Cell Migration Gateway Logo

Article navigation

Featured Articles

Cell migration: ESCRTed by integrin degradation

Cell Migration Gateway (August 2010) | doi:10.1038/cmg123

Integrin ubiquitination mediates lysosomal degradation of fibronectin–integrin complexes, thereby controlling cell migration.

A migrating fibroblast with α5β1 integrin stained in red and the lysosome-associated membrane protein LAMP1 in blue. Colocalization (magenta) indicates that a substantial fraction of the integrin molecules are transported to late endosomes or lysosomes. The nucleus is stained in green. Image courtesy of Dr. Harald Stenmark, Department of Biochemistry, Institute for Cancer Research, Oslo University Hospital, Norway.

Integrin endocytosis and recycling have important roles during cell migration; however, it is not known whether the degradation of integrin proteins is also involved. In Developmental Cell, Herald Stenmark and colleagues now report that ubiquitination of α5 integrin and binding of fibronectin to α5β1 integrin mediates lysosomal degradation of both proteins via the endosomal sorting complex required for transport (ESCRT) machinery, and that this process is required for cell migration.

In human fibroblast cells, the authors first examined the intracellular localization of fibronectin and α5β1 integrin and demonstrated that a portion of α5 integrin colocalizes with fibronectin in the multivesicular endosomes (MVEs). This suggests that fibronectin and a fraction of α5β1 integrin are trafficked together from endosomes to lysosomes for degradative processing. Furthermore, they found that endogenous α5 is ubiquitinated at the cytoplasmic lysine residues in response to fibronectin binding, and that mutation of the cytoplasmic lysine residues in α5 reduced the rate of its degradation, showing that fibronectin-mediated ubiquitination of α5 integrin promotes its lysosomal degradation.

Next, they used confocal imaging to show that both α5 and β1 integrin are localized in the lumen of MVEs upon fibronectin binding. In contrast, mutant α5 integrin could not be sorted into MVEs even after fibronectin treatment, suggesting that ubiquitination of α5 is crucial for its proper endocytic sorting. Importantly, wild-type α5 integrin colocalized with 'active' β1 integrin inside the lumen of the MVEs, indicating that endocytic sorting of integrin proteins is dependent on ligand binding. Moreover, depletion of α5 integrin using short interfering (si)RNA inhibited sorting of fibronectin to the MVEs, with no effect on its endocytosis, and expression of mutant α5 protein could not restore fibronectin sorting. This confirms that fibronectin and its receptor α5β1 integrin are trafficked together and that this depends on ubiquitination of α5 integrin.

As ubiquitinated proteins directly interact with the components of the ESCRT, the authors next tested whether ESCRT machinery is involved in α5β1 degradation. Inhibition of ESCRT complexes with siRNA resulted in accumulation of ubiquitinated α5β1 integrin on endosomal compartments, and this was enhanced in the presence of fibronectin. Furthermore, ESCRT depletion induced accumulation of fibronectin in early endosomes, which confirms that fibronectin is trafficked together with α5 integrin, and its degradation is also dependent on the ESCRT machinery. Consistent with these results, immuno-electron microscopy showed that fibronectin and α5 integrin colocalize in enlarged early endosomes in ESCRT-depleted cells.

Western blotting demonstrated that protein levels of fibronectin and α5β1 integrin accumulated in the cells upon inhibition of ESCRT machinery, whereas no change was detected in their mRNA expression. This indicates inhibition of protein degradation and is consistent with an ESCRT-dependent mechanism.

Finally, live-cell time-lapse imaging showed reduced cell migration speed in the absence of α5 integrin; transfection with siRNA-resistant wild-type α5 integrin restored the cells' speed, whereas expression of mutant α5 did not have any effect. Taken together, these results demonstrate that ubiquitination of α5 integrin mediates proper migration of fibroblast cells through trafficking and degradation of fibronectin-bound α5β1 integrin via the ESCRT pathway. Further research will help elucidate the role of integrin degradation in cancer cell migration.

Iley Ozerlat - Copyright © 2010 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission

Original Research Paper

  1. Lobert V.H. et al. Ubiquitination of α5β1 integrin controls fibroblast migration through lysosomal degradation of fibronectin–integrin complexes. Dev. Cell 19, 148–59 (2010)
    doi:10.1016/j.devcel.2010.06.010 | Article