Early etiology of Alzheimer’s disease: tipping the balance toward autophagy or endosomal dysfunction?

The presumed pathogenicity of extracellular Aβ is based on the fact that these aggregation-prone peptides are secreted into the external environment where plaques are found [133]. In this case, amyloidogenic processing of APP would result in Aβ liberation and its consequent spontaneous self-aggregation into amyloid forms of higher order, including Aβ fibrils, which precipitate in APs. Insoluble APs, however, correlate poorly with neuronal loss [45] and dementia [5], and nowadays their Aβ oligomeric precursors are considered to be more neurotoxic [160]. In AD, these soluble Aβ forms associate better with disease severity [88] and dementia [87], and display stronger correlation with synaptic loss [79]. Here, their excessive binding to synaptic membranes/receptors is believed to underlie consequent cognitive deficits [94]; however, whether all aspects of Aβ toxicity are exclusively mediated via their pathogenic influences from the extracellular environment remains an intriguing question (Fig. 1b, c).

To this end, in recent years, there is a growing awareness that the intraneuronal pool of Aβ as well may be detrimental in AD. In an APP-based murine AD model, in which enhanced oligomerization of Aβ occurs without its fibrillization, it is noted that the time of initial deterioration of synaptic function and memory coincides with intraneuronal Aβ accumulation [150]. Also, in 3xTg-AD and arcAβ mice, accumulation of intraneuronal Aβ correlates with early cognitive deficits, preceding the appearance of APs [11, 65]. In APP(SL)/PS1KI mice, in turn, internal pile up of Aβ, rather than its external deposition in APs, associates with neuronal loss [23]. In primary neurons and brains of Tg2576-AD APP mutant mice as well as in human AD brains, intracellular Aβ42 accumulates and oligomerizes within late endosomal multivesicular bodies (MVBs) of neuronal processes and synaptic compartments, where its deposition associates with their morphological abnormalization [144]. In line, in both AD and Down syndrome patients (that invariably develop features of AD neuropathology due to trisomy of chromosome 21 and thus an extra APP copy) early rises in Aβ coincide with its deposition within abnormally enlarged neuronal endosomes [19]. Dystrophic neurites and synaptic terminals in AD also prominently accumulate AVs [101], which not only can facilitate the Aβ clearance [62], but also may be an important source of amyloidogenic activity [172]. Together, this implies that part of the pathogenic mechanisms in AD could also affect the homeostasis of the cellular interior, where internalized and/or internally produced Aβ may be a factor of synapto/neurotoxicity (Fig. 1c).

Indeed, increasing evidence suggests that in neurons/cells amyloidogenic processing of APP preferentially occurs internally, e.g., within the biochemically optimal endosomal compartments, where specific protein trafficking/sorting regulators can impact the Aβ production [113, 125]. In this respect, in AD context, the role of retromer and its accessory proteins is becoming prominent. This heteropentameric adaptor protein complex, comprising two subcomplexes (the VPS35/26/29 trimer and hetero/homodimer of sorting nexins (SNXs)), facilitates protein cargo recognition and transport from endosomes to the trans Golgi network (TGN) or plasma membrane [131]. While this is primarily important in the maintenance of an active pool of lysosomal hydrolase receptors in TGN, retromer-mediated sorting also controls intracellular shuttling of other proteins, including Aβ yielding APP and BACE1 [113, 162, 163]. Notably, in AD-vulnerable entorhinal cortex, the levels of both VPS35 and VPS26 are specifically decreased [135]. The importance of this AD-related change is underscored by findings demonstrating that hemizygous deletion of VPS35 in Tg2576 mice elevates the hippocampal Aβ levels and exacerbates the AD pathology [162]. There is also strong evidence that APP sorting receptor SORLA/LR11/SORL1 (sortilin-related receptor), genetically linked to AD, is as well decreased and/or dysfunctional in this disorder [163]. SORLA controls the intracellular APP trafficking by associating with the retromer and other sorting adaptors [163]. Like in the case of VPS35, its deficiency has been implicated in enhanced Aβ production and AD pathogenesis [34]. Although there is still no clear consensus with respect to how exactly retromer-mediated intracellular sorting of APP and BACE1 affects the Aβ production, a currently prevailing idea postulates that retromer dysfunction may disrupt normal trafficking of APP and BACE1, thereby increasing the likelihood of their physical co-sequestration in endosomal vesicles, and thus promote amyloidogenesis (for a review see [113, 163]). In line, other retromer-associated sorting receptors genetically linked to increased AD risk, like, e.g., SORCS1, may play a similar role [68]. Interestingly, in addition to APP, SORLA via its N-terminal VPS10P domain may also directly bind Aβ and thus regulate its trafficking to lysosomes for degradation. This function is disturbed by an FAD mutation in SORL1, which compromises its interaction with Aβ [17]. Endosomal sorting defects related to decreased levels of phosphatidylinositol-3-phosphate (PI3P), required for proper functioning of retromer and other sorting regulators, were recently also associated with AD and shown to underlie aberrant amyloidogenic processing [93]. Finally, intracellular sequestration of cholesterol, a relevant risk factor in AD [13], can as well promote abnormal endosomal trafficking/activity of amyloidogenic proteins, thus enhancing Aβ production [81, 119, 167].

In addition to being produced within intracellular compartments, Aβ can also disturb their normal functioning. To this end, recent genome-wide genetic screen in yeast, accompanied by congruent findings in nematode glutamatergic and rat primary cortical neurons [151], identified several endocytic regulators as modifiers of Aβ42 toxicity. This among others included the ortholog of human PICALM (phosphatidylinositol binding clathrin assembly protein), a regulator of clathrin-mediated endocytic trafficking and one of the most well-established SAD risk factors [151]. The importance of endosomally localized Aβ indeed cannot be underestimated, because a failure in either degrading or secreting it may increase its local concentration within these acidic compartments, thus facilitating its oligomerization and subsequent pathogenic processes [41, 51, 144]. Accordingly, AD-associated APP “Arctic” mutation (APP_arc; E693G) favors the formation of soluble Aβ protofibrils as well as the intracellular amyloidogenic APP processing [99, 122]. Pile up of oligomeric Aβ42 in endosomes in turn may hamper cholesterol efflux from these compartments [91], thus creating a vicious cycle of self-propelling endosomal dysfunction and excessive Aβ production (see above and Fig. 1c). Localized to endosomes, Aβ42 may also affect their sorting capacity, thereby causing degradation and signaling defects [1]. Importantly, excessive levels of oligomerized Aβ42 may even compromise the physical integrity (impermeability) of endolysosomal–autophagic compartments [31, 41, 74, 144, 155, 169]. For instance, an in vivo overexpression of Aβ42 in fruit fly (Drosophila melanogaster) neurons causes progressive impairment of their degradative capacity and buildup of increasingly dysfunctional AVs [74]. Here, at early stages, AVs are protective and contribute to Aβ42 elimination, but as toxic burden increases, impaired degradation and leakage of lysosomal proteins from abnormal AVs promote neurodegeneration [74]. Taken together, intracellular Aβ accumulation may be both a cause and a consequence of an endolysosomal–autophagic dysfunction in AD (Figs. 1c, 4).

NPC is an autosomal recessive lysosomal storage disorder, caused by mutations in NPC1 or NPC2 genes and characterized by late endosomal/lysosomal accumulation of several lipid species, including unesterified cholesterol [156]. Interestingly, this fatal neurodegenerative disease displays some intriguing parallels to AD with respect to certain aspects of cellular pathology. In addition to similar exacerbating influence of cholesterol and intracellular Aβ42 deposition in endosomes [58], this also includes pronounced endolysosomal–autophagic abnormalities [36, 58, 73] as well as aberrant tau phosphorylation [78, 128]. Because neither in AD nor NPC tau is mutated, its abnormal phosphorylation in the context of these diseases therefore likely results from deregulated levels/activity of tau kinases and/or phosphatases. To this end, it is noteworthy that many of these enzymes (reviewed in [82, 83] and summarized in Table 1) take part in various cellular signaling cascades, in which endosomal membranes play an important regulatory function as signaling platforms [90, 108]. Because a similar role in cellular signaling control has recently also been ascribed to certain autophagy regulators and autophagosomal membranes [84, 85], it seems plausible to assume that anomalies in endolysosomal–autophagic system, common to both NPC and AD, may at least in part explain the aberrant activity of enzymes regulating tau phosphorylation. Notably, abnormal functioning of the endolysosomal–autophagic system, in addition, may contribute to pile up of toxic tau species by hampering their clearance (as their turnover relies on autophagy and lysosomal function) [109]. Accordingly, autophagy deficits have recently been directly implicated in tau phosphorylation and related neurodegeneration [52]. Moreover, intracellular accumulation of oligomeric Aβ, known to promote endolysosomal–autophagic defects (see above), coincides with early rises in abnormal tau phosphorylation in an APP-based AD model with endogenous (unmutated) tau, long before APs are formed [150]. Overall, this strongly supports the pathogenic relevance of disturbed intracellular trafficking/degradative homeostasis in AD in general and tau pathology in particular (Fig. 1c). Additional evidence reinforcing this concept comes from studies demonstrating that AD-related and γ-secretase-associated PSENs, may independent of their proteolytic function affect the endolysosomal–autophagic system as well as tau phosphorylation.

PSENs are primarily known to be catalytic components of the γ-secretase complex that—besides APP—cleaves many other type I transmembrane proteins, thus taking part in a wide range of cellular processes [28, 60]. However, other functions, distinct from their role in intramembrane proteolysis have been attributed to the PSENs as well, including cellular signaling, intracellular Ca²⁺ homeostasis, endolysosomal trafficking and autophagy (Fig. 2). Particularly interesting seems that, in both in vitro and in vivo settings, using various cell types, primary neuronal cultures as well as murine brain samples, PSEN deficiency was shown to result in endolysosomal–autophagic abnormalities [33, 37, 69, 97, 164]. For instance, in adult murine brain neurons, with both PSEN isoforms genetically ablated, such defects occur very early (already at 2–3 months after birth) [69]. Around the same time these mice start having synaptic and memory deficits, which worsen with age due to progressive neurodegenerative alterations, accompanied by aberrant tau phosphorylation [127], implying an important role of PSENs in all these phenomena. Additional evidence indicates that much like lack of PSENs, also certain of their FAD-linked mutations can alter the intracellular signaling, leading to pathological tau phosphorylation [7] and in vitro degeneration of primary neurons [6], in a manner independent of their catalytic activity [6, 7]. FAD-related PSEN mutations also associate with pronounced lysosomal neuropathology in AD neurons [20], which based on evidence from FAD patient fibroblasts may compromise their degradative function [69]. In line, other studies demonstrate that some of the PSEN1 FAD mutants, unlike wild-type human PSEN1 (hPSEN1) and its catalytically inactive forms, are unable to fully rescue altered Wnt signaling in PSEN-deficient cells [33] or completely alleviate defective epidermal growth factor receptor (EGFR) turnover in lysosomes, in a similar context [116]. Together, all these studies suggest that FAD-associated PSEN mutations, in addition to altering the catalytic activity of this protein, may also contribute to disease progression via additional loss of other functions. As PSEN-dependent endolysosomal–autophagic and signaling phenotypes are even more pronounced when PSENs are lacking [33, 69, 116], their overall decreased levels, per se, may also be important. In support, polymorphisms are found in the PSEN1 promotor sequence that repress transcription of PSEN1 and associate with both increased risk for AD and elevated total Aβ load [146]. Progressive lowering of PSEN1 expression in vitro, paralleled by concomitant gradual increase in Aβ42 levels (observed in another study) [115], accordingly reinforces the assumption that such mechanisms are possible and potentially relevant, also in amyloidogenesis. To this end, structural (conformational) changes in PSEN1, similar to those observed in some of its FAD mutants, were recently shown to occur in relation to SAD and aging, wherein they were proposed to underlie pathogenic amyloidogenesis [159]. Based on this, it seems tempting to speculate that in AD, altered levels, structure and activity of PSEN proteins may all be relevantly important and that their catalytic function may work together with γ-secretase-independent roles in disease promotion.

One of the first clear indications of a γ-secretase-independent role of PSEN1 in endolysosomal–autophagic system emerged from the identification of its interaction with ICAM-5 (also named telencephalin) [3]. This forebrain-specific neuronal intercellular cell adhesion molecule with an exclusive somatodendritic localization is, despite its type I transmembrane topology, not a γ-secretase substrate [37]. Instead, in PSEN1^−/− primary hippocampal neurons, ICAM-5 accumulates intracellularly in degradative organelles that are not acidified, but label positively for certain autophagic markers [37]. While these accumulations also occur in wild-type (WT) neurons, PSEN1 deficiency clearly leads to their earlier and more abundant appearance [37, 111]. Accumulation of similar degradative organelles was also noted in another study, where PSEN1 deficiency in neurons was shown to lead to a pronounced α- and β-synuclein intracellular accumulation [164]. Later, Lee and co-workers supported these seminal observations by identifying extensive accumulations of autophagic compartments in PSEN1-deficient cells, as well as in the brains of PSEN1 hypomorph or conditional knockout mice [69]. Importantly, in addition to extensive accumulation of AVs, PSEN-deficient cells were also shown to have significantly more late endosomal MVBs [33]. Although all studies agree that the observed phenomena relate to an impaired turnover of endosomal/autophagic cargo, whether lysosomal degradation per se or their fusion capacity is compromised, remains debated. To explain these deficits, two major hypotheses have been put forth, including defective lysosomal acidification and Ca²⁺ homeostasis (Fig. 3).

Lee et al. suggested a model whereby PSEN1 deficiency/FAD-related mutations would compromise a proclaimed role of endoplasmic reticulum (ER)-localized full-length PSEN1 as a critical co-factor of the oligosaccharyltransferase (OST) complex in N-glycosylation of the V0a1 subunit of the vacuolar ATPase (v-ATPase; proton pump). Allegedly, this should hamper the targeting of V0a1 to lysosomes, thus compromising the proton pump function and consequently lysosomal acidification and degradation (by impairing the activity of lysosomal hydrolases) [69]. In contrast, we as well as others failed to reproduce pronounced acidification defects [24, 97, 175], related to it disruption of lysosomal proteolysis and cathepsin D maturation [24, 175] and, critically important for the original hypothesis, defective N-glycosylation of the V0a1 subunit in PSEN-deficient cells [24, 175]. Using Drosophila melanogaster as a model, we conclusively showed that embryonic lethality caused by the lack of the V0a1 ortholog v100 could be fully rescued by a glycosylation-deficient v100 mutant, underscoring that N-glycosylation is even dispensable for the proper lysosomal targeting and function of this proton pump subunit [24]. Overall, this raises doubt if defective lysosomal acidification and the proposed mechanism, in particular, indeed primarily underlie PSEN-related lysosomal deficits. Alternatively, we originally demonstrated that lysosomal calcium storage/release, which is as well required for lysosomal function and fusion, is compromised in PSEN-deficient cells and neurons [24]; a finding later confirmed by others [96] (Fig. 3).

This phenomenon resembles the situation in NPC cells where a significant reduction in lysosomal calcium storage/release irrespective of acidification defects causes similar endolysosomal dysfunction. The initiating factor here is an aberrant sphingosine storage that instigates altered calcium homeostasis leading to secondary sphingomyelin and cholesterol storage [76]. In line, NPC disease-associated endolysosomal dysfunction can be induced by decreasing and rescued by increasing the Ca²⁺ levels [76]. Rescue of Ca²⁺ defects in PSEN-deficient cells in turn can be achieved by catalytically inactive hPSEN1 [24], underscoring a γ-secretase-independent nature of PSEN-mediated lysosomal Ca²⁺ regulation.

Toward identifying the underlying causes of this lysosomal dysfunction, our recent omics study performed on isolated plasma membranes (PMs) of PSEN-deficient cells already provide first insights. Using a novel isolation procedure, based on superparamagnetic nanoparticles, we compared the biomolecular composition of pure PMs derived from PSEN double knockout (PSENdKO) mouse embryonic fibroblasts (MEFs) vs. their WT and hPSEN1 rescued counterparts [147]. Our experiments revealed a convergent prominent surface depletion of cholesterol along with certain proteins, of which many are constituents of focal adhesion sites, lipid rafts and caveolae (or functionally related to them), in PSEN deficient cells [147]. Several of these molecules are also small GTPases involved in endosomal transport regulation [147]. As these PSEN-dependent surface alterations go along with a decrease in focal adhesion sites and caveolae, intracellular accumulation of cholesterol, caveolin-1 and the integrin-interacting CD47 protein [147, 166], we hypothesize that PSENs may regulate selective intracellular trafficking routes. Among the surface-depleted GTPases in PSENdKO MEFs, several play a role in endosomal recycling, such as RAB10, RAB11 and RAB35. RAB11 is a known interactor of PSENs [35], also recently shown to be of potential relevance to AD and amyloidogenesis [154]. RAB35, on the other hand, has mutually antagonizing roles with ADP-ribosylation factor 6 (ARF6) in endocytosis and endosomal sorting/recycling, important in regulation of cellular adhesion, migration, phagocytosis, cytokinesis and neurite outgrowth [21]. Interestingly, ARF6 has recently been found to be functionally linked as well to both APP processing and PSEN1 interactors. First, we showed that ARF6 plays a role in surface to endosome transport of BACE1 via the clathrin-independent internalization route, thereby keeping this amyloidogenic sheddase spatially separated from APP (the internalization of which is primarily clathrin dependent) until they meet in early endosomes [126]. Here, affecting ARF6 activity or expression inversely impacted on APP processing and Aβ production, thereby underscoring the important role of sorting/recycling regulation mediated by ARF6 in amyloidogenesis [126]. Second, and related to PSEN1, ARF6 also regulates the surface expression and later endosomal routing of the PSEN1 interactor ICAM-5 (which accumulates intracellularly in PSEN1^−/− neurons; see above) [37, 111]. While the exact role of ARF6 in these PSEN-related trafficking defects remains to be elucidated, it seems interesting that NPC-associated aberrant cholesterol efflux from endosomes can be induced by blocking and rescued by promoting ARF6-mediated recycling [130]. Aberrant cholesterol efflux in NPC also affects amyloidogenic APP processing by trapping APP and BACE-1 in the same endosomal compartments [81], thus further extending parallels to ARF6-mediated transport regulation in amyloidogenesis. Based on these studies it seems tempting to speculate that a defect in selective protein/membrane routing (to recycling and/or degradation) may contribute to or even underlie the endolysosomal–autophagic dysfunction observed in relation to PSEN deficiency and/or different PSEN FAD mutants (Fig. 3). As similar deficits are also noted in SAD, analogous pathogenic mechanisms, disturbing specific trafficking/degradative processes, may also operate in late-onset AD. Note, however, that depending on the context (FAD vs. SAD) the primary pathogenic factors may differ. To provide further support for this concept, in the following paragraphs we will highlight the known functional links between the autophagy and the endolysosomal trafficking regulators, which may be of potential pathogenic significance in AD.

These mechanisms may be of great importance in AD, as decreased PI3P, Vps34 and beclin 1 levels have all been reported [57, 93, 107]. Importantly, as genetic ablation of beclin 1 or Vps34 results in reduced levels of both Atg14L and UVRAG [53, 148], these changes in addition to compromising autophagy as well may affect endolysosomal trafficking [148]. Indeed, the phenotypes of beclin 1- and Vps34-deficient mice are more severe (E7.5–8.5; lethal) [173, 176] than that of other autophagy-related genes, such as Atg3 [136], Atg5 [67] and Atg16L [124] (P1; neonatal lethal). This suggests additional, autophagy-independent cellular roles of beclin 1 and Vps34, some of which likely relate to their endolysosomal trafficking function.

In line, Vps34 deficiency in sensory neurons leads to rapid neurodegeneration, primarily resulting from disruption of the endosomal and not the autophagic pathway [177]. Similarly, Vps34 downregulation in primary cortical neurons results in impaired endosomal sorting and consequent endosomal swelling [93].

Also in relation to beclin 1, recent studies demonstrate that this protein may not be exclusively involved in autophagy initiation, as originally proposed [174], but as well could play a role in endocytic trafficking regulation. For instance, beclin 1, Vps34, Vps15 and UVRAG were all shown to take part in trafficking of membrane receptors toward lysosomes [148]. Moreover, in C. elegans both beclin 1 ortholog (BEC-1) and Vps34 are pivotal in retrograde retromer-mediated endocytic sorting toward the TGN [118]. Finally, a similar role in endocytic trafficking for beclin1 has also been demonstrated in Drosophila [134].

In beclin 1^+/− mice also lysosomal abnormalities are noted, while their crossing with an APP-based AD mouse model results in higher intraneuronal and extraneuronal Aβ levels, more profound ultrastructural defects, including more severe lysosomal/autophagic abnormalities as well as increased neurodegeneration [107]. Beclin 1 overexpression in the same AD model reduces intraneuronal Aβ and extracellular plaque pathology [107], thus underscoring that turnover and/or excessive production of toxic Aβ peptides may rely on beclin 1-mediated functions. To this end, in the follow-up study, the authors show that beclin 1 transient downregulation in parallel to increasing the Aβ secretion also causes intracellular accumulation of APP and APP-CTFs [57]. Interestingly however, in beclin 1 silenced cells and in AD brains where beclin 1 and Vps34 are reduced, higher levels of autophagosomal marker LC3-II (the lipidated form of the microtubule-associated protein 1 light chain 3) are also observed [57]. As this indicates boosted (not hampered) autophagy, which would be expected should beclin 1 exclusively regulate autophagy initiation, to explain their findings Jaeger and colleagues proposed that beclin 1 may also regulate later maturation steps of autophagy. Here, defective beclin 1-dependent clearance of autophagic compartments would explain the increased levels of APP metabolites and Aβ. These findings are also consistent with a primary defect in the endolysosomal trafficking pathway: in the study of Jaeger et al., lowered beclin 1 levels parallel those of Vps34 not only in AD, but also in cells where either of these two proteins is silenced [57]. This is important, as in AD lowered levels of Vps34 and its product PI3P may disturb normal endosomal sorting of APP, thus causing its enhanced amyloidogenic processing [93]. As decreased PI3P levels also directly impact functioning of the endosomal sorting complex required for transport (ESCRT) [93], which is pivotal in MVB maturation as well as fusion of AVs with the endolysosomal system and consequent cargo degradation [121], a primary defect in the endolysosomal system provides an alternative explanation for the observed effects by Jaeger and collaegues [57].

In line, although AVs were implied as an important source of amyloidogenic activity in neurons [172], more recently Boland et al. contested this view by showing a more likely primary role of the endolysosomal system in APP processing [15]. Despite these conceptual disparities, both studies, however, largely agree with respect to the contribution of disturbed lysosomal degradation in these phenomena [15, 172]. Accordingly, in TgCRND8 AD mice, improved lysosomal degradative function, achieved through genetic ablation of endogenous lysosomal cysteine protease inhibitor cystatin B, rescues autophagic/lysosomal dysfunction and amyloid pathology, as well as related memory and cognitive deficits [170].

Taken together, all these findings further strengthen the important role of the endolysosomal system in amyloidogenesis and balanced autophagic degradation (Fig. 4).

Endolysosomal and autophagy dysfunctions in AD may also be linked via an underappreciated pathomorphological feature of this disease, namely the granulovacuolar degeneration (GVD) bodies. These intracellular, double membrane-bound organelles, with electron-dense core granules [105], occur in relation to aging and different neurodegenerative disorders, but only in AD do these autophagic-like structures [43] disseminate in an orderly hierarchical pattern, which correlates with distribution of several disease progression markers and the degree of dementia [145].

Interestingly, GVD bodies stain positively for the charged multi-vesicular body protein 2B (CHMP2B) [43]. This subunit of the ESCRT-III protein complex, involved in intraluminal vesicle (ILV) sorting in MVBs, is important in successful autophagic degradation, as CHMP2B mutations, found in a subset of FTD and amyotrophic later sclerosis (ALS) patients, compromise lysosomal degradation of AV cargo [40]. In line, depletion of other ESCRT complex components produces a similar phenotype. Here, autophagosomes and amphisomes seem to be normally formed; however, ESCRT depletion-related sorting defects impair fusion of these AVs with lysosomes and thus degradation in nascent autolysosomes [40]. Based on their resemblance to late-stage AVs, CHMP2B-positive GVD bodies were also proposed to accumulate due to a failure in autolysosome formation [43].

Interestingly, GVD bodies may also link to tau pathology. Accordingly, several prominent tau kinases, such as casein kinase 1 delta (CK1δ) [61], glycogen synthase kinase-3 beta (GSK3β) [71] and cyclin-dependent protein kinase-5 (CDK5) [100], physically associate with GVD bodies, which appear in correlation with the early accumulation of phospho-tau pathology [168]. This provides additional proof for the concept proposed by us that aberrant tau phosphorylation may in fact stem from endolysosomal–autophagic deficits and related cellular signaling and/or degradative abnormalities (see previously and Fig. 1c).

GVD bodies also contain lipid raft marker proteins, such as flotillin-1 [100]. In this respect, we reported that the PSEN1-interactor ICAM-5 associates with flotillin-1 both at the cell surface and within endosomal compartments [111]. Moreover, in “aged” (in vitro) WT primary neurons, ICAM-5 accumulations (similar to those observed in younger PSEN1^−/− neurons [37]), much like GVD bodies, stain positively for flotillin-1 as well as late endosomal MVBs [111]. These convergent pathomorphological and cell biological findings may reflect commonalities in endolysosomal dysfunction that occur in relation to aging and of relevance to AD. Here, underlying transport jamming and/or delayed cargo degradation potentially relates to either declining function (levels) of PSEN1 or another relevant trafficking deficit. Taken together, all this further strengthens the notion that autophagic and endolysosomal trafficking pathways cannot be perceived as autonomous, physically separate entities, but rather as two functional components of an integrated cellular system, the balance of which in AD may become compromised at various levels and in relation to many different factors.