Accumulation of DNA damage is a major driving force of normal cellular aging and has recently been demonstrated to hasten the development of vascular diseases such as atherosclerosis. VSMCs (vascular smooth muscle cells) are essential for vessel wall integrity and repair, and maintenance of their proliferative capacity is essential for vascular health. The signalling pathways that determine VSMC aging remain poorly defined; however, recent evidence implicates persistent DNA damage and the A-type nuclear lamins as key regulators of this process. In the present review, we discuss the importance of the nuclear lamina in the spatial organization of nuclear signalling events, including the DNA-damage response. In particular, we focus on the evidence suggesting that prelamin A accumulation interferes with nuclear spatial compartmentalization by disrupting chromatin organization and DNA-damage repair pathways to promote VSMC aging and senescence.
- DNA-damage repair
- nuclear lamina
- persistent DNA damage
- smooth muscle cell
VSMCs (vascular smooth muscle cells) are a major component of the vessel wall and function to maintain vascular tone . VSMCs normally exist in a contractile quiescent state. However, unlike skeletal and cardiac muscle, they are not terminally differentiated and display a unique ability to modulate to a proliferative and migratory phenotype in order to facilitate vessel repair in response to vascular injury [1,2]. Age is the single largest risk factor in the development of vascular disease; however, our understanding of the mechanisms driving VSMC aging are still poorly defined.
Previous studies suggest that the DDR (DNA-damage response) is a critical regulator of VSMC aging [3,4]. DNA damage is normally efficiently repaired in VSMCs by pathways that induce cell-cycle checkpoints that block damaged cells from entering mitosis. This is critical, as chromosomal breaks in mitosis promote genomic instability and tumorigenic transformation. To prevent this, prolonged DNA-damage signalling, arising from either excessive levels of DNA damage or defects in the repair process, act to induce senescence, a permanent cell-cycle arrest that safeguards against cellular transformation . However, VSMCs within atherosclerotic plaques display persistent DNA-damage signalling, telomere attrition, senescence and genomic instability [3,4]. Furthermore, plaque-derived VSMCs grow poorly, senesce early, display nuclear morphology defects and show persistent DNA-damage signalling that is reminiscent of high-passage ‘aged’ VSMCs that are approaching senescence [3,4]. This suggests that aged or diseased VSMCs are unable to efficiently repair DNA damage, leading to the accumulation of senescent cells. These senescent VSMCs have an impaired ability to modulate and repair the vessel wall, leading to an acceleration of vascular disease [3,4].
Within the vessel wall, VSMCs are exposed to a variety of exogenous stresses that induce DNA damage, with oxidative stress most prevalent . The risk factors for the development of atherosclerosis, including diabetes and lipid accumulation increase the oxidative environment of the vessel wall [6,7] and, consequently, atherosclerotic plaques display high levels of ROS (reactive oxygen species) that have the ability to directly induce SSBs (single-strand breaks) and DSBs (double-strand breaks) . In addition, we have recently shown that, in response to oxidative stress/damage, VSMCs accumulate prelamin A and that this protein accelerates the VSMC aging process . This is because prelamin A accumulation during VSMC aging alters nuclear lamina function and activates persistent DNA-damage signalling that ultimately drives VSMC senescence. Emerging evidence outlined below suggests that the integrity of the nuclear lamina is crucial for maintaining VSMC health and its dysfunction is a key event in driving VSMC aging.
The DNA-damage response
DNA is exposed to a variety of both endogenous and exogenous genotoxic stresses that induce different types of breaks and adducts, including DSBs, SSBs, mismatches and oxidative modification that need to be resolved to ensure genetic fidelity. To overcome these genetic insults, cells have evolved elaborate repair pathways collectively termed the DDR that are composed of an ever-expanding array of proteins involved either directly in DNA repair or in the signalling that facilitates these repair processes .
DNA-damage signalling pathways share common characteristics. Lesions in DNA are detected by a variety of sensors that recruit signal transducers. These in turn transmit signals to effector proteins that mediate cellular responses to the damage . Two DNA-damage-sensing complexes have been described: the MRN (Mre11–Rad50–Nbs1) complex that detects DSBs in G1-phase, and the 9-1-1 (Rad9–Rad1–Hus1) complex that detects lesions during DNA replication in S-phase [10,11]. These MRN and 9-1-1 complexes in turn recruit other factors, including the signal transducers ATM (ataxia telangiectasia mutated) and ATR (ATM- and Rad3-related) respectively to the lesion. ATM and ATR belong to the phosphoinositide 3-kinase like family of serine/threonine kinases. A large body of evidence suggests that ATM functions in the DSB-repair pathway, whereas ATR is pivotal during SSB and replicative DNA-damage repair [12,13]. Finally, these transducers phosphorylate and activate the downstream effector kinases Chk1 and Chk2 (checkpoint kinase 1 and 2). Classically, Chk1 is activated by ATR and Chk2 is activated by ATM to mediate the cellular responses to DNA damage, including DNA repair, senescence and apoptosis .
A key feature of the DDR is its spatial compartmentalization. Two forms of nuclear compartmentalization are observed during the DDR. In the first instance, repair and signalling proteins, including the MRN complex and ATM/ATR, are recruited to sites of DNA damage, where they participate in lesion repair . The second form of compartmentalization is observed at sites that are distant from DNA lesions. Several intranuclear structures, including PML (promyelocytic leukaemia protein) NBs (nuclear bodies) and the nucleolus have been implicated in the DDR and undergo reorganization in response to DNA damage. For example, PML NBs have been proposed to sense DNA lesions and are known to reorganize to abut the sites of damage [15–17]. Importantly, several proteins involved in DNA-damage repair, including Nbs1, are components of PML NBs and, in the presence of DNA damage, they are rapidly released from this NB and relocalize to the sites of DNA damage in order to facilitate lesion repair [18–20]. Furthermore, several studies have also shown that PML NBs relocate to nucleolar cap structures in the presence of DNA damage [21–23]. Importantly, these nucleolar caps sequester Mdm2 (murine double minute 2) at the nucleolus to activate DNA-damage-induced p53 signalling [24,25]. Failure of these proteins to localize correctly in the presence of DNA damage, either at repair foci or at sites distant to the lesion, results in inefficient DNA repair that promotes genomic instability. Although spatial reorganization of the nucleus in response to DNA damage is essential for efficient repair, our understanding of the factors that mediate this process is lacking. However, the filamentous nuclear lamina is a good candidate for providing a structural framework to facilitate nuclear compartmentalization, and emerging evidence supports this notion .
The nuclear lamina and nuclear organization
The nuclear lamina is composed of the A-type (lamins A/C) and B-type (lamins B1 and B2) lamins, as well as a host of lamin-binding proteins. Lamins are intermediate filament proteins that form a filamentous meshwork that underlies the INM (inner nuclear membrane). This ‘lamina’ functions to organize and provide structural support to the NE (nuclear envelope); however, the filamentous meshwork produced by the lamins extends beyond the NE and is present throughout the nucleoplasm . Importantly, disruption of the nuclear lamina gives rise to a group of divergent tissue-specific disorders, collectively termed the laminopathies, many of which manifest with aging-like phenotypes. These same disorders, including EDMD (Emery–Dreifuss muscular dystrophy), can also be caused by disruption of lamin A INM-binding partners, including emerin and nesprins 1 and 2 . These observations have focused the laminopathy field towards investigating the role of the A-type lamins at the INM, leaving our understanding of the intranuclear roles of this complex filamentous network in its infancy .
Indeed, within the nucleus, lamins bind and anchor a variety of proteins that regulate chromatin and telomere organization, splicing and nuclear signalling complexes [30–33] (Table 1). A rapidly expanding number of proteins have been shown to associate with the nuclear lamina, either directly or indirectly via interactions with lamina-associated proteins, suggesting that this filamentous network is essential for nuclear organization. For example, the A-type lamina has been implicated in the spatial organization of key signalling proteins that regulate cell-cycle progression. Fibroblasts lacking lamins A/C display proliferation defects that arise, at least in part, because of the failure of the Rb (retinoblastoma) protein to localize to nuclear speckles . In addition, the nuclear lamina can associate indirectly, via lamin-binding proteins, with signalling molecules to spatially and temporally regulate their activity. For example, we have recently described the lamin A-binding protein nesprin-2 as a nuclear ERK (extracellular-signal-regulated kinase) 1/2 scaffold protein that tethers these kinases at PML NBs in VSMCs . Although the importance of the nuclear lamina in the organization of this complex remains to be confirmed, nesprin-2, PML and ERK1/2 have all been described to associate with the nuclear lamina [36–38]. Importantly, disruption of this complex deregulates nuclear ERK1/2 activity and increases VMSC proliferation, confirming the importance of correct spatial regulation of nuclear ERK1/2 signalling . Intriguingly, PML NBs and ERK1/2 have been implicated in the DDR, suggesting that the nesprin-2–ERK–PML complex is potentially a novel component of the DDR in VSMCs, though this possibility remains to be tested [18,39,40]. Although our understanding of the spatial organization of nuclear signalling events remains limited, it is easy to envisage how compartmentalization may confine signalling activity to a specific subset of targets and how loss of this organization will disrupt these signalling events. For example, evidence dictates that signalling compartmentalization has particular importance during the DDR and, tantalizingly, recent evidence directly implicates the nuclear lamina in spatial organization of the DDR [4,32].
Prelamin A accumulation is a hallmark of VSMC aging
The importance of the nuclear lamina in the cardiovascular system has been highlighted by the laminopathy syndromes, where patients display a variety of diseases ranging from dilated cardiomyopathy in EDMD to atherosclerosis in the premature aging syndrome HGPS (Hutchinson–Gilford progeria syndrome) [41,42]. These patients display defects in lamin A processing and accumulate a truncated form of prelamin A called progerin . Importantly, VSMCs are grossly affected in patients with HGPS, who die of severe premature atherosclerosis in their second decade of life . Fibroblasts derived from HGPS fibroblasts exhibit specific nuclear defects, including lobulations and blebbing. Importantly, these same defects have been observed in aged VSMCs and arise as a result of the accumulation of the lamin A precursor protein prelamin A . Prelamin A is cleaved to mature lamin A by the processing enzyme FACE1 (farnesylated proteins-converting enzyme 1) and we have identified down-regulation of FACE1 by VSMCs as the prelude to prelamin A accumulation . Importantly, overexpression of prelamin A in normal proliferative VSMCs disrupts efficient DNA-damage repair and induces persistent DNA-damage signalling that ultimately drives VSMCs into premature senescence . These data suggest that this pathway of prelamin A accumulation and persistent DNA-damage signalling reinforces the senescence response during VSMC aging. In agreement with this notion, we have shown that this pathway is activated in response to other factors that mediate VSMC aging, including ROS, suggesting that signals promoting VSMC aging converge on this pathway to initiate VSMC senescence and aging .
How does prelamin A accumulation induce VSMC aging?
Although our understanding of the mechanisms whereby prelamin A induces VSMC aging remains limited, two possible mechanisms have been proposed from studies of the premature aging syndrome HGPS. These mechanisms are not mutually exclusive and, in combination, they explain both the nuclear morphology and DNA-repair defects observed during VSMC aging.
In the first hypothesis, prelamin A is proposed to interfere with lamin A associations at the NE, resulting in destabilization of nuclear spatial organization (Figure 1). Indeed, prelamin A has a higher binding affinity for the lamin A-interacting protein SUN1 (Table 1), than does lamin A . Surprisingly, despite its similarity to prelamin A, progerin interacts poorly with SUN1 and SUN2, suggesting that subtle differences exist between the interactions and modes of action of prelamin A and progerin . However, either prelamin A or progerin accumulation interferes with the connection between the NE and the nuclear lamina, and the importance of this connection is two-fold: it defines the mechanical properties of the entire cell (for a detailed recent review, see ), and also tethers heterochromatin, tightly packed DNA that is essential for gene silencing at the INM . Importantly, lamin A also interacts with LAP2α (lamina-associated polypeptide 2α) and HP1α (heterochromatin protein 1α), proteins that are essential for heterochromatin organization, at the INM. Prelamin A accumulation is known to impinge on this organization and potentially promotes the loss of heterochromatin observed in aging [48–50] (Figure 1). Although the precise mechanism of this disruption is unknown, NE-associated farnesylated prelamin A displays a low affinity for LAP2α and HP1α, suggesting that prelamin A accumulation displaces these proteins from the INM  (Figure 1). Therefore prelamin A accumulation at the NE would interfere with lamin A function and potentially accelerate cellular aging by increasing the mechanosensitivity of cells and by reducing DNA stability.
The second hypothesis predicts that prelamin A interferes with the intranuclear roles of lamin A. Evidence suggests that the nuclear lamina is essential for efficient DNA repair and prelamin A or progerin accumulation interferes with these repair processes [4,32]. Although the mechanism whereby prelamin A induces DNA-damage accumulation remains unknown, emerging data implicate lamin A and progerin directly in DSB repair [51,52]. Indeed, lamin A and progerin interact with Ku70 and DNA-PKcs (catalytic subunit of DNA-dependent protein kinase) respectively [51,52] (Table 1). Surprisingly, these interactions are specific and unique for these lamin A variants, suggesting that the accumulation of progerin could interfere with the recruitment of a subset of DNA-repair proteins to the lesion. For example, Ku70 normally recruits DNA-PKcs to DSBs, which in turn phosphorylates and recruits other components of the DSB-repair machinery, including 53BP1 (p53-binding protein 1) (Figure 1). However, in the presence of progerin, DNA-PKcs is mislocalized to the cytoplasm, preventing its recruitment and compartmentalization to the lesion, resulting in inefficient DSB repair (Figure 1). In support of this notion, fibroblasts from HGPS patients display both defective DNA repair and delayed recruitment of 53BP1 to the lesion .
In addition, evidence dictates that prelamin A and progerin stabilize DNA-damage-signalling events [4,53]. For example, early-passage VSMCs display transient ATM activation in response to DSBs; however, aged VSMCs accumulate pre-lamin A and display persistent ATM activity that lowers their growth rate before senescence. One possibility is that prelamin A and progerin disrupt DNA repair, but leave DNA-damage signalling intact. Thus unrepaired lesions continually stimulate ATM autophosphorylation and drive persistent DNA-damage signalling. Our understanding of the associations between the nuclear lamina and components of DNA-damage signalling remain lacking; however, it is plausible that signalling molecules, including ATM and the nesprin-2–PML–ERK complex, are anchored to the nuclear lamina during the DDR and this is vital for DNA-damage-signalling fidelity. Importantly, lamin A and prelamin A may possess different binding affinities for components of the DDR, and therefore prelamin A may strengthen intranuclear tethering of the ATM signalling cascade. Increased tethering may further stabilize and prolong DNA damage signalling to reinforce the senescence response during VSMC aging. Further investigation of these possible mechanisms are important areas for future research (Figure 1).
Age has always been considered an unmodifiable risk factor in the development of vascular disease. However, recent advances in our understanding of the nuclear lamina have highlighted the importance of this filamentous network in nuclear signalling fidelity and genome integrity. Importantly, changes in this filamentous network drive VSMC aging and future studies focusing on identifying the factors that regulate FACE1 expression and characterizing novel lamin A/prelamin A-binding partners and their relative binding affinities may allow the development of therapies to combat age-associated vascular diseases such as atherosclerosis and pathological calcification.
D.W. holds a BHF (British Heart Foundation) Centre of Research Excellence Career Development Fellowship from KCL (King's College London).
Nuclear Envelope Disease and Chromatin Organization: A Biochemical Society Focused Meeting held at Robinson College, Cambridge, U.K., 13–15 July 2011. Organized and Edited by Juliet Ellis (King's College London, U.K.) and Sue Shackleton (Leicester, U.K.).
Abbreviations: 9-1-1, Rad9–Rad1–Hus1; 53BP1, p53-binding protein 1; ATM, ataxia telangiectasia mutated; ATR, ATM- and Rad3-related; DDR, DNA-damage response; DNA-PKcs, catalytic subunit of DNA-dependent protein kinase; DSB, double-strand break; EDMD, Emery–Dreifuss muscular dystrophy; ERK, extracellular-signal-regulated kinase; FACE1, farnesylated proteins-converting enzyme 1; HGPS, Hutchinson–Gilford progeria syndrome; HP1α, heterochromatin protein 1α; INM, inner nuclear membrane; LAP2α, lamina-associated polypeptide 2α; MRN, Mre11–Rad50–Nbs1; NB, nuclear body; NE, nuclear envelope; PML, promyelocytic leukaemia protein; Rb, retinoblastoma; ROS, reactive oxygen species; SSB, single-strand break; VSMC, vascular smooth muscle cell
- © The Authors Journal compilation © 2011 Biochemical Society