Demyelination in the spinal cord of murine globoid cell leukodystrophy (the twitcher mouse)

Deficiency of galactosyl-ceramidase in adult oligodendrocytes worsens disease severity during chronic experimental allergic encephalomyelitis

Galactosyl-ceramidase (GALC) is a ubiquitous lysosomal enzyme crucial for the correct myelination of the mammalian nervous system during early postnatal development. However, the physiological consequence of GALC deficiency in the adult brain remains unknown. In this study, we found that mice with conditional ablation of GALC activity in post-myelinating oligodendrocytes were lethally sensitized when challenged with chronic experimental allergic encephalomyelitis (EAE), in contrast with the non-lethal dysmyelination observed in Galc-ablated mice without the EAE challenge. Mechanistically, we found strong inflammatory demyelination without remyelination and an impaired fusion of lysosomes and autophagosomes with accumulation of myelin debris after a transcription factor EB-dependent increase in the lysosomal autophagosome flux. These results indicate that the physiological impact of GALC deficiency is highly influenced by the cell context (oligodendroglial vs. global expression), the presence of inflammation, and the developmental time when it happens (pre-myelination vs. post-myelination). We conclude that Galc expression in adult oligodendrocytes is crucial for the maintenance of adult central myelin and to decrease vulnerability to additional demyelinating insults.

Krabbe Disease: Prospects of Finding a Cure Using AAV Gene Therapy

Krabbe Disease (KD) is an autosomal metabolic disorder that affects both the central and peripheral nervous systems. It is caused by a functional deficiency of the lysosomal enzyme, galactocerebrosidase (GALC), resulting in an accumulation of the toxic metabolite, psychosine. Psychosine accumulation affects many different cellular pathways, leading to severe demyelination. Although there is currently no effective therapy for Krabbe disease, recent gene therapy-based approaches in animal models have indicated a promising outlook for clinical treatment. This review highlights recent findings in the pathogenesis of Krabbe disease, and evaluates AAV-based gene therapy as a promising strategy for treating this devastating pediatric disease.

Krabbe disease: New hope for an old disease

Krabbe disease (globoid cell leukodystrophy) is a lysosomal storage disease (LSD) characterized by progressive and profound demyelination. Infantile, juvenile and adult-onset forms of Krabbe disease have been described, with infantile being the most common. Children with an infantile-onset generally appear normal at birth but begin to miss developmental milestones by six months of age and die by two to four years of age. Krabbe disease is caused by a deficiency of the acid hydrolase galactosylceramidase (GALC) which is responsible for the degradation of galactosylceramides and sphingolipids, which are abundant in myelin membranes. The absence of GALC leads to the toxic accumulation of galactosylsphingosine (psychosine), a lysoderivative of galactosylceramides, in oligodendrocytes and Schwann cells resulting in demyelination of the central and peripheral nervous systems, respectively. Treatment strategies such as enzyme replacement, substrate reduction, enzyme chaperones, and gene therapy have shown promise in LSDs. Unfortunately, Krabbe disease has been relatively refractory to most single-therapy interventions. Although hematopoietic stem cell transplantation can alter the course of Krabbe disease and is the current standard-of-care, it simply slows the progression, even when initiated in pre-symptomatic children. However, the recent success of combinatorial therapeutic approaches in small animal models of Krabbe disease and the identification of new pathogenic mechanisms provide hope for the development of effective treatments for this devastating disease. This review provides a brief history of Krabbe disease and the evolution of single and combination therapeutic approaches and discusses new pathogenic mechanisms and how they might impact the development of more effective treatment strategies.

AAV-Mediated GALC Gene Therapy Rescues Alpha-Synucleinopathy in the Spinal Cord of a Leukodystrophic Lysosomal Storage Disease Mouse Model

Krabbe's disease (KD) is primarily a demyelinating disorder, but recent studies have identified the presence of neuronal protein aggregates in the brain, at least partially composed by alpha-synuclein (α-syn). The role of this protein aggregation in the pathogenesis of KD is largely unknown, but it has added KD to a growing list of lysosomal storage diseases that can be also be considered as proteinopathies. While the presence of these protein aggregates within the KD brain is now appreciated, the remainder of the central nervous system (CNS) remains uncharacterized. This study is the first to report the presence of thioflavin-S reactive inclusions throughout the spinal cord of both murine and human spinal tissue. Stereological analysis revealed the temporal and spatial accumulation of these inclusions within the neurons of the ventral spinal cord vs. those located in the dorsal cord. This study also confirmed that these thio-S positive accumulations are present within neuronal populations and are made up at least in part by α-syn in both the twitcher mouse and cord autopsied material from affected human patients. Significantly, neonatal gene therapy for galactosylceramidase, a treatment that strongly improves the survival and health of KD mice, but not bone marrow transplantation prevents the formation of these inclusions in spinal neurons. These results expand the understanding of α-syn protein aggregation within the CNS of individuals afflicted with KD and underlines the tractability of this problem via early gene therapy, with potential impact to other synucleinopathies such as PD.

Novel GALC Mutations Cause Adult-Onset Krabbe Disease With Myelopathy in Two Chinese Families: Case Reports and Literature Review

Krabbe disease (KD), also referred to as globoid cell leukodystrophy, is a rare autosomal recessive lysosomal storage disorder caused by β-galactocerebrosidase (GALC) deficiency. Most patients affected by this disease are infants, and <10% of cases suffer from adult-onset KD. In this study, two Chinese males presented with long-term progressive weakness in their limbs. Magnetic resonance imaging of the brain and spinal cord of these patients revealed lesions with abnormally high signal intensity on T2-weighted (T2W) and T2W fluid-attenuated inversion recovery images. Whole-exome sequencing was performed for both patients, and four GALC mutations were identified. Case 1 carried a novel deletion mutation (p.T633Tfs^*2) and a known missense mutation (p.T529M), while case 2 carried a novel missense mutation (p.W355C) and a known missense mutation (p.P154H). Previous literature has rarely reported myelopathy in patients with KD; in this study, we report two cases of adult-onset KD who both experienced myelopathy. We also conducted a literature review of KD and its association with myelopathy. Our findings provide a better understanding of the phenotypic and genotypic profiles associated with adult-onset KD. We recommend that physicians consider KD as a possible diagnosis in cases showing progressive motor dysfunction or gait disorder in association with typical myelopathy.

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.

Substrate reduction therapy for Krabbe's disease

Krabbe's disease (KD) is a lysosomal storage disorder in which galactosylceramide, a major glycosphingolipid of myelin, and psychosine (galactose-sphingosine) cannot be adequately metabolized because of a deficiency in galactosylceramidase. Substrate reduction therapy (SRT) has been tested in preclinical studies. The premise of SRT is to reduce the synthesis of substrates that are not adequately digested so that the substrate burden is lowered, resulting in less accumulation of unmetabolized material. SRT is used for Gaucher's disease, in which inhibitors of the terminal biosynthetic step are used. Unfortunately, an inhibitor for the final step of galactosylceramide biosynthesis, i.e., UDP glycosyltransferase 8 (a.k.a. UDP-galactose ceramide galactosyltransferase), has not been found. Approaches that inhibit an earlier biosynthetic step or that lessen the substrate burden by other means, such as genetic manipulations, have been tested in the twitcher mouse model of KD. Either as a stand-alone therapy or in combination with other approaches, SRT slowed the disease course, indicating that this approach has potential therapeutic value. For instance, in individuals with adult-onset disease, SRT theoretically could lessen the production of substrates so that residual enzymatic activity could adequately manage the lower substrate burden. In more severe forms of disease, SRT theoretically could be part of a combination therapy. However, SRT has the potential to impair normal function by reducing the synthesis of galactosylceramide to levels that impede myelin function, or SRT could have other deleterious effects. Thus, multiple issues need to be resolved before this approach is ready for testing in humans. © 2016 Wiley Periodicals, Inc.

Psychosine enhances the shedding of membrane microvesicles: Implications in demyelination in Krabbe's disease

In prior studies, our laboratory showed that psychosine accumulates and disrupts lipid rafts in brain membranes of Krabbe’s disease. A model of lipid raft disruption helped explaining psychosine’s effects on several signaling pathways important for oligodendrocyte survival and differentiation but provided more limited insight in how this sphingolipid caused demyelination. Here, we have studied how this cationic inverted coned lipid affects the fluidity, stability and structure of myelin and plasma membranes. Using a combination of cutting-edge imaging techniques in non-myelinating (red blood cell), and myelinating (oligodendrocyte) cell models, we show that psychosine is sufficient to disrupt sphingomyelin-enriched domains, increases the rigidity of localized areas in the plasma membrane, and promotes the shedding of membranous microvesicles. The same physicochemical and structural changes were measured in myelin membranes purified from the mutant mouse Twitcher, a model for Krabbe’s disease. Areas of higher rigidity were measured in Twitcher myelin and correlated with higher levels of psychosine and of myelin microvesiculation. These results expand our previous analyses and support, for the first time a pathogenic mechanism where psychosine’s toxicity in Krabbe disease involves deregulation of cell signaling not only by disruption of membrane rafts, but also by direct local destabilization and fragmentation of the membrane through microvesiculation. This model of membrane disruption may be fundamental to introduce focal weak points in the myelin sheath, and consequent diffuse demyelination in this leukodystrophy, with possible commonality to other demyelinating disorders.

Generation of a LacZ reporter transgenic mouse line for the stereological analysis of oligodendrocyte loss in galactosylceramidase deficiency

Krabbe's disease is a leukodystrophy resulting from deficiency of galactosylceramidase and the accumulation of galactosylsphingosine (psychosine) in the nervous system. Psychosine is believed to cause central demyelination by killing oligodendrocytes. Quantitative analysis of this process is lacking. To address this, we generated a new transgenic reporter twitcher line in which myelinating oligodendrocytes are genetically marked by the expression of LacZ under control of the myelin basic protein (MBP) promoter. MBP-LacZ-twitcher transgenic mice were used for unbiased stereological quantification of β-galactosidase+ oligodendrocytes in the spinal cord. As expected, we found decreased numbers of these cells in mutant cords, paralleling the severity of clinical disease. The decrease of oligodendrocytes does not correlate well with the increase of psychosine. The new MBP-LacZ-twitcher line will be a useful genetic tool for measuring changes in oligodendrocyte numbers in different regions of the mutant CNS and in preclinical trials of therapies to prevent demyelination. © 2016 Wiley Periodicals, Inc.

Mechanism of neuromuscular dysfunction in Krabbe disease

The atrophy of skeletal muscles in patients with Krabbe disease is a major debilitating manifestation that worsens their quality of life and limits the clinical efficacy of current therapies. The pathogenic mechanism triggering muscle wasting is unknown. This study examined structural, functional, and metabolic changes conducive to muscle degeneration in Krabbe disease using the murine (twitcher mouse) and canine [globoid cell leukodystrophy (GLD) dog] models. Muscle degeneration, denervation, neuromuscular [neuromuscular junction (NMJ)] abnormalities, and axonal death were investigated using the reporter transgenic twitcher-Thy1.1-yellow fluorescent protein mouse. We found that mutant muscles had significant numbers of smaller-sized muscle fibers, without signs of regeneration. Muscle growth was slow and weak in twitcher mice, with decreased maximum force. The NMJ had significant levels of activated caspase-3 but limited denervation. Mutant NMJ showed reduced surface areas and lower volumes of presynaptic terminals, with depressed nerve control, increased miniature endplate potential (MEPP) amplitude, decreased MEPP frequency, and increased rise and decay rate constants. Twitcher and GLD dog muscles had significant capacity to store psychosine, the neurotoxin that accumulates in Krabbe disease. Mechanistically, muscle defects involved the inactivation of the Akt pathway and activation of the proteasome pathway. Our work indicates that muscular dysfunction in Krabbe disease is compounded by a pathogenic mechanism involving at least the failure of NMJ function, activation of proteosome degradation, and a reduction of the Akt pathway. Akt, which is key for muscle function, may constitute a novel target to complement in therapies for Krabbe disease.

Missense mutation in mouse GALC mimics human gene defect and offers new insights into Krabbe disease

Krabbe disease is a devastating pediatric leukodystrophy caused by mutations in the galactocerebrosidase (GALC) gene. A significant subset of the infantile form of the disease is due to missense mutations that result in aberrant protein production. The currently used mouse model, twitcher, has a nonsense mutation not found in Krabbe patients, although it is similar to the human 30 kb deletion in abrogating GALC expression. Here, we identify a spontaneous mutation in GALC, GALCtwi-5J, that precisely matches the E130K missense mutation in patients with infantile Krabbe disease. GALCtwi-5J homozygotes show loss of enzymatic activity despite normal levels of precursor protein, and manifest a more severe phenotype than twitcher, with half the life span. Although neuropathological hallmarks such as gliosis, globoid cells and psychosine accumulation are present throughout the nervous system, the CNS does not manifest significant demyelination. In contrast, the PNS is severely hypomyelinated and lacks large diameter axons, suggesting primary dysmyelination, rather than a demyelinating process. Our data indicate that early demise is due to mechanisms other than myelin loss and support an important role for neuroinflammation in Krabbe disease progression. Furthermore, our results argue against a causative relationship between psychosine accumulation, white matter loss and gliosis.

MMP-3 mediates psychosine-induced globoid cell formation: implications for leukodystrophy pathology

Globoid cell leukodystrophy (GLD) or Krabbe disease, is a fatal demyelinating disease attributed to mutations in the galactocerebrosidase (GALC) gene. Loss of function mutations in GALC result in accumulation of the glycolipid intermediate, galactosylsphingosine (psychosine). Due to the cytotoxicity of psychosine, it has been hypothesized that accumulated psychosine underlie the pathophysiology of GLD. However, the cellular mechanisms of GLD pathophysiology remain unclear. Globoid cells, multinucleated microglia/macrophages in the central nervous system (CNS), are a defining characteristic of GLD. Here we report that exposure of primary glial cultures to psychosine induces the expression and the production of matrix metalloproteinase (MMP)-3 that mediated a morphological transformation of microglia into a multinucleated globoid cell type. Additionally, psychosine-induced globoid cell formation from microglia was prevented by either genetic ablation or chemical inhibition of MMP-3. These effects are microglia-specific as peripheral macrophages exposed to psychosine did not become activated or express increased levels of MMP-3. In the brain from twitcher mice, a murine model of human GLD, elevated MMP-3 expression relative to wild-type littermates was contemporaneous with disease onset and further increased with disease progression. Further, bone marrow transplantation (BMT), currently the only therapeutically beneficial treatment for GLD, did not mitigate the elevated expression of MMP-3 in twitcher mice. Hence, elevated expression of MMP-3 in GLD may promote microglial responses to psychosine that may represent an important pathophysiological process in this disease and its treatment.

Peripheral neuropathy in the Twitcher mouse involves the activation of axonal caspase 3

Infantile Krabbe disease results in the accumulation of lipid-raft-associated galactosylsphingosine (psychosine), demyelination, neurodegeneration and premature death. Recently, axonopathy has been depicted as a contributing factor in the progression of neurodegeneration in the Twitcher mouse, a bona fide mouse model of Krabbe disease. Analysis of the temporal-expression profile of MBP (myelin basic protein) isoforms showed unexpected increases of the 14, 17 and 18.5 kDa isoforms in the sciatic nerve of 1-week-old Twitcher mice, suggesting an abnormal regulation of the myelination process during early postnatal life in this mutant. Our studies showed an elevated activation of the pro-apoptotic protease caspase 3 in sciatic nerves of 15- and 30-day-old Twitcher mice, in parallel with increasing demyelination. Interestingly, while active caspase 3 was clearly contained in peripheral axons at all ages, we found no evidence of caspase accumulation in the soma of corresponding mutant spinal cord motor neurons. Furthermore, active caspase 3 was found not only in unmyelinated axons, but also in myelinated axons of the mutant sciatic nerve. These results suggest that axonal caspase activation occurs before demyelination and following a dying-back pattern. Finally, we showed that psychosine was sufficient to activate caspase 3 in motor neuronal cells in vitro in the absence of myelinating glia. Taken together, these findings indicate that degenerating mechanisms actively and specifically mediate axonal dysfunction in Krabbe disease and support the idea that psychosine is a pathogenic sphingolipid sufficient to cause axonal defects independently of demyelination.

Diffusion tensor imaging detects axonal injury and demyelination in the spinal cord and cranial nerves of a murine model of globoid cell leukodystrophy

Globoid cell leukodystrophy is an inherited neurodegenerative disorder caused by a deficiency of the lysosomal enzyme galactosylceramidase. In both human patients and the authentic murine Twitcher model, pathological findings include demyelination as well as axonal damage in both the central and peripheral nervous system. Diffusion tensor imaging (DTI) has emerged as a powerful noninvasive technique that is sensitive to these white matter disease processes. Increases in radial diffusivity (lambda perpendicular) and decreases in axial diffusivity (lambda parallel) correlate with histopathological evidence of demyelination and axonal damage, respectively. Compared to age-matched, normal littermates, DTI of optic nerve and trigeminal nerve in end-stage Twitcher mice displayed a statistically significant increase in lambda perpendicular and decrease in lambda parallel, consistent with previously characterized demyelination and axonal damage in these regions. In the Twitcher spinal cord, a statistically significant decrease in lambda parallel was identified in both the dorsal and ventrolateral white matter, relative to normal controls. These results were consistent with immunofluorescence evidence of axonal damage in these areas as detected by staining for nonphosphorylated neurofilaments (SMI32). Increase in lambda perpendicular in Twitcher spinal cord white matter relative to normal controls reached statistical significance in the dorsal columns and approached statistical significance in the ventrolateral region. Correlative reduced levels of myelin basic protein were detected by immunofluorescent staining in both these white matter regions in the Twitcher spinal cord. Fractional anisotropy, a nonspecific but sensitive indicator of white matter disease, was significantly reduced in the optic nerve, trigeminal nerve, and throughout the spinal cord white matter of Twitcher mice, relative to normal controls. This first reported application of spinal cord DTI in the setting of GLD holds potential as a noninvasive, quantitative assay of therapeutic efficacy in future treatment studies.

Galactosylsphingosine (psychosine)-induced expression of cytokine-mediated inducible nitric oxide synthases via AP-1 and C/EBP: implications for Krabbe disease

Globoid cell leukodystrophy (Krabbe disease) is characterized by the accumulation of a toxic metabolite, psychosine (galactosylsphingosine), which is a substrate for the deficient enzyme (galactocerebroside beta-galactosidase). This study underscores the possible role of psychosine in the effect of inducible nitric oxide synthase (iNOS) -derived NO in the pathophysiology of this demyelinating disease. For the first time, we provide evidence of the expression of iNOS in CNS of Krabbe patient and show that the iNOS-expressing cells in the CNS were astrocytes. Psychosine potentiated the LPS-induced production of proinflammatory cytokines (IL-1beta, IL-6, and TNF-alpha) in primary rat astrocytes and regulated the cytokine-mediated production of NO in C6 glioma and primary rat astrocyte. Psychosine induced cytokine-mediated nuclear translocation of AP-1 and C/EBP by potentiating the expression of Fra-1 and C/EBP-delta proteins. This suggests that psychosine maintained or sustained the cytokine-primed expression of iNOS by further potentiating the nuclear translocation of AP-1 and C/EBP without modulating the cytokine-mediated transcription activity of NF-kappaB. This study hypothesizes that accumulated psychosine leads to production of cytokines and iNOS expression. The ensuing excessive production of NO and ONOO- may play a role in pathogenesis of Krabbe disease.

TNF-receptor 1 deficiency fails to alter the clinical and pathological course in mice with globoid cell leukodystrophy (twitcher mice) but affords protection following LPS challenge

Twitcher mice have an autosomal recessive mutation in the gene for the lysosomal enzyme galactosylceramidase, which is the same gene that is affected in human globoid cell leukodystrophy (Krabbe's disease). The failure to digest galactosylceramide and psychosine leads to initial pathological changes in oligodendrocytes. Secondary pathological changes that include infiltrating macrophages and other inflammatory responses have been postulated to promote the disease course. TNFalpha levels are elevated in twitcher mice compared to control animals, and studies on another demyelinating disease, experimental allergic encephalomyelitis, indicate that TNF promotes pathogenesis via TNF-receptor 1 (TNF-R1). In the present study, twitcher/TNF-R1 deficient mice were generated, and the clinical and pathological course was compared between these mice and regular twitcher mice. There was no statistical evidence for any differences between these two groups of mice for all clinical (life span, weight loss, onset day of twitching) and pathological (demyelination, astrocyte gliosis, macrophage infiltration) measures that were examined. If mice were administered an intraperitoneal injection of LPS, then twitcher/TNF-R1 deficient mice had a longer [corrected] life span and a decreased [corrected] disruption to the blood-brain barrier compared to regular twitcher mice. These results showed that TNF-R1 is not sufficiently activated to affect the pathological and/or clinical signs during the natural course of this disease. However, when there is a secondary insult, TNF-R1 activation does lead to a significant acceleration of the development of clinical and pathological signs.

Murine model of genetic demyelinating disease: the twitcher mouse

Twitcher mouse is an authentic murine model of human genetic demyelinating disease, globoid cell leukodystrophy (GLD), or Krabbe disease. Since its discovery at the Jackson Laboratory (Bar Harbor, ME) this model has been used extensively for the morphological, biochemical-enzymatic studies to clarify pathogenesis and also for therapeutic manipulation of genetic demyelinating disease in humans. As a result of these studies, now we know that (1) GLD is caused by a deficiency of lysosomal enzyme galactosylceramidase, and a toxic metabolite, psychosine, accumulates in the tissue, including the nervous system, damaging myelin forming cells and resulting in secondary demyelination; (2) morphological features of demyelination and associated cellular reactions in demyelination in this mutant are similar to those seen in autoimmune or toxic demyelination; and (3) with enzyme supplementation provided by bone marrow transplantation, remyelination occurs to some extent in demyelinated fibers in both central and peripheral nervous systems of twitcher mouse.

Absence of MHC class II molecules reduces CNS demyelination, microglial/macrophage infiltration, and twitching in murine globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD) is a severe genetic demyelinating disorder with an increased number of Ia (immune response antigen) positive brain microglia/macrophages. To assess the role of aberrant Ia expression in the central nervous system (CNS), twitcher mice, which represent the murine model for GLD, were mated with Ia- transgenic mice. Compared with the Ia+ controls, Ia- twitcher mice showed a profound reduction in the severity of demyelinating lesions correlated with significantly fewer microglia/macrophages. Most importantly, Ia- twitcher mice showed significantly reduced twitching compared with ia+ twitcher mice. In contrast with experimental allergic encephalomyelitis (EAE), there was no significant amount of inflammatory T cell infiltrates, implying that T cells may not play a predominant role in this disease. These findings may have broad therapeutic implications for Alzheimer's disease, Parkinson's disease, and Huntington's disease, which display enhanced Ia expression in the CNS without obvious T cell infiltrates.

MHC class II antigen expression and T-cell infiltration in the demyelinating CNS and PNS of the twitcher mouse

The expression of the major histocompatibility complex class II antigens (Ia) was investigated in the central and peripheral nervous systems of the twitcher mouse, an authentic murine model of globoid cell leukodystrophy (Krabbe disease) in humans. In this mutant, demyelination is noted as early as postnatal day 10 in the peripheral nerve and day 20 in the spinal cord. Emergence of Ia antigen expressing cells (Ia+ cells) was largely coincident with the initiation of demyelination, suggesting the importance of local factors for the induction of Ia antigens. Ia+ cells gradually increased in number with the progression of demyelination, but reached a plateau between postnatal days 30 and 40, then rapidly decreased despite continuous demyelination in both central and peripheral nervous systems. The only exception was in the spinal cord where Ia+ cells were numerous even at postnatal day 50. The cells expressing L3T4 antigen (L3T4+ cells), which is a mouse CD4 antigen, and the interleukin-2 receptor antigen expressing cells (IL-2R+ cells), also appeared in the regions where Ia+ cells were observed. L3T4+ cells were still abundant after P45, when Ia+ and IL-2R+ cells decreased. Combined autoradiographic and immunocytochemical studies demonstrated mitotic activity in a few Ia+ cells. However, low labeling indices of Ia+ cells in both cerebrum and sciatic nerve suggest that the increase of Ia+ cells was largely due to either induction of Ia antigens on local cells or new recruitment of Ia+ cells from the circulation. Expression of Ia antigens associated with an emergence of L3T4+ or IL-2R+ cells in the demyelinating lesions may indicate involvement of immunological responses in the pathogenesis of this metabolic demyelinating disorder. Alternatively, these apparent immunological phenomena may be tentative and non-specific local reactions to degenerating tissue components such as myelin. The mechanism(s) regulating the expression of Ia antigens in this genetic metabolic demyelinating disease is yet to be determined.

Proliferation of microglia/macrophages in the demyelinating CNS and PNS of twitcher mouse

In demyelinating lesions in the central and peripheral nervous systems of twitcher mouse, a murine model of globoid cell leukodystrophy, marked increases of [3H]thymidine-labeled elements including Mac-1 immunopositive (Mac-1+) microglia/macrophages was observed. Their proliferative activities were already pronounced at the postnatal day (P) 20, an early stage of demyelination, peaked at P30 and then declined at P45, at the terminal stage of the disease. Many of the [3H]thymidine-labeled Mac-1+ cells had morphological features of ramified microglia. Macrophage-like Mac-1+ cells were less frequently labeled. The results of this study showed that (1) microglia/macrophages proliferated in the genetic demyelinating lesions, (2) many dividing Mac-1+ cells had the shape of ramified microglia and (3) the rate of proliferation declined in later stages when reactive microglia/macrophages were abundant in the lesions. The temporal events of this study suggest that signal(s) from the degenerating myelin or myelin-forming cells stimulate(s) cellular proliferation and that ramified microglia were one of the principal-dividing elements in the lesion. The biological mechanisms underlying the decline of the proliferative activity of microglia require further studies.

Accumulation of galactosylsphingosine (psychosine) does not interfere with phosphorylation and methylation of myelin basic protein in the twitcher mouse

In attempts to elucidate mechanisms of demyelination in the twitcher mouse (Twi), phosphorylation and methylation of myelin basic protein (MBP) were examined in the brainstem and spinal cord of this species. Phosphorylation of MBP in isolated myelin by an endogenous kinase and an exogenous [32P]ATP was not impaired and protein kinase C activity in the brain cytosol was not reduced. When the methylation of an arginine residue of MBP was examined in slices of the brainstem and spinal cord, using [3H]methionine as a donor of the methyl groups, no difference was found between Twi and the controls. Radioactivity of the [3H] methionine residue of MBP of Twi was also similar to that of the controls. Thus, accumulation of psychosine in Twi does not interfere with the activity of endogenous kinase, methylation of MBP, and the synthesis and transport of MBP into myelin membrane.

Metabolism of exogenous galactosylceramide in the twitcher mouse brain

The in vivo metabolism of galactosylceramide (gal-cer) in normal mice and in twitcher mice, a model of human GLD, was examined following intracerebral administration of gal-cer containing [1-14C] stearic acid. In normal mice, gal-cer was hydrolyzed to ceramide within 6 hours and ceramide was hydrolyzed to sphingosine and fatty acid. Most of the released fatty acid was immediately incorporated into other lipids. About 75% of injected gal-cer was hydrolyzed 80 hours after the injection, while in the twitcher mouse, only 17% of gal-cer was hydrolyzed. These results show that degradation of gal-cer is impaired in the twitcher mouse brain, but contradict to the fact that there was no evidence of any accumulation of gal-cer in the brain. This discrepancy may be due to the different sorting routes of biosynthesized and exogenously-administered gal-cer in the mouse brain. Most of the biosynthesized gal-cer is incorporated into myelin, while the injected gal-cer is incorporated into lysosomes.

The twitcher mouse: attenuated processes of Schwann cells in unmyelinated fibers

Morphological alterations occurring in Schwann cells of unmyelinated fibers (unmyelinated Schwann cells) were investigated in the sciatic nerve of the twicher mouse, a murine model of human globoid cell leukodystrophy. After postnatal day 10, the number of Schwann cell-axon units gradually increased and the number of unmyelinated axons per unit progressively decreased in the twitcher mouse. However, the total number of unmyelinated axons showed no significant differences between twitcher and normal mice. Thus, these alterations of unmyelinated Schwann cells in the twitcher mouse suggest that attenuated branching of cellular processes develops at an early stage and progresses together with progression of demyelination in this mutant.

The twitcher mouse: accumulation of galactosylsphingosine and pathology of the central nervous system

In the twitcher mouse, a murine model of globoid cell leukodystrophy (GLD), pathological changes of various parts of the central nervous system correlated well with the concentration of galactosylsphingosine (psychosine). The development of GLD lesions was more obvious in tracts with a more rapid progression of myelination. It was suggested that accumulation of galactosylsphingosine subsequent to myelin maturation caused suicidal death of myelin forming cells.

Decreased fatty acylation of myelin proteolipid protein in the twitcher mouse

We examined chronological changes of myelin proteins of the brainstem and spinal cord of the twitcher mouse (15, 20, and 30 days old), a murine model of human globoid cell leukodystrophy caused by a genetic deficiency of galactosylceramidase I activity. The yield of myelin was normal until postnatal day 20, whereas galactosylsphingosine (psychosine) accumulated with age in myelin. The protein profiles of myelin and the activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase in the myelin remained normal throughout the experimental period. Fatty acylation of proteolipid protein (PLP) was examined in a cell-free system by incubation of myelin with [3H]palmitic acid, CoA, and ATP, and was normal at postnatal day 15, but decreased after postnatal day 20. Decreased fatty acylation of PLP was also observed in the twitcher mouse at postnatal day 20 when the isolated myelin was incubated with [14C]palmitoyl-CoA in the absence of ATP and CoA, or the slices of brainstem and spinal cord were incubated with [3H]palmitic acid. The activity of fatty acid:CoA ligase was reduced in myelin. These data suggest that decreased acylation of PLP in twitcher mouse myelin is probably due to reduced activities for both activation and transfer of fatty acid into PLP and that metabolic disturbance is present in myelin because acylation of PLP has been shown to occur in myelin membrane. Although psychosine (200 microM) inhibited only 17% of the acylation in vitro, it may be responsible for the reduced acylation of PLP in vivo.

Alterations of oligodendrocytes and demyelination in the spinal cord of patients with mitochondrial encephalomyopathy

The spinal cords of 2 autopsied patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) were examined. Histologically, the spinal cords showed a spongy state due to the presence of distended myelinated fibers with enlarged periaxonal spaces. Ultrastructurally, the affected fibers showed extensive microvacuolation of the inner myelin sheath with occasional vesicular changes. The presence of macrophages near the degenerated myelin was a frequent finding. The stripping of myelin lamellae by macrophage was observed, with frequent appearance of denuded axons. Furthermore, prominent morphological changes were observed in oligodendrocytes. These findings indicate that demyelination, probably secondary to the degeneration of oligodendrocytes, occurs in the spinal cord of MELAS.

The twitcher mouse: accumulation of galactosylsphingosine and pathology of the sciatic nerve

Morphological and biochemical changes were investigated in the early developmental stages of sciatic nerve of the twitcher mouse, a murine model of human globoid cell leukodystrophy. The concentration of galactosylsphingosine (psychosine) and the chronological changes of the twitcher mouse peripheral nerve pathology correlated well. Galactosylsphingosine had already accumulated at birth and dramatically increased with age. Characteristic inclusions were observed in Schwann cells and macrophages of the twitcher mouse on the 5th postnatal day. Endoneurial edema developed after 10 postnatal days and the hypomyelination was pronounced at 15-20 postnatal days. These findings suggest that galactosylsphingosine is cytotoxic for myelin-forming cells and is closely related to pathogenetic events in the twitcher mouse.

Donor-derived cells in the central nervous system of twitcher mice after bone marrow transplantation

The twitcher mouse is an animal model of galactosylceramidase deficiency, comparable to Krabbe's disease, a lysosomal storage disease in humans. As in most lysosomal storage diseases, neurological deterioration is a prominent feature of the disease in these mice. Transplantation of enzymatically normal congenic bone marrow was earlier found to result in prolonged survival and increased levels of galactosylceramidase in the visceral organs of twitcher mice. It is now reported that bone marrow transplantation results in increased galactosylceramidase levels in the central nervous system (CNS). Concomitantly, the levels of psychosine, a highly toxic lipid that progressively accumulates in the CNS of untreated twitcher mice, stabilized at much lower levels in the CNS of treated twitcher mice. Histologically, a gradual disappearance of globoid cells, the histological hallmark of Krabbe's disease, and the appearance of foamy macrophages capable of metabolizing the storage product were seen in the CNS. By immunohistochemical labeling it was demonstrated that these foamy macrophages were of donor origin. The infiltration of enzymatically competent, donor-derived macrophages was accompanied by extensive remyelination in the CNS. It is concluded that after bone marrow transplantation, donor-derived macrophages infiltrate the affected brain tissue and are capable of inducing a partial reversal of the enzyme deficiency.

Neurological and neurobehavioral development of the mutant 'twitcher' mouse

The present study described the neurological and locomotor development of the mutant 'twitcher' mouse (B57BL/6J-twi), an enzymatically authentic model of globoid cell (Krabbe) leukodystrophy. Comparisons were made on a neurological developmental battery and a series of behavioral tests, including open field, rotorod, and hangtime performance. Homozygous affected (twi/twi), heterozygous carriers (+/twi) and homozygous normals (+/+) were compared. Neurological development was slowed in twi/twi with some subtler differences between +/twi and normals. Twi/twi reached all functional milestones except grasp. There was a rapid deterioration of motor indices after 20 days of age. However, most sensory markers were preserved. On hangtime, there were significant differences from normal for both twi/twi and +/twi at 15 days of age and across the 15-30 day developmental stage, with the +/twi males slightly more impaired. On the rotorod, all animals were equally unable to stay on the rod at 15 days of age and neither male nor female twi/twi showed significant development. Male +/twi lagged significantly behind male +/+. In the open field, all groups were equally inactive at 13-15 days and showed similar increases in activity, rearing, and grooming until weaning. All groups peaked immediately after weaning and declined thereafter, with twi/twi showing the lowest activity. The data were discussed in terms of the relationship between the human disease and the animal model.

Accumulation of galactosylsphingosine (psychosine) in the twitcher mouse: determination by HPLC

We developed a sensitive and simple procedure for determination of galactosylsphingosine (psychosine), using HPLC. The method involved extraction of lipids, separation by cation-exchange and C18 reverse-phase columns, and derivatization with o-phthalaldehyde. The fluorescent galactosylsphingosine was detected by HPLC. The amount of galactosylsphingosine was accurately assayed by simultaneous determination of glucosylsphingosine, as the internal standard. The detection limit was 0.5 ng/assay tube, and the quantitative range of the method was up to 750 ng. This procedure was applied to tissue from the twitcher mouse, an animal model of human globoid cell leukodystrophy, as well as tissue from normal and carrier mice. In the latter mice, a small amount of galactosylsphingosine was detected in the spinal cord (21.6-37.2 ng/100 mg wet weight) but not in the cerebrum and sciatic nerve. Marked accumulation of galactosylsphingosine was noted in the nervous tissues of the twitcher strain, even on postnatal day 4. The concentration of galactosylsphingosine was greater in the peripheral than in central nervous tissues. The spinal cord and brainstem contained more galactosylsphingosine than did the cerebrum and cerebellum. The concentration increased with age from 764 ng/100 mg in the sciatic nerve at 4 days to 5,910 ng/100 mg at 37 days. These data correlate well with the pathological changes; tissues containing higher concentrations of galactosylsphingosine show earlier and more severe pathological changes than those containing lower concentrations, thereby indicating the close link of galactosylsphingosine to the pathogenesis of the twitcher mouse.

Balo's concentric sclerosis: new observations on lesion development

A 54-year-old woman with a four-month history of progressive neurological illness was found at postmortem examination to have lesions of Balo's concentric sclerosis. Balo lesions were found in several areas scattered widely throughout the central nervous system, including the spinal cord, a previously unreported location, and were studied by histological and ultrastructural methods. Balo lesions consisted of bands of intact myelin alternating with zones of demyelination. These lesions were centered on a perivascular cuff of inflammatory cells. The center of the lesion was the oldest area with the concentric rings of demyelination decreasing in age with increasing distance from the center. The bands of intact myelin comprised mainly remyelinated fibers, were similar to those seen at the edges of chronic active multiple sclerosis plaques, and may have represented the repaired margins of preceding episodes. The occurrence of small foci of acute demyelination centered on perivascular cuffs and other changes typical of both acute and chronic active multiple sclerosis may indicate that the lesion of Balo's concentric sclerosis represents an intermediate stage in the development of an established multiple sclerosis lesion.

Heritable metabolic storage diseases

During the last decade, examinations of skin biopsies have gained increasing importance in the search for a better understanding and facilitated diagnosis of metabolic storage diseases. In addition to biochemical and tissue culture techniques, light- and electron-microscopic investigations have been recommended in a number of such disorders (1-6). There are, however, a large number of different and inhomogeneous conditions to be identified in this group. At present, approximately 300 diseases can be identified as inborn errors of metabolism by the determination of a biochemically well-established underlying defect. Moreover, a yet undetermined number of pathological conditions is accompanied by the deposition of more or less defined storage materials in cutaneous tissue elements. It appears, therefore, premature to propose a systematic dermatopathology of metabolic storage diseases, and the following contribution should be understood as an attempt to indicate possibilities and chances by quoting some representative examples.

The twitcher mouse: positive immunohistochemical staining of globoid cells with monoclonal antibody against Mac-1 antigen

The expression of Mac-1 antigen (macrophage surface antigen) was studied by the immunoperoxidase method in the nervous system of the twitcher mouse (an authentic murine model of globoid cell leukodystrophy) from day 10 to day 39, to elucidate the origin of the globoid cells. Mac-1 positive cells were seen in peripheral nervous system (PNS) at all ages examined. The positive cells in central nervous system (CNS) showed staining characteristics similar to those in PNS, but in the CNS positive cells they appeared first at day 15 and increased significantly in older mice. These cells were always predominant in the white matter. There were few positive cells in the neural parenchyma of control mice before day 10 but none thereafter. The occurrence and the preferential distribution indicate that it is quite likely that Mac-1 positive cells in the twitcher mouse represent globoid cells. Twitcher mice which were previously given colloidal carbon intravenously, had some Mac-1 positive cells which contained carbon particles both in the CNS and PNS. These facts indicate that globoid cells are the cells of monocytic lineage and at least some of them are derived from blood monocytes.

Biochemical aspects of globoid and metachromatic leukodystrophies

Galactosylceramides and sulfogalactosylceramides are characteristic lipids of the myelin sheath. Two genetically determined leukodystrophies are caused by an inability to enzymically hydrolyze these glycolipids. Thus, a deficiency of galactocerebroside beta-galactosidase results in globoid cell leukodystrophy, whereas a reduced activity of arylsulfatase A is responsible for metachromatic leukodystrophy. Besides these disorders, deficiencies of arylsulfatases A, B, C, and other sulfatases have been shown in a distinct condition called "multiple sulfatase deficiency." All of these disorders are fatal and are characterized by marked demyelination and severe mental retardation. The cause of this demyelination is not known. However, cytotoxic galactosylsphingosine and sulfogalactosylsphingosine have been suggested as the agents responsible for this demyelination. Recent immunological studies have also shown that patients with globoid and metachromatic leukodystrophies contain a mutant galactocerebroside beta-galactosidase and arylsulfatase A, respectively. The mutant enzymes have different kinetic properties compared to the enzymes from normal subjects. However, they can cross-react with antibodies to these enzymes. Since partially purified preparations of galactocerebroside beta-galactosidase and homogeneous arylsulfatase A are now available, the possibility of enzyme replacement therapy in globoid and metachromatic leukodystrophies is discussed.