Axonal pathology in Krabbe's disease: The cytoskeleton as an emerging therapeutic target.

In Krabbe's disease (KD), demyelination and myelin-independent axonal and neuronal defects contribute to the severe neuropathology. The toxic substrate that accumulates in this disease, psychosine, induces alterations in membrane lipid rafts with downstream consequences to cellular signaling pathways that include impaired protein kinase C, ERK, and AKT-glycogen synthase kinase-3ß (GSK3ß) activation. In addition to impaired recruitment of signaling proteins to lipid rafts, endocytosis and axonal transport are affected in KD. Defects in AKT-GSK3ß activation, a central pathway regulating microtubule stability, together with alterations in neurofilaments and microtubules and severely defective axonal transport, highlight the importance of the neuronal cytoskeleton in KD. This Review critically discusses these primary neuronal defects as well as new windows for action opened by their identification that may contribute to effectively correct the neuropathology that underlies this disorder. © 2016 Wiley Periodicals, Inc.

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.

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.

A galactose-free diet enriched in soy isoflavones and antioxidants results in delayed onset of symptoms of Krabbe disease in twitcher mice.

Krabbe disease or globoid cell leukodystrophy is an autosomal recessively inherited disorder caused by the deficiency of galactocerebrosidase, the lysosomal enzyme that catalyzes the hydrolysis of galactose from galactosylceramide and galactosylsphingosine (psychosine). Psychosine accumulation results in the loss of myelin and oligodendrocytes in the brain of Krabbe patients as well as twitcher mice (natural model of human Krabbe disease). The aim of the present research was to investigate in twitcher mice the potential role of a diet deficient in galactose enriched in soy isoflavones and a pool of antioxidants molecules, such as l-glutathione, coenzyme Q10, xanthophylls, in counteracting the toxic effects derived by psychosine accumulation. A second goal of this manuscript was to demonstrate suppression of the apoptotic effects of psychosine in cultured oligodendrocyte progenitor mice cells (OLP-II) with antioxidants. The affected twitcher mice began the milk-derivatives free diet on post-natal day 15 although they also received mother's milk until post-natal day 18. Nevertheless, average life span was increased 50%, from 32+/-2 to 48+/-3 days, onset of tremor was delayed 17 days (from 21 days in the untreated twitcher mice to 38 days in the treated affected mice) and the gait in the treated mice was normal until almost a week after the untreated animals died (38+/-1 days versus 32 days at death). Weight gain in the treated animals also progressed to 38 days compared with 22 days for the untreated affected twitcher mice. Protection of the OLP-II cells against psychosine was shown using the MTT test (the ability of the tetrazolium salt MTT to form a dark blue formazan product by mitochondrial dehydrogenase in viable cells) and assay of expression of p53 and TNF-related apoptosis-inducing ligand (TRAIL). The results showed a time-dependent and concentration-dependent decrease of OLP-II viability on exposure to psychosine and dose-dependent protection with the antioxidants xanthophylls and glutathione. They also demonstrated that psychosine-induced p53 induction of apoptosis and TNF-related apoptosis-inducing ligand receptors could be decreased by l-glutathione and xanthophylls. A dietary approach may constitute a promising clinical management of the late-infantile and juvenile forms of Krabbe leukodystrophy.

Apoptotic positive cells in Krabbe brain and induction of apoptosis in rat C6 glial cells by psychosine.

Globoid cell leukodystrophy (GLD) is an autosomal recessive disorder of infants, caused by deficient activity of cerebroside-beta-galactosidase resulting in loss of myelin accompanied by loss of oligodendrocytes. The loss of oligodendrocyte population is accompanied by accumulation of psychosine, which is considered as the molecule responsible for the observed pathophysiology of GLD. We were able to detect apoptotic cells by terminal dUTP nick-end labeling assay and nuclear localization of p53 in postmortem brain tissue of Krabbe's disease patients, which were not detected in the control brain. To study the role of psychosine in cell death, we investigated the effect of psychosine on C6 glial cell survival by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Similar to ceramide (43.8% loss) the galactopsychosine and glucopsychosine treatment killed up to 46.3 and 48.75% of cells, respectively. On the other hand, sphingosine had no effect. DNA laddering assay confirmed these results. Moreover, psychosine-induced detection of annexin-V positive cells supports a role for psychosine in C6 glial cell death via the apoptotic pathway. These results indicate that psychosine may play a role in apoptotic cell loss observed in GLD brain.

Psychosine-induced apoptosis in a mouse oligodendrocyte progenitor cell line is mediated by caspase activation.

Globoid cell leukodystrophy (GLD) is an inherited neurological disorder caused by the deficiency of galactocerebrosidase (GALC) activity resulting in cell death of oligodendrocytes and subsequent demyelination. The death of oligodendrocytes is accompanied by accumulation of psychosine, which is also a substrate for the GALC enzyme. In this report, we investigated the mechanism of the toxic effect of psychosine in a mouse-derived oligodendrocyte progenitor cell line (OLP-II), a precursor to the cell type most affected in GLD. Psychosine caused cytotoxicity in a dose-dependent manner. Lower concentration of psychosine (5 microM) did not significantly reduce OLP-II cell numbers. However, 50 microM psychosine induced up to 45% cell death. These results were confirmed by the Tunel assay, which is a hallmark for the detection of apoptosis. Moreover, psychosine treatment resulted in the activation/cleavage of initiator caspase-8 and -9, and effector caspase-3. These results support a role for psychosine in OLP-II cell death via an apoptotic mechanism, and suggest the involvement of caspases.

Psychosine cytotoxicity toward rat C6 glioma cells and the protective effects of phorbol ester and dimethylsulfoxide: implications for therapy in Krabbe disease.

Psychosine cytotoxicity was tested as to its effects on rat C6 glioma cells. At a low concentration--below 40 microM--psychosine appeared to stimulate cell proliferation. Above the concentration range of 40 microM-60 microM, however, it showed a cytotoxic effect. When phorbol ester (PDB) or dimethylsulfoxide (DMSO) was supplemented to cultures being exposed to psychosine, the total number of live cells, protein content and CNPase activity dramatically increased as compared with the levels in cultures treated with psychosine alone. The results of these basic studies suggest another approach as to therapy for globoid cell leukodystrophy.

[Application of neuronal cell culture in human degenerative brain disorders].

Neuronal cell culture system has been used for the study of pathochemical evaluations in human degenerative brain disorders, particularly for Krabbe's disease and neuronal ceroid lipofuscinosis. To understand the pathochemistry of Krabbe's disease, we added psychosine into neuronal cell cultures and psychosine treated cells showed the destruction of cytoskeleton and pathy intracellular changes. Electron microscopic finding showed the swelling of the mitochondria. Oligodendrocytes and Schwann cells were isolated from the brains and sciatic nerve of twitcher mouse as an authentic murine model of globoid cell leukodystrophy. Oligodendroglial cells cultured for 22 days were stained by anti-galactocerebroside antibodies. In twitcher oligodendrocyte processes were wirelike and progressively degenerated and there were few membranous expansion. Schwann cells from twitcher could not elongated their processes. These data suggest that psychosine might be important factor to result in these pathological conditions. Furthermore, we studied the effect of protease inhibitors, E-64 on dissociated primary cultures from fetal rat brain. After treated with E-64 in a concentration from 0.1-50 micrograms/ml, numerous cytoplasmic accumulations appeared in neuronal cells. These morphological pictures resemble with those of neuronal ceroid lipofuscinosis, Batten disease. We will discuss the relationship between the deficiency of catepsin H in Batten disease and inclusion bodies found in E-64.

["Toxins" and nerve--a discussion on the pathogenesis of acrylamide intoxication, giant axonal neuropathy and Krabbe disease].

A common mechanism appears to play a pathogenetic role in acrylamide intoxication, which is due to an exogenous cause, and in giant axonal neuropathy, an intrinsic disorder. In giant axonal neuropathy, a genetic defect in the processing of an intrinsic acrylamide or acrylamide-like substance may be postulated. On the other hand, the devastating pathology in Krabbe disease (a genetic disorder affecting almost exclusively the nervous system) appears to be due to the effects of psychosine, a toxic lipid produced in the nervous system. These suggest that the "toxicological approach" can be more diverse and of greater use than usually considered.

[Binding of toxic substances and suppression of their toxicities by albumin--studies on psychosine and pentachlorophenol].

Psychosine (galactosylsphingosine) potently inhibits cytochrome c oxidase (COX) activity and hemolyzes washed erythrocytes in vitro. However, no hemolytic anemia is seen in patients with Krabbe disease or its animal models in which a psychosine-degrading enzyme is missing. This "discrepancy" may be caused by albumin because, 1) albumin is synthesized only in liver and it does not enter cells once it is released into the blood stream; 2) albumin can bind psychosine and suppress its effects effectively; 3) psychosine cannot exert its toxicity in the blood, should it be present there. On the other hand, an artificial substance, pentachlorophenol (PCP), which is widely used for wood preservation, causes mitochondrial dysfunction and hemolysis in vitro, both of which can be suppressed by albumin. Hence, it appears reasonable that hemolytic anemia is extremely rare, and that a dramatic improvement was seen in patients severely intoxicated with PCP. Thus, differences in the effects of these toxins seen in vitro and in vivo seem to be well explained by the effects of albumin. These also suggest that attention to the possible effects of albumin may be rewarding in monitoring and treating some intoxications.

Dysfunction of peroxisomes in twitcher mice brain: a possible mechanism of psychosine-induced disease.

Psychosine (galactosylsphingosine) accumulates in the brain of Krabbe disease (KD) patients as well as twitcher mice, a murine model of KD, resulting in loss of oligodendrocytes and myelin. This study documents progressive loss of peroxisomal proteins/functions and induction of expression of inflammatory cytokine TNF-alpha in twitcher brain. The observed decrease in peroxisomal proteins was accompanied by decreased level of peroxisome proliferator-activated receptor-alpha (PPAR-alpha), one of the transcription factors required for expression of peroxisomal protein genes. The role of psychosine in down-regulation of PPAR-alpha activity was further supported by decreased PPAR-alpha mediated PPRE transcriptional activity in cells transfected with PPAR-alpha and PPRE reporters. The psychosine-induced down-regulation of PPAR activity and cell death was attenuated by sPLA2 inhibitor. Therefore, this study provides the first evidence of peroxisomal abnormality in a lysosomal disorder, suggesting that such dysfunction of peroxisomes may play a role in the pathogenesis of Krabbe disease.

Intrinsic resistance of neural stem cells to toxic metabolites may make them well suited for cell non-autonomous disorders: evidence from a mouse model of Krabbe leukodystrophy.

While transplanted neural stem cells (NSCs) have been shown to hold promise for cell replacement in models of a number of neurological disorders, these examples have typically been under conditions where the host cells become dysfunctional due to a cell autonomous etiology, i.e. a 'sick' cell within a relatively supportive environment. It has long been held that cell replacement in a toxic milieu would not likely be possible; donor cells would succumb in much the same way as endogenous cells had. Many metabolic diseases are characterized by this situation, suggesting that they would be poor targets for cell replacement therapies. On the other hand, models of such diseases could prove ideal for testing the capacity for cell replacement under such challenging conditions. In the twitcher (twi ) mouse -- as in patients with Krabbe or globoid cell leukodystrophy (GLD), for which it serves as an authentic model -- loss of galactocerebrosidase (GalC) activity results in the accumulation of psychosine, a toxic glycolipid. Twi mice, like children with GLD, exhibit inexorable neurological deterioration presumably as a result of dysfunctional and ultimately degenerated oligodendrocytes with loss of myelin. It is believed that GLD pathophysiology is related to a psychosine-filled environment that kills not only host oligodendrocytes but theoretically any new cells placed into that milieu. Through the implantation of NSCs into the brains of both neonatal and juvenile/young adult twi mice, we have determined that widespread oligodendrocyte replacement and remyelination is feasible. NSCs appear to be intrinsically resistant to psychosine -- more so in their undifferentiated state than when directed ex vivo to become oligodendrocytes. This resistance can be enhanced by engineering the NSCs to over-express GalC. Some twi mice grafted with such engineered NSCs had thicker white tracts and lived 2-3 times longer than expected. While their brains had detectable levels of GalC, it was probably more significant that their psychosine levels were lower than in twi mice that died at a younger age. This concept of resistance based on differentiation state extended to human NSCs which could similarly survive within the twi brain. Taken together, these results suggest a number of points regarding cellular therapies against degenerative diseases with a prominent cell non-autonomous component: Cell replacement is possible if cells resistant to the toxic environment are employed. Furthermore, an important aspect of successful treatment will likely be not only cell replacement but also cross-correction of host cells to provide them with enzyme activity and hence resistance. While oligodendrocyte replacement alone was not a sufficient treatment for GLD (even when extensive), the replacement of both cells and molecules -- e.g. with NSCs that could both become oligodendrocytes and 'pumps' for GalC -- emerges as a promising basis for a multidisciplinary strategy. Most neurological disease are complex in this way and will likely require multifaceted approaches, perhaps with NSCs serving as the 'glue'.

Peroxisomal participation in psychosine-mediated toxicity: implications for Krabbe's disease.

Psychosine (galactosylsphingosine) accumulation in globoid cell leukodystrophy (Krabbe's disease) results in the loss of myelin and oligodendrocytes. To understand the role of psychosine toxicity in Krabbe's disease, we examined the effects of psychosine on peroxisomal functions and their relationship with reactive oxygen species. Rat C(6) glial cells were treated with psychosine with and without cytokines. Peroxisomal beta-oxidation was significantly inhibited and very long chain fatty acid levels and free radicals were increased in treated cells. Furthermore, psychosine treatment decreased glutathione and ATP levels, plasmalogen content, and expression of alkyl-DHAP synthase. Brain tissue of twitcher mice (animal model of Krabbe's) had decreased beta-oxidation activity, low glutathione, and reduced plasmalogens. Psychosine treatment of rat primary oligodendrocytes inhibited peroxisomal activities. Psychosine-mediated loss of peroxisomal function and free radical production was inhibited with the antioxidant N-acetylcysteine in glial cells. Our results suggest that inhibition of peroxisomal functions and increased free radical production by psychosine may be partly responsible for oligodendrocyte and myelin loss observed in the Krabbe's brain, and that antioxidant therapy may be useful in the treatment of Krabbe's disease.

Insulin-like growth factor-1 provides protection against psychosine-induced apoptosis in cultured mouse oligodendrocyte progenitor cells using primarily the PI3K/Akt pathway.

Psychosine (galactosylsphingosine) is a toxic metabolite that accumulates in globoid cell leukodystrophy (GLD) due to the deficiency of galactocerebrosidase (GALC) activity. This results in subsequent programmed cell death of oligodendrocytes and demyelination in human patients and animal models. We investigated the potential role of insulin-like growth factor-1 (IGF-1) in modifying the apoptotic effect of psychosine in cultured mouse oligodendrocyte progenitor cells (OLP-II). We show that psychosine inhibits the phosphorylation of Akt and Erk1/Erk2 (Erk1/2), which are the main anti-apoptotic pathways of the IGF-1 receptor (IGF-1R). Although IGF-1 sustained phosphorylation of both of these pathways, it provided maximum protection to OLP-II cells from psychosine-induced cell death in a PI3K/Akt-dependent manner. The effects of IGF-1 were dose-dependent and resulted in increased IGF-1R autophosphorylation levels. Although relatively high concentrations of IGF-1 also resulted in the activation of the insulin receptor (IR), its effect was more significant on the IGF-1R.

Tissue culture model of Krabbe's disease: psychosine cytotoxicity in rat oligodendrocyte culture.

Krabbe's disease (globoid cell leukodystrophy) is a progressive cerebral degenerative disease of infancy characterized by severe myelin loss and the presence of globoid bodies in the white matter. Previous studies have suggested that psychosine is the causative agent for the pathogenesis of Krabbe's disease. In the present study, we investigated psychosine-induced injury and cell death of oligodendrocytes in enriched cultures of oligodendrocytes prepared from 3-week-old rat brain. The psychosine concentration sufficient to induce 50% cell death in oligodendrocytes was 30 micrograms/ml in the medium containing serum and 10 micrograms/ml in the serum-free medium. When oligodendrocytes were exposed to psychosine in the presence of phorbol esters, insulin, insulin-like growth factor I (IGF-I), demethylsulfoxide, or serum albumin, the survival of oligodendrocytes was greatly increased. These results indicate that psychosine cytotoxicity against oligodendrocytes is blocked by phorbol esters, insulin, and IGF-I through activation of protein kinase-C, by dimethylsulfoxide through activation of beta-galactosidase, and by albumin through its binding to psychosine.

Psychosine cytotoxicity in rat neural cell cultures and protection by phorbol ester and dimethyl sulfoxide.

In Krabbe's disease (globoid cell leukodystrophy), galactosylsphingosine (psychosine) is considered to be a causative agent of the pathology found in the nervous system of the patients. In our study, we examined the cytotoxic effect of psychosine in neural cell cultures derived from the rat nervous system. The concentration of toxic thresholds varied from cell type to cell type. The 50% of toxic doses for oligodendrocytes, astrocytes, and the sensory neurons of the dorsal root ganglia were 8, 20, and 30 micrograms/mL, respectively. Oligodendrocytes, therefore, appeared to show a higher sensitivity to psychosine than did astrocytes or neurons. When phorbol ester or DMSO was applied simultaneously with psychosine as protective agents in enriched cultures of rat oligodendrocytes, the total number of live cells and galactocerebroside-positive cells and the 2'3'-cyclic nucleotide 3'-phosphohydrolase activity in these cultures were considerably higher as compared with their levels in the experimental cultures treated with psychosine alone. These results indicate that phorbol ester and DMSO could serve as protective agents for psychosine neurotoxicity.

Molecular mechanism of psychosine-induced cell death in human oligodendrocyte cell line.

This study delineates the molecular mechanism underlying psychosine-induced oligodendroglial cell death. An immortalized human oligodendroglial cell line, MO3.13, was treated with exogenous psychosine (beta-galactosylsphingosine), a toxic metabolite that accumulates in the tissues of patients with Krabbe's disease. The mode of cell death induced by psychosine was found to be apoptotic, as revealed by different apoptotic markers viz., TUNEL, DNA fragmentation and caspase cleavage/activation. The action of psychosine was redox sensitive, as measured by changes in mitochondrial membrane potential (psidelta), and this effect of psychosine could be reversed by pre-treatment with the antioxidant molecules N-acetyl-l-cysteine or pro-cysteine. Psychosine directly affects the mitochondria as revealed by the activation of caspase 9 but not caspase 8. Up-regulation of the c-jun/c-jun N-terminal kinase pathway by psychosine leads to the induction of AP-1 and, at the same time, psychosine also down-regulates the lipopolysaccharide-induced NF-kappaB transactivation. These observations indicate that the mechanism of action of psychosine is, through the up-regulation of AP-1, a pro-apoptotic pathway as well as, through the down-regulation of the NF-kappaB pathway, an antiapoptotic pathway.