Receptor-associated protein (RAP) plays a central role in modulating Abeta deposition in APP/PS1 transgenic mice

Low to moderate ethanol exposure reduces astrocyte-induced neuroinflammatory signaling and cognitive decline in presymptomatic APP/PS1 mice

Alcohol use disorder has been associated with the development of neurodegenerative diseases, including Alzheimer's disease (AD). However, recent studies demonstrate that moderate alcohol consumption may be protective against dementia and cognitive decline. We examined astrocyte function, low-density lipoprotein (LDL) receptor-related protein 1 (LRP1), and the NF-κB p65 and IKK-α/β signaling pathways in modulating neuroinflammation and amyloid beta (Aβ) deposition. We assessed apolipoprotein E (ApoE) in the brain of APP/PS1 mice using IHC and ELISA in response to low to moderate ethanol exposure (MEE). First, to confirm the intracerebral distribution of ApoE, we co-stained with GFAP, a marker for astrocytes that biosynthesize ApoE. We sought to investigate whether the ethanol-induced upregulation of LRP1 could potentially inhibit the activity of IL-1β and TNF-α induced IKK-α/β towards NF-κB p65, resulting in a reduction of pro-inflammatory cytokines. To evaluate the actual Aβ load in the brains of APP/PS1 mice, we performed with a specific antibody Aβ (Thioflavin S) on both air- and ethanol-exposed groups, subsequently analyzing Aβ levels. We also measured glucose uptake using 18F- fluorodeoxyglucose (FDG)-positron emission tomography (PET). Finally, we investigated whether MEE induced cognitive and memory changes using the Y maze, noble object recognition test, and Morris water maze. Our findings demonstrate that MEE reduced astrocytic glial fibrillary acidic protein (GFAP) and ApoE levels in the cortex and hippocampus in presymptomatic APP/PS1 mice. Interestingly, increased LRP1 protein expression was accompanied by dampening the IKK-α/β-NF-κB p65 pathway, resulting in decreased IL-1β and TNF-α levels in male mice. Notably, female mice show reduced levels of anti-inflammatory cytokines IL-4, and IL-10 without altering IL-1β and TNF-α concentrations. In both males and females, Aβ plaques, a hallmark of AD, were reduced in the cortex and hippocampus of APP/PS1 mice exposed to ethanol starting at pre-symptomatic stage. Consistently, MEE increased FDG-PET-based brain activities and normalized cognitive and memory deficits in the APP/PS1 mice. Our findings suggest that MEE may benefit AD pathology via modulating LRP1 expression, potentially reducing neuroinflammation and attenuating Aβ deposition. Our study implies that reduced astrocyte-derived ApoE and LDL cholesterol levels are critical for attenuating AD pathology.

Somatostatin and the pathophysiology of Alzheimer's disease

Among the central features of Alzheimer's disease (AD) progression are altered levels of the neuropeptide somatostatin (SST), and the colocalisation of SST-positive interneurons (SST-INs) with amyloid-β plaques, leading to cell death. In this theoretical review, I propose a molecular model for the pathogenesis of AD based on SST-IN hypofunction and hyperactivity. Namely, hypofunctional and hyperactive SST-INs struggle to control hyperactivity in medial regions in early stages, leading to axonal Aβ production through excessive presynaptic GABAB inhibition, GABAB1a/APP complex downregulation and internalisation. Concomitantly, excessive SST-14 release accumulates near SST-INs in the form of amyloids, which bind to Aβ to form toxic mixed oligomers. This leads to differential SST-IN death through excitotoxicity, further disinhibition, SST deficits, and increased Aβ release, fibrillation and plaque formation. Aβ plaques, hyperactive networks and SST-IN distributions thereby tightly overlap in the brain. Conversely, chronic stimulation of postsynaptic SST2/4 on gulutamatergic neurons by hyperactive SST-INs promotes intense Mitogen-Activated Protein Kinase (MAPK) p38 activity, leading to somatodendritic p-tau staining and apoptosis/neurodegeneration - in agreement with a near complete overlap between p38 and neurofibrillary tangles. This model is suitable to explain some of the principal risk factors and markers of AD progression, including mitochondrial dysfunction, APOE4 genotype, sex-dependent vulnerability, overactive glial cells, dystrophic neurites, synaptic/spine losses, inter alia. Finally, the model can also shed light on qualitative aspects of AD neuropsychology, especially within the domains of spatial and declarative (episodic, semantic) memory, under an overlying pattern of contextual indiscrimination, ensemble instability, interference and generalisation.

Alzheimer's amyloid-β and tau protein accumulation is associated with decreased expression of the LDL receptor-associated protein in human brain tissue

INTRODUCTION

One of the major neuropathological features of Alzheimer's disease (AD) is the accumulation of amyloid-β (Aβ) protein in the brain. Evidence suggests that the low-density lipoprotein receptor-associated protein (RAP) binds strongly to Aβ and enhances its cellular uptake and that decreased RAP expression correlates with increased Aβ production in animal models of AD.

METHODS

The current study examined whether RAP levels change in AD human brain tissue and whether they are related to the amount of AD pathology. RAP and NeuN levels were determined by Western blot, while low-density lipoprotein receptor-related protein 1 (LRP1), tau and Aβ levels were determined by ELISA in the temporal cortex of 17 AD and 16 control cases.

RESULTS

An increase in total Aβ and insoluble and soluble tau protein was observed in AD brain tissue. In contrast, RAP levels were significantly decreased in AD brain tissue compared to controls. Correlation analysis revealed that levels of RAP correlated with both total Aβ and soluble and insoluble tau levels. Neither LRP1 nor NeuN levels were significantly altered in AD brain tissue homogenates and did not correlate with Aβ or tau protein levels.

CONCLUSION

Reduction in RAP may contribute to the accumulation and aggregation of Aβ in the AD brain.

7B2 chaperone knockout in APP model mice results in reduced plaque burden

Impairment of neuronal proteostasis is a hallmark of Alzheimer's and other neurodegenerative diseases. However, the underlying molecular mechanisms leading to pathogenic protein aggregation, and the role of secretory chaperone proteins in this process, are poorly understood. We have previously shown that the neural-and endocrine-specific secretory chaperone 7B2 potently blocks in vitro fibrillation of Aβ42. To determine whether 7B2 can function as a chaperone in vivo, we measured plaque formation and performed behavioral assays in 7B2-deficient mice in an hAPPswe/PS1dE9 Alzheimer's model mouse background. Surprisingly, immunocytochemical analysis of cortical levels of thioflavin S- and Aβ-reactive plaques showed that APP mice with a partial or complete lack of 7B2 expression exhibited a significantly lower number and burden of thioflavin S-reactive, as well as Aβ-immunoreactive, plaques. However, 7B2 knockout did not affect total brain levels of either soluble or insoluble Aβ. While hAPP model mice performed poorly in the Morris water maze, their brain 7B2 levels did not impact performance. Since 7B2 loss reduced amyloid plaque burden, we conclude that brain 7B2 can impact Aβ disposition in a manner that facilitates plaque formation. These results are reminiscent of prior findings in hAPP model mice lacking the ubiquitous secretory chaperone clusterin.

Neuronal clearance of amyloid-β by endocytic receptor LRP1

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly population. Accumulation, aggregation, and deposition of amyloid-β (Aβ) peptides generated through proteolytic cleavage of amyloid precursor protein (APP) are likely initiating events in the pathogenesis of AD. While Aβ production is accelerated in familial AD, increasing evidence indicates that impaired clearance of Aβ is responsible for late-onset AD. Because Aβ is mainly generated in neurons, these cells are predicted to have the highest risk of encountering Aβ among all cell types in the brain. However, it is still unclear whether they are also involved in Aβ clearance. Here we show that receptor-mediated endocytosis in neurons by the low-density lipoprotein receptor-related protein 1 (LRP1) plays a critical role in brain Aβ clearance. LRP1 is known to be an endocytic receptor for multiple ligands including Aβ. Conditional knock-out of Lrp1 in mouse forebrain neurons leads to increased brain Aβ levels and exacerbated amyloid plaque deposition selectively in the cortex of amyloid model APP/PS1 mice without affecting Aβ production. In vivo microdialysis studies demonstrated that Aβ clearance in brain interstitial fluid is impaired in neuronal Lrp1 knock-out mice. Because the neuronal LRP1-deletion did not affect the mRNA levels of major Aβ degrading enzymes, neprilysin and insulin-degrading enzyme, the disturbed Aβ clearance is likely due to the suppression of LRP1-mediated neuronal Aβ uptake and degradation. Together, our results demonstrate that LRP1 plays an important role in receptor-mediated clearance of Aβ and indicate that neurons not only produce but also clear Aβ.

Entorhinal cortical neurons are the primary targets of FUS mislocalization and ubiquitin aggregation in FUS transgenic rats

Ubiquitin-positive inclusion containing Fused in Sarcoma (FUS) defines a new subtype of frontotemporal lobar degeneration (FTLD). FTLD is characterized by progressive alteration in cognitions and it preferentially affects the superficial layers of frontotemporal cortex. Mutation of FUS is linked to amyotrophic lateral sclerosis and to motor neuron disease with FTLD. To examine FUS pathology in FTLD, we developed the first mammalian animal model expressing human FUS with pathogenic mutation and developing progressive loss of memory. In FUS transgenic rats, ubiquitin aggregation and FUS mislocalization were developed primarily in the entorhinal cortex of temporal lobe, particularly in the superficial layers of affected cortex. Overexpression of mutant FUS led to Golgi fragmentation and mitochondrion aggregation. Intriguingly, aggregated ubiquitin was not colocalized with either fragmented Golgi or aggregated mitochondria, and neurons with ubiquitin aggregates were deprived of endogenous TDP-43. Agonists of peroxisome proliferator-activated receptor gamma (PPAR-γ) possess anti-glial inflammation effects and are also shown to preserve the dendrite and dendritic spines of cortical neurons in culture. Here we show that rosiglitazone, a PPAR-γ agonist, rescued the dendrites and dendritic spines of neurons from FUS toxicity and preserved rats' spatial memory. Our FUS transgenic rats would be useful to the mechanistic study of cortical dementia in FTLD. As rosiglitazone is clinically used to treat diabetes, our results would encourage immediate application of PPAR-γ agonists in treating patients with cortical dementia.

Reduction of low-density lipoprotein receptor-related protein (LRP1) in hippocampal neurons does not proportionately reduce, or otherwise alter, amyloid deposition in APPswe/PS1dE9 transgenic mice

INTRODUCTION

The low-density lipoprotein receptor-related protein (LRP1) and its family members have been implicated in the pathogenesis of Alzheimer's disease. Multiple susceptibility factors converge to metabolic pathways that involve LRP1, including modulation of the processing of amyloid precursor protein (APP) and the clearance of Aβ peptide.

METHODS

We used the Cre-lox system to lower LRP1 levels in hippocampal neurons of mice that develop Alzheimer-type amyloid by crosses between mice that express Cre recombinase under the transcriptional control of the GFAP promoter, mice that harbor loxp sites in the LRP1 gene, and the APPswe/PS1dE9 transgenic model. We compared amyloid plaque numbers in APPswe/PS1dE9 mice lacking LRP1 expression in hippocampus (n = 13) to mice with normal levels of LRP1 (n = 12). Student t-test was used to test whether there were significant differences in plaque numbers and amyloid levels between the groups. A regression model was used to fit two regression lines for these groups, and to compare the rates of Aβ accumulation.

RESULTS

Immunohistochemical analyses demonstrated efficient elimination of LRP1 expression in the CA fields and dentate gyrus of the hippocampus. Within hippocampus, we observed no effect on the severity of amyloid deposition, the rate of Aβ40/42 accumulation, or the architecture of amyloid plaques when LRP1 levels were reduced.

CONCLUSIONS

Expression of LRP1 by neurons in proximity to senile amyloid plaques does not appear to play a major role in modulating the formation of these proximal deposits or in the appearance of the associated neuritic pathology.

Immunohistochemical detection of receptor-associated protein in normal human brain and Alzheimer's disease

This study is one of the few to characterize immunohistochemically the distribution and localization of Receptor-Associated Protein (RAP) in human autopsy brain. The results show prominent cortical neuronal localization. RAP is clearly identified in large neuronal dendritic/axonal processes. RAP is expressed in both large pyramidal and smaller interneurons. Occasional, much less frequent RAP is detectable in glial cells in white matter, which appear to be predominantly astrocytic. Although RAP is detectable immunohistochemically in Alzheimer's disease autopsy brain, the level of expression appears significantly reduced relative to age-matched control brains. These results suggest, at the immunohistochemical level, that there is a reduction of RAP protein in Alzheimer's disease brain (cortex). In terms of Alzheimer's disease pathophysiology, a reduction of neuronal RAP could then lead to reduced membrane expression of LRP, since RAP has also been shown to be an LRP antagonist.

Receptor-associated protein interacts with amyloid-beta peptide and promotes its cellular uptake

Brain amyloid-beta (Abeta) peptide accumulation and aggregation are critical events in the pathogenesis of Alzheimer disease. Increasing evidence has demonstrated that LRP1 is involved in Alzheimer disease pathogenesis. The physiological ligands of LRP1, including apoE, play significant roles in the cellular clearance of Abeta. The receptor-associated protein (RAP) is a specialized chaperone for members of the low density lipoprotein receptor family. RAP shares structural and receptor-binding properties with apoE. Here, we show that RAP binds to both Abeta40 and Abeta42 in a concentration-dependent manner and forms complexes with them. Fluorescence-activated cell sorter analysis showed that RAP significantly enhances the cellular internalization of Abeta in different cell types, including brain vascular smooth muscle, neuroblastoma, glioblastoma, and Chinese hamster ovary cells. This effect of RAP was confirmed by fluorescence microscopy and enzyme-linked immunosorbent assay. RAP binds to both LRP1 and heparin; however, the ability of RAP to enhance Abeta cellular uptake was blocked by heparin and heparinase treatment but not by LRP1 deficiency. Furthermore, the effects of RAP were significantly decreased in heparan sulfate proteoglycan-deficient Chinese hamster ovary cells. Our findings reveal that RAP is a novel Abeta-binding protein that promotes cellular internalization of Abeta.

Allele-specific RNAi mitigates phenotypic progression in a transgenic model of Alzheimer's disease

Despite recent advances suggesting new therapeutic targets, Alzheimer's disease (AD) remains incurable. Aberrant production and accumulation of the Abeta peptide resulting from altered processing of the amyloid precursor protein (APP) is central to the pathogenesis of disease, particularly in dominantly inherited forms of AD. Thus, modulating the production of APP is a potential route to effective AD therapy. Here, we describe the successful use of an allele-specific RNA interference (RNAi) approach targeting the Swedish variant of APP (APPsw) in a transgenic mouse model of AD. Using recombinant adeno-associated virus (rAAV), we delivered an anti-APPsw short-hairpin RNA (shRNA) to the hippocampus of AD transgenic mice (APP/PS1). In short- and long-term transduction experiments, reduced levels of APPsw transprotein were observed throughout targeted regions of the hippocampus while levels of wild-type murine APP remained unaltered. Moreover, intracellular production of transfer RNA (tRNA)-valine promoter-driven shRNAs did not lead to detectable neuronal toxicity. Finally, long-term bilateral hippocampal expression of anti-APPsw shRNA mitigated abnormal behaviors in this mouse model of AD. The difference in phenotype progression was associated with reduced levels of soluble Abeta but not with a reduced number of amyloid plaques. Our results support the development of allele-specific RNAi strategies to treat familial AD and other dominantly inherited neurodegenerative diseases.