Glucosylceramide synthase inhibition reduces ganglioside GM3 accumulation, alleviates amyloid neuropathology, and stabilizes remote contextual memory in a mouse model of Alzheimer’s disease

Background

Gangliosides are highly enriched in the brain and are critical for its normal development and function. However, in some rare neurometabolic diseases, a deficiency in lysosomal ganglioside hydrolysis is pathogenic and leads to early-onset neurodegeneration, neuroinflammation, demyelination, and dementia. Increasing evidence also suggests that more subtle ganglioside accumulation contributes to the pathogenesis of more common neurological disorders including Alzheimer’s disease (AD). Notably, ganglioside GM3 levels are elevated in the brains of AD patients and in several mouse models of AD, and plasma GM3 levels positively correlate with disease severity in AD patients.

Methods

Tg2576 AD model mice were fed chow formulated with a small molecule inhibitor of glucosylceramide synthase (GCSi) to determine whether reducing glycosphingolipid synthesis affected aberrant GM3 accumulation, amyloid burden, and disease manifestations in cognitive impairment. GM3 was measured with LC-MS, amyloid burden with ELISA and amyloid red staining, and memory was assessed using the contextual fear chamber test.

Results

GCSi mitigated soluble Aβ42 accumulation in the brains of AD model mice when treatment was started prophylactically. Remarkably, GCSi treatment also reduced soluble Aβ42 levels and amyloid plaque burden in aged (i.e., 70 weeks old) AD mice with preexisting neuropathology. Our analysis of contextual memory in Tg2576 mice showed that impairments in remote (cortical-dependent) memory consolidation preceded deficits in short-term (hippocampal-dependent) contextual memory, which was consistent with soluble Aβ42 accumulation occurring more rapidly in the cortex of AD mice compared to the hippocampus. Notably, GCSi treatment significantly stabilized remote memory consolidation in AD mice—especially in mice with enhanced cognitive training. This finding was consistent with GCSi treatment lowering aberrant GM3 accumulation in the cortex of AD mice.

Conclusions

Collectively, our results indicate that glycosphingolipids regulated by GCS are important modulators of Aβ neuropathology and that glycosphingolipid homeostasis plays a critical role in the consolidation of remote memories.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-00966-0.

Glucocerebrosidase modulates cognitive and motor activities in murine models of Parkinson's disease

Mutations in GBA1, the gene encoding glucocerebrosidase, are associated with an enhanced risk of developing synucleinopathies such as Parkinson’s disease (PD) and dementia with Lewy bodies. A higher prevalence and increased severity of motor and non-motor symptoms is observed in PD patients harboring mutant GBA1 alleles, suggesting a link between the gene or gene product and disease development. Interestingly, PD patients without mutations in GBA1 also exhibit lower levels of glucocerebrosidase activity in the central nervous system (CNS), implicating this lysosomal enzyme in disease pathogenesis. Here, we investigated whether modulation of glucocerebrosidase activity in murine models of synucleinopathy (expressing wild type Gba1) affected α-synuclein accumulation and behavioral phenotypes. Partial inhibition of glucocerebrosidase activity in PrP-A53T-SNCA mice using the covalent inhibitor conduritol-B-epoxide induced a profound increase in soluble α-synuclein in the CNS and exacerbated cognitive and motor deficits. Conversely, augmenting glucocerebrosidase activity in the Thy1-SNCA mouse model of PD delayed the progression of synucleinopathy. Adeno-associated virus-mediated expression of glucocerebrosidase in the Thy1-SNCA mouse striatum led to decrease in the levels of the proteinase K-resistant fraction of α-synuclein, amelioration of behavioral aberrations and protection from loss of striatal dopaminergic markers. These data indicate that increasing glucocerebrosidase activity can influence α-synuclein homeostasis, thereby reducing the progression of synucleinopathies. This study provides robust in vivo evidence that augmentation of CNS glucocerebrosidase activity is a potential therapeutic strategy for PD, regardless of the mutation status of GBA1.

Translational fidelity of intrathecal delivery of self-complementary AAV9-survival motor neuron 1 for spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by mutations in survival motor neuron 1 (SMN1). Previously, we showed that central nervous system (CNS) delivery of an adeno-associated viral (AAV) vector encoding SMN1 produced significant improvements in survival in a mouse model of SMA. Here, we performed a dose-response study in SMA mice to determine the levels of SMN in the spinal cord necessary for efficacy, and measured the efficiency of motor neuron transduction in the spinal cord after intrathecal delivery in pigs and nonhuman primates (NHPs). CNS injections of 5e10, 1e10, and 1e9 genome copies (gc) of self-complementary AAV9 (scAAV9)-hSMN1 into SMA mice extended their survival from 17 to 153, 70, and 18 days, respectively. Spinal cords treated with 5e10, 1e10, and 1e9 gc showed that 70-170%, 30-100%, and 10-20% of wild-type levels of SMN were attained, respectively. Furthermore, detectable SMN expression in a minimum of 30% motor neurons correlated with efficacy. A comprehensive analysis showed that intrathecal delivery of 2.5e13 gc of scAAV9-GFP transduced 25-75% of the spinal cord motor neurons in NHPs. Thus, the extent of gene expression in motor neurons necessary to confer efficacy in SMA mice could be obtained in large-animal models, justifying the continual development of gene therapy for SMA.

Silencing mutant huntingtin by adeno-associated virus-mediated RNA interference ameliorates disease manifestations in the YAC128 mouse model of Huntington's disease

Huntington's disease (HD) is a fatal autosomal dominant neurodegenerative disease caused by an increase in the number of polyglutamine residues in the huntingtin (Htt) protein. With the identification of the underlying basis of HD, therapies are being developed that reduce expression of the causative mutant Htt. RNA interference (RNAi) that seeks to selectively reduce the expression of such disease-causing agents is emerging as a potential therapeutic strategy for this and similar disorders. This study examines the merits of administering a recombinant adeno-associated viral (AAV) vector designed to deliver small interfering RNA (siRNA) that targets the degradation of the Htt transcript. The aim was to lower Htt levels and to correct the behavioral, biochemical, and neuropathological deficits shown to be associated with the YAC128 mouse model of HD. Our data demonstrate that AAV-mediated RNAi is effective at transducing greater than 80% of the cells in the striatum and partially reducing the levels (~40%) of both wild-type and mutant Htt in this region. Concomitant with these reductions are significant improvements in behavioral deficits, reduction of striatal Htt aggregates, and partial correction of the aberrant striatal transcriptional profile observed in YAC128 mice. Importantly, a partial reduction of both the mutant and wild-type Htt levels is not associated with any notable overt neurotoxicity. Collectively, these results support the continued development of AAV-mediated RNAi as a therapeutic strategy for HD.

Augmenting CNS glucocerebrosidase activity as a therapeutic strategy for parkinsonism and other Gaucher-related synucleinopathies

Mutations of GBA1, the gene encoding glucocerebrosidase, represent a common genetic risk factor for developing the synucleinopathies Parkinson disease (PD) and dementia with Lewy bodies. PD patients with or without GBA1 mutations also exhibit lower enzymatic levels of glucocerebrosidase in the central nervous system (CNS), suggesting a possible link between the enzyme and the development of the disease. Previously, we have shown that early treatment with glucocerebrosidase can modulate α-synuclein aggregation in a presymptomatic mouse model of Gaucher-related synucleinopathy (Gba1(D409V/D409V)) and ameliorate the associated cognitive deficit. To probe this link further, we have now evaluated the efficacy of augmenting glucocerebrosidase activity in the CNS of symptomatic Gba1(D409V/D409V) mice and in a transgenic mouse model overexpressing A53T α-synuclein. Adeno-associated virus-mediated expression of glucocerebrosidase in the CNS of symptomatic Gba1(D409V/D409V) mice completely corrected the aberrant accumulation of the toxic lipid glucosylsphingosine and reduced the levels of ubiquitin, tau, and proteinase K-resistant α-synuclein aggregates. Importantly, hippocampal expression of glucocerebrosidase in Gba1(D409V/D409V) mice (starting at 4 or 12 mo of age) also reversed their cognitive impairment when examined using a novel object recognition test. Correspondingly, overexpression of glucocerebrosidase in the CNS of A53T α-synuclein mice reduced the levels of soluble α-synuclein, suggesting that increasing the glycosidase activity can modulate α-synuclein processing and may modulate the progression of α-synucleinopathies. Hence, increasing glucocerebrosidase activity in the CNS represents a potential therapeutic strategy for GBA1-related and non-GBA1-associated synucleinopathies, including PD.

Disease progression in a mouse model of amyotrophic lateral sclerosis: the influence of chronic stress and corticosterone

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by motor neuron cell loss, muscular atrophy, and a shortened life span. Survival is highly variable, as some patients die within months, while others live for many years. Exposure to stress or the development of a nonoptimal stress response to disease might account for some of this variability. We show in the SOD1(G93A) mouse model of ALS that recurrent exposure to restraint stress led to an earlier onset of astrogliosis and microglial activation within the spinal cord, accelerated muscular weakness, and a significant decrease in median survival (105 vs. 122 d) when compared to nonstressed animals. Moreover, during normal disease course, ALS mice display a cacostatic stress response by developing an aberrant serum corticosterone circadian rhythm. Interestingly, we also found that higher corticosterone levels were significantly correlated with both an earlier onset of paralysis (males: r(2)=0.746; females: r(2)=0.707) and shorter survival times (males: r(2)=0.680; females: r(2)=0.552) in ALS mice. These results suggest that stress is capable of accelerating disease progression and that strategies that modulate glucocorticoid metabolism might be a viable treatment approach for ALS.

Distribution of acid sphingomyelinase in rodent and non-human primate brain after intracerebroventricular infusion

One treatment approach for lysosomal storage diseases (LSDs) is the systemic infusion of recombinant enzyme. Although this enzyme replacement is therapeutic for the viscera, many LSDs have central nervous system (CNS) components that are not adequately treated by systemic enzyme infusion. Direct intracerebroventricular (ICV) infusion of a high concentration of recombinant human acid sphingomyelinase (rhASM) into the CNS over a prolonged time frame (hours) has shown therapeutic efficacy in a mouse model of Niemann-Pick A (NP/A) disease. To evaluate whether such an approach would translate to a larger brain, rhASM was infused into the lateral ventricles of both rats and Rhesus macaques, and the resulting distribution of enzyme characterized qualitatively and quantitatively. In both species, ICV infusion of rhASM resulted in parenchymal distribution of enzyme at levels that were therapeutic in the NP/A mouse model. Enzyme distribution was global in nature and exhibited a relatively steep gradient from the cerebrospinal fluid compartment to the inner parenchyma. Additional optimization of an ICV delivery approach may provide a therapeutic option for LSDs with neurologic involvement.

Comparative analysis of acid sphingomyelinase distribution in the CNS of rats and mice following intracerebroventricular delivery

Niemann-Pick A (NPA) disease is a lysosomal storage disorder (LSD) caused by a deficiency in acid sphingomyelinase (ASM) activity. Previously, we reported that biochemical and functional abnormalities observed in ASM knockout (ASMKO) mice could be partially alleviated by intracerebroventricular (ICV) infusion of hASM. We now show that this route of delivery also results in widespread enzyme distribution throughout the rat brain and spinal cord. However, enzyme diffusion into CNS parenchyma did not occur in a linear dose-dependent fashion. Moreover, although the levels of hASM detected in the rat CNS were determined to be within the range shown to be therapeutic in ASMKO mice, the absolute amounts represented less than 1% of the total dose administered. Finally, our results also showed that similar levels of enzyme distribution are achieved across rodent species when the dose is normalized to CNS weight as opposed to whole body weight. Collectively, these data suggest that the efficacy observed following ICV delivery of hASM in ASMKO mice could be scaled to CNS of the rat.

Relationship between neuropathology and disease progression in the SOD1(G93A) ALS mouse

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive loss of upper and lower motor neurons. However, recent reports suggest an active role of non-neuronal cells in the pathogenesis of the disease. Here, we examined quantitatively the temporal development of neuropathologic features in the brain and spinal cord of a mouse model of ALS (SOD1(G93A)). Four phases of the disease were studied in both male and female SOD1(G93A) mice: presymptomatic (PRE-SYM), symptomatic (SYM), endstage (ES) and moribund (MB). Compared to their control littermates, SOD1(G93A) mice showed an increase in astrogliosis in the motor cortex, spinal cord and motor trigeminal nucleus in the SYM phase that worsened progressively in ES and MB animals. Associated with this increase in astrogliosis was a concomitant increase in motor neuron cell death in the spinal cord and motor trigeminal nucleus in both ES and MB mice, as well as in the ventrolateral thalamus in MB animals. In contrast, microglial activation was significantly increased in all the same regions but only when the mice were in the MB phase. These results suggest that astrogliosis preceded or occurred concurrently with neuronal degeneration whereas prominent microgliosis was evident later (MB stage), after significant motor neuron degeneration had occurred. Hence, our findings support a role for astrocytes in modulating the progression of non-cell autonomous degeneration of motor neurons, with microglia playing a role in clearing degenerating neurons.

AAV4-mediated expression of IGF-1 and VEGF within cellular components of the ventricular system improves survival outcome in familial ALS mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by motor neuron cell death in the cortex, brainstem, and spinal cord. Extensive efforts have been made to develop trophic factor-based therapies to enhance motor neuron survival; however, achievement of adequate therapeutic delivery to all regions of the corticospinal tract has remained a significant challenge. Here, we show that adeno-associated virus serotype 4 (AAV4)-mediated expression of insulin-like growth factor-1 (IGF-1) or vascular endothelial growth factor (VEGF)-165 in the cellular components of the ventricular system including the ependymal cell layer, choroid plexus [the primary cerebrospinal fluid (CSF)-producing cells of the central nervous system (CNS)] and spinal cord central canal leads to trophic factor delivery throughout the CNS, delayed motor decline and a significant extension of survival in SOD1(G93A) transgenic mice. Interestingly, when IGF-1- and VEGF-165-expressing AAV4 vectors were given in combination, no additional benefit in efficacy was observed suggesting that these trophic factors are acting on similar signaling pathways to modestly slow disease progression. Consistent with these findings, experiments conducted in a recently described in vitro cell culture model of ALS led to a similar result, with both IGF-1 and VEGF-165 providing significant motor neuron protection but in a nonadditive fashion. These findings support the continued investigation of trophic factor-based therapies that target the CNS as a potential treatment of ALS.

Intracerebroventricular infusion of acid sphingomyelinase corrects CNS manifestations in a mouse model of Niemann-Pick A disease

Niemann-Pick A (NPA) disease is a lysosomal storage disorder (LSD) caused by a deficiency in acid sphingomyelinase (ASM) activity. Previously, we showed that the storage pathology in the ASM knockout (ASMKO) mouse brain could be corrected by intracerebral injections of cell, gene and protein based therapies. However, except for instances where distal areas were targeted with viral vectors, correction of lysosomal storage pathology was typically limited to a region within a few millimeters from the injection site. As NPA is a global neurometabolic disease, the development of delivery strategies that maximize the distribution of the enzyme throughout the CNS is likely necessary to arrest or delay progression of the disease. To address this challenge, we evaluated the effectiveness of intracerebroventricular (ICV) delivery of recombinant human ASM into ASMKO mice. Our findings showed that ICV delivery of the enzyme led to widespread distribution of the hydrolase throughout the CNS. Moreover, a significant reduction in lysosomal accumulation of sphingomyelin was observed throughout the brain and also within the spinal cord and viscera. Importantly, we demonstrated that repeated ICV infusions of ASM were effective at improving the disease phenotype in the ASMKO mouse as indicated by a partial alleviation of the motor abnormalities. These findings support the continued exploration of ICV delivery of recombinant lysosomal enzymes as a therapeutic modality for LSDs such as NPA that manifests substrate accumulation within the CNS.

Delivery of AAV-IGF-1 to the CNS extends survival in ALS mice through modification of aberrant glial cell activity

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of the motor system. Recent work in rodent models of ALS has shown that insulin-like growth factor-1 (IGF-1) slows disease progression when delivered at disease onset. However, IGF-1's mechanism of action along the neuromuscular axis remains unclear. In this study, symptomatic ALS mice received IGF-1 through stereotaxic injection of an IGF-1-expressing viral vector to the deep cerebellar nuclei (DCN), a region of the cerebellum with extensive brain stem and spinal cord connections. We found that delivery of IGF-1 to the central nervous system (CNS) reduced ALS neuropathology, improved muscle strength, and significantly extended life span in ALS mice. To explore the mechanism of action of IGF-1, we used a newly developed in vitro model of ALS. We demonstrate that IGF-1 is potently neuroprotective and attenuates glial cell-mediated release of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO). Our results show that delivering IGF-1 to the CNS is sufficient to delay disease progression in a mouse model of familial ALS and demonstrate for the first time that IGF-1 attenuates the pathological activity of non-neuronal cells that contribute to disease progression. Our findings highlight an innovative approach for delivering IGF-1 to the CNS.