Research ArticleProgeria

Disrupting the LINC complex in smooth muscle cells reduces aortic disease in a mouse model of Hutchinson-Gilford progeria syndrome

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Science Translational Medicine  26 Sep 2018:
Vol. 10, Issue 460, eaat7163
DOI: 10.1126/scitranslmed.aat7163

Help for HGPS

Children with Hutchinson-Gilford progeria syndrome (HGPS), a rare disorder of accelerated aging caused by mutations in lamin, exhibit vascular complications. Kim et al. investigated the mechanism responsible for vascular smooth muscle cell death and aortic disease using HGPS mice. They found that smooth muscle cells expressing progerin (truncated form of lamin produced by mutations in lamin) became apoptotic under stretch. HGPS mice exhibited greater disease burden in the inner curvature of the aorta and at vascular branch regions. Disrupting the transmission of force between the cytoskeleton and nucleus abrogated apoptosis in progerin-expressing smooth muscle cells. These results highlight the role of shear stress in contributing to vascular disease in HGPS.

Abstract

Hutchinson-Gilford progeria syndrome is a disorder of premature aging in children caused by de novo mutations in LMNA that lead to the synthesis of an internally truncated form of prelamin A (commonly called progerin). The production of progerin causes multiple disease phenotypes, including an unusual vascular phenotype characterized by the loss of smooth muscle cells in the arterial media and fibrosis of the adventitia. We show that progerin expression, combined with mechanical stress, promotes smooth muscle cell death. Disrupting the linker of the nucleoskeleton and cytoskeleton (LINC) complex in smooth muscle cells ameliorates the toxic effects of progerin on smooth muscle cells and limits the accompanying adventitial fibrosis.

INTRODUCTION

Hutchinson-Gilford progeria syndrome (HGPS), the classic progeroid disorder of children, elicits multiple disease phenotypes resembling premature aging (16). Affected children appear normal at birth but soon manifest failure to thrive (gaining ~0.44 kg/year), hair loss, sclerodermatous skin, joint contractures, and a variety of dental and bone abnormalities. However, certain hallmarks of physiologic aging (arthritis, cancer, and dementia) are absent, prompting some to refer to HGPS as a “segmental aging syndrome” (7). Children with HGPS die at a mean age of 14.6 years, generally from occlusive arteriosclerotic disease in the coronary and cerebral arteries (6, 8). The classical form of HGPS is caused by a de novo mutation in LMNA that results in the synthesis of a mutant form of prelamin A, commonly called progerin (3, 9).

LMNA normally yields two alternatively spliced transcripts, one for prelamin A (the precursor to mature lamin A) and the other for lamin C (10). Prelamin A terminates with a C-terminal CaaX motif, which triggers farnesylation of a C-terminal cysteine, endoproteolytic cleavage of the last three amino acid residues, carboxyl methylation of the newly exposed farnesylcysteine, followed by the release of 15 additional amino acids by ZMPSTE24 (11). The final ZMPSTE24 cleavage step removes the C-terminal farnesylcysteine methyl ester and releases mature lamin A. The posttranslational processing of prelamin A is thought to aid in the assembly of the nuclear lamina by targeting farnesyl–prelamin A to the inner nuclear membrane. Lamin C is 74 residues shorter than mature lamin A (including six unique amino acids at its C terminus) and does not undergo any of the aforementioned posttranslational processing steps. Lamins A and C, together with lamins B1 and B2, are the building blocks of the nuclear lamina—a filamentous meshwork lining the inner nuclear membrane (12)—in somatic cells. In interphase cells, the nuclear lamina interacts with nuclear chromatin, proteins of the inner nuclear membrane, nuclear pore complexes, and indirectly with the cytoskeleton via the LINC (linker of the nucleoskeleton and cytoskeleton) complex (13). Because of these physical interactions, defects in the nuclear lamina can affect multiple nuclear-related properties such as heterochromatin organization, nuclear membrane integrity, and gene expression (1420).

The most common mutation underlying HGPS is a point mutation in LMNA codon 608 that promotes usage of an alternative splice donor site, leading to an in-frame deletion of 50 amino acids and the production of a mutant prelamin A (progerin) (3, 9). The internal deletion does not affect prelamin A’s CaaX motif but eliminates the upstream ZMPSTE24 cleavage site. Thus, progerin undergoes farnesylation and methylation but does not undergo the final endoproteolytic cleavage step mediated by ZMPSTE24, meaning that the C terminus of progerin retains its farnesylcysteine methyl ester. Progerin accumulates in the nuclear envelope (16, 21), resulting in misshapen nuclei (3), DNA damage (22, 23), increased sensitivity to mechanical strain (24, 25), and cell senescence (2628). Inhibiting the farnesylation of progerin with protein farnesyltransferase inhibitors minimizes several of these phenotypes (21, 2931).

Mice engineered to produce progerin, either globally or in specific tissues, exhibit a variety of disease phenotypes resembling those in children with HGPS: alopecia, loss of subcutaneous adipose tissue, skeletal abnormalities, retarded growth, and shortened life span (3238). About 15 years ago, Varga et al. (39) generated a transgenic mouse with a 164.4-kb bacterial artificial chromosome (BAC) clone containing four genes, including LMNA modified to contain the most common HGPS point mutation. Although the transgenic line did not manifest some of the hallmarks of progeria such as skeletal abnormalities and shortened life span, it developed vascular pathology—loss of smooth muscle cells (SMCs) in the media of the aorta along with fibrosis of the adventitia. Studies using these mice were intriguing because they seemed potentially relevant to the vascular disease observed in children with HGPS. Children with HGPS develop atherosclerotic lesions within the arterial intima (8), but SMC loss and adventitial fibrosis have also been observed (8, 40, 41). The cause of SMC loss is unknown, but it has been speculated that mechanical stress plays a role (8, 25, 39, 41).

Over the past 7 years, other groups have observed aortic disease in mouse models of HGPS (35, 38, 42), but there has been little progress in understanding the etiology of the vascular lesions. For example, given that progerin is expressed in many tissues, it is unclear why the pathology is so obvious in the aorta but seemingly absent in other tissues. Also, why does progerin lead to SMC death, and what causes adventitial fibrosis? Do mechanical forces in large arteries contribute to SMC loss? Here, we investigated these topics using both cell culture and mouse models.

RESULTS

Vascular disease is present in the aorta of LmnaG609G mice

To investigate the aortic pathology associated with HGPS, we studied a knock-in mouse model (LmnaG609G) with a mutant Lmna allele harboring the most common point mutation found in children with HGPS (38). Both LmnaG609G/+ and LmnaG609G/G609G mice developed progressive disease phenotypes (skeletal abnormalities and reduced body weight gain), but disease phenotypes appeared at an earlier age and progressed more rapidly in LmnaG609G/G609G mice (fig. S1A). Western blotting confirmed the synthesis of progerin and the absence of prelamin A in the aorta (Fig. 1A), and hematoxylin and eosin (H&E) staining revealed hallmark vascular phenotypes in the ascending thoracic aorta (loss of SMCs, adventitial thickening, and intact endothelium) (Fig. 1B). To quantify the vascular phenotype, we measured the density of SMC nuclei (nucleus per square micrometer media area) and adventitial area as a percentage of total area in 4-month-old LmnaG609G/G609G mice. In those mice, the density of SMC nuclei was reduced by >90% in the proximal ascending aorta, and the adventitial area was about threefold greater than that in wild-type (WT) mice (Fig. 1C).

Fig. 1 Mice expressing progerin develop aortic pathology.

(A) Western blot showing the synthesis of progerin but not prelamin A in aortas from LmnaG609G/+ (G609G/+) and LmnaG609G/G609G (G609G/G609G) mice. Average progerin amounts (relative to tubulin) were 1.15 and 1.43 for heterozygous and homozygous G609G mice, respectively. KO, knockout; Ab, antibody. (B) Representative H&E-stained sections of the ascending aorta from 4-month-old Lmna+/+ and LmnaG609G/G609G mice. Colored arrows identify the aortic media (black) and adventitia (yellow). Scale bars, 50 μm. (C) Bar graph showing SMC nuclei (left) and adventitial area (right) in the ascending aorta (inner curvature) of 4-month-old LmnaG609G/G609G mice (blue), compared with age-matched WT mice (black). Means ± SEM; n = 10 per group. LmnaG609G/G609G versus WT; *P < 0.001 (t test). (D) Thoracic aortas from WT (+/+), LmnaG609G/+ (G609G/+), and LmnaG609G/G609G (G609G/G609G) mice. Numbered white ovals identify locations for sections in (E). (E) H&E-stained sections of four regions of the thoracic aorta from 4-month-old Lmna+/+ and LmnaG609G/G609G mice. Colored arrows identify the adventitia (yellow) and media (black). Scale bars, 40 μm. (F) Enlarged image of the proximal ascending aorta [boxed area in (E)] showing vacuolated SMCs (black arrowheads). Scale bar, 20 μm. (G) Electron micrographs showing SMC nuclei in Lmna+/+, LmnaG609G/+, and LmnaG609G/G609G mice. Yellow arrowheads point to intranuclear vesicles in LmnaG609G/+ aortas. A duplicate image is colorized to show the nucleoplasm (blue) and cytoplasm (yellow). Red arrowhead points to a cytoplasmic vacuole in a LmnaG609G/G609G aortic SMC.

The vascular phenotype was also observed in other regions of the aorta. Disease was present in the mid-arch, proximal descending aorta, and the lower descending aorta (Fig. 1, D and E, and fig. S1B); the same pathology was observed in the brachiocephalic, right and left common carotid, and left subclavian arteries (fig. S1C). On H&E staining, the muscle layer of the aorta stained less red (Fig. 1E), and some SMCs contained vacuoles (Fig. 1F). No disease was detected in WT mice (Fig. 1E and fig. S1B) or in Zmpste24−/− mice (where there is an accumulation of the farnesylated form of WT prelamin A) (fig. S1D). The onset of aortic disease occurred after 2 months of age [aortas in 2-month-old LmnaG609G/+ and LmnaG609G/G609G mice were free of obvious pathology (fig. S2A)]. We did not observe similar pathology in the parenchyma of other tissues (kidney and skeletal muscle) (fig. S2, B and C).

LmnaG609G aortic SMCs contain intranuclear vesicles

Transmission electron microscopy identified two unusual features in LmnaG609G aortic SMCs. The first was nuclei containing multiple intranuclear vesicles (Fig. 1G and fig. S3, A and B). The vesicles were bounded by inner and outer nuclear membranes, with heterochromatin lining the inner nuclear membrane, and occasionally containing cytoplasmic material. These structures were not observed in endothelial or adventitial cells (fig. S3C). The second unusual feature was that some SMCs contained multiple cytoplasmic vesicles (Fig. 1G and fig. S3B). Most of the vesicles appeared to be surrounded by a single lipid bilayer.

Collagen type VIII synthesis is increased in the aortic adventitia of LmnaG609G mice

Previous histological studies showed increased collagen in diseased aortas (38, 39). In keeping with those findings, we observed increased collagen content in aortas of LmnaG609G/G609G mice, as judged by Verhoeff-Van Gieson (VVG) and Masson’s trichrome (MT) staining (Fig. 2A). To determine the type of collagen that accumulates in diseased aortas, we analyzed gene expression in different layers of the aorta (adventitia versus media). The layers were separated by enzymatic digestion (43) and validated by the expression of marker genes (Col1a1 for the adventitia and Acta2 for the media; Fig. 2B). The expression of Col1a1, Col3a1, Col5a1, and Col8a1 was increased in the adventitia of LmnaG609G/+ mice, with the largest increases relative to WT mice in the expression of Col1a1 and Col8a1 (3.8- and 17.2-fold, respectively). Col1a1 and Col8a1 were also increased in the aortic media, but expression was much lower than that in the adventitia. Immunofluorescence microscopy confirmed increased amounts of collagen types I and VIII protein in the aortic adventitia of LmnaG609G/+ mice (Fig. 2C).

Fig. 2 Collagen synthesis is increased in the adventitia of LmnaG609G mice.

(A) Histochemical studies showing collagen content in the adventitia of aortas from 4-month-old LmnaG609G/G609G and Lmna+/+ mice. H&E: blue, nuclei; pink, cytoplasm. VVG stain: black, elastic fibers and nuclei; red, collagen. MT stain: pink/red, cytoplasm; blue, collagen. Colored arrows identify the media (white) and adventitial (black) layers. Scale bars, 50 μm. (B) Reverse transcription polymerase chain reaction (RT-PCR) studies showing Col1a1 and Col8a1 expression in the adventitia of 12-month-old WT and LmnaG609G/+ mice (means ± SEM; n = 3). LmnaG609G/+ versus WT; *P < 0.05, **P < 0.01, and ***P < 0.001 (t test). (C) Serial frozen sections of the ascending aorta from 12-month-old WT and LmnaG609G/+ mice stained with antibodies against collagen types I, III, IV, V, and VIII (red) and CD31 (cyan), examined by confocal fluorescence microscopy. Scale bars, 50 μm.

Lamin A and progerin are highly expressed in the aorta

The toxicity of progerin is dose-related. In cultured cells, increased amounts of progerin result in more misshapen nuclei, more DNA damage, and more rapid senescence (18, 30). In mouse models, the severity of disease expression correlates with amounts of progerin expression (fig. S1A) (32, 33, 35). Given these observations, we suspected that tissues that express the highest amounts of progerin would be most likely to develop disease. Measuring progerin in tissues of LmnaG609G mice would be the most straightforward approach to test this idea; however, we were concerned that progerin expression might be influenced by the disease itself: The emergence of disease might influence progerin by affecting Lmna splicing, cell viability, or protein turnover. To circumvent this possibility, we first measured lamin A amounts in WT mice, reasoning that lamin A expression in WT mice might correlate with progerin expression in LmnaG609G mice. Lamin A expression was measured in the brain (cerebral cortex), liver, kidney, intercapsular brown adipose tissue, gonadal white adipose tissue, gallbladder, quadriceps, heart, skin, femur, and aorta (ascending, descending, and abdominal segments) by Western blotting (Fig. 3A) and normalized to tubulin, lamin C, or lamin B1 (Fig. 3, B to D). Lamin A expression in the different tissues was variable, differing as much as ~14-fold when normalized to tubulin. Low expression was found in the brain, liver, and kidney, whereas high expression was observed in the aorta, skin, and bone (Fig. 3B). Of note, the latter tissues are sites of pathology in HGPS, whereas the former tissues are not (4). The tissues with high amounts of lamin A (aorta, skin, and bone) also had a high lamin A/lamin C ratio (Fig. 3C).

Fig. 3 Lamin A is expressed at high amounts in the aorta of WT mice.

(A) Representative Western blot comparing lamin A, lamin C, and lamin B1 in different tissues from WT mice. Tubulin was measured as a loading control. Cortex, cerebral cortex; BAT, brown adipose tissue; WAT, white adipose tissue; Gb, gallbladder; ASC, ascending aorta; DESC, descending aorta; ABD, abdominal aorta. Graphs showing lamin A expression relative to tubulin (B), relative to lamin C (C), and relative to lamin B1 (D). For (B) to (D), tissues are arranged in ascending order of expression from left to right, with the expression in kidney set at a value of 1 (means ± SEM; n = 4 mice). (E) RT-PCR comparing the expression of prelamin A (black) and Lmnb1 (blue) in tissues (means ± SEM; n = 4 mice). (F) RT-PCR comparing the expression of prelamin A (black) and Lmnb1 (blue) in the media and adventitia (Adv) layers of WT mice (means ± SEM; n = 4 mice). Media versus adventitia; **P < 0.001 (t test). (G) Confocal fluorescence microscopy images showing the expression of CD31 (cyan) and lamin B1 (red) in the ascending aorta of a WT mouse. In the merged image, the adventitia is outlined by dashed white lines (see fig. S4G). Scale bar, 50 μm.

The brain, a tissue that is spared in HGPS, expresses low amounts of lamin A relative to lamin B1 (37, 44). When we ranked lamin A expression relative to lamin B1, the brain was the lowest and the aorta was the highest (Fig. 3D). Differences in expression were also evident at the transcript level (Fig. 3E). On the basis of the pattern of lamin A expression in WT mice, we predicted that we would find high amounts of progerin expression in the aorta of mice carrying the LmnaG609G allele. This was the case; the tissue with the highest amounts of progerin in LmnaG609G/+ mice was the aorta (fig. S4, A to D).

Lamin B1 amounts are low in aortic medial SMCs

A puzzling feature of the pathology in mouse models of HGPS is the loss of SMCs in the arterial media but the absence of obvious pathology in endothelial cells of the intima and cells of the adventitia. We hypothesized that the susceptibility of different cells to disease might be due to different amounts of lamin A expression. To explore this idea, we separated the media and adventitia layers and measured gene expression. Prelamin A transcript amounts were similar in the media and adventitia, but lamin B1 levels were about three- to fourfold higher in the adventitia than in the media (Fig. 3F). The higher lamin B1 expression in the adventitia was confirmed by Western blotting (fig. S4F) and immunofluorescence microscopy (Fig. 3G). The microscopy studies also showed that lamin B1 expression was higher in endothelial cells than in medial SMCs. Thus, SMCs have a far higher lamin A/lamin B1 ratio than endothelial cells and adventitial cells.

The vascular phenotype is more severe at curvatures and branches

Our experience in dissecting aortas from LmnaG609G mice suggested that the aortic pathology was not uniformly distributed. To determine the distribution of aortic disease, we sectioned the proximal aorta down the midline and stained for CD31 (an endothelial cell marker) and collagen type VIII (a marker of adventitial fibrosis). In 12-month-old LmnaG609G/+ mice and 4-month-old LmnaG609G/G609G mice, the pathology was more pronounced along the inner curvature of the ascending aorta and at major branch points (especially at the origin of the brachiocephalic artery) (Fig. 4, A to C). Quantitative studies in LmnaG609G/+ (Fig. 4, D and E) and LmnaG609G/G609G (fig. S5, A to C) mice revealed that there was more adventitial fibrosis and a greater loss of SMCs in the media along the inner curvature of the aorta. Similar changes were previously noted along the posterior wall of the ascending aorta in mice expressing human progerin, but the changes were not quantified (42). Given that our Western blot and gene expression studies showed a correlation between high lamin A expression, low lamin B1 expression, and the extent of aortic pathology, we suspected that we might find distinct patterns of lamin A and lamin B1 expression in the inner and outer curvatures of the aorta. However, this was not the case (Fig. 4F).

Fig. 4 Vascular pathology is more severe along the inner curvature of the ascending aorta and branches of the aortic arch.

(A to C) Composite fluorescence microscopy images of the ascending thoracic aorta stained with antibodies against CD31 (green) and collagen type VIII (red) in 12-month-old Lmna+/+ (A) and LmnaG609G/+ (B) mice and in a 4-month-old LmnaG609G/G609G mouse (C). The white lines identify the location where the measurements were made. BC, brachiocephalic; LCC, left common carotid; LSC, left subclavian. Scale bars, 500 μm. (D) Bar graph showing adventitial area as a percentage of total area in WT (black) and LmnaG609G/+ (white) mice at the outer and inner curvatures of the ascending aorta. (E) Bar graph showing the number of SMC nuclei relative to media area in WT (black) and LmnaG609G/+ (white) mice at the outer and inner curvatures of the ascending aorta. Means ± SEM for WT (n = 12) and LmnaG609G/+ (n = 11) mice; *P < 0.02 and **P < 0.001 (t test). (F) RT-PCR measuring prelamin A, lamin C, and Lmnb1 expression at the inner and outer curvatures in WT mice (means ± SEM; n = 4). Inner versus outer; ns, not significant; P > 0.20 (t test).

Progerin expression induces hallmark HGPS phenotypes in cultured SMCs

The fact that the aortic pathology in the HGPS mice was greater in some locations than in others led us to suspect that local biomechanical forces are relevant to disease pathogenesis. The predilection of specific regions of the mouse aorta to atherosclerotic lesions near curvatures and branch points has been attributed to disturbed blood flow and dynamic changes in shear stress (45, 46). To determine whether cells expressing progerin have increased sensitivity to mechanical forces, we developed a model system with cultured cells. Mouse SMCs stably expressing either human prelamin A or human progerin were grown on flexible polydimethylsiloxane (PDMS) membranes and exposed to repetitive biaxial stretch. In these studies, a doxycycline-inducible system was used to achieve a lamin A/lamin C profile similar to that in medial SMCs in vivo (lamin A/lamin C ratio of 3–4:1) (Fig. 3C). The expression of mature lamin A and progerin depended on doxycycline concentration in the medium, and the optimal expression for lamin A and progerin was achieved at 0.1 and 0.3 μg of doxycycline per milliliter, respectively (fig. S6A). The expression of progerin in SMCs resulted in “HGPS phenotypes”—misshapen nuclei (Fig. 5A) and markers of DNA damage [an accumulation of phosphorylated p53(Ser15) and H2AXγ] (Fig. 5B). The increased DNA damage in progerin-expressing cells could not be attributed to the higher concentration of doxycycline required to induce progerin expression because DNA damage was not observed when WT cells were incubated with up to 3 μg of doxycycline per milliliter (fig. S6B).

Fig. 5 Disrupting the LINC complex in SMCs ameliorates phenotypes elicited by progerin.

(A) Microscopy images of mouse aortic SMCs expressing prelamin A and progerin. Scale bars, 20 μm. Bar graph shows quantification of misshapen nuclei (means ± SEM; n = 3 experiments). *P < 0.02 (t test). (B) Western blot of lamin A (LA), lamin C, progerin (P), phosphorylated p53 (Phos-p53), and H2AXγ expression in cells expressing lamin A or progerin. Cells exposed to ultraviolet (UV) light were included as a control. The bar graph shows the expression of p53 phosphorylation (black) and H2AXγ (red) after 1 or 2 days (means ± SEM; n = 3 experiments). Progerin versus prelamin A; *P < 0.01 and **P < 0.001 (t test). (C) Cell protein expressed relative to static cells (means ± SEM; n = 3 experiments) in prelamin A– and progerin-expressing SMCs exposed to uniaxial strain via stretching (6 mm, 0.5 Hz) or static conditions for 1 day. Stretch versus static; *P < 0.001 (t test). (D) Microscopy images of progerin-expressing cells transduced with enhanced green fluorescent protein (EGFP)–labeled KASH2 or EGFP-labeled ext-KASH2 (both in green). Nuclei were stained with DAPI (4′,6-diamidino-2-phenylindole) (blue) and LAP2β (red; a nuclear membrane marker). Scale bars, 20 μm. Bar graph shows quantification of misshapen nuclei in prelamin A– and progerin-expressing cells (means ± SEM; n = 3 experiments). KASH2 versus ext-KASH2; *P < 0.02 (t test). (E) Western blot showing lamin A, progerin, lamin C, phosphorylated p53, KASH2, and H2AXγ in unstrained, prelamin A (WT)– or progerin (608)–expressing cells. The bar graph shows the average expression of p53 phosphorylation (white) and H2AXγ (black) after 1 day (means ± SEM; n = 3 experiments). KASH2 versus ext-KASH2; *P < 0.05 and **P < 0.01 (t test). (F) Bar graph showing relative cell protein in progerin- and prelamin A–expressing stretched SMCs transfected with KASH2 or ext-KASH2 (means ± SEM; n = 3 experiments). KASH2 versus ext-KASH2; *P < 0.001 (t test).

Progerin expression promotes cell death in stretched SMCs

To determine whether biomechanical forces could contribute to SMC loss in HGPS mice, cells were cultured on flexible PDMS membranes and exposed to repetitive biaxial stretching (25). After 24 hours, ~40% of progerin-expressing SMCs detached from the membrane, as judged by cell protein (Fig. 5C). Collection of the detached cells and staining with trypan blue showed that the cells were nonviable. In contrast, SMCs that expressed WT prelamin A were unaffected and remained viable. To assess the effects of stretch before cell death and detachment, the cells were stained with propidium iodide (PI) (25) after stretching for 2 hours. PI staining increased to ~12% in progerin-expressing SMCs, whereas it increased to only ~1% in lamin A–expressing cells (fig. S6C). Similarly, reducing lamin B1 increased H2AXγ to a larger extent in progerin-expressing SMCs than in prelamin A–expressing cells (fig. S6D). Thus, progerin expression renders SMCs more susceptible to cell death in response to mechanical stress.

Disrupting the LINC complex reduces the toxic effects of progerin in cultured SMCs

The cell nucleus is connected to the cytoskeleton by the LINC complex (13), and this complex is known to transmit biomechanical forces to the cell nucleus. Two outer nuclear membrane proteins of the complex, Nesprin1 and Nesprin2, are tethered to the cytoskeleton. The KASH (Klarsicht/Anc-1, Syne Homology) domains of Nesprin1 and Nesprin2, located within the perinuclear space, interact with the Sun domains of Sun1 and Sun2. Sun1 and Sun2 traverse the inner nuclear membrane and interact with the nuclear lamina (47). The link between the cytoskeleton and the nuclear lamina can be disrupted by overexpressing the KASH domain of Nesprin2 (KASH2). KASH2 expression occupies the binding sites on the Sun proteins and thereby prevents Sun-Nesprin interactions within the perinuclear space (48). We predicted that KASH2 expression would reduce force transmission to the nucleus and limit the death of progerin-expressing cells during repetitive stretching. To test this possibility, we generated stable cell lines expressing either a KASH2-EGFP fusion protein (KASH2) or an inactive KASH2-EGFP mutant (ext-KASH2). KASH2 prevented the nuclear shape abnormalities that appear in progerin-expressing cells (Fig. 5D) and simultaneously reduced markers of DNA damage (Fig. 5E). Of note, these effects were observed in the absence of any effects on progerin (Fig. 5E). When progerin-expressing SMCs were exposed to repetitive stretching, the expression of KASH2 increased cell survival by 71% and reduced PI staining by ~60% (Fig. 5F and fig. S6C). These studies demonstrate that KASH2-mediated disruption of force transmission to the cell nucleus limits the toxicity of progerin in cultured SMCs.

Disrupting the LINC complex in aortic SMCs ameliorates SMC loss and adventitial fibrosis

The beneficial effects of KASH2 expression in progerin-expressing SMCs prompted us to test whether KASH2 expression in the SMCs of HGPS mice would ameliorate SMC loss in the medial layer of the aorta. We introduced a Cre-conditional KASH2-EGFP transgene (49) into LmnaG609G/G609G mice harboring an SMC-specific Sm22-Cre transgene (50). We bred three groups of mice: Lmna+/+KASH2–EGFP+Sm22-Cre+ (WT), LmnaG609G/G609GKASH2–EGFP+Sm22-Cre (MUT), and LmnaG609G/G609GKASH2–EGFP+Sm22-Cre+ (MUT + KASH2). The specificity of the Sm22-Cre transgene was confirmed with mT/mG reporter mice (51). Sm22-Cre–mediated recombination occurred only in SMCs in the aortic medial layer, as judged by EGFP fluorescence (fig. S6E). KASH2 expression in the aorta of LmnaG609G mice was confirmed by Western blotting, and this did not affect progerin expression (fig. S6F). Because all mice were collected at 4 months of age (before LmnaG609G mice died of progeria disease), the study design did not permit us to determine the effects of KASH2 expression on mouse survival. However, KASH2 expression did not reduce body weights (fig. S6G).

KASH2 expression in MUT mice improved vascular pathology in the ascending aorta (inner and outer curvatures) and proximal descending thoracic aorta (Fig. 6). In the outer curvature of the ascending aorta (Fig. 6A), the density of SMC nuclei increased to 65% of that in WT mice, and adventitial fibrosis was eliminated (Fig. 6E). In the inner curvature, where the aortic pathology is more severe, the density of SMC nuclei increased about threefold but only to ~33% of the density observed in the aorta of WT mice (Fig. 6, B and F). Nevertheless, adventitial fibrosis in the ascending aorta was greatly improved (~60% decrease) (Fig. 6F). In the descending aorta, where disease is less severe, the beneficial effects of KASH2 expression were striking (Fig. 6, C and D). The density of SMC nuclei in the medial layer of MUT + KASH2 aortas was 75% as high as that in WT mice, and adventitial fibrosis was eliminated (Fig. 6G). These studies demonstrate that force transmission from the cytoskeleton to the nucleus is a contributing factor to promoting SMC loss. Moreover, our results show that improved SMC survival prevents adventitial fibrosis, indicating that adventitial fibrosis in HGPS mice is secondary to the loss of SMCs.

Fig. 6 The expression of the KASH2 domain in SMCs ameliorates aortic disease in LmnaG609G/G609G mice.

(A to D) Representative photographs of H&E-stained cross sections at the outer (A) and inner (B) curvatures of the ascending aorta and proximal descending aorta (C and D) of WT (Lmna+/+KASH2–EGFP+Sm22-Cre+), MUT (LmnaG609G/G609GKASH2–EGFP+Sm22-Cre), and MUT + KASH2 (LmnaG609G/G609GKASH2–EGFP+Sm22-Cre+) mice. Scale bars, 50 μm (A to C) and 20 μm (D). (E) Adventitial area as a percentage of total area and numbers of SMC nuclei per media area at the outer ascending thoracic aorta (means ± SEM; n = 6 per group). (F) Adventitial area and numbers of SMC nuclei at the inner ascending thoracic aorta (means ± SEM; n = 6 per group). (G) Adventitial area and numbers of SMC nuclei in the proximal descending thoracic aorta (means ± SEM; n = 6 per group). *P < 0.01 and **P < 0.001; ns, not significant defined as P > 0.40 (t test).

DISCUSSION

The vascular pathology of HGPS has puzzled physicians and biomedical investigators for decades (8, 40, 41). Despite an absence of the typical risk factors for atherosclerosis, children with HGPS succumb to heart attacks or stroke, a consequence of occlusions in the coronary and cerebral arteries (52, 53). The arterial pathology in HGPS must be caused by progerin, but the underlying mechanisms have remained unclear. In the current studies, we investigated the vascular pathology in HGPS mice and developed three insights into pathogenesis. The first relates to why the aorta develops disease while other tissues are spared. We found that the aorta produces more progerin than any other tissue—more than the skin and bone (two tissues affected by HGPS) and ~15-fold more than the kidney (an unaffected tissue). Electron micrographs of aortic SMCs of HGPS mice revealed striking abnormalities—intranuclear membrane vesicles and vacuoles in the cytoplasm. The second insight is that mechanical forces influence the distribution of vascular lesions. The vascular disease in HGPS mice is most severe along the inner curvature of the ascending aorta and at arterial branch points, where blood flow is disturbed and biomechanical forces are altered (45, 46). The third insight is that reducing force transmission to the nucleus ameliorates the toxicity of progerin. In cultured SMCs, uncoupling the LINC complex with KASH2 (and thereby blunting force transmission to the cell nucleus) resulted in less DNA damage, fewer nuclear blebs, and reduced cell death. The expression of KASH2 in SMCs in HGPS mice markedly reduced SMC loss in the aortic media and reduced adventitial fibrosis.

Both the loss of aortic medial SMCs and the accompanying adventitial fibrosis depend on the dosage of progerin. The onset of vascular disease is earlier and disease progression is more rapid in homozygous HGPS mice than in heterozygous mice. The fact that disease severity depends on progerin dosage is not particularly surprising, given earlier studies showing that the amount of progerin expression affects the frequency of misshapen nuclei in cultured fibroblasts (18, 30) and mouse experiments showing that the severity of nonvascular disease phenotypes depends on the extent of progerin expression (32, 33, 35). Of note, in humans, point mutations that increase progerin expression are associated with more severe disease (54).

Given the link between gene dosage and the severity of disease phenotypes, we suspected that we might encounter higher amounts of progerin expression in tissues that are susceptible to disease (aorta, skin, and bone) than in tissues that are spared (liver, kidney, and brain). This was the case. Of note, the higher amounts of progerin expression in the aorta, bone, and skin were not secondary to pathology in those tissues, but due to the intrinsic higher expression of lamin A in those tissues as found in WT mice (highest in skin, bone, and aorta and lowest in liver, kidney, and brain). The aorta was the third highest for lamin A expression in WT mice, behind skin and bone, but ranked first for progerin expression in HGPS mice. Those observations raise the possibility that progerin might accumulate in the aorta in HGPS mice, perhaps because of lower levels of turnover.

The distribution of vascular lesions was not uniform in the proximal aorta, suggesting that there are regional influences on lesion formation. The vascular disease was most severe along the inner curvature of the ascending aorta and at branches of the aortic arch—the very same locations where early atherosclerotic lesions develop in models of atherosclerosis. The nonrandom formation of atherosclerotic lesions is thought to be due to differences in flow patterns, where lesion formation coincides with areas of disturbed flow and oscillating low shear stress conditions—namely, at bends and branch points (45). The fact that HGPS vascular lesions develop in the same locations suggests that low shear stress contributes to the HGPS vascular phenotype. However, it is noteworthy that SMC loss and adventitial fibrosis also occur along the outer curvature of the ascending aorta (albeit to a lower extent), where shear stress and circumferential vessel strain are high. Thus, HGPS vascular lesions are not limited to regions of low shear stress and suggest that high vessel strain could also lead to SMC loss. Our in vitro mechanical stretch studies, which more closely model high vessel strain, support this proposal. In our studies, the combination of progerin expression and repetitive cell stretching resulted in greater cell toxicity than progerin expression alone.

Disrupting the LINC complex in SMCs with KASH2 expression ameliorated medial SMC loss and adventitial fibrosis throughout the aorta, but the greatest improvement was in regions with the least severe disease—the outer curvature of the ascending aorta and the descending aorta. In those regions, KASH2 expression in SMCs increased the density of medial SMCs to amounts approaching those in WT mice. Somewhat less expected was the significant reduction of adventitial fibrosis with KASH2. The Sm22-Cre transgene used to activate KASH2 expression is specific for medial SMCs in the aorta; hence, the improvement in adventitial fibrosis cannot be attributed to KASH2 expression in the mesenchymal cells of the adventitia. The simplest explanation is that collagen synthesis in the adventitia is activated in response to medial SMC death and weakening of the medial layer. In support of this idea, we observed an inverse relationship between medial SMC density and collagen type VIII staining in the aortic adventitia. Also, in earlier studies, Turlo et al. (55) observed aortic adventitial fibrosis in the setting of a genetic intervention (inactivation of β1 integrin) that caused SMC death in the aortic media.

Nearly 20 years ago, Stehbens et al. (41) described SMC loss in the aorta in two autopsy cases of “progeria” (a firm diagnosis of HGPS was not established by genetic testing). In the more severe case, the medial layer of the aorta was nearly devoid of SMCs, and immunohistochemistry studies showed increased expression of collagen types I and III in the aortic intima. More recently, Olive et al. (8) examined atherosclerotic lesions in two children with bona fide HGPS (where a diagnosis of HGPS was confirmed by genetic testing). They observed loss of vascular SMCs in the aorta of one case, but both cases had adventitial fibrosis. Staining with Picrosirius red revealed increased amounts of collagen types I and III in the aorta. In our studies, Col1a1 and Col3a1 expression were increased in the aortic adventitia of HGPS mice, but the increase in Col8a1 expression was the most impressive (17-fold greater than that in the aorta of WT mice). The expression of collagen type VIII in blood vessels is normally very low, but increased expression has been observed in the setting of vascular injury and atherosclerosis (56, 57). Further studies will be required to determine whether increased levels of collagen type VIII also occur in other diseased tissues (skin and bone).

An intriguing issue is why medial SMCs in HGPS mice die, whereas the endothelial cells in the arterial intima and the mesenchymal cells of the adventitia remain viable. We doubt that these differences can be fully explained by differences in progerin expression. We found similar amounts of prelamin A transcripts in the media and adventitia of the mouse aorta, and the amounts of lamin A protein in the aorta media were only about twofold higher than that in the adventitia. We speculate that the peculiar susceptibility of medial SMCs to cell death could relate to low amounts of lamin B1 expression. By immunofluorescence microscopy, lamin B1 expression in medial SMCs is low—far lower than that in intimal endothelial cells or adventitial cells. Gene expression and Western blotting studies confirmed those findings. Low amounts of lamin B1 expression have been reported to cause stiffer cell nuclei (58) and to induce senescence (59)—similar to phenotypes elicited by progerin. We speculate that increased rigidity of cell nuclei in medial SMCs, where progerin expression is high and lamin B1 expression is low, renders the nucleus more susceptible to nuclear membrane ruptures (20, 60), which could lead to DNA damage and cell death (61). Additional studies are required to investigate the role of lamin B1 in the increased mechanical sensitivity of SMCs.

The ultrastructural hallmark of aortic medial SMCs in HGPS mice are the intranuclear vesicles bounded by a double membrane and surrounded by heterochromatin. We suspect that these intranuclear vesicles, which are absent from intimal endothelial cells and adventitial cells, are the result of tubular invaginations of the nuclear envelope. In support of this idea, we have observed cytoplasmic contents inside the intranuclear vesicles. We do not understand the mechanism for the tubular invaginations, but one possibility is that high progerin expression leads to increased nuclear membrane synthesis (62), which, in turn, results in tubular invaginations. The consequences of deep nuclear membrane invaginations are unknown, but it is easy to imagine that these structures could disrupt chromatin architecture and gene expression. It is also conceivable that the transmission of cytoskeletal forces to intranuclear vesicles would be tangential to the surface of the nucleus (or would occur in multiple directions), increasing the likelihood of nuclear membrane ruptures. We also observed SMCs containing multiple cytoplasmic vesicles, surrounded by a single lipid bilayer. We suspect that these represented dying cells with cytoplasmic autolysosomes (63).

In mouse models of atherosclerosis driven by hyperlipidemia, the atherosclerotic lesions preferentially form at the inner curvature of the ascending aorta and at branches of the aortic arch, sites of disturbed flow and oscillating shear stress (45). Those sites are identical to sites of vascular pathology in HGPS mice, implying that shear stress is a common factor in the pathogenesis of hyperlipidemia-driven atherosclerosis and the vascular pathology in HGPS. Although atherosclerosis is generally considered to be an intimal disease, the loss of SMCs in the media and the resultant weakening of the aorta could lead indirectly to more atheromas in the intima. Of note, Clarke et al. (64) have shown that inducing vascular SMC apoptosis in Apoe−/− mice (a model of atherosclerosis) induces intense intimal inflammation and characteristics of atherosclerotic plaque vulnerability (thin fibrous cap, inflammatory foci, large necrotic core, cellular debris, and depletion of matrix content). Alternatively, it is entirely possible that progerin expression in arterial endothelial cells, accompanied by increased biomechanical forces, leads to more subtle (less visible) changes in intimal endothelial cells that promote atherogenesis (increased lipoprotein uptake, more lipoprotein oxidation, and increased macrophage infiltration) (65). Additional studies will be required to assess the impact of progerin on arterial wall cell function and atherogenesis. Such studies will likely add to our understanding of vascular disease in children with HGPS and further define the relevance of HGPS vascular pathology to ordinary atherosclerosis in adults within the general population.

MATERIALS AND METHODS

Study design

The objective of the study was to investigate the vascular phenotype in HGPS. The vascular phenotype—defined as the loss of SMCs and fibrosis of the adventitia—was examined in the thoracic aorta of LmnaG609G mice, a mouse model of HGPS. We hypothesized that high progerin expression combined with mechanical strain promote SMC loss. To test this hypothesis, we performed studies in cultured cells and mouse models. Unless otherwise specified, all cell culture studies were independently performed at least three times. Studies in animals were performed in group sizes listed in figure legends.

Mice

Zmpste24−/− and LmnaG609G/G609G mice have been described previously (38, 66). Sm22α-Cre transgenic (stock no. 017491) and mT/mG reporter (stock no. 007676) mice were purchased from The Jackson Laboratory. The Sm22α-Cre mouse strain was genotyped by PCR with forward primer 5′-CAGACACCGAAGCTACTCTCCTTCC-3′ and reverse primer 5′-CGCATAACCAGTGAAACAGCATTGC-3′ [yielding a 500–base pair (bp) product]. The mT/mG mouse strain was genotyped with a mutant forward primer 5′-TAGAGCTTGCGGAACCCTTC-3′, a WT forward primer 5′-AGGGAGCTGCAGTGGAGTAG-3′, and a common reverse primer 5′-CTTTAAGCCTGCCCAGAAGA-3′ (128-bp product for the mutant allele and 212-bp product for the WT allele). The CAG-LacZ/EGFP-KASH2 transgenic mouse (49) was genotyped with forward primer 5′-GGAGTTCGTGACCGCCGCCGGGATCACTCT-3′ and reverse primer 5′-TTTAAACGGGCCCCCTAGGTGGGAGGTGGC-3′ (yielding a ~280-bp product). Mice were housed in a specific pathogen–free barrier facility with a 12-hour light/dark cycle. The mice were provided pelleted mouse chow (NIH-31) and water ad libitum and nutritional food cups as required for supportive care. All animal studies were approved by the UCLA’s (University of California, Los Angeles) Animal Research Committee. Antibodies used for Western blotting and immunohistochemistry are reported in table S1; primers used for quantitative RT-PCR are listed in table S2.

Cells

Immortalized mouse aortic SMCs (#CRL-2797) were purchased from the American Type Culture Collection and cultured in Dulbecco’s modified Eagle’s medium (Gibco) containing 10% fetal bovine serum (HyClone), 1× nonessential amino acids (Gibco), 1 mM sodium pyruvate (Gibco), 2 mM glutamine (Gibco), and G418 (0.2 mg/ml; Gibco). The cells exhibit both synthetic and contractile phenotypes, as judged by a high proliferation rate and the absence of smooth muscle myosin heavy chain expression (67), and perinuclear distribution of vimentin (68), respectively. The cells were transduced with lentivirus by UCLA’s Vector Core Facility. To produce UV-treated cells, SMCs in six-well dishes were exposed to UV light (100 mJ/cm2) in a Stratalinker 2400 (Stratagene). Cells were transfected with 100 nM Lmnb1 siRNA (small interfering RNA) (AM16706, Ambion) using RNAiMAX (Invitrogen) according to the manufacturer’s instructions.

Stretching SMCs on PDMS membranes

Flexible PDMS membranes (1 mm thick) were prepared in 150-mm culture dishes with the Sylgard 184 silicone elastomer kit (#3097358-1004, Dow Corning). Membranes strips (7 × 0.8 cm) were activated with a plasma cleaner, treated with 2% 3-aminopropy-triethoxysilane at room temperature for 45 min, washed in ethanol, and then dried at 55°C for 30 min. The membranes were incubated with sulfo-SANPAH [sulfosuccinimidyl 6-(4′-azido-2′-nitrophenylamino)hexanoate] (0.5 mg/ml) in Hepes buffer and cross-linked with UV exposure (300 to 460 nm) for 30 s. The washed membranes were stored at 4°C in a collagen solution (100 μg/ml; 5005, PureCol; Advanced BioMatrix). Cells (1 × 105) were added to individual membrane strips in molds and incubated in media ± doxycycline for 24 hours and then clamped into a custom-built stretching device. The brackets holding the membranes were attached to an L12 linear actuator (Actuonix) controlled by a multifunction DAQ (data acquisition) device (National Instruments) and LabVIEW 2015 software (National Instruments). The membranes were stretched 6 mm at 0.5 Hz (24 hours) for the cell viability studies (trypan blue and cell protein measurements), 3 mm at 0.5 Hz (24 hours) for the DNA damage studies (Western blotting studies), and 2 mm at 0.5 Hz (2 hours) for the cell damage studies (PI studies). The milder conditions for the DNA and cell damage studies were to examine the effects of mechanical stress before cell death. To measure cell protein, membranes were washed with phosphate-buffered saline (PBS) and digested with 0.1 N NaOH, and protein content was measured with the DC protein assay kit (Bio-Rad). To stain cells with PI, we incubated membranes with freshly prepared PI (10 μg/ml; Thermo Fisher Scientific) and Hoechst 33342 (1 μg/ml; Thermo Fisher Scientific) in Dulbecco’s PBS at room temperature for 15 min. The stained cells were washed, fixed, and analyzed by fluorescence microscopy.

Separation of aortic adventitia and media layers

The adventitia and media layers of the mouse aorta were separated by enzymatic digestion (43). Cleaned aortas were incubated in PBS containing collagenase type II (1 mg/ml), soybean trypsin inhibitor (1 mg/ml), and elastase (2 mg/ml) (all from Worthington) at 37°C for 10 min. The digested aortas were placed into ice-cold PBS and cut into three segments of equal length, and the adventitia layer was unrolled from each segment. The adventitia and media (also including the endothelial cell layer) were processed immediately or frozen in liquid nitrogen for storage.

Statistical analysis

Statistical analyses were performed with Microsoft Excel for Mac 2011 and GraphPad Software. Experimental groups were analyzed by a two-tailed Student’s t test. Individual subject-level data are reported in table S3.

SUPPLEMENTARY MATERIALS

www.sciencetranslationalmedicine.org/cgi/content/full/10/460/eaat7163/DC1

Materials and Methods

Fig. S1. Pathology in LmnaG609G mice.

Fig. S2. H&E staining of aortic tissue from young mice and nonaortic tissues from older LmnaG609G/+ mice.

Fig. S3. Cell and nuclear morphology in LmnaG609G aortic SMCs.

Fig. S4. Nuclear lamin expression in the aorta of WT and LmnaG609G/+ mice.

Fig. S5. Analysis of the vascular phenotype in the ascending aorta of LmnaG609G/G609G mice.

Fig. S6. Control studies for the lamin-inducible SMC system and the Sm22-Cre–dependent activation of KASH2 expression in the aorta.

Table S1. Antibodies used for Western blotting and immunohistochemistry.

Table S2. Quantitative RT-PCR primers.

Table S3. Individual subject-level data.

Reference (69)

REFERENCES AND NOTES

Acknowledgments: We thank D. Di Carlo (UCLA) for the use of the plasma cleaner and L. Edillo for assistance with quantification of misshapen nuclei. Virus production and transduction were performed by the Integrated Molecular Technologies Core (IMTC)/UCLA Vector Core, which is supported by CURE/P30 DK041301. Funding: This work was supported by the NIH (AG047192 and AG035626). Author contributions: Designing research studies: P.H.K., S.G.Y., and L.G.F. Conducting experiments: P.H.K., J.L., P.H., Y.T., T.A.W., C.L., N.C., R.L.L., and L.G.F. Providing reagents: P.-Y.L., J.C.Y.D., and D.H. Writing the manuscript: P.H.K., S.G.Y., and L.G.F. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data associated with this study are present in the paper or the Supplementary Materials.
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