SCUBE2, where are you? Recruitment of SCUBE2 to adherens junctions preserves vascular health and integrity

This editorial refers to ‘SCUBE2 regulates adherens junction dynamics and vascular barrier function during inflammation’, by Y.-C. Lin et al., https://doi.org/10.1093/cvr/cvae132.

Endothelial cell adherens junctions (AJs) play a pivotal role in maintaining vascular integrity by regulating permeability and preserving the endothelial barrier, primarily mediated by proteins such as vascular endothelial (VE)-cadherin. VE-cadherin is a transmembrane adhesion receptor that forms AJs through calcium-dependent homodimerisation of its extracellular domain with VE-cadherin on adjacent cells. Its intracellular domain interacts with the actin cytoskeleton via catenins. Under normal physiological conditions, endothelial barrier function is essential for preserving tissue homeostasis and organ function, selectively allowing essential molecules to pass while preventing leakage of plasma proteins and infiltration of leucocytes that can trigger inflammatory responses. Disruption of AJs increases permeability, thereby facilitating leucocyte infiltration into tissues and contributing to inflammation and tissue damage.¹

Endothelial function and barrier integrity are strongly regulated by tyrosine kinases, which constitute an important area of research given the potential therapeutic implications for many diseases characterized by vascular leak. These kinases include the Src-family kinases (SFK) and Abelson kinases, some of which directly phosphorylate VE-cadherin.^2–6 VE-cadherin contains several tyrosine residues in its cytoplasmic tail, such as Y658, Y685, and Y731, that together affect multiple aspects of endothelial cell adhesion and function.³ For instance, VE-cadherin phosphorylation at Y685 regulates the interaction with C-terminal Src kinase (CSK), a cytosolic kinase that negatively regulates the activity of other SFKs and controls endothelial cell proliferation.⁴ Recent work has shown that another SFK member, Yes, localizes at AJs and phosphorylates VE-cadherin at Y658 and Y685 in response to shear stress, thus regulating VE-cadherin turnover and cell–cell adhesion.⁵ Furthermore, nitric oxide (NO) produced by endothelial nitric oxide synthase (eNOS), also plays a significant role in this regulation. NO activates SFK members (especially c-Src) through vascular endothelial growth factor (VEGF) signalling, which promotes VE-cadherin phosphorylation at Y685, thereby increasing vascular permeability.⁶ The balance between phosphorylation of VE-cadherin by various kinases and its dephosphorylation by phosphatases, including Src homology phosphatase 2 and vascular endothelial phosphotyrosine phosphatase (VE-PTP), is crucial for AJ dynamics.

Very recent research by Lin et al. has highlighted Signal peptide-CUB-epidermal growth factor-like domain-containing protein 2 (SCUBE2) as a critical player in AJs, influencing endothelial barrier function through interactions with VE-cadherin and VE-PTP (Figure 1).⁷ The authors find that SCUBE2 recruits VE-PTP to form a complex with VE-cadherin, thereby stabilizing the VE barrier by preventing VE-cadherin phosphorylation at Y685, as well as its internalisation and degradation. Depletion of SCUBE2 in endothelial cells increases permeability and compromises barrier function, which is exacerbated by exposure to VEGF or histamine, and is associated with VE-cadherin phosphorylation and internalisation. Moreover, endothelial-specific SCUBE2 knockout mice exhibit vascular leakage in the kidneys and the heart, elevated VE-cadherin phosphorylation, and increased mortality in pulmonary inflammation induced by influenza A or lipopolysaccharide. Intriguingly, pro-inflammatory cytokines like tumour necrosis factor-α and interleukin-1β, as well as VEGF or histamine, down-regulate SCUBE2 via the NF-κB pathway, which can be prevented pharmacologically with compounds like temsirolimus and resveratrol, thus preventing barrier impairment. In line with this, overexpression of SCUBE2 in endothelial cells in vitro strengthens endothelial barrier function, and reduces permeability induced by VEGF or histamine. Finally, SCUBE2 overexpression also enhances barrier function in vivo under both physiological and inflammatory conditions, resulting in reduced vascular permeability, diminished immune cell infiltration in the lungs, and improved survival rates.

Figure 1.

Localization of SCUBE2 to adherens junction preserves barrier integrity. Top: Barrier stabilisation by formation of the VE-cadherin - VE-PTP complex is facilitated by SCUBE2. Bottom: Loss of SCUBE2 diminishes the VEcadherin - VE-PTP interaction, leading to endocytosis of VE-cadherin and subsequent barrier disruption. It is unknown where SCUBE2 locates after release from the VE-cadherin - VE-PTP complex, indicated by?

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It was previously found by the same group that endothelial SCUBE2, which is primarily located at the plasma membrane, is up-regulated by hypoxia-inducible factor (HIF)1α, influencing VEGF receptor 2 (VEGFR2) signalling during ischaemia-induced angiogenesis.⁸ Because it has been shown that HIF2α promotes the association of VE-cadherin with VE-PTP under hypoxia,⁹ it is possible that also HIF2α promotes SCUBE2 expression, and that this is at least partially responsible for the enhanced association of VE-cadherin and VE-PTP under hypoxic conditions. Indeed, there are three putative hypoxia-responsive elements in the SCUBE2 promoter, suggesting a direct effect of HIFs in hypoxia-induced SCUBE2 expression. Understanding the potential crosstalk between NF-κB signalling and hypoxia-induced SCUBE2 expression and function at endothelial AJs requires further investigation, to elucidate how vascular integrity is maintained during hypoxia.

SCUBE2's ability to recruit VE-PTP and prevent VE-cadherin phosphorylation at Y685 indicates its pivotal role in endothelial barrier function, reducing vascular permeability and maintaining tissue homeostasis. It remains unknown whether the SCUBE2/VE-PTP complex also affects the phosphorylation of other VE-cadherin tyrosine residues, thereby influencing vascular permeability. Further studies are needed to fully understand the mode of action of SCUBE2, for instance to find out whether SCUBE2 actively modulates VE-PTP function, and to understand the contribution of individual protein domains of SCUBE2, such as its CUB domain or spacer region.

Moreover, SCUBE2 shows promise as a biomarker for vascular injury, potentially enabling diagnostic and prognostic applications. Similar to the release of SCUBE1 from activated platelets during events like ischaemia,¹⁰ SCUBE2 could be shed from injured vasculature and thus serve as a biomarker for compromised vascular integrity, though this requires further validation.

In conclusion, the present study underscores SCUBE2's critical role in enhancing endothelial barrier integrity through stabilisation of VE-cadherin-dependent AJs. Pharmacological upregulation of SCUBE2 holds therapeutic potential for reducing vascular permeability and inflammation, positioning SCUBE2 as a promising target and biomarker in vascular injury. However, further investigation is essential to fully explore and validate SCUBE2's therapeutic applications before clinical implementation can be considered.

Funding

We gratefully acknowledge financial support from the Landsteiner Foundation for Blood Transfusion Research (LSBR, grants #2019 and #2336), and the Dutch Research Council (NWO, grant # OCENW.M.23.018).

References

Author notes