Impact of GBA2 on Neuronopathic Gaucher’s Disease and α-Synuclein Accumulation in Medaka (Oryzias Latipes)

doi:10.21203/rs.3.rs-241251/v1

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Impact of GBA2 on Neuronopathic Gaucher’s Disease and α-Synuclein Accumulation in Medaka (Oryzias Latipes)

https://doi.org/10.21203/rs.3.rs-241251/v1

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Homozygous mutations in the lysosomal glucocerebrosidase gene, GBA1, cause Gaucher’s disease (GD), while heterozygous mutations in GBA1 are a strong risk factor for Parkinson’s disease (PD), whose pathological hallmark is intraneuronal α-synuclein (asyn) aggregates. We previously reported that GBA1 knockout (KO) medaka exhibited glucosylceramide accumulation and neuronopathic GD phenotypes, including short lifespan, non-selective neuron loss, microglial activation, and swimming abnormality, with asyn accumulation in the brains. A recent study reported that deletion of GBA2, non-lysosomal glucocerebrosidase, in a non-neuronopathic GD mouse model rescued its phenotypes. In the present study, we generated GBA2 KO medaka and examined the effect of GBA2 deletion on the phenotypes of GBA1 KO medaka. The GBA2 deletion in GBA1 KO medaka resulted in the exacerbation of glucosylceramide accumulation and no improvement in neuronopathic GD pathological changes, asyn accumulation, or swimming abnormalities. Meanwhile, though GBA2KO medaka did not show any apparent phenotypes, biochemical analysis revealed asyn accumulation in the brains. GBA2KO medaka showed a trend towards an increase in sphingolipids in the brains, which is one of the possible causes of asyn accumulation. In conclusion, this study demonstrated that the deletion of GBA2 does not rescue the pathological changes or behavioral abnormalities of GBA1 KO medaka, and GBA2 represents a novel factor affecting asyn accumulation in the brains.

Cellular & Molecular Neuroscience

GBA1

GBA2

Gaucher’s disease

Parkinson’s disease

α-synuclein

Glucocerebrosidase

Sphingolipids

Homozygous mutations in GBA1 are responsible for Gaucher’s disease (GD), the most common lysosomal storage disorder. GBA1 gene encodes the lysosomal glucocerbrosidase (GCase), and mutations in the GBA1 gene lead to a decrease in lysosomal GCase activity. The marked decrease in GCase activity causes the accumulation of the GCase substrate, glucosylceramide (GlcCer) and glucosylsphigosine (GlcSph), which is thought to cause GD (1).

GD is divided into three subtypes based on clinical manifestations: a non-neuronopathic form (type 1: GD1), an acute neuronopathic form (type 2: GD2), and a chronic neuronopathic form (type 3: GD3). GD2 and GD3 are together referred to as neuronopathic GD (nGD) (2). Recent genetic studies identified the heterozygous mutations in GBA1 as a strong risk for sporadic Parkinson’s disease (PD) and dementia with Lewy bodies, whose pathological hallmark is the disease-specific inclusion bodies, composed of α-synuclein (asyn) called Lewy bodies (3-5). Not only heterozygous mutations of GBA1 gene carriers but also GD1 patients are at an increased risk for developing PD (6, 7). Pathological analysis revealed the presence of Lewy bodies in the brains of both GD1 patients and GBA1 heterozygotes with PD (GBA1-PD), suggesting that GBA1 mutations contribute to asyn aggregation (8-12). Accumulating evidence suggests that the perturbation of sphingolipid metabolism and disruption of the autophagic-lysosomal pathway (ALP) play a key role in GBA1 mutations leading to asyn aggregation (13-20). However, the underlying mechanisms by which GBA1 mutations lead to both GD and PD have yet to be established.

Besides GBA1, mammalian GBA2 (non-lysosomal GCase), GBA3 (cytosolic GCase) and lactase-phlorizin hydrolase (LPH) have GCase activity, which hydrolyzes GlcCer into ceramide and glucose. The physiological functions of GBA2 have been investigated in several studies, but there are very few reports on GBA3. LPH is exclusively present in the plasma membrane of the small intestine and may be involved in the digestion of dietary GlcCer (21). Homozygous loss of function GBA2 mutations are responsible for hereditary spastic paraplegia (HSP; SPG46), autosomal-recessive cerebellar ataxia (ARCA) and Marinesco-Sjögren-like syndrome in humans (22-27). A previous study has reported that the deletion of GBA2 resulted in spermatogenesis abnormality but not in CNS abnormality in mice (28). However, a recent study showed that Gba2 knockout (KO) mice exhibited locomotor dysfunction despite a high phenotypic variance (29). Morpholino antisense oligonucleotide-mediated knockdown of GBA2 leads to abnormal motoneuron outgrowth and motility defect in zebrafish (25).

Interestingly, a previous study reported that the deletion of GBA2 rescued the visceral manifestations, such as hepatosplenomegaly, cytopenia and osteopenia, in a GD1 mouse model through a reduction in sphingosine (30). In addition, the deletion of GBA2 also rescues the CNS phenotypes, such as neuronal cell loss, motor coordination and lifespan, in the Niemann-Pick Type C (NPC) mouse model, which shows a decreased GBA1 activity in the brain (31). These findings raise the possibility that GBA2 works as a novel factor affecting the CNS pathology in GBA1-related disorders, including nGD and PD.

GBA1 and GBA2 are widely conserved from Caenorhabditis elegans to humans, as well as in medaka, mice and rats. We previously reported that GBA1 KO medaka survived long enough to perform pathological analysis of the disease progression, in contrast to the perinatal death observed in Gba1 KO mice (32, 33). GBA1 KO medaka develop swimming abnormalities at 2 months post-fertilization (mpf) and start to die from 3 mpf. Medaka have endogenous asyn, enabling the observation of the effects of genetic mutations or pharmacological intervention on the dynamics of asyn in the brains. We showed that GBA1 KO medaka displayed not only non-selective neuronal cell loss, neuroinflammation, and ALP dysfunction, but also asyn accumulation in the brains (33). GBA1 KO medaka are a useful model animal for investigating the pathological mechanisms underlying the CNS pathology in GBA1-related disorders.　　

In the present study, we generated GBA2 KO medaka and analyzed GBA2 single- and GBA1/GBA2 double-KO (DKO) medaka to investigate the contributions of GBA2 to nGD. We found that the deletion of GBA2 did not rescue the CNS phenotypes of GBA1 KO medaka but resulted in the perturbation of sphingolipid metabolism and an increase in the amount of asyn. The present study provides novel insights into the pathological role of GBA2 in nGD and asyn accumulation in the brains.

Generation of GBA2 KO and GBA1/GBA2 DKO medaka

The ethics statement and maintenance of medaka were described previously (33). Medaka experiments were approved by the Animal Experiments Committee of Kyoto University and conducted in accordance with national guidelines. Medaka were maintained in an aquaculture system with recirculating water at 27 °C in a 14-h light/10-h dark cycle.

Medaka of the Kyoto-cab strain, a substrain of Cab, were used in this study. The generation and characterization of GBA1-deficient medaka were reported previously (33). GBA2-deficient medaka were generated using a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) system, as reported previously (34). In brief, the cDNA sequence of medaka GBA2 was determined by reverse transcription-polymerase chain reaction and rapid amplification of cDNA ends. The medaka GBA2 gene consists of 18 exons encoding 858 amino acids. The crRNAs were designed using the CRISPR design tool (http://viewer.shigen.info/cgi-bin/crispr/crispr.cgi), and the following crRNA was used: 5’-GGAGGGCAAAGCACTGTCGGGGG-3. The crRNA and tracrRNA were constructed by Fasmac Co. (Kanagawa, Japan). The Cas9 RNA was synthesized from pCS2+hSpCas9 vector (Addgene #51815) using mMessage mMachine SP6 Kit (Thermo Fisher Scientific, Waltham, MA, USA). The RNA mixture was injected into single-cell-stage embryos. The injected founders (F₀) were raised to sexual maturity and back-crossed with wild-type (WT) to generate F₁s. The GBA2 gene of F₁s was sequenced, and novel heterozygous GBA2 mutation (GBA2^+/⁻) medaka with 21 bases deleted and 2 bases inserted into exon 5 were obtained (Additional file 1: Fig. S1). These deletions and insertions resulted in a frame shift mutation, leading to the deficiency in protein expression and enzymatic activity of GBA2 in the brain. Off-target candidates were searched for using the Medaka pattern match tool (http://viewer.shigen.info/medakavw/crisprtool/). No alterations were found in three off-target candidates located on exons.

GBA2^+/⁻ medaka were back-crossed with WT medaka at least five times and then crossed with GBA1-deficient medaka to create GBA1/GBA2 DKO medaka. Medaka brains collected by surgery were directly snap-frozen in liquid nitrogen and then stored at −80 °C until use. The GBA2^+/⁻ medaka and GBA2 KO medaka were used in the previously reported study (35).

The GBA2 enzymatic activity assay

The measurement of GBA2 enzymatic activity was carried out as described previously (36, 37). Medaka brains were homogenized in 200 µl deionized water and centrifuged at 500 × g at 4 °C for 10 min. The supernatant was collected and centrifuged at 20,000 × g at 4 °C for 20 min. The pellet was rinsed with 200 µl of 50 mM potassium phosphate buffer, pH 5.8 and centrifuged at 20,000 × g at 4 °C for 15 min. This step was repeated twice. Next, 30 µl of 50 mM potassium phosphate buffer, pH 5.8 was added to the pellet, and the pellet was resuspended. The resulted suspension was used for the enzyme assay.

The reaction mixture contained 10 µl of the suspension and 20 µl of 4.5 mM 4-methlumbeliferyl β-d-glucopyranoside (Wako, #324-37411) in 100 mM citric acid and 200 mM disodium hydrogen phosphate buffer, pH 5.8. The reaction mixture was incubated at 37 °C for 60 min with or without 0.3 mM NB-DGJ (N-deoxygalactonojirimycin, #B690500; Toronto Research Chemicals Inc., Toronto, Canada). The reaction was terminated by adding 200 µl of 0.5 M sodium carbonate buffer at pH 10.7, and the fluorescence (excitation 55 nm and emission 460 nm) was measured by Fluoroskan Ascent FL (Thermo Fisher Scientific).

Locomotor function analysis

The medaka locomotor function analysis was performed as described previously (33). Medaka were transferred to a 20-cm-diameter tank filled with water to a depth of 2 cm at room temperature. After 5 min of rest, the free-swimming distance for 5 min was analyzed using an ethovision XT 5 software (Noldus, Leesburg, VA).

Immunoblotting analysis

For the asyn, p62, LC3, neuron specific enolase (NSE) and β-actin analysis, the brains of medaka were homogenized in high-salt buffer containing 1% Triton X-100 (750 mM NaCl, 5 mM EDTA, 50 mM Tris-HCl, 1% [v/v] Triton X-100, pH 7.5). For the GBA2 analysis, the brains of medaka were homogenized in RIPA buffer (50 mM Tris-HCl, 0.15 M NaCl, 1% [v/v] Triton X-100, 0.1% [w/v] sodium dodecyl sulfate (SDS), 1% [v/v] sodium deoxycholate, pH 7.5). The homogenate was centrifuged at 20,400 × g at 4 °C for 5 min. For SDS-soluble fractions, the pellet of asyn analysis was subsequently sonicated in SDS buffer (50 mM Tris-HCl, 2% SDS, pH 7.4) followed by centrifugation at 20,400 × g at 4 °C for 5 min. The supernatant was collected, and the protein concentration was measured using a BCA protein assay kit (Pierce, Rockford, IL, USA). The supernatant was then mixed with sample buffer (1% [w/v] SDS, 12.5% [w/v] glycerol, 0.005% [w/v] bromophenol blue, 2.5% [v/v] 2-mercaptoethanol, 25 mM Tris-HCl, pH 6.6) and boiled at 95 °C for 10 min. The boiled samples containing 10 µg of protein or 0.75 µg for SDS-soluble fractions of protein were separated on 4-12% NuPAGE Bis-Tris Precast Gel (Thermo Fisher Scientific) or 10-20% SuperSep ^TMACE (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) and transferred to polyvinylidene difluoride membranes using a Trans-Blot SD semi Dry Transfer Cell (Bio-Rad Laboratories Inc., Hercules, California, USA).

To detect medaka asyn, the membranes were treated with 4% (w/v) paraformaldehyde in phosphate-buffered saline (PBS) for 30 min at room temperature before being blocked with 5% skim milk (33, 38). The following antibodies were used as the primary antibody: anti-β-actin (#A1978, 1:5000; Sigma-Aldrich Co., St. Louis, MO, USA), anti-LC3 (#PM036, 1:2000; MBL,　Nagoya, Japan), anti-NSE (#M0873, 1:500; DAKO, Carpinteria, CA, USA), anti-medaka asyn (1:10,000) (33) and anti-p62 (#PM045, 1:500; MBL). Anti-medaka GBA2 antibody was raised against 377-395 amino acids of medaka GBA2 at Sigma-Aldrich Co. (1:1000). The membrane was incubated with anti-β-actin for 60 min at room temperature or with other primary antibodies for 1 day at 4 °C. Subsequent steps were performed according to the standard method using horseradish peroxidase-conjugated secondary antibodies (1:5000; Novus, Biologicals, Littleton, CO, USA) for 1 h at room temperature. The chemiluminescent signal was detected using an Amersham Imager 600 (GE Healthcare, Chicago, IL, USA).

Confirmation of the cross-species reactivity of antibodies

To confirm the cross-species reactivity of the antibodies used in present study against medaka, the homology search for β-actin, LC3, NSE and p62 genes was performed (https://asia.ensembl.org/Oryzias_latipes/Info/Index), showing high homology of medaka β-actin, LC3, and NSE to the peptides used to raise each antibodies (93%, 93%, and 84%, respectively). Meanwhile, medaka p62 has only 45% homology to the peptide used to raise the p62 antibody. Therefore, we confirmed the cross-species reactivity of the antibody by overexpression of medaka p62 in cultured cells. The medaka p62 cDNA was cloned into pcDNA3 and expressed in HEK293T cells. A strong signal was detected at the molecular weight close to that of medaka p62 in the lanes of medaka p62-overexpressing HEK293T cells and GBA1 KO medaka brains, but not in the lane of mock transfected cells (Additional file 2: Fig. S2). The following is the brief description about this experiment. The sequence of medaka p62 cDNA was determined previously (33). The full-length medaka p62 cDNA was inserted into pcDNA3 to generate a medaka p62-expressing vector. Transfection of HEK293T cells (RIKEN Cell Bank, Tsukuba, Japan) with vector alone (pcDNA3-mock) or vector containing full-length medaka p62 (pcDNA3p62) was performed using polyethyleneimine MAX (#24765, Polysciences, Warrington, PA, USA) according to the manufacturer’s instructions. The cell lysates were obtained with RIPA buffer. The SDS-PAGE and chemiluminescent signal detection was performed as described above. The amount of loading protein was 5 μg for WT and GBA1 KO medaka brains and 0.05 μg and 0.25 μg for cell lysates, respectively.

Immunohistochemical analysis

Paraffin sections were used for immunohistochemical analysis as reported previously (33). Medaka brains were collected by surgery. The collected brains were fixed with 4% (w/v) paraformaldehyde in PBS at 4 °C for 1 day and stored in 70% ethanol until use. The fixed samples were dehydrated and embedded in paraffin using Surgipath FSC22 (Leica, Wetzlar, Germany), and sections were acquired using a Microm HM 325 (Thermo Fisher Scientific). The thickness of the sections was set at 20 µm for tyrosine hydroxylase (TH) -positive cell counting and 8 µm for other analysis. The following antibodies were used as the primary antibody: anti-TH (#MAB318, 1:1000; Merck Millipore, Burlington, MA) and anti-medaka asyn (1:2000) (33). The sections were incubated with the primary antibody at 4 °C for 1 day after blocking with 4% skim milk. Histofine (#414322; Nichirei Bioscience Inc., Tokyo, Japan) was used as the secondary antibody for diaminobenzidine staining.

Cell counting

The number of dopaminergic (DA) neurons in the middle diencephalon and noradrenergic (NA) neurons in the locus coeruleus were counted as previously described (33). The numbers of TH-positive neurons with visible nuclei were counted under the microscope (Cx41; Olympus, Tokyo, Japan).

Quantitative reverse transcription polymerase chain reaction (qRT-PCR)

RNA was isolated from medaka brains using Qiazol (QIAGEN). cDNA was synthesized using the PrimeScript RT reagent kit of Perfect Real Time (#RR037A; Takara, Kyoto, Japan). The quantification of cDNA was performed with the LightCycler 480 using LightCycler 480 SYBR GreenⅠ Master (#04887352001; Roche Diagnostics, Mannheim, Germany). The following primer sets were used: TNF-α: 5’-ATTGGAGTGAAAGGCCAGAA-3’ and 5’-ACTAATTTGAGACCGCCACG-3’; β-actin: 5’-TCCACCTTCCAGCAGATGTG-3’ and 5’-AGCATTTGCGGTGGACGAT-3’; apolipoprotein E (ApoE)-b: 5’-GACGAGAGTTGGAGACCCTGA-3’ and 5’-ACTGGTGCTTGTGGTGATGG-3’; and asyn: 5’-ATGGACGCGTTAATGAAGGGTTT-3’ and 5’-TCAGTCATCGCTGTCTTCCT -3’.

High performance liquid chromatography for the dopamine quantification

Medaka brains were homogenized in 100 µl of 0.1 M HClO₄ containing 4mM Na₂S₂O₅ and 4 mM diethylenetriaminepentaacetic acid. The supernatant by centrifugation at 20,400 × g for 5 min was used for measurement. High performance liquid chromatography (HPLC) was conducted with a mobile phase A (acetonitrile:methanol, 1000:25.9:62.9 [v/v/v], with 0.1 M phosphate, 0.05 M citrate, 4 mM sodium 1-heptanesulfonate and 0.1 mM EDTA, pH 3.0). Dopamine was detected with series coulometric detector (ESA, Inc., Chelmsford, MA, USA). Data were collected and analyzed on a CHROMELEONTM Chromatography Data Systems 6.40 (Dionex, Sunnyvale, CA, USA).

The material for the sphingolipid profile analysis

β-d-Glucopyranosyl-(1→1)-N-lauroyl-d-erythro-sphingosine (GlcCer [d18:1-C12:0]), β-d-glucopyranosyl-(1→1)-d-erythro-sphingosine-d5 (GlcSph-d5), N-lauroyl-d-erythro-sphingosine (ceramide [d18:1-C12:0]) and d-erythro-sphingosine (C17 base) (d17:1-sphingosine) were purchased from Avanti Polar Lipids (Alabaster, AL, USA). For liquid chromatography (LC)-electrospray ionization tandem mass spectrometry (ESI-MS/MS), high-performance LC-grade acetonitrile and methanol were purchased from Thermo Fisher Scientific (Waltham, MA, USA), chloroform and distilled water were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan), and ammonium formate was purchased from Sigma Aldrich, Japan.

Lipid extraction for the sphingolipid profile analysis

The frozen tissue (approximately 5 mg) was homogenized, and total lipids were extracted with a chloroform:methanol (C:M) (2:1 [v/v], 5 ml) mixture spiked with 1 pmol/mg frozen tissue of GlcCer (d18:1-C12:0), GlcSph-d5, ceramide (d18:1-C12:0) and d17:1-sphingosine as internal standards. Extracts were dried under a flow of N₂ gas and hydrolyzed for 2 h at room temperature in C:M (2:1 [v/v], 2 ml) containing 0.1 M KOH. The reaction mixture was neutralized with 7.5 µl of glacial acetic acid.

For the analysis of GlcCer, galactosylceramide (GalCer), GlcSph and galactosylsphingosine (GalSph: psychosine), the neutralized reaction mixture was subjected to Folch’s partition, and the lower phase was dried under a flow of N₂ gas. For the analysis of ceramide and sphingosine, the neutralized reaction mixture was dried under a flow of N₂ gas. The resulting lipid films were suspended in C:M (2:1, v/v) at a concentration of 100 µg frozen tissue/µl, and aliquots were subjected to LC-ESI-MS/MS.

LC-ESI-MS/MS for the sphingolipid profile analysis

LC-ESI-MS/MS was performed on an LC system Nexera X2 (SHIMADZU, Kyoto, Japan) attached to a triple-quadrupole linear ion trap mass spectrometer (QTRAP4500; SCIEX, Tokyo, Japan). The LC-ESI-MS/MS datasets were analyzed with the MultiQuant™️ (ver. 2.1) and Analyst (SCIEX) software programs. Target lipids were monitored in multiple reaction monitoring (MRM) mode using specific precursor-product ion pairs, as detailed in Additional file 3: Table S1. Peak areas were integrated and quantified relative to the associated internal standard.

GlcCer, GalCer, GlcSph and GalSph were analyzed as previously reported (15, 39, 40) by hydrophilic interaction chromatography (HILIC)-ESI-MS/MS with minor modifications. In brief, 100 µg frozen tissue/µl of the lipid extracts was diluted 10-fold with mobile phase A (acetonitrile:methanol:formic acid, 97:2:1 [v/v/v], with 5 mM ammonium formate), and aliquots (10 µl) were applied to an Atlantis silica HILIC column (2.1 mm i.d.×150 mm, particle size, 3 µm; Waters, Milford, MA) maintained at 40 °C. Samples were eluted at a flow rate of 0.15 ml/min using the following gradient of mobile phase B (methanol:water:formic acid, 89:9:1 [v/v/v], with 20 mM ammonium formate): 3.3 min, 0%; 13.4 min, 0%-35% linear gradient; 1.3 min, 35%-70% linear gradient; 3 min, 70% (washing step); 29 min, 0%, flow rate increased to 0.2 ml/min (equilibration); 1 min, 0%, flow rate decreased to 0.15 ml/min. The mass spectrometer was set to positive ion mode (ion spray voltage, 5500 V; curtain gas pressure, 30 psi; nebulizer gas pressure, 90 psi; heating gas pressure, 30 psi, temperature, 100 °C) using MRM detection for a targeted analysis. The quantitative values of GlcCer and GalCer with various chain lengths of fatty acids (C14:0, C16:0, C18:1, C18:0, C20:1, C20:0, C22:1, C22:0, C23:1, C23:0, C24:1, C24:0 and C26:1) were summarized.

Ceramide and d-erythro-sphingosine (C18 base) (sphingosine (C18 base)) were analyzed as previously reported (39) by reversed-phase liquid chromatography (RPLC)-ESI-MS/MS with minor modifications. The lipid extracts dissolved in C:M (2:1, v/v) were diluted 10-fold with mobile phase B (M:W 85:15 [v/v], 5 mM ammonium acetate) and applied to an RP column (Luna C18(2) column; 2 mm i.d.×250 mm, particle size, 3 μm; Phenomenex, Torrance, CA, USA) maintained at 36 °C and at a flow rate of 0.15 ml/min. The samples were then eluted with the following gradients of mobile phase A (methanol pure, 5 mM ammonium acetate): 2 min, 0%; 13 min, 0%-100% linear gradient; 40 min, 100% (washing step); 15 min 0% (equilibration). The mass spectrometer was set to positive ion mode (ion spray voltage, 5500 V; curtain gas pressure, 20 psi; nebulizer gas pressure, 80 psi; heating gas pressure, 40 psi, temperature, 100 °C), utilizing either MRM detection for a targeted analysis. The quantitative values of ceramide with various chain lengths of fatty acids (C16:0, C18:0, C24:1 and C24:0) were summarized.

Statistical analysis

For comparison of two groups, at wo-tailed unpaired Student’s t-test was performed. An F test was performed to evaluate the differences in variances. For comparison of three or more groups, one-way ANOVA with Tukey’s post-hoc test or Newman-Keuls multiple comparison test was performed. A Brown–Forsythe test was performed to evaluate the differences in variances. Differences with p values of less than 0.05 were considered significant. Statistical calculations were performed with GraphPad Prism Software (GraphPad) Version5.0 and 7.04.

GBA2KO medaka show no apparent phenotypes

First, we attempted to create GBA2 KO medaka using a CRISPR/Cas9 system. We successfully generated GBA2 nonsense mutant medaka via the deletion of 21 bases and insertion of 2 bases in exon 5, which resulted in a frame shift mutation (Additional file 1: Fig. S1).

There were 15 off-target candidates of crRNA used in this study, among which 3 were located on exons (Additional file 4: Table S2). We conducted direct sequencing and confirmed no mutations around these three off-target candidates in medaka carrying heterozygous mutations of the GBA2 gene.

We then analyzed the phenotypes of GBA2 KO medaka after heterozygous GBA2 mutant medaka were backcrossed with WT medaka more than five times. GBA2 KO medaka showed almost no enzymatic activity or protein expression of GBA2 in the brains, and GBA2^+/⁻ medaka showed ~58% of enzymatic activity and ~42 % of protein expression level compared with WT medaka (Fig. 1a and Additional file 5: Fig. S3). The lifespan of both GBA2 KO and GBA2^+/⁻ medaka were the same as that of WT medaka (Fig. 1b).

We next examined their locomotor function, as the loss of GBA2 causes locomotor dysfunction in mice and zebrafish (25, 29). In contrast to GBA2-deficient mice and zebrafish, GBA2 KO medaka showed a normal locomotor function at 6 mpf (Fig. 1c). Such differential effects of GBA2 loss between medaka and mice are seen also in spermatogenesis, as Gba2 KO mice show infertility due to morphological abnormality of sperm, called globozoospermia, while GBA2 KO medaka showed a normal fertilization rate (Fig. 1d). In summary, GBA2 KO medaka did not show any apparently abnormal phenotypes.

Deletion of GBA2 in GBA1 KO medaka does not rescue the CNS phenotype

The compensatory changes of GBA2 in GBA1 deficiency remain unclear given the previous studies showing conflicting data (18, 28, 41-43). Therefore, we examined GBA2 protein expression levels and enzymatic activity in the brains of WT and GBA1 deficient medaka, showing no differences in protein expression levels or enzymatic activity among the GBA1 genotypes (Additional file 6: Fig. S4a and b). To examine whether or not GBA2 functioned as a novel factor affecting nGD, we crossed GBA2 mutant medaka and GBA1 mutant medaka to generate GBA1/GBA2 DKO medaka. Recent studies have reported that the deletion of GBA2 in a GD1 mouse model and a NPC mouse model improved the phenotypes (30, 31). However, GBA1/GBA2 DKO medaka showed a shorter lifespan than GBA1 KO medaka (Fig. 2a). In addition, GBA1/GBA2 DKO medaka did not show improvement of abnormal swimming movement seen in GBA1 KO medaka (33) (Additional file 7–10: video S1–4).

We previously reported that GBA1 KO medaka showed non-selective neuronal cell loss, increased inflammatory cytokine levels and microgliosis similar to nGD patients (33). We then evaluated the neuronal cell loss by counting the number of TH-positive neurons in the middle diencephalon (DA neurons) and locus coeruleus (NA neurons). The numbers of both DA and NA neurons were decreased in GBA1/GBA2 DKO medaka, similar to GBA1 KO medaka at 3 months mpf (Fig. 2b–d). The amount of DA in the brains showed trend towards decrease in GBA1 KO and GBA1/GBA2 DKO medaka albeit not reaching statistical significance (Additional file 11: Fig. S5). We also examined the neuroinflammation in GBA1/GBA2 DKO medaka. The mRNA expression of tumor necrosis factor (TNF)-α was not markedly different between GBA1/GBA2 DKO and GBA1 KO medaka at 3 mpf (Fig. 2e). We evaluated the expression of mRNA encoding ApoE-b, a marker for microglia in teleost fish (33, 44), and confirmed a similar expression between GBA1/GBA2 DKO and GBA1 KO medaka at 3 mpf (Fig. 2f). GBA1/GBA2 DKO medaka also showed the Gaucher cell-like cells which had been seen in GBA1 KO medaka (33) (Additional file 12: Fig. S6). These results suggested that the deletion of GBA2 does not rescue any CNS phenotypes of GBA1 KO medaka.

Perturbation of sphingolipid metabolism in the brains of GBA1 and GBA2 mutant medaka

To examine why GBA2 deletion did not rescue the CNS phenotypes of GBA1 KO medaka, we analyzed the sphingolipid profiles in the medaka brains at 3 mpf using LC-ESI-MS/MS which can measure GlcCer and GalCer separately. We previously reported that GBA1 KO medaka showed the accumulation of GlcCer, similar to GBA1-deficient human and mice (33). The sphingolipid analysis showed that the amounts of GlcCer and glucosylsphingosine (GlcSph) were further increased in GBA1/GBA2 DKO medaka compared with GBA1 KO medaka (Fig. 3a and e). The amount of GlcCer tended to be increased in GBA2 KO medaka compared with WT medaka (Fig. 3a and Additional file 13: Table S3). While the amount of ceramide did not differ among the genotypes (Fig. 3c), that of GalCer was decreased in the absence of GBA1, irrespective of the GBA2 genotype (Fig. 3b). In contrast, GalSph was increased in GBA1 KO and GBA1/GBA2 DKO medaka (Fig. 3f). While the amount of GalSph was below the detection threshold in WT medaka, it was detected in GBA2 KO and GBA1^+/- medaka (Fig. 3f and Additional file 13: Table S3). GalSph is a substrate of galactocerebrosidase (GALC) and is believed to be the pathological sphingolipid involved in Krabbe’s disease (KD) (45). The amount of sphingosine, which was reported to have cellular toxicity in GD1 (30), was not decreased in GBA1/GBA2 DKO medaka compared with GBA1 KO medaka (Fig. 3d).

In summary, compared with GBA1 KO medaka, GBA1/GBA2 DKO medaka showed a significant increase in GlcCer and GlcSph, and an increasing trend in sphingosine and GalSph in the brains. Although GBA2 KO medaka did not show significant increase in the amount of GlcCer and GalSph when comparing all groups, it reached statistical significance when compared with WT medaka (Additional file 13: Table S3).

The impact of GBA2 on asyn accumulation

Recent studies have suggested that the perturbation of sphingolipid metabolism causes conformational changes in asyn, resulting in asyn accumulation (16, 18, 46). GBA1 KO medaka showed the perturbation of sphingolipid metabolism accompanied by asyn accumulation in axonal swellings containing autophagosomes, which we previously determined by electron microscopic analysis (33). The increase in asyn was seen in the Triton X-soluble fractions, but not in SDS-soluble fractions (Additional file 14: Fig. S7). Compared with WT medaka, GBA2 KO medaka showed an increased amount of asyn without any asyn-positive abnormal structures like axonal swellings seen in GBA1 KO medaka (Fig. 4a–c). GBA1/GBA2 DKO medaka showed the similar phenotypes to those of GBA1 KO medaka (Fig. 4a–c). The qRT-PCR for asyn mRNA showed no increase in asyn mRNA expression levels in GBA2 KO medaka (Additional file 15: Fig. S8). This result suggests that asyn accumulation seen in GBA2 KO medaka was the pathological process after the translation.

ALP dysfunction observed in GBA1 KO medaka but not in GBA2 KO medaka.

Perturbation of the sphingolipid metabolism may be one of the causes of the asyn accumulation in the brains of GBA2 KO medaka. Because both ALP dysfunction and the perturbation of the sphingolipid metabolism presumably leads to asyn accumulation in GBA1 KO medaka (33), we also examined the autophagic function in GBA2 KO medaka. p62 acts as a linker between microtubule-associated protein 1 light chain 3 (LC3) and poly-ubiquitinated proteins. The expression of p62 correlates with the autophagic function, while the LC3-II/I ratio correlates with the accumulation of autophagosomes (47). The expression of p62 and the LC3-II/I ratio were not markedly different in the brains of GBA2 KO medaka from WT (Fig. 5a–c). We assumed that ALP was intact in GBA2 KO medaka in contrast to GBA1 KO medaka.

This study revealed that the deletion of GBA2 in GBA1 KO medaka did not rescue the phenotypes, including the altered sphingolipid metabolism and asyn accumulation in the brains. However, GBA2 represents a novel factor affecting asyn accumulation because GBA2 KO medaka showed an increased amount of asyn in the brains, which was presumably caused by the perturbation of sphingolipid metabolism.

In GD, the accumulation of substrates of GBA1, GlcCer and GlcSph, is thought to play a central role in disease pathogenesis. At present, substrate reduction therapy to reduce these sphingolipids is available for GD1. Although the mechanisms by which GBA1 mutations cause PD are not fully understood yet, the following evidence suggests that a reduced GBA1 enzymatic activity is responsible for the increased risk of developing PD: (1) There is a negative correlation between GBA1 activity and asyn accumulation (17, 48); (2) heterozygous GBA1 mutations with lower enzymatic activities confer a higher risk of developing PD than those with higher enzymatic activities (49); (3) GBA1 overexpression ameliorates asyn accumulation in a PD mouse model (50). Furthermore, previous studies have shown that a brain-penetrating GlcCer synthase inhibitor and a GBA1 chaperone alleviated pathological asyn accumulation in the brains of PD mouse models (51, 52). These findings suggest that decreased GBA1 enzymatic activity and the resultant substrate accumulation lead to increased risk of developing PD.

In the present study, we focused on another glucocerebrosidase, GBA2, as a novel factor affecting nGD, based on the findings of a previous study that the deletion of GBA2 ameliorated the visceral phenotypes of a GD1 mouse model (30). However, contrary to our expectation, deletion of GBA2 did not rescue any CNS phenotypes of GBA1 KO medaka. In the previous study using the GD1 mouse model, the deletion of GBA2 was presumed to reduce the amount of sphingosine, thereby ameliorating the phenotypes of GD1 mice (30). However, in the present study, the analysis of the sphingolipids profile revealed that the deletion of GBA2 in GBA1 KO medaka further tended to increase sphingosine in the brains, albeit without statistical significance. This result suggests the existence of an alternative pathway that produces sphingosine other than GBA1 and GBA2. One possible explanation for the failure of the GBA2 deletion to rescue the phenotypes of GBA1 KO medaka is the fact that human, rats and medaka, but not mice, possess GBA3. GBA3 hydrolyzes GlcCer and GlcSph to produce sphingosine in GBA1/GBA2 DKO medaka but not in GBA1/GBA2 DKO mice. This pathological effect of GBA3 is supported by a report that GBA3 does not affect the pathology of GD1, whereas GD1 patients with GBA3 deficiency showed a tendency to have a milder disease severity (53). Thus, the failure to rescue the phenotypes of GBA1 KO medaka by GBA2 deletion may be relevant to human patients having GBA1/GBA2 mutations.

The present study reports, for the first time, the extensive sphingolipids profile in the brains of GBA2 KO animals. Compared with WT medaka, the amounts of GlcCer and GalSph were found to be significantly increased in GBA2 KO medaka. Accumulating evidence has shown that metabolic abnormalities of sphingolipids are closely associated with the formation of asyn pathology (16, 17, 46, 54). Under physiological conditions, asyn maintains an equilibrium state between free monomeric and membrane-bound multimeric conformation, which can be disrupted by sphingolipid metabolic perturbation (54-56). Previous studies demonstrated that GlcCer converts the physiological asyn conformers to assembly-state pathological asyn, leading to its accumulation in cultured cells (17, 46). In addition, a recent report indicated that GalSph also promotes asyn aggregation (57, 58). Based on these previous studies, an increased amount of GlcCer and/or GalSph presumably causes asyn accumulation in the brains of GBA2 KO medaka. The facts of the absence of increased asyn mRNA expression level and autophagic dysfunction in GBA2 KO medaka also support this idea. However, given that the physiological function of GBA2 is largely unknown, we cannot exclude the possibility that other mechanisms are also involved in asyn accumulation in GBA2 KO medaka. To our knowledge, the present study is also the first report showing the quantitative changes in GalSph in the brains of heterozygous GBA1 mutant animals. Although its synthetic pathway is unclear, GalSph might play some pathological role in GBA1-PD. Based on the findings of the present and previous studies, we have proposed pathological mechanisms of asyn accumulation under GBA1 or GBA2 KO backgrounds (Fig. 6). Interestingly, it was reported that the GBA2 activity declines with aging in the mouse brain (59). Given the asyn accumulation in GBA2 KO medaka, as shown in the present study, GBA2 may be a novel factor affecting asyn accumulation in the brains. Further studies are required to determine whether the perturbation of sphingolipid metabolism directly causes asyn accumulation in GBA2 KO animals and how the age-dependent decline in GBA2 activity contributes to asyn accumulation.

One limitation of the present study is that we failed to detect asyn in the Triton X-insoluble, SDS-soluble fractions in any genotypes partly because of small brains of medaka. Another limitation is that we did not detect pathological asyn aggregates on brain sections due to the lack of antibodies that recognize pathological medaka asyn, such as misfolded or phosphorylated forms of asyn.

In conclusion, the present study demonstrated that GBA2 deletion does not rescue the CNS phenotypes of GBA1 KO medaka. GBA2 KO increases the amount of asyn in the medaka brain, through sphingolipid metabolic perturbation. Further studies are required to determine whether GBA2 could be a therapeutic target of α-synucleinopathy.

ApoE: apolipoprotein E

asyn: alpha synuclein

Cas9: CRISPR-associated 9

Ceramide [d18:1-C12:0]: N-lauroyl-d-erythro-sphingosine

C:M: chloroform:methanol

CNS: central nervous system

CRIPSR: clustered regularly interspaced short palindromic repeats

DA: dopaminergic

DKO: double knockout

ESI-MS/MS: electrospray ionization tandem mass spectrometry

GalCer: galactosylceramide

GalSph: galactosylsphingosine

GALC: galactosylceramidase

GCase: glucocerebrosidase

GlcCer: glucosylceramide

GlcCer (d18:1-C12:0): β-d-glucopyranosyl-(1→1)-N-lauroyl-d-erythro-sphingosine

GlcSph: glucosylsphingosine

GlcSph-d5: β-d-glucopyranosyl-(1→1)-d-erythro-sphingosine-d5

GD: Gaucher’s disease

HILIC: hydrophilic interaction chromatography

High performance liquid chromatography: HPLC

KD: Krabbe’s disease

KO: knockout

LC: liquid chromatography

LC3: microtubule-associated protein 1 light chain 3

mpf: months post fertilization

MRM: multiple reaction monitoring

NA: noradrenergic

NB-DGJ: N-deoxygalactonojirimycin

NSE: neuron specific enolase

nGD: neuronopathic Gaucher’s disease

NPC: Niemann-Pick Type C

PBS: phosphate-buffered saline

PD: Parkinson’s disease

qRT-PCR: quantitative reverse transcription polymerase chain reaction

RPLC: reversed-phase liquid chromatography

SDS: sodium dodecyl sulfate

d17:1-Sphingosine: d-erythro-sphingosine (C17 base)

Sphingosine (C18 base): d-erythro-sphingosine (C18 base)

TH: tyrosine hydroxylase

TNF: tumor necrosis factor

WT: wild type

Acknowledgements

We thank Dr. Miki Oono, Ms. Rie Hikawa and Yasuko Matsuzawa for the technical support.

Authors’ contributions

Conceived and designed the experiments: EN, NU, HA, HY, ST, YH, RT. Performed the experiments: EN, NU, HA, MK, SM. Analyzed the data: EN, NU, HA. Contributed materials: MS, YT. Wrote the paper: EN, NU, HA, MK, ST, HY, YH, RT. All authors read and approved the final manuscript.

Funding

This work was supported by Grants-in-Aid for Scientific Research (A) [15H02540 and 18H04041]; Grant-in-Aid for Young Scientists (B) [15K19486]; Core Research for Evolutional Science and Technology, Japan Science and Technology Agency [JP16gm0710011]; Grant-in-Aid for Scientific Research on Innovative Areas [JP23111002]; and Japan Society for the Promotion of Science core to core program.

Availability of data and materials

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate

All experimental procedures used in this study followed national guidelines. The Animal Research Committee of Kyoto University granted a formal waiver of ethical approval and also granted permission.

Consent for publication

Not applicable

Competing interests

The authors declare that they have no competing interests.

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Impact of GBA2 on Neuronopathic Gaucher’s Disease and α-Synuclein Accumulation in Medaka (Oryzias Latipes)

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