Beta 1/beta 3 integrin ligation is uncoupled from ERK1/ERK2 activation in cytotoxic T lymphocytes

MMP Inhibition Preserves Integrin Ligation and FAK Activation to Induce Survival and Regeneration in RGCs Following Optic Nerve Damage

Purpose

Integrin adherence to the extracellular matrix (ECM) is essential for retinal ganglion cell (RGC) survival: damage causes production and release of ECM degrading matrix metalloproteinases (MMPs) that disrupt integrin ligation, leading to RGC death. The interplay of MMPs, integrins, and focal adhesion kinase (FAK) was studied in RGCs after optic nerve injury.

Methods

Optic nerve transection and optic nerve crush were used to study RGC survival and regeneration, respectively. Treatments were administered intravitreally or into the cut end of the optic nerve. RGC survival was assessed by fluorescence or confocal microscopy; cell counting, peptide levels, and localization were assessed by Western blot and immunohistochemistry.

Results

MMP-9 was most strongly increased and localized to RGCs after injury. Pan-MMP, MMP-2/-9, and MMP-3 inhibition all significantly enhanced RGC survival and increased RGC axon regeneration. FAK activation was decreased at 4 days postaxotomy, when apoptosis begins. FAK inhibition reduced RGC survival and abrogated the neuroprotective effects of MMP inhibition, whereas FAK activation increased RGC survival despite MMP activation. Integrin ligation with CD29 antibody or glycine-arginine-glycine-aspatate-serine (GRGDS) peptide increased RGC survival after axotomy.

Conclusions

ECM-integrin ligation promotes RGC survival and axon regeneration via FAK activation.

MHC class I and integrin ligation induce ERK activation via an mTORC2-dependent pathway

The aim of this study was to characterize the interaction between mTOR and ERK in primary endothelial cells (EC) following MHC class I and integrin ligation. Ligation of MHC class I molecules or integrins on the surface of EC leads to phosphorylation of ERK at Thr202/Tyr204. We utilized small interfering RNA (siRNA) blockade of mTOR and proteins involved in mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) to define a relationship between mTOR and ERK following MHC class I signaling. We found mTORC2 was responsible for MHC class I and integrin induced phosphorylation of ERK at Thr202/Tyr204. We corroborated these results demonstrating that long-term exposure to rapamycin also inhibited ERK pathway activation in response to MHC class I signaling. Our results demonstrate, for the first time, that engagement of either MHC class I or integrin on the surface of EC leads to ERK activation through an mTORC2-dependent pathway.

Focal adhesion kinase-related protein tyrosine kinase Pyk2 in T-cell activation and function

Pyk2 is a protein tyrosine kinase expressed primarily in brain and hematopoietic cells. It becomes activated in response to stimulation through numerous receptors, including integrins, chemokine receptors, and antigen receptors, and is found in association with src-family kinases. Although this enzyme associates with many proteins known to be important for activation and has many characteristics of a scaffolding protein, its function remains elusive. A number of studies in non-T-cells suggest that Pyk2 is important for cell spreading, cell migration, and integrin function; however, a defined role in T-cells has not been established. Here, we discuss evidence that implicates Pyk2 in directionality of signaling, which is essential to establishment of the directional killing mediated by cytotoxic lymphocytes.

Galectin-8 binds specific β1 integrins and induces polarized spreading highlighted by asymmetric lamellipodia in Jurkat T cells

Integrin-mediated encounters of T cells with extracellular cues lead these cells to adhere to a variety of substrates and acquire a spread phenotype needed for their tissue incursions. We studied the effects of galectin-8 (Gal-8), a beta-galactoside binding lectin, on Jurkat T cells. Immobilized Gal-8 bound alpha1beta1, alpha3beta1 and alpha5beta1 but not alpha2beta1 and alpha4beta1 and adhered these cells with similar kinetics to immobilized fibronectin (FN). Function-blocking experiments with monoclonal anti-integrin antibodies suggested that alpha5beta1 is the main mediator of cell adhesion to this lectin. Gal-8, but not FN, induced extensive cell spreading frequently leading to a polarized phenotype characterized by an asymmetric lamellipodial protrusion. These morphological changes involved actin cytoskeletal rearrangements controlled by PI3K, Rac-1 and ERK1/2 activity. Gal-8-induced Rac-1 activation and binding to alpha1 and alpha5 integrins have not been described in any other cellular system. Strikingly, Gal-8 was also a strong stimulus on Jurkat cells in suspension, triggering ERK1/2 activation that in most adherent cells is instead dependent on cell attachment. In addition, we found that patients with systemic lupus erythematosus (SLE), a prototypic autoimmune disorder, produce Gal-8 autoantibodies that impede both its binding to integrins and cell adhesion. These are the first function-blocking autoantibodies reported for a member of the galectin family. These results indicate that Gal-8 constitutes a novel extracellular stimulus for T cells, able to bind specific beta1 integrins and to trigger signaling pathways conducive to cell spreading. Gal-8 could modulate a wide range of T cell-driven immune processes that eventually become altered in autoimmune disorders.

Tyrosine kinase activity and remodelling of the actin cytoskeleton are co-temporally required for degranulation by cytotoxic T lymphocytes

In this study, we examined the contribution of the actin cytoskeleton to T-cell receptor (TCR)-initiated signalling in cytotoxic T lymphocytes (CTLs). We demonstrate that cytoskeletal remodelling is required for sustaining TCR-stimulated signals that lead to degranulation by CTLs. Disruption of the actin cytoskeleton in CTLs already undergoing signalling responses results in an almost immediate loss of essentially all protein tyrosine phosphorylation. This signal reversal is not restricted to tyrosine phosphorylation, as disruption of the actin cytoskeleton also reverses the phosphorylation of the more downstream serine/threonine kinase extracellular signal regulated kinase (Erk). An intact cytoskeleton and cell spreading are not sufficient for maintaining signals, as stabilization of actin filaments, at a point when peak tyrosine phosphorylation is occurring, also leads to the rapid loss of protein tyrosine phosphorylation. Disruption of tyrosine kinase activity after TCR signals are maximally induced causes the immediate reversal of tyrosine phosphorylation as well as cytoskeletal disruption, as indicated by loss of cell spreading, adhesion and CTL degranulation. Taken together, our results indicate that actin remodelling occurs co-temporally with ongoing tyrosine kinase activity, leading to CTL degranulation. We hypothesize that continuous actin remodelling is important for sustaining productive signals, even after downstream signalling molecules such as Erk have been activated, and that the actin cytoskeleton is not solely required for initiating and maintaining the T cell in contact with its stimulus.