Widespread distribution of beta-hexosaminidase activity in the brain of a Sandhoff mouse model after coinjection of adenoviral vector and mannitol

Targeting Neurological Aspects of Mucopolysaccharidosis Type II: Enzyme Replacement Therapy and Beyond

Mucopolysaccharidosis type II (MPS II) is a rare, pediatric, neurometabolic disorder due to the lack of activity of the lysosomal hydrolase iduronate 2-sulfatase (IDS), normally degrading heparan sulfate and dermatan sulfate within cell lysosomes. The deficit of activity is caused by mutations affecting the IDS gene, leading to the pathological accumulation of both glycosaminoglycans in the lysosomal compartment and in the extracellular matrix of most body districts. Although a continuum of clinical phenotypes is described, two main forms are commonly recognized-attenuated and severe-the latter being characterized by an earlier and faster clinical progression and by a progressive impairment of central nervous system (CNS) functions. However, attenuated forms have also been recently described as presenting some neurological involvement, although less deep, such as deficits of attention and hearing loss. The main treatment for the disease is represented by enzyme replacement therapy (ERT), applied in several countries since 2006, which, albeit showing partial efficacy on some peripheral organs, exhibited a very poor efficacy on bones and heart, and a total inefficacy on CNS impairment, due to the inability of the recombinant enzyme to cross the blood-brain barrier (BBB). Together with ERT, whose design enhancements, performed in the last few years, allowed a possible brain penetration of the drug through the BBB, other therapeutic approaches aimed at targeting CNS involvement in MPS II were proposed and evaluated in the last decades, such as intrathecal ERT, intracerebroventricular ERT, ex vivo gene therapy, or adeno-associated viral vector (AAV) gene therapy. The aim of this review is to summarize the main clinical aspects of MPS II in addition to current therapeutic options, with particular emphasis on the neurological ones and on the main CNS-targeted therapeutic approaches explored through the years.

Targeting the central nervous system in lysosomal storage diseases: Strategies to deliver therapeutics across the blood-brain barrier

Lysosomal storage diseases (LSDs) are multisystem inherited metabolic disorders caused by dysfunctional lysosomal activity, resulting in the accumulation of undegraded macromolecules in a variety of organs/tissues, including the central nervous system (CNS). Treatments include enzyme replacement therapy, stem/progenitor cell transplantation, and in vivo gene therapy. However, these treatments are not fully effective in treating the CNS as neither enzymes, stem cells, nor viral vectors efficiently cross the blood-brain barrier. Here, we review the latest advancements in improving delivery of different therapeutic agents to the CNS and comment upon outstanding questions in the field of neurological LSDs.

Advances in Optical Sensors of N-Acetyl-β-d-hexosaminidase (N-Acetyl-β-d-glucosaminidase)

N-Acetyl-β-d-hexosaminidases (EC 3.2.1.52) are exo-acting glycosyl hydrolases that remove N-acetyl-β-d-glucosamine (Glc-NAc) or N-acetyl-β-d-galactosamine (Gal-NAc) from the nonreducing ends of various biomolecules including oligosaccharides, glycoproteins, and glycolipids. The same enzymes are sometimes called N-acetyl-β-d-glucosaminidases, and this review article employs the shorthand descriptor HEX(NAG) to indicate that the terms HEX or NAG are used interchangeably in the literature. The wide distribution of HEX(NAG) throughout the biosphere and its intracellular location in lysosomes combine to make it an important enzyme in food science, agriculture, cell biology, medical diagnostics, and chemotherapy. For more than 50 years, researchers have employed chromogenic derivatives of N-acetyl-β-d-glucosaminide in basic assays for biomedical research and clinical chemistry. Recent conceptual and synthetic innovations in molecular fluorescence sensors, along with concurrent technical improvements in instrumentation, have produced a growing number of new fluorescent imaging and diagnostics methods. A systematic summary of the recent advances in optical sensors for HEX(NAG) is provided under the following headings: assessing kidney health, detection and treatment of infectious disease, fluorescence imaging of cancer, treatment of lysosomal disorders, and reactive probes for chemical biology. The article concludes with some comments on likely future directions.

Treatment of GM2 Gangliosidosis in Adult Sandhoff Mice using an Intravenous Self-Complementary Hexosaminidase Vector

BACKGROUND

GM2 gangliosidosis is a neurodegenerative, lysosomal storage disease caused by the deficiency of β-hexosaminidase A enzyme (HexA), an α/β-subunit heterodimer. A novel variant of the human hexosaminidase α-subunit, coded by HEXM, has previously been shown to form a stable homodimer, HexM, that hydrolyzes GM2 gangliosides (GM2) in vivo.

MATERIALS & METHODS

The current study assessed the efficacy of intravenous (IV) delivery of a self-complementary adeno-associated virus serotype 9 (scAAV9) vector incorporating the HEXM transgene, scAAV9/HEXM, including the outcomes based on the dosages provided to the Sandhoff (SD) mice. Six-week-old SD mice were injected with either 2.5E+12 vector genomes (low dose, LD) or 1.0E+13 vg (high dose, HD). We hypothesized that when examining the dosage comparison for scAAV9/HEXM in adult SD mice, the HD group would have more beneficial outcomes than the LD cohort. Assessments included survival, behavioral outcomes, vector biodistribution, and enzyme activity within the central nervous system.

RESULTS

Toxicity was observed in the HD cohort, with 8 of 14 mice dying within one month of the injection. As compared to untreated SD mice, which have typical survival of 16 weeks, the LD cohort and the remaining HD mice had a significant survival benefit with an average/median survival of 40.6/34.5 and 55.9/56.7 weeks, respectively. Significant behavioral, biochemical and molecular benefits were also observed. The second aim of the study was to investigate the effects of IV mannitol infusions on the biodistribution of the LD scAAV9/HEXM vector and the survival of the SD mice. Increases in both the biodistribution of the vector as well as the survival benefit (average/median of 41.6/49.3 weeks) were observed.

CONCLUSION

These results demonstrate the potential benefit and critical limitations of the treatment of GM2 gangliosidosis using IV delivered AAV vectors.

White Matter Pathology as a Barrier to Gangliosidosis Gene Therapy

The gangliosidoses are a family of neurodegenerative lysosomal storage diseases that have recently seen promising advances in gene therapy. White matter deficits are well established components of gangliosidosis pathology that are now receiving more attention because they are partially refractory to correction by gene therapy. After a brief synopsis of normal myelinogenesis, this review outlines current viewpoints on the origin of white matter deficits in the gangliosidoses and potential obstacles to treating them effectively by gene therapy. Dysmyelinogenesis (failure of myelin sheaths to form properly) is proposed as the predominant contributor to white matter pathology, but precise mechanistic details are not well understood. The involvement of neuronal storage deficits may extend beyond secondary demyelination (destruction of myelin due to axonal loss) and contribute to dysmyelinogenesis. Preclinical studies in animal models of the gangliosidoses have substantially improved lifespan and quality of life, leading to the initiation of several clinical trials. However, improvement of white matter pathology has lagged behind other metrics and few evidence-based explanations have been proposed to date. Research groups in the field are encouraged to include myelin-specific investigations in future gene therapy work to address this gap in knowledge.

Viral-Mediated Gene Replacement Therapy in the Developing Central Nervous System: Current Status and Future Directions

The past few years have witnessed rapid developments in viral-mediated gene replacement therapy for pediatric central nervous system neurogenetic disorders. Here, we provide pediatric neurologists with an up-to-date, comprehensive overview of these developments and note emerging trends for future research. This review presents the different types of viral vectors used in viral-mediated gene replacement therapy; the fundamental properties of viral-mediated gene replacement therapy; the challenges associated with the use of this therapy in the central nervous system; the pathway for therapy development, from translational basic science studies to clinical trials; and an overview of the therapies that have reached clinical trials in patients. Current viral platforms under investigation include adenovirus vectors, adeno-associated viral vectors, lentiviral/retroviral vectors, and herpes simplex virus type 1 vectors. This review also presents an in-depth analysis of numerous studies that investigated these viral platforms in cultured cells and in transgenic animal models for pediatric neurogenetic disorders. Viral vectors have been applied to clinical trials for many different pediatric neurogenetic disorders, including Canavan disease, metachromatic leukodystrophy, neuronal ceroid lipofuscinosis, mucopolysaccharidosis III, spinal muscular atrophy, and aromatic l-amino acid decarboxylase deficiency. Of these diseases, only spinal muscular atrophy has a viral-mediated gene replacement therapy approved for marketing. Despite significant progress in therapy development, many challenges remain. Surmounting these challenges is critical to advancing the current status of viral-mediated gene replacement therapy for pediatric central nervous system neurogenetic disorders.

Biocompatible Polymer Nanoparticles for Drug Delivery Applications in Cancer and Neurodegenerative Disorder Therapies

Polymer nanoparticles (NPs) represent one of the most innovative non-invasive approaches for drug delivery applications. NPs main objective is to convey the therapeutic molecule be they drugs, proteins, or nucleic acids directly into the target organ or tissue. Many polymers are used for the synthesis of NPs and among the currently most employed materials several biocompatible synthetic polymers, namely polylactic acid (PLA), poly lactic-co-glycolic acid (PLGA), and polyethylene glycol (PEG), can be cited. These molecules are made of simple monomers which are naturally present in the body and therefore easily excreted without being toxic. The present review addresses the different approaches that are most commonly adopted to synthetize biocompatible NPs to date, as well as the experimental strategies designed to load them with therapeutic agents. In fact, drugs may be internalized in the NPs or physically dispersed therein. In this paper the various types of biodegradable polymer NPs will be discussed with emphasis on their applications in drug delivery. Close attention will be devoted to the treatment of cancer, where both active and passive targeting is used to enhance efficacy and reduce systemic toxicity, and to diseases affecting the central nervous system, inasmuch as NPs can be modified to target specific cells or cross membrane barriers.

Efficacy of a Bicistronic Vector for Correction of Sandhoff Disease in a Mouse Model

G_M2 gangliosidoses are a family of severe neurodegenerative disorders resulting from a deficiency in the β-hexosaminidase A enzyme. These disorders include Tay-Sachs disease and Sandhoff disease, caused by mutations in the HEXA gene and HEXB gene, respectively. The HEXA and HEXB genes are required to produce the α and β subunits of the β-hexosaminidase A enzyme, respectively. Using a Sandhoff disease mouse model, we tested for the first time the potential of a comparatively lower dose (2.04 × 10¹³ vg/kg) of systemically delivered single-stranded adeno-associated virus 9 expressing both human HEXB and human HEXA cDNA under the control of a single promoter with a P2A-linked bicistronic vector design to correct the neurological phenotype. A bicistronic design allows maximal overexpression and secretion of the Hex A enzyme. Neonatal mice were injected with either this ssAAV9-HexB-P2A-HexA vector or a vehicle solution via the superficial temporal vein. An increase in survival of 56% compared with vehicle-injected controls and biochemical analysis of the brain tissue and serum revealed an increase in enzyme activity and a decrease in brain G_M2 ganglioside buildup. This is a proof-of-concept study showing the “correction efficacy” of a bicistronic AAV9 vector delivered intravenously for G_M2 gangliosidoses. Further studies with higher doses are warranted.

Murine Sialidase Neu3 facilitates GM2 degradation and bypass in mouse model of Tay-Sachs disease

Tay-Sachs disease is a severe lysosomal storage disorder caused by mutations in Hexa, the gene that encodes for the α subunit of lysosomal β-hexosaminidase A (HEXA), which converts GM2 to GM3 ganglioside. Unexpectedly, Hexa^-/- mice have a normal lifespan and show no obvious neurological impairment until at least one year of age. These mice catabolize stored GM2 ganglioside using sialidase(s) to remove sialic acid and form the glycolipid GA2, which is further processed by β-hexosaminidase B. Therefore, the presence of the sialidase (s) allows the consequences of the Hexa defect to be bypassed. To determine if the sialidase NEU3 contributes to GM2 ganglioside degradation, we generated a mouse model with combined deficiencies of HEXA and NEU3. The Hexa^-/-Neu3^-/- mice were healthy at birth, but died at 1.5 to 4.5months of age. Thin-layer chromatography and mass spectrometric analysis of the brains of Hexa^-/-Neu3^-/- mice revealed the abnormal accumulation of GM2 ganglioside. Histological and immunohistochemical analysis demonstrated cytoplasmic vacuolation in the neurons. Electron microscopic examination of the brain, kidneys and testes revealed pleomorphic inclusions of many small vesicles and complex lamellar structures. The Hexa^-/-Neu3^-/- mice exhibited progressive neurodegeneration with neuronal loss, Purkinje cell depletion, and astrogliosis. Slow movement, ataxia, and tremors were the prominent neurological abnormalities observed in these mice. Furthermore, radiographs revealed abnormalities in the skeletal bones of the Hexa^-/-Neu3^-/- mice. Thus, the Hexa^-/-Neu3^-/- mice mimic the neuropathological and clinical abnormalities of the classical early-onset Tay-Sachs patients, and provide a suitable model for the future pre-clinical testing of potential treatments for this condition.

Systemic Gene Transfer of a Hexosaminidase Variant Using an scAAV9.47 Vector Corrects GM2 Gangliosidosis in Sandhoff Mice

GM2 gangliosidosis is a group of neurodegenerative diseases caused by β-hexosaminidase A (HexA) enzyme deficiency. There is currently no cure. HexA is composed of two similar, nonidentical subunits, α and β, which must interact with the GM2 activator protein (GM2AP), a substrate-specific cofactor, to hydrolyze GM2 ganglioside. Mutations in either subunit or the activator can result in the accumulation of GM2 ganglioside within neurons throughout the central nervous system. The resulting neuronal cell death induces the primary symptoms of the disease: motor impairment, seizures, and sensory impairments. This study assesses the long-term effects of gene transfer in a Sandhoff (β-subunit knockout) mouse model. The study utilized a modified human β-hexosaminidase α-subunit (μ-subunit) that contains critical sequences from the β-subunit that enables formation of a stable homodimer (HexM) and interaction with GM2AP to hydrolyze GM2 ganglioside. We investigated a self-complementary adeno-associated viral (scAAV) vector expressing HexM, through intravenous injections of the neonatal mice. We monitored one cohort for 8 weeks and another cohort long-term for survival benefit, behavioral, biochemical, and molecular analyses. Untreated Sandhoff disease (SD) control mice reached a humane endpoint at approximately 15 weeks, whereas treated mice had a median survival age of 40 weeks, an approximate 2.5-fold survival advantage. On behavioral tests, the treated mice outperformed their knockout age-matched controls and perform similarly to the heterozygous controls. Through the enzymatic and GM2 ganglioside analyses, we observed a significant decrease in the GM2 ganglioside level, even though the enzyme levels were not significantly increased. Molecular analyses revealed a global distribution of the vector between brain and spinal cord regions. In conclusion, the neonatal delivery of a novel viral vector expressing the human HexM enzyme is effective in ameliorating the SD mouse phenotype for long-term. Our data could have implications not only for treatment of SD but also for Tay-Sachs disease (α-subunit deficiency) and similar brain disorders.

Novel Vector Design and Hexosaminidase Variant Enabling Self-Complementary Adeno-Associated Virus for the Treatment of Tay-Sachs Disease

GM2 gangliosidosis is a family of three genetic neurodegenerative disorders caused by the accumulation of GM2 ganglioside (GM2) in neuronal tissue. Two of these are due to the deficiency of the heterodimeric (α-β), "A" isoenzyme of lysosomal β-hexosaminidase (HexA). Mutations in the α-subunit (encoded by HEXA) lead to Tay-Sachs disease (TSD), whereas mutations in the β-subunit (encoded by HEXB) lead to Sandhoff disease (SD). The third form results from a deficiency of the GM2 activator protein (GM2AP), a substrate-specific cofactor for HexA. In their infantile, acute forms, these diseases rapidly progress with mental and psychomotor deterioration resulting in death by approximately 4 years of age. After gene transfer that overexpresses one of the deficient subunits, the amount of HexA heterodimer formed would empirically be limited by the availability of the other endogenous Hex subunit. The present study used a new variant of the human HexA α-subunit, μ, incorporating critical sequences from the β-subunit that produce a stable homodimer (HexM) and promote functional interactions with the GM2AP- GM2 complex. We report the design of a compact adeno-associated viral (AAV) genome using a synthetic promoter-intron combination to allow self-complementary (sc) packaging of the HEXM gene. Also, a previously published capsid mutant, AAV9.47, was used to deliver the gene to brain and spinal cord while having restricted biodistribution to the liver. The novel capsid and cassette design combination was characterized in vivo in TSD mice for its ability to efficiently transduce cells in the central nervous system when delivered intravenously in both adult and neonatal mice. This study demonstrates that the modified HexM is capable of degrading long-standing GM2 storage in mice, and it further demonstrates the potential of this novel scAAV vector design to facilitate widespread distribution of the HEXM gene or potentially other similar-sized genes to the nervous system.

Animal models of GM2 gangliosidosis: utility and limitations

GM2 gangliosidosis, a subset of lysosomal storage disorders, is caused by a deficiency of the glycohydrolase, β-N-acetylhexosaminidase, and includes the closely related Tay–Sachs and Sandhoff diseases. The enzyme deficiency prevents the normal, stepwise degradation of ganglioside, which accumulates unchecked within the cellular lysosome, particularly in neurons. As a result, individuals with GM2 gangliosidosis experience progressive neurological diseases including motor deficits, progressive weakness and hypotonia, decreased responsiveness, vision deterioration, and seizures. Mice and cats are well-established animal models for Sandhoff disease, whereas Jacob sheep are the only known laboratory animal model of Tay–Sachs disease to exhibit clinical symptoms. Since the human diseases are relatively rare, animal models are indispensable tools for further study of pathogenesis and for development of potential treatments. Though no effective treatments for gangliosidoses currently exist, animal models have been used to test promising experimental therapies. Herein, the utility and limitations of gangliosidosis animal models and how they have contributed to the development of potential new treatments are described.

Gangliosides and gangliosidoses: principles of molecular and metabolic pathogenesis

Gangliosides are the main glycolipids of neuronal plasma membranes. Their surface patterns are generated by coordinated processes, involving biosynthetic pathways of the secretory compartments, catabolic steps of the endolysosomal system, and intracellular trafficking. Inherited defects in ganglioside biosynthesis causing fatal neurodegenerative diseases have been described so far almost exclusively in mouse models, whereas inherited defects in ganglioside catabolism causing various clinical forms of GM1- and GM2-gangliosidoses have long been known. For digestion, gangliosides are endocytosed and reach intra-endosomal vesicles. At the level of late endosomes, they are depleted of membrane-stabilizing lipids like cholesterol and enriched with bis(monoacylglycero)phosphate (BMP). Lysosomal catabolism is catalyzed at acidic pH values by cationic sphingolipid activator proteins (SAPs), presenting lipids to their respective hydrolases, electrostatically attracted to the negatively charged surface of the luminal BMP-rich vesicles. Various inherited defects of ganglioside hydrolases, e.g., of β-galactosidase and β-hexosaminidases, and of GM2-activator protein, cause infantile (with tetraparesis, dementia, blindness) and different protracted clinical forms of GM1- and GM2-gangliosidoses. Mutations yielding proteins with small residual catabolic activities in the lysosome give rise to juvenile and adult clinical forms with a wide range of clinical symptomatology. Apart from patients' differences in their genetic background, clinical heterogeneity may be caused by rather diverse substrate specificities and functions of lysosomal hydrolases, multifunctional properties of SAPs, and the strong regulation of ganglioside catabolism by membrane lipids. Currently, there is no treatment available for neuronal ganglioside storage diseases. Therapeutic approaches in mouse models and patients with juvenile forms of gangliosidoses are discussed.

Intracisternal rSV40 administration provides effective pan-CNS transgene expression

Potential genetic treatments for many generalized central nervous system (CNS) diseases require transgene expression throughout the CNS. Using oxidant stress and apoptosis caused by HIV-1 envelope gp120 as a model, we studied pan-CNS neuroprotective gene delivery into the cisterna magna (CM). Recombinant SV40 vectors carrying Cu/Zn superoxide dismutase or glutathione peroxidase were injected into rat CMs following intraperitoneal administration of mannitol. Sustained transgene expression was seen in neurons throughout the CNS. On challenge, 8 weeks later with gp120 injected into the caudate putamen, significant neuroprotection was documented. Thus, intracisternal administration of antioxidant-carrying rSV40 vectors may be useful in treating widespread CNS diseases such as HIV-1-associated neurocognitive disorders characterized by oxidative stress.

Efficient CNS gene delivery by intravenous injection

We administered recombinant SV40-derived viral vectors (rSV40s) intravenously to mice with or without prior intraperitoneal injection of mannitol to deliver transgenes to the central nervous system (CNS). We detected transgene-expressing cells (mainly neurons) most prominently in the cortex and spinal cord; prior intraperitoneal mannitol injection increased CNS gene delivery tenfold. Intravenous injection of rSV40s, particularly with mannitol pretreatment, resulted in extensive expression of multiple transgenes throughout the CNS.

Targeted nonviral delivery vehicles to neural progenitor cells in the mouse subventricular zone

Targeted gene therapy can potentially minimize undesirable off-target toxicity due to specific delivery. Neuron-specific gene delivery in the central nervous system is challenging because neurons are non-dividing and also outnumbered by glial cells. One approach is to transfect dividing neural stem and progenitor cells (NSCs and NPCs, respectively). In this work, we demonstrate cell-specific gene delivery to NPCs in the brains of adult mice using a peptide-modified polymeric vector. Tet1, a 12-amino acid peptide which has been shown to bind specifically to neuronal cells, was utilized as a neuronal targeting ligand. The cationic polymer polyethylenimine (PEI) was covalently modified with polyethylene glycol (PEG) for in vivo salt stability and Tet1 for neuron targeting to yield a Tet1-PEG-PEI conjugate. When plasmid DNA encoding the reporter gene luciferase was complexed with Tet1-PEG-PEI and delivered in vivo via an injection into the lateral ventricle, Tet1-PEG-PEI complexes mediated increased luciferase expression levels in brain tissue when compared to unmodified PEI-PEG complexes. In addition, cells transfected by Tet1-PEG-PEI complexes were found to be exclusively adult NPCs whereas untargeted PEG-PEI complexes were found to transfect a heterogenous population of cells. Thus, we have demonstrated targeted, nonviral delivery of nucleic acids to adult NPCs using the Tet1 targeting ligand. These materials could potentially be used to deliver therapeutic genes for the treatment of neurodegenerative diseases.

Spontaneous appearance of Tay-Sachs disease in an animal model

Tay-Sachs disease (TSD) is a progressive neurodegenerative disorder due to an autosomal recessively inherited deficiency of beta-hexosaminidase A (Hex A). Deficiency of Hex A in TSD is caused by a defect of the alpha-subunit resulting from mutations of the HEXA gene. To date, there is no effective treatment for TSD. Animal models of genetic diseases, similar to those known to exist in humans, are valuable and essential research tools for the study of potentially effective therapies. However, there is no ideal animal model of TSD available for use in therapeutic trials. In the present study, we report an animal model (American flamingo; Phoenicopterus ruber) of TSD with Hex A deficiency occurring spontaneously in nature, with accumulation of G(M2)-ganglioside, deficiency of Hex A enzymatic activity, and a homozygous P469L mutation in exon 12 of the hexa gene. In addition, we have isolated the full-length cDNA sequence of the flamingo, which consists of 1581 nucleotides encoding a protein of 527 amino acids. Its coding sequence indicates approximately 71% identity at the nucleotide level and about 72.5% identity at the amino acid level with the encoding region of the human HEXA gene. This animal model, with many of the same features as TSD in humans, could represent a valuable resource for investigating therapy of TSD.

Early deficits in motor coordination and cognitive dysfunction in a mouse model of the neurodegenerative lysosomal storage disorder, Sandhoff disease

Mouse models of lysosomal storage diseases, including Sandhoff disease, are frequently employed to test therapies directed at the central nervous system. We backbred such mice and conducted a behavioral test battery which included sensorimotor and cognitive assessments. This is the first report of short-term memory deficits in a murine model of Sandhoff disease. We also document early onset of motor deficits using the balance beam test.

CNS-directed gene therapy for lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a group of inherited metabolic disorders usually caused by deficient activity of a single lysosomal enzyme. As most lysosomal enzymes are ubiquitously expressed, a deficiency in a single enzyme can affect multiple organ systems, including the central nervous system (CNS). At least 75% of all LSDs have a significant CNS component. Approaches such as bone marrow transplantation (BMT) or enzyme replacement therapy (ERT) can effectively treat the systemic disease associated with LSDs in some patients. However, CNS disease remains a major challenge. Gene therapy represents a promising approach for the treatment of CNS disease as it has the potential to provide a permanent source of the deficient enzyme. Direct intracranial injection of viral gene transfer vectors has resulted in reduced lysosomal storage and functional improvement in certain small (rodent) and large (canine and feline) animal models of LSDs. The addition of protein transduction domains (PTDs) to the recombinant enzymes increased the distribution of enzyme and the extent of correction. Therapeutic levels of lysosomal enzymes can also be delivered to distant sites in the brain by anterograde and retrograde axonal transport. Finally, combining disparate approaches such as BMT and CNS-directed gene therapy can increase treatment efficacy in LSDs with severe CNS disease that are refractory to more conventional approaches.

CONCLUSION

The development of gene transfer vectors that mediate persistent expression in vivo, the addition of PTDs, a better understanding of lysosomal enzyme trafficking and combining different therapies provide hope that the CNS component of LSDs can be effectively treated.

Protecting neurons from HIV-1 gp120-induced oxidant stress using both localized intracerebral and generalized intraventricular administration of antioxidant enzymes delivered by SV40-derived vectors

Human immunodeficiency virus-1 (HIV-1) is the most frequent cause of dementia in adults under 40. We sought to use gene delivery to protect from HIV-1-related neuron loss. Because HIV-1 envelope (Env) gp120 elicits oxidant stress and apoptosis in cultured neurons, we established reproducible parameters of Env-mediated neurotoxicity in vivo, then tested neuroprotection using gene delivery of antioxidant enzymes. We injected 100-500 ng mul(-1)gp120 stereotaxically into rat caudate-putamens (CP) and assayed brains for apoptosis by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) 6-h to 14-day post-injection. Peak apoptosis occurred 1 day after injection of 250 and 500 ng microl(-1)gp120. TUNEL-positive cells mostly expressed neuronal markers (NeuroTrace), although some expressed CD68 and so were most likely microglial cells. Finally, we compared neuroprotection from gp120-induced apoptosis provided by localized and generalized intra-central nervous system (CNS) gene delivery. Recombinant SV40 vectors carrying Cu/Zn superoxide dismutase (SOD1) or glutathione peroxidase (GPx1) were injected into the CP, where gp120 was administered 4-24 weeks later. Alternatively, we inoculated the vector into the lateral ventricle (LV), with or without prior intraperitoneal (i.p.) administration of mannitol. Intracerebral injection of SV(SOD1) or SV(GPx1) significantly protected neurons from gp120-induced apoptosis throughout the 24-week study. Intraventricular vector administration protected from gp120 neurotoxicity comparably, particularly if preceded by mannitol i.p. Thus, HIV-1 gp120 is neurotoxic in vivo, and intracerebral or intra-ventricular administration of rSV40 vectors carrying antioxidant enzymes is neuroprotective. These findings suggest the potential utility of both localized and widespread gene delivery in treating neuroAIDS and other CNS diseases characterized by excessive oxidative stress.

Dilation and degradation of the brain extracellular matrix enhances penetration of infused polymer nanoparticles

This study investigates methods of manipulating the brain extracellular matrix (ECM) to enhance the penetration of nanoparticle drug carriers in convection-enhanced delivery (CED). A probe was fabricated with two independent microfluidic channels to infuse, either simultaneously or sequentially, nanoparticles and ECM-modifying agents. Infusions were performed in the striatum of the normal rat brain. Monodisperse polystyrene particles with a diameter of 54 nm were used as a model nanoparticle system. Because the size of these particles is comparable to the effective pore size of the ECM, their transport may be significantly hindered compared with the transport of low molecular weight molecules. To enhance the transport of the infused nanoparticles, we attempted to increase the effective pore size of the ECM by two methods: dilating the extracellular space and degrading selected constituents of the ECM. Two methods of dilating the extracellular space were investigated: co-infusion of nanoparticles and a hyperosmolar solution of mannitol, and pre-infusion of an isotonic buffer solution followed by infusion of nanoparticles. These treatments resulted in an increase in the nanoparticle distribution volume of 51% and 123%, respectively. To degrade hyaluronan, a primary structural component of the brain ECM, a pre-infusion of hyaluronidase (20,000 U/mL) was followed after 30 min by infusion of nanoparticles. This treatment resulted in an increase in the nanoparticle distribution of 64%. Our results suggest that both dilation and enzymatic digestion can be incorporated into CED protocols to enhance nanoparticle penetration.

Optimization of adenoviral production in human embryonic kidney cells using response surface methodology

Adenoviruses are the most commonly used vectors in clinical trials for gene therapy. How to efficiently produce abundant and high-quality adenoviral vectors for therapeutic research is a challenge for biochemical engineers. A recombinant adenovirus carrying a green fluorescent protein (GFP) gene together with an anchorage-dependent 293 cell line is used as a model system for evaluating the effects of chemicals on adenoviral production in this study. Our aim is to develop a formulation to be added to an infection medium that could enhance the in vitro production of adenoviral vectors. Eleven ingredients obtained from a literature survey were screened for their stimulatory effects on adenoviral production using the 50% tissue culture infectious dose (TCID(50)) method. Among these ingredients, sucrose and mannitol when supplemented to the infection medium significantly increased adenovirus titer. Central composite design and response surface methodology were also adopted to determine the optimal concentrations of sucrose and mannitol. The formulation developed, which is composed of DMEM/F12 medium plus 0.54 M sucrose and 0.37 M mannitol, can significantly increase adenoviral production by 13-fold that of the control (DMEM/F12 medium).

Sphingolipid metabolism diseases

Human diseases caused by alterations in the metabolism of sphingolipids or glycosphingolipids are mainly disorders of the degradation of these compounds. The sphingolipidoses are a group of monogenic inherited diseases caused by defects in the system of lysosomal sphingolipid degradation, with subsequent accumulation of non-degradable storage material in one or more organs. Most sphingolipidoses are associated with high mortality. Both, the ratio of substrate influx into the lysosomes and the reduced degradative capacity can be addressed by therapeutic approaches. In addition to symptomatic treatments, the current strategies for restoration of the reduced substrate degradation within the lysosome are enzyme replacement therapy (ERT), cell-mediated therapy (CMT) including bone marrow transplantation (BMT) and cell-mediated "cross correction", gene therapy, and enzyme-enhancement therapy with chemical chaperones. The reduction of substrate influx into the lysosomes can be achieved by substrate reduction therapy. Patients suffering from the attenuated form (type 1) of Gaucher disease and from Fabry disease have been successfully treated with ERT.

Effective gene therapy in an authentic model of Tay-Sachs-related diseases

Tay-Sachs disease is a prototypic neurodegenerative disease. Lysosomal storage of GM2 ganglioside in Tay-Sachs and the related disorder, Sandhoff disease, is caused by deficiency of beta-hexosaminidase A, a heterodimeric protein. Tay-Sachs-related diseases (GM2 gangliosidoses) are incurable, but gene therapy has the potential for widespread correction of the underlying lysosomal defect by means of the secretion-recapture cellular pathway for enzymatic complementation. Sandhoff mice, lacking the beta-subunit of hexosaminidase, manifest many signs of classical human Tay-Sachs disease and, with an acute course, die before 20 weeks of age. We treated Sandhoff mice by stereotaxic intracranial inoculation of recombinant adeno-associated viral vectors encoding the complementing human beta-hexosaminidase alpha and beta subunit genes and elements, including an HIV tat sequence, to enhance protein expression and distribution. Animals survived for >1 year with sustained, widespread, and abundant enzyme delivery in the nervous system. Onset of the disease was delayed with preservation of motor function; inflammation and GM2 ganglioside storage in the brain and spinal cord was reduced. Gene delivery of beta-hexosaminidase A by using adeno-associated viral vectors has realistic potential for treating the human Tay-Sachs-related diseases.

Gene therapy for lysosomal storage diseases

Lysosomal storage diseases (LSDs) comprise a diverse group of monogenetic disorders with complex clinical phenotypes that include both systemic and central nervous system pathologies. In recent years, the identification or development of mouse models recapitulating the clinical course of the LSDs has been instrumental in evaluating therapeutic strategies. Here, we review the various gene replacement strategies for target organs affected in many LSDs and describe briefly the various vector systems employed to test how best to accomplish long-lasting therapies for these fatal disorders.

Storage solutions: treating lysosomal disorders of the brain

Many neurodegenerative diseases are characterized by the accumulation of undegradable molecules in cells or at extracellular sites in the brain. One such family of diseases is the lysosomal storage disorders, which result from defects in various aspects of lysosomal function. Until recently, there was little prospect of treating storage diseases involving the CNS. However, recent progress has been made in understanding these conditions and in translating the findings into experimental therapies. We review the developments in this field and discuss the similarities in pathological features between these diseases and some more common neurodegenerative disorders.