Discovery of the target receptor for the plasma protein antithrombin ~New developments in plasma proteinology~

A research group consisting of Professor Masahiro Nishibori (Specially Appointed Professor）, Department of Translational Research and Drug Development, Faculty of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Professor Masakiyo Sakaguchi (Department of Cell Biology, Faculty of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University), and Visiting Researcher Yohei Takahashi (Department of Translational Research and Drug Development, Faculty of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, has identified C-type lectin family 1A (CLEC1A) ¹⁾ as a specific receptor for antithrombin, a plasma protein with anti-inflammatory properties.

Antithrombin is well known for its anticoagulant effect by directly inhibiting thrombin activity, but it has been revealed that stimulating CLEC1A on human neutrophils ²⁾ cells changes the morphology of neutrophils to a spherical shape with a smooth surface, functionally inhibiting neutrophil cell death and inhibiting the production of reactive oxygen species (ROS). This action of antithrombin is unrelated to the anticoagulant effect due to the inhibition of thrombin activity, and may be involved in the anti-inflammatory effect of antithrombin preparations already in clinical use. The receptor identification method used this time is an extremely innovative screening method independently developed by Professor Sakaguchi's research group (Gao et al., iScience, 2020; Takahashi et al., J Immunol, 2021).

The results of this research were published online in the international journal Blood, Vessels, Thrombosis & Hemostasis (Blood, VTH) on October 8th. Combined with previous research by the same research group on the plasma protein histidine-rich glycoprotein (HRG), this research strongly suggests a completely new functional role and mechanism of action of plasma proteins, and is expected to bring about new developments in plasma proteinology.

◆Key points of the presentation

- Using an innovative method, a new plasma membrane receptor³⁾ for antithrombin⁴⁾ was identified.

- Antithrombin strongly regulated human neutrophils by stimulating the identified CLEC1A receptor.

- It was revealed that the antithrombin-CLEC1A system is involved in the anti-inflammatory action of antithrombin.

It is expected that the results of this research will lead to new developments in plasma proteinology.

■Presentation contents

＜Current situation＞

Antithrombin, a plasma protein, has been reported to control thrombin activity by directly binding to thrombin, as well as to have an anti-inflammatory effect. However, the actual molecular mechanism of anti-inflammatory effect of antithrombin was completely unknown.

<Research Results>

In a previous study on HRG, Professor Nishibori's research group succeeded in identifying a new HRG-specific receptor (Gao et al., iScience, 2020), and based on a similar idea, they hypothesized that antithrombin has a plasma membrane receptor related to its anti-inflammatory effect. Using a screening method developed independently by Professor Sakaguchi's research group, they conducted a search for and identification of a new receptor for antithrombin and analyzed its function. Antithrombin ligands and candidate receptor genes were co-expressed in HEK293T cells, and the candidate receptors bound to the ligand were identified by immunoprecipitation. As a result, CLEC1A was identified as a candidate receptor for antithrombin. Antithrombin stimulates CLEC1A on human neutrophil cells to change the morphology of neutrophils to a spherical shape with a smooth surface (Figure 1), and functionally suppresses neutrophil cell death and ROS production. They also demonstrated that the antithrombin-CLEC1A system is involved in the anti-inflammatory action of antithrombin. This cellular effect of antithrombin is very similar to that of HRG, which has the same CLEC1A as its receptor, suggesting a completely new functional role and mechanism of action for this plasma protein group.

<Social Significance>

This research proposes a completely new mechanism of action, cell plasma membrane receptor stimulation, for the functional role of the plasma protein antithrombin. Not only is the innovative method of receptor identification academically unique, but it is expected to bring about revolutionary changes in plasma protein science through its application to other plasma protein factors in the future.

■Supplementary information and terminology

1) CLEC1A: One of several dozen members of the CLEC superfamily.

The receptors in this group are believed to be involved in important functions such as cell adhesion, intercellular signaling, and inflammation regulation.

2) Neutrophil: A type of white blood cell in the blood that prevents infection by phagocytosing and killing bacteria and fungi that have invaded the body.

3) Plasma membrane receptor: There are specific protein structures on the cell surface that act as antennae to detect various signal molecules present outside the cell. Many of these exist in a form that penetrates the cell membrane, and are collectively called cell plasma membrane receptors.

4) Antithrombin: A glycoprotein with a molecular weight of approximately 58,000 produced in the liver and secreted into the blood. It directly binds to thrombin, which functions in the final step of the blood coagulation cascade, and inhibits its activity, exerting an anticoagulant effect.