Microglia degrade Alzheimer's amyloid-beta deposits extracellularly via digestive exophagy

Berbamine targets the FKBP12-rapamycin-binding (FRB) domain of the mTOR complex to promote microglial autophagy and ameliorate neuroinflammation in Alzheimer's disease

BACKGROUND

Berbamine (BBM), a natural bisbenzylisoquinoline alkaloid, has demonstrated promising effects in ameliorating pathological process and inflammation response in central neuronal system (CNS). Alzheimer's disease (AD), primarily characterized by amyloid-beta (Aβ)-containing extra-cellular plaques and abnormal "autophagy-brake regulation" of neuroinflammation, currently lacks of effective therapeutic options. Therapeutics of BBM on AD is indeed intriguing, and the potential targets and mechanisms are vague yet.

PURPOSE

This study is designed to elucidate the therapeutic potential of BBM on AD, focusing particularly on its ability to enhance autophagy, induce microglial M2 polarization, and to uncover the underlying molecular mechanisms and implicated targets.

METHODS

The therapeutic efficacy of BBM was systematically investigated in APP/PS1 mice, with a focus on its potential to enhance autophagy, induce M2 polarization in microglia, and facilitate the clearance of Aβ plaques. Cognitive function was rigorously assessed through a series of behavioral tests, including the Morris Water Maze and Object Location Task. Immunofluorescence was employed to visualize the spatial distribution of inflammatory cytokines and autophagic markers within the brain parenchyma. Quantitative measurements of these cytokines were obtained using enzyme-linked immunosorbent assay (ELISA). Western blotting was utilized to analyze protein profiles associated with autophagy and microglial phenotypes. Additionally, chemo-proteomics and molecular docking techniques were applied to identify the key molecular targets of BBM.

RESULTS

BBM treatment significantly ameliorated cognitive dysfunction and reduced Aβ plaque deposition in APP/PS1 transgenic mice. Notably, BBM promoted microglial polarization from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype, accompanied by attenuation of neuroinflammation. Mechanistically, BBM exerted its effects through inhibition of mTOR signaling via direct interaction with the FKBP12-rapamycin-binding domain, thereby restoring autophagic flux and facilitating M2 microglial polarization. The mTOR activator MHY1485 abrogated the beneficial effects of BBM, highlighting the pivotal role of mTOR inhibition in its mechanism of action.

CONCLUSIONS

BBM promotes M2 microglial polarization and restores autophagic flux in AD by inhibiting mTOR signaling, representing a novel dual-modulatory mechanism for AD intervention. These findings highlight BBM's ability to target mTOR and intersecting pathways, offering a promising disease-modifying therapeutic approach for AD and other neurodegenerative disorders.

The Alzheimer's Disease Gene SORL1 Regulates Lysosome Function in Human Microglia

The SORL1 gene encodes the sortilin-related receptor protein SORLA, a sorting receptor that regulates endo-lysosomal trafficking of various substrates. Loss of function variants in SORL1 are causative for Alzheimer's disease (AD) and decreased expression of SORLA has been repeatedly observed in human AD brains. SORL1 is highly expressed in the central nervous system, including in microglia, the tissue-resident immune cells of the brain. Loss of SORLA leads to enlarged lysosomes in hiPSC-derived microglia-like cells (hMGLs). However, how SORLA deficiency contributes to lysosomal dysfunction in microglia and how this contributes to AD pathogenesis is not known. In this study, we show that loss of SORLA results in decreased lysosomal degradation and lysosomal enzyme activity due to altered trafficking of lysosomal enzymes in hMGLs. Phagocytic uptake of fibrillar amyloid beta 1-42 and synaptosomes is increased in SORLA-deficient hMGLs, but due to reduced lysosomal degradation, these substrates aberrantly accumulate in lysosomes. An alternative mechanism of lysosome clearance, lysosomal exocytosis, is also impaired in SORL1-deficient microglia, which may contribute to an altered immune response. Overall, these data suggest that SORLA has an important role in the proper trafficking of lysosomal hydrolases in hMGLs, which is critical for microglial function. This further substantiates the microglial endo-lysosomal network as a potential novel pathway through which SORL1 may increase AD risk and contribute to the development of AD. Additionally, our findings may inform the development of novel lysosome and microglia-associated drug targets for AD.

Evidence suggesting that microglia make amyloid from neuronally expressed APP: a hypothesis

While APP is largely neuonally expressed, Aβ amyloid is largely produced by microglia as the clearance mechanisms for damaged membranes becomes overwhelmed.

A Sensitive and Versatile Cell-Based Assay Combines Luminescence and Trapping Approaches to Monitor Unconventional Protein Secretion

In addition to the conventional endoplasmic reticulum (ER)-Golgi secretory pathway, alternative routes are increasingly recognized for their critical roles in exporting a growing number of secreted factors. These alternative processes, collectively referred to as unconventional protein secretion (UcPS), challenge traditional views of protein and membrane trafficking. Unlike the well-characterized molecular machinery of the conventional secretory pathway, the mechanisms underlying UcPS remain poorly understood. Various UcPS pathways may involve direct transport of cytosolic proteins across the plasma membrane or the incorporation of cargo proteins into intracellular compartments redirected for secretion. Identifying the specific chaperones, transporters and fusion machinery involved in UcPS cargo recognition, selection and transport is crucial to decipher how cargo proteins are selectively or synergistically directed through multiple secretory routes. These processes can vary depending on cell type and in response to particular stress conditions or cellular demands, underscoring the need for standardized tools and methods to study UcPS. Here, we combine the sensitivity of split NanoLuc Binary Technology with the versatility of the Retention Using Selective Hooks (RUSH) system to develop a straightforward and reliable cell-based assay for investigating both conventional and unconventional protein secretion. This system allows for the identification of intracellular compartments involved in UcPS cargo trafficking. Additionally, its sensitivity enabled us to demonstrate that disease-associated mutants or variants of Tau and superoxide dismutase-1 (SOD1) show altered secretion via UcPS. Finally, we leveraged this assay to screen for Alzheimer's disease risk factors, revealing a functional link between amyloid-beta production and Tau UcPS. This robust assay provides a powerful tool for increasing our knowledge of protein secretion mechanisms in physiological and pathological contexts.

Real-Time Ratiometric pH Imaging of Macrophage Lysosomes Using the Novel pH-sensitive Probe ApHID

Lysosomes actively regulate their lumenal pH, which is necessary for optimal enzymatic activity. Endocytic processes are involved in many diseases, including Alzheimer's disease, in which sub-optimal lysosomal function has been reported. To measure acidification, pH-sensitive probes can be delivered to endosomes and lysosomes using labeled dextran polymers or proteins. However, many commercially available probes have limited sensitivity in the acidic range of lysosomes, and their fluorescence is subject to enzymatic degradation and photobleaching. Herein, we describe the preparation, characterization, and use of a novel pH-sensitive probe, ApHID, a green-emitting fluorescent dye with optimal dynamic range within the acidity of endosomes and lysosomes. ApHID has a pKa near 5, increasing brightness with acidity, and it is robustly resistant to oxidation and photobleaching. We used ApHID ratiometric imaging to measure lysosomal pH in macrophages, yielding virtually identical results when compared with fluorescein and Oregon Green. Overall, ApHID circumvents limitations presented by most commercially available pH-sensitive probes and can be useful in demanding imaging applications such as intravital imaging of tissues.

Digestive exophagy as a novel mechanism of amyloid-β degradation by microglial lysosomes

Microglia are known to be involved in the modulation of amyloid-β (Aβ) plaques in Alzheimer's disease (AD). In a recent study, Jacquet et al. describe how microglia degrade larger Aβ aggregates by forming lysosomal synapses, further implicating the microglial release of lysosomal Aβ, amongst other processes, in the growth and spread of fibrillary Aβ.