Psychosine, the cytotoxic sphingolipid that accumulates in globoid cell leukodystrophy, alters membrane architecture

Globoid cell leukodystrophy (GLD) is a neurological disease caused by deficiency of the lysosomal enzyme galactosylceramidase (GALC). In the absence of GALC, the cytotoxic glycosphingolipid, psychosine (psy), accumulates in the nervous system. Psychosine accumulation preferentially affects oligodendrocytes, leading to progressive demyelination and infiltration of activated monocytes/macrophages into the CNS. GLD is characterized by motor defects, cognitive deficits, seizures, and death by 2-5 years of age. It has been hypothesized that psychosine accumulation, primarily within lipid rafts, results in the pathogenic cascade in GLD. However, the mechanism of psychosine toxicity has yet to be elucidated. Therefore, we synthesized the enantiomer of psychosine (ent-psy) to use as a probe to distinguish between protein-psy (stereo-specific enantioselective) or membrane-psy (stereo-insensitive nonenantioselective) interactions. The enantiomer of psychosine has equal or greater toxicity compared with psy, suggesting that psy exerts its toxicity through a nonenantioselective mechanism. Finally, in this study we demonstrate that psy and ent-psy localize to lipid rafts, perturb natural and artificial membrane integrity, and inhibit protein Kinase C translocation to the plasma membrane. Although other mechanisms may play a role in disease, these data strongly suggest that psy exerts its effects primarily through membrane perturbation rather than through specific protein-psy interactions.

Axonopathy is a compounding factor in the pathogenesis of Krabbe disease

Loss-of-function of the lysosomal enzyme galactosyl-ceramidase causes the accumulation of the lipid raft-associated sphingolipid psychosine, the disruption of postnatal myelination, neurodegeneration and early death in most cases of infantile Krabbe disease. This work presents a first study towards understanding the progression of axonal defects in this disease using the Twitcher mutant mouse. Axonal swellings were detected in axons within the mutant spinal cord as early as 1 week after birth. As the disease progressed, more axonopathic profiles were found in other regions of the nervous system, including peripheral nerves and various brain areas. Isolated mutant neurons recapitulated axonal and neuronal defects in the absence of mutant myelinating glia, suggesting an autonomous neuronal defect. Psychosine was sufficient to induce axonal defects and cell death in cultures of acutely isolated neurons. Interestingly, axonopathy in young Twitcher mice occurred in the absence of demyelination and of neuronal apoptosis. Neuronal damage occurred at later stages, when mutant mice were moribund and demyelinated. Altogether, these findings suggest a progressive dying-back neuronal dysfunction in Twitcher mutants.

Multipotential neural precursors transplanted into the metachromatic leukodystrophy brain fail to generate oligodendrocytes but contribute to limit brain dysfunction

Neural stem cells appear to be best suited for regenerative therapy in neurological diseases. However, the effects of high levels of potentially toxic substances such as sulfatides--which accumulate in metachromatic leukodystrophy (MLD)--on this regenerative ability are still largely unclear. To start addressing this question, in vitro and in vivo experiments were used to examine the behavior of multipotential neural precursors exposed to abnormally high levels of sulfatides. Following transplantation of dissociated neurospheres into the brain of presymptomatic MLD pups, the majority of donor-derived cells were distributed in a caudal to rostral direction, with higher numbers in the cortex. Most if not all of the donor cells acquired an astroglial phenotype. We found no evidence of oligodendrocyte or neuronal commitment of transplanted cells in long-term-treated MLD mice (e.g. up to 1.5 years of age). This was in line with our in vitro findings of sulfatides blocking oligodendrocyte formation after induction of differentiation in sulfatide-treated epidermal growth factor/fibroblast growth factor responsive neurospheres. Transplanted MLD mice showed an improved arylsulfatase A (ARSA) activity and a significant amelioration of sulfatide metabolism, neurodegeneration and motor-learning/memory deficits. Furthermore, transplanted cells were shown to act as a source of ARSA enzyme that accumulated in endogenous brain cells, indicating the occurrence of enzyme cross-correction between transplanted and host cells. These results provide a first insight into the effect of sulfatides on the stemness properties of neural stem cells and on the effects of the MLD environment on the in vivo expectations of using neural stem cells in cell therapy.

Transgenic mice expressing a dual, CRE-inducible reporter for the analysis of axon guidance and synaptogenesis

Improved and modular tools are needed for the neuroanatomical dissection of CNS axonal tracts, and to study the cell-intrinsic and cell-extrinsic cues that govern their assembly and plasticity. Here we describe a general purpose transgenic tracer that can be used to visualize axonal tracts and synaptic terminals in any region of the embryonic neural tube or postnatal CNS, on any wild type or mutant genetic background. The construct permits CRE-inducible expression of a dicistronic axonal marker encoding two surface reporter proteins: a farnesylated GFP and the human Placental Alkaline Phosphatase (PLAP). Both proteins localize alongside the neuronal surface, permitting the concomitant detection of cell body, neurites, and presynaptic and postsynaptic sites in the same neuron. This provides a CRE-inducible dual system for imaging neural circuits in vivo, and to study their assembly and remodeling in cultured neurons, neural stem cells, and tissue explants derived from the reporter line. Unlike existing lines, this reporter does not encode a ubiquitously expressed, floxable LacZ gene, permitting the simultaneous analysis of beta galactosidase activity in mutant lines.

Design and optimization of lentiviral vectors for transfer of GALC expression in Twitcher brain

BACKGROUND

Demyelination in globoid cell leukodystrophy (GLD) is due to a deficiency of galactocerebrosidase (GALC) activity. Up to now, in vivo brain viral gene transfer of GALC showed modest impact on disease development in Twitcher mice, an animal model for GLD. Lentiviral vectors, which are highly efficient to transfer the expression of therapeutic genes in neurons and glial cells, have not been evaluated for direct cerebral therapy in GLD mice.

METHODS

Lentiviral vectors containing the untagged cDNA or the hemagglutinin (HA)-tagged cDNA for the full-length mouse GALC sequence were generated and validated in vitro. In vivo therapeutic efficacy of these vectors was evaluated by histology, biochemistry and electrophysiology after transduction of ependymal or subependymal layers in young Twitcher pups.

RESULTS

Both GALC lentiviral vectors transduced neurons, oligodendrocytes and astrocytes with efficiencies above 75% and conferred high levels of enzyme activity. GALC accumulated in lysosomes of transduced cells and was also secreted to the extracellular medium. Conditioned GALC medium was able to correct the enzyme deficiency when added to non-transduced Twitcher glial cultures. Mice that received intraventricular injections of GALC vector showed accumulation of GALC in ependymal cells but no diffusion of the enzyme from the ependymal ventricular tree into the cerebral parenchyma. Significant expression of GALC-HA was detected in neuroglioblasts when GALC-HA lentiviral vectors were injected in the subventricular zone of Twitcher mice. Life span and motor conduction in both groups of treated Twitcher mice were not significantly ameliorated.

CONCLUSIONS

Lentiviral vectors showed to be efficient for reconstitution of the GALC expression in Twitcher neural cells. GALC was able to accumulate in lysosomes as well as to enter the secretory pathway of lysosomal enzymes, two fundamental aspects for gene therapy of lysosomal storage diseases. Our in vivo results, while showing the capacity of lentiviral vectors to transfer expression of therapeutic GALC in the Twitcher brain, did not limit progression of disease in Twitchers and highlight the need to evaluate other routes of administration.

Oligodendroglial progenitor cell therapy limits central neurological deficits in mice with metachromatic leukodystrophy

This work describes the first successful oligodendrocyte-based cell therapy for presymptomatic arylsulfatase A (ARSA) null neonate mice, a murine model for human metachromatic leukodystrophy (MLD). We found that oligodendrocyte progenitors (OLPs) engrafted and survived into adulthood when transplanted in the neonatal MLD brain. Transplanted cells integrated nondisruptively, did not produce tumors, and survived as proteolipid protein- and MBP-positive postmitotic myelinating oligodendrocytes (OLs) intermingled with endogenous MLD OLs within the adult MLD white matter. Transplanted MLD mice had reduced sulfatide accumulation in the CNS, increased brain ARSA activity, and full prevention of the electrophysiological and motor deficits that characterize untreated MLD mice. Our results provide direct evidence that healthy OLPs can tolerate the neurotoxic accumulation of sulfatides that evolves during the postnatal development of the MLD brain and contribute to OL cell replacement to limit the accumulation of sulfatides and the evolution of CNS defects in this lysosomal storage disease mouse model.

Myelin deterioration in Twitcher mice: motor evoked potentials and magnetic resonance imaging as in vivo monitoring tools

We have used magnetic resonance imaging (MRI) and motor evoked potentials (MEPs) for monitoring disease progression within the CNS of the Twitcher mouse, the murine model for globoid cell leukodystrophy (GLD). GLD is a lysosomal storage disorder, resulting from galactocerebrosidase deficiency, causing central and peripheral myelin impairment, leading to death, usually during early infancy. Neuroradiological, electrophysiological, and pathological parameters of myelin maturation were evaluated in Twitcher mice between postnatal days 20 and 45. Healthy controls showed a gradual-appearance MRI T2-weighted hypointensity of the corpus callosum (CC) starting at about P30 and ending at about P37, whereas MRI of age-matched Twitcher mice showed a complete loss of the CC-related MRI signal. MEPs allowed the functional assessment of myelin maturation within corticospinal motor pathways and showed a progressive deterioration of MEPs in Twitcher mice with increased central conduction time (CCT; 5.12 +/- 0.49 msec at P27 to 6.45 +/- 1.96 msec at P32), whereas physiological CCT shortening was found in healthy controls (3.01 +/- 0.81 msec at P27 to 2.5 +/- 0.27 msec at P32). These findings were not paralleled by traditional histological stainings. Optical observation of Bielchowsky and Luxol fast blue-PAS stainings showed mild axonal/myelin deterioration of the Twitcher brain within this time frame. Our results demonstrate that serial MRI and MEP readings are sensitive evaluation tools for in vivo monitoring of dysmyelination in Twitcher mice and underscore their potential use for longitudinal evaluation of the therapeutic impact of gene and cell therapies on these animals.

Analysis of galactocerebrosidase activity in the mouse brain by a new histological staining method

Gene therapy of galactocerebrosidase (GALC) deficient mice (Twitcher mutants) requires a fast and sensitive assay to detect transduced cells in vitro and in vivo. We have developed a new rapid histochemical method that specifically detects GALC activity in situ in neural cells using 5-Br-3Cl-beta-galactopiranoside (X-Gal) in the presence of taurodeoxycholic and oleic acids to enhance suspension of the substrate at low pH. Using this method, we observed robust X-Gal staining in diverse neuronal populations and interfascicular oligodendrocytes in sections from normal mouse brain. In contrast, sections of Twitcher brain did not show a specific staining pattern in neurons or glial cells. The availability of this new sensitive and rapid in situ detection assay is fundamental for the follow-up of Twitcher mice under gene or cellular therapies to correct central GALC deficiency.

Conditionally immortalized cell lines, engineered to produce and release GABA, modulate the development of behavioral seizures

Transplantation of genetically engineered cells can provide sustained focal delivery of naturally occurring molecules, including neurotransmitters and growth factors. We have engineered immortalized mouse cortical neurons and glia to deliver GABA by driving GAD(65) expression. Engineered cell lines showed GAD(65) mRNA expression, enzymatic activity, and GABA release. In vitro, basal flux of GABA was approximately 20% of total cellular GABA. We transplanted these GABA-producing cells bilaterally into either the anterior or the posterior substantia nigra of 43 rats. The rats were subsequently kindled through an electrode placed in the entorhinal cortex. GABA-producing cells, but not beta-galactosidase-producing cells, affected kindling rates. The number of stimulations needed to reach the first stage-5 seizure and to achieve full kindling differed significantly between the anterior and posterior transplantation sites when GAD(65)-producing cells were transplanted but not when beta-galactosidase-producing cells were transplanted. Our data show that transplanted engineered cells can make and release GABA at physiologically meaningful concentrations.