Impact of disrupted cyclic stretch in intracranial aneurysms: Insights from endothelial cell transcriptomic dataset

Intracranial aneurysm (IA) rupture is a common cause of hemorrhagic stroke. The treatment of unruptured IAs is a challenging decision that requires delicate risk stratification. The rate of poor clinical outcomes after surgical intervention (aneurysm clipping) or endovascular coiling remains elevated (6.7% and 4.8%, respectively), and they do not provide an absolute guarantee to prevent IA growth and rupture. Currently, there is no pharmaceutical treatment to cure or stabilize IAs. Improving the current or developing new treatments for IA disease would require a better understanding of the cellular and molecular mechanisms occurring in the different stages of the disease. Hemodynamic forces play a critical role in IA disease. While the role of wall shear stress in IAs is well-established, the influence of cyclic circumferential stretch (CCS) still needs clarification. IAs are generally characterized by a lack of CCS. In this investigation, we sought to understand the effect of aneurysmal CCS on endothelial cell (EC) function and its potential significance in IA disease, hypothesizing that CCS can influence IA wall remodelling. RNA-seq data were generated from human umbilical vein ECs (HUVECs) exposed to physiological (6%) or aneurysmal CCS (static). We performed differential gene expression and pathway enrichment analysis. Additionally, we highlighted cell junction gene expression between static and 6% CCS to contribute to the debate about how cell junctions affect endothelium stability and integrity. Researchers in the vascular biology field may benefit from this transcriptomic profile to understand the effect of mechanical stretch on EC biology and its potential significance in vascular disease development.

EVA1A (Eva-1 Homolog A) Promotes Endothelial Apoptosis and Inflammatory Activation Under Disturbed Flow Via Regulation of Autophagy

BACKGROUND

Hemodynamic wall shear stress (WSS) exerted on the endothelium by flowing blood determines the spatial distribution of atherosclerotic lesions. Disturbed flow (DF) with a low WSS magnitude and reversing direction promotes atherosclerosis by regulating endothelial cell (EC) viability and function, whereas un-DF which is unidirectional and of high WSS magnitude is atheroprotective. Here, we study the role of EVA1A (eva-1 homolog A), a lysosome and endoplasmic reticulum-associated protein linked to autophagy and apoptosis, in WSS-regulated EC dysfunction.

METHODS

The effect of WSS on EVA1A expression was studied using porcine and mouse aortas and cultured human ECs exposed to flow. EVA1A was silenced in vitro in human ECs and in vivo in zebrafish using siRNA (small interfering RNA) and morpholinos, respectively.

RESULTS

EVA1A was induced by proatherogenic DF at both mRNA and protein levels. EVA1A silencing resulted in decreased EC apoptosis, permeability, and expression of inflammatory markers under DF. Assessment of autophagic flux using the autolysosome inhibitor, bafilomycin coupled to the autophagy markers LC3-II (microtubule-associated protein 1 light chain 3-II) and p62, revealed that EVA1A knockdown promotes autophagy when ECs are exposed to DF, but not un-DF . Blocking autophagic flux led to increased EC apoptosis in EVA1A-knockdown cells exposed to DF, suggesting that autophagy mediates the effects of DF on EC dysfunction. Mechanistically, EVA1A expression was regulated by flow direction via TWIST1 (twist basic helix-loop-helix transcription factor 1). In vivo, knockdown of EVA1A orthologue in zebrafish resulted in reduced EC apoptosis, confirming the proapoptotic role of EVA1A in the endothelium.

CONCLUSIONS

We identified EVA1A as a novel flow-sensitive gene that mediates the effects of proatherogenic DF on EC dysfunction by regulating autophagy.

JAG1-NOTCH4 mechanosensing drives atherosclerosis

Endothelial cell (EC) sensing of disturbed blood flow triggers atherosclerosis, a disease of arteries that causes heart attack and stroke, through poorly defined mechanisms. The Notch pathway plays a central role in blood vessel growth and homeostasis, but its potential role in sensing of disturbed flow has not been previously studied. Here, we show using porcine and murine arteries and cultured human coronary artery EC that disturbed flow activates the JAG1-NOTCH4 signaling pathway. Light-sheet imaging revealed enrichment of JAG1 and NOTCH4 in EC of atherosclerotic plaques, and EC-specific genetic deletion of Jag1 (Jag1ECKO) demonstrated that Jag1 promotes atherosclerosis at sites of disturbed flow. Mechanistically, single-cell RNA sequencing in Jag1ECKO mice demonstrated that Jag1 suppresses subsets of ECs that proliferate and migrate. We conclude that JAG1-NOTCH4 sensing of disturbed flow enhances atherosclerosis susceptibility by regulating EC heterogeneity and that therapeutic targeting of this pathway may treat atherosclerosis.

Aneurysmal cyclic circumferential stretch affects the expression of endothelial genes involved in vascular homeostasis

Funding Acknowledgements

Type of funding sources: None.

Background

Intracranial aneurysm (IA) rupture is a common cause of hemorrhagic stroke. IAs are present in 3 to 5% of the general population and the annual rupture rate of an IA has been estimated to 1%. Once formed, an IA can rupture immediately, grow before rupturing, or remain stable. Predicting the evolution of an unruptured IA is very difficult. Indeed, the biology of the IA wall is complex and the precise biomechanical processes leading to aneurysm wall rupture are not yet known. While the contribution of wall shear stress in IA disease has been demonstrated in several studies, the role of cyclic circumferential stretch (CCS) remains poorly understood.

Purpose

The levels of CCS in saccular IAs are unknown but are generally considered very low or even absent. The goal of our study was to investigate the transcriptional response of endothelial cells (ECs) to absence of CCS and the potential impact on the endothelium function.

Methods

Human umbilical veins ECs (HUVECs) from 6 different donors were exposed to physiological CCS levels for cerebral arteries (6% stretch) or to aneurysmal CCS (0%) for 48h using the Flexcell strain unit FX-5000T, and unbiased transcriptomics were performed. The expression of selected genes was confirmed by qPCR.

Results

As a validation of our methodology, we verified the induction of eNOS phosphorylation after exposure of the ECs to physiological CCS. Stretched EC samples display a high similarity between each other while EC samples under aneurysmal CCS were spread-out on a multidimensional scaling plot. The differential gene expression of ECs exposed to aneurysmal or physiological CCS revealed 51 up-regulated genes and 49 down-regulated genes in absence of CCS. The gene set enrichment analysis identified 70 up-regulated and 38 down-regulated pathways in HUVECs exposed to aneurysmal CCS. Up-regulated pathways were involved in oxidative stress, angiogenic and inflammatory pathways, and the down-regulated pathways in proliferation and extracellular matrix-receptor interactions. We confirmed by qPCR the down-regulation by aneurysmal CCS of Gja4 and Gja5, genes coding for the gap junction proteins Connexin37 (Cx37) and Connexin40 (Cx40), respectively. Connexins (Cxs) are crucial proteins for endothelial homeostasis, vascular function, endothelial cell cycle regulation and inflammation, and their role in the context of IA disease will be further studied.

Conclusion

Our results indicate that physiological CCS prevents large deviation in ECs and regulate the expression of genes essential for maintaining vascular homeostasis. The down-regulation of endothelial Cx37 and Cx40 under aneurysmal CSS may contribute to the endothelial dysfunction associated with IA progression. Further investigation may help to better define the role of Cxs in aneurysmal disease and to understand the significance of absence of CCS in IA disease progression.

Effects of Low and High Aneurysmal Wall Shear Stress on Endothelial Cell Behavior: Differences and Similarities

Background: Intracranial aneurysms (IAs) result from abnormal enlargement of the arterial lumen. IAs are mostly quiescent and asymptomatic, but their rupture leads to severe brain damage or death. As the evolution of IAs is hard to predict and intricates medical decision, it is essential to improve our understanding of their pathophysiology. Wall shear stress (WSS) is proposed to influence IA growth and rupture. In this study, we investigated the effects of low and supra-high aneurysmal WSS on endothelial cells (ECs).

Methods: Porcine arterial ECs were exposed for 48 h to defined levels of shear stress (2, 30, or 80 dyne/cm²) using an Ibidi flow apparatus. Immunostaining for CD31 or γ-cytoplasmic actin was performed to outline cell borders or to determine cell architecture. Geometry measurements (cell orientation, area, circularity and aspect ratio) were performed on confocal microscopy images. mRNA was extracted for RNAseq analysis.

Results: ECs exposed to low or supra-high aneurysmal WSS were more circular and had a lower aspect ratio than cells exposed to physiological flow. Furthermore, they lost the alignment in the direction of flow observed under physiological conditions. The effects of low WSS on differential gene expression were stronger than those of supra-high WSS. Gene set enrichment analysis highlighted that extracellular matrix proteins, cytoskeletal proteins and more particularly the actin protein family were among the protein classes the most affected by shear stress. Interestingly, most genes showed an opposite regulation under both types of aneurysmal WSS. Immunostainings for γ-cytoplasmic actin suggested a different organization of this cytoskeletal protein between ECs exposed to physiological and both types of aneurysmal WSS.

Conclusion: Under both aneurysmal low and supra-high WSS the typical arterial EC morphology molds to a more spherical shape. Whereas low WSS down-regulates the expression of cytoskeletal-related proteins and up-regulates extracellular matrix proteins, supra-high WSS induces opposite changes in gene expression of these protein classes. The differential regulation in EC gene expression observed under various WSS translate into a different organization of the ECs’ architecture. This adaptation of ECs to different aneurysmal WSS conditions may affect vascular remodeling in IAs.

Primary cilia control endothelial permeability by regulating expression and location of junction proteins

Aims

Wall shear stress (WSS) determines intracranial aneurysm (IA) development. Polycystic kidney disease (PKD) patients have a high IA incidence and risk of rupture. Dysfunction/absence of primary cilia in PKD endothelial cells (ECs) may impair mechano-transduction of WSS and favour vascular disorders. The molecular links between primary cilia dysfunction and IAs are unknown.

Methods and results

Wild-type and primary cilia-deficient Tg737^orpk/orpk arterial ECs were submitted to physiological (30 dynes/cm²) or aneurysmal (2 dynes/cm²) WSS, and unbiased transcriptomics were performed. Tg737^orpk/orpk ECs displayed a fivefold increase in the number of WSS-responsive genes compared to wild-type cells. Moreover, we observed a lower trans-endothelial resistance and a higher endothelial permeability, which correlated with disorganized intercellular junctions in Tg737^orpk/orpk cells. We identified ZO-1 as a central regulator of primary cilia-dependent endothelial junction integrity. Finally, clinical and histological characteristics of IAs from non-PKD and PKD patients were analysed. IAs in PKD patients were more frequently located in the middle cerebral artery (MCA) territory than in non-PKD patients. IA domes from the MCA of PKD patients appeared thinner with less collagen and reduced endothelial ZO-1 compared with IA domes from non-PKD patients.

Conclusion

Primary cilia dampen the endothelial response to aneurysmal low WSS. In absence of primary cilia, ZO-1 expression levels are reduced, which disorganizes intercellular junctions resulting in increased endothelial permeability. This altered endothelial function may not only contribute to the severity of IA disease observed in PKD patients, but may also serve as a potential diagnostic tool to determine the vulnerability of IAs.

Correlating Clinical Risk Factors and Histological Features in Ruptured and Unruptured Human Intracranial Aneurysms: The Swiss AneuX Study

Pathogenesis of intracranial aneurysm is complex and the precise biomechanical processes leading to their rupture are uncertain. The goal of our study was to characterize the aneurysmal wall histologically and to correlate histological characteristics with clinical and radiological factors used to estimate the risk of rupture. A new biobank of aneurysm domes resected at the Geneva University Hospitals (Switzerland) was used. Histological analysis revealed that unruptured aneurysms have a higher smooth muscle cell (SMC) content and a lower macrophage content than ruptured domes. These differences were associated with more collagen in unruptured samples, whereas the elastin content was not affected. Collagen content and type distribution were different between thick and thin walls of unruptured aneurysms. Classification of aneurysm domes based on histological characteristics showed that unruptured samples present organized wall rich in endothelial and SMCs compared with ruptured samples. Finally, aneurysm wall composition was altered in unruptured domes of patients presenting specific clinical factors used to predict rupture such as large dome diameter, dome irregularities, and smoking. Our study shows that the wall of aneurysm suspected to be at risk for rupture undergoes structural alterations relatively well associated with clinical and radiological factors currently used to predict this risk.

KLF4-Induced Connexin40 Expression Contributes to Arterial Endothelial Quiescence

Shear stress, a blood flow-induced frictional force, is essential in the control of endothelial cell (EC) homeostasis. High laminar shear stress (HLSS), as observed in straight parts of arteries, assures a quiescent non-activated endothelium through the induction of Krüppel-like transcription factors (KLFs). Connexin40 (Cx40)-mediated gap junctional communication is known to contribute to a healthy endothelium by propagating anti-inflammatory signals between ECs, however, the molecular basis of the transcriptional regulation of Cx40 as well as its downstream effectors remain poorly understood. Here, we show that flow-induced KLF4 regulated Cx40 expression in a mouse EC line. Chromatin immunoprecipitation in ECs revealed that KLF4 bound to three predicted KLF consensus binding sites in the Cx40 promoter. HLSS-dependent induction of Cx40 expression was confirmed in primary human ECs. The downstream effects of Cx40 modulation in ECs exposed to HLSS were elucidated by an unbiased transcriptomics approach. Cell cycle progression was identified as an important downstream target of Cx40 under HLSS. In agreement, an increase in the proportion of proliferating cell nuclear antigen (PCNA)-positive ECs and a decrease in the proportion of ECs in the G0/G1 phase were observed under HLSS after Cx40 silencing. Transfection of communication-incompetent HeLa cells with Cx40 demonstrated that the regulation of proliferation by Cx40 was not limited to ECs. Using a zebrafish model, we finally showed faster intersegmental vessel growth and branching into the dorsal longitudinal anastomotic vessel in embryos knock-out for the Cx40 orthologs Cx41.8 and Cx45.6. Most significant effects were observed in embryos with a mutant Cx41.8 encoding for a channel with reduced gap junctional function. Faster intersegmental vessel growth in Cx41.8 mutant embryos was associated with increased EC proliferation as assessed by PH3 immunostaining. Our data shows a novel evolutionary-conserved role of flow-driven KLF4-dependent Cx40 expression in endothelial quiescence that may be relevant for the control of atherosclerosis and diseases involving sprouting angiogenesis.

Role of hemodynamics in initiation/growth of intracranial aneurysms

BACKGROUND

Intracranial aneurysm (IA) is a disease of the vascular wall resulting in abnormal enlargement of the vessel lumen. It is a common pathology with a prevalence of 2%-3% in the adult population. IAs are mostly small, quiescent and asymptomatic; yet, upon rupture, severe brain damage or even death is frequently encountered. In addition to clinical factors, hemodynamic forces, mainly wall shear stress (WSS), have been associated with the initiation of IAs and possibly with their risk of rupture. However, the mechanism by which WSS contributes to aneurysm growth and rupture is not completely understood.

DESIGN

PubMed and Ovid MEDLINE databases were searched. In addition, key review articles were screened for relevant original publications.

RESULTS

Current knowledge about the relation between WSS and IA has been obtained from both computational fluid dynamic studies in patients and experimental models of IA formation and growth. It is increasingly recognized that a high wall shear stress (gradient) participates to IA formation and that both low and high WSS can drive IA growth. Primary cilia (PC) play an important role as mechanosensors as patients with polycystic kidney disease, which is characterized by the absence or dysfunction of PC, have increased risk to develop IAs as well as increased risk of rupture.

CONCLUSION

Wall shear stress is a key player in IA initiation and progression. It is involved in vascular wall remodelling and inflammation, processes underlying aneurysm pathophysiology.