Cell Biology: Research & TherapyISSN: 2324-9293

All submissions of the EM system will be redirected to Online Manuscript Submission System. Authors are requested to submit articles directly to Online Manuscript Submission System of respective journal.

Review Article, Cell Biol Res Ther Vol: 2 Issue: 2

NGF and APP Interplay: Focus on YENPTY Motif of Amyloid Precursor Protein and Y682 Residue

Basso E1* and Matrone C2
1Cell and Molecular Neuroscience Unit, Instituto de Medicina Molecular, Lisboa, Portugal
2Department of Medical Biochemistry, University of Aarhus, 8000 Aarhus C, Denmark
Corresponding author : Elisa Basso
Cell and Molecular Neuroscience Unit, Instituto de Medicina Molecular, Lisboa, Portugal
E-mail: [email protected]
Received: July 04, 2013 Accepted: November 13, 2013 Published: November 19, 2013
Citation: Basso E, Matrone C (2013) Ngf and App Interplay: Focus on Yenpty Motif of Amyloid Precursor Protein and Y682 Residue. Cell Biol: Res Ther 2:2. doi:10.4172/2324-9293.1000106

Abstract

NGF and APP Interplay: Focus on YENPTY Motif of Amyloid Precursor Protein and Y682 Residue

Cholinergic deficits originated from NGF metabolism disruption, represent one of the early changes in Alzheimer’s disease, where abnormal deposition of β-amyloid peptide (Aβ) and phosphorylated Tau define the neuropathological hallmarks of the disorder. A failure in NGF maturation can promote pro-apoptotic pathway activation, through p75 receptor; while lack of NGF signalling can generate an atypical TrkA receptor phosphorylation resulting in neuronal cell death and Aβ toxicity. These evidences suggest a complex interaction between TrkA, p75, and Aβ, whose exact cellular mechanisms remain still elusive. Here, we provide a general overview on the current knowledge on NGF and APP interplay, focusing on the events that mediate NGF signalling impairment, and, mostly, on the role of APP Tyrosine 682 phosphorylation whose absence in APPY682G mice impairs APP/TrkA interaction and leads to cholinergicneurodegeneration.

Keywords: APP; NGF; Neurodegeneration; Tyrosine phosphorylation; Alzheimer’s disease; YENPTY

Keywords

APP; NGF; Neurodegeneration; Tyrosine phosphorylation; Alzheimer’s disease; YENPTY

Abbreviations

AD: Alzheimer’s Disease; SP: Senile Plaques; Aβ: β-Amyloid Peptide; NFTs: Neurofibrillary Tangles; APP: Amyloid Precursor Protein; NGF: Nerve Growth Factor; APLP1 and APLP2: Mammalian APP-Like Protein; APPL: Amyloid Precursor Protein-Like; CNS: Central Nervous System; AICD: APP Intracellular Domain;
KPI: Kunitz Protease Inhibitor; C83: C-Terminal Fragment of 83 Amino Acids; sAPPβ: Cleavage of APP By β-Secretase; C99: C-Terminal Fragment of 99 Amino Acids; C31: C-Terminal Fragment of APP; ER: Endoplasmic Reticulum; PM: Plasma Membrane; TGN: Trans-Golgi Network; LRP: Low-Density Lipoprotein Receptor- Related Protein; LTP: Long-Term Potentiation; sAPPβ: Cleavage of APP By β-Secretase; SVZ: Subventricular Zone;
EGF: Epidermal Growth Factor; ERK: Extracellular Signal- Regulated Kinase; CDK5: Cyclin-Dependent Kinase 5; IGF-1: Insulin-Like Growth Factor 1; GSK-3β: Glycogen Synthase Kinase 3β; NF-κ: Nuclear Factor κB;
IGF2: Insulin-Like Growth Factor 2; IGF-BP2: Insulin-Like Growth Factor-Binding Protein 2; JNK: c-jun NH2-Terminal Kinase; pro-NGF: Nerve Growth Factor Precursor; p75NTR: p75 Neurotrophin Receptor; MCI: Mild Cognitive Impairment; TK: Tyr- Kinase

Background

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder representing the most common form of dementia in the elderly [1]. The major clinical hallmarks of AD include progressive impairments in memory, decision-making, and language. These characteristics are not distinguishable between rare early-onset familial cases (before 60 years of age) and the most common lateonset sporadic form (after 60 years of age) [2]. The two pathological hallmarks in AD brains are senile plaques (SPs), consisting of deposits of β-amyloid peptide (Aβ), and neurofibrillary tangles (NFTs), composed of an abnormally phosphorylated form of the cytoskeleton-associated protein, Tau, particularly in the cerebral cortex and hippocampus [3].
The purpose of this review is to summarize the main findings on amyloid precursor protein (APP) and nerve growth factor (NGF) and discuss recent emerging evidence of the interplay between NGF and APP. More specifically, we review the results obtained in a new mouse model, APPY682G mice, in which tyrosine is replaced by glycine at position 682.
APP: Processing and trafficking
Amyloid precursor protein (APP) is a member of a family of conserved type 1 membrane proteins that includes mammalian APPlike protein (APLP1) and APLP2 [4,5] and the amyloid precursor protein-like (APPL) in Drosophila. The alternative splicing of the APP transcript generates eight isoforms, three of which are most common: the 695-amino-acid form, which is expressed predominantly in the central nervous system (CNS), and the 751- and 770-amino-acid forms, which are more ubiquitously expressed [6].
The APP protein consists of a large extracellular domain, a hydrophobic transmembrane region, and a short C-terminal domain (AICD). The extracellular portion of APP contains E1 and E2 domains and a Kunitz protease inhibitor (KPI) domain that is absent in the APP695 isoform [7,8]. Notably, increased levels of APP isoforms that contain the KPI domain have been detected in AD patients [9].
APP is posttranslational processed by two major pathways (Figure 1). In the non-amyloidogenic pathway, APP is cleaved by α-secretase and γ-secretase. The cleavage of APP by α-secretase occurs in the middle of the Aβ domain at the Lys16-Leu17 bond, precluding the formation of Aβ peptides. α-secretase cleavage releases a large extracellular soluble N-terminal domain and a membranebound C-terminal fragment of 83 amino acids (C83) [10]. The C83 fragment is further cleaved by γ-secretase to release the P3 peptide and the APP intracellular domain (AICD), both of which are rapidly degraded [11].
Figure 1: Figure 1: Amyloid precursor protein processing
APP can be cleaved by two independent pathways: a non-amyloidogenic (A) and an amyloidogenic pathway (B). (A) Mainly occurs at the cell surface, and it is mediated by α-secretase, which cleaves APP in an extracellular domain named sAPPα, and an intra membrane domain, C83/CTF. The intra membrane domain is further processed by γ-secretase to produce P3 extracellular domain and APP Intracellular Domain (AICD). (B) Includes a sequential cleavage by β-secretase and γ-secretase in the APP extracellular domain to generate Aβ peptides. The cleavage by γ-secretase within the intra membrane APP domain give rise to the AICD that can translocate into the nucleus and modulate gene expression. The intra membrane domain can also be cleaved by caspases (C) to produce two fragments: a C31 domain and a Jcasp domain that is believed to be neurotoxic.
In the amyloidogenic pathway, the cleavage of APP by β-secretase [12] releases the ectodomain (sAPPβ) and produces the C-terminal fragment of 99 amino acids (C99), which remains membranebound. The C99 fragment is further cleaved by γ-secretase into two peptides: the amyloidogenic Aβ peptide, consisting of two major 40- and 42-amino-acid species (Aβ40 and Aβ42, respectively), and an intracellular product named the APP intracellular domain (AID or AICD) [12,13].
In addition to secretases, caspases (predominantly caspase-3) can directly cleave APP between Asp664 and Ser665 (based on the APP695 sequence) within the cytoplasmic tail and release a fragment that contains the last 31 amino acids of APP (C31) and Jcasp fragment between the γ- and caspase-cleavage sites [11,14,15]. Both the C31 and Jcasp fragments are found to be cytotoxic [13,16]; transgenic mice with Swedish and Indiana mutations of APP show susceptibility to seizures that could be counteracted by a D664A mutation through the disruption of the caspase cleavage site [17].
The proteolytic processing of APP has been shown to occur at various sites throughout the secretory and endocytic pathways. Although a large amount of work has been devoted to the study of APP trafficking within the endocytic pathway, the precise mechanism is still not completely understood. After synthesis, APP transits from the endoplasmic reticulum (ER) to the plasma membrane (PM) where it is modified by N- and O-linked glycosylation, tyrosine sulphation, and the phosphorylation of ectodomain and cytoplasmic residues. The majority of APP appears to be localized in the Golgi apparatus and trans-Golgi network (TGN), and only a small fraction reaches the PM [18-21].
β-secretase preferentially localizes in the Golgi apparatus and endosomes, where acidic pH favours its activity [12]. Components of the γ-secretase complex have been localized to the ER, lysosomes, and the cell surface [22], while α-secretase activity is found primarily at the cell surface [23]. According to these studies, most newly synthesized APP molecules are internalized from the PM to the endosomes and cleaved by β-secretase. APPβ fragments then return to the cell surface, or they are trafficked to the lysosome where they are cleaved by γ-secretase to produce Aβ [24]. In an alternative pathway, APP can also be internalized or undergo degradation in lysosomes [25].
What discriminates the two pathways is not clear yet, but one possibility may be that the access of α- and β-secretase to APP and therefore Aβ generation may be determined by dynamic interactions between APP and lipid rafts. According to this hypothesis, APP inside raft clusters may be cleaved by β-secretase, and APP outside rafts may undergo cleavage by α-secretase [26]. Interestingly, the type I transmembrane protein SorLA/LR11 and sortilin are members of the VPS10p-domain receptor family, which plays a role in modulating APP trafficking and processing in neurons. SorLA confines APP to the Golgi apparatus and affects its transition to the cell surface, preventing sAPPα and sAPPβ/Aβ production [27]. In contrast, APP binding to sortilin selectively increased α-secretase cleavage in neuronal processes and prevented proteolytic APP processing in intracellular compartments [28]. Similarly to SorLA/LR11, lowdensity lipoprotein receptor-related protein (LRP) binds to APP through Fe65, a cytoplasmic adaptor protein [29], and mediates Aβ clearance by the direct or indirect binding to Aβ [30,31]. The lack of LRP expression reduces APP internalization and Aβ secretion [32-34]. Relevant emerging evidence indicates a genetic association between SorLA and AD [35], and decreased expression of SorLA in vulnerable neurons in AD brains has been recently detected [36,37].
Physiological role of APP
APP belongs to an evolutionarily conserved protein family and studies of different species have indicated the conservation of functions between APP family members. Several authors have shown that APP plays a physiological role in cell adhesion, neuronal differentiation, migration, outgrowth, and synapse formation [38-43].
APP exerts its adhesion properties by binding to extracellular proteoglycans and subsequently promoting the growth of neuritis. A peptide homologous to the APP heparin-binding domain was able to block the effect of APP and to prevent neuritis outgrowth in hippocampal neurons [44,45].
An interesting role for APP in neuronal regeneration was also hypothesized. Increased APP protein levels were found in regenerating neurons in Drosophila [46,47] and various mice models after brain injury [48-50]. Similarly, transgenic mice that overexpress APP exhibited resistance to kainate-induced excitotoxicity and appeared to be protected against acute and chronic excitotoxic injury in vivo [51-53].
Evidence of the crucial role of APP in neuronal generation, differentiation, and migration has been provided by studies of APP null mice [54]. APP knockout mice exhibited diminished balance and strength and deficiencies in behaviour and long-term potentiation (LTP) [55,56]. Homozygous APP-deficient mice were smaller than age-matched controls and exhibited decreased locomotor activity, reactive gliosis, and behavioural deficit in the Morris water maze [56]. However, APP-deficient mice resulted viable and fertile, similar to APLP1 and APLP2 single-knockout mice [57]. Intriguingly, APP/ APLP2 and APLP1/APLP2 double-knockout mice showed early postnatal lethality, whereas APP/APLP1 double-knockout mice were viable, suggesting that members of the APP gene family may exert concerted control over the development of the nervous system.
Other studies failed to observe any differences in the celldeath sensitivity of cortical neurons in APP knockout mice [58], while increased vulnerability to kainate-induced epilepsy but not glutamate- and NMDA-induced toxicity was also reported [5,53,59]. Nonetheless, these findings support a role for APP in neural development and in response to brain injury.
Under physiological conditions, the majority of APP is known to be processed by α-secretase to produce the secreted N-terminal APP product sAPPα [60-62]. Strong evidence support a role for sAPPα as neuroprotective molecule [63-65], as modulator of neuronal excitability [63,66], in synaptic plasticity [67,68], in neuronal growth and branching [69-71], and as an enhancer of synaptogenesis [66,72], but the precise molecular mechanisms that underlie this function are still unclear. Interestingly, AD patients present decreased levels of sAPPα in cerebrospinal fluid [73], while infusions of sAPPα into the brain increased synaptic density and improved memory performance [74]. Furthermore, injection of sAPPα into the brain in brain-injured rats had positive effects on motor and cognitive function [75,76].
Interestingly, a role for sAPPα in adult neurogenesis has been initially suggested by Caillé et al. [39]. They found that sAPPα can bind cells in the subventricular zone (SVZ) and activate neurogenesis. Conversely, the blockade of sAPPα production by α-secretase inhibitors or downregulation of APP synthesis decreased the proliferation of epidermal growth factor (EGF)-responsive cells. Further studies by Gakhar-Koppole et al. [77] and Rohe et al. [78] established that sAPPα stimulated neurogenesis and neuritis outgrowth through increased extracellular signal-regulated kinase (ERK) phosphorylation.
sAPPα is also involved in the activation of potassium channels and in the subsequent suppression of NMDA currents to avoid excitotoxic damage in neurons [65,66,79]. The potential upregulation of sAPPα secretion under conditions of neuronal excitotoxicity has also been reported [62,80,81]. Previous studies demonstrated that sAPPα can protect neurons under hypoglycemic conditions and against glutamate neurotoxicity by activating potassium channels, such activation in turn leads to the inhibition of calcium influx and to the modulation of neuronal excitability [82,83]. Furthermore, the sensitivity to excitotoxic damage increases in neurons in APPdeficient mice, which was shown to be mediated by cyclin-dependent kinase 5 (CDK5) over-activation via calcium/calpain/p25 pathway [84]. sAPPα was recently shown to suppress physiological and stressinduced CDK5 activation through insulin-like growth factor 1 (IGF- 1) or insulin receptors via PI3K/Akt pathway activation and glycogen synthase kinase 3β (GSK-3β) inhibition [85].
Altogether, these data confirmed that sAPPα is able to act on various survival signaling pathways, including the PI3K/Akt [85-88], nuclear factor-κB (NF-κB) [86,89], and ERK pathways [90-92].
Transcriptional sAPPα activity has also been reported and sAPPα may activate antioxidative defence genes (e.g. manganese superoxide dismutase, peroxiredoxin-2, and catalase), the anti-amyloidogenic gene transthyretin, insulin-like growth factor 2 (IGF2), and IGFbinding protein 2 (IGF-BP2) [64,93]. The stress-induced activation of the c-jun NH2-terminal kinase (JNK) signaling pathway was shown to be reduced by sAPPα and after the exogenous addition of sAPPα [88,94].
NGF
Nerve growth factor (NGF) was the first member of the neurotrophin family to be discovered [95,96]. It is initially synthesized as a precursor (pro-NGF) and subsequently cleaved to release the C-terminal mature protein, a 12 kDa non-covalently linked dimer, NGF [97]. NGF exerts biological activity through its signalling receptor TrkA (i.e. a receptor tyrosine kinase of the Trk family) and p75 Neurotrophin Receptor (p75NTR) [98]. NGF was first investigated in a series of experiments that began over 60 years ago when a soluble growth factor released from sarcoma tissue was found to cause the outgrowth of fibres from sensory or sympathetic nerve cells placed nearby [95,99]. NGF is produced in the cortex, hippocampus, and olfactory regions, which represent targets for basal forebrain cholinergic innervation [100]. NGF plays a crucial role in development and differentiation, in the growth of developing neurons, in the maintenance of mature neurons, and in synaptic plasticity [101-104]. Remarkably, the crucial involvement of NGF in early chicken embryo development and particularly in the regulation of somite survival and axial rotation has been further described by the Prof. Montalcini group [105], and an emerging role for NGF in fertility has been recently reported [106].
Consistent with the relevance of NGF in development, homozygous NGF knockout mice die few weeks after birth [103,107] and NGF heterozygous knockout mice are characterized by decreased cholinergic innervation in the hippocampus, deficient memory acquisition and retention, and the loss of neurons in the peripheral nervous system [97,103].
Additionally, AD11 transgenic mice that express recombinant antibodies, which neutralize up to 50% of endogenous NGF throughout adulthood, exhibit a neurodegenerative phenotype that is similar to human sporadic forms of AD, with a loss of basal forebrain cholinergic neurons, Tau hyperphosphorylation, Aβ plaque accumulation, synaptic plasticity deficits, and an improper balance between unprocessed pro-NGF and NGF signaling [107-114].
One of the early and remarkable changes in AD pathology consists in the degeneration of cholinergic system of the basal forebrain in which NGF possesses a crucial role [115,116]. Several experimental models have been produced and characterized in order to investigate the neurotrophic properties of NGF and to test its potential use in therapy. Interestingly, in adult rats it was shown that NGF improved neurogenesis of hippocampal neurons, whereas in aged ones its infusion declined age-dependent neurodegeneration of cholinergic innervations and corrects spatial memory defects [117]. In addition, human recombinant NGF injection into the brain of adult primate prevented lesion-induced cholinergic neurodegeneration and promoted neuritis outgrowth [118]. Further evidence showed that cholinergic hypo-activity might inhibit the non-amyloidogenic pathway leading to the formation of Aβ [119].
pro-NGF and NGF
The fundamental discovery of a biological role for proneurotrophins, often in opposition to the role of their mature form, redirected many investigators to study the action of proneurotrophins [120]. Pro-NGF, the NGF precursor protein, represents the predominant form in the brain. it is released following nerve stimulation [121] and notably, pro-NGF levels increase in both aged and AD brains [122]. Pro-NGF has a trophic function; it can dose-dependently induce neuritis outgrowth, although with lower sensitivity compared with the mature form [123]. This phenomenon is less pronounced in adult brains and can lead to cell loss in aged brains [124,125]. Pro-NGF has higher affinity for p75 receptor and lower affinity for TrkA receptor compared with the mature form, and its binding to p75 receptor can mediate pro-apoptotic signaling; indeed the levels of p75 and pro-NGF increase following neuronal injury [126]. The interaction with p75 requires sortilin, a protein known to promote high-affinity binding site for pro-NGF and interestingly, sortilin levels increase in ageing brains [127].
The maturation of pro-NGF occurs both in the trans-Golgi network via furin cleavage and in the extracellular space after releasing. Alteration in pro-NGF processing and production is considered one of the early factors predisposing to AD. In this regard, a decrease in plasmin levels paralleled to an increase in MMP-9 levels, enzymes required respectively for extracellular pro-NGF maturation and degradation, induce accumulation of pro-NGF as described in AD brains [128,129]. Interestingly, these results were subsequently reproduced by injecting Aβ oligomers in mouse hippocampus [130]. Recently, a clear correlation between NGF metabolism and brain atrophy was shown by chronic inhibition of pro-NGF extracellular maturation in the medial prefrontal cortex (mPFC) of normal adult rats [130]. This inhibition produced local atrophy of the cortical cholinergic system followed by cognitive impairment, and notably, the phenotype was reversed by blockage of NGF-degrading enzyme [131]. Moreover, impairment in pro-NGF processing in Golgi has been recently discovered in transgenic mice expressing a furin cleavage-resistant form of pro-NGF. This study provides a strong and direct proof for a causal link among alterations of pro-NGF/NGF levels, excitatory/inhibitory synaptic imbalance and APP metabolism disruption [132].
An interesting insight into the role of NGF metabolism in AD came using the AD11 mouse model. As previously mentioned this mouse produces NGF recombinant antibodies (mAbαD11) and develops an AD-like neurodegenerative phenotype [113,114]. This phenotype is rescued by injecting NGF, a result that was shown to be related to the preferential binding of mAbαD11 to pro-NGF [130]. This mechanism reproduces the disruption of NGF replace with: metabolism promoting the activation of the apoptotic pathway by pro-NGF binding to p75 and sortilin [133,134]. This hypothesis was differently confirmed by crossing p75 null mice with AD11 mice where the lack of p75 completely reverted the AD-like phenotype. This result pointed on the key role of p75 in mediating neuronal toxicity through the binding to pro-NGF [133,134].
Further interesting insights on the role of pro-NGF/NGF in AD came from a study of Mufson et al. carried out in post mortem AD tissues at different stages of neurodegeneration [135]. The authors suggested that the alterations in the hippocampal NGF signaling pathways from patients with mild cognitive impairment (MCI) and with AD correlated with the increase of pro-NGF levels and activity, but not with Aβ accumulation and deposit [136]. Remarkably, a unconventional role of pro-NGF was reported by La Rosa et al. [137]; the authors showed that exogenous NGF, as well as pro-NGF, recovered LTP deficits from APP-null mice, probably through the p75-mediated activation of JNK pathway.
NGF and APP: Two molecules, one fate
A large amount of in vivo and in vitro data on the molecular link between NGF and APP has been provided by the Cattaneo and Calissano groups. Specifically, substantial interest came from the AD11 (anti-NGF) model, where the lack of NGF signaling and production resulted in a progressive AD-like neurodegeneration [108,109].
Defects in NGF receptor activity or expressions have been largely related to AD. In this regard, cortical TrkA protein expression was shown to be decreased, while p75 protein levels remained stable during the progression of AD [138]; furthermore TrkA gene expression levels were shown to be down regulated in end-stage AD patients [139]. Some authors suggested that the reduction in cortical TrkA levels, which occurs at the onset of AD, can result in an increase in pro-apoptotic p75 signaling in cholinergic neurons [127]. Other studies demonstrated that overexpression of p75 in cell lines might confer sensitivity to Aβ induced toxicity [140-143] and Aβ binding to p75 receptor in hippocampal and cortical neurons can activate downstream toxicity signalling [140,144-154], such as the transcription of c-jun mRNA, and stimulate stress-activated JNK [144]. Chemical inhibitors of JNK were shown to exert protective effects against p75-dependent neurotoxicity induced by Aβ in neuronal hybrid cells [155]. Aβ can also activate p38, JNK, and the translocation of NF-κB through the intracellular domain of the p75 receptor, mediating neuronal death via the p53 pathway [156]. Sotthibundhu et al reported a clear correlation between the increase in extracellular Aβ and the intracellular accumulation of the C-terminal peptides of the p75 receptor [157], and an increasing number of authors pointed on the role of p75 in promoting Aβ toxicity both in vitro and in vivo animal model of disease [158,159] (Figure 2).
Figure 2: NGF signalling impairment
When NGF signaling is impaired or blocked, TrkA undergoes an anomalous phosphorylation, probably mediated by CDK5 and Src kinases. Under this condition, APP is cleaved by β and γ-secretases to generate intra and extracellular accumulation of Aβ. Aβ peptides in turn trigger p75 receptor proteolytical processing producing p75-CTF and p75-ICD fragments.
In particular, an imbalance in α-, β-, and γ-secretase activity, which in turn enhances APP and p75 processing, may result in an increase in Aβ peptides and in the activation of downstream pathways leading to neuronal death.[160-163].
Intriguingly, a study by Matrone et al showed an “anomalous” NGF-independent TrkA phosphorylation in hippocampal neurons deprived of NGF for 24hrs [154]. The inhibition of TrkA phosphorylation prevented neuronal death and appeared to reduce Aβ toxicity in neurons. Moreover, neurons lacking p75 receptor resulted protected from NGF deprivation and from Aβ exposure and did not show NGF-independent TrkA phosphorylation. Matrone et al hypothesized a complex mechanism of regulation between APP and NGF, in which Aβ may switch TrkA function as from prosurvival to pro-death neuronal factor. The conversion appears to be dependent on β- and γ-secretase activation, and on the accumulation of APP and p75 intracellular peptides (Figure 2) [154]. According to this model, p75 C-terminal peptides may bind Aβ and affect TrkA signaling. These results also suggest a direct interaction between TrkA, p75, and Aβ and a possible role for the multiprotein TrkA-p75 complex in mediating apoptotic signaling under specific conditions [154,161,163]. Similarly, Harel et al. reported a pro-death role for TrkA in paediatric tumour cells of neural origin. This toxicity requires the interaction between activated TrkA and the protein product of the cerebral cavernous malformation 2 genes (CCM2) but it was not related to p75 expression and/or activity [164]. Altogether, these studies suggest that TrkA has the potential to switch from prosurvival to pro-death activity in some circumstances, but the precise mechanism and the intracellular inputs responsible for such switch require further investigation.
This crosstalk between TrkA and APP/Aβ activity was further investigated by using a new experimental model of neurodegeneration and dementia developed and characterized by the D’Adamio group at Albert Einstein University [165-169].
APPY682G mouse model
The sequence of the APP C-terminal domain contains Tyr residues (Y) at position 653 and a YENPTY motif that spans from amino acid 682 to amino acid 687 (referring to APP 695 numbering). YENPTY is a typical motif present in many Tyr-kinase (TK) receptors and non-receptor TKs. It is generally phosphorylated and represents the docking site for multiple interacting proteins [170]. According to the Y682 phosphorylation state, the YENPTY motif can bind different proteins and mediate different signaling pathways. Some proteins, such as Grb2 [171,172], Shc [171,173], Grb7, and Crk [174], interact with APP only when Y682 is phosphorylated; others, like Fe65, Fe65L1, and Fe65L2, interact with APP only when this tyrosine is not phosphorylated (Figure 3) [174]. Initial studies on the role of Y682, in neuronal degeneration and decline came from mutant APPY682G mice. The replacement of a tyrosine in position 682 by a glycine prevented phosphorylation and leaded to an unexpected shift in APP processing, with an accumulation of sAPPα as well as of APP intracellular peptides [166] (Figure 3). Moreover, Y682G mutation introduced on an APLP2 null background did not rescue early postnatal lethality or neuromuscular synaptic defects present in APLP2 null mice, suggesting a key role of Y682 in development or aging [165].
Figure 3: The impact of YENPTY domain on NGF signalling
A) NGF binding to TrkA receptor induces Tyr (Y) phosphorylation and APP/TrkA protein interaction. This phosphorylation generates a docking site for specific families of protein, such as Shc, Grb2 and Grb7. B) The absence of NGF signalling, hampers APP phosphorylation and changes APP binding partners (Fe65, F65L1, Fe65L2). These interactions modulate APP trafficking and processing, redistributing both APP and TrkA within the neurons. C) The absence of Y phosphorylation hampers APP and TrkA association resulting in TrkA perinuclear accumulation and redistributing APP towards the nonamyloidogenic pathway with accumulation of sAPPα and AID peptides.
Matrone et al. hypothesized that NGF may modulate APP signaling by promoting the phosphorylation of tyrosine residues on the APP C-terminal domain [167]. In vivo experiments performed on hippocampal slices demonstrated that NGF is able to phosphorylate APP on tyrosine residues through TrkA receptor activation, and such phosphorylation required the binding between TrkA and APP [167]. When these researchers attempted to reproduce the same experiments in APPY682G knock-in mice, NGF fails to phosphorylate its receptor, and TrkA did not bind APP [168]. The lack of the interaction between APP and TrkA was explained as due to an intracellular re-distribution of APP and TrkA receptor in neurons. APP as well as TrkA staining from APPY682G neurons appeared decreased along the neuritis with an aberrant accumulation in the perinuclear and intracellular compartments (Figure 3) [167]. Moreover, an alteration in the lysosomal morphology and in the number of Lamp1 positive vesicles from APPY682G neurons, suggested that Y682 mutation may lead to a defect in the autophagy pathway [166,175]. The dorsal root ganglia from Y682G mice, which require NGF to grow and differentiate in vitro, largely died few days after plating providing strong evidence of the intricate interplay between these two membrane proteins. Mutations of the Y682 residue of APP caused marked cholinergic neurodegeneration in mice and a reduction of TrkA receptor expression and activity [168], reminiscent with those observed from AD mice model of disease. Interestingly, cholinergic and neurotrophic support deficits appeared to be related to the premature aging-dependent decline in learning, cognitive and neuromuscular performance (Figure 4). Beside the mechanism by which Y682G mutation leads to cognitive and neuromuscular performance deficits in APPY682G mice is not yet fully understood, an intriguing possibility is that the lack of the binding of APP to adaptor proteins may impact APP sorting and processing, by contributing to the neuronal decline observed in APPY682G mice (Figure 3). Furthermore, the blockage of NGF binding to APP may affect NGF/TrkA signaling and induce a decline in cholinergic tone, leading in turn to the deficits in locomotor, muscular, and cognitive performance described in the mutant mice [168].
Figure 4: APPY682G Phenotype
APP Y682G knock-in (KI) mice, in which a tyrosine residue in position 682 is replaced by a glycine (Y682G), show an early and massive dendritic spine loss that seems to be associated to an age-dependent cognitive and learning decline and to a progressive reduction of muscular strength, and motor learning performances. Consistently to an AD like phenotype, APPY682G mice show reduced TrkA expression levels in hippocampus and septum and a significant loss in cholinergic tone. Finally Y682G mutation leads to development defects when introduced in APLP2 null background.

Conclusion

In this review we provided evidence on the crucial role of Y682 phosphorylation in the regulation of APP signaling and metabolism, its effect on neuronal degeneration and cognitive decline and ultimately its prominent involvement in AD pathogenesis. Further studies are necessary to identify proteins interacting with the YENPTY domain that may be able to influence APP trafficking and that may acquire a therapeutic interest as prognostic or diagnostic tools in humans.

Acknowledgments

We are grateful to Professor M.S. Nielsen, Prof PH Jensen, Prof. Dr. TF Outeiro and MIND Center for supporting our research. We thank BioMed Proofreading LLC for English editing.
This paper was supported by the Lunbeck Foundation (R108-A10719 and R151-2013-14806) to C.M. E.B. is supported by EC Framework 7 Marie Curie Fellowship Training Network Grant (NEURASYNC).

References
















































































































































































Track Your Manuscript