Prelamin A, Zmpste24, misshapen cell nuclei, and progeria--new evidence suggesting that protein farnesylation could be important for disease pathogenesis

Prelamin A undergoes multistep processing to yield lamin A, a structural protein of the nuclear lamina. Prelamin A terminates with a CAAX motif, which triggers farnesylation of a C-terminal cysteine (the C of the CAAX motif), endoproteolytic release of the last three amino acids (the AAX), and methylation of the newly exposed farnesylcysteine residue. In addition, prelamin A is cleaved a second time, releasing 15 more residues from the C terminus (including the farnesylcysteine methyl ester), generating mature lamin A. This second cleavage step is carried out by an endoplasmic reticulum membrane protease, ZMPSTE24. Interest in the posttranslational processing of prelamin A has increased with the recognition that certain progeroid syndromes can be caused by mutations that lead to an accumulation of farnesyl-prelamin A. Recently, we showed that a key cellular phenotype of these progeroid disorders, misshapen cell nuclei, can be ameliorated by inhibitors of protein farnesylation, suggesting a potential strategy for treating these diseases. In this article, we review the posttranslational processing of prelamin A, describe several mouse models for progeroid syndromes, explain the mutations underlying several human progeroid syndromes, and summarize recent data showing that misshapen nuclei can be ameliorated by treating cells with protein farnesyltransferase inhibitors.     

Human CaaX protease ZMPSTE24 expressed in yeast: Structure and inhibition by HIV protease inhibitors

The function and localization of proteins and peptides containing C‐terminal “CaaX” (Cys‐aliphatic‐aliphatic‐anything) sequence motifs are modulated by post‐translational attachment of isoprenyl groups to the cysteine sulfhydryl, followed by proteolytic cleavage of the aaX amino acids. The zinc metalloprotease ZMPSTE24 is one of two enzymes known to catalyze this cleavage. The only identified target of mammalian ZMPSTE24 is prelamin A, the precursor to the nuclear scaffold protein lamin A. ZMPSTE24 also cleaves prelamin A at a second site 15 residues upstream from the CaaX site. Mutations in ZMPSTE24 result in premature‐aging diseases and inhibition of ZMPSTE24 activity has been reported to be an off‐target effect of HIV protease inhibitors. We report here the expression (in yeast), purification, and crystallization of human ZMPSTE24 allowing determination of the structure to 2.0 Å resolution. Compared to previous lower resolution structures, the enhanced resolution provides: (1) a detailed view of the active site of ZMPSTE24, including water coordinating the catalytic zinc; (2) enhanced visualization of fenestrations providing access from the exterior to the interior cavity of the protein; (3) a view of the C‐terminus extending away from the main body of the protein; (4) localization of ordered lipid and detergent molecules at internal and external surfaces and also projecting through fenestrations; (5) identification of water molecules associated with the surface of the internal cavity. We also used a fluorogenic assay of the activity of purified ZMPSTE24 to demonstrate that HIV protease inhibitors directly inhibit the human enzyme in a manner indicative of a competitive mechanism.     

Inhibitors of protein geranylgeranyltransferase-I lead to prelamin A accumulation in cells by inhibiting ZMPSTE24

Protein farnesyltransferase (FTase) inhibitors, generally called "FTIs," block the farnesylation of prelamin A, inhibiting the biogenesis of mature lamin A and leading to an accumulation of prelamin A within cells. A recent report found that a GGTI, an inhibitor of protein geranylgeranyltransferase-I (GGTase-I), caused an exaggerated accumulation of prelamin A in the presence of low amounts of an FTI. This finding was interpreted as indicating that prelamin A can be alternately prenylated by GGTase-I and that inhibiting both protein prenyltransferases leads to more prelamin A accumulation than blocking FTase alone. Here, we tested an alternative hypothesis-GGTIs are not specific for GGTase-I, and they lead to prelamin A accumulation by inhibiting ZMPSTE24 (a zinc metalloprotease that converts farnesyl-prelamin A to mature lamin A). In our studies, commonly used GGTIs caused prelamin A accumulation in human fibroblasts, but the prelamin A in GGTI-treated cells exhibited a more rapid electrophoretic mobility than prelamin A from FTI-treated cells. The latter finding suggested that the prelamin A in GGTI-treated cells might be farnesylated (which would be consistent with the notion that GGTIs inhibit ZMPSTE24). Indeed, metabolic labeling studies revealed that the prelamin A in GGTI-treated fibroblasts is farnesylated. Moreover, biochemical assays of ZMPSTE24 activity showed that ZMPSTE24 is potently inhibited by a GGTI. Our studies show that GGTIs inhibit ZMPSTE24, leading to an accumulation of farnesyl-prelamin A. Thus, caution is required when interpreting the effects of GGTIs on prelamin A processing.     

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.     

Targeting RAS‐converting enzyme 1 overcomes senescence and improves progeria‐like phenotypes of ZMPSTE24 deficiency

Several progeroid disorders are caused by deficiency in the endoprotease ZMPSTE24 which leads to accumulation of prelamin A at the nuclear envelope. ZMPSTE24 cleaves prelamin A twice: at the third carboxyl‐terminal amino acid following farnesylation of a –CSIM motif; and 15 residues upstream to produce mature lamin A. The carboxyl‐terminal cleavage can also be performed by RAS‐converting enzyme 1 (RCE1) but little is known about the importance of this cleavage for the ability of prelamin A to cause disease. Here, we found that knockout of RCE1 delayed senescence and increased proliferation of ZMPSTE24‐deficient fibroblasts from a patient with non‐classical Hutchinson‐Gilford progeria syndrome (HGPS), but did not influence proliferation of classical LMNA‐mutant HGPS cells. Knockout of Rce1 in Zmpste24‐deficient mice at postnatal week 4–5 increased body weight and doubled the median survival time. The absence of Rce1 in Zmpste24‐deficient fibroblasts did not influence nuclear shape but reduced an interaction between prelamin A and AKT which activated AKT‐mTOR signaling and was required for the increased proliferation. Prelamin A levels increased in Rce1‐deficient cells due to a slower turnover rate but its localization at the nuclear rim was unaffected. These results strengthen the idea that the presence of misshapen nuclei does not prevent phenotype improvement and suggest that targeting RCE1 might be useful for treating the rare progeroid disorders associated with ZMPSTE24 deficiency.     

Molecular tools that block maturation of the nuclear lamin A and decelerate cancer cell migration

Lamin A contributes to the structure of nuclei in all mammalian cells and plays an important role in cell division and migration. Mature lamin A is derived from a farnesylated precursor protein, known as prelamin A, which undergoes post-translational cleavage catalyzed by the zinc metalloprotease STE24 (ZPMSTE24). Accumulation of farnesylated prelamin A in the nuclear envelope compromises cell division, impairs mitosis and induces an increased expression of inflammatory gene products. ZMPSTE24 has been proposed as a potential therapeutic target in oncology. A library of peptidomimetic compounds were synthesized and screened for their ability to induce accumulation of prelamin A in cancer cells and block cell migration in pancreatic ductal adenocarcinoma cells. The results of this study suggest that inhibitors of lamin A maturation may interfere with cell migration, the biological process required for cancer metastasis.     

A potent HIV protease inhibitor, darunavir, does not inhibit ZMPSTE24 or lead to an accumulation of farnesyl-prelamin A in cells

HIV protease inhibitors (HIV-PIs) are key components of highly active antiretroviral therapy, but they have been associated with adverse side effects, including partial lipodystrophy and metabolic syndrome. We recently demonstrated that a commonly used HIV-PI, lopinavir, inhibits ZMPSTE24, thereby blocking lamin A biogenesis and leading to an accumulation of prelamin A. ZMPSTE24 deficiency in humans causes an accumulation of prelamin A and leads to lipodystrophy and other disease phenotypes. Thus, an accumulation of prelamin A in the setting of HIV-PIs represents a plausible mechanism for some drug side effects. Here we show, with metabolic labeling studies, that lopinavir leads to the accumulation of the farnesylated form of prelamin A. We also tested whether a new and chemically distinct HIV-PI, darunavir, inhibits ZMPSTE24. We found that darunavir does not inhibit the biochemical activity of ZMPSTE24, nor does it lead to an accumulation of farnesyl-prelamin A in cells. This property of darunavir is potentially attractive. However, all HIV-PIs, including darunavir, are generally administered with ritonavir, an HIV-PI that is used to block the metabolism of other HIV-PIs. Ritonavir, like lopinavir, inhibits ZMPSTE24 and leads to an accumulation of prelamin A.     

核层蛋白A前体的法尼基化与衰老

编码核层蛋白A(lamin A)的LMNA基因突变导致法尼基化的核层蛋白A前体(prelamin A)不能被进一步加工成成熟的核层蛋白A,从而导致一种Hutchinson-Gilford早老症综合征(Hutchinson-Gilford progeria syndrome,HGPS)。一种更严重的早老症——限制性皮肤病(restrictive dermopathy,RD),是由于缺失核层蛋白A前体加工过程中的剪切酶ZMPSTE24引起的。ZMPSTE24的缺失阻止了法尼基化的核层蛋白A前体不能正常加工成为成熟的核层蛋白A,同时导致法尼基化的核层蛋白A前体的堆积。在HGPS和RD病人的成纤维细胞中,发现法尼基化的核层蛋白A前体都定位在核膜,从而影响细胞核膜的完整性,并导致细胞核形的异常,进而导致衰老。最近研究表明经过法尼基酰转移酶抑制剂(farnesyltransferase inhibitor,FTI)处理后的细胞的核形异常减少。同时,FTI能够改善HGPS和RD小鼠的早老症状。本文就核层蛋白A前体的法尼基化对衰老的影响有关研究进展作一综述。     

In vivo virus structures: simultaneous classification, resolution enhancement, and noise reduction in whole-cell electron tomography

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging syndrome caused by the expression and accumulation of a mutant form of lamin A, Δ50 lamin A. As a component of the cell's nucleoskeleton, lamin A plays an important role in the mechanical stabilization of the nuclear envelope and in other nuclear functions. It is largely unknown how the characteristic 50 amino acid deletion affects the conformation of the mostly intrinsically disordered tail domain of lamin A. Here we perform replica exchange molecular dynamics simulations of the tail domain and determine an ensemble of semi-stable structures. Based on these structures we show that the ZMPSTE 24 cleavage site on the precursor form of the lamin A tail domain orients itself in such a way as to facilitate cleavage during the maturation process. We confirm our simulated structures by comparing the thermodynamic properties of the ensemble structures to in vitro stability measurements. Using this combination of experimental and computational techniques, we compare the size, heterogeneity of size, thermodynamic stability of the Ig-fold, as well as the mechanisms of force-induced denaturation. Our data shows that the Δ50 lamin A tail domain is more compact and displays less heterogeneity than the mature lamin A tail domain. Altogether these results suggest that the altered structure and stability of the tail domain can explain changed protein-protein and protein-DNA interactions and may represent an etiology of the disease. Also, this study provides the first molecular structure(s) of the lamin A tail domain, which is confirmed by thermodynamic tests in experiment.     

Mislocalization of prelamin A Tyr646Phe mutant to the nuclear pore complex in human embryonic kidney 293 cells

Mature lamin A is formed after post-translational processing of prelamin A, which includes prenylation and carboxymethylation of cysteine 661 in the CaaX motif, followed by two proteolytic cleavages by zinc metalloprotease (ZMPSTE24). We expressed several prelamin A mutants, C661S (defective in prenylation), Y646F (designed to undergo prenylation but not second proteolytic cleavage), double mutant, Y646F/C661S and Y646X (mature lamin A), and the wild-type construct in human embryonic kidney (HEK-293) cells. Only the Y646F mutant co-localized with nuclear pore complex proteins, including Nup53 and Nup98, whereas the other mutants localized to the nuclear envelope rim. The cells expressing Y646F mutant also revealed abnormal nuclear morphology which was partially rescued with the farnesyl transferase inhibitors. These data suggest that the unprenylated prelamin A is not toxic to the cells. The toxicity of prenylated prelamin A may be due to its association and/or accumulation at the nuclear pore complex which could be partially reversed by farnesyl transferase inhibitors.     

Prelamin A farnesylation and progeroid syndromes

Hutchinson-Gilford progeria syndrome (HGPS) is caused by a LMNA mutation that leads to the synthesis of a mutant prelamin A that is farnesylated but cannot be further processed to mature lamin A. A more severe progeroid disorder, restrictive dermopathy (RD), is caused by the loss of the prelamin A-processing enzyme, ZMPSTE24. The absence of ZMPSTE24 prevents the endoproteolytic processing of farnesyl-prelamin A to mature lamin A and leads to the accumulation of farnesyl-prelamin A. In both HGPS and RD, the farnesyl-prelamin A is targeted to the nuclear envelope, where it interferes with the integrity of the nuclear envelope and causes misshapen cell nuclei. Recent studies have shown that the frequency of misshapen nuclei can be reduced by treating cells with a farnesyltransferase inhibitor (FTI). Also, administering an FTI to mouse models of HGPS and RD ameliorates the phenotypes of progeria. These studies have prompted interest in testing the efficacy of FTIs in children with HGPS.     

Analysis of prelamin A biogenesis reveals the nucleus to be a CaaX processing compartment

Proteins establish and maintain a distinct intracellular localization by means of targeting, retention, and retrieval signals, ensuring most proteins reside predominantly in one cellular location. The enzymes involved in the maturation of lamin A present a challenge to this paradigm. Lamin A is first synthesized as a 74-kDa precursor, prelamin A, with a C-terminal CaaX motif and undergoes a series of posttranslational modifications including CaaX processing (farnesylation, aaX cleavage and carboxylmethylation), followed by endoproteolytic cleavage by Zmpste24. Failure to cleave prelamin A results in progeria and related premature aging disorders. Evidence suggests prelamin A is imported directly into the nucleus where it is processed. Paradoxically, the processing enzymes have been shown to reside in the cytosol (farnesyltransferase), or are ER membrane proteins (Zmpste24, Rce1, and Icmt) with their active sites facing the cytosol. Here we have reexamined the cellular site of prelamin A processing, and show that the mammalian and yeast processing enzymes Zmpste24 and Icmt exhibit a dual localization to the inner nuclear membrane, as well as the ER membrane. Our findings reveal the nucleus to be a physiologically relevant location for CaaX processing, and provide insight into the biology of a protein at the center of devastating progeroid diseases.     

Biogenesis of the Saccharomyces cerevisiae Pheromone a-Factor,from Yeast Mating to Human Disease

Summary: The mating pheromone a-factor secreted by Saccharomyces cerevisiae is a farnesylated and carboxylmethylated peptide and is unusually hydrophobic compared to other extracellular signaling molecules. Mature a-factor is derived from a precursor with a C-terminal CAAX motif that directs a series of posttranslational reactions, including prenylation, endoproteolysis, and carboxylmethylation. Historically, a-factor has served as a valuable model for the discovery and functional analysis of CAAX-processing enzymes. In this review, we discuss the three modules comprising the a-factor biogenesis pathway: (i) the C-terminal CAAX-processing steps carried out by Ram1/Ram2, Ste24 or Rce1, and Ste14; (ii) two sequential N-terminal cleavage steps, mediated by Ste24 and Axl1; and (iii) export by a nonclassical mechanism, mediated by the ATP binding cassette (ABC) transporter Ste6. The small size and hydrophobicity of a-factor present both challenges and advantages for biochemical analysis, as discussed here. The enzymes involved in a-factor biogenesis are conserved from yeasts to mammals. Notably, studies of the zinc metalloprotease Ste24 in S. cerevisiae led to the discovery of its mammalian homolog ZMPSTE24, which cleaves the prenylated C-terminal tail of the nuclear scaffold protein lamin A. Mutations that alter ZMPSTE24 processing of lamin A in humans cause the premature-aging disease progeria and related progeroid disorders. Intriguingly, recent evidence suggests that the entire a-factor pathway, including all three biogenesis modules, may be used to produce a prenylated, secreted signaling molecule involved in germ cell migration in Drosophila. Thus, additional prenylated signaling molecules resembling a-factor, with as-yet-unknown roles in metazoan biology, may await discovery.     

Embryonic senescence and laminopathies in a progeroid zebrafish model

Background

Mutations that disrupt the conversion of prelamin A to mature lamin A cause the rare genetic disorder Hutchinson-Gilford progeria syndrome and a group of laminopathies. Our understanding of how A-type lamins function in vivo during early vertebrate development through aging remains limited, and would benefit from a suitable experimental model. The zebrafish has proven to be a tractable model organism for studying both development and aging at the molecular genetic level. Zebrafish show an array of senescence symptoms resembling those in humans, which can be targeted to specific aging pathways conserved in vertebrates. However, no zebrafish models bearing human premature senescence currently exist.

Principal Findings

We describe the induction of embryonic senescence and laminopathies in zebrafish harboring disturbed expressions of the lamin A gene (LMNA). Impairments in these fish arise in the skin, muscle and adipose tissue, and sometimes in the cartilage. Reduced function of lamin A/C by translational blocking of the LMNA gene induced apoptosis, cell-cycle arrest, and craniofacial abnormalities/cartilage defects. By contrast, induced cryptic splicing of LMNA, which generates the deletion of 8 amino acid residues lamin A (zlamin A-Δ8), showed embryonic senescence and S-phase accumulation/arrest. Interestingly, the abnormal muscle and lipodystrophic phenotypes were common in both cases. Hence, both decrease-of-function of lamin A/C and gain-of-function of aberrant lamin A protein induced laminopathies that are associated with mesenchymal cell lineages during zebrafish early development. Visualization of individual cells expressing zebrafish progerin (zProgerin/zlamin A-Δ37) fused to green fluorescent protein further revealed misshapen nuclear membrane. A farnesyltransferase inhibitor reduced these nuclear abnormalities and significantly prevented embryonic senescence and muscle fiber damage induced by zProgerin. Importantly, the adult Progerin fish survived and remained fertile with relatively mild phenotypes only, but had shortened lifespan with obvious distortion of body shape.

Conclusion

We generated new zebrafish models for a human premature aging disorder, and further demonstrated the utility for studying laminopathies. Premature aging could also be modeled in zebrafish embryos. This genetic model may thus provide a new platform for future drug screening as well as genetic analyses aimed at identifying modifier genes that influence not only progeria and laminopathies but also other age-associated human diseases common in vertebrates.     

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 LAP2alpha 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.     

Subcellular localization of SREBP1 depends on its interaction with the C-terminal region of wild-type and disease related A-type lamins

Lamins A and C are nuclear intermediate filament proteins expressed in most differentiated somatic cells. Previous data suggested that prelamin A, the lamin A precursor, accumulates in some lipodystrophy syndromes caused by mutations in the lamin A/C gene, and binds and inactivates the sterol regulatory element binding protein 1 (SREBP1). Here we show that, in vitro, the tail regions of prelamin A, lamin A and lamin C bind a polypeptide of SREBP1. Such interactions also occur in HeLa cells, since expression of lamin tail regions impedes nucleolar accumulation of the SREBP1 polypeptide fused to a nucleolar localization signal sequence. In addition, the tail regions of A-type lamin variants that occur in Dunnigan-type familial partial lipodystrophy of (R482W) and Hutchison Gilford progeria syndrome (?607–656) bind to the SREBP1 polypeptide in vitro, and the corresponding FLAG-tagged full-length lamin variants co-immunoprecipitate the SREBP1 polypeptide in cells. Overexpression of wild-type A-type lamins and variants favors SREBP1 polypeptide localization at the intranuclear periphery, suggesting its sequestration. Our data support the hypothesis that variation of A-type lamin protein level and spatial organization, in particular due to disease-linked mutations, influences the sequestration of SREBP1 at the nuclear envelope and thus contributes to the regulation of SREBP1 function.     

BCL10GFP fusion protein as a substrate for analysis of determinants required for Mucosa-Associated Lymphoid Tissue 1 (MALT1)-mediated cleavage

Background

MALT1 belongs to a family of paracaspase and modulates NF-κB signaling pathways through its scaffolding function and proteolytic activity. MALT1 cleaves protein substrates after a positively charged Arginine residue. BCL10, a 233 amino acids polypeptide, is identified as one of the MALT1 proteolytic substrates. MALT1 cleaves BCL10 at the C-terminal end of Arg228. A mere 5 amino acids difference between the substrate and the proteolytic product made it difficult to tell whether the cleavage event took place by using a simple western blot analysis. Here, BCL10GFP was constructed and utilized to examine the specificity and domain determinants for MALT1 cleavage in cells.

Methods

Various BCL10GFP constructs were transfected into HEK293T cell with MALT1 construct by using calcium phosphate-DNA precipitation method. Lysates of transfectants were resolved by SDS/PAGE and analyzed by western blot analysis.

Results

BCL10GFP was proteolytically processed by MALT1 as BCL10. The integrity of caspase recruitment domain (CARD) and MALT1-interacting domain on BCL10 were required for MALT1 proteolytic activity. Besides the invariant P1 cleavage site Arg228, P4 Leu225 played a role in defining BCL10 as a good substrate for MALT1.

Conclusions

We offered a way of monitoring the catalytic activity of MALT1 in HEK293T cells using BCL10GFP as a substrate. BCL10GFP can be utilized as a convenient tool for studying the determinants for efficient MALT1 cleavage in HEK293T cells