Crossing Bridges between Extra- and Intra-Cellular Events in Thoracic Aortic Aneurysms

Thoracic aortic aneurysms (TAAs) are common, life-threatening diseases and are a major cause of mortality and morbidity. Over the past decade, genetic approaches have revealed that 1) activation of the transforming growth factor beta (TGF-β) signaling, 2) alterations in the contractile apparatus of vascular smooth muscle cells (SMCs), and 3) defects in the extracellular matrix (ECM) were responsible for development of TAAs. Most recently, a fourth mechanism has been proposed in that dysfunction of mechanosensing in the aortic wall in response to hemodynamic stress may be a key driver of TAAs. Interestingly, the elastin-contractile unit, which is an anatomical and functional unit connecting extracellular elastic laminae to the intracellular SMC contractile filaments, via cell surface receptors, has been shown to play a critical role in the mechanosensing of SMCs, and many genes identified in TAAs encode for proteins along this continuum. However, it is still debated whether these four pathways converge into a common pathway. Currently, an effective therapeutic strategy based on the underlying mechanism of each type of TAAs has not been established. In this review, we will update the present knowledge on the molecular mechanism of TAAs with a focus on the signaling pathways potentially involved in the initiation of TAAs. Finally, we will evaluate current therapeutic strategies for TAAs and propose new directions for future treatment of TAAs.

protein fibrillin-1, a major component of microfibrils; structures that serves as a scaffold for elastin deposition and provides structural support and stability to elastic laminae in the aorta 3) . Heritable TAAs without syndromic features have also been reported and are classified as familial thoracic aortic aneurysms/aortic dissections (TAAD). A number of nonsyndromic TAAD genes that have been identified so far turned out to be genes involved in regulation of smooth muscle cell (SMC) contraction (reviewed in 4, 5) ). Interestingly, regardless of the cause, TAAs are often accompanied by the disruption of elastic laminae. Indeed, mutations in several genes encoding for components of elastic laminae such as fibulin-4, microfibril-associated glycoprotein 2 (MAGP2), and lysyl oxidase (LOX), a cross-linking enzyme for elastin and collagen, have also been implicated in TAAs 6-8) . The discovery of gene mutations in TAAs has rapidly progressed by introduction of next generation sequencing technology combined with human genetics studies.
Most recently, it has been proposed that dysfunction of the mechanosensing in the aortic wall in Introduction Aortic aneurysms are characterized by an abnormal enlargement of the aortic lumen, usually asymptomatic, and are associated with a high risk of mortality from dissection and/or rupture. Aortic aneurysms can occur in the portion of the aorta above the diaphragm, termed thoracic aortic aneurysms (TAAs), or in the portion below the diaphragm, termed abdominal aortic aneurysms (AAAs). Whereas AAAs have been linked to atherosclerosis and chronic inflammation (reviewed in 1, 2) ), TAAs are often associated with heritable and degenerative diseases such as Marfan syndrome (MFS) and Loeys-Dietz syndrome (LDS). MFS patients were found to have mutations in the FBN1 gene, which encodes the extracellular matrix (ECM) was the primary cause of aneurysm formation (reviewed in 14,15) ). TGF-plays important roles in embryogenesis, development and normal tissue homeostasis by affecting cell proliferation and differentiation, and extracellular matrix (ECM) synthesis. Binding of TGFligands to TGF-receptors activates downstream signaling pathways, including the phosphorylation (p-) of Smad2 and Smad3 (known as canonical pathway), leading to the translocation of Smad4 into the nucleus and the activation of transcription of Smad-targeted genes 16) . Connective tissue growth factor (CTGF) and plasminogen-activator inhibitor-1 (PAI-1) are both well known target genes downstream of this canonical pathway and are involved in aortic wall remodeling. TGFalso affects Smad-independent pathways (known as non-canonical pathways), which are the mitogen-activated protein kinase (MAPK) cascades that include extracellular signal-regulated kinase 1 and 2 (ERK1/2), Jun N-terminal kinase (JNK) and p38 17,18) .
Dysregulation of TGF-activity has been implicated in the pathogenesis of MFS 19) , and mutations in the genes encoding the TGF-receptor type (TGFBR2) and type (TGFBR1) were identified in LDS 20,21) . In MFS, it was proposed that defects in fibrillin-1 causes impaired tethering of the large latent complex (LLC), response to hemodynamics may be a key driver of pathogenesis of TAAs ( Fig. 1; reviewed in 9,10) ). In particular, abnormal mechanosensing of SMCs due to the loss of elastic laminae-SMC connections (Fig. 2) and the resultant alteration of actin cytoskeletal remodeling, play causative roles in the formation of aortic aneurysms 11) . These observations are consistent with the concept of an "elastin-contractile unit" that is involved in the mechanosensing of SMCs and maintenance of aortic wall integrity 4, 12) .
In this review, we will summarize the knowledge obtained from patients and mouse models (Table 1-3, respectively), and the underlying signaling pathways involved in pathogenesis of TAAs (Fig. 3). Finally, we will discuss current and future therapeutic strategies for TAAs.

Defective Fibrillin-1 and Activation of TGF-Signaling in TAAs
The pathogenesis of MFS in humans and mouse models was initially suggested to be due to a weakening of the aortic wall as a result of abnormal fibrillin-1 3, 13) . Subsequently, it was proposed that increased transforming growth factor beta (TGF-) signaling The aortic wall consists of three layers: intima, media and adventitia. These layers include endothelial cells (ECs), smooth muscle cells (SMCs) and elastic laminae, and fibroblasts, respectively. One risk factor for TAAs is dysfunctional mechanosensing of SMCs in the aortic wall, which leads to increased hemodynamic stress. and die postnatally within the first two weeks 35) . Mutations in genes encoding proteins in the TGF-signaling pathway, including the TGF-ligand TGFB2 36,37) , SMAD3 38) and SMAD4 39) were identified and shown to predispose affected individuals to thoracic aortic diseases. Interestingly, the causative mutations in these genes were shown to be loss-of-function mutations; however, paradoxical activation of TGF-signaling was observed in the aorta of these MFS-related mouse models.
Surprisingly, treatment of these MFS mice with TGF-neutralizing antibodies prevented progression of TAAs in some studies 40,41) , while promoting aneurysm expansion in others 42) . In addition, SMC-specific Tgfbr2 disruption in Fbn1 C1039G/ mice showed activation of the non-canonical pathway and acceleration of aneurysm growth 43) . It is interesting to note that Ltbp3 deficiency prevented the aneurysm phenotype in Fbn1 mgR/mgR mice with reduced disruption of elastic fibers and decreased Erk1/2 and Smad2/3 activation 44) . Thus, it is plausible that improper localization of the LLC to microfibrils mediated by LTBP3 contributes to progression of TAA in MFS.
Since the indentification of fibrillin-1 as a gene responsible for syndromic TAAs, substantial progress has been made in identifying the altered signaling pathways in this disease, however, the mechanism by which is composed of proTGF-dimers covalently bound to latent TGF-binding proteins (LTBPs)-1, -3, or -4 22) , to microfibrils 23) . Active TGF-is released from the LLC by activators such as integrin v 6 24, 25) , thrombospondin-1 (TSP1) 26) , matrix metalloproteinases (MMPs) 27, 28) and reactive oxygen species (ROS) 29) . It was hypothesized that mutations in fibrillin-1 disrupt binding of the LLC to fibrillin-1 and increase bioavailability of TGF-in the aortic wall 23, 30, 31) .
Several mouse models of MFS have provided some clues regarding the molecular pathogenesis of thoracic aortic diseases 32) . Fbn1 mgR/mgR mice, which have only 20% of the amount of normal fibrillin-1, were established as the first MFS mouse model 33) . Fbn1 C1039G/ mice, which harbor a disease-causing missense mutation in fibrillin-1, were also generated and recapitulated the aortic aneurysm phenotype 34) . Both types of MFS mice showed upregulation of p-Smad2/3 (canonical pathway) and p-Erk1/2 (noncanonical pathway) as well as fragmentation of elastic laminae. The severity of the aortic aneurysm, however, differed between these MFS mice; Fbn1 C1039G/ mice exhibit slowly progressing aortic root aneurysms but rarely showed dissection or rupture, whereas Fbn1 mgR/mgR mice showed a more severe phenotype with rapidly enlarging aortic root aneurysms and frequent dissections and/or ruptures. Fbn1 / mice exhibit the most severe aortic phenotype or how loss-of-function mutations in TGF-components lead to heightened TGF-activity.
which fibrillin-1 controls the bioavailability of TGFsignaling has not been determined. Additionally, it is not known whether upregulation of TGF-signaling pathway is the primary driver for TAA pathogenesis, Fibrillin-1 deficiency recapitulated vascular phenotype of Marfan syndrome in mice.
Identified the upregulation of TGFactivity in Marfan syndrome.
Identification of TGFBR1 and TGFBR2 mutation driven Marfan syndrome.
Angiotensin receptor blockade as therapeutic target in mice.
Identification of SMAD3 mutation cause aortic aneurysm.
Identification of TGFB2 mutation driven Marfan syndrome.
Disruption of Acta2 in SMCs activate ROS and NF-kB signaling, leading At1r expression.
Zhu et al 45) Guo et al 48) Wang et al 52) Guo et al 53) Kuang et al 54) Chen et al 51) Table 3. Summary of selected time points with significant findings linked to TAAs and fibulin-4, fibulin-5 and LOX mediated elastic fibers disruption.
Year Description Inactivation of LOX leads to aortic aneurysms in mice.
Fibulin-5 is essential for elastic fiber assembly.
ELN (encoding elastin protein) mutations cause aortic disease in patients with cutis laxa.
Fibulin-4 is necessary for elastic fiber formation and connective tissue development.
Mutations in FBLN4 cause aortic aneurysm.
Abnormal mechanosensing in SMCs initiate aneurysm formation in Fbln4 SMKO mice.
Maki et al 6) Yanagisawa et al 67) Nakamura et al 68) Szabo et al 80) McLaughlin et al 7) Hucthagowder et al 77) Hanada et al 79) Dasouki et al 78) Huang et al 83) Huang et al 84) Yamashiro et al 11) region of MYH11 and were predicted to decrease myosin motor activity. A rare variant of MYH11 R247C was also reported in TAAD and mice carrying homozygous Myh11 R247C were generated 47) . The Myh11 R247C/R247C mice exhibited decreased aortic contraction but no aortic aneurysms; however, they developed severe neointima formation after injury due to an increased proliferation of SMCs. In addition, ACTA2, which encodes the SMC-specific isoform of -actin, was also identified as a causal gene in familial TAAD 48) . Although Acta2 null mice did not develop aortic aneurysms, they showed compromised vascular contractile force,

Alteration of SMC Contractile Apparatus in TAAs
Familial thoracic aortic aneurysms and dissections (familial TAAD) are autosomal dominant disorders and refer to an inherited predisposition to thoracic aortic disease in the absence of syndromic features. Mutations in the MYH11 gene, which encodes for the thick filaments in the smooth muscle-specific isoform of myosin heavy chain, were identified in familial TAAD with patent ductus arteriosus (PDA) 45,46) . These mutations led to deletion of the C-terminal The ECM protein fibulin-4 regulates lysyl oxidase (LOX) activity, which plays an important role in elastic fiber assembly. Fibrillin-1 is a major component of extracellular microfibrils. Inactive TGF-is tethered onto microfibrils via latent TGF-binding proteins (LTBPs). Once TGF-is activated by one of its activators, such as MMPs, TSPs, ROS or integrin v 6, it is released from latency-associated protein (LAP) and binds to its receptor (TGFBR), thus activating subsequent signaling pathways. Angiotensin type 1 receptor (AT1R) and nitric oxide (NO) signaling pathways are involved in SMC contraction or relaxation. Integrins leads to an altered mechanotransduction signal and initiation of cellular responses such as cytoskeletal remodeling. LLC (large latent complex), SLC (small latent complex), FAK (focal adhesion kinase), ILK (integrin-linked kinase), RhoGEF (Rho guanine nucleotide exchange factor), RhoGAP (Rho GTPase-activating protein), ROCK (Rho associated kinase), MLCP (myosin light chain phosphatese), MLCK (myosin light chain kinase), PKG1 (protein kinase G 1), cGC (soluble guanylate cyclase), cGMP (cyclic guanosine monophosphate), MRTF-A (myocardin-related transcription factor A), SRF (serum response factor), JNK (c-Jun NH2-terminal kinase), ERK (extracellular signal regulated kinase), LIMK (LIM kinase), Ssh1 (slingshot-1). production, increased aortic wall stiffness and increased pulse wave velocity 59) . Such endothelial dysfunction has been reported in MFS patients 60,61) and in the Fbn1 C1039G/ mice 62) . However, how the dysregulation of endothelial cells and NO signaling contribute to the development of TAAs remains unexplored.

Disruption of Elastic Fibers in TAAs
Elasticity is provided by elastic fibers, which play a crucial role by maintaining structural integrity in the medial layer of the aorta. The major components of elastic fibers are polymerized elastin and microfibrils (consisting predominantly of fibrillin-1). Normally, monomers of elastin, known as tropoelastin, form small aggregates (known as coacervates), that are transported to and deposited onto microfibrils. These elastin aggregates are then cross-linked by lysyl oxidase (LOX) to form mature, insoluble elastic fibers 63) . Fibulins (FBLNs) play a critical role in elastic fiber assembly, and to date, seven members of the FBLN family have been identified 64,65) . Among these members, FBLN3, 4 and 5 possess high homology to each other and are involved in elastic fiber assembly 7, [66][67][68] . An immuno-electron microscopy (EM) study showed that fibulin-4 is localized on microfibrils and fibulin-5 on elastin 69) . Subsequent research revealed that fibulin-5 promotes coacervation of tropoelastin and its deposition onto microfibrils 70) by interacting with LTBP-4, thereby leading to cross-linking and elastic fiber assembly 71,72) . In addition, Lox and Loxl1 (lysyl oxidase-like protein 1) are recruited to elastic fibers in a fibulin-4-dependent and fibulin-5-depednet manner, respectively 73,74) . Inactivation or loss-of-function mutations of LOX reduces the crosslinking of collagen and elastin and causes aortic aneurysms 6, 75) .
Although elastic fiber disruption was frequently observed in the aneurysmal wall of MFS patients, MFS mouse models 76) and FBLN4 deficiency [77][78][79] , an aneurysm phenotype was uncommon in patients with mutations in ELN (encoding elastin protein) 80) . In addition, Eln deficiency in mice led to increased SMC proliferation and thickening of the aortic wall with narrowing of the lumen 81) . Similarly, aortic aneurysms were never observed in mice deficient in Fibulin-5 (Fbln5) or in cutis laxa patients with FBLN5 deficiency 67,82) . These observations suggest that a disrupted elastin core is not sufficient to cause TAAs and that elastin and microfibrils have distinct roles in protecting the vessel wall from the development of TAAs.
tone and blood flow 49) , as well as increased neointima formation after vascular injury due to proliferation of SMCs and activation of focal adhesion kinase (FAK) 50) . Furthermore, it has recently been shown that Acta2null mice have increased angiotensin (Ang ) signaling in a ligand-independent manner. Loss of SMactin led to an increase in ROS generation and an upregulation of Ang type 1a receptor (Agtr1a) expression, thereby increasing the sensitivity to Ang by 100-fold in SMCs 51) . These studies indicate that mutations in SMC contractile genes not only affect contractile force generation but also alter the intrinsic properties of SMCs.
Other mutations linked to familial TAAD are dominant negative mutations in the gene encoding for the myosin light chain kinase (MYLK), which controls SMC contraction 52) . One PRKG1 variant (p.R117Q), which encodes a type cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG1) that is activated upon binding of cGMP and controls SMC relaxation, was also identified as a causal mutation in TAAD 53) . Additionally, in the forkhead family of transcription factors, forkhead box E3 (FOXE3) mutations have been reported in TAAD. Foxe3 / mice had decreased SMC density in the aortic media and incr eased SMC apoptosis leading to dysfunction of the aortic wall 54) . Mechanistically, these mutations lead to reduction of SMC contraction.
Rare variants in MFAP5 (encoding Microfibril-Associated Glycoprotein 2, MAGP2) and MAT2A (encoding methionine adenosyltransferase 2A) have also been found in TAAD 8, 55) . MFAP5 is a component of elastic fibers and associates with the microfibrils. Although the Mfap5 knockout alone did not show an obvious phenotype, double knockout mice for Mfap5 and Mfap2, which encode an evolutionaryrelated protein known as MAGP1, caused age-dependent aortic dilation 56) . MAT2A is involved in the synthesis of s-adenosylmethionine, which serves as a methylgroup donor for methylation reaction in vivo. In both cases, the alteration of these genes caused haploinsufficiency or loss-of-function and predispose the affected individuals to TAAD.
In the aortic wall, endothelial cells (ECs) and SMCs constantly interact with each other, either directly or in a paracrine fashion. Nitric oxide (NO) is involved in vascular tone 57) and is produced from L-arginine by a calcium-dependent endothelial nitric oxide synthase (NOS-3; known as eNOS). NO regulates the degree of SMC contraction by stimulating soluble guanylyl cyclase (sGC), which generates cyclic GMP and activates protein kinase G, thereby activating myosin light chain phosphatase (MLCP) and causing SMC relaxation 58) . Endothelial dysfunction causes altered NO (Fig. 2). In wild-type aortas, extensive connections exist between the elastic laminae and SMCs via elastin extenstions and cell surface receptors, such as integrin receptors. The elastin extensions attach to the cell surface at the sites of membrane-associated dense plaques; sites where intracellular actin filaments attached to cell membrane 91) . This "elastin-contractile unit" of the aorta plays a critical role in the proper transmission of the mechanical force between elastic laminae and SMCs (Fig. 2). Disruption of genes involved in the extracellular or intracellular portion of the elastin-contractile unit have been shown to lead to aortic aneurysms in humans and mice (reviewed in 4) ). In Fbln4 SMKO aortas, mechanosensitive molecules such as early growth response 1 (Egr1), ACE and TSP1, all of which were shown to respond to pressure overload of the aorta, were highly up-regulated, and phosphorylation of cofilin was significantly decreased ( activated) in the aneurysmal wall 11) . These observations suggested that a loss of elastin-contractile units resulted in abnormal mechanosensing of the Fbln4 SMKO aortas. The fact that downregulation of Ssh1 by a phosphatidylinositol-3 kinase (PI3K) inhibitor led to an increase in phosphorylated cofilin and prevented the aneurysms expansion in Fbln4 SMKO mice, indicates that abnormal mechanosensing may be driving the aneurysmal phenotype. Similarly, it was reported that impaired microfibril-cardiomyocyte connections in Fbn1 mgR/mgR mice caused downregulation of phosphorylated FAK and affected intracellular signaling in the heart 92) . Furthermore, compound heterozygous mice for Fbn1 and Itgb1 (encoding the integrin 1 gene) developed cardiomyopathy while Fbn1 / mice appeared normal 92) . We speculate that in the aorta, the elastin-contractile units composed of elastin extensions, SMC receptors and actin filaments, form a structural and functional unit that transmits mechanical stress from the ECM to the SMCs, as well as maintains cellular tension through actin cytoskeletal remodeling.

Prospective Strategies for Treatment of TAA
Current therapeutic strategies to treat TAAs are limited to surgical endovascular procedures such as stent grafts and aortic replacements 93) . So far, effective therapeutic strategies based on the etiology of each TAA type have not been established. Further understanding of the underlying mechanism of TAAs is required to establish an effective treatment for TAAs.
In the Fbn1 C1039G/ mice, treatment with losartan (ARB) prevented aortic root enlargement from exceeding normal levels and recovered pathologic changes, such as elastic fiber fragmentation, in the medial layer 40) . The first prospective trial was reported in 2008; losar-

Loss of Elastin-Contractile Units Results in Abnormal Mechanosensing of SMCs in TAAs
SMC-specific deletion of Fbln4 in mice (Fbln4 SMKO ), showed ascending aortic aneurysms with marked disruption of elastic fibers, thickened medial wall, increased phosphorylation of ERK1/2 signaling and decreased expression of SMC differentiation markers 83) . In addition, angiotensin-converting enzyme (ACE) was highly expressed in the aneurysmal walls and subsequent activation of angiotensin signaling in the aortic wall was responsible for driving the aneurysm phenotype 84) . In this Fbln4 SMKO model, aneurysms are completely prevented by administration of an ACE inhibitor or angiotensin type 1 receptor (AT1R) blockade (ARB) within the first month of life. ARB treatment initiated after the establishment of an aneurysm did not reverse the aneurysm phenotype, indicating that the signals required for maintenance of aneurysms might be independent of angiotensin -AT1R 84) . Furthermore, the actin depolymerizing factor cofilin, which severs polymerized actin and triggers disassembly of actin fibers, was activated ( dephosphorylated) by its phosphatase slingshot-1 (Ssh1), resulting in accelerated actin remodeling 11,85) . In the Fbln4 SMKO aneurysmal wall, the ratio of monomeric actin (G-actin) to filamentous actin (F-actin) was significantly increased compared to control mice 11) . The increased G-actin potentially affects aneurysm expansion by sequestering myocardin-related transcription factor A (MRTF-A) in the cytoplasm and inhibiting its binding to the transcriptional co-activator serum response factor (SRF), which induces the transcription of SMC contractile genes, including Acta2, Myh11 and Cnn1 (calponin 1) 86) . Similarly, mice that have integrin-linked kinase (ILK) deletion in vascular SMCs (SM22Cre Ilk Fl/Fl ) showed aneurysmal dilatation, alteration in RhoA/Rho-associated protein kinase (ROCK) signaling, decreased F-actin and abnormal localization of MRTF-A 87) . ILK is located at focal adhesions and links the ECM to the actin cytoskeleton via 1-and 3-integrins. Since integrin cytoplasmic domains lack actin-binding sites and enzymatic activity, signaling is propagated through a series of linker proteins including vinculin, paxillin, talin, -actinin and kinases such as FAK and ILK (reviewed in [88][89][90] . Therefore, deletion of ILK in vascular SMCs may lead to the impaired activation of RhoA/ROCK and down-regulation of SMC contractile genes due to reduced nuclear MRTF-A.
Interestingly, disruption of elastic laminae-SMC connections was observed in Fbln4 SMKO aortas, along with a remarkable, moth eaten-like, irregular appearance of the elastin located between the SMC layers standing of the mechanobiology of SMCs will shed light on advanced therapeutic strategies based on the underlying pathophysiology of TAAs.

Acknowledgement
The authors thank Christina Papke, Rachel Sides, and Elaine Davis for critical reading of the manuscript and valuable comments. The work in Yanagisawa laboratory was supported in part by MEXT KAKENHI (Grant Number JP 17H04289), The Naito Foundation, Astellas Foundation for Research on Metabolic Disorders, and the National Institutes of Health (R01HL 106305) to HY and Grants-in-Aid for Young Scientists (B) (Grant Number 15K20898), Japan Heart Foundation Research Grant, The Inamori Foundation and Japan Foundation for Applied Enzymology to YY.

Conflicts of Interest
None.  94) . Although this trial was a small cohort study with only 18 patients, the success of using losartan to prevent TAA in MFS patients led to the initiation of randomized trials of losartan worldwide. In 2013, an openlabel, randomized controlled trial was conducted as a series of double-blind trials, which assessed 233 MFS patients over the age of 18 years. This trial reported that losartan reduced the aortic dilatation rate in the ascending aorta in patients who had not undergone aortic root replacement, and in the ascending aorta in patients who had undergone aortic root replacement 95) . Subsequent analyses revealed that MFS patients with FBN1 haploinsufficiency seem to be more responsive to losartan therapy for the inhibition of aortic root growth compared with dominant-negative patients 96) . The largest randomized trial, which enrolled 608 patients with MFS between the ages of 6 months to 25 years, demonstrated that both groups treated with losartan or atenolol ( blocker) showed a decrease in the growth of aortic root with no significant difference between the groups 97) . Recent trials from European countries comparing losartan to blockers or placebo reached similar conclusions 98,99) .
A more recent study in Fbn1 mgR/mgR mice showed that neither losartan nor TGF-neutralizing antibodies prevented aneurysm formation; however, a combination of both treatments starting at postnatal day (P)16 and P45, respectively, effectively prevented aortic aneurysms in these mice 42) . Other potential therapeutic targets that have been identified include MMPs and PI3K. Inhibition of MMP activity by doxycycline and deletion of Mmp2 gene attenuated aneurysm formation in the Fbn1 C1039G/ and Fbn1 mgR/mgR mice 100) , and two PI3K inhibitors, Wortmannin and LY294002, independently prevented TAA progression in the Fbln4 SMKO mice 11) . It is therefore likely that a multidrug regimen targeting various molecular pathways will be required to prevent TAAs.

Conclusion
For the past 20 years, hyperactivation of TGFsignaling pathways, disruption of the vascular SMCs contractile apparatus and impairment of ECM synthesis have been identified as causal events for TAAs. Molecular signaling pathways have been linked to initiation of TAAs although it is still debated whether these pathways converge into a common pathway or are independent of each other (Fig. 3). Currently, we have not established effective therapeutic strategies based on the etiology of each TAA type. Accumulating recent reports suggest that studying a better under-