Aggregates of RNA Binding Proteins and ER Chaperones Linked to Exosomes in Granulovacuolar Degeneration of the Alzheimer’s Disease Brain

Abstract

Granulovacuolar degeneration (GVD) occurs in Alzheimer's disease (AD) brain due to compromised autophagy. Endoplasmic reticulum (ER) function and RNA binding protein (RBP) homeostasis regulate autophagy. We observed that the ER chaperones Glucose -regulated protein, 78 KDa (GRP78/BiP), Sigma receptor 1 (SigR1), and Vesicle-associated membrane protein associated protein B (VAPB) were elevated in many AD patients' subicular neurons. However, those neurons which were affected by GVD showed lower chaperone levels, and there was only minor co-localization of chaperones with GVD bodies (GVBs), suggesting that neurons lacking sufficient chaperone-mediated proteostasis enter the GVD pathway. Consistent with this notion, granular, incipient pTau aggregates in human AD and pR5 tau transgenic mouse neurons were regularly co-localized with increased chaperone immunoreactivity, whereas neurons with mature neurofibrillary tangles lacked both the chaperone buildup and significant GVD. On the other hand, APP/PS1 (APPswe/PSEN1dE9) transgenic mouse hippocampal neurons that are devoid of pTau accumulation displayed only few GVBs-like vesicles, which were still accompanied by prominent chaperone buildup. Identifying a potential trigger for GVD, we found cytoplasmic accumulations of RBPs including Matrin 3 and FUS as well as stress granules in GVBs of AD patient and pR5 mouse neurons. Interestingly, we observed that GVBs containing aggregated pTau and pTDP-43 were consistently co-localized with the exosomal marker Flotillin 1 in both AD and pR5 mice. In contrast, intraneuronal 82E1-immunoreactive amyloid-␤ in human AD and APP/PS1 mice only rarely co-localized with Flotillin 1-positive exosomal vesicles. We conclude that altered chaperone-mediated ER protein homeostasis and impaired autophagy manifesting in GVD are linked to both pTau and RBP accumulation and that some GVBs might be targeted to exocytosis.

Untangling the origin and function of granulovacuolar degeneration bodies in neurodegenerative proteinopathies

“…Furthermore, a higher GVB load is found in bigenic TAPP mice that express both human tau P301L and human mutant amyloid precursor protein (APP) K670M/N671L (APP Sw ) compared to JNPL3 mice carrying only the human tau P301L transgene [54,78] (Table 2). This higher GVB frequency is likely related to the exacerbated tau pathology in TAPP mice rather than the comorbid Aβ pathology, as GVBs are absent in mouse models of pure Aβ amyloidosis by overexpression of APP Sw (Tg2576), APP Sw,Ind (J20), APP Sw/L /PS1 M146L or APP Sw /PS1 dE9 [54,67,90] although the recent detection of granular structures with an immunogenicity profile that partially overlaps with GVBs in APP Sw /PS1 dE9 mice requires further investigation [148]. Taken together, the data indicate that neither in human nor in Tg mouse brain, GVBs and extracellular Aβ pathology are associated.…”

“…A few studies have reported GVBs in tau transgenic (Tg) mouse models. GVBs are found in the brains of aged tau Tg mice expressing human mutant tau – more specifically, in mice from the strains JNPL3 [ 54 , 78 ] and pR5 [ 64 , 66 , 67 , 90 , 148 ] expressing human tau P301L (Table 2 ). In contrast, GVBs are not or only rarely found in age-matched non-Tg littermates [ 54 , 64 , 66 , 67 , 90 ].…”

“…The Thal GVD neuropathological staging system [132] is based on immunopositivity for CK1δ and CK1ε and in addition pTDP-43 [59]. pTDP-43 localizes to structures that morphologically resemble GVBs in the human [59,79,132] and mouse brain [148], but has so far not been tested in the in vitro GVB model. pTDP-43 staining in tissue overlaps with CK1δ-and CK1ε-immunolabeled GVBs [132,148], although the co-localization was not quantified.…”

“…pTDP-43 localizes to structures that morphologically resemble GVBs in the human [59,79,132] and mouse brain [148], but has so far not been tested in the in vitro GVB model. pTDP-43 staining in tissue overlaps with CK1δ-and CK1ε-immunolabeled GVBs [132,148], although the co-localization was not quantified. However, in the human brain the distribution pattern of GVBs is similar when determining the Thal GVD stage using CK1δ, CK1ε or pTDP-43 [132].…”

“…1 a-d) and CK1ε [ 33 , 67 , 143 ], CHMP2B [ 90 , 143 , 147 ], pPERK (Fig. 1 c-f), peIF2α and pIRE1α [ 43 , 67 , 143 ] and pTDP-43 [ 47 , 59 , 79 , 148 ]. In Supplementary Table 1 , an overview of primary antibodies used to detect these common GVB markers in tissue and culture is provided.…”