Inhibition of microglial β-glucocerebrosidase hampers the microglia-mediated antioxidant and protective response in neurons

Abstract

Background Homozygotic mutations in the GBA gene cause Gaucher’s disease; moreover, both patients and heterozygotic carriers have been associated with 20- to 30-fold increased risk of developing Parkinson’s disease. In homozygosis, these mutations impair the activity of β-glucocerebrosidase, the enzyme encoded by GBA, and generate a lysosomal disorder in macrophages, which changes morphology towards an engorged phenotype, considered the hallmark of Gaucher’s disease. Notwithstanding the key role of macrophages in this disease, most of the effects in the brain have been attributed to the β-glucocerebrosidase deficit in neurons, while a microglial phenotype for these mutations has never been reported. Methods We applied the bioluminescence imaging technology, immunohistochemistry and gene expression analysis to investigate the consequences of microglial β-glucocerebrosidase inhibition in the brain of reporter mice, in primary neuron/microglia cocultures and in cell lines. The use of primary cells from reporter mice allowed for the first time, to discriminate in cocultures neuronal from microglial responses consequent to the β-glucocerebrosidase inhibition; results were finally confirmed by pharmacological depletion of microglia from the brain of mice. Results Our data demonstrate the existence of a novel neuroprotective mechanism mediated by a direct microglia-to-neuron contact supported by functional actin structures. This cellular contact stimulates the nuclear factor erythroid 2-related factor 2 activity in neurons, a key signal involved in drug detoxification, redox balance, metabolism, autophagy, lysosomal biogenesis, mitochondrial dysfunctions, and neuroinflammation. The central role played by microglia in this neuronal response in vivo was proven by depletion of the lineage in the brain of reporter mice. Pharmacological inhibition of microglial β-glucocerebrosidase was proven to induce morphological changes, to turn on an anti-inflammatory/repairing pathway, and to hinder the microglia ability to activate the nuclear factor erythroid 2-related factor 2 response, thus increasing the neuronal susceptibility to neurotoxins. Conclusion This mechanism provides a possible explanation for the increased risk of neurodegeneration observed in carriers of GBA mutations and suggest novel therapeutic strategies designed to revert the microglial phenotype associated with β-glucocerebrosidase inhibition, aimed at resetting the protective microglia-to-neuron communication.

Neuronopathic GBA1 L444P mutation accelerates Glucosylsphingosine levels and formation of hippocampal alpha-synuclein inclusions

“…Neuroinflammation in GBA1 +/L444P mice compared to GBA1 +/+ mice Expression of the GBA1 L444P mutation causes systemic inflammation in addition to enhanced neuroinflammation clinically and in this mouse model (Mizukami et al, 2002;Rocha et al, 2015;Mus et al, 2019;Brunialti et al, 2021;Williams et al, 2021). Thus, ten months post-monomer or fibril injection, hippocampal brain sections were stained for MHCII and IgG to investigate inflammation.…”

Neuroinflammation and Parkinson’s Disease—From Neurodegeneration to Therapeutic Opportunities

“…Similarly, the GBA gene, which encodes the lysosomal hydrolase glucocerebrosidase (GCase), was found to affect microglial function. Following positron-emission tomography (PET) scans in Lewi body-susceptible brain regions, it was shown that GBA mutations are associated with microglial activation, which consequently fails to protect neurons [ 39 , 40 ]. The work of Keatinge and colleagues supports this role of GBA in neuroinflammation.…”

“…Nevertheless, the exact molecular mechanism affecting microglia dysfunction remains to be unveiled. Nevertheless, Brunialti et al, suggested that—due to similar features between macrophages and microglia—the same mechanisms (autophagy and lysosomal storage) might be impaired [ 40 ]. Additionally, genetic alterations in PD-associated genes, together with α-Syn aggregation and propagation, were also found as key players disrupting other vital microglial processes such as mitochondrial dysfunction [ 42 ], autophagy [ 43 , 44 ], and possibly phagocytosis [ 45 , 46 , 47 ], thereby leading to neuroinflammation and consequently PD development.…”

Glucocerebrosidase mutations and Parkinson disease

“…Upregulation of complement C1q has been noted in the brains of mice following CBE exposure (Rocha et al 2015b ). GCase deficiency may increase the sensitivity of microglia to various insults, leading to an increased risk of neurodegeneration (Brunialti et al 2021 ). Moreover, the finding of an association between HLA alleles and an increased risk of PD adds further support to the role of neuroinflammation in the disease process (International Parkinson’s Disease Genomics and Wellcome Trust Case Control 2011 ).…”