Search tips
Search criteria 


Logo of nddKargerHomeAlertsResources
Neurodegener Dis. 2012 April; 10(1-4): 60–63.
Published online 2011 December 7. doi:  10.1159/000332815
PMCID: PMC3363352

A Persistent Stress Response to Impeded Axonal Transport Leads to Accumulation of Amyloid-β in the Endoplasmic Reticulum, and Is a Probable Cause of Sporadic Alzheimer's Disease


Background and Objective

Could a normal – but persistent – stress response to impeded axonal transport lead to late-onset Alzheimer's disease (AD)? Our results offer an affirmative answer, suggesting a mechanism for the abnormal production of amyloid-β (Aβ), triggered by the slowed axonal transport at old age. We hypothesize that Aβ precursor protein (APP) is a sensor at the endoplasmic reticulum (ER) that detects, and signals to the nucleus, abnormalities in axonal transport. When persistently activated, due to chronically slowed-down transport, this signaling pathway leads to accumulation of Aβ within the ER.

Methods and Results

We tested this hypothesis with the neuronal cell line CAD. We show that, normally, a fraction of APP is transported into neurites by recruiting kinesin-1 via the adaptor protein, Fe65. Under conditions that block kinesin-1-dependent transport, APP, Fe65 and kinesin-1 accumulate in the soma, and form a complex at the ER. This complex recruits active c-Jun N-terminal kinase (JNK), which phosphorylates APP at Thr668. This phosphorylation increases the cleavage of APP by the amyloidogenic pathway, which generates Aβ within the ER lumen, and releases Fe65 into the cytoplasm. Part of the released Fe65 translocates into the nucleus, likely to initiate a gene transcription response to arrested transport. Prolonged arrest of kinesin-1-dependent transport could thus lead to accumulation and oligomerization of Aβ in the ER.


These results support a model where the APP:Fe65 complex is a sensor at the ER for detecting the increased level of kinesin-1 caused by halted transport, which signals to the nucleus, while concomitantly generating an oligomerization-prone pool of Aβ in the ER. Our hypothesis could thus explain a pathogenic mechanism in AD.

Key Words: Alzheimer's disease, Amyloid-β precursor protein, Amyloid-β peptide, Axonal transport, Kinesin, Endoplasmic reticulum stress, Protein phosphorylation, Mechanism of neurodegenerative diseases

Background and Objective

Herein, we propose a novel function of APP, which could explain why normal stress – well tolerated by the young and adult brain – could lead to intraneuronal accumulation of toxic amyloid-β (Aβ) oligomers and initiation of Alzheimer's disease (AD) in the aging brain. Old age is the major risk factor for sporadic AD [1]. The changes that accompany aging, which could trigger AD, are numerous. It was proposed that the age-related impairment of axonal transport could lead to AD, but the mechanisms are poorly defined [2]. We hypothesize that a complex of APP and Fe65, a well-characterized APP-binding protein, is a sensor localized to the endoplasmic reticulum (ER) that detects and signals to the nucleus abnormalities in axonal transport. Prolonged activation of this signaling pathway due to chronically slowed-down axonal transport – likely to occur in AD – could lead to increased production and intracellular accumulation of Aβ. We propose a model (fig. (fig.1)1) where a complex containing APP, Fe65 and kinesin-1 – normally transported into neurites – accumulates at ER when axonal transport is arrested and the level of kinesin-1 in the soma is increased. This complex recruits c-Jun N-terminal kinase (JNK), which phosphorylates APP, resulting in cleavage of the phosphorylated APP via the amyloidogenic pathway and accumulation of oligomerization-prone Aβ in the ER lumen. Concomitantly, Fe65 is released, and translocates into the nucleus to regulate expression of stress-related genes, including the AD-relevant kinase, GSK3β.

Fig. 1
The molecular mechanism of ER stress response triggered by arrested axonal transport. a The complex APP:Fe65:kinesin-1, normally targeted to neurites, accumulates at the ER when transport is arrested. b Recruitment of active JNK kinase complex by kinesin-1 ...


CAD cells, a CNS-derived neuronal cell line [3], provide an excellent system for studying APP transport and metabolism, including intracellular accumulation of Aβ [4,5,6,7,8,9,10], a process typical for neurons of the aging brain [11] that is poorly reproduced in primary cultures of embryonic neurons. Mouse hippocampal neurons were used for confirmation purposes. Immunocytochemical, biochemical, and molecular biology approaches were employed as described previously [4,5,6,7,8,9,10]. Moderate overexpression of the HA-tagged, cargo-binding, tetratricopeptide repeat (TPR) domain of the kinesin-1 light chain (KLC), HA-KLC-TPR, was used to experimentally mimic the halted axonal transport in CAD cells, when kinesin-1 accumulates in the soma.


The model of ER response to stress caused by impeded axonal transport proposed here (fig. (fig.1)1) is supported by our previous studies, as well as by the new results shown herein (fig. (fig.2,2, ,3,3, ,4).4). Key findings, pertinent to each step of the model, are listed below.

Fig. 2
Knockdown of Fe65 with siRNA in CAD cells decreases Aβ levels in the soma by approximately 50%. GFP marks cells transfected with Fe65 siRNA. The histogram on the right shows the levels of Aβ along the indicated line. Bar = 10 μm. ...
Fig. 3
Oligomeric species (anti-oligomer antibody, A11) accumulate in ER. CAD cells were transfected with HA-KLC-TPR. This construct expresses the 6 TPRs of KLC, which can simultaneously bind 2 proteins. Insets are magnifications of the cell in the lower, left ...
Fig. 4
Overexpressed KLC accumulates in the cell soma of transfected CAD cells, causing an approximately 70% increased translocation of Fe65 in the nucleus. Plane of focus is through the nucleus. Bar = 10 μm.

(1) We showed that, under normal conditions, a fraction of APP is transported into neurites by kinesin-1, recruited by Fe65, an adaptor protein that simultaneously binds the cytoplasmic domain of APP, and kinesin-1 (via the TPR domain of KLC) [12] (fig. (fig.1a1a).

(2) We showed that, blocking axonal transport by various means – e.g. osmotic stress – leads to accumulation of kinesin-1, Fe65, and APP in the soma [9], and formation of an APP:Fe65:kinesin-1 complex at the ER [13].

(3) Our previous results indicate that the APP:Fe65: kinesin-1 complex, accumulates at the ER, recruits active JNK, binds to the JNK-interacting protein-3 (JIP-3) scaffold, and phosphorylates APP (fig. (fig.1b).1b). Indeed, this APP phosphorylation is facilitated by, and requires, JIP-3 [6], Fe65, and KLC [13], and is augmented by stress [9].

(4) We hypothesize that the Fe65- and kinesin-1-dependent phosphorylation of APP facilitates its cleavage via the amyloidogenic pathway, and thus increases the generation and oligomerization of Aβ in the ER lumen (fig. (fig.1c).1c). Consistent with this scenario, we show that the amount of oligomerized Aβ that accumulates in the ER region is positively regulated by the levels of Fe65 (fig. (fig.2)2) and kinesin-1 (fig. (fig.3),3), manipulated experimentally with Fe65 siRNA, or expression of HA-KLC-TPR.

(5) We hypothesize that phosphorylation and cleavage of APP, caused by increased levels of kinesin-1 in the soma, leads to the release of Fe65, which translocates into the nucleus (fig. (fig.1c).1c). Our data show that the nuclear level of Fe65 is increased upon expression of HA-KLC-TPR (fig. (fig.44).

The proposed model is consistent with findings from other laboratories, showing that: (1) Thr668-phosphorylated APP is a better substrate for the β-secretase, BACE1 than nonphosphorylated APP [14]; (2) phosphorylation of APP at multiple residues reduces its affinity for Fe65 [15,16], and facilitates release of Fe65, and (3) Fe65 is part of a transcriptionally active complex, together with Tip60 [17,18]. Interestingly, one of the genes upregulated by Fe65 could be GSK3β; this kinase is activated in AD, and phosphorylates the microtubule-associated protein tau, leading to its release from the microtubules and enhancement of axonal transport.


The ER stress response triggered by slowed-down axonal transport described here explains both the Aβ and the tau pathology characteristic for AD. According to our model, APP is a sensor that measures the pace of axonal transport. It detects variations in kinesin-1 levels, and triggers a stress response that signals to the nucleus abnormalities in axonal transport. A persistent stress response to impeded axonal transport, causing oligomerization and accumulation of Aβ in the ER, and abnormal phosphorylation of tau, could be a pathogenic mechanism in sporadic AD. This and other studies [19,20] confer to the ER a prominent role in the disease mechanism in AD.


This work was supported by National Institutes of Health awards GM068596-05S1 (to V.M., Z.M.) and AG039668 (to Z.M.).


1. Herrup K. Reimagining Alzheimer's disease – An age-based hypothesis. J Neurosci. 2010;30:16755–16762. [PMC free article] [PubMed]
2. Muresan V, Muresan Z. Is abnormal axonal transport a cause, a contributing factor or a consequence of the neuronal pathology in Alzheimer's disease? Future Neurol. 2009;4:761–773. [PMC free article] [PubMed]
3. Qi Y, Wang JK, McMillian M, Chikaraishi DM. Characterization of a CNS cell line, CAD, in which morphological differentiation is initiated by serum deprivation. J Neurosci. 1997;17:1217–1225. [PubMed]
4. Muresan V, Varvel NH, Lamb BT, Muresan Z. The cleavage products of amyloid-beta precursor protein are sorted to distinct carrier vesicles that are independently transported within neurites. J Neurosci. 2009;29:3565–3578. [PMC free article] [PubMed]
5. Muresan Z, Muresan V. A phosphorylated, carboxy-terminal fragment of beta-amyloid precursor protein localizes to the splicing factor compartment. Hum Mol Genet. 2004;13:475–488. [PubMed]
6. Muresan Z, Muresan V. c-Jun NH2-terminal kinase-interacting protein-3 facilitates phosphorylation and controls localization of amyloid-beta precursor protein. J Neurosci. 2005;25:3741–3751. [PubMed]
7. Muresan Z, Muresan V. Coordinated transport of phosphorylated amyloid-beta precursor protein and c-Jun NH2-terminal kinase-interacting protein-1. J Cell Biol. 2005;171:615–625. [PMC free article] [PubMed]
8. Muresan Z, Muresan V. Neuritic deposits of amyloid-β peptide in a subpopulation of central nervous system-derived neuronal cells. Mol Cell Biol. 2006;26:4982–4997. [PMC free article] [PubMed]
9. Muresan Z, Muresan V. The amyloid-beta precursor protein is phosphorylated via distinct pathways during differentiation, mitosis, stress, and degeneration. Mol Biol Cell. 2007;18:3835–3844. [PMC free article] [PubMed]
10. Muresan Z, Muresan V. Seeding neuritic plaques from the distance: a possible role for brainstem neurons in the development of Alzheimer's disease pathology. Neurodegener Dis. 2008;5:250–253. [PMC free article] [PubMed]
11. Gouras GK, Tsai J, Naslund J, Vincent B, Edgar M, Checler F, Greenfield JP, Haroutunian V, Buxbaum JD, Xu H, Greengard P, Relkin NR. Intraneuronal abeta42 accumulation in human brain. Am J Pathol. 2000;156:15–20. [PubMed]
12. Muresan Z, Muresan V. The phosphorylated, carboxy-terminal fragments of amyloid-β precursor protein are targeted to multiple intraneuronal locations by destination-specific molecular motors and adaptor proteins.
13. Muresan Z, Muresan V. Kinesin-1 is a signaling protein that recruits active kinase complexes and directly participates in substrate phosphorylation.
14. Lee MS, Kao SC, Lemere CA, Xia W, Tseng HC, Zhou Y, Neve R, Ahlijanian MK, Tsai LH. APP processing is regulated by cytoplasmic phosphorylation. J Cell Biol. 2003;163:83–95. [PMC free article] [PubMed]
15. Radzimanowski J, Simon B, Sattler M, Beyreuther K, Sinning I, Wild K. Structure of the intracellular domain of the amyloid precursor protein in complex with Fe65-PTB2. EMBO Rep. 2008;9:1134–1140. [PubMed]
16. Zhou D, Zambrano N, Russo T, D'Adamio L. Phosphorylation of a tyrosine in the amyloid-beta protein precursor intracellular domain inhibits Fe65 binding and signaling. J Alzheimers Dis. 2009;16:301–307. [PubMed]
17. Baek SH, Ohgi KA, Rose DW, Koo EH, Glass CK, Rosenfeld MG. Exchange of N-CoR corepressor and Tip60 coactivator complexes links gene expression by NF-kappaB and beta-amyloid precursor protein. Cell. 2002;110:55–67. [PubMed]
18. Cao X, Sudhof TC. A transcriptionally [correction of transcriptively] active complex of APP with Fe65 and histone acetyltransferase Tip60. Science. 2001;293:115–120. [PubMed]
19. He W, Lu Y, Qahwash I, Hu XY, Chang A, Yan R. Reticulon family members modulate bace1 activity and amyloid-beta peptide generation. Nat Med. 2004;10:959–965. [PubMed]
20. Tu H, Nelson O, Bezprozvanny A, Wang Z, Lee SF, Hao YH, Serneels L, De Strooper B, Yu G, Bezprozvanny I. Presenilins form ER Ca2+ leak channels, a function disrupted by familial Alzheimer's disease-linked mutations. Cell. 2006;126:981–993. [PMC free article] [PubMed]

Articles from Neuro-Degenerative Diseases are provided here courtesy of Karger Publishers