Insulysin cleaves the APP cytoplasmic fragment at multiple sites

The amyloid peptide (Aβ) deposited in Alzheimer's disease (AD) is generated by β- and γ-secretase processing of a larger integral membrane protein precursor (APP). Intramembrane processing of APP by γ-secretase also yields an intracellular fragment, CTFγ (a.k.a. AICD), which is highly conserved and is believed to regulate the transcription of several genes including KAI-1 and GSK3β. The intracellular domain of APP is also processed by caspase to a 31 aa fragment that was shown to induce apoptosis by several groups. Although large quantities of CTFγ are generated continuously by neurons, little if any is normally detected in cell lysates, which suggests that it is very rapidly turned over in vivo. Previous studies demonstrated that insulysin (IDE), an Aβ-degrading enzyme, is responsible for cytosol-mediated CTFγ degradation in vitro. Consistent with this finding, knockout mice lacking IDE accumulate CTFγ to detectable levels in the brain, although its levels remain lower than its precursor, suggesting that it continues to be turned over in the brain. Moreover, when we treated cultured cells with IDE inhibitors, we did not observe an increase in CTFγ in cell lysates, suggesting that pathways other than IDE are also involved in CTFγ turnover. To understand CTFγ turnover further, we have mapped the IDE cleavage sites with the intention of mutating them to examine alternative pathways in future studies. Edman degradation revealed that IDE cleaves CTFγ at multiple sites to small peptides ranging from 5 to 14 aa. The cleavage sites do not reveal the existence of any sequence specificity for IDE cleavage. Understanding the turnover mechanisms of CTFγ is critical to the understanding of the signaling function of APP mediated by this fragment. The current study presents the interesting specificity of CTFγ turnover by IDE, which has been previously identified as the major degrading enzyme for Aβ as well as CTFγ. In addition, the study provides evidence for the presence of alternative CTFγ-degrading pathways in the cell.