evidence suggests that the nuclear envelope is a site of regulation of lipid metabolism but there is limited appreciation of the responsible mechanisms and molecular components within this organelle. We showed that conditional hepatocyte deletion of the inner nuclear membrane protein lamina-associated polypeptide 1 (LAP1) caused defective VLDL secretion and steatosis, including intranuclear lipid accumulation. LAP1 binds to and activates torsinA, an AAA+ ATPase that resides in the perinuclear space and continuous main ER. Deletion of torsinA from mouse hepatocytes caused even greater reductions in VLDL secretion and profound steatosis. Both of these mutant mouse lines developed hepatic steatosis and subsequent steatohepatitis on a regular chow diet in the absence of whole-body insulin resistance or obesity. Our results establish an essential role for the nuclear envelope-localized torsinA-LAP1 complex in hepatic VLDL secretion and suggest that the torsinA pathway participates in the pathophysiology of nonalcoholic fatty liver disease.

Fatty liver in lipodystrophy: A review with a focus on therapeutic perspectives of adiponectin and/or leptin replacement

Lipodystrophy is a group of clinically heterogeneous, inherited or acquired, disorders characterized by complete or partial absence of subcutaneous adipose tissue that may occur simultaneously with the pathological, ectopic, accumulation of fat in other regions of the body, including the liver. Fatty liver adds significantly to hepatic and extra-hepatic morbidity in patients with lipodystrophy. Lipodystrophy is strongly associated with severe insulin resistance and related comorbidities, such as hyperglycemia, hyperlipidemia and nonalcoholic fatty liver disease (NAFLD), but other hepatic diseases may co-exist in some types of lipodystrophy, including autoimmune hepatitis in acquired lipodystrophies, or viral hepatitis in human immunodeficiency virus (HIV)-associated lipodystrophy. The aim of this review is to summarize evidence linking lipodystrophy with hepatic disease and to provide a special focus on potential therapeutic perspectives of leptin replacement therapy and adiponectin upregulation in lipodystrophy.

Senescent Hepatocytes in Decompensated Liver Show Reduced UPRMT and Its Key Player, CLPP, Attenuates Senescence In Vitro

BACKGROUND AND AIMS

Non-dividing hepatocytes in end-stage liver disease indicates permanent growth arrest similar to senescence. Identifying senescence in vivo is often challenging and mechanisms inhibiting senescence are poorly understood. In lower organisms mitochondrial unfolded protein response (UPR^MT) helps in increasing longevity; however, its role in senescence and liver disease is poorly understood. Aim of this study was to identify hepatocyte senescence and the role of UPR^MT in cryptogenic cirrhosis.

METHODS

Doxorubicin was used to induce senescence in non-neoplastic hepatocytes (PH5CH8) and hepatoma cells (HepG2 and Huh7). Senescence-associated markers and unfolded protein response was evaluated by fluorescence microscopy, immunoblotting and gene expression. Explants/biopsies from normal, fibrosis, compensated and decompensated cirrhosis without any known etiology were examined for presence of senescence and UPR^MT by immunohistochemistry and gene expression.

RESULTS

Accumulation of senescent hepatocytes in cryptogenic cirrhosis was associated with reduced proliferation, increased expression of γH2AX and p21, together with loss of LaminB1. Dysfunctional mitochondria and compromised UPR^MT were key features of senescent hepatocytes both in vitro and also in decompensated cirrhosis. Intriguingly, compensated cirrhotic liver mounted strong UPR^MT, with high levels of mitochondrial protease, CLPP. Overexpression of CLPP inhibited senescence in vitro, by reducing mitochondrial ROS and altering oxygen consumption.

CONCLUSIONS

Our results implicate a role of hepatocyte senescence in cryptogenic cirrhosis together with a crucial role of UPR^MT in preventing hepatocyte senescence. A compromised UPR^MT may shift the fate of cirrhotic liver toward decompensation by exaggerating hepatocyte senescence. Restoring CLPP levels at least in cell culture appears as a promising strategy in mitohormesis, thereby, preventing senescence and possibly improving hepatocyte function.

Changes at the nuclear lamina alter binding of pioneer factor Foxa2 in aged liver

Increasing evidence suggests that regulation of heterochromatin at the nuclear envelope underlies metabolic disease susceptibility and age-dependent metabolic changes, but the mechanism is unknown. Here, we profile lamina-associated domains (LADs) using lamin B1 ChIP-Seq in young and old hepatocytes and find that, although lamin B1 resides at a large fraction of domains at both ages, a third of lamin B1-associated regions are bound exclusively at each age in vivo. Regions occupied by lamin B1 solely in young livers are enriched for the forkhead motif, bound by Foxa pioneer factors. We also show that Foxa2 binds more sites in Zmpste24 mutant mice, a progeroid laminopathy model, similar to increased Foxa2 occupancy in old livers. Aged and Zmpste24-deficient livers share several features, including nuclear lamina abnormalities, increased Foxa2 binding, de-repression of PPAR- and LXR-dependent gene expression, and fatty liver. In old livers, additional Foxa2 binding is correlated to loss of lamin B1 and heterochromatin (H3K9me3 occupancy) at these loci. Our observations suggest that changes at the nuclear lamina are linked to altered Foxa2 binding, enabling opening of chromatin and de-repression of genes encoding lipid synthesis and storage targets that contribute to etiology of hepatic steatosis.

Lamin A/C Maintains Exocrine Pancreas Homeostasis by Regulating Stability of RB and Activity of E2F

Lamins have important roles in nuclear structure and cell signaling. Several diseases are associated with mutations in the lamin A/C gene (LMNA in humans). Patients with familial partial lipodystrophy caused by LMNA mutations develop pancreatitis, but lamin function in the pancreas and how these mutations affect pancreatic regulation are unknown. We generated mice with inducible exocrine pancreas-specific disruption of Lmna and showed that LMNA is lost from most exocrine pancreas cells. LMNA-knockout pancreata develop endoplasmic reticulum stress with loss of acinar cell markers, increased autophagy, apoptosis, and cell proliferation, compared to CreERT2^- mice (littermate controls). Disruption of Lmna led to a phenotype that resembled chronic pancreatitis, with increased Sirius Red staining and α-smooth muscle actin in male LMNA-knockout mice compared to littermate males, but not in female mice. LMNA-knockout pancreata have reduced levels of RB and activation of E2F, based on increased expression of E2F target genes. Therefore, lamins maintain pancreatic homeostasis by regulating RB stability and E2F activity.

Lamins and Lamin-Associated Proteins in Gastrointestinal Health and Disease

The nuclear lamina is a multi-protein lattice composed of A- and B-type lamins and their associated proteins. This protein lattice associates with heterochromatin and integral inner nuclear membrane proteins, providing links among the genome, nucleoskeleton, and cytoskeleton. In the 1990s, mutations in EMD and LMNA were linked to Emery-Dreifuss muscular dystrophy. Since then, the number of diseases attributed to nuclear lamina defects, including laminopathies and other disorders, has increased to include more than 20 distinct genetic syndromes. Studies of patients and mouse genetic models have pointed to important roles for lamins and their associated proteins in the function of gastrointestinal organs, including liver and pancreas. We review the interactions and functions of the lamina in relation to the nuclear envelope and genome, the ways in which its dysfunction is thought to contribute to human disease, and possible avenues for targeted therapies.

Nuclear lamina genetic variants, including a truncated LAP2, in twins and siblings with nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is becoming the major chronic liver disease in many countries. Its pathogenesis is multifactorial, but twin and familial studies indicate significant heritability, which is not fully explained by currently known genetic susceptibility loci. Notably, mutations in genes encoding nuclear lamina proteins, including lamins, cause lipodystrophy syndromes that include NAFLD. We hypothesized that variants in lamina-associated proteins predispose to NAFLD and used a candidate gene-sequencing approach to test for variants in 10 nuclear lamina-related genes in a cohort of 37 twin and sibling pairs: 21 individuals with and 53 without NAFLD. Twelve heterozygous sequence variants were identified in four lamina-related genes (ZMPSTE24, TMPO, SREBF1, SREBF2). The majority of NAFLD patients (>90%) had at least one variant compared to <40% of controls (P < 0.0001). When only insertions/deletions and changes in conserved residues were considered, the difference between the groups was similarly striking (>80% versus <25%; P < 0.0001). Presence of a lamina variant segregated with NAFLD independently of the PNPLA3 I148M polymorphism. Several variants were found in TMPO, which encodes the lamina-associated polypeptide-2 (LAP2) that has not been associated with liver disease. One of these, a frameshift insertion that generates truncated LAP2, abrogated lamin-LAP2 binding, caused LAP2 mislocalization, altered endogenous lamin distribution, increased lipid droplet accumulation after oleic acid treatment in transfected cells, and led to cytoplasmic association with the ubiquitin-binding protein p62/SQSTM1.

CONCLUSION

Several variants in nuclear lamina-related genes were identified in a cohort of twins and siblings with NAFLD; one such variant, which results in a truncated LAP2 protein and a dramatic phenotype in cell culture, represents an association of TMPO/LAP2 variants with NAFLD and underscores the potential importance of the nuclear lamina in NAFLD. (Hepatology 2018;67:1710-1725).

Potential association of LMNA-associated generalized lipodystrophy with juvenile dermatomyositis

Background

Juvenile dermatomyositis (JDM) is an auto-immune muscle disease which presents with skin manifestations and muscle weakness. At least 10% of the patients with JDM present with acquired lipodystrophy. Laminopathies are caused by mutations in the lamin genes and cover a wide spectrum of diseases including muscular dystrophies and lipodystrophy. The p.T10I LMNA variant is associated with a phenotype of generalized lipodystrophy that has also been called atypical progeroid syndrome.

Case presentation

A previously healthy female presented with bilateral proximal lower extremity muscle weakness at age 4. She was diagnosed with JDM based on her clinical presentation, laboratory tests and magnetic resonance imaging (MRI). She had subcutaneous fat loss which started in her extremities and progressed to her whole body. At age 7, she had diabetes, hypertriglyceridemia, low leptin levels and low body fat on dual energy X-ray absorptiometry (DEXA) scan, and was diagnosed with acquired generalized lipodystrophy (AGL). Whole exome sequencing (WES) revealed a heterozygous c.29C > T; p.T10I missense pathogenic variant in LMNA, which encodes lamins A and C. Muscle biopsy confirmed JDM rather than muscular dystrophy, showing perifascicular atrophy and perivascular mononuclear cell infiltration. Immunofluroscence of skin fibroblasts confirmed nuclear atypia and fragmentation.

Conclusions

This is a unique case with p.T10I LMNA variant displaying concurrent JDM and AGL. This co-occurrence raises the intriguing possibility that LMNA, and possibly p.T10I, may have a pathogenic role in not only the occurrence of generalized lipodystrophy, but also juvenile dermatomyositis. Careful phenotypic characterization of additional patients with laminopathies as well as individuals with JDM is warranted.