Flow-dependent cellular mechanotransduction in atherosclerosis

Hemodynamics and Wall Mechanics of Vascular Graft Failure

Blood vessels are subjected to complex biomechanical loads, primarily from pressure-driven blood flow. Abnormal loading associated with vascular grafts, arising from altered hemodynamics or wall mechanics, can cause acute and progressive vascular failure and end-organ dysfunction. Perturbations to mechanobiological stimuli experienced by vascular cells contribute to remodeling of the vascular wall via activation of mechanosensitive signaling pathways and subsequent changes in gene expression and associated turnover of cells and extracellular matrix. In this review, we outline experimental and computational tools used to quantify metrics of biomechanical loading in vascular grafts and highlight those that show potential in predicting graft failure for diverse disease contexts. We include metrics derived from both fluid and solid mechanics that drive feedback loops between mechanobiological processes and changes in the biomechanical state that govern the natural history of vascular grafts. As illustrative examples, we consider application-specific coronary artery bypass grafts, peripheral vascular grafts, and tissue-engineered vascular grafts for congenital heart surgery as each of these involves unique circulatory environments, loading magnitudes, and graft materials.

Extracellular Matrix Remodeling in Vascular Disease: Defining Its Regulators and Pathological Influence

Because of structural and cellular differences (ie, degrees of matrix abundance and cross-linking, mural cell density, and adventitia), large and medium-sized vessels, in comparison to capillaries, react in a unique manner to stimuli that induce vascular disease. A stereotypical vascular injury response is ECM (extracellular matrix) remodeling that occurs particularly in larger vessels in response to injurious stimuli, such as elevated angiotensin II, hyperlipidemia, hyperglycemia, genetic deficiencies, inflammatory cell infiltration, or exposure to proinflammatory mediators. Even with substantial and prolonged vascular damage, large- and medium-sized arteries, persist, but become modified by (1) changes in vascular wall cellularity; (2) modifications in the differentiation status of endothelial cells, vascular smooth muscle cells, or adventitial stem cells (each can become activated); (3) infiltration of the vascular wall by various leukocyte types; (4) increased exposure to critical growth factors and proinflammatory mediators; and (5) marked changes in the vascular ECM, that remodels from a homeostatic, prodifferentiation ECM environment to matrices that instead promote tissue reparative responses. This latter ECM presents previously hidden matricryptic sites that bind integrins to signal vascular cells and infiltrating leukocytes (in coordination with other mediators) to proliferate, invade, secrete ECM-degrading proteinases, and deposit injury-induced matrices (predisposing to vessel wall fibrosis). In contrast, in response to similar stimuli, capillaries can undergo regression responses (rarefaction). In summary, we have described the molecular events controlling ECM remodeling in major vascular diseases as well as the differential responses of arteries versus capillaries to key mediators inducing vascular injury.

Polarized Mechanosensitive Signaling Domains Protect Arterial Endothelial Cells Against Inflammation

Endothelial cells (ECs) in the descending aorta are exposed to high laminar shear stress, which supports an anti-inflammatory phenotype that protects them from atherosclerosis. High laminar shear stress also supports flow-aligned cell elongation and front-rear polarity, but whether this is required for athero-protective signaling is unclear. Here, we show that Caveolin-1-rich microdomains become polarized at the downstream end of ECs exposed to continuous high laminar flow. These microdomains are characterized by higher membrane rigidity, filamentous actin (F-actin) and lipid accumulation. Transient receptor potential vanilloid-type 4 (Trpv4) ion channels, while ubiquitously expressed, mediate localized Ca ²⁺ entry at these microdomains where they physically interact with clustered Caveolin-1. The resultant focal bursts in Ca ²⁺ activate the anti-inflammatory factor endothelial nitric oxide synthase (eNOS) within the confines of these domains. Importantly, we find that signaling at these domains requires both cell body elongation and sustained flow. Finally, Trpv4 signaling at these domains is necessary and sufficient to suppress inflammatory gene expression. Our work reveals a novel polarized mechanosensitive signaling hub that induces an anti-inflammatory response in arterial ECs exposed to high laminar shear stress.

Disturbed Flow-Facilitated Margination and Targeting of Nanodisks Protect against Atherosclerosis

Disturbed blood flow induces endothelial pro-inflammatory responses that promote atherogenesis. Nanoparticle-based therapeutics aimed at treating endothelial inflammation in vasculature where disturbed flow occurs may provide a promising avenue to prevent atherosclerosis. By using a vertical-step flow apparatus and a microfluidic chip of vascular stenosis, herein, it is found that the disk-shaped versus the spherical nanoparticles exhibit preferential margination (localization and adhesion) to the regions with the pro-atherogenic disturbed flow. By employing a mouse model of carotid partial ligation, superior targeting and higher accumulation of the disk-shaped particles are also demonstrated within disturbed flow areas than that of the spherical particles. In hyperlipidemia mice, administration of disk-shaped particles loaded with hypomethylating agent decitabine (DAC) displays greater anti-inflammatory and anti-atherosclerotic effects compared with that of the spherical counterparts and exhibits reduced toxicity than "naked" DAC. The findings suggest that shaping nanoparticles to disk is an effective strategy for promoting their delivery to atheroprone endothelia.

Integrin Conformational Dynamics and Mechanotransduction

The function of the integrin family of receptors as central mediators of cell-extracellular matrix (ECM) and cell-cell adhesion requires a remarkable convergence of interactions and influences. Integrins must be anchored to the cytoskeleton and bound to extracellular ligands in order to provide firm adhesion, with force transmission across this linkage conferring tissue integrity. Integrin affinity to ligands is highly regulated by cell signaling pathways, altering affinity constants by 1000-fold or more, via a series of long-range conformational transitions. In this review, we first summarize basic, well-known features of integrin conformational states and then focus on new information concerning the impact of mechanical forces on these states and interstate transitions. We also discuss how these effects may impact mechansensitive cell functions and identify unanswered questions for future studies.

Development and Preliminary Testing of Porcine Blood-Derived Endothelial-like Cells for Vascular Tissue Engineering Applications: Protocol Optimisation and Seeding of Decellularised Human Saphenous Veins

Functional endothelial cells (EC) are a critical interface between blood vessels and the thrombogenic flowing blood. Disruption of this layer can lead to early thrombosis, inflammation, vessel restenosis, and, following coronary (CABG) or peripheral (PABG) artery bypass graft surgery, vein graft failure. Blood-derived ECs have shown potential for vascular tissue engineering applications. Here, we show the development and preliminary testing of a method for deriving porcine endothelial-like cells from blood obtained under clinical conditions for use in translational research. The derived cells show cobblestone morphology and expression of EC markers, similar to those seen in isolated porcine aortic ECs (PAEC), and when exposed to increasing shear stress, they remain viable and show mRNA expression of EC markers similar to PAEC. In addition, we confirm the feasibility of seeding endothelial-like cells onto a decellularised human vein scaffold with approximately 90% lumen coverage at lower passages, and show that increasing cell passage results in reduced endothelial coverage.

Endothelial Cell Plasma Membrane Biomechanics Mediates Effects of Pro-Inflammatory Factors on Endothelial Mechanosensors: Vicious Circle Formation in Atherogenic Inflammation

Chronic low-grade vascular inflammation and endothelial dysfunction significantly contribute to the pathogenesis of cardiovascular diseases. In endothelial cells (ECs), anti-inflammatory or pro-inflammatory signaling can be induced by different patterns of the fluid shear stress (SS) exerted by blood flow on ECs. Laminar blood flow with high magnitude is anti-inflammatory, while disturbed flow and laminar flow with low magnitude is pro-inflammatory. Endothelial mechanosensors are the key upstream signaling proteins in SS-induced pro- and anti-inflammatory responses. Being transmembrane proteins, mechanosensors, not only experience fluid SS but also become regulated by the biomechanical properties of the lipid bilayer and the cytoskeleton. We review the apparent effects of pro-inflammatory factors (hypoxia, oxidative stress, hypercholesterolemia, and cytokines) on the biomechanics of the lipid bilayer and the cytoskeleton. An analysis of the available data suggests that the formation of a vicious circle may occur, in which pro-inflammatory cytokines enhance and attenuate SS-induced pro-inflammatory and anti-inflammatory signaling, respectively.

Wnt2 Contributes to the Development of Atherosclerosis

Atherosclerosis, is a chronic inflammatory disease, characterized by the narrowing of the arteries resulting from the formation of intimal plaques in the wall of arteries. Yet the molecular mechanisms responsible for maintaining the development and progression of atherosclerotic lesions have not been fully defined. In this study, we show that TGF-β activates the endothelial-to-mesenchymal transition (EndMT) in cultured human aortic endothelial cells (HAECs) and this transition is dependent on the key executor of the Wnt signaling pathway in vitro. This study presents the first evidence describing the mechanistic details of the TGF-β-induced EndMT signaling pathway in HAECs by documenting the cellular transition to the mesenchymal phenotype including the expression of mesenchymal markers α-SMA and PDGFRα, and the loss of endothelial markers including VE-cadherin and CD31. Furthermore, a short hairpin RNA (shRNA) screening revealed that Wnt2 signaling is required for TGF-β-mediated EndMT of HAECs. Also, we found that LDLR^−/− mice fed on a high-fat western-type diet (21% fat, 0.2% cholesterol) expressed high levels of Wnt2 protein in atherosclerotic lesions, confirming that this signaling pathway is involved in atherosclerosis in vivo. These findings suggest that Wnt2 may contribute to atherosclerotic plaque development and this study will render Wnt2 as a potential target for therapeutic intervention aiming at controlling atherosclerosis.

CLEC-2-dependent platelet subendothelial accumulation by flow disturbance contributes to atherogenesis in mice

Rationale: Platelets play an essential role in atherosclerosis, but the underlying mechanisms remain to be addressed. This study is to investigate the role of platelets in d-flow induced vascular inflammation and the underlying mechanism. Methods: We established a disturbed blood flow (d-flow) model by partial carotid ligation (PCL) surgery using atherosclerosis-susceptible mice and wild-type mice to observe the d-flow induced platelet accumulation in the subendothelium or in the plaque by immunostaining or transmission electron microscopy. The mechanism of platelet subendothelial accumulation was further explored by specific gene knockout mice. Results: We observed presence of platelets in atherosclerotic plaques either in the atheroprone area of aortic arch or in carotid artery with d-flow using Ldlr-/- or ApoE-/- mice on high fat diet. Immunostaining showed the subendothelial accumulation of circulating platelets by d-flow in vivo. Transmission electron microscopy demonstrated the accumulation of platelets associated with monocytes in the subendothelial spaces. The subendothelial accumulation of platelet-monocyte/macrophage aggregates reached peak values at 2 days after PCL. In examining the molecules that may mediate the platelet entry, we found that deletion of platelet C-type lectin-like receptor 2 (CLEC-2) reduced the subendothelial accumulation of platelets and monocytes/macrophages by d-flow, and ameliorated plaque formation in Ldlr-/- mice on high fat diet. Supportively, CLEC-2 deficient platelets diminished their promoting effect on the migration of mouse monocyte/macrophage cell line RAW264.7. Moreover, monocyte podoplanin (PDPN), the only ligand of CLEC-2, was upregulated by d-flow, and the myeloid-specific PDPN deletion mitigated the subendothelial accumulation of platelets and monocytes/macrophages. Conclusions: Our results reveal a new CLEC-2-dependent platelet subendothelial accumulation in response to d-flow to regulate vascular inflammation.

Links between autophagy and tissue mechanics

Physical constraints, such as compression, shear stress, stretching and tension, play major roles during development, tissue homeostasis, immune responses and pathologies. Cells and organelles also face mechanical forces during migration and extravasation, and investigations into how mechanical forces are translated into a wide panel of biological responses, including changes in cell morphology, membrane transport, metabolism, energy production and gene expression, is a flourishing field. Recent studies demonstrate the role of macroautophagy in the integration of physical constraints. The aim of this Review is to summarize and discuss our knowledge of the role of macroautophagy in controlling a large panel of cell responses, from morphological and metabolic changes, to inflammation and senescence, for the integration of mechanical forces. Moreover, wherever possible, we also discuss the cell surface molecules and structures that sense mechanical forces upstream of macroautophagy.

Heparan sulfate proteoglycan glypican-1 and PECAM-1 cooperate in shear-induced endothelial nitric oxide production

This study aimed to clarify the role of glypican-1 and PECAM-1 in shear-induced nitric oxide production in endothelial cells. Atomic force microscopy pulling was used to apply force to glypican-1 and PECAM-1 on the surface of human umbilical vein endothelial cells and nitric oxide was measured using a fluorescent reporter dye. Glypican-1 pulling for 30 min stimulated nitric oxide production while PECAM-1 pulling did not. However, PECAM-1 downstream activation was necessary for the glypican-1 force-induced response. Glypican-1 knockout mice exhibited impaired flow-induced phosphorylation of eNOS without changes to PECAM-1 expression. A cooperation mechanism for the mechanotransduction of fluid shear stress to nitric oxide production was elucidated in which glypican-1 senses flow and phosphorylates PECAM-1 leading to endothelial nitric oxide synthase phosphorylation and nitric oxide production.

The Story of Wall Shear Stress in Coronary Artery Atherosclerosis: Biochemical Transport and Mechanotransduction

Coronary artery atherosclerosis is a local, multifactorial, complex disease, and the leading cause of death in the US. Complex interactions between biochemical transport and biomechanical forces influence disease growth. Wall shear stress (WSS) affects coronary artery atherosclerosis by inducing endothelial cell mechanotransduction and by controlling the near-wall transport processes involved in atherosclerosis. Each of these processes is controlled by WSS differently and therefore has complicated the interpretation of WSS in atherosclerosis. In this paper, we present a comprehensive theory for WSS in atherosclerosis. First, a short review of shear stress-mediated mechanotransduction in atherosclerosis was presented. Next, subject-specific computational fluid dynamics (CFD) simulations were performed in ten coronary artery models of diseased and healthy subjects. Biochemical-specific mass transport models were developed to study low-density lipoprotein, nitric oxide, adenosine triphosphate, oxygen, monocyte chemoattractant protein-1, and monocyte transport. The transport results were compared with WSS vectors and WSS Lagrangian coherent structures (WSS LCS). High WSS magnitude protected against atherosclerosis by increasing the production or flux of atheroprotective biochemicals and decreasing the near-wall localization of atherogenic biochemicals. Low WSS magnitude promoted atherosclerosis by increasing atherogenic biochemical localization. Finally, the attracting WSS LCS's role was more complex where it promoted or prevented atherosclerosis based on different biochemicals. We present a summary of the different pathways by which WSS influences coronary artery atherosclerosis and compare different mechanotransduction and biotransport mechanisms.

The molecular mechanism of mechanotransduction in vascular homeostasis and disease

Blood vessels are constantly exposed to mechanical stimuli such as shear stress due to flow and pulsatile stretch. The extracellular matrix maintains the structural integrity of the vessel wall and coordinates with a dynamic mechanical environment to provide cues to initiate intracellular signaling pathway(s), thereby changing cellular behaviors and functions. However, the precise role of matrix-cell interactions involved in mechanotransduction during vascular homeostasis and disease development remains to be fully determined. In this review, we introduce hemodynamics forces in blood vessels and the initial sensors of mechanical stimuli, including cell-cell junctional molecules, G-protein-coupled receptors (GPCRs), multiple ion channels, and a variety of small GTPases. We then highlight the molecular mechanotransduction events in the vessel wall triggered by laminar shear stress (LSS) and disturbed shear stress (DSS) on vascular endothelial cells (ECs), and cyclic stretch in ECs and vascular smooth muscle cells (SMCs)-both of which activate several key transcription factors. Finally, we provide a recent overview of matrix-cell interactions and mechanotransduction centered on fibronectin in ECs and thrombospondin-1 in SMCs. The results of this review suggest that abnormal mechanical cues or altered responses to mechanical stimuli in EC and SMCs serve as the molecular basis of vascular diseases such as atherosclerosis, hypertension and aortic aneurysms. Collecting evidence and advancing knowledge on the mechanotransduction in the vessel wall can lead to a new direction of therapeutic interventions for vascular diseases.

Functional lipidomics of vascular endothelial cells in response to laminar shear stress

Laminar shear stress generated by blood flow stimulates endothelial cells and activates signal transduction, which plays an important role in vascular homeostasis. Several lines of evidence indicate that membrane and intracellular lipids are involved in the signal transduction of biomechanical stresses. In this study, we performed global profiling of cellular lipids from human pulmonary artery endothelial cells (HPAEC) exposed to laminar shear stress. A total of 761 species of lipids were successfully annotated, with 198 of these species significantly changed in response to shear stress for 24 hours. Ether-linked lipids containing an alkyl moiety with a medium chain length (C11-C14) were uniquely upregulated, and the administration of their biosynthetic precursor 1-O-dodecyl-rac-glycerol attenuated phorbol 12-myristate 13-acetate (PMA) induced vascular cell adhesion molecule-1 (VCAM-1) expression. Given the pro-inflammatory and atherogenic roles of VCAM-1, our findings suggest that the induction of a specific group of lipids (ie, ether-linked lipids with medium length alkyl side chain) may confer atheroprotective and anti-inflammatory roles to vascular endothelial cells under flow conditions.

MEF2 (Myocyte Enhancer Factor 2) Is Essential for Endothelial Homeostasis and the Atheroprotective Gene Expression Program

OBJECTIVE

Atherosclerosis predominantly forms in regions of oscillatory shear stress while regions of laminar shear stress are protected. This protection is partly through the endothelium in laminar flow regions expressing an anti-inflammatory and antithrombotic gene expression program. Several molecular pathways transmitting these distinct flow patterns to the endothelium have been defined. Our objective is to define the role of the MEF2 (myocyte enhancer factor 2) family of transcription factors in promoting an atheroprotective endothelium. Approach and Results: Here, we show through endothelial-specific deletion of the 3 MEF2 factors in the endothelium, Mef2a, -c, and -d, that MEF2 is a critical regulator of vascular homeostasis. MEF2 deficiency results in systemic inflammation, hemorrhage, thrombocytopenia, leukocytosis, and rapid lethality. Transcriptome analysis reveals that MEF2 is required for normal regulation of 3 pathways implicated in determining the flow responsiveness of the endothelium. Specifically, MEF2 is required for expression of Klf2 and Klf4, 2 partially redundant factors essential for promoting an anti-inflammatory and antithrombotic endothelium. This critical requirement results in phenotypic similarities between endothelial-specific deletions of Mef2a/c/d and Klf2/4. In addition, MEF2 regulates the expression of Notch family genes, Notch1, Dll1, and Jag1, which also promote an atheroprotective endothelium. In contrast to these atheroprotective pathways, MEF2 deficiency upregulates an atherosclerosis promoting pathway through increasing the amount of TAZ (transcriptional coactivator with PDZ-binding motif).

CONCLUSIONS

Our results implicate MEF2 as a critical upstream regulator of several transcription factors responsible for gene expression programs that affect development of atherosclerosis and promote an anti-inflammatory and antithrombotic endothelium. Graphic Abstract: A graphic abstract is available for this article.

SMAD6 transduces endothelial cell flow responses required for blood vessel homeostasis

Fluid shear stress provided by blood flow instigates a transition from active blood vessel network expansion during development, to vascular homeostasis and quiescence that is important for mature blood vessel function. Here we show that SMAD6 is required for endothelial cell flow-mediated responses leading to maintenance of vascular homeostasis. Concomitant manipulation of the mechanosensor Notch1 pathway and SMAD6 expression levels revealed that SMAD6 functions downstream of ligand-induced Notch signaling and transcription regulation. Mechanistically, full-length SMAD6 protein was needed to rescue Notch loss-induced flow misalignment. Endothelial cells depleted for SMAD6 had defective barrier function accompanied by upregulation of proliferation-associated genes and down regulation of junction-associated genes. The vascular protocadherin PCDH12 was upregulated by SMAD6 and required for proper flow-mediated endothelial cell alignment, placing it downstream of SMAD6. Thus, SMAD6 is a required transducer of flow-mediated signaling inputs downstream of Notch1 and upstream of PCDH12, as vessels transition from an angiogenic phenotype to maintenance of a homeostatic phenotype.

Inflammatory Cytokines and Atherosclerotic Plaque Progression. Therapeutic Implications

PURPOSE OF THE REVIEW

Inflammatory cytokines play a major role in atherosclerotic plaque progression. This review summarizes the rationale for personalized anti-inflammatory therapy.

RECENT FINDINGS

Systemic inflammatory parameters may be used to follow the clinical outcome in primary and secondary prevention. Medical therapy, both in patients with stable cardiovascular disease, or with acute events, may be tailored taking into consideration the level and course of systemic inflammatory mediators. There is significant space for improvement in primary prevention and in the treatment of patients who have suffered from severe cardiovascular events, paying attention to not only blood pressure and cholesterol levels but also including inflammatory parameters in our clinical analysis. The potential exists to alter the course of atherosclerosis with anti-inflammatory drugs. With increased understanding of the specific mechanisms that regulate the relationship between inflammation and atherosclerosis, new, more effective and specific anti-inflammatory treatment may become available.

Stress fiber anisotropy contributes to force-mode dependent chromatin stretching and gene upregulation in living cells

Living cells and tissues experience various complex modes of forces that are important in physiology and disease. However, how different force modes impact gene expression is elusive. Here we apply local forces of different modes via a magnetic bead bound to the integrins on a cell and quantified cell stiffness, chromatin deformation, and DHFR (dihydrofolate reductase) gene transcription. In-plane stresses result in lower cell stiffness than out-of-plane stresses that lead to bead rolling along the cell long axis (i.e., alignment of actin stress fibers) or at different angles (90° or 45°). However, chromatin stretching and ensuing DHFR gene upregulation by the in-plane mode are similar to those induced by the 45° stress mode. Disrupting stress fibers abolishes differences in cell stiffness, chromatin stretching, and DHFR gene upregulation under different force modes and inhibiting myosin II decreases cell stiffness, chromatin deformation, and gene upregulation. Theoretical modeling using discrete anisotropic stress fibers recapitulates experimental results and reveals underlying mechanisms of force-mode dependence. Our findings suggest that forces impact biological responses of living cells such as gene transcription via previously underappreciated means.

Living cells and tissues experience various complex modes of forces but how different force modes impact gene expression is elusive. Here authors apply forces via magnetic beads to integrins on a cell surface and observe force-mode dependent chromatin stretching and gene upregulation in cells and identify underlying mechanisms.

Low Shear Stress Upregulates CX3CR1 Expression by Inducing VCAM-1 via the NF-κB Pathway in Vascular Endothelial Cells

Atherosclerosis is a significant cause of mortality and morbidity. Studies suggest that the chemokine receptor CX3CR1 plays a critical role in atherogenesis. Shear stress is an important mechanical force that affects blood vessel function. In this study, we investigated the effect of shear stress on CX3CR1 expression in vascular endothelial cells (VECs). First, cells were exposed to different shear stress and then CX3CR1 mRNA and protein were measured by quantitative RT-PCR and western blot analysis, respectively. CX3CR1 gene silencing was used to analyze the molecular mechanisms underlying shear stress-mediated effects on CX3CR1 expression. CX3CR1 mRNA and protein expression were significantly increased with 4.14 dyne/cm² of shear stress compared with other tested levels of shear stress. We observed a significant increase in CX3CR1 mRNA levels at 2 h and CX3CR1 protein expression at 4 h. CX3CR1-induced VCAM-1 expression in response to low shear stress by activating NF-κB signaling pathway in VECs. Our findings demonstrate that low shear stress increases CX3CR1 expression, which increases VCAM-1 expression due to elevated NF-κB activation. The current study provides evidence of the correlation between shear stress and atherosclerosis mediated by CX3CR1.

YAP and the RhoC regulator ARHGAP18, are required to mediate flow-dependent endothelial cell alignment

BACKGROUND

Vascular endothelial cell alignment in the direction of flow is an adaptive response that protects against aortic diseases such as atherosclerosis. The RhoGTPases are known to regulate this alignment. We have shown previously that ARHGAP18 in endothelial cells is a negative regulator of RhoC and its expression is essential in flow-mediated alignment. Depletion of ARHGAP18 inhibits alignment and results in the induction of a pro-inflammatory phenotype. In embryogenesis, ARHGAP18 was identified as a downstream effector of the Yes-associated protein, YAP, which regulates cell shape and size.

METHODS

We have used siRNA technology to deplete either ARHGAP18 or YAP in human endothelial cells. The in vitro studies were performed under athero-protective, laminar flow conditions. The analysis of YAP activity was also investigated, using high performance confocal imaging, in our ARHGAP18 knockout mutant mice.

RESULTS

We show here that loss of ARHGAP18, although decreasing the expression of YAP results in its nuclear localisation consistent with activation. We further show that depletion of YAP itself results in its activation as defined by an in increase in its nuclear localisation and an increase in the YAP target gene, CyR61. Depletion of YAP, similar to that observed for ARHGAP18 depletion, results in loss of endothelial cell alignment under high shear stress mediated flow and also in the activation of NFkB, as determined by p65 nuclear localisation. In contrast, ARHGAP18 overexpression results in upregulation of YAP, its phosphorylation, and a decrease in the YAP target gene Cyr61, consistent with YAP inactivation. Finally, in ARHGAP18 deleted mice, in regions where there is a loss of endothelial cell alignment, a situation associated with a priming of the cells to a pro-inflammatory phenotype, YAP shows nuclear localisation.

CONCLUSION

Our results show that YAP is downstream of ARHGAP18 in mature endothelial cells and that this pathway is involved in the athero-protective alignment of endothelial cells under laminar shear stress. ARHGAP18 depletion leads to a disruption of the junctions as seen by loss of VE-Cadherin localisation to these regions and a concomitant localisation of YAP to the nucleus.

ARHGAP18: A Flow-Responsive Gene That Regulates Endothelial Cell Alignment and Protects Against Atherosclerosis

Background

Vascular endothelial cell (EC) alignment in the direction of flow is an adaptive response that protects against aortic diseases, such as atherosclerosis. The Rho GTPases are known to regulate this alignment. Herein, we analyze the effect of ARHGAP18 on the regulation of EC alignment and examine the effect of ARHGAP18 deficiency on the development of atherosclerosis in mice.

Methods and Results

We used in vitro analysis of ECs under flow conditions together with apolipoprotein E^−/−Arhgap18^−/− double‐mutant mice to study the function of ARHGAP18 in a high‐fat diet–induced model of atherosclerosis. Depletion of ARHGAP18 inhibited the alignment of ECs in the direction of flow and promoted inflammatory phenotype, as evidenced by disrupted junctions and increased expression of nuclear factor‐κB and intercellular adhesion molecule‐1 and decreased endothelial nitric oxide synthase. Mice with double deletion in ARHGAP18 and apolipoprotein E and fed a high‐fat diet show early onset of atherosclerosis, with lesions developing in atheroprotective regions.

Conclusions

ARHGAP18 is a protective gene that maintains EC alignments in the direction of flow. Deletion of ARHGAP18 led to loss of EC ability to align and promoted atherosclerosis development.

Extracorporeal shock waves protect cardiomyocytes from doxorubicin-induced cardiomyopathy by upregulating survivin via the integrin-ILK-Akt-Sp1/p53 axis

Doxorubicin (DOX) is a widely used anti-cancer drug; however, it has limited application due to cardiotoxicity. Extracorporeal shock waves (ESW) have been suggested to treat inflammatory and ischemic diseases, but the concrete effect of ESW in DOX-induced cardiomyopathy remain obscure. After H9c2 cells were subjected to ESW (0.04 mJ/cm²), they were treated with 1 μM DOX. As a result, ESW protected cardiomyocytes from DOX-induced cell death. H9c2 cells treated with DOX downregulated p-Akt and survivin expression, whereas the ESW treatment recovered both, suggesting its anti-apoptotic effect. ESW activated integrin α_vβ₃ and α_vβ₅, cardiomyocyte mechanosensors, followed by upregulation of ILK, p-Akt and survivin levels. Further, Sp1 and p53 were determined as key transcriptional factors mediating survivin expression via Akt phosphorylation by ESW. In in vivo acute DOX-induced cardiomyopathy model, the echocardiographic results showed that group subjected to ESW recovered from acute DOX-induced cardiomyopathy; left ventricular function was improved. The immunohistochemical staining results showed increased survivin and Bcl2 expression in ESW + DOX group compared to those in the DOX-injected group. In conclusion, non-invasive shockwaves protect cardiomyocytes from DOX-induced cardiomyopathy by upregulating survivin via integrin-ILK-Akt-Sp1/p53 pathway. In vivo study proposed ESW as a new kind of specific and safe therapy against acute DOX-induced cardiomyopathy.

Darwinian evolution and cardiovascular remodeling

Mechanotransduction, MT, is an ancient evolutionary legacy existing in every living species and involving complex rearrangements of multiple proteins in response to a mechanical stress. MT includes three different interrelated processes: mechanosensation, mechanotransmission, and mechanoresponse. Each process is specifically adapted to a given tissue and stress. Both cardiac and arterial remodeling involve MT. Physiological or pathological cardiac remodeling, CR, is firstly a beneficial mechanoresponse, MR, which allows the heart to recover to a normal economy, better adapted to the new working conditions. Nevertheless, exercise-induced cardiac remodeling is more a coming-back to normal conditions than a superimposed event. On the longer term, the MR creates fibrosis which accounts, in part, for the reduced cardiac output in the CR. In the hypertension-induced arterial remodeling, arterial MR allows the vessels to maintain a normal circumferential constraint before an augmented arterial pressure. In atherogenesis: (i) The presence of atheroma in several animal species and atherosclerosis in ancient civilizations suggests more basic predispositions. (ii) The atherosclerotic plaques preferably develop at predictable arterial sites of disturbed blood flow showing that MT is involved in the initial steps of atherogenesis.

Fluid shear stress regulates the expression of Lectin-like oxidized low density lipoprotein receptor-1 via KLF2-AP-1 pathway depending on its intensity and pattern in endothelial cells

BACKGROUND AND AIMS

Vascular endothelial cells (ECs) are exposed to fluid shear stress (FSS), which modulates vascular pathophysiology. Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is crucial in endothelial dysfunction and atherosclerosis. We elucidated the mechanism regulating LOX-1 expression in ECs by FSS.

METHODS

Human umbilical vein endothelial cells were exposed to laminar shear stress (LSS) of indicated intensities using a unidirectional steady flow, or to oscillatory shear stress (OSS) using a bidirectional disturbed flow. In vivo studies were performed in a mouse model of partial carotid ligation and human pulmonary artery sections.

RESULTS

Within ECs, OSS upregulated LOX-1 expression, while LSS (20 dyne/cm²) downregulated it. We confirmed that OSS-induced LOX-1 expression was suppressed when the mechanotransduction was inhibited by knockdown of the mechanosensory complex. In addition, we demonstrated that Kruppel-like factor 2 (KLF2) has an inhibitory role on OSS-induced LOX-1 expression. Next, we determined that activator protein-1 (AP-1) was the key transcription factor inducing LOX-1 expression by OSS, which was inhibited by KLF2 overexpression. To explore whether the intensity of LSS affects LOX-1 expression, we tested three different intensities (20, 60, and 120 dyne/cm²) of LSS. We observed higher LOX-1 expression with high shear stresses of 120 dyne/cm² compared to 20 and 60 dyne/cm², with OSS-like KLF2-AP-1 signaling patterns. Furthermore, ECs within disturbed flow regions showed upregulated LOX-1 expression in vivo.

CONCLUSIONS

We concluded that LOX-1 expression on ECs is regulated via FSS depending on its intensity as well as pattern. Furthermore, this is mediated through the KLF2-AP1 pathway of mechanotransduction.

Alternative RNA splicing in the endothelium mediated in part by Rbfox2 regulates the arterial response to low flow

Low and disturbed blood flow drives the progression of arterial diseases including atherosclerosis and aneurysms. The endothelial response to flow and its interactions with recruited platelets and leukocytes determine disease progression. Here, we report widespread changes in alternative splicing of pre-mRNA in the flow-activated murine arterial endothelium in vivo. Alternative splicing was suppressed by depletion of platelets and macrophages recruited to the arterial endothelium under low and disturbed flow. Binding motifs for the Rbfox-family are enriched adjacent to many of the regulated exons. Endothelial deletion of Rbfox2, the only family member expressed in arterial endothelium, suppresses a subset of the changes in transcription and RNA splicing induced by low flow. Our data reveal an alternative splicing program activated by Rbfox2 in the endothelium on recruitment of platelets and macrophages and demonstrate its relevance in transcriptional responses during flow-driven vascular inflammation.

Ultrahigh-Resolution Optical Coherence Elastography Images Cellular-Scale Stiffness of Mouse Aorta

Cellular-scale imaging of the mechanical properties of tissue has helped to reveal the origins of disease; however, cellular-scale resolution is not readily achievable in intact tissue volumes. Here, we demonstrate volumetric imaging of Young's modulus using ultrahigh-resolution optical coherence elastography, and apply it to characterizing the stiffness of mouse aortas. We achieve isotropic resolution of better than 15 μm over a 1-mm lateral field of view through the entire depth of an intact aortic wall. We employ a method of quasi-static compression elastography that measures volumetric axial strain and uses a compliant, transparent layer to measure surface axial stress. This combination is used to estimate Young's modulus throughout the volume. We demonstrate differentiation by stiffness of individual elastic lamellae and vascular smooth muscle. We observe stiffening of the aorta in regulator of G protein signaling 5-deficient mice, a model that is linked to vascular remodeling and fibrosis. We observe increased stiffness with proximity to the heart, as well as regions with micro-structural and micro-mechanical signatures characteristic of fibrous and lipid-rich tissue. High-resolution imaging of Young's modulus with optical coherence elastography may become an important tool in vascular biology and in other fields concerned with understanding the role of mechanics within the complex three-dimensional architecture of tissue.

Quantitative trait locus mapping in mice identifies phospholipase Pla2g12a as novel atherosclerosis modifier

BACKGROUND AND AIMS

In a previous work, a female-specific atherosclerosis risk locus on chromosome (Chr) 3 was identified in an intercross of atherosclerosis-resistant FVB and atherosclerosis-susceptible C57BL/6 (B6) mice on the LDL-receptor deficient (Ldlr^-/-) background. It was the aim of the current study to identify causative genes at this locus.

METHODS

We established a congenic mouse model, where FVB.Chr3^B6/B6 mice carried an 80 Mb interval of distal Chr3 on an otherwise FVB.Ldlr^-/- background, to validate the Chr3 locus. Candidate genes were identified using genome-wide expression analyses. Differentially expressed genes were validated using quantitative PCRs in F0 and F2 mice and their functions were investigated in pathophysiologically relevant cells.

RESULTS

Fine-mapping of the Chr3 locus revealed two overlapping, yet independent subloci for female atherosclerosis susceptibility: when transmitted by grandfathers to granddaughters, the B6 risk allele increased atherosclerosis and downregulated the expression of the secreted phospholipase Pla2g12a (2.6 and 2.2 fold, respectively); when inherited by grandmothers, the B6 risk allele induced vascular cell adhesion molecule 1 (Vcam1). Down-regulation of Pla2g12a and up-regulation of Vcam1 were validated in female FVB.Chr3^B6/B6 congenic mice, which developed 2.5 greater atherosclerotic lesions compared to littermate controls (p=0.039). Pla2g12a was highly expressed in aortic endothelial cells in vivo, and knocking-down Pla2g12a expression by RNAi in cultured vascular endothelial cells or macrophages increased their adhesion to ECs in vitro.

CONCLUSIONS

Our data establish Pla2g12a as an atheroprotective candidate gene in mice, where high expression levels in ECs and macrophages may limit the recruitment and accumulation of these cells in nascent atherosclerotic lesions.

VE-Cadherin Phosphorylation Regulates Endothelial Fluid Shear Stress Responses through the Polarity Protein LGN

Fluid shear stress due to blood flow on the vascular endothelium regulates blood vessel development, remodeling, physiology, and pathology [1, 2]. A complex consisting of PECAM-1, VE-cadherin, and vascular endothelial growth factor receptors (VEGFRs) that resides at endothelial cell-cell junctions transduces signals important for flow-dependent vasodilation, blood vessel remodeling, and atherosclerosis. PECAM-1 transduces forces to activate src family kinases (SFKs), which phosphorylate and transactivate VEGFRs [3-5]. By contrast, VE-cadherin functions as an adaptor that interacts with VEGFRs through their respective cytoplasmic domains and promotes VEGFR activation in flow [6]. Indeed, shear stress triggers rapid increases in force across PECAM-1 but decreases the force across VE-cadherin, in close association with downstream signaling [5]. Interestingly, VE-cadherin cytoplasmic tyrosine Y658 can be phosphorylated by SFKs [7], which is maximally induced by low shear stress in vitro and in vivo [8]. These considerations prompted us to address the involvement of VE-cadherin cytoplasmic tyrosines in flow sensing. We found that phosphorylation of a small pool of VE-cadherin on Y658 is essential for flow sensing through the junctional complex. Y658 phosphorylation induces dissociation of p120ctn, which allows binding of the polarity protein LGN. LGN is then required for multiple flow responses in vitro and in vivo, including activation of inflammatory signaling at regions of disturbed flow, and flow-dependent vascular remodeling. Thus, endothelial flow mechanotransduction through the junctional complex is mediated by a specific pool of VE-cadherin that is phosphorylated on Y658 and bound to LGN.

Inflammation and myointimal hyperplasia. Correlation with hemodynamic forces

OBJECTIVES

The aim of our study was to correlate flow dynamics and the release of inflammatory cytokines Interleukin 1, 2, 6, TNF (Tumour Necrosis Factor) alfa, both in vitro and in vivo.

MATERIALS AND METHODS

Endothelial cells were exposed to laminar flow (6dyne/cm²) in an in vitro circulatory system and the release of Interleukin 1, 2, 6 and TNF alfa was quantified by ELISA. Interleukin 1, 2, 6 and TNF alfa release was also assessed in vein grafts implanted in the arterial circulation of Lewis rats. Arterial vein grafts were explanted at different time intervals from 3days to 12weeks after surgery. Vein grafts implanted in the arterial circulation for 4weeks, were re implanted in the venous circulation of syngenic Lewis rats, and the release of Interleukin 1, 2, 6 and TNF alfa, was assessed in an organ culture. Six vein grafts (4 occluded, 2 patent) implanted in humans as femorodistal bypass were examined for the presence of myointimal hyperplasia and perigraft inflammatory cells.

RESULTS

In vitro, endothelial cells exposed to laminar flow released an increased amount of Interleukin 1, 2, 6 and TNF alfa in comparison to endothelial cells not exposed to flow. In experimental vein grafts implanted in the arterial circulation there was an increased release of inflammatory cytokines associated to inflammatory changes in the adventitia. Once the vein grafts were re implanted in the venous circulation, the release of these cytokines diminished, while the inflammatory changes in the adventitia regressed.

CONCLUSIONS

Increased shear stress induces release of cytokines and inflammatory changes in the adventitia. These inflammatory changes can contribute to plaque progression and to un stable plaque. These findings support the use of anti-inflammatory therapy in patients prone to develop atherosclerosis and in those who had arterial reconstructive surgery.

A Protocol for Using Förster Resonance Energy Transfer (FRET)-force Biosensors to Measure Mechanical Forces across the Nuclear LINC Complex

The LINC complex has been hypothesized to be the critical structure that mediates the transfer of mechanical forces from the cytoskeleton to the nucleus. Nesprin-2G is a key component of the LINC complex that connects the actin cytoskeleton to membrane proteins (SUN domain proteins) in the perinuclear space. These membrane proteins connect to lamins inside the nucleus. Recently, a Förster Resonance Energy Transfer (FRET)-force probe was cloned into mini-Nesprin-2G (Nesprin-TS (tension sensor)) and used to measure tension across Nesprin-2G in live NIH3T3 fibroblasts. This paper describes the process of using Nesprin-TS to measure LINC complex forces in NIH3T3 fibroblasts. To extract FRET information from Nesprin-TS, an outline of how to spectrally unmix raw spectral images into acceptor and donor fluorescent channels is also presented. Using open-source software (ImageJ), images are pre-processed and transformed into ratiometric images. Finally, FRET data of Nesprin-TS is presented, along with strategies for how to compare data across different experimental groups.

2016 Russell Ross Memorial Lecture in Vascular Biology: Molecular-Cellular Mechanisms in the Progression of Atherosclerosis

Atherosclerosis is initiated by the subendothelial accumulation of apoB-lipoproteins, which initiates a sterile inflammatory response dominated by monocyte-macrophages but including all classes of innate and adaptive immune cells. These inflammatory cells, together with proliferating smooth muscle cells and extracellular matrix, promote the formation of subendothelial lesions or plaques. In the vast majority of cases, these lesions do not cause serious clinical symptoms, which is due in part to a resolution-repair response that limits tissue damage. However, a deadly minority of lesions progress to the point where they can trigger acute lumenal thrombosis, which may then cause unstable angina, myocardial infarction, sudden cardiac death, or stroke. Many of these clinically dangerous lesions have hallmarks of defective inflammation resolution, including defective clearance of dead cells (efferocytosis), necrosis, a defective scar response, and decreased levels of lipid mediators of the resolution response. Efferocytosis is both an effector arm of the resolution response and an inducer of resolution mediators, and thus its defect in advanced atherosclerosis amplifies plaque progression. Preclinical causation/treatment studies have demonstrated that replacement therapy with exogenously administered resolving mediators can improve lesional efferocytosis and prevent plaque progression. Work in this area has the potential to potentiate the cardiovascular benefits of apoB-lipoprotein-lowering therapy.

MicroRNA-210 Enhances Fibrous Cap Stability in Advanced Atherosclerotic Lesions

RATIONALE

In the search for markers and modulators of vascular disease, microRNAs (miRNAs) have emerged as potent therapeutic targets.

OBJECTIVE

To investigate miRNAs of clinical interest in patients with unstable carotid stenosis at risk of stroke.

METHODS AND RESULTS

Using patient material from the BiKE (Biobank of Karolinska Endarterectomies), we profiled miRNA expression in patients with stable versus unstable carotid plaque. A polymerase chain reaction-based miRNA array of plasma, sampled at the carotid lesion site, identified 8 deregulated miRNAs (miR-15b, miR-29c, miR-30c/d, miR-150, miR-191, miR-210, and miR-500). miR-210 was the most significantly downregulated miRNA in local plasma material. Laser capture microdissection and in situ hybridization revealed a distinct localization of miR-210 in fibrous caps. We confirmed that miR-210 directly targets the tumor suppressor gene APC (adenomatous polyposis coli), thereby affecting Wnt (Wingless-related integration site) signaling and regulating smooth muscle cell survival, as well as differentiation in advanced atherosclerotic lesions. Substantial changes in arterial miR-210 were detectable in 2 rodent models of vascular remodeling and plaque rupture. Modulating miR-210 in vitro and in vivo improved fibrous cap stability with implications for vascular disease.

CONCLUSIONS

An unstable carotid plaque at risk of stroke is characterized by low expression of miR-210. miR-210 contributes to stabilizing carotid plaques through inhibition of APC, ensuring smooth muscle cell survival. We present local delivery of miR-210 as a therapeutic approach for prevention of atherothrombotic vascular events.

Intravascular hemodynamics and coronary artery disease: New insights and clinical implications

Intracoronary hemodynamics play a pivotal role in the initiation and progression of the atherosclerotic process. Low pro-inflammatory endothelial shear stress impacts vascular physiology and leads to the occurrence of coronary artery disease and its implications.

Interaction between integrin α5 and PDE4D regulates endothelial inflammatory signalling

Atherosclerosis is primarily a disease of lipid metabolism and inflammation; however, it is also closely associated with endothelial extracellular matrix (ECM) remodelling, with fibronectin accumulating in the laminin-collagen basement membrane. To investigate how fibronectin modulates inflammation in arteries, we replaced the cytoplasmic tail of the fibronectin receptor integrin α5 with that of the collagen/laminin receptor integrin α2. This chimaera suppressed inflammatory signalling in endothelial cells on fibronectin and in knock-in mice. Fibronectin promoted inflammation by suppressing anti-inflammatory cAMP. cAMP was activated through endothelial prostacyclin secretion; however, this was ECM-independent. Instead, cells on fibronectin suppressed cAMP via enhanced phosphodiesterase (PDE) activity, through direct binding of integrin α5 to phosphodiesterase-4D5 (PDE4D5), which induced PP2A-dependent dephosphorylation of PDE4D5 on the inhibitory site Ser651. In vivo knockdown of PDE4D5 inhibited inflammation at athero-prone sites. These data elucidate a molecular mechanism linking ECM remodelling and inflammation, thereby identifying a new class of therapeutic targets.

Characterizations and Correlations of Wall Shear Stress in Aneurysmal Flow

Wall shear stress (WSS) is one of the most studied hemodynamic parameters, used in correlating blood flow to various diseases. The pulsatile nature of blood flow, along with the complex geometries of diseased arteries, produces complicated temporal and spatial WSS patterns. Moreover, WSS is a vector, which further complicates its quantification and interpretation. The goal of this study is to investigate WSS magnitude, angle, and vector changes in space and time in complex blood flow. Abdominal aortic aneurysm (AAA) was chosen as a setting to explore WSS quantification. Patient-specific computational fluid dynamics (CFD) simulations were performed in six AAAs. New WSS parameters are introduced, and the pointwise correlation among these, and more traditional WSS parameters, was explored. WSS magnitude had positive correlation with spatial/temporal gradients of WSS magnitude. This motivated the definition of relative WSS gradients. WSS vectorial gradients were highly correlated with magnitude gradients. A mix WSS spatial gradient and a mix WSS temporal gradient are proposed to equally account for variations in the WSS angle and magnitude in single measures. The important role that WSS plays in regulating near wall transport, and the high correlation among some of the WSS parameters motivates further attention in revisiting the traditional approaches used in WSS characterizations.

Cell-cell junctional mechanotransduction in endothelial remodeling

The vasculature is one of the most dynamic tissues that encounter numerous mechanical cues derived from pulsatile blood flow, blood pressure, activity of smooth muscle cells in the vessel wall, and transmigration of immune cells. The inner layer of blood and lymphatic vessels is covered by the endothelium, a monolayer of cells which separates blood from tissue, an important function that it fulfills even under the dynamic circumstances of the vascular microenvironment. In addition, remodeling of the endothelial barrier during angiogenesis and trafficking of immune cells is achieved by specific modulation of cell-cell adhesion structures between the endothelial cells. In recent years, there have been many new discoveries in the field of cellular mechanotransduction which controls the formation and destabilization of the vascular barrier. Force-induced adaptation at endothelial cell-cell adhesion structures is a crucial node in these processes that challenge the vascular barrier. One of the key examples of a force-induced molecular event is the recruitment of vinculin to the VE-cadherin complex upon pulling forces at cell-cell junctions. Here, we highlight recent advances in the current understanding of mechanotransduction responses at, and derived from, endothelial cell-cell junctions. We further discuss their importance for vascular barrier function and remodeling in development, inflammation, and vascular disease.

The F-BAR protein pacsin2 inhibits asymmetric VE-cadherin internalization from tensile adherens junctions

Vascular homoeostasis, development and disease critically depend on the regulation of endothelial cell-cell junctions. Here we uncover a new role for the F-BAR protein pacsin2 in the control of VE-cadherin-based endothelial adhesion. Pacsin2 concentrates at focal adherens junctions (FAJs) that are experiencing unbalanced actomyosin-based pulling. FAJs move in response to differences in local cytoskeletal geometry and pacsin2 is recruited consistently to the trailing end of fast-moving FAJs via a mechanism that requires an intact F-BAR domain. Photoconversion, photobleaching, immunofluorescence and super-resolution microscopy reveal polarized dynamics, and organization of junctional proteins between the front of FAJs and their trailing ends. Interestingly, pacsin2 recruitment inhibits internalization of the VE-cadherin complex from FAJ trailing ends and is important for endothelial monolayer integrity. Together, these findings reveal a novel junction protective mechanism during polarized trafficking of VE-cadherin, which supports barrier maintenance within dynamic endothelial tissue.

In vivo modulation of endothelial polarization by Apelin receptor signalling

Endothelial cells (ECs) respond to shear stress by aligning in the direction of flow. However, how ECs respond to flow in complex in vivo environments is less clear. Here we describe an endothelial-specific transgenic zebrafish line, whereby the Golgi apparatus is labelled to allow for in vivo analysis of endothelial polarization. We find that most ECs polarize within 4.5 h after the onset of vigorous blood flow and, by manipulating cardiac function, observe that flow-induced EC polarization is a dynamic and reversible process. Based on its role in EC migration, we analyse the role of Apelin signalling in EC polarization and find that it is critical for this process. Knocking down Apelin receptor function in human primary ECs also affects their polarization. Our study provides new tools to analyse the mechanisms of EC polarization in vivo and reveals an important role in this process for a signalling pathway implicated in cardiovascular disease.

Macrophages and Dendritic Cells

Atherosclerosis is a complex chronic disease. The accumulation of myeloid cells in the arterial intima, including macrophages and dendritic cells (DCs), is a feature of early stages of disease. For decades, it has been known that monocyte recruitment to the intima contributes to the burden of lesion macrophages. Yet, this paradigm may require reevaluation in light of recent advances in understanding of tissue macrophage ontogeny, their capacity for self-renewal, as well as observations that macrophages proliferate throughout atherogenesis and that self-renewal is critical for maintenance of macrophages in advanced lesions. The rate of atherosclerotic lesion formation is profoundly influenced by innate and adaptive immunity, which can be regulated locally within atherosclerotic lesions, as well as in secondary lymphoid organs, the bone marrow and the blood. DCs are important modulators of immunity. Advances in the past decade have cemented our understanding of DC subsets, functions, hematopoietic origin, gene expression patterns, transcription factors critical for differentiation, and provided new tools for study of DC biology. The functions of macrophages and DCs overlap to some extent, thus it is important to reassess the contributions of each of these myeloid cells taking into account strict criteria of cell identification, ontogeny, and determine whether their key roles are within atherosclerotic lesions or secondary lymphoid organs. This review will highlight key aspect of macrophage and DC biology, summarize how these cells participate in different stages of atherogenesis and comment on complexities, controversies, and gaps in knowledge in the field.

Endothelial-to-mesenchymal transition drives atherosclerosis progression

The molecular mechanisms responsible for the development and progression of atherosclerotic lesions have not been fully established. Here, we investigated the role played by endothelial-to-mesenchymal transition (EndMT) and its key regulator FGF receptor 1 (FGFR1) in atherosclerosis. In cultured human endothelial cells, both inflammatory cytokines and oscillatory shear stress reduced endothelial FGFR1 expression and activated TGF-β signaling. We further explored the link between disrupted FGF endothelial signaling and progression of atherosclerosis by introducing endothelial-specific deletion of FGF receptor substrate 2 α (Frs2a) in atherosclerotic (Apoe(-/-)) mice. When placed on a high-fat diet, these double-knockout mice developed atherosclerosis at a much earlier time point compared with that their Apoe(-/-) counterparts, eventually demonstrating an 84% increase in total plaque burden. Moreover, these animals exhibited extensive development of EndMT, deposition of fibronectin, and increased neointima formation. Additionally, we conducted a molecular and morphometric examination of left main coronary arteries from 43 patients with various levels of coronary disease to assess the clinical relevance of these findings. The extent of coronary atherosclerosis in this patient set strongly correlated with loss of endothelial FGFR1 expression, activation of endothelial TGF-β signaling, and the extent of EndMT. These data demonstrate a link between loss of protective endothelial FGFR signaling, development of EndMT, and progression of atherosclerosis.

FOXC2 and fluid shear stress stabilize postnatal lymphatic vasculature

Biomechanical forces, such as fluid shear stress, govern multiple aspects of endothelial cell biology. In blood vessels, disturbed flow is associated with vascular diseases, such as atherosclerosis, and promotes endothelial cell proliferation and apoptosis. Here, we identified an important role for disturbed flow in lymphatic vessels, in which it cooperates with the transcription factor FOXC2 to ensure lifelong stability of the lymphatic vasculature. In cultured lymphatic endothelial cells, FOXC2 inactivation conferred abnormal shear stress sensing, promoting junction disassembly and entry into the cell cycle. Loss of FOXC2-dependent quiescence was mediated by the Hippo pathway transcriptional coactivator TAZ and, ultimately, led to cell death. In murine models, inducible deletion of Foxc2 within the lymphatic vasculature led to cell-cell junction defects, regression of valves, and focal vascular lumen collapse, which triggered generalized lymphatic vascular dysfunction and lethality. Together, our work describes a fundamental mechanism by which FOXC2 and oscillatory shear stress maintain lymphatic endothelial cell quiescence through intercellular junction and cytoskeleton stabilization and provides an essential link between biomechanical forces and endothelial cell identity that is necessary for postnatal vessel homeostasis. As FOXC2 is mutated in lymphedema-distichiasis syndrome, our data also underscore the role of impaired mechanotransduction in the pathology of this hereditary human disease.

Molecular controls of arterial morphogenesis

Formation of arterial vasculature, here termed arteriogenesis, is a central process in embryonic vascular development as well as in adult tissues. Although the process of capillary formation, angiogenesis, is relatively well understood, much remains to be learned about arteriogenesis. Recent discoveries point to the key role played by vascular endothelial growth factor receptor 2 in control of this process and to newly identified control circuits that dramatically influence its activity. The latter can present particularly attractive targets for a new class of therapeutic agents capable of activation of this signaling cascade in a ligand-independent manner, thereby promoting arteriogenesis in diseased tissues.

Recent insights into the cellular biology of atherosclerosis

Atherosclerosis occurs in the subendothelial space (intima) of medium-sized arteries at regions of disturbed blood flow and is triggered by an interplay between endothelial dysfunction and subendothelial lipoprotein retention. Over time, this process stimulates a nonresolving inflammatory response that can cause intimal destruction, arterial thrombosis, and end-organ ischemia. Recent advances highlight important cell biological atherogenic processes, including mechanotransduction and inflammatory processes in endothelial cells, origins and contributions of lesional macrophages, and origins and phenotypic switching of lesional smooth muscle cells. These advances illustrate how in-depth mechanistic knowledge of the cellular pathobiology of atherosclerosis can lead to new ideas for therapy.

Cell-stiffness-induced mechanosignaling - a key driver of leukocyte transendothelial migration

The breaching of cellular and structural barriers by migrating cells is a driving factor in development, inflammation and tumor cell metastasis. One of the most extensively studied examples is the extravasation of activated leukocytes across the vascular endothelium, the inner lining of blood vessels. Each step of this leukocyte transendothelial migration (TEM) process is regulated by distinct endothelial adhesion receptors such as the intercellular adhesion molecule 1 (ICAM1). Adherent leukocytes exert force on these receptors, which sense mechanical cues and transform them into localized mechanosignaling in endothelial cells. In turn, the function of the mechanoreceptors is controlled by the stiffness of the endothelial cells and of the underlying substrate representing a positive-feedback loop. In this Commentary, we focus on the mechanotransduction in leukocytes and endothelial cells, which is induced in response to variations in substrate stiffness. Recent studies have described the first key proteins involved in these mechanosensitive events, allowing us to identify common regulatory mechanisms in both cell types. Finally, we discuss how endothelial cell stiffness controls the individual steps in the leukocyte TEM process. We identify endothelial cell stiffness as an important component, in addition to locally presented chemokines and adhesion receptors, which guides leukocytes to sites that permit TEM.

Endothelial permeability and VE-cadherin: a wacky comradeship

The endothelium forms a selective semi-permeable barrier controlling bidirectional transfer between blood vessel and irrigated tissues. This crucial function relies on the dynamic architecture of endothelial cell–cell junctions, and in particular, VE -cadherin-mediated contacts. VE -cadherin indeed chiefly organizes the opening and closing of the endothelial barrier, and is central in permeability changes. In this review, the way VE -cadherin-based contacts are formed and maintained is first presented, including molecular traits of its expression, partners, and signaling. In a second part, the mechanisms by which VE -cadherin adhesion can be disrupted, leading to cell–cell junction weakening and endothelial permeability increase, are described. Overall, the molecular basis for VE -cadherin control of the endothelial barrier function is of high interest for biomedical research, as vascular leakage is observed in many pathological conditions and human diseases.

Mechanobiology of myofibroblast adhesion in fibrotic cardiac disease

Fibrotic cardiac disease, a leading cause of death worldwide, manifests as substantial loss of function following maladaptive tissue remodeling. Fibrosis can affect both the heart valves and the myocardium and is characterized by the activation of fibroblasts and accumulation of extracellular matrix. Valvular interstitial cells and cardiac fibroblasts, the cell types responsible for maintenance of cardiac extracellular matrix, are sensitive to changing mechanical environments, and their ability to sense and respond to mechanical forces determines both normal development and the progression of disease. Recent studies have uncovered specific adhesion proteins and mechano-sensitive signaling pathways that contribute to the progression of fibrosis. Integrins form adhesions with the extracellular matrix, and respond to changes in substrate stiffness and extracellular matrix composition. Cadherins mechanically link neighboring cells and are likely to contribute to fibrotic disease propagation. Finally, transition to the active myofibroblast phenotype leads to maladaptive tissue remodeling and enhanced mechanotransductive signaling, forming a positive feedback loop that contributes to heart failure. This Commentary summarizes recent findings on the role of mechanotransduction through integrins and cadherins to perpetuate mechanically induced differentiation and fibrosis in the context of cardiac disease.

Animal models of surgically manipulated flow velocities to study shear stress-induced atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial tree that develops at predisposed sites, coinciding with locations that are exposed to low or oscillating shear stress. Manipulating flow velocity, and concomitantly shear stress, has proven adequate to promote endothelial activation and subsequent plaque formation in animals. In this article, we will give an overview of the animal models that have been designed to study the causal relationship between shear stress and atherosclerosis by surgically manipulating blood flow velocity profiles. These surgically manipulated models include arteriovenous fistulas, vascular grafts, arterial ligation, and perivascular devices. We review these models of manipulated blood flow velocity from an engineering and biological perspective, focusing on the shear stress profiles they induce and the vascular pathology that is observed.

Syndecan 4 is required for endothelial alignment in flow and atheroprotective signaling

Atherosclerotic plaque localization correlates with regions of disturbed flow in which endothelial cells (ECs) align poorly, whereas sustained laminar flow correlates with cell alignment in the direction of flow and resistance to atherosclerosis. We now report that in hypercholesterolemic mice, deletion of syndecan 4 (S4(-/-)) drastically increased atherosclerotic plaque burden with the appearance of plaque in normally resistant locations. Strikingly, ECs from the thoracic aortas of S4(-/-) mice were poorly aligned in the direction of the flow. Depletion of S4 in human umbilical vein endothelial cells (HUVECs) using shRNA also inhibited flow-induced alignment in vitro, which was rescued by re-expression of S4. This effect was highly specific, as flow activation of VEGF receptor 2 and NF-κB was normal. S4-depleted ECs aligned in cyclic stretch and even elongated under flow, although nondirectionally. EC alignment was previously found to have a causal role in modulating activation of inflammatory versus antiinflammatory pathways by flow. Consistent with these results, S4-depleted HUVECs in long-term laminar flow showed increased activation of proinflammatory NF-κB and decreased induction of antiinflammatory kruppel-like factor (KLF) 2 and KLF4. Thus, S4 plays a critical role in sensing flow direction to promote cell alignment and inhibit atherosclerosis.