Accumulation of glucosylceramide in the absence of the beta-glucosidase GBA2 alters cytoskeletal dynamics

Sinbaglustat ameliorates disease pathology in a murine model of GM1 gangliosidosis without affecting CNS ganglioside levels

Sinbaglustat is a brain-penetrating small molecule that inhibits the non-lysosomal glucocerebrosidase (GBA2) and, with lower potency, glucosylceramide synthase (GCS). Sinbaglustat has passed clinical phase I. Our preclinical study assessed its efficacy in a transgenic mouse model of GM1 gangliosidosis, lacking a functional β-galactosidase enzyme (Glb1-/-). Starting at 4 weeks of age, mice were either treated with a nominal dose of 10 or 300 mg/kg/day of sinbaglustat or remained untreated. Wild-type (WT) mice served as control. Body weight, clinical and neurological signs, and motor function was assessed until 17-18 weeks (4 months) and 30 weeks (7 months) of age when mice were euthanized for ex vivo assessments. In comparison to WT, Glb1-/- mice showed the expected accumulation of GM1 gangliosidosis-related sphingolipids, neuropathology, and behavioral deficits. Both dosages of sinbaglustat left GM1 and lyso GM1 levels in the brain unaffected but delayed the onset of motor impairment and progression of clinical disease in Glb1-/- mice with the higher dose being more efficacious. Histologically and immunohistochemically, both treatment groups of Glb1-/- mice displayed reduced neuronal vacuolation. Only the higher dose of sinbaglustat decreased axonal damage and astrogliosis, which was also associated with a decrease of the axonal/neuronal damage marker plasma neurofilament light at 4 months (17-18 weeks). Both doses of sinbaglustat increased the GBA2 substrate glucosylceramide (GluCer) in the brain, while only the high dose reduced GluCer and other glycosphingolipids (GSLs) in the periphery indicating additional inhibition of GCS. We conclude that sinbaglustat had a therapeutic-like effect in the GM1 gangliosidosis mouse model.

Molecular insights into sperm head shaping and its role in human male fertility

BACKGROUND

Sperm head shaping, controlled by the acrosome-acroplaxome-manchette complex, represents a significant morphological change during spermiogenesis and involves numerous proteins expressed in a spatially and temporally specific manner. Defects in sperm head shaping frequently lead to teratozoospermia concomitant with oligozoospermia and asthenozoospermia, but the pathogenic mechanism underlying sperm head shaping, and its role in male infertility, remain poorly understood.

OBJECTIVE AND RATIONALE

This review aims to summarize the mechanism underlying sperm head shaping, reveal the relationship between gene defects associated with sperm head shaping and male infertility in humans and mice, and explore potential clinical improvements in ICSI treatment.

SEARCH METHODS

We searched the PubMed database for articles published in English using the keyword 'sperm head shaping' in combination with the following terms: 'acrosome formation', 'proacrosomal vesicles (PAVs)', 'manchette', 'perinuclear theca (PT)', 'chromatin condensation', 'linker of nucleoskeleton and cytoskeleton (LINC) complex', 'histone-to-protamine (HTP) transition', 'male infertility', 'ICSI', and 'artificial oocyte activation (AOA)'. The selected publications until 1 August 2024 were critically summarized, integrated, and thoroughly discussed, and the irrelevant literature were excluded.

OUTCOMES

A total of 6823 records were retrieved. After careful screening, integrating relevant literature, and excluding articles unrelated to the topic of this review, 240 articles were ultimately included in the analysis. Firstly, we reviewed the important molecular events and structures integral to sperm head shaping, including PAV formation to fusion, acrosome attachment to the nucleus, structure and function of the manchette, PT, chromatin condensation, and HTP transition. Then, we set forth human male infertility associated with sperm head shaping and identified genes related to sperm head shaping resulting in teratozoospermia concomitant with oligozoospermia and asthenozoospermia. Finally, we summarized the outcomes of ICSI in cases of male infertility resulting from mutations in the genes associated with sperm head shaping, as well as the ICSI outcomes through AOA for infertile men with impaired sperm head.

WIDER IMPLICATIONS

Understanding the molecular mechanisms of sperm head shaping and its relationship with human male infertility holds profound clinical implications, which may contribute to risk prediction, genetic diagnosis, and the potential treatment of human male infertility.

Ceramide-mediated orchestration of oxidative stress response through filopodia-derived small extracellular vesicles

Extracellular vesicles (EVs) are shed from the plasma membrane, but the regulation and function of these EVs remain unclear. We found that oxidative stress induced by H2O2 in Hela cells stimulated filopodia formation and the secretion of EVs. EVs were small (150 nm) and labeled for CD44, indicating that they were derived from filopodia. Filopodia-derived small EVs (sEVs) were enriched with the sphingolipid ceramide, consistent with increased ceramide in the plasma membrane of filopodia. Ceramide was colocalized with neutral sphingomyelinase 2 (nSMase2) and acid sphingomyelinase (ASM), two sphingomyelinases generating ceramide at the plasma membrane. Inhibition of nSMase2 and ASM prevented oxidative stress-induced sEV shedding but only nSMase2 inhibition prevented filopodia formation. nSMase2 was S-palmitoylated and interacted with ASM in filopodia to generate ceramide for sEV shedding. sEVs contained nSMase2 and ASM and decreased the level of these two enzymes in oxidatively stressed Hela cells. A novel metabolic labeling technique for EVs showed that oxidative stress induced secretion of fluorescent sEVs labeled with NBD-ceramide. NBD-ceramide-labeled sEVs transported ceramide to mitochondria, ultimately inducing cell death in a proportion of neuronal (N2a) cells. In conclusion, using Hela cells we provide evidence that oxidative stress induces interaction of nSMase2 and ASM at filopodia, which leads to shedding of ceramide-rich sEVs that target mitochondria and propagate cell death.

From seconds to days: Neural plasticity viewed through a lipid lens

Many adult neurons are dynamically remodeled across timescales ranging from the rapid addition and removal of specific synaptic connections, to largescale structural plasticity events that reconfigure circuits over hours, days, and months. Membrane lipids, including brain-enriched sphingolipids, play crucial roles in these processes. In this review, we summarize progress at the intersection of neuronal activity, lipids, and structural remodeling. We highlight how brain activity modulates lipid metabolism to enable adaptive structural plasticity, and showcase glia as key players in membrane remodeling. These studies reveal that lipids act as critical signaling molecules that instruct the dynamic architecture of the brain.

Glial control of sphingolipid levels sculpts diurnal remodeling in a circadian circuit

Structural plasticity in the brain often necessitates dramatic remodeling of neuronal processes, with attendant reorganization of the cytoskeleton and membranes. Although cytoskeletal restructuring has been studied extensively, how lipids might orchestrate structural plasticity remains unclear. We show that specific glial cells in Drosophila produce glucocerebrosidase (GBA) to locally catabolize sphingolipids. Sphingolipid accumulation drives lysosomal dysfunction, causing gba1b mutants to harbor protein aggregates that cycle across circadian time and are regulated by neural activity, the circadian clock, and sleep. Although the vast majority of membrane lipids are stable across the day, a specific subset that is highly enriched in sphingolipids cycles daily in a gba1b-dependent fashion. Remarkably, both sphingolipid biosynthesis and degradation are required for the diurnal remodeling of circadian clock neurites, which grow and shrink across the day. Thus, dynamic sphingolipid regulation by glia enables diurnal circuit remodeling and proper circadian behavior.

Lipid Dyshomeostasis and Inherited Cerebellar Ataxia

Cerebellar ataxia is a form of ataxia that originates from dysfunction of the cerebellum, but may involve additional neurological tissues. Its clinical symptoms are mainly characterized by the absence of voluntary muscle coordination and loss of control of movement with varying manifestations due to differences in severity, in the site of cerebellar damage and in the involvement of extracerebellar tissues. Cerebellar ataxia may be sporadic, acquired, and hereditary. Hereditary ataxia accounts for the majority of cases. Hereditary ataxia has been tentatively divided into several subtypes by scientists in the field, and nearly all of them remain incurable. This is mainly because the detailed mechanisms of these cerebellar disorders are incompletely understood. To precisely diagnose and treat these diseases, studies on their molecular mechanisms have been conducted extensively in the past. Accumulating evidence has demonstrated that some common pathogenic mechanisms exist within each subtype of inherited ataxia. However, no reports have indicated whether there is a common mechanism among the different subtypes of inherited cerebellar ataxia. In this review, we summarize the available references and databases on neurological disorders characterized by cerebellar ataxia and show that a subset of genes involved in lipid homeostasis form a new group that may cause ataxic disorders through a common mechanism. This common signaling pathway can provide a valuable reference for future diagnosis and treatment of ataxic disorders.

Structure, metabolism and biological functions of steryl glycosides in mammals

Steryl glycosides (SGs) are sterols glycosylated at their 3β-hydroxy group. They are widely distributed in plants, algae, and fungi, but are relatively rare in bacteria and animals. Glycosylation of sterols, resulting in important components of the cell membrane SGs, alters their biophysical properties and confers resistance against stress by freezing or heat shock to cells. Besides, many biological functions in animals have been suggested from the observations of SG administration. Recently, cholesteryl glucosides synthesized via the transglycosidation by glucocerebrosidases (GBAs) were found in the central nervous system of animals. Identification of patients with congenital mutations in GBA genes or availability of respective animal models will enable investigation of the function of such endogenously synthesized cholesteryl glycosides by genetic approaches. In addition, mechanisms of the host immune responses against pathogenic bacterial SGs have partially been resolved. This review is focused on the biological functions of SGs in mammals taking into consideration their therapeutic applications in the future.

Effect of Expression of Human Glucosylceramidase 2 Isoforms on Lipid Profiles in COS-7 Cells

Glucosylceramide (GlcCer) is a major membrane lipid and the precursor of gangliosides. GlcCer is mainly degraded by two enzymes, lysosomal acid β-glucosidase (GBA) and nonlysosomal β-glucosidase (GBA2), which may have different isoforms because of alternative splicing. To understand which GBA2 isoforms are active and how they affect glycosphingolipid levels in cells, we expressed nine human GBA2 isoforms in COS-7 cells, confirmed their expression by qRT-PCR and Western blotting, and assayed their activity to hydrolyze 4-methylumbelliferyl-β-D-glucopyranoside (4MUG) in cell extracts. Human GBA2 isoform 1 showed high activity, while the other isoforms had activity similar to the background. Comparison of sphingolipid levels by ultra-high resolution/accurate mass spectrometry (UHRAMS) analysis showed that isoform 1 overexpression increased ceramide and decreased hexosylceramide levels. Comparison of ratios of glucosylceramides to the corresponding ceramides in the extracts indicated that GBA2 isoform 1 has broad specificity for the lipid component of glucosylceramide, suggesting that only one GBA2 isoform 1 is active and affects sphingolipid levels in the cell. Our study provides new insights into how increased breakdown of GlcCer affects cellular lipid metabolic networks.

Reconstruction of the birth of a male sex chromosome present in Atlantic herring

Understanding the evolution of sex determination mechanisms and sex chromosomes is of fundamental importance in biology. Here we have reconstructed the evolution of the sex-determining region in the Atlantic herring. The region is small and contains only three intact genes. The candidate sex-determining factor BMPR1BBY is an evolutionary innovation in the herring lineage. It encodes a truncated form of a BMP type I receptor, which originated by gene duplication and underwent rapid protein evolution. The receptor has maintained its kinase activity and has the potential to induce development of testis. The other two genes in the sex-determining region, CATSPERG and CATSPER3Y, are male beneficial genes because they encode proteins predicted to be essential for sperm to fertilize the egg.

The mechanisms underlying sex determination are astonishingly plastic. Particularly the triggers for the molecular machinery, which recalls either the male or female developmental program, are highly variable and have evolved independently and repeatedly. Fish show a huge variety of sex determination systems, including both genetic and environmental triggers. The advent of sex chromosomes is assumed to stabilize genetic sex determination. However, because sex chromosomes are notoriously cluttered with repetitive DNA and pseudogenes, the study of their evolution is hampered. Here we reconstruct the birth of a Y chromosome present in the Atlantic herring. The region is tiny (230 kb) and contains only three intact genes. The candidate male-determining gene BMPR1BBY encodes a truncated form of a BMP1B receptor, which originated by gene duplication and translocation and underwent rapid protein evolution. BMPR1BBY phosphorylates SMADs in the absence of ligand and thus has the potential to induce testis formation. The Y region also contains two genes encoding subunits of the sperm-specific Ca²⁺ channel CatSper required for male fertility. The herring Y chromosome conforms with a characteristic feature of many sex chromosomes, namely, suppressed recombination between a sex-determining factor and genes that are beneficial for the given sex. However, the herring Y differs from other sex chromosomes in that suppression of recombination is restricted to an ∼500-kb region harboring the male-specific and sex-associated regions. As a consequence, any degeneration on the herring Y chromosome is restricted to those genes located in the small region affected by suppressed recombination.

Molecular Mechanism Underlying the Action of Zona-pellucida Glycoproteins on Mouse Sperm

Mammalian oocytes are enveloped by the zona pellucida (ZP), an extracellular matrix of glycoproteins. In sperm, stimulation with ZP proteins evokes a rapid Ca²⁺ influx via the sperm-specific, pH-sensitive Ca²⁺ channel CatSper. However, the physiological role and molecular mechanisms underlying ZP-dependent activation of CatSper are unknown. Here, we delineate the sequence of ZP-signaling events in mouse sperm. We show that ZP proteins evoke a rapid intracellular pH_i increase that rests predominantly on Na⁺/H⁺ exchange by NHA1 and requires cAMP synthesis by the soluble adenylyl cyclase sAC as well as a sufficiently negative membrane potential set by the spem-specific K⁺ channel Slo3. The alkaline-activated CatSper channel translates the ZP-induced pH_i increase into a Ca²⁺ response. Our findings reveal the molecular components underlying ZP action on mouse sperm, opening up new avenues for understanding the basic principles of sperm function and, thereby, mammalian fertilization.

Truncated mutants of beta-glucosidase 2 (GBA2) are localized in the mitochondrial matrix and cause mitochondrial fragmentation

The enzyme β-glucosidase 2 (GBA2) is clinically relevant because it is targeted by the drug miglustat (Zavesca^®) and because it is involved in inherited diseases. Mutations in the GBA2 gene are associated with two neurological diseases on the ataxia-spasticity spectrum, hereditary spastic paraplegia 46 (SPG46) and Marinesco-Sjögren-like syndrome (MSS). To establish how GBA2 mutations give rise to neurological pathology, we have begun to investigate mutant forms of GBA2 encoded by disease-associated GBA2 alleles. Previously, we found that five GBA2 missense mutants and five C-terminally truncated mutants lacked enzyme activity. Here we have examined the cellular locations of wild-type (WT) and mutant forms of GBA2 by confocal and electron microscopy, using transfected cells. Similar to GBA2-WT, the D594H and M510Vfs*17 GBA2 mutants were located at the plasma membrane, whereas the C-terminally truncated mutants terminating after amino acids 233 and 339 (GBA2-233 and -339) were present in the mitochondrial matrix, induced mitochondrial fragmentation and loss of mitochondrial transmembrane potential. Deletional mutagenesis indicated that residues 161–200 are critical for the mitochondrial fragmentation of GBA2-233 and -339. Considering that the mitochondrial fragmentation induced by GBA2-233 and -339 is consistently accompanied by their localization to the mitochondrial matrix, our deletional analysis raises the possibility that that GBA2 residues 161–200 harbor an internal targeting sequence for transport to the mitochondrial matrix. Altogether, our work provides new insights into the behaviour of GBA2-WT and disease-associated forms of GBA2.

Sphingolipids controlling ciliary and microvillar function

Cilia and microvilli are membrane protrusions that extend from the surface of many different mammalian cell types. Motile cilia or flagella are only found on specialized cells, where they control cell movement or the generation of fluid flow, whereas immotile primary cilia protrude from the surface of almost every mammalian cell to detect and transduce extracellular signals. Despite these differences, all cilia consist of a microtubule core called the axoneme. Microvilli instead contain bundled linear actin filaments and are mainly localized on epithelial cells, where they modulate the absorption of nutrients. Cilia and microvilli constitute subcellular compartments with distinctive lipid and protein repertoires and specialized functions. Here, we summarize the role of sphingolipids in defining the identity and controlling the function of cilia and microvilli in mammalian cells.

Lipids in the Physiopathology of Hereditary Spastic Paraplegias

Hereditary spastic paraplegias (HSP) are a group of neurodegenerative diseases sharing spasticity in lower limbs as common symptom. There is a large clinical variability in the presentation of patients, partly underlined by the large genetic heterogeneity, with more than 60 genes responsible for HSP. Despite this large heterogeneity, the proteins with known function are supposed to be involved in a limited number of cellular compartments such as shaping of the endoplasmic reticulum or endolysosomal function. Yet, it is difficult to understand why alteration of such different cellular compartments can lead to degeneration of the axons of cortical motor neurons. A common feature that has emerged over the last decade is the alteration of lipid metabolism in this group of pathologies. This was first revealed by the identification of mutations in genes encoding proteins that have or are supposed to have enzymatic activities on lipid substrates. However, it also appears that mutations in genes affecting endoplasmic reticulum, mitochondria, or endolysosome function can lead to changes in lipid distribution or metabolism. The aim of this review is to discuss the role of lipid metabolism alterations in the physiopathology of HSP, to evaluate how such alterations contribute to neurodegenerative phenotypes, and to understand how this knowledge can help develop therapeutic strategy for HSP.

N-Butyldeoxygalactonojirimycin Induces Reversible Infertility in Male CD Rats

This study shows for the first time that an iminosugar exerts anti-spermiogenic effect, inducing reversible infertility in a species that is not related to C57BL/6 male mice. In CD rats, N-butyldeoxygalactonojirimycin (NB-DGJ) caused reversible infertility at 150 mg/kg/day when administered daily as single oral dose. NB-DGJ inhibited CD rat-derived testicular β-glucosidase 2 (GBA2) activity at 10 µM but did not inhibit CD rat-derived testicular ceramide-specific glucosyltransferase (CGT) at doses up to 1000 µM. Pharmacokinetic studies revealed that sufficient plasma levels of NB-DGJ (50 µM) were achieved to inhibit the enzyme. Fertility was blocked after 35 days of treatment and reversed one week after termination of treatment. The rapid return of fertility indicates that the major effect of NB-DGJ may be epididymal rather than testicular. Collectively, our in vitro and in vivo studies in rats suggest that iminosugars should continue to be pursued as potential lead compounds for development of oral, non-hormonal male contraceptives. The study also adds evidence that GBA2, and not CGT, is the major target for the contraceptive effect of iminosugars.

Role of β-glucosidase 2 in aberrant glycosphingolipid metabolism: model of glucocerebrosidase deficiency in zebrafish

β-glucosidases [GBA1 (glucocerebrosidase) and GBA2] are ubiquitous essential enzymes. Lysosomal GBA1 and cytosol-facing GBA2 degrade glucosylceramide (GlcCer); GBA1 deficiency causes Gaucher disease, a lysosomal storage disorder characterized by lysosomal accumulation of GlcCer, which is partly converted to glucosylsphingosine (GlcSph). GBA1 and GBA2 also may transfer glucose from GlcCer to cholesterol, yielding glucosylated cholesterol (GlcChol). Here, we aimed to clarify the role of zebrafish Gba2 in glycosphingolipid metabolism during Gba1 deficiency in zebrafish (Danio rerio), which are able to survive total Gba1 deficiency. We developed Gba1 (gba1 ^-/-), Gba2 (gba2 ^-/-), and double (gba1 ^-/- :gba2 ^-/-) zebrafish knockouts using CRISPR/Cas9 and explored the effects of both genetic and pharmacological interventions on GlcCer metabolism in individual larvae. Activity-based probes and quantification of relevant glycolipid metabolites confirmed enzyme deficiency. GlcSph increased in gba1 ^-/- larvae (0.09 pmol/fish) but did not increase more in gba1 ^-/- :gba2 ^-/- larvae. GlcCer was comparable in gba1 ^-/- and WT larvae but increased in gba2 ^-/- and gba1 ^-/- :gba2 ^-/- larvae. Independent of Gba1 status, GlcChol was low in all gba2 ^-/- larvae (0.05 vs. 0.18 pmol/fish in WT). Pharmacologic inactivation of zebrafish Gba1 comparably increased GlcSph. Inhibition of GlcCer synthase (GCS) in Gba1-deficient larvae reduced GlcCer and GlcSph, and concomitant inhibition of GCS and Gba2 with iminosugars also reduced excessive GlcChol. Finally, overexpression of human GBA1 and injection of recombinant GBA1 both decreased GlcSph. We determined that zebrafish larvae offer an attractive model to study glucosidase actions in glycosphingolipid metabolism in vivo, and we identified distinguishing characteristics of zebrafish Gba2 deficiency.

Fluoride exposure arrests the acrosome formation during spermatogenesis via down-regulated Zpbp1, Spaca1 and Dpy19l2 expression in rat testes

The exposure and health effects of fluoride are an ongoing topic that has attracted worldwide attention. Fluoride exposure disturbs the testicular development, sexual hormone levels and spermatogenesis. However, as to whether fluoride interferes with acrosome formation which is essential for production of capable spermatozoa during spermatogenesis still remains unclear. The objective was to determine the effects of fluoride on the acrosome formation and to further elucidate the potential mechanism of impaired reproductive function. For this, forty adult rats were assigned into four groups. The control group received distilled water, while the other three groups were treated with 25, 50 and 100 mg NaF/L via drinking water for 56 d, respectively. Testes were processed for total RNA extraction and western blot analysis. Three samples of each group were fixed in 2.5% glutaraldehyde solution for transmission electron microscopy analysis. From the results, we first found that fluoride decreased the expression of mRNA and protein levels of Zpbp1, Spaca1 and Dpy19l2 of seven markers during acrosome biogenesis in testes. Furthermore, fluoride damaged not only the acrosome structure, but also the structure of the nuclear lamina which was observed to be discontinuous and partially missing by transmission electron microscopy. Moreover, the results indicated that the altered structure in nuclear lamina maybe due to reduced LMNB2 expression in testis induced by fluoride. In a nutshell, fluoride exposure could restrain acrosome biogenesis during spermatogenesis and contribute to the elucidation of the underlying mechanisms of fluoride-induced male reproductive toxicity.

Ceramides in Parkinson's Disease: From Recent Evidence to New Hypotheses

Ceramides (Cer) constitute a class of lipids present in the cell membranes where they act as structural components, but they can also work as signaling molecules. Increasing genetic and biochemical evidence supports a link between deregulation of ceramide metabolism in the brain and neurodegeneration. Here, we provide an overview of the genes and cellular pathways that link Cer with Parkinson’s disease and discuss how ceramide pathobiology is gaining increasing interest in the understanding of the pathological mechanisms that contribute to the disease and in the clinical and therapeutic side.

Species-specific differences in nonlysosomal glucosylceramidase GBA2 function underlie locomotor dysfunction arising from loss-of-function mutations

The nonlysosomal glucosylceramidase β2 (GBA2) catalyzes the hydrolysis of glucosylceramide to glucose and ceramide. Mutations in the human GBA2 gene have been associated with hereditary spastic paraplegia (HSP), autosomal-recessive cerebellar ataxia (ARCA), and the Marinesco-Sjögren-like syndrome. However, the underlying molecular mechanisms are ill-defined. Here, using biochemistry, immunohistochemistry, structural modeling, and mouse genetics, we demonstrate that all but one of the spastic gait locus #46 (SPG46)-connected mutations cause a loss of GBA2 activity. We demonstrate that GBA2 proteins form oligomeric complexes and that protein-protein interactions are perturbed by some of these mutations. To study the pathogenesis of GBA2-related HSP and ARCA in vivo, we investigated GBA2-KO mice as a mammalian model system. However, these mice exhibited a high phenotypic variance and did not fully resemble the human phenotype, suggesting that mouse and human GBA2 differ in function. Whereas some GBA2-KO mice displayed a strong locomotor defect, others displayed only mild alterations of the gait pattern and no signs of cerebellar defects. On a cellular level, inhibition of GBA2 activity in isolated cerebellar neurons dramatically affected F-actin dynamics and reduced neurite outgrowth, which has been associated with the development of neurological disorders. Our results shed light on the molecular mechanism underlying the pathogenesis of GBA2-related HSP and ARCA and reveal species-specific differences in GBA2 function in vivo.

Distinguishing the differences in β-glycosylceramidase folds, dynamics, and actions informs therapeutic uses

Glycosyl hydrolases (GHs) are carbohydrate-active enzymes that hydrolyze a specific β-glycosidic bond in glycoconjugate substrates; β-glucosidases degrade glucosylceramide, a ubiquitous glycosphingolipid. GHs are grouped into structurally similar families that themselves can be grouped into clans. GH1, GH5, and GH30 glycosidases belong to clan A hydrolases with a catalytic (β/α)8 TIM barrel domain, whereas GH116 belongs to clan O with a catalytic (α/α)6 domain. In humans, GH abnormalities underlie metabolic diseases. The lysosomal enzyme glucocerebrosidase (family GH30), deficient in Gaucher disease and implicated in Parkinson disease etiology, and the cytosol-facing membrane-bound glucosylceramidase (family GH116) remove the terminal glucose from the ceramide lipid moiety. Here, we compare enzyme differences in fold, action, dynamics, and catalytic domain stabilization by binding site occupancy. We also explore other glycosidases with reported glycosylceramidase activity, including human cytosolic β-glucosidase, intestinal lactase-phlorizin hydrolase, and lysosomal galactosylceramidase. Last, we describe the successful translation of research to practice: recombinant glycosidases and glucosylceramide metabolism modulators are approved drug products (enzyme replacement therapies). Activity-based probes now facilitate the diagnosis of enzyme deficiency and screening for compounds that interact with the catalytic pocket of glycosidases. Future research may deepen the understanding of the functional variety of these enzymes and their therapeutic potential.

Biochemical Characterization of the GBA2 c.1780G>C Missense Mutation in Lymphoblastoid Cells from Patients with Spastic Ataxia

The GBA2 gene encodes the non-lysosomal glucosylceramidase (NLGase), an enzyme that catalyzes the conversion of glucosylceramide (GlcCer) to ceramide and glucose. Mutations in GBA2 have been associated with the development of neurological disorders such as autosomal recessive cerebellar ataxia, hereditary spastic paraplegia, and Marinesco-Sjogren-Like Syndrome. Our group has previously identified the GBA2 c.1780G>C [p.Asp594His] missense mutation, in a Cypriot consanguineous family with spastic ataxia. In this study, we carried out a biochemical characterization of lymphoblastoid cell lines (LCLs) derived from three patients of this family. We found that the mutation strongly reduce NLGase activity both intracellularly and at the plasma membrane level. Additionally, we observed a two-fold increase of GlcCer content in LCLs derived from patients compared to controls, with the C₁₆ lipid being the most abundant GlcCer species. Moreover, we showed that there is an apparent compensatory effect between NLGase and the lysosomal glucosylceramidase (GCase), since we found that the activity of GCase was three-fold higher in LCLs derived from patients compared to controls. We conclude that the c.1780G>C mutation results in NLGase loss of function with abolishment of the enzymatic activity and accumulation of GlcCer accompanied by a compensatory increase in GCase.

The Enigmatic Role of GBA2 in Controlling Locomotor Function

The non-lysosomal glucosylceramidase GBA2 catalyzes the hydrolysis of glucosylceramide to glucose and ceramide. Loss of GBA2 function results in accumulation of glucosylceramide. Mutations in the human GBA2 gene have been associated with hereditary spastic paraplegia (HSP) and autosomal-recessive cerebellar ataxia (ARCA). Patients suffering from these disorders exhibit impaired locomotion and neurological abnormalities. GBA2 mutations found in these patients have been proposed to impair GBA2 function. However, the molecular mechanism underlying the occurrence of mutations in the GBA2 gene and the development of locomotor dysfunction is not well-understood. In this review, we aim to summarize recent findings regarding mutations in the GBA2 gene and their impact on GBA2 function in health and disease.

A novel function for glucocerebrosidase as a regulator of sterylglucoside metabolism

BACKGROUND

Sterols are major cell membrane lipids, and in many organisms they are modified with glucose to generate sterylglucosides. Glucosylation dramatically changes the functional properties of sterols. The formation of sterylglucosides from sterols in plants, fungi, and bacteria uses UDP-glucose as a glucose donor. By contrast, sterylglucoside biosynthesis in mammals is catalyzed by the transglucosylation activity of glucocerebrosidases, with glucosylceramide acting as the glucose donor. Recent success in isolation and structural determination of sterylglucosides in the vertebrate central nervous system shows that transglucosylation also occurs in vivo. These analyses also revealed that sterylglucoside aglycons are composed of several cholesterol-related metabolites, including a plant-type sitosteryl.

SCOPE OF REVIEW

In this review, we discuss the biological functions and metabolism of sterylglucosides. We also summarize new findings from studies on the metabolism of vertebrate sterylglucosides and review the circumstances underlying the recent discovery of sterylglucosides in vertebrate brain. Finally, we discuss the role of sterylglucosides in a variety of neurodegenerative disorders such as Gaucher disease and Parkinson's disease.

MAJOR CONCLUSIONS

The biological significance of UDP-glucose-independent sterol glucosylation is still unknown, but it is plausible that glucosylation may provide sterols with novel biological functions. Even though sterol glucosylation is a simple reaction, it can dramatically change the physical properties of sterols.

GENERAL SIGNIFICANCE

Sterylglucosides may play roles in various physiological processes and in the pathogenesis of different diseases. Arriving at a better understanding of them at the organ and cellular level may open up new approaches to developing therapeutics for a variety of diseases. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Identification of a feedback loop involving β-glucosidase 2 and its product sphingosine sheds light on the molecular mechanisms in Gaucher disease

The lysosomal acid β-glucosidase GBA1 and the non-lysosomal β-glucosidase GBA2 degrade glucosylceramide (GlcCer) to glucose and ceramide in different cellular compartments. Loss of GBA2 activity and the resulting accumulation of GlcCer results in male infertility, whereas mutations in the GBA1 gene and loss of GBA1 activity cause the lipid-storage disorder Gaucher disease. However, the role of GBA2 in Gaucher disease pathology and its relationship to GBA1 is not well understood. Here, we report a GBA1-dependent down-regulation of GBA2 activity in patients with Gaucher disease. Using an experimental approach combining cell biology, biochemistry, and mass spectrometry, we show that sphingosine, the cytotoxic metabolite accumulating in Gaucher cells through the action of GBA2, directly binds to GBA2 and inhibits its activity. We propose a negative feedback loop, in which sphingosine inhibits GBA2 activity in Gaucher cells, preventing further sphingosine accumulation and, thereby, cytotoxicity. Our findings add a new chapter to the understanding of the complex molecular mechanism underlying Gaucher disease and the regulation of β-glucosidase activity in general.

Altered lysosome distribution is an early neuropathological event in neurological forms of Gaucher disease

In the lysosomal storage disorder Gaucher disease (GD), glucosylceramide (GlcCer) accumulates due to the defective activity of glucocerebrosidase. A subset of GD patients develops neuropathology. We now show mislocalization of Limp2-positive puncta and a large reduction in the number of Lamp1-positive puncta, which are associated with impaired tubulin. These changes occur at an early stage in animal models of GD, prior to development of overt symptoms and considerably earlier than neuronal loss. Altered lysosomal localization and cytoskeleton disruption precede the neuroinflammatory pathways, axonal dystrophy and neuronal loss previously characterized in neuronal forms of GD.

Characterization of the Zebrafish Homolog of β-Glucosidase 2: A Target of the Drug Miglustat

The small-molecular compound miglustat (N-butyldeoxynojirimycin, Zavesca(®)) has been approved for clinical use in type 1 Gaucher disease and Niemann-Pick type C disease, which are disorders caused by dysfunction of the endosomal-autophagic-lysosomal system. Miglustat inhibits a number of enzymes involved in glycoconjugate and glycan metabolism, including β-glucosidase 2 (GBA2), which is exceptionally sensitive to inhibition by miglustat. GBA2 is a glucosylceramide-degrading enzyme that is located on the plasma membrane/endoplasmic reticulum, and is distinct from the lysosomal enzyme glucocerebrosidase (GBA). Various strands of evidence suggest that inhibition of GBA2 contributes to the therapeutic benefits of miglustat. To further explore the pharmacology and biology of GBA2, we investigated whether the zebrafish homolog of GBA2 has similar enzymatic properties and pharmacological sensitivities to its human counterpart. We established that zebrafish has endogenous β-glucosidase activity toward lipid- and water-soluble GBA2 substrates, which can be inhibited by miglustat, N-butyldeoxygalactonojirimycin, and conduritol B epoxide. β-Glucosidase activities with highly similar characteristics were expressed in cells transfected with the zebrafish gba2 cDNA and in cells transfected with the human GBA2 cDNA. These results provide a foundation for the use of zebrafish in screening GBA2-targeting molecules, and for wider studies investigating GBA2 biology.

Lack of enzyme activity in GBA2 mutants associated with hereditary spastic paraplegia/cerebellar ataxia (SPG46)

Glucosylceramide is a membrane glycolipid made up of the sphingolipid ceramide and glucose, and has a wide intracellular distribution. Glucosylceramide is degraded to ceramide and glucose by distinct, non-homologous enzymes, including glucocerebrosidase (GBA), localized in the endolysosomal pathway, and β-glucosidase 2 (GBA2), which is associated with the plasma membrane and/or the endoplasmic reticulum. It is well established that mutations in the GBA gene result in endolysosomal glucosylceramide accumulation, which triggers Gaucher disease. In contrast, the biological significance of GBA2 is less well understood. GBA2-deficient mice present with male infertility, but humans carrying mutations in the GBA2 gene are affected with a combination of cerebellar ataxia and spastic paraplegia, as well as with thin corpus callosum and cognitive impairment (SPastic Gait locus #46, SPG46). To improve our understanding of the biochemical consequences of the GBA2 mutations, we have evaluated five nonsense and five missense GBA2 mutants for their enzyme activity. In transfected cells, the mutant forms of GBA2 were present in widely different amounts, ranging from overabundant to very minor, compared to the wild type enzyme. Nevertheless, none of the GBA2 mutant cDNAs raised the enzyme activity in transfected cells, in contrast to the wild-type enzyme. These results suggest that SPG46 patients have a severe deficit in GBA2 activity, because the GBA2 mutants are intrinsically inactive and/or reduced in amount. This assessment of the expression levels and enzyme activities of mutant forms of GBA2 offers a first insight in the biochemical basis of the complex pathologies seen in SPG46.