The Neural Cell Adhesion Molecules L1 and CHL1 Are Cleaved by BACE1 Protease in Vivo

Abstract

Background:The function and physiological substrates of BACE1 remain largely unknown. Results: Novel substrates for BACE1 were identified using large scale proteome analysis; L1 and CHL1 were validated in vivo. Conclusion: L1 and CHL1 are physiological substrates for BACE1. Significance: Identification of physiological substrates of BACE1 is important to understand its function and helps to predict potential side effects of BACE1 inhibitor drugs for Alzheimer disease.

β-Site Amyloid Precursor Protein (APP)-cleaving Enzyme 1 (BACE1)-deficient Mice Exhibit a Close Homolog of L1 (CHL1) Loss-of-function Phenotype Involving Axon Guidance Defects

“…3, A and B ing ϳ180-kDa full-length CHL1 and ϳ170-kDa CHL1 NTF (Fig. 8, A and B, left panels), consistent with previous findings (28,29). Importantly, P7 BACE1 null homogenates exhibited significantly increased intensity of the upper full-length CHL1 band, although the level of the lower CHL1 NTF band was diminished compared with wild type, suggesting that the ϳ170-kDa band did indeed represent BACE1-cleaved CHL1 NTF.…”

“…During our study, two groups reported that the neural cell adhesion molecules L1 and CHL1 are novel BACE1 substrates (28,29). Because L1 and CHL1 play important roles in neurite outgrowth and axon guidance (46), we reasoned that these molecules might be involved in the axon targeting defects we have observed in BACE1 Ϫ/Ϫ mice, at least in part.…”

“…CHL1 is an ϳ180-kDa type I transmembrane protein that is cleaved by BACE1 to produce an ϳ170-kDa secreted NTF and an ϳ10-kDa membrane-bound CTF (28,29). To investigate BACE1 processing of CHL1 in vivo, we analyzed hippocampal and olfactory bulb homogenates from P7 and adult BACE1 ϩ/ϩ and BACE1 Ϫ/Ϫ mice by immunoblot using an anti-N-terminal CHL1 antibody (Fig.…”

“…For example, members of the ephrin and Eph receptor families of axon guidance molecules were identified as BACE1 substrates in a proteomics screen using nonneuronal cell lines (27). More recently, the neural cell adhesion molecules L1 and close homolog of L1 (CHL1) were shown to be BACE1 substrates in primary neuron cultures and in the brain (28,29). The roles of L1 and CHL1 in neurite outgrowth and the ephrins and Eph receptors in axon guidance, together with the fact that BACE1 expression is elevated during the early postnatal period (19), a time of substantial axon outgrowth, guidance, and synapse formation, suggest that BACE1 Ϫ/Ϫ mice may have phenotypes related to axon outgrowth and guidance caused by deficient processing of these substrates.…”

“…Initially, a small number of BACE1 substrates were discovered through individual studies (26). However, the list of potential BACE1 substrates has recently grown considerably because of the use of traditional and novel proteomics screening methods (27)(28)(29). Recent reports suggest that molecules involved in axon guidance, neurite outgrowth, and synapse formation are BACE1 substrates.…”