Studies on the pathogenesis of Krabbe's leukodystrophy: cellular reaction of the brain to exogenous galactosylsphingosine, monogalactosyl diglyceride, and lactosylceramide

Neuron-specific ablation of the Krabbe disease gene galactosylceramidase in mice results in neurodegeneration

Krabbe disease is caused by a deficiency of the lysosomal galactosylceramidase (GALC) enzyme, which results in the accumulation of galactosylceramide (GalCer) and psychosine. In Krabbe disease, the brunt of demyelination and neurodegeneration is believed to result from the dysfunction of myelinating glia. Recent studies have shown that neuronal axons are both structurally and functionally compromised in Krabbe disease, even before demyelination, suggesting a possible neuron-autonomous role of GALC. Using a novel neuron-specific Galc knockout (CKO) model, we show that neuronal Galc deletion is sufficient to cause growth and motor coordination defects and inflammatory gliosis in mice. Furthermore, psychosine accumulates significantly in the nervous system of neuron-specific Galc-CKO. Confocal and electron microscopic analyses show profound neuro-axonal degeneration with a mild effect on myelin structure. Thus, we prove for the first time that neuronal GALC is essential to maintain and protect neuronal function independently of myelin and may directly contribute to the pathogenesis of Krabbe disease.

Waning efficacy in a long-term AAV-mediated gene therapy study in the murine model of Krabbe disease

Neonatal AAV9-gene therapy of the lysosomal enzyme galactosylceramidase (GALC) significantly ameliorates central and peripheral neuropathology, prolongs survival, and largely normalizes motor deficits in Twitcher mice. Despite these therapeutic milestones, new observations identified the presence of multiple small focal demyelinating areas in the brain after 6-8 months. These lesions are in stark contrast to the diffuse, global demyelination that affects the brain of naive Twitcher mice. Late-onset lesions exhibited lysosomal alterations with reduced expression of GALC and increased psychosine levels. Furthermore, we found that lesions were closely associated with the extravasation of plasma fibrinogen and activation of the fibrinogen-BMP-SMAD-GFAP gliotic response. Extravasation of fibrinogen correlated with tight junction disruptions of the vasculature within the lesioned areas. The lesions were surrounded by normal appearing white matter. Our study shows that the dysregulation of therapeutic GALC was likely driven by the exhaustion of therapeutic AAV episomal DNA within the lesions, paralleling the presence of proliferating oligodendrocyte progenitors and glia. We believe that this is the first demonstration of diminishing expression in vivo from an AAV gene therapy vector with detrimental effects in the brain of a lysosomal storage disease animal model. The development of this phenotype linking localized loss of GALC activity with relapsing neuropathology in the adult brain of neonatally AAV-gene therapy-treated Twitcher mice identifies and alerts to possible late-onset reductions of AAV efficacy, with implications to other genetic leukodystrophies.

Canine Models of Inherited Musculoskeletal and Neurodegenerative Diseases

Mouse models of human disease remain the bread and butter of modern biology and therapeutic discovery. Nonetheless, more often than not mouse models do not reproduce the pathophysiology of the human conditions they are designed to mimic. Naturally occurring large animal models have predominantly been found in companion animals or livestock because of their emotional or economic value to modern society and, unlike mice, often recapitulate the human disease state. In particular, numerous models have been discovered in dogs and have a fundamental role in bridging proof of concept studies in mice to human clinical trials. The present article is a review that highlights current canine models of human diseases, including Alzheimer's disease, degenerative myelopathy, neuronal ceroid lipofuscinosis, globoid cell leukodystrophy, Duchenne muscular dystrophy, mucopolysaccharidosis, and fucosidosis. The goal of the review is to discuss canine and human neurodegenerative pathophysiologic similarities, introduce the animal models, and shed light on the ability of canine models to facilitate current and future treatment trials.

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.

Psychosine-induced alterations in peroxisomes of twitcher mouse liver

Krabbe disease is a neuroinflammatory disorder in which galactosylsphingosine (psychosine) accumulates in nervous tissue. To gain insight into whether the psychosine-induced effects in nervous tissue extend to peripheral organs, we investigated the expression of cytokines and their effects on peroxisomal structure/functions in twitcher mouse liver (animal model of Krabbe disease). Immunofluorescence analysis demonstrated TNF-alpha and IL-6 expression, which was confirmed by mRNAs quantitation. Despite the presence of TNF-alpha, lipidomic analysis did not indicate a significant decrease in sphingomyelin or an increase in ceramide fractions. Ultrastructural analysis of catalase-dependent staining of liver sections showed reduced reactivity without significant changes in peroxisomal contents. This observation was confirmed by assaying catalase activity and quantitation of its mRNA, both of which were found significantly decreased in twitcher mouse liver. Western blot analysis demonstrated a generalized reduction of peroxisomal matrix and membrane proteins. These observations indicate that twitcher mouse pathobiology extends to the liver, where psychosine-induced TNF-alpha and IL-6 compromise peroxisomal structure and functions.

Globoid cell leukodystrophy (Krabbe's disease): update

The classic globoid cell leukodystrophy (Krabbe's disease) is caused by genetic defects in a lysosomal enzyme, galactosylceramidase. It is one of the two classic genetic leukodystrophies, together with metachromatic leukodystrophy. The mode of inheritance is autosomal recessive. Typically, the disease occurs among infants and takes a rapidly fatal course, but rarer late-onset forms also exist. Clinical manifestations are exclusively neurologic with prominent white-matter signs. The pathology is unique, consisting of a rapid and nearly complete disappearance of myelin and myelin-forming cells--the oligodendrocytes in the central nervous system and the Schwann cells in the peripheral nervous system, reactive astroytic gliosis, and infiltration of the unique and often multinucleated macrophages ("globoid cells") that contain strongly periodic acid-Schiff (PAS)-positive materials. A normally insignificant but highly cytotoxic metabolite, galactosylsphingosine (psychosine), is also a substrate of galactosylceramidase and is considered to play a critical role in the pathogenesis. The galactosylceramidase gene has been cloned, and a large number of disease-causing mutations have been identified. Equivalent genetic galactosylceramidase deficiency occurs in several mammalian species, such as mouse, dog, and monkey. Recently, deficiency of one of the sphingolipid activator proteins, saposin A, was demonstrated to cause a late-onset, slowly progressive globoid cell leukodystrophy at least in the mouse, with all of the phenotypic consequences of impaired degradation of galactosylceramidase substrates. Human globoid cell leukodystrophy owing to saposin A deficiency might be anticipated and should be suspected in human patients with a late-onset leukodystrophy with normal galactosylceramidase activity when other possibilities are also excluded. The only serious attempt at treating human patients is bone marrow transplantation, which can provide significant alleviation of symptoms, particularly in those patients with later-onset, more slowly progressive globoid cell leukodystrophy.

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.

Twenty five years of the "psychosine hypothesis": a personal perspective of its history and present status

Twenty five years ago in 1972, a hypothesis was introduced to explain the pathogenetic mechanism underlying the unusual cellular and biochemical characteristics of globoid cell leukodystrophy (Krabbe disease). It postulated that galactosylsphingosine (psychosine), which cannot be degraded due to the underlying genetic defect, is responsible for the very rapid loss of the oligodendrocytes and the consequent paradoxical analytical finding, the lack of accumulation of the primary substrate, galactosylceramide, in patients' brain. It took nearly ten years before the actual accumulation of psychosine was demonstrated in human Krabbe patients and also in the brain of twitcher mice, an equivalent murine mutant. Meanwhile this "psychosine hypothesis" has been extended to Gaucher disease and then to a more general hypothesis encompassing all sphingolipidoses that the "lyso-derivatives" of the primary sphingolipid substrates of the defective enzymes in respective disorders play a key role in their pathogenesis. Some of these extensions not only remain speculative without conclusive factual evidence but may eventually turn out to be an overstretching. This article attempts, from my personal perspective, at tracing historical development of the "psychosine hypothesis" and examining its current status and possible future directions.

Effects of long-chain fatty amines on the growth of ras-transformed NIH 3T3 cells

A number of aliphatic primary amines were tested for their effects on the growth of ras-transformed NIH 3T3 cells (PAP2 cells), as measured by incorporation of tritiated thymidine into DNA. Long-chain, saturated amines (C12 to C18) were growth inhibitory, whereas short-chain amines (C6, C8) were not. Farnesylamine, a branched-chain, unsaturated amine (C15), had an IC50 of 6.9 microM compared to IC50 values of 13.1 to 45.8 microM for straight-chain, saturated amines. Oleylamine, with an IC50 of 0.1 microM, was the most potent inhibitor. The long-chain amines, but not the short-chain amines, were also effective inhibitors of protein kinase C, assayed in vitro in a cell-free system. In addition, studies with indo-1-loaded PAP2 cells showed that long-chain amines induced a reversible rise in intracellular free Ca2+ concentration. Growth inhibition by the amines was positively correlated with this effect, suggesting that factors other than protein kinase C may be involved in the inhibition of growth of PAP2 cells by long-chain amines.

Biochemical pathogenesis of demyelination in globoid cell leukodystrophy (Krabbe's disease): the effects of psychosine upon oligodendroglial cell culture

The effects of psychosine on the metabolism of myelin associated glycolipids such as galactocerebroside and sulfatide in mouse brain cell cultures were investigated in order to clarify the mechanism of demyelination in globoid cell leukodystrophy (Krabbe's disease). The incorporation of 3H-galactose into cerebroside and sulfatide was studied in the presence of psychosine (1-3 micrograms/ml medium). These data indicated that psychosine inhibited the incorporation of 3H-galactose into cerebroside and sulfatide not in astroglial cell culture but in oligodendroglial cell culture. Oligodendrocytes produce myelin in the central nervous system, and cerebroside and sulfatide are major components of myelin. These results suggest that psychosine influences the lipid metabolisms of myelin and subsequently leads to the demyelination in Krabbe's disease.

Biosynthesis of galactosylsphingosine (psychosine) in the twitcher mouse

In attempts to elucidate the origin of accumulated galactosylsphingosine in the twitcher mouse, a murine model of human globoid cell leukodystrophy (Krabbe's disease), UDP-galactose:sphingosine galactosyltransferase activity was assayed in tissues from normal and twitcher mice. Among several tissues from normal, 20 day postnatal mice, the highest galactosyltransferase activity was found in the brainstem and spinal cord, followed by cerebrum, kidney and liver, in that order. Chronologically, the enzyme activity in the central nervous tissue increased with age, reached a maximum at 25 postnatal days, and declined thereafter. In the kidney and liver, however, the activity remained much the same during development. In the twitcher mouse, developmental change in the enzyme activity was similar to that seen in control mouse, but the decrease in activity in the central nervous tissue after the 25 postnatal days was more rapid. The galactosyltransferase activity and the accumulation of galactosylsphingosine in the tissue of the twitcher mouse were closely related; where and when the enzyme activity was higher, the greater was the accumulation of galactosylsphingosine in the tissue of the twitcher mouse. These results strongly suggest that the accumulated galactosylsphingosine in the twitcher mouse is synthesized mainly by UDP-galactose:sphingosine galactosyltransferase.

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.

Globoid cell leukodystrophy is a generalized galactosylsphingosine (psychosine) storage disease

Galactosylsphingosine (psychosine) in somatic organs from a patient with globoid cell leukodystrophy and from the twitcher mouse, an animal model of human globoid cell leukodystrophy was assayed. There was an abnormal accumulation of galactosylsphingosine as in nervous tissues, albeit the concentrations being lower than those in nervous tissues. Galactosylsphingosine accumulation in the kidney of the twitcher mouse increased with age. These findings indicate that globoid cell leukodystrophy is a generalized galactosylsphingosine storage disease.

Thermal and structural behavior of natural cerebroside 3-sulfate in bilayer membranes

Differential scanning calorimetry (DSC), polarizing microscopy and X-ray diffraction studies have been performed on dry and hydrated natural bovine brain sulfatides. Dry sulfatide fractions exhibit a high temperature transition (delta H = 6.6 kcal/mol sulfatide) at 87.3 degrees C. X-ray diffraction shows this transition to be associated with a hydrocarbon chain order-disorder transformation between two lamellar phases. Hydrated sulfatide dispersions undergo a complex chain order-disorder transition (delta H = 7.5 kcal/mol sulfatide) at 32 degrees C with two peak temperatures at 35 degrees C and 47 degrees C. Structural studies performed on hydrated liquid-crystal sulfatide dispersions at 75 degrees C verify the existence of a bilayer structure over the 16 wt.% to 50 wt.% phosphate buffer (pH = 7.4) range. The interbilayer separation between galactosyl-3-sulfate groups averages 48 A as the multilamellar bilayers swell with the addition of phosphate buffer. The formation of micellar phases is not observed at high water contents. The comparison of the structural characteristics of dry and hydrated sulfatides with structural data for dry and hydrated bovine brain non-sulfated glycolipid (cerebroside) is discussed in molecular terms.

The twitcher mouse: an ultrastructural study on the oligodendroglia

Morphological alterations of oligodendroglia were investigated in the spinal cord of the twitcher mouse, an authentic murine model of human globoid cell leukodystrophy (GLD) from day 5 to day 45 postnatal (p.n.). Typical inclusions were seen in the perikarya as well as the processes of oligodendroglia after day 10 with increasing frequency. The majority of the inclusions was non-crystalloid but rather needle-like or slender tubular in appearance. Ultrastructural features of cellular degeneration became first noticeable on days 25-30 in the oligodendroglial cytoplasm. These consisted of an increased number of microtubules and/or smooth cisterns, dispersed ribosomes, alteration of endoplasmic reticulum forming stacked lamellae or whorls, vesiculation or vacuolation of cytoplasm. The number of degenerating oligodendroglia increased in the older twitcher mice, so did the degenerating myelin sheath. However, even on day 45, when globoid cells became conspicuous in subpial and perivascular regions, many oligodendroglia and myelin sheaths were still well preserved. These observations suggested that oligodendroglial degeneration resulted in the degeneration of myelin sheaths but globoid cells appeared even before morphological evidence of myelin degeneration, presumably in response to the biochemical alterations resulted from the deficiency of galactosylceramidase.