Primary adult human astrocytes as an ex vivo vehicle for beta-glucuronidase delivery in the brain

Structure-specific patterns of neural stem cell engraftment after transplantation in the adult mouse brain

Transplantation of neural stem cells (NSCs) may be useful for delivering exogenous gene products to the diseased CNS. When NSCs are transplanted into the developing mouse brain, they can migrate extensively and differentiate into cells appropriate to the sites of engraftment, in response to the normal signals directing endogenous cells to their appropriate fates. Much of the prior work on NSC migration in the adult brain has examined directed migration within or toward focal areas of injury such as ischemia, brain tumors, or 6-hydroxydopamine (6-OHDA) lesions. However, treatment of many genetic disorders that affect the CNS will require widespread dissemination of the donor cells in the postnatal brain, because the lesions are typically distributed globally. We therefore tested the ability of NSCs to migrate in the unlesioned adult mouse brain after stereotaxic transplantation into several structures including the cortex and hippocampus. NSC engraftment was monitored in live animals by magnetic resonance imaging (MRI) after superparamagnetic iron oxide (SPIO) labeling of cells. Histological studies demonstrated that the cells engrafted in significantly different patterns within different regions of the brain. In the cerebral cortex, donor cells migrated in all directions from the injection site. The cells maintained an immature phenotype and cortical migration was enhanced by trypsin treatment of the cells, indicating a role for cell surface proteins. In the hippocampus, overall cell survival and migration were lower but there was evidence of neuronal differentiation. In the thalamus, the transplanted cells remained in a consolidated mass at the site of injection. These variations in pattern of engraftment should be taken into account when designing treatment approaches in nonlesion models of neurologic disease.

Improved gene delivery into neuroglial cells using a fiber-modified adenovirus vector

One impediment to treating neuronal diseases is finding ways to introduce genes into specific neuroglial cell types. Here we describe the strategy for efficient gene delivery via transferrin receptor using an adenovirus bearing a peptide mimic for transferrin. The attachment of the peptide consisted of 12 amino acids on the C-terminus of adenovirus fiber protein significantly improved entry and expression of a beta-galactosidase transgene into neuroglial cells such as astrocytes, and Schwann cells. The entry of re-targeted viruses into cells depends on the attached peptide and the transferrin receptor. Furthermore, transferrin did not affect gene delivery by the engineered adenovirus, suggesting that the effectiveness of therapeutic agents targeted to the receptor would not be diminished by competition with the abundant endogenous transferrin present in the plasma. Therefore, such transduction systems hold promise for efficient delivering gene to neuroglial cells in gene therapy protocols.

In vitro characterization of genetically modified embryonic stem cells as a therapy for murine mucopolysaccharidosis type IIIA

The mucopolysaccharidoses (MPS) are lysosomal storage disorders resulting from the impaired catabolism of glycosaminoglycans (GAG). MPS type IIIA patients have dysfunctional sulfamidase enzyme leading to lysosomal storage of the GAG heparan sulfate, severe neurological symptoms including regression in learning, behavioural abnormalities, and premature death. We have engineered mouse D3 embryonic stem (ES) cells to over-express recombinant human sulfamidase. Human sulfamidase was correctly folded and secreted 2h post-labelling as determined by immunoprecipitation and SDS-PAGE analysis of transfected ES cells. Secreted human sulfamidase present in conditioned ES cell media was able to be taken up via mannose-6-phosphate-mediated endocytosis and restored sulfamidase enzyme activity in human MPS IIIA fibroblast cell lines. ES cells underwent directed differentiation to neural precursor populations and were capable of sustained human sulfamidase over-expression at all stages. Additionally, transfected and control cells were proliferative (Ki67+) and expressed several neural markers (nestin, MAP-2, and NF160) as determined by immunofluorescence. These findings suggest the possibility of ES cell-based therapy for the treatment of neurological pathology of MPS IIIA.

Gene transfer strategies for correction of lysosomal storage disorders

Lysosomal storage diseases (LSDs) represent a large group of monogenic disorders of metabolism, which affect approximately 1 in 5000 live births. LSDs result from a single or multiple deficiency of specific lysosomal hydrolases, the enzymes responsible for the luminal catabolization of macromolecular substrates. The consequent accumulation of undigested metabolites in lysosomes leads to polysystemic dysfunction, including progressive neurologic deterioration, mental retardation, visceromegaly, blindness, and early death. In general, the residual amount of functional enzyme in lysosomes determines the severity and age at onset of the clinical symptoms, implying that even modest increases in enzyme activity might affect a cure. A key feature on which therapy for LSDs is based is the ability of soluble enzyme precursors to be secreted by one cell type and reinternalize by neighboring cells via receptor-mediated endocytosis and routed to lysosomes, where they function normally. In principle, somatic gene therapy could be the preferred treatment for LSDs if the patient's own cells could be genetically modified in vitro or in vivo to constitutively express high levels of the correcting enzyme and become the source of the enzyme in the patient. Both ex vivo and in vivo gene transfer methods have been experimented with for gene therapy of lysosomal disorders. Several of these methods have proved efficient for the transfer of genetic material into deficient cells in culture and reconstitution of enzyme activity. However, the same methods applied to humans or animal models have been giving inconsistent results, the bases of which are not fully understood. A broader knowledge of disease pathogenesis, facilitated by available, faithful animal models of LSDs, coupled to the development of better gene transfer systems as well as the understanding of vector host interactions will make somatic gene therapy for these devastating and complex diseases the most suitable therapeutic approach.

Intracerebral transplantation and successful integration of astrocytes following genetic modification with a high-capacity adenoviral vector

To investigate the ability of genetically modified astrocytes to integrate into adult rat brain, two spontaneously immortalized cell lines and the allogenic nontumorigenic glioma cell line F98 were transduced with a high-capacity adenoviral vector (HC-Adv) expressing the EGFP gene from the hCMV promoter. In organotypic slice cultures the transduced astrocytes were shown to integrate into the brain tissue. Following transplantation of the transduced astrocytes into the striatum of adult rats, the transplanted cells survived at least for 6 weeks, continuously expressed the EGFP transgene, in close neighborhood with cells of the recipient tissue executing their differentiation capacity along the glial lineage. Thus, HC-Adv transduced astrocytes are promising vehicles to locally deliver therapeutic proteins for the treatment of neurodegenerative diseases.

Transplantation of human adult astrocytes: efficiency and safety requirements for an autologous gene therapy

Ex vivo gene therapy is emerging as a promising approach for the treatment of neurodegenerative diseases and central nervous system (CNS) trauma. We have shown previously that human adult astrocytes can be expanded in vitro and can express various therapeutic transgenes (Ridet et al. [1999] Hum. Gene Ther. 10:271-280; Serguera et al. [ 2001] Mol. Ther. 3:875-881). Here, we grafted normal and lentivirally-modified human adult astrocytes into the striatum and spinal cord of nude mice to test whether they are suitable candidates for ex vivo CNS gene therapy. Transplanted cells survived for at least 2 months (longest time analyzed) and sustained transgene expression. Importantly, the absence of proliferating cell nuclear antigen (PCNA) staining, a hallmark of cell division, ascertains the safety of these cells. Thus, adult human astrocytes are a promising tool for human CNS repair; they may make autologous ex vivo gene transfer feasible, thereby avoiding the problems of immunological rejection and the side effects of immunosuppressors.

Lysosomal storage disorders: diagnostic dilemmas and prospects for therapy

PURPOSE

The main purpose of this review is to address some concerns regarding the accurate and timely diagnosis of lysosomal storage disorders (LSD).

METHODS

Using their experience in diagnosing LSD in more than 2500 individuals, the authors highlight several diagnostic difficulties and solutions and review the latest methods for early diagnosis and treatment.

RESULTS

While "classic" patients can be accurately diagnosed using relatively simple methods in an experienced laboratory, atypical patients require more detailed studies. With a few exceptions, almost all LSD can be diagnosed in leukocytes or plasma. Methods for screening all newborns without a family history of a LSD have been proposed, but such screening may require a large amount of effort for little gain.

CONCLUSIONS

With effective therapy becoming available for some LSD, early diagnosis is critically important. If the goal is to prevent serious complications related to the nervous and skeletal systems, earlier diagnosis is potentially advantageous. Accurate prognosis and assessing the need for aggressive therapy in newly diagnosed patients are problems that need further study.

Distribution of a lysosomal enzyme in the adult brain by axonal transport and by cells of the rostral migratory stream

A portion of the lysosomal enzymes produced by cells is secreted, diffuses through extracellular spaces, and can be taken up by distal cells via mannose-6-phosphate receptor-mediated endocytosis. This provides the basis for treating lysosomal storage diseases, many of which affect the CNS. Normal enzyme secreted from a cluster of genetically corrected cells has been shown to reverse storage lesions in a zone of surrounding brain tissue in mouse disease models. However, low levels of enzyme activity and reduction of storage lesions also have been observed at sites in the brain that may not be explained by a contiguous gradient of secreted enzyme diffusing away from the genetically corrected cells. No direct evidence for alternative mechanisms of enzyme transport has been shown, and little is understood about the intracellular movement of lysosomal enzymes in neurons. We investigated whether axonal transport could occur, by expressing an eukaryotic lysosomal enzyme that can be visualized in tissue sections (beta-glucuronidase) in brain structures that have defined axonal connections to other structures. This resulted in the transfer of enzyme to, and a reversal of storage lesions in, neurons that project to the gene expression site, but not in nearby structures that would have been corrected if the effect had been mediated by diffusion. In addition, transduction of cells in the subventricular zone resulted in the uptake of beta-glucuronidase by cells entering the rostral migratory stream. Gene transfer to specific neuronal circuits or cells in migratory pathways may facilitate delivery to the global brain lesions found in these disorders.

Gene therapy for the central nervous system in the lysosomal storage disorders

Although great promise has been made in the field of gene therapy, a number of difficulties must be solved before successful human studies can be completed. These issues involve safety, immunological reactions to the vectors and their transgene products, persistent transgene expression, and ability to repeat administrations of the vector safely. A major hurdle that must be overcome is the ubiquitous delivery of the transgene throughout the nervous system. Significant gene delivery to the CNS of murine models of LSD has been accomplished, but we await the successful treatment of the nervous system in a larger mammalian model of LSD. As yet there is no perfect vector that can solve all of these problems. It is likely that vector technology will evolve into hybrid vectors also using synthetic components that will increase safety and efficacy of recombinant vectors. The treatment of the CNS remains complicated, but progress is being made in this area. Clinical trials already planned will give us increasing information as to the ideal gene therapy for the CNS.

Long-term expression of beta-glucuronidase by genetically modified human neural progenitor cells grafted into the mouse central nervous system

Mucopolysaccharidosis type VII (MPS VII) is an inherited disease caused by beta-glucuronidase (beta-glu) deficiency. This deficiency results in the lysosomal accumulation of glycosaminoglycans in all tissues and affects a wide range of organs, including the central nervous system (CNS). Gene transfer is a promising approach to therapy for MPS VII because it allows extensive delivery of the enzyme to the affected tissues. We studied neurotransplantation of primary human cells to supply beta-glucuronidase to the CNS. Human neural progenitor cells (HNPC) were amplified and cotransduced with two lentiviral vectors, one encoding the green fluorescent protein and the other the human beta-glu. We show that these cells strongly expressed both transgenes in culture. When grafted into the mouse striatum, HNPC differentiated into neurons and astrocytes and expressed the two transgenes for at least 6 months. This study therefore paves the way for the treatment of MPS VII by long-term delivery of the appropriate enzyme.

Ex vivo and in vitro studies of transgene expression in rat astrocytes transduced with lentiviral vectors

Implantation of cells genetically modified to express therapeutic genes into the brain has been proposed as a potential treatment for neurodegenerative diseases. In the current study embryonic rat-derived astrocytes were cultured and transduced with a lentiviral vector expressing the reporter gene green fluorescent protein (GFP) and subsequently grafted into the adult rat brain. The proportion of GFP expressing cells was stable, albeit small (1%), at all survival times, up to 6 weeks, the longest time point studied. In parallel in vitro studies, the astrocytes were lentivirally transduced to express either one of the two isoforms of glutamate decarboxylase (GAD(65) or GAD(67)) or glial cell line-derived neurotrophic factor (GDNF). When transducing 293T cells with the two GAD vectors, released GABA could be measured using high-performance liquid chromatography. Further studies of rat astrocytes transduced with the same vectors resulted in a level of GAD activity about 10 times higher than the activity of an intact rat striatum. One hundred thousand astrocytes transduced with LV-GDNF released approximately 27 ng of GDNF per hour. Thus, taken together, our observations provide support for the use of rat astrocytes in ex vivo gene transfer of these proteins in animal models of CNS disorders, e.g., Parkinson's disease or epilepsy.