Mitochondrial dysfunction associated with glucocerebrosidase deficiency

The lysosomal hydrolase glucocerebrosidase (GCase) is encoded for by the GBA gene. Homozygous GBA mutations cause Gaucher disease (GD), a lysosomal storage disorder. Furthermore, homozygous and heterozygous GBA mutations are numerically the greatest genetic risk factor for developing Parkinson's disease (PD), the second most common neurodegenerative disorder. The loss of GCase activity results in impairment of the autophagy-lysosome pathway (ALP), which is required for the degradation of macromolecules and damaged organelles. Aberrant protein handling of α-synuclein by the ALP occurs in both GD and PD. α-synuclein is the principle component of Lewy bodies, a defining hallmark of PD. Mitochondrial dysfunction is also observed in both GD and PD. In this review we will describe how mitochondria are affected following loss of GCase activity. The pathogenic mechanisms leading to mitochondria dysfunction will also be discussed, focusing on the likely inhibition of the degradation of mitochondria by the ALP, also termed mitophagy. Other pathogenic cellular processes associated with GBA mutations that might contribute, such as the unfolding of GCase in the endoplasmic reticulum, calcium dysregulation and neuroinflammation will also be described. Impairment of the ALP and mitochondria dysfunction are common pathogenic themes between GD and PD and probably explain why GBA mutations increase the risk of developing PD that is very similar to sporadic forms of the disease

Glucocerebrosidase-associated Parkinson disease: Pathogenic mechanisms and potential drug treatments

Dysfunction of the endolysosomal system is implicated in the pathogenesis of both sporadic and familial Parkinson disease (PD). Variants in genes encoding lysosomal proteins have been estimated to be associated with more than half of PD cases. The most common genetic risk factor for PD are variants in the GBA gene, encoding the lysosomal enzyme glucocerebrosidase (GCase), which is involved in sphingolipid metabolism. In this review we will describe the clinical symptoms and pathology of GBA-PD, and how this might be affected by the type of GBA variant. The putative mechanisms by which GCase deficiency in neurons and glia might contribute to PD pathogenesis will then be discussed, with particular emphasis on the accumulation of α-synuclein aggregates and the spread of pathogenic α-synuclein species between the cell types. The dysregulation of not only sphingolipids, but also phospholipids and cholesterol in the misfolding of α-synuclein is reviewed, as are neuroinflammation and the interaction of GCase with LRRK2 protein, another important contributor to PD pathogenesis. Study of both non-manifesting GBA carriers and GBA-PD cohorts provides an opportunity to identify robust biomarkers for PD progression as well as clinical trials for potential treatments. The final part of this review will describe preclinical studies and clinical trials for increasing GCase activity or reducing toxic substrate accumulation

Glucocerebrosidase deficiency promotes release of α-synuclein fibrils from cultured neurons

Mutations in the GBA gene, which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the most important genetic risk factor for Parkinson disease (PD). GCase activity is also decreased in sporadic PD brains and with normal aging. Loss of GCase activity impairs the autophagy lysosomal pathway resulting in increased α-synuclein (α-syn) levels. Furthermore, elevated α-syn results in decreased GCase activity. While the role of α-syn in PD remains unclear, evidence indicates that aggregated α-syn fibrils are a pathogenic species in PD, passing between neurons and inducing endogenous native α-syn to aggregate; spreading pathology through the brain. We have investigated if preformed α-syn fibrils (PFFs) impair GCase activity in mouse cortical neurons and differentiated dopaminergic cells, and whether GCase deficiency in these models increased the transfer of α-syn pathology to naïve cells. Neurons treated with PFFs induced endogenous α-syn to become insoluble and phosphorylated at Ser129 to a greater extent than monomeric α-syn-treatment. PFFs, but not monomeric α-syn, inhibited lysosomal GCase activity in these cells and induced the unfolded protein response. Neurons in which GCase was inhibited by conduritol β-epoxide did not increase the amount of insoluble monomeric α-syn or its phosphorylation status. Instead the release of α-syn fibrils from GCase deficient cells was significantly increased. Co-culture studies showed that the transfer of α-syn pathology to naïve cells was greater from GCase deficient cells. This study suggests that GCase deficiency increases the spread of α-syn pathology and likely contributes to the earlier age of onset and increased cognitive decline associated with GBA-PD

The role of glucocerebrosidase in Parkinson disease pathogenesis

GBA encodes the lysosomal enzyme glucocerebrosidase (GCase), an enzyme involved in sphingolipid metabolism. Mutations in the GBA gene are numerically the most important risk factor for developing Parkinson disease (PD) accounting for at least 5% of all PD cases. Furthermore, loss of GCase activity is found in sporadic PD brains. Lysosomal dysfunction is thought to play a principal role in PD pathogenesis and in particular its effect on the metabolism of α‐synuclein. A hallmark of PD is the presence of intraneuronal protein inclusions called Lewy bodies, which are composed mainly of α‐synuclein. Cellular and animal models of GCase deficiency result in lysosomal dysfunction, and in particular the autophagy lysosome pathway, resulting in the accumulation of α‐synuclein. Some forms of mutant GCase unfold in the endoplasmic reticulum activating the unfolded protein response, which might also contribute to PD pathogenesis. It has also been suggested that accumulation of GCase substrates glucosylceramide/glucosylsphingosine may contribute to GBA‐PD pathogenesis. Mitochondrial dysfunction and neuroinflammation are associated with GCase deficiency and have also been implicated in the aetiology of PD. This review discusses these points and highlights potential treatments that might be effective in treating GCase deficiency in PD

Effects of ambroxol on the autophagy-lysosome pathway and mitochondria in primary cortical neurons

Glucocerebrosidase (GBA1) mutations are the major genetic risk factor for Parkinson's Disease (PD). The pathogenic mechanism is still unclear, but alterations in lysosomal-autophagy processes are implicated due to reduction of mutated glucocerebrosidase (GCase) in lysosomes. Wild-type GCase activity is also decreased in sporadic PD brains. Small molecule chaperones that increase lysosomal GCase activity have potential to be disease-modifying therapies for GBA1-associated and sporadic PD. Therefore we have used mouse cortical neurons to explore the effects of the chaperone ambroxol. This chaperone increased wild-type GCase mRNA, protein levels and activity, as well as increasing other lysosomal enzymes and LIMP2, the GCase transporter. Transcription factor EB (TFEB), the master regulator of the CLEAR pathway involved in lysosomal biogenesis was also increased upon ambroxol treatment. Moreover, we found macroautophagy flux blocked and exocytosis increased in neurons treated with ambroxol. We suggest that ambroxol is blocking autophagy and driving cargo towards the secretory pathway. Mitochondria content was also found to be increased by ambroxol via peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α). Our data suggest that ambroxol, besides being a GCase chaperone, also acts on other pathways, such as mitochondria, lysosomal biogenesis, and the secretory pathway

No evidence for substrate accumulation in Parkinson brains with GBA mutations

To establish whether Parkinson's disease (PD) brains previously described to have decreased glucocerebrosidase activity exhibit accumulation of the lysosomal enzyme's substrate, glucosylceramide, or other changes in lipid composition

Autophagic lysosome reformation dysfunction in glucocerebrosidase deficient cells: relevance to Parkinson disease.

Glucocerebrosidase (GBA1) gene mutations increase the risk of Parkinson disease (PD). While the cellular mechanisms associating GBA1 mutations and PD are unknown, loss of the glucocerebrosidase enzyme (GCase) activity, inhibition of autophagy and increased α-synuclein levels have been implicated. Here we show that autophagy lysosomal reformation (ALR) is compromised in cells lacking functional GCase. ALR is a cellular process controlled by mTOR which regenerates functional lysosomes from autolysosomes formed during macroautophagy. A decrease in phopho-S6K levels, a marker of mTOR activity, was observed in models of GCase deficiency, including primary mouse neurons and the PD patient derived fibroblasts with GBA1 mutations, suggesting that ALR is compromised. Importantly Rab7, a GTPase crucial for endosome-lysosome trafficking and ALR, accumulated in GCase deficient cells, supporting the notion that lysosomal recycling is impaired. Recombinant GCase treatment reversed ALR inhibition and lysosomal dysfunction. Moreover, ALR dysfunction was accompanied by impairment of macroautophagy and chaperone-mediated autophagy, increased levels of total and phosphorylated (S129) monomeric α-synuclein, evidence of amyloid oligomers and increased α-synuclein release. Concurrently, we found increased cholesterol and altered glucosylceramide homeostasis which could compromise ALR. We propose that GCase deficiency in PD inhibits lysosomal recycling. Consequently neurons are unable to maintain the pool of mature and functional lysosomes required for the autophagic clearance of α-synuclein, leading to the accumulation and spread of pathogenic α-synuclein species in the brain. Since GCase deficiency and lysosomal dysfunction occur with ageing and sporadic PD pathology, the decrease in lysosomal reformation may be a common feature in PD

Parkinson disease-linked GBA mutation effects reversed by molecular chaperones in human cell and fly models

GBA gene mutations are the greatest cause of Parkinson disease (PD). GBA encodes the lysosomal enzyme glucocerebrosidase (GCase) but the mechanisms by which loss of GCase contributes to PD remain unclear. Inhibition of autophagy and the generation of endoplasmic reticulum (ER) stress are both implicated. Mutant GCase can unfold in the ER and be degraded via the unfolded protein response, activating ER stress and reducing lysosomal GCase. Small molecule chaperones that cross the blood brain barrier help mutant GCase refold and traffic correctly to lysosomes are putative treatments for PD. We treated fibroblast cells from PD patients with heterozygous GBA mutations and Drosophila expressing human wild-type, N370S and L444P GBA with the molecular chaperones ambroxol and isofagomine. Both chaperones increased GCase levels and activity, but also GBA mRNA, in control and mutant GBA fibroblasts. Expression of mutated GBA in Drosophila resulted in dopaminergic neuronal loss, a progressive locomotor defect, abnormal aggregates in the ER and increased levels of the ER stress reporter Xbp1-EGFP. Treatment with both chaperones lowered ER stress and prevented the loss of motor function, providing proof of principle that small molecule chaperones can reverse mutant GBA-mediated ER stress in vivo and might prove effective for treating PD

Mitochondrial and lysosomal biogenesis are activated following PINK1/parkin-mediated mitophagy

Impairment of the autophagy-lysosome pathway (ALP) is implicated with the changes in α-synuclein and mitochondrial dysfunction observed in Parkinson's disease (PD). Damaged mitochondria accumulate PINK1, which then recruits parkin, resultingin ubiquitination of mitochondrial proteins. These can then be bound by the autophagic proteins p62/SQSTM1 and LC3, resulting in degradation of mitochondria by mitophagy. Mutations in PINK1 and parkin genes are a cause of familial PD. We found a significant increase in the expression of p62/SQSTM1 mRNA and protein following mitophagy induction in human neuroblastoma SH-SY5Y cells. p62 protein not only accumulated on mitochondria, but was also greatly increased in the cytosol. Increased p62/SQSMT1 expression was prevented in PINK1 knock down (KD) cells, suggesting increased p62 expression was a consequence of mitophagy induction. The transcription factors Nrf2 and TFEB, which play roles in mitochondrial and lysosomal biogenesis, respectively, can regulate p62/SQSMT1. We report that both Nrf2 and TFEB translocate to the nucleus following mitophagy induction and that the increase in p62 mRNA levels was significantly impaired in cells with Nrf2 or TFEB KD.. TFEB translocation also increased expression of itself and lysosomal proteins such as glucocerebrosidase and cathepsin D following mitophagy induction. We also report that cells with increased TFEB protein have significantly higher PGC-1α mRNA levels, a regulator of mitochondrial biogenesis, resulting in increased mitochondrial content. Our data suggests that TFEB is activated following mitophagy to maintain ALP and mitochondrial biogenesis. Therefore strategies to increase TFEB may improve both the clearance of α-synuclein and mitochondrial dysfunction in PD. This article is protected by copyright. All rights reserved

Mitochondria and Quality Control Defects in a Mouse Model of Gaucher Disease-Links to Parkinson's Disease

Mutations in the glucocerebrosidase (gba) gene cause Gaucher disease (GD), the most common lysosomal storage disorder, and increase susceptibility to Parkinson’s disease (PD). While the clinical and pathological features of idiopathic PD and PD related to gba (PD-GBA) mutations are very similar, cellular mechanisms underlying neurodegeneration in each are unclear. Using a mouse model of neuronopathic GD, we show that autophagic machinery and proteasomal machinery are defective in neurons and astrocytes lacking gba. Markers of neurodegeneration—p62/SQSTM1, ubiquitinated proteins, and insoluble α-synuclein—accumulate. Mitochondria were dysfunctional and fragmented, with impaired respiration, reduced respiratory chain complex activities, and a decreased potential maintained by reversal of the ATP synthase. Thus a primary lysosomal defect causes accumulation of dysfunctional mitochondria as a result of impaired autophagy and dysfunctional proteasomal pathways. These data provide conclusive evidence for mitochondrial dysfunction in GD and provide insight into the pathogenesis of PD and PD-GBA