RCAN1 Inhibits BACE2 Turnover by Attenuating Proteasome-Mediated BACE2 Degradation

Abstract

Amyloid-β protein (Aβ) is the main component of neuritic plaques, the pathological hallmark of Alzheimer’s disease (AD). β-site APP cleaving enzyme 1 (BACE1) is a major β-secretase contributing to Aβ generation. β-site APP cleaving enzyme 2 (BACE2), the homolog of BACE1, is not only a θ-secretase but also a conditional β-secretase. Previous studies showed that regulator of calcineurin 1 (RCAN1) is markedly increased by AD and promotes BACE1 expression. However, the role of RCAN1 in BACE2 regulation remains elusive. Here, we showed that RCAN1 increases BACE2 protein levels. Moreover, RCAN1 inhibits the turnover of BACE2 protein. Furthermore, RCAN1 attenuates proteasome-mediated BACE2 degradation, but not lysosome-mediated BACE2 degradation. Taken together, our work indicates that RCAN1 inhibits BACE2 turnover by attenuating proteasome-mediated BACE2 degradation. It advances our understanding of BACE2 regulation and provides a potential mechanism of BACE2 dysregulation in AD.

Experimental approaches for altering the expression of Abeta‐degrading enzymes

“…Aluminum chloride (Castorina et al, 2010) BACE2 cDNA-containing plasmid transfection (Sun et al, 2005) Docosahexaenoic acid (Bie et al, 2021) Lentivirus-mediated overexpression (Sun et al, 2005;Wang et al, 2020) Leptin (Niedowicz et al, 2013) Lethal-7c (let-7c) microRNA transfection (Liu et al, 2022) Lysine acetyltransferase 8 (KAT8) transfection (Chen et al, 2020) Lysosomal inhibition (Liu et al, 2013) Morphine and S-nitroso-N-acetylpenicillamine (a nitric oxide donor) (Pak et al, 2005) Nsmce1 (murine homolog of NSE1) knockdown (Gong et al, 2020) Oxidative stress (H 2 O 2 ) (Azmi et al, 2015) Regulator of calcineurin 1 (RCAN1, inhibitor of proteasomal degradation) (Qiu et al, 2020) Thy1-BACE2 expression cassette microinjection (Azkona, Amador-Arjona, et al, 2010) Decreasing of BACE2 expression APP-binding benzofuran compounds (Espeseth et al, 2005) BACE2 gene knockout (Dominguez et al, 2005;Sun et al, 2005) CRISPR/Cas9-mediated elimination of BACE2 gene (Alić et al, 2021) Mulberry extracts (Song et al, 2014) Myocyte enhancer factor 2C (MEF2C) knockdown (Ren et al, 2022) Nsmce1 overexpression (Gong et al, 2020) RNA silencing (via small interfering RNA [siRNA]) of the α7 nicotinic acetylcholine receptor (Qi et al, 2013) Transfection of siRNA targeting BACE2 (Basi et al, 2003;Casas et al, 2010;Rochin et al, 2013;Wang et al, 2020) Note: BACE2 cleaves amyloid precursor protein (APP) in a nonamyloidogenic manner, but may also cleave APP at its β-secretase site, promoting the production of Aβ. Because of uncertainty about the role of BACE2 in AD pathogenesis, both overexpression and reduced expression of BACE2 have been accomplished by various approaches in experimental systems.…”

BACE2: A Promising Neuroprotective Candidate for Alzheimer’s Disease

“…Importantly, in the AD brain, BACE2 protein levels inversely correlate with higher Braak stages [29], suggesting that low BACE2 levels portends increasing severity of AD. Although why BACE2 expression describes such a trend in the AD brain remains unclear, it is noteworthy that a recent report has identified RCAN1 (regulator of calcineurin 1) to be a regulator of BACE2 expression [30]. In their report, Qiu and colleagues demonstrated that RCAN1 promotes BACE2 expression by inhibiting its turnover by the proteasome [30].…”

“…Although why BACE2 expression describes such a trend in the AD brain remains unclear, it is noteworthy that a recent report has identified RCAN1 (regulator of calcineurin 1) to be a regulator of BACE2 expression [30]. In their report, Qiu and colleagues demonstrated that RCAN1 promotes BACE2 expression by inhibiting its turnover by the proteasome [30]. Notably, RCAN1 is highly expressed in the human brain and its expression is further increased in aged and AD brains, which BACE2 expression mirrors.…”

“…Indeed, Alic et al found that the degradation of A␤ by BACE2 occurs in LAMP2-positive compartments of neurons as exemplified by the high degree of colocalization between BACE2, its substrate A␤ 40 , and degradation product A␤ 34 with the components of the chaperone-mediated autophagy pathway [13]. BACE2 may also be degraded via the macroautophagy-lysosome pathway [46] and the ubiquitin-proteasome pathway [30]. Consistent with this, the inhibition of BACE2 degradation via lysosomal inhibitor treatment with chloroquine or NH 4 Cl in 4EB2 cells, which express both BACE2 and the human APP Swedish mutant, led to an increase in BACE2 levels and non-amyloidogenic C80 levels [46].…”

RCAN1 Reduces FBXW7β Transcription by Inhibiting Calcineurin/NFAT Signaling Pathway

“…Transient transfections were performed by using PEI-based protocol [14]. The plasmids pRCAN1 and pNFAT used in transfection were obtained from Weihong Song's lab as described previously [5,15].…”