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In a mouse model of Alzheimer disease (AD), researchers have successfully transferred a gene to deliver a growth factor that not only enhanced new neurons in the hippocampus that grew into maturity and integrated into surrounding networks, but also appeared to reverse memory deficits in symptomatic mice and helped clear amyloid beta (Abeta) plaques.
ADENO-ASSOCIATED VIRUS (AAV) gene transfer: Intracranial injection of AAV 2/1 hybrid expresses green fluorescent protein (GFP) mainly in the dentate gyrus and partly in CA3 hippocampal regions of mouse brain. In the upper panel, the projection of phase contrast and GFP images are shown. The lower panel shows the in situ fluorescence signal of GFP in the dentate gyrus and CA3 regions.
Treated animals maintained better functional memory in maze tests than their untreated counterparts, while those with more advanced symptoms showed gradually improved performance.
In a Dec. 6 paper in the Proceedings of the National Academy of Sciences, researchers at Boston University Medical Center and the University of Nebraska Medical Center in Omaha reported that they were able to deliver the fibroblast growth factor 2 (FGF2) gene bilaterally to the hippocampi of APP+presenilin-1 bigenic mice via an adeno-associated virus serotype 2/1 hybrid (AAV2/1-FGF2).
Once injected bilaterally into the hippocampus, the growth factor helped spur neuronal growth in damaged areas and resulted in memory improvements —measured by the ability of the mice to navigate the mazes. Improvements were observed after one month and continued for three to four months, said the study's principal investigator Tsuneya Ikezu, MD, PhD, professor of pharmacology and experimental therapeutics and neurology in the Laboratory of Molecular Neurotherapeutics at the Boston University School of Medicine, in a telephone interview. The study's first author was Tomomi Kiyota, PhD, an instructor in the University of Nebraska Medical Center department of pharmacology and experimental neurosciences.
“Our key finding is that promoting neurogenesis with this method can correct brain function before and after symptoms appear,” Dr. Ikezu said. “I believe this sheds an important light on whether neurogenesis can correct symptoms of AD as well as slow the development of symptoms in patients with early diseases.”
In addition to AD, strategies to enhance neurogenesis have been a promising research avenue for a number of years as potential treatments for other neurodegenerative conditions including Huntington and Parkinson diseases, he said.
Examination of hippocampal tissue, harvested after four months, revealed lower total levels of Abeta and fewer dense plaques in symptomatic mice, indicating that the growth factor activated microglial cells to partially clear unwanted debris and protein accumulations from the brain.
FGF2 occurs early in embryonic development where it supports self-renewal of nascent stem cells and helps them to become different cell types by encouraging growth of nascent neural stem cells and inhibiting differentiation. But as a potential treatment the factor has been problematic because it appears to prevent full maturation of neural cells. To get around this obstacle, the researchers used laboratory methods to enhance the promotional isoforms of the factor so that they would override those isoforms that limit differentiation and prevent newborn cells from reaching maturity.
DR. DANIEL PETERSON: “Is neurogenesis the cause or a secondary effect that results in these positive changes? This is very difficult to document, especially because it may be due to stimulating other processes in the hippocampus.”
In the study, the researchers used markers to track the development of immature neurons and proliferation. In older animals with AD symptoms, their analysis suggested that FGF2 also promoted synaptic gene expression and long-term potentiation in the hippocampus, a key brain region for learning and memory.
“This study is a positive finding for therapeutics and points us in a useful direction if we can restore neurogenesis,” said Daniel A. Peterson, PhD, associate professor of neuroscience and executive director of the Laboratory for Stem Cell and Regenerative Medicine at Rosalind Franklin University of Medicine & Science in Chicago.
But in this study, Dr. Peterson pointed out, the investigators did not show the effect in non-AD mice. That would have stimulated neurogenesis to a much higher level and made their results appears less impressive by comparison.”
“I feel the findings are solid, but using growth factor stimulation is not an entirely new approach to neurogenesis,” he told Neurology Today in a telephone interview. “When looking at therapy, gene delivery to the central nervous system is always an issue.”
According to Dr. Peterson, trials involving gene transfer technology to promote neurogenesis for other neurological diseases always face the same question: if it is possible to achieve a beneficial effect, can it be sustained for the long-term? “Before any of these techniques are ready for clinical use we need to make sure to optimize delivery, which may require use of a different vector. Do we have to repeat the procedure again in five years?”
He also emphasized the necessity of incorporating a regulatory on-off mechanism in case there are downstream problems with treatment. “This will be essential before we jump from bench to bedside.”
As it is, it remains unclear if AAV1/2 will provide a delivery mechanism with a wide enough distribution, he said, noting that other candidates include AAV5 and AAV Rh10.
“The human hippocampus is much larger than in mice, and we will need to scale treatment with this in mind. So testing in larger animals and eventually in non-human primates will be necessary. Generally, when approaching the Food and Drug Administration for approval of a clinical trial, all of these things will have to be determined, so there are a couple of things that must still be worked out.”
There is the question of whether the reported improvements were due to neurogenesis or Abeta clearance, he noted.
“Is neurogenesis the cause or a secondary effect that results in these positive changes? This is very difficult to document, especially because it may be due to stimulating other processes in the hippocampus.”
As it stands, there is still a gray area that requires better understanding of all of these possible contributing factors, Dr. Peterson said. “I would, however, consider this a very viable approach — right up there with other strategies being considered.”
FGF2 is known to be important for the maintenance of adult neurogenesis within the hippocampus, noted Oliver von Bohlen und Halbach, PhD, professor of anatomy and cell biology at the Institute of Anatomy & Cell Biology at the University of Greifswald in Germany. However in a recent study, he and his colleagues found that FGF2 appears to operate synergistically with other mechanisms and/or growth factors in the hippocampus.
In an e-mail to Neurology Today, he explained that research has also demonstrated that FGF2 may play a prominent role in mental disorders such as depression. Studies have shown that FGF2 and FGF receptor 1 mRNA expression is altered in the postmortem hippocampus of people who suffered from depression, he noted.
“Little is known about its potential role for Alzheimer disease,” he said. “CSF levels of FGF2 in patients with the disease seems to be unaltered, and there have only been suggestions, or hints, that endogenous FGF2 is not altered as well, so we have not been able to draw any conclusions about whether exogenous FGF2 might have positive effects.”
However the new study provides evidence that exogenous applied FGF2 might be helpful in Alzheimer disease, according to Dr. von Bohlen und Halbach.
“The authors have not only demonstrated that neurogenesis is enhanced and that treated animals performed better in a hippocampus-dependent learning task, but that clearance of fibrillar amyloid-beta peptide was partially enhanced,” he said.
DR. TSUNEYA IKEZU: “Our key finding is that promoting neurogenesis with this method can correct brain function before and after symptoms appear. I believe this sheds an important light on whether neurogenesis can correct symptoms of AD as well as slow the development of symptoms in patients with early diseases.”
“Further investigations are needed to get a deeper insight in any beneficial effects of virus-based application of FGF2, and mechanisms that may contribute to these effects. In the study's animal model, the effectiveness of FGF2 has been demonstrated, and this raised the hope that FGF2 treatment could also have beneficial effects in treatment of humans with Alzheimer disease.”
Werner S, Unsicker K, von Bohlen und Halbach O. Fibroblast growth factor-2 deficiency causes defects in adults hippocampal neurogenesis, which are not rescued by exogenous fibroblast growth factor-2. 2011;89:1605-1617.