There is a lot of evidence to show that inflammation accompanies damage in neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis and Huntington’s disease. The nervous system has its own unique immune cell type, call microglia. Microglia provide the first line of defense against infection, tumors and injury in the central nervous system (CNS: brain, spinal cord and optic nerve). These cells become activated in response to any of these insults, producing inflammatory substances called enzymes, cytokines and chemokines. It is also clear that microglia are activated when any cell is transplanted into the CNS such as would occur in stem cell transplant. Although microglia activation is a normal response to injury or disease, it is thought to have a negative effect on the transplants in the brain.
We plan to study the involvement of microglia in the survival of stem cell transplants into the CNS of mice with a disease similar to Parkinson’s disease. We want to see if stopping activation of microglia will allow long-term survival of stem cells transplanted into the CNS. Three approaches will be used, the first will involve a type of mouse that lacks the ability to activate microglia, while the second will involve treatment of diseased mice with two drugs that are currently FDA approved to treat other conditions, but which are known to also stop microglial activation. We will follow mice that are treated with these drugs and see how much longer transplanted cells survive compared to mice that did not receive any treatment. We will also see if the transplanted cells are still able to function normally in the treated mice and see if they are now able to help repair the damaged brain.
The use of the two drugs that are already used in humans to treat other symptoms will allow us to easily translate the results of these experiments to patients with neurodegenerative disease. Thus, this project is not only unique, but is especially relevant to the treatment of human disease in the CNS.
Statement of Benefit to California:
The State of California has made a critical investment in the future of medicine by pioneering efforts to support and conduct stem cell research for repair and regeneration in many human diseases. The first funding initiatives were directed to invest in training of scientists for stem cell research, defining basic science priorities and establishing core facilities to support stem cell research efforts. Recently, CIRM has focused on translational approaches to move research into the clinic, a move that will benefit patients in California as well as the financial future of the state.
We have concentrated on regenerative medicine in the nervous system, developing techniques to create reliable progenitor neural cells that can be used as transplants to promote repair in neurological diseases. Because all neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis, Huntington’s disease and multiple sclerosis involve inflammation and changes in immune responsiveness in the nervous system, we have focused on developing an understanding of the interaction of the immune response and neural stem cells.
In order to use stem cells in the treatment of these diseases one has to address the possibility that stem cell transplants in the central nervous system are likely to be rejected. Our team has experience in degenerative diseases of the nervous system, stem cell biology, transplantation, and of particular importance expertise in the study of immune responses in the CNS.
The number of individuals with neurodegenerative diseases in the United States and the State of California is staggering. In addition, our population is aging, which means that the incidence of neurodegenerative diseases is likely to increase ten-fold over the next ten years. The human suffering and the financial burden associated with these diseases is also staggering. Costs to both the state and federal government are likely to run into the billions. The state of California, as the most populous state with an enormously high indigent population, is now, and will continue to be disproportionately affected by both the human and financial burden of neurological disease. Efforts to reduce this burden by developing viable strategies for stem cell transplants in the nervous systems are greatly needed.
This proposal focuses on the role of microglia, the resident immune cells of the central nervous system, in the survival of stem cell transplants in a mouse model of Parkinson’s disease (PD). The applicant hypothesizes that the rejection of cells transplanted into the brain is a result of microglial activation, and that blocking this activation could promote long-term survival of grafted cells. The proposal encompasses three specific aims. In Aim 1, the applicant will use transgenic mice in which microglia can be selectively killed to assess the role of these cells in the survival of human embryonic stem cell (hESC)-derived neural stem cell (NSC) transplants. PD-like neurodegeneration will be induced in these mice prior to transplantation so that the benefits of NSC transplantation can be assessed. In Aims 2 & 3, the applicant proposes to test two drugs, approved for use in humans, for their ability to prolong the survival of NSCs following transplantation in a mouse model of PD.
Reviewers agreed that achieving a better understanding of the immune response in the central nervous system would have a major impact on possible cell transplantation strategies for neurodegenerative diseases. This proposal makes an attempt to understand the role of microglia using what some reviewers considered to be an innovative approach. They commented that the role of the immune system in rejecting stem cell transplants in the brain is understudied, and appreciated the creative use of a transgenic mouse model to address the question. Reviewers also agreed that understanding the role of microglia in the elimination of injected allogeneic tissue in the brain could be of great importance for stem cell transplantation, and could provide insights into why some human PD patients have shown benefit from cell transplants while others have not.
In spite of its strengths and potential impact, the review panel had reservations about the rationale and feasibility, particularly for the immunology aspects of the research proposal. Most importantly, they saw no convincing evidence that the proposed transgenic mouse model would allow for adequate elimination of activated microglia as stated. Reviewers noted that the strategy will only work if the targeted cells are proliferating, but that is not characteristic of mature microglia. Without evidence that this microglial ablation could nevertheless be achieved in this model, reviewers doubted the project’s potential for success. In addition, some members of the review panel were concerned that the ablation strategy is not specific to microglia, and that peripheral monocytes and macrophages would also be affected. The applicants acknowledged these peripheral effects of the proposed ablation approach, including the general hematopoietic toxicity, and present a plan to address these concerns, but reviewers were still not convinced that the overall strategy would work. The review panel was also concerned that the ablative agent might not cross the blood brain barrier, or if it does it could have unexpected effects in the brain. Finally, reviewers commented that Aims 2 & 3 lacked critical discussions, and needed a more detailed justification for the selection of drugs, and a presentation of alternative plans should the proposed experiments fail. Reviewers felt that these problems diminished the project’s feasibility.
With regard to the neurobiological aspects, reviewers had a few minor criticisms. They noted that the applicant proposes to only assess histological and biochemical outcomes in the animal model of PD - the inclusion of behavioral analysis would have strengthened the proposal. They also noted that dopamine neuron survival may not be the best outcome measure, as a recent study in a large animal model demonstrated behavioral improvement with a limited number of surviving dopamine neurons. Furthermore it may be instructive to perform stereology for glia, which could be involved in mediating these positive effects. Finally, reviewers did not think that the 90-day time point chosen to assess long-term survival of grafted cells is long enough to be relevant to human disease, and recommended that these experiments be carried out longer. These criticisms of the research plan were considered minor, however. Reviewers commented that the rationale and experimental design were strong and well-justified.
Reviewers described the principal investigator (PI) as a highly experienced and productive neurologist with an excellent publication record. They appreciated the contributions of the two co-investigators, who provide significant immunology expertise. Reviewers noted that the research team has little published expertise with hESCs, but the preliminary data is reassuring in this respect. In general, reviewers found the assembled team to be well suited to carry out the proposed research.
Overall, while reviewers appreciated that this proposal is innovative and addresses a significant aspect of the field, they questioned its feasibility, mostly because of their skepticism about the transgenic model of selective ablation.
A motion was made to move this application into Tier 1, Recommended for Funding. One reviewer pointed out that the proposal scored the second highest among three addressing PD, and is uniquely focused on an important issue of long-term rejection of stem cell transplants. This reviewer particularly appreciated the preliminary data demonstrating that the brain is not immune-privileged and that transplanted cells are eventually rejected. Although this reviewer commented that the PI and research team were very strong and could get the project done, other reviewers noted that the immunological aspects of the proposal are weak. The majority of the review panel agreed that the strategy for microglial ablation in the mouse model was flawed and seriously impacted the project’s feasibility. The motion to move this project into Tier 1 was opposed by a majority of the Grants Working Group. Because the motion was supported by more than 35% of members, supporters have exercised their right to have that position reported to the ICOC.
The minority position highlighted the importance of studying immune rejection in the brain in order to advance stem cell therapy approaches for the treatment of diseases such as Parkinson’s. It acknowledged the concerns about the feasibility of the project’s experimental approach, but proposed that the research team has the expertise to address these concerns in a productive way that could advance the field. The minority report thus recommends funding the project in spite of the risk that key experiments may prove unfeasible.