Ubiquitination within the membrane-proximal ezrin-radixin-moesin (ERM)-binding region of the L1 cell adhesion molecule

Functional Relationships between L1CAM, LC3, ATG12, and Aβ

Abnormal protein accumulations in the brain are linked to aging and the pathogenesis of dementia of various types, including Alzheimer's disease. These accumulations can be reduced by cell indigenous mechanisms. Among these is autophagy, whereby proteins are transferred to lysosomes for degradation. Autophagic dysfunction hampers the elimination of pathogenic protein aggregations that contribute to cell death. We had observed that the adhesion molecule L1 interacts with microtubule-associated protein 1 light-chain 3 (LC3), which is needed for autophagy substrate selection. L1 increases cell survival in an LC3-dependent manner via its extracellular LC3 interacting region (LIR). L1 also interacts with Aβ and reduces the Aβ plaque load in an AD model mouse. Based on these results, we investigated whether L1 could contribute to autophagy of aggregated Aβ and its clearance. We here show that L1 interacts with autophagy-related protein 12 (ATG12) via its LIR domain, whereas interaction with ubiquitin-binding protein p62/SQSTM1 does not depend on LIR. Aβ, bound to L1, is carried to the autophagosome leading to Aβ elimination. Showing that the mitophagy-related L1-70 fragment is ubiquitinated, we expect that the p62/SQSTM1 pathway also contributes to Aβ elimination. We propose that enhancing L1 functions may contribute to therapy in humans.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.