Lysosomal storage results in impaired survival but normal neurite outgrowth in dorsal root ganglion neurones from a mouse model of Sandhoff disease

Therapeutic advantages of combined gene/cell therapy strategies in a murine model of GM2 gangliosidosis

Genetic deficiency of β-N-acetylhexosaminidase (Hex) functionality leads to accumulation of GM2 ganglioside in Tay-Sachs disease and Sandhoff disease (SD), which presently lack approved therapies. Current experimental gene therapy (GT) approaches with adeno-associated viral vectors (AAVs) still pose safety and efficacy issues, supporting the search for alternative therapeutic strategies. Here we leveraged the lentiviral vector (LV)-mediated intracerebral (IC) GT platform to deliver Hex genes to the CNS and combined this strategy with bone marrow transplantation (BMT) to provide a timely, pervasive, and long-lasting source of the Hex enzyme in the CNS and periphery of SD mice. Combined therapy outperformed individual treatments in terms of lifespan extension and normalization of the neuroinflammatory/neurodegenerative phenotypes of SD mice. These benefits correlated with a time-dependent increase in Hex activity and a remarkable reduction in GM2 storage in brain tissues that single treatments failed to achieve. Our results highlight the synergic mode of action of LV-mediated IC GT and BMT, clarify the contribution of treatments to the therapeutic outcome, and inform on the realistic threshold of corrective enzymatic activity. These results have important implications for interpretation of ongoing experimental therapies and for design of more effective treatment strategies for GM2 gangliosidosis.

Novel bicistronic lentiviral vectors correct β-Hexosaminidase deficiency in neural and hematopoietic stem cells and progeny: implications for in vivo and ex vivo gene therapy of GM2 gangliosidosis

The favorable outcome of in vivo and ex vivo gene therapy approaches in several Lysosomal Storage Diseases suggests that these treatment strategies might equally benefit GM2 gangliosidosis. Tay-Sachs and Sandhoff disease (the main forms of GM2 gangliosidosis) result from mutations in either the HEXA or HEXB genes encoding, respectively, the α- or β-subunits of the lysosomal β-Hexosaminidase enzyme. In physiological conditions, α- and β-subunits combine to generate β-Hexosaminidase A (HexA, αβ) and β-Hexosaminidase B (HexB, ββ). A major impairment to establishing in vivo or ex vivo gene therapy for GM2 gangliosidosis is the need to synthesize the α- and β-subunits at high levels and with the correct stoichiometric ratio, and to safely deliver the therapeutic products to all affected tissues/organs. Here, we report the generation and in vitro validation of novel bicistronic lentiviral vectors (LVs) encoding for both the murine and human codon optimized Hexa and Hexb genes. We show that these LVs drive the safe and coordinate expression of the α- and β-subunits, leading to supranormal levels of β-Hexosaminidase activity with prevalent formation of a functional HexA in SD murine neurons and glia, murine bone marrow-derived hematopoietic stem/progenitor cells (HSPCs), and human SD fibroblasts. The restoration/overexpression of β-Hexosaminidase leads to the reduction of intracellular GM2 ganglioside storage in transduced and in cross-corrected SD murine neural progeny, indicating that the transgenic enzyme is secreted and functional. Importantly, bicistronic LVs safely and efficiently transduce human neurons/glia and CD34+ HSPCs, which are target and effector cells, respectively, in prospective in vivo and ex vivo GT approaches. We anticipate that these bicistronic LVs may overcome the current requirement of two vectors co-delivering the α- or β-subunits genes. Careful assessment of the safety and therapeutic potential of these bicistronic LVs in the SD murine model will pave the way to the clinical development of LV-based gene therapy for GM2 gangliosidosis.

Integrated Computational Analysis Highlights unique miRNA Signatures in the Subventricular Zone and Striatum of GM2 Gangliosidosis Animal Models

This work explores for the first time the potential contribution of microRNAs (miRNAs) to the pathophysiology of the GM2 gangliosidosis, a group of Lysosomal Storage Diseases. In spite of the genetic origin of GM2 gangliosidosis, the cascade of events leading from the gene/protein defects to the cell dysfunction and death is not fully elucidated. At present, there is no cure for patients. Taking advantage of the animal models of two forms of GM2 gangliosidosis, Tay-Sachs (TSD) and Sandhoff (SD) diseases, we performed a microRNA screening in the brain subventricular zone (SVZ) and striatum (STR), which feature the neurogenesis and neurodegeneration states, respectively, in adult mutant mice. We found abnormal expression of a panel of miRNAs involved in lipid metabolism, CNS development and homeostasis, and neuropathological processes, highlighting region- and disease-specific profiles of miRNA expression. Moreover, by using a computational analysis approach, we identified a unique disease- (SD or TSD) and brain region-specific (SVZ vs. STR) miRNAs signatures of predicted networks potentially related to the pathogenesis of the diseases. These results may contribute to the understanding of GM2 gangliosidosis pathophysiology, with the aim of developing effective treatments.

Abnormal differentiation of Sandhoff disease model mouse-derived multipotent stem cells toward a neural lineage

In Sandhoff disease (SD), the activity of the lysosomal hydrolytic enzyme, β-hexosaminidase (Hex), is lost due to a Hexb gene defect, which results in the abnormal accumulation of the substrate, GM2 ganglioside (GM2), in neuronal cells, causing neuronal loss, microglial activation, and astrogliosis. We established induced pluripotent stem cells from the cells of SD mice (SD-iPSCs). In the present study, we investigated the occurrence of abnormal differentiation and development of a neural lineage in the asymptomatic phase of SD in vitro using SD mouse fetus-derived neural stem cells (NSCs) and SD-iPSCs. It was assumed that the number of SD mouse fetal brain-derived NSCs was reduced and differentiation was promoted, resulting in the inhibition of differentiation into neurons and enhancement of differentiation into astrocytes. The number of SD-iPSC-derived NSCs was also reduced, suggesting that the differentiation of NSCs was promoted, resulting in the inhibition of differentiation into neurons and enhancement of that into astrocytes. This abnormal differentiation of SD-iPSCs toward a neural lineage was reduced by the glucosylceramide synthase inhibitor, miglustat. Furthermore, abnormal differentiation toward a neural lineage was reduced in SD-iPSCs with Hexb gene transfection. Therefore, differentiation ability along the time axis appears to be altered in SD mice in which the differentiation ability of NSCs is promoted and differentiation into neurons is completed earlier, while the timing of differentiation into astrocytes is accelerated. These results clarified that the abnormal differentiation of SD-iPSCs toward a neural lineage in vitro was shown to reflect the pathology of SD.

Impaired neural differentiation of induced pluripotent stem cells generated from a mouse model of Sandhoff disease

Sandhoff disease (SD) is a glycosphingolipid storage disease that arises from mutations in the Hexb gene and the resultant deficiency in β-hexosaminidase activity. This deficiency results in aberrant lysosomal accumulation of the ganglioside GM2 and related glycolipids, and progressive deterioration of the central nervous system. Dysfunctional glycolipid storage causes severe neurodegeneration through a poorly understood pathogenic mechanism. Induced pluripotent stem cell (iPSC) technology offers new opportunities for both elucidation of the pathogenesis of diseases and the development of stem cell-based therapies. Here, we report the generation of disease-specific iPSCs from a mouse model of SD. These mouse model-derived iPSCs (SD-iPSCs) exhibited pluripotent stem cell properties and significant accumulation of GM2 ganglioside. In lineage-directed differentiation studies using the stromal cell-derived inducing activity method, SD-iPSCs showed an impaired ability to differentiate into early stage neural precursors. Moreover, fewer neurons differentiated from neural precursors in SD-iPSCs than in the case of the wild type. Recovery of the Hexb gene in SD-iPSCs improved this impairment of neuronal differentiation. These results provide new insights as to understanding the complex pathogenic mechanisms of SD.

Vps18 deficiency inhibits dendritogenesis in Purkinje cells by blocking the lysosomal degradation of Lysyl Oxidase

Dendrite development occupies a central position in the formation of nervous system. However, whether lysosomal degradative function is required for dendritogenesis of neurons remains unknown. We have recently demonstrated the critical role of Vps18 in the lysosomal degradation pathway in mice. Here, we report that Vps18 deficiency severely blocks the dendrite development of Pukinje cells but not cerebral cortical neurons. Furthermore, we also demonstrate that the lysyl oxidase (Lox) protein is degraded through lysosome and accumulated in the Vps18 deficient cerebellum but not in cerebral cortices. Our results suggest that lysosome regulates dendritogenesis of Purkinje cells though degrading Lox.

DRG-targeted helper-dependent adenoviruses mediate selective gene delivery for therapeutic rescue of sensory neuronopathies in mice

Dorsal root ganglion (DRG) neuron dysfunction occurs in a variety of sensory neuronopathies for which there are currently no satisfactory treatments. Here we describe the development of a strategy to target therapeutic genes to DRG neurons for the treatment of these disorders. We genetically modified an adenovirus (Ad) to generate a helper virus (HV) that was detargeted for native adenoviral tropism and contained DRG homing peptides in the adenoviral capsid fiber protein; we used this HV to generate DRG-targeted helper-dependent Ad (HDAd). In mice, intrathecal injection of this HDAd produced a 100-fold higher transduction of DRG neurons and a markedly attenuated inflammatory response compared with unmodified HDAd. We also injected HDAd encoding the beta subunit of beta-hexosaminidase (Hexb) into Hexb-deficient mice, a model of the neuronopathy Sandhoff disease. Delivery of the DRG-targeted HDAd reinstated neuron-specific Hexb production, reversed gangliosidosis, and ameliorated peripheral sensory dysfunction. The development of DRG neuron-targeted HDAd with proven efficacy in a preclinical model may have implications for the treatment of sensory neuronopathies of diverse etiologies.

Reduced NGF secretion by Schwann cells under the high glucose condition decreases neurite outgrowth of DRG neurons

BACKGROUND

Schwann cells (SCs) have been supposed to play prominent roles in axonal regeneration under various diseases. Here, to evaluate the direct interaction between SCs and dorsal root ganglion (DRG) neurons under a diabetic condition, the effects of Schwann cell-conditioned media on neurite outgrowth of DRG neurons were investigated.

METHODS

Immortalized mouse Schwann cells (IMS) were cultured under 5.5 mM glucose (NG) or 30 mM glucose (HG) conditions for 4 days. IMS-conditioned media (IMS-media) were added to the culture media of neurons isolated from 8-week-old DDY mice. Neurons were cultured for 48 h with or without mouse recombinant NGF (mrNGF) or nerve growth factor (NGF) neutralizing antibody. The concentrations of NGF in IMS-media by ELISA and neurite outgrowth by a computed image analysis system were evaluated.

RESULTS

Neurite outgrowth was significantly enhanced by IMS-media (IMS-media (-): 177+/-177 microm, IMS-media (+): 1648+/-726). The neurite outgrowth cultured with IMS-media obtained under the HG condition was significantly reduced compared with that under the NG condition (NG: 1474+/-652, HG: 734+/-331). The NGF concentrations were significantly lower in IMS-media under the HG condition than in those under the NG condition. The accelerated neurite outgrowth by IMS-media was inhibited by NGF neutralizing antibody.

CONCLUSIONS

These results suggest that SCs play important roles in neurite outgrowth of DRG neurons, and that the decreased NGF secretion by SCs under the diabetic condition would cause a defect of axonal regeneration, resulting in the development of diabetic neuropathy.

Peripheral nervous system manifestations in a Sandhoff disease mouse model: nerve conduction, myelin structure, lipid analysis

Background

Sandhoff disease is an inherited lysosomal storage disease caused by a mutation in the gene for the β-subunit (Hexb gene) of β-hexosaminidase A (αβ) and B (ββ). The β-subunit together with the GM2 activator protein catabolize ganglioside GM2. This enzyme deficiency results in GM2 accumulation primarily in the central nervous system. To investigate how abnormal GM2 catabolism affects the peripheral nervous system in a mouse model of Sandhoff disease (Hexb-/-), we examined the electrophysiology of dissected sciatic nerves, structure of central and peripheral myelin, and lipid composition of the peripheral nervous system.

Results

We detected no significant difference in signal impulse conduction velocity or any consistent change in the frequency-dependent conduction slowing and failure between freshly dissected sciatic nerves from the Hexb+/- and Hexb-/- mice. The low-angle x-ray diffraction patterns from freshly dissected sciatic and optic nerves of Hexb+/- and Hexb-/- mice showed normal myelin periods; however, Hexb-/- mice displayed a ~10% decrease in the relative amount of compact optic nerve myelin, which is consistent with the previously established reduction in myelin-enriched lipids (cerebrosides and sulfatides) in brains of Hexb-/- mice. Finally, analysis of lipid composition revealed that GM2 content was present in the sciatic nerve of the Hexb-/- mice (undetectable in Hexb+/-).

Conclusion

Our findings demonstrate the absence of significant functional, structural, or compositional abnormalities in the peripheral nervous system of the murine model for Sandhoff disease, but do show the potential value of integrating multiple techniques to evaluate myelin structure and function in nervous system disorders.

Expression and histochemical localization of ciliary neurotrophic factor in cultured adult rat dorsal root ganglion neurons

Ciliary neurotrophic factor (CNTF) is abundantly expressed in Schwann cells in adult mammalian peripheral nerves, but not in neurons. After peripheral nerve injury, CNTF released from disrupted Schwann cells is likely to promote neuronal survival and axonal regeneration. In the present study, we examined the expression and histochemical localization of CNTF in adult rat DRG in vivo and in vitro. In contrast to the restricted expression in Schwann cells in vivo, we observed abundant CNTF mRNA and protein expression in DRG neurons after 3 h, 2, 7, and 15 days in dissociated cell culture. At later stages (7 and 15 days) of culture, CNTF immunoreactivity was detected in both neuronal cell bodies and regenerating neurites. These results suggest that CNTF is synthesized and transported to neurites in cultured DRG neurons. Since we failed to observe CNTF immunoreactivity in DRG neurons in explant culture, disruption of cell-cell interactions, rather than the culture itself, may be an inducible factor for localization of CNTF in the neurons.

Reversion of the biochemical defects in murine embryonic Sandhoff neurons using a bicistronic lentiviral vector encoding hexosaminidase alpha and beta

Sandhoff disease, a neurodegenerative disorder characterized by the intracellular accumulation of GM2 ganglioside, is caused by mutations in the hexosaminidase beta-chain gene resulting in a hexosaminidase A (alphabeta) and B (betabeta) deficiency. A bicistronic lentiviral vector encoding both the hexosaminidase alpha and beta chains (SIV.ASB) has previously been shown to correct the beta-hexosaminidase deficiency and to reduce GM2 levels both in transduced and cross-corrected human Sandhoff fibroblasts. Recent advances in determining the neuropathophysiological mechanisms in Sandhoff disease have shown a mechanistic link between GM2 accumulation, neuronal cell death, reduction of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA) activity, and axonal outgrowth. To examine the ability of the SIV.ASB vector to reverse these pathophysiological events, hippocampal neurons from embryonic Sandhoff mice were transduced with the lentivector. Normal axonal growth rates were restored, as was the rate of Ca(2+) uptake via the SERCA and the sensitivity of the neurons to thapsigargin-induced cell death, concomitant with a decrease in GM2 and GA2 levels. Thus, we have demonstrated that the bicistronic vector can reverse the biochemical defects and down-stream consequences in Sandhoff neurons, reinforcing its potential for Sandhoff disease in vivo gene therapy.

Establishment of immortalized Schwann cells from Sandhoff mice and corrective effect of recombinant human β-hexosaminidase A on the accumulated GM2 ganglioside

We have established spontaneously immortalized Schwann cell lines from dorsal root ganglia and peripheral nerves of Sandhoff mice. One of the cell lines exhibited genetically and biochemically distinct features of Sandhoff Schwann cells. The enzyme activities toward 4-methylumbelliferyl N-acetyl-beta-D-glucosamine (beta-hexosaminidases A, B, and S) and 4-methylumbelliferyl N-acetyl-beta-D-glucosamine-6-sulfate (beta-hexosaminidases A and S) were decreased, and GM2 ganglioside accumulated in lysosomes of the cells. Incorporation of recombinant human beta-hexosaminidase isozymes expressed in Chinese hamster ovary cells into the cultured Sandhoff Schwann cells via cation-independent mannose 6-phosphate receptors was found, and the incorporated beta-hexosaminidase A degraded the accumulated GM2 ganglioside. The established Sandhoff Schwann cell line is useful for investigation and development of therapies for Sandhoff disease.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.

Possible role of autoantibodies in the pathophysiology of GM2 gangliosidoses

Mice containing a disruption of the Hexb gene have provided a useful model system for the study of the human lysosomal storage disorder known as Sandhoff disease (SD). Hexb(-/-) mice rapidly develop a progressive neurologic disease of ganglioside GM2 and GA2 storage. Our study revealed that the disease states in this model are associated with the appearance of antiganglioside autoantibodies. Both elevation of serum antiganglioside autoantibodies and IgG deposition to CNS neurons were found in the advanced stages of the disease in Hexb(-/-) mice; serum transfer from these mice showed IgG binding to neurons. To determine the role of these autoantibodies, the Fc receptor gamma gene (FcR gamma) was additionally disrupted in Hexb(-/-) mice, as it plays a key role in immune complex-mediated autoimmune diseases. Clinical symptoms were improved and life spans were extended in the Hexb(-/-)FcR gamma(-/-) mice; the number of apoptotic cells was also decreased. The level of ganglioside accumulation, however, did not change. IgG deposition was also confirmed in the brain of an autopsied SD patient. Taken together, these findings suggest that the production of autoantibodies plays an important role in the pathogenesis of neuropathy in SD and therefore provides a target for novel therapies.

Synthesis, localization and externalization of galectin-1 in mature dorsal root ganglion neurons and Schwann cells

We recently confirmed that oxidized galectin-1 is a novel factor enhancing axonal growth in peripheral nerves after axotomy, but the process of extracellular release and oxidization of endogenous galectin-1 in the injured nervous tissue remains unknown. In the present study, we examined the distribution of galectin-1 in adult rat dorsal root ganglia (DRG) in vivo and in vitro. By RT-PCR analysis and in situ hybridization histochemistry, galectin-1 mRNA was detected in both DRG neurons and non-neuronal cells. Immunohistochemical analyses revealed that galectin-1 was distributed diffusely throughout the cytoplasm in smaller diameter neurons and Schwann cells in DRG sections. In contrast, the immunoreactivity for galectin-1 was detected in almost all DRG neurons from an early stage in culture (3 h after seeding) and was restricted to the surface and/or extracellular region of neurons and Schwann cells at later stages in culture. In a manner similar to the primary cultured cells, we also observed the surface and extracellular expression of this molecule in immortalized adult mouse Schwann cells (IMS32). Western blot analysis has revealed that both reduced and oxidized forms of galectin-1 were detected in culture media of DRG neurons and IMS32. These findings suggest that galectin-1 is externalized from DRG neurons and Schwann cells upon axonal injury. Some of the molecules in the extracellular milieu may be converted to the oxidized form, which lacks lectin activity but could act on neural tissue as a cytokine.

Inhibition of calcium uptake via the sarco/endoplasmic reticulum Ca2+-ATPase in a mouse model of Sandhoff disease and prevention by treatment with N-butyldeoxynojirimycin

Gangliosides are found at high levels in neuronal tissues where they play a variety of important functions. In the gangliosidoses, gangliosides accumulate because of defective activity of the lysosomal proteins responsible for their degradation, usually resulting in a rapidly progressive neurodegenerative disease. However, the molecular mechanism(s) leading from ganglioside accumulation to neurodegeneration is not known. We now examine the effect of ganglioside GM2 accumulation in a mouse model of Sandhoff disease (one of the GM2 gangliosidoses), the Hexb-/- mouse. Microsomes from Hexb-/- mouse brain showed a significant reduction in the rate of Ca2+-uptake via the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), which was prevented by feeding Hexb-/- mice with N-butyldeoxynojirimycin (NB-DNJ), an inhibitor of glycolipid synthesis that reduces GM2 storage. Changes in SERCA activity were not due to transcriptional regulation but rather because of a decrease in Vmax. Moreover, exogenously added GM2 had a similar effect on SERCA activity. The functional significance of these findings was established by the enhanced sensitivity of neurons cultured from embryonic Hexb-/- mice to cell death induced by thapsigargin, a specific SERCA inhibitor, and by the enhanced sensitivity of Hexb-/- microsomes to calcium-induced calcium release. This study suggests a mechanistic link among GM2 accumulation, reduced SERCA activity, and neuronal cell death, which may be of significance for delineating the neuropathophysiology of Sandhoff disease.

Reduced rates of axonal and dendritic growth in embryonic hippocampal neurones cultured from a mouse model of Sandhoff disease

Sandhoff disease is a lysosomal storage disease in which ganglioside GM2 accumulates because of a defective beta-subunit of beta-hexosaminidase. This disease is characterized by neurological manifestations, although the pathogenic mechanisms leading from GM2 accumulation to neuropathology are largely unknown. We now examine the viability, development and rates of neurite growth of embryonic hippocampal neurones cultured from a mouse model of Sandhoff disease, the Hexb-/- mouse. GM2 was detected by metabolic labelling at low levels in wild type (Hexb+/+) neurones, and increased by approximately three-fold in Hexb-/- neurones. Hexb-/- hippocampal neurones were as viable as their wild type counterparts and, moreover, their developmental programme was unaltered because the formation of axons and of the minor processes which eventually become dendrites was similar in Hexb-/- and Hexb+/+ neurones. In contrast, once formed, a striking difference in the rate of axonal and minor process growth was observed, with changes becoming apparent after 3 days in culture and highly significant after 5 days in culture. Analysis of various parameters of axonal growth suggested that a key reason for the decreased rate of axonal growth was because of a decrease in the formation of collateral axonal branches, the major mechanism by which hippocampal axons elongate in culture. Thus, although the developmental programme with respect to axon and minor process formation and the viability of hippocampal neurones are unaltered, a significant decrease occurs in the rate of axonal and minor process growth in Hexb-/- neurones. These results appear to be in contrast to dorsal root ganglion neurones cultured from 1-month-old Sandhoff mice, in which cell survival is impaired but normal outgrowth of neurones occurs. The possible reasons for these differences are discussed.

Plasmid-based gene transfer ameliorates visceral storage in a mouse model of Sandhoff disease

Sandhoff disease is a severe neurodegenerative disorder with visceral involvement caused by mutations in the HEXB gene coding for the beta subunit of the lysosomal hexosaminidases A and B. HEXB mutations result in the accumulation of undegraded substrates such as GM2 and GA2 in lysosomes. We evaluated the efficacy of cationic liposome-mediated plasmid gene therapy using the Sandhoff disease mouse, an animal model of a human lysosomal storage disease. The mice received a single intravenous injection of two plasmids, encoding the human alpha and beta subunits of hexosaminidase cDNAs. As a result, 10-35% of normal levels of hexosaminidase expression, theoretically therapeutic levels, were achieved in most visceral organs, but not in the brain, 3 days after injection with decreased levels by day 7. Histochemical staining confirmed widespread enzyme activity in visceral organs. Both GA2 and GM2 were reduced by almost 10% and 50%, respectively, on day 3, and by 60% and 70% on day 7 compared with untreated age-matched Sandhoff disease mice. Consistent with the biochemical results, a reduction in GM2 was observed in liver cells histologically as well. These initial findings support further development of the plasmid gene therapy against lysosomal diseases with visceral pathology.