While stem cells have always been heralded as the future of cellular replacement therapies, recent stem cell research has explored the potential “bystander” or “paramedic” effects of stem cells, which use stem cells to repair damaged cells rather than replacing them. Bystander therapies do not require the stem cells to become the type of cell that is damaged (to differentiate into this cell type), but rather can help damaged neurons by changing the host environment. The phenotype of the undifferentiated, stem cell that are still pluripotent, able to differentiate into many different cell types, may provide therapeutic benefits in Huntington’s Disease (HD) by releasing neurotrophic factors that promote neuron growth and survival and arrest the mutant huntingtin protein’s negative influence on key cellular survival and energetic pathways. This paramedic function of stem cells might be harnessed to prevent mechanisms of HD that cause harmful symptoms rather than replacing damaged cells, as an alternative approach to traditional drug therapy for the treatment of neurodegenerative diseases.
Researchers in South Korea have recently found that adipose-derived stem cells (ASCs) can serve the same “paramedic” function in HD as is observed in the mesenchymal stem cells that Dr. Nolta is currently researching at the University of California, Davis. Similar to mesenchymal stem cells, adipose-derived stem cells do not create the ethical debates that embryonic stem cells do, as they are removed from adults during elective surgery, not from embryos in vitro. But in contrast to mesenchymal stem cells, ASCs are not found naturally in the body, but are rather multipotent stem cells created by iPS, or induced pluripotent stem cells (for more information about the new technology of iPS, click here). Derived from fat tissue taken from consenting patients, ASCs have the double advantage of easy access and minimal ethical implications. Like all stem cells, ASCs have the ability to differentiate into different somatic cells, though the mechanisms of differentiation are still unclear and scientists do not know how to direct differentiation of ASCs into certain types of tissue. This study was not concerned with the differentiation of ASCs, but rather their neuroprotective abilities, such as the release of growth factors that are essential in combating many of the symptoms of HD.
How the Bystander Mechanism Works^
Current research using HD mouse models indicates that the use of fetal striatal tissue to replace the damaged striatal tissue in HD mouse models is not possible at this time, as the replacement tissue does not alter the toxicity of the mutant huntingtin protein. This is like replacing a wall that has been eaten away by termites without doing anything to remove the termites: the new wall will soon also be harmed by the pests. Even if this barrier was overcome, the use of stem cells to replace damaged tissue is hampered by many other problems, such as a lack of donor tissue and rejection by the immune system, identical problems to organ replacement. Rather than replacing damaged tissue, a different approach could use stem cells to preemptively prevent HD from harming neuronal tissue. Scientists have noted that with other diseases, stem cells often act through a bystander mechanism, preventing the symptoms of the disease from manifesting, rather than directly replacing damaged cells. This novel approach to stem cell use does not require the extensive technology of differentiation, transplantation, and incorporation with host tissue that cell replacement needs. Instead, the bystander mechanism approach of stem cells takes advantage of the cells’ ability to release factors in their pluripotent state that combat the symptoms of HD. The researchers in this study wished to test this mechanism using ASCs, knowing that the cells could not only differentiate into cells such as neurons and glial cells that could be useful in combating HD, but could also release growth factors that may slow the symptoms of HD (to learn about growth factors and their role in fighting HD, click here.
HD symptoms may be caused by the alterations the mutant huntingtin protein makes to the transcription of DNA by interfering with transcription factors (for more information about the HD’s affect on transcription, click here) Transcriptional interference and mitochondrial dysfunction are key aspects of HD pathology. Mutant huntingtin protein aggregates impede important transcription factors such as the CREB-binding protein which is essential for transcription of pathways key to cell survival. In HD, one way neural cell death is induced is by inhibiting this transcription factor. ASCs could slow neurodegeneration and neural death by releasing neurotrophic factors that help prevent premature cell death. Neurotrophic factors encourage the maintenance, growth, and survival of neurons and so serve as a counterforce to mutant huntingtin’s stimulation of cell death. Another transcription factor, PGC-1, controls the creation of mitochondria, and when it is repressed by mutant huntingtin, reduced numbers of mitochondria causes the cell to receive insufficient energy. This makes the cell susceptible to glutamate, an excitatory neurotransmitter that can harm neurons and may be involved in many of the symptoms of HD. ASCs increase PGC-1 expression by preventing the huntingtin protein from impeding its transcriptional regulation, and so prevent glutamate levels from becoming harmfully high in the neuron.
Effects of ASCs in Three Different Models^
The effect of ASC transplantation was tested in three models, a knockout rat model, a transgenic mouse model and an in vitro cell model (for more information about different types of experimental models, click here). All models showed the positive effects of ASCs. The rat model showed that the ASCs were neuroprotective, reducing neuronal death. More specifically, this was observed by comparing the size of the rats’ ventricles of the brain, which are enlarged by the loss of neural tissue in HD. The ventricles of the ASC-treated rats were smaller than those of the control HD model rats, indicating less tissue loss. Ventricle volume is closely correlated with HD progression, as larger ventricles indicate greater loss of neural tissue. By showing a decreased ventricle volume, ASC rats seem to exhibit less cell death, resulting in less loss of neural tissue. In addition, compared to the HD control mice, the mouse model with ASC transplantation had a delayed decline in motor function, had less mutant huntingtin aggregates, and lived longer.
Potential Problems with ASCs for Clinical Use^
The promising results of the three models hint at the potential of ASCs to delay the onset of HD symptoms, but there is much that still needs to be researched. It was found that the transplanted ASCs were not evenly distributed across the striatum as expected, but rather most ASCs remained at the initial site of transplantation. This may have impeded the effectiveness of the ASCs and may even be harmful in long-term models. The proliferation of the ASCs was slow once transplanted, so the documented ability of stem cells to proliferate in actual organisms does not seem to apply to ASC transplantation in HD models. This study did not test whether the same results could be achieved in model organisms that are longer lived. Whether ASCs would be effective in humans, for extended periods of time, has not yet been determined. Furthermore, the ASCs must be prevented from differentiating into cells that would be harmful in the brain. Currently, the research for directed differentiation of pluripotent cells is rudimentary, and there is particular risk associated with the spontaneous differentiation of ASCs in vivo. The potential risk of the cells to differentiate into other tissue like heart or skin cells should be tested. Another problem of stem cells is that they may be rejected by the patient’s immune system, though this problem is greatly reduced with ASCs. In this study, human ASCs were successfully injected into rat and mouse models without the aid of immunosuppressants, which is encouraging. To further prevent the risk of rejection, the patient’s own adipose cells could be used to create the ASCs. But ASCs derived from patients with the mutant huntingtin protein have yet to be tested and it is possible that these cells may be damaged or not fully effective.
Researchers have shown that ASCs may have the potential to protect mechanisms of transcription and rescue degenerating neurons by combating the detrimental actions of huntingtin aggregates through the release of growth factors. This bystander mechanism is a novel approach to using stem cells, as they have been traditionally thought of as replacement cells for damaged tissue. The value of stem cells to replace damaged tissue with healthy, fully-differentiated replacement cells cannot be dismissed, especially with new iPS technology and the ability to engineer replacement cells from the patient who is to receive them, reducing the risk of immune rejection. Unfortunately, much more research must be done before stem cells will be used in clinical therapies for cellular replacement, but a more immediate potential for stem cells is in a paramedic capacity, where differentiation and incorporation with the host tissue is not required. By influencing key events in the pathogenesis of HD, ASCs may delay the onset of harmful symptoms. Current research has shown that ASC transplantation may allow for the expression of transcription pathways that HD suppresses, reduce the number of toxic huntingtin aggregates, and decrease the extent of neuron death in mouse models. Its therapeutic use still requires much more research and exploration, and then must make the leap from animal models to human trials, but ASCs have the potential to rescue degenerating neurons and prevent HD symptoms. This ability of stem cells to not only replace damaged tissue, but also prevent tissue damage, holds promise for the treatment of HD.
For Further Reading
A. Lanctot 2011