Research Update
Authors: Robert Frederickson
Journal: Molecular Therapy (2002)

Parkinson's disease is a neurodegenerative disorder resulting from the death of dopamine-producing neurons in the substantia nigra. The result is an inbalance of excitatory and inhibitory signals that disrupts motor activity–leading to tremor, rigidity, and other symptoms. In a report appearing in the October 11, 2002, issue of Science, Matt During's group has shown that delivery of a gene expressing an enzyme involved in neurotransmitter synthesis can protect these neurons from further damage in a rat model of the disease by establishing a better balance between the opposing signals controlling movement. The authors created recombinant AAV vectors expressing glutamic acid decarboxylase (GAD), an enzyme that synthesizes the inhibitory neurotransmitter, GABA. These vectors were injected into the the brain, where subsequent immunohistochemistry revealed robust expression of the enzyme. Stimulation of the region of the brain that received the GAD-producing vectors revealed a clear release of GABA that was not seen in control rats. Moreover, expression of GAD protected dopaminergic neurons in the injected region from an excitatory neurotoxic insult and rescued the animals from a parkinsonian behavioral phenotype. The results suggest that delivery of a single gene product is sufficient to have marked effects on neuronal phenotype and underline the apparent plasticity of the mammalian nervous system, providing hope that future clinical strategies based on the approach might work in human patients. Science Online, doi:10:1126/science.1074549 The ability of transplanted neural stem cells to replace dead or damaged brain tissue has been described in several reports over the past decade, and has been promoted as a promising strategy to treat neurodegenerative disease. In another study focusing on the rescue of damaged dopaminergic neurons, Evan Snyder's group at Harvard has shown, in a report in the November 2002 issue of Nature Biotechnology, that transplanted stem cells seem to express substances that can also stabilize and protect the existing damaged neurons in vivo. The authors asked whether the stem cells are effective against the slow degeneration of brain function in mice treated with chemicals to mimic the process of aging or Parkinson's disease in people. Consistent with earlier studies, conversion of donor stem cells to a dopaminergic phenotype did contribute to recovery in the mice. Surprisingly, however, most of the recovered neurons were derived from the host. The transplanted cells thus appeared to rescue the damaged host cells, stimulating them to produce more of the enzymes essential for dopamine activity. In a second study appearing in the same issue of Nature Biotechnology, workers in the Snyder lab demonstrate the successful use of neural stem cells to treat a different type of brain injury–that characterized by extensive tissue loss, such as occurs in cerebral palsy or in stroke. The authors delivered neural stem cells into large cavities in the brains of mice that had suffered severe ischemic injury. Although delivery of the cells alone was unable to rebuild the lost tissue, the cells’ regenerative ability was much improved when they were seeded before transplantation into a biodegradable scaffold. As the scaffold degraded, the cavity filled with new, vascularized brain tissue, and numerous interconnections formed between the host brain and the transplanted stem cells. In addition, inflammation and scarring were significantly reduced. Taken together, the studies highlight the importance of the local environment to the repair of brain tissue using neuronal stem cells. Nature Biotechnology, doi:10.1038/nbt750 and doi:10.1038/nbt751 β-Thalassemia is an important genetic disease, characterized by a deficiency of the β-chain of hemoglobin. The disease results in abnormal maturation and eventual destruction of oxygen-carrying blood cells. In a new study in the October 15, 2002, issue of the Proceedings of the National Academy of Sciences USA, a research group based in Canada delivered the β-globin gene in mouse model using lentiviral vectors that achieved virtually complete transduction of hematopoietic stem cells and correction of the disorder in animals. A cure for the disease using genetically modified hematopoietic stem cells is currently being pursued by several groups. Use of retroviral vectors has given rise to problems of deleterious processing of the therapeutic gene and low titers of the viral vector. Lentiviral vectors provide a promising alternative to retroviral vectors for this application–but have come with their own set of problems. Lentiviral-based gene therapies for hematopoietic disorders have been hampered by low transduction efficiencies and position effect variegation of proviruses integrated at a single copy per transduced genome. The new study, led by Keith Humphrey, demonstrated an average of three integrations per transduced genome and achieved titers of the recombinant virus that led to transduction of 95% of the cells in the graft–providing a foundation for the initiation of clinical trials in human patients. Proc. Natl. Acad. Sci. USA Online, doi:10.1073/pnas.212507099 Gene expression profiling is a powerful way of characterizing distinct populations of cells at the molecular level. In the October 7, 2002, issue of the Proceedings of the National Academy of Sciences USA, Suslov et al. explore the transcriptional patterns that arise during the growth and differentiation of clonal populations of cultured neural stem cells. Neurospheres are clonal structures derived from the culture of neural stem cells that contain neuronal and glial cells in different stages of differentiation. Using fresh specimens from human brain, the authors generated neurosphere clones from which they prepared individual cDNA libraries. These libraries were then used to profile the expression of a select set of differentially expressed transcripts, by a combination of repetitive rounds of subtractive hybridization and microarray analysis. Gene expression in the clonal progeny was het-erogenous, and reflected different states of neural commitment–likely based on the state of the parental clone-forming cells. The authors divided the clones into four different clusters, providing a model that they contend can be used to elucidate the molecular phenotype of neural stem cell populations. Proc. Natl. Acad. Sci. USA Online, doi:10:1073/pnas.212507099 Anew report that takes a close look at CD8+ cells in HIV-infected individuals whose immune systems seem to have contained the virus may provide clues as to how to create better therapeutic vaccines. In the November 2002 issue of Nature Immunology, new research suggests that it is not the number of these cells that allows these so-called “long-term nonprogressors” to keep the virus in check, but rather differences in their activity. “Cytotoxic” CD8+ cells seek out and attack virus-infected cells. Mark Connors and colleagues found that the blood of nonprogressors contained numbers of CD8+ cells similar to those present in patients in whom the virus actively replicated. What was different, however, was that the nonprogressors’ cells were better able to proliferate when challenged with infected cells. Interestingly, CD8+ cells in these individuals also expressed higher levels of a protein called perforin, an important component of the immunological armamentarium used to kill virus-infected cells. According to Connors, the finding is particularly relevant to efforts to develop an HIV vaccine, as most current strategies target the CD8+ population in an attempt to induce greater cell-mediated immunity. A better understanding of how to avoid–or even reverse–factors that damage the function of CD8+ cells would be a boon to the development of therapeutic vaccines against this and other viruses. Nat. Immunol. Online, doi:10.1038/ni845 Research Update written by Robert Frederickson