Pivotal role of the RanBP9-cofilin pathway in Aβ-induced apoptosis and neurodegeneration

Abstract

Neurodegeneration associated with amyloid b (Ab) peptide accumulation, synaptic loss, neuroinflammation, tauopathy, and memory impairments encompass the pathophysiological features of Alzheimer's disease (AD). We previously reported that the scaffolding protein RanBP9, which is overall increased in brains of AD patients, simultaneously promotes Ab generation and focal adhesion disruption by accelerating the endocytosis of amyloid precursor protein (APP) and b1-integrin, respectively. Here, we show that RanBP9 protein levels are increased by fourfold in FAD mutant APP transgenic mice. Accordingly, RanBP9 transgenic mice demonstrate significantly increased synapse loss, neurodegeneration, gliosis, and spatial memory deficits. RanBP9 overexpression promotes apoptosis and potentiates Ab-induced neurotoxicity independent of its capacity to promote Ab generation. Conversely, RanBP9 reduction by siRNA or gene dosage mitigates Ab-induced neurotoxicity. Importantly, RanBP9 activates/dephosphorylates cofilin, a key regulator of actin dynamics and mitochondria-mediated apoptosis, and siRNA knockdown of cofilin abolishes both Ab and RanBP9-induced apoptosis. These findings implicate the RanBP9-cofilin pathway as critical therapeutic targets not only for stemming Ab generation but also antagonizing Ab-induced neurotoxicity. Cell Death and Differentiation (2012) 19, 1413-1423; doi:10.1038/cdd.2012.14; published online 24 February 2012Alzheimer's disease (AD) is a neurodegenerative disorder characterized by accumulations of the amyloid b (Ab) -peptide and hyperphosphorylated tau in senile plaques and neurofibrillary tangles, respectively. Ab is a neurotoxic peptide derived from b-and g-secretase cleavages of the amyloid precursor protein (APP) and is thought to be a critical early player in AD pathogenesis. 1 In addition to the accumulation of Ab and tau, abnormalities in the actin cytoskeleton are detected earlier during the course of AD and other neurodegenerative diseases. 2 Indeed, it has been demonstrated that Ab can induce actin/cofilin pathology associated with hyperphosphorylated tau in primary neurons and in vivo. [3][4][5] We recently demonstrated that the scaffolding protein RanBP9 interacts with the cytoplasmic tails of LRP, APP, and BACE1, and functions as a scaffold upon which APP is brought together with BACE1 and LRP. Such interactions of RanBP9 promote the endocytosis of APP and strongly increase BACE1 cleavage of APP to generate Ab in vitro and in vivo. 6,7 Conversely, siRNA knockdown of RanBP9 reduces BACE1 cleavage of APP and Ab generation, indicating that endogenous RanBP9 normally functions in this capacity. 6 In addition, a 60-kD proteolytic fragment of RanBP9 is increased by more than sixfold in brains of AD patients, and this fragment potentiates Ab generation via BACE1 processing of APP. 8 In addition to promoting Ab generation by accelerating APP endocytosis, RanBP9 also potently disrupts integrin-dependent focal adhesion signaling and assembly by accelerating b1-integrin and LRP endocytosis. 9 In th...

COPS5 (Jab1) Protein Increases β Site Processing of Amyloid Precursor Protein and Amyloid β Peptide Generation by Stabilizing RanBP9 Protein Levels

“…Consistent with this prediction, here we found increased levels of both RanBP9-FL and RanBP9-N60 in Alzheimer's brains as well as AP⌬E9 transgenic mice. We also demonstrated previously that RanBP9 protein levels were increased in J20 mice, also APP transgenic mice (37). We also demonstrated that RanBP9 scaffolds a tripartite protein complex by directly binding to LRP, APP, and BACE1, and enhances their interaction and A␤ generation (10,11).…”

“…Similarly, we have now demonstrated that COPS5 can also bind with APP, LRP, and BACE1. Further, RanBP9 accelerates endocytosis of APP, LRP, and ␤1-integrin by physically interacting with each one of them, thereby simultaneously regulating cell adhesion and A␤ generation (11,37). Because endocytosis of APP and LRP is required for A␤ generation (38 -40), COPS5-mediated increased A␤ generation may be the consequence of increased RanBP9 protein levels and APP/LRP endocytosis.…”

COPS5 Protein Overexpression Increases Amyloid Plaque Burden, Decreases Spinophilin-immunoreactive Puncta, and Exacerbates Learning and Memory Deficits in the Mouse Brain

“…Therefore, increased RanBP9 protein levels in AP⌬E9/ COPS5 mice were almost double the quantity seen in COPS5-Tg mice. This is consistent with our previous observation that RanBP9 protein levels are increased in both J20 (22) and AP⌬E9 (27) mouse brains compared with littermate controls. The exact reason for an increase in RanBP9 levels under APP overexpression conditions is not known, but such an increase was confirmed in the AD brains as well (23,27,31).…”

“…In a recent study, RanBP9 has been found to be within the clusters of RNA transcript pairs associated with markers of AD progression (18), suggesting that RanBP9 might contribute to the pathogenesis of AD. In fact, we demonstrated previously that RanBP9 is an important component of a multiprotein complex involving LRP, BACE1, and APP that robustly increased A␤ levels (19,20) and substantially reduced synaptic proteins (21)(22)(23)(24)(25) because of reduced dendritic intersections and spine density (26), consequently leading to severe deficits in learning and memory performance in AP⌬E9 mice overexpressing RanBP9 (23,25). We also recently identified COP9 constitutive photomorphogenic homolog subunit 5 (COPS5, Jab1) as a bona fide binding partner of RanBP9 (27).…”

Global Analysis of S-nitrosylation Sites in the Wild Type (APP) Transgenic Mouse Brain-Clues for Synaptic Pathology

“…siRNA knockdown of cofilin abolished both A␤ and RanBP9-induced apoptosis (122). In hippocampal neurons, fibrillar A␤ was able to alter the PAK1/LIMK1/cofilin axis and thereby actin organization (123).…”