Eliminating the synthesis of mature lamin A reduces disease phenotypes in mice carrying a Hutchinson-Gilford progeria syndrome allele

Cardiovascular Progerin Suppression and Lamin A Restoration Rescue Hutchinson-Gilford Progeria Syndrome

BACKGROUND

Hutchinson-Gilford progeria syndrome (HGPS) is a rare disorder characterized by premature aging and death mainly because of myocardial infarction, stroke, or heart failure. The disease is provoked by progerin, a variant of lamin A expressed in most differentiated cells. Patients look healthy at birth, and symptoms typically emerge in the first or second year of life. Assessing the reversibility of progerin-induced damage and the relative contribution of specific cell types is critical to determining the potential benefits of late treatment and to developing new therapies.

METHODS

We used CRISPR-Cas9 technology to generate Lmna^{HGPSrev/HGPSrev} (HGPSrev) mice engineered to ubiquitously express progerin while lacking lamin A and allowing progerin suppression and lamin A restoration in a time- and cell type-specific manner on Cre recombinase activation. We characterized the phenotype of HGPSrev mice and crossed them with Cre transgenic lines to assess the effects of suppressing progerin and restoring lamin A ubiquitously at different disease stages as well as specifically in vascular smooth muscle cells and cardiomyocytes.

RESULTS

Like patients with HGPS, HGPSrev mice appear healthy at birth and progressively develop HGPS symptoms, including failure to thrive, lipodystrophy, vascular smooth muscle cell loss, vascular fibrosis, electrocardiographic anomalies, and precocious death (median lifespan of 15 months versus 26 months in wild-type controls, P<0.0001). Ubiquitous progerin suppression and lamin A restoration significantly extended lifespan when induced in 6-month-old mildly symptomatic mice and even in severely ill animals aged 13 months, although the benefit was much more pronounced on early intervention (84.5% lifespan extension in mildly symptomatic mice, P<0.0001, and 6.7% in severely ill mice, P<0.01). It is remarkable that major vascular alterations were prevented and lifespan normalized in HGPSrev mice when progerin suppression and lamin A restoration were restricted to vascular smooth muscle cells and cardiomyocytes.

CONCLUSIONS

HGPSrev mice constitute a new experimental model for advancing knowledge of HGPS. Our findings suggest that it is never too late to treat HGPS, although benefit is much more pronounced when progerin is targeted in mice with mild symptoms. Despite the broad expression pattern of progerin and its deleterious effects in many organs, restricting its suppression to vascular smooth muscle cells and cardiomyocytes is sufficient to prevent vascular disease and normalize lifespan.

Preclinical Advances of Therapies for Laminopathies

Laminopathies are a group of rare disorders due to mutation in LMNA gene. Depending on the mutation, they may affect striated muscles, adipose tissues, nerves or are multisystemic with various accelerated ageing syndromes. Although the diverse pathomechanisms responsible for laminopathies are not fully understood, several therapeutic approaches have been evaluated in patient cells or animal models, ranging from gene therapies to cell and drug therapies. This review is focused on these therapies with a strong focus on striated muscle laminopathies and premature ageing syndromes.

Hutchinson-Gilford Progeria Syndrome: An Overview of the Molecular Mechanism, Pathophysiology and Therapeutic Approach

Lamin A/C encoded by the LMNA gene is an essential component for maintaining the nuclear structure. Mutation in the lamin A/C leads to a group of inherited disorders is known as laminopathies. In the human body, there are several mutations in the LMNA gene that have been identified. It can affect diverse organs or tissues or can be systemic, causing different diseases. In this review, we mainly focused on one of the most severe laminopathies, Hutchinson-Gilford progeria syndrome (HGPS). HGPS is an immensely uncommon, deadly, metameric ill-timed laminopathies caused by the abnormal splicing of the LMNA gene and production of an aberrant protein known as progerin. Here, we also presented the currently available data on the molecular mechanism, pathophysiology, available treatment, and future approaches to this deadly disease. Due to the production of progerin, an abnormal protein leads to an abnormality in nuclear structure, defects in DNA repair, shortening of telomere, and impairment in gene regulation which ultimately results in aging in the early stage of life. Now some treatment options are available for this disease, but a proper understanding of the molecular mechanism of this disease will help to develop a more appropriate treatment which makes it an emerging area of research.

Potential Benefits of Allogeneic Haploidentical Adipose Tissue-Derived Stromal Vascular Fraction in a Hutchinson-Gilford Progeria Syndrome Patient

Hutchinson-Gilford progeria syndrome (HGPS) is a rare, fatal, and genetic disorder in the LMNA gene encoding for prelamin A. Normally, prelamin A is processed to become lamin A protein. In HGPS patients, there is a heterozygous mutation in LMNA gene, in which there is a deletion of genetic codes responsible for 50 amino acids at the C-terminus of prelamin A. The processing of the abnormal prelamin A results in abnormal lamin A protein, called progerin, causing symptoms of accelerated early aging, probably due to the inflammaging process. It is well known that adipose tissue-derived mesenchymal stem cells (MSCs) have anti-inflammatory effects by modulating inflammatory cytokines and by extracellular vesicles. Here, we present a case of an HGPS patient who responded positively to injections of allogeneic haploidentical adipose tissue-derived stromal vascular fractions containing MSCs by showing rapid height and weight growth along with increased blood level of insulin-like growth factor 1.

Progerin accelerates atherosclerosis by inducing endoplasmic reticulum stress in vascular smooth muscle cells

Hutchinson–Gilford progeria syndrome (HGPS) is a rare genetic disorder caused by progerin, a mutant lamin A variant. HGPS patients display accelerated aging and die prematurely, typically from atherosclerosis complications. Recently, we demonstrated that progerin‐driven vascular smooth muscle cell (VSMC) loss accelerates atherosclerosis leading to premature death in apolipoprotein E‐deficient mice. However, the molecular mechanism underlying this process remains unknown. Using a transcriptomic approach, we identify here endoplasmic reticulum stress (ER) and the unfolded protein responses as drivers of VSMC death in two mouse models of HGPS exhibiting ubiquitous and VSMC‐specific progerin expression. This stress pathway was also activated in HGPS patient‐derived cells. Targeting ER stress response with a chemical chaperone delayed medial VSMC loss and inhibited atherosclerosis in both progeria models, and extended lifespan in the VSMC‐specific model. Our results identify a mechanism underlying cardiovascular disease in HGPS that could be targeted in patients. Moreover, these findings may help to understand other vascular diseases associated with VSMC death, and provide insight into aging‐dependent vascular damage related to accumulation of unprocessed toxic forms of lamin A.

Hutchinson-Gilford Progeria Syndrome-Current Status and Prospects for Gene Therapy Treatment

Hutchinson-Gilford progeria syndrome (HGPS) is one of the most severe disorders among laminopathies—a heterogeneous group of genetic diseases with a molecular background based on mutations in the LMNA gene and genes coding for interacting proteins. HGPS is characterized by the presence of aging-associated symptoms, including lack of subcutaneous fat, alopecia, swollen veins, growth retardation, age spots, joint contractures, osteoporosis, cardiovascular pathology, and death due to heart attacks and strokes in childhood. LMNA codes for two major, alternatively spliced transcripts, give rise to lamin A and lamin C proteins. Mutations in the LMNA gene alone, depending on the nature and location, may result in the expression of abnormal protein or loss of protein expression and cause at least 11 disease phenotypes, differing in severity and affected tissue. LMNA gene-related HGPS is caused by a single mutation in the LMNA gene in exon 11. The mutation c.1824C > T results in activation of the cryptic donor splice site, which leads to the synthesis of progerin protein lacking 50 amino acids. The accumulation of progerin is the reason for appearance of the phenotype. In this review, we discuss current knowledge on the molecular mechanisms underlying the development of HGPS and provide a critical analysis of current research trends in this field. We also discuss the mouse models available so far, the current status of treatment of the disease, and future prospects for the development of efficient therapies, including gene therapy for HGPS.

Vascular Smooth Muscle-Specific Progerin Expression Accelerates Atherosclerosis and Death in a Mouse Model of Hutchinson-Gilford Progeria Syndrome

Supplemental Digital Content is available in the text.

Background:

Progerin, an aberrant protein that accumulates with age, causes the rare genetic disease Hutchinson-Gilford progeria syndrome (HGPS). Patients who have HGPS exhibit ubiquitous progerin expression, accelerated aging and atherosclerosis, and die in their early teens, mainly of myocardial infarction or stroke. The mechanisms underlying progerin-induced atherosclerosis remain unexplored, in part, because of the lack of appropriate animal models.

Methods:

We generated an atherosclerosis-prone model of HGPS by crossing apolipoprotein E–deficient (Apoe^–/–) mice with Lmna^G609G/G609G mice ubiquitously expressing progerin. To induce progerin expression specifically in macrophages or vascular smooth muscle cells (VSMCs), we crossed Apoe^–/–Lmna^LCS/LCS mice with LysMCre and SM22αCre mice, respectively. Progerin expression was evaluated by polymerase chain reaction and immunofluorescence. Cardiovascular alterations were determined by immunofluorescence and histology in male mice fed normal chow or a high-fat diet. In vivo low-density lipoprotein retention was assessed by intravenous injection of fluorescently labeled human low-density lipoprotein. Cardiac electric defects were evaluated by electrocardiography.

Results:

Apoe^–/–Lmna^G609G/G609G mice with ubiquitous progerin expression exhibited a premature aging phenotype that included failure to thrive and shortened survival. In addition, high-fat diet–fed Apoe^–/–Lmna^G609G/G609G mice developed a severe vascular pathology, including medial VSMC loss and lipid retention, adventitial fibrosis, and accelerated atherosclerosis, thus resembling most aspects of cardiovascular disease observed in patients with HGPS. The same vascular alterations were also observed in Apoe^–/–Lmna^LCS/LCSSM22αCre mice expressing progerin specifically in VSMCs, but not in Apoe^–/–Lmna^LCS/LCSLysMCre mice with macrophage-specific progerin expression. Moreover, Apoe^–/–Lmna^LCS/LCSSM22αCre mice had a shortened lifespan despite the lack of any overt aging phenotype. Aortas of ubiquitously and VSMC-specific progerin-expressing mice exhibited increased retention of fluorescently labeled human low-density lipoprotein, and atheromata in both models showed vulnerable plaque features. Immunohistopathological examination indicated that Apoe^–/–Lmna^LCS/LCSSM22αCre mice, unlike Apoe^–/–Lmna^G609G/G609G mice, die of atherosclerosis-related causes.

Conclusions:

We have generated the first mouse model of progerin-induced atherosclerosis acceleration, and demonstrate that restricting progerin expression to VSMCs is sufficient to accelerate atherosclerosis, trigger plaque vulnerability, and reduce lifespan. Our results identify progerin-induced VSMC death as a major factor triggering atherosclerosis and premature death in HGPS.

Modulation of LMNA splicing as a strategy to treat prelamin A diseases

The alternatively spliced products of LMNA, lamin C and prelamin A (the precursor to lamin A), are produced in similar amounts in most tissues and have largely redundant functions. This redundancy suggests that diseases, such as Hutchinson-Gilford progeria syndrome (HGPS), that are caused by prelamin A-specific mutations could be treated by shifting the output of LMNA more toward lamin C. Here, we investigated mechanisms that regulate LMNA mRNA alternative splicing and assessed the feasibility of reducing prelamin A expression in vivo. We identified an exon 11 antisense oligonucleotide (ASO) that increased lamin C production at the expense of prelamin A when transfected into mouse and human fibroblasts. The same ASO also reduced the expression of progerin, the mutant prelamin A protein in HGPS, in fibroblasts derived from patients with HGPS. Mechanistic studies revealed that the exon 11 sequences contain binding sites for serine/arginine-rich splicing factor 2 (SRSF2), and SRSF2 knockdown lowered lamin A production in cells and in murine tissues. Moreover, administration of the exon 11 ASO reduced lamin A expression in wild-type mice and progerin expression in an HGPS mouse model. Together, these studies identify ASO-mediated reduction of prelamin A as a potential strategy to treat prelamin A-specific diseases.

A high-content imaging-based screening pipeline for the systematic identification of anti-progeroid compounds

Hutchinson-Gilford Progeria Syndrome (HGPS) is an early onset lethal premature aging disorder caused by constitutive production of progerin, a mutant form of the nuclear architectural protein lamin A. The presence of progerin causes extensive morphological, epigenetic and DNA damage related nuclear defects that ultimately disrupt tissue and organismal functions. Hypothesis-driven approaches focused on HGPS affected pathways have been used in attempts to identify druggable targets with anti-progeroid effects. Here, we report an unbiased discovery approach to HGPS by implementation of a high-throughput, high-content imaging based screening method that enables systematic identification of small molecules that prevent the formation of multiple progerin-induced aging defects. Screening a library of 2816 FDA approved drugs, we identified retinoids as a novel class of compounds that reverses aging defects in HGPS patient skin fibroblasts. These findings establish a novel approach to anti-progeroid drug discovery.

Vertical nanopillars for in situ probing of nuclear mechanics in adherent cells

The mechanical stability and deformability of the cell nucleus are crucial to many biological processes, including migration, proliferation and polarization. In vivo, the cell nucleus is frequently subjected to deformation on a variety of length and time scales, but current techniques for studying nuclear mechanics do not provide access to subnuclear deformation in live functioning cells. Here we introduce arrays of vertical nanopillars as a new method for the in situ study of nuclear deformability and the mechanical coupling between the cell membrane and the nucleus in live cells. Our measurements show that nanopillar-induced nuclear deformation is determined by nuclear stiffness, as well as opposing effects from actin and intermediate filaments. Furthermore, the depth, width and curvature of nuclear deformation can be controlled by varying the geometry of the nanopillar array. Overall, vertical nanopillar arrays constitute a novel approach for non-invasive, subcellular perturbation of nuclear mechanics and mechanotransduction in live cells.

Mice that express farnesylated versions of prelamin A in neurons develop achalasia

Neurons in the brain produce lamin C but almost no lamin A, a consequence of the removal of prelamin A transcripts by miR-9, a brain-specific microRNA. We have proposed that miR-9-mediated regulation of prelamin A in the brain could explain the absence of primary neurological disease in Hutchinson-Gilford progeria syndrome, a genetic disease caused by the synthesis of an internally truncated form of farnesyl-prelamin A (progerin). This explanation makes sense, but it is not entirely satisfying because it is unclear whether progerin-even if were expressed in neurons-would be capable of eliciting neuropathology. To address that issue, we created a new Lmna knock-in allele, Lmna(HG-C), which produces progerin transcripts lacking an miR-9 binding site. Mice harboring the Lmna(HG-C) allele produced progerin in neurons, but they had no pathology in the central nervous system. However, these mice invariably developed esophageal achalasia, and the enteric neurons and nerve fibers in gastrointestinal tract were markedly abnormal. The same disorder, achalasia, was observed in genetically modified mice that express full-length farnesyl-prelamin A in neurons (Zmpste24-deficient mice carrying two copies of a Lmna knock-in allele yielding full-length prelamin A transcripts lacking a miR-9 binding site). Our findings indicate that progerin and full-length farnesyl-prelamin A are toxic to neurons of the enteric nervous system.

LMNA-associated cardiocutaneous progeria: an inherited autosomal dominant premature aging syndrome with late onset

Hutchinson-Gilford Progeria Syndrome (HGPS) is a premature aging disorder caused by mutations in LMNA, which encodes the nuclear scaffold proteins lamin A and C. In HGPS and related progerias, processing of prelamin A is blocked at a critical step mediated by the zinc metalloprotease ZMPSTE24. LMNA-linked progerias can be grouped into two classes: (1) the processing-deficient, early onset "typical" progerias (e.g., HGPS), and (2) the processing-proficient "atypical" progeria syndromes (APS) that are later in onset. Here we describe a previously unrecognized progeria syndrome with prominent cutaneous and cardiovascular manifestations belonging to the second class. We suggest the name LMNA-associated cardiocutaneous progeria syndrome (LCPS) for this disorder. Affected patients are normal at birth but undergo progressive cutaneous changes in childhood and die in middle age of cardiovascular complications, including accelerated atherosclerosis, calcific valve disease, and cardiomyopathy. In addition, the proband demonstrated cancer susceptibility, a phenotype rarely described for LMNA-based progeria disorders. The LMNA mutation that caused LCPS in this family is a heterozygous c.899A>G (p.D300G) mutation predicted to alter the coiled-coil domain of lamin A/C. In skin fibroblasts isolated from the proband, the processing and levels of lamin A and C are normal. However, nuclear morphology is aberrant and rescued by treatment with farnesyltransferase inhibitors, as is also the case for HGPS and other laminopathies. Our findings advance knowledge of human LMNA progeria syndromes, and raise the possibility that typical and atypical progerias may converge upon a common mechanism to cause premature aging disease.

Potential therapeutic approaches for modulating expression and accumulation of defective lamin A in laminopathies and age-related diseases

Scientific understanding of the genetic components of aging has increased in recent years, with several genes being identified as playing roles in the aging process and, potentially, longevity. In particular, genes encoding components of the nuclear lamina in eukaryotes have been increasingly well characterized, owing in part to their clinical significance in age-related diseases. This review focuses on one such gene, which encodes lamin A, a key component of the nuclear lamina. Genetic variation in this gene can give rise to lethal, early-onset diseases known as laminopathies. Here, we analyze the literature and conduct computational analyses of lamin A signaling and intracellular interactions in order to examine potential mechanisms for altering or slowing down aberrant Lamin A expression and/or for restoring the ratio of normal to aberrant lamin A. The ultimate goal of such studies is to ameliorate or combat laminopathies and related diseases of aging, and we provide a discussion of current approaches in this review.

Blocking farnesylation of the prelamin A variant in Hutchinson-Gilford progeria syndrome alters the distribution of A-type lamins

Mutations in the lamin A/C gene that cause Hutchinson-Gilford progeria syndrome lead to expression of a truncated, permanently farnesylated prelamin A variant called progerin. Blocking farnesylation leads to an improvement in the abnormal nuclear morphology observed in cells expressing progerin, which is associated with a re-localization of the variant protein from the nuclear envelope to the nuclear interior. We now show that a progerin construct that cannot be farnesylated is localized primarily in intranuclear foci and that its diffusional mobility is significantly greater than that of farnesylated progerin localized predominantly at the nuclear envelope. Expression of non-farnesylated progerin in transfected cells leads to a redistribution of lamin A and lamin C away from the nuclear envelope into intranuclear foci but does not significantly affect the localization of endogenous lamin B1 at nuclear envelope. There is a similar redistribution of lamin A and lamin C into intranuclear foci in transfected cells expressing progerin in which protein farnesylation is blocked by treatment with a protein farnesyltransferase inhibitor. Blocking farnesylation of progerin can lead to a redistribution of normal A-type lamins away from the inner nuclear envelope. This may have implications for using drugs that block protein prenylation to treat children with Hutchinson-Gilford progeria syndrome. These findings also provide additional evidence that A-type and B-type lamins can form separate microdomains within the nucleus.

Nuclear mechanics in disease

Over the past two decades, the biomechanical properties of cells have emerged as key players in a broad range of cellular functions, including migration, proliferation, and differentiation. Although much of the attention has focused on the cytoskeletal networks and the cell's microenvironment, relatively little is known about the contribution of the cell nucleus. Here, we present an overview of the structural elements that determine the physical properties of the nucleus and discuss how changes in the expression of nuclear components or mutations in nuclear proteins can not only affect nuclear mechanics but also modulate cytoskeletal organization and diverse cellular functions. These findings illustrate that the nucleus is tightly integrated into the surrounding cellular structure. Consequently, changes in nuclear structure and composition are highly relevant to normal development and physiology and can contribute to many human diseases, such as muscular dystrophy, dilated cardiomyopathy, (premature) aging, and cancer.

Investigating the purpose of prelamin A processing

Lmna yields two major protein products in somatic cells, lamin C and prelamin A. Mature lamin A is produced from prelamin A by four posttranslational processing steps-farnesylation of a carboxyl-terminal cysteine, release of the last three amino acids of the protein, methylation of the farnesylcysteine, and the endoproteolytic release of the carboxyl-terminal 15 amino acids of the protein (including the farnesylcysteine methyl ester). Although the posttranslational processing of prelamin A has been conserved in vertebrate evolution, its physiologic significance remains unclear. Here we review recent studies in which we investigated prelamin A processing with Lmna knock-in mice that produce exclusively prelamin A (Lmna(PLAO)), mature lamin A (Lmna(LAO)) or nonfarnesylated prelamin A (Lmna(nPLAO)). We found that the synthesis of lamin C is dispensable in laboratory mice, that the direct production of mature lamin A (completely bypassing all prelamin A processing) causes no discernable pathology in mice, and that exclusive production of nonfarnesylated prelamin A leads to cardiomyopathy.

Autophagic degradation of farnesylated prelamin A as a therapeutic approach to lamin-linked progeria

Farnesylated prelamin A is a processing intermediate produced in the lamin A maturation pathway. Accumulation of a truncated farnesylated prelamin A form, called progerin, is a hallmark of the severe premature ageing syndrome, Hutchinson-Gilford progeria. Progerin elicits toxic effects in cells, leading to chromatin damage and cellular senescence and ultimately causes skin and endothelial defects, bone resorption, lipodystrophy and accelerated ageing. Knowledge of the mechanism underlying prelamin A turnover is critical for the development of clinically effective protein inhibitors that can avoid accumulation to toxic levels without impairing lamin A/C expression, which is essential for normal biological functions. Little is known about specific molecules that may target farnesylated prelamin A to elicit protein degradation. Here, we report the discovery of rapamycin as a novel inhibitor of progerin, which dramatically and selectively decreases protein levels through a mechanism involving autophagic degradation. Rapamycin treatment of progeria cells lowers progerin, as well as wild-type prelamin A levels, and rescues the chromatin phenotype of cultured fibroblasts, including histone methylation status and BAF and LAP2α distribution patterns. Importantly, rapamycin treatment does not affect lamin C protein levels, but increases the relative expression of the prelamin A endoprotease ZMPSTE24. Thus, rapamycin, an antibiotic belonging to the class of macrolides, previously found to increase longevity in mouse models, can serve as a therapeutic tool, to eliminate progerin, avoid farnesylated prelamin A accumulation, and restore chromatin dynamics in progeroid laminopathies.

Low and high expressing alleles of the LMNA gene: implications for laminopathy disease development

Today, there are at least a dozen different genetic disorders caused by mutations within the LMNA gene, and collectively, they are named laminopathies. Interestingly, the same mutation can cause phenotypes with different severities or even different disorders and might, in some cases, be asymptomatic. We hypothesized that one possible contributing mechanism for this phenotypic variability could be the existence of high and low expressing alleles in the LMNA locus. To investigate this hypothesis, we developed an allele-specific absolute quantification method for lamin A and lamin C transcripts using the polymorphic rs4641(C/T)LMNA coding SNP. The contribution of each allele to the total transcript level was investigated in nine informative human primary dermal fibroblast cultures from Hutchinson-Gilford progeria syndrome (HGPS) and unaffected controls. Our results show differential expression of the two alleles. The C allele is more frequently expressed and accounts for ∼70% of the lamin A and lamin C transcripts. Analysis of samples from six patients with Hutchinson-Gilford progeria syndrome showed that the c.1824C>T, p.G608G mutation is located in both the C and the T allele, which might account for the variability in phenotype seen among HGPS patients. Our method should be useful for further studies of human samples with mutations in the LMNA gene and to increase the understanding of the link between genotype and phenotype in laminopathies.

A conserved splicing mechanism of the LMNA gene controls premature aging

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disorder phenotypically characterized by many features of premature aging. Most cases of HGPS are due to a heterozygous silent mutation (c.1824C>T; p.Gly608Gly) that enhances the use of an internal 5' splice site (5'SS) in exon 11 of the LMNA pre-mRNA and leads to the production of a truncated protein (progerin) with a dominant negative effect. Here we show that HGPS mutation changes the accessibility of the 5'SS of LMNA exon 11 which is sequestered in a conserved RNA structure. Our results also reveal a regulatory role of a subset of serine-arginine (SR)-rich proteins, including serine-arginine rich splicing factor 1 (SRSF1) and SRSF6, on utilization of the 5'SS leading to lamin A or progerin production and a modulation of this regulation in the presence of the c.1824C>T mutation is shown directly on HGPS patient cells. Mutant mice carrying the equivalent mutation in the LMNA gene (c.1827C>T) also accumulate progerin and phenocopy the main cellular alterations and clinical defects of HGPS patients. RNAi-induced depletion of SRSF1 in the HGPS-like mouse embryonic fibroblasts (MEFs) allowed progerin reduction and dysmorphic nuclei phenotype correction, whereas SRSF6 depletion aggravated the HGPS-like MEF's phenotype. We demonstrate that changes in the splicing ratio between lamin A and progerin are key factors for lifespan since heterozygous mice harboring the mutation lived longer than homozygous littermates but less than the wild-type. Genetic and biochemical data together favor the view that physiological progerin production is under tight control of a conserved splicing mechanism to avoid precocious aging.

Direct synthesis of lamin A, bypassing prelamin a processing, causes misshapen nuclei in fibroblasts but no detectable pathology in mice

Lamin A, a key component of the nuclear lamina, is generated from prelamin A by four post-translational processing steps: farnesylation, endoproteolytic release of the last three amino acids of the protein, methylation of the C-terminal farnesylcysteine, and finally, endoproteolytic release of the last 15 amino acids of the protein (including the farnesylcysteine methyl ester). The last cleavage step, mediated by ZMPSTE24, releases mature lamin A. This processing scheme has been conserved through vertebrate evolution and is widely assumed to be crucial for targeting lamin A to the nuclear envelope. However, its physiologic importance has never been tested. To address this issue, we created mice with a "mature lamin A-only" allele (Lmna(LAO)), which contains a stop codon immediately after the last codon of mature lamin A. Thus, Lmna(LAO/LAO) mice synthesize mature lamin A directly, bypassing prelamin A synthesis and processing. The levels of mature lamin A in Lmna(LAO/LAO) mice were indistinguishable from those in "prelamin A-only" mice (Lmna(PLAO/PLAO)), where all of the lamin A is produced from prelamin A. Lmna(LAO/LAO) exhibited normal body weights and had no detectable disease phenotypes. A higher frequency of nuclear blebs was observed in Lmna(LAO/LAO) embryonic fibroblasts; however, the mature lamin A in the tissues of Lmna(LAO/LAO) mice was positioned normally at the nuclear rim. We conclude that prelamin A processing is dispensable in mice and that direct synthesis of mature lamin A has little if any effect on the targeting of lamin A to the nuclear rim in mouse tissues.

Progeria, the nucleolus and farnesyltransferase inhibitors

HGPS (Hutchinson-Gilford progeria syndrome) is a rare genetic disease affecting children causing them to age and die prematurely. The disease is typically due to a point mutation in the coding sequence for the nuclear intermediate-type filament protein lamin A and gives rise to a dominant-negative splice variant named progerin. Accumulation of progerin within nuclei causes disruption to nuclear structure, causes and premature replicative senescence and increases apoptosis. Now it appears that accumulation of progerin may have more widespread effects than previously thought since the demonstration that the presence and distribution of some nucleolar proteins are also adversely affected in progeria cells. One of the major breakthroughs both in the lamin field and for this syndrome is that many of the cellular defects observed in HGPS patient cells and model systems can be restored after treatment with a class of compounds known as FTIs (farnesyltransferase inhibitors). Indeed, it is demonstrated that FTI-277 is able to completely restore nucleolar antigen localization in treated progeria cells. This is encouraging news for the HGPS patients who are currently undergoing clinical trials with FTI treatment.

Altered nuclear functions in progeroid syndromes: a paradigm for aging research

Syndromes of accelerated aging could provide an entry point for identifying and dissecting the cellular pathways that are involved in the development of age-related pathologies in the general population. However, their usefulness for aging research has been controversial, as it has been argued that these diseases do not faithfully reflect the process of natural aging. Here we review recent findings on the molecular basis of two progeroid diseases, Werner syndrome (WS) and Hutchinson-Gilford progeria syndrome (HGPS), and highlight functional connections to cellular processes that may contribute to normal aging.

Effect of progerin on the accumulation of oxidized proteins in fibroblasts from Hutchinson Gilford progeria patients

The mutation responsible for Hutchinson Gilford Progeria Syndrome (HGPS) causes abnormal nuclear morphology. Previous studies show that free radicals and reactive oxygen species play major roles in the etiology and/or progression of neurodegenerative diseases and aging. This study compares oxidative stress responses between progeric and normal fibroblasts. Our data revealed higher ROS levels in HGPS cells compared to age-matched controls. In response to oxidative challenge, progeric cells showed increased mRNA levels for mitochondrial superoxide dismutase (SOD) and SOD protein content. However, this did not prevent a drop in the ATP content of progeria fibroblasts. Previous studies have shown that declines in human fibroblast ATP levels interfere with programmed cell death and promote necrotic inflammation. Notably, in our investigations the ATP content of progeria fibroblasts was only approximately 50% of that found in healthy controls. Furthermore, HGPS fibroblast analysis revealed a decrease in total caspase-like proteasome activity and in the levels of two active proteolytic complex subunits (beta(5) and beta(7)). A number of studies indicate that the molecular mechanisms causing accelerated aging in progeric patients also occur in healthy cells of older individuals. Thus, the results of this study may also help explain some of the cellular changes that accompany normal aging.

Assessing the efficacy of protein farnesyltransferase inhibitors in mouse models of progeria

Hutchinson-Gilford progeria syndrome (HGPS) is caused by the accumulation of a farnesylated form of prelamin A (progerin). Previously, we showed that blocking protein farnesylation with a farnesyltransferase inhibitor (FTI) ameliorates the disease phenotypes in mouse model of HGPS (Lmna(HG/+)). However, the interpretation of the FTI treatment studies is open to question in light of recent studies showing that mice expressing a nonfarnesylated version of progerin (Lmna(nHG/+)) develop progeria-like disease phenotypes. The fact that Lmna(nHG/+) mice manifest disease raised the possibility that the beneficial effects of an FTI in Lmna(HG/+) mice were not due to the effects of the drug on the farnesylation of progerin, but may have been due to unanticipated secondary effects of the drug on other farnesylated proteins. To address this issue, we compared the ability of an FTI to improve progeria-like disease phenotypes in both Lmna(HG/+) and Lmna(nHG/+) mice. In Lmna(HG/+) mice, the FTI reduced disease phenotypes in a highly significant manner, but the drug had no effect in Lmna(nHG/+) mice. The failure of the FTI to ameliorate disease in Lmna(nHG/+) mice supports the idea that the beneficial effects of an FTI in Lmna(HG/+) mice are due to the effect of drug on the farnesylation of progerin.

The posttranslational processing of prelamin A and disease

Human geneticists have shown that some progeroid syndromes are caused by mutations that interfere with the conversion of farnesyl-prelamin A to mature lamin A. For example, Hutchinson-Gilford progeria syndrome is caused by LMNA mutations that lead to the accumulation of a farnesylated version of prelamin A. In this review, we discuss the posttranslational modifications of prelamin A and their relevance to the pathogenesis and treatment of progeroid syndromes.

Different prelamin A forms accumulate in human fibroblasts: a study in experimental models and progeria

Lamin A is a component of the nuclear lamina mutated in a group of human inherited disorders known as laminopathies. Among laminopathies, progeroid syndromes and lipodystrophies feature accumulation of prelamin A, the precursor protein which, in normal cells, undergoes a multi-step processing to yield mature lamin A. It is of utmost importance to characterize the prelamin A form accumulated in each laminopathy, since existing evidence shows that drugs acting on protein processing can improve some pathological aspects. We report that two antibodies raised against differently modified prelamin A peptides show a clear specificity to full-length prelamin A or carboxymethylated farnesylated prelamin A, respectively. Using these antibodies, we demonstrated that inhibition of the prelamin A endoprotease ZMPSTE24 mostly elicits accumulation of full-length prelamin A in its farnesylated form, while loss of the prelamin A cleavage site causes accumulation of carboxymethylated prelamin A in progeria cells. These results suggest a major role of ZMPSTE24 in the first prelamin A cleavage step.

Progerin elicits disease phenotypes of progeria in mice whether or not it is farnesylated

Hutchinson-Gilford progeria syndrome (HGPS), a rare disease that results in what appears to be premature aging, is caused by the production of a mutant form of prelamin A known as progerin. Progerin retains a farnesyl lipid anchor at its carboxyl terminus, a modification that is thought to be important in disease pathogenesis. Inhibition of protein farnesylation improves the hallmark nuclear shape abnormalities in HGPS cells and ameliorates disease phenotypes in mice harboring a knockin HGPS mutation (LmnaHG/+). The amelioration of disease, however, is incomplete, leading us to hypothesize that nonfarnesylated progerin also might be capable of eliciting disease. To test this hypothesis, we created knockin mice expressing nonfarnesylated progerin (LmnanHG/+). LmnanHG/+ mice developed the same disease phenotypes observed in LmnaHG/+ mice, although the phenotypes were milder, and mouse embryonic fibroblasts (MEFs) derived from these mice contained fewer misshapen nuclei. The steady-state levels of progerin in LmnanHG/+ MEFs and tissues were lower, suggesting a possible explanation for the milder phenotypes. These data support the concept that inhibition of protein farnesylation in progeria could be therapeutically useful but also suggest that this approach may be limited, as progerin elicits disease phenotypes whether or not it is farnesylated.

Increasing the length of progerin's isoprenyl anchor does not worsen bone disease or survival in mice with Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is caused by the synthesis of a truncated prelamin A, commonly called progerin, that contains a carboxyl-terminal farnesyl lipid anchor. The farnesyl lipid anchor helps to target progerin to membrane surfaces at the nuclear rim, where it disrupts the integrity of the nuclear lamina and causes misshapen nuclei. Several lines of evidence have suggested that progerin's farnesyl lipid anchor is crucial for the emergence of disease phenotypes. Because a geranylgeranyl lipid is approximately 45-fold more potent than a farnesyl lipid in anchoring proteins to lipid membranes, we hypothesized that a geranylgeranylated version of progerin might be more potent in eliciting disease phenotypes. To test this hypothesis, we used gene targeting to create mice expressing geranylgeranylated progerin (Lmna(ggHG/+)). We then compared Lmna(ggHG/+) mice, side-by-side, with otherwise identical mice expressing farnesylated progerin (Lmna(HG/+)). Geranylgeranylation of progerin in Lmna(ggHG/+) cells and farnesylation of progerin in Lmna(HG/+) cells was confirmed by metabolic labeling. Contrary to our expectations, Lmna(ggHG/+) mice survived longer than Lmna(HG/+) mice. The Lmna(ggHG/+) mice also exhibited milder bone disease. The steady-state levels of progerin, relative to lamin C, were lower in Lmna(ggHG/+) mice than in Lmna(HG/+) mice, providing a potential explanation for the milder disease in Lmna(ggHG/+) mice.

Nuclear shape, mechanics, and mechanotransduction

In eukaryotic cells, the nucleus contains the genome and is the site of transcriptional regulation. The nucleus is the largest and stiffest organelle and is exposed to mechanical forces transmitted through the cytoskeleton from outside the cell and from force generation within the cell. Here, we discuss the effect of intra- and extracellular forces on nuclear shape and structure and how these force-induced changes could be implicated in nuclear mechanotransduction, ie, force-induced changes in cell signaling and gene transcription. We review mechanical studies of the nucleus and nuclear structural proteins, such as lamins. Dramatic changes in nuclear shape, organization, and stiffness are seen in cells where lamin proteins are mutated or absent, as in genetically engineered mice, RNA interference studies, or human disease. We examine the different mechanical pathways from the force-responsive cytoskeleton to the nucleus. We also highlight studies that link changes in nuclear shape with cell function during developmental, physiological, and pathological modifications. Together, these studies suggest that the nucleus itself may play an important role in the response of the cell to force.

Nuclear lamins: major factors in the structural organization and function of the nucleus and chromatin

Over the past few years it has become evident that the intermediate filament proteins, the types A and B nuclear lamins, not only provide a structural framework for the nucleus, but are also essential for many aspects of normal nuclear function. Insights into lamin-related functions have been derived from studies of the remarkably large number of disease-causing mutations in the human lamin A gene. This review provides an up-to-date overview of the functions of nuclear lamins, emphasizing their roles in epigenetics, chromatin organization, DNA replication, transcription, and DNA repair. In addition, we discuss recent evidence supporting the importance of lamins in viral infections.