Post-translational modification-regulated leukocyte adhesion and migration

Abstract

Leukocytes undergo frequent phenotypic changes and rapidly infiltrate peripheral and lymphoid tissues in order to carry out immune responses. The recruitment of circulating leukocytes into inflamed tissues depends on integrin-mediated tethering and rolling of these cells on the vascular endothelium, followed by transmigration into the tissues. This dynamic process of migration requires the coordination of large numbers of cytosolic and transmembrane proteins whose functional activities are typically regulated by post-translational modifications (PTMs). Our recent studies have shown that the lysine methyltransferase, Ezh2, critically regulates integrin signalling and governs the adhesion dynamics of leukocytes via direct methylation of talin, a key molecule that controls these processes by linking integrins to the actin cytoskeleton. In this review, we will discuss the various modes of leukocyte migration and examine how PTMs of cytoskeletal/adhesion associated proteins play fundamental roles in the dynamic regulation of leukocyte migration. Furthermore, we will discuss molecular details of the adhesion dynamics controlled by Ezh2-mediated talin methylation and the potential implications of this novel regulatory mechanism for leukocyte migration, immune responses, and pathogenic processes, such as allergic contact dermatitis and tumorigenesis.

Keywords: EZH2; adhesion; dendritic cells; migration; post-translational modifications.

Figure 1. Tri-methyl lysine mimicking talin1 mutant promotes FA turnover and rescues excessive cell spreading and defective migration phenotypes of Ezh2-deficient DCs

A. Control and Ezh2-deficient DCs expressing GFP-talin1 variants were allowed to adhere to slides coated with VCAM-1 (20 μg/ml) for 2 h. The cell areas were visualized by GFP staining and calculated using ImageJ (left) or time-lapse images were taken every 5 min for 2 h (right). “Un” indicates untransduced control. ***P <0.0001 (black asterisks: one-way ANOVA, red asterisks: between the indicated pairs, two-tailed student's t-test with equal variance). Data are represented as mean ± standard error of the mean (SEM) of cells pooled from 2-4 independent experiments. B. Control and Ezh2-deficient DCs expressing GFP-talin1 variants were allowed to adhere to VCAM-1 coated slides as in A for 2 h. FAs and F-actin were visualized by anti-paxillin (Pax, red) and Alexa Fluor^® 647 phalloidin (pseudo-colored green), respectively. Over 90% of the cells were GFP+ expressing talin variants and the GFP staining is not shown. Scale bar, 10 μm. Technical details and original data are published in reference [50].

Figure 2. Schematic model for Ezh2-regulated cell adhesion and migration through direct methylation of talin1

Ezh2 is recruited to talin1 through interaction with the cytoskeletal-reorganization effector, Vav1 and mediates talin1 methylation, thereby reducing talin1 binding to F-actin and promoting adhesion turnover. Overexpression in wild-type mature dendritic cells of a mutant talin1 that was created by replacing the lysine at position 2454 with glutamine, which cannot be methylated, but preserves the polarity of the lysine (K2454Q), results in extensive cell spreading that resembles the phenotype of Ezh2-deficient mature dendritic cells (top). In contrast, overexpression in Ezh2-deficient mature dendritic cells of a methyl-mimicking talin1 mutant that was created by replacing the K2454 residue with phenylalanine (K2454F) restores normal cell spreading and migration (bottom).