An Outside-In Switch in Integrin Signaling Caused by Chemical and Mechanical Signals in Reactive Astrocytes.

Leonardo A Pérez,Pascal Schneider,Aysha Rashid,Khalid Salaita,Andrés Rodríguez,Lisette Leyton,J Dale Combs

doi:10.3389/fcell.2021.712627

Abstract

Astrocyte reactivity is associated with poor repair capacity after injury to the brain, where chemical and physical changes occur in the damaged zone. Astrocyte surface proteins, such as integrins, are upregulated, and the release of pro-inflammatory molecules and extracellular matrix (ECM) proteins upon damage generate a stiffer matrix. Integrins play an important role in triggering a reactive phenotype in astrocytes, and we have reported that αVβ3 Integrin binds to the Thy-1 (CD90) neuronal glycoprotein, increasing astrocyte contractility and motility. Alternatively, αVβ3 Integrin senses mechanical forces generated by the increased ECM stiffness. Until now, the association between the αVβ3 Integrin mechanoreceptor response in astrocytes and changes in their reactive phenotype is unclear. To study the response to combined chemical and mechanical stress, astrocytes were stimulated with Thy-1-Protein A-coated magnetic beads and exposed to a magnetic field to generate mechanical tension. We evaluated the effect of such stimulation on cell adhesion and contraction. We also assessed traction forces and their effect on cell morphology, and integrin surface expression. Mechanical stress accelerated the response of astrocytes to Thy-1 engagement of integrin receptors, resulting in cell adhesion and contraction. Astrocyte contraction then exerted traction forces onto the ECM, inducing faster cell contractility and higher traction forces than Thy-1 alone. Therefore, cell-extrinsic chemical and mechanical signals regulate in an outside-in manner, astrocyte reactivity by inducing integrin upregulation, ligation, and signaling events that promote cell contraction. These changes in turn generate cell-intrinsic signals that increase traction forces exerted onto the ECM (inside-out). This study reveals αVβ3 Integrin mechanoreceptor as a novel target to regulate the harmful effects of reactive astrocytes in neuronal healing.

Highlights

Integrins are receptors that span the plasma membrane and anchor cells to the external environment
It is possible that mechanical stress produced through structural extracellular matrix (ECM) remodeling by proteins secreted from astrocytes or other cells after brain damage (Stichel et al, 1995; Haas et al, 1999; McKeon et al, 1999; Iseki et al, 2002; Liesi and Kauppila, 2002; Hagino et al, 2003; Hirano et al, 2004; Yiu and He, 2006; Gris et al, 2007; Falo et al, 2008; Bellin et al, 2009), could stimulate and prime the cells to respond to Thy-1
We explored a novel response of astrocytes to physical sensing of their environment, and the role of the mechanoreceptor αV β3 Integrin

Summary

Introduction

Integrins are receptors that span the plasma membrane and anchor cells to the external environment. They transmit signals in and out of the cells, allowing a fluid communication between the extracellular matrix (ECM) and the cytoskeleton. The crosstalk between these extra- and intra-cellular signals transduced by the integrin receptors occurs in both ways, in a process canonically described as outside-in and insideout signaling. Ligand-induced clustering is well known to activate integrins, increasing the affinity and avidity of the interactions and signaling (outside-in) (Oria et al, 2017). Molecular changes occurring intracellularly assist integrin activation by promoting the formation of integrinbased adhesion complexes (known as focal adhesions), and cytoskeletal changes that convey mechanical stress signals back to the integrins and the ECM (inside-out) (Mohammed et al, 2019)

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