Leanne K. Hein

Intra-articular enzyme administration for joint disease in feline mucopolysaccharidosis VI: enzyme dose and interval.

Degenerative joint changes have been reported in human mucopolysaccharidosis VI (MPS VI) and are a prominent feature of feline MPS VI. Joint disease has proven refractory to intravenous enzyme replacement therapy (ERT) in the MPS VI cat because enzyme is unable to reach cells in cartilage. In this study, enzyme was infused directly into the intraarticular space to determine whether joint tissues are able to respond to replacement enzyme. Clearance of glycosaminoglycans from chondrocytes was observed at a dose of 10 μg recombinant human N-acetylgalactosamine-4-sulfatase (rh4S), but greater clearance was observed with higher doses. The chondrocytes at the articular surface were cleared preferentially. Lysosomal vacuolation in cruciate ligament and synovial cells also decreased upon addition of rh4S. One month after injection of rh4S, a slight reaccumulation of storage was observed at the surface of the joint, but extensive reaccumulation was observed 2 mo after injection. These results indicate that by bypassing the synovium using intraarticular ERT, significant reduction in storage material in joint tissues can be achieved. Localized ERT in the joint space provides a mechanism for delivering enzyme directly to the articular cartilage and a potential therapy for joint pathology in MPS VI.

Transgenic mice expressing human glucocerebrosidase variants: utility for the study of Gaucher disease.

Gaucher disease is an autosomal recessively inherited storage disorder caused by deficiency of the lysosomal hydrolase, acid β-glucosidase. The disease manifestations seen in Gaucher patients are highly heterogeneous as is the responsiveness to therapy. The elucidation of the precise factors responsible for this heterogeneity has been challenging as the development of clinically relevant animal models of Gaucher disease has been problematic. Although numerous murine models for Gaucher disease have been described each has limitations in their specific utility. We describe here, transgenic murine models of Gaucher disease that will be particularly useful for the study of pharmacological chaperones. We have produced stable transgenic mouse strains that individually express wild type, N370S and L444P containing human acid β-glucosidase and show that each of these transgenic lines rescues the lethal phenotype characteristic of acid β-glucosidase null mice. Both the N370S and L444P transgenic models show early and progressive elevations of tissue sphingolipids with L444P mice developing progressive splenic Gaucher cell infiltration. We demonstrate the potential utility of these new transgenic models for the study of Gaucher disease pathogenesis. In addition, since these mice produce only human enzyme, they are particularly relevant for the study of pharmacological chaperones that are specifically targeted to human acid β-glucosidase and the common mutations underlying Gaucher disease.

Lipid composition of membrane rafts, isolated with and without detergent, from the spleen of a mouse model of Gaucher disease.

Biological membranes are composed of functionally relevant liquid-ordered and liquid-disordered domains that coexist. Within the liquid-ordered domains are low-density microdomains known as rafts with a unique lipid composition that is crucial for their structure and function. Lipid raft composition is altered in sphingolipid storage disorders, and here we determined the lipid composition using a detergent and detergent-free method in spleen tissue, the primary site of pathology, in a mouse model of the sphingolipid storage disorder, Gaucher disease. The accumulating lipid, glucosylceramide, was 30- and 50-fold elevated in the rafts with the detergent and detergent-free method, respectively. Secondary accumulation of di- and trihexosylceramide resided primarily in the rafts with both methods. The phospholipids distributed differently with more than half residing in the rafts with the detergent-free method and less than 10% with the detergent method, with the exception of the fully saturated species that were primarily in the rafts. Individual isoforms of sphingomyelin correlated with detergent-free extraction and more than half resided in the raft fractions. However, this correlation was not seen with the detergent extraction method as sphingomyelin species were spread across both the raft and non-raft domains. Therefore caution must be exercised when interpreting phospholipid distribution in raft domains as it differs considerably depending on the method of isolation. Importantly, both methods revealed the same lipid alterations in the raft domains in the spleen of the Gaucher disease mouse model highlighting that either method is appropriate to determine membrane lipid changes in the diseased state.

Selective reduction of bis(monoacylglycero)phosphate ameliorates the storage burden in a THP-1 macrophage model of Gaucher disease.

Bis(monoacylglycero)phosphate (BMP) assists lysosomal function by facilitating interaction of hydrolases and activator proteins with sphingolipid substrates. Impaired lysosomal degradation of the sphingolipid glucosylceramide (GC) occurs in Gaucher disease due to an inherited deficiency of acid β-glucosidase, with secondary BMP alterations. We investigated the nature of BMP accumulation and whether its correction reduced the storage burden in a THP-1 macrophage model of Gaucher disease. Using sucrose gradients and detergent solubility, 98% of BMP resided in the detergent-soluble membranes (DSM) rather than in the detergent-resistant membranes (DRM) where 73% of GC predominated. There was a 2-fold widespread elevation in BMP, including the saturated, mono- and polyunsaturated species. Linoleic acid in the culture media selectively reduced BMP from 4.2 nmol/mg to 0.49 nmol/mg (except 18:1/18:2) and prevented up to one third of GC, dihexosylceramide (DHC), and trihexosylceramide (THC) from accumulating. The 2-fold reduction in these sphingolipids occurred only in the DRM and did not reduce 18:1/16:0. However, once GC had accumulated, linoleic acid could not reverse it, DHC, or THC, despite effectively reducing BMP. These results imply a causative link for BMP in the pathobiology of Gaucher disease and demonstrate that linoleic acid can shield the cell from excessive substrate accumulation.

The α-L-iduronidase mutations R89Q and R89W result in an attenuated mucopolysaccharidosis type I clinical presentation

Mucopolysaccharidosis type I (MPS I; McKusick 25280; Hurler syndrome, Hurler-Scheie syndrome and Scheie syndrome) is caused by a deficiency in the lysosomal hydrolase, alpha-L-iduronidase (EC 3.2.1.76). MPS I patients present within a clinical spectrum bounded by the extremes of Hurler and Scheie syndromes. The alpha-L-iduronidase missense mutations R89Q and R89W were investigated and altered an important arginine residue proposed to be a nucleophile activator in the catalytic mechanism of alpha-L-iduronidase. The R89Q alpha-L-iduronidase mutation was shown to result in a reduced level of alpha-L-iduronidase protein (< or =10% of normal control) compared to a normal control level of alpha-L-iduronidase protein that was detected for the R89W alpha-L-iduronidase mutation. When taking into account alpha-L-iduronidase specific activity, the R89W mutation had a greater effect on alpha-L-iduronidase activity than the R89Q mutation. However, overall the R89W mutation produced more residual alpha-L-iduronidase activity than the R89Q mutation. This was consistent with MPS I patients, with an R89W allele, having a less severe clinical presentation compared to MPS I patients with either a double or single allelic R89Q mutation. The effects of the R89Q and R89W mutations on enzyme activity supported the proposed role of R89 as a nucleophile activator in the catalytic mechanism of alpha-L-iduronidase.

Liquid chromatography/electrospray ionisation–tandem mass spectrometry quantification of GM2 gangliosides in human peripheral cells and plasma

GM2 gangliosidosis is a group of inherited neurodegenerative disorders resulting primarily from the excessive accumulation of GM2 gangliosides (GM2) in neuronal cells. As biomarkers for categorising patients and monitoring the effectiveness of developing therapies are lacking for this group of disorders, we sought to develop methodology to quantify GM2 levels in more readily attainable patient samples such as plasma, leukocytes, and cultured skin fibroblasts. Following organic extraction, gangliosides were partitioned into the aqueous phase and isolated using C18 solid-phase extraction columns. Relative quantification of three species of GM2 was achieved using LC/ESI-MS/MS with d35GM1 18:1/18:0 as an internal standard. The assay was linear over the biological range, and all GM2 gangliosidosis patients were demarcated from controls by elevated GM2 in cultured skin fibroblast extracts. However, in leukocytes only some molecular species could be used for differentiation and in plasma only one was informative. A reduction in GM2 was easily detected in patient skin fibroblasts after a short treatment with media from normal cells enriched in secreted β-hexosaminidase. This method may show promise for measuring the effectiveness of experimental therapies for GM2 gangliosidosis by allowing quantification of a reduction in the primary storage burden.

Glycosphingolipid analysis in a naturally occurring ovine model of acute neuronopathic Gaucher disease.

Gaucher disease arises from mutations in the β-glucocerebrosidase gene which encodes an enzyme required for the lysosomal catabolism of glucosylceramide. We have identified a naturally occurring mutation in the β-glucocerebrosidase gene in sheep that leads to Gaucher disease with acute neurological symptoms. Here we have examined the clinical phenotype at birth and subsequently quantified lipids in Gaucher lamb brain, in order to characterise the disorder. Enzyme activity assessments showed that a reduction in β-glucocerebrosidase activity to 1-5% of wild-type occurs consistently across newborn Gaucher lamb brain regions. We analyzed glucosylceramide, glucosylsphingosine, bis(monoacylglycero)phosphate and ganglioside profiles in brain, liver, and spleen, and observed 30- to 130-fold higher glucosylceramide, and 500- to 2000-fold higher glucosylsphingosine concentrations in Gaucher diseased lambs compared to wild-type. Significant increases of bis(monoacylglycero)phosphate and gangliosides [GM1, GM2, GM3] concentrations were also detected in the brain. As these glycosphingolipids are involved in many cellular events, an imbalance or disruption of the cell membrane lipid homeostasis would be expected to impair normal neuronal function. To our knowledge, this is the first detailed analysis of glycosphingolipids in various brain regions in a large animal model of neuronal disease, which permits the mechanistic investigation of lipid deregulation and their contribution to neurodegenerative process.

Lipid composition of microdomains is altered in neuronopathic Gaucher disease sheep brain and spleen

Gaucher disease is a lysosomal storage disorder caused by a deficiency in glucocerebrosidase activity that leads to accumulation of glucosylceramide and glucosylsphingosine. Membrane raft microdomains are discrete, highly organized microdomains with a unique lipid composition that provide the necessary environment for specific protein-lipid and protein-protein interactions to take place. In this study we purified detergent resistant membranes (DRM; membrane rafts) from the occipital cortex and spleen from sheep affected with acute neuronopathic Gaucher disease and wild-type controls. We observed significant increases in the concentrations of glucosylceramide, hexosylsphingosine, BMP and gangliosides and decreases in the percentage of cholesterol and phosphatidylcholine leading to an altered DRM composition. Altered sphingolipid/cholesterol homeostasis would dramatically disrupt DRM architecture making them less ordered and more fluid. In addition, significant changes in the length and degree of lipid saturation within the DRM microdomains in the Gaucher brain were also observed. As these DRM microdomains are involved in many cellular events, an imbalance or disruption of the cell membrane homeostasis may impair normal cell function. This disruption of membrane raft microdomains and imbalance within the environment of cellular membranes of neuronal cells may be a key factor in initiating a cascade process leading to neurodegeneration.

Absence of α-galactosidase cross-correction in Fabry heterozygote cultured skin fibroblasts

Fabry disease (FD) is an X-linked lysosomal storage disorder resulting from deficiency of α-galactosidase A (GLA). Traditionally, heterozygotes were considered asymptomatic carriers of FD, but it is now apparent that the asymptomatic female carrier is the exception and most heterozygotes suffer significant multisystemic disease. To determine why the process of cross-correction does not occur effectively in FD heterozygotes, we investigated GLA production and secretion in cultured skin fibroblasts as well as GLA levels in plasma. The maturation of GLA was similar in FD heterozygotes and control fibroblasts, confirming that both produce the 46kDa mature form; the same as that present in control plasma. However, the proportion of GLA secreted into the culture media was substantially less than eight other lysosomal proteins. Artificial generation of FD heterozygotes in cellulo, along with another lysosomal storage disorder, mucopolysaccharidosis type II, revealed no cross-correction in the FD system, whereas MPS II fibroblasts were able to cross-correct. In plasma, GLA was present as the 46kDa mature form, which lacks the mannose 6-phosphorylated moiety and is not able to be efficiently endocytosed by affected cells. Our evidence shows that fibroblasts secrete minimal amounts of GLA and consequently normal fibroblasts are unable to cross-correct FD fibroblasts. We suggest that symptomatic FD heterozygotes arise due to the secretion of primarily the mature form, with only small amounts of the mannose 6-phosphorylated form of GLA from unaffected cells. This limits capacity for enzyme cross correction of affected cells, despite uptake of exogenous recombinant GLA.

A novel fluorescent probe reveals starvation controls the commitment of amyloid precursor protein to the lysosome

Alzheimers disease is the most important cause of dementia but there is no therapy that has been demonstrated to stop or slow disease progression. Amyloid precursor protein (APP) is the source of amyloid-β (Aβ), which aggregates in Alzheimers disease to form toxic oligomeric species. The endo-lysosomal system can clear APP and Aβ from the cell if these molecular species are trafficked through to the lysosome. Currently, there are no easy methods available for the analysis of lysosomal APP trafficking. We therefore generated a fusion protein (tandem-fluorescent, or tf-APP) that allows detection of changes in APP trafficking using accessible techniques such as flow cytometry. This permits rapid analysis or screening of genes and compounds that alter APP processing in the cell. Using our novel molecular probe, we determined that starvation induces trafficking of APP and APP-carboxy-terminal fragments (APP-CTFs) to the degradative endo-lysosomal network. In line with this finding, suppression of mTOR signalling using AZD8055 also strongly induced trafficking of APP to the endo-lysosomal system. Remarkably, activation of mTOR signalling via RHEB over-expression inhibited the starvation-induced autophagy but did not affect trafficking of tf-APP. These results show tf-APP can be used to determine how APP is trafficked through the lysosomal system of the cell. This molecular probe is therefore useful for determining the molecular mechanism behind the commitment of APP to the degradative pathway or for screening compounds that can induce this effect. This is important as clearance of APP and APP-CTF provides an important potential therapeutic strategy for Alzheimers disease.