Research Highlights

Migration and anergy: The lattice strikes again

Cooperation of Gal-3 and Cav-1 (red structures) in the generation of FA domains.
Copyright © 2008 Rockefeller University Press

GGLs can induce the formation of ordered cell surface structures because they restrict the lateral mobility of cell surface glycoproteins. Aside from Gal-3 expression, GGL formation is regulated by the expression of the N–acetylglucosaminyltransferases (particularly that of Mgat5) that synthesize -galactosides as the terminal carbohydrates of N-glycans. Two studies now provide further insight into the functional properties of GGLs.

Goetz et al. in the Journal of Cell Biology analyze the contribution of Gal-3 and the phosphorylated caveolin-1, pY14Cav1, to FA formation at the leading edge of migrating cells. Formation of FAs - membrane subdomains by which the migrating cell communicates with the extracellular matrix - requires the recruitment of integrins, caveolin-1 and the focal adhesion kinase (FAK) to a membrane subdomains. The authors found that a lack of Mgat5, GGL disruption by the competitive Gal-3 binder lactose or by siRNA-mediated Gal-3 knockdown led to defects in FA formation in mouse mammary tumor cells, shown by increased FAK turnover in FAs. Diminished recruitment and stabilization of FAK at FA domains was also observed when cells were transfected with a mutant caveolin-1 lacking its tyrosine phosphorylation site. Based on their results, Goetz et al. propose a model for FA formation, in which Gal-3 crosslinks and activates N-glycosylated 51 integrins, which subsequently recruit FAK to the cell membrane. Stabilization of FAK in FA domains by pY14Cav1, on the other hand, initiates FAK downstream signaling, FA maturation and cell motility.

Demotte et al. highlight an inhibitory function of GGLs. In a prior study, the authors had noticed that the effector activity of CTLs was decreased for several days after antigenic stimulation. Now, writing in Immunity, the authors found that in the decreased activity state, the CTL clones had lost colocalization of the TCR and CD8; instead, TCR colocalized with Gal-3. Incubation of the CTLs with the competitive Gal-3 binder N-acetyllactosamine restored TCR/CD8 colocalization and effector activity. This suggests that Gal-3 binds to the TCR N-glycans, sequestering it from from CD8 by GGL formation. However, activated human CTLs did not show increased expression of Mgat5, as was observed in the mouse model, so that other modifications of surface glycans may be responsible for the stimulus-induced anergy of CTLs. Demotte et al. report that TCR/CD8 separation also occurred on anergic human tumor-infiltrating lymphocytes, and that TCR/CD8 colocalization and effector activity were restored by N-acetyllactosamine incubation. Thus, administration of competitive Gal-3 binders may be an therapeutic option to induce a more efficient and long-lasting anti-tumor immune response.

Author: Mirko von Elstermann