Abstract
Huntington’s disease (HD) is a fatal autosomal-dominant neurodegenerative disease that affects 1 in every 10,000 individuals in the United States. HD is caused by massive brain neurodegeneration mainly due to the loss of neurons in the striatum. There is currently no cure for HD with limited treatment available. HD is caused by a trinucleotide CAG repeat expansion of the huntingtin gene (HTT), which results in the mutated huntingtin protein (HTT). Although the exact cellular function of HTT is still not fully understood, researchers have found it to be essential in embryonic development and in controlling levels of brain-derived neurotrophic factor (BDNF). BDNF is important for the survival of striatal neurons as well as the activity of the cortico-striatal synapses. A previous study done in our lab found that delivery of BDNF to the striatum of mice through genetically modified human mesenchymal stem cells (hMSCs) led to a decrease in striatal atrophy, an increase in neurogenesis, and an increased lifespan in specific HD mouse models. This study used immunosuppressive drugs to dampen the immune response from HD mice, allowing the human MSCs to survive. However, these immune suppression drugs have neurotoxic effects on the mice, making the drugs inefficient in studying human stem cell-based therapies for HD. This has led our lab to develop novel immune deficient HD mouse models for two commonly used lines, the YAC128 and R6/2 mice. Observation of these novel models revealed that these mice were surviving longer than the YAC128 and R6/2 mice, leading our lab to launch a more extensive study to characterize the models. Our primary goal for this project was to evaluate and compare the behavior and histology of the immune deficient HD mice (YAC-NSG and R6/2-NSG) to the immune competent mice and begin to investigate the role of the immune system in the disease. This study first used an in vivo imaging system to evaluate how long the immune deficient lines were able to retain the human MSCs in comparison to a commonly used immune deficient line, the NSG mouse. We found that our novel immune deficient strains were able to retain human MSCs for the same length of time as the NSG mice, indicating they are functioning as immune deficient mice. We next characterized the behavior and histology of the YAC-NSG and R6/2-NSG mice and compared them to the immune competent models. Behavioral assays included the open field assay and the rotarod assay. We found that the YAC-NSG are beginning to display mild behavioral deficits compared to the YAC128 parental strain, although more data points are required to make further conclusions. The R6/2-NSG mice are showing no significant behavioral deficits when compared to the R6/2 parental strain. We used immunohistochemistry to stain for HTT aggregates in the striatum. Our results showed that the R6/2-NSG mice did not have HTT aggregates. The staining on the YAC-NSG brains was inconclusive, as our positive control YAC128 brains had no positive staining for aggregates. We were also interested in what would occur when the immune deficient strains became immune competent, more specifically with a human immune system, and how that would affect their behavior. We therefore used human hematopoietic stem cells (HSCs) to humanize the mice with a human immune system and ran open field and rotarod behavioral assays. We found that the humanized group also may be beginning to show behavioral deficits, but again more data points are necessary. This study suggests that the novel immune deficient mice could be good models for human stem-cell based therapies and that the immune system may play an important role in the pathology of HD.