Age-related defects in the cytoskeleton signaling pathways of CD4 T cells

Abstract

It has been postulated that the cytoskeleton controls many aspects of T cell function, including activation, proliferation and apoptosis. Recent advances in our understanding of F-actin polymerization and the Ezrin-Radixin-Moesin (ERM) family of cytoskeleton signal proteins have provided new insights into immunological synapse formation during T cell activation. During aging there is a significant decline of T cell function largely attributable to declines in activation of CD4 T cells and defects in the formation of the immunological synapse. Here we discuss recent progress in the understanding of how aging alters F-actin and ERM proteins in mouse CD4 T cells, and the implications of these changes for the T cell activation process.

Figure 1. Age impairs lammellopodia formation in CD4⁺ T cells from old mice

CD4⁺ T cells from young and old CB6F1 mice were incubated for 10 min on glass cover slips covered with anti-DNP (a negative control) or anti-CD3. The cells were fixed, stained with phalloidin to detect F-actin filaments, and analyzed by confocal microscopy. The images show a field of the Z-plane level at which the T cells are in contact with the glass slide. Nearly all of the T cells from the young mice show an extended zone of contact with the glass substrate, triggered by anti-CD3. The arrows indicate two types of cells from old mice; one (left arrow) resembling T cells from young mice with lammellopodia formation, and the other (right arrow) resembling unstimulated cells as in the anti-DNP control slides.

Figure 2. Age increases CD3ζ phosphorylated isoforms associated with the cytoskeleton of CD4⁺T cells

Resting CD4⁺ T cells from young and old mice were solubilized with Brij-50, and the cytoskeletal fractions purified by centrifugation. The samples were resolved by SDS and analyzed for levels of the different CD3ζ isoforms (p16, p21 and p23) using anti-CD3ζ western blots. The bar graph represents the mean and SEM of each of the CD3ζ isoforms, normalized to the levels in young cells for each endpoint. Groups indicated by * are different at a significance level of p < 0.05.

Figure 3. General model of aging effects on F-actin signaling

During aging there is a significant increase (indicated by the red arrows) in Vav phosphorylation and increases in GTPase activity. These increases lead to activation of Rac1function that in turn increases the polymerization of G-actin to the F-actin form. Higher F-actin present in the T cells results in increases in association of CD3ζ phosphorylated isoforms to the cytoskeleton. The overall result is a decrease in TCR mobility and membrane fluidity that may diminish TCR signaling. In addition, the age-related increases in F-actin may result in its association to other signaling molecules in the cytoskeleton or increases in the links between membrane and cytoskeleton, with unknown consequences for T cell function.

Figure 4. Aging impairs ERM function

The age-related increases in Rac1 also have other consequences. The higher threshold level of Rac1 activity induces dephosphorylation of the ERM, which in turn leads to dissociation of signaling molecules, such as EBP50 and CD44, from the ERM. Some of these molecules, such as EBP50, can diminish TCR signaling when released. In addition the lack of active phosphorylated ERM could prevent formation of the distal pole complex in CD4 T cells from old mice, increasing the access of TCR negative regulators (such as CD43, CD44 and others) to interfere with the formation of the immunological synapse.