Coordinated Metabolism of Alcadein and Amyloid β-Protein Precursor Regulates FE65-dependent Gene Transactivation

Abstract

The Alcadeins (Alcs)/calsyntenins and the amyloid ␤-protein precursor (APP) associate with each other in the brain by binding via their cytoplasmic domains to X11L (the X11-like protein). We previously reported that the formation of this APP-X11L-Alc tripartite complex suppresses the metabolic cleavages of APP. We show here that the metabolism of the Alcs markedly resembles that of APP. The Alcs are subjected to a primary cleavage event that releases their extracellular domain. Alcs then undergo a secondary presenilin-dependent ␥-cleavage that leads to the secretion of the amyloid ␤-protein-like peptide and the liberation of an intracellular domain fragment (AlcICD). However, when Alc is in the tripartite complex, it escapes from these cleavages, as does APP. We also found that AlcICD suppressed the FE65-dependent gene transactivation activity of the APP intracellular domain fragment, probably because AlcICD competes with the APP intracellular domain fragment for binding to FE65. We propose that the Alcs and APP are coordinately metabolized in neurons and that their cleaved cytoplasmic fragments are reciprocally involved in the regulation of FE65-dependent gene transactivation. Any imbalance in the metabolism of Alcs and APP may influence the FE65-dependent gene transactivation, which together with increased secretion of amyloid ␤-protein may contribute to neural disorders.The deposition and accumulation of amyloid ␤-protein (A␤) 1 in the human brain are hallmarks of Alzheimer's disease (AD)(1). Amyloid ␤-protein precursor (APP) is the precursor of A␤. It has a receptor-like transmembrane protein structure that consists of an extracellular domain, a transmembrane domain, and a short carboxyl-terminal cytoplasmic domain (2). The cytoplasmic domain of APP controls its metabolism and various physiological functions by interacting with cytoplasmic adaptor proteins (3-8). One of these adaptor proteins is X11L (the X11-like protein), which associates with the cytoplasmic domain of APP and stabilizes APP metabolism (5, 9). During our previous research that aimed to reveal the molecular mechanism by which X11L regulates APP metabolism, we found that the Alcadeins, which form cadherin-related membrane protein family, are X11-and X11L-binding proteins (9). These proteins are also known as calsyntenins, which were originally isolated as postsynaptic Ca 2ϩ -binding membrane proteins, but whose functions were not identified (10, 11). The Alcadeins (Alcs) consist of two Alc␣ isoforms (Alc␣1 and Alc␣2) and Alc␤ and Alc␥, all of which are type I transmembrane proteins and contain a conserved X11L-binding motif in their single cytoplasmic domains, similar to APP (9).Alc does not directly interact with the cytoplasmic domain of APP. Rather, the association between the two molecules is bridged by the phosphotyrosine interaction domain of X11L. This results in the formation of a tripartite complex in the brain (9). The formation of this complex enhances the X11L-mediated stabilization of APP metabolism and suppresses the generation...

CASY-1, an ortholog of calsyntenins/alcadeins, is essential for learning in Caenorhabditis elegans

“…Many transmembrane proteins have been shown to undergo ectodomain shedding, the release of the extracellular domain by proteolysis (24). It has been shown that calsyntenins/alcadeins are cleaved in the extracellular region (13,16). By analogy, we reasoned that CASY-1 might also be proteolytically cleaved.…”

“…Mammalian calsyntenins/alcadeins can physically interact with APP via scaffold proteins (15), and both are coordinately metabolized in neurons (16). Calsyntenins/alcadeins and APP also show similar localization in neuritic plaques of patients with Alzheimer's diseases (AD) (15).…”

“…Similarly, the ectodomain of APL-1, a C. elegans APP homolog, is sufficient to rescue the lethality of apl-1 mutants (31). Because calsyntenin and APP form a tripartite complex via the scaffold protein X11/X11L and their proteolytic processing is coregulated (16), it may turn out that the cleaved ectodomains of calsyntenin and APP act in concert or in parallel to modulate learning and memory.…”

“…Calsyntenins and APP are coordinately metabolized in neurons (16). The extracellular region of calsyntenin-1 is released into the synaptic cleft, whereas the intracellular region, which can bind Ca 2ϩ , is internalized (13,16). Furthermore, interaction between calsyntenin-1 and kinesin-1 blocked transport of APP-containing vesicles and increased ␤-amyloid generation (17).…”

“…Biochemical studies revealed that calsyntenins/alcadeins can associate with the scaffold protein X11/X11L, which in turn associates with APP, resulting in the formation of a tripartite complex in the brain (15). Calsyntenins and APP are coordinately metabolized in neurons (16). The extracellular region of calsyntenin-1 is released into the synaptic cleft, whereas the intracellular region, which can bind Ca 2ϩ , is internalized (13,16).…”