Glucocerebrosidase Is Shaking Up the Synucleinopathies

Marina Siebert; Ellen Sidransky; Wendy Westbroek


Brain. 2014;137(5):1304-1322. 

In This Article

Therapeutics for Gaucher Disease May Have Promise for the Treatment of the Synucleinopathies

As previously mentioned in this review, studies performed on cell free systems, cell and animal models, and patient samples have demonstrated that knockdown of GBA1 expression, the introduction of GBA1 mutations, inhibition by CBE, or treatment with glucocerebroside substrate all enhance accumulation and/or oligomerization of α-synuclein (Manning-Bog et al., 2009; Cullen et al., 2011; Mazzulli et al., 2011; Sardi et al., 2011, 2013; Gegg et al., 2012; Cleeter et al., 2013; Osellame et al., 2013). On the other hand, upregulation of α-synuclein levels decrease glucocerebrosidase protein and activity levels in cell-free systems, cell and mouse models, and post-mortem brains of Parkinson's disease patients with and without GBA1 mutations (Mazzulli et al., 2011; Sardi et al., 2011, 2013; Yap et al., 2011, 2013; Gegg et al., 2012). This reciprocal relationship between glucocerebrosidase activity and α-synuclein levels has generated great interest in the potential role of Gaucher disease therapeutics for the treatment of the synucleinopathies (Sardi et al., 2013; Schapira and Gegg, 2013). Therapies for Gaucher disease, which are targeted towards augmenting glucocerebrosidase activity or decreasing glucocerebroside storage, could prove to be promising strategies for modulating α-synuclein proteostasis and its subsequent aggregation and oligomerization. This rational was supported by experimental evidence showing that viral-mediated infection into the central nervous system of a mouse model with GBA1 mutations representing neuronopathic Gaucher disease and a transgenic mouse model over-expressing A53T α-synuclein without GBA1 mutations significantly reduced α-synuclein levels (Sardi et al., 2013). This set a paradigm for augmentation of glucocerebrosidase activity as a beneficial therapeutic strategy for halting disease progression in patients with Parkinson's disease, both with and without GBA1 mutations, and even preventing the onset of Parkinson's disease in healthy individuals. In this review, we address FDA-approved and 'under development' therapeutics for Gaucher disease and their potential implications for treatment of the synucleinopathies.

The first available FDA-approved therapy for Gaucher disease was enzyme replacement therapy, which was developed at the National Institutes of Health. Patients with Gaucher disease type 1 received intravenous infusion of exogenous enzyme, which improved haematologic and visceral manifestations and reduced glucocerebroside levels (Brady et al., 1974; Barton et al., 1991). Currently, three different recombinant enzymes are commercially available, imiglucerase, taliglucerase alfa, and velaglucerase alfa. Although each of the enzymes differ in their cell system production and glycosylation pattern, the function and biodistribution of all three enzymes are comparable (Tekoah et al., 2013) (Fig. 3). As intravenous enzyme replacement therapy does not cross the blood–brain barrier it does not ameliorate neurological manifestations and would not be suitable for treatment of Parkinson's disease neuropathology (Erikson, 2001; Beck, 2007). In fact, patients with Gaucher disease undergoing enzyme replacement therapy have still gone on to develop Parkinson's disease.

Figure 3.

Therapeutic strategies to enhance glucocerebrosidase. (1 and 2) In healthy cells, wild-type glucocerebrosidase (WT GCase) is sorted to the lysosome via the endoplasmic reticulum, Golgi, and late endosomes (LE) where it will degrade its substrate glucocerebroside. (3) Mutant glucocerebrosidase is misfolded in the endoplasmic reticulum, becomes polyubiquitinated (ub) and undergoes proteasome-mediated degradation. (4) Pharmacological chaperones can stabilize mutant glucocerebrosidase and facilitate translocation to lysosomes. (5) In enzyme replacement therapy, recombinant glucocerebrosidase enzyme is delivered into the cells via the mannose-6-phosphate receptor and trafficked through the late endosomes to the lysosomes where it is able to degrade substrate. CM = Cell Membrane.

The accumulation of glucocerebroside in the lysosome can impact α-synuclein breakdown and oligomerization (Mazzulli et al., 2011), which suggests that therapeutic reduction of excessive glucocerebroside substrate could potentially be beneficial for Parkinson's disease. Therapeutic inhibition of the enzyme glucosylceramide synthase, which catalyzes the synthesis of glucocerebroside, attenuates glucocerebroside production and has been used as a form of substrate reduction therapy. Treatment of patients with Gaucher disease with two glucosylceramide synthase inhibitors, miglustat (N-butyldeoxynojirimycin) and eliglustat tartrate, resulted in visceral and hematopoietic improvement but failed to impact neurological manifestations (Lukina et al., 2010). Recently, a screening effort of novel compounds identified the compound GZ 161, which successfully reduced both glucocerebroside and glucosylsphingosine accumulation in the brain of the K14 acute neuronopathic Gaucher disease mouse model and significantly increased their lifespan (Cabrera-Salazar et al., 2012).

Another approach gaining much momentum in the field of lysosomal storage disorders is pharmacological chaperone therapy. The proper folding process of glucocerebrosidase takes place in the endoplasmic reticulum by direct interaction with endogenous cellular chaperones such as heat shock protein 90 and heat shock protein 70 (Lu et al., 2011). Studies have demonstrated that several disease-causing glucocerebrosidase mutants are misfolded and do not pass the ERAD quality control system, which leads to early proteasome-mediated degradation (Ron and Horowitz, 2005; Ron et al., 2010; Bendikov-Bar et al., 2011; Bendikov-Bar and Horowitz, 2012; Maor et al., 2013a, b). Therapy with pharmacological chaperones, which specifically bind to the newly synthesized mutant enzyme, can prevent premature ERAD and promote trafficking to the lysosome, where most mutant glucocerebrosidase proteins can exert sufficient residual enzyme activity for the breakdown of accumulated lysosomal glucocerebroside (Lieberman et al., 2009; Bendikov-Bar et al., 2013) (Fig. 3). The drawback of such a therapeutic approach is that translation of mutated glucocerebrosidase and an intact chaperone-binding site are required. Treatment will not be effective in the case of null alleles, large deletions, or mutations affecting the chaperone-binding site. Many of the pharmacological chaperones are inhibitors of glucocerebrosidase that bind to its active site. Ambroxol is a pH-dependent mixed inhibitor of glucocerebrosidase that was identified by screening of an FDA-approved drug library (Maegawa et al., 2009); it is a potent chaperone for the translocation of mutant glucocerebrosidase to lysosomes (Bendikov-Bar et al., 2011, 2013; Babajani et al., 2012; Luan et al., 2013). One limited pilot study conducted in a small group of patients with Gaucher disease indicated amelioration of clinical symptoms after ambroxol treatment (Zimran et al., 2013), but its efficacy in relevant Parkinson's disease models has not yet been evaluated. Another glucocerebrosidase inhibitor, Isofagomine, showed great efficacy in cell and mouse models of Gaucher disease, resulting in increased glucocerebrosidase protein levels and enzyme activity, reduction in levels of glucocerebroside and glucosylsphingosine, delayed neurological manifestations, and increased life span (Khanna et al., 2010; Sun et al., 2011, 2012), but improvement in clinical symptoms were not observed in a phase 2 clinical trial (Zimran, 2011). In a cell model for Parkinson's disease consisting of PC12 cells over-expressing α-synuclein and transfected with wild-type or mutant glucocerebrosidase, the efficacy of isofagomine treatment on α-synuclein levels was non-significant (Cullen et al., 2011); its efficacy in relevant in vivo models of Parkinson's disease remains to be investigated. Clinical development of these inhibitory chaperones has major obstacles as both drug dosage and the length of treatment have to be carefully optimized for high endoplasmic reticulum to lysosome chaperone activity, yet minimal lysosomal enzyme inhibition. This can be circumvented by using pharmacological chaperones that both facilitate the lysosomal translocation and residual activity of mutant glucocerebrosidase without enzyme inhibition. Recent efforts have identified promising activators that increase translocation and enzyme activity of mutant glucocerebrosidase in fibroblasts (Goldin et al., 2012; Patnaik et al., 2012).

As ERAD is a major player in the premature degradation of many glucocerebrosidase mutants, targeting proteins that regulate the proteostasis of mutated glucocerebrosidase could serve as an alternative therapy. Recently, histone deacetylase inhibitors were identified as modulators of heat shock protein 90-dependent degradation of mutated glucocerebrosidase by inhibiting the deacetylation of heat shock protein 90. Treatment resulted in increased glucocerebrosidase protein levels and enzyme activity in Gaucher disease fibroblasts (Lu et al., 2011; Yang et al., 2013). Future development of histone deacetylase inhibitors for therapeutics might prove challenging, as the target of this therapy remains broad rather than glucocerebrosidase-specific. Additionally, the exact molecular mechanism of regulation of proteostasis by histone deacetylase inhibitors remains unclear.