Fasciclin II signals new synapse formation through amyloid precursor protein and the scaffolding protein dX11/Mint

Cell adhesion molecules (CAMs) have been universally recognized for their essential roles during synapse remodeling. However, the downstream pathways activated by CAMs have remained mostly unknown. Here, we used the Drosophila larval neuromuscular junction to investigate the pathways activated by Fasciclin II (FasII), a transmembrane CAM of the Ig superfamily, during synapse remodeling. We show that the ability of FasII to stimulate or to prevent synapse formation depends on the symmetry of transmembrane FasII levels in the presynaptic and postsynaptic cell and requires the presence of the fly homolog of amyloid precursor protein (APPL). In turn, APPL is regulated by direct interactions with the PDZ (postsynaptic density-95/Discs large/zona occludens-1)-containing protein dX11/Mint/Lin-10, which also regulates synapse expansion downstream of FasII. These results provide a novel mechanism by which cell adhesion molecules are regulated and provide fresh insights into the normal operation of APP during synapse development.

Regulation of APP-dependent transcription complexes by Mint/X11s: differential functions of Mint isoforms

Mints/X11s are neuron-specific (Mints 1 and 2) and ubiquitous (Mint 3) adaptor proteins composed of isoform-specific N-terminal sequences and common C-terminal phosphotyrosine-binding (PTB) and PDZ domains. We now show that all three Mints bind to the cytoplasmic tail of amyloid-beta precursor protein (APP) and presenilins and strongly increase the levels of cellular APP in transfected cells. Immunocytochemistry revealed that in neurons, Mints 1 and 2 were colocalized with APP in the trans-Golgi network, with lower levels throughout the cell body and neurites. Using an APP-dependent transactivation assay that uses a fusion protein of APP coupled to the potent transcription factor Gal4/VP16, we examined the effects of Mints on the proteolytic processing and putative transcriptional function of APP. Although all Mints were biochemically similar, only Mints 1 and 2 but not Mint 3 strongly inhibited transactivation by APP-Gal4/VP16. Inhibition was enhanced by a mutation of the first PDZ domain and by deletion of the PDZ domains or the N-terminal sequences but abolished by inactivation of the PTB- and PDZ domains. Mint 1 also inhibited transactivation by the "precleaved" cytoplasmic tail of APP fused to Gal4/VP16, whereas Fe65 (which binds to APP as strongly as Mints) enhanced transactivation. Our data suggest that Mints 1 and 2 but not Mint 3 have a specific effect on APP function that cannot be explained simply by their interaction with presenilins and occurs at least partly after cleavage of APP. In view of their biochemical similarity, the functional differences among Mints are unexpected, suggesting that Mints 1 and 2 have a brain-specific function related to APP that is not executed by the ubiquitous Mint 3.

Molecular analysis of the X11-mLin-2/CASK complex in brain

A heterotrimeric complex containing Lin-10/X11alpha, Lin-2/CASK, and Lin-7 is evolutionarily conserved from worms to mammals. In Caenorhabditis elegans, it localizes Let-23, a receptor tyrosine kinase, to the basolateral side of vulval epithelium, a step crucial for proper vulva development. In mammals, the complex may also participate in receptor targeting in neurons. Accordingly, phosphotyrosine binding (PTB) and postsynaptic density-95/Discs large/Zona Occludens-1 domains found in X11alpha and mLin-2/CASK bind to cell-surface proteins, including amyloid precursor protein, neurexins, and syndecans. In this paper, we have further analyzed the X11alpha-mLin-2/CASK association that is mediated by a novel protein-protein interaction. We show that the mLin-2/CASK calmodulin kinase II (CKII) domain directly binds to a 63 amino acids peptide located between the Munc-18-1 binding site and the PTB domain in X11alpha. Ca2+/calmodulin association with mLin-2/CASK does not modify the X11alpha-mLin-2 interaction. A region containing the mLin-2/CASK guanylate kinase domain also interacts with X11alpha but with a lower affinity than the CKII domain. Immunostaining of X11alpha in the brain shows that the protein is expressed in areas shown previously to be positive for mLin-2/CASK staining. Together, our data demonstrate that the X11alpha-mLin-2 complex contacts many partners, creating a macrocomplex suitable for receptor targeting at the neuronal plasma membrane.